HYDROGEN STORAGE COMPOSITIONS, METHODS, AND USES THEREOF CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to co-pending U.S. Provisional Patent Application No. 62/701,344, filed on July 20, 2018, entitled“Renewable Energy Storage Via Efficient Reversible Hydrogenation of Piperidine Captured C02,” the contents of which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant no. 1748579 awarded by the National Science Foundation. The government has certain rights in the invention.

TECHNICAL FIELD

The subject matter disclosed herein is generally directed to hydrogen storage reactions, devices and uses thereof.

BACKGROUND

Many environmental and political forces are driving a shift from fossil resources to renewable energy sources.

Hydrogen (¾) has the highest gravimetric energy density of any fuel, 120 MJ/kg (LHV). However, on a volumetric basis, the energy density of the gaseous ¾ is extremely low at ambient temperature and pressure. Hence, the current state-of-the art ¾ storage units utilize either compressed gaseous hydrogen (700 bar, ca. 40 g H2/liter) or liquefied hydrogen (20 K, ca. 71 g H2/liter), which requires costly gas compression and/or cryogenic operations. Bulk storage and transport of hydrogen is of critical importance for onboard fuel cell electric vehicles (FCEV), as well as other off-board uses of hydrogen. In particular, a large market could be created for backing up surplus electricity from intermittent renewable resources including solar, wind, etc. through a regenerative hydrogen fuel cell (RHFC) system, which converts electricity to ¾, stores the ¾, and is later fed into PEM fuel cell to re-generate electric power.

Alternatively, chemical hydrogen storage which involves storing hydrogen in the form of chemical bonds, could be a safe substitute to physical hydrogen storage. The chemical hydrogen carriers can be stored from daily to seasonal duration and be transported to a long distance of hundreds of kilometers. In particular, liquid hydrogen carriers (LHCs) can be used to transport hydrogen using current infrastructure with minor modifications. The LHCs such as N-ethylcarb azole, alcohols, ammonia, toluene, and formic acid (FA) have received significant interest given the potential to be recycled. However, the formation of the start-of- the art carriers and the release of the hydrogen are not energy-efficient due to the high energy penalty associated with either uptake or release of hydrogen. From the thermodynamic point of view, the challenge is that most LHCs only favor half of the cycle.

In short and although promising, the storage of renewable energy is the major hurdle during the transition of fossil resources to renewables. As such there exists a need for compositions, methods, devices, and techniques for improved renewable energy storage.

SUMMARY

Described herein are aspects of a method including the steps of:

(a) reacting CO2 with an amine or ammonia to form an adduct comprising captured CO2; and

(b) hydrogenating the captured CO2 in the presence of a hydrogenation solvent to form formates.

In some aspects, the method further includes the step of:

(c) dehydrogenating the formates at the surface of a catalyst to generate ¾.

In some aspects, ammonia is used. In some aspects, the amine is selected from the group of: monoethanolamine, diethanolamine, triethanolamine, ethane- 1,2-diamine, 2-amino-2- methyl-1 -propanol, 2-amino-2 -methyl-1, 3, -propanediol, 2-amino-2 -hydroxymethyl -1,3- propanediol, piperazine, piperidine, 1 -methyl piperazine, 2-methyl piperazine, and combinations thereof. In some aspects, the amine is piperidine.

In some aspects, the formate formed in step (b) is a formate-piperidine adduct.

In some aspects, the hydrogenation solvent includes water, acetonitrile, or tetrahydrofuran. In some aspects, the hydrogenation solvent includes an alcohol. In some aspects, the alcohol is methanol, ethanol, propanol (e.g. 1-propanol or 2-propanol), or butanol. In some aspects, the alcohol is present at 0 percent to 100 percent by volume or wt % of the hydrogenation solvent. In some aspects, the water, acetonitrile, or tetrahydrofuran is present at 0 to 100 percent by volume or wt % of the hydrogenation solution. In some aspects, the water, acetonitrile, or tetrahydrofuran is present at 0 to 50 percent by volume or wt % of the hydrogenation solution. In some aspects, the hydrogenation solvent is composed only of alcohol. In some aspects, the hydrogenation solvent contains only water, acetonitrile, or tetrahydrofuran (i.e. is 100% water, acetonitrile, or tetrahydrofuran). In some aspects, step (c) is performed in a dehydrogenation solvent. In some aspects, the dehydrogenation solvent is the same as the hydrogenation solvent. In some aspects, the dehydrogenation solvent includes water, acetonitrile, or tetrahydrofuran. In some aspects, the dehydrogenation solvent includes an alcohol. In some aspects, the alcohol is methanol, ethanol, propanol (e.g. 1-propanol or 2-propanol), or butanol. In some aspects, the alcohol is present at 0 percent to 100 percent by volume or wt % of the hydrogenation solvent. In some aspects, the water, acetonitrile, or tetrahydrofuran is present at 0 to 100 percent by volume or wt % of the dehydrogenation solution. In some aspects, the water, acetonitrile, or tetrahydrofuran is present at 0 to 50 percent by volume or wt % of the hydrogenation solution. In some aspects, the de hydrogenation solvent is composed only of alcohol. In some aspects, the dehydrogenation solvent contains only water, acetonitrile, or tetrahydrofuran (i.e. is 100% water, acetonitrile, or tetrahydrofuran).

In some aspects, the reaction temperature of any one of steps (a), (b), (c), steps (a) and

(b), steps (a) and (c), or steps (b) and (c) ranges from about 20 degrees C to about 100 degrees C. In some aspects, the reaction temperature of step (a), step (b), or both ranges from about 20 degrees C to about 40 degrees C. In some aspects, the reaction temperature of step (a), step (b), or both is about 30 degrees C. In some aspects, the reaction temperature of step (c) ranges from about 60 degrees C to about 100 degrees C. In some aspects, the reaction temperature of step

(c) ranges from about 80 degrees C to about 90 degrees C. In some aspects, the catalyst is a homogenous catalyst. In some aspects, the catalyst is a heterogenous catalyst. In some aspects, the catalyst is composed of a metal selected from the group of: Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Rm, Yb, Lu, Hf, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, Ra, Ac, Th, Pa, U, Np, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, FI, Me, Lv, and combinations thereof. In some aspects, the catalyst is composed of a metal selected from the group of: Ni, Ru, Rh, Pd, Cu, Fe, Co, Ag, Au, Pt, and combinations thereof.

In some aspects, the catalyst is part of a hydrogen generation system. In some aspects, the hydrogen generation system provides high-purity hydrogen gas to a fuel cell.

Also described herein are aspects of a liquid hydrogen carrier that can be composed of formates comprising hydrogenated CO2, wherein the formate is formed via a method described herein. Also described herein are aspects of a liquid hydrogen carrier that can be composed of a formate-piperidine adduct having hydrogenated CO2. In some aspects, the formate-piperidine adduct is formed via a method as described herein.

Also described herein are aspects of a hydrogen capture and/or generation system configured to perform a method as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings.

FIG. 1 shows an Arrhenius plot of the hydrogenation of bicarbonate and ethyl carbonate in the presence of piperidine with 5 wt % Pd/ AC in pure water and ethanol solvents, respectively. The reaction rates at different temperatures are shown in FIGS. 7A-7B,† ESI. Reaction conditions: 1 M piperidine-captured CO2 in water or ethanol solution, 400 psi H2, 1.0 g of Pd/AC.

FIG. 2 shows a graph that can demonstrate the effect of different temperatures on the H2 releasing rate from the dehydrogenation of formate piperidine adducts. Reaction conditions: 0.1 g Pd/ AC catalyst, 1 atm initial pressure of N2, 1 M formate piperidine adducts, 20 mL aqueous solvent with 70% EtOH.

FIG. 3 shows a graph that can demonstrate the effect of different bases on the EE releasing rate from the dehydrogenation of formate piperidine adducts. Reaction conditions: 1 M formic acid mixed with 1 M of varied bases, 20 mL aqueous solutions with 70% EtOH, 0.1 g Pd/ AC catalyst, 1 atm initial pressure of N2, 100 °C.

FIG. 4 shows a graph that can demonstrate ATR-FTIR spectra recorded during the dehydrogenation of formate with MEA and piperidine, respectively, at 55 °C.

FIG. 5 shows a graph that can demonstrate results of a stability test of the Pd/ AC catalyst for 5 cycles of hydrogenation-dehydrogenation. Hydrogenation of PIPD-CO2: 70% 2- propanol, 0.1 g Pd/AC, 30 °C, 1 hour; dehydrogenation of PIPD-formic acid: 70% 2-propanol, 0.1 g Pd/ AC, 100 °C, 30 min. The spent Pd/AC catalyst was reused without regeneration.

FIG. 6 shows ¹³ C NMR spectra of CO2 derived ionic species intermediates after CO2 was captured with piperidine in water, 70% Ethanol, and 100% ethanol, respectively. CO2 capture conditions: 20 mL ethanol/water solvent, 1 M piperidine, 20 degrees C, 40 min. D8 is 1,4-dioxane as an internal standard. FIGS. 7A-7B show graphs that can demonstrate hydrogenation of piperidine captured CO2 in water (FIG. 7A), and ethanol (FIG. 7B). Reaction conditions: 0.1 g Pd/ AC catalyst, 400 psi hydrogen pressure, 20 -30 degrees C, 20 mL 1 M piperidine captured CO2 solution in parr reactor.

FIG. 8 shows an Arrhenius plot of the dehydrogenation of formate piperidine adducts over 5 wt % Pd/ AC in aqueous ethanol solutions. Reaction conditions: 0.1 g Pd/ AC catalyst, 1 atm initial pressure of N2, 1 M formate piperidine adducts, 20 ml aqueous solvent with 70% EtOH. In k was calculated by dehydrogenation rate in 5 mins.

FIG. 9 shows a graph that can demonstrate a dehydrogenation of formate piperidine adducts in water, ethanol and mixed water/ethanol solvents. Reaction conditions: 0.1 g Pd/ AC catalyst, 1 atm initial N2 pressure, 80 degrees C, 1 M formate piperidine adducts, 20 mL solvent.

FIGS. 10A-10B show graphs that can demonstrate dehydrogenation of formate piperidine adducts in aqueous solvent with (FIG. 10A) 70% of 1 -propanol and (FIG. 10B) 70% of 2-propanol. Reaction conditions: 0.1 g Pd/ AC catalyst, 1 atm initial pressure of N2, 1 M formate piperidine adducts, 20 mL solvent.

FIG. 11 shows a GC-TCD chromatogram of the gaseous products from the dehydrogenation of formate piperidine adduct in aqueous solvent with 70% EtOH. Reaction conditions: 0.1 g Pd/ AC, 100°C, 1 atm of initial pressure of N2. Note the peak at 4.387 min is an injection peak.

FIG. 12 shows a graph that can demonstrate the effect of piperidine concentration on H2 releasing rate from dehydrogenation of formate piperidine adducts. Reaction conditions: 1 M formic acid mixed with varying amounts of piperidine, 20 mL aqueous solutions with 70% EtOH, 0.1 g Pd/ AC catalyst, 1 atm initial pressure of N2, 40 degrees C.

FIG. 13 shows in-Situ ATR-FTIR spectra of Pd/ AC with flowing CO (pink lines). Procedure: 5% Pd/ AC was coated on the window of ATR, and then 20 mL/min of 10% CO/He flew through the ATR cell. The temperature increased from 25 degrees C to 55 degrees C, stayed for 1 hour, and then introduced the N2 flow at 20 mL/min.

FIG. 14 shows in-Situ ATR-FTIR spectra of adsorbed species on Pd surface during the dehydrogenation of piperidine formate adduct. 5% Pd/ AC was coated on the window of ATR, then the reactant solution (1 M piperidine formate adduct in aqueous ethanol solution with 70% EtOH) was added. FTIR spectra were recorded during the decomposition of formate at 55 °C. The negative peaks at 1589, 1375 and 1346 cm ¹ which are partially due to the intermediates still remain, this may be a joint result from consumption of formate and also certain amounts of intermediates were taken as the background at the beginning of the experiment.

FIG. 15 shows in-situ ATR-FTIR spectra of adsorbed species on Pd surface during the dehydrogenation of formate monoethanolamine adduct. 5% Pd/ AC was coated on the window of ATR, and then reactant solution (1 M monoethanolamine formate adduct in aqueous ethanol solution with 70% EtOH) was added. FTIR spectra were recorded during the decomposition of formate at 55 °C. The negative peaks at 1589, 1375 and 1346 cm ¹ which are partially due to the intermediates still remain, this may be a joint result from consumption of formate and also certain amounts of intermediates were taken as the background at the beginning of the experiment.

FIG. 16 shows ¹³ C NMR spectra of CO2 derived intermediates after capturing CO2 with PIPD, hydrogenation of PIPD-CO2 and dehydrogenation of PIPD-Formic acid. CO2 capture: 20 °C, 1 M PIPD in aqueous solvent with 70% EtOH. Hydrogenation: 20 °C, 30 mins, 0.1 g Pd/ AC. Dehydrogenation: 0.1 g Pd/AC, 100 °C, 40 mins.

FIG. 17 shows reaction Scheme I, which is a mechanism of the formation of piperidine-carbamate by capturing CO2 with piperidine and the subsequent conversion of piperidine-carbamate to the corresponding bicarbonate and ethyl carbonate salts in water and ethanol solvents, respectively.

FIG. 18 shows reaction Scheme SI, which shows (a) A general scheme of decarboxylation of formic acid on Pd surface (b) A proposed mechanism for dehydrogenation of piperidine formate adduct on Pd surface (c) A proposed mechanism for dehydrogenation of monoethanolamine formate adduct on Pd surface. In water solvent, R=H; in ethanol solvent,

R=CH CH ₂ .

FIG. 19 shows a graph that can demonstrate an ethanol co-solvent effect on the hydrogenation of ammonium bicarbonate and sodium bicarbonate. (Reaction conditions: 20 ml solvent, 0.5 M ammonium bicarbonate/sodium bicarbonate, 0.1 g Pd/ AC (5 wt%), 20°C, 2.75 MPa H ₂ ).

FIG. 20 shows C ¹³ NMR spectra of NH4HCO3 and NH4HCO2 before and after the hydrogenation reaction in ethanol solvent.

FIGS. 21A-21C show structures and graphs that can demonstrate the Effect of mass fraction of ethanol on the hydrogenation of various amine captured CO2. FIG. 21A. Different amines using in this study; FIG. 21B. Liner amines; FIG. 21C. Sterically hindered amines; FIG. 21D cyclic amines. CO2 capture conditions: 20 mL of 1 M amine water- ethanol solution, RT, 40 mins. Hydrogenation reaction conditions (Parr reactor): 100 mg Pd on carbon (5 wt%), 20 mL captured CCh-amine water-ethanol solution, 20 °C, 400 psi FL, 1 hour reaction time.

FIGS. 22A-22C show graphs that can demonstrate the effects of FL pressure (FIG. 22A), temperature (FIG. 22B), and time (FIG. 22C) on the hydrogenation of AMP captured CO2. Reaction conditions: FIG. 22A 1M AMP, 20 mL captured CO2 solution (95.6 wt% ethanol), 0.1 g Pd/ AC catalyst, 20°C, 1 h; FIG. 22B 1M AMP, 20 mL captured CO2 solution (95.6 wt% ethanol), 0.1 g Pd/ AC catalyst, 400 psi FL, 1 h; FIG. 22C 1M AMP, 20 mL captured CO2 solution (95.6 wt% ethanol), 0.1 g Pd/ AC catalyst, 30°C, 400 psi FL.

FIGS. 23A-23C can demonstrate the Formation of active intermediates due to the change of the solvent environment. FIG. 23A ¹³ C NMR spectra of different acitive intermediates in different solvents; FIG. 23B Distribution of carbonate, carbamate, bicarbonate, and ethyl carbonate before the hydrogenation of AMP captured CO2 as determined by ¹³ C NMR spectroscopy; FIG. 23C Proposed mechanism accounting for carbamate (1), ethyl carbonate (2) and bicarbonate (3) formation. CO2 capture conditions: 20 mL amine/water- ethanol, 1 M AMP, 20 °C, 40 min.

FIGS. 24A-24B can demonstrate mechanistic insight into the hydrogenation of the captured CO2 to formate. FIG. 24A In situ ATR-FTIR spectra of hydrogenation of AMP captured CO2 in ethanol; FIG. 24B Proposed pathways of hydrogenation of the bicarbonate and ethyl carbonate on Pd catalysts, R=H (in FLO) or CFLCFL (in ethanol), the steps include the insertion of bicarbonate or ethylcarbonate onto the metal surface to form the complex 1 , the hydrogenation of adsorbed complex 1 , generating the complex 2, and the final release of the formate.

FIGS. 25A-25B can demonstrate Theoretical adsorption sites and hydrogenation of the intermediate. FIG. 25A. Top and side view of favorable adsorption sites for (panel a) ethyl carbonate (panel b) formate (panle c) ethanol on Pd (111) at a coverage of 0.06 ML. FIG. 25B. Top and side views of ethyl carbonate hydrogenation to formate on Pd (111) through the Eley- Rideal reaction mechanism. Conditions are (panel a) in the absence of ethanol at a ethyl carbonate coverage of 0.06 ML, (panel b) in the presence of ethanol at a total surface coverage of 0.25 ML, and (panel c) in the presence of ethanol at a total surface coverage of 0.31 ML.

