First-Principles Investigation of Selective Oxidation of Propane on

Article pubs.acs.org/JPCC

First-Principles Investigation of Selective Oxidation of Propane on Clean and Sulfided V2O5 (010) Surfaces John M. H. Lo,*,† Zahra A. Premji,†,‡ Tom Ziegler,‡ and Peter D. Clark†,‡ †

Alberta Sulphur Research Ltd., University of Calgary, University Research Center, Unit 6-3535 Research Road N.W., Calgary, Alberta, Canada T2L 2K8 ‡ Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 ABSTRACT: Oxidative dehydrogenation of propane on single-crystal V2O5 and V2O4S surfaces has been studied by means of periodic density functional theory. Three previously proposed ODH reaction mechanisms, namely the multisite vanadyl mechanism, the vanadyl-lattice mechanism, and the multisite bridging mechanism, were investigated. Results were compared with existing data from the literature. It was found that the multisite vanadyl mechanism plays the most dominant role in propene formation, with an apparent activation energy of 27 kcal/mol, in very good agreement with the experimental value. The present study also demonstrated that the other two mechanisms are less important in the propane ODH reaction because of the kinetic hindrance arising from the H2O desorption and O2 addition at the lattice site. On the other hand, the effects of sulfur substituents on V2O5 are detrimental in general. In the presence of H2S, V2O5 could be readily sulfided, with the estimated activation energy of 30 kcal/mol and reaction energy of 2.5 kcal/mol. At moderate temperatures, a small amount of surface VO will be converted to VS, which raises the apparent activation energy associated with the multisite vanadyl mechanism by only 3 kcal/mol. Further increase of reaction temperatures, however, would lead to the formation of lattice S whose accumulation results in the deactivation of the catalyst.

1. INTRODUCTION Metal oxides are common materials employed in catalytic conversion and selective oxidation of a variety of chemicals due to their versatility in terms of acid−base properties, reducibility of metal ions, defect sites, and surface morphology.1 Examples of such processes include the CO oxidation on RuO2,2 ammoxidation of propane on Mo−V−Ox based systems,3 and selective oxidation of benzene to phenol by uranium oxide catalysts.4 Vanadium oxide is one of the most thoroughly studied systems because of its usefulness in industrial synthesis of sulfur trioxide5 and phthalic anhydride.6 Meanwhile, vanadium oxide has also been found to be a potent catalyst in the oxidative dehydrogenation (ODH) of small alkanes to olefins, a process essential to the petroleum and polymer industries. Accordingly, numerous investigations have been conducted on the ODH processes involving vanadium oxide7−11 and the results have been discussed in a number of reviews.12−15 In general, the conversion in the ODH reactions of alkanes (C2−C4) over V2O5 is low (∼20%) with an unsatisfactory selectivity (7−31% if alkane/oxygen = 1/2).13 The selectivity can be slightly improved to 40−50% by introducing suitable metal oxide supports while maintaining the loading of vanadia below monolayer coverage13 or by employing vanadium-based multicomponent oxide catalysts.16 Gaspar and Pasternak have demonstrated that H2S significantly promotes the selective oxidation of ethane to ethylene over alumina catalysts17 and the conversion of butane to butadiene in molten LiCl/KCl.18 The former approach has been further verified by Liu and Clark.19 They postulated that the H2S/O2 mixture in the feed gas initially undergoes a partial © XXXX American Chemical Society

oxidation, generating reactive sulfur species which participate in the subsequent dehydrogenation of ethane to produce ethylene and regenerate H2S. It was, however, uncertain if the gas-phase S2 (or SH or H2S2) or surface sulfur species is truly responsible for the enhanced conversion of ethane to ethylene. Moreover, it was pointed out that more oxidizing catalysts (e.g., V2O5) minimize CS2 formation and maximize the yield of ethylene.19 In order to acquire insights into the genuine functions of sulfur-containing species in the ODH reaction of alkane facilitated by H2S, a comprehensive theoretical investigation has been performed. Propane was chosen as the source of saturated hydrocarbon. Particular emphasis was put on evaluating the effects of sulfur substituents on the stability of surface intermediates, the energetics and kinetics of the ODH process leading to propylene. Three mechanistic pathways have been considered in details: (i) vanadyl-O mediated ODH reactions; (ii) lattice-O mediated ODH reactions; (iii) cooperative mechanism involving both vanadyl and lattice O sites (see Figure 1). This paper is organized as follows. The description of the V2O5 model system as well as the technical details of the computations are given in section 2. Section 3 starts with a brief description of the present consensus regarding the most recognized mechanisms of the partial oxidation of saturated hydrocarbons on V2O5 surfaces. This is then followed by the detailed analysis and discussion for each of the three above-mentioned mechanisms. Received: March 11, 2013 Revised: April 23, 2013

A

dx.doi.org/10.1021/jp4024734 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Schematic of the three major mechanisms involved in the V2O5 catalyzed ODH reaction.

the electronic structure of bulk V2O5.33 This simplified approach, using either periodic slab or finite cluster, has been adopted in a number of studies,34−39 and good results in describing the physical, chemical and catalytic properties in comparison with existing experimental data have been reported. Accordingly, a monolayer V2O5(010) slab was chosen as the surface model for vanadia catalyst. To take into account the surface relaxation due to surface cleavage, geometry optimization was performed for a V2O5 slab. All V−O bonds except vanadyl were stretched marginally (0.3−0.7%) upon surface cleavage. On the other hand, the VO bond was contracted from 1.601 to 1.593 Å, which is attributed mainly to the loss of interlayer coordination of vanadyl groups. Interestingly, all computed V−O bond lengths were approximately 1% overestimated compared to the experimental values31 regardless of the size of k-point grid. It was therefore suspected that these errors are inherited from the deficiency of the PBE functional in calculating accurate bond lengths and lattice constants.40 A (2 × 3) supercell was employed throughout the calculations in order to ensure that lateral interaction between repeating images is minimal. Preliminary calculations revealed that adsorption energies obtained at the Γ-point and using a 4 × 4 × 1 grid, respectively, differ by less than 1 kcal/mol. As a result, for the sake of computational efficiency, the Brillouin-zone sampling was done only at the Γ-point. The adsorption energies reported in this work are defined as follows:

Finally, the influences of surface sulfur on the catalytic activity and selectivity of vanadia in ODH reactions are commented on based on the results obtained in the present study.

2. METHODS OF COMPUTATION All calculations were performed using the Vienna ab initio simulation package (VASP)20−23 in which the spin-polarized density functional methodology at generalized gradient approximation with the Perdew−Burke−Ernzerhof (PBE) functional24 was employed. The one-electron orbitals were expanded in terms of plane waves with the energy cutoff of 400 eV, and the ionelectron interaction was described by the projector-augmented wave (PAW) method.25,26 The pseudopotential including the semicore 3s-shell as valence for V and the standard potentials for C, H, S, and O were used. The structures of V2O5 surface and all adsorbates were optimized using the quasi-Newton algorithm27 with the force tolerance of 0.05 eV/Å and energy tolerance of 1 × 10−4 eV. A Gaussian smearing function of 0.1 eV was utilized to facilitate the wave function convergence. The transition state geometries and the associated energy profiles were determined by the climbing-image nudged elastic band method.28−30 A vacuum layer of thickness 10 Å was inserted between slabs, and dipole correction was imposed throughout the calculations to eliminate the artificial interaction between repeating images due to the presence of adsorbate molecules. Single-crystal V2O5 was considered to possess orthorhombic symmetry with a layered structure made of VO5 square pyramids sharing edges and corners. The experimental lattice parameters are a = 11.512 Å, b = 4.368 Å, and c = 3.564 Å.31 The V2O5 sheets are held together by weak van der Waals force with the interlayer separation of 2.791 Å. Optimization for V2O5 bulk structure has been performed using the k-point meshes varying from 2 × 2 × 2 to 12 × 12 × 12. It was found that lattice constants and energy were already converged within 0.002 Å and 0.001 eV, respectively, when a 4 × 4 × 4 mesh was used. Density of state calculations employing the optimized lattice constants (a = 11.60685 Å, b = 4.67294 Å, and c = 3.57302 Å) yielded the direct band gap of 2.17 eV for bulk and 2.05 eV for monolayer (010) surface; both values agree well with the experimental band gap of 2.2 eV.32 It has been shown by Chakrabarti et al. that a single-layer V2O5 slab is capable of reproducing very closely the essential features of

Eads = EA + Esurf − EA − surf

Here EA, Esurf, and EA−surf are the DFT energies of adsorbate A, the surface, and the adsorbed A, respectively. According to this convention, a positive Eads implies a favorable adsorption.

