Method for synthesizing 5-hydroxymethylfurfural

Abstract

The invention relates to a method for dehydrating a hexose-containing feedstock, the process comprises the steps of:-preparing a catalyst by contacting a zeolite containing a series of channels in which openings are greater than or equal to 10 oxygen atoms (10MR) and having a Si / Al ratio of at least 10 with a solution comprising an organic compound having the formula: # imgabs0 # R1, R2, R3 and R4 are independently selected from linear, branched or cyclic alkyl groups having 1 to 10 carbon atoms, X-is a hydroxyl ion OH-; -contacting said dehydration catalyst with said hexose-containing feedstock in at least one solvent, said contacting being carried out at a temperature of 30-160 DEG C and a pressure of 0.0001-8.0 MPa.

Description

Technical Field

[0001] The present invention relates to a process for producing 5-hydroxymethylfurfural (5-HMF) from a feedstock containing sugars, in particular at least one hexose, preferably fructose, in the presence of at least one solvent, preferably a polar aprotic solvent, and at least one dehydration catalyst, which comprises a zeolite modified by alkali treatment in the presence of an organic compound of the tetraalkylammonium hydroxide type. Prior art

[0002] 5-Hydroxymethylfurfural (5-HMF) is a biomass-derived compound that can be modified in various ways, for example in specialty chemicals, polymers or agrochemicals.

[0003] The production of 5-HMF by dehydration of hexoses is well known to those skilled in the art and is the subject of extensive research. It is known to use Bronsted acid catalysts to accelerate the reaction. Zeolite-based catalysts are particularly effective in catalyzing the reaction.

[0004] However, the diffusion of hexoses into the pore structure of the zeolite has been shown to limit the kinetics of the reaction, as described in the publication by Dapsens et al. (“Hierarchical Sn-MFI zeolites prepared by facile top-down methods for sugar isomerisation”, Vol. 4, No. 8, p. 2302, (2014) Catalysis Science & Technology. Moreover, according to the publication by Ordomsky et al. (“The effect of solvent addition on fructose dehydration to 5-hydroxymethylfurfural in biphasic system over zeolites”, Vol. 287, pp. 68-75, (2012) Journal of Catalysis, Long-term contact of sugars with the outer surface of the zeolite leads to the formation of undesirable polymeric by-products, which are collectively referred to as humic substances, which lead to yield losses and catalyst deactivation.

[0005] Therefore, it is advantageous to reduce the restriction of hexose diffusion into the zeolite.

[0006] A publication by Rac et al. (“Hierarchical ZSM-5, Beta and USY zeolites. Acidity assessment by gas and aqueous phase calorimetry and catalytic activity in fructose dehydration reaction” in Microporous and Mesoporous Materials (2014), Vol. 194, pp. 126-134) discloses that a zeolite modified by alkali treatment in the presence of sodium hydroxide (NaOH) and tetrapropylammonium bromide is used in a method for dehydrating fructose to 5-HMF.

[0007] The disadvantage of the prior art methods is the use of NaOH and tetraalkylammonium compounds during the zeolite modification process. This is because the use of NaOH (or other alkali metal or alkaline earth metal hydroxides) will neutralize the acidity of the zeolite, and an additional step of NaOH treatment is required after the alkaline treatment. + (or other alkali metal or alkaline earth metal salts), these steps make the process very costly.

[0008] The present invention therefore aims to improve the performance of zeolite catalysts for the production of 5-HMF from sugars while limiting costs. In particular, the Applicant has found that zeolites modified by treatment in an alkaline medium in the presence of at least one organic compound of the tetraalkylammonium hydroxide type acting as a structure directing agent can be used as catalysts for the conversion of sugars into 5-HMF without the use of additional alkaline compounds, such as sodium hydroxide, to improve the performance of the catalyst for the production of 5-HMF from hexoses. Advantageously, the selectivity is improved and the formation of oligomeric by-products is limited, and the process is more economical. SUMMARY OF THE INVENTION

[0010] The present invention relates to a method for dehydrating a raw material containing hexose, the method comprising the following steps:

[0011] The catalyst is prepared by contacting a zeolite containing a series of channels in which the openings are greater than or equal to 10 oxygen atoms (10 MR) and having a Si / Al ratio of at least 10 with a solution containing an organic compound having the formula:

[0012]

[0013] R1, R2, R3 and R4 are independently selected from linear, branched or cyclic alkyl groups having 1 to 10 carbon atoms, X- is a hydroxide ion OH-;

[0014] - contacting the dehydration catalyst with the hexose-containing feedstock in at least one solvent, the contacting being carried out at a temperature of 30-160° C. and a pressure of 0.0001-8.0 MPa. DETAILED DESCRIPTION OF THE INVENTION

[0016] According to the present invention, the expressions "... -..." and "... to..." are equivalent, meaning that the limits of the interval are included in the stated value range. If this is not the case and the limits are not included in the stated range, the present invention will explain this.

