Sustainable and effective methods for biomass pretreatment and biomass cascade utilization are reviewed in this work.

Deep eutectic solvents (DESs) have attracted broad attention due to their low cost, easy preparation, low toxicity, good biological compatibility and similar characteristics to those of ionic liquids (ILs). In this study, we found that not only the glycolysis time is sharply shortened under mild reaction conditions, but also the high selectivity of monomer bis(hydroxyalkyl) terephthalate (BHET) is obtained when DESs were used as catalysts. Then, the influences of technological parameters on PET degradation were investigated and the optimization conditions were obtained. Under the optimization conditions of ethylene glycol (EG) (20 g), catalyst (n(urea)/n(ZnCl₂) 4/1, 0.25 g), PET (5 g), and atmospheric pressure at 170 °C for 30 min, the conversion of PET and selectivity of BHET were 100% and 83%, respectively. This time is equal to that taken by a supercritical method under 15.3 MPa at 450 °C. In addition, the degradation mechanism of PET wastes catalyzed by DESs is proposed through the experiments and DFT calculations. The high catalytic activity is attributed to the synergetic catalysis of H-bonds and coordination bonds formed between the DES catalyst and EG.

Glycolysis of poly(ethylene terephthalate) (PET) is a prospective way for degradation of PET to its monomer bis(hydroxyethyl) terephthalate (BHET) which can be polymerized again to form new qualified PET materials, and hence provides possibilities for a permanent loop recycling. However, most of the reported glycolysis catalysts are metal-based, leading to high cost and negative environmental impact. In this study, we developed a series of choline-based ionic liquids (ILs) without metals and applied them in the glycolysis of PET as catalysts. Choline acetate ([Ch][OAc]), which is cheaper, more biologically compatible and environmentally friendly in comparison with conventional imidazolium metal-based ILs, can achieve a comparable or even better performance than them. Under optimum conditions (PET (5.0 g), ethylene glycol (EG) (20.0 g), [Ch][OAc] (5 wt%), 180 °C, 4 h, atmospheric pressure), the yield of BHET reached up to 85.2%. Additionally, the reaction kinetics was studied and proved to be the shrinking-core model. The apparent activation energy is 131.31 kJ mol⁻¹, and the pre-exponential factor is 1.21 × 10¹³ min⁻¹. Finally, based on the experimental results and density functional theory (DFT) calculations, a possible mechanism was proposed. The promotion of the glycolysis reaction is attributed to the activation of EG by the formation of hydrogen bonds between EG and the IL.