Photocatalytic Valorization of Plastic Waste over Zinc Oxide Encapsulated in a Metal–Organic Framework

Abstract

The conversion of waste plastic into high‐value‐added chemicals is regarded as a promising approach for relieving global plastic pollution and contributing to the circular economy. Herein, a partial calcination strategy is developed to fabricate a zinc oxide/UiO66‐NH2 (ZnO/UiO66‐NH2) heterojunction, in which ZnO is encapsulated in porous UiO66‐NH2 for the photocatalytic valorization of plastic. This strategy preserves the framework structure of UiO66‐NH2, thus enabling the formation of ZnO with ultra‐small size distributed inside the skeleton. The synergistic effect of the obtained ZnO/UiO66‐NH2 heterojunction facilitates providing an efficient channel for carrier/mass transfer and guarantees structural stability. As a result, ZnO/UiO66‐NH2 exhibits high activity for converting polylactic acid (PLA) and polyvinyl chloride (PVC) into acetic acid, coupled with H2 production. This work provides a feasible strategy for rationally designing heterojunction photocatalysts, as well as an insight into understanding the process of photocatalytic valorization of plastic.

Recent advancement on photocatalytic plastic upcycling

“…44 Another key reaction condition for photocatalytic plastic upcycling is the existence/absence of oxygen (O 2 ) in the reaction system. If photocatalytic plastic upcycling is conducted under aerobic conditions, abundant reactive oxygen species (ROS), such as ˙O 2 − , ˙HO 2 − and ˙OH radicals, will be produced by the following reactions: 36,37,44,46–48 e − + O 2 → ˙O 2 − ˙O 2 − + e − + 2H + → H 2 O 2 H 2 O 2 + hν → 2˙OHH 2 O 2 + ˙O 2 − → ˙OH + O 2 + OH − e − + H 2 O 2 → ˙OH + OH − …”

“…1–33 Among these techniques, photocatalysis stands out as a cost-effective, environmentally-benign and green strategy able to upcycle plastic wastes at ambient temperature and pressure utilizing renewable sunlight. 34–79 A photocatalysis reaction conducted in mild conditions is anticipated to accurately activate target chemical bonds, while reserving the other functional groups, thus realizing high selectivity to desirable products. 34–49…”

“…34–79 A photocatalysis reaction conducted in mild conditions is anticipated to accurately activate target chemical bonds, while reserving the other functional groups, thus realizing high selectivity to desirable products. 34–49…”

“…, ZnO encapsulated in UiO66-NH 2 (ref. 46) and Ag 2 O encapsulated in Fe based MOFs 47 ), have been developed for catalytic upcycling of plastics. To date, many reviews have covered upcycling of plastic waste, 1–31,80,81 including various upcycling techniques such as the photo-electrochemical (PEC) technique, photo-thermal technique, photocatalysis technique, thermo-catalysis technique, bio-catalysis technique, pyrolysis technique, hydrogenolysis technique, solvolysis technique, hydrolysis technique, microwave-assisted technique, plasma-assisted technique, mechanical technique and combination of these techniques.…”

Advancements in catalysis for plastic resource utilization

“…In terms of efficiency, composite photocatalysts prepared using the porous MOFs material UiO66-NH 2 encapsulated with ZnO contribute to improved electron–hole separation, enhancing photocatalytic efficiency. 140,141 This, in turn, accelerates the catalytic conversion of PLA and PVC to acetic acid. The widespread application of light has also encouraged the photocatalytic cracking of plastics beyond the recycling of waste plastics, even extending to lithography processes.…”