Activity in thiophene hydrodesulfurization (HDS) and in the three routes of 2,6-dimethylaniline (DMA) decomposition was examined on Mo sulfide catalysts supported on Al 2 O 3 , Nb 2 O 5 and Nb 2 O 5 -Al 2 O 3 . Catalysts activity is enhanced when Mo phase is deposited on niobium-containing support. For HDS and for the hydrogenation route of DMA decomposition, the niobiumcontaining support strongly contributes to the catalyst activity whereas the activity of the Mo phase per Mo atom decreases with the increase of niobium amount in the support. By contrast, as for the DMA route, which leads to xylene formation (XYL), the activity of the Mo sulfide phase per Mo atom is strongly enhanced. The electronic properties of the MoS 2 phase were studied by means of IR spectroscopy of CO adsorption. Comparison of m(CO/Mo) wavenumbers reveals an upward shift when Mo sulfide phase is deposited on Nb-containing support. The modification of the electronic properties of the sulfide phase is related to an interaction Mo-Nb either through the formation of a mixed Mo-Nb sulfide phase, or through the interaction MoS 2 slabs -support whose strength depends on the support acidity. Hence, the beneficial effect for xylene formation route is attributed to a decrease of the electron density of the Mo sulfide phase that should strengthen the DMA adsorption on the sulfide phase.

The effect of high-pressure sulfidation on (Co)Mo/Al 2 O 3 catalysts was studied by means of IR spectroscopy of adsorbed CO and thiophene hydrodesulfurization (HDS). A new IR cell, called CellEx, was designed in order to characterize catalysts sulfided in situ under H 2 S/H 2 flow with pressure varying from 0.1 up to 4.0 MPa. On Mo/Al 2 O 3 , high sulfidation pressure changes neither the HDS rate nor the MoS 2 site concentration. Conversely on CoMo catalyst, sulfidation under high pressure leads to a marked increase of the thiophene HDS activity and of the Co-promoted site concentration as well as to some changes in the local structure of CoMoS sites.