Biomass valorisation over metal-based solid catalysts from nanoparticles to single atoms

Heterogeneous catalysts are vital to unlock superior efficiency, atom economy, and environmental friendliness in chemical conversions, with the size and speciation of the contained metals often playing a decisive role in the activity, selectivity and stability. This tutorial review analyses the impact of these catalyst parameters on the valorisation of biomass through hydrogenation and hydrodeoxygenation, oxidation, reforming and acid-catalysed reactions, spanning a broad spectrum of substrates including sugars and platform compounds obtained from (hemi)cellulose and lignin derivatives. It outlines multiple examples of classical structure sensitivity on nanoparticle-based materials with significant implications for the product distribution. It also shows how the recently emphasised application of metals in the form of ultrasmall nanoparticles (<2 nm), clusters and single atoms, while fulfilling superior metal utilisation and robustness, opens the door to unprecedented electronic and geometric properties. The latter can lead to facilitated activation of reactants as well as boosted selectivity control and synergy between distinct active sites in multifunctional catalysts. Based on the analysis conducted, guidelines for the selection of metals for diverse applications are put forward in terms of chemical identity and structure, and aspects that should be explored in greater depth for further improving the exploitation of metals in this research field and beyond are highlighted.