A solid catalyst and a method for preparing L-ribose

Abstract

The present invention discloses a method for preparing molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton, comprising the following steps: S11, chitosan is enriched on the surface of pectin-glucose-calcium gel particles to form a polyelectrolyte complex; S12, chitosan on the surface of the polyelectrolyte complex is solidified; S13, MoO4 2‑ Under the adsorption of chitosan, they are accumulated on the surface of chitosan; S14, hydrothermal reaction; S15, filtration, cleaning, and vacuum drying; S16, high-temperature carbonization to obtain the target product; the present invention also discloses a method for preparing L-ribose by uniformly dispersing L-arabinose and molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton in deionized water added with 4-pyridine propanol as a complexing agent; the present invention utilizes a solid catalyst to effectively adsorb L-arabinose and the complexing agent, improves the hydrophilicity of the carbon skeleton, enhances the contact between molybdenum oxide and L-arabinose, and improves the conversion efficiency of L-arabinose.

Description

Technical Field

[0001] The invention relates to the technical field of L-ribose preparation, and in particular to a method for preparing a molybdenum oxide solid catalyst dispersed and loaded by a nitrogen-doped porous carbon skeleton and a method for preparing L-ribose. Background Art

[0002] The chemical method for preparing L-ribose is more commonly used. Under the catalysis of molybdic acid and other catalysts, L-arabinose can be catalytically stereoisomerized into L-ribose. The specific reaction formula is as follows:

[0003] ;

[0004] The stereoisomerization reaction of L-arabinose into L-ribose in an aqueous solution under the action of a molybdenum-containing catalyst has great practical value. The method has a simple synthesis route, and the raw material L-arabinose is cheap and easy to obtain. The catalyst used for catalytic isomerization of L-arabinose into L-ribose can be obtained by hydrolysis and separation from natural materials such as corn fiber. The invention patent CN109232676B discloses a method for separating L-ribose, using ammonium molybdate as a catalyst. The ammonium molybdate is dispersed in the liquid phase and is difficult to recover. Summary of the invention

[0005] The purpose of the present invention is to provide a preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton. The present invention utilizes the electrostatic interaction and hydrogen bonding between chitosan, pectin and glucose to aggregate on the surface of calcium pectin gel micron particles, and then utilizes the adsorption effect after chitosan solidification under alkaline conditions to effectively adsorb molybdate on the surface of chitosan, and cooperates with calcination to obtain molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton. The molybdenum oxide loaded on the nitrogen-doped porous carbon skeleton is fully in contact with the liquid phase, thereby improving the isomerization efficiency of L-arabinose.

[0006] To solve this technical problem, the technical solution of the present invention is: a method for preparing molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton, comprising the following steps:

[0007] S11, placing pectin calcium gel microparticles in a chitosan solution, wherein the pH value of the chitosan solution is 3 to 4, and stirring, so that the chitosan is enriched on the surface of the pectin-glucose-calcium gel particles to form a polyelectrolyte complex;

[0008] S12, adjusting the pH of the solution in the polyelectrolyte complex dispersion system obtained in S11 to 7.5 to 8.0, and solidifying the chitosan on the surface of the polyelectrolyte complex;

[0009] S13, dispersing molybdate in the material of S12, wherein MoO4 2- Under the adsorption of chitosan, they accumulated on the surface of chitosan;

[0010] S14, placing the material obtained in S13 in a reactor for hydrothermal reaction;

[0011] S15, filtering and washing the solid obtained in S14, and drying in vacuo at 80°C to 100°C;

[0012] S16. Carbonizing the obtained solid at high temperature under nitrogen protection to form molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton.

[0013] The method for preparing calcium pectin gel microparticles preferably comprises the following steps:

[0014] S111, preparing a low-ester pectin and glucose mixture using deionized water, and adjusting the pH of the mixture to 3.0;

[0015] S112, heating the mixed solution obtained in S111 in boiling water;

[0016] S113, adding calcium chloride to the mixed solution heated in S112, and stirring rapidly and uniformly; placing the obtained mixed solution in an environment of 3°C to 5°C for cooling and placing for 24h to 48h to obtain calcium pectin gel;

[0017] S114, calcium pectin gel is freeze-dried, crushed, and sieved through 250-300 mesh;

[0018] S115, placing the calcium pectin powder obtained in S114 in an acid solution to swell and obtain calcium pectin gel micron particles.

