(397q) Gas Sorption and Swelling in Flexible Metal-Organic Frameworks

Metal organic frameworks (MOFs) have recently attracted considerable attention as new nanoporous materials with applications in separation, catalysis, gas capture and storage and drug delivery. MIL-53 (Al or Cr) family as a subclass of the MOFs displays a remarkable structural phase transition, often referred to as the breathing transition, due to the adsorption of guest molecules in its morphology. In the breathing phenomenon, the structure oscillates between two distinguishable phases called the large-pore (lp) phase and the narrow-pore (np) phase.
While molecular simulations can be used to compute the sorption isotherms of gas molecules in a flexible structure, such computations are expensive and applicable to a limited number of unit cells. We have developed a model based on the energetics of the system that couples guest adsorption with the host-guest interaction and structural deformation of the MOFs. The total energy of the system – the MOF plus the gas – is written as the sum of three terms: The elastic energy of the material, the fluid-phase energy, and the interaction energy between the fluid (gas) and the solid matrix. By minimizing the total energy with respect to both the fluid’s density and the displacement field of the solid we derive the governing equations for both. The equations are then solved by the finite-element method. Adsorption/desorption isotherms, along with the spatial distribution of the induced strain in the MIL-53 framework, are then computed. The proposed model can be applied to the structural transitions in materials, such as adsorption-induced structural deformation, at large scales, such as crystal scale. The model is completely general and may be used in studying of the adsorption-induced swelling of porous media by any type of gas.
When the model was applied to adsorption of CO2 in MIL-53 (Al) samples, all the reported experimental features of the phenomenon were reproduced. We computed the adsorption/desorption isotherms and volumetric strains various at different temperatures. At low pressures the system experiences contraction in which the structure transforms form the lp to the np phase, whereas at high pressures the system undergoes swelling and reverses the structural transformation from the np to the lp phase.