Atomic level characterization and sulfur resistance of unsupported W2C during dibenzothiophene hydrodesulfurization. Classical kinetic simulation of the reaction

• Stability of unsupported: presence of clean W2C after 5 days-HDS runs.
• Sulphur resistance of W2C during HDS of DBT: monolayer of adsorbed S species.
• HRTEM/EDS coupling revealed W2C platelet – texture and crystal orientation.
• HRTEM image and definition of CUS active sites: two kind of sites are defined.
• HDS of DBT presents a zero order to DBT-DDS route, first order-HYD route.

The stability of unsupported W2C tungsten carbide and its sulfur resistance during hydrodesulfurization (HDS) of dibenzothiophene (DBT) are a specificity of W2C. This material was characterized by X-ray diffraction, specific surface area measurements, HRTEM associated with EDS, lattice images, direct and reverse Fast Fourier Transforms, direct lattice fringes profiles, dynamic CO chemisorptions and elemental analysis of S, C and W. The absence of surface crystallographic layer of tungsten sulfide and the presence of a surface monolayer of “adsorbed sulfur species” (post-sulfur passivation by H2S by the end of HDS runs), after 5 days of HDS runs on unsupported W2C, are established. The repeatability of both W2C synthesis and catalytic activity with chemical stability are shown. The HDS of DBT was investigated using a fixed-bed reactor. The reaction was carried out at 613 K, under a 6 MPa total pressure of H2. The global reaction follows two parallel routes. Under the experimental conditions, DBT has a single final product along each route, leading either to biphenyl through the direct desulfurization (DDS) pathway, or to cyclohexylbenzene through the hydrogenation (HYD) pathway. Accordingly, the HDS of DBT over W2C presents a true zero order reaction to DBT along the DDS route, and a true first order reaction along the HYD route. The resulting global apparent first order rate with respect to DBT is linked to the selectivity towards these two routes. The kinetically true zero order rate along the DDS route corresponds to the saturation by DBT of all its active sites and, consequently, means that the HYD route is occurring on a second kind of sites. The presence of two kinds of sites is thus kinetically demonstrated and discussed in term of Coordinatively Unsaturated Sites (CUS).

W2C oriented platelets after 5-day HDS reaction defining coordinatively unsaturated sites on terraces, steps, kinks, edges, ledges, corners. dh k l (0.227 and 0.25 nm) correspond to those of W2C. No effect of adsorbed sulphur species can be detected on dh k l values.Figure optionsDownload as PowerPoint slide