(62c) Hemicellulose and Lignin Recovery for Ammonium Hydroxide Pretreatment Processes

During ammonium hydroxide pretreatment of lignocellulosic materials such as switchgrass, a majority of the lignin and some hemicellulose are extracted from the cellulosic matrix. The remaininig hemicellulose and cellulose are then hydrolyzed into sugars.  Although C6 sugars are easily metabolized by native yeast strains to make ethanol, for instance, C5 sugars, which largely originate from hemicellulose, are not very easily metabolized by native yeast strains, and pass through the downstream fermentation process nearly untouched. Efficient usage of C5 sugars in ethanol fermentations therefore requires access to engineered yeast strains with modified biochemical pathways, and such strains are not necessarily available to everyone without the expertise to create such strains, or without the funding to purchase licenses for them.

C5 sugars have value beyond their merit as a carbon source for ethanol-producing yeast, and can be used to make value-added products such as xylitol, succinic acid, and furfural, all of which sell for a greater unit price than ethanol. If the pretreatment process can be improved to extract more hemicellulose from the lignocellulosic matrix prior to the hydrolysis step, then perhaps a purer stream of hemicellulose might be recovered that is amenable to making value-added products. 

A process to recover lignin from ammonium hydroxide pretreatment fluids was recently developed, but extracted hemicellulose is not recovered in that method, and it is lost as a waste material.  The work described here examines the sequential application of a long-known hemicellulose extraction step involving calcium hydroxide, either before or after ammonium hydroxide pretreatment, to switchgrass, and measures the effects of the hemicellulose extraction on the amount and purity of hemicellulose recovered, the amount and purity of lignin recovered, and the behavior of the pretreated solid material during enzymatic hydrolysis.  Application of a pretreatment step that is selective for hemicellulose provides a distinct path for recovering both lignin and hemicellulose as side products, and may enable for faster or more complete enzymatic hydrolysis of the remaining cellulosic material downstream.  A high-level examination of the potential application of a preferred sequence at a pilot or larger scale is also discussed.