PNNL

Abstract

The structure, dynamics, and free energies of absorption of CO2 by a low density structure of Calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO2:TBC4 have been studied using molecular dynamics simulations. From the computed radial distribution functions, we notice that the 1:1 loading peak shows a single broad peak from the caged CO2 separation. At higher temperature, the peak is only slightly broadened from the 300 K peak and the shoulder around 11 Å is reduced. The radial distribution function of the 2:1 loading shows a prominent sharp peak around 3 Å and a second peak around 4 Å, indicating a dimer center-of-mass separation of 3 Å that is smaller than that of optimized gas-phase dimer. This result suggests that dimer is sufficiently stabilized by the interaction with the TBC4. The relative angle distributions for paired CO2 molecules are flat and do not show a preference for the crossed geometry found in the vacuum dimer at 3 Å. Angular distributions relative to the TBC4 symmetry axis show a preference for alignment tilted relative to the TBC4 axis closer to the plane of the phenyl rings of the TBC4 cage. Translational velocity autocorrelation calculations show a single peak under all conditions studied with very little change with temperature. Rotational velocity autocorrelation calculations show relatively little structure with significant tailing to low frequencies indicating rotation is hindered in the conical TBC4 cavity in the low density structure. The free energy of inclusion for CO2 in this TBC4 structure at 300 and 450 K for various loadings show the inclusion of a single CO2 in the system is favorable at -4.2 kcal/mol at 300 K and -1.5 kcal/mol at 450 K. The fully loaded 1:1 CO2:TBC4 system is slightly less favorable at -3.6 and -1.0 kcal/mol for 300 and 450 K respectively. The first CO2 added beyond 1:1 loading shows a significant drop in absorption energy to -1.8 and +1.5 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO2 at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the energy of inclusion for addition of the CO2 above 1:1 is less favorable. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.