Here, the measured concentration of WS was about 5 g/L by HPLC. Thus, in all experiments, 5 g/L, 100 mL of WS ¦ ltrate was applied. 20 wt% of PMo 12 and 120 min of reaction time were applied in this part. As shown in Fig. 3b, the TRS yield increased and then decreased with increasing the reaction temperature. The highest TRS yield of 62.98 wt% was obtained at 145 °C; the highest yield of glycolic acid was 16.45 wt% at 160 °C. Then, with further increasing the reaction temperature to 160 °C, the yield rapidly decreased to 33.63 wt%, which was 46.60% lower than that at 145 °C. On the contrary, the increase of glycolic acid was 4.92 wt% from 145 °C to 160 °C. Therefore, it can be concluded that the reaction temperature was an important factor that restricts the conversion of starch into RS, and had a great in § uence on the TRS yield, meaning that starch could not be fully hydrolyzed to RS at lower temperature. However, as the temperature increased, the oxidation of PMo 12 played a major role, and the oxidative degradation of RS was violent. In acidic solution at a high temperature, RS was the intermediate substance in the process of WS degradation, and was easily degraded in acid medium to produce insoluble humins and other byproducts (Yu et al. 2017). Basically, to prevent ineffective degradation of starch, the optimization reaction temperature is 145 °C, the sum of TRS and glycolic was 74.91 wt%. Effect of PMo 12 dosage on TRS and glycolic acid yield Apart from the effects of temperature and time, the dosage of PMo 12 also had a signi ¦ cant in § uence on the hydrolysis of WS to glucose. The acidity of PMo 12 accelerates the hydrolysis of WS, and the oxidation contributes to the cleavage of chemical bonds of WS, driving the reaction forward. Different TRS yields were obtained by changing the PMo 12 dosage (20, 30, 50, and 80 wt%) at 145 °C for 120 min. For a dosage of 20 wt%, most degradation was observed, and the TRS yield was only 62.98 wt% (Fig. 3c). Fortunately, the TRS yield increased rapidly to 73.54 wt% and 75.83 wt%, and the glycolic yield was 11.05 wt% to 12.99 wt% when the PMo 12 dosage was 30 to 50 wt%, respectively. However, the TRS yield did not increase signi ¦ cantly with further increasing the PMo 12 dosage. Hence, from the perspective of environmental protection and economy, the optimal reaction condition was 30 wt% PMo 12 , which afforded a TRS yield of 73.54 wt% and 11.04 wt% yield of glycolic acid.
Effect of reaction time on TRS and glycolic acid yield
With the optimized reaction temperature and PMo 12 dosage in hands, which were 145 °C and 30 wt%, respectively, the TRS yield was explored at different reaction times. We found that the TRS yield increased ¦ rst and then decreased with the reaction time (Fig. 3d). This was because the RS was further degraded under high temperature and acidic conditions. Simultaneously, organic degradation products such as glycolic acid, 5-HMF and formic acid were generated. With increasing the reaction time, the C–C bond cleavage effect of PMo 12 increased(Khenkin and Neumann 2008), which favored the production of levulinic acid and formic acid. Although the TRS yield was only 0.57 wt% higher at 180 min than at 120 min, the reaction time increased by 60 min. Consequently, the TRS yield was reduced. The optimized TRS yield of 73.54 wt% was obtained for 120 min reaction time. In conclusion, the optimal conditions for the
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