Monocopper Active Site for Partial Methane Oxidation in Cu

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Mono-Copper Active Site for Partial Methane Oxidation in Cu-exchanged 8MR Zeolites Ambarish R. Kulkarni, Zhi-Jian Zhao, Samira Siahrostami, Jens K. Norskov, and Felix Studt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01895 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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Mono-Copper Active Site for Partial Methane Oxidation in Cu-Exchanged 8MR Zeolites Ambarish R. Kulkarni,a,b Zhi-Jian Zhao,a,b,c Samira Siahrostami,a Jens K Nørskov,a,b Felix Studt,b,d,e*

a

SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford University, 450 Serra Mall Stanford, California 94305, USA

b

SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States c

Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative

Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China d

Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Hermannvon-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

e

Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Engesserstr. 18, 76131 Karlsruhe, Germany

E-mail: [email protected]

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Abstract Direct conversion of methane to methanol using oxygen is experiencing renewed interest owing to the availability of new natural gas resources. Copper-exchanged zeolites such as Mordenite and ZSM-5 have shown encouraging results, and di- and tri-copper species have been suggested as active sites. Recently, small 8 membered ring (8MR) zeolites including SSZ-13, -16 and -39 have been shown to be active for methane oxidation, but the active sites and reaction mechanisms in these 8MR zeolites are not known [Wulfers et al., Chem. Commun. 51 2015]. In this work, we use Density Functional Theory (DFT) calculations to systematically evaluate mono-copper species as active sites for the partial methane oxidation reaction in Cu-exchanged SSZ-13. Based on kinetic and thermodynamic arguments, we suggest that [CuIIOH]+ species in the 8MR are responsible for the experimentally observed activity. Our results successfully explain the available spectroscopic data and experimental observations including (i) necessity of water for methanol extraction and (ii) effect of Si/Al ratio on the catalyst activity. Mono-copper species have not yet been suggested as an active site for the partial methane oxidation reaction, and our results suggest that [CuIIOH]+ active site may provide complementary routes for methane activation in zeolites in addition to the known [Cu-O-Cu]+ and Cu3O3 motifs. Keywords: partial methane oxidation, [CuOH]+, density functional theory, zeolites, methanol


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1. INTRODUCTION A process that would enable the direct conversion of methane to methanol using O2 would have significant implications for the energy sector as well as the chemical industry. The main difficulty in realizing such a process lies in finding materials that are sufficiently reactive to activate the stable C-H bond of CH4, while at the same time preventing its overoxidation to CO and CO2. Inspired by the high activity and selectivity of the enzyme methane mono-oxygenase (MMO),1 heterogeneous catalytic systems consisting of isolated Cu or Fe- sites are being actively explored.2-28 Initial studies using zeolites have focused on using N2O or H2O2 for activation.2-3, 13-15 However, since the large-scale use of N2O is commercially infeasible, synthesis routes based on O2 are moving into focus. In their seminal work, Groothaert et al.4 examined a Cu-exchanged ZSM-5 and MOR for methane partial oxidation using O2.5, 8, 18, 29 Despite the differences in zeolite topologies, many similarities are prevalent, perhaps the most important being a peak close to 22,700 cm-1 observed in UV-vis spectra.4 Based on spectroscopic measurements, kinetic data and computational analysis, this feature was assigned to the μ-oxo dicopper ([Cu-O-Cu]+2) species for ZSM-5 and MOR zeolites.5-6, 14. Limitations of these materials are related to the low concentration of the active sites as well as the harsh activation protocol that is required when O2 is used as the oxidant.7, 16 One way of improving the overall performance of zeolites is to increase the concentration of the active sites. Encouraging results by Grunder et al.11, 30 have shown a significantly improved gravimetric activity of Cu-MOR where, in contrast with the previous studies, the majority of the Cu cations form a Cu3O3 cluster that has been shown to be active for methane activation. Interestingly, recent computational results suggest similarities in the methane activation mechanism for both Cu-O-Cu and Cu3O3 species.12, 31

Previously, Wulfers et al.17 have successfully synthesized a number of small 8membered ring (MR) zeolites including SSZ-13 (CHA), SAPO-34 (CHA), SSZ-39 (AEI) and SSZ-16 (AFX) that are active for methane selective oxidation. More recent report by Narsimhan et al. has shown catalytic methane conversion to methanol in many Cuexchanged zeolites including SSZ-13.32 Unlike the large-pore 12MR zeolites mentioned previously, the characteristic peak corresponding to the [Cu-O-Cu]+2 species is not


