Topological and Electronic Structure of Heterocyclic Compounds Adsorbed on Hydrotreating Catalysts

Abstract We studied the electronic structure of theadsorption of S- and N-containing aromatic compoundspresent in crude oils on MoS 2 and WS 2 clusters by means ofall-electron DFT methods. The aim of this work is tounderstand results related to the hydrotreating catalyst poi-soning by quinoline. We studied the adsorption of theorganic compounds by flat (p) and perpendicular (r)adsorption on each cluster catalyst. The calculated adsorp-tionenergiesindicatedthatp-adsorptionwasmorefavorableover r-adsorption. In the r mode, quinoline presented thelargest adsorption energy, which led to understand the poi-soning of the catalysts. We performed electron localizationfunction (ELF) studies on the molecules adsorbed on aperpendicular orientation. We showed methyl-substitutedcompounds had a weaker S-{Mo,W} bond due to sterichindrance. Furthermore, atoms-in-molecules (AIM) calcu-lations at the critical points (i.e. {S,N}-{Mo,W} interfaces)revealed a correlation between electron density and Lapla-cian of the electron density at this region and the adsorptionenergy.Ellipticity(e)studiesrevealedstructuralinformationof binding at these sites, as well as the competition betweenS- and N-containing compounds. Similarly, e showed thatmethyl-containing compounds had a very distinct characterthan non-substituted ones, thus revealing the importance ofsteric effects. Analytic tools such as ELF and AIM providecorrelations between the experimental observations andproperties. We find these studies can be further used tounderstand other catalytic phenomena.Keywords DFT ELF Ellipticity DBT Quinoline Hydrodesulfurization Poisoning1 IntroductionThe field of catalysis has responded to diminish sulfur andnitrogen content in fuels. Current laws in industrial coun-tries restrict the sulfur content in diesel below 15 ppm [1].In the hydrotreatment (HDT) process, organic compoundsin fluid phase react with a heterogeneous catalyst to removeunwanted moieties, e.g., S- and N-containing compounds.Two challenges immediately present themselves: the first isdetermining the type and structure of the chemical com-pounds in crude oils; secondly, it is essential to understandthe interaction between S- and N-containing compoundsand the catalytic surfaces which target its elimination.Aromatic S- and N-containing compounds are known tointeract in two ways with a heterogeneous catalyst. They