Acidity versus metal-induced Lewis acidity in zeolites for Friedel-Crafts acylation

Acid catalysts including Ni, Ag and Fe-loaded zeolites of different structures were prepared either via cationic exchange or impregnation techniques from pristine H-zeolites (BEA, and MFI). Their catalytic activity was evaluated in the liquid-phase Friedel-Crafts acylation of anisole with propanoic acid. It turned out that, whatever the doping procedure was, the zeolite loaded with transition metals led to considerable decrease in propanoic acid conversion, regardless of the nature or the metal content. However, the extent of this detrimental effect followed the order: Ag+ > Ni2+ > Fe3+. Pristine acidic zeolites were not only found to be the most active, but also to be the most selective toward ortho-and para-acylation products. H-ZSM-5 zeolites yielded the highest intrinsic activity, with TOF values of 0.09 h(-1). The catalyst activity proved to be essentially attributed to the density and accessibility of Bronsted acid sites, playing a key role in the activation of the reactants. Bronsted sites are proposed to be the most likely catalytic species for performing this Friedel-Crafts acylation. (C) 2016 Academie des sciences. Published by Elsevier Masson SAS.