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observed and the nature of the active site for methane oxidation in 8MR zeolites is not known.17 Herein, we use Density Functional Theory (DFT) calculations to systematically study the partial methane oxidation reaction for copper containing zeolites. Various copper clusters have been investigated experimentally so far,8, 11, 16 with di and tri-nuclear copper clusters having been subject to detailed theoretical studies.11-12, 31 Herein we focus solely on zeolites containing only one copper per active site motif. Using Cu-exchanged, low Al content CHA as our initial model system, we identify [CuIIOH]+ located in the 8MR as the active site. Similar thermodynamic and kinetic arguments are then used to explain the effect of Si/Al ratio on the catalyst reactivity. Importantly, this analysis of mono-copper actives sites in zeolites opens the possibility for other small 8MR zeolites as potential catalysts for methane oxidation.

2. RESULTS AND DISCUSSION The BEEF-vdW functional is employed throughout this contribution (details are given in the Method section).33 This Generalized Gradient Approximation (GGA) functional has been shown to quantitatively describe van der Waals interactions of molecules within zeolite pores and has also yielded accurate reaction kinetics.34-35 DFT functionals of GGA flavor, however, are frequently prone to self-interaction errors for molecular species such as O2 so that inclusion of exact exchange may be necessary.36-37 We have previously addressed this issue by comparing BEEF-vdW to HSE06/D338-39 for reactions involving O2 and found BEEF-vdW to be sufficiently accurate.31 We start by focusing on a low Al content (Si/Al = 11) CHA zeolite that has a similar Si/Al ratio as the sample studied by Wulfers et al.17 (experimental Si/Al = 12). Assuming a uniform distribution of Al atoms in the experimental sample,40-41 the low Cu/Al exchange ratio (~ 0.35) suggests that formation of di-copper or tri-copper species is unlikely.42 Although the Cu-speciation will depend on the synthesis conditions, this assumption is supported by the absence of [Cu-O-Cu]+2 signature peaks in the UV-vis measurements.17 Thus, we limit our discussion to single copper active sites as candidates for the methane activation reaction.


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The CHA structure consists of hexagonal prisms that are interconnected to form a 3D pore network of 4-, 6- and 8-membered rings (MR). Due to its importance in the NH3selective catalytic reduction (SCR), the nature of Cu species under activation and reaction conditions has been widely studied. 43-47 As shown in Figure S1, three sites are commonly observed for extra-framework cations in CHA, i.e. (i) the center of 6MR, (ii) the corner of 8MR and (iii) the center of 4MR. Previous experiments and calculations have shown the 6MR (SII) to be preferred location for bare Cu ions (Figure 1(a)),48-49 while some reports indicate 8MR to be populated by [Cu-OH]+ species under O2 activation conditions (Figure 1(b)).44-45, 47 Recent results have shown that CHA with predominantly [Cu-OH]+ species can be obtained for isolated Al sites by controlling the crystallization process.41 In agreement with previous experimental and computational studies,50 our DFT calculations show that the location of bare copper ions is energetically favorable in 6MR compared to the 8MR (+26 kJ/mol) and 4MR (+50 kJ/mol). Importantly, however, [CuIIOH]+ species are found to be slightly more stable when located in the 8MR (by 5 kJ/mol more than 6MR) as will be discussed later.

(a)

(b)

Figure 1. Location of extra-framework cations (a) in the 6MR as bare-Cu and (b) in 8MR as [Cu-OH]+. Colors scheme: Si (yellow), O (red), C (grey), H (white), Cu (brown) and Al (pink).

