Abstract:
Process for the manufacture of maleic anhydride which comprises contacting in the vapor phase a mixture of butane and molecular oxygen with a catalyst comprising in chemical combination phosphorus, vanadium, oxygen and a catalyst modifier comprising titanium combined with one or more elements selected from the group consisting of aluminum, cerium, cobalt and iron.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     This invention relates to a process for the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to a process suitable for producing maleic anhydride from saturated hydrocarbons in higher yields than heretofore possible. 
     B. The Prior Art 
     Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs. The prior art discloses a number of processes used in the conversion of organic feedstocks to maleic anhydride. 
     Of particular interest is U.S. Pat. No. 3,293,268 which teaches a process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions and in the presence of phosphorus-vanadium-oxygen catalysts. Through various improvements including the use of promoters such as cobalt, nickel, cadmium and titanium yields have been substantially increased, as exemplified in U.S. Pat. No. 4,111,963, U.S. Pat. No. 3,987,063, and U.S. Pat. No. 3,907,833. 
     Any economically feasible method of substantially reducing the operating temperature or increasing the level of selectivity in the manufacture of maleic anhydride including alternative methods to those already known could be a substantial advance in the art and is an object of this invention. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved in a process for preparing maleic anhydride wherein a mixture of an oxygen-containing gas and a saturated hydrocarbon having 4 to 10 carbon atoms is contacted with a catalyst complex comprising phosphorus, vanadium and oxygen, the improvement wherein the complex also includes a catalyst modifier comprising titanium combined with one or more elements selected from the group consisting of aluminum, cerium, cobalt and iron. 
     In the ordinary practice of this invention, the mixture of oxygen-containing gas and the hydrocarbon are reacted at about 350°-360° C. The phosphorus-to-vanadium atom ratio of the catalyst is about 1:2 to about 2:1. At least 50 atom % of the vanadium is in the tetravalent state. An effective amount of the modifier can be any amount which reduces the required operating temperature or increases the selectivity in the manufacture of maleic anhydride. A typical titanium/vanadium atom ratio, as will be seen in the examples, is 0.05-0.10. 
     For the purposes of this invention, the term &#34;catalytic activity&#34; means the ability to convert a particular feedstock, such as butane, at a particular temperature to other compounds. The term &#34;selectivity&#34; means the ratio of the moles of maleic anhydride obtained to the moles of hydrocarbon reacted. The term &#34;yield&#34; means the ratio of the moles of maleic anhydride obtained to the moles of hydrocarbon introduced into the reaction. The term &#34;space velocity&#34; means the hourly volume of the gaseous feed expressed in cubic centimeters (cc) at 60° F. and standard atmospheric pressure divided by the catalyst bulk volume expressed in cubic centimeters (cc), the term expressed as cc/cc/hour. 
     Broadly described, the catalysts used in the process of this invention to convert saturated hydrocarbons to maleic anhydride are prepared by contacting vanadium compounds, phosphorus compounds, and the modifier under such conditions that a substantial amount of tetravalent vanadium is provided to form catalyst precursors, recovering the catalyst precursors, forming the catalyst precursors into structures for use in a maleic anhydride reactor, and calcining the structured catalyst precursors to form the catalysts. 
     The vanadium compounds useful as a source of vanadium in the catalyst precursors are those known to the art to be useful for preparing catalysts to oxidize hydrocarbons. Suitable vanadium compounds include: vanadium oxides, such as vanadium pentoxide, vanadium trioxide, and the like; vanadium oxyhalides, such as vanadyl chloride, vanadyl trichloride, vanadyl dichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide, and the like; vanadium salts, such as ammonium metavanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxalate and the like. However, vanadium pentoxide is preferred. 
     As a source of phosphorus in the catalyst precursors, useful phosphorus compounds are also those well known in the art useful for preparing catalysts to oxidize hydrocarbons. Suitable phosphorus compounds include: phosphorous acid, phosphoric acids, such as metaphosphoric acid, othophosphoric acid, triphosphoric acid, pyrophosphoric acid, and the like; phosphorus oxides, such as phosphorus pentoxide and the like; phosphorus halides, such as phosphorus oxyiodide, phosphorus pentachloride, phosphorus oxybromide and the like; and organophosphorus compounds such as ethyl phosphate, methyl phosphate and the like. However, phosphorous acid is preferred. 
