Abstract
<p>Evaluation of catalytic performances of selected metal chlorides such as AlCl<sub>3</sub>, SnCl<sub>4</sub>, ZnCl<sub>2</sub>, FeCl<sub>3</sub>, InCl<sub>3</sub> and GaCl<sub>3</sub> with solid acids such as sulfated zirconia, and zeolite beta was accomplished for acetylation of anisole, toluene and naphthalene. Presence of super acidity (Lewis or Bronsted acid) is found to be indispensable for activation of substrates towards acetylation reactions. In addition, presence of redox centers would further complement with the Lewis acid sites rendering catalytic stamina against deactivation. Strength of Lewis acid basically determines the activity of the metal chlorides towards acetylation. Among the Lewis acids investigated, FeCl<sub>3</sub>, InCl<sub>3</sub> and GaCl<sub>3</sub> exhibit their catalytic behaviour mostly through redox property as is evident from the conservation of Turn over number even after first cycle. Sulfated zirconia surpasses all the acid catalysts including metal chlorides and exhibits extended catalytic activity in acetylation of anisole. The pre-eminence of sulphated zirconia over other catalytic systems is owing to the synergistic effect of Lewis and Bronsted acidity.</p>
Highlights
Synthesis of aromatic ketones possesses a high industrial significance, as it is a key intermediate in the synthesis of many of the pharmaceuticals [1] and this task is normally achieved through catalytic acylation route
Lewis acids such as FeCl3, AlCl3, BF3 and Bronsted acids such as HF, H2SO4 having super acidity have been used in a homogenous catalytic fashion through Friedel-Crafts acylation and alkylation processes for this purpose, regardless of the generation of environmentally hazardous products [1,2]
The facts deduced from the comparative evaluation of solid acids with metal chlorides as Lewis acids results in the following proposition that designing a solid acid containing, Lewis and Bronsted acid sites in particular ratio in conjunction with redox centers as promoters would be a catalyst of higher order performance for acetylation reactions
Summary
Synthesis of aromatic ketones possesses a high industrial significance, as it is a key intermediate in the synthesis of many of the pharmaceuticals [1] and this task is normally achieved through catalytic acylation route. As a measure of addressing the issue of contributions of the Lewis, Bronsted acidity along with redox centers towards acylation, a systematic investigation of catalytic acetylation activity of metal chlorides, sulfated zirconia, and zeolite beta has been accomplished using turn over number as a measure of catalytic activity. Acetylation of anisole, toluene and naphthalene was carried out with metal chlorides according to Scheme 1 and the results are presented in Tables 1, 2 and 3 respectively. It has to be take into consideration that reaction time required for attaining 90% conversion of anisole has been selected as a measure of catalytic activity. The order of catalytic activity of metal chlorides towards acetylation of anisole in accordance with Fig. 1 is given below as: AlCl3 > SnCl4 > ZnCl2 > FeCl3 >. It can readily be comprehended that the activity in acetylation of aromatics correlates well with the
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