Research Article pubs.acs.org/acscatalysis
On the Preferred Active Sites of Promoted MoS2 for Hydrodesulfurization with Minimal Organonitrogen Inhibition Srinivas Rangarajan and Manos Mavrikakis* Department of Chemical & Biological Engineering, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *
ABSTRACT: Hydrodesulfurization is a process to produce ultralow-sulfur diesel fuel. Although promoted molybdenum sulfide (MoS2) catalysts have been used industrially for several decades, the active site requirements for selective hydrodesulfurization of organosulfur compounds with minimal inhibition by organonitrogen constituents of a real gasoil feed has not been resolved. Using molecular binding energy descriptors derived from plane wave density functional theory calculations for comparative adsorption of organosulfur and organonitrogen compounds, we analyzed more than 20 potential sites on unpromoted and Ni- and Co-promoted MoS2. We find that hydrogen sulfide and ammonia are simple descriptors of adsorption of sterically unhindered organosulfur and organonitrogen compounds such as dibenzothiophene and acridine, respectively. Further, organonitrogen compounds in gasoil bind more strongly than organosulfur compounds on all sites except on sites with exposed metal atoms on the corner and sulfur edges of promoted MoS2. Consequently, these sites are proposed as required for maximum-hydrodesulfurization minimum-inhibition catalysis. KEYWORDS: promoted molybdenum sulfide, hydrodesulfurization, nitrogen inhibition, binding energy descriptors, density functional theory, active site identification
I. INTRODUCTION Hydrotreating is a process to remove heteroatoms, specifically sulfur, nitrogen, and oxygen, from organic compounds using hydrogen. A primary industrial purpose of hydrotreating is hydrodesulfurization (HDS), a process which is carried out mostly on Co- and Ni-promoted molybdenum sulfide catalysts (MoS2, with Ni- and Co-promoted catalysts referred to as NiMoS and CoMoS, respectively), leading to the reduction of sulfur content in diesel fuel to legislatively permitted levels of 15 ppm.1−3 Studies based on experiments (microscopy and spectroscopy)4−15 and density functional theory (DFT)3,16−22 have shown that these catalyst particles contain layers of molybdenum sulfide with edge metal atoms substituted by cobalt or nickel atoms which may be further decorated under industrial processing conditions by sulfur and hydrogen atoms. While the exact nature of the active site is yet to be comprehensively determined, it is generally agreed that the active sites are located along the periphery of catalyst particles and can involve the edges (both the metal and sulfur edges), corners, and brim sites (Figure 1). The edges and corners fall in the category of the coordinative unsaturated sites (CUS) formed by the removal of sulfur atoms decorating the edges of these catalyst particles by molecular hydrogen. The unsaturated Mo atoms behave as Lewis acid sites with the unoccupied d states above the Fermi level allowing for strong binding with electron-donating molecules.23 The brim sites, on the other © XXXX American Chemical Society
Figure 1. Representative particle models of NiMoS (left) with Ni atoms replacing Mo atoms on the metal edge and CoMoS (right) with Co atoms replacing Mo atoms on the sulfur edge of MoS2. The three types of sites (edge, corner, and brim) are marked for reference. Color scheme: violet, cobalt; red, nickel; cyan, molybdenum; yellow, sulfur; black, hydrogen.
hand, do not require unsaturation and possess metallic characteristics on account of the presence of one-dimensional edge states that cross the Fermi level.24 The confinement of electrons in a narrow patch near the edges is visible on atomReceived: September 24, 2016 Revised: November 26, 2016
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estimate the relative binding strengths of sulfur- and nitrogencontaining compounds. Subsequently, we use the insights to propose site requirements for hydrodesulfurization catalysts with minimal nitrogen inhibition.
