Silica-Entrapped Reactants

A = Bound Reagent SM = Starting Material AS= Spent Reagent 

Alkene Catalyst Alkene conversion (%) Selectivity towards alkylbenzene (%)  

The formation of linear alkylbenzene (LAB) by Friedel-Crafts alkylation is strongly influenced by homogeneous or heterogeneous reaction conditions (Table 3.5).7 The steric bulk of silica reduces over-alkylation and shelf life. Silica also catalyses the formation of ethers. 

SiliCycle manufactures a number of functionalized silica gels to meet the specific requirements of pharmaceutical professionals, and has 

Cabrera, Applications of silica-based monolithic HPLC columns, J. Sep. Sci., 2004, 27, 843. 

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Catalysis by sol gel doped silica-based materials has become in the last 20 years a prominent tool to synthesize a vast number of useful molecules both in the laboratory and in industrial plants.12 The underlying basic concept of all sol-gel applications is unique one or more host molecules are entrapped by a sol-gel process within the cages of an amorphous metal oxide where they are accessible to diffusible reactants through the inner pore network, which leads to chemical interactions and reactions (Figure 5.3). 

In other cases, organic modification of the sol gel cages markedly protects the entrapped molecular dopant from degradation by external reactants, as shown for instance by the entrapment of the radical 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). This is a highly active catalyst which in the NaOCl oxidation of alcohols to carbonyls in a CH2CI2-H20 biphasic system becomes highly stabilized upon sol gel entrapment in an ORMOSIL matrix it progressively loses it activity when entrapped at the external surface of commercial silica.25... 

Another magnetically recoverable catalytic silica microreactors 156 were prepared and tested in the ATH of ketones in aqueous medium [135]. Magnetite nanoparticles were coated with shells synthesised via co-polymerisation of Si(OEt)4 and the Ru-TsDPEN complex 157 functionalised with a trimethoxysilane group. The transport of the hydrophobic reactants from the water medium to an entrapped catalyst in a sol-gel matrix was ensured by the surfactant CTAC. Under these conditions, ring-substituted acetophenones were almost quantitatively reduced with HC02Na giving the corresponding alcohols in over 90 % ees within 24 h (Fig. 49). 

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