Valorisation of carbohydrate-rich food waste for synthesis of hydroxymethylfurfural (HMF)

Abstract

Food waste that amounts to over one billion tonnes per year globally is a potential renewable feedstock for biorefinery. This thesis focuses on the development of high-throughput catalytic systems to produce value-added chemicals, such as hydroxymethylfurfural (HMF), from various selected food waste that is rich in starch, cellulose, or sugars. Different homogeneous catalysts and solvents are scrutinised to pursue an efficient conversion system. The findings highlight that the derived Brønsted acidity and intrinsic Lewis acidity of metals (e.g., Sn(IV) and Al(III)) are critical for HMF selectivity, by maneuvering the kinetics of desirable tandem reactions and undesirable side reactions. These acidities depend on the electrochemical properties of metal ions, e.g., electronegativity and charge density. Adding a co-catalyst (e.g., maleic acid) to the metal-based catalytic systems is demonstrated to be a feasible means to control conversion rate and selectivity. In addition, solvents are evidenced to play an important role beyond serving as a reaction medium. Production of HMF in a binary mixture of water and acetone or acetonitrile (i.e., acetone/H₂O and acetonitrile/H₂O, respectively) is substantially faster than that in dimethyl sulfoxide/H₂O and tetrahydrofuran/H₂O. Replacing the industrial co-solvents by greener alternatives, e.g., propylene carbonate and γ-valerolactone, can further accelerate the conversion, in which HMF of 20-25 mol% can be produced from bread waste within 20 min under microwave heating at 120°C. The solvent medium interacts with the substrates and catalysts, altering their reactivity during catalysis. These research efforts elucidate the roles of different parameters, advise the design of high-performance solid catalyst for recycling purpose, and demonstrate a good potential of food waste valorisation for synthesis of value-added chemicals in real-life applications.