Tunable Rare Earth fcu-MOF Platform: Access to Adsorption Kinetics Driven Gas/Vapor Separations via Pore Size Contraction

Abstract

Reticular chemistry approach was successfully employed to deliberately construct new rare-earth (RE, i.e., Eu(3+), Tb(3+), and Y(3+)) fcu metal-organic frameworks (MOFs) with restricted window apertures. Controlled and selective access to the resultant contracted fcu-MOF pores permits the achievement of the requisite sorbate cutoff, ideal for selective adsorption kinetics based separation and/or molecular sieving of gases and vapors. Predetermined reaction conditions that permitted the formation in situ of the 12-connected RE hexanuclear molecular building block (MBB) and the establishment of the first RE-fcu-MOF platform, especially in the presence of 2-fluorobenzoic acid (2-FBA) as a modulator and a structure directing agent, were used to synthesize isostructural RE-1,4-NDC-fcu-MOFs based on a relatively bulkier 2-connected bridging ligand, namely 1,4-naphthalenedicarboxylate (1,4-NDC). The subsequent RE-1,4-NDC-fcu-MOF structural features, contracted windows/pores and high concentration of open metal sites combined with exceptional hydrothermal and chemical stabilities, yielded notable gas/solvent separation properties, driven mostly by adsorption kinetics as exemplified in this work for n-butane/methane, butanol/methanol, and butanol/water pair systems.

Selective gas capture via kinetic trapping

“…We have also considered the existence of only one kind of binding site, although in Mg-MOF-74 the displacement of CO 2 by H 2 O may involve the passing of CO 2 from the primary binding site to a secondary one, through a low-energy exchange pathway [22,24,42,43] (note though that CO 2 binds almost as strongly to the secondary site as the primary one in Mg-MOF-74 [42], indicating that energy barriers for its removal from the framework are similar to those assumed here). Nonetheless, our results agree qualitatively with existing experimental observations: water inhabits Mg-MOF-74 in preference to CO 2 in equilibrium [15,24]; CO 2 can be resident within the Mg-MOF-74 for some considerable time away from equilibrium [24]; and narrower pores lead to better gas-capture selectivity [32][33][34][35]. The nonequilibrium 'filtration' mechanism seen in Fig.…”

“…The origin of this selective capture is an emergent nonequilibrium 'filtration' mechanism that allows, within a crowded framework, certain gas types to invade more rapidly than others. We describe this gas separation mechanism and show that the residence time and the abundance of the desired gas can be increased by impeding the flow of all gases within the framework, consistent with experiments in which constriction of pore apertures in metal-organic frameworks improved the selectivity of a framework for particular gas types [10,[32][33][34][35]. Because of the model's simplicity we do not expect it to be quantitatively precise, but where comparison can be made our results agree qualitatively with experiments [15,24,31,32,36], and indicate that CO 2 can under nonequilibrium conditions occupy a substantial fraction of the framework's binding sites.…”

Gas confinement in compartmentalized coordination polymers for highly selective sorption

“…5 Even more attractive are porous materials possessing restricted guest-accessible pores in which permanent cavities are inter-connected by small apertures in a long-range order, since they often provide improved selective separations. 6,7 A different approach consists on the use of interpenetrated frameworks, which also result as an effective solution for enhancing selectivity despite the decrease in porosity. 8,9 Typically, these materials are capable of discriminating between different guests based on their size, and even subtle changes in the pore diameter can modify the selectivity.…”

Mass transport through metal organic framework membranes

“…Thus, MOFs demonstrate great potential applications in various fields. Owing to the decades' efforts of researchers, thousands of structures of MOFs have been discovered and applied in various fields, such as separation [31][32][33][34][35][36][37][38], adsorption [20,[39][40][41][42], optics [43][44][45][46][47][48][49][50], catalysis [19,[51][52][53][54][55][56], sensors [57][58][59][60][61][62][63], etc. However, most of these studies are based on MOFs powders.…”