Process for the preparation of xylitol

Abstract

The application discloses a preparation method of xylitol, and comprises the following steps: S1, putting deprotonated seaweed particles into Ni 2+ / MoO4 2‑ in a metal citrate precursor solution to form seaweed-Ni 2+ / MoO4 2‑ ; S2, introducing CO2 into a reaction kettle, heating, and synchronously pulsing a microwave; hemicellulose is converted and xylitol is accumulated; meanwhile, the seaweed-Ni 2+ / MoO4 2‑ is converted into Ni-Mo synergistic active sites; S3, cooling the reaction kettle, separating CO2, adjusting the pH to neutral, introducing H2, heating, and catalyzing xylitol to be reduced to xylitol by using the in-situ enriched Ni-Mo active sites of seaweed particles; S4, cooling and decompressing, releasing H2 to normal pressure, concentrating and crystallizing to obtain target xylitol; in the application, the seaweed particles are used as a carrier dispersed catalyst to synchronously release xylitol and convert Ni-Mo synergistic active sites under the action of CO2, and then H2 reduction is performed, so that the yield and purity of xylitol are effectively improved.

Description

Technical Field

[0001] This invention relates to the field of xylitol preparation technology, and in particular to a method for preparing xylitol. Background Technology

[0002] Xylitol is a naturally occurring five-carbon sugar alcohol with the chemical formula C5H. 12 O5. It is widely found in fruits, vegetables, and plants such as birch and corncobs, and can be produced by hydrogenation reduction after hydrolyzing plant fibers. As a sugar substitute, xylitol has a similar sweetness to sucrose but is lower in calories and has a unique refreshing taste. In addition, the human body absorbs xylitol more slowly, with a glycemic index of 7, which is only 10.77% of that of sucrose, making it suitable for diabetics.

[0003] Xylitol has been applied in multiple fields: in the food industry, it is not only the core sweetener in sugar-free chewing gum, candy, and beverages, but is also widely used in low-calorie ice cream and baked goods to replace traditional sugars; in the pharmaceutical field, its low glycemic index is used to develop foods specifically for diabetes, while it is also used as an antibacterial ingredient in nasal sprays to inhibit pathogen colonization and as a stabilizer in intravenous nutrient solutions; in the daily chemical industry, its anti-caries properties are incorporated into oral care products such as toothpaste and mouthwash, and its moisture-absorbing and moisturizing functions are applied to skin care products such as lip balms; in the industrial field, xylitol is used as a raw material to synthesize plasticizers and environmentally friendly surfactants, fully demonstrating its cross-border value in healthy consumption and green manufacturing.

[0004] Chinese patent CN120624567A discloses a two-step enzymatic method for producing xylitol from hemicellulose hydrolysate. The method uses hemicellulose hydrolysate as raw material, adding arabinose dehydrogenase, NADH oxidase, and the coenzyme NADH for a first-step fermentation until no L-arabinose residue remains in the fermentation solution, yielding fermentation solution I. Subsequently, xylose reductase and glucose dehydrogenase are added to fermentation solution I for a second fermentation, yielding a fermentation product containing xylitol. This invention utilizes inexpensive hemicellulose hydrolysate as raw material, and according to the disclosed method, both xylose conversion rate and xylitol yield can exceed 99%, with high product purity.

[0005] To improve the xylitol production process, Wang Guanbin et al. of Shandong Futian Pharmaceutical Co., Ltd. used a simulated moving bed chromatography (SSMB) apparatus to purify the second distillation liquid of xylose. This increased hydrogenation efficiency, improved the purity and yield of xylitol, and enhanced its taste. The results showed that the extract prepared using this method had a xylose purity of 89.783%, an arabinose content of 1.348%, and a chromatographic separation yield of 97.21%.

[0006] Although there are reports on the preparation of xylitol using simulated moving bed chromatography systems and biological enzymes with hemicellulose and xylose as raw materials, the core issues of crystallization efficiency and pollution in the production process still exist. Stable and continuous industrial production of xylitol needs to be achieved by addressing issues related to raw materials and preparation methods. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing xylitol. This invention uses seaweed as the raw material for preparing xylitol and alginic acid macromolecules as a spatially confined template to in-situ coordinate and assemble highly dispersed Ni 2+ / MoO4 2- seaweed-Ni 2 + -MoO4 2- Under the action of CO2 and pulsed microwaves, it is converted into Ni-Mo synergistic active sites, which can efficiently prepare xylitol and facilitate the separation of catalytic substances.

[0008] To solve this technical problem, the technical solution of the present invention is: a method for preparing xylitol, comprising the following steps: S1. Add deprotonated seaweed particles to Ni 2+ / MoO4 2- -In the solution of metal precursor citrate; Ni is enriched at the carboxyl site of alginate in seaweed particles. 2+ MoO4 2- Through Ni 2+ Enriched in seaweed particles to form seaweed-Ni 2+ -MoO4 2- ; S2, seaweed-Ni 2+ -MoO4 2- Add deionized water to the reaction vessel, heat, introduce CO2 into the reaction vessel, and simultaneously pulse microwave; CO2 strips hemicellulose from seaweed, which is then converted and accumulates xylose under the catalysis of carbonic acid derived from CO2. Simultaneously seaweed-Ni 2+ -MoO4 2- Under the influence of CO2 and pulsed microwaves, they are transformed into Ni-Mo synergistic active sites; S3. After step S2, the reaction vessel is cooled down and the carbon dioxide in the reaction vessel is separated. H2 is introduced into the reaction vessel, heated and stirred. The Ni-Mo active sites enriched in situ by the seaweed particles catalyze the hydrogenation reduction of xylose to xylitol. S4. Cool down and depressurize to release H2 to normal pressure; add activated carbon to remove impurities, centrifuge and nanofilter the reaction solution, concentrate and crystallize to obtain the target xylitol.

