2006, Volume 110A

Abstract

In our simulations of selectivity of CO 2 from CO 2 +CH 4 binary mixture, the force field parameters for CH 4 were refined by fitting the experimental data of CH 4 adsorption in Mn-MOF (Dybtsev et al., J. Am. Chem. Soc. 2004, 126, 32). However, the authors of the experimental work claimed that the extremely low uptake of CH 4 in Mn-MOF was caused by the sieving effect that excluded CH 4 (Dybtsev et al., J. Am. Chem. Soc. 2004, 126, 32). If this is true, our method for obtaining the force field parameters for CH 4 is meaningless, and thus the predicted selectivity is not credible and also is meaningless. However, it is difficult for us to understand why the window (0.45 nm) can let CO 2 (kinetic diameter 0.33 nm) pass while screen Ar and CH 4 (kinetic diameters 0.34 and 0.38 nm, respectively). On the other hand, J. K. Johnson et al. (J. Phys. Chem. B 2005, 109, 13094) found much higher CH 4 uptake in Mn-MOF using standard force fields. They had calculated zero-coverage dif-fusivity of CH 4 in Mn-MOF and found it was similar in magnitude to the diffusivity of CH 4 in silicalite (MFI), and concluded that slow diffusion of CH 4 into Mn-MOF was not likely to be a cause for the low uptake (J. Phys. Chem. B 2005, 109, 13094). Based on the above considerations, we could not get a definite conclusion on the reason of the extremely low uptake of CH 4 in Mn-MOF measured by of Dybtsev et al. As a result, our simulations concerning CO 2 selectivity from CO 2 +CH 4 mixture in Mn-MOF may not be reliable.