Modified MWW molecular sieve and preparation method thereof, xylene isomerization catalyst and preparation method and application thereof

Abstract

The invention provides a modified MWW molecular sieve and a preparation method thereof, a xylene isomerization catalyst and a preparation method and application thereof, the method adopts two structure-directing agents to synthesize spherical MWW molecular sieve raw powder formed by agglomeration of nanosheet structures, and adopts an alkaline treatment agent to perform hydrothermal treatment on the MWW molecular sieve raw powder, the MWW molecular sieve raw powder is subjected to acid pickling modification by adopting the acid treating agent, so that the acid center distribution and the pore structure of the MWW molecular sieve raw powder are effectively adjusted, the prepared xylene isomerization catalyst has optimized acid center and acid strength and proper pore structure, and when the xylene isomerization catalyst is used for ethylbenzene-containing xylene isomerization, the ethylbenzene-containing xylene isomerization effect is improved. The catalyst can improve the selectivity of p-xylene, also has high ethylbenzene conversion activity (i.e., conversion of ethylbenzene into xylene) and ethylbenzene ethyl removal activity (i.e., conversion of ethylbenzene ethyl removal into benzene), and realizes high yield of xylene or benzene.

Description

Technical Field

[0001] The present disclosure relates to the technical field of xylene isomerization, and specifically, to a modified MWW molecular sieve and a preparation method thereof, a xylene isomerization catalyst and a preparation method and application thereof. Background Art

[0002] Para-xylene (PX) is an important chemical raw material. With the development of coatings, fuels, pesticides and pharmaceuticals in recent years, the demand for PX has grown rapidly. At present, the industry generally uses xylene isomerization technology to increase the production of PX. This technology can convert ethylbenzene (EB) into xylene or deethylate to generate benzene while converting meta-xylene and o-xylene into PX. The economic efficiency of these two methods depends on the composition of the raw materials, the energy consumption of the device and the market conditions. When the mass fraction of ethylbenzene in the raw material is low, the ethylbenzene deethylation route is often used. The reaction can be completed under higher space velocity and lower hydrogen-to-hydrocarbon ratio and pressure conditions to deethylate the ethylbenzene in the raw material and convert it into benzene. When the mass fraction of ethylbenzene in the raw material is high, the deethylation route will produce a large amount of benzene as a by-product. When the price of benzene is low, the economic efficiency will significantly deteriorate. Therefore, high ethylbenzene raw materials often use the route of converting ethylbenzene into xylene.

[0003] Currently, the types of molecular sieves and acidic characteristics of the catalysts used in the two routes are significantly different, and the reaction conditions are also different.

[0004] CN100425343C discloses an ethylbenzene deethylation isomerization catalyst, the active component of which is a ZSM-5 molecular sieve, and the catalyst contains 1.0-4.5% by mass of mordenite. The catalyst is suitable for isomerization reaction of aromatic isomerization raw materials with high ethylbenzene content, and has a high ethylbenzene removal capacity under the conditions of reaction temperature of 320-400°C and reaction pressure of 0.1-1.2MPa. The small amount of mordenite contained in the catalyst has good deethylation performance and good aromatic isomerization performance.

[0005] CN102107144A discloses an ethylbenzene conversion isomerization catalyst, the active component of which is an EUO / mesoporous composite molecular sieve, wherein the mesoporous molecular sieve is an MCM series mesoporous molecular sieve or an SBA series mesoporous molecular sieve. The catalyst is used for the isomerization of C8 aromatics and has a high ethylbenzene conversion rate and a high thermodynamic approach value for p-xylene. At a reaction temperature of 350-450°C, a reaction pressure of 0.6-1.5MPa, a hydrogen-to-hydrocarbon molar ratio of 2-8, and a mass space velocity of 2-5h -1 Under the conditions, the thermodynamic equilibrium approach value of paraxylene can reach up to 99.2%, and the ethylbenzene conversion rate can reach up to 53.2%.

[0006] CN106669809A discloses a C8 aromatic isomerization catalyst and its preparation method and application, wherein the active component is a MCM-22 / Eu-1 composite molecular sieve. An organic halogen element is added during the preparation of the molecular sieve so that the MCM-22 molecular sieve is evenly wrapped around the Eu-1 molecular sieve. When the composite catalyst is used in the C8 aromatic isomerization reaction process, it has an ideal xylene isomerization function while reducing side reactions such as xylene dealkylation, and significantly reduces the loss of xylene compared to existing catalysts. The catalyst also has a high ethylbenzene conversion rate (ethylbenzene dealkylation is converted into benzene and ethane), at a reaction temperature of 370°C, a reaction pressure of 1.0MPa, and an aromatic volume space velocity of 2h -1 Under the condition of hydrogen-hydrocarbon molar ratio of 2:1, the ethylbenzene conversion rate can reach 46.8%.

[0007] However, the existing xylene isomerization catalysts cannot meet the requirements of both deethylation and ethylbenzene isomerization process routes. For raw materials with different ethylbenzene contents, catalysts with different molecular sieve components are required. Summary of the invention

[0008] The purpose of the present disclosure is to provide a modified MWW molecular sieve and a preparation method thereof, a xylene isomerization catalyst and a preparation method and application thereof. The modified MWW molecular sieve has an optimized acid center and acid strength and a suitable pore structure. When it is used for xylene isomerization reaction, it can achieve more xylene or benzene according to different raw materials and reaction conditions.

[0009] In order to achieve the above object, the present disclosure provides a method for preparing a modified MWW molecular sieve in a first aspect, the method comprising:

[0010] (1) mixing a silicon source, an aluminum source, a first structure directing agent, a second structure directing agent and water to obtain a first mixture; performing a crystallization treatment on the first mixture to obtain a raw MWW molecular sieve powder; wherein the first structure directing agent is selected from a bispiperidinyl alkane compound, and the second structure directing agent is selected from piperidinyl methylamine;

[0011] (2) contacting the MWW molecular sieve raw powder with an alkaline treatment agent and performing a hydrothermal treatment to obtain a first intermediate;

[0012] (3) contacting the first intermediate with an acidic treatment agent and performing an acid washing treatment to obtain a modified MWW molecular sieve.

[0013] Optionally, in step (1), the silicon source is selected from an inorganic silicon source, and the inorganic silicon source is selected from one or more of water glass, silicon dioxide, silica gel and white carbon black, preferably water glass and / or silicon dioxide; the aluminum source is selected from one or more of aluminum oxide, sodium aluminate, aluminum sulfate and aluminum hydroxide, preferably aluminum oxide and / or sodium aluminate; the first structure directing agent is selected from one or more of the compounds having the structure shown in the following formula (1):

[0014] In formula (1), n ​​is any integer in the range of 2 to 10, preferably any integer in the range of 4 to 6;

[0015] The second structure directing agent is selected from one or more compounds having a structure represented by the following formula (2):

[0016] In formula (2), R is selected from an alkyl group having 1 to 4 carbon atoms; preferably, R is selected from one or more of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl and tert-butyl; more preferably, R is selected from one or more of methyl, ethyl and n-butyl.

[0017] Optionally, in step (1), the first structure directing agent is selected from one or more of 1,4-dipiperidinylbutane, 1,5-dipiperidinylpentane and 1,6-dipiperidinylhexane; the second structure directing agent is selected from one or more of 1-methyl-4-piperidinylmethylamine, 1-n-butyl-4-piperidinylmethylamine and 1-ethyl-4-piperidinylmethylamine.

[0018] Optionally, in step (1), the molar ratio of the aluminum source to the silicon source is 0.01-0.1, preferably 0.04-0.75; the molar ratio of the first structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the second structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the water to the silicon source is 10-100, preferably 20-50; wherein the silicon source is calculated as SiO2, and the aluminum source is calculated as Al2O3.

[0019] Optionally, in step (1), the temperature of the crystallization treatment is 140-190° C., preferably 165-175° C.; the time is 20-120 h, preferably 25-75 h; preferably, the method further comprises mixing the first mixture with a pH adjuster to adjust the pH value of the first mixture to 8-10, wherein the pH adjuster comprises one or more of sodium hydroxide solution, potassium hydroxide solution, lithium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution and ammonia water, preferably sodium hydroxide solution.

[0020] Optionally, in step (2), the conditions for the hydrothermal treatment include: using an alkaline treatment agent as a steam source to perform hydrothermal treatment on the MWW molecular sieve raw powder; preferably, the alkaline treatment agent is selected from one or more of triethanolamine, potassium hydroxide, ammonium bicarbonate, tetrapropylammonium hydroxide and diisopropylamine, preferably triethanolamine; preferably, the molar concentration of the alkaline treatment agent is 0.01-0.08 mol / L, preferably 0.04-0.06 mol / L; relative to 1g of the MWW molecular sieve raw powder, the volume of the alkaline treatment agent is 5-30 mL; preferably, the temperature of the hydrothermal treatment is 200-450°C, preferably 350-400°C; the time is 0.5-8h, preferably 2-6h.

[0021] Optionally, in step (3), the conditions for the pickling treatment include: mixing the first intermediate with an acidic treatment agent and stirring under water bath conditions to obtain a second mixture; sequentially performing suction filtration treatment and a first drying treatment on the obtained second mixture; the acidic treatment agent is selected from one or more of an aqueous nitric acid solution, an aqueous sulfuric acid solution and an aqueous citric acid solution, preferably an aqueous citric acid solution; preferably, the acid content in the acidic treatment agent is 0.01 to 10% by weight, preferably 0.5 to 4% by weight; the volume of the acidic treatment agent is 5 to 30 mL relative to 1 g of the first intermediate; the temperature of the water bath is 50 to 90° C., preferably 60 to 80° C.; the stirring time is 2 to 10 h, preferably 4 to 6 h; the suction filtration time is 1 to 8 h, preferably 2 to 4 h; the temperature of the first drying treatment is 100 to 300° C., preferably 110 to 170° C.; and the time is 8 to 24 h, preferably 12 to 18 h.

