Solid support catalysts resin properties

Another method for generating an epoxidation catalyst on a solid support is to simply absorb or non-covalendy attach the catalyst to the solid support <06MI493>. Epoxidation of olefin 6 with mCPBA and catalyst 8 provides 7 in quantitative yields and with 89% ee. The immobilization of 8 on silica gel improves the enantioselectivity of the reaction providing 7 with 95% ee. Recycling experiments with silica-8 show a decrease in both yield and the enantiomeric excess for each cycle (45% ee after 4 cycles). This is attributed to a leaching of the catalyst from the silica gel. Two other solid supports, a Mg-Al-Cl-LDH resin (LDH) and a quaternary ammonium resin (Q-resin) were also examined. It was expected that ionic attraction between 8 and the LDH or Q-resin would allow the catalyst to remain immobilized through multiple cycles better than with silica gel. Both of these resins showed improved catalytic properties upon reuse of the catalyst (92-95% ee after 4 cycles). 

Earlier reference has been made to sulphonic acid resins and supported mineral acids as heterogeneous catalysts. The chlorination of alumina provides a strongly acidic surface, while the performance of a number of solid superacid catalysts (— Ho values up to 16) has recently been reported. However a number of mixed oxides of metals and metalloids also show useful acidic properties. 

Further simplification was attained by immobilization of the catalyst on a solid polymer (49), so that its separation from the product was reduced to mere mechanical filtration the resin bound catalyst could then be used for another run. Ofthe number of polymeric supports investigated, Merrifield (49) and Wang resins were identified as most suitable, exhibiting the same behavior and efficiency [18]. However, owing to the heterogeneous nature of the system, the enantioselectivity of the reaction decreased by 10 15% ee (Table 4.11, entries 6 12). In view ofthe swelling properties, toluene was found to be less suitable than chloroform (compare entries 6 and 7), which makes the method less environment friendly. Furthermore, a conditioning effect was observed for these systems the second run was always found to be more enantioselective than the first one by 10% ee and this level was maintained in the subsequent runs (compare entries 7 with 8 12). The latter effect stems from the... 

Esterification of organic compounds often involves multiphase catalytic reactions in which contact of liquid (organic substrate) and solid (catalyst) phases are involved. The most common esterification processes fall into the category of two phase (liquid-solid) reactions. Both slurry and fixed bed reactors can be used for ion exchange resin catalyzed esterification reactions. The overall performance of these reactors depends on the inter phase mass transfer, intrinsic kinetics of reaction, physicochemical properties and mixing of the fluid phases. For a continuous process, fixed bed reactors should be preferred, however, in fixed bed reactors small catalyst particles cause higher pressure drop. Special type of support trays may also be required to support small catalyst particles in fixed bed reactors. 

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