Study On CO₂ Hydrogenation Performance Of Hydrophobic Modified Silicalite-1 Zeolite Coupled With Fe-based Catalyst

Utilizing CO2 resources is essential to achieving China’s target of "carbon peak" by 2030 and"carbon neutral" by 2060.Progress has been made in the production of high-value-added chemicals from renewable H2 coupled with CO2,but the molecular chemical inertness of CO2 and the kinetic barriers to C-C bond formation make CO2 activation and chain growth difficult.Due to the high activity of Fe-based catalysts for both the Fischer-Tropsch synthesis(FTS)and Reverse Water Gas Shift(RWGS)reactions,the CO2-FTS pathway via Fe-based catalysts has attracted a lot of attention.Iron oxide(mainly Fe3O4)is the active phase of the RWGS reaction and iron carbide(mainly Fe5C2)is the active phase of the FTS reaction in the CO2-FTS pathway.Both reactions produce a significant amount of water as a byproduct.Water will reabsorb on the catalyst surface if it is not removed from the catalytic system in time,which will result in a number of issues:(1)limiting the reaction equilibrium;(2)covering the active sites on the catalyst surface;(3)oxidizing the active phase of the catalyst.Therefore,it is important to regulate the adsorption and desorption of by-product H2O on the catalyst surface to improve the CO2 conversion as well as the target product selectivity.In response to the above issues,our present work takes the regulation of the adsorption and desorption behavior of H2O on the surface of Fe-based catalysts as the starting point.The hydrophobic Silicalite-1 zeolite was designed and synthesized by the silylation method and their hydrophobicity was regulated by changing the time of the silylation reaction.Fe3O4 and Fe5C2 are prepared by the solvothermal and thermal reduction-carburization methods respectively.The hydrophobic Silicalite-1 zeolite was coupled with Fe3O4 and Fe5C2,respectively,and their CO2 hydrogenation behavior was investigated at H2/CO2=3/1,a reaction pressure of 3 MPa,and a reaction temperature of 320℃.The main results obtained are as follows:1.The hydrophobic Silicalite-1 zeolite was characterized as follows:water contact angle test showed that the Silicalite-1 zeolite before hydrophobic modification was 28.72° and after hydrophobic modification became 92.85°,105.41°,and 134.18°;Zeta potential and FT-IR tests showed that the number of hydroxyl groups on the surface of the hydrophobically modified Silicalite-1 zeolite was reduced and hydrophobic groups were successfully grafted onto the surface of the Silicalite-1 zeolite;XRD and N2 physical adsorption and desorption tests showed that the physical phase and weave properties of Silicalite-1 zeolite did not change significantly after hydrophobic modification,respectively.The above characterization results indicate that Silicalite-1 zeolite with adjustable hydrophobicity was successfully prepared by silylation,and the original properties of Silicalite-1 zeolite remained basically unchanged.2.Investigating the CO2 hydrogenation behaviour of Fe3O4 coupled hydrophobic Silicalite-1 zeolite.The CO2 hydrogenation behavior of Fe3O4 coupled hydrophobic Silicalite-1 zeolite was investigated in terms of the addition of hydrophobic Silicalite-1 zeolite,the hydrophobicity of the hydrophobic Silicalite-1 zeolite,the combination of the two components and the residence time of the reaction gas,respectively.The results showed that compared to Fe3O4 without hydrophobic Silicalite-1 zeolite,the addition of hydrophobic Silicalite-1 zeolite increased CO2 conversion from 16.8%to 22.5%and CO selectivity from 22.5%to 30.5%,while hydrocarbon selectivity did not change significantly.The different combinations of Fe3O4 and hydrophobic Silicalite-1 zeolite also have a significant influence on the reaction,with the best performance obtained when the two are mixed in a post-granulation manner.The above results indicate that Fe3O4 coupled with hydrophobic Silicalite-1 zeolite has a more pronounced effect on the promotion of the RWGS reaction without affecting the FTS reaction.3.Investigating the CO2 hydrogenation behaviour of Fe3O4-Fe5C2 coupled hydrophobic Silicalite-1 zeolite.The results showed that compared to Fe3O4-Fe5C2 without hydrophobic Silicalite-1 zeolite,the addition of hydrophobic Silicalite-1 zeolite increased CO2 conversion from 22.6%to 46.2%and decreased CO selectivity from 17.0%to 7.9%,the selectivity of the C5+ component was increased from 15.5%to 53.5%,while the selectivity of the C5+ olefin was increased from 4.8%to 31.8%.And the most significant improvement in CO2 conversion and reduction in CO selectivity was achieved when Fe3O4-Fe5C2 and hydrophobic Silicalite-1 zeolite were combined in separate granulation followed by blending.The above results show that the addition of hydrophobic Silicalite-1 zeolite to Fe3O4-Fe5C2 enhanced the activity of both the RWGS reaction and the FTS reaction.