The diffusivity of CO2 and N2 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 /N2 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 m2 s-1 , and N2 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 PO⋅⋅⋅H⋅⋅⋅N zwitterionic sites are much higher than those for N2 , which explains the faster diffusivity observed for N2 .
Keywords: carbon dioxide; diffusion; metal-organic frameworks; molecular dynamics; quasielastic neutron scattering.
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