(303g) Computational Study of CO2 Adsorption on Apohost and LiCl-Functionalized Zn(bdc)(ted)0.5 Metal–Organic Frameworks

Carlos Fernández-Cabán, Jorge Rosa-Raíces, Paul Meza-Morales, and María Curet-Arana

Department of Chemical Engineering, University of Puerto Rico, Mayaguez Campus, Road 108 - km 1.1, Mayaguez, PR 00681-9000, United States

Metal-organic frameworks (MOFs) are self-assembling crystalline nanoporous materials with potential applications in gas separation and storage and several other fields.^1â??3 Zn(bdc)(ted)_0.5 (Zn-DMOF) is a MOF with remarkable uptake capacity and affinity for CO₂.^4,5 Studies involving Zn-DMOF have shown that its CO₂ adsorption properties can be enhanced by tailoring its structure and chemistry through post-synthesis modification.^6,7 An alternative functionalization method, known as spontaneous thermal dispersion, involves introducing extra-framework moieties that can adjust the materialâ??s functionality for a targeted application while preserving its basic chemical and structural properties. This method was recently used to incorporate LiCl in Zn-DMOF, and resulted in a crystalline functionalized variant (Zn-DMOF-(LiCl)) featuring a dramatically altered CO₂ uptake capacity, appearance of an adsorptionâ??desorption hysteresis gap and contraction of the framework pores with respect to the apohost material.⁸

In this multiscale study, we use density functional theory (DFT) calculations and grand canonical Monte Carlo (GCMC) simulations to yield molecular-level insight into the structural and chemical features that underlie the CO₂ adsorption properties of Zn-DMOF and Zn-DMOF-(LiCl). Our computations reveal that the ionic salt residues in the functionalized variant remain bonded and interact preferably with the metallic centers of Zn-DMOF, inducing severe unit-cell deformation and altering the textural properties of the MOF as verified by comparing simulated and experimental spectroscopic data. GCMC simulations with the deformed structures are in fair agreement with experimental results over a significant portion of the pressure range studied, and confirm that the CO₂ uptake capacity of the apohost MOF undergoes drastic changes upon functionalization with LiCl, with framework-adsorbate interactions being strengthened at low pressures. We also explore the low-energy configuration space of Zn-DMOF as a function of CO₂ loading, and resolve the reported guest-induced structural changes in the apohost material.⁹Our work offers a molecular-level understanding of the structural and textural changes caused by spontaneous dispersion of an inorganic moiety in a prototypical MOF, the concomitant impact on its functionality, and the applicability of this technique in the rational design and tailoring of similar materials.

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