Theoretical Insights into the Selective Oxidation of Methane to

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Theoretical insights into the selective oxidation of methane to methanol in copper-exchanged mordenite Zhi-Jian Zhao, Ambarish Kulkarni, Laia Vilella, Jens K. Norskov, and Felix Studt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00440 • Publication Date (Web): 02 May 2016 Downloaded from http://pubs.acs.org on May 5, 2016

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Theoretical insights into the selective oxidation of methane to methanol in copper-exchanged mordenite Zhi-Jian Zhao,1,2 Ambarish Kulkarni,1 Laia Vilella,1,3 Jens K. Nørskov,1 Felix Studt1§*

[1] SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, United States and Department of Chemical Engineering, Stanford University, Stanford, CA 94305, United States.

[2] Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China [3] Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, 08193, Barcelona, Spain. § present address: Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany email: [email protected] Abstract Selective oxidation of methane to methanol is one of the most difficult chemical processes to perform. A potential group of catalysts to achieve CH4 partial oxidation are Cu-exchanged zeolites mimicking the active structure of the enzyme methane monooxygenase. However, the details of this conversion, including the structure of active site, are still under debate. In this contribution, periodic density functional theory (DFT) methods were employed to explore the molecular features of the selective oxidation of methane to methanol catalyzed by Cu exchanged mordenite (Cu-MOR). We focused on two types of previously suggested active species, which are CuOCu and CuOOCu. Our calculations indicate that the formation of CuOCu is more feasible than the CuOOCu. In addition, a much lower C-H dissociation barrier is located on the former active site, indicating that C-H bond activation is easily achieved with CuOCu. We calculated the energy barriers of all elementary steps for the entire process including catalyst activation, CH4 activation and CH3OH desorption. Our calculations are in agreement with experimental observations and present the first theoretical study examining the entire process of methane selective oxidation to methanol. Keywords methane activation, density functional theory, zeolite, mordenite, copper, methanol

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Introduction Natural gas becomes increasingly important as societies source for chemicals and

transportation fuels.1,2 In fact, methanol, a major bulk chemical and potential precursor for the production of plastics and gasoline range hydrocarbons,3 is primarily manufactured from methane. The current process, however, takes place via steam reforming and methanol synthesis and is only applicable at a large scale.4 The direct selective oxidation of methane to methanol would therefore have many advantages, specifically for the employment of stranded natural gas that is being flared nowadays.5 The selective oxidation of methane, however, is rather difficult to achieve owing to the inertness of methane and the thermodynamic exothermicity of complete methane oxidation. While nature achieves this goal through the enzyme monooxygenase,6 CH4 partial oxidation has been recently shown to be catalyzed in part by a range of Cu-exchanged zeolites.7-16 Examples include copper exchanged ZSM-5 and mordenite (MOR) that have been shown to yield methanol with 98 % selectivity.15 Recent studies suggest that the catalytic active center of Cu-MOR is a mono oxygen dicopper cluster [CuOCu]2+,13 and density functional theory (DFT) calculations employing this active site motif in Cu-ZSM-5 obtained an activation barrier of 77 kJ mol-1, in good agreement with the experimentally observed apparent activation barrier (66 kJ mol-1).17 This finding is particularly interesting as the active sites of Cu exchanged zeolites mimic the active site motif of particulate monooxygenase.18,19 Methane partial oxidation employing copper exchanged zeolites is usually performed in three stages: (1) oxidative activation of the copper exchanged zeolite through e.g. O2 or N2O at high temperatures, (2) reaction of the activated material with methane at lower temperatures, and (3) extraction of the produced methanol through water vapor.9,13,15 There are several theoretical studies exploring various aspects of methane partial oxidation,17,20-24 a complete picture of the various stages of this process as well as the formation and stability of the active site motif, however, is still missing to date. Herein we employ periodic DFT calculations to explore the molecular features of methane selective oxidation to methanol catalyzed by Cu-MOR. We do this by focusing on two types of active species that have been previously suggested,25 namely [CuOCu]2+ and [CuOOCu]2+. Our calculations represent the first theoretical investigation of the entire process of methane selective oxidation over CuMOR including all three stages. They indicate that the formation of [CuOCu]2+ is thermodynamically and kinetically preferred over [CuOOCu]2+ under O2 and N2O environment, respectively. We also find that the free energy of activating methane is


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different for these two moieties suggesting that only [CuOCu]2+ is able to activate methane at an appreciable rate. Lastly, we investigate the difficulty of extracting methanol from the catalyst and provide an explanation on how this is achieved through extraction with water vapor. 2.