2. 1 C-H Bond Activation by Mono-Copper Species We start our analysis by calculating methane activation for three different active site motifs, [CuOO]+, [CuO]+ and [CuOH]+. For all three cases, a methyl-radical like transition state is observed with C-H bond distance of 1.3 – 1.5 Å. The radical nature of


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the transition state is confirmed by analyzing the carbon-projected density of states (PDOS) and the spin-densities (Figure S2-S4). We calculate the free energy barrier of C-H abstraction at 423 K to be 155, 49 and 110 kJ/mol for [CuOO]+, [CuO]+ and [CuOH]+, respectively, as shown in Figure 2. Using these barriers, the forward rate constants of methane activation for [CuOO]+, [CuO]+ and [CuOH]+ are calculated as 6.3×10-7 s-1, 9×106 s-1 and 0.23 s-1 respectively (using transition state theory, 𝑘𝐵 𝑇 ℎ

exp (

∆𝐺

𝑘𝐵 𝑇

𝑘𝑓𝑜𝑟𝑤𝑎𝑟𝑑 =

))31. Our analysis thus renders C-H activation by [CuOO]+ implausible even

when the errors associated with DFT are considered (using the Bayesian Error Estimation approach51 we calculate rates in the range 1.8×10-4 to 3.1×10-9 s-1 for [CuOO]+). These results are consistent with the high barrier (167 kJ/mol) calculated for [Cu-OO] active sites in Lytic polysaccharide monooxygenaes enzymes.52 (d) (a)

(b) 155 kJ/mol T = 423 K

Cu-OO

Cu-O

110 kJ/mol

(c) physisorbed CH4 CH4(g)

Cu-OH

49 kJ/mol CH3 radical

Figure 2. Transition state geometries for (a) [Cu-OO]+, (b) [Cu-O]+ and (c) [Cu-OH]+ active sites in CHA. (d) The calculated transition state energies and Bayesian Error Estimates for the 3 active sites considered in this work.

It is encouraging to note that our C-H activation barriers for [Cu-OH]+ (110 kJ/mol) are in good agreement with the apparent activation energies (100 kJ/mol) reported by Narsimhan et al.32 Further, the calculated barriers for [CuOH]+ compare reasonably well with previous periodic DFT results for [Cu-O-Cu]+2/ZSM-5 (93 kJ/mol, PBE)12 and [CuO-Cu]+2/MOR (99 kJ/mol, BEEF-vdW)31. Interestingly, C-H activation barriers for [Cu-


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O-Cu]+2 and [Cu-OH]+ motifs are higher than the calculated values for [Cu3O3] species in ZSM-5 (54 kJ/mol, PBE)12 and MOR (74 kJ/mol, PBE)11 indicating that tri-copper species are more active for the C-H bond cleavage step.12 Although the results for different DFT functionals cannot be directly compared, we obtain similar barriers using the PBE (47 kJ/mol), BEEF (49 kJ/mol) and RPBE (57 kJ/mol) functionals for [CuO]+ species. 2.2 Thermodynamic Analysis of O2 Activation Having excluded [CuOO]+ as a possible active site, we continue by calculating the initial distribution of the various active site motifs after activation by O2. Experimentally, the temperature is gradually increased from RT to 450 °C and is accompanied by simultaneous removal of water.17 Although it is likely that kinetics are important during the activation process, as a first approximation, we can assume the system to be at equilibrium at the activation temperature. Figure 3 shows the Gibbs formation energies of various Cu species, including [CuO]+ and [CuOH]+, for different activation temperatures calculated according to Equation 1 𝑥

𝑦

𝑦

2

4

2

∆𝐺 = 𝐺𝑓 (𝐶𝑢𝑂𝑥 𝐻𝑦 ) − 𝐺𝑓 (𝐶𝑢) − ( − ) 𝐺(𝑂2 ) − 𝐺(𝐻2 𝑂)

(1)

where, 𝐺𝑓 (𝐶𝑢𝑂𝑥 𝐻𝑦 ) and 𝐺𝑓 (𝐶𝑢) refer to the free energies of the Cu species and the bare Cu site in CHA respectively. Similar approaches have been previously used for determining Cu-speciation in zeolites.11-12, 42


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Figure 3. Gibbs formation energies, ∆G of different species in 6MR (dashed lines) and 8MR (solid lines) at different temperatures for 5% H2O (balance O2, total pressure = 1 bar) in Cu-exchanged CHA. The purple shaded area estimates the errors in DFT energies calculated using the Bayesian Error Estimation approach.51

