VAPOR-PHASE BECKMANN REARRANGEMENT OVER SILICA-SUPPORTED TANTALUM OXIDE CATALYSTS

Silica-supported tantalum oxide catalysts (Ta oxide/SiO2 (ALK)), prepa red by chemical reaction between tantalum alkoxide and surface hydroxy l groups of SiO2, exhibited high catalytic efficiency for the vapor ph ase Beckmann rearrangement of cyclohexanone oxime to caprolactam. The oxime conversion reached 96.5%, and the selectivity for caprolactam wa s 97.5% at 573 K. The effects of precursors of tantalum oxide, physica l properties of catalyst carriers, and preparation conditions were stu died, and the factors in catalytic efficiency were discussed. SiO2 was the best carrier for the reaction among SiO2, Al2O3, TiO2, MgO, and S iO2-Al2O3. The pore structure of silica carriers, especially the high ratio (>90%) of the pore volume for fine pores with diameters from 4 t o 200 nm to total pore volume was effective. Ta2O5/SiO2 (IMP) catalyst , prepared by impregnation of silica with an aqueous TaCl5 solution an d hydrated tantalum oxide (Ta2O5 . nH2O) were less active and less sel ective. A large number of acid sites of intermediate strength, measure d by NH3-TPD method, existed on Ta2O5/SiO2 (ALK) and were effective fo r the Beckmann rearrangement. On the contrary, strong acid sites appea red on Ta2O5/SiO2 (IMP). The difference in the acid property and catal ytic performance between Ta2O5/SiO2 (ALK) and Ta2O5/SiO2 (IMP) was ass umed to be related to the difference in morphology of tantalum oxide t hat was observed by SEM and EPMA. Good initial activity and high selec tivity for caprolactam were observed over boron oxide supported on alu mina catalyst. However, the oxime conversion declined significantly. A lthough the NH3-TPD profile of B2O3/Al2O3 catalyst was similar to that of Ta2O5/SiO2 (ALK), tantalum oxide of Ta2O5/SiO2 (ALK) was more stab le than boron oxide of B2O3/Al2O3 in the course of reaction. The react ion between tantalum alkoxide vapor and the surface hydroxyl groups of SiO2 was observed by FTIR diffuse reflectance spectrometer. A new hyd roxyl group appeared on Ta2O5/SiO2 (ALK) catalyst. The unique acid pro perty and catalytic performance of Ta2O5/SiO2 (ALK) were discussed in connection with the new hydroxyl group. (C) 1994 Academic Press, Inc.