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The metal–organic frameworks (MOFs) attract interest as potential catalysts whose catalytic properties are driven by defects. Several methods have been proposed for the defects‐inducing synthesis of MOFs. However, the active species formed on the defective sites remain elusive and uncharacterized, as the spectroscopic fingerprints of these species are hidden by the regular structure signals. In this work, we have performed the synthesis of ZIF‐8 MOF with defect‐inducing procedures using fully deuterated 2‐methylimidazolate ligands to enhance the defective sites′ visibility. By combining 1H and 31P MAS NMR spectroscopy and X‐ray absorption spectroscopy, we have found evidence for the presence of different structural hydroxyl Zn−OH groups in the ZIF‐8 materials. It is demonstrated that the ZIF‐8 defect sites are represented by Zn−OH hydroxyl groups with the signals at 0.3 and −0.7 ppm in the 1H MAS NMR spectrum. These species are of basic nature and may be responsible for the catalytic activity of the ZIF‐8 material. [ABSTRACT FROM AUTHOR]

Kinetics of H/D hydrogen exchange between deuterated isobutane‐d10 and Brønsted acid sites (BAS) of three zeolite samples (H‐BEA, ZnO/H‐BEA, Zn2+/H‐BEA) were monitored with 1H MAS NMR in situ at 343–468 K. The regioselective H/D exchange in the methyl groups detected on H‐BEA can be rationalized in terms of the mechanism of indirect exchange, which involves protonation of the intermediate olefin and further hydride abstraction from the other alkane molecule by the formed carbenium ion. Loading of Zn species in the zeolite results in a decrease of the rate and an increase of the activation energy of the exchange. The loaded Zn species provide the tuning effect on the reaction occurrence, changing the mechanism from the indirect one to the mechanism of the direct exchange. [ABSTRACT FROM AUTHOR]

To clarify the issue on Brønsted acidity enhancement by the presence of zinc species in H‐ZSM‐5 zeolite (G.D. Qi, et al. Angew. Chem.‐Int. Edit. 2016, 55, 15826), the influence of Zn2+ cationic species, introduced into the zeolite, on the strength of Brønsted acid sites (BAS) has been analyzed by solid‐state NMR and FTIR techniques using trimethylphosphine (TMP) and CO as probe molecules. Comparative analysis of the acidity for H‐ZSM‐5 and Zn2+/H‐ZSM‐5 zeolites has been performed. TMP adsorbed on both zeolite samples exhibits same signals at 6.3 and −4.5 ppm in 1H and 31P MAS NMR spectra, respectively, assigned to protonated TMP. FTIR spectroscopy has shown that both zeolite samples, H‐ZSM‐5 and Zn2+/H‐ZSM‐5, display the same red frequency shift (ΔνOH⋅⋅⋅CO=−318 cm−1) of the O−H stretching band and the same blue frequency shift (ΔνCO=+32 cm−1) of the CO stretching band for CO adsorption complex with BAS. The results obtained do not provide any evidence for the existence of some specific BAS with enhanced acid strength for Zn2+‐exchanged zeolite compared to the strength inherent to pure acid‐form zeolite. It is thus inferred the strength of BAS in H‐ZSM‐5 zeolite is not affected by the modification of the zeolite with Zn2+ cationic species. [ABSTRACT FROM AUTHOR]

Coordinatively unsaturated metal sites (CUS) play an important role in catalysis by metal‐organic frameworks (MOF). Being an intrinsic part of the framework the CUS take the role of acidic sites active in industrially relevant processes such as condensation or oxidation reactions. The key step of such reactions represents the coordination of the reagents to CUS. In MOFs the mechanism of the reagent interaction with CUS is poorly understood. Herein, we characterize the interaction of a widely used acidity probe pyridine with CUS of MIL‐100(Al) MOF by means of the 2H solid‐state NMR spectroscopy. 2H NMR reveals that pyridine species, which are interacting with CUS and the ones which are coordinated to the Al−OH site, exhibit different motional behavior. 2H NMR line shape as well as T1, T2 relaxation analyses for [D5]pyridine adsorbed in MIL‐100(Al) allowed us to perform a detailed characterization of pyridine dynamics in both states including the kinetics of the exchange process between these adsorption states. [ABSTRACT FROM AUTHOR]