Methods DFT calculations were performed with the plane-wave based Vienna Ab-initio

Simulation Package, VASP. 26 , 27 The calculations employed the generalized-gradient approximation (GGA) in the form of the Bayesian error estimation functional with van der Waals correlation (BEEF-vdW).28 We note that this functional has been shown to provide a quantitative description of van der Waals interactions between molecules and zeolite pores29 as well as reaction kinetics. 30 In addition, the quality of the calculated binding energies generated by BEEF-vdW is validated through comparison to results obtained with a higher level of theory (hybrid HSE06 functional) 31,32 using copper exchanged Chabazite as the model catalyst. The calculations indicate that BEEF-vdW generally performs well when compared to HSE06 and HSE06/D3 (see supporting information Figure S1). Importantly, the BEEF errors that have been employed throughout this study are well capable of reproducing errors associated with BEEF-vdW. The interaction between the atomic cores and electrons is described by the projector augmented wave (PAW) method33,34 and the Brillouin zone is sampled by the single Γ-kpoint. The valence wave functions are expanded in a plane-wave basis with a cutoff energy of 400 eV, employing a Gaussian smearing width of 0.05 eV. All the calculations are performed spin polarized. For all intermediates, we considered two spin states, singlet and triplet. All VASP optimizations automatically converged to the most stable spin state, which was confirmed by additional calculations with a fixed spin state other than the VASP converged spin configuration. All atoms are allowed to relax during geometry optimizations until the force on each atom was less than 0.03 eV/Å. Transition states (TS) are determined by applying the climbing image nudged elastic band (CI-NEB) method 35 employing five images between the initial and final states. The TS structures obtained in this way are further refined until the forces on atomic centers reach 0.03 eV/Å. For a spin-forbidden reaction, the two-state reactivity was checked both on the singlet and triplet potential energy surfaces. Zero point energies (ZPE) and entropic contributions were calculated within the harmonic approximation.36 Spurious frequencies that are smaller than 12 cm-1 were replaced by 12 cm-1. This value is chosen as the entropy contribution from a 12 cm-1 mode approximates that from a pseudotranslational/rotational degree of freedom, as


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discussed in detail elsewhere.30, 37 Note that the harmonic oscillator model employed here has some limitations as it fails to describe the soft modes within zeolite frameworks accurately. These small errors get amplified for reactions involving high temperatures, but should be relatively minor for the temperatures employed for the process considered here. Recent methods improve on this accuracy through incorporation of anharmonic effects. All intermediates are singlet state.

38 , 39 , 40

The uncertainty associated with the GGA

approximation is obtained through an ensemble of exchange-correlation functionals representing the known computational errors of the BEEF-vdW functional.41 The MOR framework was represented by periodic orthorhombic cells. The optimized lattice constant for MOR is a = 18.34 Å, b = 20.58 Å and c = 7.58 Å, almost identical to the experimental values of a = 18.25 Å, b = 20.53 Å and c = 7.54 Å.42 Two Si atoms were replaced by two Al atoms within each unit cell, and the [Cu2Ox]2+ clusters were attached to the two [AlO4]- units keeping the zeolite charge neutral. 3.

Results and Discussion Selective oxidation of methane to methanol catalyzed by Cu exchanged zeolites

requires three separated steps:15,23 (1) oxidative activation of the catalyst at high temperatures (using either O2 or N2O as oxidants); (2) reaction of the activated Cu-zeolite with CH4 at temperatures ranging from 323 to 473 K; and (3) extraction of methanol through steaming. In the following, we will discuss these processes separately, in an attempt to gain a deeper understanding of each individual step. 3.1. Activation of the Cu exchanged zeolite As shown in Figure 1, the biggest channel in MOR is the 12 membered-ring (MR) (7.0 × 6.5 Å), which is in parallel with a compressed 8MR channel (5.7 × 2.6 Å). These two channels are connected by the so-called side pockets, 8MR window. There are four distinct tetrahedral sites: T1, T2 and T4 that are located in the 12MR channel, and T3 that is within the 8MR channel (Figure 1). DFT calculations have shown that a Cu2O2 cluster, in a µ-1,2peroxo dicopper conformation, is bound more strongly to two Al T sites separated by two Si atoms. 43 A similar binding site has also been reported to be favored by µ-oxo dicopper complexes.22 In both cases, the two Al atoms are separated by about 7-8 Å, and the two copper atoms that form the precursor of Cu2Ox are separated ~ 5Å. In this study we investigate five distinct T sites pairs in the 12MR, i.e. T1-T1; T1-T4; T2-T2; T2-T4; T4-T4, as well as one pair, T3-T3, in the 8MR, as suggested by Ref 13. In the following, we will


