Preparation Of Hierarchical Porous γ-Al₂O₃ Supported Noble Metal Catalysts And Catalytic Properties For Propane Dehydrogenation

Propylene is an important raw material for the production of three major synthetic materials:plastics,synthetic fibres and synthetic rubber.Currently,petroleum or naphtha cracking is the main source of propylene.The key to the PDH process is the development of a dehydrogenation catalyst with high stability,high activity and high resistance to carbon build-up.Therefore,the catalysts selected need to be highly resistant to sintering and carbon build-up while maintaining high catalytic activity.In this thesis,γ-Al₂O₃,which has a large specific surface area,high thermal stability and good mechanical strength,was chosen as the support.The three-dimensionally ordered macroporous mesoporous（3DOMM）structure,which has a large specific surface area and both macroporous and mesoporous structures,can modulate the surface acidic nature of the Al₂O₃ support.A series of propane dehydrogenation catalysts with multi-level pore structure and rare earth metal addition were designed and prepared by using organic polymer microspheres as macroporous templates and surfactant-saturated micelles as mesoporous templating agents within the interstices of the microsphere templates.The spatial morphology,elemental composition,phase structure and redox properties of the catalysts were investigated by various characterization methods,and the effects of the pore structure and rare earth metals on their catalytic propane dehydrogenation performance were studied.The main innovations and findings of this paper are as follows:（1）Pt Sn/γ-Al₂O₃ catalysts with different pore structures were synthesized by using a variety of templates,and the effects of ordered mesopores,ordered macropores and3DOMM pore structures on the reactivity of the catalysts were comparatively studied.Among them,the three-dimensional ordered macroporous-ordered mesoporousγ-Al₂O₃support was prepared by solvent volatilization-interface guided self-assembly method.The three-dimensional ordered macroporous framework increases the specific surface area of the catalyst and neutralizes the acidity of theγ-Al₂O₃ support;the ordered mesoporous structure increases the dispersion of active sites while increasing the specific surface area;the hierarchical porous nanostructure combines macropores The advantage of mesopores is not only the proper acidity,but also the pore structure of the pores increases the interaction between the metal and the support,which is beneficial to reduce the occurrence of side reactions and the dispersion of metal particles,thereby improving the stability of the catalyst.（2）A series of PtSnLa/3DOMM-Al₂O₃ catalysts with different La contents were prepared by the equal volume impregnation method.The addition of La has a significant effect on the catalytic performance of the Pt Sn/3DOMM-Al₂O₃catalyst for propane dehydrogenation.When the La content in the Pt Sn/3DOMM-Al₂O₃ catalyst is low（1wt%and 1.5wt%）,the acidity of the catalyst decreases slightly and the active sites can be stabilized,thereby the initial matching of metal functional groups and acidic functional groups in the Pt Sn/3DOMM-Al₂O₃ catalyst is improved,which is beneficial to the reaction.Among these catalysts,the PtSnLa（2wt%）/3DOMM-Al₂O₃ catalyst has the best conversion and selectivity.However,at high La content,the metallic character of Pt is weakened and the catalyst acidity increases significantly,breaking the previous match in the PtSnLa（2wt%）/3DOMM-Al₂O₃ catalyst,resulting in decreased catalyst activity and selectivity.The XPS results indicated that the addition of La could suppress the reduction of Sn species in the catalyst.Thermogravimetric experiments show that the proper addition of La can effectively prevent the catalyst from coking,and with the excessive loading of La,the amount of coking will increase.Appropriate addition of La in the Pt Sn/3DOMM-Al₂O₃ catalyst is beneficial to the dehydrogenation of propane,while when the addition of La is excessive,there is an opposite effect.