Synthesis Of Dimethyl Ether From CO₂Hydrogenation Over Cu-Fe-Zr/HZSM-5Catalyst

Dimethyl ether was both a kind of basic chemical material and a new type of fuel. The reduction of CO2into dimethyl ether with a high added value can solve the problems triggered by the petrochemical resources shortage and the environmental pollution. It become a hotspot on the fields of energy, environment, physics, chemistry, material. However, the development of the CO2conversion technology was limited due to the inertia of CO2. The catalyst development for the high efficient conversion of CO2and the study of the mechanism for dimethyl ether synthesis from CO2were the keys to promote the CO2conversion technology.In this paper, the Cu-Fe-Zr/HZSM-5catalyst and the synthesis of dimethyl ether from CO2were studied and the following three aspects were included in the studies.Firstly, Cu-Fe-Zr/HZSM-5catalyst, the new CO2-activated catalyst, was developed and the relationship between the structure and the catalytic performance of the catalyst was analyzed by a series of characterization methods and activity evaluation experiments. The catalyst was characterized by the X-ray diffraction(XRD), the nitrogen adsorption/desorption, the H2temperature programmed reduction(H2-TPR), the temperature programmed desorption (H2-TPD or CO2-TPD), the X-ray photoelectron spectra(XPS), the thermogravimetry/differential thermal analyzer(TG-DTA) and other analysis methods. The results showed that the pore size and the specific surface area of the catalyst can be changed by ZrO2doping. The interaction between the Cu and Fe species can be promoted by ZrO2doping. The chemical combination state of CuO was changed and the reducibility of the Cu-Fe catalyst was increased by ZrO2doping. At the same time, the adsorption performance of the Cu-Fe catalyst for H2and CO2can be controlled by changing the adsorption intensity for H2and CO2on the Cu-Fe catalyst. The reduction temperature, the reaction temperature, the reaction pressure, the space velocity and other influence factors of synthesis of dimethyl ether from CO2were investigated. The study showed that reducing at300℃, reacting at260℃and3.0MPa with a gaseous hourly space velocity(GHSV) of1500mL·gcat-1·h-1were the optimum reaction conditions for the synthesis of dimethyl ether from CO2with a28.4%conversion of CO2with64.5%selectivity for dimethyl ether, respectively. During a16h reaction process, the activity of the Cu-Fe-Zr/HZSM-5remained stable.Secondly, the intrinsic kinetics of the dimethyl ether synthesis process from CO2was researched and the intrinsic kinetic model was established in order to provide the experimental data and the theoretical basis for improving CO2conversion and dimethyl ether selectivity. The reverse water gas shift reaction, the methanol synthesis and the dehydration of methanol to dimethyl ether were the basic reactions for the intrinsic kinetics of the synthesis of dimethyl ether from CO2. The activation energy were109.16kJ·mol-1,173.72kJ·mol-1and89.91kJ·mol-1. The productions of H2CO and HXCH3OHCH3+were the rate determined steps for the methanol synthesis and the dehydration of methanol to dimethyl ether, respectively. The reaction rate of the synthesis of dimethyl ether from CO2can be increased by increasing the reaction rates of the two intermediates, H2CO and HXCH3OHCH3+.Finally, the adsorption state of CO2on Cu(111) surface and the methanol synthesis reaction were studied by using the density functional theory. To the eight adsorption configurations of CO2on the Cu(111) surface, when CO2in the horizontal form was adsorbed on the bridge site of Cu(111) surface, the adsorption energy of the adsorption configuration was lowest and the adsorption configuration was the optimal adsorption configuration. When the linear-type CO2in the horizontal form was adsorbed on the bridge site of Cu(111) surface, the adsorption energy was-8.70kJ·mol-1and CO2was physically adsorbed on Cu (111) surface. The main role of Cu was to dissociate and adsorb H2. CO2was adsorbed on Fe3O4which was regarded as a catalytic promoter. Then CO2was spilled to the interface between Cu and Fe3O4. The reaction between the CO2molecule and the dissociative adsorption of H on the Cu surface were occurred directly. The molecular simulation was used on the study of two pathways of the methanol synthesis from CO2and H2. The result indicated the methanol synthesis from CO2was carried out following the formate pathway. To the results of proving the rate determining steps of the reverse water gas shift reaction and the methanol synthesis reaction and the reaction energy barriers of the two reactions, the molecular simulation results was in accordance with the intrinsic kinetics results. Thus the experimental process can be predicted and guided by the molecular simulation.