Catalysis of Organic..

80Methylation of CatecholThe main reactions that occur in catechol methylation over strongly acidcatalysts (H-mordenite and AlF 3 ), as inferred from catalytic tests, are summarized inScheme 1. The main indication is that it is possible to achieve a good selectivity toguaiacol only under mild reaction conditions, that is, at low catechol conversion. Ifthe temperature is increased to reach a higher conversion, the selectivity to guaiacoldrops, with an increase in the formation of phenol and cresols, rather than ofmethylated catechol. Therefore, the reaction network is different from that obtainedin the case of phenol methylation over zeolites. This difference is due to the lowstability of guaiacol, which at temperatures higher than 300°C is transformed tocatechol and to phenols.OHO HH 3 CCH 3 OHOH CH 3 OHOHOHOCH 3OHOCH 3OHOHOH+OCH 3OCH 3OCH3H 3 CO HOCH 3-CH 2 OorCH3-CH 2 Ode-oxygenated compounds(phenol, o-cresol)Scheme 1. Reaction scheme in catechol methylation with strongly acid catalysts.Al/P/O catalyst, containing crystalline tridimyte AlPO 4 , possesses mildly acidicproperties (6). Figure 3 plots the effect of temperature on catalytic performance.Despite the weaker acidity as compared to zeolites, the catalyst was clearly moreactive than H-mordenite and AlF 3 . A guaiacol conversion of 42% could be reachedat 275°C; even more remarkably, the catalytic performance was maintainedsubstantially unaltered for hundreds hours reaction time (7). The selectivity toguaiacol was higher than 95% in the entire range of temperature investigated. Theslight fall of selectivity for increasing reaction temperature was due to both theformation of veratrole (the product formed by consecutive reaction upon guaiacol),and the formation of methylcathecols. Data obtained indicate that the nature of acidsites and the strength of interaction with catechol and guaiacol greatly affects thecatalytic performance. A strong interaction is finally responsible for catalystdeactivation, and for by-products formation.Experimental SectionThe following catalysts were used for reactivity tests: (a) H-mordenite zeolite, H-MOR40, shaped in extrudates, supplied by Süd-Chemie AG, and having SiO 2 /Al 2 O 3ratio equal to 40. (b) AlF 3 , supplied by Aldrich. (c) Al/P/O, prepared following the