Abstract
High-temperature (150–170 °C) pretreatment of lignocellulosic biomass with mineral acids is well established for xylan breakdown. Fe2+ is known to be a cocatalyst of this process although kinetics of its action remains unknown. The present work addresses the effect of ferrous ion concentration on sugar yield and degradation product formation from corn stover for the entire two-step treatment, including the subsequent enzymatic cellulose hydrolysis. The feedstock was impregnated with 0.5% acid and 0.75 mM iron cocatalyst, which was found to be optimal in preliminary experiments. The detailed kinetic data of acid pretreatment, with and without iron, was satisfactorily modelled with a four-step linear sequence of first-order irreversible reactions accounting for the formation of xylooligomers, xylose and furfural as intermediates to provide the values of Arrhenius activation energy. Based on this kinetic modelling, Fe2+ turned out to accelerate all four reactions, with a significant alteration of the last two steps, that is, xylose degradation. Consistent with this model, the greatest xylan conversion occurred at the highest severity tested under 170 °C/30 min with 0.75 mM Fe2+, with a total of 8% xylan remaining in the pretreated solids, whereas the operational conditions leading to the highest xylose monomer yield, 63%, were milder, 150 °C with 0.75 mM Fe2+ for 20 min. Furthermore, the subsequent enzymatic hydrolysis with the prior addition of 0.75 mM of iron(II) increased the glucose production to 56.3% from 46.3% in the control (iron-free acid). The detailed analysis indicated that conducting the process at lower temperatures yet long residence times benefits the yield of sugars. The above kinetic modelling results of Fe2+ accelerating all four reactions are in line with our previous mechanistic research showing that the pretreatment likely targets multiple chemistries in plant cell wall polymer networks, including those represented by the C–O–C and C–H bonds in cellulose, resulting in enhanced sugar solubilization and digestibility.
Highlights
Lignocellulosic biomass consists of three major components: cellulose, hemi-cellulose and lignin [1]
Chen et al reported a pretreatment with FeCl3 of rice straw with an 83% hemicellulose removal and 82% glucose yield after the subsequent enzymatic hydrolysis [26]. These findings warrant further studies on sugar product yields with iron, and the current study addresses this issue
After the dilute acid and cocatalyst impregnation, most of the sucrose was dissolved in the recirculating atmospheric pressure impregnation (RAPI) post-soak liquor, along with some water-soluble extractives
Summary
Lignocellulosic biomass consists of three major components: cellulose, hemi-cellulose and lignin [1]. It includes non-cellulosic polysaccharides (such as pectin). Pretreatment of lignocellulosic biomass with dilute mineral acids under mild conditions has been found to effectively separate and break down hemicellulose, modify the lignin network and increase cellulose accessibility [8,9]. The technology of dilute-acid pretreatment is being actively studied and applied; in 2017 alone, a number of papers have been published in the field of dilute-acid pretreatment of biomass [10,11,12,13,14,15,16,17,18]
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