Direct Observation of the Mesopores in ZSM-5 Zeolites with Hierarchical Porous Structures by Laser-Hyperpolarized ¹²⁹Xe NMR

Abstract

Mesopore-modified ZSM-5 zeolites with hierarchical porous structures (Meso-ZSM-5) have been synthesized by using tetrapropylammonium hydroxide and starch as cotemplates. One- and two-dimensional 129Xe NMR spectroscopy has been employed to study the porosity under the continuous flow of laser-hyperpolarized xenon gas. Besides the micropores, the mesoporous domains in Meso-ZSM-5 zeolites are directly observed by variable-temperature experiments. Combining with nitrogen adsorption, the influence of the Si/Al ratios on the mesopores in Meso-ZSM-5 is also investigated. The exchange of Xe atoms in different types of pores is very fast at ambient temperature. Two-dimensional exchange spectroscopy (EXSY) is used for the first time to monitor such an exchange process, and the results indicate that even at very low temperature Xe atoms still undergo much faster exchange between mesopores and micropores in Meso-ZSM-5 than in the mechanical mixture of conventional ZSM-5 and mesoporous silica. The results demonstrate that these hierarchical pores may have good connected networks that facilitate xenon diffusion and exchange. Also, this may give some indications for other molecules adsorption and diffusion in mesoporous zeolites especially in the process of catalysis.