pubs.acs.org/IC Forum Article Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity C. S. Praveen, A. P. Borosy, C. Copéret, and A. Comas-Vives* Cite This: Inorg. Chem. 2021, 60, 6865−6874 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation (PDH) reaction. In this contribution, we evaluate the catalytic activity toward PDH of tricoordinated and tetracoordinated Ga(III) sites on SiO2 by means of first-principles calculations using realistic amorphous periodic SiO2 models. We evaluated the three reaction steps in PDH, namely, the C−H activation of propane to form propyl, the β-hydride (β-H) transfer to form propene and a gallium hydride, and the H−H coupling to release H2, regenerating the initial Ga−O bond and closing the catalytic cycle. Our work shows how Brønsted−Evans−Polanyi relationships are followed to a certain extent for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga−O pairs on such sites of realistic amorphous SiO2 models. It also shows how transition-state scaling holds very well for the β-H transfer step. While highly strained sites are very reactive sites for the initial C−H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need for the right balance in strain to be an effective site for PDH. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites toward the PDH reaction and paves the way toward the design and prediction of better single-site catalysts on SiO2 for the PDH reaction. 1. INTRODUCTION (OtBu)3)3(THF)] as a molecular precursor.16 This approach The high demand of light olefins1 and the large abundance of generates tetracoordinated Ga sites, [(SiO)3Ga(XOSi)] shale gas,2 mostly constituted of light alkanes, have stimulated (X = −H or Si), according to IR, X-ray absorption near-edge interest in on-site propane dehydrogenation (PDH).1,3 PDH structure, and extended X-ray absorption fine structure involves activation of the C(sp3)−H bond of propane as a first (EXAFS) analyses. This catalyst displays high activity and step, which is still nowadays a very challenging reaction.4 selectivity towards propene as well as remarkable stability Because of the highly endothermic nature of alkane compared to Ga2O3 and other single-site catalysts based on Fe, dehydrogenation, this reaction is generally carried out at 550 Co, and Zn.24−28 PDH is proposed to involve three main °C to obtain reasonable conversion to the alkene product. The elementary steps: the C−H activation of propane on the Ga− two historical heterogeneous catalysts that are used for this O pair site, formation of a Ga−propyl intermediate and an OH reaction in industry correspond to alumina-supported PtSn group, the subsequent β-hydride elimination, forming a nanoparticles and the CrOx/Al2O3 system, also known as the hydride and propene, and the decoordination of propene Houdry or Catofin catalysts.5 Recent research developments and H−H coupling to regenerate the initial Ga−O pair. have also helped to launch a PtGa-based catalyst.6 The Cr- Previous works by Sauer et al. on ethane dehydrogenation also based catalyst is thought to have Cr(III) active sites dispersed considered the same sequence of reactions on ethane on alumina. Among alternative supported catalysts, Ga-based dehydrogenation catalyzed by Cr(III) sites.29 Other reaction materials are particularly noteworthy. For instance, Ga- steps, such as regeneration of the propyl group via σ-bond exchanged zeolites can convert light alkanes such as propane metathesis of an incoming propane molecule releasing H2 and directly into aromatics and H2 in a process proposed to involve a tandem dehydrogenation−aromatization process.7−13 Ga2O3 also promotes PDH reaction, but it suffers from fast Special Issue: Heterogeneous Interfaces through the deactivation, presumably because of reduction of the catalyst Lens of Inorganic Chemistry under reaction conditions.14,15 More recently, silica-supported Received: October 26, 2020 well-defined Ga(III) single-site catalysts have been devel- Published: February 5, 2021 oped16 using a combined approach of surface organometallic chemistry17−23 and a thermolytic precursor using [Ga(OSi- © 2021 American Chemical Society https://dx.doi.org/10.1021/acs.inorgchem.0c03135 6865 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article Figure 1. Ga sites on the amorphous model of SiO2 (I, II, III, III-mod, and V sites). regenerating propyl, are significantly more energy-demanding, the most strained sites, thus yielding an overall more efficient as we showed recently for our work on the Cr(III) system.30 catalysis. Thus, the latter step was not considered in the present study. To evaluate the catalytic activity of well-defined silica- 2. RESULTS supported single-site catalysts by first principles, cluster models 2.1. Construction of Ga(III) Sites. Ga(III)/SiO2 models have