Self Assembly Of Nanoporous Silica Shapes Synthesis Morphogenesis And Applications

Nanoporous Materials IV contains the invited lectures and peer-reviewed oral and poster contributions to be presented at the 4th International Symposium on Nanoporous Materials, which will be hosted in Niagara Falls, Ontario, Canada, June 7-10, 2005. This volume covers complementary approaches to and recent advances in the field of nanostructured materials with pore sizes larger than 1nm, such as periodic mesoporous molecular sieves (e.g., MCM-41 and SBA-15) and related materials including clays, ordered mesoporous carbons, colloidal crystal templated materials, porous polymers and sol gels. The broad range of topics covered in relation to the synthesis and characterization of ordered mesoporous materials are of great importance for advanced adsorption, catalytic, separation and environmental processes as well as for the development of nanotechnology. This volume contains over 120 contributions related to the synthesis of ordered mesoporous silicas, organosilicas, nonsiliceous inorganic materials, carbons, polymers and related materials, their characterization and applications in adsorption, catalysis and environmental clean up. * Unique contributions brings readers up-to-date on new research and application developments * Figures and tables supplement comprehensive topics * Extensive author and subject index

In the past two decades, the field of nanoporous materials has undergone significant developments. As these materials possess high specific surface areas, well-defined pore sizes, and functional sites, they show a great diversity of applications such as molecular adsorption/storage and separation, sensing, catalysis, energy storage and conversion, drug delivery, and more. Nanoporous Materials: Synthesis and Applications surveys the key developments in the synthesis of nanoporous materials in a broad range from soft porous materials—such as porous organic and metal-organic frameworks—to hard porous materials, such as porous metals and metal oxides, and the significant advances in their applications to date. Topics Include: Synthetic approaches, characterization techniques, and applications of a variety of meso- and microporous polymers and organic frameworks Advances in the synthetic control of structures along with the function exploration of this new class of organic porous materials Synthesis and applications of nanoporous metal-organic frameworks, mesoporous silica, and nanoporous glass Synthesis of mesoporous carbons by a soft- and hard-templating method and their applications for supercapacitors and membrane separations Fabrication of nanoporous semiconductor materials Structural modification and functional improvement of layered zeolites Germanates and related materials with open-frameworks

Porous materials have long been a research interest due to their practical importance in traditional chemical industries such as catalysis and separation processes. The successful synthesis of porous materials requires further understanding of the fundamental physics that govern the formation of these materials. In this thesis, we apply molecular modeling methods and develop novel models to study the formation mechanism of ordered porous materials. The improved understanding provides an opportunity to rational control pore size, pore shape, surface reactivity and may lead to new design of tailor-made materials. To attain detailed structural evolution of silicate materials, an atomistic model with explicitly representation of silicon and oxygen atoms is developed. Our model is based on rigid tetrahedra (representing SiO4) occupying the sites of a body centered cubic (bcc) lattice. The model serves as the base model to study the formation of silica materials. We first carried out Monte Carlo simulations to describe the polymerization process of silica without template molecules starting from a solution of silicic acid in water at pH 2. We predicted Qn evolutions during silica polymerization and good agreement was found compared with experimental data, where Qn is the fraction of Si atoms with n bridging oxygens. The model captures the basic kinetics of silica polymerization and provides structural evolution information. Next we generalize the application of this atomic lattice model to materials with tetrahedral (T) and bridging (B) atoms and apply parallel tempering Monte Carlo methods to search for ground states. We show that the atomic lattice model can be applied to silica and related materials with a rich variety of structures including known chalcogenides, zeolite analogs, and layered materials. We find that whereas canonical Monte Carlo simulations of the model consistently produce the amorphous solids studied in our previous work, parallel tempering Monte Carlo gives rise to ordered nanoporous solids. The utility of parallel tempering highlights the existence of barriers between amorphous and crystalline phases of our model. The role of template molecules during synthesis of ordered mesoporous materials was investigated. Implemented surfactant lattice model of Larson, together with atomic tetrahedral model for silica, we successfully modeled the formation of surfactant-templated mesoporous silica (MCM-41), with explicit representation of silicic acid condensation and surfactant self-assembly. Lamellar and hexagonal mesophases form spontaneously at different synthesis conditions, consistent with published experimental observations. Under conditions where silica polymerization is negligible, reversible transformation between hexagonal and lamellar phases were observed by changing synthesis temperatures. Upon long-time simulation that allows condensation of silanol groups, the inorganic phases of mesoporous structures were found with thicker walls that are amorphous and lack of crystallinity. Compared with bulk amorphous silica, the wall-domain of mesoporous silicas are less ordered withlarger fractions of three- and four-membered rings and wider ring-size distributions. It is the first molecular simulation study of explicit representations of both silicic acid condensation and surfactant self-assembly.

