Functionalized mesoporous

The mesoporous ordered silicas of different type represent the new generation of materials with unique properties. The discovery of these materials became basis for creation of new catalysts, adsorbents, sensors and supporter for other molecules. The most important way of the modifying physical and chemical properties of mesopurous silicas consist in organic components incorporation on the silica surface as part of the silicate walls or their insertion within channels of the mesopores. This ensured that interest in synthesis and study of functionalized mesoporous materials shai ply grew. In spite of it, these materials are studied insufficiently. 

We showed that these mesoporous silica materials, with variable pore sizes and susceptible surface areas for functionalization, can be utilized as good separation devices and immobilization for biomolecules, where the ones are sequestered and released depending on their size and charge, within the channels. Mesoporous silica with large-pore-size stmctures, are best suited for this purpose, since more molecules can be immobilized and the large porosity of the materials provide better access for the substrates to the immobilized molecules. The mechanism of bimolecular adsorption in the mesopore channels was suggested to be ionic interaction. On the first stage on the way of creation of chemical sensors on the basis of functionalized mesoporous silica materials for selective determination of herbicide in an environment was conducted research of sorption activity number of such materials in relation to 2,4-D. 

Apart from a few reports" on solid acid catalyzed esterification of model compounds, to our knowledge utilization of solid catalysts for biodiesel production from low quality real feedstocks have been explored only recently. 12-Tungstophosphoric acid (TPA) impregnated on hydrous zirconia was evaluated as a solid acid catalyst for biodiesel production from canola oil containing up to 20 wt % free fatty acids and was found to give ester yield of 90% at 200°C. Propylsulfonic acid-functionalized mesoporous silica catalyst for esterification of FFA in flotation beef tallow showed a superior initial catalytic activity (90% yield) relative to a... 

McEleney, K., Allen, D.P., Holliday, A.E. and Crudden, C.M. (2006) Functionalized mesoporous silicates for the removal of ruthenium from reaction mixtures. Organic Letters, 8 (13), 2663-2666. 

Fattakhova-Rohlfing, D. Wark, M. Rathousky, J. 2007. Electrode layers for electrochemical applications based on functionalized mesoporous silica films. Sens. Actual B-Chem. 126 78-81. 

Fattakhova-Rohlfing, D. Rathousky, J. Rohlfing, Y. Bartels, O. Wark, M. 2005. Functionalized mesoporous silica films as a matrix for anchoring electrochemically active guests. Langmuir 21 11320-11329. 

Similarly, Hg(n) binding to thiol-functionalized mesoporous silica for which effective access to all the binding sites (100% of SH groups com-plexed with Hg(n) was achieved in micelle-templated mesostructures with pore diameters larger than 2.0 nm, whereas incomplete filling was always observed with corresponding amorphous silica-based adsorbents.37... 

R. I. Nooney, M. Kalyanaraman, G. Kennedy and E. J. Maginn, Heavy metal remediation using functionalized mesoporous silicas with controlled macrostructure, Langmuir, 2001, 17, 528. 

Methylrheniumtrioxide (MTO) can straightforwardly be immobilized using copolymers consisting of polystyrene and polyvinyl pyridine. The authors proposed two possible structures 18 shown in Scheme 4.10. In the presence of H2O2 these catalysts are efficient and selective heterogeneous catalysts for the epoxidation of alkenes. It was shown that their activity is maintained for at least five recycling experiments [23 dj. In analogy to this work, bipyridyl-functionalized mesoporous silica can be employed for the immobilization of MTO [75]. 

Organic Functionalized Mesoporous Materials and Site-Isolation of Functional Groups... 

Figure 6. Formation of a single or double amide bond by reaction of site-isolated or otherwise aminopropyl functionalized mesoporous silica with isopthaloyl chloride [20].

Figure 8. Colorimetric determination of site isolation in 3-aminopropyl functionalized mesoporous silica by complexation to copper ions [22].

Table 3. Catalysis results for the Barbier reaction of benzaldehyde and allyl bromide in presence of homogeneous palladium catalyst (entry 1) and those prepared by complexing palladium to amine functionalized mesoporous silica (entry 2-4) [74]...

Propylsulfonic acid-functionalized mesoporous silica showed a good perfomiance in the esterification of FFAs in flotation beef tallow, lowering the content of FFAs from 1 to 0.3% at 120 °C, but ahigh methanol-to-substrate ratio (20 1 by weight) and FFA-to-catalyst ratio (9 1 by weight) and after prepurification of the feed [24]. 

Functionalized mesoporous materials with organosulfonic groups have also been used in the esterification of fatty acids with alcohols." " The increased hydrophobic character of the catalyst surface resulted in high catalytic activity... 

Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (la3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. 

Figure 1.2,Si NMR spectra of functionalized mesoporous silica, (a) Large pore mesoporous silica (50 A) with low degree of hydration and low coverage (25%). (b) Large pore mesoporous (50 A) with a higher degree of hydration and a higher coverage, (c) Small pore mesoporous silica (20 A).

Figure 2. (a) 2,Si NMR spectrum of functionalized mesoporous Sffica using supercritical C02 as the reaction medium, (b) Solubility of the functional molecules as a function of pH. 

Figure 4. Adsorption isotherms of mercury by thiol functionalized mesoporous silica, (a) HgN03. (b) CH3HgOH.

W. Yantasee, L.A. Deibler, G.E. Fryxell, C. Timchalk and Y. Lin, Screen-printed electrodes modified with functionalized mesoporous silica for voltammetric analysis of toxic metal ions, Electrochem. Commun., 7 (2005) 1170-1176. 

Yoshitake, H., Yokoi, T. and Tatsumi, T. (2003) Adsorption behavior of arsenate at transition metal cations captured by amino-functionalized mesoporous silicas. Chemistry of Materials, 15(8), 1713-21. 

Oxidative polymerization of 1,4-diethynylbenzene into highly conjugated poly(phenylene butadiynylene) within the channels of surface-functionalized mesoporous silica and alumina materials. J. Am. Chem. Soc. 124 9040-9041. 

THREE WAYS TO OBTAIN ORGANICALLY FUNCTIONALIZED MESOPOROUS SILICA PHASES... 

Three Ways to Obtain Organically Functionalized Mesoporous Silica Phases 55... 

An alternative method to synthesize organically functionalized mesoporous silica phases is the co-condensation method (also called one-pot synthesis or direct synthesis). Historically, the classical co-condensation reaction involved tetraalkoxysilane [(RO Si (predominantly TEOS or TMOS)] and a terminal organotrialkoxysilane of the type (R 0)3Si-R in the presence of structure-directing agents (Fig. 3.9, top). Meanwhile, not only bis- and multi-silylated precursors are used, that is, precursors of the type [(R 0)3Si]mR, m > 2 (Fig. 3.9, bottom), together with TMOS/TEOS, but... 

The removal of toxic oxyanions such as arsenate and chromate from contaminated effluents by means of amino-functionalized mesoporous silicas.107... 

In conclusion, functionalized mesoporous materials have proven to be efficient adsorbents for a broad spectrum of adsorptives however, it is yet unclear if they will find commercial applications as there are already established and highly developed adsorbents for many purposes on the market. 

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