Process for preparing catalysts for producing methacrylic acid

Abstract: A process for preparing a multi-component catalyst for producing methacrylic acid, containing phosphorus, molybdenum and vanadium, comprising mixing oxides of molybdenum and vanadium, water and materials for the other catalyst-constituting elements except potassium, rubidium, cesium and thallium to prepare a mixed solution, heating and reacting the mixed solution at 85°C or more for 1 to 10 hours, then cooling the mixed solution to 80°C or less, adding thereto a material for at least one element component selected from the group consisting of potassium, rubidium, cesium and thallium, adding further thereto at least one compound selected from the group consisting of ammonium nitrate, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and ammonium hydrogensulfate with the temperature of the mixed solution kept at 80°C or less, then removing water therefrom and heat-treating the residual product. According to the present invention, a catalyst which works effectively at a lower reaction temperature and yet gives methacrylic acid in higher yields than when catalysts prepared by the conventional methods are used is prepared.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for preparing a catalyst used in producing methacrylic acid by the gas-phase catalytic oxidation of methacrolein. Description of the Prior Art Hitherto, in producing methacrylic acid by the gas-phase catalytic oxidation of methacrolein, it is advantageous to keep the yield high at a low reaction temperature from standpoints such as the life of catalysts, inhibition of side reactions, cost of equipments, etc. However, when catalysts prepared by the conventional methods are used, satisfactory results are not always obtained. One possible reason for this is that the dimension of specific surface area and distribution of micropores of the catalyst which are important to the oxidation reaction have not sufficiently been controlled. In order to improve these defects, the following various trials have been made at the time of preparation of the catalyst : Addition of a carboxylic acid or polyhydric alcohol (refer to Japanese Patent Application Kokai No. 51-136615), addition of an alcohol or glycol (refer to ibid, No. 55-73347), addition of pyridine or its derivative (refer to ibid. No. 47-38591, No. 57-171444), addition of quinoline or its derivative (refer to ibid. No. 60-209258), addition of agueous ammonia or ammonium nitrate (refer to ibid. No. 57-165040), etc. However, these methods have various defects that, for example, the reaction results are not satisfactory; the catalytic activity largely lowers with the lapse of time; the reaction temperature is too high; and the heat-treatment method which is a catalyst-activating treatment is troublesome because of the use of organic substances. The catalysts thus obtained, therefore, are not satisfactory as industrial catalysts. Catalysts are known from US Patent Publication 3,998,876 which have the formula P a Mo b V c (NH₄) d X e Y f O g which can be used for the oxidation of methacrolein to methacrylic acid. The process of making this catalyst involves dissolution of ammonium paramolybdate in water, addition of phosphoric acid and arsenic acid, followed by addition of a solution of ammonium metavanadate. All of these catalyst ingredients were then heated by evaporation of the mixed solution to dryness. In EP-A-350,862 a further catalyst is disclosed suitable for catalysis of oxidation of methacrolein to methacrylic acid, having the formula P a Mo b V c Fe d Ce e (NH₄) f X g Y h Z i O j . The catalyst is produced by dissolving ammonium paramolybdate and ammonium metavanadate and potassium nitrate in water, adding phosphoric acid and antimony trioxide. After heating of this mixture, further ingredients are added. These are cupric nitrate, cerium dioxide and ferric nitrate. This mixture is then evaporated to dryness. An object of the present invention is to provide a novel process for preparing a catalyst for advantageously producing methacrylic acid from methacrolein. In order to improve the conventional catalyst-preparation methods, the present inventors have studied with particular notice given to the physical properties of the catalyst, and as a result have found a novel process for preparing a catalyst which works effectively at a lower reaction temperature and yet gives methacrylic acid in higher yields than when catalysts prepared by the conventional methods are used. The present invention relates to a process for preparing a multi-component catalyst for producing methacrylic acid, containing phosphorus, molybdenum and vanadium, comprising mixing oxides of molybdenum and vanadium, water and materials for the other catalyst-constituting elements except