resolved scanning tunneling microscopy (STM) images as a bright brim on the particles.25 Molybdenum atoms on either edge of a MoS2 particle can be fully or partially replaced by Ni or Co atoms. Consequently, a multitude of potential sites can exist on these catalysts. For instance, Krebs et al.3 showed using DFT-based Wulff construction that Ni and Co promotion can lead to partial promotion on both metal and sulfur edges of the catalyst; further, Tuxen et al.26 have shown, using STM studies, the formation of vacancy sites (CUS) near the corners of CoMoS particles. Gutierrez et al.27 and Travert et al.28 have shown that experimental CO infrared (IR) spectra on promoted MoS2 lead to multiple peaks corresponding to CO adsorption on Mo, Co/ Ni, and mixed (Co−Mo or Ni−Mo) sites. Lauritsen et al.29 also reported that partial and full substitution of Mo atoms on the edges by Ni were observed in STM studies. From a synthetic point of view, promoted and unpromoted MoxW1−xS2 catalysts of various shapes, sizes, morphologies, and stacking degrees have been reported by exploiting a number of factors such as sulfidation conditions, precursors, support, and the nature of promoters to modulate HDS activitiy by engineering the concentration of the two types of active sites.27,30−41 Real gasoil feeds, however, consist of several other components including nitrogen-containing compounds that can inhibit HDS.42−50 The extent of inhibition depends on the nature and amount of sulfur- and nitrogen-containing compounds, but reduction in HDS rates by at least factors of 2−5 has been reported. This is of particular concern in the current context of “deep desulfurization”, which requires the removal of sulfur from dibenzothiophene and its alkylsubstituted forms. Despite extensive DFT and scanning tunneling microscopy (STM) studies analyzing the adsorption of organonitrogen compounds on unpromoted and promoted molybdenum sulfide,44,51−54 the inhibitory mechanism of nitrogen-containing compounds on HDS still remains unresolved. Our previous work55,56 provided a preliminary computational chemistry study of the binding of organonitrogen and hydrocarbon compounds on nickel- and cobaltpromoted molybdenum sulfide in comparison to organosulfur compounds. Specifically, we showed that inhibition by nitrogen-containing compounds is a combination of two mechanisms: (i) competitive adsorption of organonitrogen and organosulfur compounds and (ii) destabilization of HDS reaction intermediates and transition states by the presence of adsorbed organonitrogen compounds (and their hydrogenated intermediates). An optimal catalyst should have both high HDS activity and minimal inhibition by nitrogen-containing compounds. From a computational standpoint, identifying such a catalyst ideally requires developing detailed microkinetic models of the hydrotreating chemistry, taking into consideration all plausible sites of the catalyst, and thereby identifying optimal sites. In view of the computational demands of such an exercise and the large number of potential sites on the catalyst, we instead first identify those sites that are likely to be unhindered by organonitrogen compounds. Specifically, in this work, we build on these studies to explore the adsorption of sterically unhindered sulfur (e.g., dibenzothiophene) and nitrogencontaining compounds (e.g., acridine) on unpromoted molybdenum sulfide edges, use these and previous data to identify reliable descriptors of adsorption strength, and thereby elicit binding energy trends across several plausible sites on promoted and unpromoted molybdenum sulfide catalysts to
II. METHODS All electronic structure calculations were carried out on a single-layer periodic stripe model with four rows containing six metal atoms and three rows of sulfur atoms on either side of the metal rows to emulate the MoS2 trilayer, with further decoration of the edges on the top and bottom of the stripe by sulfur and hydrogen atoms. Such models are used extensively to model edge and brim sites and have been shown to successfully predict edge decoration and derive insights on the adsorption and reactions of organosulfur, organonitrogen, and hydrocarbon compounds.3,16,17,22,23,44,51,53,54,57−67 Figure 2 shows the six catalyst
Figure 2. Catalyst models: (a) equilibrium unpromoted metal edge (brim); (b) equilibrium unpromoted sulfur edge (brim); (c) equilibrium nickel-promoted metal edge (NiMoS, edge); (d) equilibrium cobalt-promoted sulfur edge (CoMoS, brim); (e) CoMoS sulfur edge with CUS-like vacancy (CoMoS CUS-like, edge); (f) unpromoted sulfur edge with edge vacancy (brim). Color scheme: violet, cobalt; red, nickel; cyan, molybdenum; yellow, sulfur; black, hydrogen. “Equilibrium” refers to the most stable structure on the basis of ab initio thermodynamics calculations. The location of coordinative unsaturation in (e) and (f) is marked by a red box.
