Unraveling the Mechanism of the Initiation Reaction of the Methanol to

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Research Article Cite This: ACS Catal. 2017, 7, 7987-7994

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Unraveling the Mechanism of the Initiation Reaction of the Methanol to Olefins Process Using ab Initio and DFT Calculations Philipp N. Plessow*,† and Felix Studt†,‡ †

Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡ Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe 76131, Germany S Supporting Information *

ABSTRACT: We report a theoretical investigation of the initiation of the methanol to olefin process, where we study the full reaction mechanism from methanol to propylene. The zeolite H-SSZ-13 is investigated with periodic density functional theory (DFT) calculations. These calculations are corrected with MP2-calculations on large (46T) cluster models, which is found to be crucial for sufficient accuracy. Our calculations clearly demonstrate that initiation via the formation of carbon monoxide is a realistic mechanism and is more likely than the methane−formaldehyde mechanism or variants thereof. A kinetic model of the autocatalytic carbon pool mechanism is employed to investigate the initiation kinetics in more detail, demonstrating that an assessment of the feasibility of an initiation reaction needs to be based on kinetic modeling of both the initiation reaction and autocatalysis. This model gives further evidence that initiation proceeds via oxidation of methanol to carbon monoxide, which subsequently forms the first carbon−carbon bond via carbonylation of methanol. The kinetic model also shows that only extremely small amounts of an olefin need to be formed for autocatalysis to start, implying that small impurities will dominate over initiation mechanisms. KEYWORDS: methanol to olefin reaction, zeolites, reaction mechanisms, density functional theory, catalysis

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INTRODUCTION Modern society relies heavily on crude oil for the production of transportation fuels and chemicals. In the future, however, their production needs to be based on alternative carbon-containing resources, which can be methane and coal or renewable sources such as biomass and CO2. The production of hydrocarbons from these feedstocks can be facilitated through the synthesis gas−methanol route, where methanol is then converted to hydrocarbons, olefins, and gasoline via the methanol to hydrocarbons (MTH), methanol to olefins (MTO), and methanol to gasoline (MTG) processes, respectively.1−4 These processes are typically run at high temperatures (350− 400 °C) employing acidic zeotype materials. The mechanistic details of these processes have been subject to numerous experimental and theoretical studies that helped establish the hydrocarbon pool mechanism.5−10 In this mechanism, hydrocarbons autocatalytically convert methanol into olefins, aromatics, and water, as depicted in a simplified way in Scheme 1 using propylene: propylene can be repeatedly methylated to form higher olefins that can then be split into two olefins, thus being an autocatalytic reaction. How this hydrocarbon pool is formed from methanol in the early stages of this process, however, is still a matter of debate, with several mechanisms and intermediates being proposed.11−25 Theoretical studies have the potential to advance our understanding of reaction mechanisms of these catalytic reactions significantly, as one can deduce the likelihood with which certain pathways dominate from their energy barrier and © XXXX American Chemical Society

Scheme 1. Simplified Part of the Alkene Cycle of the Hydrocarbon Pool Mechanism of the MTO Reaction, Illustrating Its Autocatalytic Nature and the Focus of This Work, the Initiation Reaction

the corresponding rate constant.26−33 The reliability of these calculations depends crucially on their accuracy, requiring benchmarking of the commonly used density functional theory (DFT) with higher level methods29−31 and employing a realistic model system of the catalytic material being investigated. Furthermore, consideration of entropic effects is extremely important for the reactions discussed here, since the Received: September 12, 2017

7987

DOI: 10.1021/acscatal.7b03114 ACS Catal. 2017, 7, 7987−7994

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ACS Catalysis entropic contribution is significant at high temperatures and needs to be included in an analysis of possible reaction mechanisms.26 In this work, we will focus on the initiation reaction (Scheme 1). Through comparison of the computed barriers, one can predict which of the investigated initiation mechanisms is most likely. However, to assess whether a mechanism is feasible and can explain the experimentally observed rates, one needs to model the kinetics of the reaction as well. As we will show, this requires taking into account the autocatalytic reaction that follows initiation. Consequently, an intuitive analysis of the magnitude of reaction barriers commonly used for steady-state catalytic process is shown to be inappropriate for the initiation reaction. Several zeolites have been explored for the MTO process, with H-SAPO-34 and H-ZSM5 being the most commonly used materials, but others including H-SSZ-13 are also employed.2 This study focuses on H-SSZ-1328,34−38 (H-CHA), as it is the simplest material to study theoretically due to the fact that it has only one possible Al substitution, making it computationally well defined in comparison to zeolites with more potential active sites. H-SSZ-13 has a similar reactivity in comparison to H-SAPO-34, although its deactivation through coking is more pronounced.34 H-SSZ-13 and H-SAPO-34 are isostructural with H-SAPO-34, exhibiting a slightly weaker acidity.2 As we will show later, the energetic differences between the various initiation mechanisms are rather large. We therefore expect that the conclusions obtained for H-SSZ-13 are valid for other zeolite topologies as well.