Figure 3 includes the stability of different possible adsorbates both in the 6MR and 8MR of CHA. Under these conditions, 8MR-Cu-H2O is most stable for the entire temperature range considered. At 450 °C, the temperature used to activate the samples, the stability follows the order 8MR-Cu-H2O > 6MR-Cu > 8MR-CuOH, with 6MR-Cu and 8MR-CuOH being 4.5 and 8 kJ/mol less stable than 8MR-Cu-H2O. Importantly, both 6MR-CuO and 8MR-CuO are much less stable (by 74 and 61 kJ/mol compared to 8MR-CuOH). Similar relative stabilities have been recently reported by Paolucci et al.42 using the HSE06TSvdw53 functional for CHA. The Boltzmann distribution for the various active site motifs calculated using the Gibbs formation energies at the activation temperature as a first approximation are given in Table 1. Table 1. Relative populations of different species in 6MR and 8MR at 450 °C for 5% H2O (balance O2, P = 1 bar) in CHA. The Boltzmann population of the active species, 8MR-[CuOH]+ has been highlighted for emphasis.


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Bare-Cu Cu-H2O Cu-OH Cu-O Cu-OO Cu-OOH

6MR 0.33 0.02 0.01 ~ 10-7 ~ 10-4 ~ 10-9

8MR 1.2×10-3 0.53 0.11 ~ 10-6 ~ 10-5 ~ 10-7

For a temperature of 450 °C and 5% water partial pressure, our equilibrium analysis predicts 53% of copper species to be 8MR-Cu-H2O, 33% 6MR-Cu, and 11% 8MR-Cu-OH with the remainder being distributed among all other copper species. As the distribution of various species will depend on the water content, we also examine the relative populations at other water partial pressures (Figure S5-S6). Consistent with previous experiments,42, 47 at a lower water content of 0.5%, the proportion of 8MR-CuOH remains almost the same (8%), but the 6MR-Cu increases significantly (76 %). Most notably, we calculate that there are only very few copper species of type 6MR-CuO and 8MR-CuO (< 10-4 %). Our results thus indicate the [CuO]+ will only exist in negligible amounts, and would not contribute to the experimentally observed activity of Cu-CHA. On the other hand, [CuOH]+, is calculated to be present in small quantity (< 11 %) over the entire temperature range and water partial pressures. Interestingly, this coincides with the experimental observations that only 3-9% of copper atoms in Cu-CHA are actively participating in the activation of methane.17 The presence of [CuOH]+ species after O2 activation has been previously reported using spectroscopic measurements. FTIR experiments for Cu-exchanged CHA have observed an increase in intensity of a peak at 3656 cm-1 with increasing temperature during activation, which was assigned to the Cu-OH stretching mode.47 Our calculated frequency for 8MR-[CuOH]+ of 3709 cm-1 is in reasonable agreement with the experimental value. (The vibrational frequency is affected by the presence of other Al atoms and is discussed later.) The recent work by Paolucci et al.42 suggests the formation of 8MR-[CuOH]+ species for isolated Al atoms in CHA using a combined XAS, FTIR, chemical titration and DFT calculations. Even though our analysis is based on purely thermodynamic arguments we observe that there is generally good agreement between experiments and theory, both pointing towards [CuOH]+ as the active species.


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As the formation energies of bare-Cu, Cu-H2O and Cu-OH in Figure 3 are all within ~10 kJ/mol, the distributions calculated in Table 1 are very sensitive to the relative DFT adsorption energies. We therefore performed a detailed DFT error propagation analysis using the BEEF ensemble of exchange correlation functionals51 (see SI) and concluded that while the exact species distributions are sensitive to the DFT energies, our overall conclusions remain unchanged. Thus, we propose that 8MR-[CuOH]+ is the active species responsible for the experimentally observed methane activation. These results are especially interesting, as, mono-copper species have not been previously suggested as active sites for methane activation in zeolites and complement the previously known [CuO-Cu]+2 and [Cu3O3]+2 active sites.8, 11

Scheme 1. Reaction scheme for partial methane oxidation to methanol for 8MR-[CuOH]+ active site in Cuexchanged CHA. The notation Z2Cu(Hb) refers to addition of a Brønsted hydrogen to an adjacent 2Al/Cu(II) site (Z2Cu).42