     The catalyst modifier includes titanium as well as one or more elements selected from the group consisting of aluminum, cerium, cobalt and iron. The compound or compounds must be noninterfering compounds in the sense that either in the ionized or neutral form it (they) does (do) not substantially interfere with the production of the catalyst precursor or catalysts or with the production of maleic anhydride. Preferably, the compound is a titanate of aluminum, cerium, cobalt or iron. The catalyst modifier is introduced into the catalyst at any stage of catalyst preparation prior to calcining the catalyst, but preferably it is introduced into the catalyst at the initial stages of precursor formation so as to provide an atomic ratio of titanium to vanadium of 0.05-0.01, preferably 0.07-0.09. 
     To prepare precursors to the catalysts used in the present process, a pentavalent or tetravalent vanadium compound is heated with a phosphorus compound in an acid solution along with the catalyst modifier to dissolve the starting materials. A mild reducing agent is used to provide tetravalent vanadium and/or to maintain vanadium in the tetravalent state. On the other hand, an acid with reducing properties, such as hydrogen halide acid or oxalic acid, can serve as the acid and can provide tetravalent vanadium. Phosphorous acid is preferred. The acid solution containing the phosphorus compound and the vanadium compound are heated until a blue solution is obtained, indicating that a substantial amount, i.e., greater than 50 atom percent of the vanadium is in the tetravalent state. The amount of time required to dissolve the phosphorus and vanadium compounds and to provide a substantial amount of the vanadium in the tetravalent state to form the catalyst precursors varies from batch to batch, depending upon the compounds used as starting materials and the temperature at which the compounds are heated. However, as will occur to those skilled in the art, the solution can be analyzed to insure that most of the vanadium is in the tetravalent state. 
     Although any number of phosphorus and vanadium compounds can be used to form the precursor, the atom ratio of phosphorus to vanadium in the precursor is important, since it controls the phosphorus-to-vanadium atom ratio in the final catalyst. When the precursor contains a phosphorus to vanadium atom ratio below about 1:2 or above about 2:1, the yield of maleic anhydride using the process of this invention is so low that it is not of commercial significance. It is preferred that the precursors have a phosphorus to vanadium atom ratio in the range of about 1:1 to about 1.5:1. When the catalyst is used to convert a feed that is primarily butane to maleic anhydride, it is even more preferable to have a phosphorus to vanadium atom ratio of about 1:1  to about 1.2:1, typically about 1.1:1. 
     After the precursors have been formed by heating the vanadium compounds, the phosphorus compounds and the modifier, and a substantial amount of vanadium has been reduced to the tetravalent state, it is necessary to remove most of the water in order to recover the precursor. Techniques for recovering the precursors from solution are well known to those skilled in the art. Precursors can be deposited on a carrier, such as alumina or titania, from solution, or excess water can be removed to provide the precursors. 
     After the precursors are recovered from solution, they are then formed into structures suitable for use in a maleic anhydride reactor. Techniques for forming appropriate structures from the precursors for use in a fluidized bed reactor or in a fixed tube, heat exchanger type reactor are well known to those skilled in the art. For example, the precursors can be structured for use in a fluidized bed reactor by depositing the precursors from solution on a carrier, or alternatively, the dried precursors can be comminuted for use in a fluidized bed reactor. On the other hand, the precursors can be structured for use in a fixed tube reactor by prilling or tableting the precursors. 
     After the precursors have been formed into the structures in which they will be used in the maleic anhydride reactor, they can be calcined in an oxygen-containing atmosphere, such as air, at temperatures of from about 350° C. to about 600° C. for at least about 2 hours to convert the precursors to the catalysts for use in the present process. 
     If more than about 90 atom percent of the vanadium is oxidized to pentavalent vanadium, usually caused by calcining in air at too high a temperature, the selectivity of the catalyst and the yield of maleic anhydride decrease markedly. On the other hand, oxidation of less than about 20 atom percent of the vanadium during air calcination does not seem to be more beneficial than calcination in an inert atmosphere. 
     After the precursors have been calcined to form the catalysts of this process, the catalysts can be used to convert a saturated hydrocarbon to maleic anhydride. However, the initial yield of maleic anhydride may be low; and if this is the case, the catalysts can be conditioned, as will occur to those skilled in the art by passing low concentrations of a saturated hydrocarbon in air through the catalyst for a period of time before production operations begin. It has been discovered, however, that a unique feature of this invention is that with the preferred bimetallic or multimetallic content of the catalyst the initial yield of maleic anhydride will not be lower, so that no conditioning is in fact required. 
