Catalyst for catalyzing high-concentration cellulose into ethanol and preparation method thereof

Abstract

The invention discloses a catalyst for catalyzing high-concentration cellulose into ethanol and a preparation method of the catalyst. The method comprises the following steps: (1) firstly, adding a carbon source, water-soluble nickel salt and silicon dioxide into deionized water for hydrothermal reaction; then performing high-temperature carbonization treatment on a product of the hydrothermal reaction in an inert atmosphere, and after the reaction, performing alkali washing, filtering and drying to obtain a crude catalyst body; (2) dipping the catalyst crude body in a platinum salt solution, and after dipping, filtering and drying to obtain a catalyst precursor; and (3) carrying out high-temperature reduction treatment on the catalyst precursor in an H2 atmosphere, cooling, and collecting to obtain the catalyst. The catalyst can directly convert high-concentration cellulose into ethanol in a pure water environment, the target product selectivity is high, and the catalyst still has good catalytic activity and target product selectivity after being recycled for multiple times.

Description

Technical Field

[0001] This invention relates to a catalyst for catalyzing the conversion of high-concentration cellulose into ethanol, and also to a method for preparing the above-mentioned catalyst. Background Technology

[0002] Bioethanol is a liquid fuel produced through biochemical conversion and distillation processes using polysaccharides from plant-based raw materials as precursors. Its high octane rating allows it to be blended with traditional gasoline. With the iterative upgrading of production technology, the raw material system has gradually expanded from primarily grain crops such as corn to non-grain resources such as agricultural waste (e.g., rice husks, crop stalks).

[0003] Currently, the mainstream technologies for bioethanol production include two main directions: bio-fermentation and syngas reforming. Bio-fermentation relies on enzyme catalysis to achieve polysaccharide hydrolysis and monosaccharide fermentation conversion, but it suffers from bottlenecks such as long reaction cycles, high enzyme preparation costs, and low carbon conversion efficiency. Syngas reforming, on the other hand, converts all components of lignocellulose into carbon-containing gases (such as CO and H2) through thermochemical gasification, and then produces ethanol through catalytic recombination. Although this route can improve the utilization rate of raw materials, it faces challenges such as difficulties in gas separation and purification and complex process chains.

[0004] In recent years, cellulose has been found to be catalytically hydrogenated into bioethanol in an aqueous medium. This process involves a series of reactions including hydrolysis, reverse aldol, and hydrogenation. The reverse aldol reaction directly acts on the glycosyl units generated from cellulose depolymerization, producing low-carbon intermediates, which are then converted into the target product via hydrogenation. Compared to traditional processes, this route offers potential advantages such as simplified procedures and high economic efficiency. However, aqueous hydrogenolysis technology is still in its early stages of development, facing challenges such as unclear catalytic mechanisms and poor synergy in multi-step reaction kinetics leading to low product selectivity, especially when processing high-concentration cellulose. Summary of the Invention

[0005] Objective of the invention: The objective of this invention is to provide a catalyst with high catalytic activity for the aqueous hydrogenolysis of cellulose to ethanol, which can not only tolerate high concentrations of cellulose, but also significantly improve the selectivity of the target product; another objective of this invention is to provide a method for preparing the above catalyst.

[0006] Technical solution: The preparation method of the catalyst according to the present invention includes the following steps: (1) First, carbon source, water-soluble nickel salt and silicon dioxide are added to deionized water for hydrothermal reaction; then the product of hydrothermal reaction is subjected to high-temperature carbonization in an inert atmosphere. After the reaction, the product is washed with alkali, filtered and dried to obtain crude catalyst. (2) The crude catalyst is impregnated in a platinum salt solution. After thorough impregnation, it is filtered and dried to obtain the catalyst precursor. (3) The catalyst precursor was subjected to high-temperature reduction treatment in H2 atmosphere, and collected after cooling to obtain the catalyst.

[0007] In step (1), the carbon source is one of glucose, fructose, or citric acid; the nickel salt is one of nickel nitrate, nickel acetate, or nickel chloride; and the silica has a particle size of 5-10 μm. The mass ratio of the carbon source, nickel salt, and silica is 1:2.5-3:1; the hydrothermal reaction temperature is 180-240℃, and the reaction time is 16-18 h. This invention uses silica as a hard template. The carbon source is carbonized during the hydrothermal process and coated onto the silica surface to form a carbon layer, while nickel metal particles are encapsulated inside the carbon layer. The inert atmosphere is N2 or Ar; the high-temperature carbonization treatment temperature is 800-900℃, the heating rate is no higher than 10℃ / min, and the holding time is 0.5-2 h. The alkaline washing solution is a 3-5M sodium hydroxide solution; the alkaline washing temperature is 80℃, and the alkaline washing time is 22-24 h. High-temperature carbonization forms a porous carbon layer structure on the surface of carbon microspheres (i.e., the shell of the carbon microspheres is porous); silica is removed by alkaline washing, thereby forming carbon microspheres with a cavity structure, which enhances the mass transfer process of the catalyst in aqueous phase reaction.

