Mg/Al mixed oxides prepared by coprecipitation and sol–gel routes: a comparison of their physico-chemical features and performances in m-cresol methylation

Abstract

Magnesium/aluminium mixed oxides were synthesised following two procedures, via: (i) coprecipitation (CP) and (ii) sol–gel (SG). A crystalline precursor with hydrotalcite-like structure developed in both cases only for Mg/Al ratios between 2 and 4. The thermal treatment of the precursors yielded the mixed oxides, which had different bulk and surface properties depending on the preparation method and the Mg/Al ratio. The calcined materials were tested in the liquid-phase alkylation of m-cresol with methanol. Relationships were found between the basic characteristics and the catalytic performance. In the range of Mg/Al ratios between 2 and 4, SG oxides had a higher degree of crystal phase than the CP ones, and different morphology. This difference corresponded to differences also in the surface basic features, which however did not influence considerably the catalytic performances. When low Mg/Al ratios were used, the differences between SG and CP samples were greater. Specifically, the CP procedure favoured segregation of oxides and formation of a large number of strong basic sites. However these sites soon deactivated under reaction conditions and did not directly contribute to the reaction. In the case of the SG procedure, a more efficient reciprocal dispersion of the two elements was achieved, with generation of specific sites of medium basic strength, which affected the catalytic performance. SG materials with low Mg/Al ratio (Mg/Al=0.15) showed an improvement of the methylation regioselectivity (e.g., decrease of the O/C-alkylation ratio and increase of the ortho/para-C-alkylation ratio) with respect to both CP and SG samples having higher Mg/Al ratios.