Propylene carbonate and γ-valerolactone as green solvents enhance Sn(iv)-catalysed hydroxymethylfurfural (HMF) production from bread waste†
Two green solvents, namely propylene carbonate (PC) and γ-valerolactone (GVL), were examined as co-solvents in the conversion of bread waste to hydroxymethylfurfural (HMF) over SnCl4 as the catalyst under microwave heating at 120 °C, and their performances were compared with water and acetone as a common solvent. The results showed that a HMF yield of ∼20 mol% was achieved at 7.5 and 20 min in the PC/H2O and GVL/H2O (1 : 1 v/v) systems, respectively, implying that the tandem reactions (starch hydrolysis, glucose isomerisation, and fructose dehydration) were efficient. The green systems played a critical role in maintaining effective Lewis acid sites, i.e., Sn4+, to a greater extent compared with acetone/H2O and water, where loss of Sn4+ from the liquid phase to colloidal SnO2 particles via hydrolysis was evidenced by X-ray diffraction analysis. In comparison, the utilisation of glucose (47–59 mol% from bread starch within 10 min) appeared as the rate-limiting step in the acetone/H2O and water systems. When comparing PC/H2O with GVL/H2O, the kinetics of overall conversion in the former was more favourable, which was associated with the high in-vessel pressure developed via liberation of CO2 from PC. In addition, dipole moment and dielectric constant of the solvents may also account for their respective performance. This study elucidates the multiple roles of the solvents and their interplay with the catalysts, and advocates the application of green solvents to facilitate catalytic conversion of biomass with a lower energy requirement.