Supported rhenium catalysts for the hydrogenation of levulinic acid derivatives: limits and potential

Levulinic acid derivatives, such as alkyl levulinates, are suitable starting reactants for the production of fuel components, namely gamma-valerolactone (GVL), alkyl valerates, pentanol, and pentylvalerate (PV). The reactions were performed in batch, without any additional solvents, by investigating the catalytic activity of several Re-based catalysts. In this way, we confirmed the crucial role of the support acidity in promoting the ring-opening of GVL and its consecutive reduction to valeric compounds. In the optimised conditions, bimetallic Re-Ru-O/HZSM-5 yielded methyl valerate (MV) and valeric acid (VA) with a productivity of 512 mmol g(metal)(-1) h(-1), one of the highest reported in the literature to date. Rhenium can also foster the reduction of valeric acid/esters to PV through the formation of 1-pentanol and its efficient esterification/transesterification with the starting material. However, we also proved that Re-based catalysts undergo leaching of the active phase in the presence of carboxylic acids, especially by working in neat with VA, thus limiting the recyclability of the catalytic material. Furthermore, the over-reduction of rhenium affects catalytic performance, suggesting not only that a pre-reduction step is unnecessary, but also that it could be detrimental for the catalyst activity.