Abstract:
Isobutyric acid or a lower alkyl ester thereof is oxidatively dehydrogenated in the vapor phase producing the corresponding α,β-olefinically unsaturated derivative by contact with a heterogeneous catalyst in the presence of molecular oxygen. The catalyst is composed of the calcined phosphates of iron, silver and chromium.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the conversion of isobutyric acid or its equivalents and lower alkyl esters thereof correspondingly to methacrylic acid or its equivalents and lower alkyl esters thereof. 
     2. Description of the Prior Art 
     There exists considerable prior art relating to the oxydehydrogenation of the lower saturated aliphatic monocarboxylic acids to produce the corresponding α,β-olefinically unsaturated acids. Early work in this area involved thermal, vapor phase oxydehydrogenation of the saturated aliphatic carboxylic acid in the presence of oxygen and iodine. This approach has not been particularly successful from a commerical standpoint. This is understandably so inasmuch as iodine is costly, exhibits extreme corrosion properties and poses considerable problems in realizing complete recovery of the comparatively large amounts thereof required in the process. The heterogeneous catalytic method for oxydehydrogenation according to the prior art appears to be the more attractive route to the commercial production of α,β-olefinically unsaturated monocarboxylic acids. The prior art heterogeneous oxydehydrogenation catalysts useful for this purpose include some heteropoly acids, such as phosphomolydic acid, optionally with tungsten and/or vanadium. Another type of catalyst included in the prior art is iron phosphate. 
     Iron phosphate subjected to calcination exists in several crystalline phases or species. It is not known at this time which species is or are catalytically active. There is evidence that the presence of certain extrinsic metal components in the catalyst serves to facilitate the formation of an active catalyst. For instance, U.S. Pat. No. 3,948,959 dicloses that an alkali or alkaline earth metal can be the extrinsic metal for this purpose. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a catalytic process is provided for the oxidative dehydrogenation of a saturated aliphatic monocarboxylic acid or lower alkyl ester thereof, such as isobutyric acid or methyl isobutyrate, to the corresponding α,β-olefinically unsaturated derivative, such as methacrylic acid or methyl methacrylate. 
     The process of this invention comprises contacting a heterogeneous catalyst at a temperature in the range of from 300° to 500° C. with a mixture of the saturated aliphatic monocarboxylic acid and molecular oxygen, said catalyst being a calcined iron, silver, chromium phosphate. The catalyst useful in the process of this invention can be further defined by the gram-atom empirical formula Fe a  Ag b  Cr c  P d  O x  wherein a is 1.0-2.0 is 0.01-1.0, c is 0.01-1. 0, d is 1.1-10.0, and x represents the number of oxygens required to satisfy the uncombined positive valences of the other elements shown in the formula. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There are a number of techniques which can be used for preparing the catalyst useful in the process of this invention. Of these, the more facile methods involve preparing the integral catalyst composition prior to calcination. This can be done by the so-called slurry method or the precipitation method. In the water method an aqueous solution of salts of the metals and phosphoric acid is first prepared and is then neutralized with an appropriate base so as to precipitate the mixed metal phosphates. The precipitate is washed and dried prior to calcination. In the alternative, one can add ammonium phosphate to a solution of the metal salts causing direct precipitation of the metal phosphates. As indicated, any water-soluble or water insoluble salt of iron, silver and chromium can be used in the preparation of the solution or suspension. The nitrate salts are suitable water soluble salts for this purpose and are preferred because of their ready availability and desirable solubility characteristics. 
     The so-called slurry method is the preferred catalyst preparation method because of its convenience. In accordance with this procedure, the aqueous solution of the iron, silver and chromium salts together with the phosphoric acid is obtained as previously noted. The solution is then heated with stirring to remove water and this is continued until the mass is so thick it cannot be stirred. The resulting residue is then broken up and heated to a moderately elevated temperature on the order of about 120° C. until the mass is completely dried. The resulting solid is sized and calcined. Suitable calcination temperatures range from 400°-550° C. Applicable periods of calcination range from 2-30 hours or more. 
     The use of a support or carrier for the catalyst is included in this invention. The support can be included in either of the catalyst preparation methods mentioned above. For instance, in the slurry method colloidal silica or any other form of silica as well as other support material such as alumina, quartz, titanium dioxide, carbon, silicon carbide, etc., can be included prior to the water removal step. Similarly, the precipitation of the metal phosphates can be accomplished in the presence of suspended particles of the support material in the alternate method described above. 
     The process of this invention can be carried out using the catalyst in the form of a fluidized bed reactor, a stirred tank reactor, or in a fixed bed or packed bed reactor or any combination of these types of reactor configurations. Because of the convenience associated with the use of a fixed bed reactor in a small scale operation, such a reactor will be exemplified herein. In the preferred mode of operation the feed to the reactor comprises a pre-heated gaseous mixture of the saturated aliphatic monocarboxylic acid, molecular oxygen, steam and inert diluent gas. A pre-heat temperature in the range of about 300° to 350° C. is customarily used. The oxydehydrogenation reaction can be carried out in the range of from 300° to 500° C. More generally a temperature of from 375° to 425° C. provides for optimum processing. 
     The mole ratio of molecular oxygen to carboxylic acid is from 0.2 to 1.5 and more preferably from 0.7 to 0.75 in the case where the carboxylic acid is isobutyric acid. Although steam is not necessary for the reaction, its presence is desirable in the feed because it is believed to act beneficially as a heat sink and in minimizing combustion of the carboxylic acidto undesirable waste products. The mole ratio of water to the carboxylic acid in the feed should be from about 8 to 20. The optimum ratio is from 12 to 15. 
     Another important parameter is the concentration of the carboxylic acid or ester reactant in the feed. This organic reactant should be present in the feed in from 0.1 to 20 mole percent. 
     From the standpoint of achieving a reasonable through-put combined with an acceptable yield, the concentration of the organic reactant in the feed is from about 3-6 mole percent. Concentration of the reactant in the feed is controlled to a large degree by the amount of inert gas present. The preferred inert gas or diluent is nitrogen although other inert gases such as carbon dioxide, helium, argon, and the like are suitable. Air is a very convenient source of oxygen plus inert diluent. 
     Another important parameter is contact time in the process of this invention. Contact or reaction time is defined for the purpose of this invention as the catalyst volume divided by the volume of gas feed per second at the reaction temperature. The catalyst volume is the bulk volume occupied by the catalyst in the reactor.The term &#34;catalyst&#34; in this sense not only includes the iron, silver, chromium phosphate itself, but also includes the support if present. Accordingly, reaction times can range from 0.05 to 3.0 seconds and more specifically in the orders of from 0.1 to 1.0 second. The reaction is preferably carried out at or near atmospheric pressure although the use of higher pressures up to about 10 atmosphere is contemplated. 
    
