Background and Summary

BACKGROUND OF THE INVENTION

A. Field of the Invention

[001] The present invention relates generally to rendering clathrate hydrate and hydrophobic confinement suited for hydrogen storage.

[002] In particular, the present disclosure relates to a more efficient hydrogen storage in clathrate hydrate by confining the clathrate hydrate structure in nanosized hydrophobic pores.

B. Description of the Related Art

[003] Hydrogen and fuel cells have been raised as sustainable energy solutions for the 21st century, providing a way to cut carbon dioxide emissions and to address concerns over the security of energy supply and climate change. Widespread application of hydrogen is however hindered by a technological gap arising from the energy inefficiency of current hydrogen storage and transport technologies involving compression and liquefaction.

[004] The safe and efficient storage of hydrogen are essential to enable hydrogen as ‘green’ energy carrier of the future. Current storage solutions are mainly based on compression and/or liquefaction of hydrogen, but they come at an energy cost. The energy penalty for hydrogen compression (at 70 MPa) and cryogenic liquefaction, amounts to up to 20% and 40%, respectively, of the stored hydrogen.

[005] Hydrogen storage in ice-like clathrate hydrates has been recognized as a potential alternative, but the high pressures (> 180 MPa) and low temperatures required for their formation have tempered the interest.

[006] By confining the clathrate hydrate structure in nanosized hydrophobic pores, this work shows that hydrogen storage can already be achieved at hydrogen pressures as low as 6 MPa. The volumetric storage capacity reached ca. 20 g H ₂ /L, or 3.0 wt.% H ₂ which is equivalent to compression at 35 MPa.

[007] Clathrate hydrates, water-based crystalline materials resembling ice for storage of hydrogen, are hindered in that formation of bulk hydrogen clathrate hydrate requires extreme pressure and temperature conditions preventing its practical application.

[008] Thus, there was a need in the art for a proper clathrate hydrate based hydrogen storage system.

[009] Present invention used hydrophobic confinement at the nanoscale, for instance nanosized hydrophobic pores, so enabling water and hydrogen to readily form clathrate phases in a reversible manner and at milder pressure and temperature.

[010] Energy-efficient and safe hydrogen storage and transportation using clathrate hydrates bridges the technological gap in the hydrogen economy.

SUMMARY OF THE INVENTION

[011] A hydrophobic reversed-phase silica material, with wide pore size distribution (1 - 20 nm) and an average pore connectivity, i.e. number of individual branches connected to a common point, greater than 3, was used to promote hydrogen clathrate hydrate formation. Confinement of the hydrate phase allowed to mitigate the high pressures needed for bulk hydrogen clathrate hydrate phases.

[012] This initial observation of a hydrogen clathrate hydrate contained in the pores of a reversed-phase silica gel in an NMR sample tube opens up perspectives for the development of safe, reliable and efficient methods for hydrogen storage.

[013] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

Some embodiments of the invention are set forth in claim format directly below:

1. A composition comprising engineered hydrogen clathrate hydrate comprised in nanoporous material.

2. A composition consisting essentially of engineered hydrogen clathrate hydrate comprised in nanoporous material. 3. A composition consisting of engineered hydrogen clathrate hydrate comprised in nanoporous material.

4. The composition according to any one of the embodiments 1 to 3, whereby the hydrogen clathrate hydrate is hosted in nanopores of the nanoporous material.

5. The composition according to any one of the embodiments 1 to 4, whereby the nanoporous material is hydrophobic.

6. The composition to any one of the embodiments 1 to 5, whereby the nanoporous hydrophobic material is a material selected of the group consisting of carbon, reversed-phase silica, porous silicones, covalent organic frameworks (COFs), silica based mesostructured cellular foams (MCFs), periodic mesoporous organosilicates (PMOs) and polyoligo-siloxysilicone (POSiSil) materials.

7. The composition to any one of the embodiments 1 to 6, whereby the nanoporous hydrophobic material is a material selected of the group consisting of carbon, reversed-phase silica, porous silicones, covalent organic frameworks (COFs), silica based mesostructured cellular foams (MCFs), periodic mesoporous organosilicates (PMOs) and polyoligo-siloxysilicone (POSiSil) materials with pore sizes of 1 to 100 nm, preferentially between 5 and 20 nm, and an average pore coordination number > 2.

8. The composition to any one of the embodiments 1 to 7, whereby hydrogen gas at pressures ranging from 20 to 100 bar and temperatures between 5 and -50 °C is loaded in the clathrate hydrate.

