JP2016168591A - Catalyst for manufacturing methacrylic acid and manufacturing method therefor, and manufacturing method of methacrylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst having a novel structure for manufacturing methacrylic acid by contact vapor phase oxidation of methacrolein.SOLUTION: There is provided a catalyst for manufacturing methacrylic acid for manufacturing methacrylic acid by contact vapor phases oxidation of methacrolein in a presence of molecular oxygen or molecular oxygen-containing gas, represented by the following general formula (I) MoABXYZO(I), and having an ε-Keggin type heteropolyacid structure having diffraction peaks at 7.7°±0.3°, 23.2°±0.3°, 25.4°±0.3°, 26.5°±0.3° and 29.8°±0.3° of a value of 2θ in a powder X-ray diffraction pattern using a Cu-Kα ray.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a catalyst for producing methacrylic acid having a novel structure, a method for producing the catalyst, and a method for producing methacrylic acid by catalytic vapor phase oxidation of methacrolein. The present invention relates to a method for producing methacrylic acid from rain.

  Methacrylic acid can be produced by catalytic gas phase oxidation of methacrolein, isobutyraldehyde, isobutyric acid and the like. As a catalyst used in this case, it is known that a catalyst having a heteropolyacid and / or salt structure thereof is effective, and many proposals have been made regarding its composition, physical properties and production method.

  These catalysts are usually produced by processes such as preparation of a raw material mixture, drying, molding, and firing. With regard to the production of a catalyst that stably exhibits high performance, many reports have been made on the composition, adjustment method, and calcination method.

  For example, Patent Document 1 proposes a material mainly composed of molybdenum and phosphorus and having a structure of a heteropolyacid and / or a salt thereof. In Patent Document 2, in preparing a heteropolyacid catalyst, a heteropolyacid salt containing at least molybdenum, phosphorus and cesium is obtained, and a catalyst raw material containing at least molybdenum and phosphorus and not containing cesium is added to the heteropolyacid salt. , A method for preparing a catalyst, characterized in that it is prepared in two steps. Further, in Patent Document 3, in preparing a molybdenum, phosphorus, vanadium, antimony, and copper-based catalyst, a catalyst raw material except for a vanadium raw material and / or an antimony raw material is dissolved or suspended in water to prepare an ammonium root. After the heat treatment for 1 to 24 hours at a temperature of 80 to 200 ° C. in the presence of vanadium, a vanadium raw material and / or an antimony raw material is added, and the heat treatment is again performed at a temperature of 80 to 200 ° C. for 1 to 24 hours, followed by firing. A method is described.

  On the other hand, Non-Patent Document 1 discloses a polyacid having ε-Keggin type molybdenum, vanadium and bismuth as main components.

Japanese Patent Laid-Open No. 06-91172 JP 05-177141 A Japanese Patent No. 3482476

Dalton Trans. , 2014, 43, 13584-13590

  However, all of these known heteropolyacid catalysts described in Patent Documents 1 to 3 have an α-kegin structure, but the reaction results of the method for producing methacrylic acid from methacrolein are similar to those of the reaction described above. It was inadequate compared with the reaction result of the method of manufacturing acrylic acid from acrolein. Despite long-term diligent studies, the improvement in the performance of conventional α-Keggin heteropolyacid catalysts can be seen as a peak. Accordingly, an object of the present invention is to provide a catalyst having a novel structure for producing methacrylic acid by catalytic gas phase oxidation of methacrolein, a method for producing the catalyst, and using methacrolein from the methacrolein. It is to provide a method for producing methacrylic acid.

  In order to solve the above-mentioned problems, the present inventors have intensively studied on a catalyst that can be suitably used for producing methacrylic acid from methacrolein. By using the salt, it was found that methacrylic acid can be produced from methacrolein, and the present invention was completed.

