Process for preparing maleic anhydride from C4 hydrocarbons
This is a division of application Ser. No. 558,738 filed Mar. 17, 1975 now U.S. Pat. No. 4,018,709. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved process for the preparation of maleic anhydride from normal C 4  hydrocarbons by the reaction of oxygen with the hydrocarbon in vapor phase over a particular novel catalyst. 
     The production of dicarboxylic acid anhydride by catalytic oxidation of hydrocarbons is well known. The current principal route for the production of maleic anhydride is the catalytic oxidation of benzene. The direct production of maleic anhydride from the C 4  hydrocarbons has been desirable in the past, but is now even more desirable in view of the particular world shortage of benzene. It can be readily appreciated that direct oxidation of C 4  hydrocarbons would be a hydrocarbon conservation, since for each mol of maleic anhydride prepared from benzene, one mole of benzene, molecular weight 78 is consumed whereas for each mol of the C 4  only 54 to 58 mol weight of hydrocarbon is consumed. The benzene process has consistently produced high conversions and selectivities. Although processes for the oxidation of aliphatic hydrocarbons are reported in the literature, there are certain defects and inadequacies in these processes such as short catalyst life and low yields of product. Furthermore, although many of the prior art methods are generically directed to &#34;aliphatic&#34; hydrocarbons, they are in all practical aspects directed to unsaturated aliphatic hydrocarbons. 
     A more desirable process for producing maleic anhydride would be a direct oxidation of n-butane. There are several advantages. Principal among these is the greater availability of n-butane as compared to n-butenes or butadiene. Also n-butenes may have higher economic petrochemical utilization than the n-butanes, which are now, often wastefully burned as cheap fuel. 
     In an early series of patents one of the present inventors developed a untique group of vanadium-phosphorus, oxidation catalysts, i.e., U.S. Pat. Nos. 3,156,705; 3,156,706; 3,255,211; 3,255,212; 2,255,213; 2,288,721; 3,351,565; 3,366,648; 3,385,796 and 3,484,384. These processes and catalysts proved highly efficient in the oxidation of n-butenes to maleic anhydride. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that vanadium-phosphorus-oxygen complex type catalyst modified with a particular component, Me, is excellent oxidation catalyst for the conversion of n-C 4  hydrocarbons to maleic anhydride. Surprisingly, the present catalysts are excellent for the direct oxidation of n-butane to maleic anhydride. In addition to n-butane, n-butene, and butadiene can also be used as feeds. The catalyst contains only a minor amount of the Me component. The Me component is generally a metal or metalloid element. 
     The precise structure of the present complex catalyst has not been determined, however, the complex may be represented by formula 
     
         V P.sub.a Me.sub.b O.sub.x 
    
     wherein Me is U, W or a mixture of elements selected from the group consisting of Zn, Cr, U, W, Cd, Ni, B and Si, a is 0.90 to 1.3, b is 0.005 to 0.4 This representation is not an empirical formula and has no significance other than representing the atom ratio of the active metal components of the catalysts. The x in fact has no determinate value and can vary widely depending on the combinations within the complex. That there is oxygen present is known and the O x  is representative of this. 
     In one embodiment of the present invention the catalyst complex comprises vanadium-phosphorus, the component, Me, and an alkali or alkaline earth metal, (Alk-metal) of group IA or IIA of the periodic Table of Elements.* 
    
     This complex may be represented by the configuration 
     
         V P.sub.a Me.sub.b Alk.sub.c O.sub.x 
    
     wherein Me, a, b and x as described above and Alk is a metal selected from the group of elements of Groups IA and IIA of the Periodic Table of Elements, and c is 0.001 to 0.1. Particular Group IA and IIA elements for the present invention are Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr and Ba, even more preferably Li, Na, Mg and Ba. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The catalyst may be prepared in a number of ways. The catalyst may be prepared by dissolving the vanadium, phosphorus, Me and Group IA and IIA components (referred to herein as alk metals) in a common solvent, such as hot hydrochloric acid and thereafter depositing the solution onto a carrier. The catalyst may also be prepared by precipitating the metal compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. In some instances the catalyst may be deposited as molten metal compounds onto a carrier; however, care must be taken not to vaporize off any of the ingredients such as phosphorus. The catalyst may also be prepared by heating and mixing anhydrous forms of phosphorus acids with vanadium compounds, Me compounds, and the alk - metal compound. The catalysts may be used as either fluid bed or fixed bed catalysts. In any of the methods of preparation heat may be applied to accelerate the formation of the complex. Although some methods of catalyst preparation are preferred, any method may be used which results in the formation of the catalyst complex containing the specified ratios of vanadium, Me elements, phosphorus and alk metal. 
