Benzylation of arenes with benzyl chloride over H-Beta zeolite: effects from acidity and shape-selectivity

H-Beta zeolites with various Si/Al ratios have been prepared for the liquid-phase benzylation of various arenes with benzyl chloride (BzCl). ²⁹Si and ²⁷Al MAS NMR spectroscopy revealed the incorporation of Al into the silica framework to form catalytically active Brønsted acid sites (BAS). ¹H MAS NMR spectroscopy investigations demonstrated the BAS density increased with reducing Si/Al ratio, while the BAS strength decreased as probed by CD₃CN molecules. These H-Beta zeolites are highly selective to desired monobenzylation products depending on the shape-selectivity induced by the suitable channel system. The catalytic performance is in line with the nucleophilicity and proton affinity (PA) of arenes (xylene > toluene > benzene) on the same catalyst, typically for Friedel-Crafts reaction. A shape-selective effect has been observed to dominate the reaction, lowering the performance of mesitylene compared to xylene, as well as being selective to only the monobenzylation product. The benzylation performance is enhanced with increasing the BAS strength of H-Beta zeolites due to the formation of more aryl cation intermediates by attacking electronegative chlorine atom in BzCl. An alternative reaction mechanism based on the activation of arenes by protonating aromatic ring of BAS is proposed to explain the superior benzylation activity of BzCl with less active arenes.