Preparation of nitrogen-containing compounds and nitrogen-doped biochar via nitrogen-rich pyrolysis coupled with ammonia source impregnation

Using lignocellulose to produce nitrogen-rich bio-oil and nitrogen-doped biochar in the presence of nitrogen-carrying bodies has become a research hotspot in recent years for high-value conversion and utilization of biomass. Different nitrogen carriers significantly influence biomass pyrolysis and conversion process and products. In this study, a new strategy for the preparation of both nitrogen-rich bio-oil (high-value N-containing compounds (NCCs)) and N-doped biochar by ammonia impregnation treatment coupled with Zeolite Socony Mobil Five (HZSM-5) via N-enriched pyrolysis was proposed. The influence of amine type (formamide, urea, melamine and dicyandiamide) and concentration on the product content, distribution, properties and evolution mechanism was also explored. The results showed that the amine treatment significantly reduced the activation energy (from 23.71 kJ/mol to 12.35–20.82 kJ/mol) and N content (from 7.42% to 6.05–7.29%) due to destruction of the crystalline phase and amine volatilization but increased the amount of char formed (from 18.15% to 21.47–24.06%). The amino groups derived from four different ammonia sources (formamide, urea, melamine and dicyandiamide) reacted with biomass-derived O-containing compounds to form pyridinic- and graphitic-like biochar due to NH3 etching and activation, and the enhanced adsorption capacities decreased in the order melamine (107.95 m2/g) > urea (96.84 m2/g) > dicyandiamide (79.46 m2/g) > formamide (74.12 m2/g) > biochar (35.63 m2/g)) due to the higher specific surface areas, pore volumes, graphitization degrees and enrichment of the O/N-containing functional groups. Additionally, higher concentrations of formamide coupled with the HZSM-5 catalyst noticeably boosted the NCCs content and pyrrole selectivity. The maximum NCCs content and pyrrole selectivity were 82.97% and 52.45%, respectively, and these were obtained via catalytic pyrolysis with HZSM-5 when the formamide concentration was 15%. Furthermore, formamide preferentially generated NCCSs and pyrroles via the Maillard reaction, and urea facilitated amine formation. This research provides useful background knowledge and a novel process for transformation and recovering high value-added NCCs products by N-rich pyrolysis of ammonia impregnated biomass.