     One method to obtain catalysts which produce high yields of maleic anhydride upon oxidation of C 4  hydrocarbons is whereby the catalyst complex is formed in solution and deposited as a solution onto the carrier. According to one solution method, the vanadium is present in solution with an average valence of less than plus 5 in the finally formed complex in solution. Preferably the vanadium has an average valency of less than plus 5 at the time the solution of catalyst complex is deposited onto the carrier, if a carrier is used. The reduced vanadium with a valence of less than 5 may be obtained either by initially using a vanadium compound wherein the vanadium has a valence of less than 5 such as vanadyl chloride, or by initially using a vanadium compound with a valence of plus 5 such as V 2  O 5  and thereafter reducing to the lower valence with, for example, hydrochloric acid during the catalyst preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The vanadium compound may be dissolved in a reducing solvent, such as hydrochloric acid, which solvent functions not only to form a solvent for the reaction, but also to reduce the valence of the vanadium compound to a valence of less than 5. For example, a vanadium compound, a zinc compound, phosphorus compound and alk metal compound may be dissolved in any order in a suitable reducing solvent and the formation of the complex allowed to take place. Preferably, the vanadium compound is first dissolved in the solvent and thereafter the phosphorus, Me and alk metal compounds are added. The reaction to form the complex may be accelerated by the application of heat. The deep blue color of the solution shows the vanadium has an average valence of less than  5. The complex formed is then, without a precipitation step, deposited as a solution onto a carrier and dried. In this procedure, the vanadium has an average valence of less than plus 5, such as about plus 4, at the time it is deposited onto the carrier. Generally, the average valence of the vanadium will be between about plus 2.5 and 4.6 at the time of deposition onto the carrier. 
     When the above described solution method is employed, reducing agents for the vanadium may be either organic or inorganic. Acids such as hydrochloric, hydroiodic, hydrobromic, acetic, oxalic, malic, citric, formic and mixtures thereof such as a mixture of hydrochloric and oxalic may be used. Sulphur dioxide may be used. Less desirably, sulfuric and hydrofluoric acids may be employed. Other reducing agents which may be employed, but which have not given as desirable catalysts are organic aldehydes such as formaldehyde and acetaldehyde; alcohols such as pentaerythritol, diacetone alcohol and diethanol amine. Additional reducing agents are such as hydroxyl amines, hydrazine, and nitric oxide. Nitric acid and similar oxidizing acids which would oxidize the vanadium from a valence of 4 to 5 during the preparation of the catalyst should be avoided. Generally, the reducing agents form oxysalts of vanadium. For example, if V 2  O 5  is dissolved in hydrochloric or oxalic acid, the corresponding vanadium oxysalts are produced. These vanadium oxysalts should have as the salt forming anion an anion which is more volatile than the phosphate anion. 
     According to this method, the time at which the Me and alk metal compounds are incorporated into the solution is not critical so long as it is in solution before the catalyst complex is coated onto the carrier. The Me and alk metal compounds may be added after the vanadium compound and the phosphorous compound have been reacted or may be added either before, at the same time or after either the vanadium or phosphorus compound has been added. 
     Any vanadium, Me, phosphorus and alk metal compounds may be used as starting materials which when the compounds are combined and heated to dryness in air at a temperature of, for example, 300° - 350° C. will leave as a deposit a catalyst complex having relative proportions within the described ranges. In the solution methods, preferred are vanadium, Me, phosphorus and alk metal compounds which are essentially completely soluble in boiling aqueous hydrochloric acid at 760 mm. of mercury, containing 37 percent by weight hydrochloric acid. Generally, phosphorus compounds are used which have as the cation an ion which is more volatile than the phosphate anion, for example, H 3  PO 4 . Also, generally any vanadium, or Me compound which has an anion which is either the phosphate ion or an ion which is more volatile than the phosphate anion, for example, vanadyl chloride or zinc chloride, nickel chloride or the like may be used. 