typically been used because of the simplicity of these are constructed using a recently developed amorphous silica models and their associated low computational cost, but such model,36 which corresponds to a slab of dimensions 21.4 Å × models do not account for the expected site heterogeneity on 21.4 Å × 34.2 Å and contains 372 atoms. The silica model an amorphous support like SiO2 and cannot be used to model exposes five isolated silanol (SiOH) groups and has a surface highly strained sites that are generated upon thermal treatment SiOH density (1.1 OH nm−2) close to the density found for at high temperature.31 In fact, the high degree of heterogeneity silica partially dehydroxylated under vacuum at 700 °C has been evidenced on so-called silica-supported single-site (SiO2‑700, 0.8 OH nm−2), which is used experimentally to catalysts by luminescence spectroscopy32 and magnetic prepare well-defined Ga(III) sites and related systems.16 properties33 as well as the polydispersity of polyethylene The amorphous SiO2 model is obtained from a fully obtained on the corresponding Cr(III) systems.30,34,35 We hydroxylated amorphous silica model by direct condensation have also recently shown that the use of an amorphous SiO2 of adjacent SiOH groups and by surface reconstruction steps model36 can account for strain in the Cr(III)/SiO2 catalyst and involving SiO2 migrations. The amorphous silica model has a allows for an explanation of the reactivity of this catalytic high degree of heterogeneity, as evidenced by the large system toward olefin polymerization.37,38 In our previous study variability in the energetics associated with the dehydroxylation for the Cr(III)/SiO2 system, we have shown that there is a steps. The average Si−O distances of the siloxane bridges large variability of the reactivity of the Cr−O pairs, and those formed upon surface dehydroxylation serve as descriptors of that are more strained are significantly more reactive than the the strain present on the silica surface.36 The Ga sites are ones that are less elongated and therefore less strained and introduced to the silica model by substituting surface “SiOH” prone to react either by cleaving the C−H bond or by inserting groups by Ga3+, i.e., turning (SiO)3SiOH sites into ( the ethylene into the Cr−O bond, forming an oxachroma- SiO)3Ga sites, as previously carried out to build the cycle.37,38 corresponding Cr(III) sites.37,38 This model provides five Here, we evaluate the reactivity of isolated Ga(III) sites with types of Ga(III)/SiO2 models, models I−V. In contrast to Cr, different degrees of strain on amorphous silica toward PDH we also consider the coordination of an additional siloxane reaction using first-principles calculations. We show the high group to Ga because EXAFS data pointed to the presence of variability of the reactivity from site to site and encounter this additional siloxane bridge.16 Among the various Ga(III) Brønsted−Evans−Polanyi (BEP) relationships for the three sites, site I has one siloxane group in close contact with the Ga main reaction steps in the PDH reactions, serving as a guide center, with a distance equal to 2.465 Å, and can be considered for future screening studies. We also propose that the most as a model for [(SiO)3Ga(SiOSi)] sites. Other similar efficient sites toward the PDH reactions in the Ga(III)/SiO2 sites are built by placing a siloxane bridge at 2.3 Å for all of the system display the “right” balance of strain, where the sites II, III, IV and V sites. Geometry optimization yields the final with intermediate strain and not the most strained ones are the structures. Of them, site III yields another model for [( most efficient for PDH because they both can activate the C− SiO)3Ga(SiOSi)], with a Ga···O(Si) bond of 2.479 Å, H bond of propane effectively and are easier to regenerate than referred to as the site mod-III, which is 2.4 kcal·mol−1 less 6866 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article Scheme 1. Proposed Reaction Mechanism for the Dehydrogenation of Propane on Ga(III)/SiO2 Sites stable than the initial site III (which does not contain a of the geometrical characteristics for each Ga site are given in siloxane bound to Ga). For the latter two structures, the Ga··· detail (Figures S1−S6), i.e., Ga−O distances, O−Ga−O O(Si) bond is rather long, ranging from 2.465 to 2.479 Å, angles, and O−Ga−O−O dihedral angles. possibly because of the high constraint imposed by siloxane 2.2. Evaluation of PDH on the Selected Ga(III)/SiO2 groups near the Ga single site. Therefore, in order to consider a Sites. On these selected sites, we evaluate the PDH pathway less constrained and shorter Ga···O bond, which might also be (Scheme 1),39 which involves (1) C−H bond activation of present on the real system, we added a siloxane ligand