MgO-Catalyzed Oxidative Coupling

Subscriber access provided by UNIV OF BARCELONA

Article

Gas-phase Reaction Network of Li/MgO-Catalyzed Oxidative Coupling of Methane and Oxidative Dehydrogenation of Ethane Liangfeng Luo, Rui You, Yiming Liu, Jiuzhong Yang, Yanan Zhu, Wu Wen, Yang Pan, Fei Qi, and Weixin Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04728 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Catalysis

Gas-phase Reaction Network of Li/MgO-Catalyzed Oxidative Coupling of Methane and Oxidative Dehydrogenation of Ethane Liangfeng Luo1, Rui You1, Yiming Liu1, Jiuzhong Yang2, Yanan Zhu2, Wu Wen2, Yang Pan2*, Fei Qi3, Weixin Huang1* 1 Hefei

National Laboratory for Physical Sciences at the Microscale, Key Laboratory

of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China 2

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China

3 Key

Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong University, Shanghai 200240, China

1

ACS Paragon Plus Environment

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

Page 2 of 25

Abstract: Gas-phase reaction network involving reactive intermediates such as alkyl and alkyl peroxide radicals and alkyl hydroperoxides have been long remaining as a mystery in oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODHE) reactions. Herein we report direct observations of gas-phase reactive intermediates including CH2, CH3, C2H5, CH3OO, C2H5OO, CH3OOH and C2H5OOH during OCM and ODHE reactions over Li/MgO catalysts with an online synchrotron VUV photoionization mass spectrometer connected to a catalytic reactor with adjustable distances between the catalyst bed and the sampling nozzle of mass spectrometer. Secondary reactions of these reactive intermediates in the gas phase are elucidated and the reaction network of OCM and ODHE reactions is established. These results greatly deepen the mechanistic understanding of OCM and ODHE reactions necessary for developing efficient catalysts and designing proper reactors. Keywords: Reaction mechanism; Intermediate; Radial; Surface reaction; Mass spectroscopy

2


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

ACS Catalysis

1. Introduction Methane, the major component of natural gas, shale gas, biogas and gas hydrate, is envisioned as a primary and clean source of hydrocarbon for useful chemicals and liquid fuels in the foreseeable future.1,2 Methane conversion into value-added products has been attracting great interest in heterogeneous catalysis,3 but meanwhile remains as a great challenge because of its high stability.4,5 Nowadays the industrial transformation of methane into useful chemicals and liquid fuels is only feasible indirectly via synthesis gas,6 and the direct alternatives have proven difficult to control because of low yields, selectivity, and productivity.7 An intriguing direct and nonoxidative conversion route of methane to ethylene, aromatics, and hydrogen catalyzed by latticeconfined single iron sites was recently reported but it required a very high operation temperature.8 Since the pioneering work of Keller and Bhasin,9 oxidative coupling of methane (OCM) to ethane and ethylene has been considered as an ideal direct route for valorization of methane.10 Although great endeavor has been devoted,11-14 little progress has been achieved on catalytic systems meeting the economically industrial target.15 Comprehensive understanding of OCM reaction mechanism is being recognized necessary for developing efficient catalysts and designing proper reactors. A widely-accepted mechanism of OCM reaction is that methane is activated on catalyst surfaces to generate methyl radicals (CH3·) whose secondary reactions either on the catalyst surfaces or in the gas phase lead to the products.11,14,16,17 Kinetic modeling studies predicted presences of dozens, even hundreds of reactions in the gas phase, 3


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

Page 4 of 25

involving reaction paths of methyl radicals, ethyl radicals (C2H5·), hydroxyl radicals (OH·), methyl peroxide radicals (CH3OO·), ethyl peroxide radicals (C2H5OO·),18-23 and indicated an important role of gas-phase radical reactions.24,

25

So far experimental

evidence for radical intermediates has been reported only for methyl radicals,11,24,26 but not for other radical intermediates in OCM reaction. Meanwhile, the secondary gasphase reaction network of methyl radicals is not established. Alkyl and alkyl peroxide radicals and alkyl hydroperoxides have also been proposed as important gas-phase intermediates and experimentally identified in hydrocarbon combustion reactions.27-30 The relevant reaction mechanism is proposed as the following: hydrocarbon molecules undergo hydrogen abstraction reactions to from alkyl radicals to initiate the hydrocarbon combustion processes, and alkyl radicals can add to O2 molecules to form corresponding alkyl peroxide radicals, and the alkyl peroxide radicals can undergo H abstraction reactions with hydrocarbon molecules to form corresponding alkyl hydroperoxides, and then the alkyl hydroperoxides can decompose to generate hydroxyl radicals, oxygenated intermediates or products. Synchrotron VUV photoionization mass spectrometry (SVUV-PIMS) housed in National Synchrotron Radiation Laboratory (NSRL) (Hefei, China) has demonstrated a powerful ability in detecting gas-phase radicals.26,31-33 An in situ catalytic reactor was designed and set up to connect the SVUV-PIMS that allows adjustable distances between the catalyst bed and the sampling nozzle of the mass spectrometer (Figure 1) for investigations of gas-phase reactive species in heterogeneous catalytic reactions. Herein, we report the results of OCM and oxidative dehydrogenation of ethane (ODHE) 4


