A surface silicon-rich binderless zsm-5 molecular sieve catalyst, its preparation method and application

Abstract

The present application relates to the technical field of molecular sieve preparation, and provides a surface silicon-rich binderless ZSM-5 molecular sieve catalyst and a preparation method and application thereof.The overall SiO2 / Al2O3 of the ZSM-5 molecular sieve catalyst is 86-203, the SiO2 / Al2O3 of a surface layer with a depth of 2-10 nm is 8-33% higher than the overall SiO2 / Al2O3; the acid strength measured by using Hammett indicator n-butylamine titration method is H0≤+2.27, and the content of surface acid sites is ≤0.02 mmol / g; and the ZSM-5 molecular sieve catalyst is a shaped body without amorphous phase.The surface silicon-rich binderless ZSM-5 molecular sieve catalyst provided by the present application does not contain inert adhesive, has extremely weak surface acidity, can improve the catalytic effect, and reduces the generation of by-products.In addition, the preparation method provided by the present application is simple in process and suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of molecular sieve preparation technology, and more specifically, to a surface-rich silica binder-free ZSM-5 molecular sieve catalyst, its preparation method, and its application. Background Technology

[0002] Molecular sieves are a class of inorganic microporous materials with regular and uniform pore structures, and they have wide applications in adsorption, catalysis, and ion exchange. Among them, ZSM-5 molecular sieve has a three-dimensional ten-membered ring interlocking pore system with a pore diameter of... ZSM-5 molecular sieves, due to their tunable framework composition, high specific surface area and adsorption capacity, shape selectivity of pore and cage structures, and high thermal and chemical stability, are now widely used in catalytic cracking, toluene disproportionation, xylene isomerization, and methanol-to-gasoline conversion.

[0003] The acidity of ZSM-5 molecular sieves exists simultaneously on its outer surface and within its pores. However, the acidic sites on the surface lack shape-selective catalysis capabilities, leading to side reactions or surface carbon accumulation. Modifying the outer surface of ZSM-5 molecular sieves with silica can passivate the surface acidity, which is beneficial for improving catalytic selectivity and catalytic lifetime. Patent CN115646537B discloses an embedded catalyst and its preparation method, which passivates the acidity of the molecular sieve's outer surface by epitaxial growth of metal oxides and silicon species. Patent CN111569935B discloses a core-shell structure catalyst with a ZSM-5 molecular sieve core and an S-1 (i.e., pure silicon ZSM-5) molecular sieve shell. This catalyst is prepared by epitaxial growth, and the acid-free shell can effectively improve the selectivity for the alkylation reaction of toluene and methanol to produce p-xylene. However, the catalysts prepared in the aforementioned patents are all powder catalysts. For industrial applications, these powder catalysts must be prepared into shaped bodies with a certain mechanical strength. This process typically involves mixing a binder (such as alumina or silica) with molecular sieve powder and then extruding the mixture. The binder usually accounts for 30-50% of the total catalyst mass. It not only dilutes the active centers of the molecular sieve but also covers the surface of the molecular sieve, clogging the pores and shielding some catalytic sites. Therefore, removing the inert binder component from the catalyst is beneficial for improving its catalytic performance. Summary of the Invention

[0004] The purpose of this invention is to provide a surface-rich silica-free ZSM-5 molecular sieve catalyst, its preparation method, and its application, in order to solve the technical problems of inert binders in existing ZSM-5 molecular sieve shaped catalysts and non-shape-selective catalysis caused by strong surface acidity.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a surface-rich, binder-free ZSM-5 molecular sieve catalyst, wherein the overall SiO2 / Al2O3 ratio of the ZSM-5 molecular sieve catalyst is 86-203, and the surface SiO2 / Al2O3 ratio at a depth of 2-10 nm is 8-33% higher than the overall SiO2 / Al2O3 ratio; the acid strength measured using Hammett indicator titration with n-butylamine is H0≤+2.27, and the content of surface acidic sites is ≤0.02mmol / g; the ZSM-5 molecular sieve catalyst is a shaped body without an amorphous phase.

[0007] This invention provides a shaped catalyst based on ZSM-5 molecular sieve, which is more suitable for industrial production compared to powdered ZSM-5 molecular sieve catalysts. The catalyst provided by this invention not only does not contain inert binders but also possesses extremely weak surface acidity, which can improve catalytic efficiency and reduce byproduct formation.

[0008] In this invention, the surface SiO2 / Al2O3 with a depth of 2-10 nm refers to the average thickness of SiO2 / Al2O3 extending inward from the outer surface of the ZSM-5 molecular sieve catalyst by 2-10 nm.

[0009] In this invention, the method for determining the content of surface acidic sites with an acid strength of H0 ≤ +2.27 using the Hammett indicator-n-butylamine titration method includes: using 4-aminoazobenzene (pKa = +2.27) as an indicator, titrating the surface acidic sites of the ZSM-5 molecular sieve catalyst with n-butylamine; and determining the amount of acid with an acid strength of H0 ≤ +2.27 by the amount of n-butylamine consumed when reaching the indicator equivalence point. The framework aluminum directly related to the surface acidity tested by the Hammett indicator-n-butylamine titration method includes only the framework aluminum present in 1-2 atomic layers on the surface of the ZSM-5 molecular sieve catalyst material. The test results of the Hammett indicator-n-butylamine titration method show that the surface acidity of the surface-rich, binder-free ZSM-5 molecular sieve catalyst provided by this invention is less than 0.02 mmol / g, indicating extremely weak surface acidity.

