Method for preparing methyl lactate from biomass sugar by metal-loaded titanium silicalite molecular sieve in near-critical methanol

Abstract

The present invention discloses a method for preparing methyl lactate by catalytic conversion of biomass sugars with metal-loaded titanium silicate molecular sieve in near-critical methanol. The steps of the method are as follows: 1) Add biomass sugars and methanol into a high-temperature and high-pressure reactor, where the mass concentration of biomass sugars is 10-200 g / L, and then add a metal-loaded titanium silicate molecular sieve catalyst. The mass ratio of biomass sugars to the metal-loaded titanium silicate molecular sieve is 5:1 - 1:3; 2) Heat up to 140-220 °C and react for 0.5-36 h; 3) After the reaction is completed, cool to room temperature, filter, and the filtrate is rectified to obtain a methyl lactate product, and the methanol is recycled; the filter residue is the used metal-loaded titanium silicate molecular sieve, which is directly recycled after being washed with methanol and dried. The present invention uses a metal-loaded titanium silicate molecular sieve as a catalyst, which not only has a very high yield of methyl lactate, but also has high catalyst stability and good reusability, so it has a very broad industrial application prospect.

Description

Technical Field

[0001] The present invention relates to a method for preparing methyl lactate from biomass sugar by a metal-loaded titanium silicate molecular sieve in near-critical methanol. Background Art

[0002] Biomass is not only a renewable resource that is inexhaustible and renewable, but also a carbon-neutral carrier. How to utilize biomass resources on a large scale, efficiently, greenly, and economically is a huge challenge facing us. Starting from biomass, almost all basic organic chemical raw materials can be produced, and many products have shown good economic efficiency. With the soaring oil prices, the prospects of biomass-based chemicals are becoming increasingly broad.

[0003] Sugar is a polyhydroxy aldehyde or ketone compound, which is similar to the combination of carbon and water in terms of chemical formula representation, so it is also called carbohydrate. It can be divided into monosaccharides, disaccharides, polysaccharides, and conjugated sugars according to the number of structural units. Lignocellulose is a composite material mainly composed of cellulose, hemicellulose, and lignin. In different types of lignocellulose, the composition of the three components is different. Usually, cellulose accounts for 30-50 wt%, hemicellulose accounts for 20-35 wt%, and lignin accounts for 15-30 wt%. Biomass sugar includes cellulose and hemicellulose in lignocellulose, as well as derived polysaccharides and monosaccharides such as starch, xylose, fructose, glucose, and sucrose. Biomass sugar is a cheap and easily available chemical raw material, so the research on preparing high-value-added chemicals from biomass sugar has attracted much attention.

[0004] Methyl lactate (CAS No.: 547-64-8), the structural formula is shown as follows:

[0005]

[0006] Methyl lactate is a chiral compound. The methyl lactate in the present invention refers to racemic methyl lactate (DL-methyl lactate). Methyl lactate is a colorless liquid, flammable, and soluble in water, ethanol, and organic solvents. Methyl lactate is an important biomass-based platform compound. In addition to being used as a synthesis raw material for the biodegradable material polylactic acid, it is also a green solvent with excellent solubility performance and has extremely broad application prospects.

[0007] The preparation methods of methyl lactate mainly include biological fermentation and chemical catalytic alcoholysis. In biological fermentation, lactic acid needs to be prepared first and then esterified with methanol. Currently, the traditional microbial fermentation process is still used in industry to prepare lactic acid first and then esterify it. This process has problems such as low production efficiency, long cycle, and a large amount of waste liquid. Compared with the biological fermentation method, chemical catalytic alcoholysis has advantages such as fast reaction speed, high volumetric yield, and suitability for large-scale continuous production. Therefore, the heterogeneous catalytic alcoholysis of biomass to prepare methyl lactate, which is in line with the concept of green catalysis, has become one of the research hotspots in the field of biomass directed chemical conversion in recent years.

