(375c) Adsorption of Organonitrogen Compounds on Promoted Molybdenum Sulfide Catalysts: A DFT Study on Nimos and Comos

Hydrodenitrogenation (HDN) of heteroaromatic nitrogen-containing compounds is concomitant with hydrodesulfurization (HDS) of organosulfur compounds in crude oil fractions such as vacuum gas oil. Organonitrogen compounds, consequently, inhibit HDS by competitively adsorbing on relevant active sites. On promoted molybdenum sulfide, which is the most commonly used class of catalysts for hydrotreating, the active sites have been shown to be located on or close to the two types of exposed edges – metal and sulfur [1]. These edges are further ‘decorated’ by sulfur atoms and sulfohydryl groups depending on the reaction conditions. The detailed mechanism of HDS and the inhibition by organonitrogen compounds have, however, not been completely resolved to date. Adsorption (and, hence, inhibition) can depend on the nature of the promoter (Ni, Co, etc.), basicity of the adsorbate, steric hindrance, and the specific catalyst edge and its decoration. In this study, we computationally investigate plausible modes and strength of adsorption of basic and non-basic heteroaromatic nitrogen-containing compounds such as derivatives of pyridine and pyrrole vis-à-vis those of thiophene and dibenzothiophene. Specifically, we identify favored adsorption sites of these classes of organic compounds and estimate their adsorption equilibrium constants on Ni- and Co- promoted molybdenum sulfide (NiMoS and CoMoS) to provide insights into the inhibition of HDS.

Periodic DFT and vdW-DF methods [2] have been used to systematically study the adsorption of over twenty compounds relevant to hydrotreating on the most stable catalyst configurations for NiMoS and CoMoS under typical HDS reaction conditions, viz., nickel-promoted metal edge with bare Ni atoms and cobalt-promoted sulfur edge with 50% coverage of S along with the presence of sulfohydryl groups. It is seen that basic compounds adsorb stronger than thiophene on both types of edges, adsorption is stronger on Ni than on Co except for large basic molecules, adsorption on Co-edges involves the Brønsted proton, and the dominant adsorption on Ni and Co is through the edge and brim sites respectively, while dispersion forces can significantly alter both the strength and mode of adsorption.

The catalyst models, chemisorption modes of different compounds, and the effect of van der Waal’s dispersion interaction will be presented and discussed.

References:

1. Topsøe, H. Appl. Catalysis A: General (2007), 322, 3 – 8
2. Klimeš, J. et al. Phys. Rev. B (2011), 83, 195131

Acknowledgements: Financial support from BP is gratefully acknowledged.