[0022] Optionally, the method further comprises, before the hydrothermal treatment in step (2), subjecting the MWW molecular sieve raw powder to an ammonium exchange treatment with an ammonium salt solution, and washing and performing a first calcination treatment on the product obtained by the ammonium exchange treatment to obtain a hydrogen-type MWW molecular sieve raw powder; preferably, the temperature of the ammonium exchange treatment is 40 to 100°C, preferably 50 to 90°C; the time is 1 to 4 hours, preferably 2 to 4 hours; the number of exchanges is 2 to 4 times; the temperature of the first calcination treatment is 480 to 640°C, preferably 500 to 540°C; the time is 6 to 24 hours, preferably 8 to 12 hours.

[0023] Optionally, the molar concentration of the ammonium salt solution is 0.01 to 0.08 mol / L, preferably 0.04 to 0.06 mol / L; relative to 1 g of the MWW molecular sieve raw powder, the volume of the ammonium salt solution is 5 to 30 mL; the ammonium salt solution is selected from one or more of ammonium chloride solution, ammonium nitrate solution, ammonium bicarbonate solution, ammonium iodide solution and ammonium bromide solution.

[0024] The second aspect of the present disclosure provides a modified MWW molecular sieve prepared by the method described in the first aspect of the present disclosure.

[0025] Optionally, the molar ratio of SiO2 to Al2O3 in the modified MWW molecular sieve is 10 to 100, preferably 25 to 50; preferably, the molecular sieve particles of the modified MWW molecular sieve are spherical particles agglomerated from nanosheets; optionally, the average particle size of the spherical particles is 1 to 10 μm, preferably 1.5 to 4.5 μm; the average pore size is 2 to 8 nm, preferably 2 to 6 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; preferably, the ratio of the B acid amount to the L acid amount of the modified MWW molecular sieve measured by pyridine adsorption infrared method at 350°C is 1 to 2, preferably 1.2 to 1.8.

[0026] The third aspect of the present disclosure provides a method for preparing a xylene isomerization catalyst, comprising the following steps: mixing the modified MWW molecular sieve described in the second aspect of the present disclosure with a binder to obtain a third mixture, and then sequentially performing a molding treatment, a second drying treatment, and a second calcination treatment to obtain a second intermediate; S2, contacting the second intermediate with a solution containing a Group VIII metal compound, performing an impregnation treatment, and then sequentially performing a third drying treatment, an activation treatment, and a reduction treatment.

[0027] Optionally, in step S1, the weight ratio of the modified MWW molecular sieve to the binder is 1:(0.43~19), preferably 1:(1~5.7); optionally, the binder is selected from one or more of alumina, aluminum sol, titanium aluminum colloid and aluminum hydroxide, preferably alumina; the conditions of the molding treatment include: mixing and molding the third mixture with a peptizing agent solution, the concentration of the peptizing agent solution is 1~5 weight%; relative to 1g of the third mixture, the volume of the peptizing agent solution is 0.5~1.5mL; optionally, the peptizing agent solution includes one or more of nitric acid aqueous solution, citric acid aqueous solution, oxalic acid aqueous solution and phosphoric acid aqueous solution, preferably nitric acid aqueous solution; the temperature of the second drying treatment is 100~300℃, preferably 120~180℃; the time is 8~24h, preferably 12~24h; the temperature of the second calcination treatment is 400~600℃, preferably 540~580℃; the time is 2~24h, preferably 4~12h.

[0028] Optionally, in step S2, the content of Group VIII metal in the solution containing Group VIII metal compound is 0.01-0.5g, preferably 0.05-0.35g; relative to 1g of the second intermediate, the volume of the solution containing Group VIII metal compound is 0.5-2mL; the Group VIII metal compound is selected from one or more of chloroplatinic acid, hexaamineplatinum, nickel nitrate and ruthenium nitrate, preferably chloroplatinic acid; the temperature of the third drying treatment is 120-160°C, and the time is 2-4h; the temperature of the activation treatment is 120-160°C, and the time is 8-12h; the temperature of the reduction treatment is 300-380°C, and the time is 1-4h; the reduction treatment is carried out in the presence of a reducing gas, and the reducing gas is selected from one or more of hydrogen, hydrogen-containing nitrogen and hydrogen-containing helium, preferably hydrogen.

[0029] The fourth aspect of the present disclosure provides a xylene isomerization catalyst prepared by the method described in the third aspect of the present disclosure.

[0030] Optionally, the xylene isomerization catalyst includes a carrier and a metal active component loaded on the carrier; the carrier contains the modified MWW molecular sieve and an inorganic refractory oxide, and the metal active component includes a Group VIII metal; based on the dry weight of the carrier, the content of the modified MWW molecular sieve calculated as SiO2 is 5 to 70 weight %, the content of the inorganic refractory oxide calculated as Al2O3 is 30 to 95 weight %, and the content of the metal active component calculated as a metal element is 0.01 to 0.5 weight %.

[0031] Optionally, based on the dry weight of the carrier, the content of the modified MWW molecular sieve calculated as SiO2 is 15-50 weight%, the content of the inorganic refractory oxide calculated as Al2O3 is 50-85 weight%, and the content of the metal active component calculated as the metal element is 0.05-0.35 weight%; wherein the inorganic refractory oxide is selected from one or more of alumina, aluminum sol, titanium aluminum colloid and aluminum hydroxide, preferably alumina; the Group VIII metal is selected from one or more of platinum, palladium, nickel and ruthenium, preferably platinum.

[0032] Optionally, the average pore size of the xylene isomerization catalyst is 2 to 8 nm, preferably 2 to 7 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; the ratio of the amount of B acid to the amount of L acid of the xylene isomerization catalyst measured at 350°C by pyridine adsorption infrared method is 1 to 2, preferably 1.2 to 1.8.

[0033] A fifth aspect of the present disclosure provides an application of the dimethyl isomerization catalyst described in the fourth aspect of the present disclosure in a xylene isomerization reaction.

[0034] Optionally, the method comprises the following steps: in the presence of hydrogen, contacting a xylene feedstock with the xylene isomerization catalyst to carry out an isomerization reaction, wherein the xylene feedstock contains ethylbenzene.

[0035] Optionally, based on the weight of the xylene raw material, when the ethylbenzene content is 8 to 20% by weight, the conditions for the isomerization reaction include: a temperature of 280 to 380° C., preferably 300 to 370° C.; a pressure of 0.2 to 2.2 MPa, preferably 0.4 to 1.6 MPa; a hydrogen / hydrocarbon molar ratio of 1.0 to 6.5, preferably 4.0 to 6.0; a feed weight hourly space velocity of 2 to 10 h -1 , preferably 3 to 5 hours -1 ; and / or, based on the weight of the xylene raw material, when the ethylbenzene content is less than 8% by weight, the conditions of the isomerization reaction include: temperature of 300-400°C, preferably 320-390°C; pressure of 0.2-2.0Mpa, preferably 0.4-1.8Mpa; hydrogen / hydrocarbon molar ratio of 0.1-6.0, preferably 1.0-3.0; feed weight hourly space velocity of 2-10h -1 , preferably 8 to 10 hours -1 .

[0036] Through the above technical scheme, the present disclosure provides a modified MWW molecular sieve and a preparation method thereof, a xylene isomerization catalyst and a preparation method and application thereof, wherein the method uses two structure-directing agents to synthesize spherical MWW molecular sieve raw powder formed by agglomeration of nanosheets, and uses an alkaline treatment agent to hydrothermally treat the MWW molecular sieve raw powder, and uses an acid treatment agent to acid-wash and modify the MWW molecular sieve raw powder, effectively adjusting the acid center distribution and pore structure of the MWW molecular sieve, so that the prepared modified MWW molecular sieve has an optimized acid center and acid strength and a suitable pore structure. The xylene isomerization catalyst is prepared by using the modified MWW molecular sieve to obtain a xylene isomerization catalyst with a high selectivity for xylene, and the xylene isomerization catalyst also has a high ethylbenzene conversion activity (i.e., ethylbenzene is converted into xylene) and ethylbenzene deethylation activity (i.e., ethylbenzene is deethylated and converted into benzene), and when it is used for the isomerization of xylene containing ethylbenzene, more xylene or benzene can be produced.

[0037] Other features and advantages of the present disclosure will be described in detail in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0039] Figure 1 is a SEM image of the Z-1 molecular sieve prepared in Example 1 of the present disclosure;

[0040] Figure 2 is the XRD pattern of the Z-1 molecular sieve prepared in Example 1 of the present disclosure;

[0041] Figure 3 It is a SEM picture of the DZ-1 molecular sieve prepared in Comparative Example 7 of the present disclosure. DETAILED DESCRIPTION

[0042] The specific embodiments of the present disclosure are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure, and are not used to limit the present disclosure.

[0043] The first aspect of the present disclosure provides a method for preparing a modified MWW molecular sieve, the method comprising:

[0044] (1) mixing a silicon source, an aluminum source, a first structure directing agent, a second structure directing agent and water to obtain a first mixture; performing a crystallization treatment on the first mixture to obtain a raw MWW molecular sieve powder; wherein the first structure directing agent is selected from a bispiperidinyl alkane compound, and the second structure directing agent is selected from piperidinyl methylamine;

[0045] (2) contacting the MWW molecular sieve raw powder with an alkaline treatment agent and performing a hydrothermal treatment to obtain a first intermediate;

[0046] (3) contacting the first intermediate with an acidic treatment agent and performing an acid washing treatment to obtain a modified MWW molecular sieve.

[0047] The method for preparing the modified MWW molecular sieve provided by the present invention adopts two structure-directing agents to synthesize spherical MWW molecular sieve raw powder formed by agglomeration of nanosheets. The MWW molecular sieve raw powder has a suitable silicon-aluminum ratio and a large contact area, which is beneficial to improving the mass transfer efficiency; the prepared MWW molecular sieve raw powder is subjected to hydrothermal treatment with an alkaline treatment agent, so as to adjust the acidity of the molecular sieve, increase the content and strength of the B acid site, and optimize the accessibility of the B acid center of the molecular sieve; the MWW molecular sieve raw powder is subjected to acid washing treatment with an acidic treatment agent, so as to remove the residual non-skeleton structure generated by the hydrothermal treatment, make the pore structure of the MWW molecular sieve raw powder regular, increase the total pore volume of the molecular sieve, and make the prepared modified MWW molecular sieve have an optimized acid center and acid strength as well as a suitable pore structure.