to the propane on the Ga site, forming a Ga−propyl intermediate and Ga−O III-site, modeled by H3Si−O−SiH3 to evaluate the an OH group (step 1), (2) subsequent β-H transfer, forming a effect on the reactivity on this tetracoordinated Ga site, with a hydride and propene (step 2), and (3) H−H coupling (step 3) shorter bond between Ga and siloxane. For the latter system, following the decoordination of propene to regenerate the the additional Ga−O bond is 2.036 Å long, and the group has a initial Ga sites. binding energy to the Ga center equal to −18.4 kcal·mol−1, 2.3. Step 1: C−H Bond Activation of Propane. Among with respect to the free ligand in the gas phase. the evaluated tricoordinated Ga−O pairs II−O3, III−O2, V− Site III is the most stable one among those that we O2, and V−O3, the Ga−O pairs involving site V are the most constructed. The relative stabilities of the initial tricoordinated reactive ones, in line with the previous finding regarding the sites are given in Table S1. However, because of the C−H activation of ethylene and propene on analogous Cr construction method that we adopted (vide supra) and the sites.37,38 fact that there are many different degrees of strain on the Si− In Table 1, we have summarized the energy barrier heights OH groups of SiO2, in our opinion, this does not rule out the (ΔE⧧) and reaction energies (ΔE) for the C−H activation of fact that sites “less stable” based on this construction are not propane (in kcal mol−1) on the evaluated Ga−O pairs. From formed in the real case. At the end, most Si−OH groups react with molecular Ga and generate Ga single sites after Table 1. Energy Barrier Heights (ΔE⧧) and Reaction calcination. Energies (ΔE) for the C−H Activation of Propane (in kcal· We also investigate the reactivity of selected tricoordinated mol−1) on the Evaluated Ga−O Pairs Ga(III) sites toward propane for the II−O3, III−O2, V−O2, and V−O3 Ga−O pairs in view of their higher reactivity as Ga−O pair ΔE⧧ (kcal·mol−1) ΔE (kcal·mol−1) found in our previous study on the related Cr(III) sites.37 In I−O3 25.1 −22.9 addition, we also investigate the tetracoordinated Ga(III)−O II−O3 25.5 1.3 pairs: I−O3 and III-mod−O2. Figure 1 shows all of the III−O2 21.8 −23.8 constructed tricoordinated and tetracoordinated Ga sites on V−O2 14.5 −65.0 the amorphous SiO2 model. In the Supporting Information, all V−O3 18.5 −25.2 6867 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article the table, it is clear that, among all of the Ga−O pairs, the C− relationship. This BEP relationship is depicted as a graph in H activation process on V−O2 is highly exoenergetic, with a Figure S7. reaction energy of −65.0 kcal·mol−1, and it is associated with 2.4. β-H Transfer and Propene Decoordination. an energy barrier of 14.5 kcal·mol−1. The V−O3 Ga−O pair is Following the C−H bond activation step, which yields Ga− associated with a significantly lower exoenergetic reaction alkyl and O−H groups, the next step corresponds to a β-H energy of −25.2 kcal·mol−1 and a higher energy barrier of 18.5 transfer, which forms Ga−H and a propene coordinated to the kcal·mol−1. The next most reactive site for this first C−H Ga center. The relative energy barrier (Table 2) for this step is activation step of propane is III−O2, with a thermodynami- cally favorable reaction energy of −23.8 kcal·mol−1 and an Table 2. Energy Barrier Heights (ΔE⧧) and Reaction energy barrier of 21.8 kcal·mol−1. Finally, the C−H activation Energies (ΔE) for the β-H Elimination Step (in kcal·mol−1) of propane on the last evaluated tricoordinated Ga−O pair, II− on the Evaluated Ga−O Pairs O3, displays a reaction energy and an energy barrier of 1.3 and Ga−O pair ΔE⧧ (kcal·mol−1) ΔE (kcal·mol−1) 25.5 kcal·mol−1, respectively. We then evaluated the reactivity I−O3 49.1 20.4 on tetracoordinated Ga sites, in particular on the I−O3 sites II−O3 51.8 23.1 and the II-mod−O2 Ga−O sites. The tetracoordinated Ga−O III−O2 41.9 15.6 pairs I−O3 and III-mod−O2 contain an additional siloxane V−O2 49.0 26.6 bridge coordinating to Ga at 2.465 and 2.479 Å, respectively, V−O3 41.7 13.4 and the Ga−O distance is significantly elongated in the transition state (TS) with values of 2.759 and 3.410 Å, while the distances are 3.121 and 4.183 Å in the resulting Ga−alkyl significantly higher than that for the C−H activation of species. The C−H activation on the Ga−O I−O3 pair has an propane, and this step is highly endoenergetic. In this case, the energy barrier equal to 25.1 kcal·mol−1 and a reaction energy relative energy barriers take values ranging from 41.7 to 51.8 exoenergetic by 22.9 kcal·mol−1. Therefore, it presents a kcal·mol−1, while the reaction energies are all endothermic, relatively high energy barrier but a rather favorable reaction with values varying between +13.4 and +26.6 kcal·mol−1. In