Atom efficient Friedel–Crafts acylation of toluene with propionic anhydride over solid mesoporous superacid UDCaT-5

Abstract

Friedel–Crafts acylation is ubiquitous in industry and is typically carried out by using more than stoichiometric quantities of homogeneous catalysts. This creates pollution. In this work, acylation of toluene was studied in liquid phase with propionic anhydride with a variety of solid superacids to produce 4′-methylpropiophenone (4′-MPP). The solid superacids were modified versions of zirconia, namely, UDCaT-4, UDCaT-5 and UDCaT-6 developed in our laboratory; amongst which UDCaT-5 was the most active, selective and robust catalyst. The effects of various reaction parameters on the rate of reaction and selectivity were investigated to deduce the intrinsic kinetics of the reaction. The reaction is free from any external mass transfer as well as intraparticle diffusion limitations and is intrinsically kinetically controlled. The acylation conditions were: temperature 180 °C, toluene to propionic anhydride molar ratio 5:1, catalyst loading 0.06 g cm^-3, speed of agitation 1000 rpm, under autogenous pressure in a stainless steel autoclave reactor. Propionic acid generated in situ also reacts sequentially with toluene to give 4′-MPP. A conversion of 62% of priopionic anyhydride is obtained after 3 h, with 100% mono-acylated product containing 67% 4′-MPP. Water is the only co-product of the overall reaction. A suitable kinetic model was developed. The reactions were carried out without using any solvent in order to make the process cleaner and greener.