Utilizing Highly Reactive Lewis Pairs Generated by Oxygen Vacancies in the Cu3Mo2O9 Solid Catalyst for Cycloaddition of CO2 to 1,2-Propanediol

Abstract

This work emphasizes generating highly reactive Lewis pair sites on CuMo oxides for CO₂ activation and utilization in the cyclization reaction to produce propylene carbonate from 1,2-propanediol. The CuMo oxides were synthesized by enabling the oxygen vacancies that enhance the catalytically active sites, resulting in the formation of metastable cations (Mo⁵⁺ and Cu¹⁺) and oxygen vacancies. Under ethanol–PEG-400 medium, the pure phase of Cu₃Mo₂O₉ obtained at 500 °C exposed maximum defects without any secondary phase compared to other screened catalysts. The experimental and theoretical investigations provide evidence for determining and correlating the characteristics of active sites with catalytic performance. The catalysts were extensively characterized along with density functional theory (DFT) studies, which revealed the presence of defect centers as one of the key factors in the enhanced activity. From the chemical bonding analysis, i.e., Crystal Orbital Hamiltonian Population (COHP) and Electron Localization Function (ELF), the CO₂ molecule is known to form a strong chemisorption interaction with the catalyst surface that is facilitated by the oxygen vacancy/Lewis pairs. The Cu–Mo oxide catalyst achieved 99% conversion of 1,2-propanediol and 97% yield of propylene carbonate, outperforming previously reported catalysts. Thus, Cu–Mo oxide was shown to be highly efficient catalyst with good recyclability for 1,2-propanediol and the CO₂ reaction.