Preparation method of pyromellitic acid based on modified catalyst

Abstract

The invention provides a preparation method of pyromellitic acid based on a modified catalyst, and belongs to the technical field of pyromellitic acid materials, and the preparation method comprises the following steps: adding a pore-forming agent, an aluminum salt and a template in the process of preparing titanium dioxide by a sol-gel method, carrying out hydrolysis to obtain gel, standing, aging, drying and calcining to form particles; and dipping in a vanadium-tungsten mixed solution to form a precursor solution, loading the precursor solution on an inert carrier, and roasting to obtain the modified catalyst. Filling a gas phase oxidation reaction fixed bed with the modified catalyst, introducing a durene and air mixture, and carrying out catalytic oxidation, hydrolysis, filtration, crystallization, separation and drying to obtain the pyromellitic acid. The prepared catalyst is high in catalytic activity and good in thermal stability and is used for preparing pyromellitic acid, and the obtained product is high in yield and good in purity.

Description

Technical Field

[0001] The invention relates to the technical field of pyromellitic acid materials, and in particular to a preparation method of pyromellitic acid based on a modified catalyst. Background Art

[0002] Pyromellitic acid (Pyromellitic Acid) is an important intermediate, typically produced by hydrolysis of pyromellitic dianhydride. Currently, the production of Pyromellitic Acid in China typically utilizes a fixed-bed tubular reactor and an inert carrier-supported catalyst. Durenes are catalytically oxidized to produce crude Pyromellitic Acid, which is then hydrolyzed and recrystallized to yield refined Pyromellitic Acid. The vapor-phase catalytic oxidation of durenes involves a series of reactions, often accompanied by numerous side reactions that result in low Pyromellitic Acid yields. Therefore, the development of highly selective and active catalysts is crucial.

[0003] Existing modifications to pyromellitic acid catalysts have primarily focused on the introduction of additives. While additives can alter the acidity and alkalinity of the primary catalyst, affecting reaction depth and redox activity, the catalytic mechanism suggests that the primary catalyst, vanadium pentoxide, is unstable and requires a titanium dioxide surface support. The Brønsted acidic sites on the vanadium pentoxide surface protonate the oxygen atoms in the carboxyl groups, reducing the activation energy of the dehydration reaction. The Lewis acidic sites on the titanium dioxide surface adsorb the carbonyl oxygen in the carboxylic acid, synergistically promoting the dehydration reaction with the Brønsted acidic sites on the vanadium pentoxide. Furthermore, the additives act synergistically with the primary catalyst, acting as the core active component of the catalyst and adsorbed on the titanium dioxide surface. As shown by the catalytic mechanism, simply changing the type and amount of additives has limited potential for increasing catalyst activity and stability. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing pyromellitic acid based on a modified catalyst, wherein a catalyst prepared by using modified titanium dioxide loaded with vanadium tungsten oxide as an active component is used in the preparation of pyromellitic acid to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing pyromellitic acid based on a modified catalyst, characterized in that it comprises the following steps:

[0007] S1. Prepare solution:

[0008] The titanium source is dissolved in organic alcohol to obtain solution A; distilled water and a pore-forming agent are added to the organic alcohol for ultrasonic dissolution, and the pH value is adjusted with acid to obtain solution B;

[0009] S2. Slowly add solution B to solution A and stir to react to obtain a first precursor sol-gel solution;

[0010] S3. The aluminum salt is added to the first precursor sol-gel solution of step S2, and the reaction is stirred to obtain a second precursor sol-gel solution;

[0011] S4. The template is added to the second precursor sol-gel solution of step S3, stirred, allowed to soak, then filtered and dried to obtain a doped TiO2 powder;

[0012] S5. The doped TiO2 powder was calcined in stages to obtain Al-TiO2 particles;

[0013] S6. The vanadium source and the additive are dissolved in an oxalic acid solution to obtain a mixed solution;

[0014] S7. The Al-TiO2 particles in step S5 are ultrasonically immersed in the mixed solution of step S6 to obtain a third precursor solution;

[0015] S8. The third precursor solution of step S7 is loaded on an inert carrier and calcined to obtain a modified catalyst;

[0016] S9. Using durenyl and air as raw materials, a gas-phase catalytic oxidation reaction is carried out based on the modified catalyst of step S8. The obtained pyromellitic dianhydride is dissolved in water, and after hydrolysis, filtration, crystallization, separation, and drying, the finished product of pyromellitic acid is obtained.