3. RESULTS AND DISCUSSION 3.1. ODH Reactions on Single-Crystal Vanadia Surfaces. The reaction kinetics and mechanistic details of partial oxidation of small alkanes by vanadia-based catalysts have been the subject of numerous studies. Reported data and proposed mechanisms have been summarized and discussed in several review articles.13−15 It is generally accepted that the ODH reaction proceeds via the Mars van Krevelan (MK) mechanism consisting of: (1) physisorption of alkane, (2) C−H bond B



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Figure 2. Structure of V2O5 surface. O(1), O(2), and O(3) denote the vanadyl, bridging, and triccordinated O sites, respectively.

activation by surface oxygen, (3) β-H elimination to generate olefin, and (4) reoxidation of the catalyst by O2.15 However, the identities of the active sites on vanadia responsible for the reaction are still not fully understood. Whereas many pieces of experimental41−44 and theoretical35,37−39,45 evidence have been collected favoring the vanadyl oxygen playing the most crucial role in the oxidation of hydrocarbons, 18O isotopic tracer and kinetic analysis plus computational works have pointed out the importance of lattice oxygen in C−H bond activation.7,9,34,46 In general, there are two dominant mechanisms for the catalytic partial oxidation of small alkane molecules on singlecrystal V2O5 surfaces. The first one (mechanism 1) describes the homolytic C−H cleavage involving only vanadyl oxygen sites.37,39 In this mechanism, hydrogen atoms are abstracted by two neighboring vanadyl oxygens sequentially, liberating the corresponding olefin and a H2O molecule. It also explains the formation of other oxygenated products such as alcohols, alkoxides or aldehydes.39,45 The second mechanism (mechanism 2) resembles the former one in the initial step where C−H activation occurs on a vanadyl oxygen site; however, it proposes that the subsequent hydrogen abstraction takes place from a terminal CH3 group to a lattice oxygen instead of a nearby VO site.35,38 The water formation is achieved by hydrogen migration on the surface. Alternatively, Fu et al. also proposed the third mechanism (mechanism 3) in which the ODH process proceeds entirely on bridging oxygen sites.37 This pathway, nevertheless, was found to be less kinetically demanding yet more endothermic than the first mechanism. In order to acquire a complete understanding of the possible influences of O−S substitution on the catalytic performance of V2O5 on the ODH reactions, these three mechanisms, which incorporate vanadyl and briding O sites, were considered

respectively in the present work. In particular, attention was paid upon how surface S species, either vanadyl or lattice, modifies the reaction kinetics of the ODH reactions of propane. 3.2. Sulfidation of Vanadia. The sulfidation of bulk vanadia has been studied by means of temperature-programmed sulfiding and reduction techniques.47 It was proposed that in high temperature regime (650−1273 K), V5+ in vanadia is first reduced to V3+ by sequential O−S exchange and V−S bond rupture; the V2O3 thus formed is further transformed into V2S3 after prolonged exposure to H2S. At conventional activation temperatures, however, bulk V2O5 is only partially sulfided when exposed to 15% vol. H2S, yielding a stoichiometry of about V2O3.3S0.3. Accordingly, a surface model in which only one O in (2 × 3) V2O5 supercell is substituted by S was employed to represent the sulfided bulk vanadia catalyst. There are three types of O sites, as shown in Figure 2, where O−S substitution may occur. The present calculations yielded the reaction energies of substitution of 2.47, 13.6−14.2, and 14.9−15.5 kcal/mol at vanadyl O(1), bicoordinated O(2), and tricoordinated O(3) sites, respectively. This trend is in line with the computed nucleophilicity where O(1) site is the most active toward H abstraction of alkane.35 For the substitution at O(1), the resulting VS bond is stretched to 2.168 Å compared to 1.593 Å for the VO bond; the relatively long VS bond distance (compared to 2.26 Å, the sum of covalent radii of V and S)5 suggests a much reduced double-bond character of VS bond which may destabilize the surface. On the other hand, the substitutions at O(2) and O(3) are much less favorable because of the deformation of the laminar structure of vanadia induced by the long V−S bonds (cf. 2.235−2.457 Å; about 25% expansion). Consequently, only the potential energy profile connected to VS was explored and the results are illustrated in Figure 3. The C



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Figure 3. Energy profile for the O−S substitution at the O(1) site on V2O5 surface. The red channel describes the stepwise dissociative adsorption of H2S while the blue channel describes the concerted H2S addition on VO.

just −0.6 kcal/mol, suggesting that propane only weakly, if not at all, physisorbed on V2O5. This result agrees qualitatively with the observations by Fu et al.37 and Gilardoni et al.35 that no evidence is found for propane adsorption near O(1) or O(2) sites. Nevertheless, these authors pointed out that the unfavorable nondissociative adsorption of propane may result from the inherited deficiency of the DFT/GGA methodologies in describing van der Waals-type interactions; yet the high operation temperature of the ODH reaction offsets this weak physisorption very readily. There are two possible pathways of H abstraction at a O1 site in which either the methyl or methylene H is transferred to V O. The latter one was found to be more favorable in kinetics; the transition state TS1 lies 27.7 kcal/mol above the free propane with a stretched O−H bond (1.390 Å, see Figure 6). The activation barrier is close to the value reported by Fu et al. although the O−H bond distances differ by about 20%.37 The subsequent adsorption of isopropyl radical to a nearby VO site only needs to overcome a small barrier (0.95 kcal/mol), giving a coadsorbed isopropoxide-hydroxyl intermediate O3 which is 20.84 kcal/mol more stable than free propane. The isopropoxide then tranfers one of its methyl H to the hydroxyl group and liberates propene. The reaction is highly endothermic, with a reaction energy of about 27.5 kcal/mol and an activation energy of 30.93 kcal/mol. At the transition state TS3, the C−H bond is stretched to 1.246 Å while the O−H bond is 1.542 Å. A similar reaction barrier has also been reported by Fu et al., but they predicted a more stable mode for adsorbed H2O.37 The propene in O4 is only weakly bound (Eads = 1.59 kcal/mol) and can be readily removed to yield adsorbed H2O (O5) at the O(1) site. The desorption of H2O from O5 followed by reoxidation by O2 completes the first phase of the vanadyl mechanism for the ODH reaction. The former step is a highly endothermic process because of the generation of an oxygen vacancy at a O(1) position. The computed reaction energy is larger than 42 kcal/ mol, suggesting that this step is formidable from an energy perspective. The inclusion of a V2O5 sublayer in the slab model significantly reduces this energy barrier to about 30 kcal/mol by means of interlayer V−O−V coordination. The subsequent

