States and Dynamic Behavior of Protons and Water Molecules in H3PW12O40 Pseudoliquid Phase Analyzed by Solid-State MAS NMR

Abstract

The states and dynamic behavior of acidic protons and water molecules in solid H3PW12O40·nH2O (0 < n < 6), which would be closely related to its pseudoliquid phase catalysis, were quantitatively elucidated by the comprehensive application of 31P, 1H, and 17O magic-angle spinning (MAS) NMR. 1H and 17O MAS NMR were sensitive to the local environment in the pseudoliquid phase (e.g., hydrogen bonding), and 31P MAS NMR was effective especially to quantify the states of the protons. At 173 K, several peaks appeared in the 31P MAS NMR spectra that were reasonably assigned to phosphorus atoms in polyanions (PW12O403-) having different numbers of acidic proton(s) directly attached to them. Hence, the acidic protons reside either directly on the polyanions as “isolated acidic protons” or as H3O+ or H5O2+, and the amounts of these species were determined as a function of water content. The relative intensity of the 31P MAS NMR peaks obeyed binomial distribution for all the range of 0 < n < 6, which shows that the isolated acidic protons and protonated water molecules (H3O+ and H5O2+) are distributed uniformly (i.e., randomly) in the solid. The distribution of these species was well explained by the random removal of water from the solid bulk. At 298 K, the 31P MAS NMR peaks coalesced, revealing that the isolated acidic protons, which are the origin of the strong acidity in the pseudoliquid phase, migrate between the neighboring polyanions much faster than catalytic reactions. This is the first quantitative observation of the protons in hydrated heteropolyacids by spectroscopic methods.