[0017] For the present invention, various parameter ranges of a given step, such as pressure ranges and temperature ranges, can be used alone or in combination. For example, for the present invention, a preferred pressure value range can be combined with a more preferred temperature value range.

[0018] The specific embodiments of the present invention will be described below. They can be used alone or in combination with each other. In the case where it is technically feasible, such combination is not limited.

[0019] raw material

[0020] The raw material used in the method of the present invention contains hexose.

[0021] Hexose-containing starting materials are understood to mean that the hexoses can be in monomeric form (monosaccharides) or can be a unit component of disaccharides, oligosaccharides or polysaccharides. Saccharides are compounds which are also called sugars.

[0022] The hexose is preferably fructose or a fructoside unit.

[0023] In one embodiment, the raw material comprises free fructose alone or in admixture with any sugars, or comprises any oligosaccharide or polysaccharide raw material containing one or more fructoside units capable of releasing fructose by one or more hydrolysis steps, which may be used in admixture with other sugars. The raw material treated in the method is preferably crystallized fructose, a syrup containing fructose and glucose, crystallized sucrose or sucrose syrup.

[0024] The fructose-containing feedstock advantageously comprises fructose in monomeric, oligomeric or polymeric form.

[0025] A feedstock containing a mixture of free fructose and any sugars refers to, for example, a high fructose corn syrup type syrup containing fructose and glucose in varying proportions (e.g., glucose / fructose weight or molar ratios of 58 / 42, 45 / 55, 10 / 90).

[0026] Syrup is understood to mean a solution of sugar, in particular saccharide, in water in a concentration of at least 30% by weight, preferably at least 50% by weight, preferably at least 70% by weight.

[0027] The feedstock may comprise saccharides containing one or more fructoside units and one or more non-fructoside units, from which fructose may be released by one or more hydrolysis steps, such as oligosaccharides and polysaccharides in which at least one monosaccharide unit is fructose, for example, feedstocks such as sucrose, kestose, fructan, oligofructan and inulin.

[0028] The carbohydrate feedstock is advantageously capable of being hydrolyzed by glycosides to release monomeric fructose, wherein the fructose produced can be converted into 5-HMF.

[0029] The oligosaccharide preferably has the following empirical formula: 6m H 10m+2 O 5m+1 )(C 5n H 8n+2 O 4n+1 ), wherein m and n are integers, the sum of which is 2 to 6. The monosaccharide units constituting the oligosaccharide are the same or different, and at least one of them has the formula (C 6m H 10m+2 O 5m+1 ) is fructose. By extension, the polysaccharide preferably has the empirical formula (C 6m H 10m+2 O 5m+1 )(C 5n H 8n+2 O 4n+1 ), where m and n are integers, the sum of which is greater than or equal to 7.

[0030] Preparation of dehydration catalyst

[0031] The dehydration catalyst is prepared from a zeolite having a series of channels with openings greater than or equal to 10 oxygen atoms (10 MR) and a Si / Al molar ratio of at least 10.

[0032] Advantageously, this starting zeolite is referred to as the parent zeolite.

[0033] In one embodiment, the zeolite has a series of channels with an opening equal to 12 oxygen atoms (12 MR).

[0034] Preferably, it is a zeolite of the FAU or EMT structure type. Very preferably, it is a dealuminated faujasite known as USY (Ultra Stable Y), which advantageously has a molecular mass less than lattice parameters.

[0035] In a preferred embodiment, the Si / Al molar ratio of the parent zeolite is 10-50, preferably 12-35.

