Processing of Cellulose for the Advancement of Biofuels

Abstract

The enzymatic degradation of cellulose polymers is currently a rate-limiting step in the bioconversion of biomass to biofuels. Cellulose polymers self assemble to form crystalline structures stabilized by a complex network of intermolecular interactions such as hydrogen bonding. The network of interactions in crystalline cellulose (cellulose nanostructure) poses an energy barrier that limits enzymatic degradation as apparent from the activity of Cel5H. To improve the degradability of cellulose the intermolecular interactions must be disrupted. The interactions of the cellulose nanostructure prevent solubilization by water and most other common solvents, but some organic solvents aid degradation of cellulose suggesting they influence cellulose nanostructure. The objective of this work is to understand the influence of solvents on cellulose nanostructure with the goal of improving the degradability of cellulose nanostructure using solvents. To understand solvent interaction with cellulose, phosphoric acid was used to first solubilize cellulose (PAS cellulose) followed by adding an organic liquid or water to wash the phosphate from the system. The Flory Huggins theory was used to predict wash liquids that could favorably interact with cellulose. A favorable wash liquid was predicted to prevent the reformation of crystalline domains to yield a disrupted cellulose nanostructure, which should be more degradable. Low molecular weight alcohols and glycols were calculated to be favorable wash liquids.

Washing PAS cellulose with the predicted favorable liquids yielded semi-transparent gel-like materials compared to the opaque white precipitate formed when water or unfavorable solvents were used in the wash. Fractal analysis of small angle neutron scattering (SANS) of these apparent gels indicated cellulose polymers likely have the properties of clustered rods. This partial disruption increased degradability relative to the water washed PAS cellulose. The apparent rod-like cellulose nanostructures suggested the presence of intra and interpolymer hydrogen bonding. Characterization of the hydrogen bonding network by Fourier transform infrared resonance (FTIR) indicated the gel-like material formed by ethanol washes was the result of heterogeneous interpolymer hydrogen bond cross-links.

The interactions leading to gel-like materials were evaluated using Hansen solubility parameters, which predicted mixtures of ethanol and water may be most effective for disrupting cellulose nanostructure. Fractal analysis by SANS indicated 40 % ethanol/water was most effective. Similar results were obtained when 40 % ethanol was used to disrupt the cellulose nanostructure in municipal office waste (MOW). Ethanol washes increased the degradability of MOW by at least 30 % relative to conventional water washing. This is significant because increased degradability of MOW could further the development of cellulosic biofuels by reducing the amount of enzyme required to digest the material.