On the difference between gas- and liquid-phase hydrotreating test reactions

In industrial practice, hydrotreating of oil fractions is carried out in ei ther a gas-phase process or a trickle flow process. We previously noticed t hat a remarkable difference exists between the relative activity of mixed s ulfide catalysts in gas-phase and liquid-phase hydrodesulfurization (HDS) r eactions. In the literature, however, no satisfying explanation with respec t to the possible fundamental differences between these reactions can be fo und. In this paper, we report an elaborate investigation on the effect of r eaction conditions, type of reactant and type of the catalyst on the occurr ence of differences between the relative activity, i.e. ranking, of mixed s ulfide catalysts in gas-phase and liquid-phase reactions. Striking differences were observed between the ranking of nitrilo-triacetic acid (NTA) and conventionally prepared NiMo catalysts in thiophene gas-pha se HDS and liquid-phase dibenzothiophene (DBT) HDS. Importantly, these diff erences did not depend on the nature of the reacting sulfur-containing comp ound. This allows the generalisation that NTA-based Ni(Mo) catalysts are re latively more active in gas-phase HDS reactions, whereas conventionally pre pared NiMo catalysts are relatively more active in liquid-phase HDS reactio ns. An analogous behaviour was observed for low- and high-temperature sulfi ded NiW/gamma -Al2O3 catalysts, of which the latter is much more active in gas-phase HDS reactions and the former is more active in liquid-phase HDS r eactions. It is concluded that this so-called 'gas-liquid-phase controversy ' is a generic phenomenon in hydrotreating reactions over metal sulfide cat alysts. It was verified that mass transfer limitations do not play a role i n this matter. The active sites of stacked slabs of the type II catalysts are more affecte d than those of type I catalysts, in which the active phase is in a more cl ose interaction with the support. It is proposed that the phenomenon is rel ated to a non-selective competitive adsorption of the a-polar solvent molec ules on sites protruding from the catalyst surface. Apparently, the proximi ty of the ionic surface of the alumina support hinders the adsorption of th e a-polar hydrocarbon molecules on the non-stacked systems, whereas the sul fur- and nitrogen-containing molecules are not so much affected in their ad sorption behaviour on these active sites. (C) 2001 Elsevier Science B.V. Al l rights reserved.