Cascade catalytic system and method for producing mononuclear aromatic hydrocarbon through biomass pyrolysis

Abstract

The invention provides a cascade catalytic system and method for producing mononuclear aromatic hydrocarbon through biomass pyrolysis, and belongs to the technical field of biomass catalytic conversion. The cascade catalytic system is composed of two catalyst components with synergistic functions, wherein a first-stage catalyst is a composite metal oxide ZnMOx and is used for performing efficient deoxidation and preliminary cracking on biomass pyrolysis volatile components; the second-stage catalyst is a ZSM-5 molecular sieve loaded with active metal and is used for carrying out deep shape-selective aromatization on the preliminarily converted intermediate product to generate the monocyclic aromatic hydrocarbon. The excellent deoxidation performance of the composite metal oxide and the unique shape-selective aromatization performance of the metal modified ZSM-5 molecular sieve are subjected to cascade coupling, and the problems that a single catalyst system is low in selectivity and prone to carbon deposition and deactivation are solved. By adopting the method disclosed by the invention, the selectivity of the monocyclic aromatic hydrocarbon in the liquid-phase product can be obviously improved, meanwhile, the carbon deposition amount of the catalyst is low, and high-value utilization of biomass resources is realized.

Description

Technical Field

[0001] This invention relates to the field of biomass catalytic conversion technology, specifically to a cascade catalytic system and method for producing monocyclic aromatic hydrocarbons from biomass pyrolysis. Background Technology

[0002] Monocyclic aromatic hydrocarbons (BRAs), especially benzene, toluene, ethylbenzene, and xylene (collectively known as BTX), are among the most important basic raw materials and platform compounds in modern chemical industry, with significant market demand. They are primarily derived from the catalytic reforming of petroleum and the steam cracking of naphtha, heavily reliant on non-renewable petroleum resources. With the global pursuit of carbon neutrality and sustainable development, developing technologies for the direct preparation of BRAs from biomass is of great significance, and catalytic rapid pyrolysis has been recognized as one of the most promising key technologies for achieving this goal. However, in the process of promoting this technology to industrial applications, especially at the level of catalyst design and application, traditional methods face a series of inherent and urgent technical bottlenecks: First, while single shape-selective catalysts, such as ZSM-5 molecular sieves, can promote aromatization reactions due to their unique acidic sites and pore structure, their functional limitations are significant. On the one hand, their excessive acidity, while promoting deoxygenation, easily triggers excessive cracking of the feedstock, leading to a decrease in the yield of the target product (bio-oil) and the generation of large amounts of non-target gases and coke, resulting in a serious waste of carbon resources. On the other hand, the microporous structure of ZSM-5 (approximately 0.55 nm) significantly restricts the mass transfer of large oxygen-containing compounds (such as lignin-derived phenols) produced by biomass pyrolysis, making it difficult for these key precursors to enter the active centers inside the pores for effective conversion. This is one of the fundamental reasons for severe catalyst coking and rapid decline in activity.

[0003] Secondly, if conventional metal oxides (such as Al2O3, TiO2, ZrO2, etc.) are used as catalysts, although they can promote certain deoxygenation or cracking steps to some extent, they exhibit very low selectivity for the formation of monocyclic aromatic hydrocarbons due to their lack of precise shape selectivity. This results in a wide product distribution, making it difficult to achieve efficient and targeted synthesis of the target product, and subsequent separation costs are extremely high.

[0004] In summary, the root of the problem lies in the extreme complexity of the biomass pyrolysis products themselves. The initial volatiles from pyrolysis are a mixture containing hundreds of components, with high oxygen content and a wide range of molecular sizes. This complexity not only places stringent demands on the activity and selectivity of the catalyst, but also severely affects the overall reaction efficiency due to problems such as diffusion obstruction. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a cascade catalytic system for the production of monocyclic aromatic hydrocarbons by biomass pyrolysis. The cascade catalytic system provided by this invention can significantly improve the selectivity of monocyclic aromatic hydrocarbons in liquid products, reduce the amount of carbon deposited on the catalyst, and realize the high-value utilization of biomass resources.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a cascade catalytic system for the production of monocyclic aromatic hydrocarbons from biomass pyrolysis, comprising a first-stage catalyst and a second-stage catalyst; The first-stage catalyst is a composite metal oxide with the general chemical formula ZnMO. x M is one or more of Zr, Ga, In, Cr, Mo, Ce and Al, and the value of x is such that the oxidation state is conserved. The second-stage catalyst is a ZSM-5 molecular sieve supported with an active metal, wherein the active metal includes one or more of Ga, Zn, Fe, Mo and Zr.

[0007] Preferably, in the first-stage catalyst, the molar ratio of Zn to all M metals is 1:10 to 10:1.

[0008] Preferably, the method for preparing the composite metal oxide includes the following steps: A soluble zinc salt, a soluble M metal salt, an alkaline precipitant, and water are mixed and a precipitation reaction is carried out to obtain the precipitation reaction product. The precipitated product was calcined to obtain a composite metal oxide.

[0009] Preferably, in the second-stage catalyst, the mass of the active metal is 0.5-8% of the mass of the ZSM-5 molecular sieve; The silicon-aluminum ratio of the ZSM-5 molecular sieve is 20~800.

[0010] Preferably, the mass ratio of the first-stage catalyst to the second-stage catalyst is 1:10 to 10:1.

