Unravelling the Reactivity of Framework Lewis Acid Sites towards Methanol Activation on H‐ZSM‐5 Zeolite with Solid‐State NMR Spectroscopy

Abstract

Understanding the acid sites on zeolites is vital for the establishment of catalyst structure–activity relationship and exploration of their potential applications in heterogeneous catalysis. Here, we report the identification of active framework Lewis acid sites on ZSM‐5 zeolites. The structures of framework‐associated tri‐coordinated Al that is bonding with hydroxyl groups are determined by using one‐ dimensional (1D) 31P and two‐dimensional (2D) 31P‐{27Al} NMR spectroscopy of trimethylphosphine oxide probe molecule. 2D 13C‐{27Al} NMR correlation experiments allow the observation of favorable formation of methoxy species on the framework‐associated Al Lewis acid sites in methanol reaction at low temperature, which is corroborated by density functional theory calculations. These methoxy species contribute to the further conversion of methanol to hydrocarbons as active C1 species. The results provide new insights into the Lewis acidity of zeolites.

Accurate Measurements of NH3 Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

“…Various zeolites also exhibit FAAL species. Through ammonium hexafluorosilicate (AHFS) washing of MFI-12.5, BEA-15, FER-20, and MOR-10 was conducted to remove the FAAL species, 20 which was confirmed by the decrease in OH groups observed in the infrared (IR) spectra and 27 Al MAS NMR spectra (Figures S12 and S13). The flow-pulse NH 3 adsorption microcalorimetry on the AHFS-treated zeolites at 150 °C was measured.…”

“…19 It is noteworthy that these FAAL species commonly convert into frameworkassociated octahedral or distorted tetrahedral Al species in nondehydrated zeolites. 20 The Brønsted/FAAL Lewis acid synergy was not observed in previous NH 3 adsorption microcalorimetry measurements. 21,22 Adsorption microcalorimetry is a powerful technique to comprehensively and accurately characterize the acid distribution of zeolites.…”

“…Early microcalorimetric measurements and theoretical calculations revealed that an increase in Si–O–Al bond angles and more constrained small micropores induced larger NH 3 adsorption heats on BAS of different zeolites with at a fixed Si/Al ratio. , Meanwhile, NH 3 adsorption microcalorimetry measurements showed that a high-temperature dehydroxylation of Y zeolites results in acid sites with enhanced ammonia adsorption heats, which were associated with the formation of Lewis acidic extra-framework Al (EFAL) species. , Then, such a Brønsted/EFAL Lewis acid synergy was commonly observed to enhance the strength of BAS in various zeolite frameworks by solid-state NMR spectroscopy. , Recently, using solid-state NMR spectroscopy, a spatial proximity of BAS and tricoordinated framework-associated aluminum (FAAL) species was also observed capable of generating a small amount of super acid sites (SAS) in dehydrated H-ZSM-5 zeolite . It is noteworthy that these FAAL species commonly convert into framework-associated octahedral or distorted tetrahedral Al species in nondehydrated zeolites . The Brønsted/FAAL Lewis acid synergy was not observed in previous NH 3 adsorption microcalorimetry measurements. , …”