ZSM-5 molecular sieve catalyst as well as synthesis preparation method and application thereof

Abstract

The invention provides a ZSM-5 molecular sieve catalyst as well as a synthesis preparation method and application thereof. The synthesis preparation method comprises the following steps: adding a ZSM-5 molecular sieve into an alkali solution for alkali treatment and pore expansion; filtering the ZSM-5 molecular sieve subjected to alkali treatment, and washing with water until a washing solution is neutral; adding the washed ZSM-5 molecular sieve into an acid solution for acid passivation, and neutralizing Al-OH groups on the outer surface; carrying out centrifugal separation on the ZSM-5 molecular sieve subjected to acid passivation, removing waste liquid, and carrying out drying treatment; and roasting the dried ZSM-5 molecular sieve to obtain the hierarchical pore ZSM-5 molecular sieve catalyst with gradient distribution acidity, wherein the outer surface of the hierarchical pore ZSM-5 molecular sieve catalyst is weakly acidic and the pore channels are moderately strongly acidic. Through acid-base bifunctional pretreatment, coordinated regulation and control of a pore structure and acidity are realized, a gradient structure with weak acidity on the outer surface and medium-strong acidity in a pore channel is formed, and the conversion rate and PX selectivity of the catalyst in MTPX reaction are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of catalyst preparation, and particularly relates to a ZSM-5 molecular sieve catalyst, a synthesis and preparation method thereof, and application of the ZSM-5 molecular sieve catalyst in a reaction of toluene methanol methylation to produce paraxylene. BACKGROUND

[0002] Paraxylene (PX) is widely used in the polyester industry as an important organic chemical raw material, and its production process mainly relies on catalytic cracking reforming of naphtha. However, China is short of oil resources, there is a large gap in the PX market, and the degree of dependence on imports is high. The toluene methanol methylation to produce PX (MTPX) technology can utilize excess toluene and cheap methanol, realize clean and efficient utilization of coal resources, and alleviate oil dependence, which is of great significance to guarantee the safety of the polyester industry chain and promote the coordinated development of coal chemical industry and petroleum chemical industry.

[0003] ZSM-5 molecular sieve has good aromatization activity and shape selectivity in the methanol to aromatics due to its unique MFI three-dimensional pore structure and surface acidity, and is an important catalyst for the MTPX reaction. In the traditional ZSM-5 molecular sieve, the external surface acidity is strong, which can easily lead to excessive methylation of toluene and occurrence of side reactions, and the pore structure and acid distribution are not reasonable, which affects the diffusion of reactants and products and the reaction efficiency. Therefore, how to modify the ZSM-5 molecular sieve to optimize its pore structure and acid distribution and improve the selectivity of paraxylene and the stability of the catalyst is a technical problem to be solved in the field.

[0004] Chinese patent CN111068756B improves toluene conversion rate and xylene selectivity by loading rare earth and group VIII elements on the molecular sieve carrier, but the metal loading can block the micropores of the ZSM-5 molecular sieve, increase the molecular diffusion resistance, and the pore structure of the molecular sieve itself is not optimized, which is difficult to solve the limitation of inherent micropores on diffusion efficiency.

[0005] Chinese patent application CN117654594A optimizes the acid properties of the ZSM-5 molecular sieve by nitrogenation treatment, passivates strong acid centers, retains medium-strong acid centers, and improves the PX selectivity, but does not improve the pore structure, the micropore diffusion performance is still limited, and the uniformity of the nitrogenation treatment can affect the acid regulation effect of the external surface, which can still cause PX secondary reactions.

[0006] Chinese patent application CN110872126A forms an inert shell layer on the external surface of the molecular sieve by a secondary crystallization reaction under alkaline conditions to eliminate the external surface acid center, but the shell layer formation conditions are complex, the thickness and uniformity are difficult to control, which can easily lead to incomplete passivation of the acid center or affect the contact of the active sites, and the internal micropore structure is not modified, so the pore structure and acid properties cannot be optimized.

