A critical appraisal was made of estimation methods for the Onsager coefficients L11, L22 and L12 for binary mixture diffusion within nanopores using pure component diffusivity data input. The appraisal was based upon extensive sets of molecular dynamics simulation data on Lij for various mixtures in zeolites (MFI, AFI, TON, FAU, CHA, DDR, MOR, LTA), carbon nanotubes (CNTs: armchair and zig-zag configurations), titanosilicates (ETS-4), and metal-organic frameworks (IRMOF-1, CuBTC). The success of the Lij predictions was crucially dependent upon the estimates of the degree of correlations in molecular jumps for different guest-host combinations; these correlations were captured in Maxwell-Stefan approach by the exchange coefficients Đij. Three limiting scenarios for correlation effects were distinguished; for each of these scenarios appropriate expressions for the Lij were presented. For CNTs, correlation effects were dominant and the interaction factor, defined by α12 = L12/(L11L22)1/2, was close to unity. For cage-type zeolites such as LTA, CHA, and DDR with narrow windows separating cages, correlation effects were often, but not always, negligibly small and the assumption of uncoupled diffusion, i.e., α12=0, was a reasonable approximation provided the occupancies were not too high. In other cases such as zeolites with one-dimensional channel structures (AFI, TON), intersecting channels (MFI), cage-type zeolite with large windows (FAU), ETS-4, CuBTC, and in IRMOF-1, it was essential to have a reliable estimation of the Đij; molecular dynamics simulations underline the wide variety of factors that influence the Đij. Two situations were highlighted where estimations of the Lij failed completely. In both cases the failure was due to segregated adsorption. In adsorption of CO2-bearing mixtures in LTA and DDR zeolites, CO2 was preferentially lodged at the narrow window regions and this hindered the diffusion of partner molecules between cages. The second situation arose in MOR zeolite that had one-dimensional channels connected to side pockets. Some molecules such as methane, became preferentially lodged in the side-pockets and did not freely participate in the molecular thoroughfare. Current phenomenological models did not cater for segregation effects on mixture diffusion.

Onsager Coefficients for Binary Mixture Diffusion in Nanopores. Krishna, R., van Baten, J.M.: Chemical Engineering Science, 2008, 63[12], 3120-40