Valorization of Carbohydrates of Agricultural Residues and Food Wastes: A Key Strategy for Carbon Conservation

The complete conservation of the carbon, hydrogen, and oxygen atoms of cellulose and starch can theoretically be achieved by their conversion to levulinic and formic acids. Similarly, hemicellulose can be converted to furfural and water with 100% atom economy. The valorization of carbohydrates of agricultural residues (banana-, orange-, and potato-peels, corn leaf and root, and rice husk), potential components of raw food wastes (corn, rice, potato, lettuce, onion, leek, vegetable, bean, and tomato), and cooked food wastes (rice noodle, spaghetti, risotto, noodle, bread, and biscuit) was demonstrated in the presence of sulfuric acid at 130 °C in γ-valerolactone as the green solvent to yield levulinic and formic acids as the two main products. Levulinic acid was also produced from furfural by its transfer hydrogenation with formic acid to furfuryl alcohol in the presence of Shvo's catalyst followed by its acid catalyzed conversion to levulinic acid. Neutralization of the sulfuric acid in the reaction mixture with ammonium hydroxide resulted in an aqueous ammonium sulfate phase and a γ-valerolactone rich organic phase. Due to the salting out effect of the ammonium sulfate in the aqueous phase, levulinic acid and formic acid partitioned to the γ-valerolactone rich organic phase at >99.8% and >97.1%, respectively. The transfer hydrogenation of levulinic acid with formic acid in the presence of the Shvo's catalyst resulted in the formation of 4-hydroxyvaleric acid, which readily underwent dehydration to yield γ-valerolactone, a green and sustainable liquid. The possibility of converting C₅- and C₆-carbohydrates to levulinic acid or γ-valerolactone is a great opportunity for carbon conservation with a carbon-economy higher than that for butanol or ethanol.