[0007] The prior art is limited to single-dimensional optimization or adjustment of acid properties or modification of pore structure in improving toluene methanol methylation catalysts, lacks synergistic regulation of pore structure and acid properties, and is difficult to overcome both micropore diffusion limitation and side reactions caused by external surface acidity. SUMMARY

[0008] (One) technical problems to be solved

[0009] The present application provides a ZSM-5 molecular sieve catalyst and a synthesis and preparation method and application thereof to solve the technical problems of synergistic regulation of pore structure and acid properties, and to overcome both micropore diffusion limitation and side reactions caused by external surface acidity.

[0010] (Two) technical solutions

[0011] In order to solve the above technical problems, the present application provides a synthesis and preparation method of a ZSM-5 molecular sieve catalyst, which comprises the following steps:

[0012] S1. ZSM-5 molecular sieve is added to an alkali solution for alkali treatment and pore expansion;

[0013] S2. The ZSM-5 molecular sieve after alkali treatment is filtered and washed until the washing liquid is neutral;

[0014] S3. The ZSM-5 molecular sieve after washing is added to an acid solution for acid passivation to neutralize the external surface Al-OH groups;

[0015] S4. The ZSM-5 molecular sieve after acid passivation is centrifuged to remove waste liquid and dried;

[0016] S5. The dried ZSM-5 molecular sieve is calcined to obtain a multi-level pore ZSM-5 molecular sieve catalyst with gradient distribution of weak acid on the external surface and medium-strong acid in the channel.

[0017] Further, in step S1, the alkali is one or more of NaOH, tetrapropylammonium hydroxide and tetraethylammonium hydroxide; the concentration of the alkali solution is 0.05-0.1 mol / L; and the liquid-solid ratio of the alkali solution to the ZSM-5 molecular sieve is 50 mL:1 g.

[0018] Further, in step S1, the alkali treatment temperature is 55-65℃, and the alkali treatment time is 1-2 h.

[0019] Further, in step S3, the acid is acetic acid; the concentration of the acid solution is 0.1-0.3 mol / L; and the liquid-solid ratio of the acid solution to the ZSM-5 molecular sieve is 50 mL:1 g.

[0020] Further, in step S3, the acid passivation temperature is room temperature, and the acid passivation time is 30-60 min.

[0021] Further, in step S4, the drying temperature is 100 DEG C, and the drying time is 12h.

[0022] Further, in step S5, the calcination temperature is 550 DEG C, and the calcination time is 3h.

[0023] In addition, the application further provides a ZSM-5 molecular sieve catalyst prepared by the above method.

[0024] In addition, the application further provides application of the above ZSM-5 molecular sieve catalyst in MTPX reaction, wherein the prepared ZSM-5 molecular sieve catalyst is loaded in a fixed bed reactor, and raw gas of toluene and methanol is introduced for reaction.

[0025] Further, the reaction temperature is controlled at 450 DEG C, the reaction pressure is 0.2 MPa, the molar ratio of toluene to methanol is 1:1, and the space velocity is 1.0h -1 ; during the reaction, the reaction product is sampled and analyzed every 12h, a total of 6 times, and by means of gas chromatography analysis, the methanol conversion rate, toluene conversion rate, xylene selectivity and PX selectivity in xylene are determined according to the average value of initial reaction for 72h.

[0026] (III) Beneficial effects

[0027] The application provides a ZSM-5 molecular sieve catalyst and a synthesis and preparation method and application thereof, which comprises the following steps: adding ZSM-5 molecular sieve into an alkali solution for alkali treatment and hole expansion; filtering and washing the ZSM-5 molecular sieve after alkali treatment until the washing liquid is neutral; adding the ZSM-5 molecular sieve after water washing into an acid solution for acid passivation to neutralize Al-OH groups on the outer surface; centrifugally separating the ZSM-5 molecular sieve after acid passivation, removing waste liquid and performing drying treatment; and calcining the dried ZSM-5 molecular sieve to obtain a multi-level pore ZSM-5 molecular sieve catalyst with gradient distribution of weak acid on the outer surface and medium-strong acid in the channel.

[0028] The application realizes synergistic regulation of pore structure and acidity by acid-alkali dual-function pretreatment, forms a gradient structure of weak acid on the outer surface and medium-strong acid in the channel, and improves the conversion rate and PX selectivity of the catalyst in MTPX reaction.