[0036] If the Si / Al molar ratio of the parent zeolite is less than 10, a dealumination treatment may be performed, preferably before any step of contacting with an organic compound of the tetraalkylammonium hydroxide type, to obtain a zeolite with a Si / Al molar ratio within the desired range. This dealumination treatment comprises treating the zeolite with an acid solution or a solvent containing an organic compound capable of reacting with Al in the solution. 3+ The method is well known to those skilled in the art. Some examples of suitable acids are: HNO3, HCl, H2SO4, CH3COOH, HOOC-COOH. Examples of complexing agents include EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid) and the like in the form of their acid or Na salt. The concentration of the acid or complexing agent in the acid solution or complexing agent solution is preferably 0.1-6 mol / 1. The volume of the solution used is preferably 3-50 ml / g zeolite (dry weight of zeolite). The temperature of the dealumination treatment is preferably 20-100°C. The duration of the dealumination treatment is preferably 15 minutes to 10 hours. After the dealumination treatment, the zeolite is advantageously separated by filtration or centrifugation, dried at a temperature preferably of 80-150°C, and finally calcined at a temperature of 450°C-600°C.

[0037] The dry weight of zeolite refers to the mass of the zeolite after treatment at 1000°C.

[0038] In order to improve the catalyst performance in the process of converting sugar, preferably fructose, into 5-HMF, the zeolite (optionally after adjustment of the Si / Al ratio by dealumination) is contacted with a solution comprising an organic compound having the formula:

[0039]

[0040] R1, R2, R3 and R4 are independently selected from linear, branched or cyclic alkyl groups having 1 to 10 carbon atoms, and X- is a hydroxide ion OH-.

[0041] The organic compounds are more specifically known as tetraalkylammonium hydroxides.

[0042] The radicals R1, R2, R3 and R4 may be identical or different.

[0043] In a specific embodiment, the organic compound contains at least one group R1, R2, R3 or R4 selected from methyl, ethyl, propyl and butyl. In a preferred embodiment, the organic compound is selected from tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide.

[0044] In a preferred embodiment, the organic compound contains at least one radical R1, R2, R3 or R4 selected from the group consisting of propyl and butyl.

[0045] In a very preferred embodiment, the organic compound is selected from tetrapropylammonium hydroxide and tetrabutylammonium hydroxide.

[0046] Treating zeolites by contacting them with tetraalkylammonium can modify the acidity of the zeolites and create mesopores that are favorable for the diffusion of reactants and products of the sugar dehydration reaction.

[0047] During contact with the zeolite, the organic compound is advantageously in the form of a solution, preferably an aqueous solution. The solution preferably does not contain an alkali metal or alkaline earth metal hydroxide, such as NaOH. The use of NaOH or other alkali metal or alkaline earth metal hydroxides will neutralize the acidity of the zeolite and require an additional step of NaOH treatment after the alkaline treatment. + These additional steps make the process very costly.

[0048] In a specific embodiment, the solution may include a second organic compound having the formula:

[0049]

[0050] wherein R1, R2, R3 and R4 are independently selected from a linear, branched or cyclic alkyl group having 1 to 10 carbon atoms, - Selected from halide ions Cl - Br - and I - .

[0051] A variety of cations N (R1R2R3R4) can be used + mixture, and various anions X - mixture.

[0052] In the process according to the invention, it is important to provide the OH groups which provide alkalinity and allow the formation of mesopores in the parent zeolite. - The anion is distinguished from other anions such as halides.

[0053] Optionally, the solution may comprise an organic base, for example an amine, such as dimethylamine or trimethylamine, as a second organic compound.

[0054] The contacting of the zeolite with the organic compound and the optional second organic compound is preferably carried out in a vessel in which the solution can be stirred to ensure good mass exchange.

[0055] The volume of the solution is defined relative to the dry weight of the zeolite used (ie the parent zeolite). The volume usually selected is 3-100 ml / g zeolite, preferably 5-50 ml / g zeolite.

[0056] The solution generally comprises water as solvent, optionally mixed with another protic polar solvent.

[0057] The concentration of the base in the solution corresponds to the sum of the molar amount of N(R1R2R3R4)OH and the molar amount of the optionally added second organic compound per unit volume of the solution. The concentration of the base is advantageously 0.1-2 mol / l, preferably 0.1-0.3 mol / l.

[0058] N(R1R2R3R4) + The ratio of the concentration of to the total concentration of alkali is 1-0.1.

