Unveiling the intrinsic activity of TiO2 in HMF valorisation

[Display omitted] •TiO2 and its surface sites are active in HMF conversion under H2 and N2.•The surface of TiO2 contains exclusively Lewis acid sites.•The Lewis acid sites in TiO2 enable hydride transfer between HMF molecules.•HMF undergoes disproportionation yielding 5-MF and 2,5-DFF. In the context of 5-hydroxymethylfurfural (HMF) valorisation, TiO2 has emerged as a promising support. However, its intrinsic activity and underlying structural parameters remained undiscovered. This study elucidates the close relationship between structural and surface properties of TiO2 (e.g. crystalline size, surface area, and the number and type of acid sites) and its activity in HMF conversion. Lewis acid sites were found to be the exclusive acid sites present in anatase TiO2 samples, and their abundance correlated strongly with the overall surface area of TiO2. HMF conversion over TiO2 led to the simultaneous formation of 5-methylfurfural (5-MF) and 2,5-diformylfuran (2,5-DFF). Their formation was attributed to an intermolecular hydride transfer occurring over the Lewis acid sites. This opens up the possibility of selectively obtaining 5-MF and 2,5-DFF using solely TiO2 active sites. These findings provide valuable insights for designing TiO2-supported metal catalysts for future energy and environmental applications, aligning with green chemistry principles.