(242d) Adsorption of CO2 Using Amine Functionalized Silica

The adsorptive behavior of amorphous silica can be enhanced by the use of organic groups. Using certain organic species can increase the selectivity of solid silica. These functionalized materials can be used to capture gases reversibly using adsorption. [1] Effective design of these materials requires a method that can relate the structure of the adsorbent to its performance. This level of understanding can be achieved by using molecular simulations, as they relate the microscopic behavior of the molecules during the adsorption process to the macroscopic behavior of the system, allowing one to search for the best materials for separation purposes.

Although the silica material can be modeled as a rigid structure, the functionalized chains in the surface have to be allowed to move during the adsorption of CO₂. The tethered molecules have branched chains. Therefore, for the simulations of adsorption it is necessary to use a method capable of moving branched chains efficiently. The torsion and bending angles in the surface groups can be handled using a coupled-decoupled configurational bias algorithm [2]. Additionally, we used pregenerated Gaussian distributions for the probabilities of generating the bending and torsion angles for the grafted molecules, which are then corrected in acceptance rules [3]. For all the possible substitution sites (surface silanols), we calculate the Rosenbluth factor for replacing the hydroxyl group for the first and second bead in the chain. In our simulations, we consider as the first atom in our chains the oxygen atom bonded to the surface silica.

These models are evaluated for CO₂ adsorption using Grand Canonical Monte Carlo Simulations. We analyze the effect of physisorption on the adsorption of CO₂ using grafted amines. We demonstrate the enhanced CO₂ physisorption capabilities of the functionalized materials. We study the configurations of the amine chains during the adsorption for different degrees of functionalization. The effect of the concentration of grafted amines on the adsorption are studied in detail.

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.

[1] Choi, S.; Drese, Jeffrey H.; Jones, Christopher W., ChemSusChem 2009, 2 (9), 796-854.

[2] Martin, M. G.; Siepmann, J. I., J Phys Chem B 1999, 103 (21), 4508-4517.

[3] Martin, M. G.; Frischknecht, A. L., Mol Phys 2006, 104 (15), 2439 - 2456.