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Mechanistic Insights on the Direct Conversion of Methane into Methanol over Cu/Na–ZSM-5 Zeolite: Evidence from EPR and Solid-State NMR Jian-Feng Wu, Xu-Dong Gao, Long-Min Wu, Wei David Wang, Si-Min Yu, and Shi Bai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02898 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Catalysis

Mechanistic Insights on the Direct Conversion of Methane into Methanol over Cu/Na–ZSM-5 Zeolite: Evidence from EPR and Solid-State NMR Jian-Feng Wu*,†, Xu-Dong Gao†,‡, Long-Min Wu†, Wei David Wang†, Si-Min Yu†, Shi Bai† State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, †

People’s Republic of China ‡ CAS Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (CAS), Lanzhou 730000, People’s Republic of China ABSTRACT: The selectively direct conversion of methane to methanol on Cu-exchanged zeolites has attracts a lot of interest due to the abundant availability of methane. Detailed reaction mechanism of the transformation of methane to methanol aimed to improve the catalyst design is long expected. On the basis of EPR and solid-state NMR studies, we demonstrate that CH4 activates on the [Cu2O]2+ core of Cu/Na–ZSM-5 zeolite producing a •CH3 radical, an •OH radical. The generated methanol molecule, which formed from the combination of •CH3 and the •OH radicals, adsorbed on the Cu+ species forms the copper methoxy species (–Cu–O–CH3) and the –Al–OH group. Upon the hydration, the copper methoxy species and the adsorbed methanol transform into the free methanol. We expect that our mechanistic understanding of the methane to methanol process could pave the way for catalyst design.

KEYWORDS: methane, methanol, copper, zeolites, reaction mechanism The direct conversion of methane to methanol or its derivatives has attracted considerable research interest in the field of methane transformation1, which has been widely investigated in homogeneous2, heterogeneous3, and gas-phase4 conditions. In nature, the particulate methane monooxygenase (pMMO) enzymes have the ability to selectively convert methane into methanol at ambient temperature with copperoxygen active sites.5 Inspired by the outstanding work of nature, researchers have attempted to mimic pMMO’s activity by creating copper involved catalysts and found Cu modified zeolites or MOF could selectively convert methane to methanol under mild conditions.6 The determination of the nature of the active site and the understanding of the mechanism of methane oxidation in Cu modified zeolite are vital for developing new catalytic system for methane activation and conversion at mild condition.6b-d,6i,6k,7 DFT calculation has been used to reveal the process of direct methane to methanol.6b,7a-c,7h However, from the experimental aspect, no complete evidence has demonstrated the mechanism of how methane transforms into methanol on the surface of the Cu modified zeolite. In this contribution, we investigate the mechanism of the selectively direct oxidation of methane to methanol on the Cu/Na–ZSM-5 zeolite. Based on EPR and solid-state NMR observations, we proposed a mechanism for the selectively direct oxidation of methane to methanol on Cu/Na–ZSM-5 zeolite (Scheme 1), which includes three key steps: i) methane activates on the [Cu2O]2+ core of Cu/Na–ZSM-5 zeolite and generates methanol, while Cu2+ is reduced to Cu+; ii) the adsorbed methanol activates on the Cu+ species, forming the copper methoxy species (–Cu–O–CH3) and the –Al–OH group;

and iii) free methanol is formed by hydration of copper methoxy species and by the displacement of the adsorbed methanol.