     In this method, the catalyst complex containing vanadium, Me, phosphorus and Group I or IIA elements may be formed by simply causing the combination of each of the ingredient components in a solution or dispersion. Heat may be applied to accelerate the formation of the complex and one method of forming the complex is by causing the ingredients to react under reflux conditions at atmospheric pressure. Under reflux conditions this solution reaction generally takes about one to two hours. 
     Although the catalysts prepared by this method may be separately formed and used as pellets, it may be more economical and practical to deposit this material on a carrier such as aluminum oxide, silica or niobium oxide. Before the carrier is combined with the catalyst the solution of catalyst is preferably concentrated to a solution which contains from about 30 to 80 percent volatiles and better results have been obtained when there is from about 50 to 70 percent volatiles by weight. The carrier may be added to the catalyst solution or the catalyst solution may be poured onto the carrier. Less desirably, the Alundum or other carrier may be present during the whole course of reactions to provide the desired vanadium-oxygen phosphorus-alkali metal complex. After the catalyst complex has been coated onto the carrier, the vanadium may be converted to a more active form by heating in the presence of an oxidizing gas. 
     Another example of the catalyst preparation is to mix with heating at a temperature of about 100° to 600° C. an anhydrous phosphoric acid such as ortho-phosphoric acid, pyrophosphoric acid, triphosphoric acid or metaphosphoric acid with a vanadium compound such as vanadium pentoxide or ammonium metavanadate, a Me compound such as uranyl acetate and an alkali such as potassium chloride. After the exothermic reaction between the ingredients the catalyst may be used. The reaction mixture may be formed onto carriers or shaped into forms such as pellets prior to the reaction to form the catalyst. 
     Another example of the preparation of the catalyst complex is to dissolve the Me and alk metal compounds and a vanadium compound such as ammonium metavanadate or vanadium pentoxide in an aqueous solution of phosphoric acid. After the components have been dissolved the solution is heated until precipitation occurs. The precipitant can then be dried and used as a catalyst, or a carrier may be combined with the liquid phase either before or after the precipitation. 
     In the various methods of preparation any vanadium, Me, phosphorus and alk metal compounds may be used as starting materials which when the compounds are combined and heated to dryness in air at a temperature of, for example, 300° - 350° C. will leave as a deposit a catalyst complex having relative proportions within the above described ranges. 
     In another method a solution of the vanadium component is prepared by adding a portion of the reducing agent, such as oxalic acid and isopropanol solution to be employed, to a solution of water and phosphoric acid and heating this mixture to a temperature generally of around 50° -80° C. A vanadium compound such as V 2  O 5  is added incrementally to this heated mixture with stirring. The blue solution which indicates vanadium of average valency less than 5, is maintained by added increments of the remaining oxalic acid - isopropanol solution. After concentration of this solution, solutions of alkali and alkaline earth metals and the Me components are added to vanadium solution and this resultant solution concentrated to a paste-like consistency, which may be coated on a carrier or mixed with a carrier, heated at moderate temperatures, i.e., 250° - 500° C. for a few minutes to several hours and prepared in pellets or chips. 
     As the source of phosphorus, various phosphorus compounds may be used, such as metaphosphoric acid, triphosphoric acid, pyrophosphoric acid, ortho-phosphoric acid, phosphorus pentoxide, phosphorus oxyiodide, ethyl phosphate, methyl phosphate, amine phosphate, phosphorus pentachloride, phosphorus trichloride, phosphorus oxybromide and the like. 
     Suitable vanadium compounds useful as starting materials are compounds such as vanadium pentoxide, ammonium metavanadate, vanadium trioxide, vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadium sulfate, vanadium phosphate, vanadium tribromide, vanadyl formate, vanadyl oxalate, metavanadic acid, pyrovanadic acid, and the like. Mixtures of the various vanadium, Me, and phosphorus compounds may be used as starting materials to form the described catalyst complex. 
     The Me component is also suitably introduced by employing the various compounds thereof such as the acetates, carbonates, chlorides, bromides, oxides, hydroxides, nitrates, chromates, chromites, tellurates, sulfides, phosphates and the like. These compounds are entirely conventional and those of ordinary skill in the art know these materials and can readily determine suitable compounds to prepare the catalyst, with little, if any, experimentation. A few illustrative compounds are uranyl acetate, uranyl sulfate, zinc chloride, zinc oxalate, tungstic acid, tungsten dioxide, nickel chloride, chromium sulfate, chromium trioxide, chromium chloride, cadmium chloride, boric acid, SiO 2  (30 percent colloidal solution -- ammonia stabilized) and similar compounds. 