[0048] In one embodiment of the present disclosure, the silicon source is selected from an inorganic silicon source, and the inorganic silicon source is selected from one or more of water glass, silicon dioxide, silica gel and white carbon black, preferably water glass and / or silicon dioxide; wherein the molar ratio of H2O to SiO2 in water glass is 10 to 100, preferably 20 to 50; the molar ratio of Al2O3 to SiO2 is 0.01 to 0.1, preferably 0.05 to 0.75; the aluminum source is selected from one or more of aluminum oxide, sodium aluminate, aluminum sulfate and aluminum hydroxide, preferably aluminum oxide and / or sodium aluminate. In the above embodiment, by selecting the preferred silicon source and aluminum source, it is beneficial to improve the relative crystallinity of the molecular sieve and optimize the aggregation morphology of the molecular sieve.

[0049] In one embodiment of the present disclosure, the first structure directing agent is selected from one or more compounds having a structure represented by the following formula (1): In formula (1), n ​​is any integer in the range of 2 to 10; preferably, it is an integer in the range of 4 to 6; more preferably, the first structure directing agent is selected from one or more of 1,4-dipiperidylbutane, 1,5-dipiperidylpentane and 1,6-dipiperidylhexane. In the above embodiment, by selecting the preferred first structure directing agent, it is beneficial to guide the molecules to form a highly crystalline ordered structure.

[0050] In one embodiment of the present disclosure, the second structure directing agent is selected from one or more compounds having a structure represented by the following formula (2): In formula (2), R is selected from an alkyl group having 1 to 4 carbon atoms; preferably, R is selected from one or more of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl and tert-butyl; more preferably, it is one or more of methyl, ethyl and n-butyl; the second structure directing agent is selected from one or more of 1-methyl-4-piperidinylmethylamine, 1-n-butyl-4-piperidinylmethylamine and 1-ethyl-4-piperidinylmethylamine. In the above embodiment, by selecting the preferred second structure directing agent, it is beneficial to guide the molecular sieve to form a spatial arrangement suitable for the xylene isomerization reaction.

[0051] In one embodiment of the present disclosure, in step (1), the molar ratio of the aluminum source to the silicon source is 0.01-0.1, preferably 0.04-0.75; the molar ratio of the first structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the second structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the water to the silicon source is 10-100, preferably 20-50; wherein the silicon source is calculated as SiO2, and the aluminum source is calculated as Al2O3.

[0052] In one embodiment of the present disclosure, in step (1), the temperature of the crystallization treatment is 140-190°C, preferably 165-175°C; the time is 20-120h, preferably 25-75h. In a preferred embodiment, the method further comprises mixing the first mixture with a pH adjuster to adjust the pH value of the first mixture to 8-10, wherein the pH adjuster comprises one or more of sodium hydroxide solution, potassium hydroxide solution, lithium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution and ammonia water, preferably sodium hydroxide solution. In the above embodiment, by selecting the first mixture with a preferred ratio, it is beneficial to synthesize the MWW molecular sieve raw powder formed by the agglomeration of nanosheets with a suitable silicon-aluminum ratio.

[0053] In one embodiment of the present disclosure, in step (2), the conditions of the hydrothermal treatment include: using an alkaline treatment agent as a steam source to perform hydrothermal treatment on the MWW molecular sieve raw powder; in a preferred embodiment, the alkaline treatment agent is selected from one or more of triethanolamine, potassium hydroxide, ammonium bicarbonate, tetrapropylammonium hydroxide and diisopropylamine, preferably triethanolamine; the molar concentration of the alkaline treatment agent is 0.01-0.08 mol / L, preferably 0.04-0.06 mol / L; relative to 1g of the MWW molecular sieve raw powder, the volume of the alkaline treatment agent is 5-30 mL; the temperature of the hydrothermal treatment is 200-450°C, preferably 350-400°C; the time is 0.5-8h, preferably 2-6h.

[0054] In one embodiment of the present disclosure, in step (3), the conditions of the pickling treatment include: mixing the first intermediate with an acidic treatment agent and stirring under water bath conditions to obtain a second mixture; sequentially performing suction filtration treatment and a first drying treatment on the obtained second mixture; in a preferred embodiment, the acidic treatment agent is selected from one or more of an aqueous nitric acid solution, an aqueous sulfuric acid solution and an aqueous citric acid solution, preferably an aqueous citric acid solution; the acid content in the acidic treatment agent is 0.01 to 10% by weight, preferably 0.5 to 4% by weight; the volume of the acidic treatment agent is 5 to 30 mL relative to 1 g of the first intermediate; the temperature of the water bath is 50 to 90° C., preferably 60 to 80° C.; the stirring time is 2 to 10 h, preferably 4 to 6 h; the suction filtration time is 1 to 8 h, preferably 2 to 4 h; the temperature of the first drying treatment is 100 to 300° C., preferably 110 to 170° C.; and the time is 8 to 24 h, preferably 12 to 18 h.

[0055] In one embodiment of the present disclosure, the method further comprises, before the hydrothermal treatment in step (2), subjecting the MWW molecular sieve raw powder to an ammonium exchange treatment with an ammonium salt solution, and washing and performing a first calcination treatment on the product obtained by the ammonium exchange treatment to obtain a hydrogen-type MWW molecular sieve raw powder; in a preferred embodiment, the temperature of the ammonium exchange treatment is 40 to 100°C, preferably 50 to 90°C; the time is 1 to 4 hours, preferably 2 to 4 hours; the number of exchanges is 2 to 4 times; the temperature of the first calcination treatment is 480 to 640°C, preferably 500 to 540°C; the time is 6 to 24 hours, preferably 8 to 12 hours; optionally, the molar concentration of the ammonium salt solution is 0.01 to 0.08 mol / L, preferably 0.04 to 0.06 mol / L; relative to 1g of the MWW molecular sieve raw powder, the volume of the ammonium salt solution is 5 to 30 mL; the ammonium salt solution is selected from one or more of ammonium chloride solution, ammonium nitrate solution, ammonium bicarbonate solution, ammonium iodide solution and ammonium bromide solution.

[0056] The second aspect of the present disclosure provides a modified MWW molecular sieve prepared by the method described in the first aspect of the present disclosure.

[0057] In one embodiment of the present disclosure, the molar ratio of SiO2 to Al2O3 in the modified MWW molecular sieve is 10 to 100, preferably 25 to 50; preferably, the molecular sieve particles of the modified MWW molecular sieve are spherical particles agglomerated from nanosheets; the average particle size of the spherical particles is 1 to 10 μm, preferably 1.5 to 4.5 μm; the average pore size is 2 to 8 nm, preferably 2 to 6 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; Optionally, the ratio of the amount of B acid to the amount of L acid measured by the pyridine adsorption infrared method at 350°C of the modified MWW molecular sieve is 1 to 2, preferably 1.2 to 1.8. In the above embodiment, the modified MWW molecular sieve has a suitable silicon-aluminum ratio and pore structure, as well as a smaller particle size, which is conducive to improving the mass transfer efficiency, thereby improving the diffusion of reactant molecules and improving the reaction activity of the catalyst.

[0058] A third aspect of the present disclosure provides a method for preparing a xylene isomerization catalyst, comprising the following steps:

[0059] S1. Mixing the modified MWW molecular sieve described in the second aspect of the present disclosure with a binder to obtain a third mixture, and then sequentially performing a molding process, a second drying process, and a second calcination process to obtain a second intermediate;

[0060] S2, contacting the second intermediate with a solution containing a Group VIII metal compound, performing an impregnation treatment, and then sequentially performing a third drying treatment, an activation treatment, and a reduction treatment.

[0061] In one embodiment of the present disclosure, in step S1, the weight ratio of the modified MWW molecular sieve to the binder is 1:(0.43-19), preferably 1:(1-5.7); optionally, the binder is selected from one or more of alumina, aluminum sol, titanium aluminum colloid and aluminum hydroxide, preferably alumina; the conditions of the molding treatment include: mixing and molding the third mixture with a peptizing agent solution, the concentration of the peptizing agent solution is 1-5 weight %; relative to 1g of the third mixture, the volume of the peptizing agent solution is 0.5-1.5mL; optionally, the peptizing agent solution includes one or more of nitric acid aqueous solution, citric acid aqueous solution, oxalic acid aqueous solution and phosphoric acid aqueous solution, preferably nitric acid aqueous solution; the temperature of the second drying treatment is 100-300°C, preferably 120-180°C; the time is 8-24h, preferably 12-24h; the temperature of the second calcination treatment is 400-600°C, preferably 540-580°C; the time is 2-24h, preferably 4-12h. In the above embodiment, the third mixture is mixed with the acid solution and subjected to a molding process to obtain a molded carrier, and the molded carrier is subjected to a second drying process and a second calcination process to fully remove moisture from the molded carrier.

[0062] In one embodiment of the present disclosure, in step S2, the content of the Group VIII metal in the solution containing the Group VIII metal compound is 0.01 to 0.5 g, preferably 0.05 to 0.35 g; relative to 1 g of the second intermediate, the volume of the solution containing the Group VIII metal compound is 0.5 to 2 mL; the Group VIII metal compound is selected from one or more of chloroplatinic acid, hexaamine platinum, nickel nitrate and ruthenium nitrate, preferably chloroplatinic acid; the temperature of the third drying treatment is 120 to 160° C., and the time is 2 to 4 hours; the temperature of the activation treatment is 120 to 160° C., and the time is 8 to 12 hours; the temperature of the reduction treatment is 300 to 380° C., and the time is 1 to 4 hours; the reduction treatment is carried out in the presence of a reducing gas, and the reducing gas is selected from one or more of hydrogen, hydrogen-containing nitrogen and hydrogen-containing helium, preferably hydrogen. In the above embodiment, the preferred solution containing a Group VIII metal compound is selected for impregnation, which is beneficial to load the noble metal on the third intermediate to further optimize the acidity and acid strength of the catalyst.