Scheme 1. The proposed mechanism for the transformation of methane into methanol on the Cu/Na–ZSM-5 zeolite. The transformation of methane into methanol was conducted on Cu/Na–ZSM-5 zeolite. The detailed procedures for the catalyst preparation6a can be found in the literature (see Supporting Information for details). The synthesized Cu/Na– ZSM-5 zeolite was thoroughly characterized by spectroscopic measurements (Figures S1-S5), and the obtained results are in excellent agreement with those reported previously.8 As we used the same catalyst synthesis method as Groothaert et al.6a,

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thus, we adopted the confirmed Cu-O-Cu active sites6a in this work. The reaction of methane on Cu/Na–ZSM-5 zeolite was firstly monitored by the EPR spectroscopy (Figures S6-S7). Besides, Cu+-CO was observed during the co-reaction of methane and CO (Figure S8), indicating the formation of Cu+ ion. The reduction of Cu2+ ion to Cu+ ion upon reacting with methane (Scheme 1, Step 1 to Step 3) is confirmed. The result is good in line with previous DFT calculation6b, Cu K edge XAS measurements9 and a flow experiment at 773 K.10 Incubation of CH4 (50 mbar) with the spin-trapping agent DMPO (ca. 10 mg) at 423 K on Cu/Na–ZSM-5 zeolite led to the formation of one major radical adduct and two minor radical adducts (Figure 1a). The major radical adducts marked by asterisks (*) with the six-line spectrum (aH = 20.0 G and aN = 14.5 G) are typical DMPO/•CH3 adduct.3a,11 The minor radical adducts marked by open circles (◦) with the four-line signal (aH = aN = 14.2 G) could be DMPO/•OH adduct.3a,11b,12 The simulation of Figure 1a is shown in Figure S10 in the supporting information. Another minor radical adduct marked by solid dots (•) was also observed. This adduct comes from the background reaction of DMPO on Cu/Na–ZSM-5 (Figures 1b and S11. Detailed comparison is shown in Figure S11.). Other than these trapping signals, a background is observed in Figure 1a. The background comes from the Cu2+ ion on Cu/Na–ZSM-5 zeolite, and the whole spectrum is shown in Figure S12 in the supporting information. To further confirm that the radical signals come from the incubation of CH4 and DMPO on Cu/Na– ZSM-5 zeolite, we did extra control experiment with incubation of CH4 on Cu/Na–ZSM-5 zeolite (Figure 1c) and CH4 with DMPO on NaZSM-5 zeolite (Figure 1d). Figure 1c shows the background signal from Cu2+ on Cu/Na–ZSM-5 zeolite. Only some weak signals can be found in Figure 1d, and these signals may come from the paramagnetic impurities in DMPO.13 From the experimental aspect, we demonstrate the methane-tomethanol process on Cu/Na–ZSM-5 zeolite involving the radical mechanism (Scheme 1, Step 2). Different from previous works3a,11b,12, DMPO was used to trap radicals in the gas-solid condition. Due to the lack of solvent effect and solvated cage for the reaction of methane on Cu/Na-ZSM-5 zeolite, we think the Step 2 to Step 3 is a radical rebound-like process. The oxygen atom in Cu2O abstract an H atom from CH4 to form •OH and •CH3, if no DMPO molecule nearby, the formed •OH and •CH3 combine together to form the methanol adsorbed on the Cu+ sites. Secondly, the reaction of 13C-labeled 13CH4 (50 mbar) on Cu/Na–ZSM-5 zeolite was further investigated by the 13C highpower proton decoupling (HPDEC) magic-angle-spinning (MAS) NMR spectroscopy and the obtained results are shown in Figure 2. When the reaction was carried out at 423 K (Figure 2a), two signals appeared at 59 and 53 ppm besides the dominating signal of methane14 at -10 ppm, indicating the activation of 13CH4 on the catalyst (Scheme 1, Step 3 to Step 4). Wang et al.15 demonstrated the formation of methoxy species from methanol on zeolites, lending credence to our hypothesis (-Cu-OCH3 is formed from methanol). Upon the evacuation of the sample a) at 423 K for 30 min, methane disappeared and the decrease of the signal intensity at 53 ppm is evident, while the signal intensity at 59 ppm stays the same (Figure 2b). Combination of the above experimental results and literature data, the 53 ppm signal can be assigned to adsorbed methanol16, and the 59 ppm signal can be assigned to surface methoxy species17.