[0011] Preferably, the first-stage catalyst and the second-stage catalyst are combined in the reactor by layered loading or physical mixing.

[0012] This invention provides a method for producing monocyclic aromatic hydrocarbons by biomass pyrolysis, comprising the following steps: Biomass feedstock is pyrolyzed to obtain biomass pyrolysis gas; The biomass pyrolysis gas enters a catalytic reaction zone containing a cascade catalytic system to undergo a catalytic conversion reaction, yielding a product containing monocyclic aromatic hydrocarbons.

[0013] Preferably, the biomass raw material is lignocellulosic biomass; the pyrolysis temperature is 400~650℃.

[0014] Preferably, the temperature of the catalytic conversion reaction is 500~650℃; the mass ratio of biomass feedstock to all catalysts in the cascade catalytic system is 1:1~10.

[0015] Preferably, the monocyclic aromatic hydrocarbon includes one or more of benzene, toluene, ethylbenzene, and xylene; The product containing monocyclic aromatic hydrocarbons has a mass content of ≥60% of monocyclic aromatic hydrocarbons.

[0016] This invention provides a cascade catalytic system for the production of monocyclic aromatic hydrocarbons from biomass pyrolysis, comprising a first-stage catalyst and a second-stage catalyst; the first-stage catalyst is a composite metal oxide with the general chemical formula ZnMO. x M is one or more of Zr, Ga, In, Cr, Mo, Ce, and Al, and the value of x ensures the conservation of valence; the second-stage catalyst is a ZSM-5 molecular sieve supported on an active metal, wherein the active metal includes one or more of Ga, Zn, Fe, Mo, and Zr. Compared with the prior art, the beneficial effects of the cascade catalytic system of this invention are as follows: ① Functional Zoning and Synergistic Effect: This invention employs a bifunctional catalytic system, achieving a rational division of labor in the reaction process. Oxygen-containing compounds first undergo efficient deoxygenation and preliminary cracking at the basic sites and surface defects of the first-stage catalyst, transforming into easily aromatized intermediates. These intermediates then migrate to the acidic sites of the second-stage catalyst for precise shape-selective aromatization. The first-stage catalyst creates a more ideal reaction environment for the second-stage catalyst and provides high-quality reaction precursors. Their synergistic effect significantly enhances the directed synthesis capability and final selectivity of monocyclic aromatic hydrocarbons.

[0017] ② Suppressing side reactions and improving stability: The composite metal oxide used in the first-stage catalyst of this invention, due to its unique surface defects and tunable acid-base properties, becomes a key active center for suppressing the formation of coke precursors and polycyclic aromatic hydrocarbons. By intercepting and converting large molecular coke precursors upstream, the downstream ZSM-5 molecular sieve loaded with active metals is effectively protected, significantly reducing the overall carbon deposition rate and gaseous product yield of the system, thereby greatly improving the catalyst's operational stability and service life.

[0018] The results of the examples show that the cascade catalytic system of the present invention can achieve a total selectivity of ≥80% for aromatics, wherein the proportion of aromatics in the liquid phase products is ≥60%, and the proportion of monocyclic aromatics is ≥75%, and the carbon deposition of the catalyst is ≤4%, which extends the overall service life of the catalytic system and provides an effective way for the high-value utilization of biomass resources. Attached Figure Description

[0019] Figure 1 To prepare the ZnZrO obtained in Example 1 x XRD pattern; Figure 2 To prepare the ZnO and ZnZrO obtained in Example 1 x Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of ZrO2; Figure 3 ZnZrO x TEM and high-resolution TEM (HR-TEM) characterization results of composite oxides. Detailed Implementation

[0020] This invention provides a cascade catalytic system for the production of monocyclic aromatic hydrocarbons from biomass pyrolysis, comprising a first-stage catalyst and a second-stage catalyst; The first-stage catalyst is a composite metal oxide with the general chemical formula ZnMO. x M is one or more of Zr, Ga, In, Cr, Mo, Ce and Al, and the value of x is such that the oxidation state is conserved. The second-stage catalyst is a ZSM-5 molecular sieve supported with an active metal, wherein the active metal includes one or more of Ga, Zn, Fe, Mo and Zr.

[0021] In this invention, the first-stage catalyst is a composite metal oxide, the general chemical formula of which is ZnMO. x M is one or more of zirconium (Zr), gallium (Ga), indium (In), chromium (Cr), molybdenum (Mo), cerium (Ce) and aluminum (Al). When M is a single metal element, it is preferably Mo or Fe; when M is multiple metal elements, it is preferably a combination of Mo and Fe.

[0022] In this invention, the molar ratio of Zn to all M metals is preferably 1:10 to 10:1, more preferably 1:10, 1:8, 1:5, 1:3, 1:1, 3:1, 5:1, 8:1, or 10:1. In this invention, the particle size of the composite metal oxide is preferably 40-60 mesh. In this invention, the first-stage catalyst is responsible for the efficient preliminary cracking and deoxygenation reactions of the initial volatiles from biomass pyrolysis, such as decarboxylation and decarbonylation, converting complex macromolecular oxygen-containing compounds into smaller molecule intermediates that are more easily aromatized. The oxygen vacancies on their surface are key active sites for achieving efficient deoxygenation.

[0023] In this invention, the method for preparing the composite metal oxide preferably includes the following steps: A soluble zinc salt, a soluble M metal salt, an alkaline precipitant, and water are mixed and subjected to a precipitation reaction to obtain a precipitation product; the precipitation product is then calcined to obtain a composite metal oxide.