9. The composition to any one of the embodiments 1 to 7, whereby hydrogen gas at pressures ranging from 1 to 50 bar and temperatures between 5 and -50 °C is released from the clathrate hydrate.

10. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of water and hydrogen forced to enter the pores of the nanoporous material under pressure.

11. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of hydrogen and water forced to enter the pores of the nanoporous material by hydrogen pressure at a temperature between 5 and -50

12. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of hydrogen and water forced to enter the pores of the nanoporous material by hydrogen pressure ranging from 20 to 100 bar and at a temperature between 5 and -50 °C.

13. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of hydrogen and water forced to enter the pores of the nanoporous material by methane pressure and the methane having subsequently been replaced by hydrogen.

14. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of water forced to enter the pores of the nanoporous material by methane pressure at a temperature between 5 and -50 °C and the methane having subsequently been replaced by hydrogen.

15. The composition to any one of the embodiments 1 to 7, whereby the hydrogen clathrate hydrate is a reaction product of water forced to enter the pores of the nanoporous material by methane pressure ranging from 20 to 100 bar and at a temperature between 0 and -50 °C, and the methane having subsequently been replaced by hydrogen.

16. The composition of any one of the embodiments 1 to 15, whereby the pore connectivity of the nanoporous material is between 2 and 4, preferably between 3 and 3,5.

17. A method for storing hydrogen, comprising a composition according to any one of the embodiments 1 to 16.

18. The use of engineered hydrogen clathrate hydrate according to any one of the embodiments 1 to 16, to store hydrogen.

Detailed Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[014] The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof.

[015] Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer’s specifications, instructions etc.) are hereby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

[016] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[017] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[018] Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[019] It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to the devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[020] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[021] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[022] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[023] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[024] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

[025] It is intended that the specification and examples be considered as exemplary only. [026] Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention.

[027] Each of the claims set out a parti cular embodiment of the invention.

[028] The following terms are provided solely to aid in the understanding of the invention.

[029] Safe and energy-efficient storage of hydrogen is one of the major technological bottlenecks delaying the widespread use of hydrogen as energy carrier. Today, hydrogen is stored in gaseous form in pressurized tanks and transported with pipelines, pressurized tube trailers or as liquefied hydrogen. The energy needed for compression at 70 MPa and cryogenic liquefaction represents up to 40% of the energy contained in the stored hydrogen, which is a substantial loss. Alternative hydrogen storage principles demanding less compression and cooling energy are adsorption in nanoporous materials, formation of metal hydrides, and reversible catalytic hydrogenation- dehydrogenation of liquid organic hydrogen carrier molecules. These technologies, however, meet with challenges such as irreversibility, complexity of the processes and limited storage capacity. Hydrogen encapsulation in clathrate hydrates is an imaginative alternative. Clathrate hydrates are structures similar to ice with cages large enough to host small guest molecules. Methane encapsulation is most common. Naturally occurring methane clathrate hydrates, also referred to as methane ice’, in deep-sea sediments and in permafrost have Structure I (Fig. 1). It can also be formed synthetically by pressurizing methane up to 10 MPa in the presence of water close to its freezing point (P. Zhang, Q. Wu and C. Mu, Sci. Rep., 2017, 7, 7904) Using activated carbon to host the clathrate phase in its pores, methane hydrate is formed already at a pressure of 3.5 MPa. Interestingly, inside the hydrophobic active carbon methane hydrate formation is fast (within minutes) and reversible, which is attractive for methane storage (M. E. Casco, J. Silvestre-Albero, A. J. Ramirez-Cuesta, F. Rey, J. L. Jorda, A. Bansode, A. Urakawa, I. Peral, M. Martinez-Escandell, K. Kaneko and F. Rodriguez-Reinoso, Nat. Commun., 2015, 6, 6432).

[030] Compared to methane, storing hydrogen in clathrate hydrates is less evident. On earth, hydrogen clathrate hydrates do not occur naturally. Synthetic specimen have been synthesized at pressures of 150 - 300 MPa and temperatures far below the freezing point of water. The hydrogen storage capacity is spectacular (Table 1). Known clathrate hydrate structures can store up to 7.2 wt.% hydrogen, and values in excess of 10 wt.% have been reported in cubic ice-Ic structures formed at extreme pressure (> 2,300 MPa at 295 K) (W. L. Mao, Science, 2002, 297, 2247-2249; T. A. Strobel, C. A. Koh and E. D. Sloan, Fluid Phase Equilib., 2007, 261, 382-389 and R. Kumar, D. D. Klug, C. I. Ratcliffe, C. A. Tulk and J. A. Ripmeester, Angew. Chemie, 2013, 125, 1571-1574). Both in terms of volumetric and gravimetric hydrogen storage capacity, clathrate hydrates and ice structures bear the promise of outperforming existing technologies (Fig. 2).

