【作者】 许蕾蕾；

【导师】 郭伊荇；

【作者基本信息】 东北师范大学 ， 物理化学， 2009， 博士

【摘要】 环境友好型高效催化剂的研究与开发是绿色化学研究的一个重要方向。多金属氧酸盐(多酸)以其酸性、氧化还原性及催化性能可调控性受到广泛重视。由于多酸比表面积小且易溶于极性溶剂,将均相多酸催化剂固载化是实现其应用的有效方法,通过固载化可以极大改善多酸的催化性能。本文致力于基于多金属氧酸盐的新型微孔及介孔纳米复合催化剂的设计制备,并采用硅烷基试剂,如甲基三甲氧基硅烷、双(三乙氧基)乙基硅烷等对以上复合催化剂进行修饰改性,制备系列多功能的有机–无机杂化催化剂。以酸催化生物柴油的合成(酯化和酯交换反应)作为模型反应,分别考察了这些催化材料在非均相酸催化反应中的催化性能。具体研究内容如下:1.采用溶胶-凝胶共缩合技术制备了微孔结构酸催化剂H3PW12O40/Ta2O5,并通过ICP-AES、FT-IR、Raman、N2吸附和FESEM等测试手段对复合材料的组成、结构、形貌和表面物理化学性质进行了表征,结果表明,复合材料中催化活性组分H3PW12O40的基本结构未发生改变,催化剂的平均孔径约为2 nm,且孔径分布均匀,其比表面积比多酸有了明显提高,粒子的平均粒径约为20 nm。在酸催化乙酸乙酯的反应中,考察了催化剂用量、反应摩尔比、反应时间和多酸掺杂量对反应活性的影响。通过对不同条件的比较,反应中乙醇的转化率最高可达87.4 %,每个活性点的周转频率(TOF)为2.4×103。在采用H3PW12O40/Ta2O5催化的乙酸乙酯的合成反应中,催化剂循环使用5次后H3PW12O40的脱落量小于5 %,催化剂的活性降低小于10 %。2.采用嵌段聚合物非离子表面活性剂为结构导向剂,通过溶胶-凝胶结合程序升温溶剂热方法,一步制备了介孔结构复合催化剂H3PW12O40/Ta2O5,其中多酸的担载量为3.6~20.1 %。通过对催化剂晶相结构和表面结构的表征发现,Keggin结构单元与Ta2O5网络结构之间存在着强的相互作用,且结合牢固。此外,该类复合材料具有三维交联介孔结构,表现出优异的表面物理化学性质如较大的比表面积(106.0~126.9 m2/g)、较高的孔容(0.44~1.37 cm3/g)和较大的孔径(3.9~5.0 nm),并且,催化活性位点在复合材料中分散均匀。以上固体酸催化剂的催化性能分别通过月桂酸与乙醇的酯化反应、三棕榈酸甘油酯的酯交换反应及以豆油为原料合成生物柴油的反应进行了探究。结果表明,在温和反应条件下(常压及65 oC),H3PW12O40/Ta2O5对同时进行的酯化和酯交换反应均显示了较高的催化活性。催化过程中,对影响催化活性的诸因素如催化剂用量、反应物的摩尔比及多酸的担载量等进行了考察,并深入研究了固体酸催化酯化和酯交换反应的机理,同时对H3PW12O40/Ta2O5的循环使用情况进行了讨论。3.分别采用后合成嫁接法和共缩合的方法,以硅烷基化的介孔Ta2O5为载体,合成了不同担载量的多功能复合催化剂Ta2O5/SiO2?[H3PW12O40/R]和[H3PW12O40/R]?Ta2O5/SiO2 (R = Me or Ph)。通过疏水有机硅烷的修饰调节了介孔材料H3PW12O40/Ta2O5的表面亲/疏水平衡。以含20 %肉豆蔻酸的豆油为原料制备生物柴油的反应作为模型反应,考察了多功能杂化催化剂的催化性能,研究了催化剂制备方法、功能组分的担载量和油醇摩尔比等因素对产物脂肪酸单酯产率的影响;此外,本文还详细讨论了采用共缩合方法制备的多功能催化剂Ta2O5/SiO2?[H3PW12O40/R]具有较高催化活性的原因。在催化剂的循环使用实验中发现,经多次催化循环,多酸未发生明显的脱落现象,说明以上合成的基于多酸的杂化材料是一类潜在的非均相酸催化剂。4.与在孔道中引入疏水有机基团的修饰不同,选择不同联结链的桥联有机硅烷代替普通的硅烷,采用一步合成方法制备出了骨架中含有有机官能团的基于多酸的多功能杂化催化剂Ta2O5/Si(R)Si?H3PW12O40(R = ?Et?/?Ph?),并通过FT-IR、XPS、TEM以及N2吸附测定等表征手段对该复合物的组成、形貌、相结构以及功能组分的掺杂方式进行了表征。结果表明,在此类杂化催化剂中,功能组分如Keggin结构单元和有机功能基团与载体分别通过Ta–O–W和Ta–O–Si键作用,结果使Keggin结构单元或有机功能基团与载体结合牢固,同时还降低了催化剂表面的质子释放能,增强了催化剂的表面酸性。其次,在无机材料的骨架中引入有机官能团可使产物的三维交联孔道更均匀,从而有利于反应物及产物的传质。以豆油为原料合成生物柴油的反应中,主要讨论了该催化剂中功能组分的引入对催化活性产生的影响,并与没有修饰有机官能团的H3PW12O40/Ta2O5及修饰普通硅烷的Ta2O5/SiO2?[H3PW12O40/R]催化剂进行了比较。更多还原

