Abstract:
Compositions of the empirical formula: 
     
       Mo.sub.12 P.sub.0.1-3 Te.sub.0.01-2 M.sub.0.1-3 M&#39;.sub.0.01-3 X.sub.a 
     
      O b                                                    (I) 
     where 
     M is at least one of K, Rb and Cs; 
     M&#39; is at least one of Cu and V; 
     X is at least one of Ba, Zn, Ga, Nb, Cd, Ti, Ca, Mg, Ta, Zr, Bi, Ce, Ni, Co, Cr, Fe and Tl when a &gt;0; 
     a is a number of 0 to about 2; and 
     b is a number that satisfies the valence requirements of the other elements present, 
     are excellent catalysts for the oxidation of methacrolein to methacrylic acid.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to catalysis. In one aspect, the invention relates to novel catalytic compositions of phosphorus, molybdenum, tellurium, copper and/or vanadium and an alkali metal while in another aspect, the invention relates to a process employing these catalysts for the oxidation of methacrolein to methacrylic acid. 
     2. Description of the Prior Art 
     The art is replete with various phosphomolybdic acid catalysts useful for the oxidation of acrolein to acrylic acid. See for example U.S. Pat. Nos. 4,101,448, 4,115,441, 4,042,533 and 4,166,190. However, not only do many of these catalysts contain tungsten but most are not suitable, at least from a commercial perspective, for the oxidation of methacrolein to methacrylic acid. Of those catalysts that are suitable from a commercial perspective, none are entirely so. All are subject to improvement in methacrylic acid selectively and thermal stability. Examples of catalysts now taught to be suitable for methacrolein oxidation are those of Belgian Pat. No. 823,897, British Pat. No. 1,482,686, U.S. Pat. Nos. 3,976,688 and 4,017,423. The Belgium patent teaches a phosphomolybdic acid catalyst that can be combined with any number of optional components; the British patent teaches a P-Mo-Cu-V-W catalyst that also can be combined with optional components, U.S. Pat. No. 3,976,688 teaches a catalyst similar to the phosphomolybdic acid catalyst of the Belgium patent but containing Rb, Cs or K; and U.S. Pat. No. 4,017,423 teaches a promoted Rb-Mo-P catalyst. 
     Japanese Pat. No. 2,051,316 teaches the oxidation of methacrolein to methacrylic acid over a catalyst of Mo-P-M-Te where M is K, Rb, Cs and/or Tl. British Pat. No. 1,478,828 and Netherlands Pat. No. 7602-438 teach a similar oxidation over a catalyst of Mo-P-M-As-Cu/V-Promoter where M is Li, Na, K, Rb and/or Cs and the promoter can be among others, Te. 
     SUMMARY OF THE INVENTION 
     According to this invention, compositions of the empirical formula: 
     
         Mo.sub.12 P.sub.0.1-3 Te.sub.0.01-2 M.sub.0.1-3 M&#39;.sub.0.01-3 X.sub.a O.sub.b                                                   (I) 
    
