Autoxidation of p-cresol to p-hydroxy benzaldehyde using CoCl2, CoAPO-5 and CoAPO-11

M.P.J. Peeters, M.C.M. Busio, P. Leijten

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Abstract

CoCl2, CoAPO-5 and CoAPO-11 are active and selective catalysts for the autoxidn. of p-cresol to p-hydroxybenzaldehyde (p-HBA) in methanolic sodium hydroxide soln. at 313-353 K. The yield of p-HBA is 70-75% at approx. 95% conversion, byproducts being p-hydroxybenzyl alc. (p-HBAlc; 5%) and p-hydroxybenzyl Me ether (p-HBME; 10%) for the reactions catalyzed by CoAPO-5 and CoAPO-11. Using CoCl2 a selectivity for p-HBA of 80% can be obtained with p-HBME (10%) and p-HBAlc (5%) as byproducts. Some oligomers/polymers are formed (however, these could not be detd. by HPLC anal.). Oxidn. of p-HBAlc to p-HBA is obsd. for all three catalyst systems at these low reaction temps. The oxygen selectivity, defined as the moles of p-cresol converted per mol of oxygen consumed, is remarkably low (maximally 60%). This is due to the oxidn. 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 max. reaction rate is obsd. at a NaOH over p-cresol ratio of 3-4. A max. reaction rate is obsd. 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 (CoCl2) 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 mol. sieve. The obsd. catalytic activity mainly stems from this dissolved cobalt. [on SciFinder (R)]
Original languageEnglish
Pages (from-to)51-62
JournalApplied Catalysis. A, General
Volume118
Issue number1
DOIs
Publication statusPublished - 1994

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