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Characterization of Catalytic Materials through a Facile Approach to Probe OH Groups by Solid-State NMR

Andreev, Andrey S., Livadaris, Vincent
The Journal of Physical Chemistry C 2017 v.121 no.26 pp. 14108-14119
aluminum, ammonium, nuclear magnetic resonance spectroscopy, physical chemistry, qualitative analysis, silicon, stable isotopes, zeolites
A facile NMR approach based on dipolar filtering (DF) and spin echo (SE) is proposed to study commercially available dealuminated USY zeolites, such as H-CBV760, H-CBV720, and NH₄-CBV712 of nominal Si/Al ratio of 30, 15, and 6, respectively. The proposed ¹H DF-SE magic angle spinning (MAS) NMR approach provides the substantial suppression of water signal intensity in partly hydrated samples providing an opportunity to obtain the signal of other surface groups. ¹H DF-SE MAS NMR technique has been demonstrated its usability for quantitative (semiquantitative) analysis of dried, i.e., partly hydrated, samples. It is essential to use this approach when calcination under vacuum used as a reference procedure leads to drastic surface changes. Moreover, the technique is applicable for qualitative analysis of fully hydrated samples. This method is found to be extremely sensitive to the residual ammonium content in zeolite structure even in transformed to H form by calcination. Finally, the framework stacking faults species are found to be more pronounced in ¹H DF-SE MAS NMR spectra in hydrated state as ∼1 ppm peak that can be crucial for understanding of relationships of structure and performance. Additionally, the “standard” ²⁷Al and ²⁹Si MAS NMR approaches are also discussed in both hydrated and dried states of zeolites. ²⁹Si MAS NMR spectra demonstrate that a dependence on hydration state and the highest quantity of crystalline part is achieved in dried samples, whereas the best resolution of ²⁷Al MAS NMR spectra is obtained in a fully hydrated state. Finally, a local order of Si framework given by full width at half-maximum parameter of crystalline Q⁴₀ peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.