Elucidating Gating Effects for Hydrogen Sorption in MFU‐4‐Type Triazolate‐Based Metal–Organic Frameworks Featuring Different Pore Sizes

Abstract

A highly porous member of isoreticular MFU‐4‐type frameworks, [Zn₅Cl₄(BTDD)₃] (MFU‐4l(arge)) (H₂‐BTDD=bis(1H‐1,2,3‐triazolo[4,5‐b],[4′,5′‐i])dibenzo[1,4]dioxin), has been synthesized using ZnCl₂ and H₂‐BTDD in N,N‐dimethylformamide as a solvent. MFU‐4l represents the first example of MFU‐4‐type frameworks featuring large pore apertures of 9.1 Å. Here, MFU‐4l serves as a reference compound to evaluate the origin of unique and specific gas‐sorption properties of MFU‐4, reported previously. The latter framework features narrow‐sized pores of 2.5 Å that allow passage of sufficiently small molecules only (such as hydrogen or water), whereas molecules with larger kinetic diameters (e.g., argon or nitrogen) are excluded from uptake. The crystal structure of MFU‐4l has been solved ab initio by direct methods from 3D electron‐diffraction data acquired from a single nanosized crystal through automated electron diffraction tomography (ADT) in combination with electron‐beam precession. Independently, it has been solved using powder X‐ray diffraction. Thermogravimetric analysis (TGA) and variable‐temperature X‐ray powder diffraction (XRPD) experiments carried out on MFU‐4l indicate that it is stable up to 500 °C (N₂ atmosphere) and up to 350 °C in air. The framework adsorbs 4 wt % hydrogen at 20 bar and 77 K, which is twice the amount compared to MFU‐4. The isosteric heat of adsorption starts for low surface coverage at 5 kJ mol⁻¹ and decreases to 3.5 kJ mol⁻¹ at higher H₂ uptake. In contrast, MFU‐4 possesses a nearly constant isosteric heat of adsorption of ca. 7 kJ mol⁻¹ over a wide range of surface coverage. Moreover, MFU‐4 exhibits a H₂ desorption maximum at 71 K, which is the highest temperature ever measured for hydrogen physisorbed on metal–organic frameworks (MOFs).