Of late I have been learning about the mechanisms involved in the pretreatment of ligno-cellulosic material for hydrolysis. Here I hope to summarize what I have learned about this process. Typically wood that is good for building materials and pulping for use in paper making is bad for breaking down via hydrolysis. Building materials require durable fibers and paper making requires strong fibers. The weaker the fibers, the easier they are to break down. For this reason our current population of forest is less desirable than say poplar trees for turning into ethanol.
Selecting a favorable biomass which is easy to break down is only one of the factors which contributes to increasing the rate of cellulosic hydrolysis. Other ways include improving the effectiveness of hydrolytic agents, and increasing the accessibility of penetrating agents. I would like to devote the remainder of this summary to ways of increasing the accessibility of these hydrolyzing agents.
One of the major problems with separating the cellulose from the lignin, hemicellulose, and resins/other extractives is the nature of cellulose. Cellulose has both crystalline and amorphous parts which react differently to chemical attacks.
The cellulose is also bundled by lignin and hemicellulose which makes separating the different compounds in a plant difficult.
In order to separate the cellulose from the other constituents, it is generally a good idea to use some sort of physical mechanism to grind or crush the raw material which increases it’s total surface area. After the surface area is increased, a chemical attack proceeds which acts to separate the cellulose. In order to disrupt the hemicellulose linkages to the cellulose, either break the hydrogen bonds (swelling agents such as NaOH will do this), or break the ester cross links via acids, bases, or alcohols. In an ethanol plant the product ethanol could be used for this. Disrupting the lignin cellulose linkages can be performed in the same step as well using the same techniques for disrupting the hydrogen bonds. Surfactants can be applied as well which will change the hydrophobic-hydrophilic associations.
Once the Lignin-hemicellulose-cellulose complex is disrupted, the cellulose needs to be pretreated for hydrolysis. Because amorphous cellulose is so easily attacked by enzymes, it protects the crystalline part of the structure. If an enzyme attacks the cellulose, it has an easy time turning the amorphous region into glucose. This glucose then gets in the way of the enzyme so that it can not keep attacking the crystalline region. In order to efficiently hydrolyze the cellulose both regions need to be turned into glucose which means that the crystalline structure needs to be disrupted. This can be done in a number of ways. The amorphous cellulose can be first removed, the structure can be heated, disperse the complex in water, even applying pressure will work. The hard part is to do this efficiently.
Hydrolysis of cellulose: mechanisms of enzymatic and acid catalysis: based on a symposium jointly sponsored by the ACS Cellulose, Paper, and Textile Division, the Institute of Paper Chemistry of TAPPI, at Appleton, Wisconsin, May 17-19, 1978 / R.D. Brown, Jr., editor, L. Jurasek, editor.
