(3cq) Biomass Depolymerization Using Biphasic H2O-CO2 Mixtures

Sustainably producing concentrated solutions of monosaccharrides is a key bottleneck in the conversion of lignocellulosic biomass to biofuels or bioproducts. Most pretreatment and enzymatic hydrolysis processes are run at low-solid concentration (<10 wt%) and use chemical catalysts while high-solids enzymatic hydrolysis reactions are almost always performed with air-dried pretreatment mixtures.

Initial work during my doctoral research focused on exploring the use of biphasic H₂O-CO₂ mixtures as an alternate medium for high-solids (up to 40 wt%) pretreatment. Early studies were done in a small (25 ml) unstirred reactor using a single temperature stage. More recently, two-temperature stage pretreatment was introduced and optimized in a larger 1 L stirred reactor to take advantage of the biomass depolymerization temperature dependent reaction sequence (a short high-temperature stage at 200-210ºC was followed by a longer low-temperature stage at 160-170ºC). Optimally pretreated substrates (210ºC, 16 min for hardwood and 210ºC, 1 min for switchgrass) were used as feedstock in high-solids enzymatic hydrolysis reactions that gave glucose yields above 80% for both switchgrass and hardwood after 48 hrs of hydrolysis. Therefore, without additional chemical catalysts or any drying, two-temperature stage H₂O-CO₂ pretreatment coupled with high solids enzymatic hydrolysis can produce monosaccharide solutions of 185 gr/L and 148 gr/L for mixed hardwood and switchgrass, respectively. This suggests that H₂O-CO₂ pretreatment is an attractive alternative to chemically catalyzed processes such as dilute acid pretreatment.

Parallel to these studies, efforts are underway to better understand the relationship between the effects of pretreatment and the enzymatic depolymerization mechanisms of cellulosic substrates. A fluorescence confocal microscopy method was developed for observing and measuring the binding of cellulases in situ, for following the temporal morphological changes of cellulosic materials during their depolymerization by a commercial cellulase mixture. The Spezyme CP cellulase cocktail was supplemented with a small fraction of fluorescently labeled T. Reseii CBH I, which served as a reporter to track cellulase diffusion and binding onto the internal physical structure of various substrates (bacterial microcrystalline cellulose, cellulose filter paper and pretreated hardwood and switchgrass). Kinetic models were developed and parameters were extracted using the variation in fluorescence intensity of the substrate and the bound enzyme over time and were successfully used to predict the extent of similar reactions in solution.