High-Pressure in Situ ¹²⁹Xe NMR Spectroscopy and Computer Simulations of Breathing Transitions in the Metal–Organic Framework Ni₂(2,6-ndc)₂(dabco) (DUT-8(Ni))

Abstract

Recently, we have described the metal-organic framework Ni(2)(2,6-ndc)(2)(dabco), denoted as DUT-8(Ni) (1) (DUT = Dresden University of Technology, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane). Upon adsorption of molecules such as nitrogen and xenon, this material exhibits a pronounced gate-pressure effect which is accompanied by a large change of the specific volume. Here, we describe the use of high-pressure in situ (129)Xe NMR spectroscopy, i.e., the NMR spectroscopic measurements of xenon adsorption/desorption isotherms and isobars, to characterize this effect. It appears that the pore system of DUT-8(Ni) takes up xenon until a liquid-like state is reached. Deeper insight into the interactions between the host DUT-8(Ni) and the guest atom xenon is gained from ab initio molecular dynamics (MD) simulations. van der Waals interactions are included for the first time in these calculations on a metal-organic framework compound. MD simulations allow the identification of preferred adsorption sites for xenon as well as insight into the breathing effect at a molecular scale. Grand canonical Monte Carlo (GCMC) simulations have been performed in order to simulate adsorption isotherms. Furthermore, the favorable influence of a sample pretreatment using solvent exchange and drying with supercritical CO(2) as well as the influence of repeated pore opening/closure processes, i.e., the "aging behavior" of the compound, can be visualized by (129)Xe NMR spectroscopy.

Solid-State NMR Spectroscopy of Metal–Organic Framework Compounds (MOFs)

“…A first application of 129 Xe NMR spectroscopy to MOFs has been reported in 2006 [179]. Subsequently, other authors made use of the favorable properties of 129 Xe NMR in order to study MOFs [23,180,181,182,183]. …”

“…One way of studying the breathing transitions is by sensing the MOF lattice itself and its changes. Since the breathing transitions change the pores and/or their “entries” if the crystal structure of the MOF changes, these effects can also be explored in an indirect way by using probes like 129 Xe sensing the inner surface of the pores as has been demonstrated for the metal–organic frameworks MIL-53 [182] and DUT-8 [24,183]. …”

Challenges in first-principles NPT molecular dynamics of soft porous crystals: A case study on MIL-53(Ga)

“…Examples include zeolites 22 as well as a number of different MOFs. [23][24][25][26] FPMD also allows one to model certain electronic properties (e.g., dipole moment of adsorbed molecules 27 ) that are hardly accessible through forcefield-based MD, as well as study chemical reactions, as was recently demonstrated in studies of thermal stability of materials from the IRMOF family. 28,29 FPMD is naturally well-suited to study stimuli-induced deformations and structural transitions in flexible materials, especially when used in the isobaric ensemble to study pressure-and temperature-induced transitions.…”

Analyzing the frequency shift of physiadsorbed CO2in metal organic framework materials

“…Such understanding can either be gained through theory, using first-principles simulations, [8][9][10][11][12][13][14][15][16][17][18] or through experimental probes, such as infrared (IR) absorption and Raman scattering. [8][9][10][11]18,[20][21][22][23][24] Much progress has been made in improving the properties of MOFs. For example, it has been shown that using unsaturated metal centers, such as MOF74 with open Mg, Mn, Zn, Ni, Cu, or HKUST-1 with Cu, results in higher absorption density for hydrogen and faster absorption at small partial CO 2 pressures, the latter of which is highly desirable for CO 2 capturing applications.…”