Figure 1. Illustration of the models of the catalytic materials used in this work (H, white; Si, yellow; Al, blue; O, red; C, brown): (a) periodic model; (b) 46T model; (c) 2T model. (a) and (b) show the isolated acid site, while (c) shows the reaction of the methyl group with CO, to form a surface acetate. The highest level of theory used in this work for the respective model is also indicated for each model. (d) Differences in transition state and reaction energies between PBE-D3/ def2-TZVPP and MP2/def2-TZVPP single-point energies for the 46T cluster model. Mean absolute errors (MAEs) and mean signed errors (MSEs) are shown separately for transition states and minima.

E = E PBC(PBE‐D3) + E46T(MP2) − E46T(PBE‐D3)

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(1)

Figure 1d highlights the systematic underestimation of reaction barriers with DFT by ∼30 kJ/mol and reveals that the deviations between MP2 and GGA-DFT are in some cases even more significant (∼50 kJ/mol). The considered reaction pathways starting from methanol and resulting in the production of ethylene, propylene, and isobutene after formation of the first C−C bond are shown in Figure 2. The scheme is separated into five different blocks. Three of them are differentiated through the carbon oxidation number, the others showing C−C coupling mechanisms. We introduce the categorization by oxidation state here, as the reactions from methanol to CO exhibit the highest barrier when they are accompanied by a change in the oxidation state. All free energies are computed at 400 °C and reference pressures of 1 bar and are given relative to methanol in the gas phase and the empty zeolite (1a: G = 0), since the adsorption of MeOH was calculated to be endergonic (+14 kJ/mol at the reaction conditions). The reaction starts with methanol (oxidation state −II, pink pathway in Figure 2) reacting with an acid site to form a surface methoxy species (SMS).41,42 This intermediate can then further react with another methanol to form dimethyl ether (DME).43−45 We find that the activation barrier for the formation of the SMS group (171 kJ/mol) is significantly below the barriers required for further oxidation of methanol, in line with other theoretical work.26,46 MeOH or DME can be oxidized to oxidation state 0 (black pathway in Figure 2) via reaction with either SMS or an acid site, resulting in the formation of CH4 and H2, respectively. We find that the transition states are similar for both reactions, with the C-bound hydrogen being either protonated or methylated by the acid site (Al-OH) or SMS (Al-OMe) to release H2 or CH4, respectively, while the remaining OH group of methanol protonates another oxygen of the zeolite active site to form formaldehyde. The activation barrier for these processes are

RESULTS AND DISCUSSION In this contribution, we investigate the kinetics of the initiation reaction starting from methanol to produce the first C−C bond and olefins of the hydrocarbon pool. We employ DFT calculations with periodic boundary conditions (PBC) using the GGA PBE-D3 approach,39,40 the results of which are benchmarked and corrected using second-order Møller−Plesset perturbation theory (MP2) and CCSD(T) calculations. We employ a hierarchical cluster approach29−31 that is illustrated in Figure 1. We use periodic DFT calculations in order to capture the effect that the porous network of the actual zeolite has on the reactivity of the adsorbate. Experimental studies have found that Si/Al ratios >10 lead to greater stability during the MTO process.34,35 We therefore expect the reaction to be catalyzed by single active sites and model this situation with a single Al atom per unit cell: e.g., Si/Al = 35. Since the accuracy of this electronic structure method is limited, we also consider nonperiodic cluster models, the largest being a 46T model containing an entire pore of the H-SSZ-13 zeolite, in which the reactive center has been optimized for both minima and transition states at the PBE-D3/def2-SV(P) level of theory. For this structure, we were able to carry out single-point energy calculations at the MP2/def2-TZVPP level of theory. We have furthermore verified the accuracy of MP2 by comparison with CCSD(T) calculations on small 2T clusters. In agreement with other theoretical studies,29 we could confirm that MP2 approaches chemical accuracy (5 kJ/mol) for these kinds of systems (see the Supporting Information). Using these ab initio results to correct the periodic DFT calculations according to eq 1 (see also Figure 1), we expect that our energy profile for the C−C bond initiation pathways shown in Figure 2 for H-SSZ-13 approaches chemical accuracy. 7988

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Figure 2. (a) Free energy diagram of the possible pathways for initiation of the MTO reaction shown in (b). For important activation barriers, the value is given in kJ/mol relative to the most stable prior intermediate, as indicated by the vertical lines under these barriers. (b) Overview of the most relevant initiation mechanisms of the MTO reaction. Al-OH and Al-OMe are used to abbreviate bridging hydroxyl and methoxy groups (for example Al-OH-Si). Mechanisms are grouped into boxes according to the oxidation state of carbon. Formation of the surface methoxy group is shown explicitly only for the reaction 1a → 1b and is omitted for clarity in all subsequent reactions since it is not rate-limiting. In the reaction of 3f to 3h, the intermediate formation of Al-OEt is omitted. (c) Structure of the most relevant transition states. Formed and broken bonds are indicated with dotted lines, and distances are given in pm; the color code is as in Figure 1. Framework atoms that do not participate directly in the reaction are shown in gray.

