Autoxidation of p-cresol to p-hydroxy benzaldehyde using CoCl 2, CoAPO-5 and CoAPO-11

Abstract

CoCl 2, CoAPO-5 and CoAPO-11 are active and selective catalysts for the autoxidation of p-cresol to p-hydroxy benzaldehyde ( p-HBA) in methanolic sodium hydroxide solution at 313–353 K. The yield of p-HBA is 70–75% at approximately 95% conversion, by-products being p-hydroxy benzylalcohol ( p-HBAlc; 5%) and p-hydroxy benzyl methylether ( p-HBME; 10%) for the reactions catalysed by CoAPO-5 and CoAPO-11. Using CoCl 2 a selectivity for p-HBA of 80% can be obtained with p-HBME (10%) and p-HBAlc (5%) as by-products. Some oligomers/polymers are formed. However, these could not be determined by HPLC analysis. Oxidation of p-HBAlc to p-HBA is observed for all three catalyst systems at these low reaction temperatures. The oxygen selectivity, defined as the moles of p-cresol converted per mole of oxygen consumed, is remarkably low (maximally 60%). This is due to the oxidation of the solvent (methanol) to formic acid. The use of sodium hydroxide is undesirable from an environmental point of view, but cannot be avoided. Below a NaOH/ p-cresol ratio of 1 no reaction occurs. A maximum reaction rate is observed at a NaOH over p-cresol ratio of 3–4. A maximum reaction rate is observed at a cobalt/ p-cresol ratio of 0.6–0.7 mg/g, independent of the catalyst source. Not only the reaction rate but also the oxygen selectivity and selectivity to p-HBA show the same dependence upon the cobalt/ p-cresol ratio. This indicates a comparable reaction mechanism in the homogeneous (CoCl 2) and the supposedly heterogeneous CoAPO systems. Recycling of the catalysts at sufficiently low cobalt over p-cresol ratios indicates that the CoAPO catalysts are not stable under reaction conditions; cobalt is released from the partially dissolved CoAPO molecular sieve. The observed catalytic activity mainly stems from this dissolved cobalt.