Process for the Preparation of Unsaturated Acids From Unsaturated Aldehydes

A number of catalysts are known to be effective for the oxidation of acrolein or methacrolein to acrylic acid or methacrylic acid, respectively. However, the yields obtained using the catalysts for the preparation of methacrylic acid are low. West German Provisional Pat. No. 2,048,620 discloses catalysts containing the oxides of molybdenum, phosphorus, and arsenic for the oxidation of methacrolein and acrolein to methacrylic acid and acrylic acid, respectively. U.S. Pat. No. 3,761,516 discloses catalysts containing oxides of molybdenum, arsenic and phosphorus on a support, especially Al.sub.2 O.sub.3, having external macropores and a surface not greater than 2 m.sup.2 /g.

The present invention is a result of a search for more efficient and desirable catalysts for the production of acrylic acid and methacrylic acid. Unexpectedly higher yields of and selectivities to acrylic acid and methacrylic acid are obtained by the vapor phase oxidation of acrolein and methacrolein, respectively, with molecular oxygen in the presence of the new and useful catalysts of the present invention.

It has been discovered according to the present invention in the process for the preparation of acrylic acid or methacrylic acid by the oxidation of acrolein or methacrolein, respectively, with molecular oxygen in the vapor phase at a reaction temperature of about 200.degree. to about 500.degree. C in the presence of an oxide catalyst, and optionally in the presence of steam, the improvement comprising

Using as a catalyst a catalyst of the formula

wherein

X is a rare earth element or a mixture thereof;

Y is at least one of Ag, Tl, Rh, Pd, Ru, Pt, Cd, Al, Au, Cu, alkaline earth metal, Cl, and NH.sub.4 ;

wherein

a is 0.001 to 10;

b is 0 to 10;

c is 0.01 to 5;

d is 0.01 to 5;

x is the number of oxygens required to satisfy the valence states of the other elements present.

The surprisingly advantageous catalysts of this invention give improved yields of acrylic acid and methacrylic acid from acrolein and methacrolein, respectively, in an efficient, convenient, and economical manner at a relatively low temperature.

The most significant aspect of the present invention is the catalyst employed. The catalyst may be any of the catalysts delineated by the above formula. The catalysts can be prepared by a number of different techniques described in the art, such as coprecipitation of soluble salts and calcination of the resulting product.

The catalysts of the invention have preferred limitations on their composition. Preferred are catalysts wherein a is 0.001 to 3, catalysts wherein b is 0.001 to 3, and catalysts wherein b is zero.

Especially preferred are catalysts wherein X represents cerium, erbium, or a mixture of rare earth elements consisting essentially of Ce, La, Nd and Pr. Catalysts of particular interest are described wherein Y is silver, thallium or copper.

In the catalyst preparations, the various elements of the catalyst are combined, and the final product is calcined to obtain the catalyst. A number of methods of combining the elements of the catalyst and calcining the resultant product are known to those of skill in the art. In the broad concept of the invention, the particular method of preparing the catalysts is not critical.

There are, however, methods of preparing the catalysts that have been found to be preferred. One preferred preparation involves the preparation of the catalysts in an aqueous slurry or solution of molybdenum, arsenic, and/or phosphorus containing components, and adding the remaining components; evaporation of this aqueous mixture; and calcination of the resulting catalysts. Suitable molybdenum compounds that may be employed in the preparation of the catalysts delineated by the above formula include molybdenum trioxide, phosphomolybdic acid, molybdic acid, ammonium heptamolybdate and the like. Suitable phosphorus compounds that may be employed in the preparation of the catalysts include ortho-phosphoric acid, metaphosphoric acid, triphosphoric acid, and phosphorus halides or oxyhalides. The remaining components of the catalysts may be added as oxide, acetate, formate, sulfate, nitrate, carbonate, oxyhalide, or halide and the like.

Excellent results are obtained by refluxing phosphoric acid, an arsenic containing compound, and molybdenum trioxide, or ammonium heptamolybdate in water for about one-half hour to 3 hours, however, commercial phosphomolybdic acid may be effectively utilized; adding the remaining components to the aqueous slurry and boiling to a thick paste; drying at 110.degree. to 120.degree. C in air; and calcining the resulting catalysts.

The calcination of the catalyst usually is accomplished by heating the dry catalytic components at a temperature of about 200.degree. to about 700.degree. C. The preferred procedure of the invention is wherein the catalyst is calcined at a temperature of 325.degree. to 425.degree. C.

The reactants of the reaction of the invention are methacrolein or acrolein and oxygen. Molecular oxygen is normally supplied to the reaction in the form of air, but oxygen gas could also be employed. About 0.5 to about 4 moles of oxygen are normally added per mole of methacrolein.

The reaction temperature may vary as different catalysts are employed. Normally, temperature of about 200.degree. to about 500.degree. C are employed with temperature of 250.degree. C to 370.degree. C being preferred.

The catalyst may be used alone or a support could be employed. Suitable supports include silica, alumina, Alundum, silicon carbide, boron phosphate, zirconia and titania. The catalysts are conveniently used in a fixed-bed reactor using tablets, pellets or the like or in a fluid-bed reactor using a catalyst having a particle size of less than about 300 microns. When a fluid-bed reactor is employed, preferred catalysts are in the form of microspheroidal particles. The contact time may be as low as a fraction of a second or as high as 20 seconds or more. The reaction may be conducted at atmospheric, superatmospheric or subatmospheric pressure, with absolute pressures of about 0.5 to about 4 atmospheres being preferred.