     where 
     M is at least one of K, Rb and Cs; 
     M&#39; is at least one of Cu and V; 
     X is at least one of Ba, Zn, Ga, Nb, Cd, Ti, Ca, Mg, Ta, Zr, Bi, Ce, Ni, Co, Cr, Fe and Tl when a&gt;0; 
     a is a number of 0 to about 2; and 
     b is a number that satisfies the valence requirements of the other elements present, 
     are excellent catalysts for the oxidation of methacrolein to methacrylic acid. These catalysts demonstrate both excellent selectivity for methacrylic acid and excellent thermal stability. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Catalyst 
     As is evident from formula I, the catalytic composition of this invention is at least a six element material, i.e., a material containing molybdenum, phosphorus, tellurium, alkali metal (M), copper and/or vanadium and oxygen all in designated, proportional amounts. Preferably, the subscript value of phosphorus in formula I is about 0.8 to 1.5, of tellurium about 0.1 to 1, of alkali metal (M) about 0.15 to 2, and of copper and/or vanadium about 0.2 to 2.5. 
     Preferred catalysts of this invention are those where a is greater than 0 (a&gt;0), and preferably a number of about 0.01-2. These preferred catalysts demonstrate unusually good methacrylic acid selectivities, particularly those where X is zinc, cadmium, bismuth, titanium or barium. As is taught by formula I, certain of the components can be combinations of two or more elements, e.g., M&#39; can be a combination of copper and vanadium. In such instances, the subscript value represents the sum of the elements (e.g. for M&#39;, the sum of copper and vanadium is a number of about 0.01 to 3). Generally M, M&#39;, and X each represent but a single element. 
     Particularly preferred catalytic compositions are eight element or component (including oxygen) catalysts where M is rubidium or potassium, M&#39; is a combination of copper and vanadium, X is zinc, bismuth or cadmium, and a is about 0.05 to about 0.5. 
     The exact structure or element arrangement of these catalysts is not known but the metal and phosphorus components are present in the form of their oxides, acids or oxyacid complexes. However, the compositions of formula I are known not to be a mere physical mixture of their components but rather unique compositions where the individual components are chemically and/or physically bonded to one another. 
     The catalytic compositions of this invention can be used either in the 100% active form or in a diluted form, i.e. supported or unsupported. Suitable support materials include silica, titania, alumina, zirconia, silicon carbide, boron, various phosphates, etc., with low surface area (about 1 m 2  /g) alumina a preferred support material. If a support is used, the catalytic composition is generally present in an amount of at least about 20 weight percent, based on the combined weight of the support and the catalytic composition, and preferably in an amount of at least about 30 weight percent. 
     The catalytic compositions of this invention can be prepared in any one of a number of different methods, the particular method employed being a matter of convenience. Typically, the catalysts are prepared by mixing the appropriate catalyst ingredients in the proper proportions in an aqueous mixture, drying the resulting aqueous slurry with or without a reducing agent, and calcining the product. The ingredients can be added in any order during the preparation procedure but certain orders are preferred, particularly the mixing of the metallic ingredients prior to the addition of phosphorus (generally in the form of phosphoric acid). The ingredients employed can be the oxides, halides, nitrates, acetates or other salts of the particular metals or elements added, and particularly preferred is the use of water soluble salts of the metal components. If a support is used, the material comprising the support may be incorporated into the catalyst along with the other ingredients or the catalyst composition may be coated and/or impregnated onto or into a core. After the catalyst ingredients have been combined to form an aqueous slurry, the slurry is taken to dryness and the dried solid obtained is heated in the presence of air, nitrogen, nitric oxide or a mixture of any two or more of these gases at temperatures between about 300° and 420° C. This calcination can take place outside the catalytic reactor or an in situ activation can be utilized. Other methods of preparation are broadly taught in the art. 
     Oxidation of Methacrolein 
     In another embodiment of this invention, the compositions of formula I are highly effective catalysts for the oxidation of methacrolein to methacrylic acid. These catalytic compositions are used in the same manner as known catalytic compositions. This is a known reaction involving generally the contact of methacrolein with molecular oxygen at an elevated temperature in the presence of a catalytic amount of catalyst and optionally, steam. This particular embodiment of the invention is the use of these novel catalytic compositions within the parameters of the known art process. 
     Exemplary of this known process is the contacting of gaseous methacrolein with molecular oxygen in the presence of steam at a temperature between about 275° and about 340° C. The ratio of the reactants can vary widely with mole ratios of molecular oxygen to aldehyde of about 1 to 5 being typical. The amount of steam can also vary widely from a small amount generated in the reaction to 20 or more moles of steam per mole of aldehyde. Preferably, about 1 to 10 moles of steam per mole of aldehyde is employed in the reactant feed. In certain embodiments of this invention, recycle gas (principally N 2 , O 2 , CO 2  and CO) can be used with or instead of steam. Molecular oxygen is most conveniently added as air. 
     The oxidation reaction may be conducted in a fixed-bed, fluid-bed or transfer-line reactor using atmospheric, superatmospheric or subatmospheric pressure. The contact time of reactants over the catalyst can vary from a fraction of a second to twenty or more seconds, the exact time dependent upon other reaction conditions, such as catalyst composition, feed composition, temperature, pressure, reactor, etc. 
     Although the catalytic compositions of this invention find particular usefulness in the oxidation of methacrolein to methacrylic acid, they are also useful in other oxidation reactions. For example, these catalytic compositions are useful in the oxidation of acrolein to acrylic acid. 
    