     The catalysts of the present process are useful in a variety of reactors to convert saturated hydrocarbons to maleic anhydride. Both fluidized bed reactors and fixed tube heat exchanger type reactors are satisfactory and details of the operation of such reactors are well known to those skilled in the art. The reaction to convert saturated hydrocarbons to maleic anhydride requires only contacting the saturated hydrocarbon admixed with a free-oxygen containing gas, such as air or oxygen-enriched air, with the catalysts at elevated temperatures. The saturated hydrocarbons are contacted with the catalysts at a concentration of about 1.5 to about 10 volume percent saturated hydrocarbons at a space velocity of about 100 to 4,000 cc/cc/hour to provide maleic anhydride yields of greater than 40% at temperatures between about 350° C. and 600° C. 
     The catalysts of the present process are particularly useful in fixed tube heat exchanger type reactors. The tubes of such reactors can vary in diameter from about 1/4 inch to about 1.5 inches and the length can vary from about 6 inches to about 10 or more feet. It is desirable to have the surfaces of the reactors at relatively constant temperature, and some medium to conduct heat from the reactors is necessary to aid temperature control. Such media can be Woods metal, molten sulfur, mercury, molten lead and the like or eutectic salt baths. A metal block reactor whereby the metals surrounding the tube act as a temperature regulating body can also be used. The reactor or reaction can be iron, stainless steel, carbon steel, glass and the like. 
     Maleic anhydride prepared by using the process of this invention can be recovered by any number of means well known to those skilled in the art. For example, the maleic anhydride can be recovered by direct condensation or by absorption in suitable media with subsequent separation and purification of the anhydride. 
     The pressure in the reactor is not generally critical. Therefore, the reaction can be run at atmospheric, superatmospheric and subatmospheric pressures, although superatmospheric pressures are usually employed. 
     A large number of saturated hydrocarbons having from 4 to 10 carbon atoms can be converted to maleic anhydride using the process of the present invention. It is only necessary that hydrocarbons contain not less than 4 carbon atoms in a straight chain. As an example, the preferred saturated hydrocarbon is butane, but isobutane which does not contain 4 carbon atoms in a straight chain is not satisfactory for conversion to maleic anhydride, although its presence is not harmful. In addition to butane, other saturated hydrocarbons within the scope of this invention include the pentanes, the hexanes, the heptanes, the octanes, the nonanes, the decanes, or mixtures of any of these with or without butane. In addition to the above compounds, cyclic compound such as cyclopentane or cyclohexane are satisfactory feed materials for conversion to maleic anhydride. Also, the feedstock can be technical grade hydrocarbons containing up to about 25 weight percent of olefinically unsaturated hydrocarbons or other hydrocarbon fractions. 
     The principal product from the oxidation of the above feed materials is maleic anhydride. It should be noted that small amounts of citraconic anhydride may also be produced when the feedstock is a saturated hydrocarbon containing more than 4 carbon atoms. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is further illustrated, but not limited to the following examples: 
     EXAMPLE I 
     A two liter Parr autoclave was charged with the following: 
     1. 537 cc of deionized water (DI H 2  O) 
     2. 304.3 g of V 2  O5 
     3. 201.4 g of H 3  PO 4  soln. 
     4. 152.8 g of H 3  PO 3   
     5. 3.0 g of a surfactant having a trade name of STEROX® manufactured by Monsanto 
     6. 33.20 g of CoTiO 3   
     7. 492 cc of DI H 2  O 
     The autoclave was stirred at about 1,000 rpm, and heated with open vent to 55° C. at which time the vent was closed. Heating was continued at a setting of 80 until the autoclave reached a temperature of 91° C. at which time the setting was reduced to 49 and heating was continued until the autoclave reached a temperature of 152° C. The temperature was maintained at 153±1° C. for a period of four hours. It was then cooled rapidly under a cold stream of water to about 60° C. and the vent was cracked. The contents of the autoclave were discharged into a two quart baking dish. The slurry was blue-green. It was dried overnight at 122° C.±3° C. in an oven. The dry cake was broken into large fragments and ground in a blender to pass a #18 screen. The powder was tableted with 1 weight percent graphite, being formed into 3/16 inch tablets with an average length of 4.76 mm. These tablets were calcined at 400° C. for six hours in air. 