Excellent results are obtained using a coated catalyst consisting essentially of an inert support material having a diameter of at least 20 microns and an outer surface and a continuous coating of said active catalyst on said inert support strongly adhering to the outer surface of said support. The special coated catalyst consists of an inner support material having an outside surface and a coating of the active catalytic material on this outside surface. These catalysts can be prepared by a number of different methods.

The support material for the catalyst forms the inner core of the catalyst. This is an essentially inert support and may have substantially any particle size although a diameter of greater than 20 microns is preferred. Especially preferred in the present invention for use in a commercial reactor are those supports which are spherical and which have a diameter of about 0.2 cm. to about 2 cm. Suitable examples of essentially inert support materials include: Alundum, silica, alumina, alumina-silica, silicon carbide, titania and zirconia. Especially preferred among these supports are Alundum, silica, alumina and alumina silica.

The catalysts may contain essentially any proportions of support and catalytically active material. The limits of this relationship are only set by the relative ability of the catalyst and support material to accommodate each other. Preferred catalysts contain about 10 to about 100 percent by weight of catalytically active material based on the weight of the support.

The preparation of these coated catalysts can be accomplished by various techniques. The basic method of preparing these catalysts is by partially wetting the support material with a liquid and then contacting the support material with a powder of the catalytically active material and gently agitating the mixture until the catalyst is formed. The gentle agitation is most conveniently accomplished by placing the partially wet support in a rotating drum or jar and adding the powdered active catalytic material.

Using the catalysts of the invention in the preparation of methacrylic acid or acrylic acid, excellent yields are obtained in a convenient reaction with low amounts of byproducts.

A 20 cc. fixed-bed reactor was constructed of a 1.3 cm. stainless steel tubing. Catalysts prepared as described below were charged to the reactor and heated to the reaction temperature under a flow of air and a feed of methacrolein/air/nitrogen/steam of 1/5.7/4.6/8.7 was fed over the catalyst at an apparent contact time of 2 to 4 seconds. The reactor was run under the reaction conditions for 1 to 6 hours and the product was collected and analyzed.

The catalysts were prepared as follows:

A solution was prepared consisting of 211.88 grams of ammonium heptamolybdate, (NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, (1.2 mole Mo), 500 mls. distilled water at 60.degree. C and 7.94 grams of ammonium arsenate NH.sub.4 H.sub.2 AsO.sub.4, (0.05 mole As) as solution in 25 mls. distilled water. A white precipitate formed which was heated to about 100.degree. C for 2 hours. To this mixture was added 11.53 grams of 85% solution phosphoric acid (0.10 mole P). One-half hour later 5.0 grams of hydrazine hydrate was added. The slurry was evaporated to a thick paste, dried overnight in an oven at 110.degree. to 120.degree. C, and ground and screened to less than 80 mesh. This powder was coated on Norton 1/8 inch SA 5223 Alundum balls by taking 50 grams of Alundum, partially wetting the Alundum with 1.8 grams of water and adding 16.7 grams of active catalyst prepared above in five equal portions. During and after each addition, the Alundum was rolled in a glass jar. The powder was evenly coated onto the surface of the Alundum and the final product was dried. A hard uniform material was obtained that consisted of an inner core of the Alundum support with the continuous, strongly adhering coat of the powder on the outside surface of the support. The material was then calcined for 1 hour at 370.degree. C in 40 ml/min. air to form the active catalyst.

A solution was prepared consisting of 105.9 grams of ammonium heptamolybdate, (NH.sub.4).sub.6 Mo.sub.7 O.sub.24.4H.sub.2 O, (0.6 mole Mo), 700 mls. of distilled water at 60.degree. C and 4.0 grams of ammonium arsenate NH.sub.4 H.sub.2 AsO.sub.4, (0.025 mole As) as solution in 25 mls. of water. A white precipitate formed which was heated at 100.degree. C about one-half hour. To this mixture was added 4.4 grams of Moly Corp. rare earth chloride mixture (Product Code No. 4700) consisting of 48% CeO.sub.2, 33% La.sub.2 O.sub.3, 13% Nd.sub.2 O.sub.3, 4.5% Pr.sub.6 O.sub.1 and 1.5% other rare earth elements calculated as oxides. To this solution was added 5.8 grams of 85% solution phosphoric acid, H.sub.3 PO.sub.4 (0.05 mole P). One-half hour later 2.5 grams of hydrazine hydrate were added. The slurry was evaporated to a thick paste, dried overnight in an oven at 110.degree. to 120.degree. C, and ground and screened to less than 80 mesh size. The catalyst was then coated to a 25% active level on 1/8 inch SA 5223 Alundum balls. Calcination was the same as in Comparative Example A.

Various catalysts of the invention were prepared. The catalysts were prepared according to the procedure of Example 1, using 105.9 grams of ammonium molybdate, 700 mls. of 60.degree. C. distilled water and 4.0 grams of ammonium arsenate in solution of 25 mls. of water. The catalytic components delineated by X and/or Y were added immediately preceding the addition of 5.8 grams of 85% phosphoric acid and 2.5 grams of hydrazine hydrate. To prepare the catalysts, the following compounds and amounts were used:

This catalyst was prepared in the same manner described in Example 1, except 34.25 grams of ammonium heptamolybdate, 1.28 grams of ammonium arsenate, 1.43 grams of rare earth chloride mixture, 0.161 grams of copper acetate, 1.88 grams of 85% phosphoric acid and 0.8 grams of hydrazine hydrate were employed.

This catalyst was prepared in the same manner described in Example 8, except that 0.269 grams of silver acetate were employed.

The results of the experiments in the oxidation of methacrolein to produce methacrylic acid are shown in the TABLE below. The following definitions are used in measuring the carbon atoms in the feed and the products. ##EQU1##