Abstract
In this paper, we report on the reaction of phenol benzoylation with benzoic acid, which was carried out in the absence of solvent. The aim of this reaction is the synthesis of hydroxybenzophenones, which are important intermediates for the chemical industry. H-beta zeolites offered superior performance compared to H-Y, with a remarkably high conversion of phenol and high yields to the desired compounds, when using a stoichiometric amount of benzoic acid. It was found that the reaction mechanism did not include the intramolecular Fries rearrangement of the primary product phenyl benzoate, but indeed, the bimolecular reaction between phenyl benzoate and phenol mainly contributed to the formation of hydroxybenzophenones. The product distribution was greatly affected by the presence of Lewis-type acid sites in H-beta; it was suggested that the interaction between the aromatic ring and the electrophilic Al3+ species led to the preferred formation of o-hydroxybenzophenone, because of the decreased charge density on the C atom at the para position of the phenolic ring. H-Y zeolites were efficient than H-beta in phenyl benzoate transformation into hydroxybenzophenones.
Highlights
The acylation of phenolic compounds is an important class of reactions with several applications in the chemical industry [1,2]
Yadav et al [60] reported that the first step of the reaction, catalyzed by Cs2.5H0.5PW12O40 supported over K-10 clay, is the esterification, which is followed by Fries rearrangement towards hydroxybenzophenones (HBPs)
The number of acid sites was not exactly in line with that expected based on the Si/Al ratio; sample HB-75 had a smaller number of acid sites than HB-150, while the opposite would normally be expected
Summary
The acylation of phenolic compounds is an important class of reactions with several applications in the chemical industry [1,2]. The acylation of resorcinol is an important process for the synthesis of a precursor for the UV-light absorbent for polymers, 4-o-octyl-o-hydroxybenzophenone [3] These reactions are catalyzed by homogeneous Friedel-Crafts Lewis-type catalysts; due to the environmental problems associated with the use of these compounds, alternative heterogeneous systems have been sought and, in some cases, successfully implemented in industrial uses. There are no papers in the literature describing the use of zeolites for the benzoylation of phenol; there is only one paper reporting on phenol benzoylation with benzoic anhydride catalyzed by the H-beta zeolite [19] and one paper reporting about the benzoylation of phenol with BA using polyoxometalate catalyst [60] In this latter case, Yadav et al [60] reported that the first step of the reaction, catalyzed by Cs2.5H0.5PW12O40 supported over K-10 clay, is the esterification, which is followed by Fries rearrangement towards hydroxybenzophenones (HBPs). The direct reaction between phenol and BA is carried out, using BF3 as a Lewis-type catalyst [19]
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