Hierarchical Structure and Molecular Dynamics of Metal-Organic Framework as Characterized by Solid State NMR

Abstract

Metal-organic framework (MOF) stands out as a promising material with great potential in application areas, such as gas separation and catalysis, due to its extraordinary properties. In order to fully characterize the structure of MOFs, especially those without single crystal, Solid State NMR (SSNMR) is an indispensable tool. As a complimentary analytical technique to X-ray diffraction, SSNMR could provide detailed atomic level structure information. Meanwhile, SSNMR can characterize molecular dynamics over a wide dynamics range. In this review, selected applications of SSNMR on various MOFs are summarized and discussed.

MOFs in the time domain

“…In-situ 13 C NMR is frequently used to investigate framework motion induced by carbon-containing guests such as CO2 or hydrocarbons. [247][248][249][250][251] Sin et al investigated the effect of framework flexibility of the uptake selectivity of CO2 from a CO2/CH4 mixture. 252 Selectivity factors were calculated based on shift ratios; DUT-8(Ni)_flex is more selective than DUT-8(Ni)_rigid suggesting flexibility-enhanced selectivity.…”

Exploring Structural Disorders in Aluminum-Containing Metal–Organic Frameworks: Comparison of Solid-State 27Al NMR Powder Spectra to DFT Calculations on Bulk Periodic Structures

“…7−10 Another relevant application is the use of a multiple quantum MAS NMR to identify nonequivalent metal species in MOFs through the correlation between their quadrupolar parameters and chemical shifts. 12 Thus, we shall analyze 27 Al MAS NMR spectra of four Al-MOFs with a nominal pore diameter from 0.29 to 1.75 nm, aiming to test them against the periodic models of the MOFs obtained by electron and powder X-ray diffraction (PXRD) methods. The EFGs at Al sites in Al-MOFs are dominated by six oxygen atoms in the octahedral coordination of aluminum (Figure 1), but their relative positions and effective charges still depend on the spatial arrangement of the linkers.…”

“…Here, we take the advantage of having the NMR-active quadrupole 27 Al (spin 5/2, 100% natural abundance) to probe the coordination environment of the metal centers in Al-MOFs with solid-state NMR and thus to estimate the deviation from the expected bulk structures. The approach of observing directly the metal centers of MOFs using solid-state NMR is relatively rare, and we have found only a few of such studies in the literature. − One such application is magic angle spinning (MAS) NMR measurements of the principal components of the electric field gradient (EFG)the quadrupolar constant C Q and the asymmetry parameter η, which are then compared with the theoretical values calculated using idealized MOF structures. − Another relevant application is the use of a multiple quantum MAS NMR to identify nonequivalent metal species in MOFs through the correlation between their quadrupolar parameters and chemical shifts. − …”