[0017] Preferably, in step S1, the organic alcohol includes one or more of methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol or tert-butanol; the titanium source includes one or more of tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate or tetra-tert-butyl titanate; the pore-forming agent includes PEG (polyethylene glycol) 1000-4000; the acid includes one or more of glacial acetic acid, nitric acid or hydrochloric acid; most preferably, the organic alcohol is ethanol, the titanium source is tetrabutyl titanate, the pore-forming agent is PEG2000, and the acid is glacial acetic acid.

[0018] Preferably, in step S1, the volume ratio of the titanium source to the organic alcohol in solution A is 1:(1.5-2); the mass ratio of the pore-forming agent to the alcohol-water solvent in solution B is (0.04-0.08):1, the volume ratio of the organic alcohol to distilled water is 1:1, and the pH value is 2-4; the volume ratio of the titanium source in solution A to the distilled water in solution B is (1-2):1.

[0019] Preferably, in step S3, the aluminum salt includes one or more of aluminum nitrate, aluminum chloride, or aluminum isopropoxide; the molar ratio of the aluminum salt (calculated as aluminum) to the first precursor sol-gel solution (calculated as titanium) is Al:Ti=(0.01-0.02):1, and the reaction time is 2-3 hours; most preferably, the aluminum salt is aluminum isopropoxide.

[0020] Preferably, in step S4, the template comprises PMMA (polymethyl methacrylate) microspheres with a diameter of 200-300 nm, the mass ratio of the template to the second precursor sol-gel solution is (0.1-0.2):1, the stirring time is 2-3 hours, and the immersion time is 30-36 hours.

[0021] Preferably, in step S5, the staged calcination procedure is: the first stage has a heating rate of 5°C / min and a temperature of 200-300°C for 3 h; the second stage has a heating rate of 5°C / min and a temperature of 550-600°C for 4 h; both stages are calcined in air.

[0022] Preferably, in step S6, the vanadium source is selected from one or more of ammonium metavanadate, vanadium nitrate and sodium metavanadate; the auxiliary agent is selected from one or more of ammonium metatungstate, sodium tungstate and ammonium tungstate; most preferably, the vanadium source is ammonium metavanadate and the auxiliary agent is sodium tungstate.

[0023] Preferably, in step S7, the molar ratio of Ti:V:W in the third precursor solution is 1:(0.4-0.6):(0.045-0.055), and the ultrasonic immersion time is 4-6 hours.

[0024] Preferably, in step S8, the inert carrier is selected from one or more of α-Al2O3, silicon carbide and ceramic ring, the mass ratio of modified catalyst to inert carrier is (0.1~0.2):1, and the calcination procedure is 250~300℃, constant temperature for 1h, and then continue to heat to 500~590℃ and keep constant temperature for 4h.

[0025] Preferably, in step S9, the reaction temperature is 450-490°C, and the air rate is 5000-6000h -1 , the feed concentration of durene is 20g / m 3 .

[0026] The beneficial effects of the above technical solution of the present invention are as follows:

[0027] 1. The present invention adopts a secondary pore-forming process, successively introducing an organic polymer pore-forming agent and a PMMA template to form mesopores and macropores on the prepared titanium dioxide particles, respectively. The mesopores increase the specific surface area of the titanium dioxide particles, further increasing the vanadium loading capacity; the macropores enhance mass transfer, alleviate the clogging of the pores by carbon deposits, and improve the catalyst activity while extending the catalyst life. Compared with MOFs materials and molecular sieve pore-forming, the pore-forming agent and template used in the present invention are lower in cost.