253 and 243 kJ/mol for H2 and CH4 formation, respectively. The barriers for oxidation of the carbon atom from DME, to form Al-CH2OMe (2b), are 219 (H2) and 226 kJ/mol (CH4), revealing that oxidation via DME is more favorable than via MeOH. Whether DME oxidation is faster than MeOH oxidation depends on the gas-phase concentrations of MeOH and DME. These concentrations may reach equilibrium before the MTO process is initiated.14,47 Both reactions can produce either H2 or CH4. We calculate H2 formation to be slightly more favorable (7 kJ/mol for DME), although we point out that our expected accuracy is below 10 kJ/mol for energies,

with additional uncertainty arising through the use of the harmonic approximation to calculate entropies. The actual free energy barrier that is relevant for the kinetics also depends on the partial pressures, and the rates are proportional to the number of available sites (free acid sites vs SMS). Keeping this in mind, our analysis suggests that both pathways are feasible, with the extent to which one pathway dominates depending on the specific reaction conditions. Methane production can also occur via associative mechanisms where the CH3 transfer arises from adsorbed MeOH and DME instead of SMS. Our calculations show that these processes become more favorable 7989

DOI: 10.1021/acscatal.7b03114 ACS Catal. 2017, 7, 7987−7994

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ACS Catalysis at lower temperatures (300 kJ/mol are about 50 kJ/mol higher than those found for further oxidation of FA to MF, a difference well captured by the accuracy of our computational approach.29−31 After formation of MF (3a), the reaction proceeds via decomposition of 3a to methanol and CO (3b) with a relatively small activation barrier of 129 kJ/mol. In this concerted reaction, the methoxy group is protonated by the acid site, leading to decomposition into MeOH and an HCO+ fragment that protonates an adjacent Al-O-Si oxygen to recover the acid site while releasing CO. CO is a key intermediate in the initiation process, as it can readily react with a SMS in an SN2 fashion, leading to the surface acetate species (3c) and thus forming the first C−C bond (blue pathway in Figure 2).11,14,54−58 The reaction of CO with MeOH has been subject to numerous experimental14−16,56 and theoretical studies,11,58 the findings of which generally agree with our calculations. Ketene may be formed11,58,59 via the formation of an intermediate, where the protonated ketene (CH3CO+) is hydrogen-bound to the zeolite. In comparison to other, slow forward reactions, the surface acetate 3c is in rapid equilibrium with ketene (3e) and the more stable methyl acetate (MA) (3d) that is formed by reaction of 3c with MeOH. Both methyl and surface acetate have been observed in a recent experimental investigation using H-SAPO-34.16 Ketene (CH2CO) can be methylated by a SMS group11 to form an unstable intermediate (CH2MeCO+), which is similar to the protonated ketene

mentioned above. In the same fashion, a surface propionate species (3f) can be formed that is in equilibrium with the more stable, free methyl propionate (3g) and methyl ketene (3i). In perfect analogy, methyl ketene can again be methylated to form dimethyl ketene (3m) and the more stable related species 3j,k. Further methylation yields Al-O-CO-tBu (3n) and MeO-COtBu (3o). Formation of CC double bonds can occur by decarbonylation of the intermediate Al-O-CO-R with R = Et, iPr, tBu (labeled 3f,j,n) to form ethylene,11 propylene, and isobutene, respectively. The methyl esters Me-O-CO-R are always more stable than the corresponding zeolite species (Al-O-CO-R). They are formed by nucleophilic attack of MeOH at the carboxylic carbon, and we have found low barriers for R = Me, Et, which we expect to hold for R= iPr, tBu as well. The formation of ethylene proceeds through an SN2-type transition state that releases CO and forms an Al-OEt group, which in turn produces ethylene and AlOH. Note that this reaction has a higher barrier in comparison to the corresponding formation of propylene and isobutene. This can be rationalized, as the respective carbocations of the iPr- and tBu-substituted surface carboxylates have a higher stability and react hence through an SN1-type transition state where the CO-iPr (or CO-tBu) group dissociates as a cation from the active site, after which the C−C bond is cleaved upon release of CO. The remaining carbocation is, without further barrier, deprotonated to form the respective olefin. To summarize the role of the key intermediate CO: the key step to form CO from methanol lies in the dehydrogenation of 2d to yield 3a, which has the highest barrier in this initiation reaction (259 kJ/mol). CO methylation yields an acetate species, thus forming the first C−C bond. Further methylation via ketenes produces ethylene, propylene, and isobutene via decarbonylation of methyl esters R-CO2-OMe with R = Et, iPr, tBu. The rate for olefin formation is expected to be lowest for ethylene with a barrier of 225 kJ/mol. Formation of propylene and isobutene is expected to have comparable rates. The highest barrier (205 kJ/mol), for both the formation of propylene and isobutene from CO, is required for the methylation of ketene. MA is the resting state in the CO cycle, which explains why it has been observed experimentally,16 while the other esters and the ketenes have not. Alternatively, MA can react to olefins via keto−enol tautomerism or methylation of the carboxylic oxygen, giving 1,1-hydroxymethoxyethylene or 1,1-dimethoxyethylene, respectively. These can be further C-alkylated as in the usual olefin cycle of the MTO process. Our calculations indicate, however, that the unfavorable thermodynamics of these initial olefins (>100 kJ/mol uphill) render this alternative pathway significantly less likely (see the Supporting Information). The formation of carbenes has also been suggested in the literature.16−21 Our calculations indicate, however, that carbene is energetically extremely unfavorable (236 kJ/mol), even when it is stabilized by insertion into the Al−O bond, forming a AlCH2-O-Si structure (1e) (see the Supporting Information for details). The oxonium-ylide intermediate where carbene (CH2) is bound to DME is even more unfavorable (328 kJ/mol uphill in free energy), in agreement with previous theoretical investigations.17,18 On the basis of our findings we conclude that, out of the investigated mechanisms, dehydrogenation of MeOH to CO (via formation of H2 or CH4) and further methylation of CO via the CO mechanism to propylene and isobutene is clearly 7990

