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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.

Solvation is crucial in many chemical and electrochemical processes related to alcohol conversion on metal surfaces. Particularly, understanding the dehydrogenation mechanism of methanol on solvated Pd, Pt, and Ru surfaces could allow the design of efficient methanol fuel cells. The large computational cost related to adopting an explicit solvation approach into density functional theory can be reduced drastically by using implicit solvation methods. In this study, we use our recently developed continuum solvation model (MGCM) to elucidate the minimum number of explicit water molecules to add to the solvated methanol/metal surface systems to reproduce experimental data with an optimized balance between time and reliability. Our results stress the importance of adding two explicit water molecules, especially for the case of Ru surfaces. For this later system, we provide a first insight into the decomposition mechanism of methanol using first-principles calculations. Our predictions can be then a useful reference for future studies that aim at designing more efficient heterogeneous catalysts with solvents.

International audience; The adsorption of CO2, CH4, and N-2 at 303 K by MIL-91(Al), one of the few porous phosphonate-based-MOFs, has been investigated by combining advanced experimental and computational tools. Whereas CH4 and N-2 adsorption isotherms exhibit type I behavior, the reversible CO2 isotherm displays an unusual inflection point at low pressure. In situ X-ray powder diffraction and infrared spectroscopy showed structural changes of this small-pore MOF upon CO2 adsorption. Grand canonical Monte Carlo simulations delivered a detailed picture of the adsorption mechanisms at the microscopic level. The so-predicted arrangements of the confined CO2 molecules were supported by analysis of the in situ diffraction and infrared experiments. It was shown that while adsorbed CH4 and N-2 are located mainly in the center of the pores, CO2 molecules interact with the hydrogen-bonded POHN acidbase pairs. This results in a relatively high adsorption enthalpy for CO2 of ca. -40 kJ mol(-1), which suggests that this material might be of interest for CO2 capture at low pressure (postcombustion).