models used initially: (a) unpromoted (1010̅ ) M-edge (at the top) decorated by a single row of sulfur atoms with one hydrogen atom for every two sulfur atoms while the bottom edge is decorated by two rows of sulfur atoms,22,24 (b) unpromoted (10̅ 10) S-edge decorated by two rows of sulfur atoms and additionally one hydrogen atom for every two sulfur atoms while the bottom edge is decorated by a single row of sulfur,22 (c) nickel-promoted (101̅0) metal edge (NiMoS Medge) wherein the top row is nickel atoms without any further decoration and the bottom row is decorated by two rows of sulfur atoms,62 as studied by us previously,56 (d) cobaltpromoted (1̅010) sulfur edge (CoMoS S-edge) with the top metal row being cobalt, which is decorated further by a single row of sulfur atoms having one sulfohydryl group and the bottom metal row decorated by a single row of sulfur atoms,65 studied by us previously,55 (e) CoMoS (1̅010) S-edge with a “CUS-like” site formed by rearranging the sulfur decoration on 502
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Figure 3. Binding energy trends of (a) thiophene and dibenzothiophene with respect to the binding energy of H2S and (b) basic and (c) nonbasic organonitrogen compounds as a function of binding energy of ammonia, on six types of sites on unpromoted and promoted molybdenum sulfide. In (a), the linear trend lines are based on the complete set of data for each adsorbate and the dashed vertical line separates edge and brim adsorption modes (the specific location of the vertical line is arbitrary and is only representative of where the adsorption mode changes). In (b) and (c), solid points represent brim sites, while open points refer to edge sites and the trend lines are based on data for brim sites. All values are in eV.
the Table S1 and figures in Tables S3−S5 in the Supporting Information; these data were combined with previously calculated results on nickel-promoted metal edge and cobaltpromoted sulfur edge (brim and CUS-like new site) to identify trends among these six plausible sites for sterically unhindered sulfur- and nitrogen-containing compounds.
the top edge to create the vacancy with an associated neighboring sulfur dimer, as previously reported by us,55 and (f) unpromoted (1̅010) sulfur edge with a sulfur vacancy created by the removal of a H2S molecule reported by Nørskov and co-workers as a plausible site for hydrogenolysis of the C− S bond of thiophene.53,66 These structures have been reported as equilibrium structures on the basis of ab initio studies or have been derived from such structures to create sulfur vacancies. The adsorption on the nickel metal edge (model c) and CoMoS S-edge with CUS-like site (model e) is on the edge; adsorption on all the other sites is on the brim. All calculations were carried out using VASP,68,69 a plane wave periodic density functional theory code with generalized gradient approximation70 PAW71 potentials and the PW9172,73 exchange-correlational functional. The Brillouin zone was sampled using a 1 × 2 × 1 Monkhorst−Pack k-point sampling method.74 A Gaussian smearing of 0.05 eV was used, and the energies were extrapolated to 0 K; further, a plane wave density of 400 eV and density wave cutoff of 600 eV were used in all calculations. All atoms were relaxed, and the convergence criterion used for geometric relaxation was 0.02 eV/Å. Gasphase calculations were performed at the same level of theory in a box that has at least 10 Å of vacuum between two images in any direction with only γ-point sampling. Dipole corrections75 were included in all calculations, and spin polarization was used for calculations on NiMoS and CoMoS. With these settings, the binding energies were found to converge within 0.05 eV for even alkyl-substituted dibenzothiophene adsorption on CoMoS.55 For unpromoted molybdenum sulfide edges, tests with spin polarization led to negligible magnetic moments and less than 0.02 eV differences in total energy between including and excluding it; similar effects on energetics were observed by Vojvodic et al.76 The binding energy (BE) of adsorbates (sulfur or nitrogen containing compounds) is calculated as BE = Emolecule+stripe − Emolecule,gas − Estripe
