Journal of Physical Chemistry B, Vol.111, No.25, 7105-7113, 2007
Crystallization of AlPO4-5 aluminophosphate molecular sieve prepared in fluoride medium: A multinuclear solid-state NMR study
In the present work, multinuclear solid-state NMR techniques, combined with powder X-ray diffraction (PXRD) and infrared (IR) spectroscopy, are employed to monitor the crystallization of AlPO4-5 aluminophosphate prepared in the presence of HF under hydrothermal condition. The crystallization process is characterized by the evolution of intermediate gels, in which the long-rang ordering arrangement is probed by PXRD, revealing the threshold of the crystallization around 120 min. The appearance of P-31 signals at ca. -22 and -29 ppm due to the structural P-O-Al unit and F-19 signal at -120 ppm due to the structural F-Al-pen-O-P unit in the NMR spectra of the series gels indicates that the crystalline framework is starting to form. The onset of the crystallization is also evidenced by the presence of the pentacoordinated Al in the structural F-Al-pen-O-P unit which is considered to be associated with the ordered framework. More information about the local ordering of the gels is obtained from two-dimensional Al-27 -> P-31 heteronuclear correlation (HETCOR) and P-31/Al-27 double-resonance experiments. In combination with H-1 -> P-31 cross-polarization/magic-angle spinning (CP/MAS) experiments, two microdomains can be identified in the 120 min heated gel. A possible evolution mechanism of the gels consisting of three successive stages is proposed for the crystallization process.