    
     The following examples are illustrative of certain specific embodiments of this invention. Unless otherwise indicated, all parts and percentages are by weight. 
     SPECIFIC EMBODIMENTS 
     Catalyst Preparation 
     The catalysts here used were prepared by dissolving, with stirring, ammonium heptamolybdate in distilled water and heating the resulting solution to 30°-35° C. While continuously stirring the solution and maintaining its temperature, the alkali metal hydroxide was added followed by the halide or hydroxide of the X component. After 15 minutes, the copper acetate and/or ammonium metavanadate solutions were added followed by addition of a tellurium oxide/hydrochloric acid solution. The resulting slurry was then heated to about 70° C. for 2 hours. Phosphoric acid (H 3  PO 4 ) was the last material added after which stirring and heating was continued for 30 minutes and the pH of the slurry was adjusted to 5.6. The mixture was then evaporated to a thick paste and the catalyst precursor dried in an oven at 110°-120° C. The powder was then coated onto 1/3 in. Alundum® spheres (alumina support) so that the powder coating (i.e. the catalyst) constituted about 35 weight percent of the coated spheres. 
     As for the comparative catalyst and those catalysts not having an X and/or M component, they were prepared by essentially the same procedure except the steps of adding the X and/or M component(s) were eliminated. 
     Process Procedure and Conditions 
     The reactions were conducted in a 20 cc, downward-flow, fixed-bed reactor. All examples were performed in essentially the same manner: first, one hour at 370° C. with air flow (no feed) and second, one hour at 345° C. with air flow plus feed before the temperature was dropped to the reaction temperature. After a short stabilization period, a 15 min run was conducted to obtain sufficient reactor effluent. The off-gas rate was measured with a soap-film meter and an off-gas composition was determined at the end of the run with the aid of a Perkin-Elmer 154 gas chromatograph. At the end of the recovery run, the entire scrubber liquid was diluted with distilled water to about 100 g. A weighed amount of methanol was used as an internal standard in a 20 g aliquot of the dilute solution. A one microliter sample was then analyzed in a Varian Model 3700 gas chromatograph fitted with a flame ionization detector and a Chromsorb 107 column, 60/80 mesh. Total amounts of organic acids were determined by titrating 25 cc of the liquid with 0.1 N sodium hydroxide. The split between methacrylic, acrylic and acetic acid was determined from the gas chromatographic analysis. 
     The following process conditions were employed: 
     Pressure--atmospheric 
     Run Time--15 min 
     Contact Time--˜3.2 sec 
     Feed Ratio--methacrolein/water/air: 1/4/10.5 
     Temperature (°C.)˜315 
     The results of these experiments are reported in Table I. 
     
                                           TABLE I__________________________________________________________________________METHACROLEIN OXIDATION            Reactor   MAA.sup.2                          MA.sup.3                               Acetic    MAA.sup.4                                             CarbonExampleCatalyst Composition            Temp. (C)                  WWH.sup.1                      Yield                          (Unreac.)                               Acid                                   CO.sub.2                                      CO Sel.                                             Balance__________________________________________________________________________1    Mo.sub.12 P Rb Te.sub..5            305   0.108                      68.2                          14.7 5.1 5.3                                      4.4                                         80.0                                              99.3Cu.sub..25 O.sub.b2    Mo.sub.12 P Rb Te.sub..5            315   0.108                      71.9                           5.7 6.8 7.3                                      5.8                                         76.3                                              99.5Cu.sub..25 O.sub.b3    Mo.sub.12 P Rb Te.sub..5            315   0.111                      68.9                          12.6 5.1 6.8                                      4.8                                         78.9                                             100.3Cu.sub..25 V.sub..5 O.sub.b4    Mo.sub.12 P Rb Te.sub..25            325   0.110                      68.3                          15.0 4.8 5.7                                      4.2                                         80.4                                             100.6Cu.sub..25 V.sub..5 Bi.sub..25 O.sub.bA    Mo.sub.12 P Te.sub..5            305   0.106                      68.3                           4.2 8.2 9.3                                      8.6                                         71.3                                             105.1Cu.sub..25 O.sub.bB    Mo.sub.12 P Rb Te.sub..5 O.sub.b            315   0.096                      66.0                          19.7 3.7 4.8                                      4.1                                         82.2                                             100.2C    Mo.sub.12 P Rb Te.sub..5 As.sub..25            315   0.098                      67.5                          17.0 3.3 5.9                                      3.5                                         81.3                                             102.6V.sub..5 O.sub.b__________________________________________________________________________ .sup.1 WWH = Weight of Methacrolein/Weight of Catalyst/Hour .sup.2 Methacrylic Acid Yield = Moles of Methacrylic Acid Recovered × 100/Moles of Methacrolein Fed .sup.3 Unreacted Methacrolein .sup.4 Methacrylic Acid Selectivity = Moles of Methacrylic Acid Recovered × 100/Moles of Methacrolein Reacted 
    
     The data of Table I show the excellent yields of methacrylic acid obtained by the use of the catalysts of this invention. In comparison, the comparative catalysts consistently gave lower yields although some were able to match or even exceed the selectivity to methacrylic acid. Example A employed a catalyst free of alkali metal (M), Example B employed a catalyst free of Cu or V (M&#39;), and Example C employed a catalyst where As is a component (representative of the catalysts of British Pat. No. 1,478,828). 
     Although the invention has been described in considerable detail through the preceding examples, these examples are for the purpose of illustration only and it is understood that variations and modifications can be made by one skilled in the art without departing from the spirit and scope of the invention.