Silica-bound catalysts

Silica-bound catalyst Packed in HPLC column ... 

Table 6.12 Reusable, silica bound catalysts for the asymmetric transfer hydrogenation of imines.

Synthetic protocols for the construction of heterocyclic systems employing silica-bound catalysts... 

C.ii.g. Inorganic Support Silica- bound catalyst can easily be prepared either by... 

Bisdiazaphospholanes have been linked to organic resins via the correspond-ingtetraacyl fluoride derivatives (Scheme 2.147) [69]. Compared with the relevant homogeneous catalysts, comparable regio- and enantioselectivities were observed in the AHF of styrene, vinyl acetate, 0-silyl protected allyl ethers, and dihydro-furane. Excellent recyclability with only trace levels of rhodium leaching were reported under batch or flow conditions. In contrast, the silica-bound catalyst I exhibited poorer enantioselectivities. 

Cationic palladium(II) complexes are homogeneous catalysts for both intramolecular and inter-molecular hydroamination reactions.267 Palladium species immobilized on silica can be prepared by the simple addition of alkyl- or hydroxopalladium(II) complexes to partially dehydroxylated silica. The silica-bound species are more stable than their molecular precursors and are efficient catalysts for the cyclization of aminoalkynes.268... 

Corma, A. and Garcia, H. (2006) Silica-bound homogenous catalysts as recoverable and reusable catalysts in organic synthesis. Advanced Synthesis and Catalysis, 348 (12-13), 1391-1412. 

Similarly Silica-Bound Co(salen) 37 (Scheme 10) [69] was also effectively used in the HKR of styrene oxide (Scheme 11) and 4-hydroxy-1-butene oxide (Scheme 12). The immobilized catalysts were adapted to a continuous flow process for the generation of reaction products in high yield and ee, requiring only very simple techniques for product purification (Scheme 13). 

Both alumina and silica gel are more stable physically than the common polystyrene supports. The alumina-bound catalysts are particularly promising because of their higher activity and higher selectivity compared with the silica gel-bound catalysts. Alumina also is stable in alkali. The alumina-bound catalysts 32 and 33 worked well for reaction of 1-bromooctane with concentrated aqueous sodium cyanide 118). 

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

In recent years, the process has been modified to increase the yield of lower olefines, too. Continually improved since then, especially in the mid-1960s with the replacement of the original silica-alumina catalyst by a zeolite. The catalyst is now typically a zeolite Y, bound in a clay matrix. The feed is vaporized and contacted in a pipeline reactor with concurrently flowing microspheroidal catalyst particles. The catalyst is then separated from the hydrocarbon products and is continuously regenerated by burning off the coke in a fluidized bed. The process is licensed by UOP several hundred units are in operation worldwide. See also HS-FCC. 

Richmond, M. K., S. L. Scott, G. P. A. Yap, and H. Alper, Preparation of Dinuclear Phosphine-Bridged Palladium (II) Species and Their Silica-Bound Analogues as Catalysts for the Cyclization of Aminoalkynes , Organometallics, 21 (16), 3395 (2002). 

Corma A, Garcia H. Silica-bound homogeneous catalysts as recoverable and reuseable catalysts in organic synthesis. Adv. Synth. Catal. 2006 348 1391-1412. 

Tantalasilsequioxanes based upon LH3 = cy7Si70i2H3 ((164), Scheme 34) have been synthesized as structural models of silica-bound heterogeneous catalysts.333 Compounds LTa(NMe2)2 and [H2NMe2][TaL2] were synthesized from Ta(NMe2)5 (Scheme 34). 

A similar, physically bound CuBr catalyst on a silica gel support (L-29) was also employed for MMA polymerization.149 The polymers had narrow MWDs (MJMn 1.3), but the molecular weights were higher than the calculated values. The recycled catalysts have a lower activity but lead to better control of molecular weights i.e., the Mn agreed well with the calculated values, and the MWDs were narrower MJ Mn 1.2). The physically supported catalysts were further employed for the synthesis of end-functionalized polymers.150 When physically supported silica gel catalysts are packed into a continuous column reactor, a controlled polymerization is possible.151... 

The vapor phase synthesis of methacrylic acid from propionic acid and formaldehyde was studied [42]. In particular, the choice of alkali metal cation and loading were evaluated for their effect on the activity and selectivity of silica supported catalysts. Experiments were carried out in 0.5 in. (o.d.) quartz reactors equipped with 0.125 in. thermowells. Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid. Silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. The alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectivity to methacrylic acid show the opposite trend, Cs > K. > Na > Li. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized [42]. 

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