Study On The Preparation,characterization And Catalytic Performance Of Transition Metal-doped Zn-Al Spinel Catalysts For Dehydrogenation Of Propane Without Oxygen

Propane dehydrogenation to produce propylene has important basic research value and industrial application significance,which provides us with a potential,efficient and low-carbon new way to use alkane.However,the direct conversion of propane to propylene is a thermodynamically difficult reaction,the activation of C-H bond is relatively difficult,and the product propylene is active,then,side reactions such as deep dehydrogenation and carbon deposition are easy to occur.So the design and development of efficient catalyst is the key for propane dehydrogenation to produce propylene.Compared with Pt-based catalysts and Cr O_x catalysts commonly used in industrial propane dehydrogenation,Zn-based catalysts are non-toxic,harmless and inexpensive,so they are widely used in catalytic reactions.As a kind of face-centered cubic oxide of AB₂O₄,Zn-Al spinel can effectively prevent the loss of Zn,and the Zn-Al spinels obtained by different preparation methods have different structures.But single spinel catalyst cannot meet the needs of efficient catalysis,in order to further improve the catalytic performance of the samples,the electronic structure and geometric configuration of the catalysts are regulated by doping transition metals with more valence states.Moreover,the catalyst with the best doping effect was modified with alkali metal and alkali earth metal.In this thesis work,the effects of different synthesis methods and temperatures on the structure and catalytic performance of spinel oxide catalysts were explored starting from the preparation of Zn Al₂O₄.It was found that Zn Al₂O₄ synthesized by hydrothermal method at 600℃has high specific surface area and uniform and stable hydrothermal mesoporous structure,and the active center is dispersed and not agglutinated,so the catalytic performance is the best.Then,the electronic structure and geometric configuration of the catalyst were regulated by doping different transition metal M,and the best Ni doped Zn_1-xNi_xAl₂O₄ was selected to explore the effect of different doping amounts.It was found that a small amount of Ni doping can improve the content of acid with medium strength in the catalyst,increase the proportion of surface adsorbed oxygen species O^-.The initial conversion of propane was 61%,the stable conversion was 22%,and the selectivity was 85%.Alkaline additives were introduced into the catalysts with the best Ni doping activity to reduce the acid sites of the catalysts so as to reduce the side reactions such as propane cracking and carbon deposition.After exploring different additives and introducing amount,it was found that the introduction of a small amount of additives Mg can reduce the acid site on the catalyst surface,and enhance the proportion of medium strength acid content in Zn_0.8-yMg_yNi_0.2Al₂O₄ catalyst,improve the proportion of adsorbed oxygen content on the catalyst surface,and can also play a role in fixing Zn²⁺.When Mg doping amount of y was 0.1,the catalyst gave the best performance,the initial propane conversion was 63%,the stable propane conversion was 25%,and the propene selectivity was 93%.