[0019] The present invention utilizes pectin and glucose to prepare calcium gel particles with stable structure. Based on the electrostatic interaction and hydrogen bonding between pectin and chitosan, polar groups such as amino groups and hydroxyl groups are contained in chitosan molecules, and hydrogen bonds can be formed with hydroxyl groups in glucose molecules. Amino groups, hydroxyl groups, N-acetylamino groups and the like on the chitosan molecular chain can participate in the formation of intramolecular and intermolecular hydrogen bonds. Therefore, the interaction between chitosan and pectin calcium gel micron particles is stable, chitosan is evenly distributed on the surface of gel particles, and molybdate radicals are further evenly distributed.

[0020] The mass ratio of calcium pectin powder, chitosan and ammonium molybdate to form calcium pectin gel microparticles by soaking and absorbing water is preferably (10 to 15): (10 to 12): 1. The present invention effectively forms the dispersion and distribution of chitosan on the surface of the gel particles by controlling the dosage of calcium pectin gel microparticles and chitosan. At the same time, the calcium pectin gel microparticles, due to their own gel properties, cooperate with the adsorption of chitosan to effectively achieve the distribution of molybdate along with chitosan in the shallow surface layer of the overall solid structure, which is convenient for the contact of molybdenum oxide with L-arabinose in the liquid phase.

[0021] The preferred process conditions of the hydrothermal reaction in step S14 are: temperature of 150° C. to 180° C.; reaction time of 10 h to 12 h. In the present invention, the molybdate adsorbed on the surface of chitosan is converted into a precursor distributed on a porous spherical carbon skeleton through a hydrothermal reaction.

[0022] The preferred process conditions for high temperature carbonization in step S16 are: 650°C to 750°C, and the time is 2 hours to 4 hours. In the present invention, the molybdenum oxide and the carbon skeleton are firmly combined on the surface of the nitrogen-doped porous spherical carbon skeleton under the action of high temperature calcination at a relatively high temperature, and the molybdenum oxide reduces the solubility of the molybdenum oxide in the aqueous solution through carbon insertion and carbon substitution.

[0023] The purpose of the present invention is to provide a method for preparing L-ribose. The present invention utilizes molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton to effectively adsorb L-arabinose and a complexing agent, and enhances the contact between molybdenum oxide and L-arabinose by improving the hydrophilicity of the carbon skeleton, thereby improving the conversion efficiency of L-arabinose.

[0024] To solve this technical problem, the technical solution of the present invention is: a method for preparing L-ribose, comprising the following steps:

[0025] S1, uniformly dispersing L-arabinose and molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework in deionized water to which 4-pyridinepropanol as a complexing agent is added;

[0026] S2, isomerization reaction catalyzed by molybdenum oxide dispersed and supported by nitrogen-doped porous carbon framework;

[0027] Adjust the reaction temperature to 90°C to 100°C and react for 4 to 6 hours;

[0028] During the isomerization reaction, the nitrogen-doped porous carbon framework adsorbed 4-pyridinepropanol and L-arabinose to the solid-liquid interface of molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework;

[0029] S3. After the reaction is completed, the molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework is filtered to obtain an isomerization reaction sugar solution.

[0030] Preferably, the mass ratio of 4-pyridinepropanol to L-arabinose added in step S1 is (0.8% to 1.2%): 1. In the present invention, 4-pyridinepropanol and L-arabinose are adsorbed by the nitrogen-doped porous carbon framework and enriched at the solid-liquid interface between the nitrogen-doped porous carbon framework with good hydrophilicity and water, effectively improving the complexation of 4-pyridinepropanol with molybdenum atoms and promoting the isomerization of L-arabinose.

[0031] The preferred process conditions for separating L-arabinose from the isomerization reaction sugar solution are as follows:

[0032] S4, concentrating the isomerization reaction sugar solution obtained in S3;

[0033] S5. Methanol is added to the concentrated solution obtained in S4, and the concentrated solution with methanol is allowed to stand at 0°C to 3°C for 20 to 24 hours to crystallize L-arabinose, and the solution is filtered to obtain L-arabinose. The present invention crystallizes L-arabinose in the isomerized solution to facilitate the subsequent separation and purification of L-ribose.

