Transformation of the polysaccharide fraction of corn stove into organic acids catalysed by Lewis acid catalyst in a subcritical water reaction medium

Abstract

The transition from today’s fossil-based economy towards a more sustainable economy lays in the transformation of biomass resources into high-value-added chemicals. The potential of the polysaccharide fraction of the biomass to produce different chemical building blocks has been highligthted by the US Department of Energy. Among these building blocks, organic acids present a huge potential since they can be converted into a great variety of chemical compounds. The hydrolysis of the polysaccharide fraction of biomass, followed by sugar degradation, can result in the formation of different organic acids. Among them, in this study, production selectivity towards levulinic and lactic acid has been explored by using a mixture of Lewis acid catalysts. The chemical route for levulinic acid production is well known and it consists on the dehydration of glucose to HMF and further rehydration to produce levulinic and formic acids: On the contrary, conversion of cellulose into lactic involves mainly a reto-aldol condensation reaction pathway: In this work, a combination of Al3+ and Sn2+ chloride salts were explored to convert the polysaccharide fraction of corn stover in water under subcritical conditions as reaction medium. Two different temperatures were tried, 190 ºC and 220 ºC at a catalyst concentration of 20 % of equiweight fraction of both salts considering the glucan fraction of corn stover. Organic acid yields were evalauted in different ways. Lactic acid yield was evaluated considering the polysaccharide fraction of corn stover (cellulose + hemicellulose), but levulinic acid yield was determined considering only the celullose fraction. Previous studies with pure sugar monomers, glucose and xylose, showed that the xylose Al3+/Sn2+ catalyzed-system yielded lactic acid but no levulinic acid. On the contray, in case of glucose, both organic acids could be produced according to the above routes. These results encourage the use of corn stover as a source of valuable chemicals such as organic acids. Further studies should be performed for a deeper analysis in terms of the selectivity of the process.