Propane ammoxidation over gallium-modified MFI zeolites

Abstract

This is the first report indicating that the presence of Brønsted sites, acting in synergy with redox sites in Ga-modified MFI zeolites, can catalyse the ammoxidation of propane. Two catalysts with gallium loadings of 2 and 0.3 wt.% were prepared by ion-exchange of MFI zeolites with Si/Al ratios equal to 33 and 150, respectively. The integrity of the zeolites was essentially preserved following ion-exchange, as demonstrated by surface area, porosity, and crystallinity measurements. In situ 13C and 15N MAS NMR studies, using propane-2- 13C and 15NH 3 as labelled reactants suggest that Ga-modified H-MFI catalysts promote the formation of C–N bonds when propane is reacted in the presence of ammonia and oxygen (C 3H 8:NH 3:O 2 molar ratio=1 :3 :2 , T=673 K ). Conventional microcatalytic reactor results confirm the conclusions of the in situ MAS NMR investigations. Propane is converted to mainly propene and acrylonitrile, with minimal production of CO x (CO and CO 2), in the temperature range 723–773 K. Increasing the temperature to 773 K worsens the selectivity to acrylonitrile, whereas increasing the molar ratio NH 3:C 3H 8 from 2 to 4 increases the selectivity to acrylonitrile and decreases conversion. Both the conversion and the selectivity to acrylonitrile increase when the molar ratio O 2:C 3H 8 is increased from 0.3 (sub-stoichiometric) to 2 (stoichiometric). A new reaction pathways is proposed in which a cyclic protonated pseudo-cyclopropane (PPCP) transition state is responsible for the low temperature activation of propane.