Methacrylic acid catalyst as well as preparation method and application thereof

Abstract

The invention provides a methacrylic acid catalyst, the chemical general formula of which is PxMoyVzOw [-COOH] n, P, Mo and V are active components, and the molar ratio of x: y: z: w: n is (0.8-2.0): (10.0-12.0): (0.5-2.5): (30.0-50.0): (1.0-6.0). When the methacrylic acid catalyst is used for selectively oxidizing methylacrolein, the methylacrolein conversion rate and the methacrylic acid selectivity are high, and the stability is excellent. And the preparation method of the methacrylic acid catalyst is simple, convenient and low in cost.

Description

Technical Field

[0001] This invention relates to the field of catalytic oxidation technology, specifically to a methacrylic acid catalyst, its preparation method, and its application. Background Technology

[0002] Methacrylic acid (MAA) is an important chemical intermediate, industrially used to prepare coatings, textile additives, adhesives, resins, synthetic rubber, functional polymers, and polymer additives. It can also undergo esterification with methanol to produce methyl methacrylate (MMA). MMA can further polymerize to produce polymethyl methacrylate (PMMA), a widely used chemical product in instrument parts, automotive lights, and optical lenses. MAA can also be combined with monomers containing sulfonyl, epoxy, amino, hydroxyl, or ether groups to synthesize specialty methacrylic acid esters. These specialty methacrylic acid esters are highly economical and have a wide range of applications, including coatings, adhesives, resin modifiers, crosslinking agents, synthetic rubber, and contact lenses.

[0003] Currently, mature processes for producing MMA include the acetone cyanohydrin method, the modified acetone cyanohydrin method (MGC method), the isobutylene oxidation method (C4 method), and the ethylene method. The isobutylene oxidation method, due to its high efficiency and environmental friendliness, has gradually become the preferred technology route for major companies worldwide. This method includes two types: the three-step isobutylene process and the two-step isobutylene process. In the three-step isobutylene process, isobutylene is selectively oxidized in air to produce methacrolein (MAL), which is then selectively oxidized in air to produce methacrylic acid (MAA), and finally esterified with methanol to obtain MMA. This technology route avoids the use of toxic raw materials, is mature, and meets current green development requirements. The core lies in the development of high-performance catalysts for the oxidation of isobutylene to methacrolein and the oxidation of methacrolein to methacrylic acid. The oxidation of MAL to MAA is a gas-phase reaction, using a phosphomolybdenum catalyst through multiple oxidation reactions to produce MAA, with both MAA conversion and MMA yield exceeding 70%. The two-step isobutylene process does not involve a methacrylic acid step, avoiding side reactions such as methacrylic acid polymerization, resulting in high atom economy and simple equipment. The catalyst uses a Pd / Pb / Bi noble metal system, resulting in high manufacturing costs. Due to the high oxygen-to-olefin ratio required for the reaction, using air as the feed gas cannot meet the requirements. Therefore, the feed composition consists of methacrolein and oxygen, which is prone to flash explosions, posing a high risk to process control and making operation difficult. Furthermore, the reaction is a liquid-phase reaction with a low space velocity, resulting in a longer residence time compared to gas-phase reactions. The methacrolein in the feed is highly reactive and prone to deep polymerization, making product quality difficult to control. Therefore, the three-step oxidation process for isobutylene holds a significant advantage.

[0004] In the three-step process of isobutylene oxidation to methacrylic acid (MAA), heteropolyacid catalysts, especially P-Mo system heteropolyacid catalysts, are most widely used. Industrially, Keggin-type heteropolyacids are the main type, with reaction temperatures typically ranging from 280 to 320°C, contact times from 2 to 6 seconds, MAL conversion rates reaching 75%, and lifespans from 2 to 3 years. However, the catalytic performance of P-Mo heteropolyacid catalysts still needs improvement.

[0005] In response, Chinese patent document CN113893879A discloses a heteropolyacid catalyst and its preparation method. The catalyst is synthesized in an alcohol-water azeotrope, with the Na₂O content in the catalyst not exceeding 10 ppm. This catalyst, when used in the oxidation of methacrolein, significantly improves catalytic performance. However, the stability of this heteropolyacid catalyst needs further improvement.