[031] Such extreme pressure and temperature conditions render hydrogen storage in bulk clathrate hydrates and ice unrealistic for practical application. Organic molecules such as sodium dodecylsulfate, cyclopentane or tetrahydrofuran promote the formation of hydrogen hydrates under significantly milder conditions (5 - 12 MPa and 270 - 279.6 K), but these molecules occupy cages of the clathrate structure at the expense of the hydrogen storage. Similarly, clathrate hydrates grow more easily when occluding mixtures of hydrogen with other gases such as carbon dioxide or mixtures of methane and ethane, but again at the expense of hydrogen storage (H. P. Veluswamy, R. Kumar and P. Linga, Appl. Energy, 2014, 122, 112-132). The key question is whether principles other than pressure, temperature or adjuvant molecules can be used to favor hydrogen encapsulation at high storage capacity.

[032] Present invention demonstrated that that pores of reversed-phase silica gel, known from chromatography, favour hydrogen clathrate hydrate formation inside. Formation and encapsulation of hydrogen molecules in small and large cages of a Structure I clathrate hydrate was observed with in situ NMR spectroscopy. The volumetric hydrogen storage capacity reached 20.4 g/L at 261 K and 6 MPa hydrogen pressure. Tuning the surface chemistry and porosity of the confining material and temperature and pressure, hydrogen storage capacities up to 66 g/L, equivalent to hydrogen gas compressed at 130 MPa, is within reach. Energy-efficient and safe hydrogen storage and transportation using clathrate hydrates could help bridge the technological gap in the hydrogen economy.

[033] EXAMPLES

[034] Synthesis of confined CH ₄ clathrate hydrate [035] We opted for a C8-reversed phase silica gel (Supelclean™ LC-8 - Sigma- Aldrich Co. LLC) as a hydrophobic host material (Fig. 5-7). Reverse-phase silica gel particles were packed in a sapphire NMR tube and 120 pL of deuterated water (D2O) was added. This water phase did not wet the hydrophobic silica and remained on top (Fig. 3). Pressurizing the tube to 6 MPa with methane gas, the deuterated water penetrated the interstitial spaces and the pores of the silica host. The sample tube was subsequently sealed and cooled to 243 K, heated again to room temperature and cooled a second time to 261 K (Fig. 8). This cooling-heating-cooling procedure resulted in accelerated methane clathrate hydrate formation. The structure of the obtained clathrate (Structure I) was determined wide-angle X-ray scattering (Fig. 9), recorded at the DUBBLE beamline (ESRF, Grenoble). Encapsulation of methane molecules in the clathrate cages was monitored in situ with ^! H and ^!3 C NMR spectroscopy. Decomposition of the ^1! H NMR spectrum of the confined methane clathrate hydrate showed two resonances at 1.00 ppm and 0.58 ppm, assigned respectively to methane molecules in the small (5 ¹² ) and large (5 ¹² 6 ² ) cages of a Structure I type clathrate. This assignment was supported by the corresponding ¹³ C NMR spectrum, showing two distinct signals at -4.8 ppm and -8.7 ppm (Fig. 10). Already after 15 minutes, ^J H NMR showed the formation of a significant amount of methane hydrate. The observed pressure drop at this stage indicated the methane content of the clathrate was 6.13 wt.% of the water content of the sample (deuterated and non-deuterated residual water). The calculations are outlined below. Methane storage further increased to 10.64 wt.% after 6 h.

[036] Decomposition of the ^J H NMR spectrum of the confined methane hydrate phase revealed a small-to-large cage distribution of 0.56, which is larger than the theoretical ratio of 0.33. Provided the small (5 ¹² ) and large (5 ¹² 6 ² ) cages of the Structure I clathrate hydrate all accommodate one methane molecule, the confined clathrate structure should have an excess of small cages. Structure I clathrates have a body-centered cubic lattice structure, with a lattice constant of 1.2 nm. For a continuous methane clathrate phase growing throughout the silica pore network with an average diameter of 5.5 nm, the excess of small cavities suggests the phase to be systematically terminated by small cages along the pore walls. Such interpretation is in line with literature reporting pentameric water structures near hydrophobic surfaces promoting enclathration of adsorbed methane molecules and preferential clustering of water molecules around dissolved methane molecules in 5 ¹² -like hydrate cages (J. Miyawaki, T. Kanda, T. Suzuki, T. Okui, Y. Maeda and K. Kaneko, J. Phys. Chem. B, 1998, 102, 2187-2192; P. S. R. Prasad, B. S. Kiran and K. Sowjanya, RSC Adv., 2020, 10, 17795-17804 and N. N. Nguyen, A. V. Nguyen, K. M. Steel, L. X. Dang and M. Galib, J. Phys. Chem. C, 2017, 121, 3830-3840).

