Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters

Article pubs.acs.org/JACS

Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters Stabilized in NU-1000 Metal−Organic Framework Takaaki Ikuno,†,⊗ Jian Zheng,‡,⊗ Aleksei Vjunov,‡,∇,⊗ Maricruz Sanchez-Sanchez,† Manuel A. Ortuño,# Dale R. Pahls,# John L. Fulton,‡ Donald M. Camaioni,‡ Zhanyong Li,§ Debmalya Ray,# B. Layla Mehdi,‡ Nigel D. Browning,‡,⊥ Omar K. Farha,§,∥ Joseph T. Hupp,§ Christopher J. Cramer,# Laura Gagliardi,# and Johannes A. Lercher*,†,‡ †

Department of Chemistry and Catalysis Research Institute, Technische Universität München, 85748 Garching, Germany Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States # Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, Minneapolis, Minnesota 55455, United States ⊥ Materials Science and Engineering, University of Washington, Seattle, Washington 98195, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal−organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/ activity relations in the Cu-NU-1000 catalytic system. The Culoaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ∼15% Cu+ and ∼85% Cu2+. The oxidation of methane on Cu-NU1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu2+ to Cu+. The products, methanol, dimethyl ether, and CO2, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45−60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

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INTRODUCTION

catalysts is the strong adsorption of methanol on the hydrophilic zeolite. Here, we describe the use of the NU-1000 metal−organic framework (MOF), 4 Zr6 (μ 3 -O) 4 (μ3 -OH) 4 (OH) 4 (OH 2 ) 4 (1,3,6,8-tetrakis(p-benzoate)pyrene)2, as a support (Figure 1) for Cu-oxo clusters. This MOF is chosen because it has welldefined Zr-based nodes that provide reactive sites for postsynthetic deposition of metal ions, e.g., through atomic layer deposition (ALD).4,5 While MOFs have previously been demonstrated to be active for a range of catalytic reactions, e.g., selective hydrogenation of 1-octene 6 and ethylene, 7 oxidative dehydrogenation of propane,8 oxidation of CO to CO2,9 oxidation of organic substrates,10 hydrodesulfurization reactions,11 and activation of ethane,12 their use as catalysts for partial oxidation of methane has not been attempted.

The catalyzed conversion of shale gas-derived light hydrocarbons, e.g., methane to methanol, for further application as automotive fuels and/or bulk chemicals is especially attractive in light of improved methods of hydrocarbon extraction.1 However, the lack of a transportation infrastructure or an efficient method to upgrade gas to liquid hydrocarbons at or near the well site often leads to flaring of large quantities of natural gas, causing both environmental damage and economic loss. While methane can be converted to methanol at very high temperatures either in the presence of water vapor or via a methane-syngas-methanol process,2 a direct methane oxidation pathway to methanol at low temperatures would have a substantial advantage over the existing routes. In the past decade, several groups have reported that the formation of Cu nanoclusters in the micropores of zeolites leads to catalysts that are active and selective for methane oxidation at moderate temperatures (150−200 °C).3 The main limitation of such © 2017 American Chemical Society

Received: March 24, 2017 Published: June 14, 2017 10294

DOI: 10.1021/jacs.7b02936 J. Am. Chem. Soc. 2017, 139, 10294−10301

Article

Journal of the American Chemical Society

Figure 2. HAADF-STEM images of the polycrystalline NU-1000 (a) and Cu-NU-1000 with the view of the a-axis. The bright areas in (a) represent (Zr6) nodes, and those in (b) represent the Cu-deposited (Zr6) nodes with the pores between them. (c) View of the parent NU1000 crystal structure looking down the a-axis. Blue, red, gray, and light gray spheres represent Zr, O, C, and H atoms, respectively.

Cu X-ray Absorption Near-Edge Structure (XANES) Analysis. Cu-XANES measurements were performed to determine the electronic structure of Cu in Cu-NU-1000 during the course of the reaction, i.e., ambient conditions, activation in O2, and methane loading. Figure 3 shows the normalized Cu-XANES spectra for Cu-NU-1000 as well as reference compounds, Cu2O, CuO, aqueous Cu2+, Cu(OH)2 and Cu foil.

Figure 1. Views of the NU-1000 crystal structure looking down the c axis (left) and a/b axes (right), with a blowup of the Zr6 node (Zr6(μ3O)4(μ3-OH)4(OH)4(OH2)4) on which the Cu is deposited.

We describe in this Article the synthesis, characterization, and application of Cu-NU-1000 as a direct methane-to-methanol oxidation catalyst. In addition to exploring the chemical reactivity, in situ X-ray absorption spectroscopy (XAS) under catalytic conditions has been used to follow the modification of the Cu species and directly probe the structure/activity properties of the Cu-NU-1000 system. The insights from this first investigation of a MOF material as a catalyst for methane oxidation lay the foundation for the development of subsequent generations of materials.

