Oxidation bismuth molybdate catalyst

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. 

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). 

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. 

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on silica. 

Multicomponent Bismuth Molybdate Catalyst A Highly Functionalized Catalyst System for the Selective Oxidation of Olefin... 

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. 11. Comparison of the amount of l6Oi2a7,ice incorporated into the oxidation products over various multicomponent bismuth molybdate catalysts. Open columns, amount of whole 16Oia iCC in the catalyst shaded columns, amount of l6Oiaurce in the Bi2MoiO,2 phase solid columns, total amount of l60 incorporated into the oxidation products , oxygen conversion was 80% and the others were 60%. (a) Tricomponent system, Mo-Bi-M(II)-0, and tetra-component system, Mo-Bi-M(Il)-M (II)-) without M(III). (b) Tetracomponent system, Mo-Bi-M(II)-M(III)-0 (41).

Interestingly, the replacement of a part of Co2+ in the Mo BiiConO, by another divalent cation, Ni2+ or Mg2+, does not improve the degree of participation of lattice oxide ion at all and only the lattice oxide ion in the bismuth molybdate phase is active in the MonBiiCosNijO or MonBhCoitMgjO, catalyst. It is noteworthy that the situation mentioned above corresponds exactly to the catalytic activity of the tri- and tetracomponent bismuth molybdate catalysts. 

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

III. Stability of the Multicomponent Bismuth Molybdate Catalyst Depending on the Bulk Diffusion of Oxide Ion... 

The kinetics of the parallel formation of by-products is generally similar to the kinetics of the main reaction (i.e. rate independent of the oxygen pressure and first order with respect to propene), which presumes identical reaction steps. A study of the origin of acetaldehyde and formaldehyde carried out by Gorshkov et al. [144] is of interest in this connection. 14C-Labelled propene was oxidized over a bismuth molybdate catalyst at 460° C the results showed that acrolein, acetaldehyde and formaldehyde are formed via a symmetrical intermediate, presumably one and the same intermediate. The study, moreover, shows that acetaldehyde is exclusively formed from this intermediate, while formaldehyde may also be formed from the aldehyde group of acrolein and the methyl group of acetaldehyde. 

The hypothesis of a bifunctional mechanism involving allyl radical formation and oxygen incorporation on distinct sites is advocated by Haber et al. [147,152], This hypothesis is particularly based on experiments with Mo03, Bi203 and mechanical mixtures of these oxides, which are compared with bismuth molybdate catalysts. The reaction was carried out in cyclic operation (alternating feeds of oxygen and of propene diluted with nitrogen). The results are collected in Table 5. The authors con-... 

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