Direct Synthesis Of Diethyl Carbonate From CO₂ And Ethanol Over ZrO₂ Catalysts

Carbon dioxide, which is blamed for the global green house effect, has attracted researchers'attention for its utilization. The process for direct synthesis of diethyl carbonate (DEC) from CO₂ and ethanol not only produces economic chemicals with high value, but also improves environment by using CO₂ as recycled carbon resource. In this paper, direct synthesis of DEC from CO₂ and ethanol over ZrO₂ was studied, including the method of catalyst preparation and the mechanism investigation.ZrO₂ is an effective catalyst for direct synthesis of diethyl carbonate from CO₂ and ethanol. When CO₂ and ethanol were charged with molar ratio of 238/427 mmol, 0.45 mmol DEC with trace amount of acetaldehyde and acetal can be obtained after reacting for 2 h at 160℃, and the selectivity was above 90%. The reaction of CO₂ with alkyl and aromatic alcohols were also investigated. The reactions of methanol, 1-propanol and 1-butanol generated carbonates. However, carbonates were not detected in the reactions with phenol and benzyl alcohol.The catalytic properties of ZrO₂ were closely related with the acidic and basic sites on the surface. As the calcination temperature increased, the strength of acid on the surface of ZrO₂ decreased, and the strength of base increased. ZrO₂ calcined at 800℃had better activity as appropriate acid and basic sites existed on the surface. The crystalline structure of ZrO₂ had no effect on the reaction. ZrO₂ modified by H3PO4 was more reactive at lower reaction temperature. The presence of P changed the structure of ZrO₂ from monoclinic to tetragonal, and the acid strength on the surface did not change, but the base strength increased.In situ infrared spectroscopy was used to investigate the mechanism of synthesis of diethyl carbonate from methanol and CO₂ over ZrO₂. The O atom of ethanol coordinates with OH present at the catalyst surface to form ethoxide and simultaneously proton is released, which reacts with hydroxyl group on the ZrO₂ surface to produce water. Dissociative carbon dioxide inserts into the Zr-OCH2CH3 to form ethyl carbonate group (Zr-OCOOCH2CH3) as a mediate species. DEC is formed by reaction of the ethyl carbonate species with ethanol.