Fabrication And Performance Of Zeolites Encapsulating PtZn Catalysts For Propane Dehydrogenation Towards High Heat Sink

Active fuel cooling is an effective way to overcome thermal barrier for hypersonic flight.Propane dehydrogenation（PDH）,with high heat sink and high volumetric calorific value,is one of the candidate endothermic processes for high speed flight.However,residence time of fuel is very short,which requiring a high weight hourly space velocity（WHSV）of 500 h^-1,owing to the size of the cooling channels and the mass of vehicle.The current industrial PDH catalysts work under a WHSV less than 10h^-1.Therefore,the main challenge is to design PDH catalysts with high space velocity under harsh condition.It is mainly for the demands of high temperatures,ultra-short residence time and reuse property in active cooling applications.A new structure of zeolite-encapsulated PtZn intermetallic compounds was designed.On the basis of thermal stable Silicalite-1 zeolite encapsulating Ptcatalysts,the structure-performance relationship between active sites and propane dehydrogenation was studied.The electronic effect of subsurface modulation of PtGa alloy,and geometrical effect of isolated Ptsites in PtZn intermetallic compounds on PDH,were analyzed and summarized by combining theoretical calculations and experiments.The scaling relationship between catalytic activity and propylene selectivity of Pt-based alloy was inductively verified.The optimized PtZn@S-1 showed high propylene selectivity above 99%at 600℃and kept stable for 90 h.A new method for atomic dispersed Ptanchored by Zn O_x clusters in zeolites was developed.By modulating Zn density in zeolites,low-nuclear Zn O_xclusters were designed and prepared to anchor Ptspecies.This kind of structure simultaneously increased the exposure ratio and intrinsic activity of the Ptactive sites.The optimized Pt@2Zn-MFI exhibited a high specific activity up to 29.0 mole _C3H6 mole _Pt^-1 s^-1 at 600℃with a high WHSV of 600 h^-1.It is more than 30 times higher than industrial PDH catalysts.A new strategy of metal regioselectively encapsulated in zeolites was proposed.A dual template-assisted synthesis method was developed for the synthesis of finned Silicalite-1 zeolite encapsulated PtZn catalysts.The epitaxial growth of high crystalline fin-like zeolite（30-60 nm）significantly reduced the diffusion length based on low surface barrier.The high crystallinity of microporous structure also ensured thermal stability of Ptnanoclusters under high temperature.The growth mechanism of finned zeolite was investigated.And seed-directed secondary growth method was developed to regioselectively encapsulate Ptsites in epitaxial zeolites.It could further increase the accessibility of noble metal Ptclusters and enhance the utilization of active sites in high-temperature catalytic reactions.The optimized PtZn@Fin-12h showed a high specific activity up to 31.7 mole _C3H6 mole _Pt^-1 s^-1 at 600℃with a high WHSV of 600 h^-1 under a pure propane gas flow.And It remained above 65%of initial activity after a10 h test,achieving efficient and stable PDH with high space velocity.