余承忠             

 

 

论文题目：嵌段共聚物导向下新型介孔分子筛材料的合成与表征 

作者简介：余承忠，男,1969年05月出生，1999年09月师从于复旦大学赵东元教授，于2002年07月获博士学位。

                  

摘  要

 

1992年Mobil公司研究人员利用表面活性剂的自组装性质和模板作用，成功地合成了由表面活性剂—氧化硅（铝）构成的无机-有机杂化材料，脱除表面活性剂后得到了具有均匀纳米孔、大比表面积M41S类介孔材料。由于具有高的表面积和孔容，易于调变的规整纳米孔(如孔径、维数等)，丰富而易于设计的表面基团，以及可调控的宏观形貌（如膜、纤维、球等）,介孔材料在大分子分离、生物传感器、催化、吸附、微电子、光学以及制备新型纳米材料等领域显示了诱人的应用前景。但M41S介孔分子筛水热稳定性不高，且所使用的有机模板剂以及有机硅源价格昂贵，限制了其进一步应用。由商品化三嵌段聚醚高分子表面活性剂聚环氧乙烷-聚环氧丙烷-聚环氧乙烷（PEO-PPO-PEO）导向而成的有序度高、大孔径的（5~30 nm）厚壁介孔材料SBA-15的出现，为介孔材料的发展注入了新的活力。为了使介孔材料向功能化和经济实用方向发展，寻找新的模板剂，合成具有新结构、新性能的介孔材料，探索模板剂结构与介孔材料性能之间的关系，成为目前材料研究领域的热点，无论在理论上还是在实际应用中都具有重要的意义。

本论文选择嵌段共聚物模板剂导向合成新型介孔材料这一课题为研究内容，结合模板化学、溶胶-凝胶化学、自组装化学、胶体化学等研究方法，利用多种现代化表征技术，合成了一系列不同结构（立方、六方、层状、囊泡）、不同组成（二氧化硅、碳）、不同孔径大小、不同形貌的新型介孔材料，探索了嵌段共聚物的结构与其导向合成的介孔材料性质之间的关系，以及溶液中介孔粉体材料生长的规律，对介孔材料合成过程中温度、离子强度等因素的影响进行了系统的研究。

选择比PPO疏水性更强的聚环氧丁烷（PBO）为嵌段共聚物的疏水链段，研究了6个商品化三嵌段PEO-PBO-PEO以及一个二嵌段PEO-PBO非离子型共聚物表面活性剂导向下介孔二氧化硅材料的合成。以EO₃₉BO₄₇EO₃₉三嵌段高分子表面活性剂为结构导向剂，在水相中合成了有序度高、水热稳定性高、立方体心结构（Im3m）的介孔分子筛FDU-1，其孔径达到12 nm，是目前世界上孔径最大的立方结构的介孔材料。在EO₄₃BO₁₄EO₄₃导向下也可合成具有相同立方结构的介孔分子筛，但其孔径较小(2.4 nm)。利用三嵌段高分子EO₁₇BO₁₄EO₁₇和两嵌段高分子EO₁₇BO₁₀均可以合成出具有六方结构的介孔氧化硅材料，其孔径分别为3.8 和6.0 nm。在低温(0 °C)和强酸性条件下(6M HCl)，选用具有较大疏水/亲水链段比的非离子嵌段共聚物EO₁₅BO₄₅EO₁₅为模板剂，首次合成出大孔径(10 nm)的介孔氧化硅囊泡，通过改变硅源与模板剂（Si/EO₁₅BO₄₅EO₁₅）的比例以及其它反应条件，观察到由囊泡向六角相的转变。对于嵌段共聚物EO₁₃BO₁₁EO₁₃和EO₃₄BO₁₁EO₃₄，两者均具有较大的临界胶束浓度(CMC)，在其导向下仅制备得到无序排列的介孔氧化硅材料，但孔径分布比较均匀，分别为2.4 和2.0 nm。由以上结果并结合先前的实验现象可以得出如下结论：1）介孔材料的孔径大小主要取决于模板剂疏水链段的分子量，与介孔材料的结构有一定的关系；2）对于具有相同疏水链段分子量的嵌段共聚物而言，PBO导向而成的介孔材料的孔径要远远大于PPO导向而成的介孔材料；3）对于相同疏水链段而言，二嵌段共聚物导向而成的介孔材料的孔径要远远大于三嵌段共聚物导向而成的介孔材料的孔径。通过比较三嵌段共聚物的CMC以及它们导向而成的介孔二氧化硅材料的结构，综合文献报道，我们提出了可以用嵌段共聚物的CMC以及临界胶束温度（CMT）为指标，判断其在水溶液中导向制备介孔二氧化硅材料的结构，并进行合成路线的设计。

