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Metathetic oxidation of 2-butenes to acetaldehyde by molecular oxygen using the single-site olefin metathesis catalyst (#SiO)2Mo(=O)2 Frédéric Le Quéméner, Jean-Marie Basset, Samir Barman, Nicolas Merle, Maha A. Aljuhani, Manoja K Samantaray, Youssef Saih, Kai C. Szeto, Aimery De Mallmann, Yury Minenkov, Kuo-Wei Huang, Luigi Cavallo, and Mostafa Taoufik ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01767 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018
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ACS Catalysis
Metathetic oxidation of 2-butenes to acetaldehyde by molecular oxygen using the single-site olefin metathesis catalyst (≡SiO)2Mo(=O)2. Frédéric Le Quéméner,†,¦ Samir Barman,‡,¦ Nicolas Merle,§ Maha A. Aljuhani,‡ Manoja K. Samantaray,‡ Youssef Saih,‡ Kai C. Szeto,† Aimery De Mallmann,† Yury Minenkov, #,‡ Kuo-Wei Huang,‡,* Luigi Cavallo,‡,* Mostafa Taoufik†,* and Jean-Marie Basset‡,* †
, Laboratoire de Chimie, Catalyse, Polymères et Procédés, UMR 5265 CNRS/ESCPE-Lyon/UCBL, ESCPE Lyon, F-30843, Boulevard du 11 Novembre 1918, F-69616 Villeurbanne Cedex, France. ‡ , Physical Sciences and Engineering, KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia § , University of Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181, UCCS – Unité de Catalyse et Chimie du Solide, F-59000 Lille, France. #, Moscow Institute of Physics and Technology, Institutskiy Pereulok 9, Dolgoprudny, Moscow Region 141700, Russia ABSTRACT: The catalytic oxidation of cis-2-butene and propylene with molecular oxygen in the presence of a well-defined surface coordination compound, (≡SiO)2Mo(=O)2, affords acetaldehyde. Using a cis-2-butene/O2 feed at 350-400 °C, the reaction yields a conversion of approximately 10% and an acetaldehyde selectivity of approximately 70%. This performance is maintained up to an experimental time of 20 hours in a continuous flow reactor. The Mo(bis-oxo) surface compound was fully characterized by multiple spectroscopic techniques as well as surface microanalysis. The results from quantum mechanics calculations indicate that the reaction proceeds via [2+2] cycloaddition/cycloelimination steps with the formation of metalla-oxacyclobutane intermediates, analogous to the Chauvin mechanism in olefin metathesis.
KEYWORDS: Metathetic oxidation; acetaldehyde; surface organometallic chemistry; molybdenum catalyst; silicasupported catalyst; DFT calculations.
INTRODUCTION The environmental challenges facing our planet in terms of energy have put increasing pressure on the scientific community to discover more sustainable alternatives to existing technologies, many of which were developed almost a century ago. One example is the effort to replace the energy-intensive Haber-Bosch process, which is currently in use for the synthesis of ammonia, with more efficient technology.1 Acetaldehyde is one of the most important aldehyde as a building block for various important products, such as pyridine derivatives, vinyl acetate, penta-erythritol, crotonaldehyde and resins.2,3 The dominating industrial process responsible for the annual production of 800,000 tons of acetaldehyde is the Wacker process, involving oxidation of ethylene in an aqueous solution of CuCl2/PdCl2 as catalysts (Figure 1a).4,5 Expensive Pd is used to oxidize ethylene to acetaldehyde by reducing Pd(II) to Pd(0), followed by the reoxidation of Pd(0) to Pd(II) by CuCl2 in a co-catalytic cycle. Industrially, the two catalytic cycles can operate in a single reactor or in two separate reactors depending on the purity of the reactant gas feed. The higher cost of the two-reactor method is balanced by using less expensive dilute gases.3 This homogeneous process generates highly corrosive HCl upon the catalytic reaction which imposes the use of devoted materials (normally ceramic coated titanium) for the construction of the reactor and the plant. Furthermore, the homogeneous nature of the reaction also complicates the regeneration of the catalysts. Neverthe-
less this process is quite reliable and has been used for decades. On the other hand, oxidative transformation of alkenes into carbonyl compounds is considered a widely used synthetic strategy to cleave the C=C bonds and to build C=O functionalities. While the heme and non-heme oxygenase can oxidize olefins with molecular oxygen under mild conditions,6-8 the use of ozone and stoichiometric amount of reductive work-up reagents, such as Zn and dimethylsulfide, or a combination of other stoichiometric oxidants, such as KMnO4 and OsO4/HIO4, are still the common practice in synthetic chemistry.9 Using O2 as the sole oxidant is economically and environmentally attractive, but literature examples are scarce and the reactant scopes are limited.10-12 To the best of our knowledge, no direct catalytic oxidation of internal or α-olefins to aldehydes via single-step catalysis using molecular oxygen by metathesis has been reported.