That is, the present invention
(1) A catalyst for producing methacrylic acid for producing methacrylic acid by catalytic gas phase oxidation of methacrolein in the presence of molecular oxygen or a molecular oxygen-containing gas,
It is represented by the following general formula (I),
_{Mo a A b B c X d} Y e Z f O g (I)
(Wherein Mo is molybdenum,
A is at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel;
B is at least one element selected from the group consisting of bismuth, zinc, manganese, iron, cobalt and nickel;
X is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium;
Y is at least one element selected from the group consisting of phosphorus, arsenic, antimony, germanium, bismuth and selenium,
Z is at least one element selected from the group consisting of iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium, rhodium, ruthenium, tellurium, tungsten, silicon, aluminum, cerium, magnesium, boron, and ammonium. Or an ion,
a, b, c, d, e, and f represent the atomic ratio of each element. When b = 1, 0 <a <5.0, 0 <c <2.0, 0 ≦ d <3. 0, 0 ≦ e <1.0, 0 ≦ f <2.0, and g is a number determined by the oxidation state of each element)
In the powder X-ray diffraction pattern using Cu-Kα rays, the values of 2θ are 7.7 ° ± 0.3 °, 23.2 ° ± 0.3 °, 25.4 ° ± 0.3 °, 26. A catalyst for producing methacrylic acid having an ε-Keggin type heteropolyacid structure having diffraction peaks at 5 ° ± 0.3 ° and 29.8 ° ± 0.3 °,
(2) The catalyst for methacrylic acid production according to (1), wherein A is vanadium,
(3) The catalyst for methacrylic acid production according to (1) or (2), wherein B is bismuth,
(4) The method for producing a catalyst for producing methacrylic acid according to any one of (1) to (3), comprising the following steps:
Step (A): a compound containing molybdenum; a compound containing at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel; and bismuth, zinc, manganese, iron, cobalt and nickel A compound containing at least one element selected from the group consisting of: a compound containing at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium if necessary; phosphorus, arsenic, antimony, if necessary A compound containing at least one element selected from the group consisting of germanium, bismuth and selenium; and, if necessary, iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium, rhodium, ruthenium, tellurium, tungsten, Silicon Miniumu, cerium, magnesium, at least one element or compound containing ions selected from the group consisting of boron and ammonium, mixed with water to prepare an aqueous solution or aqueous dispersion of these compounds process,
Step (B): A step of obtaining a solid content by hydrothermal synthesis using the aqueous solution or aqueous dispersion obtained in Step (A),
Step (C): a step of firing the solid content obtained in the step (B),
(5) The method for producing a methacrylic acid production catalyst according to (4), wherein the temperature in hydrothermal synthesis is 80 ° C to 200 ° C, and the pressure is a saturated vapor pressure at the temperature,
(6) In the presence of the catalyst for producing methacrylic acid according to any one of (1) to (3), catalytic gas phase oxidation reaction with molecular oxygen or molecular oxygen-containing gas of methacrolein is performed. A process for producing methacrylic acid,
About.

  Methacrylic acid can be produced from methacrolein by using a heteropolyacid catalyst having an ε-Keggin structure that satisfies the specific composition formula of the present invention.

2 is a measurement result of powder X-ray diffraction by Cu—Kα rays of the catalyst before calcination according to Example 1. FIG. It is a measurement result of the powder X-ray diffraction by the Cu-K alpha ray of the catalyst after the calcination concerning Example 1 and comparative example 1.

  As structures of heteropolyacids and salts thereof, a plurality of structures such as Keggin type, Dawson type and Anderson type are known. A typical Keggin-type heteropolyacid refers to a structure in which three regular octahedrons share an apex oxygen atom as a structural unit, and four structural units coordinate to a tetrahedron centered on a heteroatom. α-Keggin heteropolyacids share four oxygen atoms with four structural units at the corners of the tetrahedron, and are coordinated at approximately Td symmetry positions (Td is classified by the Shane-Fries notation). It is one of the point groups in which the target molecule forms a tetrahedral shape).

  On the other hand, the ε-Keggin type heteropolyacid refers to one in which four structural units are each rotated from the α-Keggin type by 60 ° around the C3 axis. ε-Keggin type heteropolyacid has a value of 2θ of 7.7 ° ± 0.3 °, 23.2 ° ± 0.3 °, 25.4 ° in a powder X-ray diffraction pattern using Cu—Kα ray. This can be confirmed by having diffraction peaks at ± 0.3 °, 26.5 ° ± 0.3 °, and 29.8 ° ± 0.3 °.