[0034] The preferred process conditions for separating L-ribose in the filtrate after step S5 are as follows:

[0035] S6, concentrating the sugar methanol-water mixture obtained in S5, and adding water to dilute it;

[0036] Under nitrogen protection, aniline and ethanol are added and mixed evenly, and sulfuric acid is added as a catalyst to convert L-ribose into L-ribosylamine; L-ribosylamine is allowed to stand at low temperature to precipitate, and L-ribosylamine is converted into target L-ribose.

[0037] The preferred molar ratio of L-arabinose to S12 molybdate is (25 to 30):1.

[0038] By adopting the above technical solution, the beneficial effects of the present invention are:

[0039] In the present invention, calcium pectin gel micron particles are added to a chitosan solution, and chitosan and pectin form hydrogen bonds through the hydroxyl and amino groups therebetween, and -COOH in the pectin molecule is deprotonated to form -COO - NH3 with chitosan + The polyelectrolyte complex is formed by electrostatic action as a skeleton to effectively enrich chitosan in an orderly manner on the surface of calcium pectin gel microparticles; then, the physical property of chitosan that dissolves in an acid solution and solidifies in an alkaline solution is utilized, and the solidified chitosan uses its own adsorption to adsorb molybdate on the surface of the chitosan. Since the chitosan endosome is filled with calcium pectin gel microparticles to form space occupying, the uniform distribution of molybdate in the chitosan layer is promoted, wherein the interaction between chitosan and pectin is as follows:

[0040] ;

[0041] After that, with the help of high-temperature calcination, molybdenum is distributed on the surface of the chitosan skeleton carbonized into a carbon skeleton, thereby improving the dispersion of molybdenum atoms. In addition, due to the presence of nitrogen in the in-situ structure of the carbon skeleton derived from chitosan and its own porosity, when the obtained molybdenum oxide dispersed and loaded by the nitrogen-doped porous carbon skeleton is dispersed in an aqueous solution, L-arabinose and 4-pyridinepropanol in the aqueous solution will effectively surround and contact the molybdenum atoms loaded on the carbon skeleton with the help of the porous structure and hydrophilicity of the nitrogen-doped porous carbon skeleton, forming a rich and continuous solid-liquid interface, which is beneficial to the diffusion and mass transfer of reactants and products, ensuring sufficient contact between the solid catalyst and the liquid phase, and improving the isomerization efficiency of L-arabinose.

[0042] L-arabinose undergoes isomerization reaction catalyzed by molybdate catalyst, and the solution after the reaction contains a large amount of unreacted L-arabinose, a part of L-ribose and a small amount of L-xylose, L-lyxose and tetrose. During the isomerization process, molybdenum forms a complex with the hydroxyl groups of the C-1 and C-2 positions on the L-arabinose molecule, and then intramolecular exchange of hydrogen atoms on C-1 and C-2 occurs. The molybdenum oxide immobilized on the nitrogen-doped porous carbon skeleton cooperates with 4-pyridine propanol to promote the isomerization of L-arabinose to L-ribose; and compared with the molybdenum oxide used alone, the molybdenum oxide fixed on the nitrogen-doped porous carbon skeleton in the present invention, on the one hand, is more evenly dispersed as a catalyst. Under the load of the nitrogen-doped porous carbon skeleton with significantly improved hydrophilicity, the stability of the molybdenum oxide and the probability of the molybdenum oxide contacting with L-arabinose and 4-pyridine propanol under the adsorption of the nitrogen-doped porous carbon skeleton are also significantly improved, which effectively improves the isomerization rate and the yield of L-ribose; and the molybdenum oxide dispersed and loaded on the nitrogen-doped porous carbon skeleton prepared by the present invention has a stable structure, which is convenient for filtering and recovering the catalyst, and the catalyst structure is stable, which inhibits the dissolution of the molybdenum oxide in the aqueous solution, and is convenient for repeated use. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is an XRD spectrum of molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework obtained in Example 2 of the present invention;

[0044] Figure 2 This is the nitrogen adsorption-desorption curve of molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton obtained in Example 2 of the present invention;