FIGS. 26A-26D show a summary of the techno-economic analysis. The plant which is assumed to be located in North America at a commercial scale of producing 12769.4 kg/h (102 kt/year) of Ca(HCOO)2. FIG. 26A, Breakdown of total capital cost. FIG. 26B, Breakdown of average operating cost per year. FIG. 26C, Net Present Value(NPV). FIG. 26D, Payout period sensitivity to CO2, Ca(CHOO)2 and ¾ prices.

FIG. 27 shows ¹³ C NMR spectroscopy of different acitive intermediates in the EDA captured CO2 solution. 1.9 g CO2 was captured by 20 mL of 1M EDA. EDA stands for ethylenediamine.

FIG. 28 shows ¹³ C NMR spectroscopy of EDA-carbamate. EDA carbamate solid was dissolved in water before the ¹³ C NMR mesurement, D (dioxane) as internal standard, E as ethanol. Only carbamate is formed in the pure ethanol solution.

FIG. 29 shows a hypothesized hydrogen-bonded structure for primary and secondary amines with hydroxyl group leading to decreased pKa but destabilized carbamate formation due to the sterically effect.

FIG. 30 shows a graph that can demonstrate the absorption rate of CO2 in amine (MEA, DEA, TEA, AMP, PZ)/water solution. (Solvent (H2O) 20 mL, concentration of amine is 1 M, capture temperature is 20 °C).

FIG. 31 shows a graph that can demonstrate the results from stability testing of the Pd/ AC catalyst under captured CO2 hydrogenation conditions (50 ml autoclave parr reactor, 1M AMP CO2 capture solution, 20 mL 95.6 wt% EtOH, 0.1 g Pd NPS on carbon (5 wt%), 400 psi EE , 20 °C, 1 h). The Pd NPS were washed with distilled water and dried under N2 at 40°C after each cycle.

FIG. 32 shows the XRD pattern of the Pd/ AC catalyst before (fresh catalyst) and after (spent catalyst) 5 times of hydrogenation reaction (reaction conditions were the same as with

FIG. 31)

FIGS. 33A-33B show TEM images of the Pd/ AC catalyst before (fresh catalyst) and after (spent catalyst) 5 cycle of AMP captured CO2 hydrogenation. CO2 capture conditions: 20 mL amine/water-ethanol (95.6 wt% EtOH) solution, 1 M AMP, 20 ^° C, 40 min. Reaction conditions: 50 mL autoclave parr reactor, 20 mL of 1 M AMP captured CO2 solution, 0.1 g Pd NPS on carbon (5 wt%), 400 psi ¾ , 20 °C, 1 h. The Pd NPS were washed with distilled water and dried under N2 at 40°C after each cycle.

FIG. 34 shows ¹ HNMR spectra of AMP after the hydrogenation. Reaction conditions: 20 mL of 1 M AMP aqueous solution, 0.1 g Pd NPS on carbon (5 wt%), 400 psi ¾.

FIG. 35 shows a graph that can demonstrate the effect of organic co-solvent on the formate yields after the hydrogenation of AMP captured CO2. CO2 capture conditions: 20 mL amine/water-cosolvent solution, 1 M AMP, 20 ^° C, 40 min. Reaction conditions: 50 mL autoclave parr reactor, 20 mL of 1 M AMP captured CO2 solution, 0.1 g Pd NPS on carbon (5 wt%), 400 psi H 2 , 20 °C, 1 h.

FIG. 36 shows spectra that can demonstrate the effect of co-solvent (methanol and water) on the distribution of carbonate, bicarbonate and methyl carbonate. CO2 capture conditions: 20 mL amine/water-methanol solution, 1 M AMP, 20 ^° C, 40 min.

FIG. 37 shows a graph that can demonstrate DRIFTS of the white solid formed when AMP captured CO2 in acetone. CO2 capture conditions: 20 mL amine/acetone solution, 1 M AMP, 20 ^° C, 40 min.

FIG. 38 shows a graph that can demonstrate Formate concentration change with time in water and anhydrous ethanol. Formate concentration as a function of time in the hydrogenation of AMP captured CO2 fit to first order kinetics ([formate]=[initial formate]- exp(-kt) [formate]. Reaction conditions: 0.1 g Pd NPs on carbon (5 wt%), 20 mL captured CO2 amine/water/ethanol solution, 20 °C, 400 psi FL, 10-50 min reaction time.

FIG. 39 shows a graph that can demonstrate the initial reaction rate of CO2 hydrogenation (ro) at t=0 can be plotted as functions of initial concentration of carbon dioxide. The fitting parameter k _eff , which is the effective rate constant of the reaction, was also extracted from the linear fitting ro=A¾7[C0 ₂ ]o. Reaction conditions: 0.1 g Pd NPs on carbon (5 wt%), 20 mL captured CO2 amine/water/ethanol solution, 20 °C, 400 psi FL.

FIG. 40 shows a graph that can demonstrate the calculation for determining the reaction activation energy. Reaction conditions: 0.1 g Pd Nps on carbon (5 wt%), 20 mL captured CO2 amine/water/ethanol solution, 20 °C, 400 psi FL.

FIG. 41 shows ¹³ C{ ¹ FT} NMR spectra for the reaction time effect on the distribution of intermediates and products after the CO2 hydrogenation reaction. CO2 capture conditions: 20 mL amine/water-ethanol (70 wt% EtOH) solution, 1 M AMP, 20 ^° C, 40 min. Reaction conditions: 50 mL autoclave parr reactor, 20 mL of 1 M AMP captured CO2 solution, 0.1 g Pd NPS on carbon (5 wt%), 400 psi FL , 20 °C.

FIG. 42 shows a process flow diagram for the production of calcium formate.

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is to be understood that this disclosure is not limited to particular embodiments described, and as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.

All publications and patents cited in this specification are cited to disclose and describe the methods and/or materials in connection with which the publications are cited. All such publications and patents are herein incorporated by references as if each individual publication or patent were specifically and individually indicated to be incorporated by reference. Such incorporation by reference is expressly limited to the methods and/or materials described in the cited publications and patents and does not extend to any lexicographical definitions from the cited publications and patents. Any lexicographical definition in the publications and patents cited that is not also expressly repeated in the instant application should not be treated as such and should not be read as defining any terms appearing in the accompanying claims. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present disclosure is not entitled to antedate such publication by virtue of prior disclosure. Further, the dates of publication provided could be different from the actual publication dates that may need to be independently confirmed.

As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure. Any recited method can be carried out in the order of events recited or in any other order that is logically possible.

Where a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g. the phrase“x to y” includes the range from‘x’ to‘y’ as well as the range greater than‘x’ and less than‘y’. The range can also be expressed as an upper limit, e.g.‘about x, y, z, or less’ and should be interpreted to include the specific ranges of‘about x’,‘about y’, and‘about z’ as well as the ranges of‘less than x’, less than y’, and‘less than z’ . Likewise, the phrase‘about x, y, z, or greater’ should be interpreted to include the specific ranges of‘about x’,‘about y’, and‘about z’ as well as the ranges of‘greater than x’, greater than y’, and‘greater than z’. In addition, the phrase“about‘x’ to‘y’”, where‘x’ and‘y’ are numerical values, includes“about‘x’ to about‘y’”.

It should be noted that ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as“about” that particular value in addition to the value itself. For example, if the value“10” is disclosed, then“about 10” is also disclosed. Ranges can be expressed herein as from“about” one particular value, and/or to“about” another particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. For example, if the value“about 10” is disclosed, then“10” is also disclosed.

It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub ranges (e.g., about 0.5% to about 1.1%; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.

As used in the specification and the appended claims, the singular forms“a,”“an,” and “the” include plural referents unless the context clearly dictates otherwise.

As used herein, "about," "approximately,"“substantially,” and the like, when used in connection with a numerical variable, can generally refers to the value of the variable and to all values of the variable that are within the experimental error (e.g., within the 95% confidence interval for the mean) or within +/- 10% of the indicated value, whichever is greater. As used herein, the terms“about,”“approximate,”“at or about,” and“substantially” can mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or“at or about” whether or not expressly stated to be such. It is understood that where“about,”“approximate,” or“at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, organic chemistry, chemical engineering, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence where this is logically possible unless the context clearly dictates otherwise.

Definitions

As used herein,“amino” and“amine,” are art-recognized and refer to both substituted and unsubstituted amines, e.g., a moiety that can be represented by the general formula: wherein, R, R', and R" each independently represent a hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted carbonyl,— (CH2) _m— R'", or R and R' taken together with the N atom to which they are attached complete a heterocycle having from 3 to 14 atoms in the ring structure; R'" represents a hydroxy group, substituted or unsubstituted carbonyl group, an aryl, a cycloalkyl ring, a cycloalkenyl ring, a heterocycle, or a polycycle; and m is zero or an integer ranging from 1 to 8. In preferred embodiments, only one of R and R' can be a carbonyl, e.g., R and R' together with the nitrogen do not form an imide. In preferred embodiments, R and R' (and optionally R") each independently represent a hydrogen atom, substituted or unsubstituted alkyl, a substituted or unsubstituted alkenyl, or— (0¾) _ih— R'". Thus, the term‘alkylamine’ as used herein refers to an amine group, as defined above, having a substituted or unsubstituted alkyl attached thereto (i.e. at least one of R, R', or R" is an alkyl group).

As used herein,“alkyl,” refers to the radical of saturated aliphatic groups, including straight-chain alkyl, alkenyl, or alkynyl groups, branched-chain alkyl, cycloalkyl (alicyclic), alkyl substituted cycloalkylgroups, and cycloalkyl substituted alkyl. In preferred embodiments, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone (e.g., Ci- C30 for straight chains, C3-C30 for branched chains), preferably 20 or fewer, more preferably 15 or fewer, most preferably 10 or fewer. Likewise, preferred cycloalkyls have from 3-10 carbon atoms in their ring structure, and more preferably have 5, 6 or 7 carbons in the ring structure. The term“alkyl” (or“lower alkyl”) as used throughout the specification, examples, and claims is intended to include both“unsubstituted alkyls” and“substituted alkyls,” the latter of which refers to alkyl moieties having one or more substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. Such substituents include, but are not limited to, halogen, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, a phosphinate, amino, amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, aralkyl, or an aromatic or heteroaromatic moiety.

Unless the number of carbons is otherwise specified,“lower alkyl” as used herein means an alkyl group, as defined above, but having from one to ten carbons, more preferably from one to six carbon atoms in its backbone structure. Likewise,“lower alkenyl” and“lower alkynyl” have similar chain lengths. Throughout the application, preferred alkyl groups are lower alkyls. In preferred embodiments, a substituent designated herein as alkyl is a lower alkyl.

“Alkyl” includes one or more substitutions at one or more carbon atoms of the hydrocarbon radical as well as heteroalkyls. Suitable substituents include, but are not limited to, halogens, such as fluorine, chlorine, bromine, or iodine; hydroxyl;— NRR', wherein R and R' are independently hydrogen, alkyl, or aryl, and wherein the nitrogen atom is optionally quaternized; — SR, wherein R is hydrogen, alkyl, or aryl; — CN; — NO2; — COOH; carboxylate;— COR,— COOR, or— CON(R) ₂ , wherein R is hydrogen, alkyl, or aryl; azide, aralkyl, alkoxyl, imino, phosphonate, phosphinate, silyl, ether, sulfonyl, sulfonamido, heterocyclyl, aromatic or heteroaromatic moieties, haloalkyl (such as— CF ₃ ,— CH _2— CF ₃ ,— CCI ₃ );— CN;— NCOCOCH ₂ CH ₂ ,— NCOCOCHCH;— NCS; and combinations thereof.

It will be understood by those skilled in the art that the moieties substituted on the hydrocarbon chain can themselves be substituted, if appropriate. For instance, the substituents of a substituted alkyl may include halogen, hydroxy, nitro, thiols, amino, azido, imino, amido, phosphoryl (including phosphonate and phosphinate), sulfonyl (including sulfate, sulfonamido, sulfamoyl and sulfonate), and silyl groups, as well as ethers, alkylthios, carbonyls (including ketones, aldehydes, carboxylates, and esters), haloalkyls,— CN and the like. Cycloalkyls can be substituted in the same manner.

As used herein,“alkenyl” and“alkynyl” refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but that contain at least one double or triple bond, respectively. The term“substituted alkenyl” refers to alkenyl moieties having one or more substituents replacing one or more hydrogen atoms on one or more carbons of the hydrocarbon backbone. Such substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl,— CN, aryl, heteroaryl, and combinations thereof. The term“substituted alkynyl” refers to alkynyl moieties having one or more substituents replacing one or more hydrogen atoms on one or more carbons of the hydrocarbon backbone. Such substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl,— CN, aryl, heteroaryl, and combinations thereof.

As used herein,“arylalkyl,” refers to an alkyl group that is substituted with a substituted or unsubstituted aryl or heteroaryl group.

As used herein,“alkylaryl,” as used herein, refers to an aryl group (e.g., an aromatic or hetero aromatic group), substituted with a substituted or unsubstituted alkyl group. As used interchangeably herein, the terms “amide” or “amido” refer to both “unsubstituted amido” and“substituted amido” and are represented by the general formula:

wherein, E is absent, or E is substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted aralkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted aryl, substituted or unsubstituted heteroaryl, substituted or unsubstituted heterocyclyl, wherein independently of E, R and R' each independently represent a hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted carbonyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocyclyl, substituted or unsubstituted alkylaryl, substituted or unsubstituted arylalkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl,— (CH2) _m— R'", or R and R' taken together with the N atom to which they are attached complete a heterocycle having from 3 to 14 atoms in the ring structure; R'" represents a hydroxy group, substituted or unsubstituted carbonyl group, an aryl, a cycloalkyl ring, a cycloalkenyl ring, a heterocycle, or a polycycle; and m is zero or an integer ranging from 1 to 8. In preferred embodiments, only one of R and R' can be a carbonyl, e.g., R and R' together with the nitrogen do not form an imide. In preferred embodiments, R and R' each independently represent a hydrogen atom, substituted or unsubstituted alkyl, a substituted or unsubstituted alkenyl, or— (CH2) _m— R'". When E is oxygen, a carbamate is formed. The carbamate cannot be attached to another chemical species, such as to form an oxygen-oxygen bond, or other unstable bonds, as understood by one of ordinary skill in the art.

As used herein,“attached” can refer to covalent or non-covalent interaction between two or more molecules. Non-covalent interactions can include ionic bonds, electrostatic interactions, van der Walls forces, dipole-dipole interactions, dipole-induced-dipole interactions, London dispersion forces, hydrogen bonding, halogen bonding, electromagnetic interactions, p-p interactions, cation-p interactions, anion-p interactions, polar p-interactions, and hydrophobic effects.

As used herein,“aryl” refers to C5-C26-membered aromatic, fused aromatic, fused heterocyclic, or biaromatic ring systems. Broadly defined,“aryl,” as used herein, includes 5-, 6-, 7-, 8-, 9-, 10-, 14-, 18-, and 24-membered single-ring aromatic groups, for example, benzene, naphthalene, anthracene, phenanthrene, chrysene, pyrene, corannulene, coronene, etc. “Aryl” further encompasses polycyclic ring systems having two or more cyclic rings in which two or more carbons are common to two adjoining rings (i.e.,“fused rings”) wherein at least one of the rings is aromatic, e.g., the other cyclic ring or rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocycles. The term“substituted aryl” refers to an aryl group, wherein one or more hydrogen atoms on one or more aromatic rings are substituted with one or more substituents including, but not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxy, carbonyl (such as a ketone, aldehyde, carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, imino, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl (such as CF ₃ , — CH _2— CF ₃ , — CCI ₃ ), — CN, aryl, heteroaryl, and combinations thereof.

As used herein,“carbonyl,” is art-recognized and includes such moieties as can be represented by the general formula: wherein X is a bond, or represents an oxygen or a sulfur, and R represents a hydrogen, a substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocyclyl, substituted or unsubstituted alkylaryl, substituted or unsubstituted arylalkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl,— (CH2) _m— R", or a pharmaceutical acceptable salt, R' represents a hydrogen, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted heterocyclyl, substituted or unsubstituted alkylaryl, substituted or unsubstituted arylalkyl, substituted or unsubstituted aryl, or substituted or unsubstituted heteroaryl or— (CH2) _m— R"; R" represents a hydroxy group, substituted or unsubstituted carbonyl group, an aryl, a cycloalkyl ring, a cycloalkenyl ring, a heterocycle, or a polycycle; and m is zero or an integer ranging from 1 to 8. Where X is oxygen and R is defined as above, the moiety is also referred to as a carboxyl group. When X is oxygen and R is hydrogen, the formula represents a‘carboxylic acid’ . Where X is oxygen and R' is hydrogen, the formula represents a‘formate’ . Where X is oxygen and R or R' is not hydrogen, the formula represents an“ester”. In general, where the oxygen atom of the above formula is replaced by a sulfur atom, the formula represents a‘thiocarbonyl’ group. Where X is sulfur and R or R' is not hydrogen, the formula represents a‘thioester.’ Where X is sulfur and R is hydrogen, the formula represents a‘thiocarboxylic acid.’ Where X is sulfur and R' is hydrogen, the formula represents a‘thioformate.’ Where X is a bond and R is not hydrogen, the above formula represents a‘ketone.’ Where X is a bond and R is hydrogen, the above formula represents an ‘aldehyde.’

