Propylene Oxidation and Ammoxidation

The oxidation and ammoxidation of propylene (H2CHC = CH3) to acrolein (H2CHC = CHO) and acrylonitrile (H2CHC = CN) respectively, constitute major chemical intermediates. Conversion of propylene to acrolein is conducted in an oxidising environment, while conversion of propylene to acrylonitrile requires both molecular O2 and NH3. The industrial catalyst employed for these oxi-dation/ammoxidation reactions is a bulk Bi-Mo-0 mixed oxide. The alpha-Bi2 (M0O4) 3 phase is one of the most active phases and its Raman spectrum is shown in Fig. 17.7. An operando Raman-GC spectroscopy investigation of propylene 

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 

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N-allyl species and two hydrogen abstractions account for acrylonitrile formation. Thus, although allyl radicals are probably not the selective intermediate in propylene oxidation and ammoxidation, they can form acrolein or acrylonitrile via these selective O- or N-allyl intermediates. 

Burrington, J. D., Kartisek, T, and Grasselli, R. Surface intermediates in selective propylene oxidation and ammoxidation over heterogeneous molybdate and anti-monate catalysts. J Cato/ 87, 363-380 (1984). 

Other important uses of stannic oxide are as a putty powder for polishing marble, granite, glass, and plastic lenses and as a catalyst. The most widely used heterogeneous tin catalysts are those based on binary oxide systems with stannic oxide for use in organic oxidation reactions. The tin—antimony oxide system is particularly selective in the oxidation and ammoxidation of propylene to acrolein, acryHc acid, and acrylonitrile. Research has been conducted for many years on the catalytic properties of stannic oxide and its effectiveness in catalyzing the oxidation of carbon monoxide at below 150°C has been described (25). 

Ammoxidation of propylene is considered under oxidation reactions because it is thought that a common allylic intermediate is formed in both the oxidation and ammoxidation of propylene to acrolein and to acrylonitrile, respectively. 

Among the oxide catalysts, bismuth molybdates that catalyse selective oxidation and ammoxidation of propylene to yield acrolein and acrylonitrile have received considerable attention (Grasselli Burrington, 1981) ... 

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis Robert K. Grasselli and James D. Burrington... 

Catalytic oxidation and ammoxidation of lower olefins to produce a,/3-unsaturated aldehyde or nitrile are widely industrialized as the fundamental unit process of petrochemistry. Propylene is oxidized to acrolein, most of which is further oxidized to acrylic acid. Recently, the reaction was extended to isobutylene to form methacrylic acid via methacrolein. Ammoxidation of propylene to produce acrylonitrile has also grown into a worldwide industry. 

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... 

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. 

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. 

The creation of selective catalysts for such complex reactions seems to be an especially difficult problem. Nevertheless, surprisingly, selective catalysts have been developed for complex reactions, which can be exemplified by the oxidation and ammoxidation of propylene, oxidation of butene and even butane to maleic anhydride (which requires seven oxygen atoms). Such reactions are usually performed over V and Mo oxide systems [4, 6, 8-10]. High selectivity of these systems is presumably provided by a special structure of the catalyst surface that allows control... 

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis Robert K. Grasselli and James D. Burrington Mechanism of Hydrocarbon Synthesis over Fischer-Tropsch Catalysts P. Biloen and W. M. H. Sachtler Surface Reactions and Selectivity in Electrocatalysis... 

Tin(IV) oxide is used in various heterogeneous catalyst mixtures, e.g. Sn02/V20s for oxidation of arenes to carboxylic acids and anhydrides, and Sn02/Sb20s for selective oxidation and ammoxidation of propylene to acrolein, acrylic acid, and acrylonitrile. 

Bismuth Molybdate Catalysts. The Raman spectra of the bismuth molybdates, with Bi/Mo stoichiometric ratios between 0.67 and 14, have been examined using the FLS approach (see Section 3.2). " The bismuth molybdates fall into an unusual class of compounds, the ternary bismuth oxide systems Bi-M-0 (where M = Mo, W, V, Nb, and Ta) which exhibit a variety of interesting physical and chemical properties. Of commercial importance, the bismuth molybdates are heterogeneous catalysts for selective oxidations and ammoxidations (the Sohio process), for example, propylene ( 311 ) to acrolein (C3H4O) by oxidation or to acrylonitrile (C3H3N) by arrunoxidation. ... 

Selective Oxidation and Ammoxidation of Propylene by Heterogeneous Catalysis... 

All selective oxidation and ammoxidation catalysts possess redox properties. They must be capable not only of reduction during the formation of acrolein or acrylonitrile, but also subsequent catalyst reoxidation in which gaseous oxygen becomes incorporated into the lattice as to replenish catalyst vacancies (Scheme 2). As mentioned earlier, the incorporation of such redox properties into solid state metal oxides was one of the salient working hypotheses on which the development of the Sohio ammoxidation process was based (2). Later, Keulks (70) confirmed the involvement of lattice oxygen in propylene oxidation by using as a vapor phase oxidant. The results showed that the incorporation of O into the acrolein (and CO2) increases with time (Fig. 11), which is consistent with the above redox mechanism. 

SELECTIVE OXIDATION AND AMMOXIDATION OF PROPYLENE ACID SITE ... 

Scheme 11. Mechanism for selective oxidation and ammoxidation of propylene over bismuth molybdate. From (27).

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