rate determining step corresponds to the H abstraction of H2S by VO in which H2S is partially coordinated to V, with the V···S and H···S bond distances of 3.240 and 2.340 Å, respectively. The calculated activation energy of 34.8 kcal/mol with respect to adsorbed H2S is larger than the value (cf. 24 kcal/mol) estimated for the H2S decomposition on crystalline V2O5.48 This is partly attributed to the significant reorganization of the coordination shell around V. This process can also occur in a stepwise fashion where the first H transfer is followed by SH association to V. The first step is monotonic increasing in energy and the second step possesses a late transition state; the lower effective activation energy (30.4 kcal/mol) renders this approach more energetically favorable. The second H transfer between vicinal OH and SH groups overcomes a small barrier to produce H2O that desorbs from the surface and generate VS. It is worth mentioning that the small overall reaction energy for vanadyl O substitution (2.47 kcal/mol) is consistent with the finding from the X-ray diffraction study by Bonné et al. according to which the H2S uptake already takes place at 293 K leading to a noticeable color change of the V2O5 sample.47 3.3. Multisite Vanadyl Mechanism Involving Surface VO/VS Groups. Using periodic slab calculations based on DFT, Fu et al. have demonstrated that the C−H bond activation at a vanadyl O(1) site of V2O5 is the most energetically favorable initial step of the ODH reaction of propane.37 The isopropyl radical then adsorbs on a neighboring O(1) site and undergoes the second H abstraction, forming propene and either surface hydroxyls or water. The mechanism is summarized in Figure 4 (the red panel). It is worth noting that the mechanism proposed by Fu et al. only accounts for the formation of the first propene and adsorbed water on the surface; no information regarding the subsequent reoxidation and regeneration of the catalyst plus the formation of the second propene was provided. In order to fill the void, a mechanism connecting the surface dioxo species, which is the initial step of surface reoxidation, and the regenerated catalyst was proposed (the blue panel in Figure 4). Figure 5 illustrates the computed energy profile based on the phase 1 of the vanadyl mechanism. The cycle starts with the molecularly adsorbed propane (O1) whose adsorption energy is D



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Figure 4. Two-phase vanadyl mechanism of the ODH reaction of propane. Phase 1 (red) concerns the formation of the first propene and reoxidation of the suraface by O2, while phase 2 (blue) concerns the formation of the second propene and regeneration of the catalyst.

site to form an isopropanol intermediate O7 with the activation energy of 27.13 kcal/mol which is comparable to that for the multiple-site counterpart. Nguyen et al.49 and Fu et al.37 have investigated this reaction pathway respectively, and a similar reaction barrier (23.3 and 29.0 kcal/mol respectively) was obtained. The O−H bond at the transition state TS5 is 0.978 Å, whereas the C−O bond distance is 2.806 Å, indicating that the hydroxyl group is formed prior to the adsorption of the isopropyl radical. The isopropanol intermediate O7 can transfer its methyl hydrogen to either a nearby lattice oxygen site or the vanadyl hydroxyl that yields H2O directly. Nguyen et al. proposed a stepwise mechanism where the methyl hydrogen is first abstracted by a secondary VO followed by migration to the hydroxyl group; they obtained the overall activation energy of

adsorption of O2 at the oxygen vacancy is exothermic by about 59 kcal/mol and requires an activation energy of 16 kcal/mol. Despite a significant exotherm, the large activation energy required for the stepwise reoxidation of the surface renders this pathway highly unlikely. Consequently, an alternative reaction channel where H2O desorption and O2 association occur simultaneously was considered. The associated reaction barrier (6.98 kcal/mol) is much smaller than that of the previous channel, and this can be attributed to the presence of an intersystem crossing where the energetic triplet O5 with V in +3 oxidation state is rapidly quenched to the singlet O6 in which V is in a more favorable +5 oxidation state. In addition to the multiple-site H-abstraction that leads to the formation of O3, propane can adsorb dissociatively onto a O(1) E



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Figure 5. Potential energy profiles for the phase 1 of the vanadyl mechanism on V2O5. (Red) Reaction channel involving multiple O(1) site. (Blue) Reaction channel involving isopropanol intermediate. Energy values are defined with reference to clean V2O5 surface.

Starting from this point, the potential energy surfaces through the “propoxide” and “isopropylthiol” intermediates, respectively, were computed and are depicted in Figure 7. Compared to the V2O5 counterpart, the initial physisorption of propane on the V2O4S surface (S1) is less favorable, possibly attributed to the larger atomic size of S and its more diffuse electron density that leads to a greater intermolecular repulsion. In spite of the less favorable adsorption, the C−H bond activation of propane by VS is found to be more kinetically feasible, having the reaction barrier of 26.11 kcal/mol. The S−H bond at the transition state TS1S is 1.403 Å which is similar to the O−H bond in TS1. The isopropyl radical (S2) rapidly adsorbs on a neighboring VO group by overcoming a small energy barrier (2.58 kcal/mol) and yields the isopropoxide-thiol intermediate S3. The stronger lateral repulsion due to a bigger thiol group causes S3 to be 3.37 kcal/mol less stable than O3, and the barrier associated with the subsequent H abstraction is also increased to 33.82 kcal/mol. As indicated in Figure 8, the S−H bond in TS3S is stretched to 2.173 Å while the O−H bond in TS3 is only 1.542 Å. For the same reason, the propene molecule becomes more readily eliminated from S4 with the energy input of only 3.51 kcal/mol. On the contrary, the similar process on a hydroxylated V2O5 surface requires the energy input of 4.58 kcal/ mol (see Figure 5). The reoxidation of the H2S adsorbed surface to form O6 takes place in a similar way to that on the H2O adsorbed surface, but the forward reaction barrier is remarkably increased to 11.22 kcal/mol ascribed to the enhanced repulsion between H2S and the incoming O2 molecule, as well as the weak H2S coordination on the V3+ site that destabilizes the transition state TS4S. Similar to the C−H activation that leads to S2, the dissociative adsorption of propane on VS to form an isopropyl thiol intermediate S6 is more kinetically favorable than the formation of O7. The computed activation energy of 25.72 kcal/mol is marginally smaller than the activation energy of 26.11 kcal/mol for the formation of S2. Surprisingly, the subsequent intramolecular H transfer (TS6S) that yields coadsorbed propene and H2S on a single V site is strongly impeded by the large barrier (34.76 kcal/mol) and reaction energy (22.97 kcal/mol). It is therefore anticipated that the alcohol intermediate path does not contribute as significantly as the isopropoxide path does to the phase 1 cycle of the ODH reaction on the sulfided vanadia surface.

Figure 6. Transition state structures relevant to the phase 1 of the vanadyl mechanism on V2O5.

22.5 kcal/mol associated with this pathway.49 A similar mechanism in which a concerted H abstraction by the vanadyl hydroxyl has been considered in the present study. The computed transition state TS6 was located whose reaction barrier is only 17.90 kcal/mol. It can thus be seen from Figure 5 that the “propoxide” and “isopropanol” pathways are kinetically competitive and the first C−H bond dissociation is the apparent rate determining step in both cases. Yet the former channel shows a thermodynamic preference because of the highly stable O3 intermediate. Within the framework of the above-mentioned vanadyl mechanisms, the influences to the catalytic activity of V2O5 brought by sulfurization could be evaluated by replacing the surface vanadyl group VO with thiovanadyl group VS. F



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Figure 7. Potential energy profiles for the phase 1 of the vanadyl mechanism on V2O4S. (Red) Reaction channel involving multiple S(1) site. (Blue) Reaction channel involving isopropyl thiol intermediate. Energy values are defined with reference to clean V2O5 surface.

