Field of Art

The present invention relates to an amino-sulfonic acid-functionalized graphene and to its use as a catalyst for producing solketal from glycerol and esters from carboxylic acids and alcohols.

Background Art

Glycerol can be produced from biomass fermentation, and as a by-product of propylene synthesis or soap production. Furthermore, it is obtained at 10 vol.% as a side-product from biodiesel synthesis, during the transesterification of triglycerides from vegetable oils and animal fat. Such activities create a sustainable supply of glycerol, rendering it a particularly attractive renewable carbon source if effective methods for its upgrade are identified.

Glycerol derivatives, such as ethers, esters, diols, and acetals, are important synthons for a broad range of industrial processes, related to fuels, plastics, and fine chemicals. Glycerol’s acetalization to solketal (a branched oxygen -containing compound; (2,2-dimethyl-l,3-dioxolan-4-yl)methanol), has gained considerable interest because of its broad application in cosmetics, pharmaceutics, food additives, polymers, tobacco, and petrochemicals.

Solketal production is typically performed by homogeneous, non-recyclable acid catalysts, such as sulphuric acid, hydrochloric acid, or p-toluenesulfonic acids, via the acetalization of glycerol with acetone. Heterogeneous catalysts are being produced, based on zeolites, metal substituted mesostructured silica, zirconia, mixed metal oxides, metal phosphates, and sulfonic acid functionalized carbons, resins, and polymers. Although such catalysts are particularly interesting in terms of their reusability, significant challenges remain because their production rates are substantially lower than those obtained rising the industrial benchmark catalyst of H2SO4, while low selectivity, poor thermal stability, limited recyclability, use of hazardous solvents, and need for high temperatures pose further limitations.

It is the aim of the present invention to provide a heterogeneous catalyst showing at least comparable specific productivity with that of sulfuric acid while providing the benefits typically connected with the heterogeneous catalysis.

Disclosure of the invention

The aim of the invention was achieved by the development of a graphene-based catalyst containing an amino-sulfonic acid covalently bound to graphene via its amino group. In a first aspect, the present invention relates to graphene functionalized with amino -sulfonic acid moieties of formula -NH-(CH2)n-SC>3H, wherein n is 1 to 5, preferably 1-3, most preferably 2, wherein the amino-sulfonic acid moieties are covalently bound to graphene.

Preferably, the graphene functionalized with amino-sulfonic acid moieties of formula -NH-(CH2) _n - SO3H contains at least 1.5 at.% of sulfur, preferably at least 2 at.% of sulfur, yet more preferably at least 2 at.% of sulfur.

The atomic % (at.%) of individual elements are determined by X-ray photoelectron spectroscopy.

The functionalized graphene of the invention further contains residual fluorine atoms, because, in the present invention, the amino-sulfonic acid-functionalized graphene is prepared from graphite fluoride. According to X-ray photoelectron spectroscopy, fluorine atoms may amount up to 15 at. %, more preferably up to 7 at.%, and more preferably less than 2 at. %. The fluorine atoms are present in an amount of at least 0.5 at.%.

The functionalized graphene of the invention contains nitrogen atoms which are present in the aminosulfonic groups. Further nitrogen-containing groups may be present, which are introduced by the reaction of the molecules of solvent (for example, where the solvent is DMF, NMP, or DMA) with the starting graphite fluoride or with the intermediate graphene fluoride. In some embodiments, the content of nitrogen in the functionalized graphene of the invention is between equal to the sulfur content and five times the sulfur content, expressed in at.%; preferably the content of nitrogen is between twice to five times the sulfur content. In some embodiments, the content of nitrogen in the functionalized graphene of the invention is from 1.5 at. % to 10 at. %, more preferably from 5 at.% to 10 at. %.

The amino-sulfonic acid-functionalized graphene may be prepared by the following steps: a) providing graphite fluoride, b) subjecting the graphite fluoride to sonication and/or mechanical treatment, c) reacting the graphene fluoride with an amino-sulfonic acid of formula NH2-(CH2) _n -SO3H, wherein n is 1 to 5, preferably 1-3, most preferably 2 to form the amino-sulfonic acid-functionalized graphene.