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focus our detailed analysis on a Cu active site hosted on the T1-T4 pair modeled within a conventional unit cell of MOR as an example. We will then compare key elementary steps across different sites. A larger unit cell, which is the conventional cell duplicated in c direction, was employed due to the fact that several [Cu2Ox]2+ clusters hosted on other sites are parallel with c axis.

Figure 1. Framework of siliceous MOR. Color: pink – T1 site; Green – T2 site; Yellow – T3 site; Blue – T4 site; Red – oxygen atom. The T sites can be either Si or Al atoms.

Activation by N2O: It has been shown experimentally that copper exchanged MOR can be activated to form [Cu-O-Cu]2- using N2O as the oxidant.13 We calculate the activation barrier of Cu-O-Cu from 2 Cu and N2O to 75 kJ mol-1 in free energy. The process is strongly exothermic by -178 kJ mol-1 at 523 K, yielding a Cu-O-Cu in a triplet spin state that is 40 kJ mol-1 more stable than the corresponding closed-shell singlet (Table 1), with a structure similar to that reported earlier in ZSM-5 (Figure 2).22 Previous studies suggest that the unpaired electrons are located on antibonding π interactions of Cu dxy,yz and O px,y atomic orbitals, rendering this reaction step spin forbidden.20 Although the barrier on the triplet potential energy surface (PES) has not been explicitly calculated, we expect it to be considerably higher than the barrier on the singlet PES due to the instability of the triplet initial state compared to its singlet counterpart (the corresponding triplet bare Cu pair and N2O(g) are +151 and +522 kJ mol-1 less stable than the singlet states, respectively, so that


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both are significantly higher than the barrier on singlet PES, 41 kJ mol-1). We hence chose to focus on the singlet barrier for the following discussion.44 Further oxidation of Cu-O-Cu to Cu-O-O-Cu using N2O is still exothermic (∆G523K = 43 kJ mol-1) yet accompanied by an insurmountable free energy barrier of 244 kJ mol-1. Experimentally activation of Cu-MOR is found to proceed by dosing 5% N2O for 5 min at 523 K suggesting the formation of Cu-O-Cu rather than Cu-O-O-Cu.

Figure 2. Free energy profile of CuOCu and COOCu formation from Cu-MOR and N2O at 523K. The spin states of intermediates and transition states are: CuCu-singlet; TS@75singlet; TS@66-triplet; CuOOCu-triplet.


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Activation by O2: Activation of the copper cluster can also be achieved using O2 as oxidant.13 While this process can be conducted at the same temperature (T = 523 K), much longer reaction times are needed for full activation (usually overnight). Activation using O2 yields Cu-O-O-Cu in the first reaction step and is highly exothermic (∆G523K = -151 kJ mol1

). Interestingly, the Cu-O-Cu complex is lower in energy, reference to bare Cu-MOR and

1/2 O2(g), than Cu-O-O-Cu as can be inferred from Figure 3, so that the former will dominate from thermodynamic considerations. In order to form CuOCu, an O-O bond scission step is necessary. Our attempts show that direct O-O bond scission, forming two Cu-O species from Cu-O-O-Cu, is not possible within our zeolite model because the two O atoms reforms O-O bond during the optimization, due to the fact that two Cu-O are too close to each other. It means that the formation of two Cu-O species requires simultaneous separation of the two Cu sites, i.e. diffusion of Cu-O to a site which is away from the other Cu-O. However, the diffusion of Cu-O requires a new Al T sites, since Cu-O does not bind to a Si T site. Thus, in order to describe the diffusion process, our model need to be revised and more Al T sites need to be added. It arises another question: how to identify the distribution of Al site. Moreover, in reality the distribution of Al is more like randomly in the framework. It indicates that an ensemble average, which can be achieved by e.g. quantum molecular dynamics or CarParrinello molecular dynamics in frameworks with different Al distributions, is a more suitable way to estimate the diffusion of Cu-O compared to a static DFT calculation with the diffusion barriers on a specific framework structure. While the explicit calculation of the formation mechanism of Cu-O-Cu from Cu-O-OCu is limited by our zeolite model, we note that Cu ions45 as well as lattice oxygen ions46 are able to diffuse within the zeolite framework. Judging from the length of the activation period, we assume that the system equilibrates, thus being mainly comprised of Cu-O-Cu as this motif is lower in energy than Cu-O-O-Cu. This has also been verified experimentally where a similar UV-vis feature developed at 22,200 cm-1 has been observed after both O2 and N2O activation.13