[0024] In this invention, the soluble zinc salt is preferably Zn(NO3)2·6H2O, and the soluble M metal salt is preferably a nitrate of M metal, specifically preferably one or more of zirconium oxynitrate pentahydrate, gallium nitrate hydrate, indium nitrate hydrate, chromium nitrate nonahydrate, ammonium heptamolybdate, cerium nitrate hexahydrate, and aluminum nitrate nonahydrate. In this invention, the molar ratio of the soluble zinc salt to the soluble M metal salt is determined based on the molar ratio of Zn to M metal in the composite metal oxide.

[0025] In this invention, the alkaline precipitant is preferably (NH4)2CO3, and is preferably added in the form of an aqueous solution. The concentration of the aqueous solution of the alkaline precipitant is preferably 0.4~1 mol / L, more preferably 0.6~0.8 mol / L. In this invention, the amount of alkaline precipitant added is adjusted to control the pH of the precipitation reaction solution to 9.5~10. In this invention, the precipitation reaction is preferably carried out under stirring conditions, the temperature of the precipitation reaction is preferably 70℃, and the time is preferably 3 hours. After the precipitation reaction, the product is preferably allowed to stand for aging, and the standing aging time is preferably 20 hours. In this invention, the obtained precipitation reaction product is preferably subjected to solid-liquid separation, washing, and drying. The solid-liquid separation is preferably filtration or centrifugation, the washing is preferably washing to neutrality, and the drying is preferably freeze-drying at a temperature preferably -40℃.

[0026] In this invention, the calcination atmosphere is preferably air, the calcination temperature is preferably 450~850℃, more preferably 500~600℃, the calcination time is preferably 4~5h, and the heating rate to the calcination temperature is preferably 10℃ / min. After calcination, the resulting solid product is preferably tableted, crushed, and sieved, with the sieve mesh size being 40~60 mesh.

[0027] In this invention, the active metal in the ZSM-5 molecular sieve supported on the second-stage catalyst includes one or more of Ga, Zn, Fe, Mo, and Zr, preferably a combination of Fe, Ga, or Zn. In this invention, the ZSM-5 molecular sieve is an acidic zeolite molecular sieve with an MFI topology. In this invention, the silica-to-alumina ratio (molar ratio of silica to alumina) of the ZSM-5 molecular sieve is preferably 20-800, more preferably 20-600, and even more preferably 20-300, specifically 27, 36, 46, 85, 200, or 300. In this invention, the molecular sieves with the above-preferred silica-to-alumina ratios exhibit superior monocyclic aromatic hydrocarbon selectivity and lower carbon deposition rates during long-term reactions.

[0028] In this invention, the active metal in the second-stage catalyst is preferably 0.5-8% of the mass of the ZSM-5 molecular sieve, more preferably 1-6%, and even more preferably 2-4%. In this invention, the particle size of the ZSM-5 molecular sieve loaded with the active metal is preferably 40-60 mesh. In this invention, the introduction of the active metal optimizes the acidic sites of the molecular sieve. The second-stage catalyst, utilizing its regular microporous structure and Brønsted / Lønsted acid synergistic sites, precisely synthesizes monocyclic aromatic hydrocarbons from the intermediate products of the first-stage catalyst through Diels-Alder condensation, oligomerization, cyclization, and aromatization reactions, while suppressing the formation of polycyclic aromatic hydrocarbons.

[0029] In this invention, the preparation method of the ZSM-5 molecular sieve loaded with active metal preferably includes the following steps: A precursor solution was obtained by ultrasonic dispersion and stirring of an aqueous solution of a soluble active metal salt and HZSM-5 molecular sieve. The precursor solution was dried and calcined to obtain ZSM-5 molecular sieve loaded with active metal.

[0030] In this invention, the molar concentration of the soluble active metal salt aqueous solution is preferably 0~0.1 mol / L, more preferably 0.02~0.08 mol / L, and even more preferably 0.05 mol / L. In this invention, the ultrasonic dispersion frequency is preferably 40 kHz, and the time is preferably 30 min. In this invention, the stirring treatment temperature is preferably 40℃~80℃, more preferably 60℃, and the time is preferably 10~20 h, more preferably 12~18 h. In this invention, the drying temperature is preferably 80~120℃, more preferably 100℃, and the time is preferably 8~20 h, more preferably 10~15 h; the calcination atmosphere is preferably air, the calcination temperature is preferably 400~800℃, more preferably 400~600℃, and the time is preferably 4~8 h, more preferably 5~6 h.

[0031] In this invention, the mass ratio of the first-stage catalyst to the second-stage catalyst is preferably 1:10 to 10:1, more preferably 1:10, 1:8, 1:5, 1:3, 1:1, 3:1, 5:1, 8:1, or 10:1. In this invention, the first-stage catalyst and the second-stage catalyst are preferably combined in the reactor by layered loading or physical mixing. In this invention, when the first-stage catalyst and the second-stage catalyst are layered, the first-stage catalyst is located upstream of the biomass pyrolysis gas and the second-stage catalyst is located downstream of the biomass pyrolysis gas. In this invention, when the first-stage catalyst and the second-stage catalyst are physically mixed, the physical mixing is preferably performed by first forming and granulating the first-stage catalyst and the second-stage catalyst separately, and then mechanically mixing them; or preferably by first mechanically mixing the raw materials of the first-stage catalyst and the second-stage catalyst, and then forming and granulating them. In this invention, after the first-stage catalyst and the second-stage catalyst are physically mixed, the particle size of the resulting mixed catalyst is preferably 20 to 80 mesh, more preferably 40 to 60 mesh.

