Tunability parameters, inorganic

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

Many density-dependent properties of H2O, such as viscosity, polarity (dielectric constant s changes from 74 to 2), heat capacity at constant pressure (which is infinite at the critical point), ion product and solvent power can be tuned for specific requirements by setting the correct temperature and pressure, and they show significant changes near the critical point (Figure 25.2). Several studies have demonstrated that the transition from sub- to supercritical conditions also affects the elementary steps in reaction mechanisms, and radical intermediates are favoured over ionic species. Another consequence is that subcritical water shows potential for acid catalysis. Reactions can be run either under non-polar/aprotic or polar/pH controlled conditions (water can take part in these reactions). Consequently, non-polar compounds like aromatics become soluble whereas inorganic salts precipitate. Therefore, the properties of water as a solvent are tunable over much wider parameter ranges than for most other compounds. 

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