Method for improving selectivity of light aromatic hydrocarbon in biomass product catalyzed and pyrolyzed by zeolite molecular sieve catalyst

Abstract

The invention belongs to the technical field of biomass energy utilization, and particularly relates to a method for improving the selectivity of light aromatic hydrocarbon in a biomass product through catalytic pyrolysis of a zeolite molecular sieve catalyst. The content of medium-strong acid sites in the zeolite molecular sieve catalyst is found to have relatively strong positive correlation with the selectivity of light aromatic hydrocarbons prepared during catalytic pyrolysis of biomass, so that when the zeolite molecular sieve catalyst is used for catalytic pyrolysis of biomass, the content of the medium-strong acid sites in the zeolite molecular sieve catalyst can be increased by increasing the proportion of the medium-strong acid sites in the zeolite molecular sieve catalyst; the selectivity of light aromatic hydrocarbon in the catalytic pyrolysis product is improved.

Description

Technical Field

[0001] This application belongs to the field of biomass energy utilization technology, and more specifically, relates to a method for improving the selectivity of light aromatic hydrocarbons in the biomass products catalytically pyrolyzed by zeolite molecular sieve catalysts. Background Technology

[0002] Biomass, as the only renewable carbon-containing resource, is crucial for achieving carbon neutrality. Developing cost-effective, environmentally friendly, and efficient biomass conversion technologies is essential for the comprehensive utilization of waste biomass resources and the sustainable development of the biorefining industry. Pyrolysis, due to its high conversion efficiency and economic advantages, is considered a highly promising approach, rapidly decomposing biomass and its derivatives (such as lignin) into liquid fuels and high-value chemicals. However, the resulting bio-oils typically contain a high proportion of oxygenated compounds, resulting in low calorific value, high viscosity, poor stability, and corrosiveness, limiting their direct application in industrial processes. Therefore, developing efficient catalytic upgrading strategies to reduce the oxygen content of bio-oils and improve the selectivity of light aromatics has become a key research focus.

[0003] In recent years, molecular sieve catalysts have demonstrated unique advantages in the catalytic upgrading of biomass pyrolysis products due to their ordered microporous framework structure, tunable acidity, and excellent thermal stability. They can promote key reactions such as cracking, deoxygenation, aromatization, and alkylation, thereby significantly increasing the yield of light aromatics. Existing methods for preparing high-value products from biomass pyrolysis using molecular sieve catalysis include Chinese invention patent CN116970415A, "A method for rapidly preparing aromatics from biomass cotton stalks after granulation with ZSM-5 molecular sieves." This method uses cotton stalks as raw material, which are crushed, impurity removed, and dried for later use. The stalks are then mixed with ZSM-5 molecular sieves in a certain proportion and granulated. The resulting mixed particles are pyrolyzed at 350-600 °C under a nitrogen atmosphere. The pyrolysis vapor is condensed to obtain pyrolysis oil and non-condensable gas. This method effectively improves the pyrolysis efficiency of cotton stalks and significantly increases the aromatic content in the pyrolysis oil through a simple physical granulation process. However, this method still has certain limitations in improving the selectivity of light aromatic products. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of this application is to provide a method for improving the selectivity of zeolite molecular sieve catalysts in the catalytic pyrolysis of biomass to produce light aromatics, aiming to solve the technical problems of low deoxygenation efficiency and poor selectivity of light aromatics obtained by molecular sieve catalysts in the catalytic pyrolysis of biomass in the prior art.

[0005] To achieve the above objectives, in a first aspect, this application provides a method for improving the selectivity of light aromatics in the biomass products catalytically decomposed by a zeolite molecular sieve catalyst. When using a zeolite molecular sieve catalyst to catalyze the pyrolysis of biomass, the selectivity of light aromatics in the catalytic pyrolysis products is improved by increasing the proportion of medium-strong acid sites in the zeolite molecular sieve catalyst.

