Heteroatom-assisted oxygen vacancies in cerium oxide catalysts for efficient synthesis of dimethyl carbonate from CO2 and methanol

Abstract

The direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol is a green synthetic route owing to the nontoxicity of starting materials and synthetic protocol. One of the key challenges in this approach resides in designing effective catalysts which could activate CO₂ and methanol efficiently leading to a high yield of DMC. In this aspect, heteroatom (N and S) assisted CeO₂ nanorod materials (S-CeO₂-NR and N-CeO₂-NR) and pristine CeO₂-NR were synthesized to modulate the surface oxygen vacancies, Ce³⁺ concentration, acidity and basicity. The nanomaterials were characterized thoroughly by the combination of spectroscopic (Raman, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS)), microscopic (high-resolution transmission electron microscopy (HR-TEM)) and surface analytical techniques (N₂-physisorption, CO₂- and NH₃-temperature programmed desorption (TPD)). Among the CeO₂ nanomaterials including commercial CeO₂-NP, the N-CeO₂-NR outperformed all, exhibiting the highest yield of DMC under moderate reaction conditions in the order of N-CeO₂-NR (113.3 mmol g_cat⁻¹) > S-CeO₂-NR (49.7 mmol g_cat⁻¹) > CeO₂-NR (28.8 mmol g_cat⁻¹) > CeO₂-NP (0.8 mmol g_cat⁻¹) with 100% DMC selectivity. The enhanced efficiency of heteroatom-assisted materials (N-CeO₂-NR and S-CeO₂-NR) can be ascribed to their superior surface acidity, basicity, Ce³⁺ concentration, and oxygen vacancy concentrations in comparison to analogous materials, as supported by TPD, XPS and Raman analysis respectively.