Theoretical Investigation on Denitrification Mechanism of Piperidine: Effects of Methylation Versus Protonation on C–N Bond Activation

The denitrification mechanisms of piperidine, 2-methylpiperidine (2-MP), and 2,6-dimethylpiperidine (2,6-DMP) on MoP(001) were studied using self-consistent periodic density functional theory (DFT). These molecules located at Mo top site via the N atom and the molecular planes parallel with MoP surface. The energy barrier of the C–N bond cleavage of piperidinium (protonated piperidine) was much lower than that of the direct C–N bond cleavage of piperidine, indicating of a promoting effect of the protonation on the C–N bond cleavage, and the same trend was also found for 2-MP and 2,6-DMP. Our calculations suggested that the α-position methylation would not facilitate the hydrogenolysis pathway (protonation and subsequent C–N bond cleavage) of piperidine, in accordance with the experimental results. Both the protonation and α-position methylation could promote the C–N bond cleavage of piperidine on MoP(001), and the positive effect of protonation on C–N bond activation was more significant than that of the methylation.