[0008] Taking fructose as an example, the reaction equation for the catalytic conversion of biomass sugar to prepare methyl lactate is as follows:

[0009]

[0010] From the perspective of the reaction mechanism, the reaction steps for the alcoholysis of biomass to prepare methyl lactate are as follows: Biomass is first hydrolyzed to pentose and hexose, then retro-aldol condensation occurs to obtain triose, and finally dehydration, esterification, and isomerization generate methyl lactate.

[0011] This reaction can adopt two methods: homogeneous catalysis and heterogeneous catalysis.

[0012] For homogeneous catalysis, the catalysts currently used mainly include metal chlorides and mixed L-acid systems (such as InCl3, SnCl2, InCl3-SnCl2, SnCl4-NaOH, SnCl2-NiCl2, etc.). The disadvantages of this method are that these homogeneous catalysts are difficult to recycle and there are problems with product separation, which is not suitable for large-scale production.

[0013] The key to the heterogeneous catalytic alcoholysis of biomass to prepare methyl lactate is the development of solid catalysts. The solid catalysts reported in the literature include silicoaluminophosphates (such as Snβ, Nb / HUSY, Sn-MWW, Zr-SBA-15), MOF materials (such as Mg-MOF-74), metal oxides (such as NiO, La2O3, Cr2O3, TiO2, MgO), carbon-silicon composites, etc. All of these solid catalysts have a common defect, that is, the catalyst has poor reusability. When the catalyst is directly reused, the yield of methyl lactate decreases significantly. Most of the literature uses calcination treatment, which has led to the research on the heterogeneous catalytic alcoholysis of biomass sugar to prepare methyl lactate remaining at the laboratory stage and being difficult to realize industrial application.

[0014] In the early 1980s, ENI Company in Italy successfully developed the heteroatom molecular sieve TS-1 with an MFI structure in which titanium atoms are isomorphously substituted in the framework. TS-1 titanium silicate molecular sieve has excellent catalytic performance in liquid-phase selective oxidation reactions using hydrogen peroxide as the oxidant, and has attracted wide attention due to its unique shape-selective catalytic oxidation performance, mild reaction conditions and environmental friendliness. Research over the past 20 years has found that TS-1 can efficiently catalyze the oxidation of a series of organic compounds, and the catalytic application of titanium silicate molecular sieve is called a new milestone in the field of molecular sieve catalysis. One of the great advantages of titanium silicate molecular sieve is its particularly good hydrothermal stability, which is an advantage that the previous silica-alumina molecular sieves do not have. The research on the heterogeneous catalytic alcoholysis of biomass sugars to prepare methyl lactate has been carried out in pure methanol, but water is inevitable in the actual industrial process for three reasons: First, the solubility of biomass sugars in pure methanol is very small, and water needs to be added to increase its solubility for continuous high-pressure feeding (a small amount of water can greatly increase the solubility of biomass sugars in methanol); Second, both the alcoholysis and polymerization side reactions of sugar produce water; Third, directly using a high-concentration aqueous solution of biomass sugars as the reaction raw material can greatly reduce the energy consumption of sugar dehydration and the raw material cost. Except for the invention patent recently applied by the inventors of this patent (Lv Xiuyang, Jiang Yuxi, Lv Xilei, Wei Xiwen, Ren Aotian. A method for preparing methyl lactate by catalytic conversion of biomass sugars with titanium silicate molecular sieve in near-critical methanol, application number: 202310152278X, application date: February 14, 2023), there is no report on the catalytic conversion of biomass sugars with titanium silicate molecular sieve to prepare methyl lactate.

[0015] In the invention patent recently applied by the inventors of this patent (application number: 202310152278X), titanium silicate molecular sieve is used as the catalyst, which has high catalyst stability and good reusability, but the highest yield of methyl lactate is only 53.6%, and in most cases it is below 50%, and the carbon utilization rate is low. Therefore, the yield of methyl lactate needs to be improved urgently to further enhance its industrial application prospects. Summary of the Invention

[0016] The purpose of the present invention is to provide a method for preparing methyl lactate by catalytic conversion of biomass sugars with metal-loaded titanium silicate molecular sieve in near-critical methanol.

