Direct aromatic C-N bond cleavage evidenced in the hydrodenitrogenation of2,6-dimethylaniline over cobalt-promoted Mo/Al2O3 sulfide catalysts: A reactivity and FTIR study

The hydrodenitrogenation of 2,6-dimethylaniline (DMA) was studied over a se ries of sulfided Co(0-4.7%)-Mo(8.7%)/Al2O3 catalysts at 573 K under 4 MPa t otal pressure and 0-56 kPa H2S partial pressure. Two NiMo samples were test ed for comparison. The reaction network presents three parallel routes: dea romatization of DMA followed by either hydrogenation-elimination to dimethy lcyclohexenes and dimethylcyclohexanes, or NH3 elimination to m-xylene, and disproportionation of DMA to 2-methylaniline and 2,4,6-trimethylaniline. W e demonstrate that part of the xylene is formed by direct aromatic carbon-n itrogen bond cleavage through a nucleophilic substitution involving hydride species. On CoMo catalysts in the presence of H2S, the amount of extra xyl ene is independent of Co content, while the dearomatization is promoted. Wi thout H2S, this special substitution reaction is most important on the Mo c atalyst, and strikingly Co acts as a poison. FT-IR spectroscopy of adsorbed carbon monoxide evidences a new type of sites on the sulfided catalysts af ter a mild hydrogen treatment. We propose that a site configuration located exclusively on unpromoted Mo atoms highly depleted in sulfur is responsibl e for the direct denitrogenation route. The NiMo couple behaves differently : xylene formation is independent of Ni content, which means that the speci fic Mo sites for direct C-N bond rupture are poisoned by nickel, even in th e presence of H2S. The location of Co and Ni on the MoS2 slabs then appears different. (C) 2001 Academic Press.