Functionalized mesoporous silicas

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... 

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]. 

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]. 

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 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... 

Thiol-functionalized mesoporous silicas not only serve as a basis for sulfonic acid-functionalized phases, but can be used for several other applications, for instance as adsorbents. Thiol-functionalized silicas can also be prepared as small nanoparticles with spherical morphologies by employing modified Stober reaction conditions.174 The high affinity of these compounds for thiophilic heavy metals, especially toxic Hg2+ ions, has been demonstrated by several authors.175-178... 

Another photochemically based gating system is the one by Brinker and coworkers in which the cis-trans isomerization of azobenzene was applied.69 (Triethoxysilylpropylureido)azobenzene was co-condensed with TEOS by evaporation-induced self-assembly to yield a functionalized mesoporous silica thin film. The propylureidoazobenzene groups inside of the mesopores could then be iso-merized from their stable elongated trans structure to the more compact cis geometry upon UV irradiation. The trans geometry could be recovered by thermal treatment. The authors suggested that the results of the transitions from trans to cis could lead to an increase in pore size of approximately 0.7 nm and an increase of the polarity from 0 to 3 D. This could be applied to regulation of ion transport however, they did not confirm such expectations.69... 

---