Mesoporous grafting

Abstract A review of the thermolytic molecular precursor (TMP) method for the generation of multi-component oxide materials is presented. Various adaptations of the TMP method that allow for the preparation of a wide range of materials are described. Further, the generation of isolated catalytic centers (via grafting techniques) and mesoporous materials (via use of organic templates) is simimarized. The implications for syntheses of new catalysts, catalyst supports, nanoparticles, mesoporous oxides, and other novel materials are discussed. 

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. 

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. 

Fig. 4.26 Photo-controlled drug storage and release from mesoporous silica using photodimerization ofcoumarin grafted only at the pore outlet. Reprinted with permission from [221], M. Fujiwara et at., Chem. Mater. 2003, 75, 3385. 2003, American Chemical Society.

In this work, we have grafted the mesoporous SBA-15 materials with 3-aminopropyl trimethoxysilane [10], In a subsequent step, the aminopropyl moiety was reacted with glutardialdehyde, which reacts also with amino groups of the enzymes chloroperoxidase (CPO) and glucose oxidase (GOx) (Scheme 1). 

The mesoporous molecular sieve SBA-15 has been functionalized with aminopropyl moieties via grafting. Further treatment of the 3-aminopropyl-modified material with glutardialdehyde (GA) results in GA-ATS-SBA-15. The modified silica materials were characterized by NMR and IR spectroscopy as well elemental analysis confirming the successful modification. Furthermore, the elemental analysis suggests that two of three amino moieties of the 3-aminopropyl modified material react further with... 

The mesoporous materials reported above are usually prepared from relatively expensive surfactants. Some of them have poor hydrothermal stability. Furthermore, the MCM-41 host structure has a one-dimensional pore system with consequent pore blockage and diffusion limitations. Shan et al. (52) reported the synthesis of a three-dimensional and randomly connected mesoporous titano-silicate (Ti-TUD-1, mesopore wall thickness = 2.5-4 nm, surface area — 700-1000 m2/g, tunable pore size —4.5-5.7 nm) from triethanolamine (TEA). Ti-TUD-1 showed higher activity (about 5.6 times) for cyclohexene epoxidation than the framework-substituted Ti-MCM-41. Its activity was similar to that of the Ti-grafted MCM-41 (52). 

For such reasons, the following section considers in more detail some of the most significant results obtained by our team on the epoxidation with TBHP of unsaturated FAMEs over mesoporous titanium-grafted silicates. In these examples, the epoxidation tests were carried out either in ethyl acetate, which could be even obtained, in principle, from renewable sources and which is relatively less harmful than other polar non-protic solvents, or under solvent-free conditions. 

It is worthwhile pointing out another issue at this juncture. The low molecular-mass selectors can be grafted onto the surface of mesoporous silica (usually equal to 10 nm pore diameter) in a relatively high molar selector concentration due to their restricted space requirements (yielding typical selector loadings of about 0.4mmolg CSPcorrespondingtoca. 1.3 p,molm ). As a consequence, arelatively... 

Grafting a modified cinchona alkaloid to hexagonal mesoporous molecular sieve SBA-15 afforded catalyst (27) with excellent activity. 1-Phenyl-1-propene was converted to the corresponding diol in 98% yield (98% ee), while trans-stilbene yielded the desired product in 97% yield (99% ee) [92]. Other examples in this field are the utilization of microencapsulated osmium tetroxide by Kobayashi [93] and the application of continuous dihydroxylation mns in chemzyme membrane reactors described by Woltinger [94]. 

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