potassium, rubidium, cesium and thallium to prepare a mixed solution, heating and reacting the mixed solution at 85°C or more for 1 to 10 hours, then cooling the mixed solution to 80°C or less, adding thereto a material for at least one element component selected from the group consisting of potassium, rubidium, cesium and thallium, adding further thereto at least one compound selected from the group consisting of ammonium nitrate, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and ammonium hydrogensulfate with the temperature of the mixed solution kept at 80°C or less, then removing water therefrom and heat-treating the residual product. The catalyst for producing methacrylic acid obtained according to this process has a composition represented by the general formula: P a Mo b V c Cu d X e Y f (NH₄) g O h wherein P, Mo, V, Cu, NH₄ and O are phosphorus, molybdenum, vanadium, copper, ammonium group and oxygen, respectively, X is at least one element selected from the group consisting of potassium, rubidium, cesium and thallium, Y is at least one element selected from the group consisting of silver, magnesium, zinc, arsenic, germanium, silicon, tungsten, boron, bismuth, chromium, lanthanum, barium, antimony, iron, zirconium, tellurium and cerium, and a, b, c, d, e, f and h are an atomic ratio of each element, and when b is 12, a is 0.5 to 3, c is 0.01 to 3, d is 0.01 to 2, e is 0.01 to 2 and f is 0 to 5, h is the number of oxygen atoms necessary to satisfy the valence of each component, and g is the number of ammonium groups being 0.01 to 2. Materials for molybdenum and vanadium components used to prepare the catalyst include molybdenum trioxide, molybdic acid and other oxides. Particularly, good results are obtained when molybdenum trioxide and vanadium pentoxide are used. As materials for other catalyst-constituting elements, the oxides, carbonates, acetates, hydroxides, etc. of the elements can be used in combination.. In practicing the present invention, first, materials for catalyst-constituting elements (comprising at least materials for molybdenum, vanadium and phosphorus) except materials for potassium, rubidium, cesium and/or thallium is dissolved or dispersed in water. As materials for the foregoing catalyst-constituting elements, in addition to the oxides of molybdenum and vanadium and phosphorus compounds, there are used for example the oxides, carbonates, acetates, hydroxides, etc. of copper, silver, magnesium, zinc, arsenic, germanium, silicon, tungsten, boron, bismuth, chromium, lanthanum, barium, antimony, iron, zirconium, tellurium and cerium. Next, the aqueous solution or dispersion of the materials for catalyst is heated for 1 to 10 hours at 85°C or more, preferably 90°C or more. When the heating temperature is less than 85°C or the heating time is less than 1 hour, it is difficult for the resulting catalyst to take a heteropoly-acid structure effective for reaction. If the heating time is longer than the above value, a particular improvement in the effect is not observed. Some of the components can be added to the reaction mixture part way through the heating step, especially materials which are compounds of the optional elements mentioned above. After heating, the mixed solution containing the materials for catalyst is cooled to 80°C or less, preferably 35° to 70°C, and materials for potassium, rubidium, cesium and/or thallium components are added. Next, at the temperature of the mixed solution of 80°C or less, preferably 40° to 70°C, ammonium nitrate, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and/or ammonium hydrogensulfate are added. When the materials for potassium, rubidium, cesium and/or thallium components and the above ammonium compounds are added at a liquid temperature exceeding 80°C, it is difficult to prepare a catalyst producing methacrylic acid in high yields. The amount of ammonium nitrate, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and/or ammonium hydrogensulfate is preferably 0.5 to 30 wt.%, particularly preferably 1 to 20 wt.% based on the total weight of the materials for the catalyst. When the amount of the ammonium compound is less than the above value, the performance of the catalyst can be insufficient and if it is more than the above value, a particular improvement in the effect is not observed. Next, on heat-treating the mixture thus obtained, the desired catalyst is obtained. It is desirable to carry out the heat-treatment, for example, at a temperature of 300° to 430°C while streaming air and/or a gas containing 5 vol.