Investigations On Structure-performance Relationship Of Pt- And Gallium Oxide-Based Catalysts For Propane Dehydrogenation

Propylene is an important build block in the petrochemical industry with abundant downstream products.For the past few years,the revolution of the shale gas technology in the U.S.has led to a large increase in the supply of light alkanes such as ethane.The production of ethylene has been changing from naphtha cracking to ethane cracking,so that the capacity growth of by-producing propylene from naphtha cracking has slowed down.However,propane dehydrogenation as a directional propylene production technology using cheap propane as feedstock has developed rapidly.Pt-and chromium oxide-based catalysts are used in the industrial propane dehydrogenation process.The former suffers from high cost,carbon depositon and sintering,while the latter is limited by high toxicity,fast deactivation and frequent regeneration.Therefore,the development of cheap and non-toxic alternatives is of great significance.In this thesis,a series of efficient Pt-and gallium oxide-based catalysts were designed and synthesized based on the modulation of the coordination structure at the active site for propane dehydrogenation.The conventional Pt₃M alloys are limited by the scaling relationship in propane dehydrogenation.The Al₂O₃-supported Pt/Cu single atom alloy was synthesized via an incipient wetness co-impregnation method with a high Cu/Pt atomic ratio using an atom dilution strategy.The thermodynamically stable Pt/Cu single atom alloys break the scaling relationship in propane dehydrogenation and exhibit a stability of up to 120 h and a 90%propylene selectivity during propane dehydrogenation at 520°C.Furthermore,to inhibit the severe sintering of Pt Cu alloy nanoparticles at high temperatures,Pt Cu alloy nanoparticles with tunable Pt/Cu atomic ratios were encapsulated into the S-1 zeolite without frameowork Al atoms by a two-step method named of electrostatic adsorption plus hydrothermal.The dilution effect of Cu on Pt improves the geometric isolation of Pt atoms in Pt Cu bimetallic nanoparticles,which enhances the selectivity of propylene during propane dehydrogenation.The confinement effect of microporous structure of S-1 zeolite on metal particles makes the Pt Cu random alloy with a suitable Pt/Cu atomic ratio exhibit a good stability during propane dehydrogenation at 550°C.Gallium oxide-based catalysts can be a potential alternative for the industrial propane dehydrogenation.The positive effect of H₂ cofeeding on the dehydrogenation activity of the Ga₂O₃/Al₂O₃ catalyst was investigated.H₂ induces a partial reduction of Ga₂O₃ to form a Ga O_x surface rich in O vacancies,accompanied by dynamic gallium hydrides formed by the homolytic dissociated adsorption of H₂ over the Ga O_x surface.This gallium hydride participates in constituting a new efficient active site for propane dehydrogenation,which not only improves the ability of the catalyst to activate the C-H bond,but also inhibits the deep dehydrogenation of the product propylene.Compared with propane dehydrogenation without H₂ cofeeding,the rate of propylene production doubled at an inlet H₂/C₃H₈ ratio of 2.0.Moreover,the reaction mechanism of propane dehydrogenation on Ga O_x/Al₂O₃ under H₂ cofeeding was investigated by kinetic analysis.The whole process can be divided into two parts that proceed simultaneously.On the one hand,the formation of gallium hydride by homolytic dissociated adsorption of H₂ on the O vacancy-rich Ga O_x surface is a fast equilibrium process.On the other hand,the resulted gallium hydride acts as a new active site to catalyze the propane dehydrogenation reaction more efficiently.