[0063] The fourth aspect of the present disclosure provides a xylene isomerization catalyst prepared by the method described in the third aspect of the present disclosure.

[0064] In one embodiment of the present disclosure, the xylene isomerization catalyst comprises a carrier and a metal active component supported on the carrier; the carrier comprises the modified MWW molecular sieve and an inorganic refractory oxide, and the metal active component comprises a Group VIII metal; based on the dry weight of the carrier, the content of the modified MWW molecular sieve is 5 to 70% by weight in terms of SiO2, the content of the inorganic refractory oxide is 30 to 95% by weight in terms of Al2O3, and the content of the metal active component is 0.01 to 0.5% by weight in terms of metal element. The Group VIII metal in the metal active component exists in the form of, but is not limited to, a simple substance, an oxide or a sulfide.

[0065] The xylene isomerization catalyst provided by the present disclosure has optimized acid centers, acid strength and pore volume. When used in the xylene isomerization reaction, it exhibits optimized catalytic performance and can improve the selectivity for xylene. It also has high ethylbenzene conversion activity (i.e., ethylbenzene is converted into xylene) and ethylbenzene deethylation activity (i.e., ethylbenzene is deethylated into benzene), thereby achieving high production of xylene or benzene.

[0066] In one embodiment of the present disclosure, based on the dry weight of the carrier, the content of the modified MWW molecular sieve calculated as SiO2 is 15-50 weight%, the content of the inorganic refractory oxide calculated as Al2O3 is 50-85 weight%, and the content of the metal active component calculated as the metal element is 0.05-0.35 weight%; the inorganic refractory oxide is selected from one or more of alumina, aluminum sol, titanium aluminum sol and aluminum hydroxide, preferably alumina; the Group VIII metal is selected from one or more of platinum, palladium, nickel and ruthenium, preferably platinum.

[0067] In one embodiment of the present disclosure, the average pore size of the xylene isomerization catalyst is 2 to 8 nm, preferably 2 to 7 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; the ratio of the B acid amount to the L acid amount of the xylene isomerization catalyst measured by pyridine adsorption infrared method at 350°C is 1 to 2, preferably 1.2 to 1.8. In the above embodiment, by selecting a catalyst with a preferred composition and acid strength, the catalytic activity and selectivity of the catalyst can be further improved.

[0068] A fifth aspect of the present disclosure provides an application of the dimethyl isomerization catalyst described in the fourth aspect of the present disclosure in a xylene isomerization reaction.

[0069] The dimethyl isomerization catalyst provided by the present disclosure exhibits optimized catalytic performance when used in a xylene isomerization reaction, can improve the selectivity for p-xylene, and simultaneously improve the ethylbenzene conversion activity and the ethylbenzene deethylation activity, thereby achieving a higher production of xylene or benzene.

[0070] The catalyst disclosed in the present invention has both ethylbenzene conversion activity and ethylbenzene deethylation activity, and can flexibly control the reaction products according to the composition of the xylene feedstock containing ethylbenzene, the reaction temperature, pressure and hydrogen / hydrocarbon molar ratio, so as to achieve more xylene or benzene. In one embodiment of the present invention, in the presence of hydrogen, the xylene feedstock is contacted with the xylene isomerization catalyst for isomerization reaction, wherein the xylene feedstock contains ethylbenzene. In one embodiment, when the ethylbenzene content is 8 to 20% by weight based on the weight of the xylene feedstock, the conditions of the isomerization reaction include: temperature of 280 to 380°C, preferably 300 to 370°C; pressure of 0.2 to 2.2 MPa, preferably 0.4 to 1.6 MPa; hydrogen / hydrocarbon molar ratio of 1.0 to 6.5, preferably 4.0 to 6.0; weight hourly space velocity of the feed of 2 to 10 h -1 , preferably 3 to 5 hours -1 In the above embodiment, the ethylbenzene content in the xylene raw material is relatively high, and the temperature, pressure and hydrogen / hydrocarbon molar ratio of the above reaction are used to improve the ethylbenzene conversion activity of the catalyst and achieve more xylene production.

[0071] In another embodiment, when the ethylbenzene content is less than 8% by weight based on the weight of the xylene raw material, the conditions of the isomerization reaction include: temperature of 300-400°C, preferably 320-390°C; pressure of 0.2-2.0 MPa, preferably 0.4-1.8 MPa; hydrogen / hydrocarbon molar ratio of 0.1-6.0, preferably 1.0-3.0; feed weight hourly space velocity of 2-10 h -1 , preferably 8 to 10 hours -1 In the above embodiment, the ethylbenzene content in the xylene raw material is relatively low, and the temperature, pressure and hydrogen / hydrocarbon molar ratio of the above reaction are used to improve the ethylbenzene deethylation activity of the catalyst and achieve more benzene production.

[0072] The present disclosure is further illustrated by the following examples, but the present disclosure is not limited thereto. The raw materials used in the examples can be obtained through commercial sources. Among them, in the following examples and comparative examples of the present disclosure, the chemical composition of the molecular sieve and the catalyst samples was determined by using a 3013 X-ray fluorescence spectrometer of Rigaku Electric Co., Ltd., Japan, to calculate the silicon-aluminum ratio of the molecular sieve and the catalyst, a tungsten target, an excitation voltage of 40 kV, and an excitation current of 50 mA;

[0073] The total specific surface area of ​​the molecular sieve samples and the average pore size and total pore volume of the catalyst samples were calculated using the BET method.

[0074] The average particle size of the molecular sieve samples was determined using LS13 320XR-laser diffraction particle size analyzer;

[0075] The average thickness of the molecular sieve sample was measured by a scanning electron microscope. The specific method was as follows: the sample was dispersed in ethanol as a suspension and then placed in a scanning electron microscope for measurement. The thickness of the particles was marked using Nano Measurer 1.2 software. Multiple photos were collected, and the total amount of collected data was greater than 300. The average value was taken to obtain the average thickness of the sample.

[0076] The content of molecular sieve, inorganic refractory oxide and Group VIII metal in the catalyst was tested by XRF analyzer, the instrument model was Rigaku 3271E;

[0077] The acid content of B acid and L acid was measured by 2,6-di-tert-butylpyridine adsorption infrared acid method using a Vertex 70 instrument produced by Bruker Instruments. The specific method is as follows: the catalyst is pressed into a 10 mg / cm 2 The thin slice was placed in an infrared cell with a CaF2 window, first evacuated at 400°C, then lowered to 150°C to adsorb 2,6-di-tert-butylpyridine for 15 minutes, then evacuated for 1 hour, lowered to room temperature to collect spectra, and calculated the acid amounts of B acid and L acid. See Applied Catalysis A: General, 294, 2005: 92.

[0078] The p-xylene equilibrium achievement rate, xylene yield, ethylbenzene conversion rate and benzene yield were calculated by the gas chromatography peak area normalization method. The specific test method is: add the peak areas of all components to get the total area, divide the peak area of ​​each component by the total area to get the content of this component.

[0079] Example 1

[0080] The modified MWW molecular sieve was prepared by the following steps:

[0081] (1) First, 22.5 g of 1,5-dipiperidinylpentane (a first structure directing agent, with a molar mass of 250 g / mol) was dissolved in 337.5 g of water to form a solution, and 5 g of sodium aluminate (aluminum source) and 2 g of solid sodium hydroxide (alkali source) were slowly added thereto under stirring conditions, and stirred for 12 h. Then, 45 g of silicon dioxide (silicon source) was added thereto and stirred for 18 h to form a liquid sol with a uniform phase. 11.5 g of 1-methyl-4-piperidinylmethylamine (a second structure directing agent, with a molar mass of 128 g / mol) was added to the above liquid sol and mixed evenly to obtain a first mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source was 0.04, the molar ratio of the first structure directing agent to the silicon source was 0.12, the molar ratio of the second structure directing agent to the silicon source was 0.12, the molar ratio of water to the silicon source was 25, the silicon source was calculated as SiO2, and the aluminum source was calculated as Al2O3. The first mixture is placed in a reactor and sealed with a cover, and the reactor is placed in an oven for crystallization treatment at a temperature of 175° C. for 35 hours to obtain MWW molecular sieve raw powder;

[0082] (2) The MWW molecular sieve raw powder is subjected to ammonium exchange treatment with an aqueous solution of ammonium nitrate, and the obtained material is washed and subjected to a first calcination treatment to obtain a hydrogen-type MWW molecular sieve raw powder; the temperature of the ammonium exchange treatment is 90°C, the time is 4 hours, and the number of exchanges is 4 times; the temperature of the first calcination treatment is 500°C, and the time is 10 hours; the molar concentration of the aqueous solution of ammonium nitrate is 0.06 mol / L, and the volume of the aqueous solution of ammonium nitrate is 30 mL relative to 1 g of the MWW molecular sieve raw powder. Using triethanolamine solution (alkaline treatment agent) as a steam source, the obtained hydrogen-type MWW molecular sieve raw powder is subjected to a hydrothermal treatment to obtain a first intermediate; the molar concentration of the triethanolamine solution is 0.04 mol / L, and the volume of the triethanolamine solution is 30 mL relative to 1 g of the hydrogen-type MWW molecular sieve raw powder, and the temperature of the hydrothermal treatment is 350°C and the time is 6 hours;

[0083] (3) Using citric acid aqueous solution as an acidic treatment agent, the first intermediate is subjected to an acid washing treatment, wherein the conditions for the acid washing treatment include: mixing the citric acid aqueous solution with the first intermediate, and stirring them uniformly in a water bath, and sequentially subjecting the obtained second mixture to a suction filtration treatment and a first drying treatment; the acid content in the citric acid aqueous solution is 2 wt %, and the volume of the citric acid aqueous solution is 30 mL relative to 1 g of the first intermediate; the temperature of the water bath is 60° C., the stirring time is 5 h, the suction filtration time is 2 h, and the temperature of the first drying treatment is 110° C. for 18 h, thereby obtaining a modified MWW molecular sieve 1, denoted as Z-1, whose structure is shown in Tables 1 and Figures 1-2 shown.

