In situ FTIR Studies of Ethanol Conversion on Zr-KIT-5 and Zr-KIT-6 Catalysts

Zr-KIT-5 and Zr-KIT-6 have been found to be active for acid catalyzed reactions such as Prins reaction, Meerwein–Ponndorf–Verley (MPV) reduction, Hantzsch reaction, alkylation, alcohol dehydration, and lignin depolimerization. In this latter reaction, Zr-KIT-5 has been particularly found to be more active than Zr-KIT-6 (at the same level of Zr content) and commercial H-ZSM-5 (Si/Al=11.5) catalysts. Here, we have used model ethanol conversion on Zr-KIT-5, Zr-KIT-6, and H-ZSM-5 catalysts to assess their acid properties. In situ FTIR-MS temperature programmed desorption (TPD) of acid site probes (e.g., pyridine) and operando FTIR-GC during ethanol conversion at intermediate temperatures (300-450 °C) showed that Zr-KIT-6 and H-ZSM-5 catalysts are primarily composed of Lewis and Brønsted acid sites, respectively. On both catalysts, ethylene, the unimolecular dehydration product, was found to be the main reaction product; however, diethyl ether/acetaldehyde and diethyl ether/C4 olefins by-products formed as a result of alcohol bimolecular dehydration, alcohol dehydrogenation, and olefin dimerization on Zr-KIT-6 and H-ZSM-5, respectively. At the same Zr content, operando FTIR-GC of Zr-KIT-5 showed that this catalyst contains an acid site density significantly larger than that of Zr-KIT-6 and consequently resulted in a significantly higher catalytic activity. In situ pyridine-TPD and FTIR results showed apparent similar Lewis and Brønsted acid sites distribution, however, product selectivities with significant C4 olefin production resembled those of the H-ZSM-5 catalyst. These results point out to a synergy between Lewis and strong Brønsted acid sites in Zr-KIT-5. In situ FTIR suggested, however, that only a small fraction of these acid sites are responsible for ethanol conversion. Possible ethanol conversion mechanisms will be discussed.