[0010] According to some embodiments of the present invention, the surface SiO2 / Al2O3 ratio of the ZSM-5 molecular sieve catalyst is 8.1% to 32.5% higher than the overall SiO2 / Al2O3 ratio.

[0011] According to some embodiments of the present invention, the XRD pattern of the ZSM-5 molecular sieve catalyst contains only ZSM-5 molecular sieve and does not contain an amorphous phase.

[0012] According to some embodiments of the present invention, the surface SiO2 / Al2O3 ratio of the ZSM-5 molecular sieve catalyst is 93 to 257.

[0013] According to some embodiments of the present invention, the ZSM-5 molecular sieve catalyst has an acid strength of H0≤+2.27 and a surface acidic site content of 0.008~0.02mmol / g, as determined by Hammett indicator-n-butylamine titration.

[0014] According to some embodiments of the present invention, the SEM morphology of the ZSM-5 molecular sieve catalyst is a microcrystalline aggregate of 100-200 nm, which does not contain an amorphous phase.

[0015] According to some embodiments of the present invention, the ZSM-5 molecular sieve catalyst has a catalytic cracking conversion rate of less than 0.4% for the probe molecule triisopropylbenzene.

[0016] The surface acidity of ZSM-5 molecular sieve catalysts can be qualitatively analyzed by using triisopropylbenzene pyrolysis as a probe reaction. The probe molecule, triisopropylbenzene, has a diameter of approximately 0.95 nm, while the pore size of the ZSM-5 molecular sieve is approximately 0.55 nm. Therefore, triisopropylbenzene cannot diffuse into the pores of the ZSM-5 molecular sieve and can only be catalytically pyrolyzed by the acidic sites on its surface. The surface-silica-rich, binder-free ZSM-5 molecular sieve catalyst provided by this invention exhibits a triisopropylbenzene pyrolysis conversion rate of less than 0.4% at 200°C and 250°C, indicating extremely weak surface acidity.

[0017] According to some embodiments of the present invention, the crushing strength of the ZSM-5 molecular sieve catalyst is 89 to 122 N / cm.

[0018] According to some embodiments of the present invention, the ZSM-5 molecular sieve catalyst is a columnar shaped body.

[0019] According to some embodiments of the present invention, the cross-section of the column is selected from at least one of circular, four-leaf clover, and three-leaf clover shapes.

[0020] According to some embodiments of the present invention, the maximum radial dimension of the cross section of the column is 0.2 to 0.5 cm, and the length of the column is 0.5 to 1.0 cm.

[0021] In a second aspect, the present invention provides a method for preparing the ZSM-5 molecular sieve catalyst described in the first aspect, comprising: obtaining a mixture comprising ZSM-5 molecular sieve powder, a silica-type binder, a binding aid, an organic amine, and a first portion of water; extruding the mixture into strips; pretreating the strips in a sealed environment at 80–100°C for 2–4 h; after removing the sealed environment, subjecting the strips to a first drying and a first calcination to obtain a molecular sieve precursor; mixing the molecular sieve precursor, the organic template agent, and the second portion of water, and then sequentially performing crystallization treatment, a second calcination, ammonium exchange, and a third calcination to obtain the ZSM-5 molecular sieve catalyst.

[0022] The preparation method provided by this invention uses ZSM-5 molecular sieve powder, silica-type binder, binder aid, and organic amine as raw materials. First, a moist columnar molded body is prepared. During a sealed pretreatment process, the organic amine induces the silica-type binder to generate a certain amount of ZSM-5 molecular sieve crystal nuclei. Subsequently, the sealed conditions are lifted, and the body is dried and calcined to obtain the molecular sieve precursor. Then, the binder component in the molecular sieve precursor is converted into ZSM-5 molecular sieve through a hydrothermal secondary crystallization method. The acidity of the molecular sieve originates from the aluminum atoms on its framework. During the secondary crystallization of the molecular sieve precursor, the silica-type binder transforms into a high-silica ZSM-5 molecular sieve shell covering the surface of the parent ZSM-5 molecular sieve, forming a silicon-enriched layer. This passivates the surface acidity of the parent ZSM-5 molecular sieve and also eliminates the binder component in the catalyst.

[0023] The preparation method provided by this invention transforms the binder component into a molecular sieve, achieving silicon enrichment on the surface of the parent molecular sieve, thereby passivating the acidity of the catalyst surface. The process is simple and suitable for industrial production.

[0024] In this invention, the particle size of the ZSM-5 molecular sieve powder can be the common size of molecular sieve powder raw materials, which is readily available to those skilled in the art and will not be described in detail here.

[0025] In this invention, the silica-type adhesive can be selected from those commonly used in the art.

[0026] According to some embodiments of the present invention, the silica-type binder includes at least one of solid silicone, silica sol, and fumed silica.

[0027] According to some embodiments of the present invention, the organic amine includes at least one selected from ethylamine, ethylenediamine, propylamine, n-butylamine, triethylamine, and tetrapropylammonium hydroxide.

[0028] In the preparation method provided by this invention, the organic template agent can be a commonly used organic template agent in the synthesis of ZSM-5 molecular sieve.