     The alk-metal may suitably be introduced as compounds such as alkali and alkaline earth metal salts with examples being lithium acetate, lithium bromide, lithium carbonate, lithium chloride, lithium hydroxide, lithium iodide, lithium oxide, lithium sulfate, lithium orthophosphate, lithium metavanadate, potassium sulfate, potassium chloride, potassium hydroxide, sodium chloride, sodium hydroxide, rubidium nitrate, cesium chloride, beryllium nitrate, beryllium sulfate, magnesium sulfate, magnesium bromide, magnesium carbonate, calcium carbonate, calcium chromite, strontium chloride, strontium chromate, barium acetate, barium chlorate, radium carbonate and the like. Mixtures of two or more alk metal compounds may be used, such as a mixture of lithium hydroxide and sodium chloride or a mixture of lithium chloride and potassium chloride. Preferred alk-metal elements are lithium, sodium and potassium, and mixtures thereof, with lithium being particularly preferred. When the above described solution method of catalyst preparation is employed, the alkali metal compound will suitably be an alkali metal compound which either has a phosphate anion as the anion, that is a compound such as lithium phosphate, or a compound which has an anion which is more volatile than the phosphate anion. 
     The function of the Group IA element is not completely understood that superior results are obtained when the catalyst contains these elements. Longer useful catalyst life has been observed when the IA element is present, probably due, at least in part, to the partially stabilizing effect of the alkali on phosphorus, and Me compounds. 
     The atomic ratio of the total atoms of Group IA elements to vanadium should be about 0.003 to 0.08 atom of alkali per atom of vanadium. The preferred amount of alkali is about 0.01 to 0.04 atom per atom of vanadium. 
     A catalyst support, if used, provides not only the required surface for the catalyst, but gives physical strength and stablity to the catalyst material. The carrier or support normally has a low surface area, as usually measured, from about 0.110 to about 5 square meters per gram. A desirable form of carrier is one which has a dense non-absorbing center and a rough enough surface to aid in retaining the catalyst adhered thereto during handling and under reaction conditions. The carrier may vary in size but generally is from about 21/2 mesh to about 10 mesh in the Tyler Standard screen size. Alundum particles as large as 1/4 inch are satisfactory. Carriers much smaller than 10 to 12 mesh normally cause an undesirable pressure drop in the reactor, unless the catalysts are being used in a fluid bed apparatus. Very useful carriers are Alundum and silicon carbide or Carborundum. Any of the Alundums or other inert alumina carriers or low surface may be used. Likewise, a variety of silicon carbides may be employed. Silica gel may be used. 
     Other materials which can serve as carriers are Nb 2  O 5 , WO 3 , Sb 2  O 3  and mixtures of these and other supports. The support material is not necessarily inert, that is, the particular support may cause an increase in the catalyst efficiency by its chemical or physical nature or both. 
     The amount of the catalyst complex on the carrier is usually in the range of about 15 to about 95 weight percent of the total weight of complex plus carrier and preferably in the range of 50 to 90 weight percent and more preferably at least about 60 weight percent on the carrier. The amount of the catalyst complex deposited on the carrier should be enough to substantially coat the surface of the carrier and this normally is obtained with the ranges set forth above. With more absorbent carriers, larger amounts of material will be required to obtain essentially complete coverage of the carrier. In a fixed bed process the final particle size of the catalyst particles which are coated on a carrier will also preferably be about 21/2 to about 10 mesh size. The carriers may be of a variety of shapes, the preferred shape of the carriers is in the shape of cylinders or spheres. Although more economical use of the catalyst on a carrier in fixed beds is obtained, as has been mentioned, the catalyst may be employed in fluid bed systems. Of course, the particle size of the catalyst used in fluidized beds is quite small, usually varying from about 10 to about 150 microns, and in such systems the catalyst normally will not be provided with a carrier but will be formed into the particle size after drying from solution. 
     Inert diluents may be present in the catalyst, but the combined weight of the active ingredients, e.g., vanadium, oxygen, phosphorus, Me, and alk metal should preferably consist essentially of at least about 50 weight percent of the composition which is coated on the carrier, if any, and preferably these components are at least about 75 weight percent of the composition coated on the carrier, and more preferably at least about 95 weight percent. 
