GB2037604A - Process for producing methacrylic acid - Google Patents

Abstract

A process for producing methacrylic acid by vapour phase catalytic reaction of methacrolein and oxygen, at an elevated temperature, in a fixed bed reactor, and in the presence of a catalyst containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen. The catalyst has a large number of micro pores of an average pore radius of at least 2000 ANGSTROM and is prepared by mixing compounds containing the constituent elements of the catalyst with each of the aqueous medium; concentrating and/or drying the resultant mixture to form a clayey solid or powder; admixing the resultant clayey solid or powder with 1 to 10% by weight, based on the weight of the dry solid or powder, of an organic substance; moulding the resultant admixture to form a moulded catalyst composition, and calcining the moulded catalyst composition.

Description

SPECIFICATION Process for producing methacrylic acid The present invention relates to a process for producing methacrylic acid by catalytic oxidation of methacrolein. More specifically, it relates to a process for producing methacrylic acid in the vapor phase reaction of methacrolein and molecular oxygen, at an elevated temperature, in a fixed bed reactor, in the presence of an improved catalyst containing molybdenum, phosphorous, vanadium, the alkali metals and oxygen.

It has been known in the art that methacrylic acid is produced by the vapor phase reaction of methacrolein and molecular oxygen in the presence of catalysts, at an elevated temperature. The known catalysts for producing methacrylic acid contain molybdenum, phosphorus, vanadium, the alkali metals and oxygen, and are disclosed for example, in Japanese Patent Laid-Open Applications (KOKAI) No. 50-82013 (1975), 50-96522 (1975), 51-113818 (1976), 51-115413 (1976) and 52-36619(1977). These known catalysts containing molybdenum, phosphorous, vanadium, the alkali metals and oxygen are generally prepared by first mixing compounds containing the constituent elements of the catalyst, such as molybdenum compounds, phosphorous compounds, vanadium compounds, alkali metal compounds and the like, with each other in an aqueous medium; concentrating and/or drying the resultant mixture to form a clayey solid or powders; molding the clayey solid or powders, and; then, calcining the molded catalyst.

However, since the above-mentioned clayey solid or powders are non-sticky and have poor moldability, it is difficult to prepare catalysts having a crushing strength of, for example, approximately 3 kglcatalyst or more, which strength is sufficient to allow the practical use of the catalysts in fixed bed reactors even in the case where the catalysts are molded or shaped by means of, for example, an extruder, a rotary granulator, a tableting machine and the like, followed by a calcination. In the case where catalysts are molded by means of a tableting machine, a molded catalyst composition having a relatively high mechanical strength can be prepared by adjusting a molding machine or by modifying the molding operation even if materials having a poor moldability are utilized.However, the characteristics of the catalysts, for example, the reactivity of the methacrolein, the selectivity to the methacrylic acid and the like, decrease as the mechanical strength of the catalysts increase. In a case where mechanical strengths are imparted to catalysts which are sufficientto allow the use of the catalysts in fixed bed reactors, there is a problem that the yield of the methacrylic acid remarkably decreases compared to the case where the conventional catalyst is used.

Accordingly, the objects of the present invention are to obviate the aforementioned problems of the conventional processes for producing methzcrylic acid from methacrolein in the presence of a catalyst containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, and to provide a process for producing methacrylic acid by catalytic oxidation of methacrolein, at a high yield, in the presence of an improved catalyst containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, and having a mechanical strength sufficient to practically use the catalyst in a fixed bed reactor.

Other objects and advantages of the present invention will be apparent from the following description.

In accordance with the present invention, there is provided a process for producing methacrylic acid comprising the step of reacting methacrolein with molecular oxygen, at an elevated temperature, under a vapor phase, in a fixed bed reactor, in the presence of a catalyst containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, and having a large number of micro pores of an average pore radius of at least 2000 A, said catalyst being prepared by the steps of: mixing compounds containing the constituent elements of the catalyst with each other in an aqueous medium; concentrating and/or drying the resultant mixture to form a clayey solid or powders; admixing the resultant clayey solid or powders with 1 to 10 % by weight, based on the weight of the dry solid or pow ders, of an organic substance; molding or shaping the resultant admixture to form a molded catalyst composition, and; calcining the molded catalyst.

