Site-specific deactivation pathways of mixed oxide catalyst in presence of oxygenates

Advancements in catalyst design and catalytic processes are critical to successfully develop new technologies for biomass conversion to aviation fuel. Understanding catalyst deactivation mechanisms and developing corresponding mitigation strategies are important to improve the catalyst lifetime which impacts process economics by affecting the cost of catalyst replacement and overall process efficiency. Using bioethanol as feedstock, we recently developed a pathway to produce sustainable aviation fuel via ketone intermediates. The mixed ketones (C₃-C₇ ketones) obtained in this process undergoes self and cross aldol condensation over Pd promoted MgO-Al₂O₃ (hydrotalcite) catalyst and generate a pool of C₈-C₁₈ ketones. While the catalyst demonstrated >100 h stability on time on stream when model ketones were used as feedstock, it rapidly deactivated with real ketone feedstock containing other small oxygenates. Herein we focused on understanding the potential deactivation modes of the Pd/MgO-Al₂O₃ catalyst in presence of different oxygenates. The role of different impurities in catalyst deactivation was studied by performing the condensation reaction in presence of selective impurities. Among the various oxygenates containing different functionalities such as ester, acid, ketones, or alcohol, only ester compounds demonstrated strong deactivation effect with complete deactivation observed within ~6 h. The presence of as low as 0.1 wt% of ester in the feedstock was enough to cause the complete catalyst deactivation. The product profile analysis indicated that ester molecules selectively deactivate the ketone condensation sites while direct hydrogenation on Pd site remained constant. Varying the carbon chain length of esters depicted a minor impact on the deactivation profile. Condensation activity was restored when the spent catalyst was treated at 500 °C under N₂ atmosphere indicating the deactivation was not due to the coke deposition as removal of coke generally needs oxidative atmosphere. Spectroscopic and microscopic characterization of the spent catalyst along with control reaction studies indicated that the deactivation is caused by strong adsorption of ester on the condensation site (base site). The underlying reasons for specific adsorption of ester as opposed to other oxygenates on MgO-Al₂O₃ surface will be discussed herein.