[0009] The preferred preparation process for deprotonated seaweed particles is as follows: Dry and pulverize seaweed and add it to sodium hydroxide solution. Stir for 0.5 to 1.5 hours. The carboxyl groups in the alginic acid of the seaweed are deprotonated and converted into carboxylate groups.

[0010] Ni is preferred 2+ / MoO4 2- The preparation process of the citric acid metal precursor solution is as follows: Add sodium citrate to deionized water, stir to dissolve, and then add nickel ion source and molybdate source in sequence; Adjust the pH to 8.3 to 8.7; Continue stirring for 20 to 40 minutes to obtain Ni. 2+ / MoO4 2- -Citrate metal precursor solution.

[0011] In preferred step S1, the seaweed particles, based on the mass of dry seaweed powder, are compared with Ni. 2+ / MoO4 2- - The ratio of the amount of citrate metal precursor solution used is (6g to 8g): 50ml.

[0012] In the preferred step S1, seaweed-Ni is generated. 2+ -MoO4 2- The reaction conditions are: The reaction temperature is 40℃ to 60℃, and the mixture is stirred for 1 to 3 hours.

[0013] In the preferred step S2, the amount of deionized water used is based on a mass ratio of dried seaweed to deionized water of 1:(4 to 6). The reaction time for step S2 is 0.5 h to 1.5 h; The process parameters for pulsed microwave are as follows: pulse width is 10-100μs, and pulse repetition frequency is 100-1000 Hz.

[0014] The preferred solid phase collected in step S4 includes the seaweed-Ni obtained in step S1. 2+ -MoO4 2- After steps S2 and S3, the ionic coordination state is transformed into a supported Ni-Mo synergistic active phase.

[0015] The solid residue obtained after centrifugation in step S4 is preferably calcined under an argon atmosphere to obtain a Ni-MoO2 synergistic catalyst for catalytic hydrogenation.

[0016] The preferred process steps and parameters for calcining solid residue are as follows: The solid residue was calcined under an argon atmosphere, and the organic components in the solid residue were carbonized and volatilized. Subsequently, a mixture of argon and oxygen gas was introduced into the tubular furnace, cooled down, and then calcined again to obtain the Ni-MoO2 synergistic catalyst.

[0017] In the preferred step S2, CO2 is introduced until the pressure inside the reactor is 7.5 MPa to 8.5 MPa; In step S3, H2 is introduced until the pressure inside the reactor is 4.5 MPa to 5.5 MPa.

[0018] By adopting the above technical solution, the beneficial effects of the present invention are: This invention incorporates deprotonated seaweed particles into Ni 2+ / MoO4 2- - In the citrate metal precursor solution, Ni is enriched through the carboxyl sites of alginate in the seaweed particles. 2+ After that, MoO4 2- Through Ni 2+ Enriched in seaweed particles to form seaweed-Ni 2+ -MoO4 2- Then seaweed-Ni 2+ -MoO4 2- Deionized water is added to the reactor, heated, and CO2 is introduced into the reactor simultaneously with pulsed microwaves. The CO2 strips hemicellulose from the seaweed, which is then converted and accumulates xylose under the catalysis of carbonic acid derived from the CO2. Simultaneously, seaweed-Ni... 2+ -MoO4 2- Under the action of CO2 and pulsed microwaves, the xylose is converted into Ni-Mo synergistic active sites, meaning that the present invention also achieves a uniform distribution of Ni-Mo synergistic active sites during the preparation of xylose. In order to prevent further hydrolysis of xylose, which is not conducive to the reduction and conversion of xylitol, CO2 in the reaction vessel is separated after cooling, H2 is introduced into the reaction vessel, heated, and stirred. The Ni-Mo active sites enriched in situ by the seaweed particles catalyze the hydrogenation reduction of xylose to xylitol. This invention utilizes supercritical carbon dioxide to facilitate the hydrolysis of hemicellulose in seaweed powder. Supercritical carbon dioxide (temperature > 31.1 ℃, pressure > 7.38 MPa) is a critical state of carbon dioxide between liquid and gaseous states, possessing excellent solubility and dispersibility. Supercritical carbon dioxide can efficiently penetrate and swell the seaweed cell wall, exposing the xylose-containing hemicellulose. Simultaneously, carbon dioxide dissolves in water to form carbonic acid, which, as a mild acid catalyst, promotes the hydrolysis of hemicellulose to xylose and inhibits the formation of byproducts such as glucose.