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do-Wittig reactions with molecular oxygen to afford a metal oxo and an aldehyde.16 To achieve single-site catalysis,17 which could lead to high selectivity in the product distribution, we decided that the active site of the catalyst would consist of a Mo-oxo functionality supported on silica. We chose (O=)Mo(OtBu)4 complex18 as the precursor and successfully transformed it into a silicasupported catalyst that was able to promote the conversion of propylene or 2-butene to acetaldehyde via molecular oxygen with high selectivity. Spectroscopic characterization of the developed catalyst unambiguously showed that it corresponded to well-defined (≡SiO)2Mo(=O)2. The results from quantum mechanics calculations indicated that the most likely reaction mechanism was the “metathetic” oxidation of the C=C double bonds with molecular O2.
RESULTS AND DISCUSSION Catalyst preparation and characterization. The strategy we followed to obtain a bis-grafted Mo center is shown in Scheme 1. Briefly, we used (O=)Mo(OtBu)4 (1) as precursor, and silica dehydroxylated at 200 °C (SiO2-(200))19-21 as the support. Then, the targeted bipodal Mo bis-oxo species (1b) was obtained by grafting (O=)Mo(OtBu)4 onto SiO2-(200) to afford 1a followed by thermolysis of 1a to 1b.
Figure 1. Comparison of the catalytic cycles for the oxidation of olefins into aldehydes, with the reactions mechanisms: (a) Bimetallic Wacker cycle for the oxidation of ethylene to acetaldehyde; (b) Cross metathesis of ethylene and 2-butene to propylene; (c) Metathetic oxidation of 2-butene and molecular oxygen to acetaldehyde; (d) Cycle based on a silica-supported Mo(bis-oxo) singleatom species for the metathetic oxidation of 2-butene to acetaldehyde.
Specifically, (O=)Mo(OtBu)4 reacted readily with SiO2-(200) at room temperature with pentane as the solvent (Scheme 1). After extensive washing and evacuation of the volatiles, a slightly yellow material was obtained, which we characterized as 1a. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy revealed consumption of the isolated silanols of SiO2-(200) (•(SiO-H)) at 3,747 cm-1 (Figure 2a). New peaks that corresponded to ν(C-H) of the tert-butoxy fragments also appeared. The broad adsorption from 3,700 to 3,100 cm-1 was related to H-bonding interactions between the tert-butoxy ligands and the remaining surface silanols.
To find a sustainable alternative to the Wacker process we started from the analogy between olefin metathesis and the unreported “metathetic oxidation” of olefin by molecular oxygen (Figure 1). In olefin metathesis, the double bond of 2butene can be cleaved by ethylene to yield propylene via the Chauvin mechanism (Figure 1b). Therefore, we hypothesized that the double bond of 2-butene could react with the double bond of oxygen to afford 2 moles of acetaldehyde during metathetic oxidation (Figure 1c). Figure 2. (a) DRIFT spectra of silica dehydroxylated at 200 °C (black), 1a [(•SiO)2Mo(=O)(OtBu)2] (red), and 1b [(≡SiO)2Mo(=O)2 (blue); (b) Solid-state 1HNMR and (c) 13C NMR spectra of 1a.
