Efficiencies of acid catalysts in the hydrolysis of lignocellulosic biomass over a range of combined severity factors

Dicarboxylic organic acids have properties that differ from those of sulfuric acid during hydrolysis of lignocellulose. To investigate the effects of different acid catalysts on the hydrolysis and degradation of biomass compounds over a range of thermochemical pretreatments, maleic, oxalic and sulfuric acids were each used at the same combined severity factor (CSF) values during hydrolysis. Xylose and glucose concentrations in hydrolysates were highest with maleic acid. Oxalic acid gave the next highest followed by sulfuric acid. This ranking was particularly true at low CSF values. The concentrations of glucose and xylose increased with oxalic and sulfuric acid pretreatments as the CSF increased, but they never attained the levels observed with maleic acid. Among sulfuric, oxalic and maleic acid treatments, the amount of xylose released as xylooligosaccharide was highest with sulfuric acid. The fraction of xylooligosaccharide was lowest with the maleic acid and the oligosaccharide fraction with oxalic acid fell in between. Furfural and hydroxymethyl furfural levels were also highest with maleic acid. In subsequent fermentations with pretreated biomass, the ethanol concentration was maximal at 19.2 g/l at CSF 1.9 when maleic acid was used as the pretreatment catalyst. This corresponded to an ethanol volumetric production rate of 0.27 g ethanol/l per h. This was the same condition showing the highest xylose production in following pretreatment with various acid catalysts. These findings suggest that maleic and oxalic dicarboxylic acids degrade hemicelluloses more efficiently than does sulfuric acid.