[0006] Chinese patent document CN104001542A discloses a method for preparing a catalyst for the oxidation of methacrolein to methacrylic acid. The method involves stepwise mixing of phosphomolybdic acid with metal oxides (such as vanadium pentoxide, ferric oxide, copper oxide, nickel dioxide, manganese dioxide, cobalt oxide, titanium dioxide, and zinc oxide) in a liquid phase, resulting in numerous active sites on the surface of heteropolyanions. Cesium is then added to obtain the catalyst. This method exhibits high catalytic activity and a long catalyst lifetime for the oxidation of methacrolein to methacrylic acid. However, the preparation process of this catalyst is relatively long, and its stability needs improvement.

[0007] Chinese patent document CN102553624A discloses a method for preparing a catalyst for the synthesis of methacrylic acid. The method employs a stepwise precipitation method using ionic liquids and metal salts to prepare a heteropolyacid compound catalyst. A molybdenum-containing compound and a vanadium-containing compound are reacted with phosphoric acid to obtain a solution or slurry A. An imidazole ionic liquid is prepared as solution B, and an alkali metal nitrate, transition metal nitrate, and rare earth nitrate are prepared as a mixed aqueous solution C. B and C are added to A through stepwise precipitation and aging. The catalyst is then concentrated, dried, shaped, and calcined to obtain the final catalyst. The catalyst prepared by this method exhibits high activity and selectivity for the gas-phase oxidation of methacrolein to methacrylic acid, and also possesses high mechanical strength and good thermochemical stability. However, the preparation process is relatively long, and the catalytic performance needs further improvement.

[0008] Chinese patent document CN114471530A discloses a composite catalyst for the production of methacrylic acid, which is a catalyst for the one-step direct preparation of methacrylic acid from isobutylene. This catalyst contains two different active components: Mo-Bi metal oxide and mesoporous Mo-P heteropolyacid, which can selectively oxidize isobutylene to methacrolein and further oxidize methacrolein to methacrylic acid, respectively. The catalyst preparation method consists of two steps: after obtaining Mo-Bi metal oxide particles, they are further dispersed in a Mo-P heteropolyacid precursor solution containing a surfactant and a silica precursor to obtain the composite catalyst. The catalyst heat treatment method also consists of two steps: calcination in an inert gas atmosphere and an oxidizing gas atmosphere, respectively, ultimately yielding a catalyst with good thermal stability. This catalyst can achieve the one-step direct oxidation of isobutylene to methacrylic acid, exhibiting good reactivity and thermal stability. However, the catalyst preparation process requires the introduction of a template agent, and the preparation process is lengthy and costly; therefore, its catalytic performance for the selective catalysis of methacrolein needs further improvement.

[0009] Chinese patent document CN1843628A discloses a molding method for a heteropolyacid salt catalyst for the selective oxidation of methacrolein to methacrylic acid. This method involves adding at least one organic auxiliary agent and a thermally conductive diluent to catalyst powder, followed by molding to prepare the finished catalyst. The resulting catalyst exhibits high mechanical strength, long service life, and high activity and selectivity. However, this catalyst has a complex composition, a cumbersome preparation process, and high cost, and its catalytic performance for the selective catalysis of methacrolein needs further improvement.

[0010] Chinese patent document CN100490973A discloses a catalyst for the selective oxidation of methacrolein to methacrylic acid and a method for catalytic gas-phase oxidation of methacrolein to methacrylic acid. The catalyst is a heteropolyacid salt containing molybdenum, phosphorus, potassium, antimony, copper, and arsenic, exhibiting high catalytic activity and stability, and a long service life. In the presence of molecular oxygen and diluting gas, the conversion rate of methacrolein is greater than 85%, and the selectivity for methacrylic acid is above 87%. However, the catalyst has a complex composition and high preparation cost, and the conversion rate of methacrolein and the selectivity for methacrylic acid need further improvement. Summary of the Invention

[0011] In view of this, the present invention provides a methacrylic acid catalyst, its preparation method, and its application. When used for the selective oxidation of methacrolein, this methacrylic acid catalyst not only exhibits high methacrolein conversion and methacrylic acid selectivity, but also excellent stability. Furthermore, the preparation method of this methacrylic acid catalyst is simple, convenient, and low in cost.

[0012] To achieve this objective, the present invention adopts the following technical solution:

[0013] A methacrylic acid catalyst with the general chemical formula P x Mo y V z O w [-COOH] n P, Mo, and V are the active components, and the molar ratio of x:y:z:w:n is (0.8–2.0):(10.0–12.0):(0.5–2.5):(30.0–50.0):(1.0–6.0).

[0014] In one alternative implementation, the molar ratio of x:y:z:w:n is (0.8–1.5):(10.5–12.0):(0.5–1.5):(35.0–45.0):(1.0–5.0).

[0015] This invention also provides a method for preparing a methacrylic acid catalyst, comprising the following steps:

[0016] Phosphoric acid was added to a mixed solution of a molybdenum-containing compound and a vanadium-containing compound to carry out a first reaction; after the first reaction was completed, a hydroxyl-containing carboxylic acid was added to carry out a second reaction; after the second reaction was completed, the pH of the system was adjusted to 2.0-6.5 with ammonia water, and after crystallization and drying, the methacrylic acid catalyst was obtained.

[0017] In one alternative implementation, the pH of the system is adjusted to 2.8–6.0 using ammonia.

[0018] In one optional embodiment, the hydroxyl-containing carboxylic acid is selected from at least one of citric acid, malic acid, tartaric acid, salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, and caffeic acid.

[0019] In one optional embodiment, the molar ratio of molybdenum in the molybdenum-containing compound to the carboxyl group in the hydroxyl-containing carboxylic acid is (10.0–12.0):(0.5–6.5); preferably (10.5–12.0):(0.5–5.5).

[0020] In one optional embodiment, the molar ratio of molybdenum in the molybdenum-containing compound to vanadium in the vanadium-containing compound is (10.0–12.0):(0.2–3.0); preferably (10.5–12.0):(0.3–2.0).

[0021] In one optional embodiment, the molar ratio of molybdenum in the molybdenum-containing compound to phosphorus in the phosphoric acid is (10.0–12.0):(0.8–2.0); preferably (10.5–12.0):(1.0–1.5).

[0022] In one alternative embodiment, the molybdenum-containing compound is selected from at least one of molybdenum trioxide, ammonium heptamolybdate, ammonium tetramolybdate, and ammonium molybdate.

[0023] In one alternative embodiment, the vanadium-containing compound is selected from at least one of vanadium pentoxide, ammonium metavanadate, and ammonium vanadate.

[0024] In one alternative embodiment, the phosphoric acid is concentrated phosphoric acid (85 wt% phosphoric acid).

[0025] In one optional embodiment, the temperature of the first reaction is 50–90°C and the time is 2–6 hours; preferably, the temperature of the first reaction is 55–70°C and the time is 3–5 hours.

[0026] In one optional embodiment, the temperature of the second reaction is 60–100°C and the time is 2–8 hours; preferably, the temperature of the second reaction is 75–85°C and the time is 2–5 hours.

[0027] In one optional embodiment, the crystallization temperature is 150–300°C and the time is 12–48 h; preferably, the crystallization temperature is 160–250°C and the time is 20–30 h.

[0028] In one optional embodiment, the drying temperature is 80–200°C and the time is 8–48 h; preferably, the drying temperature is 80–150°C and the time is 20–40 h.

[0029] The present invention also provides the application of the above-mentioned methacrylic acid catalyst or the methacrylic acid catalyst prepared by the above-mentioned method in the selective catalytic oxidation of methacrolein to prepare methacrylic acid.

[0030] In one optional embodiment, the selective catalytic oxidation of methacrolein to prepare methacrylic acid is carried out at a reaction temperature of 290–350°C and a space velocity of 1000–4000 h⁻¹. -1 .