[0029] According to some embodiments of the present invention, the organic template agent includes one or more of ethylamine, ethylenediamine, propylamine, n-butylamine, triethylamine, and tetrapropylammonium hydroxide.

[0030] According to some embodiments of the present invention, the mass ratio of silica-type binder (calculated as SiO2) to ZSM-5 molecular sieve powder is 0.15 to 0.20.

[0031] According to some embodiments of the present invention, the mass ratio of the organic amine to the silica-type binder (calculated as SiO2) is 0.015 to 0.1.

[0032] According to some embodiments of the present invention, the mass ratio of the first portion of water to ZSM-5 molecular sieve powder is 0.15 to 0.25.

[0033] According to some embodiments of the present invention, the molar ratio of the organic template agent to the silica-type binder calculated as SiO2 is 0.1 to 0.2.

[0034] According to some embodiments of the present invention, the molar ratio of the second portion of water to the silica-type binder calculated as SiO2 is 30 to 50.

[0035] In this invention, the binder mainly plays the role of ensuring the smooth progress of the extrusion molding process, and various commonly used binders that are beneficial to extrusion molding can be selected.

[0036] According to some embodiments of the present invention, the adhesive additive includes organic adhesive additives and optionally ionic adhesive additives.

[0037] In this invention, the organic binder can be selected from those commonly used in the art.

[0038] According to some embodiments of the present invention, the organic binder includes at least one of methylcellulose, hydroxypropyl methylcellulose, guar gum powder, and polyethylene glycol.

[0039] According to some embodiments of the present invention, the ionic binder includes at least one of alkali metal compounds.

[0040] According to some embodiments of the present invention, the ionic adhesive includes at least one of potassium chloride, potassium hydroxide, sodium silicate, and sodium chloride.

[0041] According to some embodiments of the present invention, the mass ratio of the organic binder to the ZSM-5 molecular sieve powder is 0.01 to 0.05.

[0042] According to some embodiments of the present invention, the mass ratio of the ionic binder to the ZSM-5 molecular sieve powder is 0 to 0.1, preferably 0.04 to 0.08.

[0043] According to some embodiments of the present invention, a columnar molded body is obtained by extrusion molding.

[0044] According to some embodiments of the present invention, the cross-section of the column is selected from at least one of circular, four-leaf clover, and three-leaf clover shapes.

[0045] According to some embodiments of the present invention, the maximum radial dimension of the cross section of the column is 0.2 to 0.5 cm, and the length of the column is 0.5 to 1.0 cm.

[0046] According to some embodiments of the present invention, the temperature of the first drying is 80-100°C, and the drying time is 8-12 hours.

[0047] According to some embodiments of the present invention, the temperature of the first calcination is 600-650°C, and the calcination time is 6-12 hours.

[0048] According to some embodiments of the present invention, the crystallization treatment temperature is 180-200°C, and the crystallization treatment time is 1-2 days.

[0049] According to some embodiments of the present invention, after the crystallization treatment, the material is first washed and then dried, and then calcined.

[0050] According to some embodiments of the present invention, the temperature of the second drying is 80-100°C, and the drying time is 8-12 hours.

[0051] According to some embodiments of the present invention, the temperature of the second calcination is 600-650°C, and the calcination time is 6-12 hours.

[0052] According to some embodiments of the present invention, after the ammonium exchange, a third drying process is performed, followed by a third roasting process.

[0053] According to some embodiments of the present invention, the temperature of the third calcination is 500-600°C, and the time of the third calcination is 4-8 hours.

[0054] According to some embodiments of the present invention, the ammonium exchange includes: mixing with an ammonium solution, exchanging at 60-80°C for 2-4 hours, and washing after solid-liquid separation.

[0055] According to some embodiments of the present invention, the ammonium solution contains NH4 + The molar concentration is 1–3 mol / L.

[0056] According to some embodiments of the present invention, the ammonium solution includes at least one of ammonium nitrate aqueous solution, ammonium chloride aqueous solution, ammonium sulfate aqueous solution, and ammonium acetate aqueous solution.

[0057] According to some embodiments of the present invention, the ammonium exchange is performed 2 to 4 times.

[0058] Thirdly, the present invention provides the application of the surface-rich silica-free ZSM-5 molecular sieve catalyst described in the first aspect or the surface-rich silica-free ZSM-5 molecular sieve catalyst prepared by the preparation method described in the second aspect in the catalytic conversion of light hydrocarbons, such as the catalytic conversion of isobutane.

[0059] The surface-rich silica-free ZSM-5 molecular sieve catalyst provided by this invention has extremely weak surface acidity, which can inhibit the non-selective conversion of light hydrocarbons on the outer surface of the catalyst and reduce the generation of by-products.

[0060] The beneficial effects of this invention are at least as follows:

[0061] The surface-rich silica-free ZSM-5 molecular sieve catalyst provided by this invention contains no inert binder and exhibits extremely weak surface acidity, which can enhance catalytic efficiency and reduce byproduct formation. Furthermore, the preparation method provided by this invention is simple and suitable for industrial production. Attached Figure Description

[0062] Figure 1 X-ray powder diffraction patterns of the catalysts prepared in each example and comparative example (where: 1-C1, 2-C2, 3-C3, 4-C4, 5-D1, 6-D2, 7-D3, 8-D4, 9-D5).

