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

Abstract

As a result of the increasing gap between the supply and demand of butadiene, the catalytic coupling of ethanol has regained the attention of the scientific and industrial community as an “on‐purpose” production route to butadiene. The most promising systems are based on bifunctional catalysts that comprise metal sites that can dehydrogenate ethanol to acetaldehyde and Lewis acid sites that catalyze the aldol condensation between two aldehyde molecules and the Meerwein–Ponndorf–Verley reduction of the intermediate crotonaldehyde. Here, we investigate the role of Ag in an established Ag‐Zr‐BEA catalyst using modulated operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry experiments. We obtain insights into the complex reaction network that involves several consecutive and parallel reactions. Based on our investigations, we formulate suggestions for catalyst optimization.

Mechanistic Insights into One-Step Catalytic Conversion of Ethanol to Butadiene over Bifunctional Zn–Y/Beta Zeolite

“…As a very demanding reaction, the ETB conversion involves several key reaction steps, and each reaction step is catalyzed by a certain type of functional site. According to recent studies, Ag promoted Zr-Beta or Ta-Beta catalysts could also exhibit good activity in the ETB conversion. ,,, For discussing the difference in the reaction mechanism, the properties of the Zn–Y/Beta and Ag–Zr/Beta catalysts are first compared. The most significant difference between these two catalyst systems lies in the nature of the active sites and their interactions.…”

“…Through extensive debates in the past decades, several key reaction steps are now generally accepted (Scheme ), including (i) the dehydrogenation of ethanol to acetaldehyde, (ii) the aldol condensation of acetaldehyde to acetaldol, (iii) the dehydration of acetaldol to crotonaldehyde, (iv) the Meerwein–Ponndorf–Verley (MPV) reduction of crotonaldehyde to crotyl alcohol, and (v) the dehydration of crotyl alcohol to butadiene. , On the basis of this reaction route, two independent catalytic cycles of a one-step process were proposed by Sushkevich and Ivanova, i.e., dehydrogenation of ethanol into acetaldehyde over metal sites and acetaldehyde/ethanol transformation into butadiene at Lewis acid sites . This is in line with the recent reports of Hermans and co-workers, where they suggested that Lewis acid sites in Zr-Beta or Ta-Beta catalysts were responsible for the acetaldehyde coupling, , while Ag metal only promoted the ethanol dehydrogenation to acetaldehyde . In addition, Zn cations in the talc-Zn-modified catalysts were proved to accelerate the rate of ethanol dehydrogenation to acetaldehyde but had no influence on the formation of crotonaldehyde …”

Ethanol-to-butadiene: the reaction and its catalysts

“…Surface ethoxy species were also detected on other transition metal oxide catalysts during IR-TPSR experiments with ethanol, leading to similar conclusions. [118][119][120] It should be noted that studies of the sort have observed that surface ethoxy species are also intermediates in the formation of ethylene from ethanol. 121 Therefore, ethoxy species must be correlated with the formation of acetaldehyde alone to confirm their involvement in mechanisms found in Scheme 4.…”

Recent Advances in Catalysts for the Conversion of Ethanol to Butadiene

“…Using operando diffuse reflectance infrared Fourier transform spectroscopy and mass spectrometry, Muller et al found that Zr and Ta sites on Beta are isolated and are predominantly Lewis acidic. [124] These metals provided the essential active sites for the coupling and dehydration steps. Two-dimensional (2-D) layered zeolites with uniform microporous layers and high external surface areas combine the advantages of zeolites and mesoporous materials.…”