Dimethyl carbonate synthesis from CO2 and methanol over CeO2-ZrO2 catalyst

Mn-doped CeO2 derived from Ce-MOF porous nanoribbons as highly active catalysts for the synthesis of dimethyl carbonate from CO2 and methanol

It is desirable but challenging to develop highly-efficient catalysts for the direct synthesis of dimethyl carbonate (DMC) from methanol and CO2. The vacancy-mediated incorporation of heteroatom into surface reconstruction is an efficient method of defect engineering for enhancing the catalytic properties. In this work, manganese-doped cerium oxide porous nanoribbons (Mn/CeO2-BTC) were prepared derived from a Ce-BTC by a sacrificial template approach. It is found that the catalytic activity of Mn/CeO2-BTC catalysts can be readily controlled by varying the amount of Mn dopants and the as-synthesized 0.1-Mn/CeO2-BTC exhibited an outstanding activity for the synthesis of DMC from CO2 and methanol, which reached a high DMC yield (6.53 mmolDMC/gcat.) without any dehydrating agents. Based on characterization results, the enhanced performance may be attributed to the defective structures caused by Mn doping and the porous nanoribbons of the CeO2 crystals, which provide more surface oxygen vacancies and acidic-basic sites, favoring adsorption and activation of CO2 and methanol.

Uncovering the role of yttrium in a cerium-based binary oxide in the catalytic conversion of carbon dioxide and methanol to dimethyl carbonate

Cerium(IV) oxide (CeO2)-based materials are effective catalysts for the synthesis of dimethyl carbonate (DMC) from carbon dioxide (CO2) and methanol (CH3OH). Herein, 5% Y-CeO2 was synthesized by the co-precipitation method. It forms a solid solution structure, which leads to the highest concentration of oxygen vacancies. The Y-VO-Ce active site created by Y3+ doping enhances the adsorption and activation of CO2 based on moderately passivating CH3OH adsorption. Consequently, 5% Y-CeO2 exhibited the highest CH3OH conversion rate of 0.8% and a DMC yield of 15 mmol⋅(g cat)-1, which is 1.4 times of pure CeO2 (reacting in a stainless-steel autoclave at 140 °C with a stirring speed of 1000 r⋅min-1 and an initial pressure of 3.0 MPa for 2 h). An adsorption test and in situ diffuse reflectance infrared Fourier transform spectroscopy showed that 5% Y-CeO2 could effectively inhibit the formation of triple-bonded methoxy species, and promote the formation of bidentate carbonate and bridged methoxy intermediates, which is conducive to the improvement of reaction activity.

Elucidating the Role of Surface Ce4+ and Oxygen Vacancies of CeO2 in the Direct Synthesis of Dimethyl Carbonate from CO2 and Methanol

Cerium dioxide (CeO₂) was pretreated with reduction and reoxidation under different conditions in order to elucidate the role of surface Ce⁴⁺ and oxygen vacancies in the catalytic activity for direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol. The corresponding catalysts were comprehensively characterized using N₂ physisorption, XRD, TEM, XPS, TPD, and CO₂-FTIR. The results indicated that reduction treatment promotes the conversion of Ce⁴⁺ to Ce³⁺ and improves the concentration of surface oxygen vacancies, while reoxidation treatment facilitates the conversion of Ce³⁺ to Ce⁴⁺ and decreases the concentration of surface oxygen vacancies. The catalytic activity was linear with the number of moderate acidic/basic sites. The surface Ce⁴⁺ rather than oxygen vacancies, as Lewis acid sites, promoted the adsorption of CO₂ and the formation of active bidentate carbonates. The number of moderate basic sites and the catalytic activity were positively correlated with the surface concentration of Ce⁴⁺ but negatively correlated with the surface concentration of oxygen vacancies. The surface Ce⁴⁺ and lattice oxygen were active Lewis acid and base sites respectively for CeO₂ catalyst, while surface oxygen vacancy and lattice oxygen were active Lewis acid and base sites, respectively, for metal-doped CeO₂ catalysts. This may result from the different natures of oxygen vacancies in CeO₂ and metal-doped CeO₂ catalysts.