possible weakness of DFT methodology in evaluating van der Waals interaction; but the increased endothermicity compared to O1 is in line with the expected, stronger steric repulsion arising from the adsorbed dioxygen molecule. The hydrogen transfer from adsorbed propane to the dioxygen molecule has a surprisingly high reaction barrier of 41.57 kcal/ mol, in contrast to the C−H bond activation by either VO or VS where the activation energy is merely 26−27 kcal/mol. This phenomenon can be accounted for by the O2 dissociation that accompanies the methylene H migration from propane. At the transition state TS7 (see Figure 10), the O−O distance is 2.155 Å, a value significantly larger than the normal bond distances of 1.21 and 1.48 Å for OO and O−O bonds, respectively. This implies that an essential portion of the activation energy is consumed by dissociation of dioxygen while a O−H bond is partially formed (with the C−H bond length of 1.550 Å). This C−H bond dissociation constitutes the most energy-demanding step in the entire reaction cycle of the ODH reaction for propane. After the H abstraction, the isopropyl radical in O10 adsorbs on either a neighboring VO site that yields O11 or the other O site of V−O2 that leads to O14. The former configuration is more energetically favorable as the lateral interaction between hydroxyl and isopropoxide groups is minimal. The two methyl groups of the isopropoxide O11 are 2.29 Å from the OH; one of these H atoms migrates to the hydroxyl via the transition state TS8 with a relatively high activation energy of 36.25 kcal/mol. At TS8, the H atom is 1.250 and 1.467 Å from C and OH respectively. The propene and water thus formed then desorb successively to regenerate the V2O5 surface; this process only requires a small energy input of 2.58 kcal/mol. There exists an alternative channel connecting O10 and O13. The coadsorption of H and isopropyl radical on V−O2 yields O14 which lies 29.71 kcal/mol above O11. Similar to other H migration steps described above, the transformation of O14 to O13 possesses a high barrier of 33.78 kcal/mol where the transition state TS9 has a well stretched O−H bond of 1.909 Å. The instability of TS9 may be partly attributed to the radical nature of isopropyl (C−O distance ∼2.918 Å) and V−OH (V− O distance ∼1.863 Å) fragments. Owing to the large difference in stability of O14 and O11, and the exceptionally high forward reaction barrier for the H abstraction from O14, this reaction channel is expected to play merely a minor role in the phase 2 of the ODH reaction.

Figure 8. Transition state structures relevant to the phase 1 of the vanadyl mechanism on V2O4S.

3.4. Second Phase of Propane ODH Reaction. While the formation of the first propene molecule in the vanadia-catalyzed ODH reaction of propane has been studied extensively, no theoretical investigation has been reported on phase 2 in which the reduced V2O4 surface and second propene formed. In the present work, the corresponding phase 2 potential energy surface has been calculated and is shown in Figure 9. It is expected that the crucial step of the ODH reaction involving the hydrogen migration from a propane molecule takes place on the adsorbed dioxygen in O6 so as to eliminate the excess oxygen. As in the case of propane adsorption on VO, the physisorption of propane on V−O2 (O9) is endothermic by 3.72 kcal/mol with respect to free propane. This energy value may not reflect the actual nature of adsorption because of the G



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Figure 9. Potential energy profile for the phase 2 of the vanadyl mechanism on V2O5 and V2O4S surfaces.

contributions from zero-point energy and thermal energy corrections are omitted. Despite this deficiency, the energy profiles obtained in this work are still expected to be capable of describing all the important features of propane ODH reactions with acceptable accuracy. Another point worth-mentioning is the rate-limiting step, i.e., TS7, in the phase 2 of the propane ODH reaction. When Figures 5, 7, and 9 are compared, it is seen that TS7 is marginally lower in energy than TS1 and TS1S which correspond to the ratedetermining step in phase 1 of the ODH reaction on V2O5 and V2O4S surfaces, respectively. Meanwhile, all other intermediates excluding TS9 lie well below the zero energy reference (i.e., free propane). It therefore implies that the initial C−H bond activation on V2O5/V2O4S is the overall rate-determining step in the vanadyl mechanism; when sufficient thermal energy required to overcome TS1 and TS1S is provided (for example, by means of increasing the reaction temperature), the ODH reaction can occur seamlessly. The total thermal energy accessible to O9 (i.e., energy released when it is formed plus the external energy input to the system) would be enough to agitate it and generate O10 that proceeds further in the reaction cycle. 3.5. Single-Site Vanadyl Mechanism Involving both VO and Lattice O/S. The possibility of second H abstraction from isopropoxide intermediate O3 has been explored by Gilardoni et al. using a finite cluster model.35 It was reported in their work that O(2) does not serve as a good nucleophile because of the mismatch of orbital orientation, whereas O(3) is more active than O(1) in abstracting H from the terminal CH3 of O3. This idea was examined later by Fu et al.37 and Redfern et al.38 respectively employing periodic slab models. Contrary to the observations by Gilardoni, it was found that surface OH and O(2) are in similar reactivity toward the C−H bond breaking of O3.37 Because of this, the reaction channel in which lattice O(2) sites participate in the acceptance and migration of H’s was taken into consideration in the present work.

Figure 10. Transition state structures relevant to the phase 2 of the vanadyl mechanism.

It is noticed from Figure 9 that the computed reaction energy for the ODH reaction of propane 2C3H8 + O2 → 2C3H6 + 2H 2O

is 43.07 kcal/mol which is smaller than the experimental reaction enthalpy of 56.40 kcal/mol in standard conditions because the H



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Figure 11. Computed reaction profiles for the vanadyl mechanisms involving both O(1) and O(2) sites on V2O5 (red) and V2O4S (blue) surfaces. Energy values are defined with reference to clean V2O5 surface.

mechanism, which will be discussed in the following section. In order to direct O18 to phase 2 of the vanadyl mechanism, a reaction channel has been proposed in which a vanadyl O migrates to the O(2) vacancy, leaving an O(1) vacancy which then uptakes O2 to form O6. Calculations, however, revealed that it is highly energetic (approximately 30 to 43 kcal/mol above O18) and not accessible under normal conditions, although the oxygen migration step itself is kinetically possible (with the activation energy of only 12.77 kcal/mol). As this channel plays only a very minimal role in the propane ODH reaction, if any at all, it is excluded from the discussion presented here. It is also noticed from Figure 11 that the O18 → O25 transformation is the rate-determining step in the vanadyl mechanism involving lattice O sites. This agrees with the conclusion by Redfern et al. according to which the lattice oxygen vacancy on the V2O5 surface is the species of highest energy on the propane ODH reaction pathway.38 Also, it is in line with the results from the study carried out by Hermann et al. using a large V20O62H24 model that the oxygen vacancy energy for O(2) is largest among all oxygen sites.36 Consequently, this mechanism is expected to participate to a lesser extent than the vanadyl mechanism involving only O(1) sites in the propane ODH reaction unless a high operation temperature is provided. The sulfur analog of this mechanism has also been explored starting from the V2O4S surface where an O(2) site is replaced by S. The resulting structure is less stable than V2O5 by 13.57 kcal/ mol. The significant difference in stability is ascribed to the larger atomic size of the sulfur atom relative to oxygen which induces a surface deformation around the S site. The V−S(2) bond is 2.233 Å compared to 1.811 Å for the V−O(2) bond, and the sulfur atom relaxes downward by 0.665 Å from the surface, all of which suggest weakened V−S(2) bonds and a more reactive sulfided surface. The physisorption of propane on this sulfided surface is endothermic by 4.06 kcal/mol, and the binding mode of propane (S1′) is like O1 except that the O−H coordination bonds are slightly shorter. The H abstraction by the VO group takes place through the transition state TS1S′ with the associated

The initial steps in this mechanism are identical to those in the vanadyl mechanism at O(1) sites until the formation of O3. Other than to the nearby OH group, the second H abstraction is accomplished by the O(2) site bonded to the V where the isopropoxide group resides. The estimated reaction barrier is 36.77 kcal/mol which is similar to the value (41.2 kcal/mol) for the same process mediated by a O(3) site.38 The rather high activation energy is likely the consequence of a remarkable relaxation of the surface at the transition state TS10, as can be seen in Figure 12, and the moderate nucleophilicity of the bridging O(2) atoms. After the desorption of propene from O14, the hydroxyl at O(1) in the intermediate O15 undergoes a barrierless rotation until the H atom points toward the O(2)-H group (O17). From this configuration, the H at O(1) migrates to O(2)−H to from a weakly bound H2O at O(2) position. The corresponding transition state TS11 lies 17.65 kcal/mol above O17, and the process is endothermic by 5.59 kcal/mol. At TS11, the migrating H is 1.192 and 1.291 Å from O(1) and O(2)−H, respectively. This barrier is significantly lower than those associated with C−H bond activations due partly to the formation of a thermodynamically stable H2O molecule. As in the O5 → O6 transformation, the desorption of H2O at the O(2) can take place in two ways. In one way, the desorption of H2O generates a O(2) vacancy which is then filled by an incoming O2 molecule yielding the dioxygen intermediate O25. This pathway possesses a very high activation energy (∼30 kcal/ mol) because the oxygen vacancy at the O(2) site is highly unstable. In the other way, the desorption of H2O and adsorption of O2 occur simultaneously. Due to the steric constraint around the O(2) site, the O2 molecule approaches more likely from the bottom side of the V2O5 surface. Along this path the transition state TS12 was located which is 28.99 kcal/mol above O18. As seen in Figure 12, H2O is almost fully desorbed while O2 is loosely bound to two V atoms at TS12. This is the weak V−O coordination that stabilizes TS12 and lowers the activation energy. Note that the reoxidation of the O(2) vacancy leads to O25 which connects the vanadyl mechanism with the bridging-O I



