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.