[0006] Preferably, the proportion of medium-strong acid sites in the zeolite molecular sieve catalyst is defined as the proportion of the amount of medium-strong acid sites desorbed in the ammonia-programmed temperature desorption test results to the total acid content of the catalyst, wherein the amount of medium-strong acid sites desorbed is the amount of NH3 desorbed in the temperature range of 250~400℃ calculated by peak area.

[0007] More preferably, the molecular sieve catalyst contains at least 15% medium-strong acid sites, a total acid content of at least 500 µmol / g, and a mesopore volume of at least 0.2 cm³. 3 / g.

[0008] Preferably, the zeolite molecular sieve is ZSM-5, ZSM-11, or β-zeolite.

[0009] Preferably, in preparing the molecular sieve catalyst, the mesoporous structure, total acid content, and proportion of medium-strong acid sites of the molecular sieve catalyst are controlled and improved by comprehensively adjusting one or more of the following: silicon-aluminum ratio, template agent type, template agent dosage, type of supported metal, and amount of supported metal.

[0010] More preferably, the method for increasing the proportion of medium-strong acid sites in the molecular sieve catalyst includes the following steps; (1) Prepare a silica-alumina gel solution containing silicon source, aluminum source and alkali source, and adjust the molar ratio of silicon source to aluminum source to preset the total acid content of molecular sieve; control the silica-alumina ratio in the range of 20-30 to ensure that the molecular sieve catalyst has a suitable and sufficient total acid content; (2) Add a template agent to the silica-alumina gel solution described in step (1), stir, and crystallize under hydrothermal conditions; use its structure-guiding effect to regulate the distribution of aluminum atoms in the framework, thereby introducing mesopores and achieving preliminary regulation of acid strength distribution; after crystallization, after solid-liquid separation, the separated solid phase is washed, dried, ground, and calcined to remove the template agent to obtain a mesoporous molecular sieve; the amount of template agent added is 5-15 wt% of the mass of SiO2 in the silicon source, so as to avoid the formation of large clusters due to the enhanced intermolecular interaction caused by excessive template agent addition, which would affect the regulation of mesoporous structure and acid strength distribution; (3) The obtained molecular sieve catalyst was used to catalyze the pyrolysis of biomass to prepare light aromatics.

[0011] More preferably, between steps (2) and (3) there is an additional step: loading at least one metal selected from Group IIIA, Group VIII or Group IIB onto the mesoporous molecular sieve of step (2) to obtain a molecular sieve catalyst, wherein the loading amount of the metal is 1-3 wt%, to avoid the reduction of the total acid content of the catalyst, pore blockage and the decrease of the proportion of medium and strong acid sites caused by excessive loading.

[0012] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art: (1) During the experiment, the present invention accidentally discovered that the content of medium-strong acid in zeolite molecular sieve catalyst has a strong positive correlation with the selectivity of light aromatics prepared by catalytic pyrolysis of biomass. Based on this, a method to improve the selectivity of light aromatics prepared by catalytic pyrolysis of biomass using molecular sieve catalyst is proposed, which improves the selectivity of light aromatics by increasing the proportion of medium-strong acid in molecular sieve catalyst.

[0013] (2) The method for improving the selectivity of light aromatics proposed in this invention is a synergistic regulation strategy. By comprehensively adjusting one or more of the following: the silica-alumina ratio, the type and amount of template agent, and the type and loading of metal, precise control of the mesoporous structure, total acid content, and proportion of medium-strong acid sites of the molecular sieve is achieved. In the preferred embodiment, the total acid content of the molecular sieve is preset by adjusting the silica-alumina ratio, and green and renewable template agents such as starch, cellulose, and sucrose are introduced to construct the mesoporous structure and initially guide the acid strength distribution. Then, the proportion of medium-strong acid sites is finely optimized to be greater than or equal to 20% by an appropriate amount of metal loading, thereby obtaining a metal-loaded mesoporous molecular sieve catalyst with controllable medium-strong acid distribution. This catalyst exhibits excellent performance in biomass catalytic pyrolysis, not only significantly promoting deoxygenation and aromatization reactions and improving the selectivity and yield of light aromatics, but also effectively inhibiting carbon deposition and extending catalyst life.

