A mechanism for the selective epimerization of the glucose mannose pair by Mo-based compounds: towards catalyst optimization

Abstract

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.