[0059] The final pH of the solution, ie the pH at the end of the treatment, is preferably 10-14.

[0060] The alkali treatment is advantageously carried out at a temperature of 20-90°C, preferably 50-70°C.

[0061] The duration of the treatment is preferably from 15 minutes to 10 hours, preferably from 1 to 4 hours.

[0062] When the treatment is finished, the zeolite (also called treated zeolite) is separated from the solution, advantageously by filtration or centrifugation.

[0063] Preferably, the zeolite resulting from the treatment (or final ion exchange) is separated, then dried at a temperature of 80-150° C. (optionally by applying reduced pressure) and finally calcined at a temperature of 500-650° C. in a nitrogen or air stream. For safety reasons (avoiding violent oxidation reactions of organic compounds), the temperature may be raised first in a nitrogen stream and then in an air stream. The duration of the final isothermal step is from 1 to 12 hours.

[0064] The dehydration catalyst according to the method of the present invention comprises a treated zeolite obtained after alkali treatment, preferably separation, drying and calcination. The zeolite obtained after alkali treatment, preferably separation, drying and calcination may be optionally mixed with a binder to form the dehydration catalyst.

[0065] Implementation of the dehydration step

[0066] The method comprises a dehydration step. The step of dehydrating the sugars is carried out by contacting the hexose-containing feedstock with a dehydration catalyst in at least one solvent, preferably an organic solvent.

[0067] The process according to the invention is advantageously a process for producing 5-hydroxymethylfurfural from a hexose-containing feedstock.

[0068] The dehydration step of the process according to the present invention can be carried out continuously or batchwise by contacting the hexose-containing feedstock with a dehydration catalyst in at least one solvent.

[0069] The step of contacting the hexose-containing raw material with a dehydration catalyst in at least one solvent is carried out at a temperature of 30-160°C, preferably 40-140°C, preferably 50-120°C, more preferably 80-120°C, very preferably 90-120°C, and at a pressure of 0.0001-8.0 MPa, preferably 0.001-5.0 MPa, more preferably 0.01-3.0 MPa.

[0070] The reaction medium comprises the hexose-containing feedstock, at least one solvent (preferably an organic solvent) and a dehydration catalyst.

[0071] The solvent is preferably a polar aprotic solvent. The polar aprotic solvent is advantageously selected from butan-2-one, acetone, acetic anhydride, N, N, N', N'-tetramethylurea, benzonitrile, acetonitrile, methyl ethyl ketone, propionitrile, hexamethylphosphoramide, nitrobenzene, nitromethane, N, N--dimethylformamide, N, N-dimethylacetamide, sulfolane, N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate and γ-valerolactone. Preferably, the polar aprotic solvent is selected from acetone, hexamethylphosphoramide, N, N-dimethylformamide, sulfolane, N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate and γ-valerolactone.

[0072] Very preferably, the polar aprotic solvent is dimethyl sulfoxide (DMSO).

[0073] The advantage of using a polar aprotic solvent in the process is that the produced 5-HMF does not undergo rehydration; rehydration of 5-HMF in a solvent such as water, for example, results in a reduction in the process selectivity to 5-HMF. Therefore, the use of a polar aprotic solvent can achieve the best process selectivity to 5-HMF.

[0074] Aprotic solvents are understood to mean molecules acting as solvents in which all hydrogen atoms are located on carbon atoms.

[0075] Polar solvents are understood to mean molecules which act as solvents and whose dipole moment μ (expressed in debye units) at 25° C. has a value greater than or equal to 2.00 according to databases known to the person skilled in the art.

[0076] Thus, a polar aprotic solvent is understood to mean a molecule acting as a solvent in which all hydrogen atoms are located on carbon atoms and whose dipole moment μ (expressed in debye units) at 25° C. has a value greater than or equal to 2.00, according to databases known to the person skilled in the art.

[0077] The feedstock is introduced into the reaction medium at a solvent / feedstock weight ratio of 0.1-200.0, preferably 0.3-100.0, more preferably 1.0-50.0.

[0078] The dehydration step is advantageously fed by a hexose-containing feedstock in solid or liquid form.The feeding of the feedstock can be carried out according to several modes of introduction of the feedstock.

