Catalysis of Organic..

Ardizzi et al. 79type acidity at the surface of AlF 3 , either by interaction of water or of catechol withthe Lewis sites. With this catalyst, however, the deactivation was more relevant thanwith the zeolite. One reason for the deactivation effect may be related to the stronginteraction of catechol and guaiacol with the active sites. This also may be the reasonfor the low stability of guaiacol; the high effective permanence time at the adsorbedstate, or its slow diffusion, may favor its further transformation to phenol. Thishypothesis was confirmed by directly feeding guaiacol, in the absence of methanol.Figure 2 compares the distribution of products obtained when guaiacol was fedin the absence of methanol, at two levels of temperature and with the AlF 3 catalyst.Results were taken at the very beginning of the reaction time, that is, beforedeactivation caused a rapid decline of conversion. At 300°C, the main products ofguaiacol transformation were catechol, veratrol and methylguaiacols. The amount ofde-oxygenated compounds (phenol and cresols) was negligible. Therefore, underthese conditions the main reaction was the methylation of guaiacol onto anothermolecule of guaiacol, to generate catechol and either veratrol or methylguaiacols.Moreover, either the intramolecular alkylation, or the intermolecular alkylation ofguaiacol on catechol, generated methylcatechols (mainly 3-methylcatechol). It isworth noting that the sum of selectivity to veratrol and methylguaiacols was equal tothe selectivity to catechol; this confirms that bimolecular alkylation reactionsoccurred, and that catechol was the co-product in the formation of veratrol andmethylguaiacols. The situation was different at 390°C; besides catechol, mainproducts were those of C-alkylation (methylcatechols and methylguaiacols), ratherthan veratrole. The selectivity to de-oxygenated compounds (mainly phenol, withminor amounts of cresols) was relevant; therefore under such conditions thetransformation of guaiacol to phenol + formaldehyde gave an important contributionto guaiacol conversion.60Selectivity, %453015catecholveratrolmethylcatecholsmethylguaiacolsde-oxygenated0Figure 2. Initial selectivity to the products at 300°C (white bars) and at 390°C (blackbars), in the transformation of guaiacol. Catalyst AlF 3 .