Catalysis Today, Vol.333, 147-153, 2019
Effect of grinding on the physico-chemical properties of Mg-Al hydrotalcite and its performance as a catalyst for Baeyer-Villiger oxidation of cyclohexanone
Hydrotalcite material of molar ratio Mg/Al=3 was subjected to grinding in a planetary mill for various periods of time ranging from 10 to 180 min. The samples were characterized with XRD, SEM, XRF, AAS, FTIR, XPS, N-2 adsorption at - 196 degrees C, laser diffraction particle sizing, and contact angle measurements. Physico-chemical characterization revealed that already the shortest time of grinding resulted in an abrupt destruction of plate-like morphology, fall of hydrotalcite specific surface area, and change of grain sizing. Further treatment had much lesser effect on these parameters. In contrast, with increasing grinding time, a gradual deterioration of the materials crystallinity was observed, coupled with progressing segregation of Al to the surface. Initially, grinding induced increase of surface hydrophilicity, followed by progressing hydrophobization associated with the accumulation of Al at the surface. The effect of grinding on catalytic performance of the studied materials in the Baeyer-Villiger oxidation of cyclohexanone to e-caprolactone with H2O2/acetonitrile depended on the interplay between the catalyst crystallinity and hydrophilic/hydrophobic properties. An optimum activity was obtained for intermediate grinding time, over the catalyst whose crystal size has been reduced, and whose surface was characterized by maximum hydrophilicity. Recycling experiment shows that deactivation of the catalyst caused by Mg leaching can be remedied by catalyst regeneration in Mg(OH)(2) solution.