[0079] In a first embodiment, the raw material is optionally introduced in solid form using a suitable device that can control the flow of the raw material. In a non-limiting manner, the device can be an endless screw or pneumatic system for conveying solid particles. In a non-limiting manner, for raw materials of oligosaccharide or polysaccharide types, this embodiment is preferred.

[0080] The introduction of a solid form of a starting material corresponding to sucrose, kestose or inulin (from which fructose is released by hydrolysis) is one possibility. The introduction can be carried out one or more times, successively or continuously.

[0081] In a second embodiment, the feedstock is introduced in liquid form as a solution in a solvent (referred to as an additional solvent) using a pump that can control the introduction flow of the solution containing the feedstock. The additional solvent allows the sugar (or saccharide) in the feedstock to dissolve.

[0082] The additional solvent can be selected from aprotic or protic polar solvents. The additional solvent is preferably selected from butanone-2-one, acetone, acetic anhydride, N,N,N',N'-tetramethylurea, benzonitrile, acetonitrile, methyl ethyl ketone, propionitrile, hexamethylphosphoramide, nitrobenzene, nitromethane, N,N-dimethylformamide, N,N-dimethylacetamide, sulfolane, N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate, γ-valerolactone, water, methanol, ethanol, formic acid and acetic acid.

[0083] The additional solvent selected from the group of polar aprotic or protic solvents is preferably acetone, hexamethylphosphoramide, N,N-dimethylformamide, sulfolane, N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate, γ-valerolactone, water, methanol and ethanol, preferably selected from the group consisting of N,N-dimethylformamide, sulfolane, N-methylpyrrolidone, dimethyl sulfoxide, water and methanol, and the additional solvent is very preferably selected from the group consisting of water and dimethyl sulfoxide.

[0084] In the present embodiment where an additional solvent is present, the final concentration of 5-HMF cannot exceed the solubility limit of the sugar in the additional solvent, which is modified by the molar mass ratio of fructose to 5-HMF and by diluting the feedstock with the mass of polar aprotic solvent. Therefore, the choice of additional solvent is critical to achieving a high final concentration of 5-HMF.

[0085] The content of at least one additional solvent is 0% to 50% by weight, preferably 0% to 35% by weight, more preferably 0% to 25% by weight, very preferably 0% to 20% by weight, relative to the total weight of solvents in the reaction medium.

[0086] This embodiment is particularly suitable for feedstocks of the monosaccharide type or even oligosaccharide type that can be dissolved in high concentrations in the additional solvent.

[0087] The raw material corresponding to fructose syrup or fructose and glucose syrup (high fructose corn syrup type) is preferably gradually introduced via a pump. The introduction can be performed one or more times sequentially or continuously so that the instantaneous fructose concentration remains less than or equal to 5.0% by weight.

[0088] The concentration of 5-HMF or fructose is understood to be the ratio of the weight of 5-HMF or fructose, respectively, relative to the weight of the reaction medium.

[0089] In the continuous operation of the method of the present invention, the hourly mass flow rate (weight flow rate of raw materials / weight of catalyst) is 0.01h-1 to 5.0h- 1 , preferably 0.02h- 1 To 2.0h- 1 .

[0090] Regardless of which embodiment is adopted for the dehydration step of the process, the water formed and therefore present in the reaction mixture is preferably removed, preferably continuously, by any method known to a person skilled in the art, in order to maintain a water content of less than 30.0% by weight, preferably less than 20.0% by weight, more preferably less than 15.0% by weight and very preferably less than 10.0% by weight, relative to the total weight of the solvent.

[0091] Advantageously, the 5-HMF production process is carried out, in particular by controlling the instantaneous concentration in combination with the use of a zeolite modified according to the catalyst preparation step of the invention, which makes it possible to obtain a good conversion of the fructose involved, as well as an excellent selectivity in favor of 5-HMF and an improved productivity of 5-HMF.

[0092] Thus, the selectivity, yield and productivity obtained by implementing the process of the invention make it possible, for example, to obtain a final weight concentration of 5-HMF of greater than 3.5% by weight. The process according to the invention advantageously makes it possible to obtain a final weight concentration of 5-HMF of greater than 5.0% by weight, preferably greater than 10% by weight, very preferably greater than 15% by weight, relative to the weight of the reaction medium.