Figure 1. Spin-trapping evidence for the radical mechanism of the methane to methanol process on Cu/Na–ZSM-5 zeolite. All reactions were carried out at 423 K by in situ heating for about 5 min in EPR spectrometer. EPR spectrum of a) DMPO/•CH3 and DMPO/•OH was recorded by the incubation of DMPO with CH4 on Cu/Na–ZSM-5 zeolite. The control experiments were performed by b) incubation of DMPO alone on Cu/Na–ZSM-5 zeolite, c) incubation of CH4 alone on Cu/Na–ZSM-5 zeolite, d) incubation of DMPO with CH4 on NaZSM-5 zeolite. The background in spectra a), b), and c) are come from Cu2+ signal in Cu/Na–ZSM-5 zeolite. Hyperfine splitting constants for DMPO/•CH3 adduct are aH = 20.0 G and aN = 14.5 G, while for DMPO/•OH adduct are aH = aN = 14.2 G. The g-value for DMPO/•CH3 and DMPO/•OH is g = 2.0068. All the spectra in Figure 1 provide qualitative results. The figures with y-scaling are shown in Figure S9. The DMPO/•CH3 and DMPO/•OH adducts can maintain its signals for about 15 min under in situ heating. The signals intensity of DMPO/•CH3 and DMPO/•OH are not the same, which may due to •OH radicals have acidity in nature and •OH radicals are not stable as •CH3 radicals on basic Cu/Na–ZSM-5 zeolite.

These assignments were also supported by the strong spinning sidebands of sample b) record at a low spin rate of 2.5 kHz (Figure S13 in the SI). Previously, surface methoxy species was detected on Cu modified catalysts6g,7g,18, however, there are some controversies about its location (connected to the oxygen on the Cu-O-Cu sites18 or bonded to the oxygen of the Brönsted acid sites6g). The location of surface species is important to reveal the reaction mechanism. After the hydration of sample b) (Figure 2c), it can be seen that the surface methoxy species at 59 ppm and the adsorbed methanol at 53 ppm transferred into free methanol at 50 ppm19 (Scheme 1, Step 5). The formation of methanol was also observed after methane activation on Cu modified zeolites by extraction with a suitable solvent or mixed solvents.6a,6d,7e,7g At the same time, the formation of small amount of surface formate species (171 ppm20) and carbon dioxide (125 ppm21) were also observed. The NMR based selectivity of methanol is 97.7% (Figure S14), which is very close to the reported data 98%6a. The appearance of surface formate species at 171 ppm suggested that the surface methoxy species should bond to Cu rather than to the framework of zeolite.22 Other experimental evidences for the formation of copper methoxy species are shown in Figures S15-S18. In addition, the formation of H2 was observed during the reaction

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ACS Catalysis of methane on Cu/Na–ZSM-5 zeolite (Figure S19). This result is good in line with findings from van Bokhoven Group.7f,7g

Figure 2. Solid-state NMR evidence for the copper methoxy species formation upon methane activation on Cu/Na–ZSM-5 zeolite. The 13C HPDEC MAS NMR (a-c) spectra were recorded for: a) the reaction of 13CH4 on Cu/Na–ZSM-5 zeolite at 423 K for 30 min; b) the evacuation of the sample a) at 423 K for 30 min; and c) the hydration of sample b) at room temperature. The asterisk indicates the spinning sideband.