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

ACS Catalysis

reactions studied with such a system. In addition to CH3 radicals, gas-phase reactive intermediates including CH2, C2H5, CH3OO, C2H5OO, CH3OOH and C2H5OOH were directly observed for the first time. Secondary reactions of these reactive intermediates in the gas phase are elucidated and the reaction network of OCM and ODHE reactions are established.

2. Experimental MgO (AR, > 98.5%) and Li2CO3 (AR, >98.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Distilled water was made by Water Purifier lab pure water system. All gases were with ultrahigh purity and were obtained from Nanjing Shangyuan Industrial Gas Factory, China. A Li-doped MgO catalyst with calculated Li amount of 5.6 wt% (denoted as 5.6%Li/MgO) was prepared by addition of 2.5 g MgO (AR, >98.5, Sinopharm Chemical Reagent CO. Ltd) and calculated amounts of Li2CO3 (AR, >98.5, Sinopharm Chemical Reagent CO. Ltd) into 100 mL deionized water. The mixture was adequately stirred at 60 C for 4 hours and vacuum-filtered. The acquired precipitate was dried at 60 C and then calcined in Muffle furnace at 800 C for 4 hours with a heating rate of 2 C/min under air to prepare the Li/MgO catalyst. Compositions of catalysts were analyzed by an Optima 7300 DV inductively coupled plasma atomic emission spectrometer (ICP-AES). N2 adsorption-desorption isotherms were measured on a Beckman Coulter SA3100 surface area analyzer. Catalysts were degassed at 300 ºC in N2 atmosphere prior to the measurements. XRD 5


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

Page 6 of 25

spectra were acquired on a Philips X’Pert PRO SUPER X-ray diffractometer with a Nifiltered Cu K x-ray source (wavelength: 0.15418 nm) operating at 40 kV and 50 mA. Transmission electron microscopy (TEM) measurements were performed on a JEOL2100F microscope. Catalytic performances of 5.6%-Li/MgO catalyst (40-60 mesh, grain sizes of approximate 250~420 μm) were evaluated in OCM and ODHE reactions with an ambient-pressure fixed-bed flow reactor. The composition of effluent gas was analyzed with two online GC-14 gas chromatographs. One was equipped with carbon-zeolite column and TCD detector for the separation and detection of CH4, CO and CO2, and the other with Porapak Q column and H2-flame ionization detector for the separation and detection of C2H4 and C2H6. The catalyst was activated in pure O2 (flow rate: 50 mL/min) at 450 C for 2 hours and then switched to the reactants. In oxidative coupling of methane reaction, 1 g catalyst was used and the reactants consisted of 8% CH4 and 4% O2 balanced with Ar (flow rate: 150 mL/min). Steady-state catalytic performances were measured at 650, 700 and 750 C. CH4 conversions were calculated from the change in CH4 concentration in the inlet and outlet gases, and selectivity of C2H4 and C2H6 were calculated the twice amount of produced C2H4 or C2H6 divided by the amount of converted CH4. In oxidative dehydrogenation of ethane reaction, 0.5 g catalyst was used and the reactants consisted of 20% C2H6 and 10% O2 balanced with Ar (flow rate: 100 mL/min). Steady-state catalytic performances were measured at 600, 650, 700 and 750 C. C2H6 conversions were calculated from the change in C2H6 concentration in the inlet and outlet gases, and C2H4 selectivity were calculated the 6


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

ACS Catalysis

amount of produced C2H4 divided by the amount of converted C2H6. The gas-phase components during OCM and ODHE reactions were detected via SVUV-PIMS measurements which were performed on the combustion station of National Synchrotron Radiation Laboratory (Hefei, China). As shown in Figure 1, a quartz catalytic reactor was connected to the SVUV-PIMS system, where the catalyst bed was placed 27 mm, 65 mm and 137mm from the sampling nozzle of the mass spectrometer. Before being loaded in the quartz reactor, the as-prepared 5.6%-Li/MgO powders were pressed into thin wafers and broken into approximately 2╳2 mm chips. 0.25 g catalyst (2╳2 mm) was activated firstly in pure O2 (flow rate: 50 mL/min) at 450 C for 2 hours and then switched to the reactants at a total pressure of 3 Torr. For OCM reaction, the reactants consist of 16% CH4 and 8% O2 balanced with Ar (total flow rate: 200 mL/min). For ODHE reaction, the reactants consist of 20% C2H6 and 10% O2 balanced with Ar (total flow rate: 200 mL/min). After the reactions reached steady state at the 750 oC, the composition of the effluent gas was analyzed by the online SVUVPIMS. A homemade time-of-flight mass spectrometer (TOF-MS) with a mass resolution of 3000 was used, and the m/z value was calibrated based on the flight time of hydrogen (m/z=2.016), water (m/z=18.011), carbon monoxide (27.995), oxygen (m/z=31.990), and carbon dioxide (m/z=43.990).

