Dilute phosphoric and oxalic acids as pretreatments for woody biomass prior to enzymatic hydrolysis

Using sulfuric acid as a pretreatment prior to enzymatic hydrolysis and ethanol fermentation presents two significant problems; excessive degradation of biomass carbohydrates and the disposal of solid waste (gypsum) after neutralization with lime. Phosphoric acid is not a strong mineral acid, and it can be neutralized with ammonia and used as a nutrient for the yeast. Oxalic acid is a strong organic acid but weaker than sulfuric acid. This acid can be neutralized with ammonia and left in the system. But if toxic to the yeast it can be precipitated as calcium oxalate, removed, and burnt to recover lime, which can be recycled. When using either of these two acids no generation of solid waste occurs.; Hybrid poplar (Populus deltoides x Populus nigra) was pretreated using phosphoric or oxalic acid in two types of solvents: water and organosolv, using 70% (v/v) methanol. Aqueous phosphoric acid concentrations of 0.1 M, 0.05 M and 0.025 M were used. For organosolv phosphoric acid pretreatments and all oxalic acid pretreatments only 0.1 M and 0.05 M were used. The temperature range used was 150-190{dollar}spcirc{dollar}C, with pretreatment times of 5-90 min. The results indicate that phosphoric acid is an effective pretreatment for biomass prior to enzymatic hydrolysis. Lower acid concentrations performed as well as 0.1 M, but they required higher temperatures (175{dollar}spcirc{dollar}C or above) and longer reactions times (20 min. or more). SEM micrographs show that phosphoric acid is a mild pretreatment causing less degradation than sulfuric acid or even oxalic acid. The organosolv phosphoric acid pretreatments were particularly successful. The high degree of delignification resulted in a fiberized woody material. Aqueous oxalic acid pretreatments at 175{dollar}spcirc{dollar}C resulted in a high ethanol production, but the acid was very corrosive. Organosolv oxalic acid pretreatments were performed at higher temperatures without corrosion problems. The pH of the organosolv oxalic acid spent liquor was higher than when using water only. The results with organosolv oxalic acid compared unfavorably with the results of organosolv phosphoric acid.; The crystallinity index was higher for the pretreated samples than for the untreated hybrid poplar. No correlation was found between crystallinity index and maximum ethanol production. The degree of delignification of the organosolv pretreatments is highly correlated with maximum ethanol production, as well as with the amount of hemicelluloses hydrolyzed during the pretreatment.; The combined severity parameter appears to be a good prediction factor for maximum ethanol production of organosolv phosphoric acid pretreatments.