Study On Preparation And Catalytic Application Of Metal Catalysts Encapsulated In Porous Molecular Sieves

Zeolites encapsulated metal catalysts have unique advantages in regulating reaction selectivity and improving the stability of metal species.On the one hand,the rich pore structure of zeolites provides the possibility to achieve selective conversion.Zeolites have diverse composition and tunable acidity and alkalinity,which can be used as catalyst carriers and provide acid-base active sites.The cavities and/or pores of zeolites are suitable for encapsulating metal nanoparticles.The oxygen atoms in the framework of zeolites or the cations at nonskeletal position can stabilize the metal nanoparticles in the zeolites pores.At present,the research about zeolites encapsulated metal catalysts focuses on the encapsulation of metal single-atoms/clusters/nanoparticles by microporous molecular sieves.For reactions involving bulky molecules,the kinetic diameter of bulky molecules is much larger than the pore diameter of microporous zeolites,resulting in a low catalytic efficiency and coke deposition.Even if the molecular dynamics diameter of the reactant is small enough to enter the pore channel of the zeolite,the narrow micropores make the diffusion process of the reactant from the outer surface to the inner metal/acid active site also bear large diffusion resistance,which makes the concentration of the reactant near the active site far lower than its concentration in the bulk phase,leading to the catalytic efficiency far lower than the theoretical expectation.Compared with microporous zeolites,the introduction of mesopores or macropores can greatly improve the diffusion rate of reaction species and the accessibility of metal/acid active sites.However,at present,the preparation of hierarchical zeolites confined metal catalysts is still deficient.This is because the crystallization of zeolites and the confinement of metal species is a complex process,and the introduction of mesoporous template agent can easily lead to phase separation,which leads to the failure of synthesis.In view of the challenges in confinement of metal within hierarchical zeolites,this paper designed a precursor complex of carbon template-silicon source-metal species to realize the two-step synthesis of confinement of metals within hierarchical zeolites.Detailed characterizations were employed to study the synthesis conditions,morphology and formation mechanism of such catalysts.The details are as follows（1）Use the abundant phosphine coordination in phytic acid to anchor the metal species,mix the phytic acid-metal complex and silica sol evenly,and then carbonize at high temperature to obtain metal@carbon@silica（M@C@SiO₂）composite precursor,the amorphous silicon dioxide in the precursor dissolves and crystallizes to form silicate-1（S-1）zeolites under alkaline hydrothermal conditions.The carbon template and the metal species on it are coated by S-1 zeolites.After air calcination,the carbon template is removed to form the metal catalyst encapsulated by hierarchical S-1 zeolites（M@S-1）.In this work,the systhesis condition（the amount of carbon source,water content,T carbonization temperature and silica sol content）was studied.The optimized conditions（phytic acid:17.5 g,T碳化温度=300^oC,silica sol content:20 g,water content:10 g）was studied and synthesized hierarchical zeolites confined metals.（2）Based on the synthesis of M@C@SiO₂,the evolution of metal species in precursor,the relationship between P content in precursor and metal dispersion,and crystallization mechanism of M@C@SiO₂ was studied.Ni@S-1 catalyst with high Ni loading（16 wt%）,high dispersion and small particle size（6.7 nm）was prepared by double restriction strategy（zeolites wall restriction and strong metal-support interaction）.In the hydrogenation of levulinic acid,16%Ni@S-1shows higher catalyticefficiency than Ni catalyst supported by microporous S-1.（3）The deactivation of rhodium-based catalyst is an urgent problem in hydroformylation.In this context,the rhodium-cobalt bimetallic catalyst encapsulated by hierarchical S-1 zeolites was prepared by the above strategy.The introduction of Co significantly improved the recycle stability of the catalyst.In the case of semi-conversion,the turn of frequency（TOF）remained 4800 h^-1 after five cycles.In contrast,the TOF value of Rh@S-1 decreased from 5000 h^-1 to 1500h^-1 after three cycles.The developed strategy of encapsulating metal catalysts with in hierarchical zeolites can realize the general preparation of metals with high loading and high dispersion.Moreover,there is a strong interaction between metal and zeolites,which is conducive to the high temperature stability of metal particles.The rich pore structure of hierarchical zeolites provides fast diffusion and mass transfer channels for the substrate molecules.The strategy of packaging high loading metal catalysts with in hierarchical zeolites developed in this paper lays the foundation for the preparation of high loading metal catalysts in industrial applications.