ABSTRACT
Abstract Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects. (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion. (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the –O-species in directing the regio-selectivity of the C-methylation into the ortho position. Introduction The literature on basic-and acid-catalyzed alkylation of phenol and of its derivatives is wide [1,2], since this class of reactions finds industrial application for the synthesis of several intermediates: 2-methylphenol as a monomer for the synthesis of epoxy cresol novolac resin; 2,5-dimethylphenol as an intermediate for the synthesis of antiseptics, dyes and antioxidants; 2,6-dimethylphenol used for the manufacture of polyphenylenoxide resins, and 2,3,6-trimethylphenol as a starting material for the synthesis of vitamin E. The nature of the products obtained in phenol methylation is affected by the surface characteristics of the catalyst, since catalysts having acid features address the electrophilic substitution in the ortho and para positions with respect to the hydroxy group (steric effects in confined environments may however affect the ortho/para-Calkylation ratio), while with basic catalysts the ortho positions become the
preferred ones, due to the repulsive effect between the aromatic ring and oxygen atoms of the metal oxide. This repulsive effect forces the phenolate to adopt a vertical position with respect to the catalyst surface, making the para position further from the active sites and hence less available for nucleophilic attack onto activated methanol [3]. In the present work we compare the performance in the gas-phase m-cresol methylation of Mg/Al and Mg/Fe mixed oxides, both prepared starting from hydrotalcite precursors. The peculiarities of these two systems are (i) the presence of only basic sites in the case of Mg/Al/O, and (ii) the co-existence of Lewis acid and basic sites in the case of Mg/Fe/O [4]. Results and Discussion Mg/Al/O and Mg/Fe/O catalysts have been prepared by thermal treatment of the corresponding hydrotalcite precursors. Main characteristics of the catalysts are reported in Table 1. Specifically, the surface area of the calcined materials, and the amount of acid and of basic sites, as determined by pyridine and by carbon dioxide adsorption, respectively, are reported. For reference, the main characteristics of MgO, Al2O3 and Fe2O3 are also given. In both Mg/Al/O and Mg/Fe/O a solid solution of either Al3+ or Fe3+ in MgO (periclase) develops, in which the replacement of trivalent cations for Mg2+ in the lattice co-generates cationic vacancies. Oxygen anions having unsaturated ligands act as strongly nucleophilic sites, while O anions coordinated to both Mg and to the guest cation are less basic, due to the higher electronegative property of Al and Fe as compared to that of Mg. Furthermore, the presence of acid sites in the Mg/Fe/O catalyst can be associated to the coordinative properties of Fe [4].
