Characterization and mechanistic insights into coke formation on biochar-based catalysts under microwave-assisted biomass pyrolysis

Coke formation is a critical factor contributing to catalyst deactivation during biomass catalytic pyrolysis. In this study, we systematically investigated the characteristics of coke on biochar-based catalysts (BC) during microwave-assisted catalytic pyrolysis of biomass. The morphology, pore structure, thermal stability, and composition of coke were thoroughly analyzed. Our findings revealed that both the degree of graphitization and the extent of coke formation on BC increased with rising reaction temperatures and prolonged reaction durations. Specifically, the BET surface area of fresh BC decreased from 108.44 m²/g to 56.26 m²/g after being subjected to 650°C for 80 minutes. Correspondingly, the proportion of inert coke (C₃) increased markedly from 15.90 % to 63.80 %. Notably, crystal-like coke structures were observed on the surface of BC. To quantify and evaluate variations in coke formation, we introduced a novel metric termed the “coke level,” which provides a graphical and quantitative assessment of both the amount and nature of coke present on the catalyst. Soluble coke generated under high-temperature and prolonged reaction conditions predominantly consisted of phenolic compounds. Microwaves can excite plasma, which in turn activates coke, inhibits its growth, and promotes its reaction with the surrounding environment. For instance, after treatment at 650°C for 80 minutes, the coke deposition on BC was only 3.99 wt%. These results demonstrate that the microwave field significantly enhances the catalyst’s service life and economic viability by reducing coke accumulation and improving catalyst efficiency. • The morphology of coke in microwave field is like carbon cube and nanotube. • 650 ℃-80 min microwave field biochar-based catalyst coke deposition is only 3.99 wt%. • “Coke level” can effectively measure the amount and quality of coke in microwave field. • The mechanism of coke formation on biochar-based catalysts in microwave field was proposed.