     EXAMPLES 2 THROUGH 81 
     Examples 2 through 81 were prepared in the same general manner as example 1 except that the atom ratios of the catalyst components were varied as indicated in the column of Table I under &#34;catalyst description&#34;. In this column, the number following each component of the catalyst indicates the atomic ratio of that component based on vanadium. Catalysts were tested at the space velocities (SV) indicated in Table I. 
     
                                           TABLE I__________________________________________________________________________       CompCatalyst    Ex   Bath Conv     Selec.                               YieldDescription (CE) Temp %    SV  %    %__________________________________________________________________________Co.06Ti.06P1.08VOx        1   411.0                 79.40                      1480                          68.60                               54.40Co.10Ti.10P1.12VOx        2   412.0                 79.50                      1469                          67.40                               53.60        3   400.0                 79.20                      1156                          70.40                               55.70Co.10Ti.10P1.08VOx        4   374.0                 79.20                      1473                          67.60                               53.50        5   401.0                 80.30                      1158                          69.60                               55.90Co.08Ti.08P1.08VOx        6   409.0                 78.80                      1460                          67.60                               53.50        7   399.0                 79.60                      1159                          70.60                               56.20Co.10Ti.10P1.08VOx        8   393.0                 79.10                      1148                          72.40                               57.20        9   431.0                 77.60                      1164                          64.50                               50.00Co.10Ti.10P1.09VOx       10   413.0                 78.40                      1458                          67.40                               52.80       11   450.0                 68.10                      2628                          56.40                               38.50Co.03Ti.03P1.02VOx       12   396.0                 79.90                      1160                          70.40                               56.30Co.064P1.08VOx       13 CE            423.0                 79.50                      1455                          64.70                               51.40NH4CoPo4.H2OCo.064P1.08VOx       14 CE            412.0                 79.70                      1461                          69.70                               55.50NH4CoPO4.H2O       15 CE            428.0                 72.00                      2595                          67.00                               48.20Co.128P1.12VOx       16 CE            401.0                 78.90                      1464                          70.10                               55.30       17 CE            424.0                 70.40                      2623                          67.90                               47.70Co.064P1.06VOx       18 CE            363.0                 79.80                      1422                          66.50                               53.00no TiFe.10Ti.10P1.08VOx       19   391.0                 79.20                      1151                          72.70                               57.50Fe.10Ti.10P1.08VOx       20   401.0                 79.50                      1168                          70.20                               55.80Fe.10Ti.10P1.08VOx       21   419.0                 79.50                      1460                          68.30                               54.20       22   450.0                 59.70                      2636                          58.10                               34.70Fe.10Ti.10P1.08VOx       23   401.0                 79.40                      1161                          70.50                               55.904G start at 1150       24   408.0                 79.60                      1476                          67.80                               53.90Fe.10Ti.10P1.08VOx       25   406.0                 79.40                      1447                          68.00                               53.904G start at 1450       26   396.0                 79.00                      1157                          70.60                               55.80       27   424.0                 71.70                      2604                          63.30                               45.30Fe.025Ti.025P1.08VOx       28   408.0                 78.10                      1161                          69.10                               53.901MFe.10Ti.10P1.08VOx       29   406.0                 79.80                      1169                          71.00                               56.60       30   414.0                 78.20                      1472                          69.10                               54.00       31   432.0                 70.60                      2641                          63.60                               44.90Fe.10Ti.10P1.08VOx       32   398.0                 79.30                      1180                          71.00                               56.30Fe.10Ti.10P1.08VOx       33   405.0                 79.20                      1161                          71.40                               56.50       34   447.0                 78.90                      1168                          60.50                               47.70Fe.10Ti.10P1.08VOx       35   404.0                 80.40                      1162                          70.00                               56.20       36   450.0                 64.30                      2396                          53.00                               34.00Fe.10Ti.10P1.08VOx       37   407.0                 79.40                      1121                          70.50                               55.90       38   439.0                 75.50                      1126                          62.30                               47.00       39   441.0                 77.90                      1086                          60.10                               46.80Fe.10Ti.10P1.08VOx       40   408.0                 79.10                      1181                          67.70                               53.501000 g calcn lotFe.10Ti.10P1.08VOx       41   416.0                 79.30                      1450                          