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 (PW12O40 3-) 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.

Synthesis of heteropolyacid (H3PW12O40)/SiO2 nanoparticles and their catalytic properties

“…In the case of the HPA/SiO 2 , the down field shifts of the peak and some line broadening were observed. This behavior is similar to that described earlier; it can be associated to a partial dehydration of the heteropolyacid [26][27][28]. During the immobilization of heteropolyacid (H 3 PW 12 O 40 ) into the silica by solgel reaction, heteropolyacid crystallites should lose some of their water molecules due to the interaction with silica support.…”

“…2 ) [26]. Therefore, the protonated water molecules of heteropolyacid are very important to its catalytic and electronic applications [26,39].…”

Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

“…The spectra of supported H 3 PW showed similar shifts to −13.2 ppm, as well as other broad signals at −11.5 and −11.0 ppm. As it is known in the literature, 35 the higher calcination temperature, the lower hydration degree of the Keggin structure, limited by stability temperature for the Keggin anion. Thus, anhydrous H 3 PW exhibited a single signal around −10 to −11 ppm.…”

Effects of Novel Supports on the Physical and Catalytic Properties of Tungstophosphoric Acid for Alcohol Dehydration Reactions

“…In general, two HPW peaks appear in the spectra, one at -15 ppm which corresponds to the intact Keggin structures and one at -13 to -14 ppm with a different structure (Fig. 3) [15,35]; (b) lacunary and/or dimeric species formed as a result of partial decomposition of HPW [36,37]; and (c) HPW species with different levels of hydration [38,39]. For the studies attributing the downfield shifts to lacunary and/or dimeric species of HPW, it is more often the case that partial decomposition of HPW had occurred during catalyst preparation.…”