JP2000070721A - Production of methacrylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing methacrylic acid by which reaction can stably be continued over a long period of time. SOLUTION: Methacrylic acid is produced by the vapor phase catalytic oxidation of methacrolein with molecular oxygen using a fixed bed tube type reactor packed with a catalyst contg. at least Mo, P and As. The catalyst packed part consists of plural catalyst layers divided from the gaseous starting material inlet side toward the outlet side and the catalyst has a higher ratio of P to Mo and a lower ratio of As to Mo toward the outlet side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a process for producing methacrylic acid by gas phase catalytic oxidation of methacrolein with molecular oxygen using a fixed-bed tubular reactor packed with a catalyst containing at least molybdenum, phosphorus and arsenic. On how to do it.

[0002]

2. Description of the Related Art Numerous proposals have been made as catalysts for gas phase catalytic oxidation of methacrolein. For example, JP
-37614, JP-A-53-68718,
JP-A-54-103819, JP-A-57-120
JP-A-547, JP-A-60-239439 and JP-A-2-240043 disclose catalysts containing molybdenum and phosphorus as main components. From a structural point of view, these are considered to have a heteropolyacid structure mainly composed of phosphomolybdic acid or a salt thereof.

The greatest disadvantage of these heteropolyacid catalysts is that they have a short life as described in JP-A-57-72937. It has been observed that when the process is continued, some of the constituent elements are decomposed into oxides. This is because heteropolyacid compounds are generally thermally unstable.

Japanese Patent Application Laid-Open No. Sho 59-20243 discloses that a phosphorus molybdate compound scatters phosphorus in a catalyst as it reacts and moves to a reaction gas outlet side. Therefore, the inside of a reaction tube is divided into a plurality of catalyst layers. A method is disclosed in which a catalyst layer having a high phosphorus content is filled in a catalyst layer on the source gas inlet side and a catalyst having a low phosphorus content is filled in a catalyst layer on the outlet side. However, in this method, it is difficult to control the phosphorus content to an optimum range over a long period of time, and it is difficult to use the phosphorus stably for a long period of time.
Furthermore, adjusting the phosphorus content alone may reduce the catalytic activity and / or methacrylic acid selectivity, and is not sufficient for industrial use.

[0005] Japanese Patent Application Laid-Open No. 4-210937 discloses that a plurality of reaction zones having different activities are provided in a plurality of reaction zones which are divided into two or more layers in a tube axis direction in a fixed-bed multitubular reactor. A method is disclosed in which the catalyst is filled so that the activity becomes higher from the raw material gas inlet side toward the outlet side, thereby suppressing heat storage in a hot spot portion in the catalyst layer. The publication also proposes changing the composition ratio of catalyst constituent elements other than molybdenum and phosphorus as a method for preparing catalysts having different activities. For example, in Example 11 of the publication, when a molybdenum-phosphorus-arsenic-based catalyst was used, a catalyst having a low arsenic content was charged into the raw material gas inlet side and a catalyst having a high arsenic content was charged into the outlet side to perform the reaction. It is shown. However, in the case of a molybdenum-phosphorus-arsenic-based catalyst, it was confirmed that not only phosphorus but also a heteropolyacid compound having arsenic as a hetero atom was simultaneously decomposed during the reaction, and the latter decomposition was more remarkable. Thus, when the material gas is filled so that the arsenic content increases from the inlet side to the outlet side as in this example, a stable continuous reaction cannot be expected for a long period of time.

[0006]

Accordingly, the present invention provides
An object of the present invention is to provide a method for producing methacrylic acid, which can stably continue the reaction for a long period of time.

[0007]

The present inventors have studied the state of the catalyst during the gas phase catalytic oxidation reaction of methacrolein using a catalyst containing at least molybdenum, phosphorus and arsenic. It was confirmed that decomposition of a heteropolyacid compound having arsenic as a hetero atom occurred. This decomposition occurred more toward the raw material gas inlet side of the catalyst layer, and it was presumed that this was the cause of the decrease in the activity of the catalyst. As a result of further investigation, a plurality of catalysts having different phosphorus composition ratios and arsenic composition ratios were prepared, and a catalyst having a higher phosphorus composition ratio and a lower arsenic composition ratio from the raw material gas inlet side toward the outlet side was prepared. It has been found that the reaction can be stably continued without lowering the activity for a long period of time by filling.