Table 1. List of energy difference between singlet and triplet spin state for key intermediates during methane selectively oxidation to methanol in Cu-MOR and the calculated bader charge Spin State


Bader Charge47

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System

Adsorbate

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Most stable state

∆Ea

Cuc 0.68, 0.70

Bare Cu pair

no adsorbate

singlet

151

Cu-O-Cu

no adsorbate

triplet

-40 b

O

H

CH3

1.03, 1.01

-0.75

0.94, 0.91

-1.13

0.59

0.13

-1.08

0.62

0.14

0.67

0.64

CH3+H(*)

triplet

N.A.

CH3(*)+H(*)

singlet

84

0.96, 0.80

CH3OH(*)

singlet

164

0.67, 0.70

-1.13

no adsorbate

triplet

-17

0.96, 0.97

-0.31, -0.29

CH3+H(*)

triplet

N.A.b

0.86, 0.91

-0.68, -0.39

0.62

0.15

CH3(*)+H(*)

singlet

23

0.90, 0.92

-0.69, -0.45

0.67

-0.04

OCH3(*)+OH(*) triplet -48 0.99, 1.00 a -1 ∆E defines as Etriplet – Esinglet, unit is kJ mol b Singlet state is not stable c Bader charge at ~0.7 was assigned to Cu(I) and ~0.9 was assigned to Cu(II).

-0.93, -1.00

0.64

0.50

Cu-O-O-Cu

Figure 3. Formation energy of Cu-O-Cu and Cu-O-O-Cu as a function of temperature in O2 environment. The reference to calculate the formation energy is un-oxidized Cu-MOR and xO2(g) (x = 0.5 or 1, for Cu-O-Cu or Cu-O-O-Cu respectively) at a standard pressure of O2. The formation energies are calculated according to: ∆

. The shaded pink and blue areas correspond to the standard deviations obtained

through the ensemble error estimations.

3.2. Methane activation by Cu2Ox (x = 1, 2) At lower temperature, e.g. in the range of 323-473 K,13 methane can be selectively oxidized


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to methanol by activated Cu-MOR. The following is a detailed discussion of the elementary steps of the oxidation process using Cu-O-Cu and Cu-O-O-Cu as the active site motif.

Elementary steps on Cu-O-Cu: We calculated methane oxidation to methanol using Cu-O-Cu as the active site. The reaction sequence starts with the dissociation of a C-H bond of methane requiring a free energy barrier of 99 kJ mol-1 at 398 K (Figure 4). This activation barrier is comparable to the value reported in the literature (108 kJ mol-1, assuming the same entropy correction (+31 kJ mol-1) is applied).17 The resulting CH3 corresponds to a radical type planar intermediate that is stabilized by the newly formed OH group, (this stabilization amounts to 201 kJ mol-1 when compared to the free CH3 radical in a 12MR of MOR) rendering this reaction step endothermic by 76 kJ mol-1 at 398K. The CH3 radical can then react further forming a C-O bond with the OH group in Cu-OH-Cu, thus directly forming the final product CH3OH. This reaction step is strongly exothermic (∆G398K = -96 kJ mol-1) with a rather low barrier of 10 kJ mol-1 (Figure 4). Alternatively, the CH3 radical can also migrate to one of the Cu atoms (Figure 4) forming a Cu-CH3 bond. This step is also exothermic (∆G398K = -78 kJ mol-1) and essentially barrierless. Subsequently, the Cu bound methyl group can react with the OH group forming CH3OH, via a 79 kJ mol-1 kinetic barrier. The formed methanol is strongly adsorbed onto the copper cluster and we will discuss the final desorption step later on.

