Effect of H3PO4 and NaOH additives on the Co-carbonization of cellulose and N-containing compounds to produce N-doped chars

Abstract

N-doped carbons have significant applications in electronics, materials, and as tools for environmental services. However, few studies focus on N-containing compounds’ carbonization strategies to increase char N retention. For this purpose, H3PO4 and NaOH were used as additives during the co-carbonization of cellulose and four N-containing compounds (lysine, melamine, chitosan, and dicyandiamide (DCD)). Thermogravimetric and Py-GC/MS studies showed significant interactions between the N-containing compounds and cellulose with acid and basic additives. In all cases, an increase in char yield was observed. Acid-base treated samples were carbonized in a spoon reactor at 500 °C, and the resulting chars were thoroughly characterized. The maximum char yields were obtained in the presence of H3PO4 due to the impact of this additive on cellulose dehydration. N’s content in the char product varied from 2 to 12 wt. % depending on the N source. N conversion efficiency increases in the presence of H3PO4. In the case of chitosan, we obtained N conversion efficiencies as high as 77 % in acidic conditions. The deconvoluted XPS N 1s spectra showed that each acid-treated char contained a dominant pyrrolic N form, likely due to the acid’s effect on forming furanic compounds. NaOH-treated chars were richer in pyridinic structures. The first step in N-integration to biochar is the Maillard reaction. Carbonyls and amines react to form glycosylamine and the Amadori product. These intermediates form a Schiff’s base of hydroxymethyl furfural in acidic conditions. In basic conditions, reductones are produced. These are important mediators of melanoidins. The melanoidins are subjected to aromatization and polycondensation reactions to form N-containing polyaromatic ring systems at high temperatures.