2.3 Reaction Mechanism for 8MR-[CuOH]+ The reaction scheme and free energy diagram for partial methane oxidation for 8MR-[CuOH]+ active site is shown in Scheme 1 and Figure 4. (Free energy diagrams for CuO and CuOO as the active site are shown in the SI). As discussed previously, the process starts by C-H bond cleavage of CH4 via a 110 kJ/mol free energy barrier yielding Cu-H2O and a CH3 radical that preferably binds to the Cu atom (Figure 4) rather than the zeolite framework (unfavorable by ~ 173 kJ/mol). Although, a direct insertion of the CH3 radical into the H2O molecule is possible, the high activation energy (175 kJ/mol, Figure S10) indicates this path to be unlikely. In contrast, the addition of CH3 radical to the Cu atom has a negligible barrier and [Cu-H2O-CH3]+ is easily formed (denoted as 1, Scheme 1).


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Once bound to Cu, further transfer of CH3 to form methanol has a high barrier and is infeasible. Thus, our results show that after methane activation, the Cu atoms probably exist as [Cu-H2O-CH3]+ species in the 8MR. As addition of water / steam is required for methanol extraction experimentally, we explored the possibility that methanol formation may be mediated by additional water molecules that are available during the water regeneration phase. Recognizing the high mobility of hydrated (or ammoniated) Cu species during O2 activation and NH3 SCR reaction,45,

54-55

we performed ab-intio molecular

dynamics (AIMD) simulations with 0, 1 and 2 additional water molecules (Figure S11). When extra water molecules are not added, our AIMD results show that [Cu-H2O-CH3]+ remains attached to the framework and has low mobility. However, when the Cu atom is hydrated by one extra water molecule, [Cu-2(H2O)-CH3]+ (denoted as 2, Scheme 1) detaches from the framework and is free to diffuse in the zeolite framework. This is further evidenced by geometry optimizations, which show that [Cu-2(H2O)-CH3]+ species is ~ 22.6 kJ/mol more favorable in the zeolite pore than attached to the framework Al. Here, we use the harmonic approximation to calculate the free energies of the “mobile” species, and we acknowledge that entropy contributions are underestimated. However, as various degrees of freedom available for the solvated species, we do not consider other possible configurations within the CHA framework (see SI).

Figure 4. Free energy diagram for partial methane oxidation to methanol for 8MR-[CuOH]+ active site in Cu-exchanged CHA. The direct insertion of CH3 radical into Cu-H2O has barrier of 175 kJ/mol and is unfavorable (red curve). The Cu-H2O-CH3 species is formed (1) that remains attached to the framework.


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During the methanol generation phase, (1) is hydrated to form the mobile Cu-2H2O-CH3 species (2) that can diffuse in the CHA pore. The final step involves the reaction of 2 to form an adjacent Brønsted hydrogen and methanol, which is subsequently released. The notation Z2Cu(Hb) refers to addition of a Brønsted hydrogen to an adjacent 2Al/Cu(II) site (Z2Cu).42 Further details are available in Table S2.

Given the possibility for the diffusion of the hydrated Cu species, we explore another route for methanol formation, where one H atom from mobile Cu-hydrated species reacts to from an adjacent Brønsted site. Based on the energy diagram, this pathway is significantly more favorable. The actual mechanism for this transfer will depend on the distribution of Al sites in the zeolite sample, and thus a complete kinetic analysis of the various solvated species is beyond the scope of this work.

2.4 Effect of Si/Al Ratio Having established the nature of the active site for methane activation in SSZ-13, we now consider the experimentally observed lower activity of the Si/Al = 6 sample.17 To explore the effect of Al distribution, we now extend our results to the CHA model system with 2Al atoms/unit-cell (Si/Al = 5). Unlike the previous case of 1Al/unit-cell (equivalent T atoms), the position of the 2nd Al atom is not uniquely determined. To comprehensively evaluate the reaction in CHA, we considered all 7 possibilities for 2nd Al substitution that do not violate Lowenstein’s rule, and examined the reaction in 6MR and 8MR for each configuration. For each Al configuration, only the most favorable position of the Brønsted hydrogen was considered for determining the transition state. Our analysis (see Supporting Information) suggests that when 2 Al atoms are in close proximity, free energy barriers of 96 – 118 kJ/mol are obtained for [Cu-OH]+ active sites. For different Al positions, we calculate the vibrational frequency of the Cu-OH stretching mode as 3598 – 3717 cm-1, which is in good agreement with the experimental value (3656 cm-1).47These results indicate that the active site remains the same for the high Al (Si/Al = 6) SSZ-13 sample reported by Wulfers et al.17 Although the kinetics of C-H activation don’t change appreciably, the relative populations of Cu species are significantly different for Si/Al = 5. The statistical analysis of Bates et al.46 suggests that increasing the Al content in CHA increases the probability of