[0014] (3) The application of green template agent in the preferred embodiment of the present invention not only reduces the cost of molecular sieve synthesis, but also improves the sustainability and economy of the method, showing broad application prospects. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the process of adjusting the proportion of medium-strong acid sites in the molecular sieve catalyst and using it to improve the selectivity of light aromatics in the catalytic pyrolysis of biomass, according to an embodiment of this application.

[0016] Figure 2 The images show X-ray diffraction (XRD) patterns of mesoporous ZSM-5 catalysts with different silicon-to-aluminum ratios prepared in Examples 1 to 5 of this application.

[0017] Figure 3This is a graph showing the relationship between the proportion of medium-strong acid sites in the mesoporous ZSM-5 catalysts prepared in Examples 9 to 14 of this application and the selectivity of light aromatics in the lignin catalytic pyrolysis products under their action. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0019] During the experiment, the applicant accidentally discovered that the content of medium-strong acid sites in the zeolite molecular sieve catalyst showed a strong positive correlation with the selectivity of light aromatics prepared during the catalytic pyrolysis of biomass. Therefore, this invention provides a method to improve the selectivity of light aromatics in the catalytic pyrolysis products of biomass using a zeolite molecular sieve catalyst. Specifically, when using a zeolite molecular sieve catalyst to catalyze the pyrolysis of biomass, the selectivity of light aromatics in the catalytic pyrolysis products is improved by increasing the proportion of medium-strong acid sites in the zeolite molecular sieve catalyst.

[0020] The proportion of medium-strong acid sites in the zeolite molecular sieve catalyst of the present invention is defined as the proportion of the amount of medium-strong acid sites desorbed in the total acid content (desorption amount) of the catalyst in the ammonia-programmed temperature desorption (NH3-TPD) test results. The amount of medium-strong acid sites desorbed is the amount of NH3 desorbed in the temperature range of 250~400℃ calculated by peak area.

[0021] In a preferred embodiment, the proportion of medium-strong acid sites in the molecular sieve catalyst is greater than or equal to 15%, the total acid content is greater than or equal to 500 µmol / g, and the mesopore volume is greater than or equal to 0.2 cm³. 3 The catalytic pyrolysis products exhibit high selectivity for light aromatics. More preferably, the molecular sieve catalyst contains at least 20% medium-strong acid sites, has a total acid content of at least 900 µmol / g, and a mesopore volume of at least 0.3 cm³. 3 / g.

[0022] The zeolite molecular sieves of the present invention include, but are not limited to, ZSM-5, ZSM-11 or β zeolite; preferably, the zeolite molecular sieves are metal-supported ZSM-5, ZSM-11 or β zeolite, etc., and the metal is preferably one or more of Zn, Ni, Pt and Ga.

[0023] The biomass described in this invention includes, but is not limited to, trees, straw, fruit shells, algae, cellulose, lignin, etc.

[0024] In some embodiments, in order to increase the proportion of medium-strong acid sites in the zeolite molecular sieve catalyst, the molecular sieve catalyst can be prepared by comprehensively adjusting one or more of the following: the silica-alumina ratio, the type of template agent, the amount of template agent, the type of metal supported, and the amount of metal supported. This can achieve the regulation and improvement of the mesoporous structure, total acid content, and proportion of medium-strong acid sites in the molecular sieve catalyst.

[0025] In some embodiments, methods for increasing the proportion of medium-strong acid sites in the molecular sieve catalyst, such as Figure 1 As shown, it includes the following steps; (1) Prepare a silica-alumina gel solution containing a silicon source, an aluminum source, and an alkaline source, and adjust the molar ratio (silicon-alumina ratio) of the silicon source to the aluminum source to preset the total acid content of the molecular sieve; the silicon-alumina ratio is in the range of 20-30, preferably 20-25, to ensure that the molecular sieve catalyst has a suitable and sufficient total acid content; (2) Add template agent to the silica-alumina gel solution in step (1), stir and crystallize under hydrothermal conditions; use its structure guiding effect to regulate the distribution of aluminum atoms in the framework, so as to achieve preliminary regulation of acid strength distribution while introducing mesopores; after crystallization, after solid-liquid separation, the separated solid phase is washed, dried, ground and calcined to remove template agent to obtain mesoporous molecular sieve; the amount of template agent added is 5-15 wt% of the mass of SiO2 in the silicon source, preferably 8-12 wt%, to avoid the formation of large clusters due to enhanced intermolecular interaction caused by excessive amount of template agent, which would affect the regulation of mesoporous structure and acid strength distribution; (3) The obtained molecular sieve catalyst was used to catalyze the pyrolysis of biomass to prepare light aromatics.

