Adsorption\nand Dynamics in Hierarchical Metal–Organic\nFrameworks

Abstract

Adsorption and dynamics in hierarchical\nmetal–organic frameworks\nare investigated by means of molecular simulation. The models of hierarchical\nporous solids are obtained by carving mesopores of different diameters\nout of a crystal of Cu-BTC (model A) or by inserting a microporous\nparticle of Cu-BTC in an amorphous silica mesopore (model B). We show\nthat the nitrogen adsorption isotherms at 77 K for the solids corresponding\nto model A can be described as a linear combination of reference adsorption\nisotherms for pure microporous and mesoporous solids. In contrast,\nthe adsorption isotherms for model B cannot be described accurately\nas a sum of reference microporous and mesoporous adsorption isotherms.\nThe inserted particle acts as a constriction which helps nucleate\nthe liquid phase within the mesopore so that no capillary condensation\nhysteresis is observed. The dynamics of nitrogen adsorbed at 77 K\ninside the porosity of the hierarchical solids is also investigated.\nThe Fickian regime is reached at long times which are not attainable\nwith molecular dynamics simulations. At higher temperature, the faster\nself-diffusion makes it possible to obtain the diffusivity of the\nadsorbate. Nitrogen adsorbed in the microporosity of the hierarchical\nporous solids has a self-diffusion coefficient close to that of nitrogen\nadsorbed in pure Cu-BTC. In contrast, diffusion in the mesoporosity\nis faster than in the microporosity so that the overall diffusivity\nis faster than in pure Cu-BTC.