Synthesis And Characterization Of Nobel Metal Supported On Micro-mesoporous Molecular Sieves

Noble metal (such as Pt,Pd,Rh,etc.)supported zeolite have excellent hydrodearomatization and hydrodesulfurization activity,which have been widely used as catalysts in the petrochemical industry. The uniform pore structure of the zeolite prevents the growth and agglomeration of metal particles during catalyst preparation by encaging the metal particles within the zeolite pore,which can increase specific surface areas of active site.Simultaneously, those active metal sites are accessible only to the molecules that have diameters small enough to diffuse to the active sites, while larger molecules are excluded from the zeolite pore, however, they are susceptible to sulfur poisoning even at concentrations of a few parts per million Noble metals will be encapsulated in zeolite NaA channels with 4? aperture.Zeolite NaA opening will be subsequently adjusted by ion-exchanged and Chemical vapor deposition (CVD) to a proper size to exclude sulfur compounds and prevent from contacting noble metals, which makes sulfur-tolerant catalyst design come true.But its small pore width and external surface area limits its application.While mesoporous molecular sieves possess smooth mass transfer channels and large external surface area.Herein we will prepare NaA/MCM-41 loaded with platinum bimodal pore molecular sieves and mesopore zeolite NaA catalysts. Then hydrogen molecules can readily diffuse in and out of the micro-cage, dissociatively adsorb, activate on metal particles and migrate mesopore surface into via hydrogen spillover that provides enough external surface area for larger molecules reaction.In this paper, based on sol-gel and self-assembly theory, micro-mesoporous molecular sieves have been synthesized by different two methods.(1)Nanosized NaA zeolite loaded with platinum was synthesized.NaA/MCM-41 composite materials were subsequently obtained from nanosized zeolite NaA precursor solutions by overgrowth or nanoparticles self-assembly. (2)Mesoporous zeolite NaA was directly synthesized by addinging organosilane surfactant into the initial synthesis mixture and characterized with XRD, N2 adsorption-desorption, SEM, TEM and particle size distribution etc. The results obtained from experiments and studies were given in following.1 Pt/NaA-zeolite could be synthesized by incorporating different Pt(NH3)4Cl2 content precursor into the initial systhesis mixture and characterized with XRD, particle size analysis and SEM methods.The research has found that platinum precursors would have an impact on the crystal morphology of the final zeolite.The framework structure of zeolite A progressively could disappear and the particle size could decrease as the platinum content was increased.When n(Pt(NH3)4Cl2)/n(Al2O3) in initial synthesis composition was 0.076, the particle size of zeolite A would ranged between 100 and 300nm.When n(Pt(NH3)4Cl2)/n(Al2O3) in initial synthesis composition was 0.15, the uniform nanosized particles less than 100nm could be obtained.2 Round spherical zeolite NaA particles with diameter 200nm could be obtained by the addition of tetramethylammonium hydroxide(TMAOH) into the initial systhesis mixture zeolite A,when the ratio of n(H2O)/n(SiO2) was between 34 and 54,stiring time was 24h and crystallization time varied from 16h to 24h. But the yield of nanosized zeolite A is low.3 With a two-step synthesis strategy, NaA/MCM-41 composite molecular sieves have been prepared by overgrowth or nanoparticles self-assembly from as-synthesized nanosized zeolite A supported platinum precursor in the presence of cetyltrimethylammonium bromide surfactant solution and adding silica source under adjusting systematic pH. The experiments turned out that crystallization of zeolite NaA and MCM-41 worked in a competitive rather than a cooperative manner.N2 adsoption-desorption isotherms for NaA/MCM-41 samples in this manner exhibited a typical irreversible type IV isotherm with two separate hysteresis loops,which correspond to framework-confined mesopores and secondary interparticles mesopores.The BET specific surface areas increase as the amount of TEOS was increased in the second step synthesis mixture.The BET specific surface area was 326m2/g when n(SiO2)/n(Al2O3) by the addition of TEOS was 5.15.SEM and TEM showed composite molecular sieves presented packed shape and had mesoporous channels,which are different from mechanical mixture. 4 Mesoporous zeolite NaA was synthesized by a direct synthesis route adding vinyltriethoxysilane(A-151), octadecyldimethyl (3-trimethoxysilyl propyl) ammonium chloride (TPOAC) or tetradecyldimethyl (3-trimethoxy silylpropyl) ammonium chloride (TPTAC) amphiphilic organosilanes as a mesopore-directing agent into conventional alkaline zeolite NaA synthesis mixtures. The zeolite products were characterized by a complementary combination of X-ray diffraction (XRD), nitrogen sorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).XRD and nitrogen sorption results showed the present method is suitable as a direct synthesis route to highly mesoporous zeolites. nitrogen sorption analysis proved the BET specific surface area of zeolite NaA at optimum condition was 77.5 m2/g and pore width was over 20nm using A-151 as a mesoporous-directing agent;the BET specific surface area of zeolite NaA at optimum condition was 256 m2/g and pore width with narrow distribution was 4nm using TPOAC as a mesoporous-directing agent; the BET specific surface area of zeolite NaA at optimum condition was 244 m2/g and pore width with narrow distribution was 4nm using TPTAC as a mesoporous-directing agent.SEM and TEM analysis results revealed zeolite NaA contained intercrystalline void space by A-151 as a mesoporous-directing agent; zeolite NaA surface appeared disordered wormhole-like mesoporous using TPOAC and TPTAC as mesoporous-directing agents,and the short-range correlation intracrystalline mesopores stretch to zeolite NaA surface.