Design Of Methane Dry Reforming Catalysts And Its Application In Industrial Tail Gas Conversion

In recent years,natural gas consumption has been increasing among the energy sources of coal,petroleum and other fossil fuel resources.As the main component of natural gas,methane is an important chemical with high storage and low cost.However,the industrial value of methane has not been fully developed.There have been a number of methods to directly convert methane into chemicals,including methane coupling to olefins,methane dehydroaromatization to aromatics and the direct oxidation of methane to methanol.Regrettably,there lacks an economical and efficient way to convert methane,which makes these processes not reach the standards of industrial use.The indirect method to firstly convert methane into syngas and then fabricate the chemicals via Fischer-Tropsch and methanol synthesis by syngas platform is still the most economical way.Among them,steam reforming of methane?SRM?has been applied in industry,though it has many drawbacks,such as high water consumption,high H/C ratio of syngas and the release of CO₂ into atmosphere by RWGS.The partial oxidation of methane?PO?is difficult to be controlled,and using molecular O₂ as oxidant may brings about safety problems.Dry reforming of methane?DRM?is a process for methane to syngas by CO₂ consumption,which can not only uses pure methane but also gases rich in methane,such as tail gas from indirect liquefaction of coal and coke oven gas.Due to the addition of CO₂,the carbon content could be promoted compared to SRM.This reaction is of great significance in the efficient and comprehensive utilization of resources and environmental protection.Both methane and CO₂ are inert molecules,therefore,DRM requires temperature higher than 700? in order to achieve high conversion and selectivity due to the limitation of thermodynamics.Ni-based catalysts are widely used catalysts for DRM but the sintering of metal paticles and coke deposition are the reasons why the Ni-based catalysts deactive.?1?Firstly,we synthesized MCM-41 with uniform open straight pore structure and high specific area,and uniformly impregnated nickel nitrate onto MCM-41 pore channel by ethanol using the capillary effect of straight channel and thus nickel particles are well distributed on MCM-41.The Ni load on this MCM-41 with confined structure is 10 wt%.XRD shows no apparent diffraction peak for Ni,which indicatives the high dispersion of Ni particles.TEM shows 2 nm Ni particles distributed in the pores of MCM-41.TPR demonstrates its relatively high reduction temperature and illustrates the strong interaction between NiO and silicon oxide.The decrease in the conversion of methane?72%,close to the equilibrium conversion?was not observed after 200 h run of reaction at 700? with 45000 mL?g^-1?h^-1 GHSV,with H₂/CO molar ratio kept at0.87.TEM after reaction shows the average particle size is 3-4 nm and no apparent Ni sintering or wrapped by carbon was observed.TGA and TPO demonstrate the low coking rate of this catalyst and the coke is easy to be gasified.Therefore,this catalyst with Ni confined in MCM-41 straight channel is an excellent catalyst with anti-sintering and anti-coking properties.?2?We developed a facile and one-pot synthesis method for the encapsulation of substantial amounts of small confined metallic Ni particles?2 nm;loading of 4.9 wt%?into a 4.3 nm mesoporous dendritic SiO₂ channel.The Si-O-Ni skeleton was formed by the reaction of the Si-O-Si precursor?produced by TEOS hydrolysis?and nickelocene with the aid of CTAC in a two-phase hexane and water system.This approach resulted in a Ni/SiO₂ catalyst with high reactivity and stability?for at least 200 h?towards the dry reforming of methane at high temperatures?700??.This work provides a new approach for the development of anti-sintering and carbon-resistant base metal catalysts working at high temperatures.The dTG curves combined with XRD,TEM and Raman characterization results show that the surface of Ni/SiO₂?IM?catalyst has both amorphous carbon and graphitized carbon,while the surface of Ni/SiO₂?OP?catalyst has only amorphous carbon.Amorphous carbon easily reacts with CO₂ to form CO,so that the Ni/SiO₂?OP?catalyst maintains high reaction activity.DFT study indicated that C atoms on small-size Ni clusters should exist in the form of single adsorbed C atoms,while C atoms adsorbed on large-size Ni clusters can form C nanotubes and finally render the Ni cluster inactive.This explains our experimental phenomenon that the confined structure of Ni has better activity and stability at high temperature.In this newly developed synthesis method,in-situ generation of Ni-O-Si groups allows Ni nanoparticles to be encapsulated and confined within the three-dimensional mesoporous SiO₂,preventing the sintering of Ni nanoparticles and carbon deposition under the conditions of dry reforming.?3?Ni-based catalysts can be deactivated due to coking.This is because of the accumulation of C*from C-H scission on the catalyst surfaces.C*can be taken away from the system in the form of CO after combining with O*and thus the coking and deactivation of the catalyst is suppressed and the dissociation of methane is promoted.d-band center can describe the adsorption ability of transition metals for O atom.Firstly,we calculated the d-band center values of Ni-M alloys when Ni atoms are partly substitued by other metals M?from?B,?B,?B,?,?A,?A,M=W,Mo,Ga,Sn,Re,In,Fe,Co,Rh,Pd,Ru,Cu,Ag,Au,?and the relationship between methane mean conversion and d-band center values are present as the volcano curve.We predict NiIr to be an excellent alloy for DRM,because its d-band center value is located at the optimum point of the volcano curve.Further adjustment of the Ni/Ir ratio was performed and the best ratio was selected and used to investigate the structure-activity relationship of NiIr/MgAl₂O₄ dry and bi-reforming of methane.We found that the addition of Ir could promote the dispersion of Ni particles and form NiIr alloy and suppress coking during reaction?the coking site is mainly Ni particles?.In addition,the NiIr alloy was not covered by coke and thus its high activity and stability remained.The optimal NiIr/MgAl₂O₄ was applied in bi-reforming of coal-to-oil tail gas and CH₄,CO₂ and H₂O in coke oven gas.The syngas can be obtained with adjustable H/C ratio?about 2?,which can be used in the subsequent methanol synthesis.Not only could we increase the yield of methanol?about 50%?,but also decrease the energy consumption of compressors to a large extent.This work is of scientific significance of guiding the screening and synthesis of methane dry reforming catalysts with theoretical calculations and the results have broad industrial application prospects.