energy. The II-mod−O3 Ga−O pair corresponds to a this case, the most reactive Ga−O pairs are V−O3 and III− modification of the II−O3 site, in which a siloxane group is O2, with similar energy barriers and reaction energies (cf. closer to the Ga center by ca. 0.2 Å with respect to the II−O3 Table 2): ΔE⧧ values equal to 41.7 and 41.9 kcal·mol−1 and site. Nevertheless, the TS corresponding to the C−H ΔE values equal to +13.4 and +15.6 kcal·mol−1, respectively. It activation is the same in both cases, which thus have the is interesting to note that, once the C−H activation has taken same energy. Therefore, although the respective starting place on the V−O2 Ga−O pair, β-H transfer is more energy- minima of II−O3 and II-mod−O3 are different, both demanding (ΔE⧧ = 49.0 kcal·mol−1 and ΔE= 26.6 kcal·mol−1) structures lead to the same TSs and also to the same products. than when the C−H activation of propane takes place on the As mentioned earlier, formation of the Ga−O siloxane bond V−O3 Ga−O pair (ΔE⧧ = 41.7 kcal·mol−1 and ΔE = 13.4 kcal· from II−O3 to II-mod−O3 is endoenergetic by only 2.4 kcal· mol−1). The other possible Ga−O pairs are I−O3 and II−O3; mol−1. Thus, the energy barrier and reaction energy of the C− they are associated with rather high energy barriers of 49.1 and 51.8 kcal·mol−1 with reaction energies of 20.4 and 23.1 kcal· H activation of propane are practically the same for II-mod− mol−1, respectively. The correlation between the energy O3 and II−O3, being only slightly more feasible for the former barriers and reaction energies for the β-H transfer step for all than for the latter. Finally, we evaluated the Ga−O III−O2 site of the evaluated sites is similar to that for the C−H activation with the siloxane ligand, H3Si−O−SiH3, bonded to Ga. In this previous reaction step (R2 = 0.76). case, the reaction energy of the C−H activation is −23.4 kcal· We also evaluated the energy for the decoordination of mol−1, which is similar to the same site without the additional propene in all of the evaluated Ga sites. In all cases, it is an siloxane group bonded to Ga. Thus, overall it is likely that, if endoenergetic step by 10.2, 7.0, 15.5, 14.4, and 18.0 kcal·mol−1 present in the Ga@SiO2 catalytic structure, the siloxane bridge on the I−O3, II−O3, III−O2, V−O2, and V−O3 Ga−O pairs. bonded to Ga does not significantly impact the C−H bond It is worth mentioning that this step is exoergic in Gibbs free activation step. Recently, Das et al. has reported the effect of energy (vide infra). siloxane ring strain on the formation of coordinately For this reaction step, however, we found that the TS scaling unsaturated metal sites on silica.40 They have reported that relationship holds very well (R2 = 0.997; Figure 2). This at low temperature, because of the dominance of large siloxane relationship relates the energy of a given TS and its product, in rings, normally tetracoordinated Ga(III) sites are stabilized. which the energies of both structures are referenced with On the other hand, when silica is pretreated over 700 °C, respect to the initial reactant molecule and catalyst,42,43 in our tricoordinated Ga(III) sites are more plausible to be stabilized case propane and the respective initial Ga sites. because of the preferred formation of small siloxane rings. The 2.5. H−H Coupling and H2 Formation. The last step lower coordinated sites are demonstrated to be more reactive corresponds to coupling of the hydride and the proton bonded for the C−H activation, consistent with the higher activity for to Ga and O, respectively, which regenerates the initial the tricoordinated Ga(III) sites. In addition, also tricoordi- catalytic site along with H2. In this case, the most reactive Ga− nated sites of different Ga2O3-based catalysts have been O pair is II−O3, with the energy barrier for H−H coupling proposed as their active catalytic centers.41 Finally, it is worth being equal to 27.3 kcal·mol−1, in an endoenergetic step of 7.7 mentioning that there is a certain correlation (R2 = 0.77) kcal·mol−1. Three Ga−O pairs, namely, sites I−O3, III−O2, between the activation energy of the TS and reaction energy, and V−O3, have similar reactivity. The reaction energies are i.e., the BEP relationship when considering the following sites: endoenergetic and within 31.8−33.2 kcal·mol−1, while the I−O3, II−O3, III−O2, V−O2, and V−O3. The II-mod−O2 energy barriers for H−H coupling on these three sites are Ga−O pair was not considered in the evaluation of the BEP within 43.1−48.9 kcal·mol−1. Finally, the Ga−O V−O2 pair, 6868 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article TDTS appears after the TDI, ∂E is the energy difference between these two steps, whereas when it is the reverse, ΔG of the reaction (ΔGr) is added to this difference, where the l TTDTS − ITDI energetic span model (∂E) follows the equation o o ∂E = m o oTTDTS − ITDI + ΔGr n On the basis of the energetic span, we can then calculate the TOF of the reaction of interest by using the expression kBT −∂E / RT TOF = e h Figure 2. TS scaling for the β-H transfer step. ETS versus Eproduct (both The former equation holds for exergonic reactions, leading in in kJ·mol−1) of the β-H transfer step with respect to the energy of the this case to a positive TOF value. Within this model, the TOF propane molecule and the respective initial Ga sites. is understood as the catalytic flux, in analogy with Ohm’s law in electric circuits.45 A positive TOF is found for exergonic which was the most reactive site for the C−H activation of reactions, meaning that the catalytic flux goes forward, whereas propane, has a high energy barrier for H−H coupling (73.8 for endergonic reactions, the TOF is negative because the kcal·mol−1) in a reaction step endothermic by 64.6 kcal·mol−1. catalytic flux goes backward. In order to obtain the BEP relationship for this reaction step, Nevertheless, experimentally the TOF is defined differently. the opposite reaction to the H−H coupling step, i.e., H2 Because it is based on the conversion to products, it will always cleavage, needs to be evaluated. In this case, the BEP be a positive quantity. Indeed, for the Ga(III)/SiO2 catalyst, relationship between ΔE⧧ and ΔE of H2 cleavage is only the reported initial experimental TOF is equal to 20.4 mol of followed to a certain extent (R2 = 0.69). Nevertheless, in this propene per mole of Ga per hour under a kinetic regime (ca. case, there is a very good correlation between ΔE⧧ and ΔE conversion of 10%), despite the reaction being endergonic among for the forward reaction; however, it is difficult to experimentally at 550 °C and 1 bar. Thus, in order to compare interpret the physical meaning behind this correlation (Table the catalytic activity for the evaluated sites to the experimental 3). data in a semiquantitative way, we will make use the above- mentioned equation even though the ΔGr term is positive in Table 3. Energy Barrier Heights (ΔE⧧) and Reaction our case. For a full discussion of how we apply the TOF model Energies (ΔE) for the H−H Coupling Reaction (in kcal· for the current case, we refer the reader to the Supporting mol−1) on the Evaluated Ga−O Pairs Information. We also refer to the work of Shaik and Kozuch, who developed the energetic span model, in which the Ga−O pair ΔE⧧ (kcal·mol−1) ΔE (kcal·mol−1) meaning of the TOF within the model is discussed in depth.45 I−O3 48.9 33.1 In any case, when using the rigorous application of the II−O3 27.3 7.7 energetic span model, the trend of the reactivity found between III−O2 44.4 31.8 the different sites stays the same as the one described here. For V−O2 73.8 64.6 the Ga−O pair II−O3, the highest TS (TDS) in the energy V−O3 43.1 33.2 profile corresponds to the β-H transfer step; it is located 79.3 kcal·mol−1 above the initial reactants, which are the most stable 2.6. Overall Catalytic Cycles for PDH on the Selected species of the catalytic cycle. Thus, in this case, the energetic Sites. Finally, we can evaluate the overall reactivity in the span is equal to 79.3 kcal·mol−1 and the calculated TOF would dehydrogenation of propane for all of the evaluated sites, be equal to 4.57 × 10−5 h−1. Therefore, this Ga−O pair would considering the three reaction steps previously described. The be inactive. Another Ga−O pair site that is unreactive is the Gibbs energy profiles for all of the Ga−O pairs (I−O3, II−O3, V−O2 Ga−O pair but for a different reason. In this case, the III−O2, V−O2, and V−O3) are shown in Figure 3. The graph initial C−H activation of propane is the TDTS, being located shows indeed a significant variability among the five evaluated at 29.6 kcal·mol−1 with respect to the initial reactants, in a sites. On the basis of the obtained Gibbs energy profile, we can significantly exoergic step due to the significant release of compare the reactivity between the different sites. strain, with the corresponding product being located at −53.1 Overall, the calculated reaction free energy is endergonic at kcal·mol−1 with respect to the same reference, with the latter 550 °C and 1 bar by 7.4 kcal·mol−1, in good agreement with species being the TDI of the catalytic cycle. Overall, the thermodynamics limitations of the PDH reaction because considering the energy of the TDTS and TDI and the reaction at this temperature the equilibrium conversion for propane is energy, because in this case the TDI appears after the TDTS, still of ca. 30% at 550 °C and 1 bar.44 In order to compare the the energetic span is equal to 90.1 kcal·mol−1 for the Ga−O catalytic activity of the different sites, we have used the pair V−O2. Thus, this