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ACS Catalysis most favorable. While these conclusions can be safely drawn from the difference in activation barriers, the question of whether a mechanism is reasonable at all, e.g. is compatible with experimental kinetics, requires a kinetic analysis. Importantly, the MTO initiation is not a catalytic steady-state reaction but only produces small amounts of olefins that then initiate the autocatalytic carbon-pool mechanism.5−7 To judge whether an MTO initiation mechanism to form the autocatalytic carbon pool is relevant, one therefore needs to model both initiation and autocatalysis. A full mechanistic investigation and kinetic model of the entire hydrocarbon pool mechanism is beyond the scope of this work and would furthermore require reactor modeling.60−63 Instead, we will address the more fundamental question of how the interplay of initiation and autocatalysis determines catalytic turnover in an autocatalytic reaction. To this end, we consider a simple two-step kinetic model that captures the essential characteristics: namely, a process for initiation that generates the autocatalytic species and the autocatalytic process itself. The model consists only of the stoichiometric reaction

Figure 3. Results of the kinetic model for the autocatalytic reaction of methanol to propylene: (a) conversion of methanol using activation barriers of 259 and 156 kJ/mol for initiation and autocatalysis; (b) half-lifetime of methanol as a function of variations in the barrier of the autocatalytic reaction.

fast increase in conversion.69 This is often classified by the time after which 50% conversion has been obtained, t1/2. We will use this measure to investigate the effect of initiation and autocatalysis on conversion. Interestingly, a variation of the activation barrier for initiation has an effect much smaller than that of the autocatalytic reaction. An analytical expression for t1/2 (see the Supporting Information) shows that t1/2 varies only linearly with the activation barrier for initiation, while it depends exponentially on that for the autocatalytic reaction, as shown in Figure 3b. Our kinetic model allows us to draw the following conclusions. (1) Investigations of initiation kinetics or activation barriers alone will not conclusively determine whether an initiation mechanism is fast enough to start up autocatalysis within a reasonable time span. The initiation mechanism can hence not be evaluated properly without discussing the autocatalytic follow-up reaction. (2) On the basis of an approximate barrier for autocatalysis, the kinetic model reveals that the highest observed barrier in our initiation reaction (ΔG400 °C = 259 kJ/mol) does indeed represent a realistic scenario due to the autocatalytic nature of the process. The obtained t1/2 is in fact smaller than expected: e.g., the rate of propylene conversion would be overestimated. This can be attributed to our simplified approximation of the autocatalytic hydrocarbon pool reactions that we have not studied in detail here, where we assume that propylene methylation constitutes the rate-determining step. As shown in Figure 3b, a higher autocatalytic barrier will exponentially increase the initiation period. Finally, we will consider the role of impurities on the initiation kinetics. Our kinetic model has two regimes, where first initiation produces propylene and at some point autocatalysis is faster and the initiation can be neglected. Obviously, if sufficient propylene (or other catalytically active impurities) for autocatalysis is available, no initiation kinetics are required. We have repeated our kinetic analysis above with different initial pressures of propylene, p0 (Figure 4), which we use here to model impurities in the feed gas. Initial pressures p0 > 10−8 bar have a significant effect on the kinetics, essentially making an initiation reaction obsolete, since autocatalysis is sufficiently fast from the beginning. If one studies autocatalysis without initiation, e.g. only on the basis of initial propylene impurities, the dependence of t1/2 on the pressure of the impurity is logarithmic: t1/2 ∝ ln p0 . Therefore, even substantial

