Gas phase Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam over mesoporous, microporous and amorphous Nb2O5/silica catalysts: A comparative study

The gas phase Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam was investigated over niobium containing mesoporous (Nb-MCM-41, Nb-HMS and Nb-SBA-15), microporous (Nb-Beta zeolite) and amorphous (Nb impregnated on SiO2) catalysts. A relation between catalytic activities, structural and the acidic properties was derived. The catalysts Nb2O5/SiO2, Nb-MCM-41, Nb-SBA-15, and Nb-HMS resulted in higher cyclohexanone oxime conversion and caprolactam selectivity compared with Nb-Beta zeolite. Almost 100% cyclohexanone oxime conversion and up to 98% caprolactam selectivity were achieved over Nb2O5/SiO2 catalyst. Further, the catalytic activity results of Nb-MCM-41 are fairly similar to Nb2O5/SiO2. The high activity of both catalysts is related to vicinal silanol groups and silanol nests with weak acidity and negligible Brønsted acid sites. The physico-chemical properties of the investigated catalysts were characterized by various techniques including X-ray diffraction, N2 physisorption, Fourier transform infrared spectroscopy (FTIR), UV–vis diffuse reflectance, atomic emission spectroscopy (ICP-AES), and Raman spectroscopy. Catalyst acidity was measured by NH3-TPD and pyridine adsorbed FTIR spectroscopy. The structural characterization results revealed that Nb was incorporated into the mesoporous and microporous materials framework.