Molecular identification and quantification of defect sites in metal-organic frameworks with NMR probe molecules

Abstract

The defects in metal-organic frameworks (MOFs) can dramatically alter their pore structure and chemical properties. However, it has been a great challenge to characterize the molecular structure of defects, especially when the defects are distributed irregularly in the lattice. In this work, we applied a characterization strategy based on solid-state nuclear magnetic resonance (NMR) to assess the chemistry of defects. This strategy takes advantage of the coordination-sensitive phosphorus probe molecules, e.g., trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO), that can distinguish the subtle differences in the acidity of defects. A variety of local chemical environments have been identified in defective and ideal MOF lattices. The geometric dimension of defects can also be evaluated by using the homologs of probe molecules with different sizes. In addition, our method provides a reliable way to quantify the density of defect sites, which comes together with the molecular details of local pore environments. The comprehensive solid-state NMR strategy can be of great value for a better understanding of MOF structures and for guiding the design of MOFs with desired catalytic or adsorption properties.

Structural and Acidic Characteristics of Multiple Zr Defect Sites in UiO-66 Metal–Organic Frameworks

“…The state-of-the-art probe-assisted NMR approaches have been widely demonstrated to be a reliable, sensitive, and practical tool for probing detailed local chemical structures as well as qualitative and quantitative acidic features (e.g., type, strength, distribution, and concentration) of active sites in various catalytic materials. ,,− Herein, keep in mind that different Zr defect sites normally manifest Lewis acidity with subtle distinctions. Meanwhile, the interaction of TMP molecule with various Lewis acid sites usually span a wide range of 31 P chemical shifts (δ 31 P) from ca.…”

Nanoanalytical Insights into the Stability, Intracellular Fate, and Biotransformation of Metal–Organic Frameworks

“…Note that the recoveries of <100% indicate the presence of defects in the MOF lattice. Defects have recently been shown to have pivotal importance in the (bio)­catalytic activity of MOFs , and might act as reactive sites during transformation reactions (vide infra).…”

Adsorption Characteristics of Refrigerants for Thermochemical Energy Storage in Metal–Organic Frameworks

“…This considerable discrepancy might potentially arise from crystal defects present in the experimental sample, which could simultaneously decrease framework density while increasing the pore size and volume within the adsorbent. These defects, typically in the form of missing organic ligands, have been extensively documented in existing literature. , To understand the disparity between experimental and simulated adsorption isotherms, we introduced different concentrations of defects into the UiO-66 model, as outlined in the methodology. In Figure d), a schematic depiction of full UiO-66 contrasts with a sample incorporating structural defects, illustrating their influence on the adsorption isotherm.…”