Influence of sulfiding agent on the chemical-physical properties and catalytic pathway of CoMoSx catalysts in the HDT process of o-xylene.

The European Fuels Quality Directive 2009/30/EC governs several elements of fuel quality, primarily linked to air pollutant emissions and biofuels sustainability. In particular, the Directive states that: i) sulphur content in petrol and diesel must be lower than 10 ppm, ii) a 6% target reduction for the greenhouse gas intensity of fuel is mandatory, iii) greenhouse gases emission from biofuels must be 50-60% lower than from the fossil fuel they replace. This modern scenario, introduces new challenges in refinery and bio-refinery. In particular, hydrotreating (HDT) has become a key process aimed at the production of “high-quality” fuels and bio-fuels. Catalysts of interest in the HDT are usually composed by molybdenum or tungsten sulphide active phase deposited on high surface area support, promoted by cobalt or nickel. In the present study, the effect of sulfiding agent (H2S or DMDS) on the chemical-physical properties, acidic character and catalytic behavior of CoMoSx catalysts has been investigated. Catalysts have been characterized by the means of X-ray Diffraction (XRD), FTIR spectroscopy, chemisorption techniques (using NH3 and CO as probe molecules), Raman spectroscopy and XPS study, while the catalytic tests have been performed in o-xylene saturated pressure, at 350°C and 11bar of total pressure. The catalytic study in the hydrotreating of o-xylene model compound effectively allowed measuring the hydrogenating activity and the acidic activity of HDT catalysts, approaching operating conditions of the industrial process. Then, the sulfiding procedure affects the active nature of "active site" and the extension of active phase on the catalyst surface. Moreover, a clear relationship between the acid strength of differently sulfided CoMo catalysts and their catalytic behavior in the o-xylene HDT test has been ascertained.