Ferrous and Ferric Ion-Facilitated Dilute Acid Pretreatment of Lignocellulosic Biomass under Anaerobic or Aerobic Conditions: Observations of Fe Valence Interchange and the Role of Fenton Reaction.

Hui Wei,Peter N Ciesielski,Melvin P Tucker,Xiaowen Chen,Bryon S Donohoe,Min Zhang,Wei Wang,Michael E Himmel

doi:10.3390/molecules25061427

Hui Wei, Peter N Ciesielski + Show 6 more

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https://doi.org/10.3390/molecules25061427

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Abstract

Ferrous ion co-catalyst enhancement of dilute-acid (DA) pretreatment of biomass is a promising technology for increasing the release of sugars from recalcitrant lignocellulosic biomass. However, due to the reductive status of ferrous ion and its susceptibility to oxidation with exposure to atmosphere, its effective application presumably requires anaerobic aqueous conditions created by nitrogen gas-purging, which adds extra costs. The objective of this study was to assess the effectiveness of oxidative iron ion, (i.e., ferric ion) as a co-catalyst in DA pretreatment of biomass, using an anaerobic chamber to strictly control exposure to oxygen during setup and post-pretreatment analyses. Remarkably, the ferric ions were found to be as efficient as ferrous ions in enhancing sugar release during DA pretreatment of biomass, which may be attributed to the observation that a major portion of the initial ferric ions were converted to ferrous during pretreatment. Furthermore, the detection of hydrogen peroxide in the liquors after DA/Fe ion pretreatment suggests that Fenton reaction chemistry was likely involved in DA/Fe ion pretreatments of biomass, contributing to the observed ferric and ferrous interchanges during pretreatment. These results help define the extent and specification requirements for applying iron ions as co-catalysts in DA pretreatments of biomass.

Highlights

Lignocellulosic biomass is largely composed of three major constituents: cellulose, hemicellulose and lignin [1]
We extended the transmission electron microscopy (TEM) analysis to visualize ultrastructural modifications to the cell wall resulting from the anaerobic dilute acid (DA)/Fe3+ ion pretreatment process
The study revealed that aerobic conditions partially compromise the efficiency of DA/Fe3+ and DA/Fe2+ pretreatments of biomass, but this compromise can be offset by increasing the concentration of Fe co-catalysts into the range of 5 to 10 mM

Summary

Introduction

Lignocellulosic biomass is largely composed of three major constituents: cellulose, hemicellulose and lignin [1]. The efficient conversion of lignocellulosic biomass into fermentable monosaccharides still remains a difficult task due to the recalcitrance of biomass to deconstruction [7]. As one of the leading biomass conversion technologies, pretreatment of lignocellulosic biomass with dilute acid (DA) at mild conditions effectively liberates hemicellulose, modifies lignin networks, and increases cellulose accessibility [8,9]. DA pretreatment presents several challenges as well as opportunities for optimization, including improving sulfuric acid recovery, lowering the need for neutralization of pretreated biomass prior to enzymatic saccharification, and reducing the need for acid resistant reactors with expensive metallurgy. Precise process control such as adjusting the pretreatment severity is required to minimize the potential degradation of desired

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