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.