Discriminating molybdate and tungstate layered materials by operando Raman spectroscopy at the solid-liquid interface

In order to perform the characterization of liquid phase reactions catalysed by solid materials, several methodologies were developed using Raman spectroscopy. By following both the liquid and the solid-liquid interface, several layered materials could be discriminated. Indeed, molybdates or tungstates layered mixed oxides (with Nb or Ta oxides) show acidic properties that can catalyse several reactions including the conversion of polyalcohols. Therefore, these catalysts were extensively characterized not only initially to highlight their acidity and their capacity to favour molecular intercalation within their interlayers, but also to evaluate and monitor the catalysts under operando conditions. Layered molybdates, and HNbMoO₆ in particular, proved to outperform the equivalent tungstates in hexanediol cyclodehydration, while tantalates were very similar to niobates. Our results show that while the moderate acidity of the molybdates is sufficient to perform this reaction, the overall catalytic activity is rather directly influenced by the accessibility of the reactant to the active sites in the interlayer.