Metal-organic frameworks (MOFs) are porous materials built from inorganic nodes connected by organic linkers that are increasingly being used for applications in gas adsorption and storage owing to their high porosities and crystallinity. However, the structural flexibility of these materials, due to the relatively weaker coordination links between the inorganic and organic units, allows for structural deformations of the framework in the presence of guest molecules, thereby making it challenging to model the adsorption behavior of these materials. Here, we have employed force-field-based Monte Carlo simulations in the NguestNhostPT-Gibbs ensemble to investigate the adsorption of N,N-dimethylformamide (DMF) and p-xylene in MOF-5 and its linker-functionalized analogue, IRMOF-3. In this ensemble, only guest molecules are allowed to transfer between reservoir phase and MOF phase, the volume of the reservoir phase can change only isotropically, whereas changes in individual elements of the H matrix allow for both volume and shape fluctuations for the MOF phase. MOF-5 is built from inorganic Zn4O nodes connected by 1,4-benzenedicarboxylate (BDC) linkers while the inorganic nodes in IRMOF-3 are coordinated by 2-amino-1,4-benzenedicarboxylate (NH2-BDC) linkers. We observe lower onset pressures for sorbate uptake in IRMOF-3 as compared to MOF-5 owing to favorable interactions of the sorbates with the linker amine groups in IRMOF-3 as evidenced from their respective radial and spatial distribution plots. The isotherms for p-xylene adsorption in MOF-5 and IRMOF-3 at 295 K indicate a “pore-filling” adsorption mechanism in these MOFs with negligible framework deformation. Remarkably, these isoreticular MOFs exhibit different DMF adsorption characteristics. While the isotherm for DMF adsorption in MOF-5 at 295 K shows a steep increase in the DMF uptake corresponding to a “pore-filling” adsorption mechanism, the DMF adsorption isotherm in IRMOF-3 at 295 K shows a gradual increase in the DMF uptake which is significantly influenced by the framework deformation. At P/Po = 0.001, we observe an approximately 30% reduction in the unit cell volume and a change in the unit cell shape from cubic to triclinic in order to maximize the DMF uptake. We also observe a bimodal distribution of the sorbate loading in individual pores of the MOF, thereby pointing to the existence of two types of pores resulting from preferred linker orientations identified in the MOF.

DMF and xylene adsorption in flexible IRMOFs: A molecular simulation study

ACS Fall 2021

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