Catalyst Type and Concentration

In order to evaluate acetic acid and sulfuric acid as catalysts, ethanol organosolv pretreatment of OPEFB was carried out at 210 ◦C for 120 min with S/L ratio of 1:20. Both acids were added to the solvent until an initial pH of 3 was achieved. The amount of acetic acid added was 0.32 g acid/g substrate, while the use of sulfuric acid was much lower, i.e., 0.0013 g acid/g sample. The effect of catalyst type on lignin purity and recovery is presented in Figure 1a. The addition of acetic acid and sulfuric acid did not have a significant effect on the lignin purity, which were 65.70 ± 4.10 and 68.25 ± 3.50%, respectively, based on the dry weight. Lignin recovery from the two pretreatment conditions also showed no significant difference (53.02 ± 0.46% for acetic acid and 48 ± 2.91% for sulfuric acid). Since the amount of acetic acid added was higher than that of sulfuric acid, sulfuric acid is thus considered more efficient as a catalyst.

A further experiment was carried out where the amount of acetic acid added to the solvent was equal to the amount of sulfuric acid added to reach pH 3. The amount of both acids added was thus 0.0013 g/g sample. The final pH of the solvent with acetic acid was 4.66. Results from this experiment are presented in Figure 1b. Organosolv pretreatment with sulfuric acid showed better performance with significantly higher lignin purity (68.25 ± 3.50%) and lignin recovery (48.00 ± 2.91%) than those obtained when acetic acid was the used catalyst. The lignin purity of OPEFB treated with acetic acid as catalyst was lower by 9.17% while the recovery was lower by 17.85%. As sulfuric acid was more efficient regarding lignin purity and recovery, it was therefore employed for all subsequent pretreatment experiments.

**Figure 1.** Effect of (**a**) catalysts type (added in different concentration to reach pH 3) and (**b**) catalyst concentration (added in the same concentration, 0.13% (w/w)) on lignin purity (*p*-value = 0.444 and *p*-value = 0.035, respectively) and recovery *(p*-value = 0.076 and *p*-value = 0.007, respectively). Error bars represent ± 2× the standard deviation.