Figure 4. Free energy profile and corresponding intermediates and transition state structures of methane conversion to methanol catalyzed by Cu-O-Cu within the mordenite framework.


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The red line corresponds to the direct formation of methanol from CH3 and OH. All energies are relative to gas-phase methane and the Cu-O-Cu motif. Error bars are obtained using the BEEF-vdW ensemble of exchange-correlation functionals. Spin states of the intermediates and transition states are: CH3+H*: triplet, CH3*+H*: singlet; CH3OH*: singlet; TS@99 triplet; TS@79 triplet; TS@86 singlet; TS@88 singlet.

From the free energy diagram presented in Figure 4 one can deduce that methane activation is the most difficult step of the reaction sequence, apart from the final release of methanol as will be discussed later on. We calculate the forward rate of this reaction using:

with

(1)

being the rate constant of the forward reaction, the methane pressure and

the number of available sites (Cu-O-Cu) where methane can be activated. The forward rate constant is:

(

∆#$%&' " )* + ℎ

(2)

with kB being the Boltzmann constant, h the Plank constant, T the reaction temperature and ∆, the free energy barrier to activate C-H bond in methane. Using the activation free

barrier of 99 kJ mol-1 we get a reaction rate of 0.79 s-1 per -.- at 398 K. We used the BEEF error ensemble to estimate how the uncertainty of our calculations of the activation barrier translates to uncertainties in reaction rates. We estimate the lower bound of this rate to 0.004 s-1 using this method (see SI for further details) and conclude that this rate is sufficiently high facilitate the conversion of methane at appreciable rates. In addition, we calculated that Cu-O-Cu is lower in energy than Cu-O-O-Cu, thus leading to being close to one at the start of the reaction for Cu-O-Cu (see Figure 3).

Elementary steps on Cu-O-O-Cu: We calculate the C-H activation step on Cu-O-O-Cu to be similar to that discussed above for Cu-O-Cu. Although several different final states after C-H bond dissociation have been included in our calculations (See supporting information for details), all reaction path proceed through the same intermediate, i.e. the formation of a Cu-O-OH-Cu and a CH3 radical that is stabilized by the hydrogen of OH. Most importantly, we find this structure to be highly unstable (∆G398K = +168 kJ mol-1, see Figure 5). As a comparison, the kinetic barrier for methane activation on Cu-O-Cu is only 99 kJ mol-1. While we were not able to locate the transition state, we point out that the high ACS Paragon Plus Environment

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energy of this reaction step renders this reaction highly unlikely at temperatures around 398 K as can be seen by the reaction rate. Using equations 1 and 2 with site = CuOOCu and applying the lower bound of the

kinetic barrier which is the reaction energy, -..- results in 7.7 × 10-10 s-1. Even with the uncertainty associated with DFT we calculate 6.7 × 10-7 s-1 as an upper bound (see SI for details). Moreover, we note that -..- is very small as most of the active sites consist as Cu-O-Cu (see Figure 3).

Figure 5 Free energy profile and corresponding intermediates during CH4 activation catalyzed by Cu-O-O-Cu. Spin states of the intermediates and transition states are: CH3+H*triplet; CH3*+H*-singlet; OCH3*+OH*-triplet


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3.3. CH3OH desorption As shown above, methane is easily converted to adsorbed methanol on the Cu-O-Cu moiety. Adsorption of CH3OH, however, is quite strong so that it cannot desorb thermally from Cu-O-Cu, as an increased reaction temperature leads to its further oxidation to CO2.15 Our DFT results confirm this finding and we obtain an adsorption free energy of -50 kJ mol-1 at 398 K. Note that this corresponds to an adsorption enthalpy of -103 kJ mol-1. The strong binding is also reflected by a rather short Cu-O bond distance of 1.92 Å. Since thermal desorption can only be facilitated at prohibitively high temperatures, methanol extraction from the catalytic system is achieved through steaming with H2O. DFT calculations of this process are shown in Figure 6.

60 60

*

50

∆G398K (kJ/mol)

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H2O*

40 40

41 -CH3OH

2020

00

-CH3OH

CH3OH*

+H2O

-8 CH3OH* +H2O*

-20 -20

+H2O -CH3OH

2H2O*

-4

Reaction Coordinate

Figure 6. Removal of adsorbed methanol from Cu-MOR with and without water. The diagram shows free energies at 398 K and a standard pressure of 1 bar of both, methanol and water. All intermediates are singlet state.

