Catalytic Transformation of Ethanol to 1,3-Butadiene over MgO/SiO2 Catalyst

Abstract Increasing concerns regarding global warming, which is caused by growing CO2emissions, have led to efforts focused on discovering alternatives to petroleum for energy and commodity chemical production. (Bio)ethanol has been seen as a platform molecule with increasing production and versatility for upgrading to various high-value fuels and chemicals. Among those high-value chemicals is 1,3-butadiene (1,3-BD), which has demonstrated widespread applications in polymer synthesis and as an organic chemistry intermediate. Its conventional methods of production rely on oil as a feedstock, hence suggesting the need for alternative and more sustainable routes. Interest in the catalytic conversion of ethanol to 1,3-BD, introduced in the 1940s by Lebedev, has been revived and is now focused on the development of selective catalysts, thus minimizing the need for the high cost separation between 1,3-BD and other (by)products, such as C2 and C4 olefins and oxygenates. The main components of the catalyst for this system are MgO and SiO2, where its reactivity and selectivity depend heavily on the method of preparation. This system is still at an early stage of development, with a lot of disagreements on structure of the catalyst, optimum ratio of Mg:Si, reactive intermediates, reaction mechanisms, and kinetics. Reaction mechanism was studied intensively using both theoretical (DFT) and experimental (spectroscopy) methods. Initial screening of the reaction mechanism using DFT with MgO defect site, i.e. kink, demonstrated that aldol condensation is more viable thermodynamically than Prins condensation. In the reaction mechanism, dehydrogenation of ethanol to acetaldehyde, an important reactive intermediate, is shown to be the rate-determining step (RDS) of the reaction. Comparison of the potential energy barrier also shows that acetaldol, the product of acetaldehyde self-aldolisation, dehydration competes with its hydrogenation with an ethanol molecule. This mechanistic study is also supported by comprehensive in-situ DRIFTS. MgO/SiO2 catalyst is synthesized using a wet-kneading method, with equivalent oxide mass ratio and thoroughly characterized with HS-LEIS, DRIFTS, and XRD. Chemical probing was also done with different probe molecules, such as pyridine, NH3, CO2, and methanol. Combination of several reactants and intermediate shows that acetaldehyde is spontaneously transformed to crotonaldehyde under constant reactant flow, while in-situ ethanol DRIFTS requires contribution from the gas-phase ethanol to make 1,3-BD. Furthermore, the crotonaldehyde does not transform to 1,3-BD under inert flow, it requires the presence of ethanol to complete the transformation to 1,3-BD. The resulting catalyst was extensively probed and characterized, revealing a silica-rich surface, where comparison with incipient wetness impregnation catalyst shows a rather Mg-rich surface. Surface silicate that is formed is confirmed by in-situ DRIFTS, where new OH groups were formed. The basicity of the catalyst also varies significantly with different methods of preparation and calcination temperature. All strong, medium, and weak basic sites were found on the catalysts surface. More superior performance, however, is shown to be enforced by lower amount of strong basic sites. Ammonia probing reveals the presence of both open and closed Lewis acid sites (LAS) and limited amount of Brønsted acid sites (BAS). Pyridine, on the other hand, could not identify any BAS, which is due to its larger molecule size. This further demonstrates that the LAS on the catalyst is much more accessible than the BAS.Promotion of the catalyst with transition metal was shown to have a significant enhancement on the reactivity. Since the RDS was determined to be the dehydrogenation of ethanol, transition metal sites lower this barrier, and shift the RDS. Zn and Cu, two very promising ethanol dehydrogenation catalysts, were separately impregnated on the uncalcined wet-kneaded MgO/SiO2 support at low loadings, 2.5 and 1%, respectively. The catalysts were thoroughly characterized using in-situUV-Vis, methanol operando DRIFTS, in-situ XANES and EXAFS, TEM, TPSR, and in-situ DRIFTS. Cu(II) exists as a surface species coordinated in a tetrahedral geometry, where it has 0.8 (or ~1) nearest neighbor, i.e. number of Cu-O-Cu bonds. The transition metal also possesses Cu-O-Mg bond, hinting to formation of solid solution. Similar interaction was also observed for Zn, suggesting the stronger interaction with Mg, instead of Si. This structural change affects the basicity and acidity of the catalyst. Both CO2 and methanol probing with DRIFTS show that the promoted catalysts have less affinity with CO2,while the BAS was eliminated, replaced with another distinct LAS. Redox capability was also modified, shown by the enhanced strength of the redox site in expense of its reduced quantity. During the reaction, Cu(II) is reduced to Cu(0) via an intermediate Cu species, before the catalyst deactivates after long hours of experiment. Zn, on the other hand, maintained its structure even after extensively tested.