The term „graphite fluoride“ or “fluorinated graphite” includes fluorographite, graphite fluoride, fluorinated graphite, and exfoliated forms of these materials. Fluorinated graphites are also available under the name poly(carbon monofluoride), carbon monofluoride, or poly(carbon fluoride). The initial content of fluorine in the starting fluorinated graphite is typically at least 40 at. %, more preferably at least 45 or at least 50 at. %, relative to the total atoms present in the sample and determined by X-ray photoelectron spectroscopy (XPS) using an Al-Ka source.

The step of sonication yields a mixture containing fluorinated graphene and/or exfoliated fluorinated graphite particles. Sonication is typically carried out at a frequency range of 20 kHz to 100 kHz and for a period of at least 2 hours, more preferably of at least 3 hours, even more preferably at least 4 hours.

Step c) is performed in a solvent. Preferably, step c) is carried out in the presence of a base.

In step c), the reaction mixture is preferably heated to at least 90 °C for at least 12 hours.

The solvent in steps b) and c) is preferably a polar solvent or a mixture of a polar and a non-polar solvent. The solvent may preferably be selected from dimethylformamide (DMF), dimethylsulfoxide (DMSO), JV-methyl -2 -pyrrolidone (NMP), N,N-dimethylacetamide (DMA), water, glycols such as ethylene glycol, and mixtures thereof. Less polar or non-polar solvents such as acetonitrile, benzene, toluene, or chlorobenzene may be used in combination with a polar organic solvent (for example DMF, NMP, DMSO, DMA).

After step c), the product may be separated from the reaction mixture by known techniques such as centrifugation, sedimentation, or filtration.

In a preferred embodiment, the product from step c) is further subjected to dialysis against water.

The term „graphene“ encompasses single-layer graphene, as well as materials comprising single-layer graphene in a mixture with moieties (e.g., flakes) or particles containing a plurality of graphene layers.

In another aspect, the invention includes use of graphene functionalized with amino -sulfonic acid moieties of formula -NH-(CH2) _n -SO3H, wherein n is 1 to 5, preferably 1-3, most preferably 2, wherein the amino-sulfonic acid moieties are covalently bound to graphene, as a catalyst.

More specifically the invention includes use of graphene functionalized with amino -sulfonic acid moieties of formula -NH-(CH2) _n -SO3H, wherein n is 1 to 5, preferably 1-3, most preferably 2, wherein the amino-sulfonic acid moieties are covalently bound to graphene as a catalyst of the reaction of glycerol to solketal. Additionally, the invention includes use of graphene functionalized with amino-sulfonic acid moieties of formula -NH-(CH2)n-SC>3H, wherein n is 1 to 5, preferably 1-3, most preferably 2, wherein the aminosulfonic acid moieties are covalently bound to graphene, as a catalyst for the esterification of carboxylic acids with alcohols.

In yet another aspect, the invention includes a method of producing solketal, comprising the step of reacting glycerol with acetone, wherein the reaction is catalyzed by the graphene functionalized with amino-sulfonic acid moieties of formula -NH-(CH2)n-SC>3H, wherein n is 1 to 5, preferably 1-3, most preferably 2.

Glycerol and acetone are preferably used in molar ratios of 1 : 1 to 1: 10, preferably 1 : 2 to 1:5. Even at a low ratio of acetone to glycerol, a high conversion of glycerol is reached, using the catalyst of the invention.

The reaction may be carried out in a solvent-free setting.

The amount of catalyst used is at least 0.05 mass %, relative to glycerol, preferably at least 0.1 mass %, more preferably at least 0.5 mass %. (mass % = percent by weight).

In yet another aspect, the invention includes a method of producing biodiesel, comprising the step of reacting carboxylic acid (containing 6 to 30 carbons) with an alcohol (containing 1 to 10 carbons), wherein the reaction is catalyzed by the graphene functionalized with amino-sulfonic acid moieties of formula -NH-(CH2) _n -SO3H, wherein n is 1 to 5, preferably 1 to 3, most preferably 2.