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hydrogen bond formation in S11 is likely the consequence of the long V−S and S−H bonds that render the thiol group more flexible and reduce the separation between O bridges on the surface. The subsequent H transfer from the hydroxyl group to the thiol group at S11 leads to the formation of S12 where H2S is weakly coordinated to two VO groups. The corresponding transition state TS8S is approximately 16.57 kcal/mol more energetic than S11. In this configuration, the migrating H is located 1.315 and 1.575 Å from VO and S−H, respectively. Its structure is looser than TS11 whose O−H bonds are much shorter (see Figure 12). It is interesting to note that the transformation S11 → S12 is exothermic by almost 3 kcal/mol while the transformation O17 → O18 is endothermic by 5.59 kcal/mol. The contrasting trend can be understood in terms of the compromise between the stabilization due to formation of H2O/H2S and the weakening of lattice V−O/V−S bonds upon the H migration. Since the lattice V−S bonds are weaker than the lattice V−O bonds, arising from the poor orbital overlap, the former factor dominates, resulting in a small, overall stabilization effect in the formation of S12. On the other hand, despite the fact that the formation of H2O is thermodynamically favorable, it is not sufficient to compensate for the destabilization due to the large degree of rupture of the stable lattice V−O bonds, which, as a result, makes the transformation O17 → O18 slightly endothermic. As the oxygen counterpart, the weakly bound H2S can be readily displaced by O2 molecule which attacks from the bottom side of the vanadia surface. The resulting transition state TS9S resembles TS12 in a way such that H2S is fully dissociated before the oxygen molecule starts to coordinate to the VO groups at the bridging position. A noticeable difference between these transition state configurations is that the V−O coordination bond lengths in TS9S are about 0.4 Å shorter than those in TS12. Meanwhile, the O−O bond is stretched to 1.302 Å, indicating that the O−O π bond is partially broken at the transition state, leaving the p orbitals of oxygen that participate in the newly formed surface bonds with V. 3.6. Multisite Bridging-O Mechanism Involving Lattice O/S Sites. The first phase of propane ODH reaction on lattice O(2) sites of V2O5 has been studied by Fu et al. and it was reported that the energy profile is very similar to that for the reaction on VO sites although it lies 3−10 kcal/mol higher in energy in general.37 Their work also showed that the most favorable first C−H bond activation occurs between two lattice O(2) sites, yet the rate limiting step corresponds to the concerted elimination of propene and formation of surface hydroxyl groups. Because of these factors, the investigation performed in this study started with the physisorption of propane on a lattice O(2) site followed by the H abstraction by a neighboring O(2) site. As in the case of the O(1) site, the adsorption of propane on O(2) is endothermic; the magnitude of 5.83 kcal/mol indicates that such adsorption is more repulsive than the adsorption at O(1), which is also reflected by the O−H bond distances in these configurations (2.4 Å in O19 versus 2.1 Å in O1). From this intermediate, the C−H bond activation gives rise to the isopropoxide species O20 at the O(2) position. The associated reaction barrier is found to be 30.83 kcal/mol with respect to free propane. This value agrees well with the calculation results (30.4 kcal/mol) by Fu et al;37 however, the structure of TS13 is more reactant-like in which the migrating H is 1.291 and 1.713 Å from C and O, respectively. The hydroxyl H in O20 then transfers to a neighboring O(2) yielding an isopropoxide-hydroxyl intermedi-

barrier of 26.11 kcal/mol which is similar to that for TS1. The isopropyl radical then adsorbs on a neighboring VO site to form S3′ by overcoming a small reaction barrier of 2.58 kcal/mol, which is larger than the energy barrier for the same step on the V2O5 surface due to the sulfur substitution that modifies the charge and nucleophilicity of the VO site. The isopropoxide in S3′ can then donate a methyl H to either the nearby V−OH to form adsorbed H2O or the bridging S to form a thiol group. The former path was not considered in detail as it produces only H2O and propene, and does not regenerate the clean V2O5 surface at the end of the reaction cycle. The migration of a methyl H of S3′ to the bridging S yields the thiol intermediate S8. Surprisingly, the process is highly endothermic by 36.73 kcal/mol and the corresponding transition state TS7S lies 38.36 kcal/mol above S3′, contrasting the observations for the analogous steps on V2O5 where the formation of bridging hydroxyl is only endothermic by about 28 kcal/mol. The enhanced instability of S8 relative to S3′ can be explained by the fact that the sulfur atom moves upward in order to accept the H atom at TS7S as shown in figure 12. This movement causes the

Figure 12. Transition state structures relevant to the vanadyl mechanisms described in Figure 11.

lattice expansion around the bridging S and raises the transition state energy significantly. Because of the increased steric interaction, the intermediate S8 is more destabilized with respect to S3′ than O14 with respect to O3. The hydroxyl group at S8 then rotates about the VO bond via the configurations S9 and S10 until the H atom points toward the thiol group. In the meantime, the thiol H bends downward and forms a hydrogen bond with the neighboring O bridge; this secondary interaction stabilizes the intermediate S11 by approximately 10 kcal/mol. Interestingly, such stabilization effect is absent at O17. It is evident by comparing the structures of O17 and S11 that the distance between the bridging hydroxyl H and O bridge in O17 is 2.446 Å but that between the thiol H and O bridge in S11 is only 2.067 Å. The feasibility of the J



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Figure 13. Potential energy surfaces involved in the multisite bridging-O/bridging-S mechanisms for the first phase of the propane ODH reaction. (Red) The reaction channel for the V2O5 surface. (Blue) The reaction channel for the V2O4S surface where propane adsorbs on a lattice S(2) site. (Black) The reaction channel for V2O4S surface where propane adsorbs on a lattice O(2) site next to S(2). Energy values are defined with reference to clean V2O5 surface.

The O−H and C−H bond distances at the transition state TS18 are relatively long (1.915 and 1.769 Å, respectively) compared to those at TS13. The isopropyl radical then adsorbs on the other O atom and forms the hydroxyl-isopropoxide intermediate O27 which is 12.36 kcal/mol more stable than O26. Because of the close proximity of the hydroxyl group and the adjacent O(2) site, a hydrogen bond could be formed between them by bending the OH bond toward the surface. The resulting hydrogen bond is 1.582 Å. Meanwhile, the downward bending of the OH group opens up a larger space for the nearby isopropoxide to rotate such that the methyl H to be transferred is oriented directly toward the surface hydroxyl. These structural alterations lead to an additional stabilization of O28 by 14.68 kcal/mol. From this configuration, the methyl H is passed on to the surface hydroxyl, through the transition state TS19, to produce H2O and a physisorbed propene on the O(2) site. These molecules then desorb sequentially, without noticeable barriers, to regenerate a clean V2O5 surface (O29 → O30 → products). It can be concluded from Figures 13 and 15 that the ratedetermining step of the bridging-O mechanism of the propane ODH reaction is the reoxidation of the surface, i.e., O12 → O25, where the transition state TS17 is about 60.5 kcal/mol less stable with respect to free propane. This barrier is significantly larger than the rate-limiting step of the vanadyl mechanism, indicating that the contribution of the bridging-O mechanism to the propane ODH reaction is very much limited within the formation of the first propene (the first phase of the cycle). Only at high temperature regime could the second phase starts to kick in and make noticeable contribution to the propene production. For the sulfided surface where a lattice O is substituted by S, there are two possible adsorption modes of propane that have been considered in this work. The propane molecule could either adsorb directly on the bridging S or a bridging O adjacent to S. In both cases, it ends up having the lattice S removed from the surface as H2S and the vacancy refilled by O2. The major difference between these proposed reaction channels is the role