[0019] The basic process of uniformly distributed Ni-Mo synergistic active sites in seaweed particles in this invention is as follows: Alginate has stronger ion exchange activity than citrate. Alginate reacts with nickel ions to form gel-like nickel alginate, and the gel-like nickel alginate adsorbs molybdate ions through surface coordination and electrostatic adsorption. Subsequently, in step S2, the reducing property of hydroxyl groups in sodium alginate is utilized to adsorb nickel ions Ni... 2+The reaction reduces to Ni; the reaction process of molybdate ions is as follows: In step S1, the molybdate ions adsorbed on the surface of nickel alginate are also reduced by the hydroxyl group in sodium alginate, thus reducing the molybdate ions to MoO4. 2- The hexavalent molybdenum in the xylose is reduced to tetravalent molybdenum to obtain MoO2. Subsequently, in conjunction with step S3, the hydrogen gas H2 introduced during the xylose hydrogenation reaction first forms hydrogen radicals under the catalysis of Ni: H2 → H·. Then, with the help of oxygen vacancies on the surface of molybdenum dioxide and the electron-deficient state of molybdenum dioxide created by the electronegativity difference between Ni and Mo, the generated hydrogen radicals H· are transferred from Ni to the surface of MoO2, promoting the conversion of hydrogen gas into hydrogen radicals. The generated hydrogen radicals can then efficiently participate in the aldehyde hydrogenation reduction reaction of xylose CH2OH(CHOH)3CHO to xylitol CH2OH(CHOH)3CH2OH, reducing the aldehyde group in xylose to a hydroxyl group and converting xylose into xylitol. In addition, the hydrogen radicals generated in the reaction can further participate in the reduction and generation process of Ni and MoO2, ensuring the generation of Ni and MoO2.

[0020] This invention uses seaweed granules as a raw material for preparing xylitol, and simultaneously utilizes the alginate and Ni in seaweed. 2+ Furthermore, the interaction between molybdate and algae particles effectively enriched Ni and Mo as catalytic sites, forming a uniform distribution of catalytic sites and recovering them from ionic dispersion in solution to solid phase recovery at the end of the reaction. This facilitated the separation of the catalyst from the reaction solution, effectively improving catalyst recovery. At the same time, the catalyst was uniformly dispersed in the reaction system, increasing the conversion rate of xylose and improving the purity of xylitol. Attached Figure Description

[0021] Figure 1 This is a schematic flowchart of the xylitol preparation method in this invention; Figure 2 The FT-IR spectrum of xylitol obtained in Example 1 of this invention; Figure 3 The image shows the XRD pattern of the solid residue obtained after step S4 in Example 1 of this invention. Figure 4 The image shows the XRD pattern of the Ni-MoO2 synergistic catalyst obtained by calcining the solid residue obtained in step S4 of Example 1 of this invention. Detailed Implementation

[0022] To further explain the technical solution of the present invention, the present invention will be described in detail below through specific embodiments.

[0023] Example 1

[0024] This embodiment discloses a method for preparing xylitol, including the following steps: S1. Add deprotonated seaweed particles to Ni 2+ / MoO4 2- -In the solution of metal precursor citrate; Ni enrichment under the confinement of carboxyl sites in alginate 2+ MoO4 2- Through Ni 2+ Aggregates on seaweed particles to form seaweed-Ni 2+ -MoO4 2- Deionized water washing removes unbound substances; The seaweed in this embodiment is brown algae.

[0025] In this embodiment, seaweed-Ni 2+ -MoO4 2- The assembly process is as follows: In this embodiment, the activated brown algae powder undergoes the following reaction in the metal ion complex solution: sodium alginate (NaAlg) reacts with nickel ions to form a complex [NiC6H5O7]. - The reaction produces gel-like nickel alginate, as shown in the following equation: Ni(Alg)₂: 2NaAlg + [NiC₆H₅O₇] - → Ni(Alg)2 + 2Na + + C6H5O7 3- The resulting gel-like nickel alginate (Ni(Alg)2) and molybdate (MoO4) 2- Adsorption occurs, with molybdate ions and nickel ions linked through surface coordination or electrostatic adsorption.

[0026] S2, seaweed-Ni 2+ -MoO4 2- Deionized water was added to the reactor, and the mixture was heated to 140 °C with a stirring speed of 200 rpm. 99.9% pure CO2 was introduced into the reactor until the pressure reached 7.5 MPa. Simultaneous pulsed microwaves were used with a pulse width of 40 μs, a pulse repetition frequency of 500 Hz, and a microwave power of 5 W / g (based on the mass of dry brown algae powder). The reaction was carried out for 1 hour. CO2 stripped hemicellulose from the algae, which was then converted to xylose under the catalysis of carbonic acid derived from CO2. Simultaneously, algae-Ni... 2+ -MoO4 2- Under the influence of CO2 and pulsed microwaves, it is transformed into Ni-Mo synergistic active sites; In step S2 of this embodiment, the amount of deionized water used is based on a mass ratio of dried seaweed to deionized water of 1:5. S3. First, cool the reactor to 60 ℃ to reduce the pressure in the high-pressure reactor. Then, open the reactor exhaust valve to further reduce the pressure to atmospheric pressure. Stir for 20 min to separate carbon dioxide from the mixture in step S2 (the solubility of carbon dioxide at 140 ℃ and 7.5 MPa is 26.4 g / kg, while the solubility of carbon dioxide at 60 ℃ and atmospheric pressure of 0.1 MPa is about 0.58 g / kg. Removing carbon dioxide can reduce its impact on subsequent reactions). Add sodium bicarbonate to the reactor until the pH in the reactor is 7±0.2, and introduce 99.9% pure H2 into the reactor. After the gas flow from the exhaust valve stabilizes, close the exhaust valve. Set the reaction temperature to 140 ℃, the stirring speed to 200 rpm, and introduce H2 into the reactor until the pressure inside the reactor is 5 MPa. React for 4 h and use the Ni-Mo active sites enriched in situ by seaweed particles to hydrogenate xylose to xylitol. S4. First, cool the mixture to 50 ℃ to reduce the pressure in the high-pressure reactor. Then, open the reactor exhaust valve to further reduce the pressure to atmospheric pressure. Add granular activated carbon to the mixture to remove impurities. Centrifuge the mixture to remove the solid residue at the bottom. Perform nanofiltration on the solution after removing the residue using a 200 Da nanofiltration membrane (Koch SR3D) at a pressure of 1 MPa. Xylitol (152.15 Da) will pass through while impurities such as magnesium alginate (520.58 Da) will be retained. Vacuum evaporate and concentrate the solution at 60 ℃ and 0.08-0.09 MPa until xylitol crystals appear. Then, add anhydrous ethanol at 5 times the volume of the mixture to reduce the solubility of xylitol. Slowly cool the mixture to 5 ℃ to allow the xylitol crystals to crystallize further. Finally, collect the obtained xylitol crystals and wash them with deionized water at 5 ℃ to obtain high-purity xylitol crystals.

