Highly selective catalytic conversion of raw sugar and sugarcane bagasse to lactic acid over YbCl3, ErCl3, and CeCl3 Lewis acid catalysts without alkaline in a hot-compressed water reaction system

Abstract

To improve the process efficiency for lactic acid production, catalytic conversion of raw cane sugar and sugarcane bagasse (SCB) to lactic acid under subcritical water condition over Lanthanide catalysts was investigated. The effect of different types of Lanthanide (III) ion catalyst namely YbCl3, ErCl3, and CeCl3 on enhancement of lactic acid (LA) yield, selectivity, turnover number [TON] and turnover frequency [TOF] was studied in the aqueous solution. The results were compared with conventional catalytic reaction for lactic acid production in strongly alkaline solution. For raw sugar-to-lactic acid conversion, ErCl3 catalyst exhibited greatest performance at 240 °C for 15 min and achieve the highest TOF and TON with 91.8% LA yield (91.79%theoretical yield) and 90.5% selectivity. In case of SCB conversion, aqueous ammonia solution (LHWAA) was the best pretreatment to eliminate lignin and hemicellulose and attain cellulose-rich fraction. The LHWAA pretreated SCB conversion to lactic acid over YbCl3 at 240 °C for 15 min achieved supreme performance for highest TOF and TON and accomplished 98.7% LA yield (88.81% theoretical yield) and 94.2% selectivity with trace amount of formic acid, acetic acid and levulinic acid as side products. The findings provided crucial information for a combined cellulose hydrolysis step with catalytic conversion of mono- and polysaccharides to lactic acid in a very shorter time relative to conventional catalytic process and biotechnological fermentation process. Lanthanide catalysts, especially ErCl3 and YbCl3, were found as promising alternative environmentally friendly catalysts providing a potential cost-efficient process of lactic acid from bioresources in a larger scale production.