Graphite oxide- and graphene oxide-supported catalysts for microwave-assisted glucose isomerisation in water

Abstract

Graphite (G), graphite oxide (GIO), and graphene oxide (GO) were evaluated for the first time as carbon-aceous supports to synthesise heterogeneous Lewis acid catalysts, via simple AlCl3 pretreatment followed by one-step thermal modification. The GIO-and GO-supported Al catalysts were active towards catalytic isomerisation of glucose in water as the greenest solvent. The highest fructose yield of 34.6 mol% was achieved under microwave heating at 140 degrees C for 20 min. The major active sites were characterised as amorphous Al hydroxides (e.g., ss-Al(OH)(3), gamma-Al(OH)(3), and gamma-AlO(OH)) with octahedral coordination, as revealed by Al-27 NMR, XPS, SEM, TEM-EDX, Raman, ESR, and XRD analyses. The transformation of octahedral Al to pentahedral/tetrahedral coordination was observed when the activation temperature increased. Oxygen-containing functional groups on the GIO and GO surfaces, e.g., C-O-C, -OH, and -COOH, contributed to the formation of microwave-absorbing active sites. In contrast, the G-supported catalyst may contain microwave-transparent Al hydroxides, accounting for its low catalytic activity under microwave irradiation. This study elucidates the significance of the surface chemistry of carbonaceous supports in generating active species for a Lewis acid-driven reaction. The revealed intertwined relationships among modification conditions, physicochemical properties, and catalytic performance will be useful for designing effective carbon-supported catalysts for sustainable biorefinery.