[0093] Obtained products and analysis methods thereof

[0094] The product selectively obtained by the conversion process according to the invention is 5-hydroxymethylfurfural (5-HMF). At the end of the reaction carried out in the process according to the invention, the reaction medium is analyzed by high performance liquid chromatography (HPLC) to quantify the compounds present in the reaction medium, such as fructose and 5-HMF, and to quantify undesirable products, such as levulinic acid, formic acid and any sugar-containing co-products. The feedstock conversion, fructose and 5-HMF content are then calculated.

[0095] Humic matter is quantified by the difference in carbon balance from the carbon initially introduced. Example

[0096] In the following examples, fructose used as a raw material was commercially available fructose and was used without further purification.

[0097] Dimethyl sulfoxide (represented by DMSO in the examples) used as a polar aprotic solvent is commercially available and was used without further purification.

[0098] The weight concentrations of the components in the reaction mixture are determined by high performance liquid chromatography (HPLC). Aliquots, i.e. samples, of the reaction mixture are periodically removed in order to assess their composition by HPLC. HMF yields below 100% are associated with only partial conversion of fructose or with the formation of undesirable products (humic substances, polymers, etc.).

[0099] Fructose conversion rate (ConV FRU ) is defined as the ratio of the molar concentration of converted fructose to the molar concentration of fructoside units present in the starting feedstock, expressed in %.

[0100] 5-HMF yield HMF ) is defined as the ratio of the molar concentration of 5-HMF found in the sample to the molar concentration of fructoside units present only in the starting material, expressed in %.

[0101] Example 1: Preparation of dehydration catalyst

[0102] The various samples were prepared by the same procedure except that the nature of the parent zeolite and tetraalkylammonium hydroxide were varied.

[0103] The parent zeolite CBV720 used had a Si / Al molar ratio of 15.7 and a lattice parameter of

[0104] The parent zeolite CBV760 used has a Si / Al molar ratio of 27.2 and a lattice parameter of

[0105] Prepare 300 ml of a 0.2 M aqueous solution of tetraalkylammonium hydroxide with distilled water. When the solution reaches 65°C, add 10 g of the parent zeolite and stir the mixture at 400 rpm for 2 hours at 65°C. Quench the solution in ice water, filter, and wash 3 to 4 times with 300 ml portions of distilled water and centrifuge at 8000-10000 rpm for 3 minutes. Dry the recovered solid in an oven at 120°C overnight. Grind the dried solid and let stand at 200°C for 2 hours (1.5°C min -1 ) and then calcined at 550 °C for 4 h (1 °C min -1 ).

[0106] A comparative solid was prepared according to the prior art (BC) using CBV760 as the parent zeolite and treated with NaOH and tetrapropylammonium bromide.

[0107] The following table summarizes the parent zeolites and organic compounds used in the preparation of each sample:

[0108] Table 1

[0109] solid Parent zeolite Organic compound (tetraalkylammonium) A1 CBV720 Tetramethylammonium hydroxide A2 CBV720 Tetraethylammonium hydroxide A3 CBV720 Tetrapropylammonium hydroxide A4 CBV720 Tetrabutylammonium hydroxide B 1 CBV760 Tetramethylammonium hydroxide B2 CBV760 Tetraethylammonium hydroxide B3 CBV760 Tetrapropylammonium hydroxide BC CBV760 Tetrapropylammonium bromide B4 CBV760 Tetrabutylammonium hydroxide

[0110] The following table lists the characteristics (specific surface area, micropore volume, mesopore volume, crystallinity and Brønsted acidity) of different parent zeolites and calculated samples:

[0111] Table 2

[0112]

[0113] ND=Not Sense

[0114] BET specific surface area (S BET , unit m 2 The molecular weight (MW) of the raw materials was determined by nitrogen adsorption according to ASTM D 3663-78, which was based on the Brunauer-Emmett-Teller method described in the journal "Journal of the American Chemical Society", 1938, 60, 309.

[0115] The micropore volume is determined by the t-plot method using the Harkins-Jura method, where a linear regression is performed on the t-plot in the range between t = 0.35 nm and 0.5 nm. The total pore volume corresponds to the adsorption volume when the pores are saturated (N2 relative pressure of 0.99). The mesopore volume is the difference between the total pore volume and the micropore volume.