[0026] In a preferred embodiment, between steps (2) and (3) there is an additional step: loading at least one metal selected from Group IIIA, Group VIII or Group IIB onto the mesoporous molecular sieve of step (2) to obtain a molecular sieve catalyst, wherein the loading amount of the metal is 1-3 wt%, to avoid the reduction of the total acid content of the catalyst, pore blockage and reduction of the proportion of medium and strong acid sites caused by excessive loading, and then using the obtained molecular sieve catalyst for catalytic pyrolysis of biomass to prepare light aromatics.

[0027] This invention achieves the directional construction of mesoporous structure and medium-strong acid sites of molecular sieve catalysts by precisely controlling the type and amount of loaded metals and by the synergistic regulatory effects of steps (1) and (2).

[0028] In some embodiments, the template agent is a green organic template agent; the green organic template agent is one or more of soluble starch, cellulose, and sucrose. The template agent, while introducing mesopores, also provides preliminary regulation of the acidity distribution. Soluble starch refers to a type of starch that can dissolve rapidly in water to form a transparent or translucent solution, and can be any conventional soluble starch available on the market. Preferably, the green organic template agent is a soluble starch, which can achieve a high total acidity while also having a high mesopore volume.

[0029] In some embodiments, the supported metal is one or more of Zn, Ni, Pt, and Ga, which can form electron-deficient Lewis acid sites in the molecular sieve catalyst, thereby enhancing Lewis acidity and simultaneously modulating Brenstein acidity, and has been found to optimize the proportion of medium-strong acid sites in the molecular sieve catalyst.

[0030] In some embodiments, the metal is loaded using an over-impregnation method or an ion exchange method.

[0031] In a preferred embodiment, the silicon source in step (1) can be one or more of tetraethyl orthosilicate, silica sol, and water glass; the aluminum source can be one or more of aluminum sulfate, sodium aluminate, and aluminum nitrate; and the alkali source can be one or more of sodium hydroxide and potassium hydroxide. The hydrothermal crystallization in step (2) specifically involves a hydrothermal reaction at 150-180 °C for 24-72 h; the calcination temperature is 500-600 °C, and the calcination time is 4-6 h.

[0032] In a preferred embodiment, the impregnation method specifically involves adding the prepared molecular sieve to an ethanol solution containing the desired metal salt, stirring thoroughly, and impregnating at a constant temperature for 6-12 hours. After drying, it is then calcined at 400-550 °C for 2-5 hours to achieve loading of the active component. The ion exchange method specifically involves placing the molecular sieve in a metal salt solution, stirring at 60-90 °C for 2-6 hours, undergoing multiple exchanges, washing, and drying, and then calcining at 450-550 °C for 3-5 hours to achieve partial or complete cation exchange.

[0033] The embodiments of the present invention are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally in accordance with conventional conditions.

[0034] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0035] The embodiments of this application are described below with reference to the accompanying drawings.

[0036] Example 1 Step 1): Dissolve 0.68 g of sodium aluminate in 45 ml of deionized water, add 20.3 g of tetrapropylammonium hydroxide (TPAOH) solution to form an aluminate solution, and stir at 600 rpm until a clear aluminate solution is formed. Then, using a separatory funnel, slowly add 26 g of tetraethyl orthosilicate (TEOS) dropwise to the above solution, ensuring thorough mixing, and continue stirring at 600 rpm for 5 h to obtain a homogeneous and transparent silica-alumina gel solution. The SiO2 / Al2O3 molar ratio in the silica-alumina gel solution is 30.

