Studies of the Raman spectra of cyclic and acyclic molecules: Combination and prediction spectrum methods

Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran by Visible Light-Driven Photocatalysis over In Situ Substrate-Sensitized Titania

Solar energy-driven processes for biomass valorization are priority for the growing industrialized society. To address this challenge, efficient visible light-active photocatalyst for the selective oxidation of biomass-derived platform chemical is highly desirable. Herein, selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was achieved by visible light-driven photocatalysis over titania. Pristine titania is photocatalytically inactive under visible light, so an unconventional approach was employed for the visible light (λ=515 nm) sensitization of titania via a formation of a visible light-absorbing complex of HMF (substrate) on the titania surface. Surface-complexation of HMF on titania mediated ligand-to-metal charge transfer (LMCT) under visible light, which efficiently catalyzed the oxidation of HMF to DFF. A high DFF selectivity of 87 % was achieved with 59 % HMF conversion after 4 h of illumination. The apparent quantum yield obtained for DFF production was calculated to be 6.3 %. It was proposed that the dissociative interaction of hydroxyl groups of HMF and the titania surface is responsible for the surface-complex formation. When the hydroxyl groups of titania were modified via surface-fluorination or calcination the oxidation of HMF was inhibited under visible light, signifying that hydroxyl groups are decisive for photocatalytic activity.

Origin of 5-hydroxymethylfurfural stability in water/dimethyl sulfoxide mixtures

In the present work, we combined vibrational spectroscopy with electronic structure calculations to understand the solvation of HMF in DMSO, water, and DMSO/water mixtures and to provide insights into the observed hindrance of HMF rehydration and aldol condensation reactions if it is dissolved in DMSO/water mixtures. To achieve this goal, the attenuated total reflection FTIR spectra of a wide composition range of binary and ternary mixtures were measured, analyzed, and compared to the findings of ab initio DFT calculations. The effect of solvent on the HMF C-O and O-H vibrational modes reveals significant differences that are ascribed to different intermolecular interactions between HMF and DMSO or water. We also found that DMSO binds to HMF more strongly than water, and interactions with the HMF hydroxyl group are stronger than those with the HMF carbonyl group. We also showed the preferential solvation of HMF C-O groups by DMSO if HMF is dissolved in DMSO/water mixed solvent. Frontier molecular orbital theory was used to examine the influence of the solvent on side reactions. The results show that HMF solvation by DMSO increases its LUMO energy, which reduces its susceptibility to nucleophilic attack and minimizes undesirable hydration and humin-formation reactions. This result, together with the preferential solvation of HMF by DMSO, provide an explanation for the enhanced HMF stability in DMSO/water mixtures observed experimentally.