In the end, a 1H-27Al TRAPDOR technique23 was adopted to investigate the reaction of methane on Cu/Na–ZSM-5 zeolite. In Figure 3a, the Hahn-echo 1H MAS NMR signals for the fresh Cu/NaZSM-5 zeolite at 4.0 and 1.8 ppm can be assigned to the protons of Brönsted acid sites and –Si–OH groups, respectively.24 After the reaction of CH4 at 423 K for 1.5 h followed by the removal of unreacted CH4 upon evacuation, a weak signal at 2.5 ppm shows in Figure 3b. The difference spectrum of the 1H peak at 2.5 ppm is assigned to the protons of –Al–OH group (Figure 3c).24 This assignment was further confirmed by the control experiment (Figure S20). Meanwhile, upon the formation of –Al–OH group, the 1H signal intensity at 4.0 ppm was increased (Figure 3b), indicating the formation of the copper methoxy species (–Cu–O–CH3) and adsorbed methanol upon the dissociative adsorption of CH4 (Scheme 1, Step 3 to Step 5). Thus, we were able to identify the formation of the –Al–OH group during the methane to methanol process by TRAPDOR experiments.

We infer the high selectivity of methanol should be attributed to the unique nature of the Cu/Na–ZSM-5 zeolite: (i) The electron deficiency of Cu+ ions result in the electron density transfer from the methyl group of copper methoxy species and adsorbed methanol to Cu+ ions. Hence, the low electron density on the methyl of copper methoxy species and adsorbed methanol protects themselves from the over-oxidation. Similarly, Periana et al. believed that methyl bisulfate can be stabilized in a strong oxidizing atmosphere is due to the electron-withdrawing bisulfate group.25 (ii) The Cu oxidation sites are space isolated by the zeolite framework, which prevents from the over-oxidation by directly contact the initial products (copper methoxy species and adsorbed methanol). (iii) The copper methoxy species and adsorbed methanol, chemisorbed or physically strongly absorbed on Cu+ ions, are difficult to diffuse to other oxidation sites and then take place deep oxidation. In conclusion, we report the mechanism of the highly selective transformation of methane into methanol on a coppermodified Cu/Na–ZSM-5 zeolite at the temperature of 423 K. EPR investigation indicates that the reduction of Cu2+ ion to Cu+ ion took place during the transformation of methane into methanol on Cu/Na–ZSM-5 zeolite. In situ EPR spin-trapping evidence demonstrates that a •CH3 radical and an •OH radical are formed during the methane-to-methanol process on Cu/Na– ZSM-5 zeolite. Solid-state NMR investigation confirms that copper methoxy species and adsorbed methanol are formed on Cu/Na–ZSM-5 zeolite after methane activation at 423 K. Upon hydration, the surface methoxy species and adsorbed methanol transferred into free methanol. Our results reveal the evolution of the active sites and the formation of copper methoxy species and adsorbed methanol upon methane activation on Cu/Na– ZSM-5 zeolite. We expect that our mechanistic understanding of the methane to methanol process (Scheme 1) on Cu/Na– ZSM-5 zeolite, then, could potentially shed light on CH4 oxidation in pMMO and the rational design of biomimetic catalytic systems.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *J.F.W.: e-mail, [email protected]

ORCID Jian-Feng Wu: 0000-0003-4444-2639

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

Figure 3. Evidences of AlOH formation from TRAPDOR experiments. Hahn-echo 1H NMR spectra without 27Al NMR irradiation (orange lines, a) and 1H-27Al TRAPDOR spectra with 27Al NMR irradiation (green lines, b) of methane activation on Cu/NaZSM-5 zeolite. The difference spectrum of a) and b) is shown in c). 1.5 h heating time makes the –Al-OH more evidence.

This work was financially supported by the National Natural Science Foundation of China (No. 21803027), Fundamental Research Funds for the Central Universities (lzujbky-2018-k08 and lzujbky-2019-cd02), and the Natural Science Foundation of Gansu Province (18JR3RA302). We thank Ms. Jiahui Yang (Bruker Corporation) and Miss Yunfei Bai (Lanzhou University) for helpful discussion.

Supporting Information Detailed experimental procedures, characterization data (PXRD pattern, 1H, 27Al, and 29Si solid-state NMR spectra, and EPR spectrum) for the prepared Cu/Na–ZSM-5 zeolite, EPR spin-

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trapping result, Solid-state NMR results, MS data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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