The term“substituted carbonyl” refers to a carbonyl, as defined above, wherein one or more hydrogen atoms in R, R' or a group to which the moiety

is attached, are independently substituted. Such substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl,— CN, aryl, heteroaryl, and combinations thereof.

The term“carboxyl” is as defined above for the formula

and is defined more specifically by the formula— R ^lv COOH, wherein R ^lv is an alkyl, alkenyl, alkynyl, cycloalkyl, heterocyclyl, alkylaryl, arylalkyl, aryl, or heteroaryl. In preferred embodiments, a straight chain or branched chain alkyl, alkenyl, and alkynyl have 30 or fewer carbon atoms in its backbone (e.g., Ci-C3o for straight chain alkyl, C3-C3o for branched chain alkyl, C2-C3o for straight chain alkenyl and alkynyl, C3-C3o for branched chain alkenyl and alkynyl), preferably 20 or fewer, more preferably 15 or fewer, most preferably 10 or fewer. Likewise, preferred cycloalkyls, heterocycles, aryls and heteroaryls have from 3-10 carbon atoms in their ring structure, and more preferably have 5, 6 or 7 carbons in the ring structure. The term“substituted carboxyl” refers to a carboxyl, as defined above, wherein one or more hydrogen atoms in R are substituted. Such substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, carbonyl (such as a carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl,— CN, aryl, heteroaryl, and combinations thereof.

As used interchangeably herein,“heterocycle,”“heterocyclic” and“heterocyclyl” refer to a cyclic radical attached via a ring carbon or nitrogen atom of a monocyclic or bicyclic ring containing 3-10 ring atoms, and preferably from 5-6 ring atoms, consisting of carbon and one to four heteroatoms each selected from the group consisting of non-peroxide oxygen, sulfur, and N(Y) wherein Y is absent or is H, O, Ci-Cio alkyl, phenyl or benzyl, and optionally containing 1-3 double bonds and optionally substituted with one or more substituents. Heterocyclyl are distinguished from heteroaryl by definition. Examples of heterocycles include, but are not limited to piperazinyl, piperidinyl, piperidonyl, 4-piperidonyl, dihydrofuro[2,3-b]tetrahydrofuran, morpholinyl, piperazinyl, piperidinyl, piperidonyl, 4- piperidonyl, piperonyl, pyranyl, 2H-pyrrolyl, 4H-quinolizinyl, quinuclidinyl, tetrahydrofuranyl, 6H-l,2,5-thiadiazinyl. Heterocyclic groups can optionally be substituted with one or more substituents as defined above for alkyl and aryl.

As used herein,“heteroaryl” refers to C5-C26-membered aromatic, fused aromatic, biaromatic ring systems, or combinations thereof, in which one or more carbon atoms on one or more aromatic ring structures have been substituted with a heteroatom. Suitable heteroatoms include, but are not limited to, oxygen, sulfur, and nitrogen. Broadly defined,“heteroaryl,” as used herein, includes 5-, 6-, 7-, 8-, 9-, 10-, 14-, 18-, and 24-membered single-ring aromatic groups that may include from one to four heteroatoms, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, tetrazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine, and the like. The heteroaryl group may also be referred to as“aryl heterocycles” or“heteroaromatics”.“Heteroaryl” further encompasses polycyclic ring systems having two or more rings in which two or more carbons are common to two adjoining rings (i.e.,“fused rings”) wherein at least one of the rings is heteroaromatic, e.g., the other cyclic ring or rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls, heterocycles, or combinations thereof. Examples of heteroaryl rings include, but are not limited to, benzimidazolyl, benzofuranyl, benzothiofuranyl, benzothiophenyl, benzoxazolyl, benzoxazolinyl, benzthiazolyl, benztriazolyl, benztetrazolyl, benzisoxazolyl, benzisothiazolyl, benzimidazolinyl, carbazolyl, 4aH-carbazolyl, carbolinyl, chromanyl, chromenyl, cinnolinyl, decahydroquinolinyl, 2H,6H- 1,5,2-dithiazinyl, furanyl, furazanyl, imidazolidinyl, imidazolinyl, imidazolyl, lH-indazolyl, indolenyl, indolinyl, indolizinyl, indolyl, 3H-indolyl, isatinoyl, isobenzofuranyl, isochromanyl, isoindazolyl, isoindolinyl, isoindolyl, isoquinolinyl, isothiazolyl, isoxazolyl, methylenedioxyphenyl, naphthyridinyl, octahydroisoquinolinyl, 1,2,3-oxadiazolyl, 1,2,4- oxadiazolyl, 1,2,5-oxadiazolyl, 1,3,4-oxadiazolyl, oxazolidinyl, oxazolyl, oxindolyl, pyrimidinyl, phenanthridinyl, phenanthrolinyl, phenazinyl, phenothiazinyl, phenoxathinyl, phenoxazinyl, phthalazinyl, pteridinyl, purinyl, pyrazinyl, pyrazolidinyl, pyrazolinyl, pyrazolyl, pyridazinyl, pyridooxazole, pyridoimidazole, pyridothi azole, pyridinyl, pyridyl, pyrimidinyl, pyrrolidinyl, pyrrolinyl, pyrrolyl, quinazolinyl, quinolinyl, quinoxalinyl, tetrahydroisoquinolinyl, tetrahydroquinolinyl, tetrazolyl, 1,2,3-thiadiazolyl, 1,2,4-thiadiazolyl, 1,2,5-thiadiazolyl, 1,3,4-thiadiazolyl, thianthrenyl, thiazolyl, thienyl, thienothiazolyl, thienooxazolyl, thienoimidazolyl, thiophenyl and xanthenyl. One or more of the rings can be substituted as defined for“substituted heteroaryl”. The term“substituted heteroaryl” refers to a heteroaryl group in which one or more hydrogen atoms on one or more heteroaromatic rings are substituted with one or more substituents including, but not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxy, carbonyl (such as a ketone, aldehyde, carboxyl, alkoxycarbonyl, formyl, or an acyl), silyl, ether, ester, thiocarbonyl (such as a thioester, a thioacetate, or a thioformate), alkoxyl, phosphoryl, phosphate, phosphonate, phosphinate, amino (or quarternized amino), amido, amidine, imine, cyano, nitro, azido, sulfhydryl, imino, alkylthio, sulfate, sulfonate, sulfamoyl, sulfonamido, sulfonyl, heterocyclyl, alkylaryl, haloalkyl (such as CF3, — CH2— CF3, — CCI3), — CN, aryl, heteroaryl, and combinations thereof.

As used herein,“heteroalkyl,” refers to straight or branched chain, or cyclic carbon- containing radicals, or combinations thereof, containing at least one heteroatom. Suitable heteroatoms include, but are not limited to, O, N, Si, P and S, wherein the nitrogen, phosphorous and sulfur atoms are optionally oxidized, and the nitrogen heteroatom is optionally quaternized. Examples of saturated hydrocarbon radicals include, but are not limited to, methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl, isobutyl, sec-butyl, cyclohexyl, (cyclohexyl)methyl, cyclopropylmethyl, and homologs and isomers of, for example, n-pentyl, n-hexyl, n-heptyl, n-octyl. Examples of unsaturated alkyl groups include, but are not limited to, vinyl, 2-propenyl, crotyl, 2-isopentenyl, 2-(butadienyl), 2,4-pentadienyl, 3-(l,4- pentadienyl), ethynyl, 1- and 3-propynyl, and 3-butynyl.

The term“molecular weight”, as used herein, can generally refer to the mass or average mass of a material. If a polymer or oligomer, the molecular weight can refer to the relative average chain length or relative chain mass of the bulk polymer. In practice, the molecular weight of polymers and oligomers can be estimated or characterized in various ways including gel permeation chromatography (GPC) or capillary viscometry. GPC molecular weights are reported as the weight-average molecular weight (M _w ) as opposed to the number-average molecular weight (M _n ). Capillary viscometry provides estimates of molecular weight as the inherent viscosity determined from a dilute polymer solution using a particular set of concentration, temperature, and solvent conditions.

The term“nanoparticle” as used herein includes a nanoscale deposit of a homogenous or heterogeneous material. Nanoparticles may be regular or irregular in shape and may be formed from a plurality of co-deposited particles that form a composite nanoscale particle. Nanoparticles may be generally spherical in shape or have a composite shape formed from a plurality of co-deposited generally spherical particles. Exemplary shapes for the nanoparticles include, but are not limited to, spherical, rod, elliptical, cylindrical, disc, and the like. In some embodiments, the nanoparticles have a substantially spherical shape.

As used herein, "substantially pure" can mean an object species is the predominant species present (i.e., on a molar basis it is more abundant than any other individual species in the composition), and preferably a substantially purified fraction is a composition wherein the object species comprises about 50 percent of all species present. Generally, a substantially pure composition will comprise more than about 80 percent of all species present in the composition, more preferably more than about 85%, 90%, 95%, and 99%. Most preferably, the object species is purified to essential homogeneity (contaminant species cannot be detected in the composition by conventional detection methods) wherein the composition consists essentially of a single species.

As used herein,“substituted,” refers to all permissible substituents of the compounds or functional groups described herein. In the broadest sense, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, but are not limited to, halogens, hydroxyl groups, or any other organic groupings containing any number of carbon atoms, preferably 1-14 carbon atoms, and optionally include one or more heteroatoms such as oxygen, sulfur, or nitrogen grouping in linear, branched, or cyclic structural formats. Representative substituents include alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C ₃ -C ₂₀ cyclic, substituted C ₃ -C ₂₀ cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(lactic-co- glycolic acid), peptide, and polypeptide groups. Such alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C ₃ -C ₂₀ cyclic, substituted C ₃ -C ₂₀ cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(lactic-co- glycolic acid), peptide, and polypeptide groups can be further substituted.“Substituted,” as used herein, refers to all permissible substituents of the compounds or functional groups described herein. In the broadest sense, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, but are not limited to, halogens, hydroxyl groups, or any other organic groupings containing any number of carbon atoms, preferably 1-14 carbon atoms, and optionally include one or more heteroatoms such as oxygen, sulfur, or nitrogen grouping in linear, branched, or cyclic structural formats. Representative substituents include alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C ₃ -C ₂₀ cyclic, substituted C ₃ -C ₂₀ cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(lactic-co-glycolic acid), peptide, and polypeptide groups. Such alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, halo, hydroxyl, alkoxy, substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, phenylthio, substituted phenylthio, arylthio, substituted arylthio, cyano, isocyano, substituted isocyano, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, sulfonyl, substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl, phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl, C3-C20 cyclic, substituted C3-C20 cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(lactic-co-glycolic acid), peptide, and polypeptide groups can be further substituted.

As used herein, the term“transition metal” means an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell, typically found in Groups 3-12 of the periodic table in Periods 4-7. Exemplary, but non limiting, transition metals include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, silver, tungsten, platinum, and gold.

As used herein, the terms“weight percent,”“wt%,” and“wt. %,” which can be used interchangeably, indicate the percent by weight of a given component based on the total weight of a composition of which it is a component, unless otherwise specified. That is, unless otherwise specified, all wt% values are based on the total weight of the composition. It should be understood that the sum of wt% values for all components in a disclosed composition or formulation are equal to 100. Alternatively, if the wt% value is based on the total weight of a subset of components in a composition, it should be understood that the sum of wt% values the specified components in the disclosed composition or formulation are equal to 100.

Discussion

The worldwide installed solar photovoltaic (PV) and wind energy system capacities have surged exponentially for the past few decades. However, wind and solar power generation is highly intermittent and seasonal, resulting in serious issues including grid capacity/stability, curtailment, and supply/demand mismatch. One possible solution to the renewable electricity storage challenge is to use a regenerative hydrogen fuel cell (RHFC), which converts electricity to H2, a clean energy carrier that can be obtained from electrochemical water splitting, and stores the ¾, which is later fed into a fuel cell to regenerate electric power. Currently, hydrogen gas is commonly compressed and stored at an extremely high pressure (700 bar), leading to a high cost as well as safety concerns and logistical challenges since it is highly inflammable. Chemical hydrogen storage options, including solid-state metal hydrides or liquid organic hydrogen carriers (LOHCs), could be a safe alternative to hydrogen storage. However, the hydrogen release from these materials is strongly endothermic, typically requiring elevated temperatures of 150-500 °C, which are well above the“waste heat” temperature range of 80- 90 °C provided by a standard PEMFC.

Formic acid (HCOOH) and formates have been considered as a promising material for chemical hydrogen storage because of their high volumetric capacities, which surpass those of most other chemical hydrogen storage materials. Recently, immense progress has been made in the development of formate-based reversible ¾ storage under mild conditions. It has been suggested that the catalytic decomposition of a formate/amine adduct solution in the presence of homogeneous Ru catalysts as a practical FF storage system for direct use in fuel cells. Boddien et al. Adv. Synth. Catal.2009, 351 :2517-2520 and Loges and Beller. Angewandte Chemie. 2008:3962-3965. Hull et al. designed a reversible ¾ storage system with a homogeneous Ir catalyst, using pH to control ¾ production or consumption. Hull et al. Nat. Chem. 2012:4:383-388. Several reports also described the feasibility of using a homogeneous Ru catalyst to enable reversible HCOONa/NaHCCF-based ¾ storage to achieve a higher volumetric density. Laurenczy’s group designed a hydrogen battery system based on cesium formate/bicarbonate due to the high solubility of cesium salts. Sordakis et al. Chem. Cat. Chem. 2015, 9:813-816. However, due to the high cost arising from the use of sophisticated ligands and the limited recyclability, the homogeneous catalyst systems have not yet been ready for commercial applications.

Compared to the significant advances of homogeneously catalyzed, formate-based hydrogen storage systems, only few reports of using heterogeneous catalysts for hydrogen storage are available in the literature. Aqueous sodium formates as the ¾ storage material over palladium on reduced graphitic oxide nanosheets (Pd/r-GO) have been developed. Bi et al. Angewandte Chemie. 2012:53 : 13583-13587. Notably, the rate of hydrogen discharge is too low in these hydrogen storage systems a for practical applications. Hosono et al., Nano Lett. 2009, 9: 1045-1051. Recently, a hydrogen storage system based on ammonium bicarbonate/formate redox equilibrium in aqueous media over the heterogeneous Pd/AC catalyst was demonstrated. Su et al. ChemSusChem. 2015: 813-816. This hydrogen storage system has an exceptionally high volumetric energy density (up to 168 g ¾ per L). However, the challenge that trace amounts of CO and M¾ could be formed by the decomposition of ammonium formate at elevated temperatures cannot be completely ruled out.

With the limitations of current systems in mind, described herein is a hydrogen storage system that uses formate piperidine pieridine adduct (FPA) solutions capable of capturing CO2. Further, aspects of the hydrogen storage system are capable of fast hydrogenation of captured CO2 with piperidine to the FPA, as well as the release high-purity ¾ via rapid decomposition of FPA. In aspects of the hydrogen storage system described herein, the hydrogenation of C02 and the decomposition of FPA can both be performed under mild conditions. Also described herein are methods of making and using the hydrogen storage system described herein. Other compositions, compounds, methods, features, and advantages of the present disclosure will be or become apparent to one having ordinary skill in the art upon examination of the following drawings, detailed description, and examples. It is intended that all such additional compositions, compounds, methods, features, and advantages be included within this description, and be within the scope of the present disclosure.

An interesting LHC is aqueous solutions of formate salts since the thermodynamics for hydrogen release and uptake by formate and the corresponding bicarbonates (Eq. 1) is neither endergonic nor exergonic, providing a liquid hydrogen carrier that can both release and take up hydrogen at moderate temperatures and pressures. (K. Miiller, K. Brooks and T. Autrey, Energy and Fuels, 2018, 32, 10008-100015 and K. Miiller, K. Brooks and T. Autrey, Energy and Fuels, 2017, 31, 12603-12611.)

MHC0 ₂ l! X) MHCCK // . M stands for cations: k , Na , Ni l , , etc.

MHC0 ₂ + H _: 0 MHCCX + // , M stands for cations: K ⁴ , Na , NH ⁴ , etc.

(Eq. 1)

Formate is an environmentally benign compound and, unlike formic acid, is neutral and thus non-corrosive. With a catalyst, formate releases ¾ and is converted to bicarbonate, while bicarbonate can be hydrogenated back to formate. The equilibrium constant is approximately 1, indicating this reaction is readily reversible. To get the ¾ off from aqueous formate solutions, half the hydrogen comes from the carbon-hydrogen bond and half comes from water, while CO2 is not released. Another advantage is that chemical compression, which generates pressure by chemical reactions, could reduce the dependence upon physical compressors that is one of the most expensive components of hydrogen refueling stations. Formate dehydrogenation reaction is endothermic and entropy-controlled and thus will not have thermal runaway. As it decomposes, the released hydrogen gas can generate high pressures, which may not need expensive compressors for the first stage of gas compression.