With respect to the catalysts containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, which are in the form of a pellet, sphere and the like, having a diameter of 3 mm or more, typically3to 10 mm, and which are used in fixed bed reactors, the inventors of the present invention have found the following facts.

(1) The selectivity to the methacrylic acid is intimately related to the micro pores of the catalysts.

The selectivity to the methacrylic acid increases as the average pore radius (determined by a mercury pressure porosimeter) increases. The suitable average pore radius of the catalysts is 2000 A or more and, more preferably, 2000 to 12000 A.

(2) The moldability of the catalyst is remarkably improved by the addition of a specific amount of organic substances, such as cellulose, polyvinyl alcohol, polyethylene glycol, gelatin and the like, prior to the molding of the catalysts when the catalysts are prepared.

(3) In the case where the catalysts are molded and calcined after the above-mentioned organic substances are previously admixed with the other constituents of the catalysts, the average pore radius of 2000 A or more, more specfically, of 2000 A to 12000 A can be easily obtained.

(4) The catalysts having the above-mentioned average pore radius, which are prepared by adding the specific amount of the above-mentioned organic substances, afford a higher yield of the methacrylic acid and their mechanical strengths allow the practical use of the catalysts in fixed bed reactors.

The term "average pore radius" used herein is determined as follows. 0.5 g of a sample (catalyst) is put into a dilatometer. After the dilatometer is evacuated to 2 x 102 mmHg or less by a vacuum pump, mercury is introduced into the dilatometer and, then, the dilatometer is charged in an autoclave.

The autoclave is gradually pressurized from an atmospheric pressure up to 1500 kg/cm2 (gauge pressure) and the decrease in the level of the mercury is continuously monitored. From the correlation between the pressure change and the level change of the mercury (i.e. the decrease in the volume of the mercury), the pore distribution is measured and, thus, the average pore radius is determined (mercury pressure porosimeter).

The catalysts used in the present invention have the features that the preparation thereof is easy and the moldabilitythereofduringthe molding step is good; the reactivity of the methacrolein and the selectivity to the methacrylic acid are good when the catalysts are used, and; not only can a high yield of the methacrylic acid be obtained by the use of these catalysts, but also, the catalysts have high mechanical strengths. Therefore, methacrylic acid can be advantageously produced by using these catalysts in fixed bed reactors, as compared to the use of the known catalysts containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen.

According to the present invention, in the catalyst preparation step, organic substances, such as, for example, cellulose, polyvinyl alcohol, polyethylene glycol, gelatin, dextrin; gum arabic, cellulose esters, cellulose ethers and the like, are incorporated into the catalyst composition in an amount of from 1 to 10 % by weight, preferably 2 to 8 % by weight, prior to the molding, and this catalyst composition is then molded and calcined. The preferable organic substance used in the present invention is at least one substance selected from the group consisting of cellulose, polyvinyl alcohol, polyethylene glycol and gelatin.

The incorporation of the above-mentioned organic substance into the catalyst composition remarkably improves the moldability of the clayey solid or pow dercomposition containing molybdenum, phosphorus, vanadium, the alkali metal elements and oxygen, and therefore, the composition can be easily molded to the molded catalyst in the form of a pellet, sphere or the like. The molded catalyst composition is then calcined to form a catalyst having a great number of micro pores thereof, due to the degradation and vaporization of the organic substances.

Although the incorporation of the inorganic substances into the catalyst composition can be effected in any step prior to the molding step of the catalyst composition, it is advisable to incorporate the organic substance into the catalyst composition just before the molding step, preferably into the clayey solid or powder composition before the molding step, and more preferably, into the dried powder composition. If the amount of the organic sub stances to be added is too large, the catalysts so obtained have a low mechanical strength and are very brittle. On the other hand, if the amount of the organic substances to be added is too small, the desired effect by the addition of the organic sub stances does not appear.Thus, a suitable amount of the organic substances to be added is within the range of from 1 to 10 % by weight and, more preferably, 2 to 8 % by weight, based on the weight of the clayey solid or powder composition (dry base).