     The oxidation of the n-C 4  hydrocarbon to maleic anhydride may be accomplished by contacting, e.g., n-butane, in low concentrations in oxygen with the described catalyst. Air is entirely satisfactory as a source of oxygen, but synthetic mixtures of oxygen and diluent gases, such as nitrogen, also may be employed. Air enriched with oxygen may be employed. 
     The gaseous feed stream to the oxidation reactors normally will contain air and about 0.5 to about 2.5 mol percent hydrocarbons such as n-butane. About 1.0 to about 1.5 mol percent of the n-C 4  hydrocarbon are satisfactory for optimum yield of product for the process of this invention. Although higher concentrations may be employed, explosive hazards may be encountered. Lower concentrations of C 4 , less than about 1 percent, of course, will reduce the total yields obtained at equivalent flow rates and thus are not normally economically employed. The flow rate of the gaseous stream through the reactor may be varied within rather wide limits but a preferred range of operations is at the rate of about 50 to 300grams of C 4  per liter of catalyst per hour and more preferably about 100 to about 250 grams of C 4  per liter of catalyst per hour. Residence times of the gas stream will normally be less than about 4 seconds, more preferably less than about one second, and down to a rate where less efficient operations are obtained. The flow rates and residence times are calculated at standard conditions of 760 mm. of mercury and at 25° C. A preferred feed for the catalyst of the present invention for conversion to maleic anhydride is a n-C 4  hydrocarbon comprising a predominant amount of n-butane and more preferably at least 90 mol percent n-butane. 
     A variety of reactors will be found to be useful and multiple tube heat exchanger type reactors are quite satisfactory. The tubes of such reactors may vary in diameter from about 1/4 to about 3 inches, and the length may be varied from about 3 to about 10 or more feet. The oxidation reaction is an exothermic reaction and, therefore, relatively close control of the reaction temperature should be maintained. It is desirable to have the surface of the reactors at a relatively constant temperature and some medium to conduct heat from the reactors is necessary to aid temperature control. Such media may be Woods metal, molten sulfur, mercury, molten lead, and the like, but it has been found that eutectic salt baths are completely satisfactory. One such salt bath is a sodium nitrate-sodium nitrite-potassium nitrate eutectic constant temperature mixture. An additional method of temperature control is to use a metal block reactor whereby the metal surrounding the tube acts as a temperature regulating body. As will be recognized by the man skilled in the art, the heat exchange medium may be kept at the proper temperature by heat exchangers and the like. The reactor or reaction tubes may be iron, stainless steel, carbon-steel, nickel, glass tubes such as Vycor and the like. Both carbon-steel and nickel tubes have excellent long life under the conditions of the reactions described herein. Normally, the reactors contain a preheat zone of an inert material such as 1/4 inch Alundum pellets, inert ceramic balls, nickel balls or chips and the like, present at about one-half to one-tenth the volume of the active catalyst present. 
     The temperature of reaction may be varied within some limits, but normally the reaction should be conducted at temperatures within a rather critical range. The oxidation reaction is exothermic and once reaction is underway, the main purpose of the salt bath or other media is to conduct heat away from the walls of the reactor and control the reaction. Better operations are normally obtained when the reaction temperature employed is no greater than about 100° C. above the salt bath temperature. The temperature in the reactor, of course, will also depend to some extent upon the size of the reactor and the C 4  concentration. Under usual operating conditions, in compliance with the preferred procedure of this invention, the temperature in the center of the reactor, measured by themocouple, is about 375° to about 550° C. The range of temperature preferably employed in the reactor, measured as above, should be from about 400° to about 515° C. and the best results are ordinarily obtained at temperatures from about 420° to about 470° C. Described another way, in terms of salt bath reactors with carbon steel reactor tubes about 1.0 inch in diameter, the salt bath temperature will usually be controlled between about 350° to about 550° C. Under normal conditions, the temperature in the reactor ordinarily should not be allowed to go above about 470° C. for extended lengths of time because of decreased yields and possible deactivation of the novel catalyst of this invention. 
     The reaction may be conducted at atmospheric, super-atmospheric or below atmospheric pressure. The exit pressure will be at least slightly higher than the ambient pressure to insure a positive flow from the reaction. The pressure of the inert gases may be sufficiently high to overcome the pressure drop through the reactor. 