As can be seen, desorption of methanol from the Cu-MOR is uphill in free energy by 50 kJ mol-1 even at temperatures of 398 K. This endothermicity corresponds to a pressure of methanol of 1 bar, and methanol desorption would be thermoneutral at a methanol pressure of 2.8×10-7 bar. Co-adsorption of water is slightly exothermic by -8 kJ mol-1 (-16 kJ mol-1 for adsorption of water on a bare copper site) and reduces the desorption energy of methanol slightly (see red line in Figure 6). Exchange of methanol with a second water molecule is


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only slightly uphill making the entire desorption process downhill by -4 kJ mol-1 (blue line in Figure 6). Figure 6 hence shows how methanol desorption can be facilitated through flushing the catalytic system with water or steam.

3.4. Other location of active sites Our detailed discussion of methane oxidation to methanol discussed in sections 3.1 – 3.3 is based on the T1-T4 site as the active site for copper within mordenite. There are, however, multiple locations where Cu-O-Cu cluster can be formed. In the following we will investigate these locations and compare how they differ from T1-T4 using the activation of methane as an example. Two possible locations of the active Cu-O-Cu cluster have been suggested by Vaneldern et al.13 The Cu-O-Cu cluster can either be attached to two T4 sites separated by two T2 sites in the 12MR, or by two T3 sites separated by two T1 sites at the intersection of the side pocket within the 8MR. These assumptions were based on two following facts: (1) Al is substituted preferentially at the T3 and T4 sites, e.g. ca. 30-40 % occupancy by Al for Si/Al = 5;13 (2) similar active sites were suggested based on theoretical studies for Cu-ZSM520,43 and Cu-MOR.34 Thus, we considered 5 additional active sites fulfilling the criteria described above, i.e. two Al T sites separated by two Si sites. In the following, we will use the notation “Ta-Tb” to indicate the two Al T sites hosting the Cu-O-Cu cluster. Our investigation includes T3-T3 and T4-T4 sites (suggested by Vaneldern et al.13), as well as other three possibilities in the 12MR, including T1-T1, T2-T2 and T2-T4. We only considered reaction path via triplet state according to our T1-T4 results discussed above. The most feasible location for Cu-OCu is T3-T3 with T2-T4, T4-T4 and T1-T4 being slightly less stable by 9-19 kJ mol-1 (see SI, Table S1). T1-T1 and T2-T2 are found to be 136 and 56 kJ mol-1 higher energy than Cu-OCu located on T3-T3 due to the change of the coordination mode between Cu and lattice O (see SI for details).


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Table 2. List of reaction energies and barriers (kJ mol-1) of first C-H dissociation step on Cu-O-Cu binds to various host sites. Initial state (reference) is gas phase CH4 and MOR with Cu-O-Cu. Sites host Pore size Cu-O-Cu T1-T4a 12MR T1-T4 12MR T1-T1 12MR T2-T2 12MR T2-T4 12MR T3-T3 8MR T4-T4 12MR a calculated with (1×1×1) cell b not available

Reaction free energy / kJ mol-1 76 65 46 43 50 149 52

Free energy barrier / kJ mol-1 99 N.A.b 124 63 66 N.A.b 90

Table 2 lists the reaction free energies of methane activation for the various sites investigated. Most of the reaction free energies are in a narrow range from 43 to 76 kJ mol-1, except the one occurring on the T3-T3 site. The latter is located in an 8MR that is too small to fit the CH3 radical. This leads to a geometry where the CH3 is attached to one of the Cu atoms, with the OH group pointing to the opposite direction of CH3, and both Cu atoms dislocated away from the framework (See SI for details). We have also considered the C-H activation barrier on other active sites. In the 12MR, both the reaction free energy and activation free barrier of this step (43-65 kJ mol-1 and 63124 kJ mol-1, respectively) are similar to T1-T4 (76 and 99 kJ mol-1, respectively, see Figure 4). However, a relatively higher barrier is expected on the T3-T3 site, due to the steric hindrance of the 8MR as discussed above. Although the exact location of the transition state is not determined, the endothermicity of this reaction, 149 kJ mol-1, constitutes the lower bound. It is important to note that this is substantially higher than the reaction barriers obtained for the other active sites.