The catalyst of the present invention is a catalyst for producing methacrylic acid for producing methacrylic acid by catalytic gas phase oxidation of methacrolein in the presence of molecular oxygen or a molecular oxygen-containing gas,
It is represented by the following general formula (I),
_{Mo a A b B c X d} Y e Z f O g (I)
(Wherein Mo is molybdenum,
A is at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel;
B is at least one element selected from the group consisting of bismuth, zinc, manganese, iron, cobalt and nickel;
X is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium;
Y is at least one element selected from the group consisting of phosphorus, arsenic, antimony, germanium, bismuth and selenium,
Z is at least one element selected from the group consisting of iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium, rhodium, ruthenium, tellurium, tungsten, silicon, aluminum, cerium, magnesium, boron, and ammonium. Or an ion,
a, b, c, d, e, and f represent the atomic ratio of each element. When b = 1, 0 <a <5.0, 0 <c <2.0, 0 ≦ d <3. 0, 0 ≦ e <1.0, 0 ≦ f <2.0, and g is a number determined by the oxidation state of each element)
In the powder X-ray diffraction pattern using Cu-Kα rays, the values of 2θ are 7.7 ° ± 0.3 °, 23.2 ° ± 0.3 °, 25.4 ° ± 0.3 °, 26. A catalyst for producing methacrylic acid having an ε-Keggin type heteropolyacid structure having diffraction peaks at 5 ° ± 0.3 ° and 29.8 ° ± 0.3 °.

  In the ε-Keggin type heteropolyacid of the above general formula (I), Mo is a polyoxometalate constituent element, A is a polyoxometalate constituent element, B is a linker, X is a cationic species, Y is a polyoxometalate constituent element (central element) and can also be a cationic species, and Z is a cationic species. A is preferably vanadium and B is preferably bismuth.

The catalyst of the present invention can be prepared by a method including the following steps.
Process (A)
A compound containing molybdenum; a compound containing A (at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel); and B (from bismuth, zinc, manganese, iron, cobalt and nickel) A compound containing at least one element selected from the group consisting of: a compound containing X (at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium) if necessary; A compound containing phosphorus, arsenic, antimony, germanium, bismuth and selenium; and Z (iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium if necessary); , Rhodium, ruthenium, tellurium, tongue A compound containing at least one element or ion selected from the group consisting of copper, silicon, aluminum, cerium, magnesium, boron and ammonium) is mixed with water to prepare an aqueous solution or dispersion of these compounds. Process,
Process (B)
Using the aqueous solution or aqueous dispersion obtained in step (A) to obtain a solid content by hydrothermal synthesis,
Process (C)
A step of firing the solid content obtained in the step (B).

  The starting material compound in the method for producing the catalyst of the present invention is not particularly limited, and examples of the compound containing molybdenum include ammonium molybdate, molybdenum trioxide, molybdic acid, and sodium molybdate.

  Examples of the component A raw material include, but are not limited to, hydroxides, chlorides, sulfates, nitrates, oxides or acetates of component elements (vanadium, zinc, manganese, iron, cobalt, and nickel). Preferably, it is a compound containing vanadium, and examples thereof include vanadate (metavanadate) such as vanadate and ammonium vanadate (ammonium metavanadate), vanadium oxide, vanadium chloride and the like, and vanadyl sulfate is particularly preferable.

  Examples of the component B raw material include, but are not limited to, hydroxides, chlorides, sulfates, nitrates, oxides or acetates of component elements (bismuth, zinc, manganese, iron, cobalt, and nickel). A compound containing bismuth is preferable, and bismuth hydroxide and bismuth nitrate are particularly preferable.

  When X, Y and / or Z component raw materials are added, the step of adding is not particularly limited, and may be added as appropriate before and after hydrothermal synthesis or during firing. Examples of the X, Y and Z component raw materials include hydroxides, chlorides, sulfates, nitrates, oxides or acetates of the component elements.