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forming two “paired” Al atoms in the 6MR. Our thermodynamic analysis shows that the paired Al atoms in 6MR strongly favor the formation of bare-Cu cations that are unreactive for C-H activation, and inhibit [CuOH]+ formation in 8MR. Specifically, we predict that < 5% of the Cu cations belong to the 8MR-[CuOH]+ species (see Supporting Information). Our results are consistent with the phase diagram of Paolucci et al.,42 which was based DFT calculations and statistical analysis of different Cu/Al and Si/Al samples. Thus, we conclude that the lower performance for CHA (Si/Al = 5, 6) is due to the smaller number of available [CuOH]+ active sites, and not due an inherent decrease in reactivity. These results explain the counter-intuitive experimental observation that increasing the number of Cu cations decreases the overall gravimetric activity of SSZ-13.17 3. CONCLUSIONS In this work, we presented a comprehensive analysis of various mono-copper species as potential active sites for partial methane oxidation in Cu-exchanged SSZ-13 zeolite. Using periodic Density Functional Theory calculations combined with a thermodynamic analysis of the O2 activation process, we conclude that [CuOH]+ in the 8 membered rings (MR) is responsible for the experimental activity of Cu-exchanged SSZ-13. Our proposed reaction mechanism is consistent with the spectroscopic data and experimental observations, and successfully explains (i) the necessity of hydration during methanol extraction step and (ii) the lower activity of high Al SSZ-13 sample. The effect of Al content and distribution is especially interesting and leads to a general design principle that 6MR are detrimental for the reaction, while 8MR are desirable and favor the formation of the active [CuOH]+ species. Mono-copper species have not yet been suggested as an active site for the partial methane oxidation reaction, and our results suggest that [Cu-OH]+ active site may provide complementary routes for methane activation in zeolites in addition to the known [Cu-OCu]+ and Cu3O3 motifs.8, 11 4. METHODS All calculations are performed using the BEEF-vdW functional as implemented in the periodic QuantumEspresso code. The initial zeolite structure is obtained from the IZA database (http://www.iza-structure.org/databases/) and the lattice constants are optimized


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at an energy cutoff of 700 eV. The optimized lattice constants (a = 9.334 Å and α = 94.54°) are in reasonable agreement with experiment. All further geometry optimizations and transition state calculations were performed using fixed unit cell size at a plane wave cutoff of 500 eV. Only the Γ-point was used to sample the reciprocal space. Forces on all atoms were converged to 0.03 eV Å -1. In our calculations, all possible spin multiplicities are considered to determine the ground state electronic configuration for each adsorbate (Table S3). The Bayesian Error estimates were calculated using the methodology of Medford et al.51 The transition state was determined by using the Climbing-Image Nudged Elastic Band (CI-NEB) method.56 Zero point energy corrections and entropy contributions at the harmonic approximation are used to calculate free energies at various temperatures. The AIMD simulations are performed using BEEF-vdW functional as implemented in the Vienna Ab-initio Simulation code57 with a time step of 0.5 fs at 300 K to obtain 30 ps trajectories that are further analyzed. SUPPORTING INFORMATION Details of DFT calculations, optimized structures, thermodynamics of Cu-speciation, Bayesian error analyses and effect of relative positions of Al atoms. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS 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 the computing resources, SUNCAT and Carbon HighPerformance Computing Cluster, at SLAC National Accelerator Laboratory and Argonne National Laboratory, respectively.


