Strategic Design of a WO₃/PdO_x-Carbon Shell Composite Photocatalyst: Visible Light-Mediated Selective Hydrogenation of 5-Hydroxymethylfurfural

The development of visible light-assisted heterogeneous selective hydrogenation by utilizing solvents as a H2 source is a sustainable and energy-saving approach. Here, we have synthesized an efficient WO3/PdOx-carbon shell composite photocatalyst via the solution combustion method (SCM). The catalyst showed excellent activity for 5-hydroxymethylfurfural (HMF) hydrogenation in H2O/MeOH (3:1) under visible light (λ ≥ 420 nm) without the use of a pressurized external hydrogen support. The primary product was identified as 5-bis(hydroxymethyl)furan (BHMF) with a yield of 79.2%, which is regarded as a valuable feedstock for polymer industries. The minor hydrogenolysis product 5-methylfurfuryl alcohol (MFA) was also observed in proton NMR with a yield <20%. Our findings reveal that conversion efficiency (>95%) and selectivity (86.9%) increase drastically along with the pH range of the reaction medium, i.e., 7.5–8.2. The photocatalyst was embedded with both quantum-sized (0.6%) Pd particles (<5 nm) and PdO. After a series of experiments, we found that the uniform dimension (10 nm) of carbon shell benefits the stability of Pd particles (catalytic active site) and enhances charge carrier mobility. The photocatalyst is durable and reusable for up to more than six cycles without any significant loss in its activity. The progress of the reaction and key reactive species (•OH) were systematically investigated by proton NMR kinetic studies and electron paramagnetic resonance (EPR) spectroscopy. The possible reaction pathway was investigated and explored. These studies may contribute to serving as a cost-effective alternative method for a hydrogenating agent at an ambient condition without the use of a pressurized external hydrogen/temperature support under visible light.