[0037] Step 2): 5 wt% (0.374 g) of soluble starch (Sinopharm Chemical Reagent Co., Ltd., catalog number 10021328) was added to the gel solution as a template agent. After stirring for 5 h, the solution was placed in a hydrothermal reactor and then crystallized in a constant temperature drying oven at 180 ℃ for 48 h. After crystallization, solid-liquid separation was performed using a centrifuge, and the solid product was washed with deionized water until the filtrate was neutral. The solid product was dried in a 70 ℃ oven, then ground to obtain a uniform powder, and calcined in a muffle furnace at 550 ℃ in air atmosphere for 4 h to remove the template agent, yielding a mesoporous ZSM-5 molecular sieve with a preliminary acid strength distribution, denoted as ZSM-ST(30, 5).

[0038] Example 2 Except for the SiO2 / Al2O3 molar ratio of 25 in step 1), the other steps are the same as in Example 1, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(25, 5).

[0039] Example 3 Except for the SiO2 / Al2O3 molar ratio of 20 in step 1), the other steps are the same as in Example 1, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(20, 5).

[0040] Example 4 Except for the SiO2 / Al2O3 molar ratio of 35 in step 1), the other steps are the same as in Example 1, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(35, 5).

[0041] Example 5 Except for the SiO2 / Al2O3 molar ratio of 40 in step 1), the other steps are the same as in Example 1, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(40, 5).

[0042] The pore structure characteristics of mesoporous ZSM-5 molecular sieves were characterized using a fully automated gas adsorption analyzer; their acidity characteristics were quantitatively analyzed using NH3-TPD. Table 1 summarizes the mesopore volume and total acidity of mesoporous ZSM-5 molecular sieves with different silica-to-alumina ratios. The table shows that the ZSM-5 molecular sieve with a silica-to-alumina ratio of 20 has the highest total acidity. Experiments revealed that further reducing the silica-to-alumina ratio significantly weakens the framework stability of the molecular sieve, making it prone to structural collapse under high temperature and water vapor conditions. Furthermore, when the silica-to-alumina ratio exceeds 30, the decrease in total acidity of the molecular sieve is too large, leading to reduced activity.

[0043] Table 1. Mesopore volume and total acidity of ZSM-5 molecular sieves with different silica-to-alumina ratios

[0044] Figure 2 The XRD patterns of ZSM-5 catalysts with different silica-alumina ratios are shown. It can be seen that the ZSM-ST catalyst synthesized hydrothermally with the addition of soluble starch, like ZSM-5(Con), exhibits typical ZSM-5 molecular sieve diffraction peaks at 2θ = 7.96°, 8.83°, 23.18°, 23.99°, and 24.45°. ZSM-5(Con) is commercially available ZSM-5 purchased from the catalyst factory of Nankai University.

[0045] Example 6 Except for step 2), where the amount of soluble starch added is 10 wt%, the other steps are the same as in Example 3.

[0046] Example 7 Except for step 2), where the template agent is cellulose and the addition amount is 10 wt%, the other steps are the same as in Example 6.

[0047] Example 8 Except for step 2), where the template agent is sucrose and the amount added is 10 wt%, the other steps are the same as in Example 6.

[0048] Table 2 summarizes the mesopore volume and total acidity of mesoporous ZSM-5 molecular sieves synthesized with different template agents. As can be seen from the table, the mesoporous ZSM-5 molecular sieve synthesized using soluble starch as a template agent has the largest mesopore volume.

[0049] Table 2. Mesopore volume and total acidity of ZSM-5 molecular sieves synthesized with different template agents

[0050] Example 9 Steps 1) and 2) are the same as in Example 3.

[0051] Step 3): The obtained mesoporous ZSM-ST(20, 5) molecular sieve was applied to the catalytic pyrolysis of lignin. Specifically, 0.3 mg of lignin and 1.2 mg of catalyst were mixed evenly and placed in a pyrolyzer. The mixture was heated to 600 °C at a heating rate of 20000 °C / s under a nitrogen atmosphere and held for 30 s. The product was analyzed by gas chromatography-mass spectrometry (GC-MS).