The carboxylic acid and the alcohol are preferably used in molar ratios 1: 1 to 1:40, preferably 1: 10 to 1:20.

The reaction may be carried out in a solvent-free setting.

The amount of catalyst used is at least 1 mass %, relative to the carboxylic acid, preferably at least 5 mass %, more preferably at least 7 mass %. (mass % = percent by weight).

Brief description of Drawings

FIG. 1. Infra-red spectra of the starting fluorinated graphite, taurine, and the product from Example 1. FIG. 2. X-ray photoelectron spectra of the product of Example 1 and its thermally treated samples at 500 and 900 °C. FIG. 3. Raman spectra of the starting fluorinated graphite and the product from Example 1

FIG. 4. Thermogravimetric analysis of the product of Example 1 with evolved gas analysis (SO and

SO ₂ )

FIG. 5. X-ray photoelectron spectra of the product of Example 1 a) survey and core-level spectra for b) C Is, c) N Is and d) S 2p

Examples of carrying out the Invention

Materials and methods:

Graphite fluoride (GrF, >61 mass % in F), 2-Aminoethanesulfonic acid (Taurine, >99%), 3-Amino-l- propanesulfonic acid (Homotaurine, >99%), N,N-dimethylformamide (DMF, 99.8 %), acetone (p.a.), ethanol (p.a.), glycerol (99.9 %), hydrochloric acid (HC1,O.1N), sodium hydroxide (NaOH, 0.1N), were obtained from Sigma-Merck. All chemicals were used without further purification. Deionized water was used for all washings (conductivity <0.5 pS/cm).

X-ray photoelectron spectroscopy (XPS) was carried out with a PHI VersaProbe II (Physical Electronics) spectrometer using an Al Ka source (15 kV, 50 W). The obtained data were evaluated with the MultiPak (Ulvac - PHI, Inc.) software package. The fresh G-ASA catalyst was first treated with the catalytic reaction reagents, thoroughly washed, and then used for the XPS measurements.

Fourier transform infrared (FT-IR) spectra were recorded on an iS5 FTIR spectrometer (Thermo Nicolet) using the Smart Orbit ZnSe ATR accessory. Briefly, a droplet of ethanol dispersion of the material was placed on a ZnSe crystal and left to dry and form a film. Spectra were acquired by summing 64 scans recorded under a nitrogen gas flow through the ATR accessory. ATR and baseline correction was applied to the collected spectra. Raman spectra were recorded on a DXR Raman microscope using the 613 nm excitation line of a diode laser.

Thermogravimetric analysis (TGA; Netzsch STA 449C Jupiter thermal analyzer) was performed in synthetic air (100 cm ³ min ’). The TGA instrument was equipped with a QMS 403 Aeolos mass spectrometer for evolved gases (EGA). The measurements were carried out using an open crucible made of (X-AI2O3, from 45 °C to 1000 °C, and a heating rate of 10 K min ¹ . The EGA was focused on m/z 48 and 64 for SO, and SO2, respectively.

The conversion of glycerol and selectivity of the product was calculated based on the following equations:

Conversion of glycerol (

~ , ,, , solketal formed (mol) „

Selectivity towards solketal (%) = - glycerol converted ( - —mol -) x 100%

Turnover frequency (TOF) was calculated according to the following equation: The amount of the acidic sites was based on the titration results, which accounted for all the possible active sites and thus provided actual, not overrated, TOF numbers.

The concentration of the acidic sites of the catalyst was determined by acid-base titration. In general, an aqueous NaOH solution (0.05 M, 10 mb) was added to the catalyst (80 mg). Then the mixture was sonicated for 60 minutes and stirred at room temperature for 12 hours. The catalyst was separated by centrifugation and five milliliters of the supernatant solution was titrated with aqueous HC1 solution (0.05 M) using phenol red as an indicator.

