Abstract:
Mixed oxides of bismuth with other metals of the perovskite structure and having vacant lattice sites in the same lattice positions occupied by bismuth are disclosed as partial oxidation and ammoxidation catalysts. Such oxides are used as catalysts in the improved method of oxidizing an acyclic hydrocarbon of 1-10 carbons having at most one olefinic unsaturation by reacting the acyclic hydrocarbon in the vapor phase with oxygen in the presence of the solid catalyst to form products having carbon, hydrogen and oxygen.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to mixed oxides and to their use as catalysts for the partial oxidation or ammoxidation of hydrocarbons. 
     A number of organic materials of value are produced by oxidizing hydrocarbons with oxygen. Examples are the oxidation of propylene to acrolein, the oxidation of butane to maleic anhydride. Related processes starting with raw materials containing one oxygen only are the oxidation of methanol to formaldehyde and the oxidation of tert-butyl alcohol to methacrolein (an intermediate to methyl methacrylate). Still other processes employ both oxygen and ammonia as reactants such as the ammoxidation of propylene to acrylonitrile. An important criterion is the ability of a catalyst and a process based on the catalyst to form partial oxidation products (having carbon and both hydrogen and oxygen) without producing excessive amounts of completely oxidized products (CO and CO 2 ) or, in the case of hydrocarbon starting materials of more than one carbon, of hydrocarbons of lesser carbon number (i.e. cracking). Other important criteria is to have a process which can be manipulated to reduce the number of different partial oxidation products produced and to maximize the proportion of those produced which are desired. From propane, for example, it may be desired to maximize the production of acetaldehyde and propionaldehyde while minimizing the production of other partial oxidation products such as ethanol, propanol, methanol (except as a by-product of acetaldehyde production) and formaldehyde (except as a by-product of acetaldehyde production). 
     Mixed oxides of various types have been suggested or used for such oxidations. Examples include the following: 
     
         ______________________________________Catalyst     Process      Reference______________________________________Fe.sub.2 (MoO.sub.4).sub.3        methanol to  K. Weissermel        formaldehyde et. al, Ind.                     Org. Chem. (1978)Bi.sub.2 O.sub.3 --MoO.sub.3        propylene to K. Weissermel        acrolein     et. al, Ind.                     Org. Chem. (1978)______________________________________ 
    
     The use of catalysts of perovskite structure has been suggested by R. J. H. Voorhoeve et. al. in Science, vol. 195, pp. 827 et. seq. (1977) and R. J. H. Voorhoeve, pp. 129-180 of Advanced Materials in Catalysis (J. L. Burton et. al. eds., Academic Press, 1977). Perovskites are a well-studied class of crystals having two types of cation lattice sites, having A ions occupying dodecahedral coordination sites (12-coordination sites) and B ions occupying octahedral coordination sites (6-coordination sites). Examples of perovskites containing barium, rare earths (e.g. La) and tellurium are disclosed in H. J. Schitterhelm et. al, Z. Anorg. Alleg. Chem., vol. 425, pp. 175 et. seq. (1976) and G. Rauser et. al., Z. Anorg. Alleg. Chem., vol. 429, pp. 181 et. seq. (1977). These references disclose several classes of perovskites having B-site vacancies. 
     In A. W. Sleight&#39;s bismuth-containing scheelite catalysts, the presence of vacancies (also called defects) and bismuth in 8-coordination sites has been said to contribute to the activity in the partial oxidation of propylene, possibly by stabilizing a pi-allyl intermediate species. 
     The search, however, continues for new mixed oxides useful as catalysts for partial oxidation and ammoxidation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention includes a mixed oxide of the perovskite structure of the formula 
     