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REFERENCES 1. Solomon, E. I.; Sarangi, R.; Woertink, J. S.; Augustine, A. J.; Yoon, J.; Ghosh, S., Accounts of Chemical Research 2007, 40 (7), 581-591. 2. Panov, G. I.; Sobolev, V. I.; Dubkov, K. A.; Parmon, V. N.; Ovanesyan, N. S.; Shilov, A. E.; Shteinman, A. A., Reaction Kinetics and Catalysis Letters 1997, 61 (2), 251-258. 3. Dubkov, K. A.; Sobolev, V. I.; Panov, G. I., Kinetics and Catalysis 1998, 39 (1), 7279. 4. Groothaert, M. H.; Smeets, P. J.; Sels, B. F.; Jacobs, P. A.; Schoonheydt, R. A., Journal of the American Chemical Society 2005, 127 (5), 1394-1395. 5. Vanelderen, P.; Hadt, R. G.; Smeets, P. J.; Solomon, E. I.; Schoonheydt, R. A.; Sels, B. F., Journal of Catalysis 2011, 284 (2), 157-164. 6. Vanelderen, P.; Vancauwenbergh, J.; Tsai, M.-L.; Hadt, R. G.; Solomon, E. I.; Schoonheydt, R. A.; Sels, B. F., ChemPhysChem 2014, 15 (1), 91-99. 7. 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., Journal of the American Chemical Society 2015, 137 (19), 6383-6392. 8. Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance, M. A.; Sels, B. F.; Schoonheydt, R. A.; Solomon, E. I., Proceedings of the National Academy of Sciences of the United States of America 2009, 106 (45), 18908-18913. 9. Smeets, P. J.; Hadt, R. G.; Woertink, J. S.; Vanelderen, P.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I., Journal of the American Chemical Society 2010, 132 (42), 1473614738. 10. Li, G.; Pidko, E. A.; van Santen, R. A.; Feng, Z.; Li, C.; Hensen, E. J. M., Journal of Catalysis 2011, 284 (2), 194-206. 11. 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. 12. Li, G.; Vassilev, P.; Sanchez-Sanchez, M.; Lercher, J. A.; Hensen, E. J.; Pidko, E. A., Journal of Catalysis 2016, 338, 305-312. 13. Dubkov, K. A.; Sobolev, V. I.; Talsi, E. P.; Rodkin, M. A.; Watkins, N. H.; Shteinman, A. A.; Panov, G. I., Journal of Molecular Catalysis a-Chemical 1997, 123 (23), 155-161. 14. Alayon, E. M. C.; Nachtegaal, M.; Bodi, A.; van Bokhoven, J. A., ACS Catalysis 2013, 4 (1), 16-22. 15. Alayon, E. M. C.; Nachtegaal, M.; Bodi, A.; Ranocchiari, M.; van Bokhoven, J. A., Physical Chemistry Chemical Physics 2015, 17 (12), 7681-7693. 16. Tomkins, P.; Mansouri, A.; Bozbag, S. E.; Krumeich, F.; Park, M. B.; Alayon, E. M. C.; Ranocchiari, M.; van Bokhoven, J. A., Angewandte Chemie 2016, 128, 5557-5561. 17. Wulfers, M. J.; Teketel, S.; Ipek, B.; Lobo, R. F., Chemical Communications 2015, 51, 4447-4450. 18. Narsimhan, K.; Michaelis, V. K.; Mathies, G.; Gunther, W. R.; Griffin, R. G.; Román-Leshkov, Y., Journal of the American Chemical Society 2015, 137 (5), 18251832. 19. Smeets, P. J.; Woertink, J. S.; Sels, B. F.; Solomon, E. I.; Schoonheydt, R. A., Inorganic Chemistry 2010, 49 (8), 3573-3583.