That is, the present invention relates to a method for producing methacrylic acid in which methacrolein is subjected to gas phase catalytic oxidation of methacrolein with molecular oxygen using a fixed-bed tubular reactor filled with a catalyst containing at least molybdenum, phosphorus and arsenic. The catalyst-filled portion is composed of a plurality of catalyst layers divided in the direction from the raw material gas inlet side to the raw material gas outlet side. The more the raw material gas outlet side, the higher the phosphorus composition ratio (molybdenum standard) and the higher the arsenic composition ratio (molybdenum standard). The present invention relates to a method for producing methacrylic acid, which is a catalyst layer filled with a small amount of catalyst.

[0009]

DETAILED DESCRIPTION OF THE INVENTION In the present invention, methacrylic acid is produced by gas-phase catalytic oxidation of methacrolein with molecular oxygen using a fixed-bed tubular reactor packed with a catalyst containing at least molybdenum, phosphorus and arsenic. I do. As the fixed bed tube type reactor, a single tube type having only one reaction tube or a multi tube type having a plurality of reaction tubes may be used, but a fixed bed multi tube type reactor is usually used industrially.

[0010] In the present invention, the catalyst-filled portion in the reaction tube is composed of a plurality of catalyst layers divided from the raw material gas inlet side to the outlet side, and the composition ratio of phosphorus (based on molybdenum) increases toward the raw material gas outlet side. And the composition ratio of arsenic (based on molybdenum)
Is a catalyst layer filled with a small amount of catalyst. That is, regarding the composition ratio of phosphorus of the catalyst filled in each catalyst layer, the catalyst layer on the raw material gas inlet side has the smallest ratio, the catalyst layer closer to the outlet side has more, and the catalyst layer on the outlet side has the largest. I do. At the same time, the composition ratio of arsenic is highest in the catalyst layer on the inlet side of the raw material gas, is smaller in the catalyst layer closer to the outlet side, and is lowest in the catalyst layer on the outlet side.

Any number of catalyst layers having different phosphorus composition ratios and arsenic composition ratios may be used. However, if the number of catalyst layers is large, the production and filling of the catalyst become complicated, so that two to three layers are industrially preferable. The ratio of each catalyst layer is not particularly limited, but, for example, in the case of two layers, the catalyst to be filled in the catalyst layer on the raw material gas inlet side is preferably 10 to 70% by volume of the total catalyst loading.

Although the composition ratio of phosphorus and the composition ratio of arsenic in each catalyst layer are not particularly limited as long as the above-mentioned magnitude relation is satisfied, the composition ratio of phosphorus to molybdenum 12 is 0.1%.
To 3 are preferable, and the composition ratio of arsenic is preferably 0.1 to 3 with respect to molybdenum 12. The composition ratio of the constituent elements other than phosphorus and arsenic in each catalyst layer is usually the same, but may be different.

The catalyst of each catalyst layer is not particularly limited as long as it contains at least molybdenum, phosphorus and arsenic and has the ability to synthesize methacrylic acid by gas phase catalytic oxidation of methacrolein with molecular oxygen. However, those having the composition represented by the following formula are preferred. During _{Mo a P b As c Cu d} V e X f Y g O h ( wherein, Mo, represents P, As, Cu, V and O represent molybdenum, phosphorus, arsenic, copper, vanadium and oxygen, X is iron , Cobalt, nickel, zinc, magnesium, calcium, strontium, barium, titanium,
At least one element selected from the group consisting of chromium, tungsten, manganese, silver, boron, silicon, tin, lead, antimony, bismuth, niobium, tantalum, zirconium, indium, sulfur, selenium, tellurium, lanthanum and cerium; Y represents at least one element selected from the group consisting of potassium, rubidium, cesium and thallium. Where a, b, c, d, e, f,
g and h represent the atomic ratio of each element, and when a = 12,
0.1 ≦ b ≦ 3, 0.1 ≦ c ≦ 3, 0.01 ≦ d ≦ 2,
0.01 ≦ e ≦ 3, 0 ≦ f ≦ 3, 0.01 ≦ g ≦ 3, and h is an oxygen atomic ratio necessary to satisfy the atomic ratio of each component. )

The method for preparing the catalyst used in the present invention is not limited to a special method, and is not particularly limited to a well-known coprecipitation method, an evaporation to dryness method, an oxide mixing method and the like, unless there is a significant uneven distribution of components. Can be used.

