Synthesis and adsorption properties of large pore periodic mesoporous organosilicas
thesisposted on 31.01.2017 by Hao, Na
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Periodic mesoporous organosilicas (PMOs) are of scientific and technological interest because of their tunable pore structures as well as their tailored catalytic, sorption and gas storage properties. PMOs therefore present one of the most important families of organic-inorganic functional materials that are hybridized at molecular scale. The last few decades have witnessed extensive research efforts devoted to generating ordered PMOs with different mesostructures, pore sizes, and morphologies by template-directed assembly methods. In order to expand applications of PMOs into the above areas, especially sorption, it is highly desirable to develop porous materials with different compositions, adjustable pore systems and novel properties in versatile synthesis methods. The current research project is mainly concerned with this issue. In this thesis, we focused on the synthesis of PMOs with different components, structures and pore sizes by using nonionic surfactants as the structure-directing agents. Such materials obtained showed efficient adsorption capacities in different systems, and are of great theoretical and practical significance with potential applications to adsorption. The thesis is arranged as follows: Chapters 1 and 2 provide an introduction to the field of mesoporous materials and a detailed literative review on past and present work related to synthesis and characterisation of PMOs. Chapter 3 details the experimental synthesis and characterisation methods employed in this project. Chapter 4 presents the results from synthesis of large-pore phenyl-bridged PMOs. These materials were facilely synthesized by evaporation-induced self-assembly (EISA) of 1,4-bis(triethoxysily)benzene (BTEB) and triblock copolymer Pluronic F127 (EO106PO70EO106) as a template under acid conditions combined with a mixed-solvothermal treatment. The hexagonal ordered PMOs exhibit large uniform mesopores of ~ 9.9 nm in diameter after calcination at 350 C in a nitrogen atmosphere. N2 adsorption/desorption isotherms reveal imperfect mesopore channels with high surface areas (up to 1150 m2•g-1) and thick pore walls (up to 7.7 nm). The mesopores can be expanded with a decrease of acidity, as well as an increase of Pluronic F127 content. A mixed-solvothermal treatment in N,N-dimethylformamide (DMF) and water at 100 C was first used to improve the periodicity of the mesopore walls, as well as increase the wall thickness. The composites exhibit efficient adsorption capacities (2.06 mmol•g-1) for benzene, suggesting a potential adsorbent for removal of volatile organic compounds. Inorganic salts were recognized to play an important role in triggering the formation of highly ordered mesostructure. In Chapter 5, we present results from the preparation of well-ordered two-dimensional (2D) hexagonal PMOs with a high content of disulfide groups. These materials were successfully prepared by a simple metal-ion assisted amphiphilic surfactant templating process under strong acid conditions. Long-chain organic bridge silane, bis(triethoxysilylpropyl)disulfide (BTSPDS) was used as a precursor which was co-condensed with tetraethoxysilane (TEOS) to assemble with the triblock copolymer Pluronic P123 (EO20PO70EO20) template and construct the mesostructured organic-inorganic frameworks. The content of disulfide functional groups as high as 20 % was incorporated into the framework. The ordered mesoporous DS-PMO materials obtained have relatively high Brunauer-Emmett-Teller (BET) surface area (~ 580 m2•g-1), large uniform pore size (up to 6.3 nm) and thick pore walls (thickness up to 7.1 nm), because of the long-chain disulfide bridges. The metal ions such as Zn2+ formed four-coordination with two sulfides of BTSPDS and ethylene oxide moieties of P123 template, which could enhance the interaction between “soft” long disulfide groups and P123 template, thus improving the mesostructural regularity correspondingly. The disulfide-bridged PMO materials exhibit excellent hydrothermal stability in boiling water for 5 days, probably due to the thick pore walls. Excellent adsorption efficiency (~ 716 mg•g-1) for Hg2+ ions is observed, suggesting a potential application in removal of heavy metal ions in wastewater. In chapter 6 we present our results on formation of PMO hollow spheres. Large-pore PMO hollow spheres with controllable pore size and high pore volume (~ 2.5 cm3•g-1) were successfully synthesized at low temperature (~ 15 C) by using the triblock copolymer Pluronic F127 as a template and 1, 3, 5-trimethylbenzene (TMB) as a swelling agent in the presence of inorganic salt (KCl). The PMO hollow spheres are uniform and well dispersed, and have a large wall thickness. The pore size (9.8 ~ 15.1 nm) of the hollow spheres can be gradually expanded by increasing the TMB content together with a relatively high acidity. By controlling the content of hexadecyltrimethylammonium bromide (CTAB), successive structural transformation from hollow sphere to wormlike mesostructure and eventually to ordered body-centered cubic (space group of Im3m) mesostructure is observed. Our results reveal that the hydrophobicity of bis(triethoxysilyl)ethane (BTSE) and a low temperature approach lead to the slow hydrolysis rate of silica precursors, which in turn leads to the weak interaction between individual TMB/F127 micelles and silicate oligomers. Furthermore, the salting-out effect of KCl may influence the swelling capacity of individual micelles as well as decrease the critical micelle concentration and critical micelle temperature, resulting in the formation of PMO hollow spheres from the assembly of individual TMB/F127 micelles with silicate oligomers. The composites exhibit efficient adsorption capacity (703 mg•g-1) for toluene, suggesting that they are a potentially useful adsorbent for removal of volatile organic compounds. The PMO hollow spheres allow biomolecules with large molecular weight to diffuse in, and showing a superior encapsulation capacity of bovine serum albumin (BSA) molecules (~ 585 mg•g-1) over other porous materials. In chapter 7, ordered mesoporous polymer-organosilica and carbon-silica nanocomposites were synthesized through a triconstituent co-assembly strategy wherein the soluble resol polymer was used as an organic precursor, prehydrolyzed BTSE as another organic precursor, and triblock copolymer F127 as a template. After thermal curing of the resin polymer, the triblock copolymer F127 was removed by calcination at 350 ºC in N2 atmosphere. The results of characterisations show that the polymer-organosilica nanocomposites have ordered 2D hexagonal mesostructures with uniform pore size (6.2 ~ 7.3 nm), suitable surface areas (619 ~ 794 m2•g-1) and pore volumes (0.61 ~ 0.88 cm3•g-1). With increasing BTSE content, the BET surface area and pore volume reduce for the polymer-organosilica composites. The mesoporous polymer-organosilica nanocomposites have homogeneous interpenetrating frameworks, in which both polymer and organosilica synergistically support the ordered mesostructure. Combustion in air or etching in HF solution can remove carbon or silica from the carbon-silica nanocomposites and yield mesoporous pure silica or carbon frameworks. The adsorption performance of the polymer-organosilica hybrid materials for benzene was also measured. Because of the hydrophobic property of phenyl-bridging groups from resin polymer and ethylene groups in the framework, the polymer-organosilica nanocomposite shows adsorption capacity of benzene up to 2.0 mmol•g-1, which suggests that they are a potential candidate for adsorption of organic compounds. Chapter 8 provides the conclusions of this project and gives an outlook for PMOs in some key areas.