Evaluation of subcritical water reaction media for an efficient conversion of the polysaccharide fraction to lactic acid by Ca(OH)2 catalysis

Abstract

Lactic acid (LA) is an -hydroxy carboxylic acid with a chiral C at its  position. Due to the presence of two funcional groups, acid and alcohol, it presents an important chemical reactivity with a growing market in the food and pharmaceutical industries. Currently, there is gap between LA production capacity and the actual ouput due to problems associated with the up-scaling of the current industrial fermentation process from the glucose monomer1. The catalytic conversion of the polysaccharide fraction of second generation biomass into different platform chemicals offers a great potential for this biomass to be incorporated as raw material in the future biorefineries. Corn stover is a lignocellulosic biomass with an important polysaccharide fraction which makes of corn stover an ideal biomass for the production of value-added chemicals. In this work, production of LA from corn stover by alkaline catalysis has been explored by using Ca(OH)2 as catalyst in subcritical water reaciton medium. Two different temperatures were tried, 190 ºC and 220 ºC at a catalyst concentration of 0.1 M. Batch kinetics were determined in a lab-assambled reactor of 0.5 L capacity covered by a ceramic heating jacket (230 V, 4000 W, ø 95 mm, 160 mm height) to reach the selected working temperature. Corn stover was charged into the reactor at a concentration of 5 wt%. The potential of the main derived-sugars from the polysaccharide fraction of corn stover, glucose and xylose, to produce LA, was studied previous to the use of corn stover. Initial concentration of glucose and xylose was 50 mM and the reaction temperature 190 ºC. Figure 1 shows the glucose and xylose conversion as a function of reaction time. Both monomeric sugars degrade rapidly in an alkaline environment, especially when compared to the conversion rate achieved in a subcritical water reaction medium without the addition of any type of external catalyst2. The presence of key intermediates such as glyceraldehyde in the reaction medium proves that bases, such as Ca(OH)2, catalyze the retro-aldol condensation sugar degradation pathway leading to the formation of lactic acid from glucose or xylose. Furfural and HMF were no detected in significant amount in the reaction medium since it is well known that Brønsted acids are needed to catalyze the dehydration of glucose and xylose to produce HMF and furfural, respectively. The presence of retro-aldol condensation compounds leads to the formation of lactic acid as the main reaction product, with slightly higher yields on a carbon basis for xylose, 41 %, than for glucose, 37 %. In any case, the concentration of LA in the medium was similar for both sugars (3.1-3.2 g/L). Other reaction by products determined in the reaction medium were acetic and formic acids. LA production was studied by alkaline catalysis (0.1 M Ca(OH)2) by using corn stover as raw material at 190 ºC. LA yield was determined considering the polysaccharide fraction in the corn stover as potential source of lactic acid (42.1 % glucans, 25.8 % xylanes and 4.6 % arabinans). A substantial decrease in LA yield was observed for corn stover compared to the yield derived from the pure monomers (see Figure 1) with maximum yields of 5.4 % and LA concentration in the reaction medium of 2.2 g/L. To explore a potential improvement of LA yield with temperature, treatment was also conducted at 225 ºC, resulting in a maximum yield of 13.5 %, corresponding to a LA concentration of 5.5 g/L. A more in-depth study needs to be conducted to achieve higher yields by improving the previously required hydrolysis step of the polysaccharide fraction of corn stover into its monomers.