3. Results and discussion The structure and catalytic performance in the OCM reaction of 5.6%-Li/MgO catalyst were previously reported.26 The catalyst exhibits an actual Li content of 0.17 7


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

Page 8 of 25

wt%, a BET specific surface area of 2.2 m2/g and size distributions of 50-300 nm, and it achieves a CH4 conversion rate of 84.4 molmin-1m-2 and a C2 selectivity of 44.4% in OCM reaction. Figure 2A shows the SVUV-PIMS spectrum of gas-phase components of OCM reaction at 750 C acquired with photons at 10.6 eV and with a catalyst bed 137 mm from the sampling nozzle of the mass spectrometer. Evident signals appear at m/z values of 15, 28, 29, 42, 46, 47, 48 and 62. Figure 2 B-G and Figure S1 show the photoionization efficiency (PIE) spectra for these signals. The signals of m/z=15 (Figure 2B) and m/z=28 (Figure S1a) give ionization thresholds of 9.80 and 10.53 eV that agree well with the ionization thresholds of methyl radicals and ethylene (C2H4), respectively.34 The m/z=29 signal exhibits two PIE components with ionization thresholds of 9.87 and 10.53 eV. The major component with an ionization threshold of 10.53 eV comes from the C2H3D isotope of C2H4 while the minor component with an ionization threshold of 9.87 eV corresponds to a C2H5+ species previously observed as a decomposition product of unstable gas phase C2H5OO+ species.27 This indicates the presence of C2H5OO· species during OCM reaction although it was not directly observed in the present work. The m/z=42 signal also consists of two PIE components with ionization thresholds of 9.60 and 9.73 eV, and a careful examination of the mass spectrum (Figure S2) also demonstrates that it consists of two m/z signals of 42.011 and 42.043. The m/z=42.043 signal exhibits an ionization threshold of 9.73 eV and corresponds to propylene (C3H6) while the m/z=42.011 signal exhibits an ionization threshold of 9.60 eV and corresponds to ketene (CH2CO).34 CH2CO was observed as a key intermediate in a recent work of converting syngas to 8


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

ACS Catalysis

C2H4 via an OX-ZEO process.35 Its formation during OCM reaction can only arise from the gas phase coupling reaction between methylene radical (CH2) and CO and subsequently indicates the presence of highly reactive methylene radical intermediate. The observed C3H6 can also be produced by the gas phase reaction between CH2 and C2H4.36 Methylene radicals can be formed by the collisions and activation of methyl radicals on the catalyst surface in OCM reaction, but they are too reactive to be directly observed under our experimental conditions. The m/z=46 signal gives an ionization threshold of 10.0 eV (Figure S1b) and can be assigned to dimethyl ether (CH3OCH3).34 The m/z=47 signal gives an ionization threshold of 10.33 eV that agrees well with the ionization threshold of methyl peroxide radical.27 The m/z=48 signal gives an ionization threshold of 9.90 eV that agrees well with the ionization threshold of methyl hydroperoxide (CH3OOH).28 The m/z=62 signal exhibits two PIE components with ionization thresholds of 9.25 and 9.61 eV that correspond well to the ionization threshold of dimethyl ether peroxide (CH3OOCH3)

37

and ethyl hydroperoxide

(C2H5OOH), 28 respectively. As indicated by the ion intensity of the PIE spectrum, the C2H5OOH species dominates in the SVUV-PIMS spectrum and its observation also indicates the presence of C2H5OO· intermediate. The control experiments (Figure S3) showed no gas-phase signal at room temperature with the presence of catalyst or at 750 C without the presence of catalyst, which indicates all observed gas-phase species were produced by Li/MgO-catalyzed OCM reaction. Therefore, we employed the on-line SVUV-PIMS system to directly and unambiguously observe gas-phase reactive intermediates including CH3OO·, C2H5OO·, 9


