Bismuth molybdate silica supported

Oxidation in the original Sohio process941,942 was carried out over a bismuth molybdate catalyst, which was later superseded by bismuth phosphomolybdate with various amounts of additional metal ions (Ce, Co, Ni), and multicomponent metal oxides based on Mo, Fe, and Bi supported on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

Aykan (35) reported that ammoxidation of propylene occurred over a silica-supported bismuth molybdate catalyst in the absence of gas-phase oxygen, although the catalytic activity decreased rapidly with increasing catalyst reduction. The reduction process was followed by X-ray and it was found that phase changes which occurred in the catalyst and the decrease in catalytic activity corresponded quantitatively to the depletion of lattice oxygen. 

Sancier et al. (43) used oxygen-18 to examine the relative role of adsorbed versus lattice oxygen in propylene oxidation over a silica-supported bismuth molybdate catalyst as a function of temperature. At 400°C they observed the formation of predominantly acrolein[I60] rather than acrolein[I80], indicating significant participation of lattice oxygen. However, as the reaction temperature was decreased, the authors concluded that the role of adsorbed oxygen became more important. 

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... 

Sachtler (135) and Sachtler and de Boer (136) also found evidence for a symmetric intermediate from the oxidation of propylenes containing radioactive carbon over bismuth molybdate. When the 0 was at either end of the propylene molecule, half of it was found in the carbonyl group of the product acrolein. When it was the middle carbon, no was found in the carbonyl group. McCain, Gough, and Godin (137) found the same results using a bismuth phosphomolybdate supported on silica. 

Molybdate-Based Catalysts. The first catalyst commercialized by SOHIO for the propylene ammoxidation process was bismuth phosphomolybdate, Bi9PMoi2052, supported on silica (9). The catalytically active and selective component of the catalyst is bismuth molybdate. In commercial fluid-bed operation, the bismuth molybdate catalyst is supported on silica to provide hardness and attrition resistance in the fluidizing environment. Bismuth molybdate catalysts can be prepared by a coprecipitation procedure using aqueous solutions of bismuth nitrate and ammonium molybdate (10). The catal3ret is produced by drying the precipitate and heat treating the dried particles to crystallize the bismuth molybdate phase. Heat treatment temperature for bismuth molybdate catalysts is generally arovmd 500°C. 

Bismuth(lII) and molybdenum(II) acetates as mono- and homopolynuclear precursors of silica-supported bismuth molybdate catalysts... 

The original bismuth molybdate catalyst described in the early patents was supported on silica and prepared by a relatively simple procedure. 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert silica or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acrylic acid were prepared from bismuth, cobalt, iron, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic silicic acids. Preferred second-stage catatysts generally7 are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity7 and productivity7 (39,45,46). 

Bismuth cerium molybdates were prepared by coprecipitation using aqueous solutions of (NH ) Mo 02, (NH,)2Ce(N0 ), and Bi(NO ) 5H2O. The catalysts were supported on oiO (20% by weight) using an ammonium stabilized silica sol. Samples for diffraction analysis were unsupported. Samples were calcined in air at 290 and 425°C for three hours each followed by 16 hours at 500, 550, or 600 C. X-ray powder patterns were obtained using a Rlgaku D/Max-IIA X-ray diffractometer using Cu K radiation. 

---