Introduction
Butadiene is one of the most important bulk chemicals produced in the petrochemical industry. For instance, it is used as a monomeric compound for synthetic rubber. Currently, butadiene is mainly obtained from petrochemical sources, as a byproduct of the naphtha steam cracking process during ethylene production.[1] Development of a cheap and sustainable process for its production from biomass-based resources would result in reduced reliance on oil resources. In this context, the possible catalytic conversion of ethanol-to-butadiene has attracted much interest. Obviously, the choice of catalysts has a strong effect on the rate of butadiene formation, selectivity, and yield. As such, many different catalysts have been explored in recent years.[2-10] Among them, the MgO/SiO2 catalyst has been widely studied.[6]
Baltrusaitis et al. investigated the structure and reactivity of MgO/SiO2 focusing on the role of weak and strong basic and acidic sites on the surface.[6] Zhang et al. have studied the structural and surface properties of MgO/SiO2 by both experimental characterization and simulation.[7] Their X-ray analysis results showed that MgO and SiO2 maintain their native morphology, and phases corresponding to MgSiO4 were not observed. Based on such observations, they concluded that the role of SiO2 in MgO/SiO2 is to increase the structural defects of MgO and, thereby, the catalytic activity. Niiyamaet al. examined the correlation between acidity/basicity and MgO/SiO2 activity[11] and reported that butadiene formation includes dehydrogenation and dehydration reactions, where the former takes place on basic sites and the latter on acidic sites, suggesting the importance of both site types for butadiene yield.
Regarding the reaction mechanism, because of its intrinsically complex nature, several mechanisms have been proposed.[12-19] At present, the mechanism described in Scheme 1 is generally accepted. This mechanism indicates that two acetaldehyde molecules are formed as a consequence of dehydrogenation of ethanol. Subsequently, an aldol condensation reaction occurs leading to the formation of 3-hydroxybutanal. Dehydration of 3-hydroxybutanal produces crotonaldehyde, which is further hydrogenated and dehydrated to form the final product, butadiene. Although many researchers[20-28] have subsequently adopted the general features of this mechanism, different suggestions have been proposed as to what constitutes the rate-determining step in the reaction. Many undesired processes can also occur leading to the production of different byproducts such as butanol, carbon dioxide and ethylene.