Propane ammoxidation over gallium-modified MFI zeolites

This is the first report indicating that the presence of Bronsted sites, ac ting 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 equ al to 33 and 150, respectively. The integrity of the zeolites was essential ly preserved following ion-exchange, as demonstrated by surface area, poros ity, and crystallinity measurements. In situ C-13 and N-15 MAS NMR studies, using propane-2-C-13 and (NH3)-N-15 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 (C3H8:NH3: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 COx (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 rat io NH3:C3H8 from 2 to 3 increases the selectivity to acrylonitrile and decr eases conversion. Both the conversion and the selectivity to acrylonitrile increase when the molar ratio O-2:C3H8 is increased from 0.3 (sub-stoichiom etric) to 2 (stoichiometric). A new reaction pathways is proposed in which a cyclic protonated pseudo-cyclopropanr (PPCP) transition state is responsi ble for the low temperature activation of propane. (C) 2001 Elsevier Scienc e B.V. All rights reserved.