Solid-State Nuclear Magnetic Resonance Study of the Microporous Aluminophosphate AlPO₄-41

Highly crystalline aluminophosphate molecular sieve AlPO 4 -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 27 Al and 31 P magic angle spinning (MAS), two-dimensional 27 Al 5Q-MAS, and 27 Al → 31 P CP/MAS techniques. Both 31 P MAS and 27 Al 5Q-MAS spectra of the as-synthesized AlPO 4 -41 exhibit several resonances, all of them being assigned to framework atoms. Spectra are drastically modified after calcination. While the 31 P 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 γ angle, but it is a completely reversible process. 27 Al 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 1 H → 31 P CP/MAS experiments. The location of the amine in the pores of the as-synthesized AlPO 4 -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 P 112 1 ).