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

Page 10 of 25

CH3OOH and C2H5OOH during the OCM reaction, in addition to CH3·observed previously.26 The CH3OO·, C2H5OO·, CH3OOH and C2H5OOH species have been long proposed as important gas-phase intermediates in OCM reaction but not been experimentally observed due to their very high reactivity and subsequent very low concentrations.38-42 As far as we know, our results represent the first solid experimental evidence for these gas-phase intermediates in the OCM reaction. In order to understand the gas-phase reaction network of OCM reaction, it is important to identify the observed gas-phase reactive intermediates as primary or secondary intermediates. We measured the SVUV-PIMS spectra under the same reaction condition with three different catalyst bed positions (Figure 1B), i.e., 27, 65 and 137 mm, away from the sampling nozzle of the SVUV-PIMS. Prior to the reach of the steady state, concentrations of primary gas-phase intermediates undergoing subsequent gas-phase reactions will keep decreasing with the flight time and thus the sampling distance while those of secondary gas-phase intermediates will initially increase. Meanwhile, the gas-phase species at the sampling position of 27 mm are likely produced by both surface reactions and gas-phase reactions while the variations of gas phase-species at the sampling positions of 65 and 127 mm should be contributed by the gas-phase reactions. Figure 3A, Figure S2 and Figure S4 show the SVUV-PIMS spectra sampled at different distances and acquired with different photon energies, from which integrated ion intensities of detected species as a function of the sampling position were derived (Figure 3B). Methyl radicals were detected at the shortest sampling position of 27 mm 10


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

ACS Catalysis

and their intensity decreased with the flight distance in the gas phase. This unambiguously confirms gas-phase methyl radicals formed by methane activation on the catalyst surface as the primary intermediate in OCM reaction. Intensities of other observed gas-phase species vary with the sampling position in different trends. The CH3OO·, CH3OOH, C2H5OOH and CH3OOCH3 species are absent at the sampling position of 27 mm, but appear at the sampling position of 65 mm and grow at the sampling position of 137 mm. Thus these species are formed by the gas-phase reactions initiated by methyl radicals, most likely the CH3·+O2 reaction, a previously observed gas-phase reaction between photolytically-produced alkyl radicals and molecular oxygen 27. The obvious growth of signals for C2H6, C2H4, CO and CO2 with the sampling position increasing from 65 mm to 137 mm proves that gas-phase reactions occur to produce these stable products. C2H6 can be formed by the selfcoupling of methyl radicals, and CO and CO2 should result from the combustion of CH3OO·, CH3OOH, C2H5OOH and CH3OOCH3 intermediates. C2H4 can be formed either by the self-coupling of methylene radicals or by the gas-phase reactions of C2H5·, C2H5OO· and C2H5OOH intermediates. The intensities of CO and CO2 species weaken very slightly as the sampling position increases from 27 mm to 65 mm. This suggests the exclusive occurrence of gas-phase reactions during this period, in which CH3OO·, CH3OOH, C2H5OOH and CH3OOCH3 species are formed but seldom undergo the subsequent combustion reactions to produce CO and CO2. This results in the decrease of the partial pressures of CO and CO2 in the gas phase. At the sampling position of 27 mm, signals of C2H6, C2H4, CO and CO2 were 11


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

Page 12 of 25

observed but those of CH3OO·, CH3OOH, C2H5OOH and CH3OOCH3 species were not. Thus the observed CO and CO2 productions should result from oxidation of methyl radicals on the catalyst surface, instead of from gas-phase oxidation reactions. This implies that the probability of gas-phase CH3·+O2 reaction should be much less than other reactions in the near-surface regions during the OCM reaction. C2H6 and C2H4 are supposed to be produced via reactions of CH3· either on the catalyst surface or in the gas phase in the OCM reaction.43 Campbell et al.

44

reported that at least 40% of

C2H6 was produced via the gas-phase self-coupling reaction of CH3· in the Li/MgOcatalyzed OCM reaction. Since only gas-phase reactions occur between the sampling positions of 27 and 137 mm, we thus can estimate that the contributions of gas-phase reactions to the formation of C2H6, C2H4, CO and CO2 are at least 59.7%, 73.7%, 52.5% and 60.0% in the OCM reaction under our experimental conditions, respectively. The above results demonstrate the productions and involvements of ethyl radicals, ethyl peroxide radicals and ethyl hydroperoxides in the OCM reaction. Thus we employed SVUV-PIMS to study ODHE reaction catalyzed by 5.6%-Li/MgO catalyst. The catalytic performance of 5.6%-Li/MgO catalyst in ODHE reaction is shown in Figure S5. It is noteworthy that the calculated carbon mass balances were closed under our experimental conditions. Figures 4A, 4B and Figure S6 respectively show the SVUV-PIMS spectra of gas-phase components of ODHE reaction sampled at different distances and acquired with photon energies of 10.0, 10.6 and 14.2 eV. Evident signals appear at m/z values of 15, 28, 29, 42, 44, 47, 48 and 62 in the SVUV-PIMS spectra acquired with photon energy of 10.6 eV. Both PIE spectra of observed signals (Figure 12