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RESULTS AND DISCUSSION Properties of Cu-NU-1000. The Cu-NU-1000 investigated here for methane oxidation to methanol was synthesized via atomic layer deposition in metal−organic frameworks (AIM). The synthesis and characterization of Cu-NU-1000 are detailed in the Experimental Section and Supporting Information (SI). The concentration of Cu was 10 wt %, corresponding to an average of approximately 4 Cu atoms per node. The crystal structure of Cu-NU-1000 was analyzed by powder X-ray diffraction (PXRD). The pattern (Figure S2) is like that of the pristine NU-1000, except for a loss of intensity of diffraction peaks at 2θ = 7−10°, corresponding to a decrease in the coherency of planes with spacing of 0.8−1.2 nm. Recently, a difference envelope density analysis of synchrotron-based X-ray scattering data has shown that ALD of Cu in NU-1000 leads to the formation of Cu-oxo clusters within the small pores that connect the triangular and hexagonal channels.13 Textural properties obtained by N2 sorption at 77 K (Figure S17 and Table S5), show that the surface area and pore volume decreased by ∼30%, consistent with the installation of Cu-oxo clusters and in line with observations for Zn- and Ni-NU1000.4,14 Figure 2 shows high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for pristine NU-1000 and Cu-NU-1000. The lattice sites of NU1000 are not changed by Cu deposition. The internode distances of approximately 1.7 nm of both samples agree well with the theoretical values of the parent NU-1000 with a view of looking down the a-axis (Figure 2c). These results show that the crystallinity and porosity of NU-1000 are largely retained after Cu deposition.

Figure 3. Normalized Cu-XANES spectra of the Cu-NU-1000 and references are shown. The inserts demonstrate the pre-edge feature at 8977−8978 eV corresponding to the 1s→3d electronic transition for the distorted symmetry of Cu2+ as well as the 1s→4p transitions for Cu+ and Cu2+ at 8982−8984 and 8985.5 eV, respectively. The colorcoding is reported in the legend.

Cu-NU-1000 most resembles the structure of Cu in Cu(OH)2. There are several important observations in the Cu-XANES spectrum of Cu-NU-1000. First, the intensity of the pre-edge peak at 8977−8978 eV, attributed to the Cu2+ 1s→3d electronic transition,15 indicates that most of Cu is present as Cu2+ in an octahedral symmetry. As its normalized intensity is lower than that for the Cu2+ standards, we conclude that a fraction of Cu is present as Cu+. The presence of Cu+ would reduce the overall intensity of the pre-edge peak because Cu+ is a d10 ion and thus a 1s→3d transition is not allowed.16 The second important spectral feature is observed at 8982−8984 eV and is assigned to the 1s→4p electronic transition in Cu+.17 Previously Kau et al. demonstrated that the peak position for this transition also reflects the coordination number (CN) of Cu+. The 8982−8984 eV pre-edge feature corresponds to twoand three-coordinated geometries. The feature will shift to higher energy in the case of four-coordinate tetrahedral 10295


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The XANES linear combination fitting suggests that ∼9% of Cu was reduced from Cu2+ to Cu+ upon contact with methane (see SI). However, because the spectral intensity at 8977−8978 eV remained largely unchanged, we conclude that the majority of Cu ions in the MOF retains the “2+” oxidation state. Cu Extended X-ray Absorption Fine Structure (EXAFS) Analysis. Cu-EXAFS analysis was performed in order to determine the structural environment around the Cu atoms. Figure 5 shows the k2-weighted Cu-EXAFS Img[χ(R)] spectra

structure.18 Thus, we assign the peak to two- or threecoordinated Cu+. The pronounced shoulder at 8985.5 eV is attributed to the 1s→4p transition of Cu2+ present in tetragonal symmetry.19 Finally, there is no evidence of the presence of Cu0 in Cu-NU-1000 under ambient conditions. We have also performed XANES linear combination fitting to determine the concentrations of Cu+ and Cu2+ using Cu2O and Cu(OH)2 as respective references.20 The fit suggests that ∼15% of Cu is present as Cu+ and ∼85% as Cu2+. These values are important as they allow us to determine the extent of oxidation state modification during the catalytic reaction steps discussed later. Figure 4 shows the Cu-XANES spectra acquired during CuNU-1000 activation in flowing O2 (top) and subsequent

