Preparation And Application Of Bifunctional Catalyst For Hydrogen Production By Steam Reforming Of Dimethyl Ether

Hydrogen fuel cells have received widespread attention due to their advantages of high energy conversion efficiency and environmental friendliness,however,the supply of hydrogen source has not been well solved.In comparison with the major defects of on-board pure hydrogen,such as low storage density of hydrogen and poor safety performance on-board hydrogen production technology via on-site reforming of liquid fuel can not only make use of the existing gas station system to refuel,but also ensure the same mileage as the fuel vehicle,which is considered to be more reasonable and feasible.Dimethyl ether?DME?is the best choice for on-board hydrogen production due to its advantages of liquid storage density and gas fluidity,clean and non-toxic,high energy density and low cost.Compared with other hydrogen production methods,hydrogen production via steam reforming of DME?SRD?can be performed at lower temperatures with high hydrogen yield.Thus,SRD would be an ideal hydrogen source for the fuel cell vehicles.Based on the comprehensive analysis of consecutive two-step reaction characteristic of SRD and the preliminary results,in this thesis,ZnO/HZSM-5 and acid activated montmorillonite?Acid-MMT?as solid acid catalysts were physically mixed with a commercial Cu O/ZnO/Al₂O₃ catalyst to prepare an SRD bifunctional catalyst,respectively.The impacts of the structure,texture,acidity of ZnO/HZSM-5 and Acid-MMT,and their synergy effects with Cu-based catalysts on the catalytic activity,stability and product selectivity of the SRD bifunctional catalyst were systematically investigated.The main research contents are summarized as follows:?1?The parent HZSM-5 was modified with a series content of ZnO via the incipient impregnation method by using Zn?NO₃?₂ as a precursor.And the ZnO-modified HZSM-5?ZnO/HZSM-5?physically mixed with a commercial Cu/ZnO/Al₂O₃ was investigated as a bifunctional catalyst for SRD.The effects of structure,texture and acidity of ZnO/HZSM-5 on the performance of SRD were studied.The results showed that the introduction of zinc oxide had slightly effect on the structure of HZSM-5.However,it had a great influence on the type?Lewis and Br?nsted acid?and distribution?strong and weak acid sites?of acid,thus affecting the SRD performance of its corresponding bifunctional catalyst.Compared with the parent HZSM-5,ZnO/HZSM-5 based bifunctional catalyst showed higher hydrogen yield and better stability in SRD reaction.The 10 wt.%ZnO modified HZSM-5 exhibited good catalytic performance,i.e.,the DME conversion was still 100%and the hydrogen yield was about 93%at 9 h under the conditions of DME/H₂O/N₂=1/4/5,P=1 atm,T=290 ^oC,and GHSV=4000 h^-1.?2?A series of Acid-MMTs were prepared via Na-MMT treated with 20%nitric acid solution at different treatment temperature and time.And the Acid-MMTs were physically mixed with commercial Cu/ZnO/Al₂O₃ to prepare bifunctional catalysts for SRD reaction.The results showed that the structure,texture and acidity of Acid-MMTs were significantly changed compared with Na-MMT,which was dependent on the acid treatment conditions.The structure and acidity of Acid-MMTs obviously affected the SRD performance of bifunctional catalyst.The bifunctional catalyst composed with the Na-MMT activated in 20%nitric acid solution at 80 ^oC for 12 hours?Acid-MMT-80/12?and Cu/ZnO/Al₂O₃ exhibited the best SRD performance,i.e.,the DME conversion and H₂ yield reached 97%and 94%under the conditions of DME/H₂O/N₂=1/4/5,P=1 atm,T=350 ^oC,GHSV=3000 h^-1,respectively,and DME conversion and H₂ yield remained basically constant in 10 hours,which indicated the catalyst had good stability.