[0045] Figure 3 This is the pore size distribution curve of molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton obtained in Example 2 of the present invention. DETAILED DESCRIPTION

[0046] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0047] Example 1

[0048] This embodiment discloses a method for preparing molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework, comprising the following steps:

[0049] S11, placing pectin calcium gel microparticles in a chitosan solution, wherein the pH value of the chitosan solution is 3.0, and stirring, so that the chitosan is enriched on the surface of the pectin-glucose-calcium gel particles to form a polyelectrolyte complex;

[0050] S12, adjusting the pH of the solution in the polyelectrolyte complex dispersion system obtained in S11 to 8.0, and solidifying the chitosan on the surface of the polyelectrolyte complex;

[0051] S13, dispersing molybdate in the material of S12, wherein MoO4 2- Under the adsorption of chitosan, they accumulated on the surface of chitosan;

[0052] S14, placing the material obtained in S13 in a reactor for hydrothermal reaction;

[0053] S15, filter and wash the solid obtained in S14, and dry it in vacuum at 80°C for 24h;

[0054] S16. Carbonizing the obtained solid at high temperature under nitrogen protection to form molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton.

[0055] The preparation method of calcium pectin gel microparticles comprises the following steps:

[0056] S111. Prepare a low-ester pectin and glucose mixture using deionized water, wherein the mass fraction of low-ester pectin (esterification degree 35%, total galacturonic acid 88%) is 1%, and the mass fraction of glucose is 10%; adjust the pH of the mixture to 3.0;

[0057] S112, placing the mixed solution obtained in S111 in boiling water and heating for 2 minutes;

[0058] S113, adding calcium chloride to the mixed solution heated in S112, with the mass ratio of calcium ion to low-ester pectin being 30 mg:1 g, and stirring rapidly and evenly; placing the obtained mixed solution in an environment of 3° C. to 5° C. and cooling for 24 hours to obtain calcium pectin gel;

[0059] S114, calcium pectin gel is freeze-dried, crushed, and sieved through 300 mesh;

[0060] S115, placing the calcium pectin powder obtained in S114 in an acid solution to swell and obtain calcium pectin gel micron particles.

[0061] In this embodiment, the mass ratio of calcium pectin gel microparticles, chitosan and ammonium molybdate is 10: 10: 1. The present invention effectively forms a spherical skeleton structure by controlling the dosage of pectin and chitosan, and then assembles molybdate from ammonium molybdate on the surface of the spherical structure skeleton through electrostatic action, effectively dispersing and loading the subsequent molybdenum oxide.

[0062] The process conditions of the hydrothermal reaction in step S14 of this embodiment are: temperature of 150° C.; reaction time of 10 hours. The present invention uses hydrothermal reaction to convert the molybdate groups electrostatically acting on the surface of chitosan into precursors distributed on the porous spherical carbon skeleton.

[0063] The process conditions of high temperature carbonization in step S16 in this embodiment are: 650°C for 2 hours. The present invention utilizes high temperature calcination to firmly bind molybdenum oxide to the nitrogen-doped porous spherical carbon skeleton, effectively reducing the dissolution of molybdenum oxide in water, and the nitrogen derived from chitosan is in situ doped in the porous spherical carbon skeleton, effectively improving the hydrophilicity of the carbon skeleton, and promoting the formation of a rich solid-liquid interface between the aqueous solution and the molybdenum oxide dispersed and loaded by the nitrogen-doped porous carbon skeleton, thereby improving the isomerization of molybdenum to L-arabinose.

[0064] This embodiment also discloses a method for preparing L-ribose, comprising the following steps:

[0065] S1, uniformly dispersing L-arabinose and molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework in deionized water to which 4-pyridinepropanol as a complexing agent is added;

[0066] The molar ratio of L-arabinose and S12 input to molybdate was 25:1.

[0067] S2, isomerization reaction catalyzed by molybdenum oxide dispersed and supported by nitrogen-doped porous carbon framework;

[0068] The reaction temperature was adjusted to 90°C and the reaction was carried out for 4 hours;

[0069] During the isomerization reaction, the nitrogen-doped porous carbon framework adsorbed 4-pyridinepropanol and L-arabinose to the solid-liquid interface of molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework;

[0070] S3. After the reaction is completed, the molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework is filtered to obtain an isomerization reaction sugar solution.

