The balance of acid, basic and redox sites in Mg/Me-mixed oxides: The effect on catalytic performance in the gas-phase alkylation of m-cresol with methanol V. Crocellà a , G. Cerrato a , G. Magnacca a , C. Morterra a , F. Cavani b,1, * , S. Cocchi b,1 , S. Passeri b,1 , D. Scagliarini b,1 , C. Flego c , C. Perego d a Dept. of Chemistry IFM and NIS Centre of Excellence, University of Turin, Consortium INSTM (Florence), Research Unit of Turin University, Via P. Giuria 7, 10125 Torino, Italy b Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy c eni SpA, Div. R&M, CR SDM, Via Maritano 26, S. Donato Milanese, MI, Italy d eni SpA, CR Energie Non Convenzionali, Istituto Eni Donegani, Via Fauser 4, Novara, NO, Italy article info Article history: Received 30 July 2009 Revised 9 December 2009 Accepted 13 December 2009 Available online 21 January 2010 Keywords: Basic catalysis Bifunctional catalysis m-Cresol methylation Methanol decomposition Mg/Al/O Mg/Cr/O Mg/Fe/O Magnesium oxide abstract The reactivity of Mg/Me-mixed oxides (Me = Fe, Cr, Al) in m-cresol gas-phase methylation with methanol was studied, with the aim of finding relationships between catalytic performance and surface/bulk fea- tures. All systems exhibit similar surface properties (basically due to the presence of basic and Lewis-type acid sites), but the presence of extra cations in the MgO framework affects greatly the products obtained in the mentioned catalytic reaction, depending on the possible co-presence of redox capacity. In particular, (i) the Mg/Al/O system, characterized by no redox capacity, shows catalytic behaviour quite similar to that of conventional Brønsted-type acid catalysts, though with the advantage of exhibit- ing almost no deactivation during use (a rather common problem for acid catalysts activating methanol for phenol attack); (ii) the Mg/Cr/O catalyst shows a reactivity typical of a basic catalyst, with high chemo- and regio-selectivity and the favoured formation of 2,5-dimethylphenol; (iii) the Mg/Fe/O system pre- sents a catalytic behaviour closer to that of Mg/Cr/O, if referred to the distribution of phenolic products, but it also exhibits a better activity in methanol dehydrogenation to formaldehyde (that is the true elec- trophilic reactant for m-cresol ring methylation), and in the decomposition of formaldehyde into light compounds. This behaviour causes a poor selectivity to dimethylphenols with respect to converted meth- anol, whereas the selectivity with respect to converted m-cresol is not different from that shown by Mg/ Cr/O system. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction The methylation of phenol and phenol derivatives has a high industrial relevance. For instance, 2,6-xylenol is the monomer for the production of poly-(2,6-dimethyl)phenylene oxide resins, 2- methylphenol (o-cresol) is the monomer for the synthesis of epoxycresol paints, 2,5-dimethylphenol is the intermediate for the synthesis of dyes, antiseptics and antioxidants, and 2,3,6-tri- methylphenol is the starting compound for the synthesis of vita- min E [1]. The products of phenol or anisole and diphenols methylation reaction are intermediates in the production of skin- protecting agents and food additives. Starting from m-cresol, and using 2-propanol as the alkylating agent, thymol is produced, which is a commercially interesting intermediate for the synthesis of menthol. The C-alkylation of phenol derivatives is carried out either in the liquid-phase, using alkyl halides as alkylating agents, or in the gas-phase, with alcohols over acid or basic catalysts. The latter may be (supported) alkali and alkaline-earth metal oxides, (mixed) transition metal oxides, or combinations of both [2–19]. Previous reactivity experiments, carried out in liquid-phase in mild conditions, showed that this reaction may provide useful information about chemo- and regio-selectivity; they are driven by catalyst properties [35–39]. However, these kinds of tests achieved very low m-cresol conversion, so limiting the possibility of examining catalyst deactivation and the role of methanol decomposition. And in this respect, we decided to study the gas- phase methylation of m-cresol. When methanol is the co-reactant, the ring methylation of phe- nolic compounds is carried out in the gas-phase, typically at tem- peratures higher than 300 °C, and one of the major problems of the industrial process is the decomposition of methanol occurring in these conditions. Consequently, a large excess of methanol is usu- ally fed in order to reach an acceptable per-pass conversion of phenol. 0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2009.12.011 * Corresponding author. Fax: +39 051 209 3680. E-mail address: fabrizio.cavani@unibo.it (F. Cavani). 1 Also: Consortium INSTM (Florence), Research Unit of Bologna (a partner of NoE Idecat, FP6 of the EU). Journal of Catalysis 270 (2010) 125–135 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat