(787a) A Study in Lignin Inhibition for Enzymatic Hydrolysis of Woody Biomass

AIChE 2012

October 28 ~ November 2

Pittsburgh, PA

A study in lignin inhibition for enzymatic hydrolysis of woody biomass

Zhiying Yu*, Hasan Jameel, Hou-min Chang, Sunkyu Park

Department of Forest Biomaterials, North Carolina State University

Campus Box 8005, Raleigh, NC 27695-8005, USA

Presenting author: zyu2@ncsu.edu

The presence of lignin in woody biomass is a great obstacle that limits the bioconversion efficiency through enzymatic hydrolysis, especially for softwood. It has been speculated that the differences in lignin content, structure, and distribution between hardwood and softwood, will influence lignin-enzyme interactions such as non-specific adsorption of enzyme onto lignin, physical blockage of lignin that restrict the enzyme penetration into biomass structure, and chemical blockage of binding sites through lignin-carbohydrate linkages. In this research, the milled wood lignins (MWL) isolated from eucalyptus, maple and loblolly pine as well as kraft lignin (indulin AT) was investigated for their impact on enzymatic hydrolysis. The woody biomass structure was mimicked by 1) physically mixing the lignins and bleached pulps and, 2) reconstructing the woody biomass via impregnating the lignins into bleached pulps.

The adsorption kinetics of cellulase on individual bleached pulps and lignins showed that the bleached pulps adsorbed 2 to 3 folds of cellulase than lignins. MWL adsorbed 17.5-22.5% cellulase, while kraft lignin only had less than 3.5 % cellulase adsorption. When 20% lignins were physically mixed with bleached pulps for enzymatic hydrolysis, it was found that the presence of 20% MWL had a minor impact on enzymatic hydrolysis, while 20% kraft lignin showed 8.3-11.6% limitation on the hydrolysis. In contrast, when 20% lignins were impregnated into the bleached pulps, 6.2%, 12.0%, 17.6% and 59.0% decrease in 96 h hydrolysis yield was observed for reconstructed eucalyptus, maple, pine and kraft pulps, respectively. Therefore, MWL isolated from untreated hardwood and softwood was found to limit enzymatic hydrolysis due to the effect of physical blockage. A more significant impact of physical blockage of lignin was found with the increasing of guaiacyl unit of lignins in the reconstructed woody biomass with the same lignin content. Kraft lignin inhibited the enzymatic hydrolysis through slightly deactivating the enzymes and significantly blocking the accessible area.

It was speculated that the smaller molecules of lignin had a greater physical blockage effect on enzymatic hydrolysis. Thus, different molecular weight of kraft lignins was fractionated by selective pH and ultrafiltration, respectively. The lignins of bigger molecular weight were found to have less physical blockage than the lignin with smaller molecular weight. In order to investigate the effect of lignin from pretreated lignocelluloses on enzymatic hydrolysis, the cellulolytic enzyme lignins were isolated from acid pretreated and alkali pretreated hardwood and softwood pulps. The effect of acid and alkali pretreatment on lignin structure as well as the effect of hardwood and softwood lignins on enzymatic digestibility will be discussed in terms of three inhibitory mechanisms suggested above.