Kinetic analysis of Ag particle redispersion into ZSM-5 in the presence of coke using in situ XAFS

Abstract

In the present study, the redispersion behavior of Ag particles on ZSM-5 in the presence of coke was observed using in situ X-ray absorption fine structure (XAFS) spectroscopy. The dynamics of Ag particle redispersion into the ZSM-5 pores were evaluated by linear combination fitting (LCF) of the X-ray absorption near edge structure (XANES) spectra. In addition, the behavior of coke combustion in Ag-ZSM-5 was observed by thermogravimetric analysis (TGA). The rate constant of Ag particle redispersion was smaller than that of coke combustion at ≥500 °C, but it was three times larger than that of coke combustion at <500 °C. According to ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR), the Ag⁺ ions in the ZSM-5 pores improved the hydrothermal stability of Ag-ZSM-5. Therefore, a two-step regeneration process, in which the Ag particle redispersion proceeded first at low temperature (<500 °C) followed by a complete combustion of the coke at high temperature (≥500 °C) to prolong the catalyst lifetime, was used.