[0063] Figure 2 Scanning electron microscope images of the catalysts prepared in each example and comparative example.

[0064] Figure 3 The results of the catalyst prepared in Example 1 and the ZSM-5 molecular sieve Z1 catalytic cracking of triisopropylbenzene are shown.

[0065] Figure 4 The result is the catalytic conversion of isobutane using the catalyst prepared in Example 1.

[0066] Figure 5 The results are from the Z1 catalytic conversion of isobutane using ZSM-5 molecular sieve.

[0067] Figure 6 The results of isobutane conversion catalyzed by the catalyst prepared in Comparative Example 1 are shown. Detailed Implementation

[0068] To make the technical problem to be solved, the technical solution, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely for illustrating this patent and do not limit the scope of protection of this invention in any way.

[0069] Unless otherwise defined, the technical terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art. Unless otherwise specified, the reagents used in the following embodiments are conventional biochemical reagents; the raw materials, instruments, and equipment used in the following embodiments can all be obtained commercially or by existing methods; unless otherwise specified, the reagent dosages are those used in routine experimental operations; unless otherwise specified, the experimental methods are conventional methods.

[0070] The materials used in the various embodiments and comparative examples of the present invention are as follows:

[0071] (1) ZSM-5 molecular sieve Z1: self-made in the laboratory, SiO2 / Al2O3=75.

[0072] (2) ZSM-5 molecular sieve Z2: self-made in the laboratory, SiO2 / Al2O3=180.

[0073] Example 1

[0074] 300g of ZSM-5 molecular sieve Z1, 45g of solid silica gel, 12g of potassium chloride, 12g of guar gum powder, 6g of propylamine, and 75g of water were mixed and kneaded, then extruded into cylindrical shapes with a diameter of 0.3cm, a length of 0.5-1cm, and a circular cross-section. The mass ratio of the components in the cylindrical shape was ZSM-5:SiO2:KCl:guar gum powder:propylamine:H2O = 1:0.15:0.04:0.04:0.02:0.25. The cylindrical shape was placed in a sealed container and pretreated at 100℃ for 2h. After that, the sealing conditions were removed and the container was dried for 8h. Then, it was calcined in a muffle furnace at 600℃ for 8h to obtain precursor A1.

[0075] Precursor A1 was mixed with 6.64 g of propylamine and 405 g of water and placed in a hydrothermal reactor. The molar ratio of SiO2, propylamine and water involved in crystallization was 1:0.15:30. Crystallization was carried out at 180 °C for 2 days. After crystallization, the mixture was thoroughly washed with deionized water, dried at 100 °C for 8 h, and then calcined at 600 °C for 8 h in a muffle furnace to obtain precursor B1.

[0076] Precursor B1 was mixed with 6 L of ammonium nitrate solution (1 mol / L) and exchanged at 80 °C for 2 h. Solid-liquid separation and thorough washing were then performed to complete one ammonium exchange. A total of two ammonium exchanges were conducted. After ammonium exchange, the mixture was dried and then calcined at 550 °C for 8 h to obtain the molecular sieve catalyst C1.

[0077] The X-ray powder diffraction pattern of C1 is as follows: Figure 1 As shown in curve 1, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that C1 is a pure-phase ZSM-5 molecular sieve. Figure 2As shown, scanning electron microscopy revealed that C1 consists of microcrystals with a particle size of 100–200 nm and does not contain an amorphous phase, indicating that the binder has completely crystallized and transformed into ZSM-5 molecular sieve.

[0078] Example 2

[0079] 300g of ZSM-5 molecular sieve Z2, 150g of 40wt% silica sol, 21g of potassium hydroxide, 9g of hydroxypropyl methylcellulose (HPMC), 15g of n-butylamine, and 45g of water were mixed and kneaded, then extruded into columnar shapes with a diameter of 0.4cm, a length of 0.5-1cm, and a clover-shaped cross-section. The mass ratio of the components in the columnar shape was ZSM-5:SiO2:KOH:HPMC:n-butylamine:H2O = 1:0.2:0.07:0.03:0.05:0.15. The columnar shapes were placed in a sealed container and pretreated at 100℃ for 4h. After that, the sealing conditions were removed and the container was dried for 8h. Finally, the container was calcined in a muffle furnace at 600℃ for 12h to obtain precursor A2.

[0080] Precursor A2 was mixed with 14.6 g of n-butylamine and 720 g of water and placed in a hydrothermal reactor. The molar ratio of SiO2, n-butylamine and water involved in crystallization was 1:0.2:40. Crystallization was carried out at 180 °C for 2 days. After crystallization, the mixture was thoroughly washed with deionized water, dried at 100 °C for 8 h, and then calcined in a muffle furnace at 650 °C for 12 h to obtain precursor B2.

[0081] Precursor B2 was mixed with 6 L of ammonium sulfate solution (1 mol / L) and exchanged at 80 °C for 2 h. Solid-liquid separation and thorough washing were then performed to complete one ammonium exchange. A total of three ammonium exchanges were conducted. After ammonium exchange, the mixture was dried and then calcined at 550 °C for 8 h to obtain the molecular sieve catalyst C2.