The catalyst raw materials used for preparing the catalyst are not particularly limited, and nitrates, carbonates, acetates, ammonium salts, oxides, halides and the like of each element can be used in combination. For example, as a raw material of molybdenum, ammonium paramolybdate, molybdenum trioxide, molybdic acid, molybdenum chloride and the like can be used.

The catalyst used in the present invention may be a carrier-free catalyst, but may be a catalyst-active substance supported on an inert carrier such as silica, alumina, silica-alumina, silicon carbide, or diluted with these. .

When performing the gas phase catalytic oxidation by the method of the present invention, the raw material gas may contain water in addition to methacrolein and molecular oxygen, and is diluted with an inert gas such as nitrogen, water vapor or carbon dioxide. You may. Further, the raw material gas may contain a lower saturated aldehyde derived from the raw material methacrolein and the like in a small amount that does not substantially affect the present reaction.

The concentration of methacrolein in the raw material gas can be varied in a wide range, but is preferably 1 to 20% by volume, particularly preferably 3 to 10% by volume. The oxygen concentration in the source gas is defined by the molar ratio to methacrolein, and the molecular oxygen is preferably 0.3 to 4 times, more preferably 0.4 to 2.5 times, the mole of methacrolein. Although it is economically advantageous to use air as the molecular oxygen source, air enriched with pure oxygen may be used if necessary.

The reaction is carried out under normal pressure to several atmospheric pressures. The reaction temperature is usually carried out at 230 to 400 ° C.,
Preferably it is 250-350 degreeC. The contact time is usually 1.5 to 15 seconds, preferably 2 to 5 seconds.

[0020]

The present invention will be described below with reference to examples and comparative examples. In the description in Examples and Comparative Examples, "parts" represents parts by weight, and the conversion of methacrolein (hereinafter, simply referred to as reaction rate) and the selectivity of generated methacrylic acid (hereinafter, simply referred to as selectivity) are as follows. It was calculated by the following equation. Reaction rate (%) = A / B × 100 Selectivity (%) = C / A × 100 In the formula, A is the number of moles of reacted methacrolein, B is the number of moles of supplied methacrolein, and C is formed. It indicates the number of moles of methacrylic acid, and these values were determined by analyzing the raw material gas and the produced gas by gas chromatography.

Example 1 (Preparation of Catalyst 1) Ammonium paramolybdate 100
Parts, 3.3 parts of ammonium metavanadate and 7.0 parts of rubidium nitrate were dissolved in 400 parts of pure water. 8
3.8 parts of 5% by weight phosphoric acid and 12.3 of 60% by weight arsenic acid
Was dissolved in 60 parts of pure water, and 7.1 parts of tin dioxide was further added, followed by heating and stirring. Next, a solution prepared by dissolving 1.1 parts of copper nitrate in 50 parts of pure water was added, and the mixture was evaporated to dryness while heating. The obtained solid was dried at 130 ° C. for 16 hours, molded under pressure, and heat-treated at 380 ° C. for 5 hours in an air stream to obtain a catalyst. The composition of elements other than oxygen in the obtained catalyst (the same applies hereinafter) was Mo ₁₂ P _0.7 As _1.1 V _0.6 Cu _0.1 Sn ₁ Rb ₁ .

(Preparation of Catalyst 2) Catalyst 2 having the following composition was prepared in the same manner as Catalyst 1. Mo ₁₂ P _1.2 As _0.6 V _0.6 Cu _0.1 Sn ₁ Rb ₁

(Reaction Test) The reaction was carried out using a fixed-bed tube reactor having one reaction tube having an inner diameter of 14 mm. The reaction tube is charged with 8 ml of the catalyst 1 on the inlet side of the raw material gas and 12 ml of the catalyst 2 on the outlet side, and a raw material gas composed of 5% by volume of methacrolein, 10% by volume of oxygen, 20% by volume of steam and 65% by volume of nitrogen is supplied. A continuous reaction test was conducted at a reaction temperature of 290 ° C. and a contact time of 3.6 seconds. Table 1 shows the results.

Example 2 A continuous reaction test was performed in the same manner as in Example 1 except that the amount of the catalyst charged in the reactor catalyst layer was changed as shown in Table 1. Table 1 shows the results.