  Hydrothermal synthesis is a method for synthesizing substances and growing crystals in the presence of high-temperature high-pressure water. In the present invention, the hydrothermal synthesis is performed, for example, by charging the slurry liquid into an autoclave. Although the reaction can be carried out in air, it is preferred to carry out by replacing part or all of the inside of the autoclave with an inert gas such as nitrogen or helium instead of air before starting the reaction. The reaction temperature of hydrothermal synthesis is usually 80 ° C to 200 ° C, and the reaction time is usually 1 to 100 hours. The pressure in the autoclave is preferably a saturated vapor pressure, and stirring may be performed during hydrothermal synthesis. After completion of hydrothermal synthesis, the solid substance may be dried as it is, but since the unreacted substance may adversely affect the production of methacrylic acid from methacrolein, the solid substance obtained by hydrothermal synthesis is More preferably, it is filtered and washed. The product thus obtained can be used as it is as the catalyst of the present invention, but is preferably subjected to a calcination treatment. The baking treatment is carried out in an inert gas such as nitrogen or helium at 200 ° C. or higher, preferably 300 to 500 ° C. for 0.5 to 10 hours.

  The catalyst of the present invention thus obtained can be used as it is as the catalyst of the present invention, but is preferably used after being pulverized. The catalyst of the present invention can be applied to any reaction mode such as a fixed bed, a fluidized bed, and a moving bed. A coated catalyst in which a product is supported and molded, or a catalyst in which a powdered composite metal oxide is molded by a molding machine such as tableting molding can be used. For fluidized bed and moving bed reactors, it is advantageous to use a uniform catalyst of about several tens of microns prepared by adding a silica component or the like to improve wear resistance.

The catalyst of the present invention is used for the production of methacrylic acid from methacrolein by catalytic gas phase oxidation. The raw material gas composition in the catalytic gas phase oxidation reaction for producing methacrylic acid from methacrolein is not particularly limited, but methacrolein: oxygen: water vapor: diluted gas = 1: 0.1 to 10: 0 to 30: 0 to 60 ( (Molar ratio) is preferable. Here, as dilution gas, nitrogen, carbon dioxide gas, etc. are preferable. The catalytic gas phase oxidation reaction may be carried out under pressure or under reduced pressure, but generally it is preferably carried out at a pressure near atmospheric pressure. The reaction temperature is generally 200 to 400 ° C, preferably 220 to 350 ° C. The supply amount of the raw material gas is usually 100～100000Hr ^-1 in the space velocity (SV), preferably 400~30000hr ^-1.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
In the following Examples, methacrolein conversion and methacrylic acid yield are defined as follows.
Conversion rate of methacrolein (mol%) = (number of moles of reacted methacrolein) / (number of moles of supplied methacrolein) × 100
Methacrylic acid yield (mol%) = (moles of methacrylic acid produced) / (moles of methacrolein supplied) × 100

Example 1
(Preparation of catalyst)
1.471 g of ammonium molybdate was dissolved in 20 ml of distilled water at room temperature. In another container, 0.5365 g of vanadyl sulfate n-hydrate was dissolved in 20 ml of distilled water, and the resulting aqueous solution was added to the above-mentioned molybdenum aqueous solution and stirred sufficiently to change into a slurry state. Further, another container prepared by dissolving 0.68 g of bismuth nitrate pentahydrate in 1.7 ml of a mixed solution of glycerin and distilled water (volume ratio 1: 1) was prepared. The aqueous solution was added to the slurry obtained by stirring the slurry solution for 3 minutes, and further stirred for 7 minutes. The pH of the slurry was adjusted to 4.0 using a 28% aqueous ammonia solution. After bubbling the slurry liquid with nitrogen gas for 10 minutes, this slurry liquid was transferred to an autoclave having an internal volume of 100 mL while washing the beaker with distilled water so that there was no residue, and heated and stirred at 175 ° C. for 48 hours. The resulting product was allowed to cool to room temperature, filtered, washed with water (20 mL × 3), and dried at 80 ° C. overnight to obtain a granular product.