[0071] The mass ratio of 4-pyridine propanol to L-arabinose added in step S1 of this embodiment is 0.8%: 1. In the present invention, 4-pyridine propanol and L-arabinose are adsorbed by the nitrogen-doped porous carbon framework and enriched at the solid-liquid interface between the nitrogen-doped porous carbon framework and water with good hydrophilicity, effectively improving the complexation of 4-pyridine propanol with molybdenum atoms and promoting the isomerization of L-arabinose.

[0072] The process conditions for separating L-arabinose from the isomerization reaction sugar solution in this example are as follows:

[0073] S4, concentrating the isomerization reaction sugar solution obtained in S3 to a concentrated solution having a volume of 1 / 3 of the isomerization reaction sugar solution obtained in S3;

[0074] S5. Add methanol to the concentrated solution obtained in S4, with the ratio of the concentrated solution to methanol being 1 g:1 ml; and let the concentrated solution with methanol stand at 0°C to 3°C for 20 hours to crystallize L-arabinose, and filter to obtain L-arabinose. The present invention crystallizes L-arabinose in the isomerized solution to facilitate the subsequent separation and purification of L-ribose.

[0075] In this embodiment, the process conditions for separating L-ribose in the filtrate after step S5 are as follows:

[0076] S6, concentrating the sugar methanol-water mixture obtained in S5, and adding water to dilute it;

[0077] Under nitrogen protection, aniline and ethanol are added, mixed evenly, sulfuric acid is added as a catalyst, and L-ribose is converted into L-ribose amine; L-ribose amine is allowed to stand at low temperature to precipitate, and L-ribose amine is converted into target L-ribose. The present invention utilizes L-ribose to convert L-ribose amine to facilitate separation from a small amount of miscellaneous sugars such as L-xylose, L-lyxose and tetrose in the conversion solution.

[0078] The process conditions for converting L-ribosyl glucosamine into the target L-ribose are as follows:

[0079] Add water to the concentrated syrup obtained in S5 at a volume ratio of 1:1 to obtain an L-ribose solution;

[0080] The ratio of the volume of ethanol to the mass of the concentrated syrup obtained in S5 is 0.5ml: 1g;

[0081] The ratio of the mass of aniline to the mass of the concentrated syrup obtained in S5 is 0.3:1;

[0082] The solution was stirred at 25°C for 2 hours to obtain L-ribose amine after nitrogen was introduced and the pH value was adjusted to 6.4 with 0.5 mol / L H2SO4.

[0083] Formaldehyde with a mass fraction of 37% was added to L-ribose amine to convert it into L-ribose and aniline. L-ribose was crystallized in ethanol, washed, and dried to obtain the target L-ribose.

[0084] Example 2

[0085] The differences between this embodiment and Example 1 regarding the preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton are shown in Table 1; the differences between this embodiment and Example 1 regarding the preparation method of L-ribose are shown in Table 2.

[0086] Example 3

[0087] The differences between this embodiment and Example 1 regarding the preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton are shown in Table 1; the differences between this embodiment and Example 1 regarding the preparation method of L-ribose are shown in Table 2.

[0088] Example 4

[0089] The differences between this embodiment and Example 1 regarding the preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton are shown in Table 1; the differences between this embodiment and Example 1 regarding the preparation method of L-ribose are shown in Table 2.

[0090] Example 5

[0091] The differences between this embodiment and Example 1 regarding the preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton are shown in Table 1; the differences between this embodiment and Example 1 regarding the preparation method of L-ribose are shown in Table 2.

[0092] Example 6

[0093] The differences between this embodiment and Example 1 regarding the preparation method of molybdenum oxide dispersed and loaded by a nitrogen-doped porous carbon skeleton are shown in Table 1; the differences between this embodiment and Example 1 regarding the preparation method of L-ribose are shown in Table 2.

[0094] Table 1 Preparation parameters of MoO dispersed and supported by nitrogen-doped porous carbon framework

[0095]

[0096] Table 2 L-ribose preparation process parameters

[0097]

[0098] Comparative Example 1

[0099] This comparative example discloses a method for preparing L-ribose, comprising the following steps:

[0100] Dissolve L-arabinose and ammonium molybdate in deionized water, adjust the pH value to 2.5, and place in a reactor for reaction at a temperature of 90° C. for 4 hours, and the isomerization reaction is completed.

