Highly active and selective Nb modified MCM-41 catalysts for Beckmann rearrangement of cyclohexanone oxime to ɛ-caprolactam

Abstract

The niobium incorporated MCM-41 molecular sieves (Si/Nb-16, 32, 64 and 128) have been synthesized by hydrothermal method using tetradecyltrimethyl ammonium bromide as templates in the absence of auxiliary organics. A combination of various physical techniques was used for characterizing the catalysts, such as X-ray diffraction, N2 physisorption, diffuse reflectance UV–Vis (DR–UV–Vis), Fourier transmission infra red (FT-IR), atomic emission spectroscopy (ICP-AES), and 29Si MAS NMR (magic-angle spinning nuclear magnetic resonance) spectroscopy. Catalyst acidity was measured by (ammonia temperature programmed desorption) NH3-TPD and pyridine FT-IR techniques. The characterization results of DR–UV–Vis, FT-IR, and Si MAS NMR techniques confirm that Nb atoms are isolated and tetrahedrally coordinated in the framework of MCM-41. All the materials exhibit hexagonal arrangement of uniform mesopores. Structural characterization results revealed that two types of Nb oxide species are present, one is framework, and second one is surface type Nb species in all Nb-MCM-41 catalysts. Catalyst samples with higher loading of Nb have shown very good activity. The optimum temperature for this Beckmann rearrangement is 300 °C. The cyclohexanone oxime conversion was about 100% and a caprolactam selectivity of about 95% and more could be achieved. The effect of Si/Nb ratio, temperature, space velocity, etc., on catalyst activity, product selectivity and catalyst stability were discussed. The cyclohexanone oxime conversion was dependent on the temperature and space velocity. The activity at 300 °C for the various catalysts followed the order Nb-MCM-41(Si/Nb-16)>Nb-MCM-41(Si/Nb-32)>Nb-MCM-41(Si/Nb-64)>Nb-MCM-41(Si/Nb-128).