[0028] 2. The present invention doped Al when preparing titanium dioxide particles 3+ , Al 3+ Replace Ti 4+, causing lattice contraction, inhibiting grain growth, promoting the formation of anatase, and raising the phase transition temperature from anatase to rutile from 550°C to 800°C. When calcining to remove the template, the temperature can be raised to 600°C to ensure that the template is completely removed. At the same time, as the calcination temperature increases, the degree of vanadium aggregation in the catalyst increases, the crystal V2O5 becomes more obvious, and the catalyst activity also increases. The subsequent catalytic oxidation of durene to pyromellitic acid is a strongly exothermic reaction, and doping with Al 3+ It also enhances the thermal stability of the catalyst and avoids the weakening of the catalyst activity caused by the transformation of the titanium dioxide crystal form in the catalyst due to the high temperature and high heat environment.

[0029] 3. The addition of auxiliary agent tungsten in the present invention can synergistically act with vanadium to inhibit deep oxidation, reduce the occurrence of side reactions, and improve product yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above and other objects, features and advantages of the exemplary embodiments of the present invention will become readily understood by reading the following detailed description with reference to the accompanying drawings. In the accompanying drawings, several embodiments of the present invention are shown in an illustrative and non-limiting manner, and the same or corresponding reference numerals represent the same or corresponding parts, wherein:

[0031] Figure 1 is a flow chart of the present invention; DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0033] If specific experimental procedures or conditions are not specified in this protocol, the procedures or conditions described in the literature in this field can be followed. Reagents or instruments used without manufacturer information are commercially available.

[0034] Example

[0035] A method for preparing pyromellitic acid based on a modified catalyst comprises the following steps:

[0036] (1) Preparation of solution:

[0037] The titanium source is dissolved in organic alcohol to obtain solution A; distilled water and a pore-forming agent are added to the organic alcohol, dissolved by ultrasonication, and the pH value is adjusted with acid to obtain solution B;

[0038] Wherein, the organic alcohol comprises one or more of methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol or tert-butanol; the titanium source comprises one or more of tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate or tetratert-butyl titanate; the pore-forming agent comprises PEG1000-4000; the acid comprises one or more of glacial acetic acid, nitric acid or hydrochloric acid; the volume ratio of the titanium source to the organic alcohol in solution A is 1:(1.5-2), the mass ratio of the pore-forming agent to the alcohol-water solvent in solution B is (0.04-0.08):1, the volume ratio of the organic alcohol to distilled water is 1:1, the pH value is 2-4, and the volume ratio of the titanium source in solution A to the distilled water in solution B is (1-2):1;

[0039] (2) Slowly drip solution B into solution A and vigorously stir to react to obtain the first precursor sol-gel solution;

[0040] (3) adding aluminum salt to the first precursor sol-gel solution of step (2), stirring and reacting for 2 to 3 hours to obtain a second precursor sol-gel solution;

[0041] The aluminum salt includes one or more of aluminum nitrate, aluminum chloride, or aluminum isopropoxide; the molar ratio of the aluminum salt (calculated as aluminum) to the first precursor sol-gel solution (calculated as titanium) is Al:Ti=(0.01-0.02):1;

[0042] (4) adding PMMA microspheres with a diameter of 200-300 nm to the second precursor sol-gel solution of step (3), stirring for 2-3 h, letting it stand and soak for 30-36 h, then filtering and drying to obtain doped TiO2 powder, wherein the mass ratio of PMMA microspheres to the second precursor sol-gel solution is (0.1-0.2):1;

[0043] (5) The doped TiO2 powder was calcined in air in stages to obtain Al-TiO2 particles. The staged calcination procedure was as follows: the first stage was heated at a rate of 5°C / min and kept at 200-300°C for 3 h; the second stage was heated at a rate of 5°C / min and kept at 550-600°C for 4 h; both stages were calcined in air;