The composition of the produced granule was analyzed using X-ray fluorescence analysis and elemental analysis. As a result, when the atomic ratio of vanadium was 1.0, the atomic ratio of molybdenum was 2.9 and the atomic ratio of bismuth was 0.00. 7. The atomic ratio of nitrogen was 0.8, and the catalyst composition was Mo _9.8 V _3.6 Bi _2.5 (NH ₄ ) _2.9 O ₄₂ . The X-ray diffraction (using Cu-Kα ray) measurement results are shown in FIG. It was confirmed that peaks appeared at 7.7 °, 23.2 °, 25.4 °, 26.5 °, and 29.8 ° as 2θ values.
The produced sample was calcined at 300 ° C. for 6 hours under air flow, and composition analysis was performed using X-ray fluorescence analysis and elemental analysis. When the atomic ratio of vanadium was 1.0, molybdenum was obtained. The atomic ratio is 3.1, the atomic ratio of bismuth is 0.9, the atomic ratio of nitrogen is 0.8, and the composition formula is Mo _11.2 V _3.6 Bi _3.2 (NH ₄ ) _2.9 O ₄₀ . Moreover, the X-ray diffraction (Cu-Kα ray is used) measurement result is shown in FIG. It was confirmed that peaks appeared at 7.7 °, 23.5 °, 25.6 °, 26.8 °, and 30.1 ° as 2θ values.

(Catalyst evaluation test)
3.45 g of the granulated granule was uniformly mixed with 6.90 g of pulverized quartz sand. This was mixed with 20.6 g of a spherical porous alumina support (particle size: 4.0 mm) and then filled into a stainless steel reaction tube having an inner diameter of 18.4 mm, and the raw material gas composition (molar ratio) methacrolein: oxygen: water vapor: nitrogen = The reaction results were measured at 1: 2: 4: 18.6, space velocity (SV) 1200 hr-1, and reaction bath temperature 290 ° C. The results of the evaluation test are shown in Table 1.

Comparative Example 1
Raw materials having the same composition as in Example 1 (20.37 g of ammonium molybdate, 9.79 g of vanadyl sulfate n hydrate, 13.42 g of bismuth nitrate pentahydrate) were weighed and physically mixed granules were prepared. When the composition of the obtained sample was analyzed using X-ray fluorescence analysis, the atomic ratio of molybdenum was 2.8 and the atomic ratio of bismuth was 0.6 when the atomic ratio of vanadium was 1.0. It was confirmed that the sample had almost the same composition as the sample obtained in Example 1. Moreover, the X-ray diffraction (Cu-Kα ray is used) measurement result is shown in FIG. Since peaks at 7.7 °, 25.4 °, 26.5 °, and 29.8 ° could not be confirmed as 2θ values, it was confirmed that this comparative sample did not have an ε-Keggin type heteropolyacid structure. confirmed.
(Catalyst evaluation test)
The catalyst evaluation test was performed under the same conditions as in Example 1. The results of the evaluation test are shown in Table 1. From this result, it was confirmed that having a structure of ε-Keggin type heteropolyacid was significant for the methacrylic acid formation reaction.

Example 2
(Preparation of catalyst)
Product preparation, filtration / water washing, and drying steps were performed in the same manner as in Example 1 except that the amount charged was 400 times. 15.0 g of a strength improver (ceramic fiber) is uniformly mixed with 125.0 g of the granules obtained in this step, and rolling granulation using 200 g of a spherical porous alumina carrier (particle size 4.5 mm) as a binder with pure water. The coating was molded by the method. The obtained molded product was subjected to main calcination at 300 ° C. for 6 hours under air flow to obtain a target coated catalyst. This coated catalyst was subjected to composition analysis using X-ray fluorescence analysis and elemental analysis. As a result, when the atomic ratio of vanadium was 1.0, the atomic ratio of molybdenum was 3.2 and the atomic ratio of bismuth was 0.00. 7. The atomic ratio of nitrogen was 1.1, and the compositional formula was Mo _11.5 V _3.6 Bi _2.5 (NH ₄ ) _4.0 O ₄₀ . When X-ray diffraction (using Cu—Kα ray) was measured, the same graph as in Example 1 shown in FIG. 2 was obtained, and the 2θ values were 7.7 °, 23.2 °, 25.4 °, 26 Since the peaks were confirmed at .5 ° and 29.8 °, it was confirmed that the sample of the present invention had an ε-Keggin type heteropolyacid structure.

