Xerogels reactivity

Indeed, the mobility of the entrapped dopant is crucial in promoting the reactivity of the final materials. Thus, provided that the dopant molecules are at the surface and enjoy enough freedom, high porosity will certainly promote reactivity by limiting intraparticle diffusion but that will not be the case if microporous xerogels of different HLB are compared (c/. entrapped lipase and tetra-//-propy 1 am monium perruthe-nate (TPAP) where ORMOSIL with the smaller pores are more reactive). 

This fact is corroborated by the chemical reactivity of these materials R groups located at the surface are chemically accessible and highly reactive. For example, complexation of phenyl groups of a CgHsSiOj 5 xerogel by Cr(CO)3 in phenyl/silsesquioxanes is easy80 and HSiOi 5 can be transformed to siliconoxynitride by reaction in a flow of NH381. 

The nature and structure of the surface of the xerogel are important characteristics with consequences which determine its chemical reactivity and stability. 

The complexation reaction is another useful reaction for such investigation. Aromatic organosilanes are known to easily form tricarbonyl(j)6-organosilylarene)chromium(0) complexes upon reaction with Cr(CO)6 or (MeCN)3Cr(CO)3180. The complexation of the aryl groups of the hybrid xerogel of precursors 40 and 73 has been used for comparing their reactivity, as depicted in Scheme 14. Under similar experimental sol-gel conditions,... 

Vanadium pentoxide xerogels are very reactive layered host materials which can be intercalated by various means such as cation-exchange, acid-base chemistry, or redox reactions. Vanadium pentoxide xerogel was prepared by polymerization of HVO3 after a few days of reaction at room temperature. The resultant xerogel... 

Hydrolysis of a mixture of Si(OR>4 and Ti(OR)4 results in the immediate precipitation of titania, because Ti(OR)4 is much more reactive than Si(OR)4. When (R0)3Si(CH2)3C[C(Me)0]2Ti(0R)3 or (R0)3Si(CH2)3C[C(Me)0]2 2Ti(0R)2 is hydrolyzed under basic conditions, transparent and crack-free monolithic xerogels are formed instead. Mixed-oxide powdCTS with the nominal composition Ti02 SiO2 or Ti02-2Si02 and a low crystallization tendency are obtained when the organic groups were removed by calcination at 550 °C. 

This paper describes several strategies we have explored to connol the pore volume, surface area and pore size distributions of dried gels (xerogels) and the numbo and the nature of the oxidative sites and surface reactivity. Some samples were tested in ammoximation reaction with the aim to find active silicas and to correlate the catalytic performances with the preparation parameters. 

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