Figure 5. k2-weighted Cu-EXAFS Img[χ(R)] spectra of Cu-NU-1000 (black), Cu(OH)2 (orange), aqueous Cu2+ (green), and Cu2O (blue). The spectral regions most affected by the Cu−O single scattering (SS) and multiple scattering (MS) as well as Cu−Cu SS are also shown. Spectra were recorded at room temperature.

of Cu-NU-1000 and selected references. Cu foil and CuO are not shown because these standards are least relevant to the Cu structure in Cu-NU-1000. We begin the comparison to references by examining the first-shell Cu−O single scattering (SS) signal observed at R ≈ 1.4 Å in Figure 5. The Cu−O in Cu-NU-1000 has a somewhat shorter bond (peak is shifted left in the plot) and has lower amplitude compared to Cu(OH)2. This suggests multiple Cu species and agrees well with the XANES-determined distribution of ∼15% Cu+ and ∼85% Cu2+. Also, we note that Cu-NU-1000 has a Cu−Cu SS feature like in Cu(OH)2. Because this spectral feature is absent in aqueous Cu2+, which represents an isolated Cu ion species, we conclude that Cu in the MOF is most likely present as Cu clusters rather than isolated cations (one Cu per Zr6-node facet). We hypothesize that either small Cu clusters of a few Cu atoms (e.g., 2−4 atoms) or widely spaced sheets similar to Cu(OH)2 exist.22 Finally, similar to Cu(OH)2 and aqueous Cu2+, there is a significant Cu−O multiple scattering (MS) peak at R = 3.3 Å (2 × Cu−O SS, distance uncorrected for phase shift) that is characteristic of square planar Cu.23 Because the spectral features of the present sample resemble those of Cu(OH)2, we used Cu hydroxide as a model to fit the experimental Cu-EXAFS spectra and to determine the Cu−O and Cu−Cu CNs. The best fit is shown in Figure 6 and the respective fitted parameters are reported in Table 1. The fit result suggests that Cu is coordinated with four (CN = 3.5 ± 0.2) O atoms in the first shell with an average Cu−O distance of ∼1.94 Å. The presence of the multiple scattering distances at ∼3.9 Å indicates that these four O atoms are arranged in a square planar configuration. There is further an O atom with a Cu−O distance of ∼2.33 Å. There is likely a second axial O at even longer distance as in Cu(OH)2. We suggest these are Jahn−Teller24 distorted out-of-plane atoms, which is typical for square pyramidal or strongly distorted octahedral Cu

Figure 4. Normalized Cu-XANES spectra of Cu-NU-1000 during activation in oxygen (a) and methane loading (b) at 150 °C. The insets demonstrate the pre-edge feature at 8977−8978 eV corresponding to the 1s→3d electronic transition for the distorted symmetry of Cu2+ as well as the 1s→4p transitions for Cu+ and Cu2+ at 8982−8984 and 8985.5 eV, respectively. The color-coding is reported in the legend.

exposure to 1 bar CH4 (bottom) at 150 °C. We observe an evolution of the XANES pre-edge features during Cu-NU-1000 heat up to 150 °C in 2.75 mL/min O2 flow. The minor differences between the 25 and 150 °C spectra are attributed to structural distortions of the Cu species with increasing temperature. Overall, little change was observed in the CuXANES (as well as the EXAFS, see Figure S8) during 1 h activation in flowing O2 at 150 °C. Giordanino et al. reported similar observations of 400 °C oxygen-activated Cu-SSZ-13 zeolite and proposed that the minor changes are induced by the partial dehydration (removal of physisorbed water) in the vicinity of the Cu species.17 It should be noted in passing that Cu+ and Cu2+ species were concluded to be in equilibrium, because both species coexist even after treatment in O2 or He at 400 °C.17,21 When the catalyst was exposed to CH4 (Figure 4b), we observe an increase in the Cu+ 1s→4p transition intensity. 10296


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A simulated EXAFS spectrum of A (Figure S22) exhibits a Cu− O SS peak in excellent agreement with that found for Cu-NU1000, and the Cu−Cu SS region is also in overall reasonable agreement with the experimental spectrum. The length of A is about 10 Å, a value close to the diameter of the c-pore of NU-1000 (Figure 1). Thus, we envision a structure like A fitting into the c-pore (Figure 1 right) and being anchored at either end by two different nodes. A computational model was designed to mimic that environment. From periodic DFT calculations,26 we extracted a cluster model containing two [Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 (OH) 4 (OH 2 ) 4 ] 8+ nodes. The four [1,3,6,8-tetrakis(p-benzoate)pyrene]4− (TBAPy4−) linkers connecting the two nodes were simplified to [1,6-bis(p-benzoate)pyrene]2−, the eight TBAPy4− linkers that coordinate to only one node were truncated to formate groups, and finally, we removed one proton from the coordinating aquo group on the bridged face of each node so that a [Cu3(OH)4]2+ fragment could be inserted and anchored by two hydroxyl groups of each node at either end. During the optimization, the organic linkers were kept frozen to maintain the rigidity of the MOF framework. The optimized structure, A-2node, is shown in Figure 8. As surmised, the trimer fits perfectly in the pore

Figure 6. k2-weighted Cu-EXAFS Img[χ(R)] spectra of Cu-NU-1000 (black) and the Cu-NU-1000 fit (red) obtained using the Cu(OH)2 model. The spectral regions most affected by the Cu−O single scattering (SS) and multiple scattering (MS) as well as Cu−Cu SS are also shown.

