Mechanistic Study on the Lewis Acid Catalyzed Synthesis of 1,3-Butadiene over Ta-BEA Using Modulated Operando DRIFTS-MS

In recent years, the on-purpose production of 1,3-butadiene (BD) from renewable resources such as ethanol has received increased attention. In that context, Lewis acid catalysts play an important role, especially in the two-step process, i.e. when a mixture of acetaldehyde and ethanol is used as substrate. As the reaction mechanism consists of many intermediates and occurs over different catalytic functionalities, it is notoriously difficult to gain molecular-level insights into the mechanism. Here, we present a study on Lewis acidic Ta-BEA and propose a plausible reaction mechanism. We developed an operando DRIFTS-MS setup that allows for precise control and analysis of changes in the gas-phase composition as well as surface species. Using this tool, we found a surface intermediate with a vibrational frequency at 1690 cm(-1) that is only formed in the presence of both ethanol and crotonaldehyde and that is likely involved in the production of BD. Our data further suggests that a subtle control of the ratio of ethanol to acetaldehyde is crucial to keep a high ethoxy coverage and to desorb the intermediate crotyl alcohol in order to achieve high BD productivity. To the best of our knowledge, this is the first dynamic operando spectroscopic study on this re-emerging reaction.