[0031] In one alternative embodiment, the raw materials for the selective catalytic oxidation of methacrolein to prepare methacrylic acid further include air and water.

[0032] The beneficial effects of this invention are as follows:

[0033] Existing Keggin-type heteropolyacid catalysts for the selective oxidation of methacrolein generally suffer from poor stability and low conversion rates of methacrolein and selectivity for methacrylic acid. The inventors discovered that the stability of the internal crystal structure of heteropolyacid catalysts affects their oxidation performance during the reaction. Defective structures form lattice oxygen with catalytic oxidation capabilities, enhancing the oxidizing power of heteropolyacids and enabling the Redox cycle. This invention addresses the problem of insufficient exposure of defective structures in Keggin-type heteropolyacid catalysts by introducing carboxyl groups into the heteropolyacid catalyst, thereby significantly improving the catalytic activity and stability of the catalyst.

[0034] The method for preparing methacrylic acid catalyst provided by this invention involves introducing a hydroxyl-containing carboxylic acid into the pretreatment process. This pretreatment preferentially anchors the carboxyl groups at defect sites to support the localized environment, ensuring that the defect structure is retained in the final catalyst. Simultaneously, the hydroxyl groups block the formation of dispersed oxide lattice clusters, inhibiting the formation of large particle structures and thus "refining" the Keggin-type heteropolyacid catalyst. This fully exposes the defect structure, improves selective oxygen insertion capability, and achieves controlled generation of the target active defect structure. It maximizes the generation of composite phase defect sites and inhibits the formation of non-selective oxidation defect sites, achieving precise control over the types of active site defect structures in the Keggin-type heteropolyacid catalyst precursor. The final methacrylic acid catalyst P... x Mo y V z O w [-COOH] n It can effectively catalyze the selective oxidation of methacrolein (MAL) to prepare methacrylic acid (MAA), improving the selectivity and yield of methacrylic acid while ensuring the conversion rate of methacrolein, and exhibiting excellent stability. Attached Figure Description

[0035] Figure 1 The X-ray diffraction patterns are those of the methacrylic acid catalysts prepared in Example 1 and Comparative Example 1 of this invention.

[0036] Figure 2 for Figure 1 A magnified view of a portion of the image. Detailed Implementation

[0037] The present invention will now be described in detail through embodiments. It should be noted that the following embodiments are only for further illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the content of the present invention.

[0038] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0039] All raw materials involved in this invention are commercially available. Among them, molybdenum-containing compounds such as molybdenum trioxide, ammonium heptamolybdate, ammonium tetramolybdate, and ammonium molybdate, as well as vanadium-containing compounds such as vanadium pentoxide, ammonium metavanadate, and ammonium vanadate, can all meet the requirements for implementing the present invention. However, for ease of comparison, molybdenum trioxide and vanadium pentoxide are used below. The present invention is further illustrated by the following examples, but it is not intended to be limited to these examples.

[0040] Example 1

[0041] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.30 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 5.76 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia water, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 11 VO 39 [-COOH]3 catalyst.

[0042] Example 2

[0043] 43.185 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.12 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 1.92 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 2 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 10 VO 36 [-COOH] catalyst.

[0044] Example 3

[0045] 51.822 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.63 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 6.92 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 11.52 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 5 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia water, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 250 °C for 20 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain P. 2.0 Mo 12 VO 46 [-COOH]6 catalyst.

[0046] Example 4

[0047] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.30 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 6.03 g of malic acid was added to mixture B, and the mixture was stirred at 75 °C for 6 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 110 °C for 38 h to obtain PMo. 11 VO 39 [-COOH]3 catalyst.

[0048] Example 5

[0049] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.30 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 6.75 g of tartaric acid was added to mixture B, and the mixture was stirred at 85 °C for 4 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia water, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 11 VO 39 [-COOH]3 catalyst.

[0050] Example 6

[0051] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.30 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 6.21 g of salicylic acid and 8.1 g of caffeic acid were added to mixture B, and the mixture was stirred at 85 °C for 6 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 11 VO 39 [-COOH]3 catalyst.

