Preparation And Synthesis Of Silicalite-1 Confined Noble Metals And Catalytic Performance Of Propane Combustion

Light alkanes are one of the major volatile organic compounds（VOCs）emitted from mobile and stationary combustion sources such as automobiles,petrochemicals,and power plants,and are major pollutants in the atmosphere,causing short-and long-term hazards to humans.Due to the thermal and chemical stability of short carbon chains,they are more difficult to degrade than long chain molecules.With increasingly stringent emission control regulations,developing efficient catalysts has become an effective way to remove light paraffins and other pollutants.In general,ultrasmall metal nanoclusters（NCs）exhibit higher catalytic activity compared to larger metal NCs due to increased accessible active sites.However,these catalysts tend to deactivate rapidly due to the competitive adsorption of water at the active sites and high temperature sintering under harsh reaction conditions.Zeolites have regular micro-pore structure and excellent thermal stability.Encapsulating ultrasmall active metal nanoparticlesintozeolitecan prevent the migration and agglomeration of metal nanoparticles,has been proven tobe one of the most efficient strategies to enhance thedispersion and stability of MNPs.Therefore,it is difficult to achieve the requirement of encapsulating metal nanoparticles in zeolite by traditional methods such as impregnation and ion exchange.In recent years,a variety of effective methods for encapsulating metal nanoparticles in molecular sieves have been developed.The in-situone-pot synthesis method can more effectively encapsulate ultra-small-sized metal nanoparticles in zeolites.Encapsulation of ultrafine metal nanoclusters into zeolites is an effective strategy to resolve the contradiction of metal dispersion and sintering in thermal catalysis.Herein,a series of ultrafine Ru nanoclusters（<0.95 nm）enveloped in silicalite-1（S-1）zeolite catalysts were designed and prepared by a simple one-pot method and applied for catalytic total oxidation of propane.The results demonstrate that Ru₁@S-1 has excellent propane total oxidation performance.Its T₉₅ is as low as 294°C,and the turnover frequency（TOF）value is up to 5.07×10^-3s^-1,evidently higher than that of the comparison supported catalyst（Ru₁/S-1）.Importantly,Ru₁@S-1 exhibits superior thermal stability,water resistance and recyclability,which should be attributed to the confinement and shielding effect of the S-1 shell.The in-situ DRIFTS result reveals that the propane total oxidation over Ru₁@S-1 follows the Mars-van Krevelen（Mv K）mechanism,where the hydroxy from the framework of zeolite can provide the active oxygen species.A novel silicalite-1 zeolite confined rhodium-manganese bimetallic nano-cluster（1.6 nm）catalyst（Rh-Mn O_x@S-1）was designed and prepared by a simple one-pot method using a dual confinement strategy（shell confinement and strong metal-metal oxide interaction）and applied in the deep oxidation of propane.The results revealed that the temperature at 90%conversion(T₉₀)for propane over Rh-Mn O_x@S-1 was as low as 264°C,which is clearly lower than those for the conventionally supported(Rh-Mn O_x/S-1,T₉₀=369°C)and the Mn O_x-free(Rh@S-1,T₉₀=285°C)catalysts.Moreover,the turnover frequency（TOF）value of Rh-Mn O_x@S-1 at 220°C was up to15.8×10^-3 s^-1,which is approximately 5-6 times higher than those of comparable catalysts.Most importantly,Rh-Mn O_x@S-1 also exhibited excellent high temperature thermal stability,water resistance and recyclability,which can be attributed to the shielding effect of the zeolite shell and the synergistic electronic-structure interaction between the Rh and Mn O_x species.The in situ diffuse reflectance infrared Fourier transform spectroscopy（in situ DRIFTS）analysis indicates that the C₃H₈ deep oxidation over Rh-Mn O_x@S-1 catalyst follows the Mars-van Krevelen（Mv K）mechanism,the adsorbed C₃H₈ species reacted with the activated oxygen species from Rh-Mn O_x interfaces.The dual confinement strategy developed in this study provides a new way to design high performance catalysts for VOC removal under harsh reaction conditions.