The catalytic pathways of hydrohalogenation over metal-free nitrogen-doped carbon nanotubes

Nitrogen-doped carbon nanotubes (N-CNTs) are found to be active as one novel heterogeneous catalyst for acetylene hydrochlorination reaction, possessing good activity (TOF=2.3×10^-3 s^-1) and high selectivity (>98 %). Compared to toxic and energy-consuming conventional catalysts, such as HgCl₂, N-CNTs are more favorable in terms of sustainability, because of their thermo-stability, metal-free make up, and the wide availability of bulk CNT. Coupling X-ray photoelectron spectroscopy and density functional theory computations (DFT), the main active source and reaction pathway are shown. Good linearity between the quaternary nitrogen content and conversion is revealed. DFT study shows that the nitrogen doping enhanced the formation of the covalent bond between C₂H₂ and NCNT compared with the undoped CNT, and therefore promoted the addition reaction of the C₂H₂ and HCl into C₂H ₃Cl. Hg, Mr. Bad Guy: Metal-free nitrogen-doped carbon nanotubes (CNTs) are employed as catalysts for C₂H₂ hydrochlorination; an essential reaction for poly(vinylchloride) (PVC) production currently done with toxic mercury catalysts. The catalysts combine high activity and excellent CH₂CHCl selectivity. The quaternary nitrogen species play a role analogous to vacancies in metal cation complexes, and increase the probability of C₂H₂ chemisorption and the formation of a C₂H₂-pseudo-metal complex.