     In one utilization of the present catalyst compositions, the oxidation is carried out at 15 to 100 psig, preferably about 20 to 50 psig and more preferably about 25 to 40 psig, which is disclosed and claimed in a commonly assigned patent application of Bruno J. Barone and Stone D. Cooley Ser. No. 558,737 filed Mar. 15, 1975. 
     Operating under pressure as described above, the temperature in the center of the reactor, measured by themocouple is about 375° to about 550° C, with the preferred temperature range for operating according to the present invention being 430° to 480° C. and the best results are ordinarily obtained at temperatures from about 430° to about 455° C. Described another way, in terms of salt bath reactors with carbon steel reactor tubes about 1.0 inch in diameter, the salt bath temperature will usually be controlled between about 325° to about 455° C. Under these conditions, the temperature in the reactor ordinarily should not be allowed to go above about 410° C for extended lengths of time because of decreased yields and possible deactivation of the novel catalyst of this invention. 
     The maleic anhydride may be recovered by a number of ways well known to those skilled in the art. For example, the recovery may be by adsorption in suitable media, with subsequent separation and purification of the maleic anhydride. 
    
    
     EXAMPLES 
     In the following examples, the reactor comprised a 4-tube cylindrical brass block (8 inches O.D. × 18 inches) reactor made of alloy 360. The block was heated by two 2500 watt (220 volt) cartridge heaters controlled by means of a 25 amp. thermoelectric proportional controller with automatic reset. Prior to its insulation, the block was tightly wound with a coil of 3/8 inch copper tubing. This external coil was connected to a manifold containing water and air inlets for cooling of the reactor block. The reactors were made of a 304 stainless steel tube, 1.315 inches O.D. and 1.049 inches I.D., 231/2 inches long, containing a centered 1/8inch O.D. stainless steel thermocouple well. The lower end of the reactor was packed with a 1 inch bed of 3 mm pyrex beads. The next 12 inches of the reactor were packed with catalyst (1/8inch × 1/8inch pellets or 6-12 mesh granules) followed by about a 10  inches bed of 3 mm pyrex beads. The gas streams are separately metered into a common line entering the top of the reactor. The reaction vapors are passed through two 2000 ml gas scrubbing bottles containing 800 ml of water. The vapors from the scrubbers then go through a wet test meter and are vented. The inlet gases are sampled before entering the reactor and after the water scrubbers. The feed is normally 0.5 to 1.8 mol percent C 4 , e.g., n-butane, in air, adjusted to maintain a desired temperature. In addition, operating temperature can be further controlled by dilution of the air with an inert gas. 
     The inlet gases and water scrubbed outlet gases were analyzed by gas chromatography using the peak area method. Butane, carbon dioxide and any olefins or diolefins present in the gas streams were determined using a 1/4 inch column with a 5 feet foresection, containing 13 weight percent vacuum pump oil on 35/80 mesh chromosorb, followed by a 40 feet sectiion containing 26 weight percent of a 70-30 volume ratio of propylene carbonate to 2,4-dimethylsulfolane on 35/80 mesh chromosorb. The analysis was conducted at room temperature with hydrogen as the carrier gas (100 ml/minute). Carbon monoxide was analyzed on a 1/4inch column with a 3 feet foresection of activated carbon followed by a 6 feet section of 40/50 mesh 5A molecular sieves. This analysis was run at 35° C with helium as the carrier gas (20 psi). 
     The water scrub solutions were combined and diluted to 3000 ml in a volumetric flask. An aliquot of this solution was titrated with 0.1 N sodium hydroxide solution to determine maleic acid (first end point) and weak acids in solution and titrated to determine the carbonyls, using hydroxylamine hydrochloride. 
     TYPICAL CATALYST PREPARATION 
     A catalyst having the representative formula VP 1 .5 Zn 0 .19 O x  for oxidation of n-butane to maleic anhydride was prepared as follows: 145.44g of vanadium pentoxide (V 2  O 5 ) was added to 1500 milliliters of 37 percent hydrochloric acid. The mixture was refluxed slowly and after the initial reaction 212.14g of 85 percent phosphoric acid (H 3  PO 4 ) was added and the mixture refluxed for about 13 hours. After a blue solution was obtained, showing that the vanadium had an average valence of less than plus five, 41.43g of ZnCl 2  was added to the solution and the mixture again refluxed. The resulting deep blue solution was concentrated to a paste like consistency and dried in an oven at 130° C. The dried catalyst was chipped, calcined at 350° C for 2 hours, ground to 60 mesh and tableted. 