[0027] In this embodiment, the mass ratio of activated carbon to dried seaweed is 0.1:1.

[0028] In this embodiment, the reaction solution contains xylose, other sugars produced by hemicellulose hydrolysis, small amounts of inorganic salts, and macromolecular impurities such as alginate, in addition to the target product xylitol. First, pigments and solid residues are removed by activated carbon adsorption and centrifugation. Then, a 200 Da nanofiltration membrane is used to separate the mixture, retaining alginate, oligosaccharides, and other macromolecular impurities, and initially enriching xylitol. Subsequently, the filtrate is concentrated by vacuum evaporation, and anhydrous ethanol is added to reduce the solubility of xylitol. Under low-temperature conditions, xylitol preferentially crystallizes out, while xylose and other soluble small-molecule sugars mainly remain in the mother liquor, thus achieving further purification and separation of xylitol. This purification process utilizes the differences in molecular size, solubility, and crystallization behavior of different components to achieve separation. Nanofiltration mainly removes macromolecular impurities and concentrates the target product, while the crystallization step selectively precipitates xylitol from the sugar mixture.

[0029] In this embodiment, brown algae is dried and crushed to produce brown algae powder with a particle size of no more than 1 mm. The dried brown algae powder is added to a 0.1 M NaOH solution and stirred for 1 h to fully deprotonate the carboxyl groups of the brown algae cell wall, thereby enhancing the activity of the brown algae. After the stirring reaction is complete, the solution is adjusted to neutral with 0.1 M hydrochloric acid to obtain deprotonated seaweed particles. The ratio of dried brown algae powder to NaOH solution is 1 g: 10 ml.

[0030] In this embodiment, Ni 2+ / MoO4 2- The preparation process of the citric acid metal precursor solution is as follows: Add 1.2 g of sodium citrate dihydrate to 500 ml of deionized water and stir until completely dissolved to obtain a sodium citrate solution. Then, while stirring at 200 rpm, slowly add 1 g of nickel sulfate hexahydrate to the sodium citrate solution in increments of 0.2 g. After complete addition and stirring for 30 min, slowly add 0.3 g of sodium molybdate dihydrate in increments of 0.1 g. After complete addition and stirring for 60 min, adjust the pH of the mixture to 8.5 ± 0.2 using 0.1 M NaOH solution and continue stirring for 30 min to obtain Ni. 2+ / MoO4 2- -Citrate metal precursor solution.

[0031] In this embodiment, Ni 2+ / MoO4 2- Ni in metal citrate precursor solution 2+ MoO4 2- The molar ratio of sodium citrate dihydrate to sodium citrate dihydrate is 1:0.33:1.07.

[0032] In this embodiment, the seaweed particles in step S1 are calculated by weight of dry seaweed powder and Ni 2+ / MoO4 2- - The volume ratio of citrate metal precursor solution is 7 g: 50 ml; In step S1, seaweed-Ni is generated. 2+ -MoO4 2- The reaction conditions are: The reaction temperature was 50℃, and the mixture was stirred for 2 hours.

[0033] The solid phase collected in step S4 includes the seaweed-Ni obtained in step S1. 2+ -MoO4 2- After steps S2 and S3, the dispersed ionic coordination state is transformed into a fixed Ni-Mo synergistic active phase.

[0034] In this embodiment, the solid residue obtained after centrifugation in step S4 is placed in a ceramic boat and calcined in a tube furnace at 500 °C for 1.5 h under an argon atmosphere. This allows the organic components of the solid residue to carbonize and volatilize, while the activated carbon further reduces the Ni-Mo synergistic active phase to a Ni-MoO2 synergistic catalyst. Subsequently, a mixture of argon and oxygen (volume ratio 98:2) is introduced into the tube furnace, and the temperature is lowered to 350 °C before calcination continues for another 1.5 h. This allows the carbon coated on the surface of Ni-MoO2 after carbonization to be converted into CO or CO2 and removed. The residue is then washed with deionized water to remove inorganic salt impurities and dried to obtain a Ni-MoO2 synergistic catalyst that can be directly used for catalytic hydrogenation reactions.

