A Remarkable Effect of Aluminum on the Novel and Efficient Aqueous-Phase Hydrogenation of Levulinic Acid into γ-Valerolactone Using Water-Soluble Platinum Catalysts Modified with Nitrogen-Containing Ligands

Abstract

The catalytic performance of novel water-soluble platinum catalysts modified with various nitrogen-containing and phosphine ligands in the hydrogenation reaction of levulinic acid (LA) into γ-valerolactone (GVL) has been studied in environmentally attractive, green, aqueous monophasic systems. The presence of the Lewis acid aluminum enormously increases the catalytic activity of water-soluble platinum catalysts modified with nitrogen-containing ligands in the LA hydrogenation reaction and high catalytic activities up to 3540 TOF’s per hour with a quantitative selectivity towards GVL have been achieved using Na2PtCl6·6H2O precursors modified with the bidentate bathophenanthrolinedisulfonic acid disodium salt (BPhDS) ligand and low amounts of AlCl3·6H2O promotors (molar ratio of AlCl3·6H2O/Pt = 0.17) in aqueous media. This unprecedented increase in catalytic activity with aluminum promotors for water-soluble transition metal catalytic systems in aqueous-phase hydrogenation reactions has not been described until now in the literature. The apparent activation energy of platinum catalyst modified with the monodentate nitrilotriacetic acid trisodium salt ligand in aqueous medium was calculated and amounts to a relative low value of 73.04 kJ mol−1 when one considers that in the LA hydrogenation reaction this catalyst reduces a less reactive keto group into alcohol functionality. A recycling experiment of the Pt/BPhDS/Al catalyst from the aqueous monophasic LA hydrogenation reaction mixture followed by biphasic recovery of the catalyst in active form from organic reaction products by extraction and simple phase separation of an aqueous/organic two-phase system formed after addition of diethyl ether has shown that the Pt/BPhDS/Al catalyst is stable without loss of activity and selectivity in a consecutive run.