3MeOH → propylene + 3H 2O

that occurs both during initiation and autocatalysis, however with different rates. For the activation barriers of the initiation reaction, we have taken step 2d → 3a with Ginitiation = 259 kJ/ mol. For the autocatalytic reaction, on the basis of prior investigations,28,33 we will use the activation barrier for methylation of propylene. We have calculated the activation barrier for propylene methylation in the stepwise mechanism using our approach to an activation free energy of Gautocatalytic = 156 kJ/mol at 400 °C. This calculated barrier (ΔH⧧ = 50 kJ/ mol) is about 25 kJ/mol higher than typical values reported in the literature using GGA functionals,26,28,64 in line with the usual underestimation of barriers using DFT-GGA (see also Figure 1). During initiation, no reactant concentration changes significantly, so that we take the initiation reaction to be zeroth order in all species. A detailed kinetic analysis of olefin alkylation, the assumed rate-determining step for autocatalysis, has shown that the effective reaction order in methanol depends on the reaction conditions, such as temperature and pressure.26 Experimentally one usually finds zeroth-order dependence in methanol and first-order dependence in the olefin.65−68 Our kinetic model for the autocatalytic reaction is first order in propylene, and both initiation and autocatalysis are zeroth order in methanol: rinitiation = c0

⎛ G ⎞ kBT exp⎜ − initiation ⎟ h kBT ⎠ ⎝

rautocatalytic = [propylene]

⎛ Gautocatalytic ⎞ kBT exp⎜ − ⎟ h kBT ⎠ ⎝

(2)

(3)

Here, kB is Boltzmann’s constant, h is Planck’s constant, T is the temperature, and c0 is the reference concentration and the rates are given per active site of the catalyst. As we discuss in the Supporting Information, the conclusions are not very sensitive with respect to the precise functional form of the kinetic model, as long as it contains an autocatalytic step that is first order in propylene. Our solution of the initiation kinetics shows the characteristic shape of autocatalytic reactions (Figure 3a) with negligible product concentration until a sudden “light-off” that leads to a 7991

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the CO pathway (∼50 kJ/mol higher), they can be dismissed with high confidence. Our calculations targeted H-SSZ-13 catalysts. Although we expect other zeolite morphologies and/ or aluminophosphates to behave similarly regarding the initiation mechanism, this would still need to be confirmed explicitly.

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COMPUTATIONAL DETAILS Periodic DFT calculations were carried out with the projectoraugmented-wave (PAW) method using the VASP program package in version 5.4 as well as the standard VASP-PAWs.70,71 The Brillouin zone was sampled at the Γ point, and Gaussian smearing with a width of 0.1 eV was used. An energy cutoff of 400 eV was used for the expansion of the wave function in the plane wave basis set and 800 eV for a change in the volume of the unit cell in the optimization of cells. The obtained lengths of the lattice vectors of the unit cell of H-SSZ-13 are 13.625, 13.625, and 15.067 Å. The PBE functional with Grimme’s dispersion correction (PBE-D3) was used in all calculations.39,40 Harmonic force constants have been obtained from a central finite difference scheme where, in addition to the adsorbate, only part of the zeolite, the involved oxygen atom, and the adjacent Al and Si atoms have been included. All transition states have been verified to contain an imaginary harmonic frequency corresponding to the transition vector of the reaction. Furthermore, the connectivity of the transition states has been verified by small distortions along the transition vector followed by optimization toward the minima. Because the harmonic approximation can lead to inaccurate entropies for low-frequency vibration, all frequencies of adsorbates on the zeolite were raised to 10 cm−1 if they were below this value. Additional, second imaginary frequencies of transition states occurred in two cases, resulting from extremely soft vibrational modes, and small numerical inaccuracies of the finite difference scheme for force constants, and these were also raised to 10 cm−1. The free energy resulting from a harmonic vibration of 10 cm−1 corresponds approximately to the contribution of one degree of freedom for free translation/rotation, and it thus serves as an estimate of the lowest possible limit where the harmonic approximation is applicable. Nonperiodic calculations were carried out with the Turbomole program package72 with the RI approximations and appropriate auxiliary basis sets.73−76 MP2 and CCSD(T) calculations use the frozen core approximation with orbitals below −3Eh frozen. The 46T cluster model was cut from HSSZ-13 to contain an entire pore, and terminating, dangling Si− O bonds were replaced by fluorine atoms (Si−F) with frozen F positions. A termination with F instead of the often employed H was used, because it is expected to be chemically more similar to an O-Si(OR)3 moiety. Generally, Si−F bonds are chemically inert, while Si−H bonds are typically very reactive. In test calculations on small clusters, no significant difference in reaction energies between termination with H or F was observed. All geometry optimizations were carried out at the PBE-D3/dhf-SV(P) level of theory. Parts of the cluster model that are distant from the active site were frozen after initial optimization of the structure of 1a, and all free atoms were reoptimized for all investigated minima and transition states. Generally, all frozen atoms are at least five covalent bonds away from Al as in Al−O−Si−O−Si−X and artificial, direct interaction with terminating atoms is avoided. To improve the computational performance, MP2 calculations employ the

Figure 4. Conversion of methanol as a function of time for different initial pressures of propylene (impurities) using the computed activation barriers for initiation and autocatalysis.

variations in the partial pressure of the impurity would only slightly affect t1/2. This logarithmic dependence can be observed in Figure 4, where the initial impurity of propylene dominates over the initiation reaction for p0 > 10−7 bar. Increasing p0 by factors of 10 to 10−6 bar and 10−5 bar leads to a constant shift by Δt. On the other hand, t1/2 again depends exponentially on the barrier for autocatalysis (see the Supporting Information for a detailed analysis). We therefore conclude that only extremely low concentrations of reactive impurities (e.g., olefins, CO) are required to start the autocatalytic process.