4.

Conclusions We investigated the mechanism of methane selective oxidation to methanol over Cu-

MOR using density functional theory calculations. We considered Cu-O-Cu and Cu-OO-Cu as active site motifs and find that Cu-O-Cu is more likely to be present after the activation process based on thermodynamic considerations. In addition, our calculations suggest that methane activation is only feasible for Cu-O-Cu whereas Cu-OO-Cu has a low rate of ACS Paragon Plus Environment

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methane conversion. In fact, methane activation by Cu-O-Cu has a free energy barrier of only 99 kJ mol-1, which corresponds to a rate of 0.8 s-1. We considered various positions for the location of the Cu-O-Cu cluster and find similar barriers for methane activation in all cases except for the 8MR, which we believe is due to steric hindrance in this small pore. The formed CH3OH binds rather strongly to Cu-MOR leading to a desorption free energy of 50 kJ mol-1. This energy is reduced through co-adsorption of H2O so that methanol can be effectively removed through treatment with water.

Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments. We gratefully acknowledge the support from the U.S. Department of Energy, Office of Sciences, Office of Basic Energy Sciences to the SUNCAT Center for Interface Science and Catalysis. We also acknowledge generous computing resources, SUNCAT and Carbon High-Performance Computing Cluster, at SLAC National Accelerator Laboratory and Argonne National Laboratory, respectively.


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Reference 1

Hughes, L.; Rudolph, J. Curr. Opin. Env. Sust. 2011, 3, 225-234.

2

Sloan, E. D. Nature 2003, 426, 353-363.

3

Olah, G. A. Angew. Chem. Int. Ed. 2005, 44, 2636-2639.

4

van Bennekom, J. G.; Venderbosch, R. H.; Winkelman, J. G. M.; Wilbers, E.; Assink, D.; Lemmens, K. P. J.; Heeres, H. J. Chem. Eng. Sci. 2013, 87, 204-208.

5

Hammond, C.; Forde, M. M.; Ab Rahim, M. H.; Thetford, A.; He, Q.; Jenkins, R. L.; Dimitratos, N.; Lopez-Sanchez, J. A.; Dummer, N. F.; Murphy, D. M.; Carley, A. F.; Taylor, S. H.; Willock, D. J.; Stangland, E. E.; Kang, J.; Hagen, H.; Kiely, C. J.; Hutchings, G. J. Angew. Chem. Int. Ed. 2012, 51, 5129-5133.

6

Bordeaux, M.; Galarneau, A.; Drone, J. Angew. Chem. Int. Ed. 2012, 51, 10712-10723.

7

Vanelderen, P.; Vancauwenbergh, J.; Sels, B. F.; Schoonheydt, R. A. Coord. Chem. Rev. 2013, 257, 483-494.

8

Smeets, P. J.; Woertink, J. S.; Sels, B. F.; Solomon, E. I.; Schoonheydt, R. A. Inorg. Chem. 2010, 49, 3573-3583.

9

Vanelderen, P.; Hadt, R. G.; Smeets, P. J.; Solomon, E. I.; Schoonheydt, R. A.; Sels, B. F. J. Catal. 2011, 284, 157-164.

10

Alayon, E. M. C.; Nachtegaal, M.; Bodi, A.; Ranocchiari, M.; van Bokhoven, J. A. Phys. Chem. Chem. Phys. 2015, 17, 7681-7693.

11

Smeets, P. J.; Hadt, R. G.; Woertink, J. S.; Vanelderen, P.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I. J. Am. Chem. Soc. 2010, 132, 14736-14738.

12

Alayon, E. M. C.; Nachtegaal, M.; Bodi, A.; van Bokhoven, J. A. ACS Catal. 2014, 4, 16-22.

13

Vanelderen, P.; Snyder, B. E. R.; Tsai, M.-L.; Hadt, R. G.; Vancauwenbergh, J.; Coussens, O.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I. J. Am. Chem. Soc. 2015, 137, 6383-6392.

14

Borfecchia, E.; Lomachenko, K. A.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A. V.; Bordiga, S.; Lamberti, C. Chem. Sci. 2014, 6, 548-563.