     Other catalyst shown below were prepared in the same manner with additional or different component compounds being added when the ZnCl 2  was added and in the appropriate amounts to give the atom ratios shown. 
     The results of the testing of the catalyst are set forth in the Table below: 
     
                                           TABLE                                   
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                             Head Mole %                                  
Catalyst Composition                                                      
                    Temp., °C                                      
                             Pressure                                     
                                  n-C.sub.4                               
                                       GHSV Mole % M.A. M.A. Output       
Example                                                                   
     Atom Ratio     Block                                                 
                        Hot Spot                                          
                             psig Feed V/V/H                              
                                            Conv.                         
                                                Select.                   
                                                    Yield                 
                                                        g/lit.cat/hr.     
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1    VP.sub.1.12 O.sub.x                                                  
                    415 451  0    0.930                                   
                                        847 87  53  46  15.3              
                    380 450  20   1.272                                   
                                        850 82  52  43  22.9              
2    VP.sub.1.15 Zn.sub.0.19 Si.sub.0.167 O.sub.x                         
                    410 467  0    0.947                                   
                                       1115 76  62  47  21.8              
                    388 456  35   1.104                                   
                                       1346 80  73  58  38.0              
3    VP.sub.1.17 Zn.sub.0.19 Cr.sub.0.01 B.sub.0.01 O.sub.x               
                    390 464  0    1.031                                   
                                       1218 80  60  48  26.6              
                    350 465  30   1.321                                   
                                       1342 85  63  54  41.6              
4    VP.sub.1.15 U.sub.0.25 B.sub.0.01 O.sub.x                            
                    360 460  0    1.161                                   
                                       1347 81  50  41  27.8              
                    325 453  20   1.253                                   
                                       1348 79  54  43  31.6              
5    VP.sub.1.1 W.sub.0.06 O.sub.x                                        
                    405 466  0    1.038                                   
                                        582 79  54  42  11.3              
                    380 467  30   0.787                                   
                                       1217 85  58  49  20.7              
6    VP.sub.1.2 W.sub.0.06 O.sub.x                                        
                    410 477  0    1.205                                   
                                        757 83  56  47  18.7              
                    370 460  35   1.464                                   
                                        838 84  64  53  28.6              
7    VP.sub.1.1 U.sub.0.25 O.sub.x                                        
                    360 460  0    1.034                                   
                                       1346 83  43  36  21.8              
8    VP.sub.1.15 W.sub.0.06 Zn.sub.0.1 O.sub.x                            
                    405 461  0    0.781                                   
                                        705 82  54  44  10.7              
                    375 455  25   0.841                                   
                                       1116 78  60  47  19.2              
9    VP.sub.1.15 Ni.sub.0.19 Cr.sub.0.005 Li.sub.0.01 O.sub.x             
                    390 457  0    1.040                                   
                                       1116 82  59  49  24.8              
                    340 464  30   1.159                                   
                                       1340 84  61  51  34.9              
10   VP.sub.1.15 U.sub.0.25 Cr.sub.0.005 Li.sub.0.01 O.sub.x              
                    360 459  0    1.136                                   
                                       1346 80  55  44  29.5              
                    335 458  30   0.797                                   
                                       7860 85  54  46  42.9              
11   VP.sub.1.175 Cd.sub.0.19 Cr.sub.0.005 Li.sub.0.001 O.sub.x           
                    410 469  0    0.979                                   
                                        892 82  54  45  17.0              
                    375 468  30   1.237                                   
                                       1348 86  61  53  38.5              
12   VP.sub.1.2 W.sub.0.06 Cr.sub.0.005 Li.sub.0.01 O.sub.x               
                    410 461  0    0.760                                   
                                       1117 83  57  47  17.5              
                    393 463  30   1.336                                   
                                       1209 83  61  51  35.9              
13   VP.sub.1.15 Ni.sub.0.19 W.sub.0.03 B.sub.0.005 O.sub.x               
                    390 460  0    0.805                                   
                                       1339 84  55  46  21.9              
                    345 468  20   1.189                                   
                                       1347 86  57  49  34.4              
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