Silica-based classical compounds

The preparation of low-density silica materials by hydrolysis of tetraethylorthosih-cate in the presence of cationic surfactants, namely, hexadecyltrimethylammonium bromide, was patented by Chiola et al. in 1969 [129]. However, the structural properties of these materials were not assessed. Di Renzo et al. [130] reported in 1997 that the patented preparation indeed results in the formation of mesoporous sihcas of the MCM-41 type, which were later rediscovered by researchers from Mobil Research and Development Corporation. 

In 1990, Kuroda and coworkers [131] reported the synthesis of hexagonaUy arranged mesoporous silicas, which were obtained from kanemite in the presence of hexadecyltrimethylammonium bromide. These materials were prepared by exchange of the sodium ions in kanemite, a crystalline layered siUcate, by hexadecyltrimethylammonium cations in aqueous solution and subsequent calcination to remove the organic template. Owing to the proposed synthesis mechanism, the novel materials were named FSM-16 folded sheet materials). The transformation of kanemite in the presence of templates with different alkyl chain lengths into mesoporous materials with different pore size was later reported by Inagaki et al. [132]. 

KIT-5 is a porous silica material with properties similar to those of SBA-16 the mesopores are ordered in a cubic face-centered Fm-3m symmetry. Similarly to SBA-16, KIT-5 is synthesized in a ternary water, n-butyl alcohol, and Pluronic F127 surfactant system with emphasis on a low HCl concentration in aqueous solution [147]. FDU-12 also represents a cubic (Fm-3m) mesostructure with a large 

KIT-6 [150] is analogous to MCM-48 and possesses a three-dimensional Ia-3d structure. The gyroid structure, which can be interpreted as two interwoven cyhndrical channel systems, results in adsorption properties very similar to those observed for two-dimensional hexagonal systems. In contrast to MCM-48, these two intertwined systems of relatively large channels in KIT-6 can also be connected through irregular micropores present in the mesopore walls analogous to those present in SBA-15. 

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Although inorganic, monolithic columns attracted considerable attention in the last 10 years, the preparation of silica-based monoliths does not yet offer the broad chemical variety of precursors and porogens for specific adjustment of separation compared with their organic counterpart. The preparation of silica monoliths uses the classical sol-gel process of hydrolysis and polycondensation of organosilicium compounds. 

The TLC separation of proper diastereoisomers using classical systems is well presented in the papers of Palamareva and coworkers [19-29], who showed that the separation is based on the adsorption mechanism. They demonstrated that TLC on silica gel can be considered as a method for the assignment of the relative configurations to some aliphatic diastereomeric compounds. 

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