[037] Formation of Hi clathrate hydrate

[038] The silica material containing the methane clathrate hydrate was cooled in dry ice (ca. 195 K) and flushed four times with hydrogen gas at 8 MPa to eliminate extraneous methane. Dry ice was used to create a stable, low-temperature environment for flushing the sample, without risking damage to the NMR probe or spectrometer in case of sample hardware malfunction. The clathrate was kept in dry ice at 8 MPa hydrogen pressure for increasing periods of time, and changes to the composition monitored with ^! H and ¹³ C NMR spectroscopy (Fig. 4). The NMR spectra were recorded at 261 K.

[039] After 2 h, a new sharp ^J H NMR signal emerged near 5 ppm (Fig. 4), which was assigned to enclathrated hydrogen molecules. Decomposition of the ^J H NMR spectrum revealed this signal to have two components at 5.03 and 4.83 ppm, respectively. These two contributions were tentatively assigned to hydrogen molecules in the large and the small cages of Structure I hydrogen clathrate hydrate, respectively, based on a series of control measurements (Figs. 11-22). The intensity of the signals increased over time and became very prominent after 14 h. This increase in hydrogen- related ' H NMR signals paralleled a decrease of the ^! H NMR signals attributed to methane molecules in the clathrate cages at 1.00 and 0.58 ppm, suggesting exchange of methane for hydrogen (Fig. 23). Quantification of the NMR signals of ^J H nuclei of hydrogen and methane revealed even more details about the exchange. In both cages, methane molecules are exchanged by more than one hydrogen molecule. After 14 h, on average, 1.1 and 1.7 hydrogen molecules were found in the small (5 ¹² ) and large (5 ¹² 6 ² ) cages of the Structure I clathrate hydrate, respectively. Multiple hydrogen occupancy in small 5 ¹² cages of Structure I type clathrates has been reported at high pressures (> 35 MPa) and upon addition of adjuvant molecules such as ethane or propane to stabilize the required lattice expansion (D. Y. Koh, H. Kang, J. Jeon, Y. H. Ahn, Y. Park, H. Kim and H. Lee, J. Phys. Chem. C, 2014, 118, 3324-3330). Double occupancy of hydrogen in large 51262 cages of a Structure I type hydrate has previously been observed in binary H ₂ -CH ₄ hydrates, but only at pressures of 70 MPa (R. G. Grim, P. B. Kerkar, M. Shebowich, M. Arias, E. D. Sloan, C. A. Koh and A. K. Sum, J. Phys. Chem. C, 2012, 116, 18557-18563). The obtained silica-confined clathrate hydrate structure obtained contained ca. 3.0 wt.% hydrogen, or 19.5 g/L on volumetric basis.

[040] In another experiment, methane clathrate hydrate, prepared in the reversed- phase silica at 26 IK and 6 MPa methane pressure, was depressurized to 0.3 MPa and subsequently repressurized with hydrogen gas at 6 MPa while keeping the temperature at 261 K. From the hydrogen uptake, the hydrogen content of the clathrate was estimated at 2.5 wt.%. Releasing the pressure at 0.3 MPa the stored hydrogen was evacuated from the structure. Repressurizing with hydrogen for a second time resulted in the uptake of even more hydrogen, reaching a hydrogen content of 3.1 wt.%. This is comparable the hydrogen content of bulk Structure I hydrogen clathrate hydrates (Table 1). This experiment shows that once the clathrate hydrate phase has been made, loading and unloading with hydrogen can be achieved merely by pressurizing and depressurizing the system, opening up perspectives for practical application.

[041] A gravimetric hydrogen storage capacity of 3.1 wt.% in the clathrate phase corresponds to a volumetric hydrogen capacity of 20.4 g/L. Such hydrogen capacity, stored in a clathrate hydrate at 6 MPa hydrogen pressure, is equivalent to the storage capacity of a hydrogen tank compressed at 35 MPa. Full occupation of all cages of the clathrate structure with two hydrogen molecules would increase the hydrogen storage capacity to 3.8 wt.% (Table 1). Our results showcase that hydrophobic confinement generates a pseudo-pressure stabilizing hydrogen clathrate structures that can be reversibly filled with hydrogen molecules at very high density.