[0084] The xylene isomerization catalyst was prepared by the following steps:

[0085] S1. 5 g of the modified MWW molecular sieve obtained in the above step was uniformly mixed with 15 g of alumina (binder) (the weight ratio of the modified MWW molecular sieve to alumina was 1:3), 20 mL of nitric acid aqueous solution was added to the obtained 20 g of the third mixture to form a viscous mixture, and the mixture was extruded to obtain a strip-shaped product, and the strip-shaped product was subjected to a second drying treatment at 120° C. for 24 h, and then pelletized, and subjected to a second calcination treatment at 540° C. for 12 h to obtain a second intermediate;

[0086] (3) 20 g of the second intermediate was impregnated with 20 mL of an aqueous solution of chloroplatinic acid containing 0.05 g of platinum (the volume of the aqueous solution of chloroplatinic acid was 1 mL relative to 1 g of the second intermediate), and then subjected to a third drying treatment at 120° C. for 2 h, an activation treatment at 160° C. for 8 h, and a reduction treatment at 300° C. for 4 h under hydrogen to obtain a xylene isomerization catalyst 1, denoted as C-1, whose structure and composition are shown in Table 2.

[0087] According to Table 1 and Figure 1 It can be seen that the Z-1 molecular sieve presents spherical particles formed by the aggregation of nano-flakes, which has a large mass transfer efficiency and is conducive to the diffusion of reactant molecules; according to Figure 2 The XRD spectrum has peaks near 2θ=6.6, 7.2, 8.0, 9.7, 20.19, and 23.7°, indicating that the Z-1 molecular sieve has an MWW structure. According to the data in Table 2, the prepared catalyst C-1 has a suitable silicon-aluminum ratio and acid strength, and the B / L ratio is 1.32, which is beneficial to improving the isomerization activity of the catalyst.

[0088] Example 2

[0089] The same as Example 1, the only difference is that in step (1), 4g of 1,6-dipiperidinylhexane (the first structure directing agent, whose molar mass is 264g / mol), 320.6g of water, 5g of sodium aluminate (aluminum source), 187.5g of water glass (silicon source, wherein the SiO2 mass concentration is 24wt%, and the molar ratio of SiO2 to Na2O is 3.1), and 12.8g of 1-n-butyl-4-piperidinylmethylamine (the second structure directing agent, whose molar mass is 170g / mol) are mixed uniformly to obtain a first mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source is 0.04, the molar ratio of the first structure directing agent to the silicon source is 0.02, the molar ratio of the second structure directing agent to the silicon source is 0.1, and the molar ratio of water to the silicon source is 33. The crystallization temperature is 165°C and the time is 25h to obtain MWW molecular sieve raw powder;

[0090] In step (2), the volume of the ammonium nitrate aqueous solution is 5 mL relative to 1 g of the MWW molecular sieve raw powder; the volume of the triethanolamine solution is 5 mL relative to 1 g of the hydrogen-type MWW molecular sieve raw powder, wherein the molar concentration of the triethanolamine solution is 0.05 mol / L;

[0091] In step (3), the acid content in the citric acid aqueous solution is 0.5% by weight, and the volume of the citric acid aqueous solution is 5 mL relative to 1 g of the first intermediate; the temperature of the water bath is 80° C., the stirring time is 4 h, and the suction filtration time is 4 h; a modified MWW molecular sieve 2 is obtained, denoted as Z-2, and its structure is shown in Table 1;

[0092] In step S1, 5 g of the modified MWW molecular sieve and 14 g of alumina (binder) are mixed evenly (the weight ratio of the modified MWW molecular sieve to alumina is 1:2.8), 19 mL of nitric acid aqueous solution is added to the obtained 19 g of the third mixture to form a viscous mixture, and extrusion molding is performed to obtain a strip-shaped object;

[0093] In step S2, 20 g of the second intermediate was impregnated with 20 mL of an aqueous solution of chloroplatinic acid containing 0.06 g of platinum, and finally a xylene isomerization catalyst 2 was prepared, which was denoted as C-2. The structure and composition are shown in Table 2.

[0094] Example 3

[0095] The same as Example 1, the only difference is that in step (1), 7g of 1,4-dipiperidinylbutane (the first structure directing agent, whose molar mass is 236g / mol), 472.5g of water, 3g of alumina (aluminum source), 6g of solid sodium hydroxide (alkali source), 45g of silicon dioxide (silicon source), and 8.5g of 1-ethyl-4-piperidinylmethylamine (the second structure directing agent, whose molar mass is 142g / mol) are mixed uniformly to obtain a first mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source is 0.04, the molar ratio of the first structure directing agent to the silicon source is 0.04, the molar ratio of the second structure directing agent to the silicon source is 0.08, and the molar ratio of water to the silicon source is 35. The crystallization temperature is 165°C and the time is 25h to obtain MWW molecular sieve raw powder;

[0096] In step (2), the molar concentration of the triethanolamine solution is 0.05 mol / L, and the volume of the triethanolamine solution is 15 mL relative to 1 g of hydrogen-type MWW molecular sieve raw powder; the temperature of the hydrothermal treatment is 400° C., and the time is 2 h;

[0097] In step (3), the acid content in the citric acid aqueous solution is 4% by weight, and the volume of the citric acid aqueous solution is 20 mL relative to 1 g of the first intermediate; the temperature of the water bath is 70° C., the stirring time is 6 h, and the suction filtration time is 3 h; a modified MWW molecular sieve 3 is obtained, denoted as Z-3, and its structure is shown in Table 1;

[0098] In step S1, 5 g of the modified MWW molecular sieve and 10 g of alumina (binder) are mixed evenly (the weight ratio of the modified MWW molecular sieve to alumina is 1:2), 20 mL of nitric acid aqueous solution is added to the obtained 17 g of the third mixture to form a viscous mixture, and extrusion molding is performed to obtain a strip-shaped object;

[0099] In step S2, 20 g of the second intermediate was impregnated with 20 mL of an aqueous solution of chloroplatinic acid containing 0.03 g of platinum, and finally a xylene isomerization catalyst 3 was prepared, which was denoted as C-3. The structure and composition are shown in Table 2.

[0100] Example 4

[0101] The same as Example 1, the only difference is that in step (1), 11.3 g of 1,5-dipiperidinylpentane (a first structure directing agent, whose molar mass is 250 g / mol), 320.6 g of water, 3 g of alumina (aluminum source), 187.5 g of water glass (silicon source, wherein the SiO2 mass concentration is 24 wt%, and the molar ratio of SiO2 to Na2O is 3.1), and 7.7 g of 1-n-butyl-4-piperidinylmethylamine (a second structure directing agent, whose molar mass is 170 g / mol) are mixed uniformly to obtain a first mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source is 0.04, the molar ratio of the first structure directing agent to the silicon source is 0.06, the molar ratio of the second structure directing agent to the silicon source is 0.06, and the molar ratio of water to the silicon source is 33. The crystallization temperature is 165°C and the time is 25h, and the original MWW molecular sieve powder is obtained; and the modified MWW molecular sieve 4 is further obtained, which is denoted as Z-4 and has a structure as shown in Table 1;

[0102] Finally, xylene isomerization catalyst 4 was prepared, denoted as C-4, and its structure and composition are shown in Table 2.

[0103] Example 5

[0104] The same as Example 1, the only difference is that in step (1), 15.8 g of 1,6-dipiperidinylhexane (a first structure directing agent, whose molar mass is 264 g / mol), 320.6 g of water, 3 g of alumina (aluminum source), 187.5 g of water glass (silicon source, wherein the SiO2 mass concentration is 24 wt%, and the molar ratio of SiO2 to Na2O is 3.1), and 2.1 g of 1-ethyl-4-piperidinylmethylamine (a second structure directing agent, whose molar mass is 142 g / mol) are mixed uniformly to obtain a first mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source is 0.04, the molar ratio of the first structure directing agent to the silicon source is 0.08, the molar ratio of the second structure directing agent to the silicon source is 0.02, and the molar ratio of water to the silicon source is 33. The crystallization temperature was 165°C and the time was 25 h, and the original MWW molecular sieve powder was obtained, and the modified MWW molecular sieve 5 was further obtained, which was recorded as Z-5 and has a structure as shown in Table 1;

[0105] In step (3), the citric acid aqueous solution is replaced with the same amount of sulfuric acid aqueous solution, and finally a xylene isomerization catalyst 5 is prepared, which is denoted as C-5. Its structure and composition are shown in Table 2.

[0106] Example 6

[0107] The same as Example 1, the only difference is that in step (1), the molar ratio of the first structure directing agent to the silicon source in the first mixture is 0.22, and finally the xylene isomerization catalyst 6 is prepared, denoted as C-6, and its structure and composition are shown in Table 2.

[0108] Example 7

[0109] The same as Example 1, the only difference is that in step (1), the molar ratio of the first structure directing agent to the silicon source in the first mixture is 0.5, and finally the xylene isomerization catalyst 7 is prepared, denoted as C-7, and its structure and composition are shown in Table 2.

[0110] Example 8

[0111] The same as Example 1, the only difference is that in step (1), the molar ratio of the second structure directing agent to the silicon source in the first mixture is 0.22, and finally the xylene isomerization catalyst 8 is prepared, denoted as C-8, and its structure and composition are shown in Table 2.

[0112] Example 9

[0113] The same as Example 1, the only difference is that in step (1), the molar ratio of the second structure directing agent to the silicon source in the first mixture is 0.5, and finally the xylene isomerization catalyst 9 is prepared, denoted as C-9, and its structure and composition are shown in Table 2.

[0114] Example 10

[0115] The same as Example 1, the only difference is that the alkaline treatment agent triethanolamine in step (2) is replaced by the same amount of sodium carbonate solution, and finally a xylene isomerization catalyst 10 is prepared, denoted as C-10, whose structure and composition are shown in Table 2.

[0116] Embodiment 11

[0117] The same as Example 1, the only difference is that in step (2), the molar concentration of the triethanolamine solution is 0.2 mol / L, and finally a xylene isomerization catalyst 11 is prepared, denoted as C-11, and its structure and composition are shown in Table 2.