Specific productivity was calculated according to the following equation, providing unequivocal reactions rates, free from any inaccuracies in active site estimations, and thus more appropriate for direct comparisons:

„ . . . . qlycerol converted (mmol)

Specific productivity = - total catalyst amount ( —g) - x reacti : -on ti : -me — (n)

NMR spectra of the esterification products were recorded on a 400 MHz NMR JEOL spectrometer.

The yield of the esterification reaction was calculated by analyzing the methyl esters in the reaction mixture using quantitative ’H NMR. The methoxy group in the methyl esters at 3.7 ppm (singlet) and the a-carbonyl methylene groups present in the fatty ester derivatives at 2.3 ppm (triplet) are chosen for integration. CDC I , solutions of a known amount of carboxylic acid and methyl esters were used for calibration. The transesterification yield (T) was obtained directly from the area (4) of the selected signals: where 4 1 and 42 are the areas of the methoxy and the methylene protons, respectively.

Example 1: Synthesis of taurine functionalized graphene at 130 °C using K2CO3 as base

The catalyst (G-ASA) was prepared by the sulfonation of GrF using taurine in DMF. In a typical procedure, 1 g of GrF (32 mmol in C-F units) was dispersed in 48 m of DMF in a round-bottom glass flask, stirred for 3 days, and then sonicated (Bandelin Sonorex, DT 255Htype, frequency 35 kHz, power 640 W, effective power 160 W) for 4 h. Taurine (4 g, 32 mmol) and K2CO3 (5.3 g, 1.2 molar excess with respect to taurine) were dissolved separately in 6 m of ultrapure water. The GrF dispersion in DMF was mixed with a K2CO3 solution in a round bottom flask and then added to the taurine solution. K2CO3 was added to secure basic conditions for keeping taurine’s amino group deprotonated and nucleophilic. The mixture was immediately heated to 130 °C under magnetic stirring at 300 rpm for 24 h in an oil bath connected to a reflux condenser. After the mixture cooled, the solid was isolated and washed by centrifugation at 20000 ref for 8 min. The precipitate was washed several times via centrifugation with solvents (2x hot DMF, 1 xDMF, 1 xhot acetone, 2x acetone, 3 x ethanol, 2x ultrapure water, 2x HC1 (2%), and 3 x ultrapure water) until the conductivity was below 200 pS/cm. The precipitate was finally redispersed in water and subjected to dialysis for one week until the surrounding water conductivity was below 10 pS/cm. Finally, the dispersion was acidified by 25 wt.% sulfuric acid to secure that all acidic sites are protonated, and finally washed via centrifugation cycles with methanol followed by freeze-drying, and this material was used for characterization and further experiments.

Fourier-transform infrared (FT-IR) spectroscopy of the starting GrF (Figure 1) showed bands of the CF and CF2 bonds at 1200 cm ¹ and 1305 cm respectively, while, after the reaction, the SO3H group in G-ASA gave rise to the characteristic bands at 1190 cm ¹ and 1035 cm ¹ corresponding to symmetric O=S=O and SO , stretching, respectively. The broad nature of the 1190 cm band is attributed to the contribution of the evolved sp ² carbon network, which also gives rise to the 1569 cm ¹ band. Any contribution from CF groups around 1200 cm ¹ in G-ASA is excluded since X-ray photoelectron spectroscopy (XPS) confirmed that almost all F atoms have been eliminated (Figure 2). A broad feature in the region between 3600 cm ¹ and 2900 cm ¹ arises from the presence of N-H and C-H groups and H-O-H molecules.

The Raman spectrum of G-ASA (Figure 3) showed the two characteristic graphene vibrations, the G- band at around 1580 cm (vibration of E2 _g symmetry in graphene), and the D-band at 1350 cm ¹ due to aromatic ring vibrations adjacent to sp ³ carbon centers bonded with taurine and other defects. Moreover, the 2D band at around 2670 cm ¹ from the overtone of the D-band is be attributed to the double resonance transition in few-layered graphene, which is only Raman active in the presence of defects, i.e., surface functionalization in this case . The broad character of the D-band and the high ID/IG ratio of 1.21 suggests the high functionalization degree in G-ASA.

