**12. Conclusions on biomass transformation with ruthenium catalysts**

Ruthenium, a late transition element, provides catalytic pathways that make it highly promising for catalysts applied for biomass conversion. Biomass, a globally available resource, is a sustainable feedstock for producing platform chemicals, that could substitute the current fossil-based platform chemicals in the chemical industry. However, in order to implement further processes in small and large-scale biorefineries, more efficient transformations will be required. Here, the distinct catalytic functions provided by ruthenium and ruthenium complexes could open new pathways. Biomass largely consists of complex molecules that comprise oxygen and other heteroatoms. Catalytic transformations need to accommodate for these heteroatoms, because molecules with heteroatoms tend to adsorb strongly to catalytic sites possibly causing substrate- or product inhibition. However, the preferential adsorption of chemical moieties associated with heteroatoms on the catalytically active site can be exploited for directing catalytic transformations. The principle has been explored for the consecutive hydrogenation of unsaturated moieties on a molecular assemble line. In this context, it is useful considering the concept of orthogonal catalytic functions, where a catalyst preferentially binds and transforms a selected chemical entity without hindering other catalysts that may be added for realising preceding or subsequent catalytic transformations.

In this context, the catalytically highly active element ruthenium embodies unique features. Ruthenium does not form binary hydrides that are stable under usual catalytic conditions. Nevertheless, metallic ruthenium can dissociate molecular hydrogen. Under an atmosphere of hydrogen, the surface of metallic ruthenium is covered with hydrogen atoms. Adsorption states and chemical reactivity of this hydrogen is well understood. Desorption of a fraction of the

hydrogen provides the empty coordination sites necessary for co-adsorption of reactant molecules. Typically following a Langmuir-Hinshelwood-type mechanism, hydrogen atoms can be transferred to unsaturated moieties. Remarkably, ruthenium can also form and cleave C-C, C-O and C-N bonds. Combined with its strong propensity for hydrogenation, this ability gives rise to hydrogenation, hydrogenolysis and hydrodeoxygenation transformations that make ruthenium catalysts so interesting for biomass conversion. Noteworthy are the distinct catalytic transformations that can be realised with ruthenium catalysts. Selected examples for intriguing transformations of biomolecules and bio-derived molecules have been discussed above.

Understanding the interaction of adsorbed molecules with ruthenium surfaces, the nature of adsorption states, binding energies and structures of the adsorption complexes lies at the heart of rational design of catalysts that are specific for the conversion of the chosen chemical entity in biomass. It is anticipated that new transformations will be realised based on the unique catalytic functions provided by heterogeneous and homogeneous ruthenium catalysts. Serving as important tools for the synthetic chemist, these transformations will bolster the use of biomass as sustainable feedstock for the chemical industry.
