Water Confinement in Faujasite Cages: A Deuteron NMR Investigation in a Wide Temperature Range. 1. Low Temperature Spectra

Abstract

Deuteron NMR spectra were measured for D2O confined in NaX, NaY, and DY faujasites with various D2O loadings at temperatures ranging from T = 70 K to T = 200 K with the aim to study the molecular mobility of confined water as a function of Si/Al ratio and loading. The recorded spectra were fitted with linear combinations of representative spectral components. At low loading, with the number of water molecules per unit cell close to the abundance of sodium cations, a component related to π-jumps of water deuterons about the 2-fold symmetry axis dominated. For loadings at levels 3 times and 5 times higher than the initial loading level, Pake dublets due to rigid water deuterons dominated the recorded spectra. A set of the quadrupole coupling constant values of localized water deuterons was derived from the analysis of the Pake dublets. Their values were attributed to deuteron positions corresponding to the locations at oxygen atoms in the faujasite framework and locations within hydrogen-bonded water clusters inside faujasite cages. The contributions of the different spectral components were observed to change with increasing temperature according to the Arrhenius law with a characteristic dynamic crossover point at T = 165 K. Below T = 165 K a spectral component was observed whose contribution changed with temperature, yielding the activation energy of about 2 kJ/mol, characteristic for jumps between inversion-related water positions in clusters.

Unexpected structural/motional mode of water intercalated into an α‐crystalline zirconium phosphate deduced by 31P and 2H solid‐state MAS NMR spectra

“…Potentially, only the 2 H MAS NMR experiments, [12,13] particularly the measurements of deuterium quadrupole coupling constants (DQCC), can provide reliable information in this context, which are very sensitive to the molecular mobility, decreasing in the presence of motions. [14,15] Herein, we report the 1 H, 31 P and 2 H solid-state MAS NMR study of the crystalline layered zirconium phosphate, Zr (HPO 4 ) 2 Á0.35D 2 O (1-D 2 O), which shows an unexpected structural/motional mode of the water.…”

Translational and Rotational Dynamics of Molecules Confined in Zeolite Nanocages by Means of Deuteron NMR

“…Therefore, results above and below it are labeled as HT and LT, respectively (Table ). The rotation of water molecules may be executed by tetrahedral jumps between inversion related orientations, proposed as responsible for effects observed in low-temperature spectra …”

“…Alternatively broad spectra below T S are measured with QE sequence. Molecules become localized at T S , and its value indicates the value of the binding strength to zeolite framework …”

“…Down to T S high mobility of molecules takes place and narrow spectra observed. Below T S , a substantial, stepwise broadening of deuteron spectra appears and magnetization recovery becomes nonexponential, 33 as moelcules are localized at fixed positions. Generally, NMR spectra are sensitive to molecular reorientations on the NMR time scale, 34,35 which is reflected in the condition for the narrowing of spectra expressed as δτ c ∼ 1, where δ is the width of a given spectrum and τ c is the temperature-dependent correlation time.…”

Dynamics of molecules confined in nanocages. A deuteron NMR study at high temperature

“…Below T S a substantial, step-wise broadening of deuteron spectra appears, and magnetization recovery becomes nonexponential. 12 Generally, NMR spectra are sensitive to molecular reorientations on the NMR time scale, 13,14 which is reflected in the condition for the narrowing of spectra expressed as δτ c ∼ 1, where δ is the width of a given spectrum and τ c is the temperature-dependent correlation time. For a typical width of a deuteron spectrum, δ = 135 kHz for C Q = 180 kHz, the condition δτ c ∼ 1 leads to the correlation time τ c = 10 −6 s. For τ c ≫ 10 −6 s deuterons are considered to be immobile and the Pake doublet is observed.…”