[0052] Example 7

[0053] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.37 L of deionized water at 60 °C with stirring to obtain mixture A. 5.19 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 4 h to obtain mixture B. 3.84 g of citric acid and 2.25 g of tartaric acid were added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 160 °C for 30 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain P. 1.5 Mo 11 VO 41 [-COOH]3 catalyst.

[0054] Example 8

[0055] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.27 L of deionized water at 60 °C with stirring to obtain mixture A. 2.77 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 2 h to obtain mixture B. 1.92 g of citric acid, 2.01 g of malic acid, and 2.25 g of tartaric acid were added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain P. 0.8 Mo 11 VO 38 [-COOH]3 catalyst.

[0056] Example 9

[0057] 47.5 g of molybdenum trioxide and 1.365 g of vanadium pentoxide were weighed and dissolved in 2.22 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 5.76 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 100 °C for 38 h to obtain PMo. 11 V 0.5 O 38 [-COOH]3 catalyst.

[0058] Example 10

[0059] 47.5 g of molybdenum trioxide and 6.825 g of vanadium pentoxide were weighed and dissolved in 2.47 L of deionized water by heating to 60 °C with stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 5.76 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 11 V 2.5 O 43 [-COOH]3 catalyst.

[0060] Example 11

[0061] 49.66 g of molybdenum trioxide and 4.914 g of vanadium pentoxide were weighed and dissolved in 2.60 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 6.228 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 55 °C for 5 h to obtain mixture B. 9.6 g of citric acid was added to mixture B, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 2.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 30 h to obtain the catalyst precursor. The catalyst precursor was dried at 150 °C for 40 h to obtain P. 1.8 Mo 11.5 V 1.8 O 46 [-COOH]4 catalyst.

[0062] Example 12

[0063] 47.5 g of molybdenum trioxide and 3.549 g of vanadium pentoxide were weighed and dissolved in 2.51 L of deionized water by heating to 60 °C and stirring to obtain mixture A. 4.15 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 85 °C for 2 h to obtain mixture B. 5.76 g of citric acid was added to mixture B, and the mixture was stirred at 90 °C for 2 h to obtain a homogeneous mixture. The pH was adjusted to 6.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 20 h to obtain the catalyst precursor. The catalyst precursor was dried at 80 °C for 40 h to obtain P. 1.2 Mo 11.8 V 1.3 O 43 [-COOH]3 catalyst.

[0064] Comparative Example 1

[0065] This comparative example is similar to Example 1, except that citric acid was not added in this comparative example, and PMo was finally obtained. 11 VO 39 catalyst.

[0066] Comparative Example 2

[0067] This comparative example is similar to Example 1, except that 2.70g of oxalic acid was used instead of 5.76g of citric acid in this comparative example, ultimately yielding PMo. 11 VO 39 [-COOH]3 catalyst.

[0068] Comparative Example 3

[0069] This comparative example is similar to Example 1, except that 0.14g of ethanol was used instead of 5.76g of citric acid in this comparative example, ultimately yielding PMo. 11 VO 39 catalyst.

[0070] Comparative Example 4

[0071] This comparative example is similar to Example 1, except that 0.14g ethanol and 2.70g oxalic acid were used instead of 5.76g citric acid (i.e., 0.14g ethanol and 2.70g oxalic acid were added to mixture B, and then stirred at 85°C for 3 hours), ultimately yielding PMo. 11 VO 39 [-COOH]3 catalyst.

[0072] Comparative Example 5

[0073] 47.5 g of molybdenum trioxide and 2.73 g of vanadium pentoxide were weighed and dissolved in 2.3 L of deionized water at 60 °C with stirring to obtain mixture A. 3.46 g of phosphoric acid (85.0% by mass) was added to mixture A, and the mixture was stirred at 80 °C for 3 h to obtain mixture B. 0.14 g of ethanol was added to mixture B, and the mixture was stirred at 60 °C for 1 h to obtain a homogeneous mixture. Then, 1.38 g of formic acid was added, and the mixture was stirred at 85 °C for 3 h to obtain a homogeneous mixture. The pH was adjusted to 3.0 with ammonia, and stirring was continued for 0.5 h to obtain catalyst precursor solution C. Catalyst precursor solution C was crystallized at 180 °C for 24 h to obtain the catalyst precursor. The catalyst precursor was dried at 120 °C for 36 h to obtain PMo. 11 VO 39 [-COOH]3 catalyst.