ate O21 with the activation energy of 13.33 kcal/mol. Fu et al., on the other hand, predicted a smaller barrier (6.7 kcal/mol) for this step although the transition state structure is almost identical to TS14 (see Figure 14). Interestingly, both studies yielded the same overall reaction energy of O21 with respect to free propane (9.60 kcal/mol). From O21, a methyl H can be transferred to a nearby available O(2) site to produce propene and the second hydroxyl group. The present work obtained the transition state TS15 in which H is weakly bound to both an O(2) site and an O(3) site simultaneously. The activation energy of 18.69 kcal/mol is smaller than 22.0 kcal/mol as reported by Fu et al.37 for the same process. The resulting propene-dihydroxyl intermediate O22 is about 5.08 kcal/mol less stable than O21. The subsequent endothermic desorption of propene (by about 2 kcal/mol) leaves the dihydroxyl intermediate O23 which undergoes a coupling reaction to form H2O (O24). This step was found to have a relatively high barrier (19.68 kcal/mol) and is highly endothermic by 14.91 kcal/mol due to the surface reconstruction necessary for the two adjacent hydroxyl groups to come close to allow for the proton transfer. The H2O molecule thus formed is displaced by an incoming O2; this step is extremely exothermic by 31.64 kcal/mol but the associated forward barrier is so large (28.99 kcal/mol) that it renders this reoxidation kinetically limited. The direct desorption of H2O from O24 followed by O2 addition has also been investigated, but this pathway has an even higher reaction barrier (greater than 45 kcal/mol) and is therefore entirely inaccessible even at high reaction temperature. The second phase of the bridging-O mechanism is similar to that of the vanadyl mechanism except that only the coadsorption of H and isopropyl on the bridging O2 unit was considered. The channel involving the hydroxyl on the bridging O2 and an isopropoxide on an adjacent O(2) site was ignored because of the short distance between these sites that creates a significant steric constraint and prevents the further H migration. The adsorbed propane on the bridging O2 (O26) undergoes the H abstraction by overcoming a large forward reaction barrier of 21.71 kcal/mol. K



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and sulfided surface despite the fact that the actual activation energy is merely 26.74 kcal/mol, a value comparable to 25.00 kcal/mol for the oxygen analog. The adsorption of isopropyl radical to the thiol group yields the isopropyl thiol intermediate S14 which is 7.24 kcal/mol more stable than S13. Unlike S6 which could undergo the methyl H migration to S intramolecularly, such a process is not plausible for S14 because of the steric constraint that hinders the rotation of the isopropyl group necessary for the transfer of a methyl H to S. The spatial restriction could be relaxed by transferring the thiol H to a neighboring O(2) site. This process is exothermic by 7.97 kcal/ mol and has a small reaction barrier of 2.58 kcal/mol. The structure of the transition state TS11S is illustrated in Figure 16. Once the H migrates to the neighboring O(2) site to form a hydroxyl, the isopropyl group in S15 bends up (to about 28.8° from the surface normal) while the bridging S relaxes outward in order to create more space for the isopropyl group to rotate about the S−C bond and transfer one of its methyl H to the bridging S. This motion leads to the transition state TS12S and the associated activation energy of 39.92 kcal/mol which is much larger than the corresponding barrier on the V2O5 surface (18.69 kcal/mol, see Figure 13). The resulting weakly bound propene molecule desorbs readily, leaving the hydroxyl-thiol intermediate S17 which is 17.25 kcal/mol less stable than S15. Finally, the H on hydroxyl could migrate back to the thiol group to form surface H2S S18 which is subsequently displaced by O2 to yield O25. This step possesses no well-defined transition state and is endothermic by 19.30 cal/mol. The reaction channel starting from the adsorption of propane on the bridging O site adjacent to a lattice S also leads to S18. Along this path, the physisorbed propane S19 is 7.20 kcal/mol more favorable than S13 as the bridging O is less repulsive than the bridging S upon propane adsorption. Accordingly, the transition state TS13S associated with the first C−H bond activation of the absorbed propane is 6.11 kcal/mol lower in energy than TS10S though the actual reaction barrier is slightly higher (27.83 kcal/mol versus 26.74 kcal/mol). Both TS10S and TS13S look alike yet the S−H bond in the former structure is much longer than the O−H bond in the latter structure (1.413 Å versus 1.003 Å). The formation of the isopropanol intermediate

Figure 14. Transition state structures related to the bridging-O mechanism.

of S; in the first channel, S serves as the active site which activates the C−H bond of propane, while in the second channel, S serves as the site which accepts the transferred H. The physisorption of propane on a lattice S, as a similar adsorption on VS, is highly unfavored because of the stronger electrostatic repulsion exerted by the S atom. The process is endothermic by 11.20 kcal/mol compared to 5.83 kcal/mol for the adsorption of propane on a lattice O (as shown in Figure 13). The unstable physisorbed propane makes the first H abstraction process very energy demanding; the transition state TS10S is 37.94 kcal/mol higher in energy with respect to the free propane

Figure 15. Potential energy surface for the second phase of the multisite bridging-O mechanism. L



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to the thiol group to generate S18 over the product-like transition state TS16S that lies 19.48 kcal/mol above S24. The direct H transfer from the isopropyl group to thiol in S22 was also considered. Due to the steric restriction imposed by the orthogonal orientation of the isopropyl group relative to the bridging thiol, the isopropyl group has to first rotate by 90 degrees so that a methyl group points toward the thiol. This rotation costs 6.85 kcal/mol of energy. The resulting structure affords the H migration from the methyl group closest to the thiol to the S atom that yields H2S adsorbed at the bridging site. The optimized transition state possesses the transferring H atom being 1.663 and 1.747 Å from C and S, respectively. Nevertheless, this process is both energetically and kinetically hindered; the reaction energy of this step is 23.82 kcal/mol, whereas the activation barrier is 37.94 kcal/mol. This results in the transition state lying almost 5 kcal/mol above TS15S, rendering this channel very rate-limiting and less accessible compared to the previous one. The first phase of the bridging-O/S mechanism, in which the S substituent is explicitly involved, ends when the adsorbed H2S in S18 is displaced by an O2 molecule. Due to the weak nature of the bridging V−S bonds, such a process is less energy demanding than the corresponding displacement reaction of O24. It is found that the activation barrier is only about 19.42 kcal/mol while the reaction exothermicity is close to 47 kcal/mol, although the incoming O2 is far from the V sites (about 3.8 Å) at the transition state TS17S (see Figure 17). Once the dioxygen intermediate O25 is formed, the second propane molecule could adsorb on it and trigger the second phase of the ODH reaction.

Figure 16. Transition state structures related to the bridging-S mechanism.

