A new mechanism — key for an improved synthesis of 2,6-di-tert-butylphenol Original Research Article

An improved process for the synthesis of 2,6-di-tert-butylphenol (2,6-DTBP) is based on an older patent, which describes the alkylation of 2-tert-butylphenol (2-TBP) with isobutene in the presence of tris(2-tert-butylphenolate)-aluminium at low temperatures and under low pressure. The traditional mechanism of an acid–catalysed alkylation of phenols with alkenes, catalysed by aluminium tris-(phenolate) between 100 and 125 °C at pressures up to 25 bar, does not allow an explanation of several differences in the behaviour of the two catalysts, for example, the different temperature dependence of their activity and selectivity. These phenomena can be explained on the basis of a proposed new mechanism. It is assumed that a monomeric, coordinatively unsaturated aluminium tris-(2-tert-butylphenolate) complex is responsible for the alkylation. During the catalytic cycle this complex is transformed into an aluminium bis(2-tert-butylphenolate)-(2,6-di-tert-butylphenolate) complex, followed by the expulsion of the more voluminous 2,6-di-tert-butylphenolate ligand by 2-tert-butylphenol. As long as an excess of the latter is present, the initial catalyst will form again. By using these ideas and working with low amounts of catalyst at low temperatures in the presence of an excess of isobutene or of selected aliphatic solvents, improvements in yield and selectivity of the 2,6-di-tert-butylphenol-formation could be achieved. Lowering the amount of catalyst was associated with higher conversion of 2-tert-butylphenol, reduction of production costs, formation of smaller amounts of by-products as well as diminution of the environmental problems. The latter are caused by the need to remove aluminium- and phenol-containing residues. These are formed during the decomposition of the catalyst, which is necessary before the distillative work-up of the reaction mixtures can take place.