The average pore radius of the micro pores on the surface of the catalysts is important in the present invention. In the case where catalysts having an average pore radius less than 2000 A are used in the present invention, methacrylic acid cannot be pro duced ata high yield even when the catalysts are prepared by the addition of the above-mentioned organic substances. The average pore radius of the catalysts greatly depends on the mechanical strength of the molded catalyst composition before calcination. If the mechanical strength of the molded catalysts is too high, the average pore radius is small.In order to prepare catalysts having an average pore radius of at least 2000 A and, preferably, 2000 to 12000 A, the catalysts may first be molded in such a manner that the crushing strength of the molded catalysts will fall within the range of from 1 to 7 kg/molded catalyst composition and, preferably, 1.5 to 6 kg/molded catalyst composition by using a molding machine, for example, a tableting machine, an extruder, a rotary granulator and the like, followed by the calcination It should be noted that, when the molded catalyst composition is wet, it is first dried and then, the crushing strength is measured.When the molded catalyst composition having the above-mentioned crushing strength and containing the specified amount of the organic substances is calcined, the mechanical strength of the catalyst increases compared to that of the molded catalyst, and a catalyst suitable for use in fixed bed reactors and having the above-mentioned average pore radius can be easily prepared.

The calcination can be generally carried out at a temperature of from 300 to 450"C and, more preferably, from 350 to 430"C, under an atmosphere of an oxygen-containing gas such as air. If the calcining temperature is either too high or too low, the performances of the catalyst decrease.

Although the compositions of the catalysts used in the present invention, containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, are not critically limited, the compositions of the catalysts having the following general formula are preferable in order to produce methacrylic acid art a high yield.

MOaPbVcAd Oe wherein Mo is molybdenum, P is phosphorus, V is vanadium, A is the alkali metals, preferably at least one metal selected from the group consisting of potassium, cesium and rubidium and 0 is oxygen; the subscripts a, bc, d and e represent the number of atoms, and when a is 12, b = 0.5 to 3, c = 0.1 to 2, d = 0.5 to 3 and e is the number which is required by, the total valance of the other atoms. Although, in the above-mentioned general formula, 0.01 to 2 atoms, in total, of at least one element selected from A, Al, * Ba, Ca, Cu, Mg, Mn, Pb, Sr, Te and the like, based on 12 atoms of Mo, are further present in the catalysts, the desired effect by the addition of the above mentioned organic substances and the average pore radius of the catalysts are not adversely affected.

Compounds containing the constituent atoms of the catalysts used, as starting materials, in the prep aration of the catalysts, such as, for example, molybdenum compounds, phosphorus compounds, vanadium compounds and the alkali metal com opounds, can be any compounds which are gener ally used for the preparation of the conventional catalysts containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen.Examples of such compounds are molybdenum compounds, such as molybdenum trioxide, 12- molybdophosphoric acid, 18-molybdodiphosphoric acid, ammonium molybdate and the like; phos phoric compounds, such as the ammonium salts of orthoDhosphoric acid, metaphosphoric acid, phos phorous acid, phosphoric acid and the like; van adium compounds, such as vanadium pentoxide, ammonium metavanadate, vanadyl sulfate, van adium tetrachloride and the like, and; nitrates, sul fates, carbonates, chlorides, of the alkali metals, such as potassium, cesium and rubidium, and the like.

The catalyst employed in the present invention is generally prepared in the following manner. The compounds containing the constituent elements of the catalyst, for example, molybdenum compounds, phosphorus compounds, vanadium compounds and alkali metal compounds, are mixed with each other in an aqueous medium and the resultant mixture is then concentrated and/or dried to form a clayey solid or powders. In to the clayey solid or powders thus formed, a given amount of the aforementioned organic substance or substances is added and mixed, and then, the mixture is molded and calcined.

Atypical example of the catalyst preparation is as follows.