[0052] Example 10 Except for step 2), where the starch addition amount is 10 wt%, the other steps are the same as in Example 9, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(20, 10).

[0053] Example 11 Except for step 2), where the starch addition amount is 15 wt%, the other steps are the same as in Example 9, and the resulting ZSM-5 molecular sieve is denoted as ZSM-ST(20, 15).

[0054] In NH3-TPD analysis, the temperature of the desorption peak indicates the acid strength, while the area of ​​the desorption peak indicates the acid content. Low-temperature desorption peaks (<250℃) correspond to weak acid sites, medium-temperature desorption peaks (250-400℃) correspond to medium-strong acid sites, and high-temperature desorption peaks (>400℃) correspond to strong acid sites. The proportion of medium-strong acid sites is defined as the percentage of desorption from medium-strong acid sites to the total acid content of the catalyst, and the desorption from medium-strong acid sites is the area corresponding to the medium-temperature desorption peak.

[0055] The selectivity of light aromatic hydrocarbons is defined as the percentage of the peak area of ​​benzene, toluene, and xylene in GC-MS analysis relative to the total peak area of ​​all products.

[0056] Table 3 summarizes the mesopore volume, acidity characterization, and light aromatic hydrocarbon selectivity of mesoporous ZSM-5 molecular sieves with different starch addition amounts. As can be seen from the table, the ZSM-ST(20, 10) catalyst exhibits the highest total acidity and the highest proportion of medium-strong acid sites, as well as the highest light aromatic hydrocarbon selectivity.

[0057] Table 3. Mesopore volume, acidity characterization, and light aromatic hydrocarbon selectivity of ZSM-5 molecular sieves with different starch additions.

[0058] Example 12 Steps 1) and 2) are the same as in Example 10.

[0059] Step 3): Using the excess impregnation method, 0.455 g of Zn(NO3)2∙6H2O was placed in 100 ml of anhydrous ethanol, followed by the addition of 10 g of ZSM-ST(20, 10) support, resulting in a zinc loading of 1 wt%. The mixture was stirred continuously at 70 ℃ until a homogeneous paste-like mixture was formed. The mixture was then dried in an oven at 105 ℃ for 24 h. The dried catalyst was then removed and thoroughly ground using an agate mortar until a uniform powder with a particle diameter of less than 60 μm was obtained. Subsequently, it was calcined in a muffle furnace at 550 ℃ in air for 4 h. Zinc-supported mesoporous ZSM-5 molecular sieve was obtained, denoted as 1%Zn / ZSM-ST(20, 10).

[0060] Step 4): The obtained 1% Zn / ZSM-ST(20, 10) was applied to the catalytic pyrolysis of lignin. Specifically, 0.3 mg of lignin and 1.2 mg of catalyst were mixed evenly and placed in a pyrolyzer. The mixture was heated to 600 °C at a heating rate of 20000 °C / s under a nitrogen atmosphere and held for 30 s. The product was analyzed by gas chromatography-mass spectrometry (GC-MS).

[0061] Example 13 Except for step 3), where the zinc loading is 2 wt%, the other steps are the same as in Example 12. The resulting zinc-loaded mesoporous ZSM-5 molecular sieve is denoted as 2%Zn / ZSM-ST(20, 10).

[0062] Example 14 Except for step 3), where the zinc loading is 3 wt%, the other steps are the same as in Example 12. The resulting zinc-loaded mesoporous ZSM-5 molecular sieve is denoted as 3%Zn / ZSM-ST(20, 10).

[0063] Table 4 summarizes the mesopore volume, acidity characterization, and light aromatic hydrocarbon selectivity of ZSM-ST(20, 10) molecular sieves with different zinc loadings. As can be seen from the table, the 2%Zn / ZSM-ST(20, 10) catalyst exhibits the highest total acidity and the highest proportion of medium-strong acid sites, as well as the highest selectivity for light aromatic hydrocarbons.

