*3.2. E*ff*ect of Temperature and Retention Time*

Three different temperatures, namely, 180 ◦C, 210 ◦C, and 220 ◦C were employed for organosolv pretreatment of OPEFB for 120 min with S/L ratio of 1:20 and using sulfuric acid as catalyst. The lignin purity and recovery were significantly higher when the temperature of the pretreatment was increased from 180 ◦C to 210 ◦C (Figure 2a). However, when the temperature was 220 ◦C, no further improvement on lignin purity and recovery were observed. Hence, 210 ◦C was chosen for further pretreatment experiments.

**Figure 2.** Effect of (**a**) temperature (for 120 min and using solid-to-liquid (S/L) ratio of 1:20) and (**b**) retention time (at temperature of 210 ◦C and S/L ratio of 1:20) on lignin purity (*p*-value = 0.001 and *p*-value = 0.001, respectively) and recovery (*p*-value = 0 and *p*-value = 0, respectively). Error bars represent ± 2× the standard deviation.

To investigate the effect of retention time, the pretreatment was conducted for 30, 60, and 120 min. Increasing the retention time from 30 to 60 min resulted in higher lignin purity (from 44.09 ± 0.41 to 66.73 ± 2.55) and recovery (from 20.52 ± 1.27 to 45.35 ± 0.00%) (Figure 2b). However, no significant improvements were observed by extending the pretreatment to 120 min.
