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

Abstract

We present a detailed solid‐state NMR characterization of the molecular dynamics of tert‐butyl alcohol (TBA) confined inside breathing metal‐organic framework (MOF) MIL‐53(Al). 27Al MAS NMR has demonstrated that TBA adsorption induces the iX phase of MIL‐53 material with partially shrunk channels. 2H solid‐state NMR has shown that the adsorbed alcohol exhibits anisotropic rotations of the methyl groups around two C3 axes and librations of the molecule as a whole about the axis passing through the TBA C−O bond. These librations are realized by two distinct ways: fast molecule orientation change during the translational jump diffusion along the channel with characteristic time τD of about 10−9 s at 300 K; slow local librations at a single coordination site, representing framework hydroxyl groups, with τl≈10−6 s at 300 K. Self‐diffusion coefficient of the alcohol in the MOF has been estimated: D=3.4×10−10 m2 s−1 at 300 K. It has been inferred that both the framework flexibility and the interaction with framework hydroxyl groups define the dynamics of TBA confined in the channels of MIL‐53 (Al).

Thermodynamic, Thermal, and Structural Stability of Bimetallic MIL-53 (Al1–xCrx)

“…For MIL-53 (Cr), spherical particles (∼150 nm) slightly smaller than previously reported sizes are seen (see Figure h) . Such particle size decrease is typical for MOFs synthesized in acid-free environments. − In short, the samples with higher Al content tend to present better morphology. Such morphological variations as a function of cation composition were also observed on other bimetallic MOFs. − …”

“…Subsequently, the precipitate (∼1 g) was kept in 20 mL of DMF overnight to maximize the removal of unreacted BDC ligands. The product was further washed with 50 mL of MeOH to exchange the residual DMF followed by drying at 150 °C under vacuum for 12 h. MIL-53 (Cr) was synthesized according to the approach reported by Serre et al Specifically, Cr­(NO 3 ) 3 ·9H 2 O, H 2 BDC, and H 2 O with a molar ratio of 1:1:280 were mixed under vigorous stirring and transferred to a Teflon-lined stainless steel autoclave for hydrothermal synthesis at 220 °C for 72 h. The relatively high synthesis temperature used in this study may lead to formation of bulk oxides outside of MOF pores, and postsynthesis centrifugation was applied to maximize the oxide removal. , The bimetallic MIL-53 (Al 1– x Cr x ) samples ( x = 0.25, 0.5, and 0.75) were synthesized by using the same method as that of MIL-53 (Cr). The same purification and activation processes were used for all samples.…”

On phosphorus chemical shift anisotropy in metal (IV) phosphates and phosphonates: A 31P NMR experimental study

“…Here, the main attention of researchers is generally focused on interpretation of isotropic phosphorus chemical shifts, determined in 31 P MAS NMR experiments at high spinning rates. [ 6–18 ] These chemical shifts provide identification of phosphorus‐containing groups, including information about their local structures. Since the second NMR parameter, describing local electron environments of phosphorus nuclei, is the chemical shift anisotropy [ 19 ] ( 31 PΔ CS ), in the present work, we report 31 PΔ CS values measured for the layered compounds listed in Table 1, which were synthesized in our group and completely characterized earlier by solid‐state NMR.…”