site is also inactive, with a calculated energetic span model.45 In this model, the turnover frequency TOF equal to 6.05 × 10−8 h−1. The III−O2 and I−O3 Ga−O (TOF) of a catalytic cycle is a function of the energetic span pairs present rather similar Gibbs energy profiles. They present (∂E), which depends on the energy of the TOF-determining similar midrange relative energy barriers for the C−H transition state (TDTS), which in a simplified view is the TS activation, β-H transfer, and H−H coupling steps: 56.0 versus with the highest energy in the Gibbs energy profile, and the 48.6 kcal·mol−1, 48.2 versus 44.5 kcal·mol−1, and 43.3 versus TOF-determining intermediate (TDI), which is generally the 48.1 kcal·mol−1. For these two sites, the calculated energetic most stable intermediate in the energy profile. Whenever the span is equal to 63.9 and 67.2 kcal·mol−1, which would 6869 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article Figure 3. Gibbs energy profile of the PDH reaction on the five evaluated Ga−O pairs of sites (I−O3, II−O3, III−O2, V−O2, and V−O3). The reference Gibbs energy for the energy profile is the sum of the respective energies of the site and the energy of the propane molecule in the gas phase. Figure 4. Top (a) and side (b) views of the Ga V site, with the corresponding labeling of the O sites. TS corresponding to the C−H activation of propane (c), β-H transfer (d), and H−H coupling of the V−O3 Ga−O pair (e) and their key geometrical features (in Å). correspond to TOFs equal to 0.58 and 0.08 h−1, respectively; TDTS of the catalytic cycle. This C−H activation relative thus, both sites would be active in the PDH reaction. Finally, energy barrier value is similar to the one that we found for the the most active Ga−O pair among all of the evaluated sites V−O2 Ga−O pair (29.6 kcal·mol−1). Nevertheless, in this case, would be the V−O3 one. This site presents a rather feasible the product of the C−H activation is exergonic but to a C−H activation step at 823.15 K, with a relative low energy significantly less extent than that for the V−O2 pair: −10.3 barrier equal to 31.7 kcal·mol−1. The corresponding TS is the versus −53.1 kcal·mol−1. The subsequent β-H transfer and H− 6870 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article H coupling also present affordable relative energy barriers at 3. CONCLUSIONS 550 °C: 40.3 and 41.3 kcal·mol−1, respectively. In this case, the Isolated Ga(III) sites dispersed on silica are rather active and TDI appears after the TDTS; it corresponds to the gallium selective catalysts for the PDH reaction. After construction of hydride species, with a relative energy equal to −19.8 kcal· the Ga(III) sites on SiO2 amorphous periodic models, we have mol−1. In this case, the energetic span is equal to 58.9 kcal· evaluated the reactivity of a variety of Ga−O pairs with mol−1, corresponding to a TOF equal to 12.5 h−1, which is very different degrees of strain. For the selected sites, we evaluated similar to the TOF obtained experimentally for the PDH three reaction steps, namely, the C−H activation of propane, reaction: 20.4 h−1.16 Despite all of the approximations and β-H transfer step, and H−H coupling. We considered tri- and considerations made to calculate the energetic span and the tetracoordinated Ga with one additional siloxane group resulting TOF, it is fair to conclude that V−O3 is the most coordinated to the Ga center because these are the proposed active among all of the evaluated sites in PDH. A graphical initial catalytic sites in the silica-supported well-defined Ga(III) representation of the Ga site V with the corresponding labeling PDH catalyst. For the tetracoordinated sites, the additional of the O sites is given in Figure 4a,b, whereas the three TSs for siloxane group coordinated to Ga does not seem to play a key the Ga V−O3 pairC−H activation of propane, β-H transfer, role in the PDH reaction on the evaluated catalytic system. and H−H couplingand its key geometrical features are After the C−H activation step of propane, the Ga···O shown in parts c−e of Figure 4, respectively. Further optimized interaction between the Ga center and the O site of the structures along the PDH pathway for the III−O2 and I−O3 siloxane group is lost, and its effect on the energetics is rather sites are also depicted in Figure S8. small. For the three evaluated reaction steps, we have found Geometrically, this V−O3 Ga−O pair has a Ga−O distance that the BEP relationship holds to certain extent for the C−H significantly elongated, being equal to 1.854 Å. Nevertheless, it and H−H activation steps. In addition, the TS scaling holds is not elongated as the V−O2 pair, in which the distance is very well for the β-H transfer step. This is rather interesting equal to 1.880 Å. Therefore, on the basis of the