Role of Maillard reactions in co-pyrolysis of lignite and spent coffee grounds: Fixation of nitrogen in semicoke and improved adsorption of phenols.

Semicoke derived from lignite and biomass has been utilized in soil remediation and activated sludge treatment. Nitrogen-doping treatment has been utilized to improve adsorption capacity of semicoke for phenols (such as humic acids). Known as nitrogen-rich biomass residue, spent coffee grounds (SCG) may serve as nitrogen resources when co-pyrolyzed with lignite. But impacts of SCG on structure and adsorption capacity of semicoke in co-pyrolysis have not been confirmed. In consideration of proteins in SCG, nitrogen fixation and the impacting mechanism have not been sufficiently confirmed. In this work, nitrogen fixation between lignite and SCG during co-pyrolysis were investigated by thermogravimetric analysis (TG) and pyrolysis experiments. Corresponding mechanism were speculated and confirmed by in-situ diffuse reflectance infrared Fourier transformation spectra (DRIFT) and mass spectrum (MS). Impacts of nitrogen fixation on adsorption of phenols on semicoke were investigated by X-ray photoelectron spectroscopy (XPS), adsorption kinetics analysis and density functional theory (DFT) calculations. Compared to theoretical values, higher contents of N have been obtained in co-pyrolyzed semicoke (N: 2.89%; pyridinic N: 44.86%), which implied N fixation and cyclization reactions. Fixation mechanism can be explained by formation of Amadori/Heyns intermediates in Maillard reactions. Release of volatiles and capture of NH 3 are crucial in 340–440 °C range, which result in nitrogen fixation. Then formation of pyrrolic and pyridinic N can be explained by cyclization reactions and decomposition of Amadori/Heyns intermediates in 550 °C pyrolysis. Enrichment of pyridinic N, pyrrolic N and C O might enhance adsorption of phenols by offering binding sites for formation of hydrogen bonds. Both C, N fixation and improved adsorption capacity (27.80 mg/g) have been realized for 7:3–550 semicoke. Consequently, doping treatment with typical nitrogen-rich biomass (SCG) may be feasible to enhance adsorption of phenols onto semicoke, based on selection of raw materials and pyrolysis conditions. • N-rich biomass residue and lignite were co-pyrolyzed for nitrogen fixation. • Role of Maillard reactions in nitrogen fixation was emphasized. • Co-pyrolysis conditions can be selected based on structure-activity relationship. [ABSTRACT FROM AUTHOR]