Ca/ZSM-5 catalysts for the methanol-to-hydrocarbons reaction: Activity – Selectivity trade-off?

Establishing structure–activity relationships for the methanol-to-hydrocarbons reaction can help to identify better catalysts and optimal reaction conditions. Modification of ZSM-5 catalysts with alkaline-earth cations such as Ca increases the selectivity to C₃₊ olefins, which has been linked to a change in the effective pore geometry, affecting the formation of particular hydrocarbon pool (HCP) components. To further understand the catalytic properties of Ca/ZSM-5, we estimated the methanol conversion rate constants and deactivation coefficients. The kinetic experiments were carried out at different contact times with and without water in the feed. Ca/ZSM-5 is substantially less active than HZSM-5 and Na/ZSM-5 in a wide range of conditions. The decreased activity can be correlated to a different composition of the HCP due to the presence of Ca, which is supported by step-response IR experiments using various feed mixtures (methanol, methanol + water, water). Ca/ZSM-5 converts methanol significantly slower than HZSM-5 and Na/ZSM-5 but with a higher C₃₊ olefins selectivity, pointing to the delicate balance between activity and selectivity in these materials.