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

ACS Catalysis

S7) and enlarged SVUV-PIMS spectra (Figure S8) confirm the presence of gas-phase reactive intermediates similar to the case of OCM reaction, including CH3·, C2H4, C2H6, CH2CO, C3H6, CO2, CH3OO·, CH3OOH and C2H5OOH. The control experiments (Figure S9) showed no gas-phase signal at room temperature with the presence of catalyst or at 750 C without the presence of catalyst, which indicates all observed gasphase species were produced by Li/MgO-catalyzed ODHE reaction. However, in the SVUV-PIMS spectrum sampled at 27 mm and acquired with photon energies of 10.0 eV, a strong signal appears at m/z=29 and the corresponding PIE spectrum (Figure 4C) gives its threshold of 8.35 eV, corresponding to that of ethyl radical

29.

This result

demonstrates the direct observation of ethyl radicals during the ODHE reaction. Figure 4D displays integrated ion intensities of detected species in the SVUVPIMS spectra as a function of the sampling position for ODHE reaction. Both signals of ethyl radicals and methyl radicals decrease with the sampling position lengthening, confirming them as the primary intermediates. Thus ethane can be activated on the catalyst surface via the hemolytic cleavage of both C-H bond and C-C bond respectively to produce ethyl radicals and methyl radicals. The C2H5· signal almost disappears at the sample position of 65 mm while the CH3· signal is still clearly visible. This indicates that the C2H5· radicals are much more reactive than the CH3· radicals under the ODHE reaction condition. The CO2 formation was observed at the sampling position of 27 mm in ODHE reaction but the formation of alkyl peroxides radicals and alkyl hydroperoxides were not. Thus the observed CO2 production should result from oxidation reactions of ethyl 13


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

Page 14 of 25

and methyl radicals on the catalyst surface, instead of from gas-phase oxidation reactions. This suggests that the probability of gas-phase C2H5·+O2 and CH3·+O2 reactions should be much less than other reactions in the near-surface regions during ODHE reaction, similar to the case of OCM reaction. We can also estimate that the contributions of gas phase reactions to the formation of C2H4 and CO2 are at least 35.4% and 72.8% in ODHE reaction under our experimental conditions, respectively. It can be seen that the gas-phase contribution to C2H4 production is less in the ODHE reaction than in the OCM reaction, which can be attributed to the production of C2H4 via the secondary reaction of C2H5· radicals on the catalyst surface. The above SVUV-PIMS results of OCM and ODHE reactions demonstrate that both reactions exhibit similar reaction mechanisms. The differences come from the fact that only CH3· radicals are the primary intermediate in the OCM reaction and C2H5· radicals are the secondary intermediate while both CH3· and C2H5· radicals are the primary intermediates in the ODHE reaction. With the above SVUV-PIMS characterization results, we postulate an elementary reaction network of OCM and ODHE reactions (Figure 5) with directly-observed reactive gas-phase intermediates of alkyl radicals, alkyl peroxide radicals and alkyl hydroperoxides except methylene and ethyl peroxide radicals. The presence of methylene radicals can be indicated by the observations of CH2CO and C3H6 products, and the presence of ethyl peroxide radicals can be indicated by the observations of C2H5+ and C2H5OOH intermediates. An important different between the reactivity of methyl and ethyl radicals is that ethyl radicals can undergo surface reactions to produce ethylene. The proposed elementary 14


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

ACS Catalysis

reaction network provides so far the most reliable fundamental understanding of reactions mechanisms of OCM and ODHE reactions.

4. Conclusions In summary, employing an online SVUV-PIMS with high sensitivity and mass resolution connected to a catalytic reactor with adjustable distances between the catalyst bed and the sampling nozzle of mass spectrometer, we have successfully observed various gas-phase reactive intermediates, including methyl radicals, ethyl radicals, methyl peroxide radicals, ethyl peroxide radicals, methyl hydroperoxide and ethyl hydroperoxide, in the OCM and ODHE reactions, and established their gas-phase reaction network. These results greatly deepen fundamental understanding of OCM and ODHE reactions and are of great value for developing efficient catalysts and designing proper reactors.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (WH); [email protected] (YP). Notes The authors declare no competing financial interests.