[0082] The X-ray powder diffraction pattern of C2 is as follows: Figure 1 As shown in curve 2, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that C2 is a pure-phase ZSM-5 molecular sieve. Figure 2 As shown, scanning electron microscopy revealed that C2 consists of microcrystals with a particle size of 100–200 nm and does not contain an amorphous phase, indicating that the binder has completely crystallized and transformed into ZSM-5 molecular sieve.

[0083] Example 3

[0084] 300g of ZSM-5 molecular sieve Z1, 45g of silica, 24g of sodium chloride, 3g of methylcellulose (MC), 4.5g of ethylamine, and 75g of water were mixed and kneaded to form a columnar shape with a diameter of 0.3cm, a length of 0.5-1cm, and a clover-shaped cross-section. The mass ratio of the components in the columnar shape was ZSM-5:SiO2:NaCl:MC:ethylamine:H2O = 1:0.15:0.08:0.01:0.015:0.25. The columnar shape was placed in a sealed container and pretreated at 100℃ for 4h. After that, the sealing conditions were removed and the container was dried for 8h. Then, it was calcined in a muffle furnace at 600℃ for 12h to obtain precursor A3.

[0085] Precursor A3 was mixed with 3.4 g of ethylamine and 675 g of water and placed in a hydrothermal reactor. The molar ratio of SiO2, ethylamine and water involved in crystallization was 1:0.1:50. Crystallization was carried out at 180 °C for 1 day. After crystallization, the mixture was thoroughly washed with deionized water, dried at 80 °C for 12 h, and then calcined at 600 °C for 8 h in a muffle furnace to obtain precursor B3.

[0086] Precursor B3 was mixed with 6 L of ammonium chloride solution (1 mol / L) and exchanged at 80 °C for 2 h. Solid-liquid separation and thorough washing were then performed to complete one ammonium exchange. A total of four ammonium exchanges were conducted. After ammonium exchange, the mixture was dried and then calcined at 550 °C for 8 h to obtain the molecular sieve catalyst C3.

[0087] The X-ray powder diffraction pattern of C3 is as follows: Figure 1 As shown in curve 3, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that C3 is a pure-phase ZSM-5 molecular sieve. Figure 2 As shown, scanning electron microscopy revealed that C3 consists of microcrystals with a particle size of 100–200 nm and does not contain an amorphous phase, indicating that the binder has completely crystallized and transformed into ZSM-5 molecular sieve.

[0088] Example 4

[0089] 300g of ZSM-5 molecular sieve Z2, 150g of 40wt% silica sol, 15g of polyethylene glycol (PEG), 75g of 40wt% tetrapropylammonium hydroxide (TPAOH) solution, and 75g of water were mixed and extruded into cylindrical shapes with a diameter of 0.5cm, a length of 0.5-1cm, and a circular cross-section. The mass ratio of the components in the cylindrical shape was ZSM-5:SiO2:PEG:TPAOH:H2O = 1:0.2:0.05:0.10:0.25. The cylindrical shape was placed in a sealed container and pretreated at 80℃ for 2h. After that, the sealing conditions were removed and the container was dried for 8h. Then, it was calcined in a muffle furnace at 600℃ for 12h to obtain precursor A4.

[0090] Precursor A4 was mixed with 81.3 g of 25 wt% tetrapropylammonium hydroxide solution and 480 g of water and placed in a hydrothermal reactor. The molar ratio of SiO2, TPAOH and water involved in crystallization was 1:0.1:30. Crystallization was carried out at 180 °C for 1 day. After crystallization, the mixture was thoroughly washed with deionized water, dried at 80 °C for 12 h, and then calcined at 600 °C for 8 h in a muffle furnace to obtain precursor B4.

[0091] Precursor B4 was mixed with 6 L of ammonium acetate solution (1 mol / L) and exchanged at 80 °C for 2 h. Solid-liquid separation and thorough washing were then performed to complete the first ammonium exchange. A total of two ammonium exchanges were conducted. After ammonium exchange, the mixture was dried and then calcined at 550 °C for 8 h to obtain the molecular sieve catalyst C4.

[0092] The X-ray powder diffraction pattern of C4 is as follows: Figure 1 As shown in curve 4, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that C4 is a pure-phase ZSM-5 molecular sieve. Figure 2 As shown, scanning electron microscopy revealed that C4 consists of microcrystals with a particle size of 100–200 nm and does not contain an amorphous phase, indicating that the binder has completely crystallized and transformed into ZSM-5 molecular sieve.

[0093] Comparative Example 1

[0094] The preparation method of molecular sieve catalyst D1 is the same as in Example 1, except that 6g of propylamine is not added during the preparation of precursor A1.

[0095] The X-ray powder diffraction pattern of D1 is as follows: Figure 1 As shown in curve 5, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be seen that D1 is a mixture of ZSM-5 molecular sieve and an amorphous phase. Figure 2 As shown, scanning electron microscopy revealed that D1 consisted of ZSM-5 crystals and an amorphous phase, indicating that the binder was not completely crystallized into ZSM-5 molecular sieves. This is because propylamine can induce the binder to transform into ZSM-5 crystal nuclei during pretreatment, and ZSM-5 crystal nuclei are crucial for the complete crystallization of the binder into ZSM-5 molecular sieves.

[0096] Comparative Example 2

[0097] The preparation method of molecular sieve catalyst D2 is the same as in Example 1, except that instead of pretreating the columnar material in a sealed container at 100°C for 2 hours, the columnar material is directly dried for 8 hours and then calcined in a muffle furnace at 600°C for 8 hours.

