Direct Aromatic C-N Bond Cleavage Evidenced in the Hydrodenitrogenation of 2,6-Dimethylaniline over Cobalt-Promoted Mo/Al_2O_3 Sulfide Catalysts:A Reactivity and FT-IR Study

摘要

The hydrodenitrogenation of 2,6-dimethylaniline (DMA) was studied over a series of sulfided Co(0-4.7%)-Mo(8.7%)/Al_2O_3 cata-lysts at 573 K under 4 MPa total pressure and 0-56 kPa H_2S par-tial pressure. Two NiMo samples were tested for comparison. The reaction network presents three parallel routes: dearomatization of DMA followed by either hydrogenation-elimination to dimethylcy-clohexenes and dimethylcyclohexanes, or NH_3 elimination to m-xylene, and disproportionation of DMA to 2-methylaniline and 2,4,6-trimethylaniline. We demonstrate that part of the xylene is formed by direct aromatic carbon-nitrogen bond cleavage through a nucleophilic substitution involving hydride species. On CoMo cata-lysts, in the presence of H_2S, the amount of extra xylene is indepen-dent of Co content, while the dearomatization is promoted. Without H_2S, this special substitution reaction is most important on the Mo catalyst, and strikingly Co acts as a poison. FT-IR spectroscopy of adsorbed carbon monoxide evidences a new type of sites on the sul-fided catalysts after a mild hydrogen treatment. We propose that a site configuration located exclusively on unpromoted Mo atoms highly depleted in sulfur is responsible for the direct denitrogena-tion route. The NiMo couple behaves differently: xylene formation is independent of Ni content, which means that the specific Mo sites for direct C-N bond rupture are poisoned by nickel, even in the presence of H_2S. The location of Co and Ni on the MoS_2 slabs then appears different.