[0044] (6) dissolving the vanadium source and the auxiliary agent in the oxalic acid solution to obtain a mixed solution;

[0045] Wherein, the vanadium source is selected from one or more of ammonium metavanadate, vanadium nitrate and sodium metavanadate; the auxiliary agent is selected from one or more of ammonium metatungstate, sodium tungstate and ammonium tungstate;

[0046] (7) ultrasonically immersing the Al-TiO2 particles in the mixed solution of step (6) for 4 to 6 hours to obtain a third precursor solution, wherein the molar ratio of Ti:V:W is 1:(0.4-0.6):(0.045-0.055);

[0047] (8) The third precursor solution of step (7) is loaded on an inert carrier to obtain a modified catalyst. The mass ratio of the modified catalyst to the inert carrier is (0.1-0.2):1. The calcination procedure is 250-300°C, constant temperature for 1 hour, and then continued to heat to 500-590°C and constant temperature for 4 hours.

[0048] (9) Using durene and air as raw materials, a gas phase catalytic oxidation reaction is carried out based on the modified catalyst of step (8), the reaction temperature is 450-490 ° C, and the air rate is 5000-6000h -1 , the feed concentration of durene is 20g / m 3 The pyromellitic dianhydride obtained by the reaction is dissolved in water, hydrolyzed, filtered, crystallized, separated, and dried to obtain the finished product pyromellitic acid.

[0049] Conventional modification processes for the preparation of pyromellitic acid catalysts mostly focus on optimizing the type and dosage of additives to change the acidity and alkalinity of the catalyst, affect the reaction depth, and affect the redox ability of the active components. However, from the perspective of the catalytic mechanism of the catalyst, the main catalyst vanadium pentoxide has poor stability and needs to be loaded on the surface of titanium dioxide. The additives and the main catalyst work synergistically and are adsorbed on the surface of titanium dioxide as the core active components of the catalyst. It is generally believed that forming a two-dimensional monolayer vanadium agent load on titanium dioxide can obtain higher catalytic activity, otherwise CO2 will be generated. Therefore, this scheme makes improvements from the perspective of constructing titanium dioxide particles with high specific surface area and high dispersion.

[0050] The present invention uses a mixture of organic polymer and PMMA template to form pores. The organic polymer decomposes during the low-temperature calcination stage to form mesopores. When the calcination temperature is further increased, the PMMA template decomposes to form macropores. The mesopores increase the loading amount of the core active components of the catalyst, thereby increasing the activity of the catalyst. The macropores enhance mass transfer, relieve carbon deposits from blocking the pores, and extend the service life of the catalyst.

[0051] Anatase titanium dioxide maintains high dispersion of the core active ingredients on its surface, preventing agglomeration and exposing more active sites. However, the anatase phase is unstable and begins to irreversibly transform to rutile at temperatures exceeding 550°C. If this phase transition occurs, the proportion of the anatase phase decreases, while the proportion of rutile increases. Rutile titanium dioxide has poor dispersion of the core active ingredients adsorbed on its surface, leading to agglomeration. Furthermore, rutile titanium dioxide readily forms V / Ti mixed crystals with vanadium ions, disrupting the valence ratio and valence state of the vanadium ions and resulting in decreased catalyst activity. During calcination to remove the template, in order to ensure that the template is completely removed, the calcination temperature is usually increased to 600°C, at which time the proportion of rutile titanium dioxide gradually increases; during the calcination process to form vanadium tungsten oxide crystals, increasing the calcination temperature is beneficial to improving the catalyst activity and increasing the conversion of tetramethylbenzene, so increasing the phase transition temperature of titanium dioxide is beneficial to improving the catalyst activity; in the reaction of catalytic oxidation of tetramethylbenzene to prepare isophthalic acid, although the reaction temperature is generally lower than 500°C, the reaction is a highly exothermic reaction, so preparing a catalyst with a higher phase transition temperature can improve the thermal stability of the catalyst and avoid catalyst deactivation caused by the increase in temperature in the reaction bed. When the present invention adopts the sol-gel method to prepare titanium dioxide, Al is introduced. 3+ Doping, Al 3+ Radius ratio Ti 4 + Small, replaces Ti in the lattice 4+ Leaving residual space, the lattice distortion of the doped titanium dioxide increases, and the increase in lattice energy makes anatase stable. 3+ Within a certain doping concentration, the specific surface area of titanium dioxide can be increased, thereby increasing the loading capacity of vanadium compounds.