Scheme 1. Schematic illustration of the preparation steps for the single-site (≡SiO)2Mo(=O)2 catalysts. Because olefin metathesis occurs with molybdenum (Mo) complexes, either in solution or after grafting to a support,13,14 we chose to test this metal for “metathetic” oxidation. Indeed, supported Mo-oxo species can be activated by olefins during the initiation step, yielding Mo-carbene species that are active in olefin metathesis,15 and these metal carbenes undergo pseu-
Qualitative gas chromatography (GC) analysis of the filtrate after washing indicated the presence of tBuOH, which originated from silanolysis of the tert-butoxy Mo fragments. Elemental analysis of the resulting material indicated Mo and C contents of 3.47 and 3.64 wt %, respectively. This C/Mo molar ratio (i.e., 8.4) is close to the expected value of 8 for 1a. Moreover, the 1H magic angle spinning (MAS) and 13C crosspolarization magic angle spinning (CP-MAS) nuclear magnetic resonance (NMR) (Figures 2b and 2c) data confirmed the presence of Mo tert-butoxy fragments based on a 1H peak at 1.4 ppm and 13C peaks at 29 and 71 ppm. The results obtained
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ACS Catalysis from different spectroscopic methods as well as the elemental analysis suggest that the reaction of [O=Mo(OtBu)4] with SiO2-(200) proceeded by Mo-O cleavage along with tBuOH release, leading to the bipodal surface species [(•SiO)2Mo(=O)(OtBu)2] (1a) (Scheme 1). Thermolysis of supported complex 1a at 250 °C (for 2h) under high vacuum (10-5 mbar) triggered elimination of the H atom from one of the methyl groups of 1a to the β oxygen atom to form the bipodal Mo oxohydroxotert-butoxide intermediate, [(•SiO)2Mo(=O)(OH)(OtBu)], with the release of 0.85 equivalent of isobutene/grafted Mo, (gas chromatography; proposed mechanism in Figure S1). Further heating of this intermediate led to α-H abstraction, which quantitatively released tBuOH (Figure S1), and formation of species (1b) that we characterized as the silica-supported single-site catalyst [(≡SiO)2Mo(=O)2](see later).
The Raman spectrum of 1b (Figure S2) contained broad Raman features at 400-500 and 800-900 cm-1as well as a smaller feature at 610 cm-1, corresponding to the various vibrational modes of the siloxane bridges. Importantly, we observed a strong band that was centered at 986 cm-1, which we attributed to a combination of the stretching vibrations of terminally bound bis-oxo ligands in 1b.22 Furthermore, no features attributed to Mo-O-Mo were observed at 270, 720, or 805 cm-1, suggesting the absence of oligomeric Mo species. The diffuse reflectance ultraviolet-visible (UV-Vis) spectrum of 1b (Figure S3) indicated characteristic ligand-to-metal charge transfer centers at 212 and 240 nm, corresponding to a band gap value (Eg) of 3.9 eV. Although the reported band gap value for isolated, perfectly tetrahedral MoO4 units (Na2MoO4) is 4.7 eV, a distortion of the tetrahedral geometry may lower the band gap value (Eg of Al2(MoO4)3 = 4.2 eV).23
The DRIFT analysis of 1b revealed the disappearance of the alkyl vibrational bands (3,000-2,800 cm-1), which was accompanied by the re-appearance of isolated silanol groups at 3,747 cm-1 (Figure 2a).
We also studied the structure of the supported species 1b using X-ray absorption spectroscopy (XAS), X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy (Figure 3 and Table 1). The XAS data suggested that 1b can be described by isolated [(•SiO)2Mo(=O)2] units with a tetrahedral structure, probably deformed due to the heterogeneity of the silica support and variation in the O-Mo-O angles.15 The XANES analysis of 1b (Figure 3a) indicated an intense pre-edge peak at 20,006.1 ± 0.3 eV, which is characteristic of a dipoleforbidden 1s→4d transition. Although this transition is forbidden, the formation of molecular orbitals mixing Mo 4d and 5p orbitals with orbitals of the ligands allows the appearance of a pre-edge peak when Mo(VI) has a tetrahedral or quasitetrahedral symmetry. In contrast, the pre-edge signal is very weak in complexes with octahedral or pseudo-octahedral symmetry.24,25 The Mo K-edge corresponds to the dipoleallowed 1s→5p transition, which is typically sensitive to both the oxidation state of Mo and the bond covalence. The transition was positioned at 20,017 ± 1 eV (the maximum of the first derivative of the edge), and the energy at the half-step height was 20,014 ± 1 eV, indicating a +VI formal oxidation state for Mo.26 This result confirms that the molybdenum in 1b was Mo(VI) with tetrahedral or nearly tetrahedral symmetry. Fitting of the EXAFS signal (Table 1) suggested the following coordination sphere for Mo: (i) ca. two oxygen atoms at 1.705(10) Å, which were attributed to two Mo=O oxo ligands and (ii) ca. two oxygen atoms at 1.870(15) Å, which were assigned to “surface siloxide ligands”. These bond lengths for the Mo=O and Mo-O bonds are in the range of those observed for bis-oxo siloxy Mo molecular complexes (Table S1 in the Supporting Information).27-30 Similar parameters were obtained when fitting the k2.χ(k) spectrum. The fit could be improved by adding a layer of further back-scatterers with ca. one oxygen and two silicon atoms at 2.39(4) and 3.27(5) Å, respectively, due to a surface oxygen atom from a siloxane bridge of the silica support and the silicon atoms of the surface siloxide ligands. Therefore, this EXAFS study is in agreement with the (•SiO)2Mo(=O)2 structure for 1b.
