Design and Synthesis of Metal-Organic Framework-Based Photocatalysts and Their Application in Upcycling of Plastic

Plastic is a widely used product in our daily lives due to its light weight, chemical resistance and excellent electrical insulation properties. Since the 1950s, more than 8.3 billion tons of plastic have been produced, approximately 80% of which becomes plastic waste in the natural environment. Due to the strong chemical inertness of plastic waste, its spontaneous degradation process takes centuries, resulting in a severe global environmental problem known as "white pollution."

In recent years, photocatalysis, as an environmentally friendly and energy-saving technology that can upcycle plastic waste into valuable fuels, has received increasing attention. However, the vast majority of existing photocatalysts have low activity in relation to plastic upcycling, which can be ascribed to their high carrier recombination rate and low quantum efficiency. Metal-organic frameworks (MOFs) have attracted the attention of researchers recently due to their high surface area, modifiable functional groups and tuneable structure. These unique characteristics enable MOFs materials to exhibit excellent performance in the fields of photocatalytically splitting H₂O, degrading organic pollutants and reducing gas. To the best of our knowledge, there have been no reports of MOFs as photocatalysts being used for plastic upcycling.

The main objective of this thesis is to introduce innovative methodologies employed in designing and constructing heterojunction photocatalysts for converting plastics. Diverse plastics, such as polyethylene glycol (PEG), polylactic acid (PLA) and polyvinyl chloride (PVC), are utilised to evaluate the catalytic efficacy of synthesised photocatalysts. A detailed explanation of the correlation between the structural characteristics of MOF-based composite photocatalysts and their photocatalytic activity is provided. Furthermore, this project also involves conducting an in-depth study of the pathways and mechanisms for photocatalytic plastic upcycling.

We first evaluate the feasibility of MOF-based composite materials for the photocatalytic conversion of PEG. Specifically, we propose a photochemical synthesis approach to embed Ag₂O within the MIL(Fe)-101 to fabricate Ag₂O/MIL(Fe)-101. During the synthesis of photocatalysts, the maintained MOF structure of Ag₂O/MIL(Fe)-101 can effectively regulate Ag₂O particle size (∼6 nm). The presence of a heterojunction between Ag₂O and MIL(Fe)-101 ensures that Ag₂O/MIL(Fe)-101 has an efficient electron-hole separation rate. As a result, the resulting Ag₂O/MIL(Fe)-101 heterojunction can convert 27.5 mg PEG into acetic acid within 3 h, coupled with H₂ production. Hence, this work preliminarily verifies the possibility of MOFs photocatalysts in plastic upcycling.

Inspired by previous work, a strategy involving partial calcination is developed herein to convert Zn sites in Zn-UiO66-NH₂ into ZnO semiconductors that are encapsulated in UiO66-NH₂ pores. The intricate interplay between ZnO and UiO66-NH₂ at the interfacial level creates a pathway for efficient charge transfer while also ensuring the structural stability of ZnO/UiO66-NH₂. A biodegradable plastic-PLA is used to evaluate the catalytic performance of the ZnO/UiO66-NH₂. As a result, the ZnO/UiO66-NH₂ shows remarkable selectivity for acetic acid production during the PLA upcycling process, coupled with efficient H₂ production. Therefore, this work presents a novel approach to designing and constructing heterojunction photocatalysts for plastic upcycling.

To improve the light absorption performance of MOF-based photocatalysts, a mild pyrolysis strategy is developed to encapsulate carbon nanodots (CDs) into UiO66 pores to fabricate CDs/UiO66. The introduction of CDs enhances the solar light absorption capacity and promotes the carrier separation rate of CDs/UiO66. In this work, a non-biodegradable plastics-PVC is used as the target plastic. As a result, the as-prepared CDs/UiO66 exhibits high catalytic activity when converting PVC into acetic acid. Overall, this work provides a novel method to enhance the plastic conversion capability of MOF-based photocatalysts.