[042] The promoting effect of hydrophobic nano-confmement enables to synthesize clathrate and ice structures capable of storing even larger quantities of hydrogen (Fig. 2). Synthesis of Structure SVI clathrates would enable to store as much as 7.2 wt.% and ice-Ic structures even up to 10 wt.% of hydrogen. Key to success is the ever growing family of nanoporous hydrophobic host materials complementing carbon and reversed-phase silica. Covalent Organic Frameworks (COFs), silica based Mesostructured Cellular Foams (MCFs), Periodic Mesoporous Organosilicates (PMOs), and Polyoligo-siloxysilicones (POSiSils). By tuning size, shape and connectivity of the pores, it is hypothesized that other clathrate phases with high hydrogen storage capacity can be synthesized (Table 1). Carbon and silica based materials are prime candidates for hosting clathrate formation as they offer plenty of tuneability and chemical versatility. These materials offer endless possibilities of chemical modification of pore walls with organic functionalities where clathrate hydrate can nucleate, and take over the role of the methane clathrate precursor used in this work. Host materials are expected to provide handles to optimize kinetics, temperature and pressure.

[043] Synthesis of confined CH ₄ clathrate hydrate - Experimental details

[044] CH ₄ clathrate hydrate formation was carried out in a 5 mm, pressureresistant sapphire NMR tube. The sapphire tube was packed with 148 mg of hydrophobic silica host material, with an estimated inter-particle void volume of 90 pL. D2O (120 pL, 99.9 atom % D, Merck) was added to fill up both the void volume and about 40% of the available pore volume (factoring in the 23 pL of H ₂ O already present in 148 mg of sample). Although the use of D2O can slightly impact the temperature and pressure conditions for hydrate formation, D2O was preferred over H ₂ O because it enables to more easily discriminate between water and CH ₄ , H ₂ , residual H ₂ O and host material background. Next, the tube was sealed and pressurized to 6 MPa with ¹³ C-enriched CH ₄ (Sigma- Aldrich, 99% enrichment), forcing the water into the hydrophobic pores and cavities of the porous host material. The pressurized tube was then transferred to the NMR spectrometer where the temperature was lowered from 277 K to 268 K, 258 K and 243 K, respectively, while simultaneously measuring the ^J H and ¹³ C spectra of the confined system. Following these initial cooling steps, the sample temperature was raised to room temperature (295 K) and the system was equilibrated for 60 minutes (inside the spectrometer) before finally lowering the temperature again to 261 K and initiating hydrate formation.

[045] Quantification of CH ₄ consumption during hydrate formation using Pitzer’s correlation

[046] “At any given time, the number of moles of the gas that has been consumed for hydrate formation (UH) is the difference between the number of moles of the gas at time t = 0 present in the gas phase (no,o) of the set-up and the number of moles of the gas present in the gas phase of the set-up at time t = t (no.t) and is given by:

[047]

[048] Where P is the pressure (bar), V the total volume of the high-pressure

NMR set-up (L), T the temperature (K), R the ideal gas constant (D.D8314 and z the compressibility factor calculated by Pitzer’s correlation:

[049]

[050]

[051]

[052]

[053]

[054]

[055]

[056]

[057]

[058]

[059]

[060]

[061]

[062]

[063] which corresponds to an overall methane storage capacity of 6.13 wt.%.

[064] NMR characterization

[065] Sample pre-treatment and clathrate hydrate formation were also characterized with *H and/or ¹³ C NMR measurements at each temperature. NMR experiments were performed on a narrow bore Broker AVANCE III 300 MHz spectrometer equipped with a BBO probe operating at a ^! H Larmor frequency of 300.13 MHz and a BCU II unit to regulate the temperature (277 - 243 K). Data collection was performed statically (non-spinning), using a 90 degree pulse with an RF strength of 83 kHz and a repetition delay of 10 s for 'H, and a 90 degree pulse with an RF strength of 36 kHz and a repetition delay of 30 s under proton decoupling for ¹³ C. Chemical shift referencing was carried out with respect to TMS, using ethylbenzene (10% in chloroform-d) as secondary reference with 5( ^! H) = 1.22 ppm and 5( ¹³ C) = 15.63 ppm (for -CH3). Probe ‘tuning and matching’ was carried out using a RF Network Analyzer (Hewlett Packard 8712C), connected to the probe through the *H pass filter, in order to obtain comparable Q-factors and maximize accuracy. Spectral decomposition and quantification of the peak area of the different components was carried out with dmFit software using a mix of Gaussian and Lorentzian lineshapes.

