Selective alkylation of phenols using solid catalysts
Alkylphenols are important industrial chemicals used in a wide range of applications. In particular, 2,6-ditertbutylphenol is an indispensable building block for anti-oxidants and light protective agents. A new solid catalyst was prepared, characterised and tested for the alkylation of phenols with alkenes in an attempt to reduce the environmental hazards associated with the aqueous wastes generated by the homogeneously catalysed alkylation reactions. The new silica gel supported aluminium phenolate catalyst was prepared by a two steps procedure, first grafting of an aluminium precursor such as aluminium trichloride or triethyl aluminium onto silica mainly through reaction with the support silanol groups, then exchange of the aluminium ligand with phenol. Catalysts exhibited mainly Lewis acidity and two types of active sites were detected. The new catalyst was successfully applied in the phenol - isobutene alkylation system. Catalysts exhibited an ortho- selectivity for the introduction of the first tertbutyl group. The selectivity of the second alkylation could be tuned by varying reaction conditions (reaction temperature, catalyst amount, alkene addition methods) and catalyst characteristics (support surface pre-treatment temperature, aluminium precursor and loading). Hence high yields of 2,4-ditertbutylphenol or moderated yields of 2,6-ditertbutylphenol were obtained. Alkylation of phenol with other alkenes and cresols alkylations were successfully catalysed by the new silica gel supported aluminium phenolate catalyst with the same selectivity. However, the diorthopropylphenol was the main dialkyl products when using propene as alkylating agent. "Greening" of the catalyst preparation by reducing the amount of solvent used was carried out without changing the selectivity and the activity of the catalyst. Reusability of the catalyst was investigated and a decrease of activity was observed. Storage of the catalyst was possible for a long time but activity and selectivity were affected.