III. RESULTS The trends of the binding energy of thiophene and dibenzothiophene (DBT) on these six sites as a function of that of hydrogen sulfide (BEH2S) are shown in Figure 3a. The calculated binding energy values vary linearly with that of hydrogen sulfide (the linear fit is given in Table S2 in the Supporting Information), with dibenzothiophene adsorbing more strongly than thiophene on all sites; given that the data set comprises multiple types of sites (unpromoted/promoted, edge/brim) with different binding modes (flat, upright, protonated, etc.), this linear trend is remarkable. The binding energy of hydrogen sulfide is, therefore, a reliable descriptor of the binding strength of unhindered organosulfur compounds. Figure 3b,c shows the binding energy trends of basic (aniline, pyridine, lutidine, quinoline, and acridine) and nonbasic (pyrrole, indole, and carbazole) organonitrogen compounds, respectively, as a function of binding energy of ammonia (BENH3). The solid (open) points correspond to adsorption on brim (edge) sites. The binding energies follow a linear trend for the brim sites (a linear fit equation is given in Table S2 in the Supporting Information) with the slope varying from 1.0 to 1.7 for basic compounds and from 0.25 to 0.4 for nonbasic compounds. The trend lines also have a mean absolute error of less than 0.05 eV for basic and 0.1 eV for nonbasic compounds. The value of the slopes for basic organonitrogen compounds increase linearly with their experimental gas-phase proton affinities (Figure S1 in the Supporting Information), consistent with the idea that these adsorbates are stabilized through the formation of a protonated complex. The binding energy values of most basic organonitrogen compounds on edge sites (Figure 3b) are significantly larger than on the brim sites; however, the trend lines of brim sites overpredict the binding energy of basic organonitrogen compounds for edge sites by at least 0.1 eV and up to 1.0 eV (for acridine on NiMoS M-edge). This overprediction increases from aniline to acridine, which can
(1)
where Emolecule+stripe is the total energy of the adsorbate molecule and the stripe (the catalyst model), Emolecule,gas is the total energy of the molecule in the gas phase, and Estripe is the total energy of the stripe only. The binding energy data and the adsorption structures on the unpromoted models are given in 503
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corresponding to nonbasic ones can be ignored for the purposes of this study. Therefore, the binding energy difference between ammonia and hydrogen sulfide can be used as a qualitative descriptor of the relative binding of organonitrogen compounds in comparison to organosulfur compounds and, thereby, of nitrogen inhibition. This can be further rationalized by noting that the slope of dibenzothiophene vs H2S (Figure 3a) is close to 1 and that of basic organonitrogen compounds vs NH3 (Figure 3b) is at least 1 (Table S2 in the Supporting Information). The intercepts in both cases are small (∼0.1 eV or smaller). This indicates that if |BENH3| > |BEH2S|, then (at least basic) organonitrogen compounds likely bind more strongly than organosulfur compounds. Figure 5a compares the binding energies of ammonia and hydrogen sulfide for the six sites. It can be seen that the brim sites (solid circles) follow a linear trend, as expected from Figure 3, and in general the binding energy of ammonia is higher than that of hydrogen sulfide (the points are below the y = x parity line). Further, the linear trend shown in Figure 5a overpredicts the binding energy of ammonia on the edge sites; clearly, therefore, brim sites bind organonitrogen compounds more strongly than edge sites relative to organosulfur compounds. This is expected because the proton affinity (basicity) of organonitrogen compounds is, in general, lower (higher) than that of organosulfur compounds. One of the edge sites, viz. the NiMoS metal edge, also binds ammonia more strongly than hydrogen sulfide; on the other hand, the other edge sitethe CoMoS CUS-like siteprefers hydrogen sulfide over ammonia, consistent with the discussion above. These binding energy descriptors were then used to probe the potential alternative sites on these catalysts. A detailed list of alternative edge and corner site models, the binding energy of H2S and NH3, and the corresponding adsorption structures are given in Tables S6−S13 and Figures S2−S4 in the Supporting Information; they were drawn from the literature (details given in the Supporting Information), and while a thorough analysis of the relative stability is not the focus of this work, we refer to previous theoretical and experimental works that argue the presence of such sites.3,26−29 Figure 5b shows a plot of comparison of the binding energies of H2S and NH3 on a variety of NiMoS and CoMoS sites, including the corner sites. These results suggest that, in almost all cases, ammonia binds more strongly than hydrogen sulfide; that is, the stronger the adsorption of H2S is on a site, so is that of ammonia. Furthermore, the binding energy of H2S on the CoMoS corner site with a vacancy is equivalent to that of NH3 on the same site; this potentially implies that organosulfur compounds bind as strongly as