【作者】 焦卫勇；

【导师】 李福祥；

【作者基本信息】 太原理工大学 ， 物理化学， 2010， 硕士

【摘要】 负载贵金属(如Pt,Pd及Rh等)沸石具有很高的加氢脱芳烃活性,在石油工业中广泛用作催化剂。在制备催化剂时,将贵金属封装在沸石均匀的纳米孔道中,有效的防止金属粒子的长大和团聚,增加活性中心的比表面积。同时沸石较小的孔径只允许小分子进入与活性中心接触,而大分子被排斥在孔口外。但这类催化剂对原料中的硫化物非常敏感,即使很低的含量(如大于5ppm)也容易使其中毒而失活。NaA沸石孔径为4?,将贵金属封装在NaA沸石的孔道中,通过孔口调变,将含硫化合物排斥于孔口之外,完全不与贵金属接触,实现了贵金属催化剂的耐硫设计。但是NaA沸石较小的孔道和外比表面积限制了它的应用,而介孔分子筛具有通畅的传质孔道和大的外比表面,因此我们将制备负载Pt的NaA/MCM-41双孔分子筛和介孔NaA沸石催化剂,氢分子可以自由进出NaA沸石孔道,在贵金属粒子上分解活化,并通过氢溢流进入介孔内,介孔为大分子反应提供了充分的表面和空间。本论文在溶胶-凝胶和自组装理论基础上,采用两种不同的方法合成微介孔复合分子筛。(1)首先合成负载Pt的纳米A型沸石,以纳米A型沸石为前驱体采用纳米自组装或者附晶生长合成NaA/MCM-41复合分子筛。(2)通过在A型沸石的合成凝胶中加入有机硅结构导向剂合成介孔NaA沸石。采用XRD、N2吸附-脱附、SEM、TEM和激光粒度分析等手段进行了表征,得到以下结论:1在合成A型沸石的体系中,加入不同量的Pt(NH3)4Cl2前驱液,通过XRD、粒度分析和SEM等手段,研究发现Pt(NH3)4Cl2能够影响A型沸石的形态和粒径,随着Pt(NH3)4Cl2加入量的增加,A型沸石的XRD衍射峰降低,粒径变小。当n (Pt(NH3)4Cl2) /n(Al2O3)为0.076时,A型沸石的粒径变为100-300nm;当n (Pt(NH3)4Cl2)/n(Al2O3)为0.15时,得到均匀的纳米颗粒,粒径为100nm以下。2在A型沸石合成体系中,加入四甲基氢氧化铵(TMAOH)结构导向剂,当n(H2O)/n(SiO2)在34～54之间,搅拌时间为24h,晶化时间为16～24h时,能够合成200nm左右的圆球状的A型沸石,但是产率较低。3采用两步晶化法,以合成的负载Pt的A型沸石为前驱物,加入十六烷基三甲基溴化铵(CTAB)模板剂和硅源,调节体系的pH,通过纳米组装或者附晶生长合成负载Pt的NaA/MCM-41复合分子筛。实验证明复合体系中,NaA沸石的晶化和MCM-41的生长没有产生协同作用,而是一个此消彼长的竞争反应。通过对孔结构分析,此法合成的NaA/MCM-41的等温线出现两个迟滞环,对应于介孔结构和二次堆砌双孔孔道,随着n(SiO2)/n(Al2O3)的增大,BET比表面积增大,当补硅量n(SiO2)/n(Al2O3)为5.15,BET比表面积为326m2/g。SEM表明复合分子筛呈包裹状,具有介孔孔道,不同于二者机械混合。4在合成A型沸石体系中,采用不同的硅源,分别加入乙烯基三乙氧基硅烷(A-151)、N,N-二甲基-N-[3-(三甲氧硅)丙基]氯化十八烷基铵(TPOAC);N,N-二甲基-N-[3-(三甲氧硅)丙基]氯化十四烷基铵(TPTAC)两性分子介孔导向剂,采用原位造孔法直接合成介孔NaA型沸石。XRD和N2吸附表明:在合成体系中,加入一定量的上述任一有机硅模板剂都能合成介孔A型沸石。N2吸附脱附分析A-151作为合成介孔A型沸石的偶联剂,A型沸石的BET比表面积为77.5m2/g,孔径为20nm以上。TPOAC作为介孔A型沸石的模板剂,A型沸石孔径分布窄,孔径集中在4nm,BET比表面为256m2/g,并且主要是介孔比表面积。TPTAC作为合成介孔A型沸石的模板剂,A型沸石孔径分布窄,孔径集中在4nm,样品的BET比表面积为244m2/g。SEM和TEM分析A-151作为合成介孔A型沸石的偶联剂,合成的A型沸石是纳米粒子的晶间空隙孔;TPOAC和TPTAC作为介孔A型沸石的模板剂,A型沸石表面呈现无序的蠕虫状介孔,这些短程相连的穿晶介孔直伸到微孔沸石表面。更多还原

【Abstract】 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.更多还原