Preparation of low-nitrogen bio-oil via biomass pyrolysis over Ni-Co bimetal-modified HZSM-5 catalysts

The presence of nitrogen-containing compounds (NCCs) in bio-oil severely limits its downstream utilization as a clean fuel. To address this issue, a catalytic strategy combining hierarchical pore construction and bimetallic active-site modification was developed for the deep denitrogenation and upgrading of biomass pyrolysis oil. Bamboo (BB) and wheat straw (WS) were selected as representative forestry and agricultural residues, respectively, and Ni-Co bimetal-modified HZSM-5 catalysts were prepared. Appropriate alkali treatment significantly improved the pore structure of HZSM-5, increasing its specific surface area from 383.9 to 409.12 m 2 /g, while Ni and Co introduction effectively regulated the distribution of acidic sites. As a result, the modified catalysts enhanced the pyrolysis of BB and WS and improved the quality of the resulting bio-oils. Over 6Ni0.5Co-HR5, aromatic selectivity reached 51.06% for BB and 50.66% for WS. GC/MS analysis showed that the catalyst promoted the transformation of nitrogen species from kinetically unstable chain-like compounds into more stable cyclic structures, facilitating their further removal. Among the catalysts, 6Ni0.5Co-HR5 exhibited the best denitrogenation performance, reducing the relative total nitrogen content in BB- and WS-derived bio-oils to 0.89% and 0.67%, respectively, corresponding to decreases of 96.34% and 93.02% compared with non-catalytic pyrolysis. This superior perf ormance was attributed to the synergistic effect of the hierarchical pore structure and Ni-Co bimetallic sites. The results demonstrate good feedstock adaptability of the catalyst and provide an effective route for producing low-nitrogen, aromatic-rich bio-oil.