Tuesday, May 3, 2011
The utilization of lignocellulosic biomass in a sugar-based biorefinery necessitates the deconstruction of the cell wall lignin-carbohydrate complex (LCC) network that further facilitates the release of fermentable sugars. These sugars can be subsequently fermented using engineered microbes to produce desired liquid fuels (e.g., ethanol, isobutanol) and chemicals (e.g., succinic acid). The disruption of the LCC network is accomplished using a thermochemical pretreatment step prior to enzymatic hydrolysis. Understanding the mechanisms by which pretreatment enhances enzymatic hydrolysis can contribute to the improvement of pretreatment methodologies and further reduction of pretreatment severity. In our previous work (Biores Technol 2010 101:8429-8438), we have characterized decomposition products formed during ammonia-based pretreatments (e.g., AFEX) as a function of the reaction conditions. We found that ammonolysis and hydrolysis reactions target ester linkages present within cell walls forming corresponding amides and acids, respectively. These ester bonds are abundant in the LCC, in the form of ferulate/diferulate esters that crosslink lignin and polysaccharides. A direct correlation between the extents of cleavage of these ester linkages with the enzymatic digestibility of the pretreated biomass was observed, revealing that these reactions can be monitored as potential markers to evaluate pretreatment effectiveness. A comprehensive kinetic study of ammonolysis and hydrolysis of model ferulate and p-coumarate esters is presented here. The extent of ammonolysis and hydrolysis reactions were determined as a function of various reaction parameters (solvent type, temperature, ammonia concentration and time). An evaluation of the conditions that maximize the extent of these reactions with minimal energy inputs was carried out.