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

Abstract

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.