Described herein are formate-based liquid hydrogen carrier (LHC) compositions that are suitable for use in reversible hydrogen storage methods and systems. In aspects, the LHC composition can be formed by reacting CO2 with an amine capable of capturing CO2 to form an adduct (such as a formate adduct), which can be subsequently reacted to dehydrogenate the adduct and release the stored hydrogen via an appropriate catalyst (see e.g. FIG. 18 Scheme Sl(b)). In some aspects the LHC can be an adduct, such as a formate adduct, that can be formed by a process described herein.

In some aspects, an adduct can be formed by reacting CO2 with an amine capable of capturing CO2 to in the presence of a hydrogenation solvent to form the formate having hydrogenated CO2. See e.g. FIG. 17, Scheme I and FIG. 18, Scheme SI (b) and (c). Suitable amines capable of capturing CO2 to form formates having captured CO2, such as a formate adduct, include primary, secondary, tertiary amines, and combinations thereof. In some aspects, the amine capable of capturing CO2 is basic. In some aspects, the amine capable of capturing CO2 is highly basic (i.e. those having a pK _a of its conjugated acid greater than 11). In some aspects, the amine can have a pK _a greater than 5, 6, 7, 8, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, or 20 or more. In some aspects, the amine can have a structure capable of destabilizing a carbamate or carbonate intermediate. In some aspects, the amines can be linear amines, sterically hindered amines, cyclic amines and combinations thereof. Suitable amines include, but are not limited to, monoethanolamine, diethanolamine, triethanolamine, ethane- 1,2-diamine, 2-amino-2-methyl- 1-propanol, 2-amino-2 -methyl-1, 3, -propanediol, 2-amino-2 -hydroxymethyl-1, 3-propanediol, piperazine, piperidine, 1 -methyl piperazine, 2-methyl piperazine, and combinations thereof. In some aspects, the amine is piperidine. In some aspects the adduct is a formate-piperidine adduct. In some aspects, the reaction of piperidine with CO2 can form a piperidine-carbamate. See e.g. FIG. 17, Scheme 1. In some aspects, the concentration of the amine can range from about 0.001 M to about 5M. In some aspects the concentration of the amine can range from about 0.001 M to about 1 M, from about 0.01 to about 1M, from about 0.1 M to about 1 M, or from about 1 to 1.5, 2, 2.5, 3, 3.5, 4, 4.5 or 5 M.

Capture and/or hydrogenation of CO2 can occur in a solvent (also referred to herein as the hydrogenation solvent) to form formate. The formate containing the captured CO2 can thus be an LHC. See e.g. FIG. 18, Scheme SI . The CO2 can be captured in a variety of species that can depend on the amine and/or the hydrogenation solvent used, among other parameters that will be appreciated by those of ordinary skill in the art. Such species can include HCO3 , CO3 ² , RNCO ² , Alkyl-CO ³ and combinations thereof.

The dehydrogenation can be completed in a solvent (also referred to herein as a dehydrogenation solvent). In some aspects, the dehydrogenation solvent can be the same as the hydrogenation solvent. Suitable dehydrogenation solvents and co-solvents can be as described with respect to the hydrogenation solvents and co-solvents described elsewhere herein. In some aspects, the dehydrogenation solvent can be different. In some aspects, the hydrogenation solvent can contain a catalyst capable of catalyzing the hydrogenation reaction. In some aspects, the dehydrogenation solvent can contain a catalyst capable of catalyzing the dehydrogenation reaction. The amine or ammonia used to capture the CO2 can be released during dehydrogenation and can be optionally recovered and reused.

In some aspects, the hydrogenation (or dehydrogenation) solvent can be water, acetonitrile, or tetrahydrofuran. In some aspects, the hydrogenation (or dehydrogenation) solvent can be 0-100 % v/v or wt% water, acetonitrile, or tetrahydrofuran. In some aspects, the hydrogenation (or dehydrogenation) solvent can range from 0 to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,

12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,

37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61,

62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86,

87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100 % v/v or wt% water, acetonitrile, or tetrahydrofuran. In some aspects, the solvent can be about 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,

13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37,

38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62,

63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87,

88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or about 100 % v/v or wt% water, acetonitrile, or tetrahydrofuran. In some aspects, the hydrogenation (or dehydrogenation) solvent can be 0- 50% v/v or wt% water, acetonitrile, or tetrahydrofuran. In some aspects, the solvent can be 0, or be about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 % v/v or wt% water, acetonitrile, or tetrahydrofuran. In some aspects, the solvent can range from about 0 to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,

25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50 % v/v or wt% water, acetonitrile, or tetrahydrofuran.

In some aspects, the hydrogenation (or dehydrogenation) solvent can include an amount of alcohol. In some aspects, the alcohol can be methanol, ethanol, propanol (e.g. 1- propanol or 2-propanol), or butanol. Other suitable alcohols may be found in J. Su, M. Lu and H. Lin, Green Chem., 2015, 17, 2769-2773 and J. Su, L. Yang, M. Lu and H. Lin, ChemSusChem, 2015, 8, 813-816. The percent by volume or wt % of alcohol can range from about 0 % to about 100 % (meaning that the alcohol is the only solvent present). In some aspects, the amount of the alcohol can range from 0 to 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,

14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100 % v/v or wt%. In some aspects, the amount of the alcohol can be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,

23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47

48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72

73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97

98, 99, or about 100 % v/v or wt%. In some aspects, the amount of alcohol can range from 50% to 100% v/v or wt%. In some aspects, the amount of alcohol can range from about 50, to about

51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75,

76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or

100 % v/v or wt%. In some aspects, the amount of alcohol can be about 50, 51, 52, 53, 54, 55,

56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80

81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, or 100 % v/v or wt%. In some aspects, the alcohol is propanol (e.g. e.g. 1 -propanol or 2-propanol). In some aspects, the alcohol is present at about 70 %.

In some aspects, the catalyst is a homogenous catalyst. In some aspects, the catalyst is a heterogenous catalyst. In some aspects, the catalyst can contain one or more metals, including but not limited to noble metals, transition metals, metal alloys, metal oxides, polymers, carbon (including but not limited to activated carbon, carbon nano-materials, etc.), graphene, graphene oxide, and combinations thereof. In some aspects, the catalyst can include a metal selected from the group of Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Rm, Yb, Lu, Hf, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, Ra, Ac, Th, Pa, U, Np, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, FI, Me, Lv, and combinations thereof. In some aspects, the catalyst is composed of a metal selected from the group of: Ni, Ru, Rh, Pd, Cu, Fe, Co, Ag, Au, Pt, and combinations thereof. Exemplary catalysts can include, but are not limited to those discussed and described in Sordakis et al., 2018. Chem Rev. 1182372-433; Onishi et al. 2019. Adv. Energy. Mat. 9(23): 1801275; Navi ani -Garcia et al. 2018. Nat. Asia Materials 10: 277-292; De et al. 2016. Energy Enviroment. Sci. 9:3314-3347; Badding et al., and ACS Omega. 2018, 3 :3501-3506. In some aspects, the catalysts is a Pd-based catalyst. In some aspects, the catalyst is a Pd/Activated carbon (AC) catalyst. In some aspects, the catalyst is a nanoparticle catalyst. The catalyst can be in any suitable form, such as film, membrane, coating layer(s), nanoparticle, nanorods, nanotubes, nanowires, rods, aerogels, etc. A disadvantage of many current hydrogen storage is that the capture of hydrogenation of CO2 and/or the dehydrogenation of an intermediate to release stored hydrogen must be performed under harsh conditions (e.g. high temperature, harsh solvents etc.). In some aspects, the capture of CO2, the dehydrogenation of the formed adduct can be performed under mild conditions. As used herein in the context of these reactions,“mild conditions” refer to a reaction temperature of less than about 100 °C.

In some aspects, the capture, hydrogenation of CO2, and adduct formation can occur at about room temperature. In some aspects, the capture, hydrogenation of CO2, and adduct formation can occur at a temperature ranging from about 20 °C to about 40 °C. In some aspects, the capture, hydrogenation of CO2, and adduct formation can occur at about 20 to about 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, 25.5, 26, 26.5, 27, 27.5, 28, 28.5, 29, 29.5, 30, 30.5,

31, 31.5, 32, 32.5, 33, 33.5, 34, 34.5, 35, 35.5, 36, 36.5, 37, 37.5, 38, 38.5, 39, 39.5, or about 40 °C. In some aspects, the capture, hydrogenation of CO2, and adduct formation can occur at about 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, 25.5, 26, 26.5, 27, 27.5, 28, 28.5, 29,

29.5, 30, 30.5, 31, 31.5, 32, 32.5, 33, 33.5, 34, 34.5, 35, 35.5, 36, 36.5, 37, 37.5, 38, 38.5, 39,

39.5, or about 40 °C. In some aspects, the capture, hydrogenation of CO2, and adduct formation can occur at about 30 °C.

In some aspects, the dehydrogenation of the adduct having stored hydrogen at the surface of a catalyst can occur at a temperature ranging from about 60 °C to about 100 °C. In some aspects, the dehydrogenation of the adduct having stored hydrogen at the surface of a catalyst can occur at a temperature ranging from about 60 to about 60.5, 61, 61.5, 62, 62.5, 63,

63.5, 64, 64.5, 65, 65.5, 66, 66.5, 67, 67.5, 68, 68.5, 69, 69.5, 70, 70.5, 71, 71.5, 72, 72.5, 73,

73.5, 74, 74.5, 75, 75.5, 76, 76.5, 77, 77.5, 78, 78.5, 79, 79.5, 80, 80.5, 81, 81.5, 82, 82.5, 83,

83.5, 84, 84.5, 85, 85.5, 86, 86.5, 87, 87.5, 88, 88.5, 89, 89.5, 90, 90.5, 91, 91.5, 92, 92.5, 93,

93.5, 94, 94.5, 95, 95.5, 96, 96.5, 97, 97.5, 98, 98.5, 99, 99.5, or 100 °C. In some aspects, the dehydrogenation of the adduct having stored hydrogen at the surface of a catalyst can occur at a temperature of about 60, 60.5, 61, 61.5, 62, 62.5, 63, 63.5, 64, 64.5, 65, 65.5, 66, 66.5, 67,

67.5, 68, 68.5, 69, 69.5, 70, 70.5, 71, 71.5, 72, 72.5, 73, 73.5, 74, 74.5, 75, 75.5, 76, 76.5, 77,

77.5, 78, 78.5, 79, 79.5, 80, 80.5, 81, 81.5, 82, 82.5, 83, 83.5, 84, 84.5, 85, 85.5, 86, 86.5, 87,

87.5, 88, 88.5, 89, 89.5, 90, 90.5, 91, 91.5, 92, 92.5, 93, 93.5, 94, 94.5, 95, 95.5, 96, 96.5, 97,

97.5, 98, 98.5, 99, 99.5, or about 100 °C. In some aspects, the dehydrogenation of the adduct having stored hydrogen at the surface of a catalyst can occur at a temperature of about 80 to about 80.5, 81, 81.5, 82, 82.5, 83, 83.5, 84, 84.5, 85, 85.5, 86, 86.5, 87, 87.5, 88, 88.5, 89, 89.5,

90 °C. The compositions and methods described herein can, in some aspects, have and/or result in a faster rate of hydrogen formation (as expressed as a lower or reduced time of hydrogen formation) as compared to conventional compositions and processes. In some aspects, the efficiency capturing and/or hydrogenation of CO2 is greater than conventional systems, which can increase the overall yield of hydrogen extracted. In some aspects, the dehydrogenation and release of hydrogen from the adduct described herein can be faster than conventional systems.

Another advantage of some aspects of the methods described herein is that they can result in a substantially pure hydrogen in the form of ¾. In some aspects, the ¾ generated by aspects of the methods described herein can be 90, 91, 92, 93, 94, 95, 96, 97, 98, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, 99.9 or 100 % pure.

Another advantage of some aspects of the compositions and methods described herein is that they are reversible. In other words, the compositions and methods described herein is that they not only can store hydrogen but they are capable of releasing it in a recyclable or renewable fashion.

The methods and compositions described herein can be used in and/or with a system configured to chemically store and release hydrogen for energy and storage production. As previously discussed, the hydrogen can be stored in the form of a liquid hydrogen carrier (e.g. an as adduct described herein), that can be subsequently reacted in the presence of an appropriate catalyst that can be present with in an energy storage/generation system to result in energy production and/or storage. Thus, the compositions and methods described herein can be appropriate for use in both continuous and“on demand” power systems. The compositions and methods described herein can be employed in the context of fuel cells for many industries including, but not limited to, automotive, aerospace, electronic device, electricity generation and transmission (e.g. within Smart Grids, etc.), as well as a variety of industrial uses that consume hydrogen such as petroleum refining, food processing, steel manufacturing and other metalworking processes, flat glass production, and fertilizer production. In some aspects, the adduct solution can store hydrogen generated from water splitting with electrical energy from renewable energy sources such as solar, geothermal etc. and thus form a“hydrogen battery”.

EXAMPLES

Now having described the embodiments of the present disclosure, in general, the following Examples describe some additional embodiments of the present disclosure. While embodiments of the present disclosure are described in connection with the following examples and the corresponding text and figures, there is no intent to limit embodiments of the present disclosure to this description. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure. The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the probes disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers ( e.g ., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

Example 1.

This Example can demonstrate various aspects of the reversible hydrogen storage system described herein.

Table 1 shows the results of the catalytic hydrogenation of piperidine captured CO2 in various aqueous ethanol solutions. After reacting for 1 hour in water at 20 °C (Table 1, entry 1), the yield of formate was 50.2%, and the corresponding turn-over frequency (TOF) was approximately 1431 h ^_1 over the activated carbon supported palladium catalyst (5 wt%, Pd/AC). Adding alcohol into a water solvent significantly improved the hydrogenation of piperidine captured CO2. For instance, the ethanol-water solution with 70 wt% ethanol exhibited a significant solvent promotion effect as a high yield of formate of about 83.6% was achieved in an hour at 20 °C, and the TOF reached up to about 3523 h ^_1 over the Pd/ AC catalyst (Table 1, entry 3). Moreover, a much higher yield of about 95.5% of formate was achieved by simply elevating the temperature from 20 °C to 30 °C (Table 1, entry 5). It was found that other alcohols also have a similar promotion effect to ethanol. At 30 °C, by switching the aqueous ethanol solvent to the aqueous 1 -propanol or the aqueous 2-propanol solvent, each containing 70 wt% alcohol, the formate yields reached about 96.4% and about 98.5%, respectively, in an hour (Table 1, entries 7 and 8).

Table 1. Hydrogenation of piperidine-captured CO2 in different aqueous solutions.

Captured CO2 Species Conversion concentration (M) Results Entry Capture ³ and Temp HCO ₃ CO ₃ ² RNCO ² Alkyl- Formate TOF ^d

Hydrogenation ^b (°C) CO ₃ yield (h ¹ )

Solvent (wt% (%)

alcohol)

T 0% alcohol 20 093 003 OOO 0 502 1431

^~ 2. 50% EtOH 20 073 OOl OOl 021 700 3303

70% EtOH 20 032 OOO 003 061 806 3523

^~ 4. 70% EtOH 25 032 OOO 003 061 87 A 4404

70% EtOH 30 032 OOO 003 061 905 5945

^~ 6. 70% EtOH 40 032 OOO 003 061 705 3083

^~ 1. 70% 1- 30 030 OOO 003 062 904 4404 propanol

^~ 8. 70% 2- 30 030 OOO 003 062 905 5504 propanol

^~ 9. 90% EtOH 20 003 OOO 003 090 804 3083

^~ Ί(l 95.6% EtOH 20 003 OOO 003 092 606 2642 L 100% EtOH 20 003 OOO 003 093 602 2202 aC0 ₂ capture conditions: 20 mL amine/water-ethanol, 1 M piperidine, 20 °C, 40 min. bHydrogenation conditions: 50 mL Parr reactor, captured CO ₂ solution (20 mL), 0.1 g Pd/ AC (5 wt%), 400 psi hydrogen, 1 hour, 20 °C except entries 4-6. ^c The captured CO ₂ species concentrations were determined by ¹³ C NMR spectroscopy d The TOFs were calculated using: moles of formate/(moles of Pd ^c 23.2%)/reaction time. The dispersion of the Pd atoms on the surface of Pd NPs is 23 2%, which is determined by carbon monoxide chemisorption.

