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

Abstract

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.

Influence of Metal Doping on the Lewis Acid Catalyzed Production of Butadiene from Ethanol Studied by using Modulated Operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Mass Spectrometry

“…To investigate the reaction mechanism, we used an operando DRIFTS–MS setup that is presented in Scheme , which we described in detail previously …”

“…In this mechanism, an ethoxy group is formed on the open site that is exchanged with an incoming CA molecule to form a “crotoxy” intermediate with a distinct IR signal at =1690 cm −1 (Structure 3 in Scheme ). This intermediate is exchanged with EtOH again to form COH . This mechanism might also explain the observed shift in the Zr 3d XPS signals in the spectra of the spent catalysts.…”

“…Recently, we studied the Lewis acidic two‐step catalyst Ta‐BEA and put forward a molecular‐level description of the reaction mechanism . The mechanism over Zr‐BEA, however, has not yet been studied in detail.…”

Recent Breakthroughs in the Conversion of Ethanol to Butadiene

“…On the contrary, subsequent kinetic studies of the ETB reaction and the Guerbet reaction have reconfirmed the original pathways, albeit on other catalysts than MgO [17,18,22]. …”

“…Another parameter that influences the reaction is the catalyst pre-treatment, usually done under inert atmosphere at high temperature to activate the material. Finally, the ratio of ethanol-to-acetaldehyde being fed in a reactor is a crucial factor of the two-step process, as it can influence the BD productivity and product distribution [18,36,41,47,48]. Since several papers have reported different optimal ratios, it is likely that this factor is governed by the nature of the catalytic system and the other reaction conditions.…”