[0101] The molar ratio of L-arabinose to ammonium molybdate was 28:1.

[0102] Decolorization and filtration: Add activated carbon to the above solution, keep warm for 30 minutes, and filter with a vacuum filter.

[0103] Ion exchange: The filtrate is passed through anion-cation-anion-cation exchange resin to remove impurities and various ions in the filtrate.

[0104] Concentration and separation: The exchanged solution is evaporated and concentrated at 70°C to a refractive index concentration of 50%. The L-ribose solution is separated using simulated moving bed technology. The separation temperature is controlled at 50°C. The solution is separated into ribose fractions and other fractions, and the ribose fraction is collected.

[0105] Concentration and crystallization: The collected ribose solution is evaporated and concentrated at 70°C to a refractive index of 70%, and then cooled and crystallized to obtain crystalline L-ribose.

[0106] Comparative Example 2

[0107] This comparative example discloses a method for preparing L-ribose. The main difference between this comparative example and Example 1 is that the solid catalyst is molybdenum trioxide.

[0108] Comparative Example 3

[0109] This comparative example discloses a method for preparing molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework, comprising the following steps:

[0110] S11, adding ammonium molybdate to a chitosan solution having a pH of 3.0, stirring, and allowing the chitosan to adsorb molybdate;

[0111] S12, dropping the chitosan solution adsorbed with molybdate obtained in S11 into a sodium hydroxide solution, and solidifying the chitosan into microspheres;

[0112] S14, dispersing the microspheres obtained in S13 in an aqueous solution with a pH of 7.5, placing the solution in a reaction kettle, and subjecting the solution to a hydrothermal reaction under the same reaction conditions as in Example 1;

[0113] S15, filtering and washing the solid obtained in S14, and vacuum drying the solid as in Example 1;

[0114] S16. Carbonizing the obtained solid at high temperature under nitrogen protection to form molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton. The carbonization conditions are the same as those in Example 1.

[0115] In this comparative example, the mass ratio of chitosan to ammonium molybdate is 10:1.

[0116] The molybdenum oxide dispersed and loaded on the nitrogen-doped porous carbon framework prepared in this comparative example is used for the preparation of L-ribose. The specific process parameters are the same as those in Example 1.

[0117] The performance indicators of molybdenum oxide dispersed and supported by nitrogen-doped porous carbon skeleton obtained in Test Examples 1 to 6 and Comparative Example 3 are shown in Table 3. The specific surface area and pore structure of the samples were characterized by nitrogen adsorption / desorption experiments. The specific surface area of ​​the samples was measured by N2 adsorption at -196 °C on a dual-channel physical microporous adsorption instrument, and the specific surface area was calculated by the BET method.

[0118] Table 3 Specific surface area and average pore size of the solid catalysts obtained in Examples 1 to 6 and Comparative Example 3

[0119]

[0120] Table 4 The content of L-ribose in the isomerization reaction solution of Examples 1 to 6 and Comparative Examples 1 to 3 and the performance indicators of the obtained L-ribose crystals

[0121]

[0122] Combining the data in Tables 1, 2, 3 and 4, it can be seen that the present invention uses an acidic solution to dissolve chitosan, and the calcium pectin gel microparticles are put into the chitosan solution. The chitosan and pectin form hydrogen bonds through the hydroxyl and amino groups therebetween, and the -COOH in the pectin molecule is deprotonated to form -COO - NH3 with chitosan + The polyelectrolyte complex is formed by electrostatic action as a skeleton to effectively enrich chitosan in an orderly manner on the surface of calcium pectin gel microparticles; then, the physical property of chitosan that dissolves in an acid solution and solidifies in an alkaline solution is utilized, and the solidified chitosan uses its own adsorption to adsorb molybdate on the surface of the chitosan. Since the chitosan endosome is filled with calcium pectin gel microparticles to form space occupying, the uniform distribution of molybdate in the chitosan layer is promoted, wherein the interaction between chitosan and pectin is as follows:

[0123] ;