In previous studies, it was considered that the promotion effect of the ethanol co-solvent can be attributed to: (1) the higher solubility of ¾ in ethanol than that in water; ¹⁵ and (2) the amount of bicarbonate and ethyl carbonate intermediate species which can be hydrogenated. Indeed, we observed an increasing trend of the formate yield as the ethanol content in the aqueous solutions increased from 0% to 70%, but the yield then decreased as the ethanol content further increased to 100%. 13C NMR characterization (ESI, FIG. 6†) found that there was only one peak located at 161.2 ppm, which was assigned to the bicarbonate/carbonate ions after capturing CO2 with piperidine in pure water.16 In the ethanol-water mixed solvent, another peak located at 159.5 ppm appeared, which was assigned to ethyl carbonate ions. In pure ethanol, only the ethyl carbonate peak appeared. This observation is well consistent with our previous report that ethyl carbonate ions appear in the NH4HCO3 aqueous solutions when adding ethanol. ¹⁴ However, the yield of formate decreased from about 83.6% in the aqueous ethanol solvent (an ethanol fraction of 70 wt%) to about 62.2% in pure ethanol, implying that an appropriate amount of water may enhance the hydrogenation performance. Interestingly, similar promotion effects by adding small amounts of water were observed in the CO2 hydrogenation reactions with homogeneous catalysts. ¹⁷ In general, under identical conditions, the maximum formate yield was obtained with the aqueous ethanol solvent at an optimal ethanol to water ratio, rather than with pure ethanol. However, the different properties of the solvents at various ethanol to water ratios likely influence the solubility of hydrogen, as well as the distribution of the bicarbonate and ethyl carbonate ions in the ethanol-water solvents, and therefore determine the optimal yield of formate.

Note that piperidine-carbamate was not observed from ex situ ¹³ C NMR characterization, although carbamate is readily formed by reacting CO2 with piperidine, a highly basic amine ¹⁸ (pKa = 11.28). Given the extended time (capturing CO2 with piperidine lasted for 40 min in this study), piperidine-carbamate could be fully converted to bicarbonate ¹⁹ or ethyl carbonate in water or ethanol, respectively (FIG. 17, Scheme 1). It was also found that the CO2 hydrogenation rates were faster with piperidine than those with AMP under identical reaction conditions. Due to its strong basicity, piperidine acts as an electron-donating ligand which reduces the bonding energy of the formates on the Pd surface, and thus could improve the hydrogenation activity by enhancing the formate desorption, if the formate desorption would be the rate-limiting step. At the same time, electron donating piperidine also decreased the electron deficient character of the Pd nanocatalysts. ²⁰ Without being bound by theory it is believed that piperidine altered the electronic states of Pd and thus promoted the hydrogenation reactions.

The temperature effect of the hydrogenation of piperidine-captured C02 is shown in Table 1 (entries 3-6). The formate yield increased with increasing reaction temperature from 20 °C to 30 °C but then decreased with further increase in the reaction temperature to 40 °C. Generally speaking, higher reaction temperatures lead to faster hydrogenation kinetics. However, from the thermodynamics point of view, elevated temperatures favor the dehydrogenation reaction and thus shift the equilibrium to hydrogen evolution, which agrees with previous work and reports. ²¹ A detailed kinetic study on the hydrogenation of bicarbonate in pure water and ethyl carbonate in pure ethanol, respectively, has been performed. Both bicarbonate and ethyl carbonate were derived from piperidine-captured CO2. As shown in FIG. 1, in the temperature range of 20-40 °C, the activation energy (Ea) is 64.1 ± 2.1 kJ mol ^-1 for the conversion of bicarbonate to formate in water, while it is slightly lower, 56.2 ± 3.2 kJ mol ^-1 , for the hydrogenation of ethyl carbonate in absolute ethanol. Unlike the comparable activation energies of both reactions, the observed rate of the hydrogenation of ethyl carbonate in ethanol was an order of magnitude higher than that of the hydrogenation of bicarbonate in water, which is likely due to the increased solubility of Fb in ethanol.

Example 2

In addition to studying the hydrogenation reactions, we also investigated the dehydrogenation of the FPA to close the hydrogen storage/evolution cycle. We conducted the dehydrogenation of the FPA (1 M in the aqueous solution with 70 wt% ethanol) in a relatively high temperature range under a N2 atmosphere at a pressure of 1 atm. As shown in FIG. 2, as the reaction temperature increased to 80 °C, the yield of hydrogen reached about 82% after 40 minutes. At 100 °C, a 92.1% yield of hydrogen was achieved after 40 min with a corresponding TOF of 9908 h ^_1 within the initial 5 min. The activation energy of the dehydrogenation was calculated to be 15 kJ mol ^-1 (ESI, FIG. 8†). By switching the aqueous ethanol solvent (70 wt% ethanol) to either pure water or absolute ethanol, however, the generation rate of H2 gas from the FPA became slower (ESI, FIG. 9† Similar to that in the hydrogenation reaction, ethanol also exhibits the co-solvent promotion effect in the dehydrogenation reaction due to the improvement of the solubility of reactants and intermediates, e.g., formates and ethyl carbonate. Whereas by using the aqueous propanol solvent containing 70 wt% alcohol, the hydrogen yield reached about 100%at 100 °C within only 30 min (ESI, FIGS. 10A-10B†) with a record fast rate (TOF = 1.21 x 104 h ^_1 within the initial 5 min) for discharging this hydrogen battery system, which results in an equivalent power density of 77.8 W kg ^-1 . Besides hydrogen, nitro-gen, and a minimal amount of C02, no other gas was detected (CO detection limit is < 1 ppm) (ESI, FIG. 11†). Thus, it was demonstrated that the same Pd/ AC catalyst was active for reversible CO2 hydrogenation/formate dehydrogenation by varying the pressure and the reaction temperature.

Example 3

It is generally accepted that adding base additives promotes both C02 hydrogenation and formic acid dehydrogenation reactions. ^{2a 7e} _’ ²² In this Example, the effect of the loading amount of piperidine on the formate dehydrogenation rate was investigated by varying the concentration of piperidine from 0 M to 5 M. A drastic increase of the hydrogen yield was observed as the concentration of piperidine increased from 0 M to 1 M, but the yield of H2 did not increase further with the increase in the piperidine concentration from 1 M to 5 M (ESI, FIG. 12†). This observation indicates a typical marginal effect of piperidine: once the formate piperidine adducts were formed, the excessive piperidine did not enhance the dehydrogenation rate. The effect of different base types with varied basicity strengths on formate dehydrogenation was also examined. As shown in FIG. 3, the dehydrogenation rates with various bases were in the order of piperidine (pKa = 11.28) ~ NaOH (pKa = 13.8) > AMP (pKa = 9.7) ~ MEA (pKa = 9.5). It seems that given the same molar ratio of formic acid to the base, the higher the pKa of the base, the faster was the dehydrogenation rate. From the thermodynamics point of view, the high pKa of the base would decrease the free energy for both hydrogenation and dehydrogenation reactions. ²³

Example 4

The decomposition of formates can involve multiple steps. In this Example, the kinetic isotope effect (KIE) measurements were used with HCOOH and DCOOH to determine the rate-limiting step and to understand the indispensable role of piperidine in facilitating the dehydrogenation (Table 2). Without being bound by theory it is believed that transient formate species adsorb on the Pd surface followed by critical formate dissociation (ESI, FIG. 18, Scheme SI†). A general scheme of the dehydrogenation of formic acid involves de carboxylation, and thus CO2 and EE are the final products. Adding an amine-like piperidine would facilitate the conversion of formate amine adducts to bicarbonates or ethyl carbonates. The deuterium kinetic isotopic effect (KIE) was higher with DC00H-piperidine-D20 (KIE = 2.1, Table 2, entry 4) than that with HC00H-piperidine-D20 (KIE = 1.1, Table 2, entry 2), showing that the cleavage of the C-H bond in formate is the rate-limiting step for the decomposition of the FPA. Note that the conjugated acid of piperidine, in association with the piperidine-H ⁺ (PIPDFE) species formed via the reaction of piperidine with formic acid, as a proton donor can also facilitate the protonation of the adsorbed formate species, leading to the formation of a Pd-bicarbonate/ethyl carbonate species during the dehydrogenation reaction. The Pd-bicarbonate/ethylcarbonate complex might undergo further desorption from the Pd surface and become the ionic species in the sol-vents. ²⁴ At elevated temperatures, bicarbonate or ethyl carbonate ions are readily decomposed to produce CO ² , which was detected in the dehydrogenation reactions at temperatures higher than 40 °C.

Example 5

To gain insight into the nature of surface intermediates during the FPA dehydrogenation reactions, the Pd/ AC catalyst samples were further characterized during the reaction by in situ ATR-FTIR. We first recorded the IR spectra of the Pd/ AC catalyst when CO flowed through the ATR cell to confirm the position of CO absorbance. A small peak was observed at about 2020 cm ^-1 (ESI, FIG. 13†), which can be assigned to linearly adsorbed CO. ²⁵ The spectra of the Pd/ AC catalyst in the reactive environment was then recorded for the dehydrogenation of the FPA. Notably, as shown in FIG. 4, no peak in the 1800-2100 cm ^-1 range (region of chemisorbed CO) ²⁵ was observed during the dehydrogenation of the FPA, which may be because piperidine suppressed the formation of CO. Boitiaux et al. also reported that piperidine exhibited a ligand effect and thus suppressed the CO formation during the hydrogenation reactions. ²⁰ This is a crucial feature because CO could occupy the active sites on the Pd catalyst surface as a poisoner and consequently deactivate the catalyst. Also, no CO formation during EE evolution is indispensable in a PEM fuel cell since a trace amount of CO would poison the Pt cathode. In contrast, the CO peak was observed during the decomposition of monoethanolamine (MEA)-formate. The above observation suggests that piperidine could inhibit the undesired reaction to form CO, while largely promoting the rate of EE generation. Note that the pKa of PIPDH+ is 11.28, which is larger than that of MEAH+ (pKa = 9.45). Therefore, the electron-donating ability of PIPD should be stronger than that of MEA. Without being bound by theory, it is believed that the stronger electron-donating ability could facilitate the CO desorption from the catalyst surface. Both the spectra of PIPD and MEA showed a negative peak at 1589 cm-1, which is assigned to the vibrations of a surface-bound formate species, ²⁶ indicating that the formate species on the catalyst surface was gradually consumed. Based on the intensity of this peak, the decomposition of the formate with PIPD was completed in 40 min since no further growth of this negative peak was detected after 40 min. As for the spectra of MEA, the intensity of the peak at 1589 cm ^-1 reached a plateau after 1 h. However, this peak is much smaller than that of piperidine which suggests that the MEA formate adduct was not completely decomposed, and instead, the reaction stopped (ESI, FIGS. 14 and 15†). We thus conclude that, due to the CO poisoning, the Pd/ AC catalyst for the dehydrogenation of MEA-formate was deactivated with a prolonged reaction time, which is consistent with the low yield of hydrogen as shown in FIG. 3.

As shown in FIGS. 14 and 15,† the negative peaks at 1375 and 1346 cm ^-1 are ascribed to the C-0 vibrations in HCCb-, CH ₃ CH ₂ C0 ₃ - and HCOCT, respectively, whose intensity increases with the reaction time. However, in the whole spectra, no C-N stretching vibration band (usually at about 1645 and 1518 cm ^-1 ) ²⁷ was observed since there was no consumption or re-formation of piperidine, which indeed acted as a co-catalyst during the reaction. Note that these carbonyl corn-pounds were likely displayed as monodentates ²⁸ on the surface of the Pd catalyst in our reaction system (FIG. 18, Scheme SI†). In contrast, in a high-temperature gas- phase reaction, the bidentate forms of formate adsorbed on the Pd surface usually appear at higher wavenumbers. ²⁵ _’ ²⁶³

After 5 cycles of hydrogenation-dehydrogenation cycling tests, the loss of catalyst activities appeared to be negligible, as shown in FIG. 5. Moreover, piperidine did not decompose at 100 °C during the dehydrogenation reaction (ESI, FIG. 16†). The excellent stability of both the Pd/ AC catalyst and the piperidine solvents suggests that the PFA-based, heterogeneously catalyzed hydrogen storage system is promising in terms of recyclability and reusability. Based on the current best H2 production rates from this study (Table 1), producing 1 kW of electric power would require 5.4 L of the 1 M piperidine formate solution or 0.69 L of the saturated piperidine formate solution (7.6 M at 25 °C), using approximately 27 g of 5 wt% Pd/AC.

Example 6

This Example demonstrates further materials and methods used in Examples 1-5.

Materials

The catalyst samples (Pd/ AC, the metal loadings are 5 wt%) were purchased from Sigma-Aldrich®. Chemicals including NaHC03 (99.5%), ethanolamine (>99%), piperazine (99%), piperidine (99%), 2-amino-2-methyl-l -propanol (95%), 1-propanol (>99%) and 2- propanol (>99%) were also purchased from Sigma-Aldrich®. Ethanol, 200 proof (absolute) was purchased from Decon™ Labs.

Reduction of bicarbonate or alkyl carbonate The experiments on the reduction of bicarbonate or alkyl carbonate were carried out in a 50 mL stirred Parr micro-reactor in the temperature range of 20 °C - 40 °C. The appropriate amount of the Pd/ AC catalyst were added into 20 mL aqueous ethanol solvent with amine captured CO2. The reactor was then sealed, purged with high purity N2 for three times, followed by charging with ¾ to the set pressure. During the reaction, mixing was achieved through an internal propeller operating at 1500 RPM. Once the set temperature was attained, the reactor was held at the set temperature for certain period. After the reaction, the reactor was cooled down to approximately 20 °C, the gas pressure was recorded. Then the reactor was vented. And the liquid was collected for chemical analysis. Typical reaction conditions are: 20 mL solvent, 1 M CO2 -amine solution (CO2 was captured and saturated with the amine solutions), 20 degrees C, 400 psi initial partial pressure of H2, 0.1 g Pd/ AC catalyst loading, 1-hour reaction time. The turnover frequency (TOF) was estimated as follows: [moles formate produced in 5 mins / (moles Pd x 23.2 %) / reaction time]. The dispersion of Pd atoms on the surface of Pd nanoparticles (NPs), which was determined by the carbon monoxide chemisorption, was 23.2%. Since the reactions were carried out in the batch reactor, we used the kinetic rate data within the first 5 minutes of each reaction to estimate the TOF.

Formate decomposition

The experiments on the formate decomposition reactions were carried out in a 50 mL three-necked round bottom flask in the heating mantle. One neck of the flask was connected to a condenser which was then further connected to a NaOH solution (10 M) trap. The trap was connected to a gas burette. The condenser was used to prevent the volatilization of liquid species, and the NaOH trap absorbed CO2 from the decomposition of formate. Before the reaction, the leakproofness of the reaction system was tested, and then the system was purged with N2 gas for 5 minutes to ensure no O2 in the reaction system. The catalytic formate decomposition for discharging hydrogen was initiated by stirring the mixture of the aqueous suspension of the Pd/ AC catalyst (0.1 g) in 20 mL formate amine adduct solution (1 M) after the reaction temperature was reached. The released CO2 and ¾ gases were introduced through the NaOH trap in which CO2 was fully absorbed, and the ¾ gas volume was monitored using the in-line gas burette. To ensure accuracy, each reaction was repeated 3 times and the data were averaged. The turnover frequency (TOF) was estimated as follows: [moles of hydrogen produced in 5 mins / (moles of Pd x 23.2 %) / reaction time].

Aqueous-phase Product Analysis

Aqueous samples collected were filtered through a 0.22 pm pore size filter for high- performance liquid chromatography (HPLC) analysis, which was performed using a Shimadzu HPLC system equipped with a dual UV-VIS Detector (Shimadzu SPD 10-AV) at 208 and 290 nm and a Refractive Index Detector (Shimadzu RID-IOA). For the analysis of formic acid and the reaction intermediates, the samples were separated in an Aminex 87-H column (Bio-Rad), using 5 mM H2S04 as the mobile phase at 0.7 mL/min flow and a column temperature of 55 °C.

NMR measurements were performed on a 2-channel 400 MHz Varian VNMRS with an ATB automation probe. For quantitative 13C NMR studies, the spectra of the sample solutions were recorded at 25 °C with the following acquisition parameters: the pulse width of 90°, the delay time between two transitions (Dl) = 25s, and the number of scans, (NS) = 1000. The time needed for quantitative experiments was approximately 7 hours. The internal standard was 1,4-Dioxane (the carbon chemical shift is at 67.19 ppm).

Gas-phase Product Analysis

After the reaction, the off gas was collected by a 0.6L Tedlar® Gas Sampling Bag and then analyzed by a Shimadzu GC-2014 gas chromatograph, equipped with a HAYESEP-N column (2.5m x l/8in x 2.1mm. SS), a HAYESEP-D column (2.5m x l/8in x 2.1mm. SS), a HAYESEP-S column (2m x l/8in x 2.1mm), a HAYESEP-D column (lm x l/8in x 2.1mm), a MOL SIEVE 5A column (3m x l/8in x 2.1mm. SS), a Carbowax column (2m x l/8in x 2.1mm) and a thermal conductivity detector (TCD), for quantitative analysis of gas phase products.

In-Situ ATR-FTIR

Attenuated total reflectance infrared (ATR-IR) was employed to obtain the information about the surface of the catalysts during the reaction. The measurements were performed using a Bruker Tensor II spectrometer and a custom-made ATR-FTIR cell. Before initiating the measurement, the catalyst sample was ground into fine powder and suspended in nanopore water (1.5 mg sample/mL water) under sonication to form ink, which was then coated onto the internal reflectance element (IRE) and dried for 1 h at 90 °C to form a thin layer. Before each measurement, the Pd/AC sample was reduced in 10% H2/Ar (40 mL/min) at 150 °C. After 1- hour reduction, the flow was switched to 40 mL/min N2 and the sample was purged for 10 min before cooling down. Then 20 pL formate amine adduct was added dropwise on the inked catalysts. Then the ATR cell was sealed after purging N2 and background spectrum of the formate amine adduct adsorbed on the catalyst was taken. During the measurement, the spectra were recorded between 4000 cm ¹ and 800 cm-1 by averaging 128 scans at a resolution of 4 cm ¹ to improve the signal to noise ratio.

