Solid-state nuclear magnetic resonance study of the microporous aluminophosphate AlPO4-41

Highly crystalline aluminophosphate molecular sieve AlPO4-41 has been synth esized using dipropylamine as the templating molecule. Materials in the as- made, calcined never rehydrated, and calcined rehydrated forms have been ch aracterized by solid-state NMR spectroscopy using Al-27 and P-31 magic angl e spinning (MAS), two-dimensional Al-27 SQ-MAS, and Al-27 --> P-31 CP/MAS t echniques. Both P-31 MAS and Al-27 SQ-MAS spectra of the as-synthesized AlP O4-41 exhibit several resonances, all of them being assigned to framework a toms. Spectra are drastically modified after calcination. While the P-31 MA S spectrum of the calcined never rehydrated solid is composed of a single b road resonance at ca. -30.5 ppm, five signals can be observed after rehydra tion. Rehydration slightly modifies the unit cell parameters, particularly the b axis and gamma angle, but it is a completely reversible process. Al-2 7 NMR spectroscopy shows that water preferentially coordinates one of the f ive nonequivalent aluminum sites of the structure and transforms it into a six-coordinated species. The position of this site in the structure has bee n unambiguously determined from P-O-Al framework connectivities and confirm ed by H-1 --> P-31 CP/MAS experiments. The location of the amine in the por es of the as-synthesized AlPO4-41 was also estimated, suggesting that a spe cific interaction with one framework oxygen could account for the special s hape of the channels (monoclinic space group P112(1)).