[0035] Example 2

[0036] This embodiment discloses a method for preparing xylitol. Except for steps S2 and S3, which are different from those in Example 1, the other steps are the same as in Example 1. Steps S2 and S3 specifically include the following steps: S2, seaweed-Ni 2+ -MoO4 2- Deionized water was added to the reactor, and the reaction was carried out for 0.5 h. CO2 stripped the hemicellulose in the seaweed, which was then converted into xylose under the catalysis of carbonic acid derived from CO2. At the same time, seaweed-Ni 2+ -MoO4 2- Under the influence of CO2 and pulsed microwaves, it is transformed into Ni-Mo synergistic active sites; S3. Introduce H2 into the reactor until the pressure inside the reactor is 5 MPa, and react for 4 h. Example 3

[0037] This embodiment discloses a method for preparing xylitol. Except for steps S2 and S3, which are different from those in Example 1, the other steps are the same as in Example 1. Steps S2 and S3 specifically include the following steps: S2, seaweed-Ni 2+ -MoO4 2- Add deionized water to the reactor, heat, and react for 1.5 h; S3. Introduce H2 into the reactor until the pressure inside the reactor reaches 5 MPa, and react for 4 h.

[0038] Example 4

[0039] This embodiment discloses a method for preparing xylitol, which uses the Ni-MoO2 co-catalyst recovered in Example 1 to catalyze the reaction of xylose to xylitol, and specifically includes the following steps: S1. Brown algae are selected as raw materials. The brown algae are dried and crushed into brown algae powder with a particle size of no more than 1 mm. The dried brown algae powder is added to 0.1 M NaOH solution and stirred for 1 h to deprotonate the carboxyl groups of the brown algae cell wall, thereby enhancing the activity of the brown algae. After the reaction is completed, 0.1 M hydrochloric acid is used to adjust the pH to neutral to obtain deprotonated seaweed particles. The ratio of dried brown algae powder to NaOH solution is 1 g: 10 ml.

[0040] S2. Deprotonated seaweed particles and deionized water are added to the reactor. The reaction temperature is set to 140 ℃, the stirring speed is 200 rpm, and 99.9% pure CO2 is introduced into the reactor until the pressure inside the reactor reaches 7.5 MPa. Simultaneous pulsed microwave is applied with a pulse width of 40 μs, a pulse repetition frequency of 500 Hz, and a microwave power of 5 W / g (based on the mass of dry brown algae powder). The reaction is carried out for 1 h. CO2 strips the hemicellulose in the seaweed and converts it into xylose under the catalysis of carbonic acid derived from CO2. S3. First, cool down to 60 ℃ to reduce the pressure in the high-pressure reactor. Then, open the reactor exhaust valve to further reduce the pressure to atmospheric pressure. Stir for 20 min to separate the carbon dioxide in the mixture from step S2 (the solubility of carbon dioxide at 140 ℃ and 7.5 MPa is 26.4 g / kg, while the solubility of carbon dioxide after cooling to 60 ℃ and atmospheric pressure 0.1 MPa is about 0.58 g / kg. Removing carbon dioxide can reduce its impact on subsequent reactions). Add sodium bicarbonate to the reactor until the pH in the reactor is 7±0.2. In this embodiment, in step S2, the amount of deionized water used is 5 ml of deionized water per gram of dried seaweed, and the recovered Ni-MoO2 synergistic catalyst from Example 1 is added to the reactor at a ratio of 0.05 g of recovered Ni-MoO2 synergistic catalyst per 10 g of dried seaweed. H2 with a purity of 99.9% is introduced into the reactor, and after the gas flow from the exhaust valve stabilizes, the exhaust valve is closed. The reaction temperature is set to 140 ℃, the stirring speed to 200 rpm, and H2 is introduced into the reactor until the pressure inside the reactor reaches 5 MPa. The reaction is carried out for 4 h, during which the Ni-MoO2 synergistic catalyst hydrogenates and reduces xylose to xylitol. S4. After completing step S3, reduce the pressure in the high-pressure reactor to atmospheric pressure and cool it to 50 °C. Filter the Ni-MoO2 synergistic catalyst, add granular activated carbon to the mixture for adsorption, and then perform nanofiltration to remove small molecule impurities from the solution after activated carbon adsorption. Vacuum evaporate and concentrate the solution at 60 °C and 0.08-0.09 MPa until xylitol crystals appear. Then slowly cool it to 5 °C to allow the xylitol crystals to crystallize further. Finally, collect the obtained xylitol crystals and wash them with deionized water at 5 °C to obtain high-purity xylitol crystals.

[0041] Comparative Example 1 This comparative example discloses a method for preparing xylitol, specifically including the following steps: S1. Select high-purity xylose with a purity of ≥98% as raw material, dissolve 40 g of high-purity xylose in 100 ml of deionized water to obtain xylose solution.

[0042] S2. Add the xylose solution obtained in step S1 to a high-pressure reactor, add 2 g of Raney nickel catalyst to the high-pressure reactor, set the reaction temperature to 140 ℃, and introduce 99.9% pure hydrogen gas into the high-pressure reactor until the pressure in the reactor is 5 MPa. The stirring rate is 200 rpm. After reacting for 4 h, a xylose-containing mixture is obtained. S3. After completing step S2, reduce the pressure in the high-pressure reactor to atmospheric pressure and cool it to 50 °C. Add granular activated carbon to the mixture for adsorption. Then, perform nanofiltration on the solution after activated carbon adsorption to remove small molecule impurities. Vacuum evaporate and concentrate the solution at 60 °C and 0.08-0.09 MPa until xylitol crystals appear. Then, slowly cool it to 5 °C to allow the xylitol crystals to crystallize further. Finally, collect the obtained xylitol crystals and wash them with deionized water at 5 °C to obtain high-purity xylitol crystals.

[0043] Comparative Example 2 This comparative example discloses a method for preparing xylitol. Compared with Example 1, this comparative example uses sugarcane bagasse as raw material, and the other steps are the same as in Example 1.

