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 Mo 6+ cation is simultaneously reduced to Mo 5+ in PMo 12 to form molybdenum blue via electron transfer(Dolbecq et al. 2010), with the concomitant solution color change to blue. Fortunately, the structure of PMo 12 is not destroyed (Chen et al. 2019; Glass et al. 2016; He and Yao 2006). Due to the oxidative property of PMo 12 , a number of degradation products can be further oxidized to compounds such as 5-HMF, formic acid, and levulinic acid. PMo 12 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 PMo 12 after three cycles and its in § uence on the TRS yield were investigated. The absorbance of PMo 12 at 700 nm was used to determine the reduction degree because both variables have a linear relationship (Fig. 5a). The absorbance curve of PMo 12 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 PMo 12 also increased, which indicates that WS was degraded. The reduction degree of molybdenum blue (reductive PMo 12 ) decreased gradually in the process of electrooxidation, being gradually oxidized and converted to oxidative PMo 12 (Fig. 5c)(Yang et al. 2019). Fig. 5d demonstrates that the TRS yield changed between the ¦ rst and third cycle of PMo 12 treatment, decreasing from 69.56 wt% for the ¦ rst cycle to 58.32 wt% for the third cycle. Nevertheless, the recovery rate of PMo 12 after the third cycle was still 80.76% of the initial value. The decreased catalytic performance of recovered PMo 12 may be due to organics adsorption by PMo 12 , which affects the oxidative degradation of WS. Fortunately, a good level of PMo 12 recycling Á performance was still maintained. The color change of PMo 12 solution during the redox process is illustrated in Fig. 6. Under the hydrothermal reaction, the WS–PMo 12 mixture changed from yellow to dark blue (Fig. 6a and Fig. 6b), indicating the reduction of PMo 12 to molybdenum blue. The Mo 6+ cation in PMo 12 is reduced by electrons from WS, and the reduction degree of PMo 12 increases. The reduction degree of a POM is de ¦ ned 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, Mo 5+ in molybdenum blue was oxidized to Mo 6+ at the anode. Simultaneously, the molybdenum blue was converted to oxidative PMo 12 . 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.
Page 8/16
Made with FlippingBook - Online magazine maker