[0098] The X-ray powder diffraction pattern of D2 is as follows: Figure 1 As shown in curve 6, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that D2 is a mixture of ZSM-5 molecular sieve and an amorphous phase. Figure 2 As shown, scanning electron microscopy revealed that D2 consisted of ZSM-5 crystals and an amorphous phase, indicating that the binder did not completely crystallize into ZSM-5 molecular sieves. This is because direct drying causes propylamine to volatilize rapidly, failing to induce sufficient ZSM-5 crystal nuclei to support the complete crystallization of the binder into ZSM-5 molecular sieves.

[0099] Comparative Example 3

[0100] The preparation method of molecular sieve catalyst D3 is the same as in Example 1, except that the columnar molded body is placed in a sealed container and pretreated at 50°C for 2 hours.

[0101] The X-ray powder diffraction pattern of D3 is as follows: Figure 1 As shown in curve 7, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that D3 is a mixture of ZSM-5 molecular sieve and an amorphous phase. Figure 2 As shown, scanning electron microscopy revealed that D3 consisted of ZSM-5 crystals and an amorphous phase, indicating that the binder did not completely crystallize into ZSM-5 molecular sieve. This is because the pretreatment temperature was too low, and propylamine could only induce a small number of ZSM-5 crystal nuclei, which was insufficient to support the complete crystallization of the binder into ZSM-5 molecular sieve.

[0102] Comparative Example 4

[0103] The preparation method of molecular sieve catalyst D4 is the same as in Example 1, except that the columnar molded body is placed in a sealed container and pretreated at 120°C for 2 hours.

[0104] The X-ray powder diffraction pattern of D4 is as follows: Figure 1 As shown in curve 8, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that D4 is a pure-phase ZSM-5 molecular sieve. Figure 2 As shown, scanning electron microscopy revealed that C4 consisted of microcrystals with a particle size of 200–300 nm and contained no amorphous phase, indicating that the binder had completely crystallized and transformed into ZSM-5 molecular sieve.

[0105] However, the overall SiO2 / Al2O3 ratio of D4 is 88, and the surface SiO2 / Al2O3 ratio is 69, indicating that D4 does not have a silicon-rich outer shell; in addition, the crushing strength of D4 is only 28 N / cm.

[0106] Based on the above results, it is possible that excessively high pretreatment temperatures cause aluminum in Z1 to dissolve and accumulate on the surface of D4 during crystallization. In addition, excessively high pretreatment temperatures have a strong corrosive effect on columnar molded bodies, resulting in excessively low crushing strength of the crystallized products.

[0107] Comparative Example 5

[0108] 120g of ZSM-5 molecular sieve Z1, 40g of solid silica gel, 4.8g of guar gum powder, and 30g of water were mixed and kneaded to form a cylindrical shape with a diameter of 0.3cm, a length of 0.5-1cm, and a circular cross-section. The mass ratio of the components in the cylindrical shape was ZSM-5:SiO2:guar gum powder:H2O = 1:0.33:0.04:0.25. The cylindrical shape was placed in an oven and dried at 100℃ for 8 hours. It was then placed in a crystallization vessel containing a mixture of 10g of triethylamine and 10g of water, with a metal mesh separating the cylindrical shape from the liquid. Crystallization was carried out at 180℃ for 4 days. After crystallization, the shape was thoroughly washed with deionized water, dried at 100℃ for 8 hours, and then calcined in a muffle furnace at 600℃ for 8 hours. It was then mixed with 2.4L of ammonium nitrate solution (1mol / L) and exchanged at 80℃ for 2 hours. Solid-liquid separation and thorough washing were then performed to complete one ammonium exchange. Two ammonium exchange processes were performed. After ammonium exchange, the mixture was dried and then calcined at 550℃ for 8 hours to obtain molecular sieve catalyst D5.

[0109] The X-ray powder diffraction pattern of D5 is as follows: Figure 1 As shown in curve 9, by comparing it with the standard diffraction pattern published by the International Molecular Sieve Association, it can be determined that D5 is a pure-phase ZSM-5 molecular sieve. Figure 2 As shown, scanning electron microscopy revealed that D5 consists of two crystal morphologies: microcrystals with a diameter of approximately 50 nm (marked region) and crystals with a diameter of over 200 nm (peripheral area of ​​the marked region), without any amorphous phase. The morphological characteristics of D5 indicate that during crystallization, the binder did not epitaxially grow along the outer surface of Z1 to form a silicon-rich shell, but instead crystallized directly to form a silicon-rich ZSM-5 region independent of Z1.

[0110] Performance Evaluation

[0111] (1) Surface SiO2 / Al2O3 analysis

[0112] The overall SiO2 / Al2O3 ratio of the molecular sieve material was determined by XRF, and the SiO2 / Al2O3 ratio at a depth of 2–10 nm on the surface of the molecular sieve material was analyzed by XPS semi-quantitative analysis. The test results are shown in Table 1.