Comparative Example 1 A continuous reaction test was carried out in the same manner as in Example 1 except that the catalyst 2 was not used and only 20 ml of the catalyst 1 was charged. Table 1 shows the results
Shown in

[Comparative Example 2] A continuous reaction test was performed in the same manner as in Example 1 except that 8 ml of the catalyst 2 was charged on the inlet side of the raw material gas and 12 ml of the catalyst 1 was charged on the outlet side. Was. Table 1 shows the results.

Comparative Example 3 (Preparation of Catalyst 3) Catalyst 3 having the following composition was prepared in the same manner as in Catalyst 1. Mo ₁₂ P ₁ As _0.8 V _0.6 Cu _0.1 Sn ₁ Rb ₁

(Reaction test) In Example 1, the catalyst 3
A continuous reaction test was performed in the same manner as in Example 1 except that only 20 ml of the mixture was filled. Table 1 shows the results.

Comparative Example 4 (Preparation of Catalyst 4) Catalyst 4 having the following composition was prepared in the same manner as in Catalyst 1. Mo ₁₂ P _1.4 As _0.8 V _0.6 Cu _0.1 Sn ₁ Rb ₁

(Reaction test) In Example 1, 8 ml of the catalyst 4 was placed on the inlet side of the raw material gas, and 12 ml of the catalyst 3 was placed on the outlet side.
A continuous reaction test was performed in the same manner as in Example 1 except that the method of filling was changed. Table 1 shows the results.

[0031]

[Table 1]

Example 3 (Preparation of Catalysts 5 and 6) Catalysts 5 and 6 were prepared in the same manner as in the preparation of Catalyst 1. Table 2 shows their compositions.

(Reaction test) In Example 1, 8 ml of the catalyst 5 was placed on the inlet side of the raw material gas, and 12 ml of the catalyst 6 was placed on the outlet side.
A continuous reaction test was performed in the same manner as in Example 1 except that the method of filling was changed. Table 2 shows the results.

Comparative Example 5 A continuous reaction test was conducted in the same manner as in Example 3 except that the catalyst 6 was not used and only 20 ml of the catalyst 5 was charged. Table 2 shows the results
Shown in

Comparative Example 6 A continuous reaction test was carried out in the same manner as in Example 3 except that the catalyst 5 was not used and only 20 ml of the catalyst 6 was charged. Table 2 shows the results.

[0036]

[Table 2]

[0037]

According to the present invention, there is provided a process for producing methacrylic acid in which gas phase catalytic oxidation of methacrolein with molecular oxygen using a fixed-bed tubular reactor filled with a catalyst containing at least molybdenum, phosphorus and arsenic. In this case, the reaction can be stably continued for a long period of time.

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Claims (2)

[Claims] 1. A method for producing methacrylic acid in which methacrolein is subjected to gas phase catalytic oxidation of methacrolein with molecular oxygen using a fixed-bed tubular reactor filled with a catalyst containing at least molybdenum, phosphorus and arsenic. Is composed of a plurality of catalyst layers divided in the direction from the raw material gas inlet side to the raw material gas outlet side. A catalyst having a higher phosphorus composition ratio (based on molybdenum) and a lower arsenic composition ratio (based on molybdenum) toward the raw material gas outlet side. A method for producing methacrylic acid, which is a packed catalyst layer. Wherein any of the catalyst in each catalyst _{layer, Mo a P b As c Cu} d V e X f Y g O h ( wherein, Mo, P, As, Cu, V and O represent molybdenum, X represents phosphorus, arsenic, copper, vanadium and oxygen, and X represents iron, cobalt, nickel, zinc, magnesium, calcium, strontium, barium, titanium,
At least one element selected from the group consisting of chromium, tungsten, manganese, silver, boron, silicon, tin, lead, antimony, bismuth, niobium, tantalum, zirconium, indium, sulfur, selenium, tellurium, lanthanum and cerium; Y represents at least one element selected from the group consisting of potassium, rubidium, cesium and thallium. Where a, b, c, d, e, f,
g and h represent the atomic ratio of each element, and when a = 12,
0.1 ≦ b ≦ 3, 0.1 ≦ c ≦ 3, 0.01 ≦ d ≦ 2,
0.01 ≦ e ≦ 3, 0 ≦ f ≦ 3, 0.01 ≦ g ≦ 3, and h is an oxygen atomic ratio necessary to satisfy the atomic ratio of each component. The method for producing methacrylic acid according to claim 1, wherein the composition has a composition represented by the following formula: