Crystallization of magnesium substituted aluminophosphate of type-36 as studied by solid-state NMR spectroscopy

The crystallization process of aluminophosphate molecular sieve MgAPO-36 is characterized by the evolution of intermediate gels. Multinuclear solid-state NMR techniques in combination with powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) reveal in detail the nature of the intermediate gels and their evolution into crystalline structure. The long-range ordering of the aged as well as heated intermediate gels are probed by PXRD, showing that the crystallization of the framework begins after heating the gels at 423 K for 1.5 h. 31P NMR indicated that the structural Al–O–P and Mg–O–P units from the ATS framework are formed at the same time. More information about the local ordering of the gels is obtained from two-dimensional 27Al → 31P HETCOR and 1H → 31P CP/MAS experiments. Two types of microstructure regions could be differentiated by partially and fully condensed P species, in which five types of P(nAl) (n = 2–4) units are identified by 31P/27Al double-resonance experiments. A possible evolution mechanism consisting of three stages is proposed for the crystallization process.