In Situ Multinuclear Magic-Angle Spinning NMR: Monitoring Crystallization of Molecular Sieve AlPO4-11 in Real Time

Molecular sieves are crystalline three-dimensional frameworks with well-defined channels and cavities. They have been widely used in industry for many applications such as gas separation/ purification, ion exchange, and catalysis. Obviously, understanding the formation mechanisms is fundamentally important. Highresolution solid-state NMR spectroscopy is a powerful method for the study of molecular sieves. However, due to technical challenges, the vast majority of the high-resolution solid-state NMR studies on molecular sieve crystallization are ex situ. In the present work, using a new commercially available NMR rotor that can withhold high pressure and high temperature, we examined the formation of molecular sieve AlPO4-11 under dry gel conversion conditions by in situ multinuclear (H-1, Al-27, P-31, and C-13) magic-angle spinning (MAS) solid-state NMR. In situ high-resolution NMR spectra obtained as a function of heating time provide much insights underlying the crystallization mechanism of AlPO4-11. Specifically, in situ Al-27 and P-31 MAS NMR along with H-1 -> P-31 crosspolarization (CP) MAS NMR were used to monitor the evolution of the local environments of framework Al and P, in situ H-1 -> C-13 CP MAS NMR to follow the behavior of the organic structure directing agent, and in situ H-1 MAS NMR to unveil the effect of water content on crystallization kinetics. The in situ MAS NMR results lead to a better understanding of the formation of AlPO4-11.