organonitrogen compounds at this site. On the equivalent NiMoS site, ammonia binds slightly more strongly. Figure 5b also shows lines of higher slope (2.5 and 4). On the basis of the linear trends, it can be inferred that the sites falling in the region between these two lines can bind large nonbasic organonitrogen compounds (carbazole and indole) more strongly than organosulfur compounds, while the region below the line with slope equal to 4 can bind even small nonbasic compounds such as pyrrole more strongly than organosulfur compounds. All sites in these two regions (slope >2.5) are brim sites. These sites will be severely poisoned by nitrogen compounds and may, therefore, not contribute to any hydrodesulfurization activity. To confirm the results for the CoMoS S-edge corner site, the adsorption of representative feed moleculesthiophene, pyridine, dibenzothiophene, and acridineon the corner sites
be rationalized on the basis of the steric influence of substituent alkyl groups or aromatic rings on the edge sites. The linear trend lines of brim sites in Figure 3c, on the other hand, tend to underpredict the binding of two nonbasic organonitrogen compoundspyrrole and indoleon edge sites by 0.3 eV while overpredicting the binding energy of carbazole by 0.1−0.2 eV. This variation among nonbasic compounds could arise due to differences in binding modes pyrrole and indole bind via carbon atoms on the heteroaromatic rings while carbazole binds via the benzene rings. The binding energy of ammonia, therefore, is not a quantitative descriptor of the binding energy trends of organonitrogen compounds. The binding energy values of organonitrogen compounds, however, monotonically increase with that of ammonia over brim and edge sites in most cases; the binding energy of ammonia, therefore, could be used as a qualitative predictor of trends. Since BENH3 and BEH2S are descriptors of the binding energies of organonitrogen and organosulfur compounds, respectively, it is pertinent to check if their difference (BENH3 − BEH2S) can potentially be used to predict the relative adsorption strengths of these compounds. Figure 4 shows this
Figure 4. Binding energy difference chart for six types of sites. The difference values plotted are the binding energy differences between organonitrogen and dibenzothiophene (DBT) and differences between ammonia (NH3) and hydrogen sulfide (H2S) in eV. Positive (negative) values indicate that DBT or H2S binds more (less) strongly than organonitrogen or ammonia, respectively. CoMoS-brim refers to the S-edge where the adsorption is preferred on the brim, while CoMoS-edge refers to the S-edge with CUS-like sites. The dashed horizontal line demarcates different sites.
comparison for the six sites, from which several observations can be made. First, the CoMoS edge (i.e., the CUS-like site) binds all organonitrogen compounds less strongly than dibenzothiophene. Second, a few other sites bind sulfurcontaining species more strongly than nitrogen-containing compounds, especially for nonbasic compounds on NiMoS. Third, the binding energy difference between ammonia and hydrogen sulfide qualitatively correlates with the trends of binding energy difference between organonitrogen compounds and dibenzothiophene; in the majority (>90%) of cases, a positive (or negative) value of one implies a positive (or negative) value of the other. Further, since basic nitrogencontaining compounds inhibit HDS more strongly than nonbasic organonitrogen compounds, several of the outliers 504
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Figure 5. (a) Comparison of binding energy values of ammonia and hydrogen sulfide. The solid (red) line is the parity line, while the dashed (black) line is the trend line for brim sites. Solid circles represent brim sites, while open circles refer to edge sites. The CoMoS CUS-like site is shown in the inset, and the location of the site is indicated by a maroon box. (b) Comparison of binding energy values of H2S and NH3 on alternative sites considered in this study. Red filled circles (triangles) are values for NiMoS edges and brim (corner) sites, and blue filled squares (diamonds) are values for CoMoS edges and brim (corner) sites. The dashed green line represents the y = x parity line. The CoMoS corner model (with CUS) is shown in the inset. The black boxes indicate the location of the site on the model. The other two dashed lines (y = 2.5x and y = 4x) demarcate sites that bind nonbasic nitrogen-containing compounds (carbazole only and all respectively) more strongly than sulfur-containing compounds on the basis of the values of the slopes in Figure 2c. All values are in eV. Color scheme: violet, cobalt; cyan, molybdenum; yellow, sulfur; black, hydrogen.
