Bifunctional ZnO/HZSM-5 Catalysts in Direct Hydrogenation of CO2 to Aromatics; Influence of Preparation Method

ZnO-incorporated HZSM-5 zeolites (ZnO/HZSM-5) were prepared through five different methods including Wet Impregnation (WI), Ion Exchange (IE), Solid-State (SS), Chemical Vapor Deposition (CVD), and Hydrothermal Synthesis (HS) and the influence of preparation method on the performance of ZnO/HZSM-5 catalysts in direct hydrogenation of CO2 to aromatics was evaluated. The addition of ZnO through IE, CVD, WI and SS methods led to decrease in the surface area and micropore volume to different extents, whereas introducing ZnO to the synthesis gel indicated slight increase in both surface area and pore volume. Experiments proved that high amount of Bronsted asidity is the main factor in enhancing CO2 conversion and aromatics synthesis. the introduction of ZnO overally decreased the weak acid sites, while the amount of strong acid sites increased. Moreover, some new medium acid sites were created in all ZnO/HZSM-5 zeolites, as detected by NH3-TPD analysis. Furthermore, UV-vis results showed that most of the ZnO particles located on the surface of ZnO/HZSM-5-HS and ZnO/HZSM-5-IE, while some ZnO clusters deposited in the pores and channels of ZnO/HZSM-5-SS and ZnO/HZSM-5-CVD catalysts. The amount of CO2 conversion followd the sequence of: ZnO/HZSM-5-HS > ZnO/HZSM-5-IE > ZnO/HZSM-5-WI > ZnO/HZSM-5-CVD > HZSM-5 > ZnO/HZSM-5-SS. However, ZnO/ZSM-5-HS prepared by hydrothermal synthesis indicated the outstanding aromatics selectivity of 57.5% as well as CO2 conversion of nearly 85% towards other ZnO-containig HZSM-5 zeolites. [GRAPHICS] .