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20. Olivos-Suarez, A. I.; Szécsényi, À.; Hensen, E. J. M.; Ruiz-Martinez, J.; Pidko, E. A.; Gascon, J., ACS Catalysis 2016, 6 (5), 2965-2981. 21. Bozbag, S. E.; Alayon, E. M. C.; Pechacek, J.; Nachtegaal, M.; Ranocchiari, M.; van Bokhoven, J. A., Catalysis Science & Technology 2016. 22. Xu, J.; Armstrong, R. D.; Shaw, G.; Dummer, N. F.; Freakley, S. J.; Taylor, S. H.; Hutchings, G. J., Catalysis Today 2016, 270, 93-100. 23. Hutchings, G. J., Topics in Catalysis 2016, 59 (8), 658-662. 24. Hammond, C.; Dimitratos, N.; Jenkins, R. L.; Lopez-Sanchez, J. A.; Kondrat, S. A.; Hasbi ab Rahim, M.; Forde, M. M.; Thetford, A.; Taylor, S. H.; Hagen, H.; Stangland, E. E.; Kang, J. H.; Moulijn, J. M.; Willock, D. J.; Hutchings, G. J., ACS Catalysis 2013, 3 (4), 689-699. 25. 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., Angewandte Chemie International Edition 2012, 51 (21), 5129-5133. 26. Hammond, C.; Jenkins, R. L.; Dimitratos, N.; Lopez-Sanchez, J. A.; ab Rahim, M. H.; Forde, M. M.; Thetford, A.; Murphy, D. M.; Hagen, H.; Stangland, E. E.; Moulijn, J. M.; Taylor, S. H.; Willock, D. J.; Hutchings, G. J., Chemistry – A European Journal 2012, 18 (49), 15735-15745. 27. Hammond, C.; Hermans, I.; Dimitratos, N., ChemCatChem 2015, 7 (3), 434-440. 28. Bozbag, S. E.; Alayon, E. M. C.; Pechacek, J.; Nachtegaal, M.; Ranocchiari, M.; van Bokhoven, J. A., Catalysis Science & Technology 2016, 6 (13), 5011-5022. 29. Beznis, N. V.; Weckhuysen, B. M.; Bitter, J. H., Catalysis Letters 2010, 138 (1-2), 14-22. 30. Grundner, S.; Luo, W.; Sanchez-Sanchez, M.; Lercher, J. A., Chemical Communications 2016, 52 (12), 2553-2556. 31. Zhao, Z.-J.; Kulkarni, A.; Vilella, L.; Nørskov, J. K.; Studt, F., ACS Catalysis 2016, 6 (6), 3760-3766. 32. Narsimhan, K.; Iyoki, K.; Dinh, K.; Román-Leshkov, Y., ACS Central Science 2016, 2 (6), 424-429. 33. Wellendorff, J.; Lundgaard, K. T.; Møgelhøj, A.; Petzold, V.; Landis, D. D.; Nørskov, J. K.; Bligaard, T.; Jacobsen, K. W., Physical Review B 2012, 85 (23), 235149. 34. Brogaard, R. Y.; Wang, C.-M.; Studt, F., ACS Catalysis 2014, 4 (12), 4504-4509. 35. Brogaard, R. Y.; Moses, P. G.; Nørskov, J. K., Catalysis Letters 2012, 142 (9), 1057-1060. 36. Cohen, A. J.; Mori-Sánchez, P.; Yang, W., Chemical Reviews 2012, 112 (1), 289320. 37. Becke, A. D., The Journal of Chemical Physics 1993, 98 (7), 5648-5652. 38. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., The Journal of Chemical Physics 2010, 132 (15), 154104. 39. Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E., The Journal of Chemical Physics 2006, 125 (22), 224106. 40. Paolucci, C.; Verma, A. A.; Bates, S. A.; Kispersky, V. F.; Miller, J. T.; Gounder, R.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F., Angewandte Chemie International Edition 2014, 53 (44), 11828-11833. 41. Di Iorio, J. R.; Gounder, R., Chemistry of Materials 2016, 28 (7), 2236-2247.


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TOC Graphic


18

Page (a)$ 19 of 24 1 2 3 4 5 6 7 8 9

ACS Catalysis (b)$


(a)

(d)

ACS Catalysis

(b)

Page 20 of 24 155 kJ/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14

T = 423 K

Cu-OO

Cu-O

110 kJ/mol

(c) physisorbed CH4

Cu-OH

ACS Paragon Plus Environment CH4(g)

49 kJ/mol CH3 radical

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ACS Catalysis


ACS Catalysis

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H

Page 23 of 24

H O CH 3ACS Cu

Catalysis

Direct insertion of CH 3 radical

1 H2 O 3 Cu

4 5 H CH3 6 7 8

H

O Cu

H

H H C• H

H H Water/ Steam H CH 3 O H O CH 3 H Cu Cu O H H Z2Cu 1 2

H

ACS Paragon Plus Environment Low Mobility

High Mobility

H O H O H CH 3 Cu

Z2Cu(H b)

O Cu

H H

O

CH 3

ACS Catalysis Page 24 of 24

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Monocopper Active Site for Partial Methane Oxidation in Cu

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