A Comparative Study of Structurally Related Homogeneous Ruthenium and Iron Catalysts for the Hydrogenation of Levulinic Acid to [gamma]-Valerolactone

The conversion of levulinic acid (LA) to [gamma]-valerolactone (GVL) was investigated by employing the homogeneous Shvo catalyst (Ru-1) and iron Knölker-type catalysts, in order to evaluate the possibility to replace ruthenium with cheap, earth-abundant iron for this catalytic reaction. While the ruthenium-catalyzed reactions readily proceed, the activating agent required for the iron complex was found to interfere with the LA. This problem could be circumvented by pre-activating the original Knölker complex (Fe-1) into the corresponding dicarbonyl mono-acetonitrile iron species (Fe-3). The pre-activated iron catalyst deactivated after a few turn-overs in transfer hydrogenation reactions with isopropyl alcohol; however, highly improved GVL yields were achieved under H2 pressure to a notable maximum of 570 turn-overs for Fe-3. Nevertheless, comparative screening experiments with various solvents and kinetic studies showed that Ru-1 is still superior over Fe-3 in terms of catalytic activity.