Ammoxidation of p-cresol to p-hydroxybenzonitrile - High-performance boria-phosphoria supported catalysts

Ammoxidation of p-cresol to p-hydroxybenzonitrile (pHBN) was studied over a large number of catalysts, mainly oxides of Pi-Mo and B-P. Supported boria -phosphoria on silica displayed superior performance. Feed composition and rate had a significant effect on catalyst performance. The best performance was measured at ammonia, air and nitrogen to cresol molar ratios of 10, 40 , 120, respectively The best performance of fresh catalyst was 63 wt.% yiel d of pHBN at P:B molar ratio ranging from 5 to 8 and total oxide content on silica between 10-18 wt.%. Polymeric deposits on the catalyst generated ex tensive deactivation. Regeneration of catalyst in a mixture of air-nitrogen and steam restored initial performance. Testing of various promoters indic ated that 0.03 wt.% Pt improved both catalyst stability and regenerability with no effect on selectivity. Cycled operation of 12 h ammoxidation and 12 h regeneration over 200 h on-stream demonstrated that an average yield of 55 wt.% pHBN could be maintained. A tentative mechanism was derived based o n reaction and adsorption measurements: p-cresol and ammonia adsorbed on me dium strength acidic sites form p-hydroxybenzylamine as intermediate produc t, p-Hydroxybenzylamine reacts with oxygen to produce pHBN. Two different r outes for formation of polymeric coke were identified: direct condensation of p-cresol on acidic sites and ammonia-induced condensation of p-hydroxybe nzaldehyde formed by p-hydroxybenzylamine oxidation. (C) 2001 Elsevier Scie nce B.V. All rights reserved.