A molar basis comparison of calcium hydroxide, sodium hydroxide, and potassium hydroxide on the pretreatment of switchgrass and miscanthus under high solids conditions

Pretreatment is an essential step in the formation of liquid biofuels. Switchgrass and miscanthus were pretreated using calcium hydroxide, potassium hydroxide, and sodium hydroxide at an equivalent hydroxyl concentration (0.46 g OH⁻/g dry biomass), at a solids content of 40%, and two temperatures (25 and 50 °C) for seven days. The cellulose, hemicellulose, and lignin composition before and after pretreatment were quantified according to the standard procedures developed by the National Renewable Energy Laboratory. After pretreatment, enzyme hydrolysis was performed at 50 °C in a shaking incubator with an enzyme loading of 60 FPU/g cellulose. Potassium and sodium hydroxide led to the largest reduction in lignin of between 30–47% with the two feedstocks and two temperature levels. Calcium hydroxide had a significantly lower amount of delignification of between 13–21%. Sodium and potassium hydroxide had a similar maximum reaction rate, except for switchgrass pretreated at 50 °C where the reaction rate was lower for potassium hydroxide. Cellulose conversion was the highest for sodium and potassium hydroxide and varied between 56.1 and 80.5% for both feedstocks at the two pretreatment temperatures investigated. Calcium hydroxide demonstrated a significantly lower cellulose conversion that varied between 23.0 and 44.9%. The results indicated that with an equivalent molar basis of OH⁻, potassium and sodium hydroxide had superior performance relative to calcium hydroxide in a high solids environment.