Thermal Stability of Aluminum-Rich ZSM-5 Zeolites and Consequences on Aromatization Reactions

The thermal stability and unique shape selectivity of the ZSM-5 structure have made this zeolite a popular choice for gas-phase petrochemical and biorenewable conversions. However, with processes such as catalytic fast pyrolysis and methane aromatization being studied at temperatures between 500 and 800 degrees C, it is imperative to better understand the dynamic changes that may occur under these conditions. Here, we study the chemistry of high aluminum content commercial ZSM-5 zeolites in the high-temperature regime. Our results suggest these catalysts thermally degrade within hours above 600 degrees C, emphasizing the importance of operating conditions on long-term catalyst performance. Detailed characterization of the thermally treated zeolites indicates that they retained the desired MFI crystallographic structure but displayed significant changes in Bronsted and Lewis acid site densities due to extensive dealumination. Depending on temperature, up to 50% of the aluminum initially present in the zeolite structure was lost to form extra-framework species that restrict the diffusion of reactants and products inside the catalyst particles. These alterations led to a 70% drop in performance for the catalyzed fast pyrolysis of cellulose. Low aluminum content ZSM-5 zeolites were more stable, suggesting a compromise must be found between reaction temperature and catalyst features to achieve high activity and long-term stability.