通过考察温度、离子强度等对合成介孔材料质量的影响，首次提出使用“盐溶”性无机盐增强有机物种自组装能力，拓宽有序介孔材料合成相区（合成温度，模板剂种类，模板剂浓度等）的新方法。利用盐效应，在低温或低模板剂浓度条件下合成出高度有序、微孔孔容小的介孔分子筛SBA-15；大大提高了利用嵌段共聚物EO₄₃BO₁₄EO₄₃，EO₁₃BO₁₁EO₁₃，EO₃₄BO₁₁EO₃₄，EO₁₇BO₁₄EO₁₇，EO₉₉PO₇₀EO₉₉和 EO₁₃₂PO₅₀EO₁₃₂等导向合成介孔材料的有序度或孔径分布。无机盐对介孔材料的形貌控制也有很大影响，利用无机盐的作用，合成了不同形貌、高度有序的介孔材料SBA-15，在低温下得到球状形貌，而在38°C时得到产率接近100%，直径在1-2 mm的介孔氧化硅棒，其孔道方向完全平行于棒的长度方向。研究了SBA-15棒状材料在溶液中的生长过程，发现具有直的棒状形貌的SBA-15介孔材料（p6mm）是由无序排列的胶体球状物生长而来的。由此提出了介孔材料在溶液中生长的相分离生长机制，把介孔粉体材料在溶液中的生长分为三个阶段：有机-无机复合相的协同组装过程；类液晶相的形成及相分离过程；由类液晶相到有序介观结构的完善过程。提出可以用自组装能量DG和比表面能F的相对大小来解释溶液中介观结构的生长以及形貌发展规律。利用胶体化学的基本概念以及相分离机制，在胶体尺度上解释了无机盐为什么会对介孔材料的形貌产生巨大的影响。

使用非离子型嵌段高分子表面活性剂EO₁₀₆PO₇₀EO₁₀₆为模板剂，利用无机盐来调变有机物种的自组装能力和无机/有机物种之间的作用力，首次对具有三维立方笼状结构的介孔分子筛SBA-16实现了形貌控制，合成了直径在毫米数量级、具有大孔径、三维孔道结构、高度有序的SBA-16球。以非离子嵌段聚合物EO₁₃₂PO₅₀EO₁₃₂为模板剂，首次合成出了高产率（100%）、形貌规则(标准十二面体)、大小均一（在1mm左右）、纯的立方相（Im3m）纳米介孔氧化硅单晶。该介孔单晶的每个晶面都可以归属为{110} 面，透射电子显微镜（Transmission electron micrographs, TEM）证实介孔晶格在整个晶面上规则排列（沿[110]方向），没有观察到明显的位错（dislocations），层错（fault planes）或孪晶（twinning）现象，确证所得到的是完美介孔单晶。

利用介孔氧化硅材料为硬模板，成功地制备出一系列不同形貌、不同结构的有序介孔碳材料。将棒状氧化硅SBA-15成功地转化为介孔碳分子筛，其形貌保持基本不变，该材料具有目前已报道的介孔碳材料中最大的比表面积和孔容（1900 m²/g，2.23 cm³g^-1）；首次对介孔碳分子筛材料的形貌进行控制，合成出高度有序，具有棒状、片状、纤维状、面包圈状的介孔碳分子筛；首次从具有立方笼状结构的介孔氧化硅材料SBA-16，FDU-1等出发，通过提高模板剂质量，扩大笼状结构的窗口尺寸以防止堵塞等手段，合成出三维有序结构的介孔碳材料。

介孔材料的合成通常在水溶液中进行，利用嵌段高分子表面活性剂丰富的相行为，将合成体系从水溶液中拓展到油/水体系以及非极性溶剂体系。结合溶胶-凝胶化学、自组装化学与乳液化学方法，合成了直径在1-4 µm，壁厚在200 nm 左右，墙壁组成为超大介孔 (直径约50 nm) 的二氧化硅空心球材料，该材料具有较大的比表面积 (674 m²/g) 和很大的孔容 (1.25 cm³/g)。利用嵌段共聚物表面活性剂在非极性溶剂中形成反相介观相（reverse mesophase）以及PEO链段容易嵌入二氧化硅墙壁的特点，制备得到多级有序排列的二氧化硅棒材料。尺寸规则（半径10nm），六方排列的二氧化硅纳米棒进一步排列成层状结构(层间距约 150 nm)， 其外观通常为膜或片状，通过选择表面活性剂及改变其浓度，纳米棒的尺寸可以在9到15 nm范围内调变。

