Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via ¹⁷O Solid-State NMR Spectroscopy

Silicoaluminophosphates (SAPOs) are microporous frameworks with Brønsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is necessary for understanding and expanding SAPO applications in heterogeneous catalysis. Solid-state ¹⁷O NMR (SSNMR) spectroscopy is an ideal tool to probe structure and formation of SAPO-based materials; the ¹⁷O quadrupolar and chemical shift interactions are exquisitely sensitive to local electronic and magnetic environments. In this work, a pure trigonal SAPO-34 molecular sieve synthesized via the dry-gel conversion (DGC) method was investigated using a combination of ¹⁷O magic-angle spinning, ¹⁷O triple-quantum magic-angle spinning, ¹⁷O­[²⁷Al] transfer of population in double-resonance, and ¹⁷O­[³¹P] rotational-echo double-resonance SSNMR spectroscopy, complemented by powder X-ray diffraction along with ²⁷Al, ²⁹Si, and ³¹P multinuclear SSNMR experiments. The four observed ¹⁷O resonances were simulated to extract NMR parameters, with each resonance assigned to individual oxygen local environments and connectivities in SAPO-34. The incorporation of oxygen from ¹⁷O-labeled water (i.e., H₂¹⁷O) during the DGC formation of pure trigonal SAPO-34 has also been investigated at various time intervals and stages of crystallization using ¹⁷O SSNMR. There is direct involvement of ¹⁷O-enriched water vapor during the DGC crystallization process of trigonal SAPO-34. The initial dry gel is amorphous, transforming to a crystalline layered AlPO₄ phase during the first hour of heating and then progressing to a semicrystalline phase after 4 h of heating; formation of the crystalline trigonal SAPO-34 product was found to be complete after 2 days