(Catalyst evaluation test)
A stainless steel reaction tube having an inner diameter of 18.4 mm was filled with 41.2 ml of the coated catalyst obtained in Example 2, and the raw material gas composition (molar ratio) methacrolein: oxygen: water vapor: nitrogen = 1: 2: 4: 18.6. The reaction results were measured at a space velocity (SV) of 300 hr-1 and a reaction bath temperature of 290 ° C. The results of the evaluation test are shown in Table 1 below. It was confirmed that the granule having the ε-Keggin type heteropolyacid structure was formed by coating, and the methacrylic acid formation reaction was significant.

Claims (6)

A catalyst for producing methacrylic acid for producing methacrylic acid by catalytic gas phase oxidation of methacrolein in the presence of molecular oxygen or a molecular oxygen-containing gas,
It is represented by the following general formula (I),
_{Mo a A b B c X d} Y e Z f O g (I)
(Wherein Mo is molybdenum,
A is at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel;
B is at least one element selected from the group consisting of bismuth, zinc, manganese, iron, cobalt and nickel;
X is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium;
Y is at least one element selected from the group consisting of phosphorus, arsenic, antimony, germanium, bismuth and selenium,
Z is at least one element selected from the group consisting of iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium, rhodium, ruthenium, tellurium, tungsten, silicon, aluminum, cerium, magnesium, boron, and ammonium. Or an ion,
a, b, c, d, e, and f represent the atomic ratio of each element. When b = 1, 0 <a <5.0, 0 <c <2.0, 0 ≦ d <3. 0, 0 ≦ e <1.0, 0 ≦ f <2.0, and g is a number determined by the oxidation state of each element)
In the powder X-ray diffraction pattern using Cu-Kα rays, the values of 2θ are 7.7 ° ± 0.3 °, 23.2 ° ± 0.3 °, 25.4 ° ± 0.3 °, 26. A catalyst for producing methacrylic acid having an ε-Keggin type heteropolyacid structure having diffraction peaks at 5 ° ± 0.3 ° and 29.8 ° ± 0.3 °.   The catalyst for methacrylic acid production according to claim 1, wherein A is vanadium.   The catalyst for methacrylic acid production according to claim 1 or 2, wherein B is bismuth. The manufacturing method of the catalyst for methacrylic acid manufacture as described in any one of Claims 1-3 characterized by including the following process:
Step (A): a compound containing molybdenum; a compound containing at least one element selected from the group consisting of vanadium, zinc, manganese, iron, cobalt and nickel; and bismuth, zinc, manganese, iron, cobalt and nickel A compound containing at least one element selected from the group consisting of: a compound containing at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium if necessary; phosphorus, arsenic, antimony, if necessary A compound containing at least one element selected from the group consisting of germanium, bismuth and selenium; and, if necessary, iron, chromium, nickel, cobalt, manganese, zinc, tin, silver, palladium, rhodium, ruthenium, tellurium, tungsten, Silicon Miniumu, cerium, magnesium, at least one element or compound containing ions selected from the group consisting of boron and ammonium, mixed with water to prepare an aqueous solution or aqueous dispersion of these compounds process,
Step (B): A step of obtaining a solid content by hydrothermal synthesis using the aqueous solution or aqueous dispersion obtained in Step (A),
Step (C): A step of baking the solid content obtained in the step (B).   The manufacturing method of the catalyst for methacrylic acid production of Claim 4 whose temperature in hydrothermal synthesis is 80 to 200 degreeC, and whose pressure is the saturated vapor pressure in this temperature.   4. Methacrylic acid is subjected to a catalytic gas phase oxidation reaction with molecular oxygen or a molecular oxygen-containing gas in the presence of the catalyst for producing methacrylic acid according to any one of claims 1 to 3. Acid production method.

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