The cocatalyst effect of 4A molecular sieve (4A-MS) on the catalytic activity of tin(II) mesotetraphenylporphyrin (SnTPP) was investigated in the aerobic Baeyer-Villiger oxidation of cyclohexanone in the presence of benzaldehyde as reductant, remarkable enhancement on the yield of e-caprolactone was achieved (from 21% to 83%). The in situ Raman spectroscopy confirmed the interaction between 4A-MS and the carbonyl group of benzaldehyde and cyclohexanone molecules. The major role of 4A-MS was to active cyclohexanone and benzaldehyde molecules by the coordination between the oxygen atom of carbonyl group and the Lewis acid site of 4A-MS. The activated benzaldehyde tended to combine dioxygen easily, which resulted in the accelerated generation rate of peroxybenzoic acid and the high-valent Sn porphyrin (PorSn(IV)=0). Accordingly, the slow generation rate of Criegee adduct intermediate by the reaction between cyclohexanone and high-valent Sn porphyrin was accelerated. A cocatalytic mechanism involving the activated cyclohexanone and benzaldehyde was proposed. (C) 2015 Elsevier B.V. All rights reserved.