New insights into Keggin-type 12-tungstophosphoric acid from 31P MAS NMR analysis of absorbed trimethylphosphine oxide and DFT calculations.

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