[0008] In step (2), the concentration of the platinum salt solution is 3-5 mmol, and the impregnation time is 12-14 h. The impregnation temperature is 65 °C, which allows the metal in the solution to be fully loaded. During the impregnation stage, a small amount of platinum ions are loaded on the surface of the carbon layer, forming bimetallic active sites together with nickel metal, thereby enhancing the hydrogenolysis ability of the catalyst.

[0009] In step (3), the high-temperature reduction process is carried out at a temperature of 400-450°C, with a heating rate not exceeding 5°C / min and a holding time of 2-3 hours. Step (3) reduces the nickel and noble metal particles loaded on the surface of the catalyst to their elemental state, thereby enhancing their catalytic activity in the hydrogenolysis of cellulose to ethanol in an aqueous medium.

[0010] The catalyst prepared by the above method includes carbon microspheres with a cavity structure, wherein the carbon layer on the surface of the carbon microspheres has a porous structure, and Ni nanoparticles and Pt nanoparticles are uniformly loaded on the carbon microspheres.

[0011] The carbon microspheres have a particle size of 5-20 μm and include an internal cavity and a porous surface structure. Ni nanoparticles and Pt nanoparticles are uniformly dispersed on the surface of the carbon layer. The loading of Ni nanoparticles is 15-16 wt% and the particle size is 10-50 nm. The loading of Pt nanoparticles is 1-1.5 wt% and the particle size is 2-10 nm. Both Ni nanoparticles and Pt nanoparticles are in elemental valence state.

[0012] The catalytic mechanism of the catalyst of this invention for the aqueous hydrogenolysis of cellulose to produce ethanol is as follows: cellulose is hydrolyzed into glucose under subcritical conditions, and ethanol is directly formed through reverse aldol condensation, selective hydrogenation, and dehydroxylation. In this process, the active sites are on the surface of the carbon layer of the catalyst, activated by electrons transferred from the inside to the outside of the carbon layer by nickel particles, mainly undergoing hydrogenation reaction, while the noble metal Pt can further enhance the hydrogenolysis ability of the catalyst. In addition, thanks to the internal and external structure brought about by the cavity, the intermediate products formed by glucose on the surface of the carbon layer can easily diffuse into the interior through the mesopores, thereby re-exposing the active sites on the outside of the carbon layer and improving the overall reaction kinetic rate. Therefore, it can still have good cellulose conversion and ethanol yield even under high concentrations of cellulose.

[0013] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The catalyst of the present invention can directly convert high concentration cellulose into ethanol in a pure water environment, with high selectivity for the target product, and still has good catalytic activity and target product selectivity after multiple cycles. Attached Figure Description

[0014] Figure 1 SEM image of the catalyst (Pt-Ni@C) prepared in Example 1; Figure 2 TEM image of the catalyst (Pt-Ni@C) prepared in Example 1; Figure 3 Mapping diagram of the catalyst (Pt-Ni@C) prepared in Example 1; Figure 4 XPS image of the catalyst (Pt-Ni@C) prepared in Example 1; Figure 5 This is a process flow diagram of the preparation method of the present invention; Figure 6 This is a mechanism diagram of the application of the catalyst of this invention in the catalytic aqueous hydrogenolysis of cellulose to prepare ethanol. Detailed Implementation

[0015] Example 1 The method for preparing the catalyst of the present invention includes the following steps: (1) Add 3g glucose, 0.03mol nickel nitrate (about 8.72g) and 3g silicon dioxide with a diameter of 5μm to 50mL deionized water, stir thoroughly and transfer to a high temperature and high pressure reactor, seal and raise to 180°C, keep warm for 16h, then filter, place the filtered solid in a tube furnace, introduce high purity N2 into the tube furnace to maintain an inert atmosphere, raise the temperature to 800°C at a heating rate of 5°C / min, keep warm for 2h; after the sample cools naturally to room temperature, take it out and place it in 5M NaOH solution for alkaline washing at 80°C for 24h, filter and dry to obtain crude catalyst; (2) The crude catalyst was uniformly dispersed in a 3 mmol platinum chloride solution and impregnated for 12 h. The impregnation temperature was 65°C. After impregnation, the catalyst was filtered and dried to obtain the catalyst precursor. (3) Place the crude catalyst in a tube furnace and introduce high-purity H2 into the tube furnace for reduction reaction. Heat the catalyst to 450°C at a heating rate of 5°C / min and react for 2 hours. After the reaction, collect the sample to obtain the catalyst (Pt-Ni@C).

