Mesoporous organosilica

Mesoporous organosilica (periodic mesoporous organosilicas, PMO) are a type of silica containing organic groups that give rise to mesoporosity. They exhibit pore size ranging from 2 nm - 50 nm, depending on the organic substituents. In contrast, zeolites exhibit pore sizes less than a nanometer. PMOs have potential applications as catalysts, adsorbents, trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors. Mesoporous organosilica (periodic mesoporous organosilicas, PMO) are a type of silica containing organic groups that give rise to mesoporosity. They exhibit pore size ranging from 2 nm - 50 nm, depending on the organic substituents. In contrast, zeolites exhibit pore sizes less than a nanometer. PMOs have potential applications as catalysts, adsorbents, trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors. The breakthrough report int this area described the use of surfactants to produce periodic mesoporous silicas (PMS) were developed in 1992 with pores larger that of zeolites. Early mesoporous organosilicas developed had organic groups attached terminally to the silica surface. They were prepared either by grafting of organic group onto the channel walls or by template-directed co-condensation. For example, by modifying the channels of PMSs with alkanethiol groups that could sequester heavy metals. However, there were some major limitations like, inhomogeneity of the pores compared to PMSs, and limited organic content (around 25% with respect to the silicon wall sites). In 1999, reports described mesoporous organosilicas with organic groups located within the pore channel walls as 'bridges' between Si centers. Since these materials had both organic and inorganic groups as integral part of the porous framework, they were considered as composites of organic and inorganic material and designated as periodic mesoporous organosilicas (PMOs). This family of porous materials had high degree of order and uniformity of pores compared to those with terminal organic groups. The framework of PMOs consists of inorganic components (polysilsesquioxanes) uniformly bridged by organic linkers. Most of the bridged polysilsesquioxane can be generically represented by the formula O1.5Si-R-SiO1.5. where R represents the organic bridging group. Each individual organic group is covalently bonded to two or more silicon atoms in the framework.The pores in the material are periodically ordered with diameter in the range 2 -30 nm. Depending on the synthetic conditions used to make mesoporous organosilicas, the mesoscale structure can either be amorphous or crystalline. Most of the mesoporous organosilicas that have been synthesized are amorphous. Although, x-ray diffraction of these materials indicate periodicity in the structure, sharp peaks in the medium scattering angle representative of crystalline materials are usually absent, except for (00l) reflections. However, few crystalline mesoporous organosilica have been reported,. The primary methods used to make mesoporous organosilicas are evaporation-induced self-assembly, surfactant-mediated synthesis, post-synthetic grafting, and co-condensation. Organosilicas with amorphous structures are typically made by functionalizing organic groups rather than directly integrating the functional groups in the framework, which produces a periodic structure. Furthermore, basic hydrolytic conditions typically produce a periodic structure because of hydrophobic and hydrophilic interactions between hydrolyzed precursors that then self-assemble. Evaporation-induced self-assembly usually causes random alignment of the material pores. This method of synthesis uses the difference in vapor pressure of solvents to vary the rate of evaporation and therefore the assembly of the organosilica framework. Surfactant-mediated synthesis has been widely used for the production of mesoporous materials in general, and PMOs specifically,. It involves the addition of a surfactant or copolymer to a specific molecular precursor. The surfactant directs the structure of the material by interacting with the precursor in such a way that is dependent on the properties of the precursor. After the bulk structure is assembled, the surfactant is removed, leaving pores, or channels, embedded in the material framework. The surfactant template can be removed by solvent extraction or ion-exchange mechanisms. An aging process is usually performed at high temperature before removal of the surfactant. During surfactant-mediated synthesis, hydrolysis and polycondensation, or co-condensation, are used to fuse precursor molecules in a framework. Acidic or basic conditions are used for the hydrolysis depending on the precursor being introduced.