Analysis of the factors influencing the oxidative degradation of WS
The factors determining the oxidative degradation of WS include reaction time, temperature, pH of the reaction system, and concentration of reactants (Girisuta et al. 2007; Mukherjee et al. 2016). We conducted single-factor experiments to select the three main factors, i.e., reaction temperature, PMo12 dosage, and reaction time, in a targeted manner, and obtained the optimal process conditions for determination of the TRS yield.
First, the standard curve of this experiment was adjusted to zero with the color reaction solution of DI water as control, and the standard working curve was drawn with the mass fraction of glucose as the abscissa and the absorbance as the ordinate (Fig. 3a). The regression equation was y = 0.0185x + 0.1074 (R2 = 0.9980), where R2 is the correlation coefficient and defines the feasibility of the method and the degree of linear relationship. Generally, R2 > 0.99 ensures an appropriate limit of error. The DNS reagent was used to determine the TRS content in the experiment.
Effect of reaction temperature on TRS and glycolic acid yield
Here, the measured concentration of WS was about 5 g/L by HPLC. Thus, in all experiments, 5 g/L, 100 mL of WS filtrate was applied. 20 wt% of PMo12 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 influence 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 PMo12 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 PMo12 dosage on TRS and glycolic acid yield
Apart from the effects of temperature and time, the dosage of PMo12 also had a significant influence on the hydrolysis of WS to glucose. The acidity of PMo12 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 PMo12 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 PMo12 dosage was 30 to 50 wt%, respectively. However, the TRS yield did not increase significantly with further increasing the PMo12 dosage. Hence, from the perspective of environmental protection and economy, the optimal reaction condition was 30 wt% PMo12, 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 PMo12 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 first 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 PMo12 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 catalytic hydrothermal process to achieve a TRS yield of 73.54 wt% were 145 °C reaction temperature, 30 wt% PMo12, and 120 min reaction time.
Mechanism of the PMo12-catalyzed oxidative degradation of starch
As shown in Table 1, the main product of starch degradation using PMo12 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).
Table 1 Products analysis of oxidation degradation of WS by PMo12
Conditions (T/°C)
|
Main products
|
TRS (wt%)
|
Glucose (wt%)
|
Glycolic acid(wt%)
|
5-HMF (wt%)
|
Formicacid(wt%)
|
140
|
46.57 ± 2.67
|
26.02 ± 2.53
|
5.34 ± 2.77
|
0.19 ± 1.08
|
0
|
145
|
62.98 ± 3.11
|
31.39 ± 2.87
|
11.93 ± 2.43
|
0.43 ± 2.13
|
0
|
150
|
58.76 ± 2.32
|
28.72 ± 2.43
|
14.18 ± 2.45
|
0.88 ± 2.19
|
0.83 ± 2.07
|
155
|
48.43 ± 3.42
|
23.46 ± 3.11
|
15.12 ± 2.54
|
1.42 ± 3.41
|
1.63 ± 1.99
|
160
|
33.63 ± 2.67
|
13.91 ± 2.87
|
16.45 ± 2.77
|
1.90 ± 3.15
|
2.49 ± 2.16
|
Notes: 5 g/L WS, 20 wt% PMo12 (PMo12/WS mass ratio), 120 min reaction time.
Fig. 4a, b display a plausible reaction mechanism for the PMo12-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 PMo12(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 PMo12 and starch macromolecules, a hydrogen bond is formed between a free –OH in the starch molecule and an O atom of Mo–O in PMo12 (Fig. 4a). Meanwhile, a proton and an electron are provided to PMo12 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 is converted to 5-HMF by isomerization and dehydration. Furthermore, part of 5-HMF continues to generate formic acid and levulinic acid via decarboxylation. On the other hand, the glycolaldehyde and erythrose are oxidative intermediate byproduct from the retro-aldol fragmentation of glucose, and are continuously oxidized to glycolic acid(Zhang et al. 2012) (Fig. 4b). The Mo6+ cation is simultaneously reduced to Mo5+ in PMo12 to form molybdenum blue via electron transfer(Dolbecq et al. 2010), with the concomitant solution color change to blue. Fortunately, the structure of PMo12 is not destroyed (Chen et al. 2019; Glass et al. 2016; He and Yao 2006). Due to the oxidative property of PMo12, a number of degradation products can be further oxidized to compounds such as 5-HMF, formic acid, and levulinic acid.
PMo12 regeneration properties
The long-term cycle stability of a catalyst is an important index to evaluate its performance, especially in industrial application. Therefore, the recovery rate of PMo12 after three cycles and its influence on the TRS yield were investigated. The absorbance of PMo12 at 700 nm was used to determine the reduction degree because both variables have a linear relationship (Fig. 5a). The absorbance curve of PMo12 in a wavelength range from 400 to 900 nm under different hydrothermal reaction times is shown in Fig. 5b. Upon increasing the reaction time from 60 min to 240 min, the reduction degree of PMo12 also increased, which indicates that WS was degraded. The reduction degree of molybdenum blue (reductive PMo12) decreased gradually in the process of electrooxidation, being gradually oxidized and converted to oxidative PMo12 (Fig. 5c)(Yang et al. 2019). Fig. 5d demonstrates that the TRS yield changed between the first and third cycle of PMo12 treatment, decreasing from 69.56 wt% for the first cycle to 58.32 wt% for the third cycle. Nevertheless, the recovery rate of PMo12 after the third cycle was still 80.76% of the initial value. The decreased catalytic performance of recovered PMo12 may be due to organics adsorption by PMo12, which affects the oxidative degradation of WS. Fortunately, a good level of PMo12 recycling performance was still maintained.
The color change of PMo12 solution during the redox process is illustrated in Fig. 6. Under the hydrothermal reaction, the WS–PMo12 mixture changed from yellow to dark blue (Fig. 6a and Fig. 6b), indicating the reduction of PMo12 to molybdenum blue. The Mo6+ cation in PMo12 is reduced by electrons from WS, and the reduction degree of PMo12 increases. The reduction degree of a POM is defined as the average number of electrons (in moles) that are transferred from the biomass to one mole of the POM anion(Liu et al. 2016; Zhao et al. 2020). A CHI660E electrochemical workstation was utilized to oxidize a molybdenum blue solution at a constant voltage of 1.0 V in the electrolysis, which was lower than the standard potential of water electrolysis (1.23 V)(Weng and Chen 2015). During electrolysis, Mo5+ in molybdenum blue was oxidized to Mo6+ at the anode. Simultaneously, the molybdenum blue was converted to oxidative PMo12. The color of the solution turned back to yellow, and WS was oxidized and degraded (Fig. 6c). A H+ from WS was transferred to the cathode, generating hydrogen.