Occurrence of elemental exchange in catalytic pyrolysis of poplar sawdust with homologous biochar catalysts doped with nitrogen.

Volatiles-char interactions during pyrolysis are well established, driven by polar oxygen-containing biochar functionalities. Adding polar nitrogen groups to biochar may alter volatiles evolution. This study explored catalytic pyrolysis of poplar sawdust using N-doped biochar (from melamine-impregnated biomass), with focus on elemental exchange of the biochar catalyst during catalyzing conversion of volatiles. The results showed that N-biochar with high nitrogen content lost 8.7 % its mass after catalysis, due to the transfer of organics to volatiles. This suppressed phenols, ketones, and aldehydes formation while generating N-containing organics in bio-oil. Such reactions led to reduction of nitrogen content from 45.9 % in fresh N-biochar to 26.7 % in spent N-biochar. This also caused deposition of oxygen/hydrogen-rich species on spent N-biochar, filling pores and rendering hydrophobic surface. The partial removal of nitrogen species also led to destruction of CN skeleton structures and regularity/interplanar spacing of g-C3N4 crystal plane in N-biochar. The in-situ infrared spectroscopic confirmed that melamine-derived nitrogens reacted with CO in biochar to form CN or with CO to form CN. Oxygen species showed high affinity to N-biochar catalysts, altering functionalities and morphology of catalysts. The results are of importance for development of method for preparation of active biochar catalysts for tailoring composition of bio-oil.