Detoxification of biomass hydrolysates by reactive membrane extraction

Abstract

Economical conversion of lignocellulosic biomass into biofuels is essential to reduce the world's dependence on fossil fuels. The typical biochemical process for biomass conversion includes a thermochemical pretreatment step to improve enzymatic cellulose hydrolysis and to release hemicellulosic sugars from the polymer matrix. However compounds that are toxic to microorganisms in subsequent fermentation steps may also be released. This work investigates the use of membrane extraction to detoxify or remove these toxic compounds from corn stover hydrolysates pretreated using dilute sulphuric acid.

Extraction of sulphuric, acetic, formic and levulinic acid as well as 5-hydroxymethylfurfural and furfural has been investigated. Octanol and oelyl alcohol were used as organic phase solvents. Alamine 336 was used as the aliphatic amine extractant. Reactive extraction of sulphuric, acetic, formic and levulinc acid was observed while 5-hydroxymethylfurfural and furfural were extracted due to their distribution in the organic solvent. Significant removal of all toxic compounds investigated was obtained as well an increase in pH from 1.0 to 5.0. As small quantities of the organic phase transferred into the hydrolysate during extraction, the toxicity of the organic phase must be considered. As it is likely that detoxification will require the use of another unit operation in combination with membrane extraction, the economical viability of the combined process must be considered.

Introduction

Development of efficient unit operations for conversion of lignocellulosic biomass into biofuels will be essential in order to replace up to 30% of the petroleum-based transportation fuels with biofuels by 2030 [1]. The economic viability of producing biofuels from biomass relies significantly on obtaining high yields of sugar from lignocellulosic biomass at low cost [2], [3]. Here the focus is on the production of bioethanol from corn stover. Corn stover is a likely near-term feedstock because it is readily available in large quantities. The main steps involved in the conversion of corn stover into bioethanol are: pretreatment, hydrolysate detoxification, enzymatic cellulose hydrolysis and co-fermentation of the sugars, and product separation and purification [4].

Lignocellulosic biomass consists of three main polymers: cellulose, hemicellulose and lignin. In the pretreatment step, biomass is treated to improve the susceptibility of the cellulose to enzymatic hydrolysis. Many different mechanical and thermochemical methods have been proposed for biomass pretreatment [5]. Dilute sulphuric acid was used to produce the material tested in this study. Dilute sulphuric acid has been shown to be effective at producing a xylose-rich hemicellulose hydrolysate liquor while enhancing cellulose enzymatic digestibility [6]. Effective dilute sulphuric acid pretreatment not only releases xylose and acetic acid, but also results in the formation of sugar degradation compounds such as 5-hydroxymethylfurfural (HMF), furfural, levulinic and formic acid and phenolic-based lignin fragments that inhibit subsequent bioconversion of the solubilized sugars into ethanol [7], [8].

The quantity of toxic compounds produced depends on the severity of the reaction (temperature, concentration and time of dilute sulphuric acid pretreatment) [9], [10]. While lower severity leads to lower concentrations of toxic compounds, it also leads to lower sugar yields. Thus optimized pretreatment and subsequent detoxification steps, to remove toxic compounds that are produced, are essential to maximize sugar yields and hence increase ethanol yields in the subsequent fermentation step.

In earlier work [7] it was shown that acetic acid may be removed by reactive hollow fibre-based membrane extraction. Schlosser et al. [11] present a summary of published studies on the use of membrane extraction for the recovery of various carboxylic acids. Membrane extraction has a number of advantages over conventional extraction. Most importantly for the detoxification of biomass hydrolysates, membrane extraction avoids the need to disperse one phase in the other thus minimizing the likelihood of entrainment of small amounts of organic phase in the aqueous hydrolysate phase. Given that the organic phase is likely to be toxic to the microorganisms used in the subsequent fermentation step, minimizing transfer of organic phase into the aqueous phase will be critical.

Alamine 336, a long chain aliphatic amine was used as the extractant while octanol was the organic solvent. Alamine 336 chemically complexes with the acetic acid present. Further, sulphuric acid present in the hydrolysate is also removed. Eyal and Canari [12] have described four major mechanisms for reactive extraction of acids by amines. Since sulphuric acid is a strong acid it is likely that it forms an ion pair with the amine in the organic phase. At pH values below the pK_a of acetic acid, the protonated form of acetic acid is extracted. Further overloading of the amine is possible by hydrogen bonding [13]. Membrane extraction was shown not only to remove acetic acid but also to increase the pH of the hydrolysate (by removal of sulphuric acid). After dilute sulphuric acid pretreatment, the pH of the hydrolysate is about 1. Increasing the pH to 5–6 is essential in order to conduct the subsequent enzymatic cellulose hydrolysis and fermentation steps.

The hydrolysate contains other toxic compounds such as HMF, furfural, formic and levulinic acid. The economic viability of a membrane extraction process may depend on the extent to which these other toxic compounds are also removed. Removal of HMF, furfural, formic and levulinic acid as well as acetic and sulphuric acid have been investigated. While formic and levulinic acid will form chemical complexes with Alamine 336, it is likely that HMF and furfural will be removed by a non-reactive mechanism. In addition, as the reactive extractant and organic phase solvent could be toxic to the microorganisms used in the subsequent fermentation step, it is essential to determine the concentration of these compounds in the hydrolysate after extraction. The results indicate that the efficiency of extraction of the different toxic compounds present in the hydrolysate varies greatly. Further, some transfer of the organic phase into the hydrolysate always occurs.