Low-pressure two-stage catalytic hydropyrolysis of lignin and lignin-derived phenolic monomers using zeolite-based bifunctional catalysts

Abstract

In the present study, a two-stage catalytic hydropyrolysis of lignin was investigated by converting lignin extracted from corn stover, loblolly pine and red oak using HZSM-5, MoO3/ZSM-5 and Ni/ZSM-5 as catalysts. Compared to the catalytic pyrolysis with inert gas, the presence of atmospheric-pressure hydrogen significantly enhanced the formation of hydrocarbon from lignin whereas dramatically reducing coke yield. In comparison to HZSM-5, MoO3/ZSM-5 increased the production of aromatic hydrocarbons, while Ni/ZSM-5 enhanced the formation of aliphatic hydrocarbons. Coke yields decreased with both MoO3/ZSM-5 and Ni/ZSM-5, which corresponds to the suppression of polyaromatic formation and increased selectivity to single ring aromatics. Corn stover lignin produced the highest yield of aromatic hydrocarbons with all three catalysts, followed by loblolly pine and red oak lignins. Particularly, up to 42.24 C% of corn stover lignin was converted into aromatic and aliphatic hydrocarbons with MoO3/ZSM-5. In this study, reaction pathways of lignin-derived pyrolysis vapor during catalytic hydropyrolysis were further studied using phenol, guaiacol, and syringol as the model compounds. Results suggest that demethoxylation followed by the hydrodeoxygenation of methoxyl radicals and subsequent alkylation of the phenolic ring to produce cresol, and dimethyl phenol as intermediates prior to the intermediate phenolics were further hydrodeoxygenated to produce benzene and alkylated benzene. MoO3/ZSM-5 showed strong ability for both demethoxylation and hydrodeoxygenation. In comparison, the ability of Ni/ZSM-5 for phenolic-hydroxyl removal was much inferior. Based on the results obtained from the conversions of lignin and the model compounds, a negative correlation between the total yields of aromatic hydrocarbons from lignin and the abundancy of phenolic methoxyls in the lignin-derived pyrolysis vapor was found. It was also found that the pyrolysis vapors of simple H, G, or S-based monomers tend to produce more aromatic hydrocarbons than the vapors composed of more complex monomers and oligomers from their corresponding H, G or S group. However, the yield differences were less significant among G and S-based monomers and oligomers.