Aluminum Redistribution in ZSM-5 Zeolite upon Interaction with Gaseous Halogens and Hydrogen Halides and Implications in Catalysis

The impact of halogens (Cl-2 and Br-2) and hydrogen halides (HCl and HBr) on the structural stability of zeolite catalysts constitutes an important yet poorly understood aspect of their performance in methane halogenation and halomethane coupling, which are pivotal steps in the halogen-mediated upgrading of natural gas. By studying the interaction of the ZSM-5 zeolite (Si/Al = 43) as a representative system with halogens and hydrogen halides in the temperature range of halogenation and coupling reactions (573-823 K) using XRD, N-2 sorption, electron microscopy, and Raman, NMR, and ex situ and in situ FTIR spectroscopy, we showed that these compounds lead to dealumination of the framework and aluminum coordination with halogens without altering the crystalline structure. Hydrogen halides lead to more extensive dealumination than molecular halogens and especially HCl over HBr. Although oxidative regeneration restores the tetrahedral aluminum sites in the case of Cl-2, Br-2, and HBr treatments, the realumination of HCl-treated samples is limited, particularly following exposure at temperatures greater than 673 K. This is rationalized by stronger Al-Cl compared to Al-Br bonds that promote halogenation and hamper realumination. The regeneration of the HBr-treated zeolite leads to Br-2 evolution, enabling ZSM-5 to function as an HBr oxidation catalyst.