% or more of oxygen. When the mixture is an aqueous solution or dispersion, it is usually desirable to carry out the heat-treatment after removal of water. The catalyst obtained according to the present invention works effectively without a carrier, but it is preferred to use the catalyst supported on or diluted with an inert carrier such as silica, alumina, silica-alumina, silicon carbide, diatomaceous earth, etc. When methacrylic acid is produced with the catalyst obtained by the present invention, a methacrolein-containing gas, a material, is used. The methacrolein concentration of the gas can be changed in a wide range, but 1 to 20% by volume, particularly 3 to 10% by volume is preferred. Methacrolein, a material, may contain small amounts of impurities such as water, a lower saturated aldehyde, etc. These impurities give substantially no effect to the reaction. As an oxygen source, the use of air is economical, but air made rich in pure oxygen may be used if necessary. The oxygen concentration of the gas, a material, is determined by the molar ratio to methacrolein. The value of this molar ratio is 0.3 to 4, particularly preferably 0.4 to 2.5. The gas, a material, may be diluted with an inert gas such as nitrogen, steam, carbon dioxide, etc. Reaction pressure is preferably normal pressure to several atmospheres. Reaction temperature is 200° to 420°C, particularly preferably 230° to 400°C. The reaction can be carried out by using either a fixed bed or fluidized bed. In the following examples and comparative examples, the conversion of methacrolein and selectivity of methacrylic acid produced are defined as follows: In the following examples and comparative examples, parts are by weight, and analyses were carried out by gas chromatography. Example 1 100 Parts of molybdenum trioxide, 2.6 parts of vanadium pentoxide and 6.7 parts of 85% phosphoric acid were added to 800 parts of pure water, and the resulting mixture was heated under reflux at 100°C for 6 hours. Thereafter, 1.2 parts of copper acetate was added, and refluxing was continued at 100°C for further 3 hours with heating. After refluxing, the mixed solution was cooled to 40°C, and 11.2 parts of cesium hydrogencarbonate dissolved in 100 parts of pure water was added, after which 5.6 parts of ammonium carbonate dissolved in 100 parts of pure water was further added with the temperature of the mixed solution kept at 40°C. The resulting mixed solution was evaporated to dryness with heating. The solid product obtained was dried at 120°C for 16 hours, shaped by applying pressure and heat-treated at 380°C for 5 hours under air stream. The composition of components except oxygen of the resulting catalyst was P₁Mo₁₂V 0.5 Cu 0.1 Cs₁(NH₄) 0.3 (catalysts described later also are represented by the composition of components except oxygen). A reactor was charged with this catalyst, and a mixed gas consisting of 5 vol.% of methacrolein, 10 vol.% of oxygen, 30 vol.% of steam and 55 vol.% of nitrogen was passed through the reactor at a reaction temperature of 285°C for a contact time of 3.6 seconds. The products were collected and analyzed by gas chromatography to find that the conversion of methacrolein was 85.8% and the selectivity of methacrylic acid was 83.9%. Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that, after the refluxing, cesium hydrogencarbonate was added with the temperature of the mixed solution kept at 100°C, and then ammonium carbonate also with the temperature of the mixed solution kept at 100°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 to find that the conversion of methacrolein was 84.1% and the selectivity of methacrylic acid was 83.8%. Comparative Example 2 A catalyst was prepared in the same manner as in Example 1 except that, after the refluxing, the mixed solution was cooled to 40°C and then ammonium carbonate was added, after which cesium hydrogencarbonate was further added with the temperature of the mixed solution kept at 40°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 to find that the conversion of methacrolein was 84.8% and the selectivity of methacrylic acid was 84.0%. Comparative Example 3 A catalyst was prepared in the same manner as in Example 1 except not adding ammonium carbonate. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 290°C. As a result, it was found that the conversion of methacrolein was 80.6% and the selectivity of methacrylic acid was 81.6%. Example 2 A catalyst having a composition, P₁Mo₁₂V 0.5 Cu 0.1 K₁Si 0.3 As 0.2 (NH₄) 0.2 , was prepared according to Example 1 except that 9.2 parts of ammonium hydrogencarbonate per 100 parts of molybdenum trioxide was added in place of ammonium carbonate. