Graphite oxide- and graphene oxide-supported catalysts for microwave-assisted glucose isomerisation in water†
Graphite (G), graphite oxide (GIO), and graphene oxide (GO) were evaluated for the first time as carbonaceous 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 °C for 20 min. The major active sites were characterised as amorphous Al hydroxides (e.g., β-Al(OH)3, γ-Al(OH)3, and γ-AlO(OH)) with octahedral coordination, as revealed by 27Al 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.