Formation mechanism of HCN and NH3 during indole pyrolysis: A theoretical DFT study

Abstract

NOx formation during coal utilisation typically derives from thermal decomposition of N-containing compounds pyrrole, which usually combines with an aromatic ring in the form of indole. NH3 and HCN are common precursors of NOx from the decomposition of N-containing compounds. In this study, possible pathways of indole pyrolysis to form HCN and NH3 are investigated using the density functional theory (DFT) method. Calculation results indicate that indole pyrolysis has two type of possible initial reactions, which are internal hydrogen transfer and hydrogen homolysis reaction, respectively. The initial reaction mode of indole has a great impact on the subsequent pyrolysis pathway. Additionally, it is shown that indole can produce two nitrogen-containing products, i.e. HCN and NH3. Five pathways will result in the formation of HCN (path-1, path-3, patha, path-b, path-c), and another two pathways will lead to the NH3 (path-2, path-4). Furthermore, among all the reaction mechanisms of indole pyrolysis, the path-1 is the optimal reaction pathway. During which, indole is converted to a diradical intermediate, then the intermediate undergoes a synergy ring-opening transition state to form a new intermediate. Afterwards, the new intermediate decomposes into CN by homolysis of the CC bond.

Recent Progress in Quantum Chemistry Modeling on the Pyrolysis Mechanisms of Lignocellulosic Biomass

“…In the recent decades, computational methods, including QC modeling, molecular dynamics (MD), ab initio molecular dynamics (AIMD), and machine learning, have been prevailing and played important roles in the mechanism studies of many fields. ,− QC modeling is currently the most popular computational method to investigate the pyrolysis mechanism of biomass. Generally, QC modeling can give relatively accurate results, and the calculated thermodynamic data are useful to build the microkinetic models.…”

Catalytic Mechanism of Calcium on the Formation of HCN during Pyrolysis of Pyrrole and Indole: A Theoretical Study

“…Indole is another important source of HCN formation in coal pyrolysis. The optimal reaction pathway for the decomposition of indole to form HCN under non-catalytic conditions has been investigated in our previous work . As shown in Figure , three key reactions are involved in the indole pyrolysis for HCN formation.…”

“…Combined experiments and molecular dynamics were employed by Laskin et al to study the indole pyrolysis mechanism, suggesting that the homolysis and internal hydrogen transfer were mainly responsible for indole pyrolysis to form HCN. Our research group also studied the possible pathways for the formation of HCN and NH 3 from indole pyrolysis through theoretical calculations . The results showed that HCN was easier to be produced than NH 3 and the optimal formation pathway of HCN included diradical intermediates, synergistic ring-opening, and C–C bond homolysis reactions.…”

Density functional theory-based investigation of HCN and NH3 formation mechanisms during phenylalanine pyrolysis

“…41 Phe pyrolysis also produces nitrogen-containing heterocyclic compounds such as pyrrole, pyridine, and indole. Wornat et al 42 stated that N-heteroaromatic compounds have good thermal stability, and Liu et al 43 found that it is difficult to form HCN and NH 3 during the pyrolysis of indole. The release of HCN during the pyrolysis of Phe is mainly due to the continued decomposition of small-molecular-weight intermediates (thermal cracking of volatiles 7 ) and not to the secondary decomposition of nitrogen-containing heterocycles.…”