[0016] The catalyst prepared in Example 1 was applied to the aqueous hydrogenolysis of cellulose to prepare ethanol. Specifically, 0.15 g of catalyst (Pt-Ni@C), 4 g of cellulose (microcrystalline cellulose), and 40 mL of deionized water were added to a high-pressure reactor. The reactor was sealed and the internal air was purged. Hydrogen gas was introduced at 5.5 MPa, and the temperature was slowly raised to 220°C and maintained for 12 h. During this period, constant stirring was maintained to ensure uniform heating and reaction. After the reaction was completed, the reaction solution was filtered and tested. The conversion rate of cellulose was 99.9%, and the carbon molar yield of the obtained ethanol was 82.1%.

[0017] pass Figure 1 and 2 It can be seen that the catalyst (Pt-Ni@C) prepared in Example 1 is a carbon microsphere with a cavity structure. The surface of the carbon microsphere has a porous structure, and Pt nanoparticles and Ni nanoparticles are uniformly dispersed on the carbon microsphere. Its specific surface area is 343 m². 2 Approximately / g.

[0018] pass Figure 3 It can be seen that in the catalyst (Pt-Ni@C) prepared in Example 1, both Pt nanoparticles and Ni nanoparticles exist in an elemental state, and the loading of Pt nanoparticles is much less than that of Ni nanoparticles. In the catalyst (Pt-Ni@C) prepared in Example 1, the loading of Pt is 1.1 wt% and the loading of Ni is 15.7 wt%.

[0019] pass Figure 6It is known that the overall catalytic process can be carried out in both the inner and outer layers of the porous carbon layer structure. After the macromolecular cellulose is hydrolyzed and reverse aldol condensed on the carbon layer surface to form small molecule ethylene glycol, it can diffuse through the pore size to the internal active sites for subsequent reactions, thereby re-exposing the external active sites and improving the overall reaction kinetic rate and catalytic efficiency. Therefore, the catalyst of this invention still has good cellulose conversion rate and ethanol yield even at high cellulose concentrations.

[0020] Based on the reaction conditions of Example 1, Examples 2-3 and Comparative Examples 1-14 changed a certain process parameter in the preparation of the catalyst, while keeping other preparation conditions unchanged, to obtain the catalysts of Examples 2-3 and Comparative Examples 1-14. The catalysts of Examples 2-3 and Comparative Examples 1-14 were applied to the process of preparing ethanol by aqueous hydrogenolysis of cellulose under the same reaction conditions as in Example 1. The corresponding cellulose conversion rate and ethanol carbon molar yield are shown in Table 1.

[0021] Table 1

[0022] As shown in Table 1, if the silica particle size or the amount added is too large, it may cause uneven carbon layer coating and collapse, thus affecting the catalytic effect. The essence of silica is actually to act as a cluster, providing sites for the carbon source in the hydrothermal process, generating hydrothermal carbon on the surface of the cluster, and then forming porous carbon in pyrolysis, and finally washing away the cluster with alkali. If the particle size is too small, the carbon layer may be too thick, and it may not be possible to completely wash away the silica with alkali. If the particle size is too large, the carbon layer coating will be uneven. If the calcination temperature is too high, it is also not conducive to the catalytic process, but if it is too low, it will not be able to ensure sufficient carbonization of the porous carbon layer. Pt plays a role in the hydrogenolysis of cellulose. If the Pt content is too low, it cannot effectively promote the hydrogenolysis of ethylene glycol to ethanol. If the Pt content is too high, due to its strong hydrogenation ability, it can easily replace the dominant position of Ni and carry out excessive hydrogenation, producing by-products. Therefore, in the catalytic system, the content of Ni is the main one, and Pt is a trace dopant. If the reduction time is too short or the reduction temperature is too low, Pt and Ni metals will not be fully reduced, and their catalytic effect will be weakened.

[0023] The catalysts of Comparative Examples 15-18 were used in the same reaction conditions as those in Example 1 for the aqueous hydrogenolysis of cellulose to produce ethanol. The corresponding cellulose conversion and ethanol carbon molar yield are shown in Table 2.

[0024] Table 2

[0025] Table 2 shows that the purpose of alkaline washing is to dissolve and remove silica from between porous carbon layers, forming a hollow carbon microsphere structure. Without alkaline washing, the number of accessible catalytic interfaces in the catalytic system decreases, leading to a decline in catalytic performance. The purpose of reduction treatment is to reduce Ni and Pt to their elemental states under H2 atmosphere; without reduction treatment, the number of active sites decreases, resulting in a decline in catalytic performance. Comparative Example 17 shows that the addition of trace amounts of Pt significantly enhances the selectivity of Ni@C for the target product. Pt / C exhibits catalytic activity for cellulose conversion but lacks selectivity for the target product.