The syntheses of silica materials provide many experimental methods that introduce different assemblies of nanoporous silica structures. Silica nanoparticles, hollow silica shells, core/shell structures and porous silica materials are examples of structures that have been synthesized in research works. A standard procedure for producing silica nanomaterials uses the established sol-gel Stöber process. Other developed methods also include solvothermal and hydrothermal techniques to prepare amorphous and crystalline silica materials. A review of the different approaches such as self-template, hard template, and soft template are described to present particular strategies that have been adapted to fabricate silica nanomaterials. The composition of silica scaffolds can include organic and inorganic states of matter. These adaptations can be utilized to achieve certain configurations during post synthetic treatments such as calcination and etching. The morphology of cubic and spherical silica particles are also studied to analyze the synthetic factors regarding the formation of shapes. Here, silica microcubes are produced after secondary precipitation of silica onto silica nanospheres. Hollow silica microcubes with different shell thicknesses are fabricated from the mSiO2@mSiO2 cubic particles through including additional silicate precursors during synthesizing the external layer followed by etching in warm water. Porous silica structures are recognized categories of materials that are applied for nanoscale storage and transport purposes. In this study, hollow silica microcubes are applied in the X-ray catalyzed hydroxylation of coumarin-3-carboxylic acid. X-ray induced enhancement occurs at the Type 1 and Type 2 proximities. Type 1 is described to be at least 100 nm away from the nanoparticle interface while Type 2 is within 100 nm from the nanoparticle interface. This review investigates current and viable methods to synthesize and apply porous silica nano-/micro-structures for research analyses.

Porous materials are of great importance in many fields due to their wide applications. An ongoing theme in this area is the tailoring of materials for specific applications. With a better understanding of the formation mechanisms, tailoring and controlling the pore structure may be achieved. The objective of this research is acquiring further understanding of the fundamental physics that govern the formation of these materials using molecular simulations. We are aiming to unravel the assembly process of silica porous materials using a semi-rigid silica tetrahedral model. This model together with reaction ensemble Monte Carlo simulations allows us to study the formation of silica nanoparticles, the initial stages of microporous material formation. A two-step formation mechanism was found to be crucial for generating the nanoparticles. A replica-exchange reaction ensemble Monte Carlo sampling together with the silica tetrahedral model is developed and applied to cross the energy barrier between amorphous silica to crystalline silica materials for searching for the ground state structure of this model. The technique involves simulating several system copies with different equilibrium constants controlling silica condensation/hydrolysis reactions, which are essential for building higher-order network structures and eventually crystals, was preformed. Different silica polymorphs including all-silica zeolite frameworks were obtained. This model shows a great potential to study the crystallization of microporous materials. We also study the formation of mesoporous materials using molecular dynamics simulations. We investigate the interplay of silica molecules and surfactants, and different mesophases such as micellar rods, hexagonal, bicontinuous and lamellar phases were obtained. Multiple charges on silicate oligomers were found to play an important role in the formation of hexagonal phases. To study the later stages of MCM-41 formation, a hybrid molecular dynamics and Monte Carlo approach is proposed. The cooperation between the physical interaction and chemical reaction can be taken into account simultaneously. Preliminary study shows that the ratio of silicate to surfactant higher than 4 is essential to the growth of MCM-41. With a further enhancement on the model, this hybrid approach will be a powerful tool to simulate the formation of MCM-41 in a large system and at a long time scale.