[0044] Comparative Example 3 This comparative example discloses a method for preparing xylitol, except for Ni 2+ - Except for the citrate metal precursor solution, the other steps are the same as in Example 1, Ni 2+ - The citric acid metal precursor solution specifically includes the following steps: Add 1.2 g of sodium citrate to 500 ml of deionized water and stir until completely dissolved to obtain a sodium citrate solution. Then, while stirring at 200 rpm, slowly add 1 g of nickel sulfate hexahydrate to the sodium citrate solution in increments of 0.2 g. After complete addition and stirring for 30 min, adjust the pH of the mixture to 8.5 ± 0.2 using 0.1 M NaOH solution and continue stirring for 30 min to obtain Ni. 2+ -Citrate metal precursor solution.

[0045] Comparative Example 4 This comparative example discloses a method for preparing xylitol, except for Ni 2+ / MoO4 2- The steps other than those for solution preparation are the same as in Example 1. Ni 2+ / MoO42- The solution specifically includes the following steps: At a stirring speed of 200 rpm, 1 g of nickel sulfate hexahydrate was slowly added to 500 ml of deionized water in increments of 0.2 g. After complete addition and stirring for 30 min, 0.3 g of sodium molybdate dihydrate was slowly added in increments of 0.1 g. After complete addition and stirring for 60 min, the pH of the mixture was adjusted to 8.5 ± 0.2 using 0.1 M NaOH solution, and stirring was continued for 30 min to obtain Ni. 2+ / MoO4 2- Solution.

[0046] Comparative Example 5 This comparative example discloses a method for preparing xylitol, specifically including the following steps: S1. Select high-purity xylose with a purity of ≥98% as raw material, dissolve 5.5 g of high-purity xylose in 100 ml of deionized water to obtain xylose solution.

[0047] S2. Add the xylose solution obtained in step S1 to the high-pressure reactor, and add 100 ml of Ni to the high-pressure reactor. 2+ / MoO4 2- - A citric acid metal precursor solution was prepared, and the reaction temperature was set at 140 °C. Hydrogen gas with a purity of 99.9% was introduced into the high-pressure reactor until the pressure in the reactor was 5 MPa. The stirring rate was 200 rpm. The reaction mixture was obtained after 4 h. S3. After completing step S2, reduce the pressure in the high-pressure reactor to atmospheric pressure and cool it to 50 °C. Add granular activated carbon to the mixture for adsorption. Then, nanofilter the solution after activated carbon adsorption to remove small molecule impurities. Vacuum evaporate and concentrate the solution at 60 °C and 0.08-0.09 MPa until crystals appear. Then, slowly cool it to 5 °C to allow the crystals to crystallize further. Finally, collect the obtained crystals and wash them with deionized water at 5 °C to obtain the product crystals.

[0048] In this comparative example, Ni 2+ / MoO4 2- The preparation process of the citric acid metal precursor solution is the same as in Example 1.

[0049] The purity, product yield, and xylitol extraction rate of xylitol prepared in all embodiments and comparative examples of this invention were tested. The purity of xylitol was determined by high performance liquid chromatography under the following conditions: amino column, acetonitrile-water 75:25 mobile phase, differential refractive index detector, column temperature 30-35 ℃, flow rate 1 ml / min, and injection volume 10-20 µl.

[0050] Product yield was calculated as follows: Xylitol yield = xylitol mass ÷ (dry seaweed mass × hemicellulose content in dry seaweed) × 100%; All brown algae raw materials used in this invention are from the same batch, and the hemicellulose content is 10%.

[0051] After step S2, the xylose content in the mixture was detected by liquid chromatography, and the xylose concentration was calculated by external standard method. Xylose conversion rate was calculated using the following method: [(Product mass * purity) ÷ (1 - 44 / 90)] ÷ Mass of xylose in the solution after S2; The solubility of xylitol in water is 90 g / 100g at 60℃ and 44 g / 100g at 5℃.

[0052] Xylitol extraction rate was calculated using the following method: Xylitol extraction rate = xylitol mass ÷ mass of dried seaweed powder used × 100%.

[0053] The purity, yield, and extraction rate of xylitol obtained in Examples 1 to 4 and Comparative Examples 1 to 5 are detailed in Table 1.

[0054] Table 1. Xylitol preparation results (test)

[0055] Comparing Examples 1 and 2, when the reaction time ratio of S2 and S3 was adjusted from 1:4 in Example 1 to 0.5:4 in Example 2, the xylose concentration in the solution after step S2 in Example 2 decreased from 12.81 g / L to 11.36 g / L, and the yield decreased from 49.19% to 43.81%. This is because the hemicellulose in seaweed cannot be fully hydrolyzed to form xylose in a shorter time, resulting in less xylose produced and a decrease in xylose content. Furthermore, the xylose hydrogenation process depends on the formation of the supported Ni-Mo synergistic active phase in step S2, but the shorter S2 reaction time cannot generate sufficient Ni-Mo synergistic active phase to support the hydrogenation reaction, leading to incomplete xylose hydrogenation and a decrease in product yield. Conversely, when the reaction time of S2 and S3 was adjusted to 1.5:4 in Example 3, the xylose concentration in the solution after S2 decreased from 12.81 g / L in Example 1 to 11.62 g / L in Example 3. The yield decreased from 49.19% in Example 1 to 44.74% in Example 3. This was because after extending the time of step S2, the xylose generated by hemicellulose hydrolysis underwent a certain degree of degradation, resulting in a reduction in the xylose content that could be used for hydrogenation reaction, thus causing a decrease in xylose content and yield.

