Adsorption and Dynamics in Hierarchical Metal-Organic Frameworks

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