The benzylation of benzene using aluminium, gallium and iron incorporated silica from rice husk ash

Abstract

Al, Ga and Fe catalysts supported on silica obtained from rice husk ash (RHA) were synthesized using the sol–gel technique at room temperature and were denoted as RHA–Al, RHA–Ga and RHA–Fe, respectively. The XRD and SEM results revealed that the amorphous and porous structure of RHA silica was retained after the incorporation of Al 3+, Ga 3+ and Fe 3+ ions. EDX analysis confirmed Al 3+, Ga 3+ and Fe 3+ ions were attached to the silica matrix and were homogeneously distributed. 27Al MAS NMR results of RHA–Al revealed the Al 3+ ions were incorporated into the silica network with tetrahedral coordination. The benzylation of benzene (Bz) and substituted benzenes with benzyl chloride (BC) were studied over the prepared catalysts. RHA–Fe showed excellent activity for the benzylation of benzene, whereas, RHA–Ga gave good selectivity towards diphenylmethane (DPM). Almost complete BC conversion and about 86% and 80% selectivity to DPM were obtained after 90 and 13 min over RHA–Ga and RHA–Fe, respectively. However, RHA–Al was found to be inactive under the reaction conditions studied. Catalytic activity of RHA–Ga with substituted benzenes showed a reverse trend to that of the conventional Friedel–Crafts reaction, i.e. benzene > toluene > ethyl benzene > anisole. The catalysts could be reused for the benzylation reaction several times without significant loss in their activity and selectivity. Kinetic studies showed that the activation energy for the benzylation of benzene was 26.4 and 17.6 kcal mol −1 over RHA–Ga and RHA–Fe. The frequency factor for RHA–Ga and RHA–Fe was found to be 5.3 × 10 14 and 1.7 × 10 10 min −1, respectively.