15


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

Page 16 of 25

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Photoionization efficiency spectra for SVUV-PIMS signals of OCM reaction, SVUVPIMS spectra for m/z=42 signal of OCM reaction, SVUV-PIMS spectra of control experiments for OCM reaction, SVUV-PIMS spectra for gas-phase components of OCM reaction, Catalytic performance of 5.6%-Li/MgO catalyst in ODHE reaction, SVUV-PIMS spectra for gas-phase components of ODHE reaction, Photoionization efficiency spectra for SVUV-PIMS signals of ODHE reaction, SVUV-PIMS spectra for m/z = 42 signal of ODHE reaction, and SVUV-PIMS spectra of control experiments for ODHE reaction. ACKNOWLEDGMENTS This work was financially supported by the National Key R & D Program of Ministry of Science and Technology of China (2017YFB0602205), the National Natural Science Foundation of China (21525313, 91745202, 21703227), the Changjiang Scholars Program of Ministry of Education of China and the Collaborative Innovation Center of Suzhou Nano Science and Technology. REFERENCES 1. 2. 3.

McFarland, E. Unconventional Chemistry for Unconventional Natural Gas. Science 2012, 338, 340-342. Boswell, R. Is Gas Hydrate Energy Within Reach? Science 2009, 325, 957958. Taifan, W.; Baltrusaitis, J. CH4 Conversion to Value Added Products: 16


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

ACS Catalysis

4.

5. 6. 7.

8.

9. 10. 11. 12.

13.

14. 15.

16. 17.

18.

19.

Potential, Limitations and Extensions of a Single Step Heterogeneous Catalysis. Appl. Catal. B 2016, 198, 525-547. Farrell, B. L.; Igenegbai, V. O.; Linic, S. A Viewpoint on Direct Methane Conversion to Ethane and Ethylene Using Oxidative Coupling on Solid Catalysts. ACS Catal. 2016, 6, 4340-4346. Pakhare, D.; Spivey, J. A Review of Dry (CO2) Reforming of Methane over Noble Metal Catalysts. Chem. Soc. Rev. 2014, 43, 7813-7837. Hickman, D. A.; Schmidt, L. D. Production of Syngas by Direct Catalytic Oxidation of Methane. Science 1993, 259, 343-346. Wang, L. S.; Tao, L. X.; Xie, M. S.; Xu, G. F.; Huang, J. S.; Xu, Y. D. Dehydrogenation and Aromatization of Methane under Non-Oxidizing Conditions. Catal. Lett. 1993, 21, 35-41. Guo, X. G.; Fang, G. Z.; Li, G.; Mao, H.; Fan, H. J.; Yu, L.; Ma, C.; Wu, X.; Deng, D. H.; Wei, M. M.; Tan, D. L.; Si, R.; Zhang, S.; Li, J. Q.; Sun, L. T.; Tang, Z. C.; Pan, X. L.; Bao, X. H. Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen. Science 2014, 344, 616-619. Keller, G. E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane. 1. Determination of Active Catalysts. J. Catal. 1982, 73, 9-19. Schwarz, H. Chemistry with Methane: Concepts Rather than Recipes. Angew. Chem. Int. Ed. 2011, 50, 10096-10115. Driscoll, D. J.; Martir, W.; Wang, J. X.; Lunsford, J. H. Formation of GasPhase Methyl Radicals over MgO. J. Am. Chem. Soc. 1985, 107, 58-63. Ito, T.; Lunsford, J. H. Synthesis of Ethylene and Ethane by Partial Oxidation of Methane over Lithium-Doped Magnesium-Oxide. Nature 1985, 314, 721722. Wang, P. W.; Zhao, G. F.; Wang, Y.; Lu, Y. MnTiO3-Driven LowTemperature Oxidative Coupling of Methane over TiO2-Doped Mn2O3Na2WO4/SiO2 Catalyst. Sci. Adv. 2017, 3, e1603180. Lunsford, J. H. The Catalytic Oxidative Coupling of Methane. Angew. Chem. Int. Ed. 1995, 34, 970-980. Zavyalova, U.; Holena, M.; Schlögl, R.; Baerns, M. Statistical Analysis of Past Catalytic Data on Oxidative Methane Coupling for New Insights into the Composition of High-Performance Catalysts. ChemCatChem 2011, 3, 19351947. Hammond, C.; Conrad, S.; Hermans, I. Oxidative Methane Upgrading. ChemSusChem 2012, 5, 1668-1686. Schwach, P.; Pan, X. L.; Bao, X. H. Direct Conversion of Methane to ValueAdded Chemicals over Heterogeneous Catalysts: Challenges and Prospects. Chem. Rev. 2017, 117, 8497-8520. Arndt, S.; Laugel, G.; Levchenko, S.; Horn, R.; Baerns, M.; Scheffler, M.; Schlögl, R.; Schomäcker, R. A Critical Assessment of Li/MgO-Based Catalysts for the Oxidative Coupling of Methane. Catal. Rev. 2011, 53, 424514. Lomonosov, V. I.; Gordienko, Y. A.; Sinev, M. Y. Kinetics of the Oxidative 17


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

20.

