Abstract:

Solid-state NMR spectroscopy was employed to investigate the coordination state of P and Al, as well as their linkages in a series of newly synthesized alunilnophosphate molecular sieve AlPO4-5 with Al/P>1. Both 31P and 27Al NMR provides evidence that Al-O-Al linkages are present in the framework of these molecular sieves, which implies that the ordering arrangement of P and Al does not obey the well-known Loewenstein rule. Apart from the usually observed NMR signals, a broad peak at ca. δ=10 is present in the 27Al MAS spectra of the calcined and dehydrated AlPO4-5 molecular sieves. Due to its large second-order quadrupole shift, an isotropic chemical shift of ca. δ=38 was estimated. Hence, we attribute this signal to 4-coordinated aluminum: Al[nAl, (4-n)P )](n=1,2,3,4). Similarly, an additional broad signal at δ=-24 was observed in the corresponding 31P MAS spectra and is due to the P coordination state in which its second coordinated shell contains Al: P[nAl, (4-n)P]second shell (n=1,2,3,4).

Key words: AlPO₄-5, Al/P>1;Al-O-Al linkage, ³¹P and ²⁷Al NMR