Table 1. Average Interatomic Distances and Debye−Waller Factors (DWF) Determined for Cu Ions in Cu-NU-1000 by Fitting the Experimental k2-Weighted Spectrum to the Cu(OH)2 Model Derived Using FEFF9a shell

CN

Cu−O1 SS Cu−O2 SS Cu−Cu SS Cu−O1 MS Cu−O1 MS

3.5 ± 0.2 1 (set)b 1.3 ± 0.3 − −

distance (Å)

DWF

± ± ± ± ±

0.0044 ± 0.0007 0.0024 ± 0.0020 0.0083 (set)b 0.011 ± 0.008 0.011 ± 0.008

1.94 2.33 2.93 3.88 3.97

0.01 0.01 0.02 0.02 0.05

a

CN = average coordination number, SS = single scattering, MS = multiple scattering. Other parameters: amplitude reduction factor (amp) = 0.83, R-factor = 0.013, and E0 = −8.6. bThe value for Cu(OH)2 (Table S2).

coordination.22,25 The fit also indicates Cu−Cu scattering at ∼2.93 Å with an average fit CN of 1.3 ± 0.3. This CN suggests that Cu is present as Cu-hydroxo dimers, trimers, or tetramers in the Cu-NU-1000. Although not shown here, the addition of Cu−Zr scattering to the EXAFS fitting did not improve the overall fit quality. It indicates that the Cu−O−Zr sequences are either few or significantly distorted. Computational Modeling of Cu-NU-1000. Having analyzed the bulk features of Cu-NU-1000 based on EXAFS data, we turn to theory to gain insight at the atomic level. All calculations described below were performed at the density functional theory (DFT) level using the M06-L functional (cf. Computational Methods in the Experimental Section). Given the strong similarities identified in the EXAFS of Cu(OH)2 and Cu-NU-1000 (Figure 5), we initially optimized a bare trinuclear cluster, [Cu(OH)2]3·2H2O, A (Figure 7). All three CuII atoms have four-coordinate, square-planar environments. The average Cu−Cu distance is 2.845 Å, similar but slightly shorter than the experimental value of 2.93 Å (Table 1).

Figure 8. DFT-optimized structure of trimer A-2node. The two Cu− Cu distances to the central metal atom are 2.968 and 2.990 Å.

bridging the two nodes. The average Cu−Cu distance is 2.979 Å, which is longer than for the bare cluster A and in excellent agreement with the reported value of 2.93 Å (Table 1). The inclusion of dispersion interactions during geometry optimization using M06-L-D3 and PBE-D3 generated similar structures with average Cu−Cu distances of 2.979 and 2.980 Å, respectively. A structure was also obtained from periodic PBE-D3 calculations. All the computational models are structurally indistinguishable to within the uncertainty in the experimental EXAFS distances (cf. Table S6). To further assess the relevance of this structure, we simulated the EXAFS spectrum of A-2node and compared it to experiment for Cu-NU-1000 (Figure 9). The Cu−O SS peaks at ∼1.4 and ∼1.9 Å are in good agreement, and the positions of the Cu−Cu SS peaks of the simulated spectrum also match well with experiment (except for the signal at ∼2.4 Å, which has a slightly longer bond in the computational model than seen in