[0026] The catalyst (Pt-Ni@C) prepared in Example 1 was applied to the aqueous hydrogenolysis of cellulose to prepare ethanol. After the first reaction, it was recovered and reused. The cellulose conversion rate and ethanol carbon molar yield after each cycle are shown in Table 3.

[0027] Table 3 shows the cycle stability results of the catalyst (Pt-Ni@C) prepared in Example 1.

[0028] As shown in Table 3, the Pt-Ni@C catalyst prepared in Example 1 exhibits excellent cycle stability. This is mainly because the active sites on the catalyst surface are synergistically activated by Pt and Ni metals on the carbon layer surface, and the carbon layer can effectively protect the metal particles from being damaged or lost during this process. In addition, the cavity structure provides more reaction interfaces for the catalytic system and improves mass transfer efficiency, further avoiding the aggregation and blockage of reaction intermediates, thereby maintaining good catalytic activity and cycle stability.

[0029] Referring to Example 1, after the first reaction of Comparative Example 15, the ethanol was recovered and used for subsequent catalytic applications, yielding an ethanol yield of 54.2%. After 8 cycles, the ethanol yield decreased to 37.8%. The above results indicate that the catalyst without alkali washing treatment only undergoes catalytic reaction on the outside of the carbon microspheres. Although the active metal is not lost, the active sites are limited and easily blocked, resulting in a decrease in catalytic activity.

[0030] The catalyst prepared in Example 1 was applied to the aqueous hydrogenolysis of cellulose to prepare ethanol. Specifically, 0.15 g of catalyst (Pt-Ni@C), 6 g of cellulose, and 40 mL of deionized water were added to a high-pressure reactor. The reactor was sealed and the internal air was purged. Hydrogen gas at 5.5 MPa was introduced, and the temperature was slowly raised to 220°C and maintained for 12 h. During this period, constant stirring was maintained to ensure uniform heating and reaction. After the reaction was completed, the reaction solution was filtered and tested. The conversion rate of cellulose was 84.7%, and the carbon molar yield of the obtained ethanol was 50.2%.

[0031] The catalyst of this invention can directly convert cellulose into ethanol in a one-pot process in a pure water environment, and can tolerate high concentrations of cellulose without affecting its catalytic activity. It has high selectivity for the target product and still has good catalytic activity and target selectivity after multiple cycles.

Claims

1. A method for preparing a catalyst for catalyzing the conversion of high-concentration cellulose into ethanol, characterized in that, Includes the following steps: (1) First, carbon source, water-soluble nickel salt and silicon dioxide are added to deionized water for hydrothermal reaction; then the product of hydrothermal reaction is subjected to high-temperature carbonization in an inert atmosphere. After the reaction, the product is washed with alkali, filtered and dried to obtain crude catalyst. (2) The crude catalyst was impregnated in a platinum salt solution. After impregnation, it was filtered and dried to obtain the catalyst precursor. (3) The catalyst precursor was subjected to high-temperature reduction treatment in H2 atmosphere, and collected after cooling to obtain the catalyst.

2. The preparation method according to claim 1, characterized in that: In step (1), the carbon source is one of glucose, fructose or citric acid; the nickel salt is one of nickel nitrate, nickel acetate or nickel chloride; and the particle size of the silica is 5~10μm.

3. The preparation method according to claim 1, characterized in that: In step (1), the mass ratio of the carbon source, nickel salt and silicon dioxide is 1:2.5~3:

1.

4. The preparation method according to claim 1, characterized in that: In step (1), the temperature of the hydrothermal reaction is 180~240℃ and the reaction time is 16~18h.

5. The preparation method according to claim 1, characterized in that: In step (1), the high-temperature carbonization process is carried out at a temperature of 800~900℃, the heating rate is not higher than 10℃ / min, and the holding time is 0.5~2h.

6. The preparation method according to claim 1, characterized in that: In step (1), the alkaline washing solution used is a 3-5M sodium hydroxide solution; the alkaline washing time is 22-24h.

7. The preparation method according to claim 1, characterized in that: In step (2), the concentration of the platinum salt solution is 3-5 mmol, and the immersion time is 12-14 h.

8. The preparation method according to claim 1, characterized in that: In step (3), the high-temperature reduction process is performed at a temperature of 400-450°C, with a heating rate of no more than 5°C / min and a holding time of 2-3 hours.

9. The catalyst prepared by the method according to claim 1, characterized in that: It includes carbon microspheres with a cavity structure, the shell of the carbon microspheres having a porous structure, and the carbon layer of the carbon microspheres being loaded with Ni nanoparticles and Pt nanoparticles.

10. The catalyst according to claim 9, characterized in that: The carbon microspheres have a Ni nanoparticle loading of 15-16 wt% and a particle size of 10-50 nm; and a Pt nanoparticle loading of 1-1.5 wt% and a particle size of 2-10 nm.

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