[0118] Example 12

[0119] The same as Example 1, the only difference is that the acidic treatment agent citric acid aqueous solution in step (3) is replaced by the same amount of hydrochloric acid solution, and finally a xylene isomerization catalyst 12 is prepared, denoted as C-12, whose structure and composition are shown in Table 2.

[0120] Embodiment 13

[0121] The same as Example 1, the only difference is that: in step (3), the third intermediate is impregnated with 20 mL of chloroplatinic acid aqueous solution containing 0.01 g of platinum, and finally a xylene isomerization catalyst 13 is prepared, denoted as C-13, whose structure and composition are shown in Table 2.

[0122] Comparative Example 1

[0123] Take 3g of commercial Eu-1 molecular sieve powder with a silicon-aluminum ratio of 30 (provided by Changling Catalyst Factory), and use 50mL of ammonium nitrate aqueous solution for ion exchange. The exchanged Z-1 is washed several times with excess deionized water and then dried. The second intermediate is hydrothermally treated with water vapor at a temperature of 360°C for 4 hours to obtain the first intermediate. The first intermediate is fully mixed with 17g of alumina, 20mL of water is added to mix to form a viscous mixture, and extruded into strips. The strips are first dried at 120°C for 6 hours, then pelletized and calcined at 540°C for 4 hours to obtain the second intermediate. After adding 300mL of water, the second intermediate is treated in a water bath at a temperature of 60°C and a filtration time of 2 hours; the second intermediate is dried at 110°C for 18 hours to obtain the third intermediate. 20 g of the third intermediate was impregnated with 20 mL of an aqueous solution of chloroplatinic acid containing 0.05 g of platinum, and a third drying treatment was performed at 120°C for 4 h. Then, an activation treatment was performed at 160°C in air for 8 h, and a reduction treatment was performed at 300°C in hydrogen for 4 h to prepare a comparative catalyst 1, denoted as D-1, whose structure and composition are shown in Table 2.

[0124] Comparative Example 2

[0125] Take 9g of commercial MOR molecular sieve powder with a silicon-aluminum ratio of 12 (provided by Fushun Catalyst Factory) and perform ion exchange with 270mL of ammonium nitrate aqueous solution, then use 270mL of triethanolamine solution for hydrothermal treatment, the molar concentration of triethanolamine solution is 0.05mol / L, the temperature of hydrothermal treatment is 350℃, and the time is 6h. -1 The first intermediate was obtained by drying at 120°C for 24 hours in a dynamic atmosphere. The first intermediate was fully mixed with 17g of alumina, 20mL of water was added to mix to form a viscous mixture, and extruded into strips. The strips were first dried at 120°C for 6h, then pelletized, and calcined at 540°C for 4h to obtain the second intermediate. After adding 400mL of water, the second intermediate was treated with a water bath at a temperature of 80°C and a filtration time of 4h; the second drying treatment was performed at 120°C for 18h to obtain the third intermediate. 20g of the third intermediate was impregnated with 20mL of a chloroplatinic acid aqueous solution containing 0.05g of platinum, and the third drying treatment was performed at 160°C for 2h, and then activated at 120°C for 12h under air, and reduced at 380°C for 1h under hydrogen to prepare comparative catalyst 2, denoted as D-2, whose structure and composition are shown in Table 2.

[0126] Comparative Example 3

[0127] Take 10g of commercial ZSM-5 molecular sieve powder with a silicon-aluminum ratio of 20 (provided by Fushun Catalyst Factory), exchange it with 50mL of ammonium nitrate aqueous solution, and then use 50mL of triethanolamine solution for hydrothermal treatment. The molar concentration of triethanolamine solution is 0.05mol / L, the temperature of hydrothermal treatment is 380℃, and the time is 4h. The fully washed molecular sieve is dried at 120℃ for 8h in a static atmosphere without air flow to obtain the first intermediate. The first intermediate is fully mixed with 10g of alumina, 20mL of water is added to mix to form a viscous mixture, and extruded into strips. The strips are first dried at 120℃ for 6h, then pelletized and calcined at 550℃ for 4h to obtain the second intermediate. After adding 200mL of water, the second intermediate is treated in a water bath at a temperature of 70℃ and the filtration time is 3h; the second intermediate is dried at 170℃ for 12h to obtain the third intermediate. 20 g of the third intermediate was impregnated with 20 mL of an aqueous solution of chloroplatinic acid containing 0.01 g of platinum, and a third drying treatment was performed at 120° C. for 3 h. Then, an activation treatment was performed at 140° C. for 10 h in air, and a reduction treatment was performed at 340° C. for 4 h in hydrogen to prepare a comparative catalyst 3, denoted as D-3, whose structure and composition are shown in Table 2.

[0128] Comparative Example 4

[0129] The same as Example 1, the only difference is that in step (2), the alkaline treatment agent triethanolamine solution is replaced with the same amount of water to prepare a comparative catalyst 4, denoted as D-4, whose structure and composition are shown in Table 2.

[0130] Comparative Example 5

[0131] The same as Example 1, the only difference is that in step (3), the acidic treatment agent citric acid aqueous solution is replaced by the same amount of water to prepare a comparative catalyst 5, denoted as D-5, whose structure and composition are shown in Table 2.

[0132] Comparative Example 6

[0133] The same as Example 1, the only difference is that in step (1), the second structure directing agent 1-methyl-4-piperidinylmethylamine is replaced by the same amount of 1,2-cyclopentanedicarboximide to obtain a MTW structure molecular sieve, denoted as MTW-1, whose structure is shown in Table 1, and finally a comparative catalyst 6 is obtained, denoted as D-6, whose structure and composition are shown in Table 2.

[0134] Comparative Example 7

[0135] (1) 22gN(CH3)3 + C6H 12 N(CH3)3 + 2Cl - (the first structure directing agent, whose molar mass is 249 g / mol) is dissolved in 337.5 g of water to form a solution, and 5 g of sodium aluminate (aluminum source) and 2 g of solid sodium hydroxide (alkali source) are slowly added thereto under stirring conditions, and the solution is stirred for 12 hours. Then, 45 g of silicon dioxide (silicon source) is added thereto and stirred for 18 hours to form a liquid sol with a uniform phase; 9 g of cycloheximide (the second structure directing agent) is added to the above liquid sol and mixed evenly to obtain a mixture with a pH of 8 to 10, wherein the molar ratio of the aluminum source to the silicon source is 0.04, the molar ratio of the first structure directing agent to the silicon source is 0.12, the molar ratio of the second structure directing agent to the silicon source is 0.12, the molar ratio of water to the silicon source is 25, the silicon source is calculated as SiO2, and the aluminum source is calculated as Al2O3. The mixture was placed in a reactor and sealed with a cover. The reactor was placed in an oven for crystallization at a temperature of 175°C for 35 hours to obtain a comparative MWW molecular sieve, designated as DZ-1, whose structure is shown in Table 1 and Figure 3 As shown;

[0136] (2) 5 g of DZ-1 and 150 mL of N(CH3)3 + C6H 12 N(CH3)3 + ·2Br -The ion exchange was carried out with an aqueous solution (molar concentration of 0.06 mol / L), the exchange temperature was 90°C, the time was 1h, the number of ion exchanges was 4, and the exchange liquid-solid ratio was 30mL / g molecular sieve; the molecular sieve after exchange was washed with excess deionized water for several times until no halogen anions were detected in the eluate, the pH range was 6-8, and the Na2O molar content of the molecular sieve after washing was 0.4%. The molecular sieve after being fully washed was heated at a volume space velocity of 50h -1 The molecular sieve is dried at 120°C for 18 hours in a dynamic atmosphere, and the dried molecular sieve is evenly mixed with 15g of alumina (binder), and 20mL of a 3% nitric acid aqueous solution is added to mix to form a viscous mixture, and the strips are extruded to obtain strips, and the strips are first dried at 120°C for 24 hours, and then pelletized, and calcined at 540°C for 12 hours to obtain a molded carrier;

[0137] (3) Using cyclohexylamine as a steam source, the shaped carrier was hydrothermally treated, the amount of cyclohexylamine used was 30 mL / g molecular sieve, the temperature of the hydrothermal treatment was 350°C, and the time was 6 h; the treated carrier was impregnated with 20 mL of a chloroplatinic acid aqueous solution containing 0.05 g of platinum, and then dried at 120°C for 2 h, activated at 160°C for 8 h, and reduced at 300°C under hydrogen for 4 h to obtain a comparative catalyst 7, denoted as D-7, whose structure and composition are shown in Table 2.

[0138] According to Table 1 and Figure 3 It can be seen that the DZ-1 molecular sieve presents a block structure formed by the accumulation of nano-flakes, with a small contact area, which is not conducive to the diffusion of reactant molecules.

[0139] Table 1 Structure and composition of modified molecular sieves

[0140]

[0141] Table 2 Structure and composition of catalyst

[0142]

[0143]

[0144] From the data in Table 1 and Table 2 above, we can see that:

[0145] In Examples 1 to 13, tetrapropylammonium hydroxide solution was used to perform hydrothermal exchange on the original powder of MWW molecular sieve, and citric acid aqueous solution was used for pickling treatment, which effectively adjusted the acid center distribution of the original powder of MWW molecular sieve, so that the obtained modified MWW molecular sieve had optimized acid center and acid strength, and had a suitable pore structure and a large total pore volume, which was beneficial to improve the mass transfer efficiency. When the modified MWW molecular sieve was used to prepare a xylene isomerization catalyst, the catalyst's para-xylene selectivity and ethylbenzene conversion activity and ethylbenzene deethylation activity could be effectively improved. The comparative examples used Eu-1, MOR or ZSM-5 molecular sieves as carriers, or only used water vapor for hydrothermal treatment, and the obtained comparative catalysts did not have optimized acid centers and acid strengths.

[0146] Test cases 1 to 20

[0147] This test is used to illustrate the xylene isomerization activity and ethylbenzene conversion activity of the catalysts prepared in Examples and Comparative Examples when converted xylene is used as a raw material.