关键词：合成，嵌段共聚物，自组装，介孔分子筛，形貌

                     Abstract

In 1992, the Mobil Scientists reported the synthesis of a new family of inorganic/organic hybrid materials by the use of self-assembled surfactant molecular aggregates as the structure-directing agents. After removing the organic species, mesoporous aluminosilicates M41S were obtained with uniform pore size and high surface areas. For the past ten years, such materials have attracted considerable attention in the areas such as separation of large molecular, biosensors, catalysis, adsorption, microelectronics, optics, and fabrication of novel nano-objects because of their uniform and adjustable pore properties (pore sizes and pore structures), rich surface functional groups, and designable morphologies (film, fiber, sphere, etc.). However, the low hydrothermal stability of M41S and the expensive surfactants and organic silica precursors employed during synthesis limited their advanced uses in such areas. The successful synthesis of highly ordered, highly stable, large pore (5-30 nm) hexagonal mesoporous silica SBA-15 materials by using commercial triblock poly (ethylene oxide)- poly (propylene oxide)- poly (ethylene oxide) (PEO-PPO-PEO) block copolymers under acidic conditions opens up new possibility in materials research. For future applications of functional and economic mesoporous materials, it is important to search new type of templates, investigate the relationship between the structure of templates and the property of resulted mesostructures, and synthesize advanced mesoporous materials with new structure and properties.

The current contribution concerns the synthesis of novel mesoporous materials templated by nonionic block copolymer surfactants. A series of new mesoporous materials with different structures (cubic, hexagonal, lamellar and vesicles), various components (silica, carbon), adjustable pore sizes (2-50 nm) and designable morphologies have successfully been synthesized by combining the concepts of sol-gel chemistry, colloid science and supramolecular self-assembly. The structure-property correlation, the temperature and ionic strength effect in the synthesis of mesoporous materials have been studied systematically in this study.

We have studied the synthesis of mesoporous silica structures by using commercial block copolymers with PEO as hydrophilic part and poly(butylene oxide) (PBO) as hydrophobic moiety, including six triblock and one diblock copolymer surfactants. Highly ordered, hydrothermally stable, caged cubic mesoporous silica structures (Im m) FDU-1 with unusual large pore size (12 nm) have been synthesized by using EO₃₉BO₄₇EO₃₉ as a structure-directing agent, this is the largest pore size for cubic silica structures reported up to date. Similar cubic silica structures have been obtained in the presence of EO₄₃BO₁₄EO₄₃ with small pore size (2.4 nm). Hexagonal silica mesostructures have been synthesized from EO₁₇BO₁₄EO₁₇ and EO₁₇BO₁₀ with pore sizes of 3.8 nm and 6.0 nm, respectively. Under low temperature (0 °C) and strong acidic condition (6M HCl), large pore (10 nm) silica vesicles have been obtained for the first time in the presence of EO₁₅BO₄₅EO₁₅ with unusual large hydrophobic/hydrophilic volume ratio; a transition from vesicle to hexagonal structure has been observed by changing the Si/copolymer ratio and other reaction parameters. For block copolymers EO₁₃BO₁₁EO₁₃ and EO₃₄BO₁₁EO₃₄ with relatively large critical micelle concentrations (CMC), only disordered silica mesostructures were obtained with relatively uniform pore size of 2.4 and 2.0 nm, respectively. Several conclusions can be drawn from this study: 1, the pore size of mesoporous materials is dependent on the molecular weight of hydrophobic part of block copolymer templates and related to the type of mesostructure; 2, for block copolymer surfactants with the same hydrophobic molecular weight, the pore size of mesoporous materials is much larger for PBO than that for PEO, 3, for the same block compositions, the pore size of mesoporous materials templated by diblock copolymer is much larger than that templated by triblock copolymer surfactants. By comparing the CMC of triblock copolymers and the resulted silica mesostructure, we proposed that the CMC and critic micelle temperature (CMT) can be used as important criteria in the design of the structure and the synthesis approach of mesoporous silica precipitated from solutions.

