Kinetic Modelling and Experimental Studies for the Effects of Fe2+ Ions on Xylan Hydrolysis with Dilute-Acid Pretreatment and Subsequent Enzymatic Hydrolysis

Abstract

High-temperature (150-170 • C) pretreatment of lignocellulosic biomass with mineral acids is well established for xylan breakdown. Fe 2+ 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, Fe 2+ 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 Fe 2+ , 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 Fe 2+ 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 Fe 2+ 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.

Effect of Dilute Acid and Alkali Pretreatments on the Catalytic Performance of Bamboo-Derived Carbonaceous Magnetic Solid Acid

“…Dilute acid pretreatment can not only effectively remove hemicellulose, but can also minimize the damage of lignin and cellulose [19]. Meanwhile, dilute alkali pretreatment can effectively expand the biomass, leaving carbohydrates (cellulose and hemicelluloses) behind, thus increasing the contact area of the solid acid [20].…”

Iron incorporation both intra- and extra-cellularly improves the yield and saccharification of switchgrass (Panicum virgatum L.) biomass

“…In contrast to glucose release, xylose release does not change in all the transgenic lines (Fig. 7b), which can be explained by the fact that, in general, the xylan in the plant cell wall is much more exposed, i.e., more easily degraded to sugar monomers (i.e., less recalcitrant) than cellulose [87]. Thus, the xylose release is very high to begin with, so there is not likely much room for further improvement; the effect of iron accumulation on the pretreatability and digestibility was more prominently reflected on the more recalcitrant part, cellulose.…”

Environmentally Friendly Approach for the Production of Glucose and High-Purity Xylooligosaccharides from Edible Biomass Byproducts

“…The xylan seemed to be fully degraded and dissolved in liquid hydrolysate at 190 • C. Then, the XGM content sharply dropped at 200 • C because it converted to HMF and furfural, which are degradation products. The pH level gradually decreased by providing more heat energy due to the cleavage of the acetyl group from the hemicellulose chain, which resulted in more acidic circumstances at higher reaction temperature conditions [37]. This means that xylan degradation might be accelerated by positive feedback from heat energy and acids during autohydrolysis.…”