[0052] Tungsten doping can enhance the weak acid sites on the catalyst surface, promote the stable adsorption of reaction intermediates, rather than further oxidation to CO2; after tungsten doping, V2O is generated on the catalyst surface. 5- The WO3 composite oxide layer, with its compactness, can isolate oxygen from direct contact with the metal active sites, reducing the rate of deep oxidation reactions. At the same time, this composite oxide layer has higher thermodynamic stability, reducing the decomposition of the active phase during the reaction.

[0053] Example 1

[0054] The preparation method of pyromellitic acid based on the modified catalyst in the above embodiment is adopted, wherein:

[0055] (1) Preparation of solution

[0056] Solution A: 1000 mL of tetrabutyl titanate dissolved in 1500 mL of anhydrous ethanol;

[0057] Solution B: Add 500 mL of distilled water to 500 mL of anhydrous ethanol, add 65 g of PEG2000, dissolve under ultrasonication, and adjust the pH to 2-4 with glacial acetic acid;

[0058] (2) Slowly drip solution B into solution A and vigorously stir to react to obtain the first precursor sol-gel solution;

[0059] (3) Add 9 g of aluminum isopropoxide to the first precursor sol-gel solution of step (2), stir and react for 3 h to obtain a second precursor sol-gel solution;

[0060] (4) Add 500 g of PMMA microspheres with a diameter of 250 nm to the second precursor sol-gel solution of step (3), stir for 3 h, let it stand and soak for 34 h, then filter and dry to obtain doped TiO2 powder;

[0061] (5) The doped TiO2 powder was calcined in air in stages to obtain Al-TiO2 particles. The calcination procedure was as follows: the first stage was heated at a rate of 5°C / min and kept at 300°C for 3 h; the second stage was heated at a rate of 5°C / min and kept at 600°C for 4 h;

[0062] (6) Dissolve 75 g of ammonium metavanadate and 19 g of sodium tungstate in 500 mL of 20% oxalic acid solution to obtain a mixed solution;

[0063] (7) 100 g of the Al-TiO2 particles from step (5) were immersed in the mixed solution from step (6) for 6 h under ultrasonic immersion to obtain a third precursor solution;

[0064] (8) The third precursor solution of step (7) was loaded on an inert carrier to obtain a modified catalyst. The mass ratio of the modified catalyst to the inert carrier was 0.2:1. The calcination procedure was 300°C, constant temperature for 1 hour, and then continued to heat to 590°C and constant temperature for 4 hours;

[0065] (9) Using durene and air as raw materials, the catalyst prepared in step (8) was filled into a fixed bed reactor to carry out gas phase catalytic oxidation reaction at a reaction temperature of 480°C and a space velocity of 5500 h -1 , the feed concentration of durene is 20g / m 3 The pyromellitic dianhydride obtained by the reaction is dissolved in water, hydrolyzed, filtered, crystallized, separated, and dried to obtain the finished product pyromellitic acid.

