Understanding the Selectivity of Selective Oxidation of Propane to Acrylic Acid on a Mo–Te–Nb–O M1 Catalyst Using Density Functional Theory

In this work,
we have performed a quantum chemical investigation for the selective
oxidation of propane toward acrylic acid on the M1 phase of a mixed
metal oxide (MMO) catalyst, consisting of Mo–Te–Nb–O.
The M1 phase of the catalyst has a complex surface structure, which
involves different arrangements of metal sites with variable oxidation
states. This complexity makes it inherently difficult to understand
its activity and selectivity in catalytic reactions. We have used
a multilayer cluster model of the main catalytically active site of
M1 and a hybrid DFT methodology to establish the minimum energy pathways
for the propane oxidation to acrylic acid via propylene, allyl alcohol,
and acrolein as the key intermediates. In addition, the reactivity
of propyl radicals toward the formation of isopropanol, which leads
the reaction toward an unselective path of CO/CO2 generation
instead of acrylic acid production, has also been depicted. We show
that the formation of isopropanol has rather a low activation barrier
and is therefore competing with the formation of propylene from the
propyl radical after C–H activation of propane. Once propylene
has formed, the allyl position can easily be activated to form acrolein,
which can be further oxidized to acrylic acid. In addition, we have
developed a more general linear scaling relation for C–H activation
chemistry to estimate activation barriers on M1 catalysts only based
on four key energetic descriptors, which are the hydrogen binding
energy (EH) on the surface site, the C–H
bond dissociation energy (EBDE) of the
reactant molecule in the gas phase, the interaction energy at transition
state structure (EintTS), and the interaction energy between metal
site and the oxygen atom of oxygenated gas molecules (EMO).