Tuning the active sites in reduced graphene oxide by hydroquinone functionalization for the aerobic oxidations of thiophenol and indane

Abstract

Sustainable reduced graphene oxide (rGO) carbocatalysts for molecular oxygen activation have been developed based on the concept that chemical reduction of graphene oxide to rGO introduces functional groups that can become active sites. Thus, a series of rGOs prepared using hydroquinone (HQ), hydrazine (HZ) and ascorbic acid (ASC) as reducing agents have been tested as catalysts in the aerobic oxidations of thiophenol and indane. In both oxidation reactions, rGO reduced by HQ (rGO-HQ) exhibited the highest catalytic activity and stability. A high selectivity of ol/one (90 %) at around 30 % indane conversion in the absence of transition metals was achieved. Reusability test showed that the activity of rGO-HQ is mostly retained in three consecutive runs. Quenching experiments indicated that the reaction proceeds with the generation of hydroxyl and carbon centered radicals. The higher activity of rGO-HQ was attributed to the covalent anchoring of some HQ molecules on rGO during the chemical reduction. This knowledge has been used to develop a HQ-functionalized Merrifield resin that in spite of its very different textural properties compared to rGO also exhibits catalytic activity for O2 activation. The present work illustrates the potential to develop rGO carbocatalysts with an increased density of active sites by selecting an appropriate reducing agent during the chemical reduction of graphene oxide.