【Abstract】 Eco-friendly and commercially visble catalyst systems have been a focus of much recent research. Polyoxometalates (POMs) have attracted significant attention because of their high acidity and favourable redox behaviour, which make them suitable for applications in size- and shape-selective catalysis. However, POMs exhibit low surface area, low pore volume, and low thermal stability, which limit their utility in many catalytic reactions. Thus, the dispersion of POMs on porous solid supports with high surface area, large pore diameter, and high specific pore volume is seen as a critical means of improving their properties and obtaining better performance in many potential heterogeneous catalytic applications.In this dissertation, the micro- and mesoporous composite catalysts based on polyoxometalates have been prepared, which were further modified with organic silanes (methyltrimethoxysilane, phenyltrimethoxysilane, 1,2-bis-(triethoxysilyl)ethane or 1,4-bis-(triethoxysilyl)beneze). The catalytic performance of the resulting materials was evaluated the biodiesel production as a model reaction,1.In this paper, varying amounts of H3PW12O40 have been supported on tantalum pentoxide (Ta2O5) via hydrolysis of tantalum pentachloride (TaCl5) in the presence of H3PW12O40 using the sol-gel method for acid catalytic reaction. All obtained amorphous materials have been characterized by ICP-AES, FT-IR spectra, Raman scattering spectroscopy, nitrogen adsorption/desorption analysis and FESEM, in order to characterize the structure integrity of the Keggin unit in as-prepared composites as well as the morphology and surface textural properties of the composites. The results indicated that the primary Keggin structure of starting H3PW12O40 in as-prepared composites remain intact after formation of the composites. They have microporosity, large BET surface area and uniform pore size. Catalytic activity of the catalysts has been evaluated by esterification of acetic acid with ethanol as a probe reaction with different catalytic reaction parameters such as H3PW12O40 loading, reaction time, catalyst dose, molar ratio of the reactants, etc. The highest conversion of ethanol is about 87.4 %, and turnover frequency is 2.4×103. The catalyst is recyclable without significant loss of activity.2.A series of mesoporous polyoxometalate-tantalum pentoxide composite catalysts, H3PW12O40/Ta2O5, with H3PW12O40 loading from 3.6 to 20.1% was prepared by a one-step sol-gel-hydrothermal route in the presence of a triblock copolymer surfactant. Bulk and surface sensitive probe testing results indicated that the primary Keggin structure remained intact after formation of the composite, and strong interaction between the Keggin unit and Ta2O5 framework existed in the composite. Additionally, the composite exhibited larger and well-distributed three-dimensionally interconnected pores (3.9~5.0 nm), larger BET surface area (106.0~126.9 m2 g-1), high porosity (0.44~1.37 cm3 g-1), and homogeneous dispersion of the Keggin unit throughout the composite. As an environmentally friendly solid acid catalyst, the catalytic performance of the H3PW12O40/Ta2O5 was evaluated in the esterification of lauric acid and myristic acid, the transesterification of tripalmitin as well as the direct use of soybean oil for biodiesel production. Regardless of the presence of free fatty acids, the H3PW12O40/Ta2O5 composite showed high reactivity and selectivity towards simultaneous esterification and transesterification under mild conditions. The catalyst can be recovered, reactivated and reused several times.3.Mesoporous Ta2O5 materials functionalized with both alkyl group and a Keggin-type heteropoly acid, Ta2O5/SiO2?[H3PW12O40/R] (R = Me or Ph), was prepared by a single step sol-gel co-condensation method followed by a hydrothermal treatment in the presence of a triblock copolymer surfactant. The catalytic performance of the resulting multifunctionalized organic-inorganic hybrid materials was evaluated by a direct use of soybean oil for biodiesel production in the presence of 20 wt% myristic acid under atmosphere refluxing, and the influences of the catalyst preparation approaches, functional component loadings, and molar ratios of oil to methanol on the catalytic activity of the Ta2O5/SiO2?[H3PW12O40/R] were studied. In addition, the recyclability of the hybrid materials was evaluated via four catalytic runs. Finally, the network structures of the hybrid materials and the functions of the incorporated alkyl groups on the catalytic activity of the materials were put forward. The experimental results indicated that the co-condensed multifunctionalized catalyst exhibiting considerably high catalytic activity compared with the alkyl-free catalyst as well as the grafted ones,because ( Ta–OH2)n+(H2PW12O40)n? species, main acid sites of the catalysts with enhanced Br?nsted acidity compared to H3PW12O40 or Ta2O5 for catalyzing esterification or transesterification, were formed at the surface of the product via Ta–O–W covalent bonds.4.Organic-inorganic hybrid catalysts by co-hydrolysis and condensation reactions of TaCl5 and bridged organosilica precursors of the type (EtO)3Si-R-Si(OEt)3 instead of (MeO)3Si-R in the presence of POM. The organic units in this case are incorporated in the three-dimensional network structure of the silica and tantalum matrix through two covalent bonds and thus distributed totally homogeneously in the pore walls. The phase structure and surface textural property of the composites were characterized by several characterization techniques including FT-IR, XPS, TEM and N2 porosimetry. The catalytic activity of the hybrid catalysts was evaluated by a direct use of soybean oil for biodiesel production. For comparison, H3PW12O40/Ta2O5 and Ta2O5/SiO2?[H3PW12O40/R] were also tested.更多还原