[0056] Example 4 uses the Ni-MoO2 synergistic catalyst prepared from the solid residue obtained in step S4 of Example 1 to catalyze the hydrogenation conversion of xylose to xylitol. Compared with Example 1, in Example 4, after S2, the concentration of xylose in the solution decreased from 12.81 g / L to 12.70 g / L, the yield decreased from 49.19% to 47.41%, and the conversion rate decreased from 99.12% to 96.27%. With a difference of only 0.11 g / L in xylose concentration, the yield and conversion rate decreased significantly, indicating that the recovered Ni-MoO2 synergistic catalyst has a certain catalytic effect. However, because it is an external catalyst, it cannot be uniformly dispersed on the seaweed surface, and after regeneration, its crystal size increases, resulting in fewer active sites for the reaction. Compared with the Ni-Mo synergistic catalyst supported on seaweed particles generated in step S2 of this invention, its performance is not as good. In the preparation of xylitol, the present invention transforms the ionic state into a solid phase during the generation and one-step reduction of xylose to xylitol, which facilitates the separation of the solid phase catalyst and effectively reduces the content of metal ions in xylitol. The recovered Ni-MoO2 co-catalyst can also be added to the xylose solution for catalytic reduction to prepare xylitol.

[0057] Compared to Example 1, Comparative Example 1 replaced the raw material with high-purity xylose, and the purity of the prepared xylitol increased from 97.81% to 98.62%, and the yield increased from 19.19% to 51.01%. Comparative Example 1, which uses high-purity xylose with higher cost and strong dependence, has higher purity and yield. Meanwhile, Example 1, which uses seaweed as raw material with lower cost, can also obtain higher purity and yield.

[0058] Compared to Example 1, Comparative Example 2 replaced the raw material with sugarcane bagasse. After S2, the concentration of xylose in the solution of the prepared xylitol increased from 12.81 g / L to 37.12 g / L, the yield decreased from 49.19% to 23.82%, and the purity decreased from 97.81% to 9.92%. This is because sugarcane bagasse lacks sodium alginate, which is necessary for the formation of Ni-Mo synergistic active sites, resulting in the absence of a catalyst during the reaction. Therefore, the xylose produced by the hydrolysis of hemicellulose in sugarcane bagasse cannot be effectively converted into xylitol.

[0059] Comparing Example 1 and Comparative Example 3, the lack of molybdate ions during the preparation of the ionic solution significantly affected the final xylitol preparation result. Compared with Example 1, the concentration of xylose in the solution of Comparative Example 3 after S2 was 12.78 g / L, which was basically the same as that of Example 1. The yield decreased from 49.19% to 46.07%, the purity decreased from 97.81% to 87.39%, and the conversion rate decreased from 99.12% to 83.22%. In contrast, in Example 1, the transfer of electrons to Ni induced by oxygen vacancies in MoO2 in the Ni-Mo synergistic active sites supported by seaweed particles increased the electron density of nickel, thereby increasing the desorption rate of the generated hydrogen free radicals, optimizing the overall reaction activity, and allowing the reaction to continue in a positive direction, thus improving the yield and purity of xylitol. However, Comparative Example 3 only had a Ni reaction cluster, which could not promote the efficient conversion of xylitol.

[0060] Comparing Example 1 and Comparative Example 4, it can be seen that sodium citrate has a significant impact on the in-situ generated Ni-MoO2 synergistic catalyst during the preparation of the ionic solution. In Comparative Example 4, after S2, the concentration of xylose in the solution was 12.84 g / L, the yield decreased from 49.19% to 40.59%, the purity decreased from 97.81% to 60.14%, and the conversion rate decreased significantly from 99.12% to 50.22%. This is because the lack of sodium citrate led to the precipitation of nickel-molybdenum metal ions to form nickel molybdate precipitate, resulting in a reduction in the catalytic effect of the reaction and a significant decrease in the yield, purity, and conversion rate of xylitol.

[0061] Compared to Example 1, in Comparative Example 5, after S2, the concentration of xylose in the solution increased from 12.81 g / L to 24.69 g / L, the yield decreased from 49.19% to 36.59%, the purity decreased from 97.81% to 11.69%, and the conversion rate decreased significantly from 99.12% to 8.37%. This is because citric acid and its degradation products have certain reducing properties, reducing trace amounts of nickel ions and molybdate ions to form Ni and MoO2. However, the catalyst generated is limited, and a large amount of xylose was not converted into xylitol.

[0062] In addition, the reaction solution after centrifugation and nanofiltration in step S4 was analyzed by ICP-MS to detect whether there were residual metal ions in the reaction solution. The results are shown in Table 2.

[0063] Table 2. ICP test results of the reaction solution obtained after centrifugation and nanofiltration from S4 (unit: ppm)

[0064] As shown in Table 2, the xylitol products prepared by the method of the present invention have low nickel and molybdenum content, which meets the national standards.

[0065] Figure 2 The image shows the infrared spectrum of xylitol prepared in Example 1 of this invention. As can be seen from the image, xylitol exhibits high activity in the 3200-3400 cm⁻¹ range. -1 A distinct hydroxyl peak appeared at 1322-1416 cm⁻¹. -1 A distinct in-plane bending characteristic peak of hydroxyl groups appeared at 1062-1224 cm⁻¹. -1 The characteristic peaks of the CO and CC skeletons appeared clearly at 1012 cm⁻¹. -1 A distinct stretching vibration peak of primary alcohol CO was observed at the point, and the above characteristic peaks correspond to the characteristic peaks of xylitol standard.