[0066] Example 2

[0067] The difference from Example 1 is that:

[0068] In step (1), the titanium source in solution A is tetraethyl titanate, the organic alcohol is methanol, and the volume of methanol is 2000 mL; in solution B, the organic alcohol is methanol, the pore-forming agent is PEG1000, and the mass of PEG1000 is 72 g;

[0069] In step (3), the aluminum salt is aluminum chloride, with a mass of 5.8 g;

[0070] Step (4) PMMA microspheres with a diameter of 300 nm and an addition amount of 400 g were stirred for 2 h, allowed to stand and soak for 30 h, and then filtered and dried to obtain doped TiO2 powder;

[0071] In step (5), the staged calcination procedure is as follows: the first stage is kept at 200 °C for 3 h; the second stage is kept at 550 °C for 4 h;

[0072] In step (6), the vanadium source is sodium metavanadate, with a mass of 78 g, and the auxiliary agent is ammonium metatungstate, with a mass of 16 g;

[0073] In step (7), the ultrasonic immersion time is 4 h;

[0074] In step (8), the mass ratio of the modified catalyst to the inert carrier is 0.1:1, and the calcination procedure is 250°C, constant temperature for 1 hour, and then continued to heat up to 500°C and constant temperature for 4 hours.

[0075] Example 3

[0076] The difference from Example 1 is that:

[0077] In step (1), the volumes of distilled water and anhydrous ethanol in solution B are both 1000 mL, and the mass of PEG2000 added is 72 g.

[0078] Step (4) PMMA microspheres with a diameter of 200 nm and an addition amount of 600 g were added and allowed to soak for 36 h;

[0079] Example 4

[0080] The difference from Example 1 is that:

[0081] In step (3), the amount of aluminum isopropoxide added was 6 g, and the reaction was stirred for 2 h;

[0082] In step (6), the amount of ammonium metavanadate is 60 g and the amount of sodium tungstate is 17 g.

[0083] Example 5

[0084] The difference from Example 1 is that:

[0085] In step (3), the amount of aluminum isopropoxide added is 12 g;

[0086] In step (6), the amount of ammonium metavanadate is 87 g and the amount of sodium tungstate is 20 g.

[0087] Comparative Example 1

[0088] The difference from Example 1 is that:

[0089] No PEG2000 was added to solution B.

[0090] Comparative Example 2

[0091] The difference from Example 1 is that:

[0092] PMMA microspheres were not added in step (4).

[0093] Comparative Example 3

[0094] The difference from Example 1 is that:

[0095] Delete step (3).

[0096] Comparative Example 4

[0097] The difference from Example 1 is that:

[0098] Sodium tungstate is not added in step (6).

[0099] Comparative Example 5

[0100] The difference from Example 1 is that:

[0101] In step (8), the calcination procedure is 300°C, constant temperature for 1 hour, and then the temperature is raised to 400°C and constant temperature for 4 hours.

[0102] After testing, the yield and purity of the pyromellitic acid prepared in Examples 1-5 and Comparative Examples 1 to 5 are shown in Table 1.

[0103] Table 1 Yield and purity test results of pyromellitic acid

[0104] Yield % purity% Example 1 97% 99.4% Example 2 95% 99.1% Example 3 93% 98.9% Example 4 92% 98.6% Example 5 98% 99.5% Comparative Example 1 73% 96.1% Comparative Example 2 76% 97.0% Comparative Example 3 74% 95.8% Comparative Example 4 77% 94.2% Comparative Example 5 78% 97.5%

