Effect of alkali metal ions on the formation mechanism of HCN during pyridine pyrolysis

Abstract

The catalytic effects of alkali metal ions (Na+ and K+) on NOx precursor formation during coal pyrolysis were investigated using the N-containing compound pyridine as a model compound. Density functional theory calculations at the B3LYP/6-31G (d, p) level of theory were conducted to elucidate the mechanism of pyridine pyrolysis and the pathways for HCN formation. The calculation results indicate that Na+ and K+ have distinct influences on different pyrolysis reactions; these alkali metal ions facilitate the initial hydrogen transfer from C1 to N and C2, whereas they hinder the other hydrogen migration reactions. Both Na+ and K+ significantly reduce the activation energies for C–C bond breakage and triple-bond formation, whereas they increase the activation energies for the isomerization reactions. The different effects essentially result from the distinct charge distributions induced by the two ions. Due to the distinct influences on the different reactions, the rate-determining steps are modulated, affecting the competitiveness of the different possible pathways of HCN formation. The formation of HCN from pyridine is promoted in the presence of Na+ and K+ because all the overall activation energies are decreased for different pathways. The calculation results agree well with previous experimental studies. Thus, the findings offer a new and promising approach to reveal the formation mechanism of NOx and facilitate the control of NOx for coal utilization.