Sustainable Conversion of Biomass to Multiwalled Carbon Nanotubes and Carbon Nanochains

The conversion of biochar, the low value byproduct of pyrolysis bio-oil production from biomass multi-walled carbon nanotubes (MWCNTs) and carbon nanochains (CNCs), is reported. It is shown that biomass can be converted to long (>30 µm) carbon nanotubes with an anomalously deep (>280 nm) stacked-cup structure. A mechanism of the transformation that is consistent with previously reported graphitization of biochar, a "non-graphitizable" carbon, is proposed, suggesting the molten metal catalyst is absorbed into the biochar by capillary action, forming graphene walls as it percolates through pore structure. Graphite is formed when the diameter of the molten catalyst droplets is large (microns), while smaller droplets (submicron) form MWCNTs and still smaller (<100 nm) form CNCs. Branching in the biochar pore structure leads to subdivision of the catalyst droplets resulting in the progression from MWCNT to CNC formation. Very long MWCNTs (>50 µm) can be formed in the absence of CNCs by transforming lignite char rather than biochar, presumably due to the elimination of smaller branching pores during coalification. CNCs, in the absence of MWCNTs, can be formed in biochar by using low concentrations of catalyst nanoparticles formed by carbon thermal reduction of a metal salt during charring. The results presented suggest that developing methods to control the porosity of the char could yield the ability to rationally synthesize carbon nanotubes with control of length, breadth and wall thickness.