of CoMoS with vacancy was considered in detail (structures and data are given in Table S14 in the Supporting Information). All compounds can adsorb in a perpendicular or parallel mode with respect to the plane of the catalyst slab. Only the parallel mode is feasible on single-layer CoMoS particles, while both modes are likely feasible on the top and middle layers of multilayered particles. All compounds prefer the perpendicular adsorption mode over the parallel mode primarily because of the lower steric hindrance when the molecule is out of the plane of the catalyst slab. Interestingly, we find (a) the binding energies of perpendicular modes of thiophene (−1.66 eV) and dibenzothiophene (−1.61 eV) are greater than those of similarly sized organonitrogen compounds, viz., pyridine (−1.61 eV) and acridine (−1.5 eV) respectively, (b) pyridine adsorbs more strongly than thiophene in the parallel mode (−1.52 eV vs −0.98 eV), and (c) acridine adsorbs less strongly than dibenzothiophene in the parallel mode (−0.66 eV vs −0.88 eV) primarily because of the steric hindrance by the aromatic rings in the approach of the nitrogen atom to the edge in acridine. In general, therefore, our proposition that corner sites are less inhibited by nitrogencontaining compounds on the basis of linear scaling trends holds. Since the slopes of pyridine, quinoline, and acridine (1.4, 1.6, and 1.8) are greater than that of dibenzothiophene (∼1), our earlier argument that |BENH3| > |BEH2S| implies inhibition by organonitrogen compounds should hold. However, since the trend lines overpredict the binding energy of basic organonitrogen compounds (Figure 3), we calculated the predicted binding energies of acridine (largest organonitrogen compound studied herein) and dibenzothiophene on all these alternative sites, correcting for the binding energy overprediction on edge sites (by adding an average value of observed overprediction in Figure 3b). The details of the procedure and the predicted binding energy values are given in Table S12 in the Supporting Information. We find that acridine should bind more strongly than dibenzothiophene on all but two sitesfully and partially promoted NiMoS sulfur edge (Figure 6). In both cases, the
Figure 6. (i) Fully and (ii) partially promoted NiMoS sulfur edge structures considered in this study based on structures reported by Krebs et al.3,57 The orange box indicates the potential edge site for adsorption.
sulfur atoms on the edge, attached to Ni atoms, tend to pair up; this results in the nickel atoms being exposed in a manner similar to that for the CUS-like sites observed on the CoMoS Sedge. Further, our predictions based on the linear scaling result in a positive value of binding energy (after correcting for expected overprediction). We, therefore, performed DFT calculations of the adsorption of acridine and dibenzothiophene on the fully promoted NiMoS sulfur edge (section 9 in the Supporting Information). We find that acridine (−0.32 eV), indeed, binds less strongly than dibenzothiophene (−0.5 eV). These two sites, potentially, are also relatively less inhibited by organonitrogen compounds.
IV. DISCUSSION Among all sites considered in this study, those with a sulfur vacancy on a promoted catalyst (CoMoS CUS-like, CoMoS/ NiMoS corner with vacancy) and partially exposed metal edge atoms of the sulfur edge of NiMoS (shown in the insets of Figure 5 and in Figure 6) would bind sterically unhindered 505
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Figure 7. Charge density difference plots of the adsorption structures of (a and b) dibenzothiophene and (c, d) acridine on CoMoS S-edge with CUS-like sites (on the left) and NiMoS S-edge (right), respectively. The sulfur and nitrogen atoms of the adsorbate are not clearly seen but are pointed to by solid black and dashed red arrows, respectively. The isosurface for the structures on CoMoS S-edge is 0.00075 e/bohr3, and that for NiMoS S-edge is 0.001 e/bohr3. The blue region represents charge accumulation, and the yellow region indicates depletion. Color scheme: violet • cobalt; red •, nickel; cyan •, molybdenum; yellow •, sulfur; blue •, nitrogen; black •, hydrogen.