A molybdenum compound is dissolved or dis persed in water, and then, aqueous solutions of a phosphorous compound, an alkali metal compound and a vanadium compound are added thereto with stirring. The mixture thus obtained by mixing the compounds containing the constituent elements of the catalyst in the presence of aqueous medium is, then, heated and concentrated or dried at a tempera ture of 100 to 2500C, for 5 to 20 hours, preferably at a temperature of 120 to 220"C, for 6 to 16 hours.After the dried material is ground to form powders, 1 to 10 % by weight, preferably 2 to 8 % by weight, based upon the dry weight of the powders, of an organic substance or substances, preferably at least one organic substance selected from the group consist ing of cellulose, polyvinyl alcohol, polyethylene glycol and gelatine, is added to and mixed with the powders. Then, the mixture is molded to form a molded catalyst composition having a crushing strength of 1 to 7 kg/molded catalyst composition, preferably 1.5 to 6 kg/molded catalyst composition. It should be noted that, when the molded catalyst composition is wet, it is first dried, and then, the i crushing strength is measured.The molded catalyst composition is then calcined at a temperature of 300 to 450 C, for3 to 20 hours, preferably at a tempera ture of 350 to 4300C, for 5 to 10 hours. Thus, a desired catalyst having a large number of micro pores of an average pore radius of at least 2000 A, preferably 2000 to 12000 A, which is suitable for use in fixed bed reactors, can be obtained. In the above mentioned catalyst preparation procedure, the mixture, which is obtained by mixing the compounds containing the constituent elements of the catalyst in the presence of aqueous medium, may be heated and concentrated to form a clayey solid, followed by the addition of the organic substance thereto.

According to the present invention, methacrolein is reacted with molecular oxygen in a vapor phase, at an elevated reaction temperature, in the presence of the catalyst prepared as mentioned above packed in a fixed bed reactor, whereby methacrylic acid is produced at a high yield. As the molecular oxygen, pure oxygen gas can, of course, be employed. However, since a high purity of oxygen is not required in the present process, air is usually employed from an economical point of view.

It is not necessary to employ, as a starting material, methacrolein having a high purity. For example, methacrolein obtained from the oxidation reaction of isobutylene, or methacrolein obtained from the oxidation reaction of hydrocarbon mixtures, such as a spent B.B. fraction containing n-butene and isobutylene, can be employed in the present invention. As is well known, the spent B.B. fraction is a residual fraction after 1,3-butadiene is extracted and separated from a C4 fraction which is a by-product during naphtha cracking. However, the use of methacrolein containing, as an impurity, a relatively large amount of unsaturated aldehydes is not advisable, because unsaturated aldehydes not only retard the reaction but, also, increase the formation of unpreferable by-products or polymers.

The catalytic process according to the present invention can be carried out in the presence of a diluent which does not adversely affect the present reaction. Examples of such diluent are nitrogen, carbon dioxide, steam or the like. Among these diluents, steam not only improves the selectivity to methacrylic acid, but also prolongs the life of the catalytic activity.

The process of the present invention can be carried out under normal pressure, elevated pressure or reduced pressure, but, in general, normal pressure can be conveniently used. The reaction temperature is preferably within the range of 250 to 400"C and, more preferably, 280 to 3800C. The contact time in a fixed bed reactor (under normal temperature and pressure) is preferably within the range of 0.1 to 20 seconds and, more preferably, 0.5 to 5 seconds. The process of the present invention is generally carried out by feeding a mixed gas containing methacrolein, molecular oxygen (e.g. air) and steam to a fixed bed reactor packed with the catalyst as mentioned above. Although the composition of the mixed gas to be fed, as a raw feed, to the ractor may be varied over a wide range, typical examples of the composition of the mixed gas are 0.5 to 7 moles and, more preferably, 1 to 5 moles, of molecular oxygen, and 1 to 30 moles and, more preferably, 2 to 10 moles, of steam, based on 1 mol of methacrolein. The methacrylic acid thus produced can be recovered according to any of the conventional techniques, such as condensation, solvent extraction and the like.

The present invention now will be further illustrated by, but is by no means limited to, the following Examples together with Comparative Examples.

In the Examples and the Comparative Examples, the conversion (%), the selectivity (%) and the yield (%) are determined by the following equations.

Moles of methacrolein reacted Conversion (%) = x 100 Moles of methacrolein fed Moles of methacrylic acid produced Selectivity (%) = x 100 Moles of methacrolein reacted Moles of methacrylic acid produced Yield (%) = x 100 Moles of methacrolein fed The average pore radius ( ) was measured by using a mercury pressure porosimeter. Each of the crushing strengths of the molded catalyst compositions and the catalysts (in terms of kg/molded catalyst composition or catalyst) was measured by using a Kiya-type harndess meter. In this test method, a sample was first place on a sample table and a load was then added onto the sample. Thus, the crushing strength was represented by the load (Kg) when the sample was crushed.Each result shown in Tables below is an average of the results obtained from 30 test samples.

