Understanding organic N interactions with cellulose during pyrolysis to produce N-doped carbonaceous materials

Abstract

N-doped carbons are widely studied due to their unique catalytic and adsorptive properties. However, the mechanism by which N from biological sources is integrated into carbonaceous materials is still poorly known. Advancing our understanding of these reactions is critical to designing N-doped carbons. The thermal behavior of cellulose, four N-containing model compounds (Lysine, Melamine, Chitosan, and Dicyandiamide (DCD), and blends of cellulose with 1–90 wt% of the N-containing compounds was studied. The prepared samples were examined by thermogravimetric analysis (TGA) and by Py-GC/MS. Our TGA results confirmed that the N act as a cross-linking agent, increasing char formation. Biochars produced in a spoon reactor were thoroughly characterized—the N content of the chars produced at 500 °C in blends of 16 wt% was between 4.3 and 12.4 wt%. N 1 s XPS confirms that most of the N in the chars produced was in the form of pyridinic N and pyrrolic N. The chars produced have a surface area between 218.5 and 364.6 m2/g. The main nitrogenated compound identified by Py-GC/MS during the pyrolysis of Melamine and DCD was NH3. In addition to ammonia, these two molecules form aromatic ring systems rich in pyridinic moieties. When melamine is pyrolyzed, the small pyridinic ring systems are removed via evaporation. In the presence of cellulose, these pyridinic rings and those formed from DCD are integrated into the char structure. The lysine and chitosan pyrolysis form aminated compounds that react with carbonyl groups and eventually form pyrrolic char. N integration to char structure occurs when the reactive cellulose carbonyl groups react with amino groups through the Maillard reaction to form Schiff’s base. A series of reactions follow, including decarboxylation, cyclization, and polymerization, creating N-heterocycle structures in char.