Methods for Preparing Nonsilica Xerogel Monoliths

The methods for preparing silica and nonsilica monoliths are principally the same [8]. However, development of the latter was limited for many years, due to high hydrolysis rates of the precursors. Subsequent developments of phase separation and replica techniques, in addition to using hydrolysis-controlling additives, made it possible to overcome this. The number of publications within this field has flourished during the last few years and some examples are presented. 

Aluminum alkoxides are very reactive and the addition of chelating agents to control both the hydrolysis and condensation rate is often required [41-43]. 

Gawel et al. present a scheme for sol-gel alumina monolith preparation starting with aluminiun alkoxides, aluminum salts, or aliuninum oxide hydroxide [8]. In the case of aliuninimi salts, they hydrolyze and condensate it into aliuninimi oxide hydroxide gels imder basic conditions [44,45]. However, since aqueous aluminum salt solutions are acidic, this requires a gradual process of increasing pH. 

Recently, Guo et al. presented a procedure for fabrication of Al203-Si02 mineral binary (mullite) and ternary (cordierite) systems using a sol-gel process accompanied by phase separation [46,47]. 

Zirconia monoliths can be prepared from alkoxy precursors however, obtaining mechanically stable zirconia monolithic materials is a challenging task [8]. Zirconium alkoxides have faster hydrolysis rates compared with aluminiun or titanium alkoxides. This is due to larger positive partial charge of the zirconium atom, which enhances nucleophilic attacks on the zirconium atoms [52]. 

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