Porous silica monoliths mesopores

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). 

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... 

The pore connectivity r of two types of silica (highly porous beads, monolithic silicas) was calculated according to the pore network model proposed by Meyers and Liapis. Nt was proportional to the particle porosity in the case of highly porous beads. The differences in the pore connectivity for both types of silica were reflected in the mass transfer kinetics in liquid phase separation processes by measuring the theoretical plate height-linear velocity dependencies. In a future study, monolithic silicas possessing different macro- as well as mesopores will be investigated and compared with the presented results. 

The pore texture of an adsorbent is a measure of how the pore system is built. The pore texture of a monolith is a coherent macropore system with mesopores as primary pores that are highly connected or accessible through the macropores. Inorganic adsorbents often show a corpuscular structure cross-linked polymers show a network structure of inter-linked hydrocarbon chains with distinct domain sizes. Porous silicas made by agglutination or solidification of silica sols in a two-phase system are aggregates of chemically bound colloidal particles (Fig. 3.25). 

Mixed solutions of cationic surfactants and nonionic poly(ethylene glycol) or block copolymers were employed for the synthesis of monolithic trimodal porous silica.[176] Lyotropic mixtures of block copolymers of different lengths with hydrophilic linear PEO chains were also applied to their nanocasting into bimodal micro-mesoporous silica to formulate the dependence of the mesopore sizes and the microporosity on the lengths or sizes of the hydrophobic and the hydrophilic blocks, but the mesostructures were worm-type in morphology and several hundred nanometers or more in size. 

Monolithic columns are another approach to provide lower pressure drops and higher rates of mass transfer. These are continuous solid columns of porous silica stationary phase instead of packed particles. Like perfusion packings, they have a bimodal pore structure (Figure 21.7). Macropores, which act as flowthrough pores, are about 2 fim in diameter. The silica skeleton contains mesopores with diameters of about 13 nm (130 A). It can be surface modified with stationary phases like Cig. The rod is shrink-wrapped in a polyetheretherketone (PEEK) plastic holder to prevent waU effects of solution flowing along the walls. The surface area of the mesopores is about 300 mVg, and the total porosity is 80%, compared with 65% for packed particles. The colunm exhibits a van Deemter curve approximating... 

Hu et al. have synthesised ordered mesoporous and macroporous carbon monoliths by template method using silica monoliths as template. In this preparation, the mesophase pitch was used as the carbon precursor [54]. The silica monolith templates were filled with a 10 wt.% solution of mesophase pitch in tetrahydrofuran solvent and then carbonised at different temperatures. The physical characteristics of these various mesoporous and macro-porous carbon are presented in Table 7.8, which demonstrates these carbons have more prominent graphitic structures as the carbonisation temperature increases. SEM images of the carbon monoliths obtained at carbonisation temperature of 700°C and 2500°C are shown in Figure 7.59. 

Comparison of the porous structure of different columns was discussed in Section 3.2 here we emphasize that with a packed column the ratio of particle size to the average interparticle pores (space) is on the level of 3-3.5 while with monolithic columns trough-pores are on the level of 6000 A and silica material is only about 1 u thick, which makes this ratio 0.5-0.2 or about 10 times smaller, thus significantly decreasing the time needed for analyte molecules to diffuse into the mesoporous space for the interaction with main surface. This allows for much faster flow rates without the loss of the dynamic equilibrium conditions (otherwise known as the slow mass transfer term (C) in the Van Deemter equation). 

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