Synergistic surface-functionalized metal-organic polyhedra for solketal synthesis from glycerol and acetone

Abstract: Biodiesel serves as a sustainable energy source that meets future energy demands while reducing greenhouse gas emissions. Its by-product, glycerol, is increasingly utilized in the synthesis of high-value compounds like solketal, derived from the reaction of glycerol and acetone. In this work, we designed Rhodium-based metal-organic polyhedra (MOP) Rh2[5- sulfo-isophthalic acid]24 (denoted as RhMOP-SO₃H) by coordinating 5-sulfo-isophthalic acid with Rh(II) paddlewheel centers. The resultant MOP shows unique structural features of combined Lewis and Brønsted acidic sites including surface-functional sulfonated groups, active rhodium metallic centers, and hierarchical pores, all contributing to its significant stability and efficacy in the ketal reaction between glycerol and acetone. The resultant RhMOP- SO3H demonstrated remarkable catalytic activity in solketal production, achieving glycerol conversion rate of up to 86% and a solketal selectivity of up to 97%. To the best of our knowledge, the specific productivity achieved the highest recorded value for any catalyst to date, reaching 7752 mmol g⁻¹ h⁻¹ when utilizing 0.1 wt% glycerol. Density functional theory (DFT) calculations reveal that the catalytic mechanism of RhMOP-SO₃H involves a nucleophilic attack of acetone on glycidol, accompanied by cyclization facilitated by the sulfonated acidic groups on the catalyst. We also explored other RhMOPs with different surface functional groups, such as amine and proton, by designing Rh2[isalophthalic acid]24 (RhMOP-H) and Rh2[5-amino-isophthalic acid]24 (RhMOP-NH₂) to explore their role in solketal catalytic activity. This work not only enhances our understanding of biodiesel by- products utilization but also contributes to the advancement of green chemical processes.