Lignin-based polyoxyethylene ether enhanced enzymatic hydrolysis of lignocelluloses by dispersing cellulase aggregates

Highlights

• Enzymatic hydrolysis lignin is used to prepare EHL–PEG.

• EHL–PEG improved glucose yield of corn stover from16.7% to 70.1%.

• EHL–PEG reduced cellulase non-productive adsorption on lignin by 88%.

• EHL–PEG dispersed cellulase aggregates from 1220 nm to 420 nm.

Abstract

Water-soluble lignin-based polyoxyethylene ether (EHL–PEG), prepared from enzymatic hydrolysis lignin (EHL) and polyethylene glycol (PEG1000), was used to improve enzymatic hydrolysis efficiency of corn stover. The glucose yield of corn stover at 72 h was increased from 16.7% to 70.1% by EHL–PEG, while increase in yield with PEG4600 alone was 52.3%. With the increase of lignin content, EHL–PEG improved enzymatic hydrolysis of microcrystalline cellulose more obvious than PEG4600. EHL–PEG could reduce at least 88% of the adsorption of cellulase on the lignin film measured by quartz crystal microbalance with dissipation monitoring (QCM-D), while reduction with PEG4600 was 43%. Cellulase aggregated at 1220 nm in acetate buffer analyzed by dynamic light scattering. EHL–PEG dispersed cellulase aggregates and formed smaller aggregates with cellulase, thereby, reduced significantly nonproductive adsorption of cellulase on lignin and enhanced enzymatic hydrolysis of lignocelluloses.

Introduction

Lignocellulosic biomass is the most abundant and renewable source on earth and can be converted to value-add chemicals or bio-fuels through the sugar platform (Himmel et al., 2007, Ragauskas et al., 2006). A pretreatment to alter and/or remove lignin and hemicellulose is necessary prior to enzymatic hydrolysis because of the recalcitrance of lignocelluloses (Zhao et al., 2012). However, lignin is usually partially removed because of the high cost of the complete delignification according to the cost accounting of pulp and paper industry (Leu and Zhu, 2013). As a result, the residual lignin acts as a physical barrier and an attractant to cellulase to reduce the adsorption of cellulase on cellulose (Kumar et al., 2012, Li et al., 2014, Selig et al., 2007). Hence, lignin has a significant negative impact on the enzymatic hydrolysis efficiency of lignocelluloses (Nakagame et al., 2011, Pan, 2008, Yu et al., 2014, Zeng et al., 2014).

Addition of surfactant (Börjesson et al., 2007a, Börjesson et al., 2007b, Eriksson et al., 2002, Kristensen et al., 2007, Li et al., 2012, Ouyang et al., 2010, Ouyang et al., 2011) and protein (Yang and Wyman, 2006) could decrease the nonproductive adsorption of cellulase on the lignin and enhance the enzymatic hydrolysis efficiency of lignocelluloses. Small molecular anionic surfactants restrained the enzymatic hydrolysis (Eriksson et al., 2002), while anionic polymeric surfactants like lignosulfonate, a byproduct from pulping spent liquor, improved the enzymatic hydrolysis (Lou et al., 2013b, Lou et al., 2014, Wang et al., 2013, Zhou et al., 2013). Non-ionic surfactants, especially polyethylene glycol (PEG) based polymer, were found to be the most effective (Eriksson et al., 2002). It had been suggested that the mechanism of enhancing the enzymatic hydrolysis of lignocelluloses by PEG included improving the stability and activity of cellulase, changing the substrate structure and reducing the nonproductive adsorption of cellulase (Börjesson et al., 2007b, Li et al., 2012, Ouyang et al., 2010). But a convincing explanation is still lacking (Eriksson et al., 2002).

Lignin is the second most abundant and renewable carbon resource on earth after cellulose. A large amount of residual lignin, generated from lignocellulosic ethanol production, is usually burned to meet the internal energy use, that has not been efficient utilization (Ragauskas et al., 2014). If enzymatic hydrolysis lignin (EHL) could be transformed to value-added materials, chemicals or fuels through an energy-effective and cost-effective route, it would greatly improve the biorefinery viability (Ragauskas et al., 2014). Lignin needs to be modified before applying as a dispersant for cement slurry, coal water slurry, pesticide suspension concentrates and so on due to its poor solubility and reactivity (Lin et al., 2014, Lou et al., 2013a, Zhou et al., 2007).

Amphiphilic lignin derivatives, prepared from acetic acid lignin and PEG-epoxide, could significantly improve the enzymatic hydrolysis efficiency of unbleached cedar pulp, but the enhancement was slightly less than that by PEG4000 (Winarni et al., 2013). Besides, residual activity of cellulase after enzymatic hydrolysis was remained at higher level by lignin derivatives than PEG4000 (Winarni et al., 2013). A method to prepare a water-soluble lignin-based polyoxyethylene ether from kraft lignin and PEG was recently proposed by our group (Lin et al., 2014), and the product contained phenylpropane unit and branched PEG hydrophilic chains. The present study is aimed to implement the comprehensive utilization of lignocellulosic biomass, as shown in Fig. 1. The residual enzymatic hydrolysis lignin (EHL) was PEGylated in order to improve the enzymatic hydrolysis efficiency of lignocelluloses.

The objective of this study is to evaluate the effect of EHL-based polyoxyethylene ether (EHL–PEG) on the enzymatic hydrolysis efficiency of corn stover. The role of EHL–PEG on the enzymatic hydrolysis of microcrystalline cellulose with different lignin contents was discussed. In order to reveal the underlying mechanism of enhancing the enzymatic hydrolysis of lignocelluloses, the effect of EHL–PEG on the nonproductive adsorption of cellulase on the lignin film and the aggregation and dispersion of cellulase was further investigated by quartz crystal microbalance with dissipation monitoring (QCM-D) and dynamic light scattering (DLS) analysis, respectively.