Example 1 200 g of 12-molybdophosphoric acid [H3(PMo12O40)29.5 H2O] was added to 500 ml of water and dissolved therein, while heating. To this solution, a solution of 5.3 g of cesium nitrate (CsNO3), 16.6 g of potassium nitrate and 4 g of ammonium metavanadate (NH4VO3) in 100 ml of water was added with stirring. After the resultant mixture was heated and concentrated at a temperature of 80"C, the mixture was dried at a temperature of 1300C for 15 hours. The dried mixture was then ground into powders or granulates having a size of less than 20 mesh (Tyler mesh) (i.e. 0.833 mm).

To 150 g of the powders thus obtained, 7.5 g of cellulose crystallite (i.e. AVICEL available from ASAHI CHEMICAL IND. LTD.) was added and then, the powder mixture containing the added cellulose crystallite was molded to form a molded catalyst composition in the form of tablets, each having a diameter of 5 mm and a height of 5 mm, by means of a tableting machine. The tableting machine was adjusted so as to form a molded catalyst composition having a crushing strength of approximately 4 Kg[molded catalyst. Since the powder mixture had a very high moldability and it was appropriately sticky, the tableting was not difficult.

The molded catalyst composition was then calcined for 5 hours, at 400 C, in an air atmosphere. The composition of the catalyst thus obtained was Mo12p1K19csoSsVo.4. It should be noted that oxygen has not been mentioned in this formula even though it is present in the catalyst.

12 ml of the catalyst thus obtained was packed into a reaction tube made of stainless steel, and having an inner diameter of 7 mm and a length of 40 mm. A mixed gas containing 4% by volume of methac rolein, 10% by volume of oxygen, 30% by volume of steam and 56% by volume of nitrogen was then fed through the reactor, at a flow rate of 150 ml/min, and was catalytically reacted at a temperature of 320"C.

The crushing strengths of the molded catalyst composition (priorto the calcination) and the calcined catalyst, the average pore radius of the catalyst and the results obtained in the reaction are shown in Table 1 below.

Comparative Example 1 A catalyst having a composition similar to that of Example 1 was prepared in a manner as described in Example 1, except that the cellulose crystallite was not added. However, the powdered catalyst composition was so non-sticky that is was not easily packed into the holes of the tableting machine and the tableted catalyst had a low crushing strength when the tablet was molded underthe same conditions as in Example 1. The crushing strength of the calcined catalyst was also low. The crushing strengths of the molded catalyst composition and the clacined catalyst, the average pore radius of the calcined catalyst, as well as the results obtained in the reaction, which was carried out in a manner as described in Example 1, are shown in Table 1 below.

Comparative Example 2 A catalyst having a composition similar to that cf Example 1 was prepared in a manner as described in Comparative Example 1, exceptthatthe cellulose crystallite was not added and that the tableting machine was adjusted so as to form a molded catalyst having a crushing strength of approximately 3 kg/molded catalyst. The moldability of the powder of the catalyst composition was as poor as that of Comparative Example 1. The crushing strengths of the molded catalyst composition and the clacined catalyst, the average pore radius of the calcined catalyst, as well as the results obtained in a catalytic reaction, which was carried out in a manner as described in Example, are shown in Table 1 below.

Comparative Example 3 A catalyst having the same composition as that of Example 1 was prepared in a manner described in Example 1, except that the tableting machine was adjusted so as to form a molded catalyst having a crushing strength of approximately 10 kg/molded catalyst. In this case, cellulose crystallite was also incorporated into the catalyst as in Example 1. The moldability of the powdered catalyst composition was good and the crushing strengths of the molded catalyst composition and the clacined catalyst were high. However, the average pore radius of the catalyst was small. The crushing strength of the molded catalyst composition, the crushing strength and the average pore radius of the calcined catalyst, as well as the results obtained in the catalytic reaction, which was carried out under the same conditions as in Example 1, are shown in Table 1 below.