[0113] Table 1

[0114]

[0115] The SiO2 / Al2O3 ratio of the molecular sieve surface layer (2–10 nm) reflects its surface acidity. Table 1 shows that the surface SiO2 / Al2O3 ratios for Z1 and Z2 are 56 and 152, respectively; while those for C1–C4 are 93, 243, 102, and 257, respectively, representing increases of 66%, 60%, 82%, and 69% compared to the parent material. This indicates that the SiO2-type binder transforms into a silicon-rich ZSM-5 molecular sieve shell covering the surface of the parent ZSM-5 molecular sieve, thus passivating the surface acidity.

[0116] (2) Crushing strength

[0117] The crushing strength of the molecular sieve material was measured using a particle strength tester, and the test results are shown in Table 2.

[0118] Table 2

[0119] Molecular sieve materials Crushing strength (N / cm) Z1 — Z2 — C1 92 C2 117 C3 89 C4 122 D1 87 D2 95 D3 91 D4 28 D5 101

[0120] (3) Surface acidity analysis

[0121] ① The surface acidity of the molecular sieve material was determined by titration with Hammett indicator and n-butylamine. The molecular sieve material sample was calcined at 500℃ for 2 hours, then transferred to a desiccator and cooled for 1 hour. Approximately 0.1 g of the sample was accurately weighed and placed in 10 dry glass test tubes. Different amounts of n-butylamine were added to the molecular sieve sample using cyclohexane as solvent, and the mixture was sonicated for 40 minutes to accelerate adsorption equilibrium. Finally, 4-aminoazobenzene indicator was added, and the acidity (H0 ≤ +2.27) was measured. The results are shown in Table 3.

[0122] Table 3

[0123]

[0124]

[0125] The experimental results in Table 3 show that the surface acidity of Z1 and Z2 is 0.304 and 0.116 mmol / g, respectively, while the surface acidity of C1-C4 is only 0.008-0.020 mmol / g. This indicates that the surface aluminum content of Z1 and Z2 is much higher than that of C1-C4, meaning that C1-C4 is a ZSM-5 molecular sieve with a silicon-rich surface.

[0126] The surface acidity of D1-D3 ranges from 0.044 to 0.050 mmol / g. This is because although the silica-type binder did not completely transform into ZSM-5 molecular sieve, it still covered the surface of the parent molecular sieve and could shield most surface acidic sites. D4 was prepared at an excessively high pretreatment temperature, which caused aluminum in Z1 to dissolve and accumulate on the surface of D4 during crystallization. Therefore, the surface acidity of D4 is closer to that of Z1. D5 was prepared by a steam-assisted gas-solid phase crystallization method. The binder did not grow epitaxially along the outer surface of Z1 to form a silicon-rich shell, but instead crystallized directly to form a silicon-rich ZSM-5 region independent of Z1. Therefore, D5 retains more surface acidity.

[0127] ② The surface acidity of binder-free ZSM-5 molecular sieve catalysts can be qualitatively analyzed by using triisopropylbenzene cracking as a probe reaction. This probe reaction is carried out in a self-made fixed-bed reactor. The reactor is loaded with 20 mg of 40-60 mesh Z1 or C1 catalyst, heated to 200℃ or 250℃, and then triisopropylbenzene is pumped in under normal pressure, with a mass hourly space velocity (WHSV) controlled at 2 h⁻¹. -1 The product composition was analyzed by post-gas chromatography, and the conversion rate of triisopropylbenzene was calculated. The results are as follows: Figure 3 As shown.

[0128] The results showed that at 200℃, the conversion rates of triisopropylbenzene by Z1 and C1 were 8.36% and 0.09%, respectively; at 250℃, the conversion rates were 13.73% and 0.37%, respectively. This indicates that the ordinary ZSM-5 molecular sieve Z1 has a certain surface acidity and can catalyze the cracking of triisopropylbenzene; while the surface-rich, binder-free ZSM-5 molecular sieve catalyst provided by this invention has only extremely weak surface acidity and is difficult to catalyze the cracking of triisopropylbenzene.

[0129] (4) Evaluation of light hydrocarbon catalytic conversion performance

[0130] Molecular sieve Z1 powder was pressed into tablets and granulated, and used as a comparative material. It, along with catalyst C1 prepared in Example 1 and catalyst D1 prepared in Comparative Example 1, were used for the catalytic conversion of isobutane.

[0131] The reaction conditions included: 10 g of catalyst and a reaction space velocity of 1 h⁻¹. -1 The reaction temperature was 550℃, the reaction pressure was 0.13 MPa, and the reaction gas was isobutane.

[0132] The compositions of the obtained products are as follows: Figure 4-6 As shown.

[0133] Depend on Figure 4 It can be seen that methane is the main byproduct, with a yield of about 5%, which comes from non-shape-selective catalysis.

[0134] Depend on Figure 5 It can be seen that the yield of the main byproduct methane is about 10%, which is significantly higher than that of catalyst C1, indicating that Z1 has more surface acidity and is more likely to undergo non-shape-selective catalysis.

[0135] Depend on Figure 6 It can be seen that the C4 content in the product increases to about 14%, significantly higher than that of catalysts C1 and Z1, indicating that D1 has lower catalytic activity. This is because D1 contains an inert amorphous phase, which blocks some of the molecular sieve channels, resulting in a lower C4 conversion rate. Meanwhile, the yield of the byproduct methane is about 8%, higher than that of catalyst C1, indicating that D1 has exposed surface acidic sites.

