Xerogels shaping

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

In addition to structural information, Li MAS NMR Tz relaxation measurements and analysis of Li line shapes have been used to probe the dynamics of the lithium ions. Holland et al. identified two different species with different mobilities (interfacial Li (longer Tz, rapid dynamics) and intercalated lithium (shorter Tz, slower dynamics)) in the elec-trochemically lithiated V2O5 xerogel matrix. Li hopping frequencies were extracted from an analysis of the Li line widths and the appearance of a quadru-polar splitting as the temperature decreased in a related system. ... 

Some of the earliest sol-gel hybrids were achieved by monomer infiltration into previously formed silica gel. Starting with a dried silica gel (xerogel), the porous shape is filled with monomer. The monomer is polymerized in situ. This is shown schematically in Figure 5. Interpenetration is achieved when the impregnating monomer polymerizes in the open pores of the rigid silica matrix. 

Depending on the procedure, materials with different shapes can be obtained but xerogels are generally obtained as powders since, either at the syneresis or at the drying step, fragmentation of the gel is not prevented. In the following part we describe attempts to control the shape of the material at the macroscopic level (formation of films or monoliths) or at the mesoscopic level (formation of mesoporous hybrid xerogels). 

Figure 2.47. Comparison of the three-dimensional shape of an aerogel and xerogel formed from a gel. Reproduced with permission from Chem. Rev. 2004, 104, 3893. Copyright 2004 American Chemical Society.

Porous inorganic oxides are made through a sol-gel process. The sol is converted into a hydrogel that is subjected to dehydration to form a porous xerogel. Special techniques have been developed to combine the sol-gel transition with a shaping to spherical particles (Fig. 3.11). 

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