Bismuth molybdates transformations

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. 

Redox Processes and Solid State Transformations In Bismuth Molybdates... 

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. 

The y phase of bismuth molybdate underwent a reversible transformation to the metastable tetragonal y" modification. This metastable modification was observed in the temperature range of 520° to 550°C and underwent an irreversible transformation to the y modification which readily formed at 700°C. The results indicated that the y modification corresponds to that reported by Blasse (83). However, refinement of the crystal data utilizing a single crystal revealed that this y modification was orthorhombic with lattice parameters a = 15.99 A, b = 15.92 A, and c = 17.43 A. An additional observation was the reversible transformation of the y modification to y at 900°C. 

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... 

The combination of these experimental findings indicate that active bismuth molybdate catalysts undergo phase transformations when exposed to reducing conditions similar to the conditions of catalysis. The phase transformations are highly dependent upon both temperature and the severity of the reducing atmosphere. However, the occurrence of solid state reactions in the catalysts suggests that the bulk structure of the catalysts plays an important role in catalytic reaction. 

LoJacono et al. (108) also utilized X-ray diffraction methods to study the structural and phase transformations which occurred in the Bi-Fe-Mo oxide system. They detected two ternary compounds containing bismuth, molybdenum, and iron. One of the compounds formed when the atomic ratio Bi/Fe/Mo = 1 1 1 the other formed when the atomic ratio Bi/Fe/Mo = 3 1 2. The X-ray data indicated a close structural relationship of the bismuth iron molybdate compounds with the scheelite structure of a-phase bismuth molybdate. Moreover, their structures were similar to compound X. The structure of the Bi/Fe/Mo = 3 1 2 compound was identical to the compound reported by Sleight and Jeitschko (107). The authors proposed that the structures of both of the compounds could be viewed as resulting from the substitution of Fe3+ in the a-phase lattice. In the Bi/Fe/Mo = 1 1 1 compound, 1 Mo6+ ion is replaced by 2 Fe3+ ions one Fe3+ ion occupies a Mo6+ site the other Fe3+ ion occupies one of the vacant bismuth sites. In the Bi/Fe/Mo = 3 1 2 compound, the Fe3+ ion replaces one Mo6+ ion while the additional Bi3+ ion occupies one of the vacant bismuth sites. 

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