Figure 3. (a) Molybdenum K-edge normalized absorption spectra of [(•SiO)2Mo(=O)(OtBu)2] (line A, red) and a metallic Mo foil (line B, blue); (b) Mo K-edge k3-weighted EXAFS for [(•SiO)2Mo(=O)2]; (c) Corresponding Fourier transform (modulus and imaginary part); solid lines: experimental; dashed lines: fit.
Table 1. EXAFS parameters for 1b.a Type of neighbor
No. of neighbors
Distance (Å)
σ2 (Å2)
Mo=O
2.1(3)
1.705(10)
0.0019(5)
Mo-OSi•
1.9b
1.870(15)
0.0032(11)
Mo-O(Si•)2
0.7(5)
2.39(4)
0.006(5)
Mo-O-Si•
1.9b
3.27(5)
0.014(12)
The errors generated by the EXAFS fitting program “RoundMidnight” are indicated in parentheses. a∆k: [2.4-15.2 Å-1] - ∆R: [0.4-3.4 Å] ([0.4-2.0 Å], when considering only the first coordination sphere); S02 = 0.68; ∆E0 = 4.5 ± 1.2 eV (the same for all shells); Fit residue: ρ = 6.7 %; Quality factor: (∆χ)2/ν = 2.96, with ν = 15 / 26 ([(∆χ)2/ν]1 = 3.46 with ν = 9 / 15, considering only the first coordination sphere: =O and –O). bShell constrained to a parameter above (2 N(=O) + N(O) = 6; N(O) = N(Si)).
Table 2. Summary of the catalytic oxidation results obtained with [(•SiO)2Mo(=O)2] (1b) using propylene as the reactant (see Table S2 for detailed selectivity).(A possible mechanism for the selectivity to acrolein is given is SI) Selectivity (%)
Temp. (oC)
Feed (%) C3=, O2, N2, He
Conv. C3=(%)
CO2
CO
CH3CHO
Acrolein
400
7.5, 10, 12.5, 70
5-6
30-32
9-10
33-35
9-11
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450
7.5, 10, 12.5, 70
12-14
31-34
11-14
25-29
9-12
450
7.5, 5, 12.5, 75
9-9.5
23-24
9-10
30-31
13-14
450
7.5, 2.5, 12.5, 77.5
5-6
21-23
10-11
28-29
10-11
Table 3. Summary of the catalytic oxidation results obtained with ((•SiO)2Mo(=O)2) (1b) using cis-2-butene as the reactant (see Table S3 for detailed selectivity). Selectivity (%)
Temp. (oC)
Feed (%) C4=, O2, N2, He
Conv. C4=(%)
CO2
350
7.5, 10, 12.5, 70
5
12-14
400
7.5, 10, 12.5, 70
9-10
17-20
6-8
66-68
3-4
450
7.5, 10, 12.5, 70
15-16
27-28
15-17
41-44
10-11
Catalytic studies. We tested the supported single-site catalyst (•SiO)2Mo(=O)2 (1b) for the oxidation of terminal and internal olefins using a continuous flow reactor. Our initial studies, in which we employed propylene as the reactant under controlled oxidation conditions in a temperature range of 400-450 °C, exhibited unprecedented results. When propylene (7.5% v/v) in the presence of O2 (10% v/v) came into contact with 1b at 400 °C (see Supporting Information for experimental details) only moderate to low propylene conversion (∼5-6%) was achieved. Nevertheless, the desired acetaldehyde oxidation product (30-35% selectivity) was produced along with CO and CO2 (40-45% combined selectivity) and acrolein (∼10% selectivity) (Table 2 and Figure S4). Other minor products including formaldehyde, acetone, ethylene, and propylene oxide were also observed. Increasing the reaction temperature to 450 °C led to a much higher propylene conversion (∼12-14%) with only a slight change in the product selectivity. Importantly, the original activity and selectivity were preserved even after a prolonged experimental time of 16 h (Figure S5), demonstrating that the catalyst was quite stable under the studied reaction conditions. The effect of the oxygen concentration in the feed on the propylene