Microwave pyrolysis of biomass model compounds for bio-oil: Formation mechanisms of the nitrogenous chemicals and DFT calculations

Abstract

Microwave pyrolysis of nitrogen-rich biomass for bio-oil is a green and sustainable pathway to realize energy conversion and produce high-value nitrogenous chemicals. This work selected three typical amino acids (serine (Ser), glutamic acid (Glu), and phenylalanine (Phe)) as nitrogenous model compounds for biomass, and the distribution characteristics and formation mechanisms of the nitrogenous chemicals in bio-oil were studied in depth. Results showed that the amino acid structure and pyrolysis temperature were the crucial factors affecting the distribution and selectivity of nitrogenous chemicals. The nitrogenous chemicals were mainly produced at 200–500 °C from the model amino acids, and the nitrogen in Ser was more prone to accumulating in bio-oil compared with Glu and Phe. At 300 °C, the model amino acids generated numerous amines and cyclic amides through decarboxylation, decarbonylation, and dehydration. Higher temperatures (400 and 500 °C) promoted amines/amides to form nitriles and N-heterocycles, with the releasing of H2. Nitriles (including propionitrile and phenylacetonitrile) were typically produced by the straight-chain dehydrogenation of amines. Ser, Glu, and Phe produced N-heterocycles (including pyrazine, pyrrole, 2-pyrrolidone, and indole) based on the cyclization and dehydration between carbonyl and amino compounds, intramolecular dehydration, and cyclization dehydrogenation of phenethylamine, respectively. Finally, the possible formation pathways of the main nitrogenous chemicals were proposed, and the kinetics and thermodynamic analyses were performed through density functional theory (DFT) calculations. The findings could provide theoretical support for the nitrogenous chemicals production from microwave pyrolysis of amino acids and biomass.