Selective Photocatalytic Oxidation of 5-Hydroxymethylfurfural using Ce doped TiO2: A Crucial Role of Defective Sites

Transforming platform compounds with conventional catalysts, such as homogeneous and heterogeneous, into valuable chemicals encounters challenges associated with entailing high temperature/pressure and reusability during the reaction. To address these challenges, photocatalysis is one of the promising approaches, especially with visible-light-driven photocatalysts for selective transformation. A series of cerium-doped TiO2 (CexTiO(2); x=0.5, 1, 3 and 5 wt %) are prepared via a simple coprecipitation method for the cocatalyst-free selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) under visible light illumination. Introducing Ce into the network to TiO2 generates impurity energy levels, reduces the band gap, and extends the spectral response range. This also causes slight distortion to the surface structure, thereby originating defective sites and various surface oxygen species, confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance studies. The HMF adsorption studies infer that HMF forms a surface complex with CeTiO2, facilitating the formation of ligand-to-metal charge transfer complex (LMCT) under visible light (similar to 420 nm), which efficiently catalyzes the conversion of HMF (54.4 %) to DFF with a selectivity of >99 %. In-situ DRIFTS and surface passivation studies provide insight into the interaction between HMF and surface acidic/basic sites along with hydroxyl groups of CeTiO2, which subsequently convert selectively to DFF.