Sustainably producing concentrated solutions of monosaccharrides from biomass 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 experiments are almost always performed with air-dried pretreatment mixtures.
This study uses a stirred 1 L reactor to study high solids (40 wt%) and high-pressure (200 bar) H2O-CO2 pretreatment of mixed hardwood and switchgrass. In addition, two-temperature stage pretreatment was introduced and optimized to take advantage of the temperature dependent biomass depolymerization reaction sequence (a short high-temperature stage at 200-210ºC for 0 to 24 min was followed by a long low-temperature stage at 160-170ºC for 60 min). Optimal glucan to glucose yields around 80% were obtained after 72 hrs of dilute hydrolysis (1 wt% glucan) for both substrates at similar pretreatment conditions (210ºC, 16 min for hardwood and 210ºC, 1 min for switchgrass, both followed by 60 min at 160ºC). These optimally pretreated substrates were used as feedstock in high-solids enzymatic hydrolysis reactions, which gave glucose yields above 80% for both hardwood and switchgrass 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 (170 gr/L of glucose and cellobiose 15gr/L of hemicellulose sugars) for mixed hardwood and 148 gr/L (115 gr/L of glucose and cellobiose 33gr/L of hemicellulose sugars) for switchgrass. This suggests that H2O-CO2 pretreatment is an attractive alternative to chemically catalyzed processes such as dilute acid pretreatment.
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