[0029] The ZSM-5 molecular sieve catalyst prepared by the acid-alkali dual-function pretreatment method has a gradient structure of weak acid on the outer surface and medium-strong acid in the channel, and the average pore size is expanded to The outer surface acid density is reduced by 40-60%. The gradient acid structure and the optimized pore size are coordinated to reduce the outer surface side reaction and promote the product diffusion. The prepared catalyst can realize the methanol conversion rate of 95%, the toluene conversion rate of 36%, the xylene selectivity of 71%, and the PX selectivity in the xylene of 93% under specific reaction conditions, and the selectivity of para-xylene and the stability of the catalyst are significantly improved. The preparation method is simple, low in cost, does not need special equipment, and has a good industrial application prospect. DETAILED DESCRIPTION

[0030] In order to make the purpose, content and advantages of the present application clearer, the specific embodiments of the present application are further described in detail below in combination with examples.

[0031] Example 1

[0032] 10g of ZSM-5 molecular sieve is added into 500mL of NaOH solution with a concentration of 0.05mol / L, stirred at 60℃ for 1h, filtered and washed with water until neutral, added into 500mL of acetic acid solution with a concentration of 0.1mol / L, treated at room temperature for 60min, centrifuged, dried at 100℃ for 12h, calcined at 550℃ in a muffle furnace for 3h, and ZSM-5 molecular sieve catalyst is obtained.

[0033] Example 2

[0034] 10g of ZSM-5 molecular sieve is added into 500mL of NaOH solution with a concentration of 0.06mol / L, stirred at 60℃ for 1.2h, filtered and washed with water until neutral, added into 500mL of acetic acid solution with a concentration of 0.15mol / L, treated at room temperature for 50min, centrifuged, dried at 100℃ for 12h, calcined at 550℃ in a muffle furnace for 3h, and ZSM-5 molecular sieve catalyst is obtained.

[0035] Example 3

[0036] 10g of ZSM-5 molecular sieve is added into 500mL of NaOH solution with a concentration of 0.07mol / L, stirred at 60℃ for 1.3h, filtered and washed with water until neutral, added into 500mL of acetic acid solution with a concentration of 0.12mol / L, treated at room temperature for 55min, centrifuged, dried at 100℃ for 12h, calcined at 550℃ in a muffle furnace for 3h, and ZSM-5 molecular sieve catalyst is obtained.

[0037] Example 4

[0038] Take 10 g of ZSM-5 molecular sieve, add to 500 mL of 0.08 mol / L NaOH solution, stir at 60°C for 1.5 h, filter and wash with water to neutral, add to 500 mL of 0.2 mol / L acetic acid solution, treat at room temperature for 45 min, centrifugal separation, dry at 100°C for 12 h, calcine in a muffle furnace at 550°C for 3 h to obtain a ZSM-5 molecular sieve catalyst.

[0039] Example 5

[0040] Take 10 g of ZSM-5 molecular sieve, add to 500 mL of 0.09 mol / L NaOH solution, stir at 60°C for 1.8 h, filter and wash with water to neutral, add to 500 mL of 0.25 mol / L acetic acid solution, treat at room temperature for 35 min, centrifugal separation, dry at 100°C for 12 h, calcine in a muffle furnace at 550°C for 3 h to obtain a ZSM-5 molecular sieve catalyst.

[0041] Example 6

[0042] Take 10 g of ZSM-5 molecular sieve, add to 500 mL of 0.1 mol / L NaOH solution, stir at 60°C for 2 h, filter and wash with water to neutral, add to 500 mL of 0.3 mol / L acetic acid solution, treat at room temperature for 30 min, centrifugal separation, dry at 100°C for 12 h, calcine in a muffle furnace at 550°C for 3 h to obtain a ZSM-5 molecular sieve catalyst.

[0043] Example 7

[0044] Take 10 g of ZSM-5 molecular sieve, add to 500 mL of 0.1 mol / L tetrapropylammonium hydroxide solution, stir at 60°C for 2 h, filter and wash with water to neutral, add to 500 mL of 0.18 mol / L acetic acid solution, treat at room temperature for 40 min, centrifugal separation, dry at 100°C for 12 h, calcine in a muffle furnace at 550°C for 3 h to obtain a ZSM-5 molecular sieve catalyst.

