Abstract:
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The selective C2 epimerization of the glucose/mannose pair on a set of Mo-based catalysts was studied
by means of density functional theory. The process, known as the Bilik reaction, encompasses a 1,2
C-shift of the C3 centers at the sugars. Molybdic acid was initially proposed as a catalyst in this reaction,
and recent experimental studies have shown that the polyoxometalate (POM) Keggin cluster H3PMo12O40
also presents a good performance. In the present work, we propose a reaction mechanism for the epimerization
on the Keggin cluster with different heteroatoms and extend it to a larger POM, H6P2Mo18O62,
and the continuous α-MoO3(010) surface. We have found that in the transition state corresponding to the
1,2 C-shift the Mo center acts as an electron buffer that promotes the transformation of the aldehyde
group in C1 into an alkoxy group and the C2 alkoxy into an aldehyde group. As a consequence, the activity
of Mo-containing compounds can be traced back to the reducibility of the Mo center and a simple microkinetic
model illustrates that this descriptor generates an activity volcano. This allows the identification of
a new POM that shall be 4.7 times more active than the parent compound. We have thus shown that continuum
models linking the properties of molecular cluster-like catalysts and oxide surfaces can be derived
and this paves the way towards a unified theory in catalysis. |