S20 is in fact endothermic by 10.32 kcal/mol, in contrast to the exothermic formation of isopropyl thiol intermediate S14. This can be accounted for by the fact that the formation of thermodynamically favorable isopropanol weakens the stable lattice V−O bonds to a larger extent than the less favorable isopropyl thiol does to the weaker V−S bonds. Therefore, the two V sites involved in S20 are expected to be more reduced than those in S14, which, as a result, renders S20 more destabilized with respect to S19. To stabilize S20, the hydroxyl H is passed on to the neighboring bridging S to form an isopropoxide thiol intermediate S21; this process releases 11.49 kcal/mol, and the H remains hydrogen-bonded to the isopropoxide group with the O−H and S−H bond distances of 1.858 and 1.401 Å, respectively. In order to open up space and adjust the orientation of the lone pair electrons for the second H abstraction, the thiol group flips over to the other side, with the overall activation energy of 14.07 kcal/mol spent mainly on removing the hydrogen bond between the isopropoxide and thiol. With this configuration (S22), the isopropyl group donates a methyl H to an available lattice O(2) site next to the thiol to form the hydroxyl-thiol intermediate S23. Despite the formation of propene, this H abstraction is both kinetically and thermodynamically hindered by the large forward reaction barrier (39.74 kcal/mol) and endothermicity (20.91 kcal/mol). It is interesting to note that a lattice O(3) site participates as a temporary H carrier during the course of the second H abstraction; the H is first transferred to the O(3) site and then passed over to the designated O(2) site. The intermediate S23 thus formed further transforms into S24 by S−H bond bending. In this process, the thiol group flips backward and reforms the hydrogen bond with the O(2) site where the isopropyl used to reside on, gaining in total 8.97 kcal/ mol of energy. Eventually, the H of the hydroxyl group migrates

Figure 17. Transition state structures related to the second half of the multisite bridging-O mechanism.

3.7. Comments on the ODH Mechanisms and the Effects of Sulfur. The computed potential energy profiles for the three proposed mechanisms discussed in the previous sections are depicted in Figures 5, 11, and 13, respectively. It is clear by comparing these energy diagrams that the multisite vanadyl mechanism plays the most dominant role in the propane ODH reactions on clean V2O5 surfaces. In this mechanism, the most energy-demanding step corresponds to the first C−H bond activation of the propane molecule by a vanadyl site, with the estimated activation energy of 27.66 kcal/mol with respect to free propane. This energy value agrees well with the M



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theoretical37,49 and experimental15,50 apparent activation energies for the propane ODH reaction, but some discrepancies between the present work and other theoretical studies in identifying the actual rate-determining step are observed. By means of periodic DFT calculations, Fu et al. showed that the multisite vanadyl mechanism possesses the rate-limiting step at the H abstraction of isopropoxide intermediate by a free vanadyl O(1) site, yet the C−H bond activation of propane on an O(1) site costs slightly less energy.37 On the contrary, Nguyen et al. using a similar approach obtained the energy profile in which the O(1) insertion into a methylene C−H bond of propane is the rate-determining step, and the transition states corresponding to the subsequent H migration from surface isopropanol are less energetic.49 Moreover, the former group predicted that the dissociative adsorption of propane yields coadsorbed isopropoxide-hydroxyl intermediate, while the latter group proposed that the C−H bond activation of propane leads to isopropanol on one O(1) site. The present study illustrated, as shown in Figure 5, that the formations of isopropoxide-hydroxyl intermediate O3 and isopropanol intermediate O7 are almost equally likely from kinetic perspective because of the similar activation barriers (difference of 0.53 kcal/mol). From the energetic viewpoint, however, O3 is more stable than O7. Since the transition states TS3 and TS6 are of similar energy, it is expected that the transformation from O7 to O8 and then O5 would take place more readily than the route from O3 to O4 and then O5, especially at lower reaction temperatures. Consequently, formation of an intermediate alcohol leading to the generation of the first propene is apparently more favorable. This conclusion contradicts the observations by Fu et al., but it could be attributed to the differences of the sizes of the surface model, the DFT functionals and the basis sets employed in these studies. The sulfur substitution of the V2O5 surface at the O(1) site brings forth some variations in the resulting energy profile for the multisite vanadyl mechanism. Like the O counterpart, the C−H bond activation of propane remains the rate-determining step; however, the transition state energy with respect to free propane is slightly increased due possibly to the smaller nucleophilicity of S compared to O. The formation of isopropylthiol intermediate S6 is marginally more favorable than isopropoxide-thiol intermediate S3 from kinetics perspective although the latter one is the thermodynamically product. Surprisingly, the subsequent H abstractions from these species to produce propene are kinetically hindered; the effect of S is manifested in particular in the alcohol route where the barrier is increased from 17.90 kcal/mol for O7 to 34.76 kcal/mol for S6. Meanwhile, S6 is more stabilized compared to O7 because of the smaller electronegativity and more diffuse valence of S that facilitate the V−S coordination and prevent the full reduction of V to +3 oxidation state. These influences result in similar forward reaction barriers of S3 and S6, and in turns, reduce the kinetic preference of the alcoholic channel. As a whole, the two channels of the vanadyl mechanism should be of comparable competence in the propane ODH reaction on the V2O4S surface. Redfern and co-workers have pointed out that the H migration of isopropoxide on the O(1) site could take place with the bridging O(2) site,38 and they have obtained an overall endothermic potential energy surface for this mechanism by means of B3LYP/6-31G* calculations on the V4O10H8 cluster. Their results suggested that the rate-determining step is desorption of H2O from the O(2) site on the triplet potential energy surface. The next highest energy transition state species

corresponds to the formation of H2O from two surface hydroxyl groups at O(1) and O(2) sites, respectively. Gilardoni et al., on the other hand, argued that the H migration to the O(2) site is less favorable compared to the O(3) because of the unfavorable orientation of the oxygen lone pairs.35 Furthermore, they estimated the activation energy of 15 kcal/mol for the second H abstraction, which is much smaller than 49.2 kcal/mol as deduced by Redfern et al. for the same step on the O(2) site.38 As seen in Figure 11, the potential energy surface computed in this work is substantially different form that of Redfern in several aspects. First of all, the current potential energy surface lies much lower in energy although a similar increasing trend is retained. All intermediates involved have the relative energy below 16 kcal/ mol. On the contrary, Redfern’s calculations predicted highly energetic intermediates along the reaction path; for instance, the adsorbed H2O (i.e., O18 in Figure 11) lies 52.3 kcal/mol above the reactants. Second, Redfern et al. predicted a large reaction barrier for the O(1) insertion of the β-H of propane (∼55 kcal/ mol), whereas the present study yielded the activation energy of only about 27 kcal/mol which is close to the experimental value. Third, in this work, the concerted H2O desorption and O2 addition were considered (i.e., O18 → O25), resulting in the transition state that constitutes the rate-determining step of the vanadyl-lattice mechanism. On the other hand, Redfern et al. described this step as a stepwise process, having the initial desorption of H2O followed by the refilling of the vacancy by an O atom. The remarkable discrepancies between these energy surfaces may arise from the different methods utilized to obtain the energy profiles and reaction energies, but the good agreement of the computed activation energy for the C−H bond activation of propane with the experimental value suggests that the PES as presented in Figure 11 should be more accurate in describing the propane ODH reaction on O(1) and O(2) sites. The effects of S on the potential energy surface for the vanadyllattice mechanism are more intriguing than the multisite vanadyl mechanism. As can be seen in Figure 11, the potential energy surface associated with the V2O4S surface is higher in energy than the O counterpart, except for the O2 addition where TS9S is lower in energy than TS12. This is due entirely to the sulfur substitution at the lattice O(2) site which significantly destabilizes the resulting surface. When the energy plot is rescaled with respect to V2O4S and free propane, it is essentially parallel to the energy surface for V2O5; the two energy profiles are separated by approximately 3 kcal/mol with the former one being higher in energy. There exist two regions, however, where the two profiles do not match. The hydroxyl-thiol species (i.e., from S8 to S10) are more destabilized compared to the hydroxylhydroxyl species (i.e., from O14 to O16). This is partly attributed to the bulky thiol group at the bridging site which induces a localized surface deformation, causing the weakening of the neighboring V−O bonds. In addition, the displacement of H2S at the lattice site by O2 occurs more readily than the analogous reaction for H2O. This can be accounted for by the relatively weak V−S bonds, implying that less energy is necessary to desorb H2S, and the strong V−O2 bonds, suggesting a large energy compensation that lowers the energy of the transition state. The biggest effect of S substitution on the vanadyl-lattice mechanism is the shift of the rate-determining step from the H2O desorption-O2 addition for V2O5 to the C−H bond activation of propane by O(1) for V2O4S. In general, this mechanism is more kinetically controlled on the V2O4S surface as most of the intermediates and transition states are highly energetic with respect to the reactants. The multisite vanadyl mechanism is N



