Multicomponent molybdate

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

Figure 10.8 The MOO line. Mild oxidation and ODH of C2 to olefins and aromatics. Linear correlation between A1 and A of catalysts. Several catalysts are represented by their A value for the same reaction (BUTD = butadiene, (M)ACO = (metha)crolein, ACRY = acrylic acid, MAA = methacrylic acid, MaA = maleic anhydride Epox = epoxide, Acet = acetaldehyde, MCM = multicomponent molybdates)...

Hur et al. (252,277,278) reported the use of alkali metal-doped MgO to catalyze the synthesis of acrylonitrile and propionitrile (278). Acrylonitrile is an important chemical, especially in the polymer industry it is generally synthesized by the ammoxidation of propene catalyzed by multicomponent bismuth molybdates (279). An alternative method of synthesis of acrylonitrile is the reaction between methanol and acetonitrile (Scheme 42). 

Synergy Effect of Multicomponent Co, Fe, and Bi Molybdates in Propene Partial Oxidation... 

Light hydrocarbons consisting of oxygen or other heteroatoms are important intermediates in the chemical industry. Selective hydrocarbon oxidation of alkenes progressed dramatically with the discovery of bismuth molybdate mixed-metal-oxide catalysts because of their high selectivity and activity (>90%). These now form the basis of very important commercial multicomponent catalysts (which may contain mixed metal oxides) for the oxidation of propylene to acrolein and ammoxidation with ammonia to acrylonitrile and to propylene oxide. 

Erom HRTEM studies, it is proposed that the majority of the bismuth molybdate phases can be derived from the fluorite structure, in which both the cation and anion vacancies can be accommodated within the fluorite framework (Buttrey et al 1987). Several industrial processes containing multicomponent bismuth molybdates may suffer loss of Mo oxides by volatilization under operating conditions, resulting in the loss of catalytic activity. Monitoring the catalyst microstructure using EM is therefore crucial to ensuring the continuity of these catalytic processes. 

Acrolein and Acrylic Acid. Acrolein and acrylic acid are manufactured by the direct catalytic air oxidation of propylene. In a related process called ammoxida-tion, heterogeneous oxidation of propylene by oxygen in the presence of ammonia yields acrylonitrile (see Section 9.5.3). Similar catalysts based mainly on metal oxides of Mo and Sb are used in all three transformations. A wide array of single-phase systems such as bismuth molybdate or uranyl antimonate and multicomponent catalysts, such as iron oxide-antimony oxide or bismuth oxide-molybdenum oxide with other metal ions (Ce, Co, Ni), may be employed.939 The first commercial process to produce acrolein through the oxidation of propylene, however, was developed by Shell applying cuprous oxide on Si-C catalyst in the presence of I2 promoter. 

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... 

During the history of a half century from the first discovery of the reaction (/) and 35 years after the industrialization (2-4), these catalytic reactions, so-called allylic oxidations of lower olefins (Table I), have been improved year by year. Drastic changes have been introduced to the catalyst composition and preparation as well as to the reaction process. As a result, the total yield of acrylic acid from propylene reaches more than 90% under industrial conditions and the single pass yield of acrylonitrile also exceeds 80% in the commercial plants. The practical catalysts employed in the commercial plants consist of complicated multicomponent metal oxide systems including bismuth molybdate or iron antimonate as the main component. These modern catalyst systems show much higher activity and selectivity... 

Multicomponent bismuth molybdate catalyst. h Heteropoly compound. 

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. 

Our recent studies have been undertaken to clarify the working mechanism of the multicomponent bismuth molybdate systems. Apart from the re-... 

II. Characterization and Working Mechanism of Multicomponent Bismuth Molybdate Catalysts... 

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

One typical way to improve the catalyst system was directed at the multi-component bismuth molybdate catalyst having scheelite structure (85), where metal cations other than molybdenum and bismuth usually have ionic radii larger than 0.9 A. It is important that the a-phase of bismuth molybdate has a distorted scheelite structure. Thus, metal molybdates of third and fourth metal elements having scheelite structure easily form mixed-metal scheelite crystals or solid solution with the a-phase of bismuth molybdates. Thus, the catalyst structure of the scheelite-type multicomponent bismuth molybdate is rather simple and composed of a single phase or double phases including many lattice vacancies. On the other hand, another type of multi-component bismuth molybdate is composed mainly of the metal cation additives having ionic radii smaller than 0.8 A. Different from the scheelite-type multicomponent bismuth molybdates, the latter catalyst system is never composed of a simple phase but is made up of many kinds of different crys-... 

Multicomponent Bismuth Molybdate Catalysts with Scheelite Structure... 

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