15

Tomkins, P.; Mansouri, A.; Bozbag, S. E.; Krumeich, F.; Park, M. B.; Alayon, E. M. C.; Ranocchiari, M.; van Bokhoven, J. A. Angew. Chem. 2016, DOI: 10.1002/ange.201511065

16

Groothaert, M. H.; Smeets, P. J.; Sels, B. F.; Jacobs, P. A.; Schoonheydt, R. A. J. Am. Chem. Soc. 2005, 127, 1394-1395.

17

Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance, M. A.; Sels, B. F.; Schoonheydt, R. A.; Solomon, E. I. Proc. Natl. Acad. Sci. 2009, 106, 18908-18913.

18

Balasubramanian, R.; Smith, S. M.; Rawat, S.; Yatsunyk, L. A.; Stemmler, T. L.; Rosenzweig, A. C. Nature 2010, 465, 115-119.

19

Balasubramanian, R.; Rosenzweig, A. C. Acc. Chem. Res. 2007, 40, 573-580.

20

Goodman, B. R.; Hass, K. C.; Schneider, W. F.; Adams, J. B. J. Phys. Chem. B 1999, 103, 10452–10460.

21

Ross P. K.; Solomon E. I. J. Am. Chem. Soc. 1990, 112, 5871-5872.


16

Page 17 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

22

Tsai, M.-L.; Hadt, R. G.; Vanelderen, P.; Sels, B. F.; Schoonheydt, R. A.; Solomon, E. I. J. Am. Chem. Soc. 2014, 136, 3522-3529.

23

Li, G.; Vassilev, P.; Sanchez-Sanchez, M.; Lercher, J. A.; Hensen, E. J. M.; Pidko, E. A. J. Catal. 2016, 338, 305–312.

24

Grundner, S.; Markovits, M. A. C.; Li, G.; Tromp, M.; Pidko, E. A.; Hensen, E. J. M.; Jentys, A.; Sanchez-Sanchez, M.; Lercher, J. A. Nat. Commun. 2015, 6, 7546.

25

Haack, P.; Limberg, C. Angew. Chem. Int. Ed. 2014, 53, 4282-4293.

26

Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251-14269.

27

Kresse, G.; Furthmüller, J. Comp. Mat. Sci. 1996, 6, 15-50.

28

Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244-13249.

29

Brogaard, R. Y.; Moses, P. G.; Nørskov, J. K. Catal. Lett. 2012, 142, 1057-1060.

30

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.

31

Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104.

32

Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E. J. Chem. Phys. 2006, 125, 224106.

33

Blöchl, P. E. Phys. Rev. B 1994, 50, 17953-17979.

34

Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758-1775.

35

Henkelman, G.; Uberuaga, B. P.; Jónsson, H. J. Chem. Phys. 2000, 113, 9901-9904.

36

McWuarrie, D. A.; Simon, J. D. Molecular Thermodynamics, Edwards Brothers, Inc. 1999.

37

Brogaard, R. Y.; Wang, C.-M.; Studt, F. ACS Catal. 2014, 4, 4504-4509.

38

Piccini, G. M.; Sauer, J. J. Chem. Theory Comput. 2013, 9, 5038

39

Piccini, G. M.; Sauer, J. J. Chem. Theory Comput. 2014, 10, 2479.

40

Bucko, T.; Hafner, J. J. Catal. 2015, 329, 32.

41

Medford, A. J.; Wellendorff, J.; Vojvodic, A.; Studt, F.; Abild-Pedersen, F.; Jacobsen, K. W.; Bligaard, T.; Nørskov, J. K. Science 2014, 345, 197-200.

42

Baerlocher Ch.; McCusker, L. B. Database of Zeolite Structures: http://www.izastructure.org/databases/

43

Vilella, L.; Studt, F. Eur. J. Inorg. Chem. 2016, 2016, 1514-1520.

44

Schröder, D.; Shaik, S.; Schwarz, H. Acc. Chem. Res. 2000, 33, 139-145.

45 46

Kwak, J. H.; Varga, T.; Peden, C. H. F.; Gao, F.; Hanson, J. C.; Szanyi, J. J. Catal. 2014, 314, 83-93. Valyon, J.; Millman, W. S.; Hall, W. K. Catal. Lett. 1994, 24, 215-225.

47

Tang, W.; Sanville, E.; Henkelman G. J. Phys.: Condens. Matter 2009, 21, 084204.


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Theoretical Insights into the Selective Oxidation of Methane to

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