Designing Of ZnO/silicalite-1 Based Catalysts And Mechanism Study For Non-Oxidative Propane Dehydrogenation

Propene,as a by-product in steam cracking and Fluid catalytic cracking,is the second-largest building block in the chemical industry.However,the current industrial processes can not fill the gap between its demand and productivity.In order to further increase the productivity of propene,other technologies for producing propene such as non-oxidative propane dehydrogenation,methanol to propene,disproportionation of ethene and butenes processes have been developed.Among those processes,non-oxidative propane dehydrogenation(PDH)is one of the on-purpose technologies to produce propene,which has already been commercialized for decades,and it is also a model reaction for studying the activation of C-H bond.Pt-based and Cr-based catalysts were commercially used in the industry processes.Although those catalysts are active and selective in PDH,they have some shortcomings related to high-cost and toxicity for Pt-based and Cr-based materials,respectively.Therefore,the academia and industry are focusing on developing alternatives.ZnO is one of the most attractive materials in PDH owing to its environmental friendliness,low-cost.It,however,has some shortcomings related to the its instability at high temperature under reduction conditions.In addition,their active sites are still unclear until now.In this thesis,we focused on the ZnO/Silicalite-1 based catalysts.The structure of active sites,their regulation,reaction mechanism and improvement of thermal stability of Zn species were studied.The main work is summarized as follow:(1)The N-doped carbon layer coated ultrasmall ZnO nanoparticles supported on Silicalite-1 catalysts were synthesized and employed in PDH after carbonization and acid leaching using ZIF-8 as the precursor of ZnO.The obtained catalyst exhibited about 90%propene selectivity at 44.4%propane conversion at 600°C after nearly 6 h on propane stream.The reason might be that the ultrasmall ZnO nanoparticles were encapsulated in the carbon layer which could hinder them from sintering,and the N species could stabilize the ultrasmall ZnO nanoparticles and prevent them from component loss at high temperatures.(2)The effect of SiO₂-based materials with different topologies on structure of Zn species was studied.The results show that ZnO nanoparticles were in-situ formed on the MCM-41 support,and the binuclear Zn-oxo species were in-situ stabilized on Silicalite-1 support.The partially reduced binuclear Zn-oxo species are the active sites in the PDH reaction.The further characterization results of the Silicalite-1 support show that,OH nests are the anchoring sites for the bi-nuclear Zn-oxo species.The temporal analysis of products and density functional theory results reveal that the first C-H bond is broken heterolytically and the second one is broken homolytically,and the latter process requires higher energy span.(3)The effect of property of Silicalite-1 on the Zn-oxo species was studied.A good positive dependence is established between catalytic performance and the amount of OH nests.The optimized catalyst showed space time yield of propene(STY_C3H6)of5.03 kg_(propene)kg^-1_(catalysts)h^-1,which is 1.54 kg_(propene)kg^-1_(catalysts)h^-1higher than that over reported Pt-based catalysts at 600^oC.The simple method(physically mixed-in-situ reduction treatment)could be extended to delaminated zeolites and other OH-rich commercial supports,such as MOR,MCM-22,Beta,Al₂O₃,TiO₂,ZrO₂,TiZrO_x,YZrO_x.(4)The effect of adding of Mg on the thermal stability of ZnO/Silicalite-1 based catalysts was studied.The catalytic performance was fully recovered for 5PDH/regeneration cycles over 1.8 wt%Mg modified ZnO/Silicalite-1 catalysts.As confirmed by IR,XPS and CO-TPR,the Zn-O-Mg structure are responsible for the improved stability of Zn species,and its reduction was suppressed during PDH.The EXAFS results showed that the single-site Zn²⁺are active sites in the PDH.