Figure 7. DFT-optimized structure of trimer A. 10297


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to 43.9 μmolcarbon/gCu‑NU‑1000, the selectivity to methanol and dimethyl ether (17.7 and 2.0 μmol, respectively) being ∼45%. A decrease in activity in the subsequent two cycles was observed, with overall yields of 30.7 and 21.8 μmolcarbon/gCu‑NU‑1000, respectively. However, the formation of CO2 decreased significantly (40% per cycle), while the methanol and dimethyl ether decreased only 14% per cycle. Thus, the selectivity toward methanol and dimethyl ether increased to 53% and 61%, respectively. When the methane-loading step was omitted (condition B), then no methanol or dimethyl ether was produced. We also tested the methane-to-methanol reaction with using 50% H2O and 50% He to desorb the products in the third step, but this produced more CO2 and less methanol and dimethyl ether (Table 2, conditions C). While total products equivalent to 60 μmolcarbon/gCu-NU-1000 were obtained in the first catalytic cycle, the combined yield of methanol dimethyl ether was only approximately 8 μmolcarbon/g (14% selectivity). We speculate that the increased amount of CO2 formed stems mainly from decarboxylation of linkers in Cu-NU-1000, since 60% of the pore volume was lost after the first cycle (Table S5) and even NU-1000 produces CO2 when subjected to the same conditions (Table 2, conditions D).28 In contrast, when 10% steam in He was used, the change in the pore volume and surface area of the Cu-NU-1000 was only 1−2%. Origin of the Cu Selectivity in Cu-NU-1000. Let us now explore the origin of the Cu activity in Cu-NU-1000 in relation to the fraction of Cu(II) that is observed to be reduced by XANES analysis. We suggest that the 15% Cu+ already present under ambient conditions does not contribute to methane conversion. Because Cu+ has only a very limited oxidation potential29 and we do not observe any formation of Cu0 via EXAFS. Only minor changes were observed in the Cu2+/Cu+ ratio after treatment at 200 °C. Upon methane loading in the second step of the process, we observed an increase in the fraction of Cu+ from 15 to 24%, which is suggested to be the consequence of a Cu(II)/methane redox couple.30 We surmise that the extent of Cu2+ reduction in Cu clusters of a few atoms via methane loading at 150 °C is thermodynamically limited.31 We, hence, conclude that the methane conversion and selectivity levels attributable to the metal deposited on the MOF nodes are achieved by just a fraction (9%) of the Cu in Cu-NU-1000. This fraction amounts to 149 μmol/g of the total 1656 μmol/g Cu species on the MOF. The stoichiometries for forming methanol and CO2 can be calculated from eqs 1 and 2, respectively, where the active Cu(II) species is represented as Cu(OH)2. The oxidations of methane to methanol and of methane to CO2 require 2 and 8 equiv of Cu(OH)2, respectively.

Figure 9. k2-weighted Cu-EXAFS Img[χ(R)] spectra for Cu-NU-1000 (black) and the A-2node (simulated, red) structure in Figure 8.

experiment). Additionally, from the k-representation we observe that the Cu−Cu and Cu−Zr scatterings appear to be in phase (see Figure S23). Catalytic Testing of Cu-NU-1000. The activity of Cu-NU1000 for methane oxidation to methanol was tested in a process consisting of three sequential reaction steps.27 First, Cu-NU1000 was pretreated in O2 flow at 200 °C to remove physisorbed water. The reactor was then cooled to 150 °C and purged with pure He. In the second step, the pretreated CuNU-1000 was exposed to a flow of CH4 at 150 °C, which causes partial reduction of Cu2+ to Cu+, as monitored by XANES. In the final step, water was introduced in the form of 10% steam in He at 135 °C to desorb the products methanol, dimethyl ether and CO2. We note that water was also produced during CH4 oxidation; however, the amount is much lower compared to the steam used in the process, and as it does not contribute to the carbon balance, we neglected its concentration. We performed three cycles of the three process steps to determine the activity and the stability of Cu-NU-1000. As shown in Table 2 (condition A), in the first cycle we observed products equivalent Table 2. Activity and Selectivity for the Oxidation of Methane by Cu-NU-1000 carbon yield (μmol/g)

carbon selectivity (%)

conditionsa

cycle

methanol

DME

CO2

totalb

methanol + DME

CO2

A

1 2 3 1 1 2 3 1 2

17.7 15.8 13.2 0 6.9 2.9 2.2 0.5 0.4

2.0 0.6 0.1 0 1.3 0.7 0.7 0.5 0.6

24.2 14.3 8.5 11.8 51.7 31.7 30.1 34.5 21.7

43.9 30.7 21.8 11.8 59.9 35.4 33.0 35.5 22.7

45 53 61 0 14 10 9 3 4

55 47 39 100 86 90 91 97 96

B C

D

CH4 + 2Cu(OH)2 ⇌ CH3OH + Cu 2O + 2H 2O

(1)

CH4 + 8Cu(OH)2 ⇌ CO2 + 4Cu 2O + 10H 2O

(2)

Thus, the first cycle requires 43.9 μmol CH4 to reduce 233 μmol Cu(OH)2 to form 19.7 μmolcarbon methanol plus dimethyl ether and 24.2 μmol CO2. As this is 84 μmol Cu(OH)2 more than the 149 μmol predicted to be converted by the XANES analysis (Figure 3), it is concluded that excess CO2 is produced during the steaming step via decarboxylation of Cu-NU-1000. We note that there are precedents for decarboxylation of carboxylic acids by Cu(II) ions32 and the proto decarboxylation of aromatic carboxylic acids is catalyzed by Cu(I) ions.33 Thus, assuming that approximately 150 μmol of Cu(OH)2 are