[066] N2 physisorption

[067] A 325 mesh reversed-phase silica with 6 nm pores, used commercially in solid-phase extraction cartridges (Merck, SupelcleanTM LC-8), served as host material for the hydrate formation experiments. The material was grafted with n-octyl groups to hydrophobize the pore wall. Residual hydroxyl groups have been removed by endcapping the surface with trimethylsilyl groups, resulting in a total carbon loading of 7 wt.%. Average pore size, surface area and pore volume were derived from N2 adsorption isotherms determined on a Micromeritics TriStar 3000 instrument. Priors to nitrogen adsorption, samples were evacuated at 393 K for 12 h. The specific surface area, external surface area and micropore volume were analyzed using the BET method and the t-plot method and yielded 389.15 m ² /g, 17.83 m ² /g and 0.774 cm ³ /g, respectively (see Fig- 5).

[068] Powder X-ray diffraction

[069] Powder X-ray diffraction (PXRD) data were collected on a STOE Stadi MP diffractometer with focusing Ge(l l l) monochromator using CuKal radiation at a wavelength of 1.54056 A. Measurement occurred between 3 and 80 °20 in Debye-Scherrer geometry with a linear position sensitive detector with a 6 °20 window. The sample was dried overnight at 333 K prior to sealing it in a 0.7 mm glass capillary (Hilgenberg) (see Fig. 6).

[070] High-resolution scanning electron microscopy

[071] High-resolution scanning electron microscopy (HR-SEM) was performed on a Nova NanoSEM 450 (FEI Eindhoven). Powder samples were dispersed on carbon tape attached to aluminum stubs and imaged without any further sample modification. High-resolution images were obtained at low voltages (2 kV) using a Centered Back Scattering detector (CBS, a new type of BSE detector) combined with Beam Deceleration Mode. The SEM images revealed a variety of particle morphologies (see Fig. 7).

[072] CH ₄ clathrate hydrate formation

[073] CH ₄ clathrate hydrate formation is demonstrated in Fig. 8 with an overview of the temperature profile and the resulting pressure changes. The duration of each step is also highlighted. The initial pressure drop from 6.0 to 5.9 MPa is attributed to adsorption phenomena and CH ₄ dissolution, and is therefore not taken into account when calculating the amount of CH ₄ consumed during hydrate formation.

[074] In situ wide-angle X-ray scattering

[075] In summary, a 1 mm quartz capillary was packed with the same hydrophobic reversed-phase silica material (Merck, SupelcleanTM LC-8) and hydrated with a known amount of water. The capillary was then attached to a portable, Swagelok based gas handling system, pressurized to 4 MPa with methane and mounted on the beamline sample holder. The temperature was then gradually lowered using a dedicated cryostream while carefully monitoring any changes to the system with in line WAXS. Once the cryostream reached a temperature of 216 K, CH ₄ hydrate formation rapidly set in (see Fig. 9).

[076] In situ NMR characterization

[077] The ’H and ¹³ C spectrum recorded after hydrate formation show the distinctive signature of CH ₄ clathrate hydrates recorded under static conditions (Fig. 10, left inset). The ^! H spectra recorded under CH ₄ pressure as function of temperature (Fig) provide additional insight into the processes leading up to CH ₄ hydrate formation. As the temperature decreases from 277 K to 268 K, the pore intruded water starts to freeze in the larger pores while still remaining liquid in the smaller ones. This difference in behaviour gives rise to splitting of the main water signals observed at 277 K (Fig. 10, right inset) into two distinct resonances at 4.5 and 8 ppm at lower temperatures (Fig. 10, main display). The resonance at 4.5 ppm corresponds to liquid water, while the resonance at 8 ppm is assigned to less mobile water molecules taking part in ice nucleation and growth. The ’H signal around 1.75 ppm (attributed to CH ₄ gas inside the pores of the host material) remains constant throughout the measurements and slightly increases with decreasing temperature, as would be expected. CH ₄ is only poorly soluble in water, but its solubility does increase with decreasing temperature, which explains the emergence of a new resonance around 0.83 ppm which is assigned to dissolved CH ₄ (268 K). As the temperature decreases, more water molecules are incorporated in the growing ice lattice (in pores of different sizes) and the water signals at 4.5 and 8 ppm decrease and broaden even more, eventually disappearing into the background (268 K -> 258 K -> 243 K). A small fraction of water, probably located in the smallest pores, does however remain liquid like, even at the lowest temperatures (243 K). At the same time, the amount of dissolved CH ₄ (at 0.83 ppm) steadily increases with decreasing temperature (268 K -> 258 K -> 243 K). At 243 K, no clathrate hydrate structure has yet been formed. This process only occurs after re-heating the system to 295 K and then cooling it down again. The initial cooling-heating pre-treatment is aimed at degassing the water through freezing and thawing. Degassing of the water effectively enhances CH ₄ dissolution. Hydrate formation essentially is a crystallization process, involving nucleation and growth, and hence requires a minimum concentration of dissolved CH ₄ before clathrate formation can take place (see Fig. 10).

