Sulfated graphene as an efficient solid catalyst for acid-catalyzed liquid reactions

Abstract

Graphene with its two-dimensional sheet of sp2-hybridized carbon is a hot topic in the fields of materials and chemistry due to its unique features. Herein, we demonstrate that sulfated graphene is an efficient solid catalyst for acid-catalyzed liquid reactions. The sulfated graphene was synthesized from a facile hydrothermal sulfonation of reduced graphene oxide with fuming sulfuric acid at 180 °C. Combined characterizations of XRD, Raman, and AFM techniques show that G-SO3H has a sheet structure (1–4 layers). IR spectroscopy shows that G-SO3H has a SO bond, and the XPS technique confirms the presence of an S element in G-SO3H. Acid–base titration indicates that the acidic concentration of sulfonic groups in the sulfated graphene is 1.2 mmol g−1. TG curves shows that the decomposition temperature (268 °C) of the sulfonic groups on the sulfated graphene is much higher than that of conventional SO3H-functionalized ordered mesoporous carbon (237 °C). Catalytic tests of the esterification of acetic acid with cyclohexanol, the esterification of acetic acid with 1-butanol, the Peckmann reaction of resorcinol with ethyl acetoacetate, and the hydration of propylene oxide show that sulfated graphene is much more active than the conventional solid acid catalysts of Amberlyst 15, OMC-SO3H, SO3H-functionalized ordered mesoporous silica (SBA-15-SO3H), graphene oxide, and reduced graphene oxide, which is attributed to the fact that the sulfated graphene almost has no limitation of mass transfer due to its unique sheet structure. Very importantly, the sulfated graphene has extraordinary recyclability in these reactions, which is attributed to the stable sulfonic groups on the sulfated graphene. The advantages, including high activities and good recyclability as well as simple preparation, are potentially important for industrial applications of the sulfated graphene as an efficient heterogeneous solid acid catalyst in the future.