2-Methylimidazole-Assisted Synthesis of Nanosized Cu₃(BTC)₂ for Controlling the Selectivity of the Catalytic Oxidation of Styrene

A nanosized Cu3(BTC)2 (HKUST-1; BTC = 1,3,5-benzenetricarboxylate) catalyst with a size range of 10–20 nm was synthesized with 2-methylimidazole (2-MI) as an effective competitive ligand and Lewis base to tune its morphology and improve its catalytic performance simultaneously. On the one hand, the addition of 2-MI can accelerate deprotonation of the bridging ligand H3BTC, thereby accelerating the rapid nucleation of crystals, which leads to a rapid decrease of the solution supersaturation and inhibits further growth of the nucleated small crystal, resulting in nanosized HKUST-1 crystals. On the other hand, the uncoordinated 2-MI in nano-HKUST-1 can improve the catalytic performance of the crystal because of its basicity. Further catalytic experiments demonstrate that nano-HKUST-1 showed high catalytic activity compared with bulk HKUST-1 in the controllable oxidation of styrene under solvent-free conditions. More importantly, both benzaldehydes and benzoic acids were successfully obtained via a facile variation of the reaction temperature or time. The benzaldehydes can be produced in 78% gas chromatographic yield after 9 h at 50 °C or 72% after 50 min at 80 °C, while benzoic acids can be obtained in moderate-to-good yield after 6 h at 80 °C. Notably, nano-HKUST-1 could be expediently recycled in four cycles without significantly affecting the yield of the desired product.