Magnesium and/or calcium-containing natural minerals as ecologically friendly catalysts for the Baeyer–Villiger oxidation of cyclohexanone with hydrogen peroxide

Abstract

Magnesium and/or calcium-containing natural minerals of basic character were tested as catalysts for the Baeyer–Villiger oxidation of cyclohexanone to ϵ-caprolactone. Two types of widely available, nontoxic and inexpensive minerals were investigated: carbonates, i.e. magnesite, dolomite and calcite (limestone), and silicates, i.e. talc, sepiolite and hectorite. The minerals were characterized with XRD, SEM, XRF, ICP OES, XPS, N2 adsorption at −196°C, laser diffraction particle sizing, and contact angle measurement. Surface basicity was determined by adsorption of organic acids with different pKa values. Mechanochemical treatment of the samples was performed in a planetary mill. The yields of ϵ-caprolactone over the as received carbonate minerals were comparable with the yield obtained for the reference Mg–Al hydrotalcite catalyst. Of the layered magnesium silicate group, the untreated hectorite gave the best performance, yielding ϵ-caprolactone in the amount similar to that obtained for the reference catalyst. The study of the effect of grinding on the catalytic performance of minerals revealed that the treatment may affect not only the mineral grain size and morphology, but also other factors relevant for the catalytic reaction, such as hydrophilic/hydrophobic properties of the mineral surface and the ease of alkaline earth leaching. The final effect of grinding depended on the interplay between all these factors. Thus, grinding-induced enhancement of catalytic performance was most pronounced for magnesite, less significant for dolomite, and practically negligible for limestone. In the series of magnesium silicates grinding increased the ϵ-caprolactone yield over sepiolite, while little effect was observed for talc and hectorite. The maximum yields of ϵ-caprolactone over ground magnesite and ground sepiolite were by factor 1.4 higher than the yield over the reference hydrotalcite catalyst. Thus, magnesite and sepiolite prove as particularly promising, eco-friendly catalysts for the Baeyer–Villiger oxidation of cyclohexanone to ϵ-caprolactone.