distance, the because, if true for other single sites based on elements other than Ga, it would allow screening of the reactivity of the Ga−O pair is reactive but not too much. Concerning the two different sites only via evaluation of the thermodynamics of the O−Ga−O angles in which the V−O3 Ga−O pair is involved, three proposed reaction steps in the PDH reaction. Thus, our they take quite different values: being equal to 106.0 and current results can serve as a basis for the future computational 138.9°. Thus, the V−O3 Ga−O pair and the site as a whole is screening of PDH silica-supported single-site catalysts, highly asymmetric because the remaining O−Ga−O angle of especially for those centers in which the β-H transfer is rather the Ga−V site is equal to 114.6°. In comparison to the other energy-demanding. Concerning the overall catalytic activity of sites, as evidenced by the O−Ga−O angle sites: site V is the the evaluated sites using the energetic span model, we have most asymmetric among all of them (see Figure S9 for a found that the strain reduces significantly the C−H activation histogram of the Ga−O distances for all of the sites). Finally, of propane. Nevertheless, if the strain is too high and the the dihedral angles in which the Ga−O pair is involved in one product of the C−H activation of propane is too stable, that of the ends are equal to 171.1 and 173.9°. The other dihedral compromises the overall catalytic activity in the dehydrogen- angle takes a value equal to 171.6°. Thus, all of the dihedral ation of propane because the subsequent β-H transfer and H− angles of this site are close to 180°, meaning the site is highly H coupling reaction steps, as well as the C−H activation of coplanar. In comparison, the other sites show coplanarity propane, become significantly more energy-demanding, similar to that of site II, and it is only slightly less coplanar than increasing the energetic span and significantly decreasing the site I, which is the most coplanar of all of the sites (all of the activity of the evaluated Ga−O pair. Thus, a compromise is dihedral angles are close to 180°). In contrast, sites III and III- needed between the strain, meaning an elongated Ga−O pair mod are quite far from coplanarity, with allof the dihedral O− for the effective cleavage of the C−H bond of propane, but not Ga−O−O angles taking values lower than 160°. too much in order to regenerate the reactive site effectively. From geometrical analysis, one could argue that both the Among all of the evaluated Ga(III)/SiO2 sites, the one Ga−O distances and O−Ga−O angles could be used as a displaying the highest catalytic activity is Ga−O V−O3, which descriptor for the C−H activation as well as the H−H has a rather elongated Ga−O bond, and it is embedded in a activation. highly asymmetric Ga(III) site close to coplanarity, as In addition, the results indicate that the V−O3 Ga−O pair evidenced by the difference in the O−Ga−O bonds and the represents a good model in order to describe the overall O−Ga−O−O dihedral angles close to 180°. activity of the Ga(III)/SiO2 catalyst in the PDH reaction. It also has an intermediate strain, yielding the highest activity. 4. COMPUTATIONAL METHODS The trend in the reactivity of the evaluated Ga−O pairs is the Density functional theory calculations based on the Gaussian and plane-wave (GPW) formalism46 were carried out using the Quickstep following: V−O3 > III−O2 > I−O3 > II−O3 > V−O2. (QS) module47 of the CP2K program package.48,49 The functional Overall, for the C−H activation of propane, the sites that are chosen was Perdew−Burke−Ernzerhof (PBE)50−52 with short-range more strained and more favorable to be cleaved had low energy Gaussian double-ζ basis sets53 optimized from molecular calculations. barriers and significantly more favored reaction energies, i.e., The energy cutoff of the auxiliary plane-wave basis set was set to 500 significantly exothermic. Conversely, for the H−H coupling Ry. The Goedecker−Teter−Hutter pseudopotentials54−56 were used. step, the Ga−O pair is formed again and thus the sites that The orbital transformation method was applied.57,58 A tetragonal were more favorable for the C−H activation of propane now simulation box of base area 21.4 Å × 21.4 Å and thickness 34.2 Å (ca. 24 Å of which corresponds to a vacuum slab added in order to avoid become less favorable for this step. In addition, if the initial C− interactions between images in the z direction) was used.36 Ground- H activation is too exothermic, this leads to very stable state structures were obtained by energy minimization with the BFGS intermediates in the Gibbs energy profile, which decreases the algorithm.59−63 The initial TS guesses were generally obtained from overall catalytic activity of that specific Ga−O pair. CI-NEB64−68 band