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 86.2% and the selectivity of methacrylic acid was 86.8%. Comparative Example 4 A catalyst was prepared in the same manner as in Example 2 except that, after the refluxing, potassium carbonate was added with the temperature of the mixed solution kept at 100°c, and then ammonium hydrogencarbonate was added with the temperature of the mixed solution kept at 100°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 to find that the conversion of methacrolein was 84.3% and the selectively of methacrylic acid was 86.6%. Comparative Example 5 A catalyst was prepared as in the same manner as in Example 2 except that, after the refluxing, the mixed solution was cooled to 40°C and ammonium hydrogencarbonate was added, after which potassium carbonate was further added with the temperature of the mixed solution kept at 40°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 85.1% and the selectivity of methacrylic acid was 86.7%. Comparative Example 6 A catalyst was prepared in the same manner as in Example 2 except not adding ammonium hydrogencarbonate. using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 280°C. As a result, it was found that the conversion of methacrolein was 79.3% and the selectivity of methacrylic acid was 84.2%. Example 3 A catalyst having a composition, P 1.5 Mo₁₂V 0.8 Cu 0.2 Rb₁Ce 0.1 Fe 0.2 Sb 0.8 (NH₄) 0.4 , was prepared according to Example 1 except that a part of ammonium carbonate was replaced by ammonium hydrogencarbonate and a mixed solution containing 2.8 parts of ammonium carbonate and 4.6 parts of ammonium hydrogencarbonate per 100 parts of molybdenum trioxide was added. In this case, antimony trioxide was used as a source of antimony. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 91.3% and the selectivity of methacrylic acid was 88.9%. Comparative Example 7 A catalyst was prepared in the same manner as in Example 3 except that, after the refluxing, rubidium acetate was added with the temperature of the mixed solution kept at 100°C, and then a mixed solution of ammonium carbonate and ammonium hydrogencarbonate was added with the temperature of the mixed solution kept at 100°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 89.7% and the selectivity of methacrylic acid was 88.8%. Comparative Example 8 A catalyst was prepared in the same manner as in Example 3 except not adding ammonium carbonate nor ammonium hydrogencarbonate. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 77.8% and the selectivity of methacrylic acid was 87.1%. Example 4 A catalyst having a composition, P 1.1 Mo₁₂V 0.8 Cu 0.2 K 0.7 Cs 0.2 Bi 0.2 Sb 0.7 (NH₄) 0.4 , was prepared according to Example 1 except that ammonium nitrate was used in place of ammonium carbonate, and 9.8 parts of ammonium nitrate per 100 parts of molybdenum trioxide was added. In this case, antimony pentoxide was used as a source of antimony. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 92.3% and the selectivity of methacrylic acid was 88.6%. Comparative Example 9 A catalyst was prepared in the same manner as in Example 4 except that, after refluxing, cesium hydrogencarbonate and potassium carbonate were added with the temperature of the mixed solution kept at 100°C, and then ammonium nitrate was added with the temperature of the mixed solution kept at 100°C. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 270°C. As a result, it was found that the conversion of methacrolein was 90.8% and the selectivity of methacrylic acid was 88.6%. Comparative Example 10 A catalyst was prepared in the same manner as in Example 4 except not adding ammonium nitrate. Using this catalyst, reaction was carried out under the same conditions as in Example 1 except that the reaction temperature was changed to 290°C. As a result, it was found that the conversion of methacrolein was 87.4% and the selectivity of methacrylic acid was 87.5%. Examples 5 to 7 Catalysts shown in Table 1 were prepared according to Example 1. In Table 1, part by weight of ammonium sulfate, ammonium hydrogencarbonate or ammonium carbonate means part by weight per 100 parts by weight of molybdenum trioxide. Solution temperature at addition means the solution temperature when materials for thallium, potassium and/or cesium components and ammonium sulfate, ammonium hydrogencarbonate or ammonium carbonate are added. Using these catalysts, reaction was carried out in the same manner as in Example 1 except that the reaction temperature was changed. The results are collectively shown in Table 1. Examples 8 to 10 Catalysts shown in Table 1 were prepared according to Example 3. In Table 1, part by weight of ammonium carbonate, ammonium sulfate and/or ammonium hydrogensulfate means part by weight per 100 parts by weight of molybdenum trioxide. Solution temperature at addition means the solution temperature when materials for potassium, cesium or rubidium component and ammonium carbonate, ammonium sulfate and/or ammonium hydrogensulfate are added. Using these catalysts, reaction was carried out in the same manner as in Example 1 except that the reaction temperature was changed. The results are collectively shown in Table 1.

