Silica support surface structure

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Pt particles remain highly dispersed in the reaction mixture during mesostructure formation. All measurements including XRD, SAXS, and TEM indicate a well-ordered silica structure. N2 physisorption measurement indicated high surface areas (523-661 m g ) and meso-sized pores (112-113 A) for the silica supports produced in the presence of different Pt particles. 

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. 

The hydrogenation catalysts can be prepared in situ, starting from the surface alkyl complex. In terms of catalytic performances, these catalysts are highly effective (Table 6.21) [150]. The best hydrogenation systems are based on silica supported dinuclear complexes, for which the structures of the active sites have not been investigated. Hydrogenation of toluene and xylenes can be achieved under similar conditions. 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

Structures of immobilized rhodium complexes on the sihca support have been proposed on the basis of the data obtained from C, P and Si MAS-NMR. NMR spectra of the rhodium-modified solid materials confirmed that trimethylsiloxide ligand was removed from the rhodium coordination sphere during the immobilization process. Formation of a new covalent bond between the rhodium organo-metallic moiety and the silica support occurs, probably with evolution of trimethylsilanol, which is rapidly converted into disiloxane (Me3Si)20. The presence of this molecule in the solution obtained after the silica surface modification process was confirmed by GCMS analysis. 

In summary, zirconocene and half-sandwich zirconium-based catalysis has been developed both in surface and solution systems. In general, the activation of Zr-POSS and silica-supported zirconium system with MAO proved inappropriate (see silsesquioxane displacement and leaching, respectively, vide supra), while the acti-vahon with BArl proved more efficient, albeit structural rearrangement complicates the activation chemistry and simple Zr(IV) cationic alkyl species are seldom the outcome of the activation. 

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