66.50                               52.70extrusions 4G0.99 g/ccCe.10Ti.10P1.08VOx       42   384.0                 79.30                      1167                          71.00                               56.30Ce.10Ti.10P1.08VOx       43   360.0                 51.20                      1151                          72.20                               37.00       44   386.0                 78.50                      1149                          69.40                               54.50Ce.10Ti.10P1.08VOx       45   393.0                 78.40                      1170                          74.10                               58.10       46   407.0                 75.20                      1176                          61.50                               46.20Ce.10Ti.10P1.08VOx       47   378.0                 79.10                      1161                          57.70                               45.60(CeO2 + TiO2)Ce.10Ti.10P1.08VOx       48   390.0                 79.50                      1155                          71.70                               57.00Ce.10Ti.10P1.08VOx       49   387.0                 79.50                      1174                          73.00                               58.00       50   395.0                 80.50                      1457                          70.40                               56.60       51   403.0                 71.70                      2656                          67.00                               48.00Ce.10Ti.10P1.08VOx       52   387.0                 79.70                      1159                          71.90                               57.30       53   439.0                 68.00                      2617                          50.10                               34.10Ce.10Ti.10P1.08VOx       54   373.0                 79.40                      1168                          73.10                               58.30100% recycle       55   382.0                 79.40                      1483                          69.20                               54.90       56   400.0                 71.50                      2651                          64.10                               45.90Ce.10Ti.10P1.08VOx       57   378.0                 79.40                      1149                          71.00                               56.40       58   398.0                 71.90                      2592                          58.90                               42.30Ce.07Ti.07P1.05VOx       59   391.0                 79.60                      1163                          71.30                               56.70       60   400.0                 79.60                      1488                          69.70                               55.50       61   430.0                 81.70                      2620                          58.10                               47.50Ce.10Ti.10P1.08VOx       62   381.0                 79.60                      1148                          72.70                               57.80       63   386.0                 86.30                      1150                          69.60                               60.00       64   399.0                 70.00                      2598                          59.70                               41.70       65   393.0                 69.60                      2605                          53.70                               37.30Ci.05P1.12VOx       66   45.0 80.10                      1427                          53.00                               42.40no TiAl.0iCo.03Ti.03P1       67   381.0                 80.30                      1159                          72.90                               58.50.08VOx      68   383.0                 83.30                      1159                          72.50                               60.30Al.13Ti.064P1       69   403.0                 78.30                      1155                          72.50                               56.80.08VOxAl.14CO.03Ti.10P1       70   397.0                 79.60                      1176                          70.50                               56.10.08VOxAl.06Ti.03P1.02VOx       71   383.0                 78.90                      1175                          71.40                               56.30Ca.10Ti.10P1.08VOx       72 CE            386.0                 80.90                      1167                          68.10                               55.10Cu.10Ti.10P1.08VOx       73 CE            390.0                 79.10                      1165                          65.60                               51.80       74   377.0                 79.30                      1169                          55.40                               43.90Pb.10Ti.10P1.08VOx       75 CE            408.0                 79.30                      1171                          47.20                               37.40Ni.064P1.074VOx       76 CE            393.0                 78.70                      1149                          72.80                               57.30(fltd) 7x   77 CE            419.0                 83.90                      1148                          64.90                               54.40P1.08VOx no add.       78   398.0                 78.90                      1465                          68.20                               53.80       79 CE            429.0                 70.10                      2604                          65.40                               45.90P1.08VOx no add.       80 CE            403.0                 79.40                      1449                          68.20                               54.10Ti.0.1P1.080Vx       81   391.0                 78.86    61.00                               48.80__________________________________________________________________________ 
    
     In assessing the data contained in Table I it is important to note that it is comparative only and is not intended to show any abstract progress in the art in terms of yield. Generally speaking those catalysts containing titanium without cobalt or cobalt without titanium did not perform as well as those catalysts containing neither cobalt nor titanium in particular, those catalysts containing titanium alone were more active and less selective. Those catalysts containing cobalt alone were less active and more selective. Wherever both cobalt and titanium were employed, a synergistic effect resulted in higher average yields. 
     It was also noted that catalysts containing titanium did not require an initial period of use (during the first 40 or 50 hours of use at higher temperatures (430°-440° C.) in order to produce maleic anhydride at a comparable rate.