[0105] The results show that compared with Examples 1 to 5, the yields of Comparative Examples 1 to 5 are significantly reduced. Comparative Examples 1 and 2 illustrate that the addition of pore-forming agents and templates can increase the specific surface area while enhancing mass transfer and reducing carbon deposition blocking the pores, which is more conducive to increasing the product yield. Comparative Example 3 illustrates that Al 3+The addition of can inhibit the transformation of titanium dioxide anatase to rutile, and the anatase crystal form of titanium dioxide after calcination accounts for a larger proportion, which enhances the dispersion of vanadium and tungsten on the surface of titanium dioxide, exposes more active sites, and increases the activity and selectivity of the catalytic oxidation reaction; Comparative Example 4 shows that the addition of tungsten can inhibit the deep oxidation of the reaction and inhibit the occurrence rate of by-products; Comparative Example 5 shows that lowering the calcination temperature will lead to a decrease in catalyst activity.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention and are not limiting. Other modifications or equivalent substitutions made to the technical solution of the present invention by ordinary technicians in this field should be included in the scope of the claims of the present invention as long as they do not depart from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing pyromellitic acid based on a modified catalyst, characterized in that: The following steps are involved: S1. Prepare solution: The titanium source is dissolved in organic alcohol to obtain solution A; distilled water and a pore-forming agent are added to the organic alcohol for ultrasonic dissolution, and the pH value is adjusted with acid to obtain solution B; S2. Slowly add solution B to solution A and stir to react to obtain a first precursor sol-gel solution; S3. The aluminum salt is added to the first precursor sol-gel solution of step S2, and the reaction is stirred to obtain a second precursor sol-gel solution; S4. The template is added to the second precursor sol-gel solution of step S3, stirred, allowed to soak, then filtered and dried to obtain a doped TiO2 powder; S5. The doped TiO2 powder was calcined in stages to obtain Al-TiO2 particles; S6. The vanadium source and the additive are dissolved in an oxalic acid solution to obtain a mixed solution; S7. The Al-TiO2 particles in step S5 are ultrasonically immersed in the mixed solution of step S6 to obtain a third precursor solution; S8. The third precursor solution of step S7 is loaded on an inert carrier and calcined to obtain a modified catalyst; S9. Using durenyl and air as raw materials, a gas-phase catalytic oxidation reaction is carried out based on the modified catalyst of step S8. The obtained pyromellitic dianhydride is dissolved in water, and after hydrolysis, filtration, crystallization, separation, and drying, the finished product of pyromellitic acid is obtained.

2. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S1, the organic alcohol includes one or more of methanol, ethanol, propanol, isopropanol, n-butanol, isobutanol or tert-butanol; the titanium source includes one or more of tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate or tetra-tert-butyl titanate; the pore-forming agent includes PEG1000-4000; and the acid includes one or more of glacial acetic acid, nitric acid or hydrochloric acid.

3. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S1, the volume ratio of the titanium source to the organic alcohol in solution A is 1:(1.5-2); the mass ratio of the pore-forming agent to the alcohol-water solvent in solution B is (0.04-0.08):1, the volume ratio of the organic alcohol to distilled water is 1:1, and the pH value is 2-4; the volume ratio of the titanium source in solution A to the distilled water in solution B is (1-2):

1.

4. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S3, the aluminum salt includes one or more of aluminum nitrate, aluminum chloride, or aluminum isopropoxide; the molar ratio of the aluminum salt (calculated as aluminum) to the first precursor sol-gel solution (calculated as titanium) is Al:Ti=(0.01-0.02):1, and the reaction time is 2-3 hours.

5. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S4, the template includes PMMA microspheres with a diameter of 200-300 nm, the mass ratio of the template to the second precursor sol-gel solution is (0.1-0.2):1, the stirring time is 2-3 hours, and the solution is allowed to stand and soak for 30-36 hours.

6. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S5, the staged calcination procedure is as follows: the first stage has a heating rate of 5°C / min and is kept at 200-300°C for 3 hours; the second stage has a heating rate of 5°C / min and is kept at 550-600°C for 4 hours; both stages are calcined in air.

7. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S6, the vanadium source is selected from one or more of ammonium metavanadate, vanadium nitrate and sodium metavanadate; and the auxiliary agent is selected from one or more of ammonium metatungstate, sodium tungstate and ammonium tungstate.

8. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S7, the molar ratio of Ti:V:W in the third precursor solution is 1:(0.4-0.6):(0.045-0.055), and the ultrasonic immersion time is 4-6 hours.

9. The method for preparing pyromellitic acid based on a modified catalyst according to claim 1, wherein In step S8, the inert carrier is selected from one or more of α-Al2O3, silicon carbide and ceramic rings, the mass ratio of the modified catalyst to the inert carrier is (0.1-0.2):1, and the calcination procedure is 250-300°C, constant temperature for 1 hour, and then continued to heat to 500-590°C and constant temperature for 4 hours.

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