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

• Published in: Green chemistry : an international journal and green chemistry resource : GC 2019, Vol.21 (16), p.4341-4353
• Main Authors: Yu, Iris K. M, Xiong, Xinni, Tsang, Daniel C. W, Ng, Yun Hau, Clark, James H, Fan, Jiajun, Zhang, Shicheng, Hu, Changwei, Ok, Yong Sik
• Format: Article
• Language: English
• Subjects: Aluminum chloride; Biorefineries; Catalysis; Catalysts; Catalytic activity; Coordination; Fructose; Functional groups; Glucose; Graphene; Graphite; Green chemistry; Hydroxides; Irradiation; Isomerization; Lewis acid; Microwave absorption; NMR; Nuclear magnetic resonance; Organic chemistry; Physicochemical properties; Pretreatment; Surface chemistry; X ray photoelectron spectroscopy
• ISSN: 1463-9262; 1463-9270
• DOI: 10.1039/c9gc00734b

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 AlCl 3 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 27 Al 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. Oxygen functionalities on GIO/GO are involved in the formation of active Al sites potentially with a microwave-absorbing ability, which facilitates glucose-fructose isomerisation in water.