Effective copper-catalyzed alkaline-oxidative pretreatment of woody biomass

Woody biomass represents a vast source of fermentable carbohydrates and offers significant advantages relative to other potential lignocellulosic feedstocks.  We recently discovered that alkaline hydrogen peroxide pretreatment catalyzed by copper(II) 2,2’-bipyridine complexes (Cu-AHP) significantly increases the enzymatic digestibility of woody feedstocks.  Subsequent optimization of Cu-AHP has led to decreased catalyst utilization, copper and base recycling, lower enzyme concentrations, and more efficient H₂O₂ utilization.  Utilizing our new, modified pretreatment method we are currently achieving glucose and xylose yields of over 90%, and the resulting hydrolysate can be readily fermented to achieve high titers of ethanol.  Furthermore, a detailed comparison of a diverse set of 36 hybrid poplar samples as well as four different species of hardwoods has established the critical factors affecting digestibility.

Mechanistic studies of the catalytic oxidation process confirmed disruption of the cell wall layers, which is associated with lignin removal and cellulose oxidation.  Interestingly, preliminary analysis of the Cu-AHP pretreated biomass revealed the presence of copper nanoparticles at the site of cell wall disruption.  Detailed analysis of the solubilized lignin via 2D-NMR revealed oxidized syringyl and guaiacyl units as well as the presence of aldehydes groups conjugated to aromatic rings.  Together these results provide important clues into the oxidative reaction mechanism.  Intriguingly, a large portion of both the solubilized and unsolubilized lignin appears to be unmodified, indicating that Cu-AHP pretreatment of woody biomass may also provide a convenient source of clean lignin amenable to subsequent valorization.