Structure of the active sites on H₃Po₄/ZrO₂ catalysts for dimethyl carbonate synthesis from methanol and carbon dioxide

The catalytic properties of H₃PO₄/ZrO₂ catalysts for dimethyl carbonate (DMC) synthesis from CH₃OH and CO₂ were investigated. The modification of ZrO₂ with H₃PO₄ promoted the activity for selective DMC formation. The characterization of H₃PO₄/ZrO₂ catalysts was performed by means of BET, XRD, ³¹P MAS NMR, LRS, DRIFT, and NH₃-TPD. From the characterization results, it was determined that ZrO₂ had Lewis acid sites and did not have Brønsted acid sites. The presence of Brønsted acid sites as well as Lewis acid sites was observed on H₃PO₄/ZrO₂ (P/Zr = 0.05). The acidity of the Brønsted sites is weak, and the site's structure is a bridged OH. Furthermore, this Brønsted site is formed by the interaction between tetragonal Zr and P. In the reaction scheme of DMC formation, the Brønsted acid site can contribute to CH₃OH activation and to the enhancement of the catalytic activity.