conversion and product selectivity was also investigated at 450 °C (Table 2 and Figure S6). At a lower oxygen to propylene ratio, the catalytic activity, especially the propylene conversion, markedly declined. Propylene conversions of ∼9% and ∼5.5% were observed when the feed gas mixture contained O2:C3H6 ratios of 5:7.5 and 2.5:7.5, respectively. This observation was accompanied by a slightly higher formation of ethylene (approximately 12-13% vs 8-9% when O2:C3H6 was 10:7.5), which may have been formed via a cracking pathway. As expected, the selectivity toward CO and CO2 decreased. The formation of CO2 and CO was primarily due to the thermal decomposition (decarboxylation and dehydration) of formic acid. In particular, CO was formed by the oxidation of formaldehyde, which was generated by [2+2] cycloaddition between α-olefins (propylene or 1-butene) and molecular oxygen (Figure S7).31 The minor quantity of acrolein might be due to C-H activation of methyl of propylene, followed by oxygen insertion and Β-H-transfer (Figure S7).
CO 2-3
CH3CHO 71
Acrolein 3-4
bined selectivity) (Table 3 and Figure 4). Approximately 10% conversion of cis-2-butene was achieved at this temperature. A decrease in the temperature to 350 °C resulted in a minor improvement in the acetaldehyde selectivity (∼71-72%) and a notable decrease in the olefin conversion (approximately 5 %, Figure 4). An increase in the temperature to 450 °C resulted in a higher olefin conversion (∼15-16%) (Figure S8), a significant drop in the acetaldehyde selectivity (41-44%), and a slight increase in the acrolein selectivity. As with propylene, the original activity using cis-2-butene as the substrate was preserved up to 20 h (Figure S9), confirming the stability of the catalyst under the studied reaction conditions. When cis-2pentene (5.9% v/v) was reacted at 400 °C in the presence of O2 (7.8% v/v) (Figure S10 and Table S4 in the Supporting Information), we also observed moderate to good selective formation of acetaldehyde (41 % selectivity), CO and CO2 (approximately 45 % combined selectivity), and other minor products including acrolein, propanal, and methacrolein.
The unexpected catalytic results that were obtained with propylene inspired us to explore the scope of the oxidation reaction with internal olefins (i.e., cis-2-butene and cis-2-pentene), and the results are summarized in Table 3. At 400 °C in the presence of O2, cis-2-butene exhibited a superior selectivity toward the formation of acetaldehyde (∼70%) as well as a reduction in the formation of CO and CO2 (∼20-25% com-
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ACS Catalysis supporting the hypothesis that the [2+2] cycloaddition between III and O2occursvia a transition state in a triplet spin state. Metallacycle IV in the singlet spin state was located 13.9 kcal/mol below III + O2. The energy of IV in the triplet spin state using the singlet spin state geometry is 34.1 kcal/mol higher than in the singlet spin state, suggesting that spin state flipping occurs during the relaxation of TS3 to IV. Cycloelimination of acetaldehyde from singlet IV regenerates I in the singlet spin state via transition state TS4 and a low energy barrier of 6.9 kcal/mol. This process closes the catalytic cycle.
