p-Cresol autoxidation using CoAPO-5 prepared by microwave heating of the precursor gel: comparison with homogeneous and biphasic reaction schemes using cobalt salt or complex catalysts

p-Cresol autoxidation to p-hydroxybenzaldehyde was performed at 60 degreesC and atmospheric pressure, using three methods: (1) quasi-heterogeneous reaction using CoAPO-5 catalyst; (2) homogeneous reaction using CoBr2 salt; and (3) biphasic organic-aqueous interfacial reaction using cobalt complexes together with a surfactant. Cobalt was proven to be the most active/selective for the p-hydroxybenzaldehyde synthesis among the transition metal salts tested. Highly crystalline and uniform CoAPO-5 was prepared using microwave heating in 40 min from the precursor gels and characterized using XRD, SEM, and UV-VIS spectroscopy. p-Cresol conversion and selectivity to p-hydroxybenzaldehyde increase with the cobalt contents and improved as the Co/p-cresol (mg/g) ratio increases to ca. 1.0. The oxygen selectivity was 82-85% for CoAPO-5, which is slightly higher than obtained with CoBr2. The highly alkaline solution employed for the reaction, however, resulted in serious metal leaching. The biphasic reaction attempted for p-cresol autoxidation resulted in poor performance due to the limits in solvent selection. The performance of p-cresol autoxidation reaction was strongly governed by the solvent and followed the order methanol > ethanol > butanol, all miscible with water. Hardly any reaction took place in non-polar solvents such as chloroform or toluene, or in 2-ethyl-1-hexanol. (C) 2002 Elsevier Science B.V. All rights reserved.