Manipulating waste-derived biochar for microwave-assisted catalysis

Abstract

Biomass is one of the most abundant renewable resources on the earth as an alternative to depleting fossil resources. Biochar is a promising carbon-neutral material derived from biomass, which can sequestrate carbon to reduce the impact on climate change. The tuneable characteristics improve its versatility and applicability in various fields, which is superior to other carbonaceous materials. This thesis focuses on manipulating waste-derived biochar as the catalyst for microwave-assisted oxidation reactions, including the degradation of refractory organic contaminants in wastewater and the valorization of glucose to platform chemicals. For the removal of organic contaminants, the research gaps are identified as the catalyst reusability and reaction mechanisms as well as the mineralization of refractory organic pollutants. The results in this thesis elucidate the high efficiency, low cost, and considerably stable features of the synthesized metal-functionalized biochar catalysts (i.e., Fe-biochar and Cu-biochar). The catalytically active sites and mechanisms are scrutinized accordingly, underlining the critical roles of biochar and microwave irradiation. As for the valuable organic acids, they are dominantly produced from non-renewable fossil resources or by low-efficiency biochemical processes in the industry. While existing studies focus on noble metal-based catalysts for sustainable production, it is important to develop earth-abundant metal-based catalysts. This thesis fabricates Cu-biochar and Mn-biochar catalysts for the oxidation of biomass-derived glucose to achieve high-efficacy production of sugary acids and shorter-chain organic acids (e.g., glycolic acid and acetic acid), respectively. This thesis highlights the contribution of microwave irradiation to the reaction, including thermal and non-thermal effects, where biochar catalysts play crucial roles by interacting with the microwave. Biochar characteristic–microwave absorption–catalytic performance relationships are established, which can guide the rational design of microwave-responsive catalysts. These research efforts demonstrate the in-depth mechanisms of the considerably high performance and minute-scale reaction accomplished by the microwave-assisted catalytic system, underscoring the potential of the low-cost biochar catalysts and the emerging microwave-assisted technology.