Design,Preparation,and Catalytic Performance Of Confined Pt Catalyst And NbS-1 Zeolite

Zeolites exhibit high catalytic performance for the reactions of small molecules.However,for the reactions that bulky molecules involve in,the sole micropores of traditional zeolites would restrict the mass transfer of the molecules,make the mass transfer become the rate controlling step.Therefore,the research of hierarchical zeolites synthesis draws much attention in the field of zeolite synthesis.Correspondingly,metal-zeolite composite materials,especially that of the zeolite crystallites encapsulating active metals with high dispersion degree,draw more attention of the researchers,as the confining effect of zeolite crystallites could improve the sintering resistance of the metals,and the intrinsic micropores of the zeolite as well as the meso/macro-pores being introduced could play the role of transporting reactants and products.For the materials with zeolites crystallites confining metal nanoparticles,their catalytic property may be strongly influenced by their structure.For instance,if the interaction between metal and zeolite is favorable for the reaction,the tight integration of metal with zeolite would be necessary,thus a structure with the zeolite crystallites embedding the metal would be ideal for catalyzing the reaction;on the other hand,if the accessibility of the metal active sites rather than the interaction contributes to the catalysis,relative separation of metal and zeolite would be ideal for catalyzing the reaction,thus the structure with the zeolite cavities encapsulating highly dispersed the metals should be designed.Moreover,introducing heteroatoms into zeolites,especially into hierarchical zeolites,to combine special active sites and a rapider mass transfer of reactants/products,is also a research hotspot.For the synthesis of NbS-1 zeolite reported in literature,the framework Nb is required to be increased and the existence of extraframework Nb is to be eliminated.In this context,this thesis devotes to develop a simple method for preparing hierarchical silicalite-1(S-1)zeolite,and on the bases,investigate effective strategies for preparing "embedding-form" as well as "cavity-form" Pt@S-1 catalysts.Moreover,this thesis also dedicates to develop more effective synthesis method for introducing more framework Nb as much as possible but no extra-framework Nb species in the NS-1 zeolite.A method for preparing hierarchical S-1 zeolite by treating the microporous S-1 zeolite with pure tetrapropylammonium hydroxide(TPAOH)solution was proposed in this thesis.This method could not only avoid the introduction of additional ions,such as Na+ and Al3+,but also obtain a filtrate that only contains TPAOH and the S-1 construction units.As a necessary and basic element,the filtrate could be used for preparing the Pt@S-1 as mentioned below.A better strategy for embedding Pt nanoparticles into hierarchical S-1 crystallites was proposed in this thesis.The strategy can be briefly described as follows:using the above mentioned filtrate containing Si source,instead of TPAOH solution,to suppress excessive dissolution of S-1 in dissolution-recrystallization process.In this case,when Pt@S-1 is prepared in the presence of Pt(NH3)4(NO3)2,the structure of a large cavity encapsulating one great Pt particle,so-called "yolk-shell" structure,reported in literature was avoided.This strategy could embed Pt nanoparticles with high dispersion into hierarchical S-1 zeolite crystallites,due to it use the filtrate containing large amounts of S-1 construction units.By using this strategy,even for the resultant "embedding-form" Pt@S-1 catalyst with 0.81 wt.%Pt,the average size of Pt nanoparticles is merely-3.9 nm.After subjected to the high temperature treatment in H2 at 750? for 2 h,the average size of Pt nanoparticles in this catalyst just increased to?4.7 nm.In the model reactions of CO oxidation and ethylene hydrogenation,the "embedding-form" Pt@S-1 catalysts showed much higher catalytic activity compared with "yolk-shell-form" and supported-form Pt catalysts reported in literature.It is found that treating the parent microporous S-1 that has average size of 200-300 nm in pure TPAOH solution,some "gullies" on crystal surface and mesopores inside the crystallites were created.After that,utilizing this kind of crystallites as starting material to perform recrystallization in the above mentioned filtrate containing S-1 nucleus,a S-1 zeolite with dozens and even hundreds of crystal cavities could be obtained.Under similar condition,but in the presence of Pt(NH3)4(NO3)2,Pt nanoparticles could be encapsulated in the numerous cavities.The mechanism accounting for the structure formation can be described as follows:the "gullies" on crystal surface and the mesopores in crystallites could provide passages for the partial silicon dissolution and recrystallization,which made the dissolution and recrystallization occur in different directions,and resulted in the S-1 with many small cavities,or the Pt@S-1 with nanoparticles being encapsulated in small cavities.Based on this knowledge and using this method,Pt nanoparticles with average particle size of?1.3 nm were successfully encapsulated into hierarchical S-1 with 1.67 wt.%Pt,Furthermore.Pt-Sn bi-metal nanoparticles were also encapsulated in hierarchical S-1 with 0.73 wt.%Pt and 0.45wt.%Sn.Compared with traditional supported-form Pt/S-1 catalyst,the "cavity-form" Pt catalyst showed much higher catalytic activity in CO oxidation as well as propane dehydrogenation.It was proposed that fast hydrolysis of niobium ethoxide is the primary reason why the NbS-1 zeolite synthesized in literature contains large amounts of extra-framework Nb species.By suppressing the fast hydrolysis,NbS-1 zeolite that scarcely contains extra-framework Nb species was successfully synthesized.Based on this success,by using desilication and dissolution-recrystallization methods treating the NbS-1 zeolite,hierarchical NbS-1 as well as multi-hollow NbS-1 were respectively obtained.Moreover,a characterization method for identifying framework Nb species with different structure in NbS-1 zeolite using UV Raman spectroscopy that is excited by lasers with suitable wavelength were proposed by this thesis.