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, [Zn(5)Cl(4)(BTDD)(3)] (MFU-4l(arge)) (H(2)-BTDD=bis(1H-1,2,3-triazolo[4,5-b],[4',5'-i])dibenzo[1,4]dioxin), has been synthesized using ZnCl(2) and H(2)-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 (N(2) 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 H(2) 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 H(2) desorption maximum at 71 K, which is the highest temperature ever measured for hydrogen physisorbed on metal-organic frameworks (MOFs).

The usable capacity of porous materials for hydrogen storage

“…For correcting the effect of the temperature gradient reference measurements using non-adsorbing reference samples have been made in the whole temperature and pressure range (see supporting information). With this technique the hydrogen uptake of 12 samples with different Additionally, references regarding the material synthesis are given a Specific surface area of the Zn compound [18] b Measured at 2 MPa structure and chemical composition was measured at seven different temperatures between 77 and 125 K up to 2.5 MPa.…”

Route to a Family of Robust, Non‐interpenetrated Metal–Organic Frameworks with pto‐like Topology

“…[24,25] The heat of adsorption shows a small decrease from 4.25 kJ mol À1 at low uptakes to 3.81 kJ mol À1 at higher uptakes, which indicates that the adsorption sites are similar regarding their interaction strength.…”

Chemistry of Metal-organic Frameworks Monitored by Advanced X-ray Diffraction and Scattering Techniques

“…Many MOF structures have been recently determined using the conventional ab-initio procedures based on the classical high-resolution synchrotron powder X-ray data with the typical approach of pattern indexing, intensity integration, structure solution and Rietveld refinement. [51][52][53][54][55][56][57][58][59][60][61][62] Herein, the more nonconventional strategies and improvement overcoming the challenges of the X-ray powder diffraction structure determination of MOFs will be overviewed.…”

“…71 The Zn and N atoms could be positioned using RED data, whereas C atoms were additionally inserted according to geometry of the imidazole ligand. ADT was employed for structure solutions of Cu-benzene-1-3-5-tricarboxylate (HKUST-1), 72 Zn-BTDD (MFU-4, BTDD = bis(1H-1,2,3-triazolo- [4,5-b], [4,5- 58 Bi-benzenetrisbenzoate (CAU-7) 73 and Zr-terephthalate (UiO-66) 74 . These methodologies can only be applied for structure determination of very limited assortment of highly stable MOF systems with the strong support of complementary methods.…”