[0064] Table 4. Mesopore volume, acidity characterization, and light aromatic selectivity of ZSM-5 molecular sieves with different zinc loadings.

[0065] Figure 3The relationship between the proportion of medium-strong acid sites in the ZSM-5 molecular sieves prepared in Examples 9-14 and the selectivity of light aromatics in the catalytic pyrolysis products of lignin is presented, showing a strong positive correlation between the two. Therefore, the selectivity of light aromatics in the catalytic pyrolysis products can be improved by controlling the proportion of medium-strong acid sites in the molecular sieve catalyst.

[0066] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for improving the selectivity of light aromatics in biomass products catalytically decomposed by zeolite molecular sieve catalysts, characterized in that, When using zeolite molecular sieve catalysts for catalytic pyrolysis of biomass, the selectivity of light aromatics in the catalytic pyrolysis products can be improved by increasing the proportion of medium-strong acid sites in the zeolite molecular sieve catalyst.

2. The method as described in claim 1, characterized in that, The proportion of medium-strong acid sites in the zeolite molecular sieve catalyst is defined as the proportion of the amount of medium-strong acid sites desorbed in the total acid content of the catalyst in the ammonia-programmed temperature desorption test results, wherein the amount of medium-strong acid sites desorbed is the amount of NH3 desorbed in the temperature range of 250~400℃ calculated by peak area.

3. The method as described in claim 1, characterized in that, The molecular sieve catalyst contains at least 15% medium-strong acid sites, a total acid content of at least 500 µmol / g, and a mesopore volume of at least 0.2 cm³. 3 / g.

4. The method as described in claim 1, characterized in that, The zeolite molecular sieve is ZSM-5, ZSM-11, or β-zeolite.

5. The method as described in claim 1, characterized in that, In preparing the molecular sieve catalyst, the mesoporous structure, total acid content, and proportion of medium-strong acid sites of the molecular sieve catalyst can be controlled and improved by comprehensively adjusting one or more of the following: silicon-aluminum ratio, template agent type, template agent dosage, type of supported metal, and amount of supported metal.

6. The method as described in claim 5, characterized in that, A method for increasing the proportion of medium-strong acid sites in the molecular sieve catalyst includes the following steps; (1) Prepare a silica-alumina gel solution containing silicon source, aluminum source and alkali source, and adjust the molar ratio of silicon source to aluminum source to the preset total acid content of molecular sieve; control the silica-alumina ratio in the range of 20-30; (2) Add a template agent to the silica-alumina gel solution described in step (1), stir, and then crystallize under hydrothermal conditions; utilize its structure-guiding effect to regulate the distribution of aluminum atoms in the framework, thereby introducing mesopores while initially regulating the acid strength distribution; after crystallization, perform solid-liquid separation, wash, dry, grind, and calcine to remove the template agent to obtain a mesoporous molecular sieve; the amount of template agent added is 5-15 wt% of the mass of SiO2 in the silicon source. (3) The obtained molecular sieve catalyst was used to catalyze the pyrolysis of biomass to prepare light aromatics.

7. The method as described in claim 6, characterized in that, The template agent in step (2) is a green organic template agent, which is one or more of soluble starch, cellulose, and sucrose; The crystallization under hydrothermal conditions in step (2) specifically involves a hydrothermal reaction at 150-180 ℃ for 24-72 h; the calcination temperature is 500-600 ℃ and the calcination time is 4-6 h.

8. The method as described in claim 6, characterized in that, The steps between steps (2) and (3) also include: The mesoporous molecular sieve described in step (2) is loaded with at least one metal selected from Group IIIA, Group VIII or Group IIB to obtain a molecular sieve catalyst, wherein the loading amount of the metal is 1-3 wt.

9. The method as described in claim 8, characterized in that, The loaded metal is one or more of Zn, Ni, Pt, and Ga.

10. The method as described in claim 8, characterized in that, The loading method for the metal is either over-impregnation or ion exchange.

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