Construction Of Metal Modified UiO-66/HZSM-5 Tandem Catalyst For Hydrogenation Of CO₂ To Aromatics

CO₂ Hydrogenation to synthesize olefin,aromatics and other high-value chemicals not only decreased the emission of greenhouse gas CO₂ and alleviated the problem of global climate change,but also decreased human dependence on fossil energy,which has been widely concerned around the world.Compared with the C₁ products,C₂₊products possess higher energy densities and values,whereas olefins and aromatics are especially desirable.Aromatics are important popular chemicals that are often used as feedstock for synthetic petrochemical products,polymers and pharmaceuticals.They are generally produced by petroleum pyrolysis and naphtha reforming.In addition,coal pyrolysis and methanol to aromatics（MTA）are also potential pathways.The development of new technologies derived from renewable resources to meet the growing demand for aromatics is particularly attractive for the production of benzene,toluene and xylene（BTX）.CO₂ hydrogenation to aromatics has become a sustainable route to solve the environmental problems caused by excessive CO₂emission and to increase the production of BTX.Currently,CO₂ hydrogenation to aromatics mainly uses CO₂-modified Fischer-Tropsch synthesis（CO₂-FTS）or methanol mediated synthesis（MeOH）routes,but is often limited by low CO₂ conversion rate,poor aromatics selectivity and catalyst deactivation.The narrow pore of HZSM-5 zeolite in the existing series catalyst severely limited the transport of molecules,resulting in the catalyst deactivation due to coking and poor stability.At present,researchers are focusing on the pore,structure and morphology of HZSM-5 zeolite.The preparation process of HZSM-5 zeolite is complicated and expensive.In conclusion,it is still a great challenge to find novel catalysts free from the bondage of HZSM-5 zeolite.In this paper,the palygorskite based HZSM-5 zeolites were prepared successfully,and the metal oxides were connected in series.The catalytic activity of the HZSM-5 zeolites for hydrogenation of CO₂ to aromatics was studied.On the other hand,the prepared catalyst can directly obtain aromatic hydrocarbons from the hydrogenation of CO₂ without passing through the HZSM-5 zeolite path,and can also effectively inhibit the occurrence of side reactions,and obtain high selectivity of aromatic hydrocarbons under atmospheric pressure.Specific research contents are as follows:1.Palygorskite-based HZSM-5 zeolite was prepared through the hydrothermal method using palygorskite as the Si source and supporter.A series tandem composite of M（Zn,Ga,In）-UiO-66/（HZSM-5）-palygorskite was obtained by the physical mixing method using metal-organic frameworks（UiO-66）as the carrier.XRD,SEM,TEM,NH₃-TPD,Py-IR,H₂-TPR,XPS,CO₂-TPD,N₂ adsorption-desorption,were used to investigate the composite’s physical and chemical properties.The calcination temperature effect of palygorskite-based HZSM-5 zeolite,Zn/Zr mole ratio,the spatial distribution between Zn-UiO-66 and palygorskite-based HZSM-5 zeolite,and reaction conditions on the catalytic performance of CO₂ hydrogenation to aromatics were investigated.Zn-UiO-66/（HZSM-5）-palygorskite showed excellent catalytic performance.CO₂ conversion was21.2%,BTX selectivity were 84.9%under optimal reaction conditions.In-situ DRIFTS and Density Functional Theory（DFT）were used to investigate and simulate the CO₂adsorption performance over the Zn-UiO-66.In addition,the catalytic reaction mechanism of CO₂ hydrogenation to aromatics was discussed.The transition state and energy barrier for CO₂ hydrogenation to aromatics were obtained by minimum energy search of the LST/QST transition state.2.Tandem catalysts composed of metal oxides and HZSM-5 zeolite are common choices for the hydrogenation of CO₂ to aromatics.However,due to the procedures for preparation of HZSM-5 are time-consuming and the reactions are also frequently inactivated resulted from the carbon deposition,most researchers are thus devoted to the modification of HZSM-5.Meanwhile,it is still a challenge to find novel catalysts free from HZSM-5 for CO₂ hydrogenation to aromatics.In this work,tandem catalysts based on metal oxides and metal-organic frameworks（MOFs）were demonstrated to have high selectivity to prepare aromatics from CO₂ without using HZSM-5.The physicochemical properties of the tandem catalysts were studied by XRD,SEM,NH₃-TPD,Py-IR,H₂-TPR,XPS,CO₂-TPD and N₂ isothermal adsorption-desorption methods.The effects of the type and the content of the metal oxides,and the reaction conditions on the catalytic performance were also investigated.The experimental results exhibited that under the optimal reaction conditions,the CO₂ conversion rate of 8%Zn-UiO-66 catalyst was25.0%,BTX was up to 76.5%.Meanwhile,the side reactions can also be effectively inhibited,and a high selectivity（67.4%）of aromatics can be achieved under atmospheric pressure.The adsorption properties of the Zn-UiO-66 catalyst for CO₂ and H₂were also studied by In-situ DRIFTS and DFT.Furthermore,the catalytic reaction mechanism of CO₂ hydrogenation to aromatics was discussed.By searching the minimum energy of the LST/QST transition state,the transition state and energy barrier of hydrogenation of CO₂to aromatics were achieved.