Atom Economical Aqueous-Phase Conversion (APC) of Biopolyols to Lactic Acid, Glycols, and Linear Alcohols Using Supported Metal Catalysts

Abstract

Conversion of renewable biopolyols to value-added chemicals such as lactic acid and glycols usually demands excess hydrogen/oxygen or harsh reaction conditions in strong alkaline medium (220–350 °C). This unfortunately promotes significant side reactions resulting in low carbon selectivity to liquid products, posing significant challenges for the development of sustainable technologies. We report here a new atom economical catalytic conversion of various biopolyols (glycerol, xylitol, mannitol, and sorbitol) to lactic acid with glycols and linear alcohols as co-products at much lower temperatures (115–160 °C) without external addition of either hydrogen or oxygen. Among various metal-based catalysts (Pt, Pd, Rh, Ru, Raney Ni, Raney Co, and Cu) evaluated, Pt/C catalyst gives the highest chemoselectivity (S > 95%) for lactic acid, glycols, and linear alcohols at 115–160 °C. An important finding is that approximately two-thirds of the hydrogen generated in situ via dehydrogenation of polyols over Pt/C catalyst is efficiently utilized for the conversion of the remaining polyols and intermediates to useful products (e.g., glycols and linear alcohols instead of gaseous products) with the remaining available hydrogen for use elsewhere in a biorefinery. The Pt/C catalyst is thus multifunctional facilitating tandem dehydrogenation/hydrogenolysis of polyols. Furthermore, it is observed that Ba2+ alkali ion promotes the activity of the Pt/C catalyst by almost 12-fold compared to other alkali promoters such as NaOH, KOH, and Ca(OH)2. In addition to being the first reported study on the conversion of C5∼C6 polyols (e.g., xylitol and sorbitol) to lactic acid at relatively low temperatures, the results also provide new insights into the mechanism of tandem catalysis of biopolyols conversion to value-added commodity chemicals.