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Unraveling the nature of active extraframework Sn species in Sn-modified ZSM-5 by solid-state NMR
Zhili Wang#, Hongyu Chen#, Yi Ji*, Yuxiang Chen, Yongzhi Zhao, Xuesong Meng, Honghai Ji, Weiya Yang, Guangjin Hou*
https://doi.org/10.1016/j.cjsc.2026.101146
Zeolite; Sn Lewis acidity; Metal-modified ZSM-5; Probe molecule; Site proximity
ABSTRACT
Modification of zeolites with metals offers an effective strategy for tailoring their acidity and catalytic activity. However, the inherent complexity of the resulting active sites hinders in-depth characterization at the molecular level. Herein, Sn-modified ZSM-5 catalysts were prepared via impregnation and evaluated for syngas conversion. Correlating the catalytic performance of differently configured samples with 119Sn MAS NMR and UV-Vis results suggests that extraframework Sn species (SnOx) act as active sites, leading to higher CO conversion and ethane selectivity. Pyridine-probed NMR spectroscopy confirms that these extraframework Sn species introduce additional Lewis acid sites (LAS). This finding is further corroborated by pyridine-adsorption FTIR, which shows a notable increase in the LAS/BAS ratio and identifies new Sn-related LASs with considerable acid strength. Notably, the superior resolution of 31P NMR spectroscopy using trimethylphosphine (TMP) as a probe molecule enables the discrimination of two distinct Lewis acidic Sn sites with different acid strengths. Moreover, 2D 31P-31P DQ-SQ correlation NMR experiments demonstrate that the active extraframework SnOx species possess two plausible forms: isolated Sn and aggregated Sn–O–Sn species within the zeolite. This work provides valuable insights into the active nature of extraframework Sn sites in Sn-modified ZSM-5, thereby contributing to the establishment of structure-property relationships for this catalytic system.
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