[0032] This invention provides a method for producing monocyclic aromatic hydrocarbons by biomass pyrolysis, comprising the following steps: Biomass feedstock is pyrolyzed to obtain biomass pyrolysis gas; The biomass pyrolysis gas enters a catalytic reaction zone containing a cascade catalytic system to undergo a catalytic conversion reaction, yielding a product containing monocyclic aromatic hydrocarbons.

[0033] This invention involves pyrolyzing biomass raw materials to obtain biomass pyrolysis gas. In this invention, the biomass raw material is preferably lignocellulosic biomass, more preferably one or more of crop straw, cotton, sawdust, rice husks, and leaves. Specifically, the crop straw can be corn straw or wheat straw. In this invention, the pyrolysis atmosphere is preferably argon, the pyrolysis temperature is preferably 500-650℃, more preferably 550-600℃, and the heating rate to the pyrolysis temperature is preferably 10℃ / min. In this invention, the main components of the biomass pyrolysis gas preferably include one or more of CO2, CO, CH4, and H2.

[0034] In this invention, the biomass pyrolysis gas enters a catalytic reaction zone containing a cascade catalytic system to undergo a catalytic conversion reaction, yielding a product containing monocyclic aromatic hydrocarbons. Preferably, the biomass pyrolysis gas enters the catalytic reaction zone containing the cascade catalytic system through an inert protective atmosphere, preferably nitrogen or Ar. The flow rate of the inert protective atmosphere can be adjusted according to the space velocity requirements, specifically 30~90 mL / min.

[0035] In this invention, the temperature of the catalytic conversion reaction is preferably 400-650℃, more preferably 500-600℃, and the heating rate to the catalytic conversion reaction is preferably 10℃ / min. In this invention, the mass ratio of the biomass feedstock to all catalysts in the cascade catalytic system is preferably 1:1-10, more preferably 1:2-8, and even more preferably 1:5. In this invention, the catalytic conversion reaction is preferably carried out under atmospheric pressure or slightly positive pressure conditions, and the pressure of the slightly positive pressure is preferably 0-0.1 MPa.

[0036] In this invention, the pyrolysis and catalytic conversion reactions are preferably carried out in the same reactor, which is preferably a fixed bed or a fluidized bed.

[0037] In this invention, the monocyclic aromatic hydrocarbon preferably includes one or more of benzene, toluene, ethylbenzene and xylene; the mass content of the monocyclic aromatic hydrocarbon in the product containing the monocyclic aromatic hydrocarbon is preferably ≥75%.

[0038] The following detailed description of the cascade catalytic system and method for producing monocyclic aromatic hydrocarbons from biomass pyrolysis, provided by the present invention, is provided in conjunction with embodiments. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0039] Preparation Example 1: First-stage catalyst (ZnZrO) x Preparation of ) A co-precipitation method was used. 10.55 g of zinc nitrate (Zn(NO3)2·6H2O) and 12.89 g of zirconium oxynitrate (ZrO(NO3)2·xH2O) were dissolved in 200 mL of deionized water at a molar ratio of 1:1 to form a mixed salt solution. The solution was vigorously stirred at 70 °C, and 1.0 M ammonium carbonate solution was added dropwise to adjust the pH to 9.5 before stopping the addition. The solution was aged at 70 °C for 3 hours, then filtered, washed, and dried at 105 °C for 12 hours. Finally, the obtained solid was calcined at 600 °C for 5 hours to obtain ZnZrO. x The composite oxide is compressed, crushed, and screened to 40-60 mesh for later use.

[0040] Obtained ZnZrO x XRD pattern as shown Figure 1 As shown, Figure 1 ZrO2 and ZnZrO are clearly shown. x Phase and crystallization characteristics of the composite oxide and ZnO: Pure ZrO2 corresponds to the characteristic diffraction peaks of both the tetragonal phase (t-ZrO2, such as the (101) crystal plane peak) and the monoclinic phase (m-ZrO2, such as the (-111) and (200) crystal plane peaks) of the #80-0784 phase; the diffraction peaks of pure ZnO match the standard card #99-0111, with sharp crystal plane peaks such as (100), (002), and (101), indicating high crystallinity. ZnZrOx The diffraction peaks of the composite oxide correspond only to the #80-0784 standard card of t-ZrO2 (mainly the (101) crystal plane peak), and no characteristic diffraction peaks of ZnO are observed. Moreover, the intensity of its diffraction peaks is weaker than that of pure ZrO2, indicating that Zn element is present in ZnZrO2. x The ZrO2 exists in a highly dispersed or amorphous form, which makes the composite oxide mainly exhibit the tetragonal ZrO2 phase, with a lower crystallinity compared to the pure ZrO2 phase.

[0041] By omitting the addition of zirconium oxynitrate and carrying out a precipitation reaction under the same conditions, ZnO is obtained.

[0042] By omitting the addition of zinc nitrate and carrying out the precipitation reaction under the same conditions, ZrO2 is obtained.

