ZSM-5 zeolites modified with Zn and their effect on the crystal size in the conversion of methanol to light aromatics (MTA)

Abstract

ZSM-5 zeolite catalysts modified with zinc were prepared by two forms of Zn incorporation the synthesis gel, and ion exchange techniques. The physico-chemical properties of zeolites were studied by XRD, N2-adsorption, NH3 temperature-programmed desorption, 27Al and 29Si MAS NMR, SEM, TEM and TGA. ZSM-5 zeolite in its acid form was exchanged using an aqueous zinc salt solution and demonstrated a significantly higher selectivity for the aromatic products in comparison with the purely acidic catalysts. The samples with distribution of ZnOH+ species are more active than the samples with ZnO sites in the zeolites. The synthesis of zeolite ZSM-5 of nanometric size resulted to present high stability and selectivity towards light aromatics. The influence of the form of zinc incorporation, the acidity and the reaction temperature had a great influence on the catalytic activity. The MTA catalyst lifetime is increased by several times due to the enhanced mesoporosity and decreased acidity. In the present work the zeolite HZSM-5 exchanged with Zn with Si/Al 25 ratio presented conversions close to 100% methanol with 32% selectivity to the BTX fraction, however, this catalyst was deactivated after 8 h of reaction with a weight hourly space velocity of 4.74 h−1 at 450 °C. On the other hand, a HZSM-5 zeolite with nanoscale crystals was found to be more stable in the MTA reaction. The nanometric catalyst showed conversions around 100% methanol after 8 h of reaction and 32.5% selectivity to the BTX fraction to 450 °C. These results clearly indicate that crystal size significantly influence the ZSM-5 lifetime and product distribution.