[078] ¹³ C NMR measurements

[079] Host material + D ₂ O + 6 MPa N2 at 277 K ( ¹³ C NMR background) is demonstrated in Fig. 11. [080] Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

[081] Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[082] Several documents are cited throughout the text of this specification. Each of the documents herein (including any manufacturer’s specifications, instructions etc.) are hereby incorporated by reference; however, there is no admission that any document cited is indeed prior art of the present invention.

Drawing Description

BRIEF DESCRIPTION OF THE DRAWINGS

[083] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[084] FIG. 1 demonstrates the cages of clathrate hydrate structures occluding gas molecules. Corners of polyhedra represent the positions of O-atoms of the water molecules making H-bonds along the edges.

[085] FIG. 2 demonstrates volumetric and gravimetric hydrogen storage capacity of existing technologies, and the potential of clathrate hydrates and ice as storage medium. [086] FIG. 3 A.& B. demonstrates a methane and hydrogen clathrate hydrate formation procedure. B. Photograph of NMR tube with the H ₂ clathrate confined in a bed of reversed-phase silica gel particles.

[087] FIG. 4 displays ¹ H NMR spectra of confined methane clathrate hydrate exposed to hydrogen gas at 8 MPa at 195 K. Signal assignments: 6.55 ppm (H ₂ gas); 5.03 ppm (H ₂ in large 5 ¹² 6 ² cages; dark, dashed area); 4.83 ppm (H ₂ in small 5 ¹² cages; light, dotted area), 4.55 ppm (residual liquid like H ₂ O), 3.0 ppm ( ^! H background), 2.2 ppm (CH ₄ gas), 1.0 ppm (CH ₄ in small 5 ¹² cages) and 0.58 ppm (CH ₄ in large 5 ¹² 6 ² cages). Zoom-in: enlarged view of 'H NMR signals of enclathrated hydrogen molecules in large (dark, dashed area) and small cages (lighter, dotted area).

[088] FIG. 5 displays N2 sorption isotherm for reversed-phase silica material (Merck, SupelcleanTM LC-8).

[089] FIG. 6 demonstrates a powder XRD diffractogram of the reversed-phase silica material (Merck, SupelcleanTM LC-8). The diffractogram shows no distinctive sharp reflections, as expected for an amorphous silica gel material. The only discernible feature is the broad hump located around 16° 20.

[090] FIG.7 demonstrates SEM images of the same reversed-phase silica host material (Merck, SupelcleanTM LC-8).

[091] FIG. 8 demonstrates a schematic overview of the temperature profile and the resulting pressure changes. The duration of each step is also highlighted. The initial pressure drop from 6.0 to 5.9 MPa is attributed to adsorption phenomena and CH ₄ dissolution, and is therefore not taken into account when calculating the amount of CH ₄ consumed during hydrate formation.

[092] FIG. 9 demonstrates a WAXS diffraction pattern showing the formation of CH ₄ hydrate inside the pores of a hydrated hydrophobized reverse-phase silica material. Reflections characteristic of CH ₄ hydrate structures are denoted by ‘C’, whereas reflections attributed to the simultaneous formation of hexagonal ice (which decrease in intensity over time) are indicated with ‘ice’. The onset of clathrate hydrate formation is clearly visible at 216 K (temperature on the cryostream read-out) and is accompanied by a noticeable decrease in methane pressure after 10 minutes.