sites can indeed be formed on MoS2 and CoMoS on the edges and the corners. The density of corner sites varies inversely with particle size, while CUS-like site density increases with the size and stacking degree of MoS2. Hensen et al.30 showed that the HDS rate increased with the stacking degree of MoS2. Lercher and co-workers27 recently showed that, for combined hydrodesulfurization of dibenzothiophene and hydrodenitrogenation of propyl aniline on NiMoS, the rate of sulfur (nitrogen) removal increased (decreased) with decreasing particle size. Furthermore, it has been shown that the hydrodesulfurization rate on nickel phosphide (Ni2P) increases with the surface area of the catalyst,31 indicating that the exposed nickel atoms (that are electronically similar to a CUS site on NiMoS) are the active sites. These observations suggest that a mechanism involving a CUS site on the corner of the catalyst particles is plausible for HDS. Further, Ho and co-workers77 have studied the inhibition effects of nitrogen compounds on real feeds and have shown that activity on sites where hydrogenation steps likely occur are inhibited by organonitrogen compounds to a greater extent than on sites where hydrogenolysis occurs. Prins and co-workers78 have also similarly shown that hydrogenation steps are inhibited to a greater extent than C−S bond scission by organonitrogen compounds. We have argued previously55 that brim sites are likely required for hydrogenation, while hydrogenolysis (primarily C−S bond scission) should preferentially occur on the CUS on CoMoS, which is consistent with these two experimental observations. In view of our results and the plausibility of CUS being active in HDS, we suggest that the sites which mimic those in Figures 5 and 6 on promoted molybdenum sulfide, in terms of electronic and geometric considerations, are required to activate sterically unhindered organosulfur compounds with minimal nitrogen inhibition. These sites include the conventional vacancy sites forming CUS and those formed by rearrangement/dimerization of sulfur atoms such as in the case of CoMoS CUS-like site and NiMoS S-edge. We, however, note that the last two sites (shown in Figures 5b and 6) do not have direct experimental evidence of presence or involvement
organosulfur compounds, such as dibenzothiophene, more strongly than organonitrogen compounds. We first turn to the kinetic feasibility of the hydrotreating chemistry on these sites. As noted earlier, the complete reaction network for hydrodesulfurization of organosulfur compounds and concomitant hydrogenolysis of nitrogen-containing compounds is likely prohibitively large, thereby precluding a detailed DFT investigation of the underlying kinetics. However, direct desulfurization is considered the major route for hydrodesulfurization for dibenzothiophene, on the basis of experimental kinetics studies, and consists of two consecutive C−S bond scission steps. We therefore calculated the barriers for these steps on the corner CUS and the CUS-like sites. We found that the formation of CUS on the corner and the adsorption of dibenzothiophene onto the edge simultaneously leading to a dimerization of sulfur atoms (thus forming the CUS-like site) have the highest energies (1.51 and 1.54 eV, respectively) relative to the gas-phase reactants and the clean edge. The C−S bond scission steps have a lower relative energy (1.25 and 1.01 eV, respectively, for the first C−S bond scission on the CUS and CUS-like sites) and are stabilized by van der Waals forces by about 0.3 eV. These energies are not prohibitive at typical hydrotreating temperatures (633−673 K), indicating that these sites are feasible from a kinetics point of view. While only a detailed microkinetic model would provide more conclusive results, we note that since the formation of these sites is the most difficult step in either case, relative adsorption strengths of organosulfur and oganonitrogen compounds once these sites are formed (or while they are formed in the case of CUS-like sites) will determine the extent of nitrogen inhibition. A detailed explanation of the experimental evidence for the direct desulfurization pathway, the potential energy surface on the two sites, and a discussion of the results are given in section 11 in the Supporting Information. There is substantial experimental evidence to suggest the importance of CUS sites in HDS and the site preferences of organonitrogen compounds. STM studies indicate that CUS 506
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ACS Catalysis
