Microwave pyrolysis-prepared engineering carbons from corn cobs and red mombin seeds towards xylene adsorption

High-quality biochars/activated carbons were prepared, optimizing individual parameters of energetically-save microwave pyrolysis (raw material loading - 20 vs. 60 g, nitrogen atmosphere - flow vs. batch, ZnCl2 activation) from two agricultural wastes - corn cobs, red mombin seeds. Most promising carbons were examined for gaseous xylene adsorption and showed higher sorption capacity (∼250–475 mgxylene· g−1) than commercial carbon (∼214 mgxylene· g−1). ZnCl2 activation of both raw materials reduces the fixed carbon content and increases volatiles in activated carbon, suggesting microwave pyrolysis of activated feedstock should take 25 min. While biochars are microporous materials with inhomogeneous low-surface mesopore/macropore network, activated carbons are highly microporous-mesoporous. ZnCl2 activation of both raw materials contributes to formation of extensive high-surface mesopore network (with pore-size < 20 nm) and enlargement of micropore-size, but does not affect the micropore volume. ZnCl2 activation increases H2 and decreases CH4 production. Microwave pyrolysis of larger raw material loading with ZnCl2 leads to CO2 increase. Best xylene adsorption capacity (475 mgxylene· g−1) was determined for activated carbon produced from 60 g loading of corn cobs in batch nitrogen atmosphere, showing the largest micropore volume, lowest surface polarity and medium rate of graphitization. Large micropore volume, low surface polarity and high rate of graphitization of carbon are xylene sorption capacity-determining factors.