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

Highly crystalline aluminophosphate molecular sieve AlPO4-41 has been synthesized using dipropylamine as the templating molecule. Materials in the as-made, calcined never rehydrated, and calcined rehydrated forms have been characterized by solid-state NMR spectroscopy using Al-27 and P-31 magic angle spinning (MAS), two-dimensional Al-27 SQ-MAS, and Al-27 --> P-31 CP/MAS techniques. Both P-31 MAS and Al-27 SQ-MAS spectra of the as-synthesized AlPO4-41 exhibit several resonances, all of them being assigned to framework atoms. Spectra are drastically modified after calcination. While the P-31 MAS spectrum of the calcined never rehydrated solid is composed of a single broad resonance at ca. -30.5 ppm, five signals can be observed after rehydration. Rehydration slightly modifies the unit cell parameters, particularly the b axis and gamma angle, but it is a completely reversible process. Al-27 NMR spectroscopy shows that water preferentially coordinates one of the five nonequivalent aluminum sites of the structure and transforms it into a six-coordinated species. The position of this site in the structure has been unambiguously determined from P-O-Al framework connectivities and confirmed by H-1 --> P-31 CP/MAS experiments. The location of the amine in the pores of the as-synthesized AlPO4-41 was also estimated, suggesting that a specific interaction with one framework oxygen could account for the special shape of the channels (monoclinic space group P112(1)).