Understanding Hydrogen in Bayer Process Emissions. 3. Hydrogen Production during the Degradation of Polyols in Sodium Hydroxide Solutions

This is the third in a series of related studies on the fundamentals of hydrogen gas production from specific classes of organic compounds in sodium hydroxide solutions. The alkaline degradation of 10 aliphatic C2–C6 polyols was investigated under anaerobic conditions in an autoclave. The evolution of hydrogen and low molecular weight carboxylates (lactate, formate, acetate, oxalate, glycerate, glycolate, pyruvate, and acrylate) during the degradation of glycerol, erythritol, xylitol, and sorbitol was studied at 275 °C for reaction times up to 300 min. All of the compounds investigated decomposed to produce approximately 2 mol of hydrogen gas per mole of polyol used. Within a common pathway, three main reactions to hydrogen production were identified: (1) hydroxide-induced formation of an aldehyde and hydride, followed by a hydride-induced β-elimination reaction with hydroxide as the leaving group; (2) the degradation of aldehyde intermediates such as glycolaldehyde (formed by retro-aldol condensation) through base-catalyzed oxidation by water; (3) the degradation of lactic acid. Among the stereoisomers studied, hydrogen production was found to be particularly sensitive to the relative stereochemistry of the hydroxyl groups, which is explained in terms of a common sequence of initial reaction steps in alkaline solution. These findings show that the alkaline degradation of polyols may produce significant amounts of hydrogen in Bayer process digestion, and if wet oxidation is used to remove organic compounds from the liquor, then the potential exists for the formation of explosive gas mixtures. The results also advance the fundamental understanding of the alkaline hydrothermal conversion of polyols to valuable products such as lactic acid.