Atoms or Molecular Substances Entrapped by Cocondensation at the Colloidization Step

1 Atoms or Molecular Substances Entrapped by Cocondensation at the Colloidization Step 

This problem can be solved by addition of the less-reactive precursor to the solution where the second, more-reactive precursor is partially prehydrolyzed, or equahzation of the reactivity by selection of precursors with different molecular structure, or by using a complexing agent to control the hydrolysis rate. For preparation of aluminum silicates with acidic functionality created by entrapment of tetracoordinated aluminum ions in the silica matrix, the prehydrolysis of the less-reactive silicon alkoxide before addition of aluminum alkoxide suppresses the early formation of Al—OH—A1 bridges and subsequent 

A modified procedure was successfully used for entrapment of Ti [74] and Zr [75] molecular species into a silica matrix. It relies on three-step colloidization, first producing partially hydrolyzed siKcon alkoxide species  

If efficient active sites shall be created by stabilizing the molecular species at the surface of a well-defined phase, as in the case of V0x/Ti02 - anatase catalysts, the better activity in NO SCR was achieved by cocondensation of the dopant alkoxide with the surface groups of preformed colloidal particles of the matrix precursor, followed by gelation [76]. A similar approach of modifying the matrix precursor sol with dopant molecules yielded well-dispersed tantalum species at the silica surface, efficient in selective oxidation of pyrimidine thioether [77]. 

The selection of methods used for entrapment of organic catalysts and modifiers in porous sohds by cocondensation depends on the chemical nature and reactivity of the immobilized catalysts. The nonreactive and functionalized organic molecules are inserted by cocondensation of preformed (OR)3Si[FG(N)] or [(OR)3Si]zFG(N) with Si(OR)4 (85, 86]. The coordinated metal complexes can be inserted by two strategies, conducting their reactions with FGs before [87] or after the cocondensation of functionalized metal alkoxides (ORjsSiFG or [(OR)3Si]zFG with Si(OR)4 [81] The first approach (for metal complexes) gives more active and leachproof catalysts, if the accessibility of active species is not strongly reduced due to unfavorable gel structure [81]. 

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