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 and copper.

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 commercial standpoint. This is understandably so inasmuch as iodine is costly, exhibits extreme corrosivity 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 olefinically unsaturated monocarboxylic acids. The prior art heterogeneous oxydehydrogenation catalysts useful for this purpose include some heteropoly acids, such as phosphomolybdic acid, optionally which 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 preparation serves to facilitate the formation of the active catalyst. For instance, U.S. Pat. No. 3,948,959 discloses that an alkali or alkaline earth metal can be the extrinsic metal component 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 thereof, 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 300-500° C. with a mixture of the saturated aliphatic monocarboxylic acid and molecular oxygen, said catalyst being a calcined copper-iron phosphate. The catalyst useful in the process of this invention can be further defined by the gram-atom empirical formula Cu a  Fe b  P c  O x  wherein a is 0.01-1.0, b is 0.1-1.0, c is 0.01-2.0 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 readily accomplished by the so-called slurry method or the precipitation method. In the latter 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 salt of iron and copper can be used in the preparation of the solution. 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 and copper 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 in the order of about 120° C. until the mass is completely dried. The resulting solid is sized and calcined. Suitable calcination temperatures range from 400-500° C. Applicable periods of calcination range from 2-30 hours. 
     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, pumice, quartz, zirconia, 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 reactors. 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 preheated gaseous mixture of the saturated aliphatic monocarboxylic acid, molecular oxygen, steam and inert diluent gas. A preheat 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 450° C. provides for optimum processing. 
     The mole ratio of molecular oxygen to carboxylic acid is from 0.5 to 1.5 and more preferably from 0.7 to 0.75 in the case where the carboxylic acid is isobutyric acid, per se. 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 acid to 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. Acetone may be included in the feed in the conversion of isobutyric acid to methacrylic acid. 
     Another important parameter is the concentration of the organic reactant in the feed. The organic reactant carboxylic acid or ester should be present in the feed in from 0.1 to 20 mole percent. From the standpoint of achieving a reasonable throughput combined with an acceptable yield, the concentration of the reactant in the feed is from about 3-6 mole percent. Concentration of 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 catalyst in this sense not only includes the copper-iron phosphate itself but also the support if present. Accordingly, reaction times can range from 0.05 to 3.0 seconds and more generally in the order 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 atmospheres is contemplated. 
    
    
     The process of this invention is further illustrated in the following specific examples. 
     EXAMPLE I 
     The slurry method was used to prepare a copper-iron-phosphate catalyst on a silica carrier. In 400 ml. of water were dissolved 120.8 g. of copper nitrate and 202 g. of iron nitrate. To this solution were added 143 g. of 85% concentrated phosphoric acid. To this mixture were then added 30 ml. of an aqueous (40% by weight solids) silica gel. Water was removed from the resulting slurry with stirring, the solid was further dried at 120° C. for 6 hours and the treated solid was calcined at 450° C. for 6 hours (in air). The final catalyst was found to be blue and had the empirical formula Cu 0 .5 Fe 0 .5 P 1 .24 O 4  /SiO 2  3.5%. 
     EXAMPLE II 
     This example illustrates the use of the catalyst described in Example I in the oxydehydrogenation of isobutyric acid to produce methacrylic acid. The procedure consisted of feeding a preheated mixture of isobutyric acid, oxygen, nitrogen and steam through a stainless steel tube of 1/2&#34; O.D. (3/8&#34; I.D.) and approximately 18&#34; in length containing the catalyst as a 12 cc. packed bed maintained at the reaction temperature. 
     The preheater consisted of a length of stainless steel tubing similar to the reactor but packed with glass beads. Any carbon dioxide formed in the course of the reaction was absorbed in an Ascarite tube protected by a calcium sulfate absorber for any uncondensed water. The condensed organic product was separated from the water, collected and analyzed by the internal standard method of gas chromatography. 
     Selectively to methacrylic acid represents the mole ratio of methacrylic acid found in the reaction effluent to that of the isobutyric acid consumed in the reaction. 
     The feed to the reactor consisted of isobutyric acid: oxygen:water:nitrogen in the corresponding mole ratio of 5.4:3.9:64.8:25.9. The reaction temperature was 430° C. and the contact time was 0.56 second. In this experiment it was found that 81.7% of the isobutyric acid was converted to other products and the selectivity to the product methacrylic acid was 67.6%. 
     EXAMPLE III 
     The procedure of Example II was followed except that the mole ratio of isobutyric acid to water in the feed was 6, the reaction temperature was 428° C. and the contact time was 0.6 second. In this experiment it was found that 75.1% of the isobutyric acid was consumed with a 63.4% selectivity to methacrylic acid. 
     EXAMPLE IV 
     The procedure of EXAMPLE III was followed using a contact time of 0.5 second and the conditions shown in Table I. The results obtained are shown in Table I. 
     
                       TABLE I______________________________________                            % SelectivityIsobutyric Acid/      Tem-     % Conversion of                            to MethacrylicWater(ML/Hour)      perature Isobutyric Acid                            Acid______________________________________8.4/20     435° C.               81.7         67.6  8.8/19.1 450° C.               71.3         66.88.8/20     435° C.               69.0         68.3______________________________________ 
    
     EXAMPLE V 
     Another catalyst was prepared according to the procedure of Example I. In 400 ml. of water were dissolved 60.4 g. of copper nitrate and 303.4 g. of ferric nitrate. To this solution were added 143 g. of 85% concentrated phosphoric acid and 30 ml. of a 40% solids aqueous silica sol. Water was removed from the resulting slurry with stirring, the solid was further dried at 120° C. for 6 hours and the thus treated solid was calcined at 450° C. for 6 hours in air. The final catalyst was found to be blue and to have the empirical formula Fe 0 .75 Cu 0 .25 P 1 .24 O 4  /SiO 2  3.5% 
     EXAMPLE VI 
     The procedure of Example II was followed with the feed ratios and reaction temperatures shown in Table II at a contact time of 0.56 second. The results obtained are given in Table II. 
     
                       TABLE II______________________________________      Tem-Isobutyric Acid/      pera-   % Conversion of                           % Selectivity toH.sub.2 O (Ml./Hour)      ture    Isobutyric Acid                           Methacrylic Acid______________________________________8.4/20     410° C.              65.2         65.78.4/20     400° C.              53.0         71.55.5/15     400° C.              49.0         85.2______________________________________