[0074] Comparative Example 6

[0075] This comparative example is similar to Example 1, except that 15.85g of ascorbic acid was used instead of 5.76g of citric acid in this comparative example, ultimately yielding PMo. 11 VO 39 catalyst.

[0076] The catalysts prepared in Example 1 and Comparative Example 1 were subjected to X-ray diffraction tests, and the specific results are as follows: Figure 1 and Figure 2 As shown, 10.5° (110 crystal plane), 18.5° (211 crystal plane), 26.3° (222 crystal plane), 30.2° (400 crystal plane), and 35.8° (332 crystal plane) are characteristic diffraction peaks of Keggin-type heteropolyacids. Figure 1 and Figure 2 It can be seen that the catalysts prepared in Example 1 and Comparative Example 1 both exhibit characteristic diffraction peaks of Keggin-type heteropolyacids, indicating that the introduction of hydroxycarboxylic acid compounds did not disrupt the structure of the heteropolyacids; meanwhile, compared with the PMo prepared in Comparative Example 1 without the addition of hydroxycarboxylic acid citric acid... 11 VO 40 Compared to the catalyst, the PMo prepared in Example 1 11 VO 40 The [-COOH]3 catalyst exhibits diffraction peaks at 47.4°, 61.8°, 64.0°, and 67.2°, and the intensity of the diffraction peak at 55.0° increases, indicating that the catalyst prepared after adding citric acid has a defective crystal structure.

[0077] The catalysts prepared in the above examples and comparative examples were evaluated for performance in a fixed-bed microreactor for the production of methacrylic acid from methacrolein. The catalyst loading was 2 g, and the molar ratio of the feedstock methacrolein:oxygen:water vapor:equilibrium gas was 1:3.8:2.6, with a space velocity of 1200 h⁻¹. -1 The reaction temperature was 340℃. The catalyst performance evaluation results are shown in Table 1.

[0078] Table 1

[0079]

[0080]

[0081] As shown in the table above, in the catalyst preparation process of this invention, after adding vanadium-containing compounds, molybdenum-containing compounds, and phosphoric acid to form a Keggin structure, the introduction of hydroxyl-containing carboxylic acids further creates more defect sites conducive to selective oxidation. From the results of the examples, the catalyst prepared without the addition of hydroxyl-containing carboxylic acids exhibits lower conversion and selectivity; when hydroxyl-containing carboxylic acids are introduced, the prepared catalyst shows higher conversion of methacrolein and higher selectivity and yield of methacrylic acid.

[0082] Catalyst lifetime test:

[0083] The catalyst prepared in Example 1 was subjected to long-term performance evaluation in a fixed-bed microreactor for the production of methacrylic acid from methacrolein. The upper and lower ends of the reaction tube were filled with inert materials, and the catalyst was packed in the middle of the tube at a loading volume of 2.0 mL. The molar ratio of raw material methacrolein:oxygen:water vapor:equilibrium gas was 1:(3.6–4.5):(2.4–3.6), and the space velocity was 1200 h⁻¹. -1 ~1800h -1 The reaction temperature was 320℃~380℃. The catalyst performance evaluation results are shown in Table 2.

[0084] Table 2

[0085]

[0086]

[0087] As can be seen from the data in the table above, after the catalyst prepared in Example 1 of the present invention has been running continuously in the microreactor for 1200 days, the conversion rate of methacrolein is not less than 85.0% and the selectivity of methacrylic acid is stable at 90.0%.

