Acidic Properties and Catalytic Activity of Natural Kaolinitic Clays for Friedel–Crafts Alkylation

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Abstract
Natural Kaolinitic clays containing transition metals Fe and Ti in their lattice have been examined for their acidic properties and catalytic activity for the alkylation of benzene with benzyl chloride. Calcination of the clays at 550 °C and subsequent acid-activation using 2 M HCl (1 M = 1 mol dm−3) rendered the kaolinite surface highly acidic with very high acid amount. These clays exhibited high catalytic activity and selectivity for the conversion of benzyl chloride to diphenylmethane. Al(III), Fe(II), and Ti(II) ions exsolved from the lattice of kaolinites by acid activation relocate in the interlamellar space and boost the Brønsted acidity of coordinated hydroxo and proposed to initiate the alkylation. In fact kaolinites having structural transition metal, on acid activation exhibited higher acid amount and enhanced catalytic activity than kaolinites devoid of transition metals and recently studied alkylation catalysts such as synthetic TiO2–SiO2–Al2O3 and Fe2O3–SiO2–Al2O3. Natural kaolinitic clays having transition metals in their lattice possess disordered structure and hence on acid activation results in the generation of larger amounts of relocatable cations, higher surface area and higher acidity compared to an ideal kaolinite. Also their acid amount and catalytic activity retained up to five successive regeneration cycles studied.

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