Attempts toward the catalyst design of HDS catalysts on the basis of identification of bonding descriptors have persisted over decades. Toulhoat et al.80 and Toulhoat and Raybaud81 used the metal−sulfur bond energy as a descriptor to derive a “volcano diagram” of the activity of HDS catalysts. Raybaud and co-workers used this methodology recently to suggest new (specifically NiMoxW1−xS) catalysts.82 Moses et al.64 also developed similar volcano plots through simple linear correlations for binding and activation barriers using the binding energy of the SH group as a descriptor. In both cases, the underlying concept is that the ideal site for a maximum HDS activity catalyst for thiophene should have an intermediate binding strength of sulfur atoms onto the active site so that stronger binding leads to H2S product inhibition, while binding that is too weak results in a high barrier for C−S bond activation. Our analysis indicates that the binding of sulfur atoms is correlated to that of nitrogen atoms; therefore, introducing nitrogen binding strength into such models is necessary and may result in a substantial shift of the peak of the volcano. 4,6-Dimethyldibenzothiophene, a sterically hindered refractory sulfur-containing compound, does not bind on the corner sites (or the CUS-like site) and prefers the brim site according to STM studies;26 a higher binding energy of sulfur compounds on the brim will, according to the trends in Figure 4, also lead to stronger binding of nitrogen-containing compounds. Therefore, cobalt- and nickel-promoted molybdenum sulfide catalysts may not have the requisite sites that can activate sterically hindered organosulfur compounds and not be inhibited by organonitrogen compounds. It is likely that other metal promoters and/or parent metals of the metal sulfide need to be considered. It has been argued that the HDS of alkylsubstituted dibenzothiophene involves hydrogenation steps to break the aromaticity of one of the rings prior to C−S bond scission.83−88 Therefore, a third propertythe binding energy of olefinic/aromatic moleculesmay be required in addition to BEH2S and BENH3 to identify likely catalysts and active sites for hydrodesulfurization of alkyl-substituted dibenzothiophenes with minimal nitrogen inhibition. In this context, it is important to note that synergistic effects have been observed for NiMoWS trimetallic catalysts.89 This was rationalized on the basis that WS2 has reportedly better hydrogenation activity in comparison to MoS2.
(such as STM studies); their similarity with the corner CUS site (Figure 5a) indicates a potential role in HDS. Further, all brim sites appear to bind organonitrogen compounds more strongly than organosulfur compounds, implying that these sites are likely strongly inhibited. The relative adsorption strengths of organosulfur and organonitrogen compounds on the brim sites can be explained on the basis of Lewis/Brønsted acidity arguments. On brim sites containing sulfohydryl groups, the hydrogen atom is acidic and its interaction with an adsorbate is proportional to the latter’s proton affinity. Basic organonitrogen compounds have a higher proton affinity than thiophene or dibenzothiophene; therefore, on such Brønsted acid sites, organonitrogen compounds will bind more strongly than organosulfur compounds. On brim sites without an acidic hydrogen, the interaction with an adsorbate will depend on the Lewis basicity of the latter. Since basic organonitrogen compounds have a higher propensity to donate electrons to Lewis acids in comparison to organosulfur compounds, they are expected to bind more strongly on such sites. Indeed, the gas-phase binding energy of basic nitrogen-containing compounds with a Lewis acid (such as BF3) is higher than that of sulfur-containing compounds (Table S15 in the Supporting Information). The relative binding energy strengths on the sites shown in Figures 5 and 6 cannot be explained using the above arguments alone, for these sites are expected to behave like Lewis acids. In such a scenario, basic organonitrogen compounds should bind more strongly than organosulfur compounds on these sites. However, sulfur-containing compounds bind onto both metal atoms of the site, while nitrogen-containing compounds bind only onto one of them, likely due to the relatively larger atomic radius of sulfur atoms (in comparison to nitrogen atoms). Figure 7 shows the charge density difference plots of the most stable adsorption structures of dibenzothiophene and acridine on CoMoS CUS-like site on the sulfur edge (S-edge) and NiMoS S-edge. Clearly, dibenzothiophene interacts with both Co (and Ni) atoms of the site, while acridine interacts with only one. This additional metal−sulfur bond leads to the higher adsorption strength of organosulfur compounds. In the absence of such geometric considerations, organonitrogen compounds should bind more strongly than organosulfur compounds. For instance, on NiMoS metal edge containing exposed Ni atoms (Table S1 in the Supporting Information), pyridine binds more strongly than thiophene and both compounds bind directly atop Ni atoms. The linear trend lines in Figure 3 form the basis of this study; however, the binding energies used capture the chemisorption interactions but are not corrected for dispersion effects. We have previously shown that the dispersion effects increase with the size of the molecule and can be significant (∼1 eV) for large organonitrogen and organosulfur compounds (acridine/dibenzothiophene and larger).55,56 The dispersion-corrected binding energies of organosulfur and organonitrogen compounds would likely not follow the linear trends observed in Figure 3. However, dispersion contributions are typically around 5−10 kJ/mol per non-hydrogen atom of the adsorbate that is close enough (
