Understanding the Loading Dependence of Adsorbate Diffusivities in Hierarchical Metal–Organic Frameworks

Using atomistic simulations,
we studied the diffusion of n-hexane in a series
of isoreticular hierarchical metal–organic
frameworks (MOFs) NU-100x. Nonmonotonic diffusivity–loading
relationships that depend on the pore sizes were observed, which can
be explained by the spatial distribution of adsorbates at different
loadings. For one of the MOFs in the series, NU-1000-M, the diffusivity–loading
relationship is almost identical to the previously reported results
of n-hexane diffusion in the hierarchical self-pillared
pentasil (SPP) zeolite. Detailed analysis revealed that the similarity
results from their similar micropore and window sizes, which was confirmed
by free-energy mapping. The effects of temperature and adsorbate chain
length on the diffusion were also studied, which supported our conclusion
that the diffusivity in hierarchical nanoporous materials is primarily
controlled by the sizes of the micropores and the connecting windows,
particularly at relatively low loadings.