Claims: A process for preparing a multi-component catalyst represented by the general formula: P a Mo b V c Cu d X e Y f (NH₄) g O h wherein P, Mo, V, Cu, NH₄ and O are phosphorus, molybdenum, vanadium, copper, ammonium group and oxygen, respectively, X is at least one element selected from the group consisting of potassium, rubidium, cesium and thallium, Y is at least one element selected from the group consisting of silver, magnesium, Zinc, arsenic, germanium, silicon, tungsten, boron, bismuth, chromium, lanthanum, barium, antimony, iron, Zirconium, tellurium and cerium, and a, b, c, d, e, f and h are an atomic ratio of each element, and when b is 12, a is 0.5 to 3, c is 0.01 to 3, d is 0.01 to 2, e is 0.01 to 2 and f is 0 to 5, and h is the number of oxygen atoms necessary to satisfy the valence of each component, and g is the number of ammonium groups being 0.01 to 2, for producing methacrylic acid, comprising mixing oxides of molybdenum and vanadium, water and materials for other catalyst-constituting elements except potassium, rubidium, cesium, and thallium to prepare a mixed solution, heating and reacting the mixed solution at 85°C or more for 1 to 10 hours, then cooling the mixed solution to 80°C or less, adding thereto a material for at least one element component selected from the group consisting of potassium, rubidium, cesium, and thallium, adding further thereto at least one compound selected from the group consisting of ammonium nitrate, ammonium carbonate, ammonium hydrogencarbonate, ammonium sulfate and ammonium hydrogensulfate with the temperature of the mixed solution kept at 80°C or less, then removing water therefrom and heat-treating the residual product. A process according to claim 1 in which molybdenum trioxide is used as starting material. A process according to any preceding claim in which vanadium pentoxide is used as starting material. A process according to any preceding claim in which compounds selected from oxides, carbonates, acetates and hydroxides of elements selected from copper, silver, magnesium, zinc, arsenic, germanium, silicon, tungsten, boron, bismuth, chromium, lanthanum, barium, antimony, iron, zirconium, tellurium and cerium are included in the said mixed solution. A process according to claim 4 in which said compounds are added to the mixed solution part way through the reaction at 85°C or more. A process according to any preceding claim in which the heat treatment is carried out at a temperature of 300° to 430°C while streaming air and/or a gas containing 5 vol.% or more of oxygen. Use of a multi-component catalyst prepared by the process of any one of claims 1 to 6 for the oxidation of methacrolein to produce methacrylic acid. Use according to claim 7 in which the oxidation is carried out at a temperature in the range 200-420°C, preferably 230 to 400°C. Use according to claim 7 or 8 in which the catalyst is in contact with a gas containing 1 to 20%, preferably 3 to 10%, by volume methacrolein and oxygen, the oxygen:methacrolein molar ratio being in the range 0.3 to 4, preferably in the range 0.4 to 2.5.

Inventors: OH-KITA MOTOMU, YAMAMOTO SHINJI, OH-KITA, MOTOMU, YAMAMOTO, SHINJI
Applicants: MITSUBISHI RAYON CO, MITSUBISHI RAYON CO., LTD.
Application year: 1991
Main classification: B01J27
IPC codes: B01J27, C07C57, C07B61, C07C51