(225a) Molecular Simulations of Tryalkoxysilane Functionalized Porous Silicates

Trialcoxysilanes are bifunctional molecules widely used to obtain self-assembled coatings by the chemisorption route in liquid solvents. However, problems arise when the method is used for porous materials due to cross-linking of alkylsilanes in solution, which lead to multilayer deposition, non-uniform surface coverage and pore blocking.

We propose a methodology for the design of models of post-synthesis functionalized silica materials. Using an energy-bias selection scheme for the possible grafting sites, we generate realistic models of the modification of the internal surface of mesoporous silica gels. The amorphous mesoporous supports were chosen for analysis in front of the most studied ordered mesoporous silicas of the type MCM-41 or SBA-15, because amorphous silica materials are considered low-cost adsorbents and could be used in a large amount of bulk applications. A hydrophobic silane was used to impregnate the internal surface to obtain oil adsorbents.

We compare the grafting densities obtained by supercritical silanization and traditional silanization to the maximum theoretical densities obtained by the proposed methodology. We analyze the decrease in grafting density with respect to flat surfaces, in terms of molecules per nm², as a result of sterical constrictions for molecules inside narrow irregular pores.

The behavior of N₂ molecules adsorbed on functionalized silicas is analyzed by means of Grand Canonical Monte Carlo (GCMC) simulations. Then, N₂ adsorption isotherms for the support and the functionalized samples between the simulation and the experimental procedures are compared. The comparison of the decrease in the surface area and pore volume serve as an indication of pore blockage in the experimental materials.

The understanding of the grafting of tryalkoxysilanes in silica adsorbents plays a key role on further optimization of the synthesis of these materials. We present our experimental adsorption isotherms and show the potential applications of this kind of materials as oil adsorbents; these results are compared to the simulated isotherms to validate the models and the methodology. 

This work was partially financed by the Spanish Government under projects CTQ2008-05370/PPQ, CTQ2011-23255 and CENIT SOST-CO2 (CEN-2008-1027). Additional support from the Catalan Government through 2009SGR-666 and Carburos Metálicos was also provided. S.B. acknowledges a Talent contract from the Catalan Government.