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 CO and N 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 CO /N separation. The calculated and measured diffusivities of both gases were shown to be of an order of magnitude sufficiently high, from 10 to 10  m  s , and N diffused faster than CO 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 CO in the region close to the specific PO⋅⋅⋅H⋅⋅⋅N zwitterionic sites are much higher than those for N , which explains the faster diffusivity observed for N .