On porous silica

Seebach has developed ImmobilizadonofTADDOLv/ith a high degree of loading on porous silica gel and appiicadons in enandoselecdve adthdon of Eq. 8.54. This catalyst leads to 85%... 

Compared with conventional type, the tentacle type exhibits a marked increase in selectivity. The results observed for preparative ion exchangers of tentacle type prompted the author to test this structural arrangement of the binding groups also for analytical materials based on porous silica. Figure 12 reveals that the tentacle-specific selectivity is fully preserved when the matrix is changed. 

Also based on porous silica support, the so-called Biomatrix phases contain a ligand that combines both hydrophilic and hydrophobic properties in the same molecule. Nonpolar analytes interact with the hydrophobic phenyl and alkyl moieties of the ligand, while the hydrophobic outer part of the ligand is supposed to be the result of hydroxyl groups that prevent the adsorption of proteins. 

In an early study of ethane adsorbed on Pt supported on porous silica glass, Clark (77) found no infrared bands on initial adsorption at room temperature, 453, or 513 K, but on hydrogenation observed absorptions near 2935 and 2870 cm1, which could be attributed to surface alkyl species. 

Large molecules that just fit into the pores can contact relatively more active sites around the wall than small molecules which only touch the wall with one side of their surface. Experimental evidence for this idea can be derived from the adsorption of polyfmethyl methacrylate) on silica with or without pores the fraction of CO groups involved in adsorption was 0.32-0.39 on nonporous but 0.46-0.49 on porous silica 2). 

Figure 4 illustrates the effect on porous silica beads. The straight diagonal line shows the expectation for reactivity control where no preferential reaction at the external surface is encountered. The TFSA data show the reagent is immobilized on the outer surface before it can migrate to the porous internal surface. There is... 

The results reported here and in earlier publications in this series suggest that cavity size and limitations to molecular motion play a dominant role in the photochemistry and photophysics of alkyl aryl ketones included in zeolites. In the case of Silicalite the size and polarity of various substituted 8-phenylpropiophenones seem to determine the efficiency of inclusion and ultimately of luminescence. The same factors, relating to size and mobility can be expected to play an important role in the use of zeolites as catalysts for other reactions, whether these are photochemical or thermal processes. In this sense studies with 8-phenylpropiophenones may lead to considerable information on adsorption sites and on the freedom (or lack of it) of molecular motion as well as on the accessibility of these sites to other reactants. Recent work from Turro s laboratory has shown that pyrene aldehyde can be used to probe the nature of inclusion sites in various zeolites (27) dibenzyl-ketones were also used as probes on porous silica (28). 

McKittrick, M.W. and Jones, C.W. (2004) Toward single-site, immobilized molecular catalysts site-isolated Tr ethylene polymerization catalysts supported on porous silica. J. Am. Chem. Soc., 126, 3052. 

The immobilization of Nd tetramethyl aluminate complexes on porous silica supports (MCM-48) was reported by Fischbach et al. [406,407]. In this study activation was accomplished by DEAC. The polymerization of IP in hexane yielded IR with high cis- 1,4-contents (> 99%). 

Yamashita H, Honda M, Harada M, et al. Preparation of titanium oxide photocatalysts anchored on porous silica glass by a metal ion-implantation method and their photo-catalytic reactivities for the degradation of 2-propanol diluted in water. J Phys Chem B 1998 102 10707-11. 

The application of relaxation time measurements to study segmental motion (in polymers) as well as diffusional chain motion is very well documented but is still a subject of study, particularly using the frequency dependence of relaxation times to test the detailed predictions of models (McBriety and Packer 1993). The anisotropy of reorientation can also be studied conveniently, and recent interest in motion of molecules on surfaces (e.g. water on porous silica) has been investigated with great sueeess (Gladden 1993). Since the dipolar interaction is usually both intermolecular and intramolecular, the relaxation of spin- /2 nuclei (e.g. H) in the same molecule as a quadrupolar nucleus (e.g. H) can permit a complete study of reorientation and translation at a microscopic level (Schmidt-Rohr and Spiess 1994). 

Tayot J-L, Tardy M (1980) Isolation of cholera toxin by affinity chromatography on porous silica beads with covalently coupled ganglioside Gmi- In Adv. Exp. Med. Biol. 125 471 -478. 

Zaborsky O, Ogletree R (1974) Immobilization of enzymes on porous silica supports. J Biochem Biophys Res Cooun 61 210-216... 

Here, we will examine the photochemistry of Fe(CO)s adsorbed on the surface of porous silica (4,5). Using IR and UV-visible spectroscopy to monitor photoproduct formation, we find that surface functional groups play a key role in determining the outcome of photochemical reactions in this system. The effects of surface coverage and surface temperature are particularly important. We will discuss these effects in detail, and we will propose a mechanism for the participation of silica surface groups in the photochemical reactions of Fe(CO)s. Finally, the results of our experiments on porous silica will be compared to the results of recent experiments on the photochemistry of Fe(CO)s adsorbed onto other surfaces. 

The formation of the yellow solid product only at high surface coverage and its instability upon pumping away Fe(C0)5 is consistent with the following mechanism. The Fe(CO)s overlayers on porous silica are reasonably well-described as a liquid phase condensed... 

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