(189cf) Molecular Simulation of CO2 Absorption into MCM-41 Porous Material Filled with PDMS Solvent

Although CO₂ loadings into porous materials could be large, such as in MCM-41, the CO₂/N₂ loading selectivity in MCM-41 is small (~ 5 at 298 K), which is partly due to the large pore volume of MCM-41 available for N₂ adsorption. This low selectivity impedes the use of MCM-41 in CO₂/N₂ separation processes. In this work, we investigate a way to improve CO₂/N₂ loading selectivity by filling poly(dimethylsiloxane) PDMS solvent into the MCM-41 porous material. By using a smooth MCM-41 potential model, it was found that the simulated CO₂, N₂, and H₂ loadings into MCM-41 are close to the experimental data. At ambient condition, the PDMS solvent density filled in MCM-41 is 15% smaller than in the neat PDMS. Both CO₂ and N₂ loadings in MCM-41 filled with PDMS become smaller than their loadings in the empty MCM-41; however, it was found that the CO₂ loading decreases by 1.45 times and N₂ decreases by 3 times. These results lead to CO₂/N₂ loading selectivity in MCM-41 filled with PDMS solvent increasing by 2 times compared with CO₂/N₂ loading selectivity in the empty MCM-41. In the empty MCM-41, simulation snap shots show that at 298 K and 5 bar and 10 bar, 20% of the N₂ molecules are adsorbed in the regions away from the MCM-41 pore wall, while the remaining 80% of the N₂ molecules are adsorbed in the region close to the wall. For CO₂ at 298 K and 1.25 bar and 2.5 bar, only 7% of the CO₂ molecules are adsorbed in the region away from the wall and the remaining 93% of the CO₂ molecules are adsorbed close to the wall. These findings suggest that more N₂ molecules are adsorbed in the region away from the MCM-41 wall compared with CO₂. After PDMS is filled in the MCM-41, it will occupy the pores of the MCM-41 and will decrease gas loadings. On the other hand, simulation snap shots show that the PDMS in MCM-41 could also absorb both CO₂ and N₂ gases. The overall effect is that PDMS will decrease less CO₂ loading compared with N₂ loading and as a result increase the CO₂/N₂ loading selectivity. Finally, we will also show the H₂ adsorption in the MCM-41 filled with PDMS solvent.