         A.sub.2-x Bi.sub.2x/3 □.sub.x/3 C.sub.2 O.sub.6 
    
     wherein A is an alkaline earth metal of atomic number between 20 and 56, C is at least one metal having a number average valence of 4 and x is between about 0.01 and about 1, with metals C and A having ionic radii satisfying the Goldschmidt Tolerance ratio for the perovskite structure, with A and Bi occupying dodecahedral coordination sites and C occupying octahedral coordination sites. 
     The invention also includes an improvement in a method of oxidizing an acyclic hydrocarbon of 1-10 carbons having at most one olefinic unsaturation by reacting the acyclic hydrocarbon with oxygen in the presence of a solid catalyst to form products having carbon, hydrogen and oxygen; wherein the catalyst is such a mixed oxide. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The mixed oxides of the present invention are of the perovskite structure having an alkaline earth metal which is barium, strontium, or calcium, preferably barium in the A-site or dodecahedral coordination site. Trivalent bismuth is also present in the A-site and vacancies are present in the A-site in ratios given by the formula ABi 2x/3  □ x/3  where A is the alkaline earth metal and x is between about 0.01 and about 1, preferably between about 0.01 and about 0.3. The presence of bismuth and vacancies in the A-site has been found to be a necessary prerequisite for catalytic activity. 
     The C-site or octahedral coordination (6-coordination) site may be occupied by any metal or combination of metals having ionic radius within the range satisfying the Goldschmidt Tolerance Ratio (given the radius of the alkaline earth metal in the A-site) for the perovskite structure and having a number average valence (as described below) of 4. Representative classes include: 
     (a) equal parts of a divalent metal and a hexavalent metal such as MgTe, 
     (b) equal parts of a trivalent metal and a pentavalent metal such as Bi +3  Bi +5 , 
     (c) three parts of a pentavalent metal and one part of a monovalent metal such as Bi 4 .5 Li 1 .5, and 
     (d) 2/3 parts of a trivalent metal, 1 part of a hexavalent metal and 1/3 part vacant octahedral coordination sites, all parts being by atoms. 
     In determining the &#34;number average valence&#34; of metals in the octahedral coordination site, it is intended to count vacant sites as having 0 valence so that, for case d, the average is computed as follows: ##EQU1## 
     The mixed oxides of the present invention can be prepared by combining the various metals in the desired proportions by atoms in the form of oxides or precursor compounds, convertable oxides, such as the carbonates, acetates, formates, nitrates, sulfates or sulfites. The oxides or precursors are ground and mixed together and heated in air (at least where any precursors were used) so as to form intimately mixed oxides which form the perovskite structure. 
    
    
     EXAMPLES 1-7 
     Preparation of Mixed Oxides 
     Six mixed oxides were prepared by combining the oxides and/or carbonates or other precursors of barium, bismuth and, in some cases, other metals such as tellurium, magnesium, lithium and tantalum. These precursors were combined in proportions giving the relative atom % of barium and bismuth (as a percentage of total metal) shown in Table 1 for (C1), (2), (3), (4), (5) and (6). The remaining atom % was tellurium in (C1)(27.3%) and (2)(27.6%); was magnesium and tellurium (25.3% each) in (3) and was lithium (12.7%) in (5). Mixed oxides (4) and (C6) had exclusively barium and bismuth as metals. 
     Each of the mixtures was heated 3 times to 600°-950° C. for 24 hours per cycle and ground before each heating cycle. The heating was conducted in air in all examples. 
     Each mixed oxide was then examined by powder x-ray diffraction techniques with monochromatic CuK radiation (wavelength 15.416 nm) and the crystallographic ordering detected by the appearance of superstructure reflections. It was determined that such mixed oxide had assumed a perovskite configuration as described in F. S. Galasso, Structure, Properties and Preparation of Perovskite Type Compounds (Pergamon Press 1969) with barium or barium and some bismuth in the dodecahedral coordination sites as indicated by the column &#34;A-site Metals&#34; in Table 1. Where bismuth was present in these sites, vacancies were also present in the A-sites at an apparent ratio of A-site bismuth to A-site vacancies of about 2:1 (as indicated by the single asterisks). In addition, metals present in the octahedral coordination sites of the perovskite structure were noted as indicated by the column &#34;B-site Metals&#34; in Table 1. As indicated by asterisks, about one-sixth of the B-sites were determined to be vacant in (C1) and (2), as indicated by the double asterisks. 
     It will thus be appreciated that only the four mixed oxides having bismuth and single asterisks had dodecahedral coordination sites occupied in part by bismuth and in part vacant. Among these four, (2), (4) and (5) had additional bismuth in B-sites and (2) had additional vacancies in B-sites. (C1) and (C6) were controls which lacked bismuth and vacancies in the A-sites, although (C1) did have bismuth and vacancies in B-sites. The narrowness of these distinctions can be seen by comparing the overall composition of (4) with (C6). 
     For comparison a lead-bismuth-molybdenum oxide of the scheelite structure was prepared in accordance with A. W. Sleight et. al., J. Catal., vol. 35, pp. 401 et. seq. (1974) and A. W. Sleight pp. 181-208 of Advanced Materials in Catalysts (J. L. Burton et. al. eds., Academic Press 1977). Its stoichiometry was about Pb 0 .85 Bi 0 .1 MoO 4  and it was determined to have a scheelite structure with lead and bismuth being in 8-coordination sites (A-sites) and molybdenum being in tetrahedral coordination sites (B-sites). 
     
