Scheme 1. Reaction mechanism on the conversion of ethanol into
1,3-butadiene.
A number of computational studies on the reaction mechanism of
ethanol-to-butadiene conversion have been
reported.[7,25-28] For examples, Chieregatoet al. performed a computational modeling coupled with an
experimental study on a MgO cluster to investigate the reactivity of
ethanol.[24] As a model for MgO nanocrystals,
(MgO)10 or (MgO)16 clusters were used.
They suggested that, based on their experimental and computational
results, crotyl alcohol and 3-buten-1-ol are the key intermediates and
precursors for butadiene formation. Zhang et al. carried out a
periodic DFT study on the aldol condensation reaction on
MgO.[25] According to their results, the barrier
height of the C-C coupling step in the aldol condensation reaction was
computed to be lower than the proton transfer step. Baltrusaitiset al. also used periodic DFT methods to explore the
ethanol-to-butadiene reaction mechanism.[26]
To improve catalytic performance, a mixture of metal oxides has been
suggested.[3, 29] The introduction of ZnO has been
reported to enhance the catalytic performance of
MgO/SiO2[4], which is of interest
for investigating details of the performance at the molecular level.
There have been several computational mechanistic studies on some metal
oxides such as MgO,[25,26]ZrO2,[27]Zr-SBA-15[28], but to our knowledge, no study has
dealt with a detailed computational investigation on full mechanistic
aspects described in Scheme 1. Moreover, the atomistic mechanism on the
enhanced catalytic performance by ZnO as a catalyst for the
ethanol-to-butadiene conversion has still been elusive. In designing new
catalysts for ethanol-to-butadiene conversion, inhibition of undesired
side reactions is very important;[1] however, no
study on the reaction mechanisms of the side reactions at the molecular
level has been reported. Thus, in the present paper, we report results
of our computational investigation on each step of the complex reaction
conversion of ethanol to butadiene catalyzed by MgO or ZnO, to
understand how the different acidic and basic properties of catalysts
affect the reaction mechanism. In addition to the main reaction, we
consider also a side reaction leading to the formation of ethylene by
the dehydration of ethanol.