Water Dynamics in Na₃PW₁₂O₄₀·nH₂O:  A ²H-NMR and ³¹P-NMR Investigation

Abstract

The dynamics of water molecules adsorbed in the secondary lattice structure of sodium dodecatungstophosphate (Na 3 PW 12 O 40 ‚nH 2 O) has been investigated by 31 P-NMR and 2 H-NMR at different temperatures and hydration levels (n). 2 H-NMR spectra show that for T e 173 K only one type of adsorbed water is present. In the temperature interval between 173 and 293 K three differently adsorbed and mutually exchanging water components are observed. The chemical exchange range and population ratios of the differently chemisorbed water molecules were determined by computer simulation of 2 H-NMR spectral profiles. The activation energy of the exchange process and the shape of 2 H-NMR spectra indicate that the different water components correspond to twice hydrogen bound, mono hydrogen bound, and not bound D 2 O molecules. A further investigation of samples at different hydration levels has been performed, at room temperature (293 K), by 31 P-NMR spectroscopy. The line width (∆(H)) and the longitudinal relaxation time (T 1 ) of the 31 P lines have been measured using both H 2 O and D 2 O for hydration. The experimental data have been used to estimate the contribution to ∆(H) and T 1 due to the protons-phosphorus dipolar interactions: ∆(H) and T 1 (H) respectively. These parameters, combined with the values of population ratios of the differently adsorbed water molecules, allow knowledge of the correlation times of the water molecules' motions.

Guest Controlled Rotational Dynamics of Terephthalate Phenylenes in Metal–Organic Framework MIL-53(Al): Effect of Different Xylene Loadings

“…2 H NMR spectroscopy is a well-established and versatile tool to probe experimentally molecular mobility in different physical states including liquids, 28 molecular crystals, 29−32 polymers, 33−35 and acid catalysts. 36,37 It is especially adapted to study the dynamical phenomena in systems based on porous materials such as zeolites 38−40 or metal−organic frameworks. 16,18,20 2 H NMR allows us to probe the dynamics of deuterated molecules occluded in the porous materials within a broad time scale, ranging between 10 −3 and 10 −10 s. Here we demonstrate that transitions between LP and NP crystalline states in MIL-53(Al) can be displayed and controlled by analyzing the evolution of rotational dynamics of terephthalate phenylenes with temperature.…”

Dynamic studies of Ormosil membranes

“…They should correspond to the presence of two kinds of water molecules with different motions, or also some charged deuterons clusters (D 3 O + , D 2 O þ 5 ) as it was also observed by deuterium NMR of pure POM [28][29][30]. At 245 K, half of the remaining signal disappears, and a much broader signal appears.…”

“…Similar behavior was also already reported in pure hydrated and deuterated POM [28], and associated to the presence of mobile deuterons (mobile water), mobile [D 3 O] + and immobile deuterons (water molecules without mobility). The ratio of these different species is related to the hydration ratio of the POM [29,30].…”

The Keggin-type potassium/hydronium 12-tungstophosphate, K2.4(H3O)0.6PO4W12O36

“…Heteropoly compounds of tungsten and molybdenum and their salts, such as M 3 PO 4 W 12 O 36 (M = H + , NH 4 + , K + ; Ito et al, 1997;Uchida et al, 2000;Benedetto et al, 1996;Song et al, 2002), K 3 PO 4 Mo 12 O 36 and (NH 4 ) x K y PO 4 Mo 12 O 36 (Boeyens et al, 1976) have drawn much attention as catalysts owing to their versatility and environmental compatibility. Many scientists have made great efforts to investigate their applications in industry and pharmaceutical chemistry.…”