[093] FIG. 10 is a graphic that shows the ¹³ C spectrum of the sample before (black trace) and after (grey trace) CH ₄ hydrate formation on the left inset. Main display: Evolution of the ’ ¹ H NMR spectrum of the confined aqueous system under CH ₄ pressure as function of temperature. Each spectrum is decomposed and the different contributions are identified and highlighted. Drop-down lines (dotted line) were added for clarity. The spectra recorded at 268, 258 and 243 K (during the initial cooling phase) comprise of 5 resonances emerging at 8 (immobilized H ₂ O), 4.5 (liquid like H ₂ O), 3.0 ('H background), 1.75 (CH ₄ gas) and 0.83 ppm (dissolved CH ₄ ). Upon hydrate formation at 261 K, two new signals emerge at 1.0 ppm and 0.58 ppm, which are assigned to CH ₄ confined to the small (5 ¹² ) and large (5 ¹² 6 ² ) cages of the SI hydrate structure, respectively. The residual trace, originating from the difference between the recorded spectrum (=experiment) and the theoretical fit based on the spectral decomposition, is shown on the baseline of each spectrum (dashed line). The sharp signal near 3.5 ppm originates from the frequency offset and is highlighted with an asterisk. Right inset: 'H NMR spectrum of the confined aqueous system at 277 K under an initial CH ₄ pressure of 6 MPa.

[094] FIG. 11 demonstrates the ¹³ C NMR background of host material + D2O, pressurized to 6 MPa with N2 (at 277 K) to enable water intrusion without the need for CH ₄ or H ₂ . No distinguishable resonances are observed.

[095] FIG. 12 demonstrates ¹³ C NMR spectra of the confined aqueous system at 6 MPa CH ₄ and temperatures varying from 277 to 243 K (pre-treatment phase) and 261 K, after CH ₄ hydrate formation. In the absence of the CH ₄ hydrate structure, only one resonance is observed around -8.9 ppm (displaying a small downfield shift with decreasing temperature) which is attributed to CH ₄ gas confined to the (interstitial) cavities and pores of the host material. CH ₄ hydrate formation at 261 K clearly introduces new ¹³ C resonances. [096] FIG. 13 demonstrates a ¹³ C NMR spectrum recorded at 261 K and 5.6 MPa of CH ₄ , after initial pretreatment and subsequent CH ₄ hydrate formation. The different spectral components are highlighted.

[097] FIG. 14 demonstrates the DmFit fitting parameters for the decomposed 1 ³ C spectrum at 261 K and 5.6 MPa CH ₄ after CH ₄ hydrate formation.

[098] FIG. 15 demonstrates a ¹³ C NMR spectrum (recorded at 261 K) after exposing the confined CH ₄ hydrate structure to hydrogen gas (8 MPa) during 14 h. The different spectral components are highlighted.

[099] FIG. 16 demonstrates DmFit fitting parameters for the decomposed ¹³ C spectrum recorded at 261 K and 8 MPa H ₂ at the end of the 14 h exposure to hydrogen gas.

[0100] FIG. 17 demonstrates a static ’H NMR spectrum of the host material recorded at 261 K under atmospheric pressure (0.1 MPa). The different spectral components are highlighted.

[0101] FIG. 18 demonstrates the DmFit fitting parameters for the decomposed (static) ’H spectrum of the host material recorded at 261 K under atmospheric pressure (0.1 MPa).

[0102] FIG. 19 demonstrates ’H NMR spectrum recorded at 277 K and 6 MPa CH ₄ at the beginning of the experiment (before the pretreatment phase). The different spectral components are highlighted.

[0103] FIG. 20 demonstrates DmFit fitting parameters for the decomposed 'H spectrum of the confined aqueous system recorded with at 6 MPa CH ₄ and 277 K.

[0104] FIG. 21 demonstrates a comparison of the ’H spectra of the confined CH ₄ hydrate after a 14 h exposure to H ₂ at 8 MPa (black trace) and to N2 at 6 MPa (dashed grey trace) in dry ice, recorded at 261 K. The absence of a sharp resonance around 5 ppm in the case of exposure to N ₂ evidences the hydrogen related nature of this resonance. [0105] FIG. 22 shows a comparison of the 'H spectra of the silica host-D2O(- CH ₄ )-H ₂ system at 8 MPa H ₂ and 195 K (dry ice) with (black trace) and without prior CH ₄ hydrate formation (dashed grey trace). Both samples underwent the same hydrogen exposure procedure. The absence of a sharp resonance around 5 ppm without prior CH ₄ formation substantiates the assignment of this sharp resonance to enclathrated H ₂ molecules (as opposed to H ₂ gas or potentially even dissolved H ₂ ).

[0106] FIG. 23 shows the evolution of the peak area of the main 'H contributions with increasing exposure to and exchange with hydrogen gas at 8 MPa. Closed symbols are used for signals related to water and CH ₄ , whereas open symbols represent signals assigned to H ₂ .