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., H2 17O) 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.

Water‐Induced Structural Dynamic Process in Molecular Sieves under Mild Hydrothermal Conditions: Ship‐in‐a‐Bottle Strategy for Acidity Identification and Catalyst Modification

“…Surprisingly, the 17 O MAS NMR spectra exhibited in Figure 1 a clearly demonstrate two broad resonance signals at 21 and 48 ppm, attributing to the framework 17 O atoms in the Si‐O‐Al and P‐O‐Al species as the linking oxygen confirmed by 2D 17 O MQMAS spectrum (Figure 1 b). [29, 42, 43] These unexpected results imply that 17 O atoms from H 2 17 O are incorporated into molecular‐sieve framework, suggesting that part of the tetrahedrally coordinated structure of SAPO‐34 may undergo a dynamic T‐O‐T bonds breaking by hydrolysis when interacting with water vapor, but in a following step, dehydration reaction completely recovers the framework immediately. After this dynamic procedure, no terminal hydroxy groups are newly generated and the crystallinity remained unchanged, comparing with the untreated sample (Figure 1 c, Figure S3 and S4), while the incorporated and retained 17 O atoms on the steamed SAPO‐34 reveal the reversible hydration‐dehydration process (Figure 1 d).…”

17O NMR spectroscopy of crystalline microporous materials

“…52 This method has been extended to the synthesis of other microporous materials including phosphates, such as SAPO-18, SAPO-34 and AlPO-11, and MOFs, such as Al-MIL-53 and HKUST-1. 53,54,55,56,57 As DGC uses the minimum amount of solvent required for a successful synthesis, this makes it a potentially useful and cost-effective approach for producing 17 additional PTFE cup which is then placed inside the autoclave so that the reagents are not in direct contact with the liquid water, before the autoclave is sealed and heated under autogenous pressure at temperatures usually between 150-200 °C. 52,53,54,55 This approach often leads to uniform enrichment (as for example, seen for MIL-53, 25,54 where carboxyl and hydroxyl O species are both efficiently enriched).…”

“…For example, in the 17 O MQMAS spectrum of zeolite CHA in Figure 3d, 37 55 and to probe the cation distribution in SAPO-34. 56 In recent years, there has been growing interest in the use of calculations to predict NMR parameters alongside experiment to aid assignment and interpretation of spectral lineshapes. 9,10 In most cases, these are carried out using density functional theory (DFT), owing to the balance of efficiency and accuracy that it provides, with many studies performed using periodic planewave codes 90 to exploit the inherent translational symmetry of crystalline solids.…”

“…For microporous materials, it is important to accurately account for dispersion forces in the calculations, and semi-empirical dispersion correction schemes are often added to the commonly-used PBE functional. 10,91 In the absence of these, particularly flexible materials can adopt different and often much more open conformations than expected (as 25,51,54,56,66,92 There is, however, considerably more scope for closer interaction between 17 O NMR experiments and computation with recent improvements in hardware and software, and with the growing expertise within the scientific community. relationships (if present) can then be used to determine detailed structural information directly from an NMR spectrum.…”

Kinetic and Thermodynamic Studies for Zeolites Synthesis

“…The initial dry gel is amorphous, transforming to a crystalline layered phase during the first hour of heating and then progressing to a semicrystalline phase after heating; formation of the crystalline trigonal product was complete (Sutrisno et al, 2018). All types of zeolites with different properties were hydrothermally synthesized as in the procedure reported in the literatures (Rahman et al, 2009, Abdullah andShareef, 2015).…”