[0045] Example 8

[0046] Take 10 g of ZSM-5 molecular sieve, add to 500 mL of 0.1 mol / L tetraethylammonium hydroxide solution, stir at 60°C for 2 h, filter and wash with water to neutral, add to 500 mL of 0.28 mol / L acetic acid solution, treat at room temperature for 38 min, centrifugal separation, dry at 100°C for 12 h, calcine in a muffle furnace at 550°C for 3 h to obtain a ZSM-5 molecular sieve catalyst.

[0047] Comparative Example 1

[0048] The original ZSM-5 molecular sieve (SiO2 / Al2O3=50) without acid-base bifunctional pretreatment was directly used in MTPX reaction.

[0049] The catalysts prepared in the above eight examples and one comparative example were applied to MTPX reaction, and the reaction conditions were as follows: reaction temperature 450℃, reaction pressure 0.2MPa, molar ratio of toluene to methanol 1:1, space velocity 1.0h-1. -1 The reaction results are shown in the following table:

[0050]

[0051] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the technical principles of the present application, a number of improvements and modifications can be made, and these improvements and modifications should also be considered as the protection scope of the present application.

Claims

1. A method for synthesizing and preparing a ZSM-5 molecular sieve catalyst, characterized in that, The synthesis preparation method includes the following steps: S1. Add ZSM-5 molecular sieve to an alkaline solution for alkaline treatment to expand pores; S2. Wash the ZSM-5 molecular sieve after alkali treatment with water until the washing solution is neutral. S3. Add the washed ZSM-5 molecular sieve to an acid solution for acid passivation to neutralize the Al-OH groups on the outer surface; S4. Centrifuge the acid-passivated ZSM-5 molecular sieve to remove waste liquid and then dry it. S5. The dried ZSM-5 molecular sieve is calcined to obtain a hierarchical porous ZSM-5 molecular sieve catalyst with a gradient distribution of acidity from weak acidity on the outer surface to medium-strong acidity in the pores.

2. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S1, the alkali is one or more of NaOH, tetrapropylammonium hydroxide, and tetraethylammonium hydroxide; the concentration of the alkali solution is 0.05-0.1 mol / L; and the liquid-solid ratio of the alkali solution to the ZSM-5 molecular sieve is 50 mL: 1 g.

3. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S1, the alkali treatment temperature is 55-65℃ and the alkali treatment time is 1-2 hours.

4. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S3, the acid is acetic acid; the concentration of the acid solution is 0.1-0.3 mol / L; and the liquid-solid ratio of the acid solution to the ZSM-5 molecular sieve is 50 mL: 1 g.

5. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S3, the acid passivation temperature is room temperature, and the acid passivation time is 30–60 min.

6. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S4, the drying temperature is 100℃ and the drying time is 12h.

7. The method for synthesizing and preparing ZSM-5 molecular sieve catalyst as described in claim 1, characterized in that, In step S5, the calcination temperature is 550℃ and the calcination time is 3 hours.

8. A ZSM-5 molecular sieve catalyst, characterized in that, The ZSM-5 molecular sieve catalyst was prepared by the method described in any one of claims 1 to 7.

9. The application of the ZSM-5 molecular sieve catalyst according to claim 8 in the MTPX reaction, characterized in that, The synthesized ZSM-5 molecular sieve catalyst was packed into a fixed-bed reactor, and toluene and methanol feed gas were introduced to carry out the reaction.

10. The application of the ZSM-5 molecular sieve catalyst as described in claim 9 in the MTPX reaction, characterized in that, The reaction temperature was controlled at 450℃, the reaction pressure was 0.2MPa, the molar ratio of toluene to methanol was 1:1, and the space velocity was 1.0h⁻¹. During the reaction, the reaction products were sampled and analyzed every 12 hours, for a total of 6 samplings. The methanol conversion rate, toluene conversion rate, xylene selectivity, and PX selectivity in xylene were determined by gas chromatography based on the average value of the initial 72 hours of reaction.

Images