[0043] ZnO, ZnZrO x Scanning electron microscope (SEM) and transmission electron microscope (TEM) images of ZrO2 are as follows: Figure 2 As shown, Figure 2 The three images above represent ZnO and ZnZrO, respectively. x Scanning electron microscope (SEM) images of ZnO and ZrO2. The three images below are of ZnO and ZnZrO, respectively. x Transmission electron microscopy (TEM) images of ZrO2. (From...) Figure 2 It can be seen that pure-phase ZnO appears as large flocculent agglomerates under SEM, and TEM further shows that it consists of irregular blocky particles with a size of tens of nanometers, indicating a high degree of agglomeration; ZnZrO x The composite oxide exhibits a denser SEM morphology and finer particle size. TEM reveals a porous aggregate structure formed by loosely aggregated nano-sized particles, with particle size significantly smaller than that of pure ZnO. Pure ZrO2, on the other hand, shows relatively uniform, spherical aggregates on SEM, while on TEM it appears as a tightly packed state of fine particles. Although its particle size is smaller than ZnO, the density of its aggregate structure is higher than that of ZnZrO. x Overall, ZnZrO x The microstructure of the composite oxide exhibits both fine particle and loose porous characteristics, which are significantly different from the agglomeration morphology and particle size of the two pure phase oxides.

[0044] Figure 3 ZnZrO x TEM and high-resolution TEM (HR-TEM) characterization results of the composite oxides: Figure 3 The image in (a) is a TEM image (scale bar 50 nm), showing a loose and porous microstructure formed by the aggregation of nanoparticles. The selected area electron diffraction (SAED) pattern in the inset shows ring-shaped diffraction spots, indicating that the composite oxide has a polycrystalline structure, which matches the phase characteristics of tetragonal ZrO2 in the previous XRD. Figure 3(b) is an HRTEM image (scale bar 5 nm), clearly showing two sets of lattice fringes: the fringe with d = 0.311 nm corresponds to the (-111) crystal plane of ZrO2, and the fringe with d = 0.276 nm corresponds to the (100) crystal plane of ZnO. This indicates that ZnZrO x The composite oxide contains both ZrO2 and ZnO crystal phases. Combined with the previous XRD results showing that the characteristic peaks of ZnO were not obvious, this further confirms that ZnO exists in the composite oxide in the form of highly dispersed small crystallites.

[0045] Preparation Example 2: First-stage catalyst (ZnGaO) x Preparation of ) Following the method described in Example 1, 12.89 g of zirconium oxynitrate was replaced with 10.23 g of gallium nitrate (Ga(NO3)3·xH2O), and all other steps were the same, ultimately yielding ZnGaO. x The composite oxide is processed to a mesh size of 40-60 for later use.

[0046] Preparation Example 3: First-stage catalyst (ZnCeO) x Preparation of ) Zinc-cerium composite bimetallic oxide (ZnCeO) x The precursor was synthesized using a co-precipitation method. First, cerium nitrate hexahydrate (Ce(NO3)3·5H2O, 7.164 g) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 1.227 g) were dissolved in 100 mL of deionized water to prepare solution A. Then, ammonium carbonate ((NH4)2CO3) was dissolved in deionized water to prepare a precipitant solution B with a concentration of 0.413 M. Under continuous stirring, solutions A and B were simultaneously added dropwise to 30 mL of deionized water at a constant temperature of 70 °C, and the pH of the system was precisely adjusted to 10.0 ± 0.1. The resulting mixture was stirred continuously at 70 °C for 3 h, and then allowed to stand for 20 h. The precipitate was repeatedly washed with deionized water and ethanol, and centrifuged until the supernatant was neutral. The resulting solid was then freeze-dried. The obtained precursor was calcined in static air at 550 °C for 4 h to obtain a white powder. Finally, the product is successively ground, compressed, crushed, and sieved to obtain a particle size fraction of 40-60 mesh.

[0047] Preparation Example 4: First-stage catalyst (ZnMoO) x Preparation of ) Zinc-molybdenum composite bimetallic oxide (ZnMoO) x The synthesis was carried out by a coprecipitation method. First, ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O) was prepared. 24Solution A was prepared by dissolving 2.913 g of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 1.227 g) in 100 mL of deionized water. Subsequently, ammonium carbonate ((NH4)2CO3) was dissolved in deionized water to prepare a 0.413 M precipitant solution B. Under continuous stirring, solutions A and B were simultaneously added dropwise to 30 mL of deionized water at a constant temperature of 70 °C, and the pH of the system was precisely adjusted to 10.0 ± 0.1. The resulting mixture was stirred continuously at 70 °C for 3 h, and then allowed to stand for 20 h. The precipitate was repeatedly washed with deionized water and ethanol, and centrifuged until the supernatant was neutral. The solid obtained by centrifugation was then freeze-dried. The resulting precursor was calcined in static air at 550 °C for 4 h to obtain a white powder. Finally, the product was successively ground, tableted, crushed, and sieved to obtain a particle size fraction of 40-60 mesh.

