Study On High Efficiency Catalyst For Hydrogen Production From Dimethyl Ether Steam Reforming

Proton exchange membrane fuel cell?PEMFC?is considered the ideal power source for fuel cell vehicles due to its high energy conversion efficiency and environmental benignity.However,challenges still exists for the commercialization of PEMFC,in which the supply of stable high quality hydrogen source is the main obstacles.Among the available hydrogen carriers for hydrogen production,dimethyl ether?DME?is believed to be a promising candidate due to its mild reaction condition,non-toxicity or harmlessness,storage and transportation compatible,which is similar to liquefied petroleum gas.In this view,the problem of PEMFC hydrogen supply can be solved by the dimethyl ether steam reforming?DME SR?perfectly.For DME SR,the development of efficient catalysts is crucial to improving the catalytic performance.Whereas,there are many deficiencies for the commonly used catalysts,for example,the thermal stability of Cu-based catalysts and carbon deposition resistance of solid acid catalysts need to be improved to gain better catalytic activity and longer life.Based on the above problems,in this work,mesoporous Cu-SiO2-Al₂O₃ and hierarchical or nanosized H-ZSM-5 are employed for DME SR.The pore structure,coordination structure,acid strength,acid type,and morphology of these catalysts are investigated carefully.Various techniques including XRD,Nitrogen adsorption-desorption,N2 O chemisorption,H₂-TPR,NH₃-TPD,SEM,TEM,FT-IR,Pyridine-FT-IR,TGA,ICP-OES and 27 Al MAS NMR are employed to characterize the prepared catalysts.Firstly,the thermodynamic equilibrium of DME SR is simulated by Aspen Plus software.Results show that when the reaction temperature is above 250°C and steam/DME ratio is above 3,carbon deposition is thermodynamically unfavorable,and the equilibrium conversion of DME is 100%.In addition,the increase of steam/DME and decrease of reaction temperature is helpful to improve the H₂ yield and simultaneously reduce the content of CO in the product.Mesoporous Cu-SiO₂-Al₂O₃ bifunctional catalysts are synthesized via a modified evaporation induced self-assembled?EISA?method,and the effect of calcination temperature,Cu content,and Si content on the structures and properties of the catalysts are investigated.The results show that the enhancement of both of calcination temperature and copper content are conducive to the formation of more stable CuAl2O4 with higher activity of methanol steam reforming,but will have a negative impact on the construction of ordered mesoporous structure,and lead to the reduction of total acid amount.As such,the mesoporous bifunctional catalyst calcinated at 700°C with 7.5% Cu content exhibits high DME conversion of 100% and H₂ yield of 84% at the reaction temperature of 400°C.The addition of Si is found effective in preserving the ordered mesoporous structure,enhancing the dispersion and thermal stable of Cu,and increasing the amount of total acid and Lewis acid sites,which is beneficial for the hydrolysis of DME.In this work,the 7.5Cu-17.5SiO2-Al₂O₃ catalyst that synthesized by one-pot EISA strategy exhibits the highest performance with the DME conversion of 100%,H₂ yield of 90%,and the relatively low CO selectivity of 15%.It also shows excellent regeneration performance as an excellent catalyst for DME SR.Hierarchical H-ZSM-5 zeolites with low SiO2/Al₂O₃ ratios of 40 and 30 are firstly synthesized via a developed steam-assisted crystallization?SAC?process,and the effect of templates on the structures and properties of the catalysts are investigated.The results show that the soft template P123 is easy to dissolve out of precursor gel in the strong basic environment created by ethylenediamine,resulting in phase separation between template and precursor gel,leading to a poor mesoporosity of the H-ZSM-5 zeolites.However,when PEG is used as the template the mesopore structure of H-ZSM-5 zeolite can be created.Furthermore,the larger the PEG molecular weight,the more favorable to the distribution of pore structure.Sucrose is an excellent mesoporous template.The hierarchical H-ZSM-5 zeolites with high crystallinity and rich mesoporous structure can be synthesized with the addition of ethylenediamine.It possesses a relatively wide pore size distributionand large pore volume of more than 0.32cm3/g.Therefore,combined with the hierarchical structure,CuZnPrCe/HZ30-Suc?1?-72-EDA composite exhibits higher catalytic stability than convertional CuZnPrCe/H-ZSM-5?30?catalyst.Zn-incorporated nanosized H-ZSM-5 zeolites?Zn@nHZ?are synthesized by a novel and facile one-pot route using zeolitic imidazolate framework-8?ZIF-8?as the template.The effect of crystallization time and amount of ZIF-8 on the morphology,textural,and acid property of Zn@nHZ are systematically investigated.The results show that the growth of Zn@nHZ involves two successive stages: dissolution-impregnation stage and then crystallization stage.After complete crystallization,the obtained Zn@nHZ30-0.3 forms regular globularsparse aggregates consisting of elliptic nanoparticles of ⁵0 nm,along with homogeneously dispersed Zn.The incorporation of appropriate amount of ZIF-8 effectively facilitates the formation of Lewis acid sites and increases the total acidity amounts.Further increase of ZIF-8 is beneficial for the enhancement of the number of active sites for methanol reforming,but decreases the crystallinity of zeolite,simultaneously leads to the decline of the amount of Lewis acid sites and total acidity.Therefore,combined with the acid characteristics and nanosized structure,Zn@nHZ30-0.9 exhibits a high catalytic activity as well as enhanced catalytic stability during the 25 h test: the DME conversion is kept above 96% under severe reaction temperature of 490°C.