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SUMMARY AND CONCLUSION In summary, we investigated the initiation reaction of the MTO process using combined ab initio/DFT calculations for H-SSZ13. Our results identified the CO pathway as the most likely mechanism, a result that was further analyzed and confirmed through a two-step kinetic model of the autocatalytic nature of this process. In this pathway, methanol is dehydrogenated to CO via the intermediate formaldehyde, thus changing the oxidation states from −II (methanol) to 0 (formaldehyde) and +II (CO). This mechanism is in agreement with species that have been observed in recent experiments for H-SAPO-34: dimethoxymethane, methyl acetate, and surface acetates.16 We find that changes in the oxidation states exhibit the highest barriers of the overall pathway, with the highest barrier being found for oxidation of formaldehyde to CO (259 kJ/mol). While such high barriers are often dismissed as being prohibitively high for a catalytic process, we showed through an approximate kinetic model that takes the non-steady-state autocatalytic nature of this process into account that these barriers are indeed realistic and lead to initiation kinetics in line with what is expected from experiments. While the kinetics of the MTO reaction in a real reactor are obviously more complex than our two-step kinetic model of a batch process, we believe that our results give qualitative insight into the reaction kinetics. We have found that the concentrations of catalytic species required to initiate autocatalysis are extremely low. The time until autocatalysis “light off” (t1/2) depends only logarithmically on this concentration. This suggests that impurities should be kept below 10−7 bar for experiments investigating the initiation reaction. While many theoretical studies rely exclusively on calculations at the DFT-GGA level, we stress here that our approach of benchmarking DFT calculations to MP2 and CCSD(T) calculations is expected to approach chemical accuracy (5 kJ/ mol). As the alternative pathways considered in this study have reaction barriers significantly higher than those identified for 7992

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Research Article

ACS Catalysis

(17) Lesthaeghe, D.; Van Speybroeck, V.; Marin, G. B.; Waroquier, M. Angew. Chem., Int. Ed. 2006, 45, 1714−1719. (18) Lesthaeghe, D.; Van Speybroeck, V.; Marin, G. B.; Waroquier, M. Chem. Phys. Lett. 2006, 417, 309−315. (19) Sinclair, P. E.; Catlow, C. R. A. J. Phys. Chem. B 1997, 101, 295− 298. (20) Yamazaki, H.; Shima, H.; Imai, H.; Yokoi, T.; Tatsumi, T.; Kondo, J. N. Angew. Chem., Int. Ed. 2011, 50, 1853−6. (21) Blaszkowski, S. R.; vanSanten, R. A. J. Am. Chem. Soc. 1997, 119, 5020−5027. (22) Comas-Vives, A.; Valla, M.; Coperet, C.; Sautet, P. ACS Cent. Sci. 2015, 1, 313−9. (23) Song, W.; Marcus, D. M.; Fu, H.; Ehresmann, J. O.; Haw, J. F. J. Am. Chem. Soc. 2002, 124, 3844. (24) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. J. Am. Chem. Soc. 2003, 125, 15260−7. (25) Wu, X.; Xu, S.; Zhang, W.; Huang, J.; Li, J.; Yu, B.; Wei, Y.; Liu, Z. Angew. Chem., Int. Ed. 2017, 56, 9039−9043. (26) Brogaard, R. Y.; Henry, R.; Schuurman, Y.; Medford, A. J.; Moses, P. G.; Beato, P.; Svelle, S.; Nørskov, J. K.; Olsbye, U. J. Catal. 2014, 314, 159−169. (27) Van Speybroeck, V.; Hemelsoet, K.; Joos, L.; Waroquier, M.; Bell, R. G.; Catlow, C. R. Chem. Soc. Rev. 2015, 44, 7044−111. (28) Wang, C. M.; Wang, Y. D.; Du, Y. J.; Yang, G.; Xie, Z. K. Catal. Sci. Technol. 2015, 5, 4354−4364. (29) Svelle, S.; Tuma, C.; Rozanska, X.; Kerber, T.; Sauer, J. J. Am. Chem. Soc. 2009, 131, 816−25. (30) Hansen, N.; Kerber, T.; Sauer, J.; Bell, A. T.; Keil, F. J. J. Am. Chem. Soc. 2010, 132, 11525−38. (31) Gomes, J.; Zimmerman, P. M.; Head-Gordon, M.; Bell, A. T. J. Phys. Chem. C 2012, 116, 15406−15414. (32) Gomes, J.; Head-Gordon, M.; Bell, A. T. J. Phys. Chem. C 2014, 118, 21409−21419. (33) Wang, S.; Wei, Z. H.; Chen, Y. Y.; Qin, Z. F.; Ma, H.; Dong, M.; Fan, W. B.; Wang, J. G. ACS Catal. 2015, 5, 1131−1144. (34) Bleken, F.; Bjorgen, M.; Palumbo, L.; Bordiga, S.; Svelle, S.; Lillerud, K. P.; Olsbye, U. Top. Catal. 2009, 52, 218−228. (35) Zhu, Q.; Kondo, J. N.; Ohnuma, R.; Kubota, Y.; Yamaguchi, M.; Tatsumi, T. Microporous Mesoporous Mater. 2008, 112, 153−161. (36) Hemelsoet, K.; Nollet, A.; Vandichel, M.; Lesthaeghe, D.; Van Speybroeck, V.; Waroquier, M. ChemCatChem 2009, 1, 373−378. (37) Yuen, L.-T.; Zones, S.; Harris, T.; Gallegos, E.; Auroux, A. Microporous Mater. 1994, 2, 105−117. (38) Park, J. W.; Lee, J. Y.; Kim, K. S.; Hong, S. B.; Seo, G. Appl. Catal., A 2008, 339, 36−44. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396−1396. (40) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (41) Ghorbanpour, A.; Rimer, J. D.; Grabow, L. C. ACS Catal. 2016, 6, 2287−2298. (42) Moses, P. G.; Nørskov, J. K. ACS Catal. 2013, 3, 735−745. (43) Blaszkowski, S. R.; vanSanten, R. A. J. Am. Chem. Soc. 1996, 118, 5152−5153. (44) Jones, A. J.; Iglesia, E. Angew. Chem., Int. Ed. 2014, 53, 12177− 81. (45) Carr, R. T.; Neurock, M.; Iglesia, E. J. Catal. 2011, 278, 78−93. (46) Boronat, M.; Martinez, C.; Corma, A. Phys. Chem. Chem. Phys. 2011, 13, 2603−12. (47) Martinez-Espin, J. S.; Mortén, M.; Janssens, T. V. W.; Svelle, S.; Beato, P.; Olsbye, U. Catal. Sci. Technol. 2017, 7, 2700−2716. (48) Hutchings, G. J.; Gottschalk, F.; Hunter, R. Ind. Eng. Chem. Res. 1987, 26, 635−637. (49) Dewaele, O.; Geers, V. L.; Froment, G. F.; Marin, G. B. Chem. Eng. Sci. 1999, 54, 4385−4395. (50) Hwang, A.; Kumar, M.; Rimer, J. D.; Bhan, A. J. Catal. 2017, 346, 154−160.

