Fabrication Of Integrated Catalysts Based On Bio-templates For CO₂ Hydrogenation

Nowadays,the massive anthropogenic emissions of greenhouse gas,especially CO2 has brought serious environmental problems.In order to reduce environmental impacts,many of the world countries made plans to reduce the CO2 emissions,and recently our country announced to reach carbon neutrality by 2060.The utilization of CO2 is one of the most effective ways to reduce the CO2 on climate change impact.Thermohydrogenation of CO2 with the help of green H2 into value-added chemicals and fuels has received extensive research interests.Bifunctional catalyst has been widely used for converting CO2 to high value-added chemicals and liquid fuels.The bifunctional catalysts commonly comprise metallic active sites for methanol synthesis reaction and acid sites on zeolite for the C-C coupling process.This research is aiming at transforming CO2 into hydrocarbons such as C1 products(e.g.,CO,CH4,CH3OH),light olefins,and aromatics.The catalytic activity of metallic sites,the acidity and porosity of zeolite,and the configuration manners of two active sites are the critical factors affecting the catalytic activity of bifunctional catalyst.Herein,efficient catalysts for CO2 hydrogenation were integrated with ZSM-5 prepared by using bio-templates(bioZSM-5),and the derived bifunctional catalysts exhibited high catalytic activity.Moreover,the catalytic activity of metal oxides,the porosity and acidity features of the ZSM-5(density,type,and strength),and the proximity of the two components effects on the catalytic activity of bifunctional catalysts were well studied.Firstly,UiO-66 has been exploited as precursors for the preparation of Zr-based solid solutions(such as ZnO-ZrO2,Co3O4-ZrO2,and CuO-ZrO2)via pyrolysis of Schiff base-modified UiO-66.The Schiff base serves as an effective bridge to dope secondary metal ions into UiO-66 frameworks with controlled amounts.And the selectivity of C1 hydrogenation products can be facilely tuned by simply changing the loading metal ions.The main products of ZnO-ZrO2,Co3O4-ZrO2,and CuO-ZrO2 were methanol,methane and CO,with the selectivity of 70%,92.5%,and 86.7%,respectively.The Schiff basemodified method benefits the synergistic effect between M and Zr in MxOy-ZrO2 solid solutions would enhance the catalytic performance of CO2 hydrocarbon.Secondly,the bio-ZSM-5 with multiple mesostructures and moderate acid sites was synthesized by using rice husk and Luffa sponge as a bio-template.It was found that the TPAOH(tetrapropylammonium hydroxide)amount,Si/Al ratio,and silicon content in biomass were three critical factors affecting the crystallinity and porosity of the bioZSM-5.The Pd/ZnO/bio-ZSM-5 bifunctional catalysts were applied in CO2 hydrogenation to DME and showed high catalytic activity compared with Pd/ZnO/comZSM-5.The results showed that the proximity between the bio-ZSM-5 and Pd/ZnO of Pd/ZnO/bio-ZSM-5 bifunctional catalyst significantly affected the DME selectivity,and appropriate proximity benefited the catalytic activity.The current results indicated that the closest proximity of the bifunctional catalyst was unfavorable for DME production(selectivity<3%),because of which low-valent Zn cations from Pd/ZnO could displace the Br(?)nsted acid sites on bio-ZSM-5 by ion exchange.Thirdly,the post-modification methods including the generation of inter-crystalline mesopores system by adding organosilane to the zeolite precursors of bio-ZSM-5(bioZSM-5-meso)and selectively decreasing the density of external Br(?)nsted acid sites of the bio-ZSM-5 via TEOS(tetraethyl orthosilicate)modification(bio-ZSM-5-Si)were studied.The post-modified bio-ZSM-5-Si was integrated with ZnZrOx solid solution nanoparticles,and the derived ZnZrOx&bio-ZSM-5-Si catalysts exhibited high C2=-C4=selectivity of 64.4%and total C2-C4 selectivity of 94.5%,but relatively low CH4 selectivity(5.5%)and aromatic selectivity(<1%),which was higher than the comZSM-5 for CO2 hydrogenation.The introduction of inter-crystalline mesopores increased the quantity of strong acid,which was detrimental to the stability for CO2 hydrogenation.Furthermore,in order to study the proximity-dependent selectivity of bifunctional catalyst deeply,we assembled the ZnZrOx and bio-ZSM-5 nanoparticles precisely on the rice husk scaffold.The proximity of oxide and zeolite can be facilely tuned by changing the amount of rice husk and the effect of different proximity on the integrated ZnZrOx&bio-ZSM-5 catalyst for CO2 hydrogenation to aromatics was systemically studied.The results showed that the proximity affects the mass-transfer during the reaction and only nano-or micro-scale proximity may benefit for catalytic activity and stability of the bifunctional catalyst.The relationship between proximity index(P)and aromatics selectivity(S)was S(%)=e(1/p-0.05)2/280*1/1+e(1/p+3.2)*100.The integrated manner benefited catalytic activity of ZnZrOx&bio-ZSM-5 catalyst for CO2 hydrogenation to aromatics,and the CO2 conversion up was up to 12.2%with aromatics selectivity of 75%.