A study of carbon-supported molybdenum hydrodesulfurization catalysts

The merits of carbon as a potential support for catalysts used in the hydroprocessing of heavy hydrocarbon liquids are explored. Of particular interest are the low susceptibility of carbon-supported catalysts to coking reactions compared to alumina-supported catalysts, and the relatively higher activities for hydrodesulfurization which have been found in carbon-supported catalysts.; A furnace black (Monarch-700) was the primary carbon used as the catalyst support in this investigation. Samples of the as-received Monarch-700 were subjected to widely varying thermal and chemical pretreatments. Additionally, two polymer-derived carbons (Ambersorb-340 and Ambersorb-348), an activated carbon (Norit-R-X3-Extra), and a graphitized carbon (HSAG-16), were used as supports in the preparation of selected catalysts. The carbon supports' surface charges (as measured by the electrophoretic mobility as a function of pH) were determined by electrophoresis.; A range of carbon-supported molybdenum sulfide catalysts was prepared and their activities were measured by determining rates of hydrodesulfurization of thiophene at 400{dollar}spcirc{dollar}C in a continuous flow fixed-bed microreactor operated slightly above atmospheric pressure.; Equilibrium adsorption of ammonium heptamolybdate, molybdenum acetylacetonate, and molybdenum oxalate aqueous solutions on different carbon supports resulted in widely varying catalyst uptakes. The results were rationalized on the basis of varying degrees of electrostatic interaction between the catalyst precursor and the supports. This interaction was found to be dependent on the surface charge of the support (which in turn depends on both the pH of the catalyst precursor solution and the surface chemistry of the support) as well as the magnitude and type of charge of the ionic precursor. Part of this study, in conjunction with a critical analysis of the literature, illustrates the heretofore mostly neglected issue that, for achieving controlled catalyst uptake and a high degree of catalyst dispersion or both, it was not sufficient to create adsorption (or catalyst anchoring) sites on the carbon support surface; these must also be made accessible and receptive to the catalyst precursor. If the isoelectric point of the support is known, the precursor can be chosen or the solution pH can be modified, or both, to favor catalyst precursor-support interaction (e.g., adsorption) and thus maximize initial catalyst dispersion.