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(1) Three different mechanisms that account for the transformation of propane to propene over a clean V2O5 surface have been investigated in detail. According to the results presented in Figures 5, 11, and 13, the multisite vanadyl mechanism is the most kinetically favorable reaction route in which the rate-determining step refers to the O(1) insertion into a methylene C−H bonds of propane. The corresponding reaction barrier of 27.66 kcal/mol demonstrates an excellent agreement with the experimental apparent activation energy of 20−30 kcal/mol.15,50 An alternative pathway involving the isopropanol intermediate was also explored. Unlike the observations by Fu et al.,37 this channel was found to possess a slightly lower rate-limiting step and can lead directly to H2O adsorbed on V. The vanadyl-lattice mechanism, which is a variant of the multisite vanadyl mechanism, is of similar feasibility. Most of the involved intermediates lie below TS1, the highest energy species in the vanadyl mechanism. The only exception is TS12 which corresponds to the concerted H2O desorption and O2 addition at the O(2) site. It thus forms the rate-determining step of the vanadyl-lattice mechanism and is responsible for the less dominant role in the propane ODH reactions. The lattice mechanism, in which propane transforms into propene on the lattice O(2) site exclusively, is the less dominant reaction channel because of the highly energetic intermediates as well as the huge reaction barrier at the formation of dioxo species O25. Consequently, the following preference of channels participating in the propane ODH reactions can be deduced: multisite vanadyl > vanadyl-lattice > lattice. (2) To complete the picture of reaction cycles involved in propane ODH reactions, mechanisms accounting for the formation of the second propene molecule were proposed for the three above-mentioned mechanisms, respectively. As can be seen in Figures 9 and 15, these mechanisms (i.e., the second phase of the ODH reaction according to Figure 4) are more kinetically feasible than the corresponding mechanisms for the formation of the first propane (the first phase). This observation agrees with the experimental fact that catalyst regeneration is not as temperature-dependent as the production of propene. (3) The effects of sulfur on the catalytic activity of V2O5 toward propane ODH reactions have been studied by computing the potential energy surfaces for the three mechanisms utilizing a V2O4S surface model. The results show that in all three mechanisms the presence of sulfur hinders the formation of propene to different extents. While S at the O(1) site increases the apparent activation energy by about 3 kcal/mol, the S substitution at the lattice O(2) site causes significant increments in both the relative energies of most intermediates and the apparent activation energies associated with the vanadyl-lattice and lattice mechanisms. Overall, the vanadyl-lattice and lattice mechanisms are expected to be suppressed on the sulfided vanadia surfaces, and propene would be formed only through the multisite vanadyl mechanism. (4) Finally, a simple mechanism for the sulfur substitution of surface O on V2O5 by the reaction with H2S was proposed. As illustrated by Figure 3, the consecutive H migration from H2S to VO has the overall reaction barrier of 33.84 kcal/mol, whereas the concerted approach requires an activation energy of 34.83 kcal/mol. The formation of VS as a whole is a thermodynamically plausible process because of the relatively small reaction energy (2.47 kcal/mol). The analogous substitutions at other O sites, however, are very energy-demanding and are thus not favorable under normal conditions.

therefore more favorable and is expected to dominate the propane transformation on this surface. On the other hand, this mechanism on the V2O5 surface should be viable as the multisite vanadyl mechanism since they have a similar apparent activation energy for the formation of the first propene. However, the reoxidation step in the vanadyl-lattice mechanism would occur at a much slower rate owing to the high reaction barrier at TS12. To the authors’ knowledge, only Fu et al. have reported the potential energy surface regarding the propane ODH reaction occurring exclusively on O(2) sites.37 They pointed out that while the initial C−H bond activation of propane takes place more slowly at the O(2) site than at the O(1) site, the second C− H bond breaking is about 100 times faster, as estimated by statistical thermodynamic calculations including entropic effect. The results obtained in this work, as depicted in Figure 13, agree very well with Fu’s data except that the dihydroxyl species O22 and O23 were found to be 6−10 kcal/mol more stable. The current work also explored the formation of H2O and the reoxidation of the surface at the lattice site, which were excluded in Fu’s investigation. As seen in Figure 13, the O2 addition at the O(2) site is in fact the most energy-demanding step where the transition state TS17 is approximately 60 kcal/mol above the reactants. Moreover, by comparing Figures 5 and 13, it can be observed that the potential energy surface for this mechanism lies above that for the multisite vanadyl mechanism over the entire range of the reaction till the dioxo intermediate O6/O25. Because of these factors, it is anticipated that the lattice mechanism does not contribute as much as the multisite vanadyl mechanism, especially at low reaction temperature. In addition, the formation of propane on the O(2) site may lead to surface hydroxyl groups (i.e., O23) whose conversion to H2O, and the subsequent surface reoxidation are difficult to achieve. This may therefore cause the accumulation of hydroxyl groups which forces the propane dehydrogenation to take place at the O(1) sites. Furthermore, a higher reaction temperature may be required to remove the hydroxyl groups by H2O desorption at the O(2) sites or H hopping to nearby V−O(1)H sites, forming V−O(1)H2 (i.e., O5) which can be more readily displaced. Analogous to the multisite vanadyl and multisite vanadyllattice mechanisms, the substitution by S on V2O5 shifts the potential energy surface for the lattice mechanism upward because of the structural deformation induced by the bulky S atom at the lattice site. As explained in the previous sections, there are two possible routes leading to the adsorbed H2S (i.e., S18). Both channels are strongly affected by the presence of sulfur. For the channel starting from dissociative adsorption of propane on the lattice S, the first and second C−H bond activation become strongly kinetically prohibited. The transition states associated with these steps are ∼40 kcal/mol above the reactants, and thus are accessible only at relatively high reaction temperatures. Meanwhile, for the channel starting from the C−H bond breaking at the O(2) site adjacent to S, only the second H abstraction is affected significantly because the steps before that are similar to those occurring on the V2O5 surface. The reaction barrier for the formation of dioxo species O25 from S18 is not large (∼20 kcal/mol) indeed, but this step is rate-determining in the whole mechanism because of the high energy of the transition state TS17S.

4. CONCLUSIONS The following conclusions can be made according to the discussions presented above. O



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(5) Combining all of the data presented in the present work, it can be deduced that in the propane ODH reaction, where a small amount of H2S is present in the feed gas, most VO groups participate in the propane conversion, whereas a small portion reacts with H2S to form VS as the former process is more competitive. The VS groups thus formed are also reactive toward producing propene albeit less kinetically favorable. Therefore, the overall rate of propane formation is not much influenced by H2S at the moderate reaction temperatures. Any VS group formed could catalyze the propane dehydrogenation with itself being converted to VO and H2S by O2. However, when the reaction temperature is increased, more VO will be transformed into VS, and the S substituents at lattice sites start to emerge. Since the first phase propane ODH reactions on these lattice S sites are very kinetically demanding, the reaction channels on VO sites, and any existing VS sites, will become more dominant. As a consequence, sulfur may start to accumulate on the catalyst surface, reducing the total number of accessible active sites and deactivating the catalyst. The adverse effect of sulfur substitution may be eliminated by increasing the partial pressure of O2 (or decreasing the H2S content) in the feed gas which facilitates the reoxidation step and removes residual sulfur on the surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.M.H.L. would like to acknowledge the Industrial Research and Development Fellowship by Natural Sciences and Engineering Research Council of Canada (NSERC) and Alberta Sulphur Research Ltd. The computing facility was provided by the Western Canada Research Grid and the University of Calgary.

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First-Principles Investigation of Selective Oxidation of Propane on

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