Diffusion of Carbon Dioxide and Nitrogen in the Small-Pore Titanium Bis(phosphonate) Metal-Organic Framework MIL-91 (Ti): A Combination of Quasielastic Neutron Scattering Measurements and Molecular Dynamics Simulations

The diffusivity of CO2 and N-2 in the small-pore titanium-based bis(phosphonate) metal-organic framework MIL-91(Ti) was explored by using a combination of quasielastic neutron scattering measurements and molecular dynamics simulations. These two techniques were used to determine the loading dependence of the self-diffusivity, corrected and transport diffusivities of these two gases to complement our previously reported thermodynamics study, which revealed that this material was a promising candidate for CO2/N-2 separation. The calculated and measured diffusivities of both gases were shown to be of an order of magnitude sufficiently high, from 10(-9) to 10(-10)m(2)s(-1), and N-2 diffused faster than CO2 through the small channel of MIL-91(Ti). Consequently, the separation process does not involve any kinetic-driven limitations. This study further revealed that the global diffusion mechanism involves motions of gases along the channels by a jump sequence, and the residence times for CO2 in the region close to the specific POHN zwitterionic sites are much higher than those for N-2, which explains the faster diffusivity observed for N-2.