By studying the temperature and salt effect in the synthesis of mesoporous materials with block copolymers, we proposed that the use of “salting-out” inorganic salts can dramatically widen the syntheses domain (in temperature, surfactant concentration, etc.) and broaden the range of surfactants that can be utilized to produce highly ordered mesostructures. By utilizing this concept, highly ordered mesoporous silica SBA-15 materials with low microporosity have been synthesized under low temperature and low surfactant concentrations; the quality of resulted mesostructure or pore size distribution can be greatly improved in the cases of EO₄₃BO₁₄EO_43, EO₁₃BO₁₁EO_13, EO₃₄BO₁₁EO_34, EO₁₇BO₁₄EO_17, EO₉₉PO₇₀EO₉₉ and EO₁₃₂PO₅₀EO₁₃₂ block copolymers. Inorganic salts also have great influence in the morphology control of mesoporous materials templated by nonionic block copolymers. By using inorganic salts, we have synthesized SBA-15 with spherical morphology at low temperature, while highly ordered SBA-15 materials with 100% rod-like morphologies were obtained at relatively higher temperature. These straight rods are quite uniform in length (1-2 mm), the mesopore channels are parallel to the long axes of the rods. By studying the particle growth of these SBA-15 rods, a phase separation mechanism of the formation of mesoporous powders in solution is proposed. The formation of mesoporous powders can be divided into three stages: 1, cooperative assembly in solution to give of inorganic-organic composites; 2, further condensation of inorganic species to give a liquid crystal like phase and further liquid-liquid phase separation; and 3, the formation of final mesostructure from separated liquid crystal like phase. The competition between the total energy of self-assembly DG and the surface tension F can be related to the formation of final mesostructure and morphology. Based on general concepts in sol-gel chemistry and the proposed phase separation mechanism, the effects of inorganic salts on the morphology of mesoporous materials can be interpreted at the colloidal level.

Highly ordered cubic mesoporous silica SBA-16 with millimeter spherical morphology (2-4 mm in diameter) was successfully synthesized by use of inorganic salts. We have also reported the first synthesis of large pore (7.4 nm), cubic mesoporous silica single crystals with exclusively uniform rhombdodecahedron shapes (~ 1 mm) and ~100% crystal yield in the presence of EO₁₃₂PO₅₀EO₁₃₂ templates. These 12 faces can be indexed to {110} planes. For thin sections of each crystal face, the TEM image along only [110] direction is observed, moreover, at each crystal face, the diagonal of the rectangular repeating unit is parallel to the crystal edge, in accordance with an ideal crystal model with Im3m symmetry. The unit cell is propagated throughout the faceted particles without twinning or apparent dislocations and fault planes, unambiguously confirming that the particles are perfect single crystals.

By utilizing mesoporous silica structures as the hard templates, mesoporous carbons with controlled morphologies and structures have been synthesized. We have synthesized highly ordered hexagonal mesoporous carbon materials with fiber-like, plate-like, rod-like and donut-like morphologies by using SBA-15 silica templates. N₂ sorption analysis results reveal that these mesoporous carbon materials with controlled morphologies have very large surface area (up to 1900 m²g^-1) and pore volume (up to 2.23 cm³g^-1), suggesting that these carbon materials are very valuable in future applications. Moreover, cubic mesoporous carbon materials have been successfully synthesized from SBA-16 and FDU-1 silica templates with cage-like structures by methods of improving the quality of mesoporous templates and enlarging the window size of caged pore structures.

We have extended previous work from water-rich phases to oil-rich phases. By the combination of sol-gel chemistry and reverse emulsion chemistry, siliceous hollow spheres (1-4 µm) with ultra large mesopore wall structures (~ 50 nm in pore diameter, ~ 200 nm in wall thickness) have been synthesized with a high surface area (674 m²/g) and pore volume (1.25 cm³/g). In non-polar solvent systems, a reverse amphiphilic mesophases approach to create hierarchically arrayed silica nanorods has been developed. The cylinder nanorods with uniform diameter (10 nm) generated from highly ordered two-dimensional (2D) reverse hexagonal mesophases are aligned within lamellar macrostructures (~ 150 nm). The diameter size of silica nanorods is small and can be adjusted from 9 to 15 nm by using different surfactants containing PEO moiety and their concentrations. The calcined material has a relatively small surface area of 275 m²/g. This approach may provide a new pathway to the fabrication of nano-objects that might be applied in nanoscale devices.

Key words: synthesis, block copolymers, self-assembly, mesoporous molecular sieves,     morphology

　回主页