                       TABLE 1______________________________________MIXED OXIDESMixed Oxides% Ba % Bi(atom % of Metal A-site Metals                        B-site Metals______________________________________C1)  Ba.sub.6 Bi.sub.2 Te.sub.3 O.sub.18                Ba.sub.6    Bi.sub.2 Te.sub.3 **54.5% 18.2%2)   Ba.sub.5.55 Bi.sub.2.3 Te.sub.3 O.sub.18                Ba.sub.5.55 Bi.sub.0.3 *                            Bi.sub.2 Te.sub.3 **51.2% 21.2%3)   Ba.sub.5.55 Bi.sub.0.3 Mg.sub.3 Te.sub.3 O.sub.18                Ba.sub.5.55 Bi.sub.0.3 *                            Mg.sub.3 Te.sub.346.8% 2.5%4)   Ba.sub.5.55 Bi.sub.6.3 O.sub.18                Ba.sub.5.55 Bi.sub.0.3 *                            Bi.sub.3.sup.+3 Bi.sub.3.sup.+546.8% 53.2%5)   Ba.sub.5.55 Bi.sub.4.8 Li.sub.1.5 O.sub.18                Ba.sub.5.55 Bi.sub.0.3 *                            Bi.sub.4.5 Li.sub.1.546.8% 40.5%C6)  Ba.sub.6 Bi.sub.6 O.sub.18                Ba.sub.6    Bi.sub.3.sup.+3 Bi.sub.3.sup.+550% 50%C7)  Pb.sub.0.85 Bi.sub.0.1 MoO.sub.4                Pb.sub.1.7 Bi.sub.0.2 ***                            Mo.sub.2-- 5.1%______________________________________ *0.05 Asite vacancies per 1.85 Ba (onefortieth of sites vacant) **Onesixth of Bsites vacant ***0.1 Asite vacancies per 1.7 Pb and 0.2 Bi (onetwentieth of sites vacant) 
    
     EXAMPLE 8 
     Oxidation of Propane 
     The seven mixed oxides prepared in Examples 1-7 were tested for catalytic activity to partially oxidize propane to products of one, two or three carbons with hydrogen and oxygen. 
     The reactor system employed comprised essentially a reactor tube containing the catalyst. The tube was mounted in a controlled temperature furnace. One end of the tube was fed with the starting materials and at the other end the reaction products were withdrawn. The reactor was mounted to permit the upward flow of the reactants over a catalyst of one of Examples 1-8 with the products exiting into a gas chromatograph. Gas phase samples were collected by air actuated sample valves in 1 cc loops. All the lines after the reactor were heated to prevent condensation. Samples were alternately collected before and after passing over the catalyst. This provided the reactant versus product analysis necessary for mass-balancing the product stream. 
     A separately controlled furnace surrounded the concentric up-flow/plug-flow reactor. The temperature of the reactor was controlled by a set of manually operated potentiometers. These potentiometers were sequenced in turn by a clock-stepping switch. In this way a suitable sequence of temperatures was programmed. At each temperature, an exit followed by an inlet analysis was performed. 
     In a typical study the temperature was raised from room temperature to 300° C. under reactant feed. Samples were taken at several temperatures between 300° C. and 525° C. The product analysis was visually inspected to look for partial oxidation products. As soon as substantial conversion (usually of the limiting reactant-oxygen) was seen, the temperature was held constant for two or three analyses. Then the temperature was lowered again, in 20° increments, to 300° C. This last stage was usually done automatically. Following this, the specific partial oxidation activity was estimated both visually from the gas chromatographic analysis and by comparing the computer integrated gas chromatographic peaks. If warranted, the activity was checked at specific temperatures. Thermal restoration of activity was confirmed. The catalyst was removed from the furnace after cooling and replaced with a new catalyst to be studied. 
     Analysis was performed by a two column gas chromatographic unit. One column (molecular sieve) was used for the low boiling reactants and products (N 2 , O 2 , CH 4 , CO) while the second column (10 feet or 3.67 m of PORAPAC Q packing at temperature ranging from 50° C. at the inlet to 210° C. at the outlet) was used for high boiling products. In addition to a dual pen recorder, an online computer (EAI) integrated the sample gas chromatographic peaks. 
     The results of tests with each mixed oxide at several temperatures are summarized in Table 2. In order to explain the terms used, a detailed analysis of one experiment is shown in Table 3. In this experiment a feed of 51.3 mole % propane, 23.4 mole % oxygen and 25.3 mole % nitrogen was passed through catalyst (2) at 390° C. and the materials indicated in Table 3 were detected in the exit. The distribution by mole % is shown in the first line. It can be seen that, of the 51.3% propane fed, 20.6% was converted [(51.3-40.7)/51.3]. The moles of carbon-containing products were then normalized based upon number of carbons and compared to the converted propane to give selectivities as indicated in the second line. The methane, ethylene, ethane and propylene (or whichever of these were observed in an effluent) are together considered to be the selectivity to cracking (3+12=15%). The methanol, acetaldehyde and acrolein are together considered to be partial oxidation products (16+12+ 8=36%). Of the partial oxidation products, the proportions of each product is considered to be the specific selectivity indicated in the following lines. 
     It can be seen, by comparing Table 3 with the fourth column of Table 2, how Table 3 summarizes the experiment. Similar calculations were used for each other column in Table 2. 
     
