Distinct impact of oxidation reactions on the evolution of pore structures during H3PO4 activation of aliphatic-rich poplar and aromatic-rich lignin

Cracking is an essential route for creating porous structures in activation. Introducing an oxidant in activation induces oxidation reactions, which might strengthen cracking to create additional porous structures. To investigate the potential effect of O 2 on the development of activated carbon (AC) pore structures, poplar and lignin were activated with H 3 PO 4 at 400 °C in either a N 2 or O 2 atmosphere. The results demonstrated that polymerization reactions induced by H 3 PO 4 formed AC-poplar-N 2 with a much higher yield (52.3 %) than pyrolysis (27.6 %). However, the presence of O 2 substantially reduced the mass yield of AC-poplar-O 2 to 22.8 %. Unexpectedly, the oxidation reactions also decreased the specific surface area from 1783.3 m 2 /g in N 2 to 1099.1 m 2 /g in O 2 . This was due to the oxidation of aldehyde/ketone intermediates to carboxyl species, which are essentially inert in the polymerization catalyzed with H 3 PO 4 . Excessive oxidation in activating aliphatic-rich poplar was undesirable, but highly desirable for activating lignin to create pores. Lignin has rigid benzene ring structures, showing much higher resistivity towards oxidation, and thus, its presence did not affect the yield of the resulting AC. However, erosion of the skeleton of lignin from oxidation did take place, forming AC-lignin-O 2 of much larger specific surface area than AC-lignin-N 2 (933.9 versus 393.7 m 2 /g) and more mesopores (60 % versus 36 %) for more efficient adsorption of tetracycline. In-situ IR characterization of the activation process showed that the activation with O 2 presence formed more oxygen-containing species (i.e., -OH, C=O, and C-O-C). Still, polymerization played a less essential role than oxidation in forming porous structures with lignin as a feedstock.