[0048] Preparation Example 5: First-stage catalyst (ZnInO) x Preparation of ) Zinc-indium bimetallic oxide (ZnInO) x The precursor was synthesized using a co-precipitation method. First, indium nitrate hydrate (In(NO3)3·xH2O, 4.963 g) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 1.227 g) were dissolved in 100 mL of deionized water to prepare solution A. Then, ammonium carbonate ((NH4)2CO3) was dissolved in deionized water to prepare a precipitant solution B with a concentration of 0.413 M. Under continuous stirring, solutions A and B were simultaneously added dropwise to 30 mL of deionized water at a constant temperature of 70 °C, and the pH of the system was precisely adjusted to 10.0 ± 0.1. The resulting mixture was stirred continuously at 70 °C for 3 h, and then allowed to stand for 20 h. The precipitate was repeatedly washed with deionized water and ethanol, and centrifuged until the supernatant was neutral. The resulting solid was then freeze-dried. The obtained precursor was calcined in static air at 550 °C for 4 h to obtain a white powder. Finally, the product is successively ground, compressed, crushed, and sieved to obtain a particle size fraction of 40-60 mesh.

[0049] Preparation Example 6: First-stage catalyst (ZnCrO) x Preparation of ) Zinc-chromium composite bimetallic oxide (ZnCrO) xThe precursor was synthesized using a co-precipitation method. First, chromium nitrate nonahydrate (Cr(NO3)3·9H2O, 6.602 g) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 1.227 g) were dissolved in 100 mL of deionized water to prepare solution A. Then, ammonium carbonate ((NH4)2CO3) was dissolved in deionized water to prepare a precipitant solution B with a concentration of 0.413 M. Under continuous stirring, solutions A and B were simultaneously added dropwise to 30 mL of deionized water at a constant temperature of 70 °C, and the pH of the system was precisely adjusted to 10.0 ± 0.1. The resulting mixture was stirred continuously at 70 °C for 3 h, and then allowed to stand for 20 h. The precipitate was repeatedly washed with deionized water and ethanol, and centrifuged until the supernatant was neutral. The resulting solid was then freeze-dried. The obtained precursor was calcined in static air at 550 °C for 4 h to obtain a white powder. Finally, the product is successively ground, compressed, crushed, and sieved to obtain a particle size fraction of 40-60 mesh.

[0050] Preparation Example 7: First-stage catalyst (ZnAlO) x Preparation of ) Zinc-aluminum composite bimetallic oxide (ZnAlO) x The precursor was synthesized using a co-precipitation method. First, aluminum nitrate hydrate (Al(NO3)3·9H2O, 6.189 g) and zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 1.227 g) were dissolved in 100 mL of deionized water to prepare solution A. Then, ammonium carbonate ((NH4)2CO3) was dissolved in deionized water to prepare a precipitant solution B with a concentration of 0.413 M. Under continuous stirring, solutions A and B were simultaneously added dropwise to 30 mL of deionized water at a constant temperature of 70 °C, and the pH of the system was precisely adjusted to 10.0 ± 0.1. The resulting mixture was stirred continuously at 70 °C for 3 h, and then allowed to stand for 20 h. The precipitate was repeatedly washed with deionized water and ethanol, and centrifuged until the supernatant was neutral. The resulting solid was then freeze-dried. The obtained precursor was calcined in static air at 550 °C for 4 h to obtain a white powder. Finally, the product is successively ground, compressed, crushed, and sieved to obtain a particle size fraction of 40-60 mesh.

[0051] Preparation Example 8: Preparation of Second-Stage Catalyst (Fe / ZSM-5) Using an impregnation method, 1.62 g of ferric nitrate (Fe(NO3)3·9H2O) was dissolved in 50 mL of deionized water. 20 g of ZSM-5 molecular sieve (SiO2 / Al2O3=30) was added to the solution. After ultrasonic dispersion for 30 minutes, the mixture was stirred at 60 °C for 12 hours. The mixture was then dried at 105 °C for 12 hours and finally calcined at 550 °C for 4 hours to obtain an iron-loaded Fe / ZSM-5 catalyst with a Fe loading of 1%. The catalyst was then processed to a mesh size of 40–60 for later use.

[0052] Preparation Example 9: Preparation of Secondary Catalyst (Ga / ZSM-5) Using an impregnation method, 1.19 g of gallium nitrate (Ga(NO3)3·9H2O) was dissolved in 50 mL of deionized water. 20 g of ZSM-5 molecular sieve (SiO2 / Al2O3=30) was added to the solution, and the mixture was ultrasonically dispersed for 30 minutes, then stirred at 60 °C for 12 hours. The mixture was then dried at 105 °C for 12 hours, and finally calcined at 550 °C for 4 hours to obtain an iron-loaded Ga / ZSM-5 catalyst with a Ga loading of 1%, which was then processed to a mesh size of 40-60 for later use.

[0053] Preparation Example 10: Preparation of Second-Stage Catalyst (Mo / ZSM-5) Using the impregnation method, 0.37 g of ammonium molybdate (NH4)6Mo7O was taken. 24 • 4H₂O was dissolved in 50 mL of deionized water. 20 g of ZSM-5 molecular sieve (SiO₂ / Al₂O₃ = 30) was added to the above solution. After ultrasonic dispersion for 30 minutes, the mixture was stirred at 60 °C for 12 hours. Subsequently, the mixture was dried at 105 °C for 12 hours and finally calcined at 550 °C for 4 hours to obtain an iron-loaded Mo / ZSM-5 catalyst with a Mo loading of 1%. The catalyst was then processed to a mesh size of 40-60 for later use.