[0124] After that, with high-temperature calcination, molybdenum is distributed on the surface of the chitosan skeleton carbonized into a carbon skeleton, thereby improving the dispersion of molybdenum atoms. In addition, due to the presence of nitrogen in the original structure of the carbon skeleton derived from chitosan and its own porosity, when the obtained molybdenum oxide dispersed and loaded by the nitrogen-doped porous carbon skeleton is dispersed in an aqueous solution, the L-arabinose and 4-pyridinepropanol in the aqueous solution will effectively surround and contact the molybdenum atoms loaded on the carbon skeleton with the help of the porous structure and hydrophilicity of the nitrogen-doped porous carbon skeleton, forming a rich and continuous solid-liquid interface, which is beneficial to the diffusion and mass transfer of reactants and products, ensuring sufficient contact between the solid catalyst and the liquid phase, and improving the isomerization efficiency of L-arabinose. Figure 3The XRD spectrum of the solid catalyst obtained in Example 2 shows that the solid catalyst has broad peaks at 2θ of 24° and 43°, which belong to the (002) and (100) planes of the graphite structure. The diffraction peaks at 2θ of 12° and 28° correspond to the (110) and (021) crystal planes of MoO3, respectively. Figure 1 The N2 adsorption-desorption curves and Figure 2 The pore size distribution curve shows the specific surface area and pore size distribution data of the solid catalyst obtained in Example 2. The pore size of the solid catalyst obtained in the present invention is between 15 nm and 19 nm. It can be seen that the present invention has prepared a mesoporous material with rich pores, thereby having a good adsorption effect.

[0125] L-arabinose undergoes isomerization reaction catalyzed by molybdate catalyst, and the solution after the reaction contains a large amount of unreacted L-arabinose, a part of L-ribose and a small amount of L-xylose, L-lyxose and tetrose. By comparing Comparative Example 1 with Examples 1 to 6, it can be seen that the method for preparing L-ribose proposed in the present invention controls the distribution of molybdenum oxide. On the one hand, the molybdenum oxide particles used as a catalyst are more evenly dispersed, and under the load of the nitrogen-doped porous carbon skeleton with significantly improved hydrophilicity, the stability of molybdenum oxide and the probability of contact between molybdenum oxide and L-arabinose and 4-pyridinepropanol under the adsorption of the nitrogen-doped porous carbon skeleton are also significantly improved, effectively improving the isomerization rate and the yield of L-ribose; By comparing Comparative Examples 2 and 3 with Examples 1 to 6, it can be seen that the present invention uses calcium pectin gel particles as the core and chitosan as the shell, and then controls the distribution position of molybdenum oxide through hydrothermal and high-temperature carbonization of molybdenum oxide through chitosan adsorbed molybdate The solid catalyst, compared with Comparative Example 2 in which molybdenum trioxide is directly used as a catalyst, and Comparative Example 3 directly uses chitosan adsorbed with molybdate to form a spherical load, the molybdenum atoms of the solid catalyst obtained in Comparative Example 3 are evenly distributed in the chitosan spheres, and the solid catalysts in Comparative Examples 2 and 3 are uniformly distributed in the chitosan spheres. The contact between molybdenum atoms and the solution is limited by its own structure, so the molybdenum oxide dispersed and loaded by the nitrogen-doped porous carbon skeleton prepared by the present invention is more likely to contact L-arabinose and 4-pyridine propanol. Therefore, the present invention proposes a method for preparing L-ribose with a high conversion rate, and the solid catalyst used in the present invention has a stable structure, which is convenient for filtering and recovering the catalyst. The high-temperature carbonization process inhibits the dissolution of molybdenum oxide in the aqueous solution. After using the molybdenum oxide dispersed and loaded by the nitrogen-doped porous carbon skeleton prepared in Example 1 for 10 times, the L-ribose content in the reaction solution is maintained at more than 30%, and the purity of L-ribose is maintained at 97%. After the solid catalyst obtained in Comparative Example 3 is repeatedly used for 5 times, the L-ribose content in the reaction solution drops to 25%. The main reason is that the molybdenum atoms in the solid catalyst obtained in Comparative Example 3 are limited by their own skeleton structure, and the pores of the catalyst may be blocked as the catalyst is used, reducing the contact area with the reaction solution. Therefore, the solid catalyst obtained by the present invention can be repeatedly and efficiently used in the preparation of L-ribose.

