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

Abstract

We use solid-state 27 Al NMR spectroscopy to test four aluminum-based metal−organic framework (MOF) materi-alsICR-2, ICR-4, ICR-6, and ICR-7against their structural models obtained from electron and X-ray diffraction. The lineshape analysis of the 27 Al NMR spectra reveals varying degrees of disorders in the Al coordination environment, depending on the sample, which we were able to parameterize in terms of the extended Czjzek model (ECM) of the electric field gradient (EFG) distribution. The model's degree-of-disorder parameter ε was found to correlate inversely with the crystallite size of the MOFs. This suggests that the EFG distribution is related to a high surfaceto-volume ratio of the particles and can be ascribed to the deviation of molecular arrangements near the surface from the bulk crystalline order. The ECM's parameters for a fixed part of the EFG tensor, a quadrupolar coupling constant C Q,0 and asymmetry parameter η 0 , were compared to the EFG parameters calculated on fully periodic models using density functional theory (DFT). The DFT-calculated C Q was found to exceed C Q,0 by 25−60%, depending on the MOF. Although this result finds support in the literature, an explanation of such apparent spectral narrowing is still lacking.

Defect Induced Structural Transition and Lipase Immobilization in Mesoporous Aluminum Metal‐Organic Frameworks

“…However, despite these advancements, the strong coordination interactions in MOFs present challenges during growth, often resulting in the formation of defective crystals with various defects [7] . These defects, including linker vacancies, missing inorganic nodes, dislocations, and stacking faults, significantly impact the material‘s characteristics and utility, necessitating advanced characterization techniques such as high‐resolution AFM, SEM, CFM (3D imaging), N 2 sorption isotherm, PXRD, DFT computations, solid‐state NMR, 1 H‐NMR, and XPS for their comprehensive analysis [8–12] . Understanding and controlling defect formation is imperative for optimizing MOF performance across various applications.…”

Solid-state 13C-NMR spectroscopic determination of side-chain mobilities in zirconium-based metal–organic frameworks

“…The framework structure of MOFs can be studied in general by 1 H, 13 C, or 17 O magic-angle spinning (MAS) NMR spectroscopy (Loiseau et al, 2005;Klein et al, 2012;Lucier et al, 2018;Bignami et al, 2018;Brunner and Rauche, 2020). Furthermore, nuclei such as 27 Al (Jiang et al, 2010;Lieder et al, 2010;Petrov et al, 2020), 71 Ga (Volkringer et al, 2007;Hajjar et al, 2011), 45 Sc (Mowat et al, 2011), and others (He et al, 2014) can be used in order to detect the environment of the central metal atom. NMR spectroscopy also allows the study of host or guest interactions with adsorbed species (Wong et al, 2019;Witherspoon et al, 2018;Yan et al, 2017;Wittmann et al, 2019).…”