Tungsten disulfide catalysts from tetraalkylammonium thiotungstates by ex situ activation, their properties and HDS activity

The role of carbon coming from tetraalkylammonium thiotungstate precursors during the ex situ activation of WS2 catalysts was herein studied through the comparison of two different thiosalts, ammonium tetrathiotungstate (ATT, without carbon) and cetyltrimethylammonium tetrathiotungstate (CTATT, with carbon). The influence on the resulting WS2 catalysts, of a N-2 vs a H2S/H-2 (20% H2S) atmosphere during ex situ activation was also evaluated. Freshly obtained catalysts were characterized by N-2 adsorption-desorption isotherms, X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX), while their catalytic properties were evaluated in the hydrodesulfurization of dibenzothiophene at 3.1 MPa and 623 K. No significant difference was found in the textural and morphological properties of the final WS2 catalysts derived from thermal decomposition of ATT under N-2 flow, compared to 20% H2S/H-2 flow. In contrast, the textural, morphological and catalytic properties of WS2 catalysts derived from CTATT were strongly influenced by the nature of the atmosphere. Samples of WS2 derived from ATT showed an absence of porosity, while WS2 catalysts formed from CTATT presented a mesoporous organization characterized by type IV N-2 isotherms. XRD and TEM showed that the use of CTATT induces a strong destacking of the WS2 particles and the excessive formation of amorphous carbon, thus yielding less active WS2 catalysts which confirm the negative role of carbon-containing tetraalkylammonium precursors in the activation of WS2 catalysts, as opposed to their positive influence in the activation of MoS2 catalysts. The very high activity of the WS2 catalyst obtained from ATT is attributed to an optimized incorporation of structural carbon inside the WS2 structure. Finally, selectivity results show the depleted hydrogenating character of the WS2 catalysts formed from CTATT. (C) 2012 Elsevier B.V. All rights reserved.