Claims

1. A method for preparing molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon framework, characterized in that: The following steps are involved: S11, placing the calcium pectin gel microparticles in a chitosan solution, wherein the pH value of the chitosan solution is 3 to 4, and stirring, so that the chitosan is enriched on the surface of the calcium pectin gel microparticles to form a polyelectrolyte complex; S12, adjusting the pH of the solution in the polyelectrolyte complex dispersion system obtained in S11 to 7.5 to 8.0, and solidifying the chitosan on the surface of the polyelectrolyte complex; S13, dispersing molybdate in the material of S12, wherein MoO4 2- Under the adsorption of chitosan, they accumulated on the surface of chitosan; S14, placing the material obtained in S13 in a reactor for hydrothermal reaction; S15, filtering and washing the solid obtained in S14, and drying in vacuo at 80°C to 100°C; S16, carbonizing the obtained solid at high temperature under nitrogen protection to form molybdenum oxide dispersed and supported by a nitrogen-doped porous carbon skeleton; The preparation method of calcium pectin gel microparticles comprises the following steps: S111, preparing a low-ester pectin and glucose mixture using deionized water, and adjusting the pH of the mixture to 3.0; S112, heating the mixed solution obtained in S111 in boiling water; S113, adding calcium chloride to the mixed solution heated in S112, and stirring rapidly and uniformly; placing the obtained mixed solution in an environment of 3°C to 5°C to cool and place to obtain calcium pectin gel; S114, calcium pectin gel is freeze-dried, crushed, and sieved through 250-300 mesh; S115, placing the calcium pectin powder obtained in S114 in an acid solution to swell to obtain calcium pectin gel micron particles; The mass ratio of calcium pectin powder, chitosan and ammonium molybdate is (10 to 15): (10 to 12): 1; The hydrothermal reaction process conditions of step S14 are: temperature of 150° C. to 180° C.; reaction time of 10 h to 12 h.

2. The preparation method according to claim 1, characterized in that: The process conditions of high temperature carbonization in step S16 are: 650° C. to 750° C., and the time is 2 hours to 4 hours.

3. A method for preparing L-ribose, characterized in that: The following steps are involved: S1, uniformly dispersing L-arabinose and the nitrogen-doped porous carbon framework prepared in claim 1 and the molybdenum oxide dispersed and loaded in deionized water added with 4-pyridinepropanol as a complexing agent; The mass ratio of 4-pyridinepropanol to L-arabinose added in step S1 is (0.8% to 1.2%): 1; S2, isomerization reaction catalyzed by molybdenum oxide dispersed and supported by nitrogen-doped porous carbon framework; Adjust the reaction temperature to 90°C to 100°C and react for 4 to 6 hours; During the isomerization reaction, the nitrogen-doped porous carbon framework adsorbed 4-pyridinepropanol and L-arabinose to the solid-liquid interface of molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework; S3. After the reaction is completed, the molybdenum oxide dispersed and supported by the nitrogen-doped porous carbon framework is filtered to obtain an isomerization reaction sugar solution.

4. The method for preparing L-ribose according to claim 3, characterized in that: The process conditions for separating L-arabinose from the isomerization reaction sugar solution are as follows: S4, concentrating the isomerization reaction sugar solution obtained in S3; S5. Add methanol to the concentrated solution obtained in S4, and let the concentrated solution with methanol stand at 0°C to 3°C for 20 to 24 hours to allow L-arabinose to crystallize and precipitate. Filter and obtain L-arabinose.

5. The method for preparing L-ribose according to claim 4, characterized in that: The process conditions for separating L-ribose in the filtrate after step S5 are as follows: S6, concentrating the sugar methanol-water mixture obtained in S5, and adding water to dilute it; Under nitrogen protection, aniline and ethanol are added and mixed evenly, and sulfuric acid is added as a catalyst to convert L-ribose into L-ribosylamine; L-ribosylamine is allowed to stand at low temperature to precipitate, and L-ribosylamine is converted into target L-ribose.

6. The method for preparing L-ribose according to claim 3, characterized in that: The molar ratio of L-arabinose and S12 input molybdate is (25 to 30):1.

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