Comparative Example 4 A catalyst was prepared in a manner as described in Example 1. However, in this Comparative Exam ple, 22.5 g of cellulose crystallite was added to 150 g of the powdered catalyst composition, so that the addition amount ofthe cellulose crystallite was 15% by weight, based on the weight of the powdered catalyst composition, and the tableting machine was adjusted so as to form a molded catalyst having a crushing strength of approximately 3 Kg/molded catalyst. In this case, although the moldability of the powdered catalyst composition was good and the average pore radius of the catalyst was large, the crushing strength of the catalyst was remarkably low.The crushing strength of the molded catalyst composition, the average pore radius and the crushing strength of the catalyst, as well as the results obtained in the catalytic reaction, which was carried out under the same reaction conditions as in Example 1, are shown in Table 1 below.

Examples 2 to 4 Catalysts were prepared in a manner as described in Example 1, except that polyvinyl alcohol (polymerization degree: approximately 500), polyethylene glycol (average molecular weight: approximately 400) and gelatin were used instead of the cellulose crystallite, in an amount of 5% by weight based on the weight of the powdered catalytic composition, and that the molding was carried out under such a condition that the crushing strength of the molded catalyst became 2 to 3 kg/molded catalyst. In all cases where the polyvinyl alcohol, the polyethylene glycol and the gelatin were used, the moldability of the powdered catalyst compositions containing these organic substances was good and the catalysts had mechanical strengths sufficient to allow their use in a fixed bed reactor.The results obtained in the catalytic reactions, which were carried out under the same conditions as in Example 1, as well as the crushing strengths of the molded catalyst composition and the average pore radius and the crushing strengths of the catalysts are shown in Table 1 below.

Table 1 Crushing strength Organic substance (kg/molded catalyst Average composition or catalyst) pore Conversion Selectivity Yield radius of to of Addition Molded of methacrolein methacrylic methacrylic Compound amount catalyst Catalyst catalyst acid acid (wt%) composition (A) (%) (%) (%) Example 1 Cellulose 5 4.2 4.9 5610 85.3 86.3 73.6 " 2 Polyvinyl- 5 2.5 4.5 3010 90.3 85.2 76.9 alcohol " 3 Poly- 5 2.6 3.6 3290 92.6 83.3 77.1 ethylene glycol " 4 Gelatin 5 3.1 5.1 3230 91.3 84.2 76.9 Comparative Example 1 None 0 3.1 2.9 1930 82.5 74.8 61.7 " 2 None 0 0.5 1.0 2110 85.2 76.5 65.5 " 3 Cellulose 5 9.8 11.5 1100 75.2 65.3 49.1 " 4 Cellulose 15 3.2 1.2 4550 80.2 85.5 68.6 Catalyst composition:Mo12 P1 K1 -e Cs0.3 V,, Form of catalyst: Tablet (5 mma x 5 mmH) Reaction temperature: 320"C Contact time: 4.8 sec.

Examples 5 to 9 Powdered catalyst compositions having the compositions listed in Table 2 below were prepared in a manneras described in Example 1. Cellulose crystallite was added to each catalyst composition thus prepared in an amount set forth in Table 2 below and, then, the molded catalyst composition having the crushing strength listed in Table 2 in the form of a cylinder with a diameter of 5 mm and a height of 5 mm was moled in a tableting machine. The molded catalyst compositions thus obtained were calcined to form the catalysts in a manner described in Example 1. Rubidium nitrate (RbNo3) was used, as the rubidium source, in Examples 8 and 9. In Example 9, the catalyst was calcined at a temperature of 410"C. By using the catalysts thus prepared, catalystic reactions were carried out under the same conditions as in Example 1, except that the reaction temp erature was 310"C in Example 9.

The crushing strengths of the molded catalyst compositions and the catalysts are shown in Table 2 below. The average pore radii of the catalysts and the results ofthe catalytic reaction are shown in Table 3 below.