Example 7 In Examples 1-6 it was at least demonstrated that a highly efficient reversible hydrogen storage approach can be realized based on the piperidine formate adducts, which are produced by the hydrogenation of piperidine-captured CO2, inaqueous alcohol solutions. As for hydrogen charging, piperidine-captured CO2 shows that the superior hydrogenation reactivity, about 95.5% formate yield, could be obtained in the ethanol-water solution (70 wt% alcohol) with 400 psi H2 after reacting for 1 hour at 30 °C. The kinetic rate of the reverse reaction, hydrogen discharging via dehydrogenation of the piperidine formate adduct in aqueous alcohol solutions, was also fast. The yield of high-purity H2 reached about 100% in 40 min at 100 °C. The impurities such as CO, NH3 or piperidine were not detected in the discharged EE. The deuterium kinetic isotopic study found that the cleavage of the C-H bond in the formate is the rate-limiting step. The mechanistic study by in situ ATR-FTIR characterization discovered that piperidine improves both hydrogenation and dehydrogenation reactivity and no surface bound CO was formed during the dehydrogenation reactions. It was also observed that the Pd/ AC catalyst is highly stable and easy to handle and recycle, and so is piperidine. The storage of renewable energy can thus be realized through the“hydrogen battery”, in which the piperidine formate adduct solutions store the hydrogen generated via water splitting with electrical energy from renewable resources such as solar, wind, geothermal energy etc.

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It was observed that the alcohol co-solvent strongly influences the kinetics of hydrogenation, as shown in FIG. 19. By comparing the hydrogenation efficiency of ammonium bicarbonate ((NH4HCO3) and sodium bicarbonate (NaHCC ) with different ratios of water/ethanol mixed solvent solutions, it was found that ethanol favors the hydrogenation of NH4HCO3, while the hydrogenation of NaHCC was suppressed with ethanol. The hydrogenation efficiency of NH4HCO3 was 10 times higher than that of NaHCC under the otherwise identical reaction conditions. The ¹³ C NMR spectra of NH4HCO3 and NH4HCO2 before and after the hydrogenation in ethanol solvent, respectively, revealed two types of signals (FIG. 20): 1) signals at 160.2 ppm assigned to ethyl carbonate ions; 2) signals at 171.8 ppm originated from formate ions. F. Mani et al. Green Chem., 2006, 8, 995-1000. Ethyl carbonate ions were detected only in the presence of both ammonium ions and ethanol.

Example 9

1. Introduction

The Intergovernmental Panel on Climate Change (IPCC) has prominently featured the potential of coupling bioenergy with carbon capture and storage (BECCS) as a critical negative-CCk-emissions technology for achieving energy and climate goals by the end of the century. ^{1 2} BECCS provides a way of removing and sequestering CO2 from air via the biomass carbon intermediates. CO2 initially assimilated by biomass is released in biorefineries through combustion, gasification or fermentation of that biomass to generate usable energy (electricity, hydrocarbons, EE, etc). This carbon-neutral biogenic CO2, the waste by-product in biorefmeries, becomes carbon-negative after being re-captured and stored underground. BECCS is believed to have the potential to remove up to 3.3 Gt of CO2 per year (t = metric ton or tonne) while supplies as much a 170 EJ yr ^-1 of renewable bioenergy ³ . However, the wide deployment of BECCS encounters many challenges such as excessive land and water usage and high cost of CCS ^{4 5} . Therefore, the sustainability of BECCS relies heavily on intelligent management of the supply chain and the cost reduction. Here, using a portion of high-purity biogenic CO2 as the raw carbon material to produce value-added chemicals could substantially improve the BECCS process economy as the profits from manufacturing CCk-derived chemical products could subsidize the BECCS costs.

CO2 can be transformed into a variety of value-added substances including methane, methanol, formaldehyde, formic acid, and organic carbonates. ^6-14 Among all possible candidates of products, formic acid or formate is an attractive Cl commodity product used as a preservative and antibacterial agent in animal feed and an eco-friendly acid for leather processing. Moreover, the surging demand for renewable energy storage materials may create a potentially huge market for formic acid and formate salts, which are heavily researched as the hydrogen carriers. ¹⁵ BECCS could supply endless cheap CO ₂ feedstocks for formate production even though the amount of CO ₂ fixed in chemicals is negligible compared to the CO ₂ emissions. However, by leveraging the existing infrastructure of BECCS, the production cost of formate chemicals from biomass CO ₂ may be low enough to guarantee a high profit margin of the chemical products. In industry, scrubbing CO ₂ gas with aqueous amine solvents is a mature carbon capture technology, in which the separation of CO ₂ from solvents is the most costly step. Therefore, if the captured CO ₂ in liquid amine solvents could be directly used for producing formates, the costs of CO ₂ separation, compression, and sequestration can be saved.

Today, CO ₂ hydrogenation in the liquid phase is dominated by homogeneous catalysis. Despite the fact that homogeneous catalysts exhibit remarkable efficiencies for the hydrogenation of CO ₂ to formic acid/formates, most homogeneous processes do not captivate industrial attention due to 1) the use of expensive but non-robust homogeneous catalysts; 2) the use of peculiar reaction conditions (requires excess ligand(s) and solvents in extreme conditions); ¹⁶ and 3) the difficulty of catalyst separation. ¹⁷ The majority of manufactured products in chemical industry are enabled by heterogeneous catalysis. However, the development of heterogeneous catalysts for hydrogenation of CO ₂ has been sluggish because of the inferior kinetics. ^{18 19} So far the slow kinetics still plague the heterogeneously catalytic hydrogenation of CO ₂ , with the reaction mechanisms poorly understood. Therefore, there is potential to develop efficient and cost-effective heterogeneous catalysts with much-enhanced activity, stability, and recyclability for industrial production of formates from CO ₂ hydrogenation.

It has been demonstrated the feasibility of a CO ₂ utilization strategy using ammonia captured CO ₂ , in the form of ammonium bicarbonate, as the feedstock to be hydrogenated to ammonium formate over a carbon supported Pd catalyst. ²⁰ It was found that an ethanol co solvent significantly improves the hydrogenation kinetic rate. In this Example, the CO ₂ absorbents where changed from ammonia to amines since the latter are industrially more widely used. ²¹ Note that ethanol-water solvents can be obtained from bio-refineries at a low cost. In this Example, a strategy of integrating biorefinery, CO ₂ capture, and hydrogenation of captured CO ₂ to produce formate chemicals is demonstrated.

2. Results and discussion 2.1 Effect of amine structure

Alkanolamines are widely used as the absorbents for CO2 capture. The structures of alkanolamines have a significant effect on their capture behavior. To compare the effects on the CO2 capture due to various properties of amines, such as chemical structure, solubility, functional groups, and so on, three groups of amines as model compounds were selected: (1) linear amines; (2) sterically hindered amines; and (3) cyclic amines (structures are shown in FIG. 21A). After capturing CO2 with different amines, CO2 hydrogenation studies were performed, where CO2 was captured with various amines in the ethanol -water solutions _. Upon using a monoamine, including monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA), a volcano shape of the formate yield was observed, while the formate yield by using the ethane diamine (EDA) shows a monotonically decreasing trend (FIG. 21B), with increasing the ethanol content in the ethanol-water solvents. The two amine groups in an EDA molecule increase the capacity of capturing CO2, resulting in a formate yield that is twice as large as that with MEA in water due to the increased concentration of the active species for hydrogenation, i.e., bicarbonate. However, increasing the weight percentage of ethanol in the aqueous solvent decreased the formate yield as ethanol stabilizes the EDA-carbamate complex. The ¹³ C NMR spectra in FIG. 27 show that the amount of stable carbamate increased as the ethanol content in the solvents increased from 0% to 70%. The EDA-carbamate complex was precipated in 100% ethanol and its NMR spectra exhibits the characteristic peak of carbamate at 165 ppm (FIG. 28). Contrarily, MEA and DEA may form a ring structure through the intramolecular hydrogen bond ²² between the hydroxyl group and the amine group (FIG. 29), which destabilizes the MEA-carbamate or DEA-carbamate complex.

Among the sterically hindered amines, 2 -Amino-2 -m ethyl- 1 -propanol (AMP) gives the highest formate yield due to its high pKa, steric effects, and also a good solubility in ethanol (FIG. 21C). Chakraborty el al. ¹ ' proposed that substitution at the R-carbon atom results in the interaction of the P _M0 and P _M6 * methyl group orbitals with the lone pair of the nitrogen. This interaction reduces the charge at the nitrogen, making it a softer base which results in a weakening of the N-H bond, and allows a greater level of hydrolysis or alcoholysis by the hydroxide (hard base) in solution. Thus, the sterically hindered effect would be expected to enhance the hydrogenation reactivity through a shift from the inactive carbamate to the active species, i.e., bicarbonate and alkylcarbonate. As for the cyclic amines, the formate yield of piperidine (PIPD) > 2-methylpiperazine (2-MPZ) > piperazine (PZ) > 1-methylpiperazine (1- MPZ) was observed (FIG. 21D). Though the PIPD showed higher activity ²⁴ than AMP on the hydrogenation of the captured CO2, AMP is still prior to PIPD since it is nontoxic and one of the most common amines used in CO2 capture industrially.

2.2 Hydrogenation of amine captured CO2 in ethanol-water solvents

It was demonstrated that adding ethanol as a co-solvent substantially increased the 5 kinetic rates of hydrogenation of ammonium carbonate/carbamates. Therefore, in this Example, ethanol was again used as a co-solvent but switched ammonia to amines. Of these commonly used amines, AMP has the sterically hindered structure and by stoichiometry, it can react with CO2 at a ratio of - 1.0 mole of CO2 per mole of amine (FIG. 30). The promotional effect of an ethanol co-solvent on the yield of formates was significant in the catalytic 10 hydrogenation of AMP-captured CO2 in ethanol -water solutions. As shown in Table SI, the yield of formate was 19.8% after a 1-hour hydrogenation reaction with a TOF of 303 h ¹ on the activated carbon supported palladium catalyst (5% Pd/ AC) in water at 20 °C (Table SI entry 1). When increasing the weight percentage of ethanol in the solvent, the formate yield and the TOF gradually increased to the maximum values, -50.5% and 777 h ¹ , respectively, with the 15 azeotropic aqueous ethanol solvent (95.6 wt% ethanol) (Table 3 entry 8). Note that amine- ethanol-water solutions are more viscous than aqueous amine solutions.

Table 3. Distribution of the CO2 derived intermediates and hydrogenation of AMP captured CO2 at various mass fraction of ethanol in water.

Capture ¹³¹ and

Hydrogenation ^[bI Captured CO2 species concentration (M) ^[cI Conversion results

Entry

Solvent HCOi CO3 ² RNCO ² Ethyl-CC>2 ^~ Formate Yield oN _[d ^I (wt % EtOH) Bicarbonate Carbonate Carbamate Ethyl Carbonate (%)

1 0 0.93 0.03 0.00 0 19.8 303

2 30 0.90 0.01 0.01 0.04 23.1 365

3 50 0.73 0.01 0.01 0.21 34.1 538

4 60 0.45 0.00 0.03 0.48 35.7 564

5 70 0.32 0.00 0.03 0.61 42.5 650

6 80 0.18 0.00 0.03 0.75 47.8 722

7 90 0.03 0.00 0.03 0.90 49.4 753

8 95.6 0.01 0.00 0.03 0.92 50.5 777 9 100 0.00 0.00 0.03 0.93 38.1 586

[a] CO2 capture conditions: 20 ml amine/water-ethanol, 1 M AMP, 20 ^° C, 40 min.

[b] Hydrogenation conditions: 50 ml autoclave parr reactor, CO2 capture solution (20 mL), 0.1 g Pd NPS on carbon (5 wt%), 400 psi hydrogen, 20 ^° C, 1 h.

[c] The captured CO2 species concentrations were determined by ^CpH} NMR spectroscopy.

[d] TON was calculated by: [moles formate/(moles Pd ^x 23.2 %)]. 23.2% represents the dispersion of Pd atoms on the surface of Pd NPs determined by carbon monoxide chemisorption.

However, it ws observed the decrease of formate yield in pure ethanol, -38.1%, as compared to that in azeotropic ethanol, -50.5% (Table 3 entries 8 and 9), which may be 20 attributed to the low solubility of AMP -formate in pure ethanol and therefore, the desorption

of formate from the Pd surface may become the rate-limiting step. ²⁵ Ethanol-water azeotrope, which is readily available from distilling bio-ethanol, costs much less than absolute ethanol. Therefore, using ethanol-water azeotrope as the amine solvent to capture and hydrogenate CO2 from a biorefinery is potentially cost-effective from the process intensification point of view. Table 4 shows the results of catalytic hydrogenation of AMP captured CO2 in ethanol- water azeotrope with different catalysts. It was found that only the Pd catalysts showed an enhanced catalytic activity and other transition metals such as supported Ru, Rh, Pt, and Ni were almost inactive under the ambient temperature. It was also observed that, among the Pd catalysts, carbon support is superior to other supports such as AI2O3 and CaCC . It is commonly accepted that carbon has a hydrogen storage nature. ^26-28

Table 4. Catalytic hydrogenation of AMP captured CO2 over different catalysts ^[a]

Conversion Result

Entry Catalyst ¹¹⁵¹ Formate

Yield (%)

1 Pd/Al ₂ 0 ₃ 12.5 191

2 Ni/AC 0 0

3 Pt/AC 0 0

4 Ru/AC 0 0

5 Rh/AC 0 0

6 Pd/CaCOs 0 0

7 Pd/AC 50.5 777

[a] 1M AMP with a CO capture capacity (mole CCb/mole amine) of 0.96.

[b] Reaction conditions: 20 ml capture solution, 0.1 g catalyst (5 wt % metal loading on the support), 400 psi ¾, 20 °C, one hour. Solvent (for both capture and hydrogenation) is 95.6 wt % ethanol (ethanol azeotrope).

[c] TON = [total moles formate formed/(total moles of metal ^x metal dispersion)]. The metal dispersion see Table 5.

Table 5. H2 Pulse Chemisorption analysis results

Sample ^[aI

Element Metal Dispersion

(R)

Pd/AC Pd 23.3%

Pt/AC Pt 42.0%

Ru/AC Ru 27.3%

Rh/AC Rh 24.3%

Ni/AC Ni 16.5%

Pd/Al ₂ 0 ₃ Pd 13.6 %

Pd/CaC0 ₃ Pd 10.3 %

Pd/BaS0 ₄ Pd 5.8 %

The initial ¾ pressure (FIG. 22A), reaction temperature (FIG. 21B), and reaction time (FIG. 21C) are also key factors that influence the hydrogenation of AMP captured CO2. Under the optimized conditions, at a higher ¾ gas pressure of 400 psi, the formate yield reached ~ 100% in 2.6 hours at a near-ambient temperature, 30 °C, and the corresponding TOF reached

1272 h ¹ . Therefore, it is possible to integrate the CO2 capture process, which is usually operated at approximately 60 °C in a CCS process, with the hydrogenation process without a need for additional energy consumption for heating.

Our results suggest that the hydrogenation of amine captured CO2 over the heterogeneous Pd/ AC catalyst can be more efficient than the homogeneous counterpart. ²⁹ The superior stability of the Pd/ AC catalyst and amine agent at near ambient temperatures in our CO2 capture and hydrogenation reaction is particularly an advantage. As shown in FIG. 31, the spent Pd/ AC catalyst had negligible activity loss as compared to the fresh catalyst after five times repeated reactions without regenerating the catalysts. The XRD and TEM (FIGS. 32 and 33A-33B) characterization also showed that the spent Pd catalysts have no obvious sintering and aggregation compared to the fresh catalyst. Since our process is carried out at low temperatures (20-30 °C) and in the presence of Eh, neither thermal nor oxidative degradation of AMP is observed (FIG. 34).

2.3 Mechanistic study

The ¹³ C NMR spectra, as shown in FIG. 23 A, disclosed only one peak at 161.2 ppm as CO2 was captured by AMP in water, which is assigned to the bicarbonate/carbonate ion pair. In contrast, an additional peak signal at 159.5 ppm was detected with adding ethanol as a co solvent, which is consistent with our previous observation that this peak indicates the formation of ethyl carbonate ions. ²⁰ Similar to what was observed in the hydrogenation of ammonium carbonate, the compositions of the CO2 derived intermediates changed with the ethanol content in the hydrogenation of AMP captured CO2 (FIG. 23B). The proposed mechanism of the formation of bicarbonate or ethyl carbonate intermediates is shown in FIG. 23C. CO2 was captured by AMP to generate the carbamate first (step l). ^{30 31} In the presence of water and alcohol, the carbamate can be transformed to ethyl carbonate (step 2) and bicarbonate (step 3), respectively.

