Catalyst silica-grafted

Figure 3.12. Silica grafted catalysts for hydroformylation and hydrogenation, the immobilisation is based on hydrogen bonding between the surface and the sulfonate group (supported hydrogen-bonded (SHB)...

Table 42.7 Examples of enantioselective hydrogenations with silica-grafted catalysts.

Figure 8.19 General mechanism for CO cycloaddition with epoxides over silica-grafted catalysts (adapted from Ref [72]).

Figure 3.13. Silica grafted ruthenium catalysts with the counterion also immobilised via hydrogen bonds...

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

Duchateau and coworkers have employed the hydroxysilsesquioxane (c-C5H9)7Si80i2(0H) and triphenylsilanol as model supports for silica-grafted olefin polymerization catalysts [5]. Treatment of [Cp MCfi] (M=Ti, Zr Cp"=C5H3(SiMe3)2)... 

Recently, we extended the approach to dendrimers immobilized on silica. For this purpose, the first generation (and also larger generations) DAB den-drimer was functionalized with urea propyl-(SiOR)3 groups that via an easy process were grafted on silica. The catalyst was noncovalently anchored by simply adding a solution of a metal complex with ancillary ligands utilized with the typical the complementary motif (Fig. 7) [29]. 

Examples of the application of recyclable solid base catalysts are far fewer than for solid acids [103]. This is probably because acid-catalyzed reactions are much more common in the production of commodity chemicals. The various categories of solid bases that have been reported are analogous to the solid acids described in the preceding sections and include anionic clays, basic zeolites and mesoporous silicas grafted with pendant organic bases. 

As an example. Table 6.2 compares the performance of Ti-MCM-41 [9] with a mesoporous silica-grafted Ti and a microporous Ti silicalite (TS-1) for propene epoxidation with EBHP. With TS-1, the pore diameter is 5.5 A, which imposes restrictions and limits the size of the molecules to be oxidized the diffusion of the oxidant is inhibited, and hence the conversion attained is less than with the mesoporous materials. The difference between the mesoporous catalyst and TS-1 is even greater when the epoxidation of propene is carried out with CHP. The affinity for propene is increased by conferring hydrophobic properties to the surface, by means of silylation. 

In this respect, Brunei and co-workers compared grafted AMP-MTS [44] silica-grafted aminopropyl materials (AMPS) [45]. They found that AMP-MTS efficiently catalyzed the reaction of ethyl cyanoacetate and benzaldehyde, that the catalysts could be easily recovered and reused after a simple regeneration step, and that the activity was a simple linear function of catalyst loading within the limits 0.8-1.9 mmol g -the highest loading achievable with these materials. Tertiary amines attached via displacement of a halogen substituent with piperidine were... 

Baiker and co-workers reported that the complex tranj-[Ru(Cl)2(dppe)2] (3) (dppe = l,2-bis(diphenylphosphino)ethane) catalyzes the formation of DMF very efficiently, despite the insolubility of the complex in SCCO2 [31]. They were even able to use a heterogenized silica-grafted ruthenium complex, which was obtained from the five-coordinated precursor 4 and tetraalkoxysi-lanes in a sol-gel process [32]. With these catalysts, the reaction occurs in the liquid dimcarb phase. The fact that high rates can be achieved with such heterogenized metal complexes makes them attractive materials for other processes involving SCFs or compressed gases. 

Fig. 9 Carbon silica composite catalyst design, based on Sn-grafted Si-MCM-41 and intraporous carbon with mild Brpnsted acid sites. Adapted from [66]...

Silica grafted imidazolium-based ionic liquids efficient heterogeneous catalysts for chemical fixation of COj to a cyclic carbonate. Energy Environ. Sci.,... 

Inexpensive sodium percarbonate can be used, both as oxidant and base [135], for the asymmetric epoxidation of a,p-unsaturated ketones using silica-grafted PLL 93 [136]. Tang and co-workers [136] have described the best results with a silica-grafted PLL with = 45. Under these conditions, the epoxidized products were obtained with 70-93% ee and 40-94% yield (Scheme 12.19). The catalyst can be recovered and reused with no erosion in the observed ee s. 

Ribeiro, S.M., Serra, A.C., Gonsalves, A.M.D. 2011. Silica grafted polyethylenimine as heterogeneous catalyst for condensation reactions. Applied Catalysis A General 399(1-2) 126-133. 

Gelbard, G., Breton, R, Quenard, M., et al. (2000). Epoxidation of Cyclohexene with Polymethacrylate-Based Peroxotungstic Catalysts, J. Mol. Catal. A Chem., 153, pp. 7-18 Gelbard, G., Gauducheau, T., Vidal, E., et al. (2002). Epoxidation with Peroxotungstic Acid Immobilised onto Silica-Grafted Phosphoramides, J. Mol. Catal. A Chem., 182-183, pp. 257-266. 

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