calculations. TS structure optimizations were 6871 https://dx.doi.org/10.1021/acs.inorgchem.0c03135 Inorg. Chem. 2021, 60, 6865−6874 Inorganic Chemistry pubs.acs.org/IC Forum Article performed using the dimer method69,70 with the conjugate gradient (6) Searles, K.; Chan, K. W.; Mendes Burak, J. A.; Zemlyanov, D.; optimizer and the two-point-based line search. In a few cases, in Safonova, O.; Copéret, C. Highly Productive Propane Dehydrogen- addition to the correct imaginary frequency along the reaction ation Catalyst Using Silica-Supported Ga−Pt Nanoparticles Gen- coordinate, minor imaginary components were obtained that could erated from Single-Sites. J. Am. Chem. Soc. 2018, 140 (37), 11674− not be avoided. However, they are expected to have a minimal impact 11679. on the reported energies. ■ (7) Kitagawa, H.; Sendoda, Y.; Ono, Y. Transformation of propane into aromatic hydrocarbons over ZSM-5 zeolites. J. Catal. 1986, 101 ASSOCIATED CONTENT (1), 12−18. * sı Supporting Information (8) Thomas, J. M.; Liu, X. S. 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Ga2O3/HZSM-5 propane ■ aromatization catalysts: Formation of active centers via solid-state reaction. J. Catal. 1990, 126 (1), 267−278. AUTHOR INFORMATION (12) Meitzner, G. D.; Iglesia, E.; Baumgartner, J. E.; Huang, E. S. Corresponding Author The Chemical State of Gallium in Working Alkane Dehydrocyclodi- A. Comas-Vives − Department of Chemistry, Universitat merization Catalysts. In situ Gallium K-Edge X-Ray Absorption Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spectroscopy. J. Catal. 1993, 140 (1), 209−225. (13) Fricke, R.; Kosslick, H.; Lischke, G.; Richter, M. Incorporation Catalonia, Spain; orcid.org/0000-0002-7002-1582; of Gallium into Zeolites: Syntheses, Properties and Catalytic Email: Aleix.Comas@uab.cat Application. Chem. Rev. 2000, 100 (6), 2303−2406. Authors (14) Copéret, C. C−H Bond Activation and Organometallic Intermediates on Isolated Metal Centers on Oxide Surfaces. Chem. C. S. Praveen − Department of Chemistry and Applied Rev. 2010, 110 (2), 656−680. Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland (15) Copéret, C.; Estes, D. P.; Larmier, K.; Searles, K. Isolated A. P. Borosy − Department of Chemistry and Applied Surface Hydrides: Formation, Structure, and Reactivity. Chem. Rev. Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland 2016, 116 (15), 8463−8505. C. Copéret − Department of Chemistry and Applied (16) Searles, K.; Siddiqi, G.; Safonova, O. V.; Copéret, C. Silica- Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland; supported isolated gallium sites as highly active, selective and stable orcid.org/0000-0001-9660-3890 propane dehydrogenation catalysts. Chem. Sci. 2017, 8 (4), 2661− Complete contact information is available at: 2666. (17) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, https://pubs.acs.org/10.1021/acs.inorgchem.0c03135 J.-M. Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry. Angew. Chem., Int. Ed. Notes 2003, 42, 156−181. The authors declare no competing financial interest. ■ (18) Copéret, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Núñez-Zarur, F.; Zhizhko, P. A. ACKNOWLEDGMENTS Surface Organometallic and Coordination Chemistry toward Single- A.P.B. acknowledges support from a BNF Project (PvB, DFE Site Heterogeneous Catalysts: Strategies, Methods, Structures, and 381 014). C.S.P. thanks the SINERGIA project (SNF Project Activities. Chem. Rev. 2016, 116, 323. CRSII2-154448) and DST-India for an INSPIRE Faculty (19) Copéret, C.; Fedorov, A.; Zhizhko, P. A. Surface Organo- metallic Chemistry: Paving the Way Beyond Well-Defined Supported Fellowship with Award IFA-18 PH217 for funding. A.C.-V. Organometallics and Single-Site Catalysis. Catal. Lett. 2017, 147 (9), acknowledges the Holcim Stiftung Foundation, the Spanish 2247−2259. MEC, the European Social Fund (Ramon y Cajal Fellowship: (20) Copéret, C.; Chabanas, M.; Petroff Saint-Arroman, R.; Basset, RyC-2016-19930), and the Spanish “Ministerio de Ciencia, J.-M. Homogeneous and Heterogeneous Catalysis: Bridging the Gap Innovación y Universidades” (PGC2018-100818-A-I00) for through Surface Organometallic Chemistry. Angew. Chem., Int. Ed. financial support. ■ 2003, 42 (2), 156−181. (21) Pelletier, J. D. A.; Basset, J.-M. Catalysis by Design: Well- REFERENCES Defined Single-Site Heterogeneous Catalysts. Acc. Chem. Res. 2016, (1) Sattler, J. J. H. B.; Ruiz-Martinez, J.; Santillan-Jimenez, E.; 49 (4), 664−677. Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on (22) Wegener, S. L.; Marks, T. J.; Stair, P. C. Design Strategies for Metals and Metal Oxides. Chem. Rev. 2014, 114 (20), 10613−10653. the Molecular Level Synthesis of Supported Catalysts. Acc. Chem. Res. (2) Malakoff, D. The gas surge. 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