Porous silica surfaces

The patterned amine materials have been used to construct CGC-inspired sites that were evaluated in the catalytic polymerization of ethylene after activation with MAO. The complexes assembled on a porous silica surface using this methodology are more active than previously reported materials prepared on densely-loaded amine surfaces. This increased activity further suggests the isolated, unique nature of the metal centers. Work is continuing in our laboratory to further characterize the nature of the active sites, as well as to obtain more detailed kinetic data on the catalysts. The patterning methodology is also being applied to the creation of immobilized catalysts for small molecule reactions, such as Heck and Suzuki catalysis. 

CP dynamics has also been very useful technique for estimation of the connectivities in the porous solids. Classic example in this case would be a clear distinction between Q2, Q3 and Q4 silicate sites in the molecular sieves on the basis of not only the chemical shift but also Tcp times which, as a result of no protons connected directly are much longer for Q4 sites (Si(OSi)4). Introduction of guest species in the pores has a dramatic effect on the CP-dynamics of the different silica sites. Thus, in the MCM-41 impregnated with the mixed metal clusters/counter-cation species, presence of embedded organic counter-cation lead to a significant reduction of Tcp and Tiph due to the presence of a different source of protons at the porous silica surface.21... 

Fluorescence techniques can also be used to study solid surfaces. A fractal approach has been used to interpret rhodamine 6G probed morphology of porous silica surfaces . Migration of excitation agrees with a one step energy transfer mechanism. 

Majority of studies concerning organic compounds chemisorption kinetics on oxides surface are performed for the reaction mechanisms described by Schemes 1, 8 or 15 and mainly, for the first mechanism of organosilicon compounds interaction with OH groups of non-porous silica surface. 

This model is intended for description of alkylchlorosilanes and alkoxysilanes reaction kinetics with the OH groups of dehydrated non-porous silica surface [106,107]... 

The fractal nature of the porous silica surface is clearely demonstrated by the fact that neither a two-dimensional nor a three-dimensional model could be applied to simulate the time-correlated fluorescence decay curves. The observed fractal dimensions are in very good agreement with other published results. 

In much of the definitive IR work on the silica surface researchers have chosen to work with fumed silica. This choice was mainly for experimental reasons (the ease of preparing the self-supporting disk), but also because it minimizes another important issue — the nature of porous silica surface. A major advance in the past decade has been in the controlled synthesis of many sUica polymorphs with variable pore size. Accordingly, the past decade has seen a renewed enthusiasm for the study of porous silicas, their reaction with chemical probes, and H2-D2 exchange reactions. An increasing body of evidence indicates that the basic silica structure is similar in both cases, but that accessibihty and derivatization of the porous silicas can stericaUy alter the process and the kinetics of the reactions. 

Porous silica surface contains both silanol (Si—OH) and siloxane groups (Si—O—Si). Silanols are considered to be strong sites for adsorption, while silox-anes are hydrophobic sites [92,96], Silanols may be isolated, vicinal, or geminate and may be linked by hydrogen bonds to the surface water. Figure 19 shows the silica surface in a schematic way. 

The three apolar phases show L-type concave adsorption isotherms. The two polar phases show S-type convex isotherms [12]. The notable difference is the CTAB adsorption on bare silica. This adsorption is a proof that hydrophobic interaction is not the only driving force for CTAB adsorption on the porous silica surface. Silanol-quatemary ammonium interactions are likely. They induce a cooperative adsorption of more CTAB molecules by hydrophobic interaction. 

Havard and Wilson [77] describe pore measurement on meso-porous silica surface area standard powders. They presented pore size distributions based on the modeUess method and the Kelvin equation based on open ended cylinders and spheres with co-ordination numbers of 4, 6 and 8. The isotherm can be used to calibrate BET apparatus over the whole range (samples are available from the British National Physical Laboratory). 

The kinetics of the adsorption of PS at a porous silica surface was studied under 0 conditions (cyclohexane at 35°C) by Kawaguchi and coworkers [112,113]. For small PS molecules (M = 96,400), with a size much smaller than that of the pores, it took less than 10 h to obtain a (quasi)-plateau value. For large PS (A/ = 355,000) the size of the polymer was half the size of the pores and a plateau was obtained only after 35 h. Adsorption of a mixture of the small and large PS molecules showed an interesting behavior. The rate of diffusion of the small molecules into the pores was much faster than that of the large moleeules. As a result, flie adsorption of the small molecules obtained a maximum in about 8 h (Fig. 33). After that period the large molecules replaced some of the small molecules. Pseudoplateau values were obtained after 25 h. All these experiments were carried out below saturation, and the solution concentration was varied between its initial concentration and zero. 

Grafting of Ordered Polymer on Porous Silica Surface... 

---