def2-TZVPP basis set except for the terminating F atoms, where the dhf-SV(P) set was used.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b03114. Cartesian coordinates of optimized structures, detailed description of the kinetic model, benchmark calculations on the 2T clusters, analysis of the free energies at different temperatures, and data used to compute free energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.N.P.: [email protected]. ORCID

Philipp N. Plessow: 0000-0001-9913-4049 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support by the state of BadenWürttemberg through bwHPC (bwunicluster and JUSTUS, RV bw16G001 and bw17D011). Financial support from the Helmholtz Association is also gratefully acknowledged.

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ABBREVIATIONS DME, dimethyl ether; FA, formaldehyde; DMM, dimethoxymethane; MF, methyl formate; MA, methyl acetate; SMS, surface methoxy species

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REFERENCES

(1) Hemelsoet, K.; Van der Mynsbrugge, J.; De Wispelaere, K.; Waroquier, M.; Van Speybroeck, V. ChemPhysChem 2013, 14, 1526− 45. (2) Olsbye, U.; Svelle, S.; Bjorgen, M.; Beato, P.; Janssens, T. V.; Joensen, F.; Bordiga, S.; Lillerud, K. P. Angew. Chem., Int. Ed. 2012, 51, 5810−31. (3) Stöcker, M. Microporous Mesoporous Mater. 1999, 29, 3−48. (4) Olsbye, U.; Svelle, S.; Lillerud, K. P.; Wei, Z. H.; Chen, Y. Y.; Li, J. F.; Wang, J. G.; Fan, W. B. Chem. Soc. Rev. 2015, 44, 7155−76. (5) Wang, C.-M.; Wang, Y.-D.; Xie, Z.-K. J. Catal. 2013, 301, 8−19. (6) Svelle, S.; Joensen, F.; Nerlov, J.; Olsbye, U.; Lillerud, K. P.; Kolboe, S.; Bjorgen, M. J. Am. Chem. Soc. 2006, 128, 14770−14771. (7) Bjorgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U. J. Catal. 2007, 249, 195−207. (8) Dahl, I. M.; Kolboe, S. Catal. Lett. 1993, 20, 329−336. (9) Dahl, I. M.; Kolboe, S. J. Catal. 1994, 149, 458−464. (10) Dahl, I. M.; Kolboe, S. J. Catal. 1996, 161, 304−309. (11) Tajima, N.; Tsuneda, T.; Toyama, F.; Hirao, K. J. Am. Chem. Soc. 1998, 120, 8222−8229. (12) Salehirad, F.; Anderson, M. W. J. Catal. 1996, 164, 301−314. (13) Li, J. F.; Wei, Z. H.; Chen, Y. Y.; Jing, B. Q.; He, Y.; Dong, M.; Jiao, H. J.; Li, X. K.; Qin, Z. F.; Wang, J. G.; Fan, W. B. J. Catal. 2014, 317, 277−283. (14) Liu, Y.; Muller, S.; Berger, D.; Jelic, J.; Reuter, K.; Tonigold, M.; Sanchez-Sanchez, M.; Lercher, J. A. Angew. Chem., Int. Ed. 2016, 55, 5723−6. (15) Chen, X. Y.; Neidig, M. L.; Tuinstra, R.; Malek, A. J. Phys. Chem. Lett. 2010, 1, 3012−3015. (16) Chowdhury, A. D.; Houben, K.; Whiting, G. T.; Mokhtar, M.; Asiri, A. M.; Al-Thabaiti, S. A.; Basahel, S. N.; Baldus, M.; Weckhuysen, B. M. Angew. Chem., Int. Ed. 2016, 55, 15840−15845. 7993

