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

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

Computational Mechanism of Methyl Levulinate Conversion to γ-Valerolactone on UiO-66 Metal Organic Frameworks

Metal-organic frameworks (MOFs) are gaining importance in the field of biomass conversion and valorization due to their porosity, well-defined active sites, and broad tunability. But for a proper catalyst design, we first need detailed insight of the system at the atomic level. Herein, we present the reaction mechanism of methyl levulinate to γ-valerolactone on Zr-based UiO-66 by means of periodic density functional theory (DFT). We demonstrate the role of Zr-based nodes in the catalytic transfer hydrogenation (CTH) and cyclization steps. From there, we perform a computational screening to reveal key catalyst modifications to improve the process, such as node doping and linker exchange. © 2022 American Chemical Society. All rights reserved