21. 22. 23.

24.

25.

26.

27.

28. 29.

30.

31.

32.

Page 18 of 25

Coupling of Methane in the Presence of Model Catalysts. Kinet. Catal. 2013, 54, 451-462. Ahari, J. S.; Zarrinpashne, S.; Sadeghi, M. T. Micro-Kinetic Modeling of OCM Reactions over Mn/Na2WO4/SiO2 Catalyst. Fuel Process. Technol. 2013, 115, 79-87. Simon, Y.; Baronnet, F.; Marquaire, P. M. Kinetic Modeling of the Oxidative Coupling of Methane. Ind. Eng. Chem. Res. 2007, 46, 1914-1922. Sun, J.; Thybaut, J. W.; Marin, G. B. Microkinetics of Methane Oxidative Coupling. Catal. Today. 2008, 137, 90-102. Farooji, N. R.; Vatani, A.; Mokhtari, S. Kinetic Simulation of Oxidative Coupling of Methane over Perovskite Catalyst by Genetic Algorithm: Mechanistic Aspects. J. Nat. Gas. Chem. 2010, 19, 385-392. Schwarz, H.; Geske, M.; Goldsmith, C. F.; Schlögl, R.; Horn, R. Fuel-Rich Methane Oxidation in a High-Pressure Flow Reactor Studied by Optical-Fiber Laser-Induced Fluorescence, Multi-Species Sampling Profile Measurements and Detailed Kinetic Simulations. Combust. Flame 2014, 161, 1688-1700. Beck, B.; Fleischer, V.; Arndt, S.; Hevia, M. G.; Urakawa, A.; Hugo, P.; Schomäcker, R. Oxidative Coupling of Methane-a Complex Surface/Gas Phase Mechanism with Strong Impact on the Reaction Engineering. Catal. Today 2014, 228, 212-218. Luo, L. F.; Tang, X. F.; Wang, W. D.; Wang, Y.; Sun, S. B.; Qi, F.; Huang, W. X. Methyl Radicals in Oxidative Coupling of Methane Directly Confirmed by Synchrotron VUV Photoionization Mass Spectroscopy. Sci. Rep. 2013, 3, 1625. Meloni, G.; Zou, P.; Stephen Klippenstein, J.; Ahmed, M.; Leone, S. R.; Taatjes, C. A.; Osborn, D. L. Energy-Resolved Photoionization of Alkylperoxy Radicals and the Stability of Their Cations. J. Am. Chem. Soc. 2006, 128, 13559-13567. Pilling, M. J.; Robertson, S. H.; Seakins, P. W. Elementary Radical Reactions and Autoignition. J. Chem. Soc. Faraday Trans. 1995, 91, 4179-4188. Battin-Leclerc, F.; Herbinet, O.; Glaude, P.-A.; Fournet, R.; Zhou, Z.; Deng, L.; Guo, H.; Xie, M.; Qi, F. Experimental Confirmation of the LowTemperature Oxidation Scheme of Alkanes. Angew. Chem. Int. Ed. 2010, 49, 3169-3172. Zhang, X.; Ye, L.; Li, Y.; Zhang, Y.; Cao, C.; Yang, J.; Zhou, Z.; Huang, Z.; Qi, F. Acetaldehyde Oxidation at Low and Intermediate Temperatures: An Experimental and Kinetic Modeling Investigation. Combustion and Flame 2018, 191, 431-441. Li, Y. Y.; Qi, F. Recent Applications of Synchrotron VUV Photoionization Mass Spectrometry: Insight into Combustion Chemistry. Acc. Chem. Res. 2010, 43, 68-78. Zhou, Z.; Du, X.; Yang, J.; Wang, Y.; Li, C.; Wei, S.; Du, L.; Li, Y.; Qi, F.; Wang, Q. The Vacuum Ultraviolet Beamline/Endstations at NSRL Dedicated to Combustion Research. J. Synchrotron Rad. 2016, 23, 1035-1045. 18


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

ACS Catalysis

33.

34.

35.

36.

37.

38.

39.

40.

41.

42. 43. 44.

