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Mesoporous organosilica
Mesoporous organosilica and periodic mesoporous organosilicas (PMO) are a class of silica based mesoporous materials containing organic groups as integral part of their structures. Mesoporous materials have been defined as porous materials with pore size ranging from 2 nm - 50 nm. Compared to early mesoporous materials, mesoporous organosilicas have much more useful properties. The organic structures in their framework provide adjustable reactive surfaces and are also responsible for uniform pore size, higher mechanical strength, and hydrothermal stability of the material. Due to these properties, they have potential applications as catalysts, adsorbents, trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors.
==1-Background and significance ==
Since zeolites, an older group of porous materials, could only be synthesized with pore sizes less than a nanometer, they were not very useful for surface interactions with larger structures. It was considered a huge breakthrough when the first periodic mesoporous silicas (PMS) were developed in 1992 with pores larger that of zeolites. These materials allowed for better host-guest chemistry with larger structures such as polymers, nanoscale clusters and wires, and enhanced diffusion rates of the structures within the pores.
The early mesoporous materials designed were pure silicates and aluminosilicates. However, their noncrystalline nature, lower thermal and mechanical stability, and broader pore-size distributions compared to zeolites were the major shortcomings. Very quickly there was a surge of interest in making such porous materials with organic groups attached to their surfaces, as this kind of functionalization would confer them useful and tunable properties. The initial 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 bind to toxic heavy metals, useful mesoporous organosilicas were prepared. 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).
Later in 1999, three independent research groups, Ozin ''et al.'', Stein ''et al.'', and Inagaki ''et al.'' reported the synthesis of a new class of mesoporous materials. These were 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.〔
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