²H Solid‐State NMR Spectroscopy Reveals the Dynamics of a Pyridine Probe Interacting with Coordinatively Unsaturated Metal Sites of MIL‐100(Al) Metal–Organic Frameworks

Abstract

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.

Elucidating structure and dynamics of crystalline α‐zirconium phosphates intercalated with water and methanol by multinuclear solid‐state MAS NMR: A comprehensive NMR approach

“…The 31 P{ 1 H}, 13 C{ 1 H}, The standard single pulse (direct nuclear excitation) and/or cross polarization (CP) pulse sequences were applied for both 31 P and 13 C nuclei at time delays needed for the full spin-lattice relaxation. The contact times of 5 ms and 6 ms were adjusted for the 13 C and 31 P nuclei, respectively, to observe the regular proton-carbon and proton-phosphorus CP MAS NMR spectra. 31 P CSA values were obtained through the full data analysis of the NMR spectra with a standard Bruker program package exampled in Figure S1.…”

“…[10,11] This technique is particularly useful for studies of intercalated phosphates, [12] where the behavior of deuterated guests can be well characterized by experiments on the 2 H nuclei. [13][14][15][16] Recently, by conducting 2 H and 31 P{ 1 H} solid-state MAS NMR experiments on a crystalline layered α-zirconium phosphate system, Zr (HPO 4 ) 2 Á0.35 D 2 O (1-D 2 O), including 31 P and 2 H T 1 measurements, [17] it has been revealed that there exists an unusual dynamic/bonding mode of the water, which is hydrogen bonded to the zirconium-coordinated oxygen and experiences a fast rotation (on the time scale of the NMR Larmor frequency) around this bond (Figure 1). At temperatures >253 K, the intercalated water unprecedentedly shows two spectrally distinguished deuterons.…”

“…In a continuation of this work, we report here a full study using 31 P, 2 H, 1 H, and 13 C solid-state MAS NMR techniques for the comprehensive characterizations of crystalline compounds 1-D 2 O-CD 3 OD and 1-CD 3 OD (Table 1), intercalated by water and/or methanol. As earlier, the choice of D 2 O is explained by the fact that the water is an important structural element of the α-zirconium phosphates.…”

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films

“…rotational motions of pyridine on Al 3+ or Al-OH sites or the effects of adsorbates on the backbone. 122,123 This has been also successfully employed in different frameworks, [124][125][126] sometimes being able to discriminate between competing adsorbing molecules of very similar nature. 127 Thus, beyond the use of well-known basic probe molecules to study MOF acidity (e.g.…”

Molecular Mobility of Tert‐butyl Alcohol Confined in a Breathing MIL‐53 (Al) Metal‐Organic Framework

“…Despite a notable number of the catalytic processes successfully tested for MOFs, [4b,7] the dynamics properties of the reactants involved remain unknown for the most of the cases. In fact, with a very few exceptions, [8] the experimental observation of the molecular mobility in MOFs was limited, so far, to relatively simple hydrocarbons [9] and small molecules such H 2 , [10] CO 2 [11] or water. [12] In addition, certain MOFs are capable of "breathing" by reversible change of the pore size upon guest molecules adsorption.…”