Thermogravimetric analysis (TGA) of G-ASA revealed the presence of surface organic species. The mass loss above 200 °C is attributed to the loss of sulfur-containing covalently-bonded moieties due to taurine’s sulfonic acid functional groups, as verified by the emission of SO and SO2 gasses (Figure 4), and further confirmed by X-ray photoelectron spectroscopy (XPS) analysis by the elimination of sulfur from G-ASA after its thermal treatment at 500 °C (Figure 2). Based on the mass loss between 200-550 °C, the amount of taurine groups in the sample was 24.4 mass % (and 71.2 % carbon after 500 °C), corresponding to 2.1 mmol g ¹ of SO3H and 1 sulfonic acid (or 1 taurine) unit per 28.7 carbon atoms of the graphitic skeleton, indicating a functionalization degree of 3.5 %.

The surface chemical states of the product obtained from Example 1 were probed by XPS (Figure 5), according to which the product contained a high N content of 8.7 % and was practically fluorine-free (Table 1). Indeed, deconvolution of the high-resolution C Is XPS profile (Figure 5b) showed that it consisted mostly of sp ² -hybridized carbons (C=C) and other components at higher binding energies (BEs), corresponding to sp ³ carbons, C-S, C-N, and C-0 bonds. The N Is core-level XPS spectrum (Figure 5c) showed three components at BEs of 399, 400.1, and 401.6 eV, assigned to the secondary non-protonated amine (C-NH-C), to the related hydrogen bonding configurations, and the protonated secondary amine groups, respectively. In the HR-XPS S 2p core level spectrum (Figure 5d), the presence of the -SO3H species is evidenced by the doublet of S 2p ^1/2 (168.87 eV) and S 2p ^3/2 (167.72 eV) due to spin-orbit splitting, which was assigned to the sulfonic acid groups, while the small component at 169.81 eV corresponds to SO4.

Table 1. Atomic contents as obtained from X-ray photoelectron spectroscopy analysis for the starting graphite fluoride and for the product of Example 1.

Example 2: Synthesis of homotaurine functionalized graphene at 130 °C using K2CO3 as base

The same procedure as in Example 1 was followed, using homotaurine instead of taurine.

X-ray photoelectron spectroscopy on the product of this example showed that the reaction with homotaurine resulted in the introduction of N atoms (8.0 at. %), S atoms (4.6 at. %) in the product, after 24 h of reaction, and in significant loss of fluorine atoms from 50.5 at. % of the starting fluorinated graphite down to 0.9 at. % (Table 2).

Table 2. Atomic contents as obtained from X-ray photoelectron spectroscopy analysis for the product of Example 2.

Example 3: Synthesis of taurine functionalized graphene at 130 °C using triethylamine as base

The same procedure as in Example 1 was followed, but instead of using K2CO3, it was used Triethylamine. X-ray photoelectron spectroscopy on the product of this example showed that the reaction with Triethylamine resulted in the introduction of N atoms (7.4 at. %), S atoms (2.5 at. %) in the product, after 24 h of reaction, and in significant loss of fluorine atoms from 50.5 at. % of the starting fluorinated graphite down to 3.1 at. % (Table 3).

Table 3. Atomic contents as obtained from X-ray photoelectron spectroscopy analysis for the product of Example 3.

Example 4: Synthesis of taurine functionalized graphene at 100 °C using triethylamine as base

The same procedure as in Example 1 was followed, but instead of using K2CO3 at 130 °C, it was used Triethylamine at 100 °C. X-ray photoelectron spectroscopy on the product of this example showed that the reaction at 100 °C resulted in the introduction of N atoms (6.7 at. %), S atoms (1.6 at. %) in the product, after 24 h of reaction, and significant loss of fluorine atoms from 50.5 at. % of the starting fluorinated graphite down to 6.5 at. % (Table 3).

Table 4. Atomic contents as obtained from X-ray photoelectron spectroscopy analysis for the product of Example 4.