Example 10 A powdered catalyst composition containing molybdenum, phosphorus, vanadium and potassium in the amounts listed in Table 2 below was prepared in a manner as described in Example 1.7% by weight, based on the weight of the powdered catalyst composition, of cellulose crystallite and 15 ml of water were added to 150 g the powdered catalyst composition to thereby form a clayey catalyst composition, which in turn was extruded into strands by means of an extruder. The extruded strands were then granulated to form a molded catalyst composition in the form of spheres, each having a diameter of 3 to 5 mm, by means of a rotary granulator (MARUMERIZER, available from FUJI POWDAL K.K.). The spheres thus obtained were then calcined for 5 hours, at a temperature of 390"C, under an air atmosphere, to thereby form the catalyst.

The crushing strengths of the molded catalyst composition and the catalyst are shown in Table 2 below, and the average pore radius of the catalyst is shown in Table 3 below.

The catalystic reaction was carried out under the same conditions as described in Example 1, except that the reaction temperature was 300"C. The results are shown in Table 3 below.

Table 2 Crushing strength Composition of catalyst Addition (kg/molded catalyst Example (atomic ratid) amount of composition or catalyst) No. cellulose Molded Mo P V K Cs Rb (wt%) Catalyst Catalyst Composition 5 12 1.5 1 2 0 0 4 3.5 4.8 6 12 1.5 1 2 0 0 4 6 6.4 7 12 1 0.5 0 1.5 0 5 2.5 3.9 8 12 1 0.5 0 0 2 5 3.2 4.2 9 12 1 0.7 2 0 0.2 5 2.3 4.5 10 12 1 0.4 2 0 0 7 1.5 3.2 * Oxygen is omitted even though it is present Table3 Selectivity Example Average Reaction Conversion to Yield of No. pore radius temperature of methacrylic methacrylic of catalyst j methacrolein acid acid (A) ( C) (%) (%) (%) 5 3010 320 95.6 80.3 76.8 6 2260 320 89.0 79.6 70.9 7 3520 320 93.8 83.2 78.0 8 3100 320 94.9 81.1 77.0 9 3550 310 93.6 83.3 78.1 10 4220 300 90.8 85.6 77.7

Claims (9)

1. A process for producing methacrylic acid comprising the step of reacting methacrolein with molecular oxygen, at an elevated temperature, under a vapour phase, in a fixed bed reactor, in the presence of a catalyst containing molybdenum, phosphorus, vanadium, the alkali metals and oxygen, and having a large number of micro pores of an average pore radius of at least 2000 , said catalyst being prepared by the steps of: mixing compounds containing the constituent elements of the catalyst with each other in an aqueous medium; concentrating and/or drying the resultant mixture to form a clayey solid or powders; admixing the resultant clayey solid or powders with 1 to 10% by weight, based on the weight of the dry solid or powders, of an organic substance; molding the resultant admixture to form a molded catalyst composition, and; calcining the molded catalyst composition.

2. A process as claimed in claim 1, wherein said organic substance is at least one substance selected from the group consisting of cellulose, polyvinyl alcohol, polyethylene glycol and gelatin.

3. A process as claimed in claim 1, wherein said molded catalyst has a crushing strength of 1 to 7 kg/molded catalyst.

4. A process as claimed in claim 1, wherein said organic substance is at least one substance selected from the group consisting of cellulose, polyvinyl alcohol, polyethylene glycol and gelatin, and said molded catalyst has a crushing strength of 1 to 7 kg/molded catalyst composition.

5. A process as claimed in claim 1, wherein said reaction temperature is within the range of from 250 to 4000C.

6. A process as claimed in claim 1, wherein the mol ratio of said molecular oxygen to said methacrolein is within the range of from 0.5 to 7.

7. A process as claimed in claim 1, wherein the average pore radius of said catalyst is within the range of from 2000 to 12000 A.

8. A process as claimed in claim 1, wherein said catalyst has the general formula MOe Pb Vc Ad Oe wherein Mo is molybdenum, P is phosphorus, V is vanadium, A is at least one metal selected from the group consisting of potassium, cesium and rubidium and 0 is oxygen; the subscripts a, b, c, d and e represent the number of atoms, and when a is 12, b = 0.5 to 3, c = 0.1 to 2, d = 0.5 to 3 and e is the number which is required by the total valance of the other atoms.

9. A process for producing methacrylic acid substantially as hereinbefore described in any one of the Examples.