[0066] Figure 3 The image shows the XRD pattern of the solid phase collected in step S4 of Example 1. The figure shows obvious characteristic peaks of Ni at 44.2°, 51.6° and 76.8°, and characteristic peaks of molybdenum oxide at 26.8° and 37.6°. It can be seen that the Ni-MoO2 synergistic catalyst was collected after step S4.

[0067] Figure 4 The image shows the XRD pattern of the Ni-MoO2 synergistic catalyst obtained by calcination and recovery of the supported Ni-Mo synergistic active phase collected in step S4 of Example 1, used in Example 4. The figure shows obvious characteristic peaks of Ni at 44.2°, 51.6° and 76.8°, and characteristic peaks of MoO2 at 26.8° and 37.6°. and Figure 3 In comparison, the Ni-MoO2 synergistic catalyst obtained by calcination has fewer impurity peaks and a smoother overall baseline. Furthermore, the full width at half maximum (FWHM) of the regenerated Ni-MoO2 synergistic catalyst has increased. This is because the regenerated Ni-MoO2 undergoes high-temperature calcination, which removes impurities, resulting in fewer impurity peaks and a cleaner test background. The broadened full width at half maximum (FWHM) in the XRD pattern of the Ni-MoO2 synergistic catalyst is caused by the slight increase in the grain size of Ni and MoO2 after high-temperature calcination.

Claims

1. A method for preparing xylitol, characterized in that: Includes the following steps: S1. Add deprotonated seaweed particles to Ni 2+ / MoO4 2- -In the solution of metal precursor citrate; Ni is enriched at the carboxyl site of alginate in seaweed particles. 2+ MoO4 2- Through Ni 2+ Enriched in seaweed particles to form seaweed-Ni 2 + -MoO4 2- ; The preparation process of deprotonated seaweed particles is as follows: Dry and pulverize seaweed and add it to sodium hydroxide solution. Stir for 0.5 to 1.5 hours. The carboxyl groups in the alginic acid of the seaweed are deprotonated and converted into carboxylate groups. Ni 2+ / MoO4 2- The preparation process of the citric acid metal precursor solution is as follows: Add sodium citrate to deionized water, stir to dissolve, and then add nickel ion source and molybdate source in sequence; Adjust the pH to 8.3 to 8.7; Continue stirring for 20 to 40 minutes to obtain Ni. 2+ / MoO4 2- -Citrate metal precursor solution; The seaweed is brown algae; S2, seaweed-Ni 2+ -MoO4 2- Add deionized water to the reactor, heat, and introduce supercritical CO2 into the reactor simultaneously with pulsed microwaves; The process parameters for pulsed microwave are as follows: pulse width is 10-100 μs, and pulse repetition frequency is 100-1000 Hz; Supercritical CO2 strips hemicellulose from seaweed, which is then converted and accumulates xylose under the catalysis of carbonic acid derived from supercritical CO2. Simultaneously seaweed-Ni 2+ -MoO4 2- Under the action of supercritical CO2 and pulsed microwaves, they are transformed into Ni-Mo synergistic active sites; S3. After step S2, the reaction vessel is cooled down and the carbon dioxide in the reaction vessel is separated. H2 is introduced into the reaction vessel, heated and stirred. The Ni-Mo active sites enriched in situ by the seaweed particles catalyze the hydrogenation reduction of xylose to xylitol. S4. Cool down and depressurize to release H2 to normal pressure; add activated carbon to remove impurities, centrifuge and nanofilter the reaction solution, concentrate and crystallize to obtain the target xylitol.

2. The method for preparing xylitol according to claim 1, characterized in that: In step S1, the seaweed particles, based on the mass of dry seaweed powder, are mixed with Ni. 2+ / MoO4 2- - The ratio of the amount of citrate metal precursor solution used is (6g to 8g): 50ml.

3. The method for preparing xylitol according to claim 1, characterized in that: In step S1, seaweed-Ni is generated. 2+ -MoO4 2- The reaction conditions are: The reaction temperature is 40℃ to 60℃, and the mixture is stirred for 1 to 3 hours.

4. The method for preparing xylitol according to claim 1, characterized in that: In step S2, the amount of deionized water used is based on a mass ratio of dried seaweed to deionized water of 1:(4 to 6). The reaction time for step S2 is 0.5 h to 1.5 h.

5. The method for preparing xylitol according to claim 1, characterized in that: The solid phase collected in step S4 includes the seaweed-Ni obtained in step S1. 2+ -MoO4 2- After steps S2 and S3, the ionic coordination state is transformed into immobilized Ni-Mo synergistic active sites.

6. The method for preparing xylitol according to claim 5, characterized in that: The solid residue obtained after centrifugation in step S4 is calcined under an argon atmosphere to obtain a Ni-MoO2 synergistic catalyst for catalytic hydrogenation.

7. The method for preparing xylitol according to claim 6, characterized in that: The process steps and parameters for calcining solid residue are as follows: The solid residue was calcined under an argon atmosphere, and the organic components in the solid residue were carbonized and volatilized. Subsequently, a mixture of argon and oxygen gas was introduced into the tubular furnace, cooled down, and then calcined again to obtain the Ni-MoO2 synergistic catalyst.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step S2, CO2 is introduced until the pressure inside the reactor is 7.5 MPa to 8.5 MPa; In step S3, H2 is introduced until the pressure inside the reactor is 4.5 MPa to 5.5 MPa.

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