                       TABLE 2______________________________________Oxidation of Propane______________________________________Catalyst    C1     C1     2    2    2    2    2______________________________________Temperature 350    480    365  390  425  475  490HC/O.sub.2 (x:1)       2      2      2    2    2    2    2HC Conversion             28.3 20.8 20.7 42.2 47.5(%)Selectivity toCO                        26   43   38   23   22CO.sub.2                  6    6    6    1    1Cracking                  30   15   20   62   66insignificantPart. Oxid.               30   36   36   14   11Spec. Sel. toCH.sub.3 OH               34   45   32   9    6CH.sub.2 O                --   --   --   --   --CH.sub.3 CH.sub.2 OH      --   --   --   --   --CH.sub.3 CHO              20   34   32   16   8CH.sub.3 CH.sub.2 CHO     --   --   --   21   45CH.sub.2 ═CHCHO       9    21   36   54   41HCOOH                     10   --   --   --   --CH.sub.3 CH.sub.2 COOH    30   --   --   --   --______________________________________Catalyst   3       3      3     4    4     4______________________________________Temperature °C.      445     405    375   465  430   412HC/O.sub.2 2       2      2     2    2     2HC Conversion      37      28     26    27   23    23Selectivity toCO         24      30     34    43   50    50CO.sub.2   4       5      6     6    9     8Cracking   55      40     32    26   16    14Part. Oxid.      17      25     28    25   25    28Spec, Sel. toCH.sub.3 OH      29      44     45    39   70    58CH.sub.2 O --      --     --    --   --    --CH.sub.3 CH.sub.2 OH      3       4      2     --   --    --CH.sub.3 CHO      23      27     35    16   30    42CH.sub.3 CH.sub.2 CHO      24      9      7     20   --    --CH.sub.2 ═CHCHO      21      16     11    25   --    --______________________________________Catalyst   5            5    C6         C7______________________________________Temperature °C.      475          442  not        480HC/O.sub.2 2            2    activated  2HC Conversion      34           30              33Selectivity toCO         29           31              33CO.sub.2   5            5               4Cracking   51           44              47Part. Oxid.      15           20              16Spec. Sel. toCH.sub.3 OH      36           42              55CH.sub.2 O --           --              --CH.sub.3 CH.sub.2 OH      2            2               --CH.sub.3 CHO      21           22              44CH.sub.3 CH.sub.2 CHO      27           11              14CH.sub.2 ═CHCHO      14           23              2______________________________________ 
    
     
                                           TABLE 3__________________________________________________________________________Oxidation of PropaneWith Oxide (2) at 390° C.__________________________________________________________________________Component N.sub.2      CO  CO.sub.2                CH.sub.4                      C.sub.2 H.sub.4__________________________________________________________________________Exit  25.3 13.8          2.0   0.9   1.9Stream(mole %)Selectivity --   43   6     3    12__________________________________________________________________________Component C.sub.3 H.sub.8      H.sub.2 O          CH.sub.3 OH                CH.sub.3 CHO                      CH.sub.2 ═CHCHO__________________________________________________________________________Exit  40.7 21.3          5.0   1.9   0.8Stream(mole %)Selectivity --   --  16    12     8Specific --   --  45    34    21Selectivity__________________________________________________________________________