[0054] Example 1: Cascade catalytic pyrolysis of pine wood (fixed bed, layered packing) 2.0g ZnZrO x The catalyst and 2.0 g Fe / ZSM-5 catalyst were layered and packed in a fixed-bed reactor, ZnZrO x The catalyst was located upstream. Pine wood powder was dried at 60°C for later use. The catalyst bed was heated to 500°C under an argon atmosphere (10 mL / min). The catalyst was then subjected to a mass space velocity (MSV) of 4.0 h⁻¹. -1 0.6 g of pine wood powder was fed into a pyrolysis zone at 550 °C at a controlled rate. The resulting biomass pyrolysis gas entered a catalytic bed for catalytic conversion. The pyrolysis gas after the reaction was condensed at -45 °C using a condenser, and the liquid product was collected. The residual solid was biochar. Gas chromatography-mass spectrometry (GC-MS) was used for qualitative and quantitative analysis of the liquid products, and the selectivity and yield of monocyclic aromatic hydrocarbons were calculated.

[0055] Tests showed that the selectivity for monocyclic aromatic hydrocarbons (BTEX) in the organic liquid phase products reached 79.1%, the carbon yield was 27.5%, the biochar yield was 23.1%, and the carbon yield of catalyst carbon deposits was only 3.8% (relative to biomass feedstock).

[0056] Example 2: Cascade catalytic pyrolysis of corn stalks (fixed bed, physical mixing) 2.0g ZnGaO x The catalyst and 2.0 g of Fe / ZSM-5 catalyst were physically mixed and then packed into a fixed-bed reactor. 0.6 g of corn stalks were crushed to 60-80 mesh and dried at 60°C for later use. The reaction conditions were the same as in Example 1.

[0057] Tests showed that the selectivity for monocyclic aromatic hydrocarbons in the organic liquid products reached 75.8%, the carbon yield was 25.2%, the biochar yield was 24.5%, and the carbon yield from catalyst carbon deposits was 4.1%.

[0058] Example 3: Cascade catalytic pyrolysis of rice husks (fluidized bed) ZnZrO x The catalyst was mixed with Fe / ZSM-5 catalyst at a mass ratio of 1:1.5 and used as the bed material in the fluidized bed reactor. Rice husks were crushed to 20-80 mesh for later use. 4.0 g of catalyst and 0.6 g of rice husks were pyrolyzed in a circulating fluidized bed at a reaction temperature of 650 °C under a nitrogen atmosphere and a flow rate of 10 mL / min.

[0059] Tests showed that the yield of monocyclic aromatic hydrocarbons remained stable at around 26%, while the biochar yield was 22.8%. After 40 cycles, the catalyst showed no significant performance degradation, demonstrating excellent stability.

[0060] Comparative Example 1 Referring to Example 1, the difference is that only 4.0g of unmodified ZSM-5 molecular sieve (SiO2 / Al2O3=30) was packed into the reactor.

[0061] Tests showed that the selectivity for monocyclic aromatic hydrocarbons in the organic liquid products was only 48.3%, with a carbon yield of 16.5%. The carbon yield from catalyst coking was as high as 16.2%.

[0062] Comparative Example 2 Referring to Example 1, the difference is that only 4.0 g of the Fe / ZSM-5 catalyst prepared in Preparation Example 3 was loaded into the reactor.

[0063] Tests showed that the selectivity for monocyclic aromatic hydrocarbons in the organic liquid products was 56.7%, and the carbon yield was 19.8%. The carbon yield from catalyst coking was 13.5%.

[0064] Comparative Example 3 Referring to Example 1, the difference is that only 4.0 g of the ZnZrO prepared in Preparation Example 1 was loaded into the reactor. x catalyst.

[0065] Tests showed that almost no aromatics were detected in the liquid product; it mainly consisted of small-molecule oxygen-containing compounds, with an aromatic yield of less than 5%.

[0066] As can be seen from Example 1 and Comparative Examples 1-3, the cascade catalytic system used in this invention (Example 1) can significantly improve the yield and selectivity of monocyclic aromatic hydrocarbons and significantly inhibit carbon deposition compared with any single-component catalyst (Comparative Examples 1, 2, 3), proving that there is a key synergistic effect between the first-stage catalyst and the second-stage catalyst.

[0067] Comparing Example 1 and Comparative Example 2, it can be seen that the first-stage catalyst ZnZrO x The introduction of this catalyst increased the yield of monocyclic aromatic hydrocarbons by about 39% and reduced the amount of carbon deposits by about 72%, clarifying the indispensable role of the first-stage catalyst in the initial quality improvement and protection of downstream molecular sieves.

[0068] Comparing Comparative Example 1 and Comparative Example 2, it can be seen that metal (Fe) modification of ZSM-5 helps to improve the yield of aromatics, but its effect is far less than that of the cascade system of the present invention.

[0069] Example 4 The specific process for producing monocyclic aromatic hydrocarbons by biomass pyrolysis using the catalysts in Table 1 is as follows: First, select one compound catalyst from S1 to S22, weigh 1.2 g and place it in the lower part of the quartz tube; then, place quartz wool in the tube, and weigh 0.6 g of wool stalks and place them in the upper suspended tray. First, at room temperature, purge the atmosphere of the reaction system with inert nitrogen gas; then, under normal pressure, introduce nitrogen gas and raise the temperature of the reaction tube to 500℃ at a heating rate of 10℃ / min, and maintain this temperature for 30 min, while simultaneously collecting the liquid and gaseous products. The biomass pyrolysis zone and the catalyst bed are heated separately using resistance wire temperature control, with the temperature of both the biomass pyrolysis zone and the catalyst bed maintained at 500℃. During the reaction, the product gas passing through the catalyst bed passes through a secondary cold trap to obtain the liquid product; the non-condensable gaseous product is collected using a gas bag; after the reaction, the coke from the biomass pyrolysis is weighed. The liquid product is analyzed using gas chromatography-mass spectrometry (GC-MS) to obtain component information, and quantified by GC. The absolute weight of the liquid product is measured using a balance. The composition information of the non-condensable gases was obtained using a gas chromatography-mass spectrometry (GC-MS) scheme. The mass of the gaseous products was obtained by subtracting the sum of the masses of coke and liquid products from the mass of biomass. To accurately evaluate the performance of the catalysts, the performance experiments for each catalyst were repeated three times, and the average value was taken. The results are listed in Table 2.