Figure 4. Conversion and selectivity as a function of time-onstream for the cis-2-butene oxidation over (•SiO)2Mo(=O)2 (1b) at 350 °C (up) and at 400 °C (down). Reaction conditions: Cat. wt: 200 mg; Feed
Computational studies. Density functional theory (DFT) calculations were performed to obtain a more complete understanding of the reaction pathway (see Section 7 in Supporting Information, for computational details and Figure S11). The energetics of all the transformations for the oxidation of cis-2butene are reported in Figure 5. The reaction starts with the conversion of the initial silica-supported Mo(VI) bis-oxo species (I in Figure 5) into the metallacyclobutane-like intermediate (II) via [2+2] cycloaddition of 2-butene with one of the W=O bonds. This step is endergonic by 22.9 kcal/mol and requires 32.5 kcal/mol of Gibbs free energy of activation to move through transition state TS1. An intermediate that corresponds to the coordination of cis-2-butene to I does not play a role in the reaction kinetics because it is less stable by 4.4 kcal/mol than the infinitely separated cis-2-butene and I. Cyclo-elimination from II via transition state TS2 (Figure S12) at 40.7 kcal/mol liberates an acetaldehyde molecule and leads to Mo-oxocarbene III. This step is endergonic by 7.2 kcal/mol. The catalytic cycle is completed by reaction of III with molecular oxygen. The transformation occurs through transition state TS3 and involves a moderate activation barrier of 24.2 kcal/mol. The reactants (i.e., III and molecular oxygen) are in the singlet and triplet spin states, respectively. Therefore, TS3 has a triplet spin state. Our attempts to locate TS3 with a singlet spin state failed because the geometry optimizations always led to metallacycle intermediate IV. Further, the energy of TS3 in the singlet spin state using the triplet spin state geometry is 22.4 kcal/mol higher than in the triplet spin state,
Figure 5. (a) Reaction pathway for oxidation of cis-2-butene to acetaldehyde by O2 and catalyzed by a model of [(•SiO)2Mo(=O)2](1b). The DFT-calculated ∆G (kcal/mol) values are reported in blue near the structure labels. (b and c) Geometry of transition states TS2 and TS3 with selected distances in Å.
Overall, the oxidation of cis-2-butene to two acetaldehyde molecules is strongly exergonic with a Gibbs free energy change of -84.2 kcal/mol. Because the reaction is performed in a flow reactor, we consider the kinetics of the two metathesis events separately, and no equilibrium condition among the reactants, products, and intermediates can be established. The first metathesis event from I to III has an overall energy change of 40.7 kcal/mol from I + cis-2-butene to the cycloelimination transition state,TS2 (Figure 5b).This energy change corresponds to a reaction half-time of ∼6 seconds at 350 °C, which is consistent with the experimental conditions. The second metathesis event from III to I has an overall energy change of 24.2 kcal/mol from III + O2 to the [2+2] cycloaddition transition state TS3 (Figure 5c). Thus, the energy change of the first metathesis event is lower than that of the second metathesis event, making the former event the rate determining step. Finally, DFT calculations indicate very
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similar reactivity for the cis and trans isomers of butene (1 kcal/mol in difference, Figure S13).
CONCLUSION Our catalytic results confirmed our starting hypothesis that high temperatures can promote metathetic oxidation of olefins by molecular oxygen using [(•SiO)2Mo(=O)2]. In this study, cis-2-butene was oxidized by O2to acetaldehyde with a selectivity higher that 70 % at 10 % conversion using a silicasupported single-site catalyst [(≡SiO)2Mo(=O)2]. Our DFT calculations indicate that the reaction occurred via “metathetic” oxidation with the formation of metallacycle intermediates. Using a Mo bis-oxo species to selectively cleave an olefinic double bond to yield the corresponding aldehyde as the product is a new reaction in the field of oxidation. No previously reported single-site catalytic system has been capable of promoting the direct oxidation of propylene or cis-2-butene to acetaldehyde via O2. Therefore, our results introduce new perspectives for organic synthesis and green chemistry. Metathetic oxidation is a simple approach for producing aldehydes that avoid the expensive bimetallic system of the Wacker process or the tedious steps of hydroformylation, which are both large-scale industrial processes.
ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:
Additional information on the general procedures and the preparation of 1a and 1b; Raman, UV-vis DRS and X-ray absorption spectra of 1b; Catalysis and computational details.
AUTHOR INFORMATION Corresponding Author K.W.H.
[email protected] L. C.
[email protected] M. T.
[email protected] J.M.B.
[email protected] Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manu¦ script. / These authors contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGEMENTS The Competitive Research Grant Program, supported by the KAUST Research Fund (project # 2174 CGR3) is gratefully acknowledged for financial support. We wish to thank Olivier Mathon for his help during the recording of the XAS spectra on BM23 beam-line at ESRF (experiment code IN-986). For computer time, this study employed the resources of the KAUST Supercomputing Laboratory (KSL).
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