Surface molybdate catalyst, mechanism

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... 

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. 

The mechanism for the oxidation of methanol to formaldehyde on iron molybdate catalysts (illustrated in Figure 4) envisages the H-abstraction and electron transfer of surface methoxy species desorption of the products and reoxidation of metal sites complete the cycle. 

The results from the infrared studies and from the GC analysis show that the reaction of methane with the ferric molybdate catalysts gives methanol, formaJdehyde, carbon dioxide, and carbon monoxide as final products. The IR spectra also indicate the formation of methoxy, surface dioxymethylene, surface formate species, and adsorbed formaldehyde. Based on these observations, a mechanism was proposed to account for all intermediates and final products and is shown in Figure 5. Since the surface structure of the catalysts is not known, the surface is represented by a straight line in the scheme. 

Using in situ FT-IR spectroscopy, the gas phase products and the principal intermediates involved in the catalytic conversion of methane over ferric molybdate catalysts were identified and the reaction mechanism was proposed. In the absence of an oxidizing agent, methane reacts with the oxygen of the catalyst to produce methoxy species, which is an important intermediate for methanol formation. Further oxidation of the methoxy groups results in the formation of surface dioxymethylene, adsorbed formaldehyde, and surface formate species. The decomposition of surface dioxymethylene and surface formate species will give carbon oxides and hydrogen. 

It has generally been assumed that the outermost surfaces of bulk bismuth-molybdate catalysts are just an extension of one of its bulk crystal planes. According to this model, the mechanism has been proposed that molecular O2 dissociatively chemisorbs on the surface bismuth sites, where it becomes incorporated into the bulk lattice and the propylene chemisorbs and reacts on the surface Mo sites where it is oxidised by oxygen supplied from the bismuth-molybdate bulk lattice. The exchange of gas-phase molecular O2 with lattice O in bismuth-molybdate catalysts has been confirmed with Raman studies using isotopically labeled 02. Recent LEIS spectroscopy analyses of the outermost surface of bismuth-molybdate, however, revealed that its surface is enriched in Mo sites and that instead of Bi, the... 

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

The structural coherence of these two phases allows for facile migration of lattice oxygen. Since the Fe-Co-Mo-0 phase is not selective for the propylene (amm)oxidation reaction, the promoting effect of the phase must be a result of its specialized function of reoxidizing the catalytically active Bi-Mo-0 phase. The criticality of specialization of functions in the complex Fe-Co-Bi-Mo-0 catalyst is further manifested in bismuth molybdate, which lacks the redox capability of Fe-Co-Mo-0 but which uniquely carries out the required steps of the surface reaction mechanism of selective alkene (amm)oxidation (see below). 

---