[0136] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A binder-free ZSM-5 molecular sieve catalyst with a surface rich in silicon, characterized in that, The overall SiO2 / Al2O3 ratio of the ZSM-5 molecular sieve catalyst is 86–203, and the surface SiO2 / Al2O3 ratio at a depth of 2–10 nm is 8–33% higher than the overall SiO2 / Al2O3 ratio. The acid strength measured by Hammett indicator titration with n-butylamine is H0 ≤ +2.27, and the content of surface acidic sites is ≤0.02 mmol / g. The ZSM-5 molecular sieve catalyst is a shaped body without an amorphous phase.

2. The ZSM-5 molecular sieve catalyst according to claim 1, characterized in that, The surface SiO2 / Al2O3 ratio of the ZSM-5 molecular sieve catalyst is 93–257. And / or, the SEM morphology of the ZSM-5 molecular sieve catalyst is a microcrystalline aggregate of 100-200 nm, which does not contain an amorphous phase; And / or, the ZSM-5 molecular sieve catalyst has a catalytic cracking conversion rate of less than 0.4% for the probe molecule triisopropylbenzene.

3. The ZSM-5 molecular sieve catalyst according to claim 1 or 2, characterized in that, The crushing strength of the ZSM-5 molecular sieve catalyst is 89–122 N / cm; And / or, the ZSM-5 molecular sieve catalyst is a columnar shaped body; preferably, the maximum radial dimension of the cross section of the column is 0.2 to 0.5 cm, and the length of the column is 0.5 to 1.0 cm.

4. The method for preparing the ZSM-5 molecular sieve catalyst according to any one of claims 1-3, characterized in that, include: A mixture comprising ZSM-5 molecular sieve powder, silica-type binder, binder aid, organic amine and water in the first part is obtained, and the mixture is extruded into strips; The molded body is pretreated in a sealed environment at 80-100°C for 2-4 hours. After the sealed environment is removed, it is dried and calcined to obtain a molecular sieve precursor. The molecular sieve precursor, organic template agent, and second part of water are mixed and then subjected to crystallization treatment, second calcination, ammonium exchange, and third calcination in sequence to obtain the ZSM-5 molecular sieve catalyst.

5. The preparation method according to claim 4, characterized in that, The silica-type binder includes at least one of solid silica gel, silica sol, and fumed silica. And / or, the organic amine includes at least one selected from ethylamine, ethylenediamine, propylamine, n-butylamine, triethylamine, and tetrapropylammonium hydroxide; And / or, the organic template agent includes one or more of ethylamine, ethylenediamine, propylamine, n-butylamine, triethylamine, and tetrapropylammonium hydroxide.

6. The preparation method according to claim 4 or 5, characterized in that, The mass ratio of silica-type binder (calculated as SiO2) to ZSM-5 molecular sieve powder is 0.15–0.

20. And / or, the mass ratio of the organic amine to the silica-type binder (calculated as SiO2) is 0.015 to 0.1; And / or, the mass ratio of the first portion of water to ZSM-5 molecular sieve powder is 0.15 to 0.25; And / or, the molar ratio of the organic template agent to the silica-type binder based on SiO2 is 0.1 to 0.2; And / or, the molar ratio of the second portion of water to the silica-type binder (calculated as SiO2) is 30 to 50.

7. The preparation method according to any one of claims 4-6, characterized in that, The bonding aids include organic bonding aids and optional ionic bonding aids; Preferred. The organic binder includes at least one of methylcellulose, hydroxypropyl methylcellulose, guar gum, and polyethylene glycol. And / or, the ionic bonding agent includes at least one of alkali metal compounds; preferably, the ionic bonding agent includes at least one of potassium chloride, potassium hydroxide, sodium silicate, and sodium chloride; And / or, the mass ratio of the organic binder to the ZSM-5 molecular sieve powder is 0.01 to 0.05; And / or, the mass ratio of the ionic binder to the ZSM-5 molecular sieve powder is 0 to 0.1, preferably 0.04 to 0.

08.

8. The preparation method according to any one of claims 4-7, characterized in that, The columnar body is obtained by extrusion molding; preferably, the maximum radial dimension of the cross section of the column is 0.2 to 0.5 cm, and the length of the column is 0.5 to 1.0 cm. And / or, the temperature of the first roasting is 600-650°C, and the time of the first roasting is 6-12 hours.

9. The preparation method according to any one of claims 4-8, characterized in that, The crystallization treatment temperature is 180–200°C, and the crystallization treatment time is 1–2 days; And / or, after the crystallization treatment, the material is first washed and then dried, and then calcined. And / or, the temperature of the second calcination is 600-650°C, and the time of the second calcination is 6-12 hours; And / or, the ammonium exchange includes: mixing with an ammonium solution, exchanging at 60-80°C for 2-4 hours, and washing after solid-liquid separation; Preferably, the ammonium solution contains NH4 + The molar concentration is 1–3 mol / L; And / or, the ammonium exchange is performed 2 to 4 times; And / or, after the ammonium exchange, a third drying process is performed, followed by a third calcination. And / or, the temperature of the third calcination is 500-600°C, and the time of the third calcination is 4-8 hours.

10. The application of the surface-rich silica-free ZSM-5 molecular sieve catalyst according to any one of claims 1-3 or the surface-rich silica-free ZSM-5 molecular sieve catalyst prepared by the preparation method according to any one of claims 4-9 in the catalytic conversion of light hydrocarbons.

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