Besides ethanol, it was further found that other alcohol solvents also promoted the formation of alkyl carbonates, while the pKa values of these solvents influenced on the hydrogenation of alkyl carbonates. As shown in FIG. 35, among the pure alcohol solvents, the formate yield decreases in the order of: 2-propanol > 1-propanol > ethanol » methanol ( ¹³ C NMR spectra disclosed methyl carbonate peak in FIG. 36). Furthermore, it was demonstrated that the AMP-carbamate itself has no hydrogenation reactivity since it is unable to be converted to alkyl carbonates or bicarbonates in pure acetone (FIG. 37). To better understand the co solvent effect of ethanol on the rate of hydrogenating AMP captured CO2, the kinetic studies in pure water, as well as in pure ethanol, were performed (FIGS. 38, 39, and 40). With the same initial CO2 concentration, the hydrogenation of CO2 was always faster in absolute ethanol than in water.

To better understand the co-solvent effect of ethanol on the rate of hydrogenating AMP captured CO2, the kinetic studies in pure water, as well as in pure ethanol, were performed. As shown in the FIG. 38, the production rates of formate via either bicarbonate or ethyl carbonate intermediates fit well with the first-order reaction kinetics regardless of the different initial CO2 concentrations or solvent compositions. With the same initial CO2 concentration, the hydrogenation of CO2 was always faster in absolute ethanol than in water. FIG. 39 shows that the initial rates of the hydrogenation of CO2 were linearly related to the initial CO2 concentrations. By taking the hydrogen pressure into account, the overall rate of this pseudo- first order reaction can be interpreted in the following form,

x = k ^' [C0 ₂ ] = k [C0 ₂ ] [H ₂ ] ^a

where k ^' is the overall effective rate constant, and k is the intrinsic rate constant of hydrogenation, which decouples the influence of the hydrogen pressure. The hydrogen concentration in the both solvents can be obtained from Henry’s law ([¾] = KaPm). Note that the Henry’s constant of ¾ in ethanol is an order of magnitude larger than that in water at ambient temperature. The effective rate constant of hydrogenation via ethyl carbonate (2.2x 10 ⁴ s ¹ ) is larger than that via bicarbonate ( 1 2 ^/ 10 ⁴ s ¹ ). The activation energy was determined by using the Arrhenius equation {k = A exp( E _a /R )), as shown in FIG. 40. In the temperature range of 20-40 °C, the activation energy (A,) is 31.9±2.1 kJ/mol in the conversion of bicarbonate to formate in water while it is 118.9±3.2 kJ/mol in the conversion of ethyl carbonate to formate in absolute ethanol. The higher energy barrier of hydrogenation of ethyl carbonate can be counteracted by the higher ¾ solubility in ethanol. ² At the same time, a high activation energy also implies a high sensitivity to a temperature change in a hydrogenation reaction. Therefore, the kinetic rate of hydrogenation of ethyl carbonate could increase substantially at elevated temperatures.

To elucidate the reaction mechanism, the ¹³ C NMR spectra of the liquid-phase products after the hydrogenation of AMP captured CO2 in the ethanol-water solvent at different reaction times was examined. As shown in FIG. 41, the peak located at 170 ppm is assigned to formate. When increasing the reaction time from 0.5 h to 4 h, the amount of formate (peak F) increased while those of both bicarbonate/carbonate (peak B) and ethyl carbonate (peak Ci, C2 _, and C3) decreased. Moreover, the selectivity of the hydrogenation of amine captured CO2 was 100% as the formate was the only detected product after the reaction. This direct evidence re confirmed that both ethyl carbonate and bicarbonate are readily hydrogenated to formate over the Pd/ AC catalyst. Our in-situ ATR-FTIR results (FIG. 24A) shows how the intermediates and products adsorbed on the catalyst surface changed over time. The increasing peaks at around 1600, 1360, 1300 and 1150 cm ¹ with increasing time are vas(C0 ₂ ), 5(CH), vs(C0 ₂ ) and vs(CO), respectively, which can be assigned to monodentate formate on Pd/AC surface. The peaks at 1400 cm ¹ and 1670 cm ¹ (vas(C0 ₃ ) and vs(C0 ₃ ), respectively) decreased with increasing time, which are assigned to ethyl carbonate on Pd/ AC surface. The decreasing trend of these peaks indicates the consumption of ethyl carbonate during the hydrogenation. Herein, a possible reaction pathway was proposed according to the surface species on the catalyst surface and applying the theory of the insertion of bicarbonate or ethyl carbonate onto the metal surface ^{32 33} to form the complex 1, as shown in FIG. 24B. The adsorbed complex 1 can be hydrogenated by ¾ and release an ethanol molecule, generating the complex 2. For both pathways in water and ethanol, the basicity of amine, i.e., AMP, facilitates the b elimination of bicarbonate or ethyl carbonate species on the catalyst surface. The protonated AMP can protonate the complex 1 to form 2 in the presence of ¾. The addition of base and alcohol decreases the free energy of hydrogenation of CO2. Thus, from a thermodynamic point of view, hydrogenation of CO2 is favorable when adding a base and alcohol solvents.

2.4 Density Functional Theory Study

Theoretical investigations examining the possible mechanisms for the hydrogenation of C0 ₂ to formate via an ethyl carbonate intermediate on Pd(l l l) in the presence of the ethanol solvent are reported here. To understand the mechanism, the adsorption geometries of the reacting species and their interaction with the surrounding adspecies on Pd(l l l) was first modeled. Then the hydrogenation reaction of ethyl carbonate to formate under the Eley-Rideal and Langmuir-Hinshelwood mechanism was evaluated. ^34-36

(1) Adspecies adsorption configurations and energies on Pd(lll)

Ethyl carbonate preferentially binds to Pd(l l l) in a bidentate structure with two oxygen atoms bonded to Pd top sites with the center carbon over a bridge site, as shown in FIG. 25A (panel a). At a coverage of 0.06 ML, ethyl carbonate on Pd(l 11) has an adsorption energy of - 3.12 eV. Formate on Pd(l 11) adopts an analogous configuration to ethyl carbonate (FIG. 25A (panel b)) with a slightly stronger adsorption energy. The configuration resembles those reported in literature. ³⁷ _’ ³⁸ Ethanol adsorbs to Pd(l 11) through a lone electron pair on a top site, ³⁹ illustrated in FIG. 25A (panel c), with a much weaker binding energy of -0.85 eV as compared to ethyl carbonate and formate. This is due to the closed shell nature of gas phase ethanol versus the formate and ethyl carbonate gas phase radicals. Hydrogen on Pd(l l l) adsorbs nearly iso- energetically at the fee and hep sites with an adsorption energy of ~4.1 eV relative to ¾ in the gas phase , consistent with the literature. ⁴⁰ The strong adsorption energy of hydrogen on palladium is consistent with the good catalytic hydrogenation activity. ⁴¹ " ⁴⁴ Carbon dioxide adsorbs to Pd(l 11) with an adsorption energy of -0.28 eV with a surface-to-carbon distance of around 3.02 A, in agreement with previous studies. ^{45 46} Together with results of the influence of surface coverage on adsorption energies for ethyl carbonate, formate, and ethanol and solvent effects on the stability of ethyl carbonate, it was demonstrated that the adspecies interaction is much higher for ethyl carbonate as compared to other adsorbates.

(2) Hydrogenation of ethyl carbonate to formate

The hydrogenation of ethyl carbonate into ethanol and formate on Pd(l l l) was first examined using an Eley-Riedel (E-R) mechanism. As shown in FIG. 25B and Eq. 2, adsorbed ethyl carbonate will react with H ₂ on Pd(l 11).

[C ₃ H ₅ 0 ₃ ](a _ds) + H ₂ (g) [HCOOfeasT C ₂ H ₅ OH(g) (Eq. 2)

In this E-R reaction pathway, molecular hydrogen approaches ethyl carbonate and the H-H bond length in hydrogen increases along with the Cl -03 bond in ethyl carbonate (FIG. 25A( panel a)) in such a way that hydrogen addition and Cl -03 bond scission occurs simultaneously. The reaction energy, DE _ΐch , at a low total adspecies coverage (0.06 ML) is -0.63 eV (FIG. 25B (panel a)). This assumes either that there is no ethanol adsorbed on the surface or, if adsorbed, they are too far from the reacting species such that there is no effect of ethanol on reaction energy. In the final state, formate remains bound to the surface whereas ethanol desorbs. The reaction energy for the hydrogenation of ethyl carbonate in the presence of ethanol (FIG. 25B (panels b, c) ) increases the reaction energy by -0.3 eV when the total adspecies coverage is increased from 0.06 ML to 0.25 ML. This is likely a result of the adspecies lateral interactions between the adspecies and of the steric hindrance, causing the reaction to become less favorable. However, even under the crowded surface conditions, the E-R mechanism for ethyl carbonate hydrogenation is still exothermic. On the other hand, it is noted that the energetic favorability of the Langmuir-Hinshelwood (L-H) mechanism is not likely to be kinetically favorable due to the unstable geometries required for the reaction to occur. The current DFT- based results suggest that ethyl carbonate can readily hydrogenate to formate on Pd(l l l) through an E-R mechanism.

2.5 Techno-economic assessment

In order to evaluate the commercialization protential of our technology, the techno- economic assessment of the designed process of producing calcium formate (Ca(HCOO) ₂ ), as shown in FIG. 42, is performed. Calcium formate is widely used as feed or food additives ^{47 48} that are twice as the price of the industrial-grade Ca(HCOO)2. The plant which is assumed to be located in North America at a commercial scale of producing 12769.4 kg/h (102 kt/yr) of Ca(HCOO)2. The process is simulated through Aspen Plus V9 using the ELECNRTL method with the experimental kinetic data and the synchronized scaled-up reaction conditions. The most recent stream prices in the market were used for the techno-economic analysis, ^{49 50} as shown in Table 6. Conservatively, the price of Ca(CHOO)2 is referred to the price of the industrial-grade Ca(CHOO)2 _, ~ 640.46 $/tonne.

Table 6. Stream Price

Stream ID Source Destination basis Price Unite

AMP ABSORBER Mass 64.74 $/ kg ^a _’ ^b

CACL2 MIXER2 Mass 140 $/ tonne ^c

CAO MIXER4 Mass 80 $/ tonne ^c

C02 ABSORBER Mass 45.5 $/ tonne ⁸

H2 MIXER Mass 1818.18 $/ tonne ⁹

NAOH MIXER3 Mass 300 $/ tonne ^c

CAC2H204 FILTER1 Mass 650 $/ tonne ^c CA02H2 FILTER3 Mass 100 $/ tonne ^c NACL FILTER2 Mass 85 $/ tonne ^c

Note: ^a Online information available at: https://www.sigmaaldrich.com; ^b Chicago Board of

Trade (CBOT); Online information available at: https://www.alibaba.com

FIG. 26A shows the distributions of total capital cost and average operating cost/year. The equipment cost is the dominant part of the capital cost. This is because Reactors 1-3 for the reduction of CO2 must be operated at a high EE gas pressure of 400 psi that raises total direct cost and equipment cost significantly, as shown in Table 7. Among these costs, the raw materials cost represents 86.36% of the total operating cost (FIG. 26B), implying that looking for rock-bottom raw materials is critical for making the plant more profitable. To this end, using the captured CO2 from biorefmeries directly is a plus. A project net present value (NPV) is positive after 4 years (FIG. 26C), indicating that our process based on the heterogeneous catalysis will make the plant profitable.

Table 7. Equipment _

Area Name Componen Compone Total Equipme Equipme Installe t Name nt Type Direct nt Cost nt d

Cost Weight Weight

_ (USD) (USD) LBS LBS

Miscellaneous FILTER2 EF 34300 18400 0 1390 Flowsheet Area TUBULA

R

Miscellaneous MIXER4 c 0 0 0 0

Flowsheet Area Miscellaneous REACTOR DAT 3.87E+0 3.37E+06 1.35E+06 1.47E+0 Flowsheet Area 1 REACTO 6 6

R

Miscellaneous MIXER3 C 0 0 0 0

Flowsheet Area

Miscellaneous MIXER C 0 0 0 0

Flowsheet Area

Miscellaneous ABSORBE DTW 567400 286800 78900 125800 Flowsheet Area R-tower TOWER

Miscellaneous REACTOR DAT 315900 154900 22500 43771 Flowsheet Area 6 REACTO

R

Miscellaneous FILTER3 EF 33600 17800 0 1390 Flowsheet Area TUBULA

R

Miscellaneous MIXER2 C 0 0 0 0

Flowsheet Area

Miscellaneous FILTER1 EF 34300 18400 0 1390 Flowsheet Area TUBULA

R

Miscellaneous REACTOR DAT 3.39E+0 2.93E+06 1.15E+06 1.26E+0 Flowsheet Area 2 REACTO 6 6

R

Miscellaneous FLASH- DVT 152400 49800 21100 34818 Flowsheet Area flash vessel CYLINDE

R

Miscellaneous REACTOR DAT 357000 193700 28700 49466 Flowsheet Area 7 REACTO

R

Miscellaneous REACTOR DAT 383800 168400 28800 50972 Flowsheet Area 5 REACTO

R

Miscellaneous PUMP DCP 109000 61100 3400 10360 Flowsheet Area CENTRIF

Miscellaneous REACTOR DAT 3.25E+0 2.80E+06 1.09E+06 1.20E+0 Flowsheet Area 3 REACTO 6 6

R

Miscellaneous PUMP2 DCP 83200 49700 2700 6295 Flowsheet Area CENTRIF

Miscellaneous COOLER DHE 157000 50500 16900 40091 Flowsheet Area TEMA

EXCH

In order to know the elastic stability of this project during operation, the effects of the prices of the main raw materials (FE, CO2) and product (Ca(CHOO)2) on the payout period were investigated. The payout period increases with increasing the CO2 price. As shown in FIG. 26D, the payout period sensitivity with respect to the price of CO2 is much higher than that with respect to the FE price. Luckily, the price of CO2 is projected to decrease with time. ⁵¹ In the current study, the fluctuation of the prices of CO2 and EE cannot lead to unprofitable operation. FIG. 26D also depicts the sensitivity of the payout period with respect to the price of Ca(CHOO)2. The result indicates that the payout period will be shorter if the price of Ca(CHOO)2 increases.

In conclusion, this Example can demonstrate the feasibility of a novel CO2 utilization strategy by integrating with biorefmeries. With amine captured CO2 as the feedstock, the formate chemicals were produced via hydrogenation over the Pd/AC catalyst in aqueous ethanol media. At the optimized reaction conditions, a -100 % formate yield was obtained from the hydrogenation of aminomethyl propanol captured CO2 in an azeotropic ethanol solution with 2.75 MPa EE in 3 hours at 30 ^° C. Among the CO2 derived ionic intermediates, bicarbonate and ethyl carbonate ions are liable to hydrogenation, while carbamate itself has no hydrogenation reactivity unless it is converted into bicarbonate or alkylcarbonate via hydrolysis or alcoholysis. The DFT results show that hydrogenation of CO2 to formate via ethyl carbonate on Pd (111) can proceed through the Eley-Rideal mechanism. By achieving a high yield of formate and rapid conversion of CO2 at the same time, this novel BECCU process could potentially be scaled up. The techno-economic analysis of a designed process for the production of calcium formate (Ca(HCOO)2) indicates a project net present value (NPV). Our findings may open a door for CO2 utilization since this new process has the potential to integrate with biorefmeries to produce value-added carbon-neutral chemicals, as well as to leverage the existing industrial infrastructure for commercialization.

3. The process flow in detail

Absorption process

According to the capacity of the plant, about 8872.38 kg/h of CO2 and 406.37 kg/h of EE are required as the main feeds. In our experiments, 1 mole of AMP in an aqueous solution or different proportion of ethanol and water co-solvent can capture 0.98 or 0.96 mole of CO2, respectively. This process is designed to capture 0.9 mole of CO2 per mole of AMP where the concentration of AMP in the azeotropic aqueous ethanol solvent is up to 3.5 M. The absorbent contained 17.74 % (mol fraction) of AMP and the azeotropic aqueous ethanol solvent (95.6 wt% ethanol) and is fed to the first stage of the absorber in the flow of 1183.91 kmol/hr to absorb CO2 (8872.38 kmol/hr) from the 11 stage of the absorber. The mole flow of CO2 in the gas-out stream is negligible, implying that nearly all the CO2 is absorbed via the counter- current absorption process. The chemical absorption process is exothermic thus the additional heat evaporates a portion of ethanol and water at the top of the absorber, resulting in the loss of the solvent. After the gaseous ethanol is cooled down, it is pumped to the high-pressure Reactor 1 together with the liquid from the bottom of the absorber. CO2 is removed by using the ethanol solution of AMP, where the chemical reaction is reversible, as shown in the following chemical equation,

AMP + C02 <® AMP - C0 ₂

Reaction stage

The outlet liquid (stream 1) from the absorber is pumped for the hydrogenation process in the presence of the heterogeneous palladium on activated carbon (Pd/AC, Pd 10 %) in the reactor vessels. At a higher ¾ gas pressure of 400 psi and 30 °C, the hydrogenation of CO2 captured by AMP -bioethanol solvent is represented and completed by 3 serial kinetic reactors (RCSTR) in which the average residence time is 3 hr. The formation of FA-AMP adduct is simplified as,

AMP - C0 ₂ + H ₂ ® AMP - HCOOH

The conversion of CO2 reaches 97.3 % in reactor 3. H2 is excess (300 kmol/hr) to ensure the complete conversion of CO2.