    
     The process of this invention is further illustrated in the following specific examples. 
     EXAMPLE 1 
     This example illustrates the use of the slurry method for preparation of the catalyst on a silica carrier. A slurry of 202.5 g ferric nitrate, 20.72 g of chromium nitrate, 12.55 g of silver nitrate in 500 ml of distilled water was mixed with 43.0 ml of concentrated phosphoric acid. The slurry was dried at 120° C. with stirring and the solid product was calcined at 450° C. for 6 hours after it had been mixed with 15.0 ml of a 40% aqueous silica sol (Ludox 40 AS, Dupont), the final catalyst composition was found to be Fe 1 .0 Ag 0 .15 Cr 0 .1 P 1 .31 O x  /SiO 2  3.5%. 
     EXAMPLE 2 
     The catalyst described in Example 1 was sized to 12-20 mesh particles. Fifteen cc of this material were mixed with 30 cc of quartz chips (diluent) and the mixture was packed in a fixed bed reactor which was a 1&#34; diameter stainless steel tube. Reactions were carried out using several reaction temperatures, mole ratios of water: isobutyric acid and mole percent isobutyric acid in the feed as shown in the following table. 
     
                       TABLE______________________________________  Water/Isobutyric                MoleReaction  Acid          IsobutyricTemp.  Mole Ratio    Acid %    % Conv. % Sel.______________________________________395° C.  15            4.5       88.7    72.5405    15            4.5       91.4    71.7405    15            3.9       96.0    66.2405    15            4.4       90.8    70.5390    12            5.3       91.6    73.0390    10            6.6       84.2    74.5______________________________________ 
    
     The percent conversion of isobutyric acid (% conv.) and percent selectivity to methacrylic acid (% sel.) are also given in the table. The feed consisted of isobutyric acid/oxygen/steam/nitrogen in the molar ratio 5/3.5/75/16.5 with a GHSV of 2000.