[0017] The steps of the method are as follows:

[0018] 1) Add biomass sugars and methanol into a high-temperature and high-pressure reactor, the mass concentration of biomass sugars is

[0019] 10 - 200 g / L, and then add the metal-loaded titanium silicate molecular sieve catalyst, and the mass ratio of biomass sugars to the metal-loaded titanium silicate molecular sieve is 5:1 - 1:3;

[0020] 2) Heat up to 140 - 220 °C and react for 0.5 - 36 h;

[0021] 3) After the reaction is completed, cool to room temperature, filter, and the filtrate is rectified to obtain methyl lactate product, and methanol is recycled; the filter residue is used metal-loaded titanium silicate molecular sieve, which is directly recycled after being washed with methanol and dried.

[0022] It should be noted that the metal-loaded titanium silicate molecular sieve catalyst used in the present invention can be a freshly prepared metal-loaded titanium silicate molecular sieve catalyst or a used metal-loaded titanium silicate molecular sieve recovered through step 3). The method for recovering the catalyst in the present invention is simple, and the catalytic activity of the recovered metal-loaded titanium silicate molecular sieve catalyst is not significantly reduced compared with the fresh catalyst. According to the examples of the present invention, the catalyst can be reused 22 times, and the molar yield of the reaction product methyl lactate is still comparable to that of the first fresh catalyst. Therefore, due to the design of the process steps and the selection of process conditions, the catalyst of the present invention can be reused multiple times, and because the catalyst recovery method is simple, the use cost of the catalyst is very low in the whole process. In an optional embodiment of the present invention, a fresh catalyst is used at the beginning of the reaction, and the used catalyst recovered in subsequent batches is used until the molar yield of the reaction product methyl lactate is reduced below the set value.

[0023] As a preferred embodiment of the present invention, the biomass sugar in step 1) of the present invention is one or more of fructose, glucose,

[0024] xylose, sucrose, lactose, maltose, cellobiose, dextran, inulin, xylan, etc., among which fructose, glucose, and xylose are monosaccharides, sucrose, lactose, maltose, and cellobiose are disaccharides, and dextran, inulin, and xylan are polysaccharides.

[0025] As a preferred embodiment of the present invention, the mass concentration of the biomass sugar is preferably 20 - 150 g / L.

[0026] As a preferred embodiment of the present invention, the metal-loaded titanium silicalite molecular sieves are In-TS-1, Co-TS-1, Fe-TS-1, Mg-TS-1, Ni-TS-1, Sn-TS-1, Cu-TS-1, Sr-TS-1, Zn-TS-1, Sb-TS-1, Pb-TS-1, Mn-TS-1, Cd-TS-1, Ca-TS-1, Ba-TS-1, In-TS-2, Sn-TS-2, Co-TS-2, Fe-TS-2, In-Sn-TS-1, In-Ni-TS-1, In-Mg-TS-1, Sn-Co-TS-1, Sn-Mg-TS-1, In-Sn-TS-2, where In-TS-1, Co-TS-1, Fe-TS-1, Mg-TS-1, Ni-TS-1, Sn-TS-1, Cu-TS-1, Sr-TS-1, Zn-TS-1, Sb-TS-1, Pb-TS-1, Mn-TS-1, Cd-TS-1, Ca-TS-1, Ba-TS-1, In-TS-2, Sn-TS-2, Co-TS-2, Fe-TS-2 are single-metal-loaded titanium silicalite molecular sieves, and In-Sn-TS-1, In-Ni-TS-1, In-Mg-TS-1, Sn-Co-TS-1, Sn-Mg-TS-1, In-Sn-TS-2 are double-metal-loaded titanium silicalite molecular sieves; the preferred titanium silicalite molecular sieves as carriers are TS-1 and TS-2; the mass ratio of the total metal atoms in the metal-loaded titanium silicalite molecular sieve to the carrier is 0.2-3%.