[0088] The catalyst prepared in Comparative Example 1 was subjected to long-term performance evaluation in a fixed-bed microreactor for the production of methacrylic acid from methacrolein. The upper and lower ends of the reaction tube were filled with inert materials, and the catalyst was loaded in the middle of the tube at a loading volume of 2.0 mL. The molar ratio of raw material methacrolein:oxygen:water vapor:equilibrium gas was 1:(3.6–4.5):(2.4–3.6), and the space velocity was 1200 h⁻¹. -1 ~1800h -1 The reaction temperature was 320℃~380℃. The catalyst performance evaluation results are shown in Table 3.

[0089] Table 3

[0090]

[0091] As can be seen from the data in the table above, the performance of the catalyst provided in Comparative Example 1 decreased after 5 days of reaction in the microreactor, and the conversion rate of methacrolein and the selectivity of methacrylic acid decreased to 71% after 100 days of reaction.

[0092] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

Claims

1. A methacrylic acid catalyst, characterized in that, Its general chemical formula is P x Mo y V z O w [-COOH] n P, Mo, and V are the active components, and the molar ratio of x:y:z:w:n is (0.8–2.0):(10.0–12.0):(0.5–2.5):(30.0–50.0):(1.0–6.0).

2. The methacrylic acid catalyst according to claim 1, characterized in that, The molar ratio of x:y:z:w:n is (0.8~1.5):(10.5~12.0):(0.5~1.5):(35.0~45.0):(1.0~5.0).

3. A method for preparing a methacrylic acid catalyst, characterized in that, Includes the following steps: Phosphoric acid is added to a mixed solution of a molybdenum-containing compound and a vanadium-containing compound to carry out a first reaction; after the first reaction is completed, a hydroxyl-containing carboxylic acid is added to carry out a second reaction; after the second reaction is completed, the pH of the system is adjusted to 2.0-6.5, and after crystallization and drying, the methacrylic acid catalyst is obtained.

4. The preparation method according to claim 3, characterized in that, The hydroxyl-containing carboxylic acid is selected from at least one of citric acid, malic acid, tartaric acid, salicylic acid, m-hydroxybenzoic acid, p-hydroxybenzoic acid, and caffeic acid.

5. The preparation method according to claim 3 or 4, characterized in that, The molar ratio of molybdenum in the molybdenum-containing compound to the carboxyl group in the hydroxyl-containing carboxylic acid is (10.0–12.0):(0.5–6.5); preferably (10.5–12.0):(0.5–5.5).

6. The preparation method according to claim 3, characterized in that, The molar ratio of molybdenum in the molybdenum-containing compound to vanadium in the vanadium-containing compound is (10.0–12.0):(0.2–3.0); preferably (10.5–12.0):(0.3–2.0).

7. The preparation method according to claim 3, characterized in that, The molar ratio of molybdenum in the molybdenum-containing compound to phosphorus in the phosphoric acid is (10.0–12.0):(0.8–2.0); preferably (10.5–12.0):(1.0–1.5).

8. The preparation method according to claim 3, characterized in that, The molybdenum-containing compound is selected from at least one of molybdenum trioxide, ammonium heptamolybdate, ammonium tetramolybdate, and ammonium molybdate; The vanadium-containing compound is selected from at least one of vanadium pentoxide, ammonium metavanadate, and ammonium vanadate. The phosphoric acid mentioned is concentrated phosphoric acid.

9. The preparation method according to claim 3, characterized in that, The temperature of the first reaction is 50–90°C, and the time is 2–6 hours; preferably, the temperature of the first reaction is 55–70°C, and the time is 3–5 hours. The temperature of the second reaction is 60–100°C, and the time is 2–8 hours; preferably, the temperature of the second reaction is 75–85°C, and the time is 2–5 hours. The crystallization temperature is 150–300°C and the time is 12–48 h; preferably, the crystallization temperature is 160–250°C and the time is 20–30 h. The drying temperature is 80–200°C, and the drying time is 8–48 h; preferably, the drying temperature is 80–150°C, and the drying time is 20–40 h.

10. The application of the methacrylic acid catalyst prepared by the method of claim 1 or 2 or any one of claims 3-9 in the selective catalytic oxidation of methacrolein to prepare methacrylic acid.

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