Conditions: (A) Cu-NU-1000 with activation in O2 at 200 °C for 3 h, CH4 loading at 150 °C for 3 h, and steam-assisted product desorption in the flow of 10% H2O and 90% He for 2 h at 135 °C; (B) same conditions as A except methane loading step was skipped; (C) CuNU-1000 with activation in O2 at 150 °C for 1 h, CH4 loading at 150 °C for 3 h, and steam-assisted product desorption in the flow of 50% H2O and 50% He for 30 min at 135 °C; (D) same conditions as C except using NU-1000 in place of Cu-NU-1000. bThe values are calculated on the basis of all the collected products. a

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ALD chamber, which was held at 110 °C for 30 min to remove any physisorbed water before dosing with the Cu precursor. A cylinder containing Cu(dmap)2 was held at 100 °C, and each of its pulses followed the time sequence of t1−t2−t3, where t1 is the precursor pulse time, t2 is the substrate exposure time, and t3 is the N2 purge time (t1 = 1 s, t2 = t3 = 240 s). To ensure full metalation of the Zr6 sites throughout the microcrystals, the Cu(dmap)2 pulsing cycle was repeated 80 times before exposure of the MOF to H2O pulses, using the same time sequence as for the Cu pulse (t1 = 0.015 s, t2 = t3 = 120 s). The concentration of Cu in Cu-NU-1000 samples was determined from inductively coupled plasma−atomic emission spectroscopy (ICPAES) measurements. X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) Measurements. The EXAFS measurement experiments took place at the Pacific Northwest Consortium/X-ray Science Division (PNC/XSD) bending-magnet beamline at Sector 20 of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). All experiments were carried out in transmission mode with a focused beam (0.7 × 0.6 mm) delivering 1010 photons through the sample. A harmonic rejection mirror was used to reduce the effects of harmonics. A Cu foil was placed downstream of the sample cell, as a reference to calibrate the photon energy of each spectrum. The XAFS reaction cell used in this work is shown in the SI. The cell is based on a HiP medium-pressure Hastelloy tee modified to house the four-sample holder and allow gas flow-through capability. Glassy-carbon discs (thickness = 0.75 mm, diameter = 5 mm) were used as the X-ray windows. The Cu-NU-1000 samples were pressed (0.25 ton) into pellets that were ∼0.5 mm thick, and the pellets were held in the sample holder (see SI). Prior to data acquisition, all gas lines were purged with dry He, and then with dry O2 or CH4, for activation and loading, respectively. A flow rate of 2.75 mL/min was used during data acquisition. Data analysis and background removal were performed using ATHENA and ARTEMIS programs from the iXAFS software package.34 The Fourier transform of the k-space EXAFS data (both the real and imaginary parts of χ(R) were fitted to theoretical models derived using the FEFF9 code.35 EXAFS spectra of calculated Cu clusters were simulated using ab initio scattering theory by applying approximate global disorder parameters, σ2, corresponding to 300 K (e.g., Figure 9). The computed coordinates were used to generate the primary input for the ab initio EXAFS scattering code (FEFF9)35 that includes all the single and multiple scattering paths out to 6 Å, resulting in several hundred scattering paths for each Cu atom in the structure. An approximate treatment of the bond disorder at 300 K is applied by setting a universal value of the DWF, σ2 = 0.005. The obtained spectra for each Cu atom in the cluster are then averaged, and an overall E0 is applied to match experimental values (oscillations in χ(k) converge at k = 0). While the global DWF is a good estimate of the first shell disorder, it is an overestimation of the order in the higher shells which manifests as an overprediction of these amplitudes, although the atom positions predicted by the theory are correctly represented. Thermogravimetric Analysis (TGA). TGA and differential scanning calorimetry (TG-DSC) analyses were performed on a SENSYS EVO TG-DSC (SETARAM Instrumentation) at the ramp rate of 3 K/min. N2-Sorption. N2-physisorption for surface area and pore volumes was obtained on a Micromeritics ASAP 2020 instrument. The carbon slit pore model with a NL-DFT method was used. Single-point adsorption close to P/P0 = 0.99 was used to determine the total pore volume. Powder X-ray Diffraction (PXRD). PXRD patterns of Cu-NU1000 were recorded on EMPYREAN (PANalytical) with Cu Kα radiation (λ = 1.5406 Å, 45 kV, 40 mA). Scanning Transmission Electron Microscopy (STEM). The samples were infiltrated in LR White acrylic resin (Electron Microscopy Sciences, Hatfield, PA) and polymerized at 60 °C for 24 h. The embedded material was sectioned to a 50 nm thickness on Leica ultramicrotome (Ultracut E) using a Diatome diamond knife. The microtomed samples were placed on 200-mesh lacey-carbon-coated Cu

