Catalytic Fast Hydropyrolysis of Lignin at Atmospheric Pressure Using Nickel-Decorated Carbon Nanotube Reactors

This study presents an atmospheric-pressure catalytic hydropyrolysis approach for lignin using nickel-decorated carbon nanotube (CNT) nanoreactors, achieving performance typically reserved for high-pressure processes. A robust compartmentalized quantification method was developed to resolve the spatial distribution of Ni across two distinct catalyst architectures: one with Ni particles confined within the inner nanotube channels and another localized on the external CNT surfaces. These microreactors, synthesized via vacuum-assisted wetness impregnation and a solvent-free mechanochemical approach, achieved internal and external Ni localization selectivities of 71.5% and 97.5%, respectively. Hydropyrolysis of mixed wheat straw and poplar lignin at 650 °C revealed that confined catalysts outperformed external counterparts. At a 20:1 catalyst-to-lignin mass ratio, nanoconfinement enabled a 12.5 wt % total yield and 4.6 wt % aromatic yield, with nearly 40% selectivity toward BTX. Furthermore, selectivity for long-chain alkanes (C10+) reached 30%, while oxygen content was reduced to 3.1 wt %, increasing the lower heating value to 42.5 MJ/kg, levels comparable to commercial jet fuels. Although four-cycle stability tests showed consistent yields, a decline in micropore volume suggests nanoscale coking requires further long-term durability assessment. Significantly, this work establishes a safe and efficient pathway for the decentralized production of hydrocarbon-rich bio-oils from abundant lignin residues by confining active sites within CNT channels.