Single reagent treatment and degradation of switchgrass using iron(III)chloride: The effects on hemicellulose, cellulose and lignin

Highlights: • FeCl₃ (2% w/w) is the most effective among FeCl₃, MgCl₂ and CaCl₂. • FeCl₃ impregnated and micro waved switchgrass gave 31.5% TRS yield. • FeCl₃ impregnated and heat treated switchgrass gave 28.6% TRS yield. • Interactions between FeCl₃ and lignin is the possible cause of catalytic activity. -- Abstract: The possibility of using abundant metal chlorides as a single reagent for pretreatment and degradation of switchgrass biomass was studied. Three metal chlorides: MgCl₂, CaCl₂ and FeCl₃ were tested in the initial phase and FeCl₃ at 2% w/w loading was found as the most effective reagent for treatment and hydrolysis of switchgrass in hot compressed water. Therefore, FeCl₃ was selected for the detailed study and the heat treatment at 140–160 °C or microwaving for 6–10 min was required to activate the FeCl₃ after impregnation of switchgrass. These samples could be hydrolyzed in water at 160 °C, without additional catalyst or reagent to produce reducing sugars. In the hydrolysis experiments, FeCl₃ impregnated and micro waved switchgrass produced slightly higher total reducing sugar (TRS) yields of 31.5%; whereas the samples heated at 140 °C after FeCl₃ impregnation produced 28.6% TRS yield. In a thermogravimetric analysis study, the decomposition temperature of lignin fraction showed the highest shift (ΔT_decomp ~ - 42 °C) after adding 2% w/w FeCl₃ and microwaving for 10 min, indicating that strong interactions between FeCl₃ and lignin is likely the primary cause of enhancement in polysaccharide degradation.