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

Abstract

Solid‐state NMR experiments on 2H, 31P, 13C, and 1H nuclei, including 31P T1, 1H T1, and 1H T1ρ measurements, as well as on the kinetics of proton‐phosphorus cross‐polarization have been performed to characterize the crystalline and amorphous α‐zirconium phosphates, which were intercalated with D2O and/or CD3OD. The 13C{1H} CP MAS NMR experiment performed for compound 1‐CD3OD (Zr (HPO4)2.0.2CD3OD) with carbon cross‐polarization via protons of phosphate groups has provided a prove that the methanol was intercalated into the interlayer spaces of this compound. The variable‐temperature 2H solid‐echo MAS NMR spectra of intercalated compounds demonstrated that the methanol molecules, in contrast to the mobile water, were immobile, keeping, however, free CD3 rotations around the C3‐axis. It has been demonstrated that the intercalated species, D2O and CD3OD, do not affect the high‐frequency motions of the phosphate groups. By utilizing local structural models that satisfy the constraints of the experimental data, it has been suggested that the immobile methanol molecules are located in the cavity between two neighboring layers of the zirconium phosphates. Thus, the present work illustrates the reliable criteria in a comprehensive NMR approach to structural and dynamic studies of such systems.

Hydronium Ions on the Surface of a Partially Hydrolyzed α-Zirconium Phosphate: Solid-State 2H and 31P MAS NMR Evidence

“…These magnitudes are very similar to those reported earlier for crystalline or partially disordered deuterium-free zirconium phosphates. 3 Thus, the CP experiments do not support the presence of DPO 4 groups in the sample. As in the case of the zirconium phosphates studied early, 3 the phosphorus spin−lattice relaxation in compound 1-D 2 Oh/dr can be represented by a stretched exponential, f(t) = exp(−t/T 1 ) β , showing a relatively short 31 P T 1 time (Table 1), typical for deuterium-f ree partially disordered zirconium phosphates.…”

“…The 31 P{ 1 H} MAS NMR spectrum of 1-D 2 Oh/dr in Figure 3B (top) shows only one resonance at δ(iso) = −22.5 ppm, which corresponds well to the water-free zirconium phosphate groups. 3,14 Thus, the potential possibility of the species H 2 PO 4 − and PO 4 3− manifesting phosphorus resonance signals around ≤−10 and ≤−30 ppm, respectively, 22−24 can be ruled out.…”

“…It has previously been reported that the presence of DPO 4 groups can be masked in 31 P MAS NMR by fast proton−deuterium exchange between the HPO 4 and DPO 4 species. 3 In addition, 31 P chemical shifts, characterizing the interlayer and surface phosphate groups, can be undistinguished. This is particularly important when, for example, only the surface phosphate groups contain deuterium.…”

“…Nevertheless, as has been generally accepted, the hydrolysis is initiated by a direct hydrolytic attack of the surface phosphate groups, yielding Zr­(H 2 O) and Zr­(OH) species after the loss of phosphate groups. Here we report the solid-state 1 H, 2 H, and 31 P­{ 1 H} magic-angle-spinning (MAS) NMR data collected for a partially hydrolyzed α-zirconium phosphate, 1-D 2 Oh , and its completely dried analogue, 1-D 2 Oh/dr , prepared by particular methods with structure disorders for 36–38% (see the Supporting Information, SI). Our results have led to the first experimental observation of D 3 O + ions in zirconium phosphates, which is important for understanding their catalytic properties. − We have used D 2 O for the experiments to effectively apply the solid-state 2 H NMR technique, − particularly its MAS version.…”

“…As mentioned above, a possible proton–deuterium exchange between D 2 O and the acidic protons of the phosphate groups can lead to the presence of zirconium D PO 4 groups [see possibility (a)]. It has previously been reported that the presence of D PO 4 groups can be masked in 31 P MAS NMR by fast proton–deuterium exchange between the H PO 4 and D PO 4 species . In addition, 31 P chemical shifts, characterizing the interlayer and surface phosphate groups, can be undistinguished.…”