A two-stage C5 selective hydrolysis on soybean hulls for xylose separation and value-added cellulose applications

Abstract

Soy hulls from dehulling of soybeans are typically disposed of with soymeal or cattle feed. The hulls contain about 38.8% cellulose and 23.8% hemicelluloses and less than 4% lignin. The low lignin content, large volume availability, and being a “captive” feedstock make soy hulls an affordable raw material to produce C5 sugars such as arabinose and xylose from hemicelluloses. In this work, dilute acid hydrolysis of soy hulls using acid concentrations less than 1% (w/w) in solution and at different temperatures (125 °C, 140 °C, and 155 °C) was investigated to generate the kinetics data for sugar and degradation product release and study selectivity towards arabinose and xylose. The primary goal was to produce a hydrolysate rich in C5 sugars with minimal glucose and degradation products. Lower acid concentration (0.4% w/w) at 140 °C and lesser reaction time favoured selectivity towards arabinose release, while xylose release needed higher acid (0.6 to 0.8%) and longer time at the same temperature. From the kinetics data, a two-stage process was devised to achieve two separate hydrolysate streams rich in arabinose (5.00 ± 0.15 g dm−3, 78.1% of total C5 sugars) and xylose (17.20 ± 0.71 g dm−3, 87.7% of total C5 sugars). The xylose-rich stream was used to isolate xylose (powder form) under ambient process conditions using our patented process. The residual soy hulls, post the two-stage hydrolysis, showed high crystallinity with morphology analogous to microcrystalline cellulose, thus making them an effective starting material for high-value cellulose applications such as microcrystalline cellulose and polymer composites.