Aerogel stabilization

Stabilization of activated oxidoreductases on time scales of months to years has historically been challenging, and the lack of success in this regard has limited the industrial implementation of redox enzymes to applications that do not require long lifetimes. However, as mentioned in the Introduction, some possibility of improved stability has arisen from immobilization of enzymes in hydrophilic cages formed by silica sol—gels and aerogels, primarily for sensor applications.The tradeoff of this approach is expected to be a lowering of current density because... 

Carbon dioxide is the drying agent of choice if the goal is to stabilize kinetically constrained structure, and materials prepared by this low-temperature route are referred to by some people as carbogeh In general, carbogels are also different from aerogels in surface functionality, in particular hydrophilicily. 

The synthesis of paratolunitrile (PTN) and terephtalonitrile (TPN) by reaction of paraxylene with nitrogen monoxide was studied over a series of aerogel chromium oxide alumina catalysts. The stabilization of the active phase was interpreted on the basis of Cr O support interactions. Kinetic studies show that the reaction follows a "redox" mechanism for the formation of PTN and a Langmuir Hinshelwood mechanism for the production of TPN. 

Fig. 3. Comparison of thermal stability for xerogels and aerogels prepared with a molar ratio Zr/Si = 0.05.

Different chitosan-based catalysts are compared in Table 3. The reactions were performed at 70°C due to the thermal stability of the catalyst. The first ones focus on the influence of the drying procedure in comparison with an uncatalyzed reaction and a known heterogeneous catalyst (Table 3, entries 1-5). Lyophilized chitosan (Cl) does not display any activity when the aerogel is as efficient as the functionalised silica (C3). This result illustrates the accessibility to the amino groups of chitosan in its aerogel form. 

Considering the hydrophilic properties of the support, the effectiveness of polysaccharide aerogel microspheres as catalyst support was evidenced in the so-called Supported Aqueous Phase Catalysis [131]. The stability of the catalyst obtained was investigated in terms of textural stability and catalytic activity in the reaction of substitution of an allyl carbonate with morpholine catalyzed by the hydrosoluble Pd (TPPTS)3 complex [132]. 

Ti02 and Z1O2 aerogels were successfully employed as supports for ruthenium supported catalysts. These catalysts were found to be very active for wet air oxidation of p-HBZ acid. Furthermore, the use of nitric acid as hydrolysis agent and Ru(N0)(N03)3 as metal precursor leads to the more efficient catalyst. When these conditions were used, ZrOa enhances the catalytic properties of the ruthenium phase. This result points out the important role of the support on the properties of the dispersed metal phase. Further investigations to improve BET surface area and thermal stability of the support are in progress. 

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