Example 5: Solketal synthesis using the product from Example 1 (1:4 mole ratio of Glycerol: Acetone)

1 g of glycerol and 2.52 g of acetone (1:4 molar ratio) were placed into a 25 mL round bottom flask and magnetically stirred at room temperature until it formed a homogenous phase. 0. 1-0.5 mass% of product from Example 1 was added to the above mixture, and the stirring continued for 1 h. The product was analyzed by a GC (Agilent 7820A), equipped with a flame ionization detector (FID). Analysis of the reaction products showed 96.5 % glycerol conversion and 96.8 % selectivity for solketal, corresponding to specific productivity of 2094 mmol g ¹ h ¹ (Table 5). To probe the actual activity of the catalyst we performed the reaction with low catalyst loading (0.1 mass%) affording a specific productivity of 7508 mmol g ’h ¹ and a TOF value of 1735 h ¹ . Other catalysts and their performance are also given in

Table 5 for comparison.

Table 5. Solketal synthesis by the product from example 1 with 1:4 glycerol to acetone mole ratio, comparison with catalysts known in the state of art Example 6: Solketal synthesis using the product from Example 1 (1:2 mole ratio of Glycerol: Acetone)

The same procedure as in Example 5 was followed, but instead of using a 1 :4 ratio of glycerol to acetone, 1:2 ratio was used. Glycerol conversion reached 99.9 % even when decreasing the mole ratio of glycerol to acetone to 1:2 with a specific productivity value of 2167 mmolg ^_1 h ^_1 and TOF value of 539 h ¹ (Table 6).

Table 6. Solketal synthesis by the product from example 1 with 1:2 glycerol to acetone mole ratio

Example 7: Solketal synthesis using the product from Example 2

The same procedure as in Example 5 was followed, but instead of using the product from Example 1, it was used the product from Example 2. By using the product from Example 2 Glycerol conversion reached 82.5 % with solketal selectivity of 91.2 % having a specific productivity value of 1790 mmol g ^-1 h ^-1 and TOF value of 510 h ¹ (Table 7).

Table 7. Solketal synthesis by the product from example 2

Example 8: Catalytic solketal production recyclability study using the product from Example 1

1 g of glycerol and 2.52 g of acetone were taken into a 25 mL round bottom flask and magnetically stirred at room temperature until a homogenous phase was formed. 0.5 mass% product from Example 1 was quickly added to the above mixture, and the stirring continued for 1 h. The product was analyzed by a GC and the used catalyst was recovered by simple centrifugation and washed with acetone several times to remove the impurities adsorbed on the catalyst. The sample was protonated by washing with 25 % sulfuric acid and then washed with methanol to remove excess acid. The final precipitate was then dried at 60 °C overnight before using it for the next cycle. After three cycles of Glycerol acetalization, the product from Example 1 did not show any activity loss and product selectivity (Table 8).

Table 8. Recyclability of the product from Example 1 for solketal synthesis.

Example 9: Biodiesel synthesis using the product from Example 1

The product from Example 1 (10 mg) and carboxylic acid (0.5 mmol) were mixed in a 2 ml screw-top vial and sonicated for 30 seconds. Then, under an N2 atmosphere, methanol (dry, 0.4 ml, ratio methanol/carboxylic acid 20: 1) was added and the vial was closed with the screw top. The mixture was sonicated for another 30 seconds, then heated at 60 °C for 4 h. The product was then separated from the catalyst by centrifugation (15 000 rpm) for 5 minutes. The supernatant was kept at room temperature for two days and allowed solvent to evaporate for direct NMR analysis. Esterification of palmitic acid, stearic acid, and oleic acid with methanol showed 100% yield (based on NMR, Table 9) after a 4 h reaction at 60°C.

Table 9. Esterification of carboxylic acid using the product from Example 1

Example 10: Biodiesel synthesis using the product from Example 2

The same procedure as in Example 8 was followed, but instead of using the product from Example 1, it was used product from Example 2. Esterification of oleic acid with methanol showed an 80% yield (based on NMR, Table 10) after a 4 h reaction at 60°C.

Table 10. Esterification of carboxylic acid using the product from Example 2