Unveiling the Pivotal Role of Ce Coordination Structures and Their Surface Arrangements in Governing 2-Cyanopyridine Hydrolysis for Direct Dimethyl Carbonate Synthesis from CO2 and Methanol

Abstract

The direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol presents a promising alternative to conventional methods that use toxic chemicals, but its yield is limited by equilibrium. Coupling this reaction with 2-cyanopyridine (2-Cp) hydrolysis over CeO₂-based catalysts was found to significantly boost the DMC yield by removing water. Our recent study has revealed that methanol is the key species being activated by surface Ce sites to produce DMC. The reactivity of surface methoxy species toward CO₂ varies greatly with their configuration, which is determined by the Ce coordination structures. A similar challenge remains in understanding the CeO₂ surface feature governing the hydrolysis of 2-Cp to 2-picolinamide (2-PA). Herein, CeO₂ nanocrystallites with well-defined (111), (110), and (100) surfaces were used to study the effects of Ce coordination structures and their arrangements in this reaction and coupled DMC synthesis. We found that the synergistic adsorption of 2-Cp via cyano-N and pyridine-N on (111) and (110) surfaces enables nucleophilic addition of lattice oxygen, producing imino-like N with stronger Lewis basicity, which in turn facilitates hydrolysis. The (111) surface outperforms the (110) surface due to its unique Ce coordination structure and arrangement, which allows more 2-Cp activation and easier 2-PA desorption. Notably, the (111)-enclosed octahedral CeO₂ used herein outperforms the reported pristine CeO₂ catalysts in this coupled reaction. In contrast, this synergistic adsorption/activation does not occur on the (100) surface, leading to low activity. These findings provide insights for designing CeO₂-based catalysts for CO₂ conversion with alcohols and amines using 2-Cp as a dehydrant.