SESSION 1: CATALYSIS AND ADSORPTION IN MOFS

Selective Gate-Driven Mechanism of Water Diffusion in MFU-4 Metal-Organic Framework

Instituto de Tecnología Química, UPV-CSIC, Valencia (Spain)

Keywords: molecular dynamics, gate adsorption, water, metal-organic framework

Metal-organic frameworks (MOFs) have been many times pointed out as adsorbants displaying the so called gate-driven adsorption mechanism. Such mechanism implies, most of the times, two localised parts in the MOF with differentiated mechanical resistances, the harder –well extended– provides the robust framework required for structural integrity, and the softer –more localised– presents the mobility (gate) that is triggered by the adsorbate diffusion.

How selective are these gates is a matter of considerable importance in order to assess the MOF performance in separation processes. But, separation does not require gates, as exemplified by zeolites, active carbons, organic polymeric membranes, as well as the vast majority of MOFs whose separation ability does not rely on the presence of gates. The question arising is, what is –if any– the unique aspect of separation based on gates?

The structural peculiarities of MFU-4 (metal-framework Ulm number 4) feature a very robust framework containing an interconnected system of alternating large (11.9 Å) and small (2.5 Å) spherical cavities making a octahedrally (6:6) connected micropore. Small pores are very flexible and originate a unique 6-entrance gate of cubic shape made of 8 chlorine ligands (edge = 4.4 Å) interacting through van der Waals interactions. The flexibility of the small pore arises from the weak Cl---Cl interactions, allowing any vertex (Cl) to move, producing a large distortion leading to gate opening.

In previous studies^[1,2],different cases of chemical interactions,with CO₂, N₂ and Xe adsorbates, were studied. The present study will illustrate how water leads to a new way of interaction with specific consequences on diffusion and uptake.

[1] Sastre, van den Bergh, Kapteijn, Denysenko, Volkmer, Unveiling the mechanism of selective gate-drivendiffusion of CO2 over N2 in MFU-4 metal–organicframework, Dalton Trans., 43, 9612, 2014
[2] Bunzen, Kolbe, Kalytta-Mewes, Sastre, Brunner, Volkmer, Achieving Large Volumetric Gas Storage Capacity in Metal−Organic Frameworks by Kinetic Trapping: Xenon Loading in MFU-4, J. Am. Chem. Soc. 140, 10191, 2018