The catalytic performances of silica supported aluminum, gallium and indium for the tert-butylation of aromatics

Abstract

RHA-Al, RHA-Ga and RHA-In catalysts were synthesized by the direct incorporation of aluminum, gallium and indium ions, respectively, into rice husk ash (RHA) silica at room temperature using the sol–gel method. The prepared catalysts were characterized by physicochemical methods, viz. N2-adsorption, XRD, FT-IR, SEM-EDX, ICP-MS and Solid-state NMR. These catalysts were used to study the tert-butylation of benzene (Bz) and some substituted benzenes with tert-butyl chloride (TBC). The reaction was proposed to proceed initially through the radical mechanism and subsequently the tert-butyl cation was formed, which in turn attacks the benzene ring for the formation of tert-butyl benzene (TBB) and di-tert-butyl benzene (DTBB) via the SN1 mechanism (main reaction). However, a proton elimination reaction (side reaction) also occurred, resulting in the formation of isobutene dimers (IBD) and isobutene trimers (IBT). The extent of these side products were found to decrease significantly with time, indicating the reversibility of the oligomerization reactions. The catalysts were stable against leaching and were reusable several times but with observable drop in catalytic activity. RHA-Ga lost almost 20% of its activity after each run, whereas, RHA-In was stable until the 3rd run and then lost ~13% of its activity at the 5th run. The deactivation was suggested to be induced by the poisoning effect of the bulky side products.