You, R.; Zhang, X.; Luo, L.; Pan, Y.; Pan, H.; Yang, J.; Wu, L.; Zheng, X.; Jin, Y.; Huang, W. NbOx/CeO2-Rods Catalysts for Oxidative Dehydrogenation of Propane: Nb–CeO2 Interaction and Reaction Mechanism. J. Catal. 2017, 348, 189-199. Linstrom, P. J.; Mallard, W. G.; NIST Chemistry Webbook, National Institute of Standards and Technology, MD, 2008; number 69, http://webbook.nist.gov, accessed on 05/06/2018. Jiao, F.; Li, J. J.; Pan, X. L.; Xiao, J. P.; Li, H. B.; Ma, H.; Wei, M. M.; Pan, Y.; Zhou, Z. Y.; Li, M. R.; Miao, S.; Li, J.; Zhu, Y. F.; Xiao, D.; He, T.; Yang, J. H.; Qi, F.; Fu, Q.; Bao, X. H. Selective Conversion of Syngas to Light Olefins. Science 2016, 351, 1065-1068. Martin, G. A.; Mirodatos, C. Evidence of Carbene Formation in Oxidative Coupling of Methane over Lithium-Promoted Magnesium-Oxide. J. Chem. Soc. Chem. Comm. 1987, 1393-1394. Kimura, K.; Osafune, K. Photoelectron Spectroscopic Study of Skew Compounds. III. N,N'-Dimethylhydrazine, Dimethyl Peroxide, and Dimethyl Disulfide. B. Chem. Soc. Jpn. 1975, 48, 2421-2427. Cavani, F.; Trifiro, F. The Oxidative Dehydrogenation of Ethane and Propane as an Alternative Way for the Production of Light Olefins. Catal. Today 1995, 24, 307-313. Lin, C. H.; Campbell, K. D.; Wang, J. X.; Lunsford, J. H. Oxidative Dimerization of Methane over Lanthanum Oxide. J. Phys. Chem. 1986, 90, 534-537. Shi, C.; Hatano, M.; Lunsford, J. H. A Kinetic Model for the Oxidative Coupling of Methane over Li+/MgO Catalysts. Catal. Today 1992, 13, 191199. Nelson, P. F.; Cant, N. W. Oxidation of C2 Hydrocarbon Products during the Oxidative Coupling of Methane over a Li/MgO Catalyst. J. Phys. Chem. 1990, 94, 3756-3761. Lin, C. H.; Wang, J. X.; Lunsford, J. H. Oxidative Dimerization of Methane over Sodium-Promoted Calcium-Oxide. J. Catal. 1988, 111, 302-316. Sun, J.; Thybaut, J. W.; Marin, G. B. Microkinetics of Methane Oxidative Coupling. Catal. Today 2008, 137, 90-102. Campbell, K. D.; Morales, E.; Lunsford, J. H. Gas-Phase Coupling of Methyl Radicals During the Catalytic Partial Oxidation of Methane. J. Am. Chem. Soc. 1987, 109, 7900-7901.

19


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

Page 20 of 25

Figure 1. (A) Schematic setup of an in-situ catalytic reactor connected with the SVUV-PIMS system. (B) Schematic illustration of adjustable distances between SVUV-PIMS sampling nozzle and catalyst bed in the quartz reactor.

20


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

ACS Catalysis

Figure 2. (A) SVUV-PIMS spectrum of gas phase components of OCM reaction catalyzed by 5.6%-Li/MgO at 750 C acquired with photons at 10.6 eV and at a sampling position of 137 mm. Corresponding photoionization efficiency spectra of signals m/z = 15 (B), 29 (C), 42 (D), 47 (E), 48 (F) and 62 (G).

21


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

Page 22 of 25

Figure 3. (A) SVUV-PIMS spectra of gas phase components of OCM reaction catalyzed by 5.6%-Li/MgO at 750 C acquired with photon energy of 10.6 eV at different sampling distances. (B) Integrated ion intensities of various components detected during OCM reaction in the SVUV-PIMS spectra acquired with photon energies of 10.6 eV (Fig. 2A and fig. S3), 11.8 and 14.2 eV (fig. S4) at different sampling positions.

22


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

ACS Catalysis

Figure 4. SVUV-PIMS spectra of gas phase components of ODHE reaction catalyzed by 5.6%-Li/MgO at 750 C acquired with photon energies of (A) 10.0 eV and (B) 10.6 eV at different sampling positions. (C) Photoionization efficiency spectrum of m/z = 29 in the SVUV-PIMS spectrum at a sampling position of 27 mm. (D) Relatively integrated ion intensities of various components detected during ODHE reaction in the SVUV-PIMS spectra acquired with photon energies of 10.0 eV (Fig. 3A), 10.6 eV (Fig. 3B and fig. S8) and 14.2 eV (fig. S6) at different sampling positions.

23


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

Page 24 of 25

Figure 5. Proposed reaction network of OCM and ODHE reactions. Molecules in black, red and blue colors respectively represent stable reactants/products, directlyobserved reactive intermediates and indirectly-observed reactive intermediates.

24


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

ACS Catalysis

TOC

25


MgO-Catalyzed Oxidative Coupling

Recommend Documents