converted during the methane loading step, which transform 33.5 μmol CH4 to 19.7 μmol methanol plus dimethyl ether and 13.8 μmol CO2, the 10.4 μmol CO2 is attributed to the decarboxylation of linkers. Indeed, in line with this estimate, 11.8 μmol/g of CO2 was observed as a sole product during steam treatment in 10% H2O and 90% He in a blank test (Table 2, condition B) in which the catalyst was not contacted with methane. Taking this into account, the selectivity for methane oxidation to methanol and dimethyl ether in the first cycle is as high as 56%, which is comparable to the selectivity measured in the subsequent cycles. As the yields of CO2 in the second and third cycles are much lower than in the first cycle, we conclude that decarboxylation of linkers becomes less significant in subsequent cycles. It should be noted that nevertheless a partial deactivation of the catalyst with each cycle is evident from the decrease in methanol and DME yield from 19.7 to 13.3 μmol/g. We tentatively attribute this to the deactivation of a fraction of the active Cu species. The evidence raises interesting questions about why only a small fraction (∼0.5%) of the available carboxylates seems susceptible to degradation. It might be, for example, that a minor fraction of carboxylates in Cu-NU-1000 feature metal coordination that differs from that indicated by the single-crystal X-ray structure of the parent material.

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CONCLUSION Cu oxide clusters deposited on the ZrO2 nodes of NU-1000 by AIM catalyze partial oxidation of methane. The state of the Cu was determined by XAS analysis to contain ∼15% Cu+ and ∼85% Cu2+ under ambient conditions. The average structure of the Cu is square planar coordinated by four O atoms (Cu−O ≈ 1.94 Å) with an additional one or two out-of-plane Jahn−Tellerdistorted O atoms, one of them has Cu−O ≈ 2.33 Å). A Cu− Cu distance of ∼2.93 Å was observed and the coordination number of Cu was determined at 1.3 ± 0.3. A combined EXAFS and DFT study indicates that the cluster that predominates in Cu-NU-1000 is likely to be a trimeric Cu-hydroxide-like structure that bridges two nodes across the c-pore of the MOF. Catalyst pretreatment in O2 flow does not change the Cu2+/ Cu+ ratio. Subsequent loading with CH4 at 150 °C, however, leads to a gradual increase in the fraction of Cu+, from 15 to 24%, indicative of methane activation. While the activity of CuNU-1000 for methane oxidation to methanol indicates that a significant fraction of the Cu atoms appears to be spectators, active Cu species deposited in NU-1000 convert methane at 150 °C with a 45−60% selectivity to methanol and dimethyl ether, the remainder being CO2. Excess CO2 produced during the first cycle derives from partial decarboxylation of the NU1000 linkers under reaction conditions.

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EXPERIMENTAL SECTION

Chemicals. High-purity (99.995%) helium, oxygen, and methane gases were purchased from Matheson and used without further purification. Deuterated dimethyl sulfoxide (DMSO-d6) was obtained from Cambridge Isotope Laboratory and deuterated sulfuric acid D2SO4 (d2) (96−98% solution in D2O) was obtained from SigmaAldrich. Materials Syntheses. The NU-1000 MOF as well as the Cucontaining Cu-NU-1000 were synthesized via atomic layer deposition similar to the literature procedure reported elsewhere.4 Bis(dimethylamino-2-propoxy)copper(II), Cu(dmap)2 (reactant A, Figure S1), was chosen as the Cu precursor. Room-temperature deionized H2O was used as the co-reactant (reactant B). In a typical experiment, a custom-made stainless steel powder sample holder containing microcrystalline NU-1000 (60.0 mg, 0.028 mmol) was placed in the 10299



