Multinuclear MAS NMR study of the microporous aluminophosphate AlPO 4-17 and the related silicoaluminophosphate SAPO-17

Abstract

AlPO 4-17 and SAPO-17 in their as-synthesized, calcined, and calcined and subsequently rehydrated forms have been studied by 27Al and 31P MAS NMR. Pronounced structural changes caused by template removal and rehydration can unambiguously be attributed to a change in the coordination number (on calcination: 5 → 4, on rehydration/dehydration: 4 ↔ 6) of part of the framework aluminium atoms. The different resonance lines can be assigned to crystallographically inequivalent sites present in the modified ERI framework. 27Al quadrupolar coupling parameters of the two aluminium sites in the calcined AlPO 4-17 ( C Q = 4.4 MHz and 2.1 MHz) were determined by recording spectra at different field strengths. The isotropic chemical shifts were obtained from rotation sidebands of the (± 3 2 , ± 1 2 ) satellite transitions. 2D 27Al nutation MAS NMR was used to corroborate the line assignment for the as-synthesized and the rehydrated AlPO 4-17. By using 13C MAS NMR it was shown that the occluded template (cyclohexylamine) is present as ions. For the first time, a splitting of the 29Si NMR line caused by crystallographically inequivalent sites in a SAPO-type material is reported. The line splitting of 4.3 ppm is comparable with that observed for isostructural aluminosilicate erionite.