[0116] The relative crystallinity of the zeolite is determined by X-ray diffraction. To assess the crystallinity, the diffraction lines between the 2θ angles of 10°-70° are integrated. The ratio between the baseline corrected integral and the baseline uncorrected integral is calculated. The ratio is then normalized by the ratio of the reference zeolite (parent zeolite before treatment).

[0117] Bronsted acids are understood to mean acids which release protons H in the reaction medium. + Lewis acids are understood to mean compounds which are able to accept an electron pair.

[0118] The concentration of Brønsted and Lewis acid sites in the zeolite was determined by pyridine adsorption. The zeolite was first dehydrated at 450°C under reduced pressure and then exposed to pyridine vapor at 150°C. Excess pyridine was removed under reduced pressure. The concentration of Brønsted acid sites was determined by transmission IR spectroscopy by integrating the band at 1540 cm-1; the intensity was converted to mol / g by adopting an extinction coefficient of 1.65 cm / μmol. The concentration of Lewis acid sites was determined by integrating the band at 1450 cm-1 and adopting an extinction coefficient of 1.57 μmol / g.

[0119] Table 3 shows that the tetraalkylammonium hydroxide treatment results in good preservation of the micropore volume of the zeolite, especially in the case of tetrapropylammonium hydroxide and tetrabutylammonium hydroxide (A3, A4, B3 and B4). In all cases, the mesopore volume increases significantly.

[0120] The crystallinity was well maintained after treatment except for B1. The acidity was close to or higher than that of the parent zeolite except for B1.

[0121] The treatments without NaOH and with tetraalkylammonium hydroxide resulted in zeolites with increased acidity compared to the prior art (BC).

[0122] Example 2: Conversion of fructose to 5-HMF using the solids from Example 1

[0123] 2 g of crystalline fructose and 20 g of DMSO were loaded into a 100 mL three-necked flask equipped with a condenser. The mixture was heated to 90 ° C under stirring. Once this temperature was reached, 1.3 g of catalyst (i.e., the solid prepared according to Example 1) was added, which marked the beginning of the dehydration reaction. The mixture was then kept at this temperature under stirring. Samples were taken at different reaction times and analyzed by HPLC to quantify the concentrations of fructose and 5-HMF, and to determine the yield of HMF and the conversion rate of fructose.

[0124] The results (fructose conversion and HMF yield after 2 and 6 hours of reaction time) are collated in the following table:

[0125] Table 3

[0126]

[0127]

[0128] These results show that zeolites prepared with tetraalkylammonium hydroxides can convert fructose. The reaction kinetics are particularly accelerated for solids prepared with tetrapropylammonium hydroxide and tetrabutylammonium hydroxide (A3, A4, B3 and B4) (higher fructose conversion and HMF yield after 2 and 6 hours reaction time compared to the parent zeolites).

Claims

1. A method for dehydrating a raw material containing hexose, the method comprising the following steps: - a dehydration catalyst is prepared by contacting a zeolite containing a series of channels with openings greater than or equal to 10 oxygen atoms (10 MR) and a Si / Al molar ratio of at least 10 with a solution containing an organic compound having the formula: R1, R2, R3 and R4 are independently selected from linear, branched or cyclic alkyl groups having 1 to 10 carbon atoms, X- is a hydroxide ion OH-; - contacting the dehydration catalyst with the hexose-containing feedstock in at least one solvent, the contacting being carried out at a temperature of 30-160° C. and a pressure of 0.0001-8.0 MPa.

2. The method of claim 1, wherein the hexose is fructose or a fructoside unit.

3. A method as claimed in any one of the preceding claims, wherein the zeolite has a series of channels with an opening equal to 12 oxygen atoms (12 MR).

4. The method of claim 3, wherein the zeolite is of FAU or EMT structure type.

5. The method of claim 4, wherein the zeolite is USY zeolite.

6. A process as claimed in any one of the preceding claims, wherein the zeolite has a Si / Al ratio of 10-50, preferably 12-35.

7. A method as claimed in any one of the preceding claims, wherein the organic compound comprises at least one group selected from methyl, ethyl, propyl or butyl.

8. The method of claim 7, wherein the organic compound is selected from tetrapropylammonium hydroxide and tetrabutylammonium hydroxide.

9. The method of any one of the preceding claims, wherein the solution comprising an organic compound is free of NaOH.

10. A process as claimed in any preceding claim, wherein the solvent is a polar aprotic solvent.

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