Year of Publication

2015

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Stephen E. Rankin

Abstract

Lignocellulosic biomass is a sustainable and renewable energy resource that can be converted to fuels and other commodity chemicals, but this conversion is currently limited by its recalcitrance to enzymatic degradation. Because of this recalcitrance, the major challenges in the commercialization of enzymatic hydrolysis processes are the relatively low hydrolysis rates, limited cellulose conversion under some conditions, and high cost of enzymes.

Enzymatic hydrolysis is influenced by the structure of the biomass after pretreatment and the mode of enzyme action, but has also been shown to be enhanced by surfactant additives. The objective of this work was to elucidate the mechanism of hydrolysis by studying the activity of cellulase enzymes and the effects of non-ionic surfactant Tween-80 on the interactions of model cellulose (varying in surface morphology and crystallinity) and lignin films with cellulases. The primary tool used to measure the binding and activity of cellulase enzymes derived from Trichoderma reesei was a quartz crystal microbalance with dissipation monitoring (QCM-D). The nonionic surfactant Tween-80 was found to reduce the adsorption of cellulases onto all types of cellulose films. Tween-80 had no significant effect on hydrolysis of amorphous LiCl/DMAc cellulose films whereas the hydrolysis rate decreased with increase in Tween-80 concentration for type II crystalline NMMO cellulose films. On lignin, co-adsorption of Tween-80 and cellulase resulted in an apparent net reduction in the amount of cellulase adsorbed on lignin. Sequential adsorption experiments suggested that Tween-80 was able to reduce and displace adsorbed cellulases. Thus, Tween-80 was found to compete effectively with cellulase enzymes for binding to hydrophobic surfaces such as lignin without significantly impeding hydrolysis of cellulose, which explains how it is able to enhance overall conversion for bulk biomass hydrolysis.

To gain fundamental understanding of the hydrolysis process, a kinetic model based on the processive action of cellulase enzymes was developed and applied to QCM-D data. The model makes a distinction between surface cellulose units and bulk sites that only become accessible as hydrolysis proceeds. The model predictions during binding and enzymatic hydrolysis under various scenarios are discussed along with future possible work.

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