(138b) A DFT Study on the Site Requirements for Hydrodesulfurization with Minimal Nitrogen Inhibition on (un)Promoted Molybdenum Sulfides

Authors: 
Rangarajan, S., University of Wisconsin-Madison
Mavrikakis, M., University of Wisconsin-Madison

Hydrodesulfurization (HDS) is an essential step in petroleum processing to reduce sulfur content in transportation fuels, specifically diesel, to legislatively mandated levels. Industrially, this reaction is carried out using nickel (NiMoS) and cobalt (CoMoS) promoted molybdenum sulfide catalysts. Organonitrogen compounds such as acridine and carbazole in the feed typically inhibit HDS through competitive adsorption and a parallel hydrodenitrogenation reaction; this effect is more pronounced for larger sulfur containing compounds. Consequently, it is imperative to identify efficient HDS catalysts that are minimally inhibited by nitrogen containing compounds.

In this study, we perform DFT calculations on six potential active sties on the metal and sulfur edges of MoS2, NiMoS, and CoMoS to show that the binding energy of sterically unhindered sulfur containing compounds (such as thiophene and dibenzothiophene) is linearly correlated with that of H2S while the binding energy of organonitrogen compounds correlates with that of NH3. Therefore, the relative binding energy of NH3 and H2S is a descriptor of HDS inhibition by organonitrogen compounds. Using this descriptor, we screen about twenty other plausible sites on MoS2, NiMoS and CoMoS to identify the site requirements of a potential active site that catalyzes HDS but prevents competitive adsorption by organonitrogen compounds.  We predominantly find that sites that bind H2S strongly, thereby allowing for activation of organosulfur compounds, also tend to bind NH3 (therefore organonitrogen compounds) competitively while some sites, such as a coordinatively unsaturated site on CoMoS sulfur edge, preferentially bind H2S over NH3.