New non-zeolitic Nb-based catalysts for the gas-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam

Abstract

Niobium oxide (Nb2O5) supported on different oxides were investigated as catalysts for the gas-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam. Niobium supported on SiO2, SiO2Al2O3, TiO2 and ZrO2 were prepared by the incipient wetness impregnation method. The physico-chemical properties of all the catalysts were characterized by various techniques including X-ray diffraction (XRD), N2 physisorption, atomic emission spectroscopy (ICP–AES), scanning electron microscope (SEM), Fourier transmission infra red (FTIR) and Raman spectroscopy. The acidity of the catalysts was determined by measurements using NH3-TPD and FTIR of adsorbed pyridine. All these techniques revealed that niobium was well dispersed on the surface. The addition of niobium oxide on the SiO2 support resulted in very weakly up to medium acid sites, but only strongly reduced or almost negligible Brønsted acidity was identified. A cyclohexanone oxime conversion of about 100% and up to 98% caprolactam selectivities were achieved in the presence of Nb2O5/SiO2 catalyst depending on the reaction conditions. The catalytic performance of the different catalysts follow the order Nb2O5/SiO2>Nb2O5/SiO2Al2O3>Nb2O5/ZrO2>Nb2O5/TiO2. The effects of temperature, pressure, and solvents have been investigated. Based on these investigations, it can be conclude that in a comparison with the very expensive Nb loaded mesoporous MCM-41 catalyst, which is suitable for Beckmann rearrangement, can be replaced efficiently by very cheaply available Nb2O5-silica gel catalyst.