Catalysis of Organic..

78Methylation of Catecholthe use of heterogeneous acid catalysts in the gas-phase methylation of catechol,for the synthesis of guaiacol.Results and DiscussionFigure 1 plots the results obtained in the gas-phase methylation of catechol, with theH-mordenite catalyst. The main reaction product at low temperature and lowcatechol conversion was guaiacol, the selectivity of which however decreased whenthe reaction temperature was increased, with a corresponding higher formation ofphenol and, at a minor extent, of 3-methylcatechol, o-cresol and p-cresol. This trendis substantially different from the one observed in the methylation of phenolcatalyzed by zeolites (3,4). In the latter case, in fact, anisole is the prevailing productat low temperature, but the decrease of its selectivity for increasing temperatures ismainly due to the intramolecular rearrangement to o-cresol (5). The latter becomesthe prevailing product, together with polymethylated phenols, at high temperature.Also the direct C-alkylation contributes to the formation of cresols andpolymethylated phenols. On the contrary, in our case the product of intramolecularrearrangement of guaiacol (3-methylcatechol), was produced with low selectivity.Phenol formed by loss of one formaldehyde molecule from guaiacol. The selectivityto veratrol was nil, and that to 4-methylcatechol was less than 0.5%.100Conversion, selectivity, %80604020Guaiacol3-MethylcatecholCatechol convPhenolp-Cresolo-Cresol0250 300 350 400 450Temperature, °CFigure 1. Catechol conversion and selectivity to guaiacol, o-cresol, phenol, p-cresoland 3-methylcatechol as functions of temperature. Catalyst H-mordenite. Feed:catechol and methanol (1/10 molar feed ratio).Analogous results were obtained with AlF 3 ; the prevailing product at lowtemperature was guaiacol, the selectivity of which decreased in favour of phenol, andof other minor products. It is worth noting that the similar catalytic performanceobtained for the H-mordenite and AlF 3 suggests similarity also in the nature of activesites. This is in favour of the generation, under reaction conditions, of a Brønsted-