Abstract
M.Sc. (Chemistry)
New Ru(II) complexes bearing pyridinyl-imine ligands have been used in the hydrogenation of biomass derived levulinic acid (LA) to γ-valerolactone (GVL).
The ligands, L1-L4 (E)-4-((pyridin-2-ylmethylene)amino)benzoic acid (L1), (E)-4-(((pyridin-2-ylmethylene)amino)methyl)benzoic acid (L2), (E)-4-((quinolin-2-ylmethylene)amino)benzoic acid (L3) and (E)-4-((pyridin-2-ylimino)methyl)benzoic acid (L4) were synthesized via the Schiff-base condensation reaction. All the ligands were characterized by 1H and 13C{1H} NMR spectroscopy, infra-red spectroscopy, high resolution mass spectrometry and elemental analysis. Treatment of these compounds with [RuCl2(COD)]n and [RuCl2(p-cymene)]2 under appropriate reaction conditions resulted in the formation of four new complexes (1–4). These complexes were also characterized by 1H and 13C{1H} NMR spectroscopy, infra-red spectroscopy, high resolution mass spectrometry and elemental analysis.
Complexes 1-4 were evaluated as pre-catalysts in the solvent-free hydrogenation of biomass derived levulinic acid to γ-valerolactone by using formic acid (FA) as the source of hydrogen. The presence of a base was required for the reaction to proceed with a catalyst loading of 0.1 mol %. The use of the inorganic base, KOH, resulted in the formation of the 4-hydroxyvaleric acid (4-HVA) intermediate along with GVL, however, the use of the organic base, triethylamine, resulted in the formation of GVL only. Pre-catalysts 3 and 4 gave full LA conversions with 100 % selectivity to GVL. In addition to this, pre-catalysts 3 and 4 were recycled up to a third run while maintaining 100 % LA conversion and 100 % GVL selectivity. However, their activities decreased for the fourth and fifth runs, even though they still maintained 100 % GVL selectivity. In-situ NMR studies performed using pre-catalyst 1 showed that the formic acid decomposed to form the hydrogen gas needed for the...