[0070] Table 1 Specific application conditions for different catalysts

[0071] Table 2 Performance test data of different catalysts

[0072] Table 2 lists benzene, toluene, ethylbenzene, and xylene as monocyclic aromatic hydrocarbons. Combining the catalyst variable design data in Table 1 with the performance data in Table 2, the overall trend is as follows: Under conventional conditions of an A / B mass ratio of 0.5, calcination at 550℃, packing method ①, and catalytic / pyrolysis at 500℃, the coke yield after pyrolysis for most catalysts (S1 and most grouped samples) remained stable at 28%–30%, and the CO2 content was concentrated between 45% and 84%, showing balanced performance. However, when the variables were adjusted, the SiO2 / Al2O3 ratio of component B was increased (e.g., reaching 300 in S6), component B was replaced with Fe / ZSM-5 (e.g., in S7), or component A was selected as ZnCeO. xWhen using composite oxides, the yield of liquid-phase products (up to 64%), the proportion of aromatics (up to 93%), and the yield of monocyclic aromatics (up to 30.3%) are significantly improved, demonstrating the promoting effect of component matching on the formation of target products. Conversely, when the mass ratio of A to B deviates from 0.5 (e.g., 0, ∞), the calcination temperature is increased to 800℃, the packing method is changed to mixed state, or the catalytic / pyrolysis temperature deviates from 500℃ (e.g., 400℃, 600℃), the yield of liquid-phase products generally decreases (down to 35%), the carbon yield of catalyst coking increases significantly (up to 4.01%), and the yield of monocyclic aromatics decreases simultaneously (down to 6.4%). Among them, S28 has the worst overall performance due to multiple variables deviating from the conventional conditions. Overall, it can be seen that the component compatibility of the catalyst, the reaction conditions and the target performance (liquid-phase yield, aromatic selectivity) are positively correlated, and negatively correlated with the formation of coking. Extreme variable adjustments can easily lead to performance deterioration, while reasonable component matching and mild reaction conditions are more conducive to optimizing the catalytic effect.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cascade catalytic system for producing monocyclic aromatic hydrocarbons from biomass pyrolysis, characterized in that, Including first-stage catalyst and second-stage catalyst; The first-stage catalyst is a composite metal oxide with the general chemical formula ZnMO. x M is one or more of Zr, Ga, In, Cr, Mo, Ce and Al, and the value of x is such that the oxidation state is conserved. The second-stage catalyst is a ZSM-5 molecular sieve supported with an active metal, wherein the active metal includes one or more of Ga, Zn, Fe, Mo and Zr.

2. The cascade catalytic system according to claim 1, characterized in that, In the first-stage catalyst, the molar ratio of Zn to all M metals is 1:10 to 10:

1.

3. The cascade catalyst according to claim 1 or 2, characterized in that, The method for preparing the composite metal oxide includes the following steps: A soluble zinc salt, a soluble M metal salt, an alkaline precipitant, and water are mixed and a precipitation reaction is carried out to obtain the precipitation reaction product. The precipitated product was calcined to obtain a composite metal oxide.

4. The cascade catalytic system according to claim 1, characterized in that, In the second-stage catalyst, the mass of the active metal is 0.5-8% of the mass of the ZSM-5 molecular sieve; The silicon-aluminum ratio of the ZSM-5 molecular sieve is 20~800.

5. The cascade catalytic system according to any one of claims 1, 2, or 4, characterized in that, The mass ratio of the first-stage catalyst to the second-stage catalyst is 1:10 to 10:

1.

6. The cascade catalytic system according to claim 1, characterized in that, The first-stage catalyst and the second-stage catalyst are combined in the reactor by layering or physical mixing.

7. A method for producing monocyclic aromatic hydrocarbons by biomass pyrolysis, characterized in that, Includes the following steps: Biomass feedstock is pyrolyzed to obtain biomass pyrolysis gas; The biomass pyrolysis gas enters a catalytic reaction zone containing the cascade catalytic system described in any one of claims 1 to 6, and undergoes a catalytic conversion reaction to obtain a product containing monocyclic aromatic hydrocarbons.

8. The method according to claim 7, characterized in that, The biomass raw material is lignocellulose biomass; the pyrolysis temperature is 400~650℃.

9. The method according to claim 7, characterized in that, The temperature of the catalytic conversion reaction is 400~650℃; the mass ratio of the biomass feedstock to all catalysts in the cascade catalytic system is 1:1~10.

10. The method according to claim 7, characterized in that, The monocyclic aromatic hydrocarbons include one or more of benzene, toluene, ethylbenzene, and xylene; The product containing monocyclic aromatic hydrocarbons has a mass content of ≥75% of monocyclic aromatic hydrocarbons.

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