Catalytic depolymerization of polyester plastics toward closed-loop recycling and upcycling

With the escalating utilization of plastic products, global attention has been increasingly drawn to environmental pollution and recycling challenges stemming from plastic waste. Against this backdrop, biodegradable plastics have emerged as viable alternatives owing to their sustainability and capacity for biodegradation. Polylactic acid (PLA) presently commands the largest market share among biodegradable plastics, finding extensive application in products such as thin films, medical materials, and biodegradable straws. However, the widespread adoption of PLA is hindered by challenges such as high cost, low recycling rates, and complete degradation to H₂O and CO₂ in natural conditions. Therefore, it is imperative and time-sensitive to explore solutions for the depolymerization and re/upcycling of PLA waste plastics. This review comprehensively outlines the current landscape of PLA recycling methods, emphasizing the advantages and significance of chemical re/upcycling. The subsequent exploration encompasses recent breakthroughs and technical obstacles inherent in diverse chemical depolymerization methods. Ultimately, this review accentuates the impediments and forthcoming possibilities in the realm of PLA plastics, emphasizing the pursuit of closed-loop recycling and upcycling.

Aromatics and cyclic-hydrocarbons are the significant components of jet fuel with high energy-density. However, conventional technologies for bio-fuel production cannot produce these products without further aromatization and isomerization. In this work, renewable liquid fuel with high content of aromatics and cyclic-hydrocarbons was obtained through aqueous catalytic conversion of biomass sorbitol over Ni-HZSM-5/SBA-15 catalyst. Texture characteristics of the catalyst were determined by physisorption of N2, which indicated its bimodal pore structures were microporous (HZSM-5, pore width: 0.56 nm) and mesoporous (SBA-15, pore width: 8 nm). The surface acidity included weak and strong acid sites, predominantly Lewis type, and was further confirmed by the NH3-TPD and Py-IR analysis. The catalytic performances were tested in a fixed-bed reactor under the conditions of 593 K, WHSV of 0.75 h−1, GHSV of 2500 h−1 and 4.0 MPa of hydrogen pressure, whereby oil yield of 40.4 wt. % with aromatics and cyclic-hydrocarbons content of 80.0% was obtained.

A series of MCM-41 supported Ni catalysts with high metal dispersion was successfully synthesized by simple co-impregnation using proper ethylene glycol (EG). The acquired Ni-based catalysts performed the outstanding hydrogenolysis activity of guaiacol. The effects of the synthesis parameters including drying temperature, calcination temperature, and metal loading on the physical properties of NiO nanoparticles were investigated through the use of X-ray diffraction (XRD). The drying temperature was found to significantly influence the particle sizes of NiO supported on MCM-41, but the calcination temperature and metal loading had less influence. Interestingly, the small particle size (≤3.3 nm) and the high dispersion of NiO particles were also obtained for co-impregnation on the mixed support (MCM-41:HZSM-5 = 1:1), similar to that on the single MCM-41 support, leading to excellent hydrogenation activity at low temperature. The guaiacol conversion could reach 97.9% at 150 °C, and the catalytic activity was comparative with that of noble metal catalysts. The hydrodeoxygenation (HDO) performance was also promoted by the introduction of acidic HZSM-5 zeolite and an 84.1% yield of cyclohexane at 240 °C was achieved. These findings demonstrate potential applications for the future in promoting and improving industrial catalyst performance.