New Insights into Keggin‐Type 12‐Tungstophosphoric Acid from 31P MAS NMR Analysis of Absorbed Trimethylphosphine Oxide and DFT Calculations

The acid and transport properties of the anhydrous Keggin‐type 12‐tungstophosphoric acid (H₃PW₁₂O₄₀; HPW) have been studied by solid‐state ³¹P magic‐angle spinning NMR of absorbed trimethylphosphine oxide (TMPO) in conjunction with DFT calculations. Accordingly, ³¹P NMR resonances arising from various protonated complexes, such as TMPOH⁺ and (TMPO)₂H⁺ adducts, could be unambiguously identified. It was found that thermal pretreatment of the sample at elevated temperatures (≥423 K) is a prerequisite for ensuring complete penetration of the TMPO guest probe molecule into HPW particles. Transport of the TMPO absorbate into the matrix of the HPW adsorbent was found to invoke a desorption/absorption process associated with the (TMPO)₂H⁺ adducts. Consequently, three types of protonic acid sites with distinct superacid strengths, which correspond to ³¹P chemical shifts of 92.1, 89.4, and 87.7 ppm, were observed for HPW samples loaded with less than three molecules of TMPO per Keggin unit. Together with detailed DFT calculations, these results support the scenario that the TMPOH⁺ complexes are associated with protons located at three different terminal oxygen (O_d) sites of the PW₁₂O₄₀³⁻ polyanions. Upon increasing the TMPO loading to >3.0 molecules per Keggin unit, abrupt decreases in acid strength and the corresponding structural variations were attributed to the change in secondary structure of the pseudoliquid phase of HPW in the presence of excessive guest absorbate.