Bismuth molybdate catalyst propylene

In 1957 Standard Oil of Ohio (Sohio) discovered bismuth molybdate catalysts capable of producing high yields of acrolein at high propylene conversions (>90%) and at low pressures (12). Over the next 30 years much industrial and academic research and development was devoted to improving these catalysts, which are used in the production processes for acrolein, acryUc acid, and acrylonitrile. AH commercial acrolein manufacturing processes known today are based on propylene oxidation and use bismuth molybdate based catalysts. 

Many key improvements and enhancements to the bismuth molybdate based propylene oxidation catalysts have occurred over the past thirty years. These are outlined in the following tabulation. 

Fig. 2. Mechanism of selective ammoxidation and oxidation of propylene over bismuth molybdate catalysts. (31).

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

The following data given in Tables 16.15, 16.16 and 16.17 on the oxidation of propylene over bismuth molybdate catalyst were obtained at three temperatures, 350,375, and 390°C (Watts, 1994). 

Figure 11.26 Plot of the position sensitivity of the degree of conversion for a set of 48 bismuth-molybdate catalysts (same batch) in propylene to acrolein conversion in a Stage II 48-fold-screening reactor (reaction conditions 2% hydrocarbon in air at GHSV of 3000 h-1, column no. 8 contains only inert carrier material).

Acrolein Production. Adams et al. [/. Catalysis, 3,379 (1964)] studied the catalytic oxidation of propylene on bismuth molybdate catalyst to form acrolein. With a feed of propylene and oxygen and reaction at 460°C, the following three reactions occur. 

Fig. I. Mechanism of selective oxidation of propylene to acrolein over bismuth molybdate catalyst by Burrington et al. (19).

In spite of the accumulated mechanistic investigations, it still seems difficult to explain why multicomponent bismuth molybdate catalysts show much better performances in both the oxidation and the ammoxidation of propylene and isobutylene. The catalytic activity has been increased almost 100 times compared to the simple binary oxide catalysts to result in the lowering of the reaction temperatures 60 80°C. The selectivities to the partially oxidized products have been also improved remarkably, corresponding to the improvements of the catalyst composition and reaction conditions. The reaction mechanism shown in Figs. 1 and 2 have been partly examined on the multicomponent bismuth molybdate catalysts. However, there has been no evidence to suggest different mechanisms on the multicomponent bismuth molybdate catalysts. 

Catalytic oxidation of propylene to acrolein was first discovered by the Shell group in 1948 on Cu20 catalyst (/). Both oxidation and ammoxidation were industrialized by the epoch-making discovery of bismuth molybdate catalyst by SOHIO (2-4). The bismuth molybdate catalyst was first reported in the form of a heteropoly compound supported on Si02, Bi P,Mo,2052/Si02 having Keggin structure but it was not the sole active species for the reactions. Several kinds of binary oxides between molybdenum trioxide and bismuth oxide have been known, as shown in the phase... 

In the 1960s, a number of binary oxides, including molybdenum, tellurium, and antimony, were found to be active for the reactions and some of them were actually used in commercial reactors. Typical commercial catalysts are Fe-Sb-O by Nitto Chemical Ind. Co. (62 -64) and U-Sb-O by SOHIO (65-67), and the former is still industrially used for the ammoxidation of propylene after repeated improvements. Several investigations were reported for the iron-antimony (68-72) and antimony-uranium oxide catalysts (73-75), but more investigations were directed at the bismuth molybdate catalysts. The accumulated investigations for these simple binary oxide catalysts are summarized in the preceding reviews (5-8). 

Typical Reaction Conditions for the Oxidation and Ammoxidation of Propylene on the Simple and Multicomponent Bismuth Molybdate Catalyst°... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

Choosing divalent and trivalent cations and determining the composition is the most important in designing the multicomponent bismuth molybdate catalyst system. Catalytic activities of typical tri- and tetracomponent bismuth molybdate catalysts having multiphase structure were reported for the oxidation of propylene to form acrolein (35, 36, 40-43, 97, 98). A typical example of the activity test is shown in Fig. 6. Summarizing the results shown in Fig. 6 and reported previously (30, 43, 44), the following trends are generally found. 

Fig. 6. Catalytic activity of multicomponent bismuth molybdate catalysts in the oxidation of propylene (41).

Oxidation of propylene to form acrolein depends on the first order of propylene and is independent of oxygen on multicomponent bismuth molybdate catalysts under the usual reaction conditions. The observed kinetics is the same with simple bismuth molybdates and suggests that the oxidation of propylene proceeds via the similar reaction scheme reported for simple molybdates, the slow step being the abstraction of allylic hydrogen (9-15, 19, 20). However, the reaction sometimes depends on the partial pressure of oxygen under lower temperature and lower oxygen pressure (41, 42). 

