Abstract
Ph.D. (Chemistry)
Several phosphorus containing ligand complexes of ruthenium(II) were synthesised and investigated as catalysts for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL). The outcomes of these studies are reported in this thesis and summarized as per the chapter studies.
Pyrazolylphosphite and pyrazolylphosphinite ruthenium(II) complexes (1-8) were prepared according to literature procedure. Complexes 1-8 were used as pre-catalysts for solvent-free conversion of LA to GVL by molecular hydrogenation and formic acid-mediated hydrogenation. The catalyst precursors were found to be more efficient when the source of hydrogen was molecular hydrogen as compared to formic acid. In situ NMR studies of reactions involving formic acid as a hydrogen source indicated that the initial step in the reaction involves the decomposition of formic acid to CO2 and H2. Recyclability studies showed that the catalyst can be used up to three times without significant loss of activity and selectivity. However, mercury poisoning experiments revealed that a cocktail of active species (both homogeneous and heterogeneous) were responsible for the hydrogenation reactions.
Compounds 4-(diphenylphosphino)benzoic acid (L1), 3-(diphenylphosphino)-propanoic acid (L2) were reacted with [Ru(p-cymene)Cl2]2 in equimolar amounts to obtain the new complexes 9 and 10 respectively. Compounds 9 and 10 were further reacted with Zn(OAc)2 in 2:1 mole ratio to form hetero-hexanuclear complexes (11 and 12) containing four ruthenium and two zinc centres. Complexes 11 and 12 were formed through coordination of ruthenium to a phosphine, while the phosphines’...