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grids (Ted Pella) and imaged with an aberration-corrected JEOL 200F operating at 200 keV. The HAADF nominal probe size was ∼0.1 nm. 1 H NMR. Spectra were recorded on a Bruker 500 FT-NMR spectrometer. 1H chemical shifts are reported in ppm with the residual solvent resonances as the reference. MOF samples (∼2−5 mg) were dissolved in 4−6 drops of concentrated D2SO4 and then diluted with DMSO-d6 before analysis. Catalytic Testing. The activity of Cu-NU-1000 for methane oxidation to methanol was tested at atmospheric pressure. The reaction was performed in a stainless-steel plug flow reactor with a 4 mm inner diameter. The catalytic reaction included three consecutive steps: (1) Cu-NU-1000 activation in O2, (2) Cu-NU-1000 loading with CH4, and (3) water-steam purge necessary for product desorption. In a typical experiment, 20 mg of Cu-NU-1000 was loaded in the reactor and activated in O2 flow (16 mL min−1) at 200 °C, followed by flushing in He. CH4 was loaded subsequently in the flow (16 mL min−1) as a mixture of 90% CH4 and 10% He for up to 4 h at 150 °C. After cooling to 135 °C in He flow, steam-assisted product desorption was performed in either 50/50 or 10/90 mixture of H2O/He purged at a flow rate of 20 mL/min for up to 2 h. The oxidation products were identified and quantified using online mass spectrometry and by monitoring the time-dependent evolution of signals at m/z 31, 44 and 46, characteristic for methanol, CO2 and dimethyl ether, respectively. The He signal (m/z = 4) was used as an internal standard. Selectivity and yield were determined from the integral of the product concentrations as a function of time. Dimethyl ether was assumed to have been formed by condensation of two molecules of methanol via loss of water. Computational Methods. All calculations were performed at the DFT level using the M06-L functional36 as implemented in Gaussian 09.37 The M06-L functional performs well for medium-range electron correlation effects, transition metal chemistry,38 and the modeling of zeolites.38c For selected structures longer-range dispersion interactions were included by adding the D3 correction39 to both the M06-L and PBE40 levels of theory. Numerical integrations were performed with an ultrafine grid. To reduce computational cost, an automatic densityfitting set generated by the Gaussian program was used. The 6-31G(d) basis set was used for H, C, and O;41 the SDD pseudopotential and its associated double-ζ basis set was employed for Cu and Zr.42 During optimization, the Zr6-node and Cu clusters were relaxed while the linkers were kept frozen. All Cu(II)3-clusters were calculated in the high-spin state (quartet). Minima were confirmed by frequency calculations for all species (except for A-2node) at the same level of theory (298.15 K). Periodic calculations were performed using the Vienna Ab Initio Simulation Package (VASP)43 employing the generalized gradient approximation exchange correlation functional PBE40 with the D3 correction.39 A plane wave kinetic energy cut off of 520 eV and a 1×1×1 k-point grid was used for the Brillouin Zone sampling. An energy convergence criteria of 10−6 eV and a force convergence criteria of 0.05 eV/Å were used. All calculations were performed considering spin polarization.

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Article

AUTHOR INFORMATION

Corresponding Author

*[email protected] or [email protected] ORCID

Jian Zheng: 0000-0003-2054-9482 Manuel A. Ortuño: 0000-0002-6175-3941 Donald M. Camaioni: 0000-0002-2213-0960 Zhanyong Li: 0000-0002-3230-5955 Omar K. Farha: 0000-0002-9904-9845 Joseph T. Hupp: 0000-0003-3982-9812 Christopher J. Cramer: 0000-0001-5048-1859 Laura Gagliardi: 0000-0001-5227-1396 Johannes A. Lercher: 0000-0002-2495-1404 Present Address ∇

A.V.: BASF Corporation, 100 Park Ave., Florham Park, NJ 07932

Author Contributions ⊗

T.I., J.Z., and A.V. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support for this work from the Inorganometallic Catalyst Design Center, an EFRC funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (DE-SC0012702). This research used resources of the APS, which is a DOE Office of Science, Office of Basic Energy Sciences User Facility. Sector 20 operations at the APS are supported by the U.S. DOE (DEAC02-06CH11357) and the Canadian Light Source. We thank Dr. Mahalingam Balasubramanian (APS X-ray Science Division) for assisting in the XAFS experiments. The STEM work was performed using the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL), a multiprogram national laboratory operated by Battelle for the U.S. DOE. B.L.M. acknowledges support by PNNL’s Laboratory Directed Research and Development (LDRD) program. N.D.B. acknowledges support by PNNL’s Chemical Imaging Initiative LDRD Program. We acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported within this paper. Finally, we acknowledge Kiley Schmidt for the design of the cover art accompanying this paper.

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ASSOCIATED CONTENT

REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02936. PXRD and STEM characterizations, the design of the XAFS cell, details of XAFS fitting analysis, the NU-1000 and Cu-NU-1000 stability analysis under reaction conditions, 1H NMR spectrum of Cu-NU-1000 dissolved in D2SO4/DMSO-d6, DFT-optimized clusters Cun-NU1000 (n = 1−3), and Periodic DFT-optimized Cu3-NU1000 unit cell Cartesian coordinates, including Figures S1−S26 and Tables S1−S6 (PDF) 10300


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Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters

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