Fig. 9. Arrhenius plots of the oxidation of propylene over various multicomponent bismuth molybdate catalysts.

Incorporation of lattice oxide ion into the oxidized products has been systematically investigated in the oxidation of propylene to acrolein using 99.1% 1802 tracer on a series of tri- and tetracomponent bismuth molybdate catalysts in our laboratory (35, 36, 40-42). 

Fig. 14. The selectivity forming acrolein in the oxidation of propylene over the supported bismuth molybdate catalysts (52). (O) Bi2Mo3Oi2/CoMo04 ( )...

The industrial process for the preparation of acrylonitrile from propylene and ammonia uses a bismuth molybdate catalyst (equation 91). 

Heterogeneous oxidative processes operate at high temperatures (250-450 6C) and are useful for the synthesis of acrolein and acrylic acid from propylene over bismuth molybdate catalysts, the synthesis of maleic and phthalic anhydrides from the oxidation of benzene (or C4 compounds) and naphthalene (or o-xylene) respectively over vanadium oxide,101 arid the synthesis of ethylene oxide from ethylene over silver catalysts.102... 

In 1959, Idol (2), and in 1962, Callahan et al. (2) reported that bismuth/molybdenum catalysts produced acrolein from propylene in higher yields than that obtained in the cuprous oxide system. The authors also found that the bismuth/molybdenum catalysts produced butadiene from butene and, probably more importantly, observed that a mixture of propylene, ammonia, and air yielded acrylonitrile. The bismuth/molybdenum catalysts now more commonly known as bismuth molybdate catalysts were brought to commercial realization by the Standard Oil of Ohio Company (SOHIO), and the vapor-phase oxidation and ammoxidation processes which they developed are now utilized worldwide. 

Aykan (35) reported that ammoxidation of propylene occurred over a silica-supported bismuth molybdate catalyst in the absence of gas-phase oxygen, although the catalytic activity decreased rapidly with increasing catalyst reduction. The reduction process was followed by X-ray and it was found that phase changes which occurred in the catalyst and the decrease in catalytic activity corresponded quantitatively to the depletion of lattice oxygen. 

Peacock et al. (37) showed that a bismuth molybdate catalyst can be reduced with propylene and that the oxygen appearing in the gaseous products (acrolein, carbon dioxide, and water) can be quantitatively replaced in the lattice. The amount of oxygen removed during reduction corresponds to the participation of many sublayers of oxide ions. 

Sancier et al. (43) used oxygen-18 to examine the relative role of adsorbed versus lattice oxygen in propylene oxidation over a silica-supported bismuth molybdate catalyst as a function of temperature. At 400°C they observed the formation of predominantly acrolein[I60] rather than acrolein[I80], indicating significant participation of lattice oxygen. However, as the reaction temperature was decreased, the authors concluded that the role of adsorbed oxygen became more important. 

Christie et al. (45) and Pendleton and Taylor (46) have recently reported the results of propylene oxidation over bismuth molybdate and mixed oxides of tin and antimony and of uranium and antimony in the presence of gas-phase oxygen-18. Their work indicated that for each catalyst, the lattice was the only direct source of the oxygen in acrolein and that lattice and/or gas-phase oxygen is used in carbon dioxide formation. The oxygen anion mobility appeared to be greater in the bismuth molybdate catalyst than in the other two. 

As a catalyst for propylene oxidation, Bi203 itself has fairly low activity and yields primarily the products of complete oxidation. Pure molybdenum trioxide has an even lower activity, but is fairly selective. In combination, however, remarkable activity and selectivity for propylene oxidation is obtained. Although industrial catalysts contain silica and phosphate as well as Bi203 and Mo03, many fundamental studies have employed catalysts containing only bismuth and molybdenum oxides in an attempt to determine the structure of the catalytically active phase. As a result of such studies, it is now known that bismuth molybdate catalysts display their superior properties only if the catalyst composition lies within the composition range of Bi/Mo = f to f (atomic ratio). 

The establishment of the structures and thermal transformations of the catalytically active phases of bismuth molybdate resulted in research directed toward investigating the stability of the structure under reducing conditions. Fattore et al. (38) investigated an unsupported bismuth molybdate catalyst with composition Bi2032.66M0O3 during propylene... 

Silica supported bismuth molybdate catalysts were examined by Dalin et al. (85) during extended steady-state ammoxidation of propylene. X-Ray diffraction measurements indicated that a significant alteration in the phase composition of supported catalysts occurred during operation. The a and /3 phases of bismuth molybdate, which were originally present, transformed into the y phase of bismuth molybdate as described by the equation... 

An alumina supported bismuth molybdate catalyst with a bismuth to molybdenum atomic ratio equal to one was examined by high-temperature X-ray diffraction techniques during propylene oxidation (86). Ac-... 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

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