Site isolated catalyst

Site Isolation of Polymeric Catalysts. A common method to site isolate catalysts from the products of a reaction is to attach the catalyst to a polymer that can be precipitated at the end of a reaction. " Catalysts attached to a polymer are generally not site isolated from other catalysts attached to a different polymer such that only one catalyst can be added to a reaction mixture. The use of PDMS thimbles solves this problem. ... 

Final Notes. Small molecules, organometallic catalysts, inorganic catalysts, and polymeric catalysts were all successfully site isolated at levels up to >99.998% using PDMS thimbles. When site isolating catalysts or reagents, the most important parameters to consider are the solubilities and rates of diffusion within the swollen PDMS matrix and the solubilities in the organic solvents on the interior and exterior of thimbles. 

Only the surface layers of the catalyst soHd ate generaHy thought to participate in the reaction (125,133). This implies that while the bulk of the catalyst may have an oxidation state of 4+ under reactor conditions, the oxidation state of the surface vanadium may be very different. It has been postulated that both V" " and V " oxidation states exist on the surface of the catalyst, the latter arising from oxygen chemisorption (133). Phosphoms enrichment is also observed at the surface of the catalyst (125,126). The exact role of this excess surface phosphoms is not weH understood, but it may play a role in active site isolation and consequently, the oxidation state of the surface vanadium. 

The viabiUty of using site-isolated Ta(V) centers for cyclohexene epoxi-dation was explored by grafting ( PrO)2Ta[OSi(O Bu)3]3 onto a mesoporous silica material [83]. After calcinations, the material formed is less active and selective in the oxidation of cyclohexene than the surface-supported Ti(IV) catalysts using organic peroxides however, the site-isolated Ta(V) catalysts are more active under aqueous conditions. 

The polymers were converted to supported catalysts corresponding to homogeneous complexes of cobalt, rhodium and titanium. The cobalt catalyst exhibited no reactivity in a Fischer-Tropsch reaction, but was effective in promoting hydroformylation, as was a rhodium analog. A polymer bound titanocene catalyst maintained as much as a 40-fold activity over homogeneous titanocene in hydrogenations. The enhanced activity indicated better site isolation even without crosslinking. 

Isolated Organometallic Catalysts on Silica Supports - Towards Single-Site Solid Catalysts... 

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

A chiral diphosphine ligand was bound to silica via carbamate links and was used for enantioselective hydrogenation.178 The activity of the neutral catalyst decreased when the loading was increased. It clearly indicates the formation of catalytically inactive chlorine-bridged dimers. At the same time, the cationic diphosphine-Rh catalysts had no tendency to interact with each other (site isolation).179 New cross-linked chiral transition-metal-complexing polymers were used for the chemo- and enantioselective epoxidation of olefins.180... 

When only the enzyme was entrapped in the sol-gel, the rhodium complexes poisoned the enzyme, resulting in low yields, whereas when both enzyme and complex were trapped in separate sol-gel mixtures, good yields were achieved. In one particular case, this procedure resulted in an increase in yield of 1-pentyl undecano-ate from 1 -undecenoic acid and 1 -pentanol by a factor of 6.5, indicating successful site isolation of the mutually interfering catalysts. 

Other Cascade Reactions with Incompatible Catalysts - Polydimethylsiloxane (PDMS) Thimbles for Generic Site Isolation... 

The same approach has also been used in a reaction cascade involving 4-dimethy-laminopyridine (DMAP) and an acid catalyst [35], These two catalysts are mutually incompatible as the add quenches the DMAP, but site isolation using a PDMS thimble enables the cascade to proceed successfully (Figure 5.3). 

Layered inorganic solids have been used for site isolation, for example, nickel phosphine complexes confined within the interlayer spaces of sepiolite have been used as olefin hydrogenation catalysts [63], and similarly there has been the encapsulation of metal complexes into zirconium phosphates [64], The principal idea is illustrated in Figure 5.8. The metal complex can be encapsulated by covalent means (a) or by non-covalent interactions (b). 

Formation of inactive dimeric species. Site isolation is the solution to this problem, which is not possible using homogeneous catalyst. 

The authors postulated that on these materials complete site isolation is not mandatory in order to have active and selective titania-silica epoxidation catalysts . The 100% selectivity of the dinuclear, silica-supported... 

These reactions require a lot more research to reach fruition, most specifically in the field of long-term catalyst stability and selectivity. Furthermore, in many instances the reaction mechanism and the active catalytic site are still poorly understood. Issues such as the importance of site isolation and phase cooperation... 

NANOPOROUS AND NANOSTRUCTURED CATALYSTS Self-Assembly AND Co-Assembly of Multiple Site-Isolated Catalytic Sites on Mesoporous Materials... 

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