[0027] As a preferred embodiment of the present invention, the metal-loaded titanium silicalite molecular sieve is prepared by the hydrothermal method. Typically but not limitedly, the metal-loaded titanium silicalite molecular sieve can be prepared by the following method (the essential difference in the preparation of TS-1 and TS-2 is the use of different organic templates):

[0028] S1. Dissolve the metal-loaded soluble salt in water to form a metal salt solution;

[0029] S2. Add the silicon source and the organic template to the above metal salt solution and stir to mix, then add the titanium source, the organic template and water and stir to mix to prepare a gel;

[0030] S3. Hydrothermal synthesis: Transfer the gel to a stainless steel autoclave with a polytetrafluoroethylene liner and crystallize for several days at 130-220°C; thoroughly wash the solid sample with deionized water, dry, and calcine to obtain the metal-loaded titanium silicalite molecular sieve.

[0031] As a preferred embodiment of the present invention, the mass ratio of the biomass sugar to the metal-loaded titanium silicalite molecular sieve is preferably 2:1-1:2.

[0032] As a preferred embodiment of the present invention, the reaction temperature in step 2) of the present invention is preferably 160-200°C, and the reaction time is preferably 1-24 h; the reaction kettle is in a vapor-liquid two-phase state, and the reaction pressure is close to the saturated vapor pressure of methanol at the reaction temperature.

[0033] The method proposed by the present invention has at least the following advantages:

[0034] 1) The metal-loaded titanium silicalite molecular sieve catalyst used as a solid catalyst has high stability and good reusability;

[0035] 2) The yield of methyl lactate is very high, up to 71.6%.

[0036] 3) It is easy to realize continuous fixed-bed operation and has broad industrial application prospects.

[0037] In summary, this method finds that the metal-loaded titanium silicalite molecular sieve can be used as a long-acting solid catalyst for the heterogeneous catalytic alcoholysis of biomass sugars to prepare methyl lactate. The yield of methyl lactate is very high, the catalyst has high stability and good reusability, and it is easy to realize continuous fixed-bed operation, with broad industrial application prospects. The present invention will greatly promote the industrialization process of the chemical preparation of biomass-based methyl lactate. Brief Description of the Drawings

[0038] Attached Figure 1 is a simplified process flow diagram of the preparation of methyl lactate from biomass sugars by a metal-loaded titanium silicalite molecular sieve in near-critical methanol. Detailed Embodiments

[0039] The labeling method of the single-metal-loaded titanium silicalite molecular sieve of the present invention: M-TS-X%, where M is the metal, TS is the titanium silicalite molecular sieve, and X is the mass ratio of the metal atom to the carrier; the labeling method of the double-metal-loaded titanium silicalite molecular sieve: M1-M2-TS-X1%-X2%, where M1 and M2 are different metals, TS is the titanium silicalite molecular sieve, X1 is the mass ratio of the M1 metal atom to the carrier, and X2 is the mass ratio of the M2 metal atom to the carrier. The metal-loaded titanium silicalite molecular sieve described in the present invention is prepared by a hydrothermal method, specifically as follows:

[0040] Preparation of Single-Metal-Loaded Titanium Silicalite Molecular Sieve

[0041] Taking In-TS-1-1% as an example, its synthesis steps are as follows. Dissolve 0.1659 g of indium chloride (InCl3) in 14.8 mL of deionized water and stir evenly. Then, add 15.63 g of tetraethyl orthosilicate (TEOS) and 6.72 g of tetrapropylammonium hydroxide (TPAOH) to the above solution and stir for 5 hours. Next, add 0.3191 g of tetrabutyl titanate (TBOT), 6.1 g of isopropyl alcohol (IPA), 5.48 g of TPAOH, and 12.2 mL of deionized water to the flask and stir for 2 hours. Subsequently, mix the two solutions and stir for 2 hours. The resulting gel composition is 1TEOS:0.0125TBOT:0.01InCl3:0.2TPAOH:20H2O. Transfer the gel to a stainless-steel autoclave with a polytetrafluoroethylene liner and crystallize at 170 °C for 3 days. Thoroughly wash the solid sample with deionized water, dry at 70 °C, and calcine at 550 °C for 6 hours to obtain In-TS-1-1%.

