Oxygen coordination of aluminum cations in dehydrated AlPW12O40 investigated by solid-state NMR spectroscopy

Dehydrated aluminumdodecatungstophosphate (AlPW12O40) was investigated by 27Al MAS NMR spectroscopy in magnetic fields of 9.4 and 17.6 T. Two kinds of octahedrally coordinated (δ27Al = 1–2 ppm, CQ = 2.4–2.5 MHz, and δ27Al = 4–6 ppm, CQ = 6.0–6.5 MHz), two kinds of penta-coordinated (δ27Al = 24–30 ppm, CQ = 7.0–7.3 MHz, and δ27Al = 44 ppm, CQ = 7.5–8.7 MHz), and a small amount of tetrahedrally coordinated aluminum cations (δ27Al = 65 ppm, CQ = 8.2–8.5 MHz) were found. Comparison with 1H MAS NMR spectroscopic studies indicates that the dehydroxylation of aluminum OH groups (δ1H = 4.2 ppm and 5.6 ppm) at temperatures of 473 and 573 K is accompanied by a partially irreversible change of the oxygen coordination of aluminum cations.