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 Bi-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 63wt.% yield of pHBN at P:B molar ratio ranging from 5 to 8 and total oxide content on silica between 10-18wt.%. Polymeric deposits on the catalyst generated extensive deactivation. Regeneration of catalyst in a mixture of air-nitrogen and steam restored initial performance. Testing of various promoters indicated that 0.03wt.% 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 55wt.% pHBN could be maintained. A tentative mechanism was derived based on reaction and adsorption measurements: p-cresol and ammonia adsorbed on medium strength acidic sites form p-hydroxybenzylamine as intermediate product. p-Hydroxybenzylamine reacts with oxygen to produce pHBN. Two different routes for formation of polymeric coke were identified: direct condensation of p-cresol on acidic sites and ammonia-induced condensation of p-hydroxybenzaldehyde formed by p-hydroxybenzylamine oxidation.