[0148] In a continuous flow fixed bed micro hydrogenation device, 2 g of catalyst was loaded and the catalyst was evaluated using actual industrial conversion type xylene feedstock. The composition weight percentage is shown in Table 3. Reaction conditions: temperature 300 ° C, pressure 0.4 MPa, feed mass space velocity 3 h -1 , the hydrogen / hydrocarbon molar ratio is 4.5. The reaction results of the catalysts of various embodiments and comparative examples in this test example are shown in Table 4. The xylene yield (Xy), ethylbenzene conversion (EBc) and benzene yield (By) are calculated by the following formulas (1) to (4):

[0149]

[0150]

[0151]

[0152]

[0153] Here, PX represents p-xylene, and X represents xylene.

[0154] Table 3

[0155]

[0156] Among them, C8 NA represents a non-aromatic hydrocarbon having eight carbon atoms, B represents benzene, Tol represents toluene, EB represents ethylbenzene, PX represents p-xylene, MX represents m-xylene, and OX represents o-xylene.

[0157] Table 4

[0158]

[0159]

[0160] From the results in Tables 3 to 4 above, it can be seen that, compared with Comparative Examples 1 to 7, the xylene isomerization catalysts prepared in Examples 1 to 13 have optimized acid centers, larger specific surface areas and total pore volumes, and the xylene isomerization activity of the catalysts is significantly improved. When used for conversion-type xylene isomerization reactions, high para-xylene selectivity and ethylbenzene conversion activity can be maintained, and more xylene or benzene can be produced for different raw materials and reaction conditions.

[0161] Among them, by comparing the data of Example 1 with those of Examples 6 to 7, it can be seen that in Example 1, the catalyst is prepared under the condition that the molar ratio of the first structure directing agent to the silicon source in the first mixture is 0.02 to 0.12, and the prepared catalyst has a more optimized acid strength and B acid site. When it is used for the conversion-type xylene isomerization reaction, a higher p-xylene equilibrium achievement rate, xylene yield and ethylbenzene conversion rate can be obtained;

[0162] Comparison of the data of Example 1 with Examples 8 to 9 shows that in Example 1, the catalyst is prepared under the condition that the molar ratio of the second structure directing agent to the silicon source in the first mixture is 0.02 to 0.12. The prepared catalyst has a more optimized acid strength and B acid site. When it is used in the conversion-type xylene isomerization reaction, a higher p-xylene equilibrium achievement rate, xylene yield and ethylbenzene conversion rate can be obtained;

[0163] Comparison of the data of Example 1 with Examples 10 to 11 shows that when Example 1 uses triethanolamine as the alkaline treatment agent and the molar concentration of triethanolamine is 0.04 to 0.06 mol / L, the prepared catalyst has better acidity and a larger specific surface area. When it is used in the conversion-type xylene isomerization reaction, it can maintain a higher selectivity for p-xylene and also has a higher ethylbenzene conversion activity.

[0164] Comparison of the data of Example 1 and Example 12 shows that when Example 1 uses citric acid aqueous solution as the acidic treatment agent, the prepared catalyst has an optimized total pore volume. When it is used in the conversion-type xylene isomerization reaction, the catalytic reaction activity is improved, and more p-xylene and xylene can be produced;

[0165] By comparing the data of Example 1 with that of Example 13, it can be seen that when Example 1 adopts the implementation method in which the content of Group VIII metal in the solution containing Group VIII metal compound is 0.05-0.35 g, the prepared catalyst has more excellent xylene isomerization activity and ethylbenzene conversion activity, achieving more production of paraxylene and xylene.

[0166] Test cases 21 to 40

[0167] The same as test example 1, the only difference is that deethylated xylene is used as the raw material, and its composition weight percentage is shown in Table 5. Reaction conditions: temperature 360°C, pressure 0.5MPa, feed mass space velocity 8h -1 , the hydrogen / hydrocarbon molar ratio is 2.1, and the reaction results are shown in Table 6.

[0168] Table 5

[0169]

[0170] Among them, C8 NA represents a non-aromatic hydrocarbon having eight carbon atoms, B represents benzene, Tol represents toluene, EB represents ethylbenzene, PX represents p-xylene, MX represents m-xylene, and OX represents o-xylene.

[0171] Table 6

[0172]

[0173]

[0174] From the results of Tables 5 to 6 above, it can be seen that, compared with Comparative Examples 1 to 7, the xylene isomerization catalysts prepared in Examples 1 to 13 have optimized acid centers, larger specific surface areas and total pore volumes, and the xylene isomerization activity of the catalysts is significantly improved. When used for deethylation-type xylene isomerization reactions, high paraxylene selectivity and ethylbenzene deethylation activity can be maintained, and more xylene or benzene can be produced for different raw materials and reaction conditions.

[0175] Among them, the comparison of the data of Example 1 with Examples 6 to 7 shows that in Example 1, the catalyst is prepared under the condition that the molar ratio of the first structure directing agent to the silicon source in the first mixture is 0.02 to 0.12. The prepared catalyst has a more optimized acid strength and B acid site. When it is used for the deethylated xylene isomerization reaction, a higher p-xylene equilibrium achievement rate and benzene yield can be obtained;

[0176] Comparison of the data of Example 1 with Examples 8 to 9 shows that in Example 1, the catalyst is prepared under the condition that the molar ratio of the second structure directing agent to the silicon source in the first mixture is 0.02 to 0.12. The prepared catalyst has a more optimized acid strength and B acid site. When it is used for the deethylated xylene isomerization reaction, a higher p-xylene equilibrium achievement rate and benzene yield can be obtained;

[0177] Comparison of the data of Example 1 with Examples 10 to 11 shows that when Example 1 uses triethanolamine as the alkaline treatment agent and the molar concentration of triethanolamine is 0.04 to 0.06 mol / L, the prepared catalyst has better acidity and a larger specific surface area. When it is used for the deethylation type xylene isomerization reaction, it can maintain a high selectivity for p-xylene and also has a high deethylation activity for ethylbenzene.

[0178] Comparison of the data of Example 1 and Example 12 shows that when Example 1 uses citric acid aqueous solution as the acidic treatment agent, the prepared catalyst has an optimized total pore volume. When it is used in the deethylated xylene isomerization reaction, the catalytic reaction activity can be improved, and more p-xylene and benzene can be produced.

[0179] Comparison of the data of Example 1 and Example 13 shows that when Example 1 adopts the implementation method in which the content of the Group VIII metal compound in the solution containing the Group VIII metal compound is 0.05 to 0.35 g, the prepared catalyst has more excellent xylene isomerization activity and ethylbenzene deethylation activity, achieving greater production of paraxylene and benzene.

[0180] From the results of the data in Table 4 and Table 6, it can be seen that the catalysts prepared in Examples 1 to 13 of the present disclosure have high selectivity for p-xylene, high ethylbenzene conversion activity (i.e., ethylbenzene is converted into xylene) and ethylbenzene deethylation activity (i.e., ethylbenzene deethylation is converted into benzene), and can achieve more xylene or benzene for different raw materials and reaction conditions. However, the comparative catalysts prepared in Comparative Examples 1 to 7 cannot simultaneously meet the two process routes of ethylbenzene conversion isomerization and ethylbenzene deethylation. Although the comparative catalysts prepared in Comparative Examples 1 to 2 have high ethylbenzene conversion activity, when they are used for deethylation type xylene isomerization reaction, the xylene isomerization activity and ethylbenzene deethylation activity are significantly reduced, and p-xylene and benzene cannot be produced more; although the comparative catalyst prepared in Comparative Example 3 has high ethylbenzene deethylation activity, when it is used for conversion type xylene isomerization reaction, the xylene isomerization activity is significantly reduced, and p-xylene cannot be produced more. The catalysts 1 to 13 prepared in the present disclosure can flexibly control the reaction products according to the raw materials with different ethylbenzene contents and reaction conditions, thereby achieving more production of paraxylene while producing more xylene or benzene.

[0181] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings; however, the present disclosure is not limited to the specific details in the above embodiments. Within the technical concept of the present disclosure, a variety of simple modifications can be made to the technical solution of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0182] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations. In addition, the various different embodiments of the present disclosure can also be combined arbitrarily, as long as they do not violate the ideas of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for preparing a modified MWW molecular sieve, characterized in that: The method comprises: (1) mixing a silicon source, an aluminum source, a first structure directing agent, a second structure directing agent and water to obtain a first mixture; performing a crystallization treatment on the first mixture to obtain a raw MWW molecular sieve powder; wherein the first structure directing agent is selected from a bispiperidinyl alkane compound, and the second structure directing agent is selected from piperidinyl methylamine; (2) contacting the MWW molecular sieve raw powder with an alkaline treatment agent and performing a hydrothermal treatment to obtain a first intermediate; (3) contacting the first intermediate with an acidic treatment agent and performing an acid washing treatment to obtain a modified MWW molecular sieve.

2. The method according to claim 1, characterized in that In step (1), the silicon source is selected from an inorganic silicon source, and the inorganic silicon source is selected from one or more of water glass, silicon dioxide, silica gel and white carbon black, preferably water glass and / or silicon dioxide; The aluminum source is selected from one or more of aluminum oxide, sodium aluminate, aluminum sulfate and aluminum hydroxide, preferably aluminum oxide and / or sodium aluminate; The first structure directing agent is selected from one or more compounds having a structure represented by the following formula (1): In formula (1), n ​​is any integer in the range of 2 to 10, preferably any integer in the range of 4 to 6; The second structure directing agent is selected from one or more compounds having a structure represented by the following formula (2): In formula (2), R is selected from an alkyl group having 1 to 4 carbon atoms; preferably, R is selected from one or more of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl and tert-butyl; more preferably, R is selected from one or more of methyl, ethyl and n-butyl.

3. The method according to claim 2, characterized in that In step (1), the first structure directing agent is selected from one or more of 1,4-dipiperidinylbutane, 1,5-dipiperidinylpentane and 1,6-dipiperidinylhexane; the second structure directing agent is selected from one or more of 1-methyl-4-piperidinylmethylamine, 1-n-butyl-4-piperidinylmethylamine and 1-ethyl-4-piperidinylmethylamine.

