catalytic hydrothermal process to achieve a TRS yield of 73.54 wt% were 145 °C reaction temperature, 30 wt% PMo 12 , and 120 min reaction time. Mechanism of the PMo 12 -catalyzed oxidative degradation of starch As shown in Table 1, the main product of starch degradation using PMo 12 was TRS, and a small number of byproducts (e.g., glycolic acid, 5-HMF, formic acid, and levulinic acid) were detected by HPLC(Deng et al. 2012). Table1 Products analysis of oxidation degradation of WS by PMo 12
Conditions (T/ °C)
Main products TRS (wt%)
Glucose (wt%)
Glycolic acid(wt%)
5-HMF (wt%)
Formicacid(wt%)
140
46.57 ± 2.67 62.98 ± 3.11 58.76 ± 2.32 48.43 ± 3.42 33.63 ± 2.67
26.02 ± 2.53 5.34 ± 2.77
0.19 ± 1.08 0
145
31.39 ± 2.87 11.93 ± 2.43
0.43 ± 2.13 0
150
28.72 ± 2.43 14.18 ± 2.45
0.88 ± 2.19 0.83 ± 2.07
155
23.46 ± 3.11 15.12 ± 2.54
1.42 ± 3.41 1.63 ± 1.99
160
13.91 ± 2.87 16.45 ± 2.77
1.90 ± 3.15 2.49 ± 2.16
Notes: 5 g/L WS, 20 wt% PMo 12 (PMo 12 /WS mass ratio), 120 min reaction time. Fig. 4a, b display a plausible reaction mechanism for the PMo 12 -catalyzed oxidative degradation of starch. The branched macro-molecule amylopectin and the linear macromolecule amylose, which consists of crystalline and amorphous lamellae of starch, form a semi-crystalline structure in the starch granule(Farias et al. 2020). It means that starch contains a large number of α -1, 4 glycosidic bonds and fewer α -1, 6 glycosidic bonds. The –O–H, C–O–C, and C–C bonds in starch molecular chain are cleaved due to the strong Brønsted acidity and oxidation properties of PMo 12 (Li et al. 2012; Liu et al. 2014a; Liu et al. 2014b). Thus, H + ions penetrate into the starch molecule in the hydrothermal reaction process, cleaving the α -1, 4 and α -1, 6 glycosidic bonds. Moreover, the amorphous region of starch likely undergoes cleavage reaction affording the hydrolysate RS, which is mainly composed of monosaccharides. In the oxidative degradation stage of PMo 12 and starch macromolecules, a hydrogen bond is formed between a free –OH in the starch molecule and an O atom of Mo–O in PMo 12 (Fig. 4a). Meanwhile, a proton and an electron are provided to PMo 12 by the starch molecule. Then, an O–H covalent bond is formed between free –OH and Mo–O at high temperature. On the one hand, the glucose
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