[0042] Taking In-TS-2-1% as an example, its synthesis steps are as follows. Dissolve 0.1659 g of InCl3 in 14.8 mL of deionized water and stir evenly. Then, add 15.63 g of TEOS and 8.57 g of tetrabutylammonium hydroxide (TBAOH) to the above solution and stir for 5 hours. Next, add 0.3191 g of TBOT, 6.1 g of IPA, 6.91 g of TBAOH, and 12.2 mL of deionized water to the flask and stir for 2 hours. Subsequently, mix the two solutions and stir for 2 hours. The resulting gel composition is 1TEOS:0.0125TBOT:0.01InCl3:0.2TBAOH:20H2O. Transfer the gel to a stainless-steel autoclave with a polytetrafluoroethylene liner and crystallize at 170 °C for 3 days. Thoroughly wash the solid sample with deionized water, dry at 70 °C, and calcine at 550 °C for 6 hours to obtain In-TS-2-1%.

[0043] Preparation of Bimetal-Loaded Titanium Silicate Molecular Sieve

[0044] Taking In-Sn-TS-1-1%-1% as an example, its synthesis steps are as follows. Dissolve 0.1659 g of InCl3 and 0.1422 g of stannous chloride (SnCl2) in 14.8 mL of deionized water and stir evenly. Then, add 15.63 g of TEOS and 6.72 g of TPAOH to the above solution and stir for 5 hours. Next, add 0.3191 g of TBOT, 6.1 g of IPA, 5.48 g of TPAOH, and 12.2 mL of deionized water to the flask and stir for 2 hours. Subsequently, mix the two solutions and stir for 2 hours to obtain a gel with a composition of 1TEOS:0.0125TBOT:0.01InCl3:0.01SnCl2:0.2TPAOH:20H2O. Transfer the gel to a stainless steel autoclave with a PTFE liner and crystallize at 170 °C for 3 days. Thoroughly wash the solid sample with deionized water, dry at 70 °C, and calcine at 550 °C for 6 hours to obtain In-Sn-TS-1-1%-1%.

[0045] Taking In-Sn-TS-2-1%-1% as an example, its synthesis steps are as follows. Dissolve 0.1659 g of InCl3 and 0.1422 g of SnCl2 in 14.8 mL of deionized water and stir evenly. Then, add 15.63 g of TEOS and 8.57 g of TBAOH to the above solution and stir for 5 hours. Next, add 0.3191 g of TBOT, 6.1 g of IPA, 6.91 g of TBAOH, and 12.2 mL of deionized water to the flask and stir for 2 hours. Subsequently, mix the two solutions and stir for 2 hours to obtain a gel with a composition of 1TEOS:0.0125TBOT:0.01InCl3:0.01SnCl2:0.2TBAOH:20H2O. Transfer the gel to a stainless steel autoclave with a PTFE liner and crystallize at 170 °C for 3 days. Thoroughly wash the solid sample with deionized water, dry at 70 °C, and calcine at 550 °C for 6 hours to obtain In-Sn-TS-2-1%-1%.

[0046] In this invention, GC-FID is used for quantitative analysis. The specific analysis conditions are as follows: The chromatographic column is an Agilent HP-5 capillary column (30 m × 0.32 mm × 0.25 μm), injection temperature: 250 °C; injection volume: 1 μL; FID detection temperature: 320 °C; programmed temperature rise: maintain at 40 °C for 2 minutes, then rise at a rate of 5 °C / min to 100 °C, and then rise at 20 °C / min to 280 °C and maintain for 2 minutes.

