Density Functional Theory Studies On Direct Synthesis Of Dimethyl Carbonate From CO₂ And Methanol Over ZrO₂ Catalysts

The burning of massive fossil fuels such as coal and oil has caused a sharp increase in the concentration of CO₂ in the atmosphere,aggravated the greenhouse effect and posed a serious threat to the global ecological environment.CO₂ capture and conversion has become a hot issue,and its catalytic conversion into chemicals and fuels is of great significance for sustainable development today.Dimethyl carbonate(DMC)is an important green chemical,which can be used as a synthetic intermediate,solvent and gasoline additive,etc.Direct synthesis of DMC from CO₂ and methanol is an effective way to realize resource utilization.In this paper,the reaction mechanism of direct synthesis of DMC from CO₂ and methanol on the ZrO₂ surface was studied by density functional theory(DFT)calculation and experiments.The influence of ZrO₂ crystal phase,surface hydroxyl group,hetero atom doping and supported Cu clusters on the reaction was discussed,providing theoretical guidance for the design and development of efficient ZrO₂ based catalyst.The main research contents and conclusions are as follows:(1)The effects of the clean cubic(c),tetragonal(t),monoclinic(m)ZrO₂catalysts on direct synthesis of DMC from CO₂ and methanol were investigated by DFT method.The results show that the optimal path on the three catalysts is^<sup>*CH₃OCO intermediate path,and the activity order is t-ZrO₂>m-ZrO₂>c-ZrO₂,which is mainly attributed to the difference of surface Lewis acid-base properties and electronic structures.Namely,the stronger the surface Lewis acid-base is and the smaller the band gap is,the higher the activity is.The electron difference density results show that compared with m-ZrO₂ and c-ZrO₂,there are more charge transfer between Zr atom and neighboring O atom on the ZrO₂ surface.(2)The influences of clean and hydroxylated t-ZrO₂ and m-ZrO₂ on direct synthesis of DMC from CO₂ and methanol were compared by DFT calculation and experimental methods.The results show that the electron density on hydroxylated ZrO₂ surfaces are rearranged and the reaction path is changed.DMC is generated by the reaction between^<sup>*CH₃OCOO and^<sup>*CH₃ species.m-ZrO₂ has higher catalytic activity than t-ZrO₂.^<sup>*CH₃OCOO species are formed at base sites on the hydroxylated ZrO₂ surface,and the strength and amount of the base sites on m-ZrO₂ and t-ZrO₂ surfaces are almost the same.^<sup>*CH₃ species are formed at acid sites,and the formation rate is positively correlated with the strength and amount of Br(?)nsted acid sites.The results show that compared with the clean m-ZrO₂ surface,the catalytic activity of hydroxylated m-ZrO₂ is mainly affected by Br(?)nsted acid sites.(3)The effects of on Si,Mg-doped hydroxylated m-ZrO₂(SHM and MHM)on direct synthesis of DMC from CO₂ and methanol were investigated by DFT method.The results show that the optimal pathway on SHM is^*CH₃OCO intermediate pathway,while two parallel pathways of^*CH₃OCO intermediates and^*CH₃ intermediates on MHM.Compared with hydroxylated m-ZrO₂,the doping of Si atom enhances the catalytic activity,while the doping of Mg atom does the opposite.The results show that the doping of Si atom enhances the surface Br(?)nsted acidity and effectively regulates the catalyst electronic structures,improving the catalytic activity.(4)The effects of m-ZrO₂ with Br(?)nsted acid sites supported Cu₄ clusters(H(Cu₄))on direct synthesis of DMC from CO₂ and methanol was studied by DFT method.The results show that the two pathways of^*CH₃OCO intermediate pathway and^*CH₃ intermediate pathway simultaneously proceed on H(Cu₄)surface.Compared with hydroxylated m-ZrO₂,methyl formation at Br(?)nsted acid sites on H(Cu₄)surface is favorable for the synthesis of DMC.Compared with the catalyst with unloaded Cu₄ clusters,there are strong charge transfer between the loaded Cu₄ clusters and H(Cu₄)surface,which promotes the surface charge rearrangement and reduces the band gap,thus improving the catalytic activity.The excellent performance of H(Cu₄)surface is attributed to the role of Br(?)nsted acid sites and the adjustment of surface electronic structure by Cu₄ clusters.