4. The method according to claim 1, characterized in that: In step (1), the molar ratio of the aluminum source to the silicon source is 0.01-0.1, preferably 0.04-0.75; the molar ratio of the first structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the second structure directing agent to the silicon source is 0.01-0.22, preferably 0.02-0.12; the molar ratio of the water to the silicon source is 10-100, preferably 20-50; wherein the silicon source is calculated as SiO2, and the aluminum source is calculated as Al2O3.

5. The method according to claim 1, characterized in that In step (1), the crystallization treatment temperature is 140-190° C., preferably 165-175° C., and the time is 20-120 hours, preferably 25-75 hours; Preferably, the method further comprises mixing the first mixture with a pH adjuster to adjust the pH value of the first mixture to 8-10, wherein the pH adjuster comprises one or more of sodium hydroxide solution, potassium hydroxide solution, lithium hydroxide solution, sodium carbonate solution, sodium bicarbonate solution and ammonia water, preferably sodium hydroxide solution.

6. The method according to claim 1, characterized in that In step (2), the conditions of the hydrothermal treatment include: using an alkaline treatment agent as a steam source to perform hydrothermal treatment on the MWW molecular sieve raw powder; Preferably, the alkaline treatment agent is selected from one or more of triethanolamine, potassium hydroxide, ammonium bicarbonate, tetrapropylammonium hydroxide and diisopropylamine, preferably triethanolamine; Preferably, the molar concentration of the alkaline treatment agent is 0.01 to 0.08 mol / L, preferably 0.04 to 0.06 mol / L; relative to 1 g of the MWW molecular sieve raw powder, the volume of the alkaline treatment agent is 5 to 30 mL; Preferably, the temperature of the hydrothermal treatment is 200-450° C., preferably 350-400° C.; the time is 0.5-8 h, preferably 2-6 h.

7. The method according to claim 1, characterized in that In step (3), the conditions of the acid washing treatment include: mixing the first intermediate with the acidic treatment agent and stirring in a water bath to obtain a second mixture; and sequentially performing a suction filtration treatment and a first drying treatment on the obtained second mixture; Preferably, the acidic treatment agent is selected from one or more of nitric acid aqueous solution, sulfuric acid aqueous solution and citric acid aqueous solution, preferably citric acid aqueous solution; the acid content in the acidic treatment agent is 0.01-10% by weight, preferably 0.5-4% by weight; relative to 1g of the first intermediate, the volume of the acidic treatment agent is 5-30mL; the temperature of the water bath is 50-90°C, preferably 60-80°C; the stirring time is 2-10h, preferably 4-6h; the time of the suction filtration treatment is 1-8h, preferably 2-4h; the temperature of the first drying treatment is 100-300°C, preferably 110-170°C; the time is 8-24h, preferably 12-18h.

8. The method according to claim 1, characterized in that The method further comprises, before the hydrothermal treatment in step (2), subjecting the MWW molecular sieve raw powder to an ammonium exchange treatment with an ammonium salt solution, and washing and first calcining the product obtained by the ammonium exchange treatment to obtain a hydrogen-type MWW molecular sieve raw powder; preferably, the temperature of the ammonium exchange treatment is 40 to 100° C., preferably 50 to 90° C.; the time is 1 to 4 hours, preferably 2 to 4 hours; the number of exchanges is 2 to 4 times; the temperature of the first calcination treatment is 480 to 640° C., preferably 500 to 540° C.; the time is 6 to 24 hours, preferably 8 to 12 hours; Optionally, the molar concentration of the ammonium salt solution is 0.01 to 0.08 mol / L, preferably 0.04 to 0.06 mol / L; relative to 1 g of the MWW molecular sieve raw powder, the volume of the ammonium salt solution is 5 to 30 mL; the ammonium salt solution is selected from one or more of ammonium chloride solution, ammonium nitrate solution, ammonium bicarbonate solution, ammonium iodide solution and ammonium bromide solution.

9. The modified MWW molecular sieve prepared by the method according to any one of claims 1 to 8.

10. The modified MWW molecular sieve according to claim 9, characterized in that: The molar ratio of SiO2 to Al2O3 in the modified MWW molecular sieve is 10 to 100, preferably 25 to 50; Preferably, the molecular sieve particles of the modified MWW molecular sieve are spherical particles formed by agglomeration of nanosheets; optionally, the average particle size of the spherical particles is 1 to 10 μm, preferably 1.5 to 4.5 μm; the average pore size is 2 to 8 nm, preferably 2 to 6 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; Preferably, the ratio of the B acid content to the L acid content of the modified MWW molecular sieve measured at 350° C. by pyridine adsorption infrared method is 1 to 2, preferably 1.2 to 1.

8.

11. A method for preparing a xylene isomerization catalyst, characterized in that: The following steps are involved: S1. Mixing the modified MWW molecular sieve according to claim 9 or 10 with a binder to obtain a third mixture, and then sequentially performing a molding process, a second drying process, and a second calcination process to obtain a second intermediate; S2, contacting the second intermediate with a solution containing a Group VIII metal compound, performing an impregnation treatment, and then sequentially performing a third drying treatment, an activation treatment, and a reduction treatment.

12. The method according to claim 11, characterized in that In step S1, the weight ratio of the modified MWW molecular sieve to the binder is 1:(0.43-19), preferably 1:(1-5.7); optionally, the binder is selected from one or more of alumina, aluminum sol, titanium aluminum colloid and aluminum hydroxide, preferably alumina; The molding process conditions include: mixing the third mixture with a peptizing agent solution and molding, wherein the concentration of the peptizing agent solution is 1 to 5 weight %; The volume of the peptizing agent solution is 0.5-1.5 mL relative to 1 g of the third mixture; optionally, the peptizing agent solution includes one or more of nitric acid aqueous solution, citric acid aqueous solution, oxalic acid aqueous solution and phosphoric acid aqueous solution, preferably nitric acid aqueous solution; The temperature of the second drying treatment is 100-300°C, preferably 120-180°C; the time is 8-24h, preferably 12-24h; the temperature of the second calcination treatment is 400-600°C, preferably 540-580°C; the time is 2-24h, preferably 4-12h.

13. The method according to claim 11, characterized in that In step S2, the content of the group VIII metal in the solution containing the group VIII metal compound is 0.01 to 0.5 g, preferably 0.05 to 0.35 g; the volume of the solution containing the group VIII metal compound is 0.5 to 2 mL relative to 1 g of the second intermediate; the group VIII metal compound is selected from one or more of chloroplatinic acid, hexaamineplatinum, nickel nitrate and ruthenium nitrate, preferably chloroplatinic acid; The temperature of the third drying treatment is 120-160°C, and the time is 2-4 hours; the temperature of the activation treatment is 120-160°C, and the time is 8-12 hours; the temperature of the reduction treatment is 300-380°C, and the time is 1-4 hours; the reduction treatment is carried out in the presence of a reducing gas, and the reducing gas is selected from one or more of hydrogen, hydrogen-containing nitrogen and hydrogen-containing helium, preferably hydrogen.

14. A xylene isomerization catalyst prepared according to the method according to any one of claims 11 to 13.

15. The xylene isomerization catalyst according to claim 14, characterized in that: The xylene isomerization catalyst includes a carrier and a metal active component loaded on the carrier; the carrier contains the modified MWW molecular sieve and an inorganic refractory oxide, and the metal active component includes a Group VIII metal; based on the dry weight of the carrier, the content of the modified MWW molecular sieve calculated as SiO2 is 5 to 70 weight %, the content of the inorganic refractory oxide calculated as Al2O3 is 30 to 95 weight %, and the content of the metal active component calculated as a metal element is 0.01 to 0.5 weight %.

16. The xylene isomerization catalyst according to claim 15, characterized in that: Based on the dry weight of the carrier, the content of the modified MWW molecular sieve calculated as SiO2 is 15-50 weight %, the content of the inorganic refractory oxide calculated as Al2O3 is 50-85 weight %, and the content of the metal active component calculated as metal element is 0.05-0.35 weight %; Among them, the inorganic refractory oxide is selected from one or more of alumina, aluminum sol, titanium aluminum colloid and aluminum hydroxide, preferably alumina; the Group VIII metal is selected from one or more of platinum, palladium, nickel and ruthenium, preferably platinum.

17. The xylene isomerization catalyst according to claim 15, characterized in that: The average pore size of the xylene isomerization catalyst is 2 to 8 nm, preferably 2 to 7 nm; the total pore volume is 0.4 to 1 cm 3 / g, preferably 0.5 to 0.8 cm 3 / g; Preferably, the ratio of the amount of B acid to the amount of L acid of the xylene isomerization catalyst measured at 350° C. by pyridine adsorption infrared method is 1 to 2, preferably 1.2 to 1.

8.

18. Use of the dimethyl isomerization catalyst according to any one of claims 14 to 17 in a xylene isomerization reaction.

19. The use according to claim 18, characterized in that The following steps are involved: In the presence of hydrogen, a xylene feedstock is brought into contact with the xylene isomerization catalyst to perform an isomerization reaction, wherein the xylene feedstock contains ethylbenzene; Optionally, when the ethylbenzene content is 8 to 20% by weight based on the weight of the xylene raw material, the isomerization reaction conditions include: a temperature of 280 to 380° C., preferably 300 to 370° C.; The pressure is 0.2-2.2Mpa, preferably 0.4-1.6Mpa; the hydrogen / hydrocarbon molar ratio is 1.0-6.5, preferably 4.0-6.0; the weight hourly space velocity of the feed is 2-10h -1 , preferably 3 to 5 hours -1 and / or, When the ethylbenzene content is less than 8% by weight based on the weight of the xylene raw material, the isomerization reaction conditions include: temperature of 300-400° C., preferably 320-390° C.; pressure of 0.2-2.0 MPa, preferably 0.4-1.8 MPa; hydrogen / hydrocarbon molar ratio of 0.1-6.0, preferably 1.0-3.0; The weight hourly space velocity of the feed is 2 to 10 h -1 , preferably 8 to 10 hours -1 .

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