[0047] The molar yield of methyl lactate in this invention is based on the molar yield of biomass sugar. The calculation formula is as follows:

[0048]

[0049] Effect of Process Conditions in Example 1 on the Yield of Methyl Lactate Prepared by the Catalytic Conversion of Biomass Sugar with Metal-Loaded Titanium Silicate Molecular Sieve in Near-Critical Methanol

[0050] According to the process flow as Figure 1 shown, 300 mL of methanol, biomass sugar, and metal-loaded titanium silicate molecular sieve catalyst were successively added to a 500 mL high-temperature and high-pressure reactor with stirring. Stirring was started, and the temperature was heated to 140 - 220 °C, and the reaction time was 0.5 - 36 h. After the reaction, it was cooled to room temperature, filtered, and the filtrate (the molar yield of methyl lactate was obtained by GC analysis and calculation after sampling) was rectified to obtain the methyl lactate product, and the methanol was recycled. The filter residue was the used metal-loaded titanium silicate molecular sieve, which was directly recycled after being washed with methanol and dried. The experimental results under different conditions are shown in Table 1 below.

[0051] Table 1

[0052]

[0053]

[0054]

[0055] Note: Experimental No. 68 is a reference experiment with titanium silicate molecular sieve TS-1 as the catalyst, the same as Experimental No. 1 in Example 1 of the invention patent (application number: 202310152278X).

[0056] Example 2 Reusability Test of Metal-Loaded Titanium Silicate Molecular Sieve

[0057] According to the process flow as Figure 1 shown, a 500 mL high-temperature and high-pressure reactor with stirring was used to conduct a catalyst reusability experiment on the used metal-loaded titanium silicate molecular sieve in Experimental No. 1 of Example 1. The post-treatment process of the used metal-loaded titanium silicate molecular sieve: after the reaction, it was cooled to room temperature, filtered, and the filter residue was the used metal-loaded titanium silicate molecular sieve, which was repeatedly washed 3 times with methanol (90 mL each time), placed in an oven at 60 °C and dried, and then directly used for the next reuse experiment. The experimental results are shown in Table 2 below.

[0058] Table 2

[0059]

[0060]

[0061] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for preparing methyl lactate by catalytic reaction of biomass sugar with metal-loaded titanium silicate molecular sieve in near-critical methanol, characterized in that, The steps of the method are as follows: 1) Add biomass sugar and methanol into a high-temperature and high-pressure reactor. The mass concentration of the biomass sugar is 10-200 g / L, and then add the metal-loaded titanium silicalite catalyst In-TS-1. The mass ratio of the biomass sugar to the metal-loaded titanium silicalite is 5:1-1:3; 2) Heat up to 140-220 °C and react for 0.5-36 h; 3) After the reaction is completed, cool to room temperature, filter. The filtrate is rectified to obtain methyl lactate product, and the methanol is recycled; the filter residue is the used metal-loaded titanium silicalite, which is directly recycled after being washed with methanol and dried.

2. The method for preparing methyl lactate by catalytic conversion of biomass sugar with metal-loaded titanium silicalite molecular sieve in near-critical methanol according to claim 1, wherein The biomass sugar described in step 1) is one or a mixture of more than one of fructose, glucose, xylose, sucrose, lactose, maltose, cellobiose, dextran, inulin, xylan.

3. The method for preparing methyl lactate by catalytic conversion of biomass sugar with metal-loaded titanium silicalite molecular sieve in near-critical methanol according to claim 1, wherein The mass concentration of the biomass sugar described in step 1) is 20-150 g / L.

4. A method for preparing methyl lactate by catalytic conversion of biomass sugars with metal-loaded titanium silicalite molecular sieve in near-critical methanol according to claim 1, characterized in that In the metal-loaded titanium silicalite described in step 1), the mass ratio of the total metal atoms to the carrier is 0.2-3%.

5. A method for preparing methyl lactate by catalytic conversion of biomass sugar with metal-loaded titanium silicate molecular sieve in near-critical methanol according to claim 1, characterized in that The mass ratio of the biomass sugar to the metal-loaded titanium silicalite described in step 1) is 2:1-1:2.

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