| Dimethyl ether (DME) is a clean fuel, using as domestic fuel and power fuel alternative to liquefied petroleum gas and diesel fuel, and it is also an important chemical intermediates, which exhibition a wide range of application prospects. CO2 is a main greenhouse gas, and the CO2 hydrogenation to DME is an important way to convert CO2, which can reduce the pollution of CO2, and ease the energy shortage. However, CO2 molecule was very stable, and the conversion of the CO2 was difficult. It is a innovation to study the efficient catalysts and use the CO2 to react after activation.In the present paper, the modification of the Cu-based catalysts for CO2 hydrogenation to DME were studied, and the plasma is introduced to active the reactant. The following three aspects are studied:Firstly, Cu-Fe/HZSM-5 bifunctional catalysts were modified with La, Ce and were applied for the direct synthesis of DME from CO2 and H2. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, H2-temperature programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that La and Ce significantly decreased the outer-shell electron density of Cu and improved the reduction ability of the Cu-Fe catalyst in comparison to the Cu-Fe-Zr catalyst, which may increase the selectivity for DME. The Cu-Fe-Ce catalyst had a greater specific surface area than the Cu-Fe-La catalyst. This promoted CuO dispersion and decreased CuO crystallite size, which increased both the DME selectivity and the CO2 conversion. The catalysts were stable for 15 h.Secondly, a series of Cu-Fe-Ce catalysts with various CeO2 doping were prepared via the homogeneous precipitation method, and mechanically mixed with HZSM-5. Their feasibility and performance for the synthesis of DME via CO2 hydrogenation in a one-step process were evaluated. The formed stable solid solution after the Cu-Fe catalyst modified with CeO2 promoted the CuO dispersion, reduced the CuO crystallite size, decreased the reduction temperature of highly dispersed CuO, modified the specific surface area of the Cu-Fe-Ce catalyst. The addition of CeO2 to Cu-Fe catalyst modified the amount of acid sites and acid types, which in turn promoted the catalytic performance of CO2 hydrogenation to DME. The CO2 conversion and DME selectivity were 20.9%, and 63.1%, respectively, when the CO2 hydrogenation to DME using Cu-Fe-Ce/HZSM-5 catalyst with 3.0 wt% CeO2 was carried out at V(H2)/V(CO2) = 4,260 ℃, and 3.0 MPa with a gaseous hourly space velocity of 1500 mL·gcat-1·h-1.Finally, CO2 was activated by a high votage discharge plasma before feed to the fixed bed reactor, hydrogenated on the Cu-Fe-Ce/HZSM-5 with a CeO2 amount of 3.0 wt%, and the intrinsic kinetics of the hydrogenation process were studied. Compared with the using Cu-Fe-Ce/HZSM-5 catalyst alone, the CO2 conversion and the DME selectivity in a plasma activation combined hydrogenation process were increased 16.3%, and 10.1%, respectively, indicating that the plasma activation enhanced the reaction performance of CO2. The intrinsic kinetics study found that the activation energy of the methanol synthesis, the reverse water gas shift reaction, and the methanol dehydration to DME were 149.34 kJ·mol-1,75.47 kJ·mol-1, and 73.18 kJ·mol-1, respectively, which was 13.05 kJ·mol-1,46.20 kJ·mol-1, and 13.33 kJ·mol-1 lower than that using Cu-Fe-Ce/HZSM-5 catalyst alone, indicating that plasma activation can reduce the activation energy of the reaction to improve the yield of DME. |
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