A review on the recent advances in the production of carbon nanotubes and carbon nanofibers via microwave-assisted pyrolysis of biomass

Biomass is a renewable hydrocarbon resource utilized to generate bioenergy, green chemicals, and carbon materials. Biomass is also promising source material for advanced carbon materials. This review reports the latest advancements and research activities in the synthesis of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) via microwave-assisted pyrolysis, covering various aspects of the formation mechanism and application. The large-scale poly-generation of carbon nanomaterials, high-value fine chemicals, and fuel gases via microwave pyrolysis can be promoted using selected biomass feedstock and by controlling reaction parameters. Cellulose is reported as the active bio-component accountable for the generation of CNTs under microwave. Although a self-extrusion mechanism has been proposed and elucidated in the literature, a recent study showed that the yield of CNTs has a linear correlation with the monosaccharides generated during biomass thermal decomposition. Selective heating and generation of hotspots were reported to promote the extrusion of volatiles. When combined with the temperature gradient of biomass particles, specific to microwave heating, and the presence of inherent inorganic species in biomass, microwave irradiation promoted the formation and growth of CNTs. Based on this technical review, the knowledge gaps and the possible future direction for generating carbon nanotubes from lignocellulosic biomass were identified.