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

Abstract

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 NH 4 -CBV712 of nominal Si/Al ratio of 30, 15, and 6, respectively. The proposed 1 H 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. 1 H 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 1 H 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” 27 Al and 29 Si MAS NMR approaches are also discussed in both hydrated and dried states of zeolites. 29 Si 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 27 Al 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 Q 4 0 peak correlates with increasing relative Al content, which is responsible for the distortion of zeolite structure.