Amine-grafted silicas

Amine grafted silicas are promising sorbents for selective adsorption, especially for applications in enviromnental control and catalysis. For example, Lasperas et al. (1997) and Angeletti et al. (1988) used amine function-modified... 

Figure 6.11. Adsorption of CO2 on amine grafted silica with and without water vapor, showing that the amount adsorbed is doubled in the presence of water (Huang et al., 2003).

Preparation of Grafted Amine-Functionalized Silica. Silica was pressed, cmshed, and sieved to 40-60 mesh particles and calcined at 550°C overnight. To anhydrous toluene (40 mL), silica (1.0 g) was added under N2 and stirred for an hour. Aminopropyltriethoxysilane (APS 1.0 g, 5.58 mmol) was syringed into mixture and stirred for 24 hours at room temperature under nitrogen. The functionalized silica was then filtered, washed with toluene three times, and dried under vacuum at 50°C. 

Table 38.1. Ninhydrin assays for surface amines using grafted silicas. Determined from grafting synthesis and elemental analysis. "Typical standard deviations were...

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Figure 1.29 TEM images of Janus silica nanoparticles (diameter 100 nm) in which the amine grafted area is decorated by gold nanocolloids, the remaining area being functionalized by methyl groups. (Reproduced from ref. 54, with permission.)...

Park s group has explored hyperbranched graft chemistry of aziridine on amine functionalized silica surfaces (Eqs. 18 and 19) [74]. Using silicon and silica substrates that have been functionalized with (3-aminopropyl)dieth-oxymethylsilane as initiator substrates, hyperbranched polyethyleneimine grafts were synthesized using aziridine monomers. 

Hicks JC, Dabestani R, Buchanan AC III, Jones CW (2006) Spacing and site isolation of amine groups in 3-aminopropyl-grafted silica materials the role of protecting groups. Chem Mater 18 5022... 

The Michael 1,4-addition of methylene activated compounds to various a,p-unsaturated carbonyl compounds in water was promoted by amines grafted on silica gel by a post-modification method. Since the reaction was carried out in a triphasic system, due to low solubility of organic substrates, internal heating by microwave irradiation (100 W) was exploited. In the model reaction between ethyl 2-oxocyclopentanecarboxylate (52) and 3-buten-2-one (53) (Scheme 3.16), the temperature was ramped for 2 min from room temperature to 80 °C. 

The synthesis of 281 (X = CN) from the same reactants was achieved by ultrasonic irradiation without catalyst (03MI488, 04MI562), and under MW irradiation in the presence of sodium bromide or piperidine under solvent-free conditions (04TL8625, 01MI742) as well as heterogeneous amines grafted on silica such as N,N-diethylamino-propylated silica (06H889). 

Figure 1.3 Surface structures of amine and/or carboxylic acid grafted silica catalysts. The organosilanes used for functionalization were aminopropyltrimethoxysilane (AP) and t-butyl 2-methyl-3-(triethoxysilyl)propanoate...

Anchored amine materials can be prepared through a number of synthetic methodologies. Because of the potential importance of these materials to organic synthesis, a ninhydrin assay was developed as a rapid laboratory determination of available surface amines. The assay agreed well with expected values for aminopropyltriethoxysilane grafted onto commercial silica. The assay also distinguished between reactive amines and protonated or poisoned surface amines on co-condensed SBA-15 materials. 

Figure 2. Scheme illustrating (A) amine-silanol and (B) amine-amine interactions and (C) single site or site-isolated 3-aminopropyl groups grafted onto mesoporous silica [20]. 

The functionalization of silica with PPM and the subsequent immobilization of [Rh(COD)2]BF4 has been achieved by Pugin and Blaser [49]. The PPM ligand was immobilized by grafting the linker, 3-trimethoxysilylpropyl isocyanate, to the silica surface (the silicas used include Grace 332, Merck 100, Merck 60, Merck 40). The isocyanate moiety then reacts with the amine functionality of the PPM ligand to afford an amide linkage (35, Figure 5.17). Introduction of a solution of [Rh(COD)2]... 

Tsubokawa et al. (44) have reported the grafting of polymeric cationic species of vinyl butyl ether or 2-methyl-2-oxazoline onto aminated silica particles via the termination. However, it is ordinarily difficult to apply the surface termination reac-... 

Base catalysis is one of the less-well developed areas of heterogeneous catalysis. We have developed novel bases derived from amines via the one-step process outlined above. A range of supported amines have been prepared and evaluated in a series of reactions We have also investigated the nature of the amine groups attached to the surface in comparison with those formed by grafting onto pre-formed silica. While many workers have studied the use of basic catalysts for the Knoevenagel condensation of aldehydes, with three articles on the use of MCM derivatives[ 12], little has been done on the more demanding condensation of ketones. 

---