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A systematic investigation has been carried out, aiming at elucidating several aspects of the gas/solid methylation of phenol over high Si/Al ratio BEA-structured zeolite in protonated form. The catalysts have been characterized by several techniques, such as XRD, SEM, BET, ICP, FT-IR, TGA, micro-calorimetry and modelling by ab initio calculations. The correlation between these characteristics and kinetics and mechanistic features of the catalytic reaction, as well as of catalyst deactivation, showed that these zeolites are very active for the present reaction, leading to cresols and anisole as primary products. As catalyst deactivation proceeds, the selectivity to cresols and anisole increases substantially, together with a rapid decrease of selectivity to poly-alkylated species. Catalyst surface acidity is prevalently made of medium-to-low-strength silanolsbased acid sites of Brønsted type. High-strength Lewis acid sites are either almost absent, especially when metal cations partially substitute for protons, or play a role essentially in catalyst deactivation. Stacking faults in the zeolite framework, generated by the 2 intergrowth of at least two BEA polymorphs, lead to an increase of the concentration of silanols-based acid sites. Deactivation is essentially due to the interaction of phenol and oxygenated products with the strong Lewis acid sites. For time-on stream values longer than a few hours, self oligomerisation-cyclisation of methanol to olefins and aromatics, followed by further alkylation to aromatic C atoms, contributes more significantly to catalyst deactivation. At higher temperature all the zeolites deactivate at a comparable rate, whereas at lower temperature initial catalytic activity is higher for larger crystal size zeolite, due to the longer diffusion time of bulkier coke precursors within zeolite pores. At any conversion level and at any temperature the anisole/cresols ratio is systematically lower for the larger crystal size zeolite, since the secondary transformations of anisole to cresols, by both intra-molecular rearrangement and inter-molecular alkylation of phenol, is favoured by the longer residence time of anisole within the zeolite pores.

This contribution reports about an in situ FT-IR investigation and the catalytic reactivity of Mg/Me(3+) mixed oxides (Me = Cr, Fe, or Al; Mg/Me = 2, atomic ratio) in the gas-phase methylation of phenol with methanol. It is the second of two papers concerning the mentioned systems, and its purpose is twofold: to confute the classic and not accurate theory concerning the reaction mechanism, and to propose a novel interpretation based on the combined use of catalytic tests and in situ molecular spectroscopy. Results here reported highlight that: (i) the reaction mechanism in phenol methylation, when catalysed by basic systems, is not a classical electrophylic substitution, as generally reported in the literature, but proceeds through the formation of formaldehyde as an intermediate, and (ii) the catalytic behaviour in respect to both methanol and phenol reactants is strictly dependent on catalyst features. Although all investigated systems exhibit a basic-type behaviour with regard to phenol, which dissociates to yield an adsorbed phenolate species, the distribution of phenolic compounds obtained with the Mg/Al/O catalyst was that typically observed with acid catalysts, with prevailing formation of anisole when the reaction was carried out below 350 degrees C and of mono and poly-C-alkylated compounds when the reaction temperature was above 350 degrees C. On the contrary, the reactivity shown by both Mg/Fe/O and Mg/Cr/O systems was that reported in the literature as typical of mixed oxides possessing basic features. The extent of methanol decomposition into light compounds was maximum in the case of Mg/Fe/O catalysts, because of the pronounced redox behaviour typical of Fe(3+) species, whereas neither methanol dehydrogenation nor decomposition were ever observed with Mg/Al/O up to 400 degrees C. Reactivity tests and spectroscopic experiments hinted for methanol dehydrogenation to formaldehyde as the first step in the ring-methylation of phenol with Mg/Cr/O and Mg/Fe/O: in that case, o-cresol and 2,6-xylenol were the only reaction products. But, with Mg/Al/O systems, for which no methanol dehydrogenation occurred, the formation of anisole was due to the synergistic effect of stronger basic features and the presence of Lewis acidic sites, that facilitate the reaction between phenol and methanol after activation over the two different types of catalytic sites.