Noble Metal Supported On Micro-/Meso-Porous Composite Core-shell Materials:Design, Synthesis And Application To One Pot Reaction

Porous core shell materials have been widely studied duo to their potential applications in catalysis, separation, electrochemistry, bio-medicine, and so forth. Different compositions with various functionalities can be organically assembled into the core and shell as a monolithic composite. On the other hand, one-pot tandem or cascade reactions have also received intensive research interests in green and sustainable chemistry owing to their advantages in terms of atom economic efficiency, free separation for intermediates as well as process intensification. However, one pot tandem reaction usually needs a multifunctional catalyst to catalyze the multi-step reaction. It is clearly seen that the challenge in one pot reaction is how to design actively multifunctional catalyst. In this thesis, we designed and synthesized a series of novel micro-/mesoporous core-shell structured composite materials for supporting noble metal nanoparticles as bi-or multi-functional catalysts used in one pot tandem reaction.In chapter land2, the prolegomenon and reagents and the method of materials characterization were presented, respectively.In chapter3, a novel micro-/mesoporous core-shell structure material (TS-1@MS) was prepared through oriented assembly of mesoporous silica on premade titanosilicalite TS-1using triblock copolymer surfactant P123as template in an acid-free system. When pH value of the P123/NaCl/H2O/EtOH/TEOS/TS-1synthesis system was controlled at the range of4.0-5.5, the crystalline particles of TS-1were negatively charged, whereas P123micelles were partially protonated. Thus induced an electrostatic interaction between inorganic zeolite and organic micelles probably via enhanced hydrogen bonding, and made an oriented assembly of silica oligomer or silica-micelle composite on the surface of P123/TS-1and further condensation. Otherwise, the silica was self-assembled in a phase separation manner, or failed to form a mesophase. Using this method, we have been successfully fabricated a series of micro-/mesoporous composite core-shell materials with other zeolites (silicalite-1, ZSM-5, Ti-MWW) as core and large pore mesoporous silica as shell. The mesopores in silica shell were of wormhole-like and interconnected with the micropores in TS-1. When as-synthesized core/shell composites were hydrothermally post-treated, the pore volume, specific surface area and pore size of shell increased with increasing temperature. The shell thickness can be conveniently adjusted in the range of30to90nm by changing synthesis time, temperature, and the amount of silica source added, and also the particle size was changed from350nm to500nm with increase the shell thickness. The research results have been published on "Microporous Mesoporous Mater.".In chapter4, TS@MS was used as a support for incorporating gold nanoparticles (Au NPs) inside the mesoporous silica channels, leading a bifunctional catalyst (Au/TS-1@MS) which catalyzes propylene direct gas phase epoxidation with H2and O2actively and selectively. Au/TS-1@MS showed a constant propylene conversion (3.7%) and PO selectivity up to87%at473K for132h of time on stream, indicating a high stability because of the confining effect of mesopores on Au NPs. The related research results have been published on "Scientia Sinica Chimica".In chapter5, a new core-shell structured TS-1@mesocarbon (TS-1@MC) material with mesoporous carbon as shell and microporous TS-1titanosilicalite as core was fabricated through a nanocasting and selective silica etching strategy. The faithful replica structure was constructed from the composite of TS-1@mesosilica and carbon when tetrapropylammonium hydroxide (TPAOH) was employed to selectively remove the amorphous mesosilica shell while the core zeolite crystal structure was not destroyed. Contrarily, the protective effect was not seen when sodium hydroxide (NaOH) or hydrogen fluoride (HF) was employed as a silica-leaching agent. The obtained TS-1@MC had a bimodal pore structure consisting of2.9nm mesopores in carbon shell and0.51nm micropores in TS-1core. Its specific surface area and total pore volume reached883m2g-1and0.63cm3g-1, respectively. TS-1@MC was used as the support to load palladium nanoparticles (Pd NPs) in carbon shell. Having an average particle size approximately2nm, the Pd NPs were highly dispersed and confined in the mesopores of the carbon shell. Pd/TS-1@MC thus obtained served as efficient tandem catalyst in the direct epoxidation of propylene with H2and O2in liquid phase. The research results have been published on "J. Mater. Chem ".In chapter6, a trimodal hierarchical yolk-shell material consisting of TS-1core and mesoporous carbon shell (YS-TS-1@MC) was successfully synthesized by using TS-1@mesosilica as hard template, sucrose as carbon source and organic base tetrapropylammonium hydroxide (TPAOH) as silica etching agent. The resultant YS-TS-1@MC contains the micropores (0.51nm) in TS-1core, the mesopores (2.9nm) in carbon shell as well as a void or a stack pore between TS-1fragments (TS-1intercrystal mesopores,-18.4nm). Under the rigorous etching conditions, the crystalline structure of TS-1core was well retained. The YS-TS-1@MC served as a good support for palladium nano-particles (Pd NPs) or Rh(OH)x species, giving rise to efficient bifunctional catalysts for the tandem reactions including one-pot synthesis of propylene oxide or amides in liquid phase. The related research results have been published on "Chin. Chem. Lett.".In chapter7, a monodispersed center radially fibrous silica encapsulated TS-1zeolite (TS-1@KCC-1) has been fabricated in a microemulsion system for the first time. TS-1@KCC-1can be easily synthesized in a wide range of temperature and time, and the shell thickness can be conveniently adjusted by changing the ratio of TEOS to TS-1. The special structure of this kind of material should be attributed to the microemulsion synthetic system. Supporting with the Rh(OH)3species, this novel core-shell structured material serves as a robust bifunctional catalyst for one-pot synthesis of benzamide from benzaldehyde, ammonia and hydrogen peroxide, in which the aldehyde ammoximation and oxime rearrangement occur in a tandem way. Rh(OH)3/TS-1@KCC-1can also be used to other aldehydes direct ammoximation and rearrangement to fabricated primary amide with NH3and H2O2. The reusability of Rh(OH)3/TS-1@KCC-1was superior duo to it can be run five times and the activity and selectivity without decreased apparently. TS-1@KCC-1is a good support for Rh(OH)x species with high hydrothermal and mechanical stability. The research results have been published on "Chem. Commun." and "Chin. J. Catal.".In the last chapter, the conclusions of this thesis were summarized and the outlook for continuous research and the development of this field was given.