The products achieve the gas-liquid separation in a flash column at 30 °C where the unconverted H2 is recycled (stream H2-RECYC) for the next-generation hydrogenation of CO2. In order to facilitate the calculation, stream H2-RECYC disconnects the Mixer but will be recycled. The liquid phase that contains AMP -HCOOH is subjected to the decomposition by ion precipitation in the next stage.

As the heterogeneous catalyst can be recovered directly in-situ, it has been assumed that the Pd/ AC is renewed once every two years. The cost of Pd/ AC is 72.40 $/h, ⁷ namely 5.67

$/tonne Ca(CH02)2.

Separation stage for the recovery of Ca(HCOO) ₂

The separation of formate can be achieved by precipitating the formate with CaCF (294.3 kmol/hr). The reaction liquid (stream 7) is mixed with CaCF and undergoing the precipitation reaction in reactor 5, where the calcium formate (Ca(CHOO)2) forms the solid in the azeotropic aqueous ethanol solvent. At the same time, the AMP-HCl forms. The liquid and solid mixture is filtered and the Ca(HCOO)2 is obtained.

2AMP - HCOOH + CaCl ₂ ® Ca(HCOO) ₂ (s) + 2AMP-HC1

Recoveries of AMP and the azeotropic aqueous ethanol solvent

AMP-HCl is recovered from filtrate by the introduction of NaOH (398.4 kmol/hr) in the reactor 6 where NaCl forms the solid in the azeotropic aqueous ethanol solvent. ² Then the liquid and solid mixture is filtered and the NaCl is obtained. The extra H2O is produced during the removal of chlorine. Therefore the partial removal of water is necessary for the formation of the azeotropic aqueous ethanol solvent with AMP in it. The removal of water is carried out in reactor 7 where CaO (199.1 kmol/hr) is introduced in it. The solid by-product (Ca(OH)2) is filtered and the filtrate is ready for the next absorption of CO2. In order to facilitate the calculation, the filtrate disconnects the absorber but will be recycled.

AMP-HC1 + NaOH ® NaCl (s) + AMP + H ₂ 0

H ₂ 0 + CaO ® Ca(OH) ₂ (s)

Since AMP and the azeotropic aqueous ethanol solvent used can be totally recovered, it has been assumed that the AMP stream is renewed once every ten years. The AMP and azeotropic aqueous ethanol solvent reserves will cost 49.39 $/h, namely 3.87 $/tonne Ca(CHOO)2.

4. Methods

Experimental Section

Materials: The catalyst samples Pd/AC (5 wt% and 10 wt%), Pd/CaCC , Pd/BaS0 ₄ , Pd/AkC , Ru/AC, Pt/AC, Rh/AC (5 wt%) were purchased from Sigma-Aldrich®. Ni/AC (5 wt%) were prepared by an impregnation method. The chemicals samples ethanolamine (>99%), piperazine (99%), diethanolamine (>98%), triethanolamine (98%), and 2-amino-2- methyl-1 -propanol (95 %) ethane- 1,2-diamine (99%), 2-amino-2methyl-l, 3-propanediol (99%) and 2-amino-2-hydroxymethyl-l, 3-propanediol (99%), 1-methyl piperazine, and 2- methyl piperazine (2-MPZ) were also purchased from Sigma-Aldrich®.

CO ₂ capture with an amine: CO2 capture was carried out in a 50 mL flask with a magnetic stirring system at 500 RPM with 20 mL amine solution (1M) in an ethanol-water solvent (0-100% wt% ethanol). The flask was then charged with 150 ml/min CO2 gas for 40 mins maintained at 20 °C with a water bath. The amount of CO2 absorbed was determined by the weight difference using an analytical balance.

Hydrogenation of amine captured CO ₂ : The low-temperature hydrogenation of amine captured CO2 experiments were carried out in a 50 mL stirred Parr micro-reactor. The appropriate amounts of CO2 amine solution and catalyst were added to the reactor. The reactor was then sealed, purged with high purity nitrogen three times, and then charged with the ¾ to the desired pressure. During the reaction, mixing was achieved through an internal propeller operating at 1520 RPM. Once the set temperature was attained, the reactor was held at the set temperature for a period and then quenched in an ice bath to lower the temperature quickly. The reactor was cooled to approximately 20 °C, and then the gas pressure was recorded and vented. The reactor was immediately broken down, and the liquid was collected for analysis. The standard reaction conditions are: 20 mL captured CCk-amine solution, 20 - 80 °C, 400 psi (¾), and 100 mg catalyst loading, 1 h. Formate yield was calculated on a carbon basis and defined as follows:

^x C atoms in product

Yield of formate [% - C] = - 1 - x i 00

f ^fl °l _rea , _{tant ct} a gei ^{x C atom s m} reactant ^)

Product analysis: Aqueous samples collected were filtered through a 0.22 pm pore size filter for high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometer (ESI-MS) analysis. HPLC analysis was performed using a Shimadzu HPLC system equipped with a dual UV-VIS Detector (Shimadzu SPD 10-AV) at 208 and 290 nm and a Refractive Index Detector (Shimadzu RID-10A). For analysis of organic acids and reaction intermediates, the samples were separated in an Aminex 87-H column from Bio-Rad, using 5 mM H2SO4 as the mobile phase at 0.7 mL/min flow and a column temperature of 55 °C. All samples for ESI-MS analysis were diluted with a base solution containing 0.1 wt% triethylamine, and the analysis was performed using a Waters Micromass ZQ quadrupole mass spectrometer.

Catalyst characterization:

TEM: Transmission Electron Microscope was done on Hitachi S-4700 II Scanning Electron Microscope operated at 200 kV. The samples were dispersed in 1 -butanol, and a drop of the suspension was placed on lacey carbon supported on 300 mesh copper grids.

XRD: The crystalline structure and the size of nano-catalysts were characterized by a PANalytical X'Pert PRO diffractometer (Cu Ka radiation, l = 0.15418 nm) at 45 kV and 40 mA.

NMR: The ¹³ C{ ¹ H} NMR spectra were obtained with a 2-channel 400 MHz Varian VNMRS spectrometer with an automated triple broadband (ATB) probe. 1,4-Dioxane (67.19 ppm) was used as the internal standard. The NMR spectra of the sample solutions before or after the reactions were recorded for 12 hours to obtain high-resolution signals.

Pulse chemisorption on catalyst sample: chemisorption was done using a Micromeritics Autochem II 2920 analyzer. The sample was heated under the inert flow of helium (50 mL/min) at 350°C for 60 min to remove adsorbed moisture. Then the sample was reduced by 10% ¾ in Ar at 250°C for 1 h, followed by helium purge at the same temperature for another 1 h to remove the physical absorbed ¾ on the surface of the catalyst. The CO-pulse chemisorption experiments were carried out at 40°C using Helium gas with a flow rate of 50 mL/min as a carrier gas. With recording (0.2 seconds), the defined amount (0.5mL) of (10% CO in helium) is pulsed to the reactor in Helium carrier gas on. The above step was repeated until desorption peaks reach the saturation value. In practice, the pulsation was terminated when two consecutive CO peaks resulted in an equal amount of CO observed according to the peak area. Between the pulses, the reactor was kept under 50 mL/min Helium flow. Moreover, the stoichiometry of metal to CO selected is 1.

ATR-FTIR: Attenuated total reflectance infrared (ATR-IR) was employed to obtain information about the surface of the catalysts during the reaction. The measurements were performed using a Bruker Tensor II spectrometer and a custom-made ATR-FTIR cell. Before initiating the measurement, the catalyst sample was ground into fine powder and suspended in nanopore water (1.5 mg sample/mL water) under sonication to form ink, which was then coated onto the internal reflectance element (IRE) and dried for 1 h at 90 °C to form a thin layer. Before each measurement, the Pd/ AC sample was reduced in 10% H2/Ar (40 mL/min) at 150°C. After 1-hour reduction, the flow was switched to 40 mL/min N2 and the sample were purged for 10 min before cooling down. Then 20 pL amine captured CO2 solution were added dropwise on the inked catalysts. Then the ATR cell was sealed after purging ¾ and background spectrum of the amine captured CO2 solution adsorped on the catalyst was taken. During the measurement, spectra were recorded between 4000 cm ¹ and 800 cm ¹ by averaging 128 scans at a resolution of 4 cm ¹ to improve the signal to noise ratio.

Computational Details

The density functional theory (DFT) -based calculations reported in this work were performed within density functional theory (DFT) using Vienna Ab Initio Simulation Package (VASP 5.4.1). ⁵² _’ ⁵³ The projector augmented wave (PAW) method ⁵⁴ _’ ⁵⁵ was used for the treatment of the core electrons. The pseudopotential used for C, H, O and Pd atoms are the ones released in Apr 2002, Jun 2001, Apr 2002 and Jan 2005 respectively. We used the Perdew- Burke-Ernzerhof (PBE) ⁵⁶ exchange-correlation functional for initial screening of adsorption sites of ethyl carbonate on Pd(l 11). All other calculations were performed using the optB86b- vdW ⁵⁷ _’ ⁵⁸ exchange-correlation functional to accurately account for van der Waal interactions. The kinetic energy cutoff for the plane wave basis set was set to 400 eV. The Pd(l 11) surface was modeled using a p(4><4) supercell with four atomic layers. During geometric optimizations, the bottom two layers were fixed and top two layers were allowed to relax. The bulk lattice constant for Pd was found to be 3.940 A and 3.905 A for the PBE and optB86b-vdW functionals, respectively. Both in good agreement with the experimental value of 3.88 A. ^59-61 A vacuum spacing of 15 A was used to separate the surface from periodic images along the surface normal direction. A (3 ><3 >< 1) Monkhorst-Pack grid ⁶² was used to sample the Brillion zone in all the calculations. Total electronic energies were converged until changes across subsequent iterations were less than 10 ⁵ eV. The conjugate gradient algorithm was used for atomic optimizations until the forces in all relaxed dimensions were less than 0.03 eV/A.

The formate is modelled as a radical under the theoretical framework which has been a common practice in the previous reported literature using periodic DFT. ^{37 46 63} Gomes et c//. ³ com pared the adsorption of the formate radical and ion on Cu metal clusters. The adsorption configuration was nearly similar for both species and the energy difference was within -0.31 eV. The presence of an ionic charge may affect the adsorption energetics to some extent, but once adsorbed on the surface the interatomic interactions between the adspecies are expected to be mostly van der Waals forces due to replenishment of the charges from the surface. Therefore, it is expected the effect of charge on reaction energies to be insignificant. Also, because of the computational limitations in treating the ions in periodic systems, no calculations where system was charged were performed. As such these species were treated as radicals which is commonly done in literature. ³⁷ _’ ⁴⁶ _’ ⁶³ Since ethyl carbonate has a similar chemical structure to formate (with an ethoxy group bonded to carbon atom in place of single hydrogen), the same treatment for these species can be considered.

The adsorption energy on a per molecule basis, £ _ads , for individual adspecies (no surface mixtures) is calculated using Eq. 3.

t _otai represents the total energy of the system (surface with adsorbates), Ep _d fiii ₎ is the total energy of clean Pd(l 11), E _gas is the total energy of the adsorbate in the gas phase, and N is the number of molecules adsorbed on Pd(l 11). E _ads represents the adsorption strength where all the molecules adsorbing on the surface are similar. Additionally, an average adsorption energy per molecule, E _ads , is defined for systems where ethyl carbonate and ethanol are co adsorbed calculated using Eq. 4.

E _E is the energy of ethyl carbonate in the gas phase, E _Et0B is the energy of ethanol in the gas phase, and N _E and lV _Et0H are the number of ethyl carbonate and ethanol, respectively, adsorbed on Pd(l 11).

The actual reaction occurs under a liquid ethanol environment. Therefore, it was estimated the average adsorption strength using Eq. 4 when ethyl carbonate and ethanol are co adsorbed. The surface coverage mentioned in the paper is the total coverage of all the co- adsorbed species unless otherwise mentioned. The coverage, Q by the definition, is given as the number of adsorbed species per number of surface atoms in a unit cell expressed in monolayers (ML). Coverage effects in the presented calculations were considered through the addition of molecules to the p(4><4) surface rather than resizing the supercell. A 1 :3 and 1 :4 ratio of ethyl carbonate to ethanol was used at total coverages of 0.25 ML and 0.31 ML respectively. At any time during the reaction, it was assumed that there will a greater number of ethanol molecules on the surface as compared to the intermediates formed during the reaction.

The reaction energy, DE _ΐch , is calculated using Eq. 5;

^Erxn— £fmal ^— ^initial (5)

£ _fi nai are the total energies of the final and initial states, respectively, of the reaction under consideration.

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63. Gomes, J. R. B. & Gomes, J. A. N. F. Adsorption of the formate species on copper surfaces: a DFT study. Surf. Sci. 432, 279-290 (1999). Further attributes, features, and embodiments of the present invention can be understood by reference to the following numbered aspects of the disclosed invention. Reference to disclosure in any of the preceding aspects is applicable to any preceding numbered aspect and to any combination of any number of preceding aspects, as recognized by appropriate antecedent disclosure in any combination of preceding aspects that can be made. The following numbered aspects are provided:

1. In an aspect described is a method comprising:

(a) reacting CO2 with an amine or ammonia to form an adduct comprising captured CO2; and

(b) hydrogenating the captured CO2 in the presence of a hydrogenation solvent to form formates.

2. The method of aspect 1, further comprising:

(c) dehydrogenating the formates with a catalyst to generate H2.

3. The method of any of aspects 1-2, wherein the amine is selected from the group consisting of: monoethanolamine, diethanolamine, triethanolamine, ethane- 1,2-diamine, 2- amino-2-methyl-l -propanol, 2-amino-2-methyl-l, 3, -propanediol, 2-amino-2 -hydroxymethyl- 1, 3-propanediol, piperazine, piperidine, 1-methyl piperazine, 2-methyl piperazine, and combinations thereof.

4. The method of any of aspects 1-3, wherein the amine is piperidine.

5. The method of any of aspects 1-4, wherein the formates formed in step (b) is a formate-pipridine adduct.

6. The method of any of aspects 1-5, wherein the hydrogenation solvent comprises water, acetonitrile, or tetrahydrofuran.

7. The method of any of aspects 1-6, wherein in the hydrogenation solvent comprises an alcohol.

8. The method of any of aspects 1-7, wherein the alcohol is be methanol, ethanol, propanol (e.g. 1-propanol or 2-propanol), or butanol.

9. The method of any of any of aspects 1-8, wherein the alcohol is present at about 0 percent to 100 percent by volume of the hydrogenation solution.

10. The method of any of aspects 1-9, wherein the water, acetonitrile, or tetrahydrofuran is present at up to 100 percent by volume of the hydrogenation solution.

11. The method of any of aspects 1-10, wherein the hydrogenation solvent consists of an alcohol. 12. The method of any aspects 1-11, wherein the hydrogenation solvent consists of water, acetonitrile, or tetrahydrofuran.

13. The method of any one of any of aspects 1-12 wherein the reaction temperature of any one of steps (a), (b), (c), steps (a) and (b), steps (a) and (c), or steps (b) and (c) ranges from about 20 degrees C to about 100 degrees C.

14. The method of any of aspects 1-13, wherein the reaction temperature of step (a), step (b), or both ranges from about 20 degrees C to about 40 degrees C.

15. The method of any of aspects 1-14, wherein the reaction temperature of step (a), step (b), or both is about 30 degrees C.

16. The method of any of aspects 1-15, wherein the reaction temperature of step (c) ranges from about 60 degrees C to about 100 degrees C.

17. The method of any of aspects 1-16, wherein the reaction temperature of step (c) ranges from about 80 degrees C to about 90 degrees C.

18. The method of any of aspects 1-17, wherein the catalyst is a homogenous catalyst.

19. The method of any of aspects 1-18, wherein the catalyst is a heterogenous catalyst.

20. The method of any of aspects 1-19, wherein the catalyst comprises a metal selected from the group consisting of: Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Rm, Yb, Lu, Hf, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, Ra, Ac, Th, Pa, U, Np, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, FI, Me, Lv, and combinations thereof.

22. The method of any of aspects 1-20, wherein the catalyst is part of a hydrogen generation system.

23. The method of aspect 22, wherein the hydrogen generation system is capable of providing high-purity hydrogen gas to a fuel cell.

24. A liquid hydrogen carrier comprising:

an adduct comprising formates, wherein the adduct is formed via a method as in any of aspects 1-23.

25. A liquid hydrogen carrier comprising:

a formate-piperidine adduct comprising hydrogenated CO2.

26. The liquid hydrogen carrier of aspect 25, wherein the formate-piperidine adduct is formed via a method as in any of aspects 1-23. 27. A hydrogen capture and/or generation system configured to perform a method as in any of aspects 1-23.