DOI: 10.1021/acscatal.7b03114 ACS Catal. 2017, 7, 7987−7994

Research Article

ACS Catalysis (51) Wei, Z.; Chen, Y.-Y.; Li, J.; Wang, P.; Jing, B.; He, Y.; Dong, M.; Jiao, H.; Qin, Z.; Wang, J.; Fan, W. Catal. Sci. Technol. 2016, 6, 5526− 5533. (52) Hutchings, G. J.; Gottschalk, F.; Hall, M. V. M; Hunter, R. J. Chem. Soc., Faraday Trans. 1 1987, 83, 571−583. (53) Wei, Z.; Chen, Y.-Y.; Li, J.; Guo, W.; Wang, S.; Dong, M.; Qin, Z.; Wang, J.; Jiao, H.; Fan, W. J. Phys. Chem. C 2016, 120, 6075−6087. (54) Boronat, M.; Martinez-Sanchez, C.; Law, D.; Corma, A. J. Am. Chem. Soc. 2008, 130, 16316−23. (55) Cheung, P.; Bhan, A.; Sunley, G. J.; Iglesia, E. Angew. Chem., Int. Ed. 2006, 45, 1617−20. (56) He, T.; Ren, P.; Liu, X.; Xu, S.; Han, X.; Bao, X. Chem. Commun. 2015, 51, 16868−70. (57) Bhan, A.; Allian, A. D.; Sunley, G. J.; Law, D. J.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 4919−4924. (58) Rasmussen, D. B.; Christensen, J. M.; Temel, B.; Studt, F.; Moses, P. G.; Rossmeisl, J.; Riisager, A.; Jensen, A. D. Angew. Chem., Int. Ed. 2015, 54, 7261−4. (59) Jiao, F.; Li, J.; Pan, X.; Xiao, J.; Li, H.; Ma, H.; Wei, M.; Pan, Y.; Zhou, Z.; Li, M.; Miao, S.; Li, J.; Zhu, Y.; Xiao, D.; He, T.; Yang, J.; Qi, F.; Fu, Q.; Bao, X. Science 2016, 351, 1065−8. (60) Kumar, P.; Thybaut, J. W.; Svelle, S.; Olsbye, U.; Marin, G. B. Ind. Eng. Chem. Res. 2013, 52, 1491−1507. (61) Janssens, T. V. W.; Svelle, S.; Olsbye, U. J. Catal. 2013, 308, 122−130. (62) Kumar, P.; Thybaut, J. W.; Teketel, S.; Svelle, S.; Beato, P.; Olsbye, U.; Marin, G. B. Catal. Today 2013, 215, 224−232. (63) Pérez-Uriarte, P.; Ateka, A.; Aguayo, A. T.; Gayubo, A. G.; Bilbao, J. Chem. Eng. J. 2016, 302, 801−810. (64) Brogaard, R. Y.; Wang, C.-M.; Studt, F. ACS Catal. 2014, 4, 4504−4509. (65) Svelle, S.; Ronning, P.; Olsbye, U.; Kolboe, S. J. Catal. 2005, 234, 385−400. (66) Svelle, S.; Rønning, P. O.; Kolboe, S. J. Catal. 2004, 224, 115− 123. (67) Hill, I. M.; Hashimi, S. A.; Bhan, A. J. Catal. 2012, 285, 115− 123. (68) Hill, I. M.; Ng, Y. S.; Bhan, A. ACS Catal. 2012, 2, 1742−1748. (69) Janssens, T. V. W.; Svelle, S.; Olsbye, U. J. Catal. 2013, 308, 122−130. (70) Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (71) Kresse, G.; Furthmuller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (72) TURBOMOLE V7.1 2016, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007, TURBOMOLE GmbH, since 2007; available from http://www. turbomole.com/ ,accessed on 10/13/2017. (73) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057−65. (74) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297−305. (75) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (76) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577.

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DOI: 10.1021/acscatal.7b03114 ACS Catal. 2017, 7, 7987−7994