Porous silica monoliths

The induction of phase separation in the course of sol-gel transition is a simple and ultimate principle of the macropore structure control. The formation of heterogeneous structures (separated phase domains) should take place in advance but not too earlier than the transition of the whole reaction system from fluid to solid. Sol-gel transition is a dynamic freezing process by cross-linking reactions. 

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Bronstein. L.M. Polarz, S. Smarsly, B. Antonietti, M. Sub-nanometer noble-metal particle host synthesis in porous silica monoliths. Adv. Mater. 2001, 13. 1333. 

Kobayashi, H. Tanaka, N. Ishizuka, N. Minakuchi, H. Nakanishi, K. The characterization of porous silica monolithic columns in HPLC and CEC. Chromatography 2000, 21, 404-405. 

In recent years, the interest in using porous silica and polymer-based monolithic stationary-phase media for ion chromatographic separations of inorganic and organic ions has increased.As compared to particle bed columns, monolithic columns represent a single piece of porous cross-linked polymer or porous silica. Monoliths are made in different formats as porous rods, generated in thin capillaries or made as thin membrane or disks. 

Le Bideau, J. Miah, M. Y Vioux, A. Fajula, F. Galarneau, A., Bimodal Porous Silica Monoliths Obtained by Phase Separation in Non-Aqueous Media. 

The growth of MOF nanoparticles inside porous silica monolith is a promising approach for catalysis applications. Sachse et al. demonstrated the successful formation of HKUST-1 crystals inside silica monolith [79]. Macro/mesoporous silica monolith was synthesized using TEOS as a Si02 precursor and ammonia as a catalyst. For the subsequent MOF formation, the as formed monolith was immersed in HKUST-1 precursor solution using DMSO as a solvent. The employment of DMSO instead of classical solvents (ethanol and water mixture) for the synthesis of HKUST-1 crystals led to slower nucleation of smaller crystals in the mesopores of monolith rather than at the outer surface. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

While only a few reports concern the in situ preparation of monolithic CEC columns from silica, much more has been done with porous polymer monoliths and a wide variety of approaches differing in both the chemistry of the monomers and the preparation technique is currently available. Obviously, free radical polymerization is easier to handle than the sol-gel transition accompanied by a large decrease in volume. 

Another group working on silica monoliths is the one of A. and M. Kuehn [41]. Their Continuous-Bed-Silica (CB-Silica) is a highly porous monolith having meso- and micropores. The structure of the CB-Silica is very porous and con-... 

The basic studies dealing with the preparation of continuous porous silica materials date back to 1991 [76-78]. Two years later, Nakanishi and Saga applied for a patent describing the fabrication of monolithic silica rods for chromatographic application [79-81], whereas a second protocol for the preparation of continuous silica rods was independently filed by Merck KGaA in Germany [82]. 

First comprehensive investigations with respect to the properties of continuous porous silica rods were, however, carried out by Tanaka and Fields in 1996 [33,34,83], who reported on two different methods for the preparation of silica monoliths. 

Monolith Column—Porous silica column prepared in situ to completely fill the column tube with a fully porous silica foam skelton. After the organic polymer support is heated off, the silica surface is silylated in place to product bonded-phase surface. Column is high resolution and can be used at high flow rates with relatively low back-pressure (see Chapter 16). 

Other than beads, porous polymer monoliths, which were photopolymerized in a COC chip, were used for solid-phase extraction. It is known that priming polymeric surfaces is not as simple as priming silica surfaces, which use a common surface primer agent, TMPM. Therefore, the grafting method as initiated by UV should be used to attach the polymer monoliths [588]. A similar strategy was used for sample pre-concentration of PAHs (e.g., pyrene). Pyrene (900 nM) was first concentrated by 400-fold in 24% ACN before switching to 56% ACN for CEC separation (see Figure 5.5) [148]. 

Fields reported that continuous silica xerogels prepared from potassium silicate solutions could be used as highly permeable support media, and exhibit reasonable chromatographic efficiency in HPLC [23]. Minakuchi et al. reported the preparation and evaluation of continuous porous-silica columns that provide a much higher column efficiency in HPLC than do conventional columns packed with particles [13-16,18], The monolithic columns prepared in a capillary can also be used in CEC. 

Porous polymer monolith in fused-silica capillary [340]... 

Another approach is the use of monolithic columns consisting of silica based rods of bimodal pore structure. They contain macropores (-1-2 pm) and smaller mesopores ( 10-20nm) [38]. The macropores allow for low backpressure at high flow rates. The mesopores provide the needed surface area for interactions between the solute and stationary phase. The macropores result in higher total porosity as compared to porous silica particles. Flow rates of 5 mL/min can be tolerated on a 10-cm column without an appreciable loss in... 

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