Online Kinetics Study of Oxidative Coupling of Methane over La2O3

Oct 30, 2018 - High-resolution temperature-dependent product rates of OCM (CO2, ... as the activation energy barriers above their onset temperatures a...
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Research Article Cite This: ACS Catal. 2018, 8, 11761−11772

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Online Kinetics Study of Oxidative Coupling of Methane over La2O3 for Methane Activation: What Is Behind the Distinguished Light-off Temperatures? Zebang Liu,†,‡,§ Jerry Pui Ho Li,† Evgeny Vovk,† Yan Zhu,∥ Shenggang Li,∥ Shibin Wang,∥ Alexander P. van Bavel,⊥ and Yong Yang*,†

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School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai, 201210, People’s Republic of China ‡ Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 101407, People’s Republic of China ∥ CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, People’s Republic of China ⊥ Shell Technology Centre Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands S Supporting Information *

ABSTRACT: Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly to valuable C2 products (ethane and ethylene). The main difficulties from further investigation of this reaction are due to the nature of its high operating temperature and the severe reaction exothermicity. In this work, an especially designed online characterization setup is applied for this reaction, which achieved both precise bed temperature control and real-time product measurement. High-resolution temperature-dependent product rates of OCM (CO2, ethane, and ethylene) were obtained, and their behavior vs reaction conditions as well as the activation energy barriers above their onset temperatures are clearly differentiated over a recently reported high performance nanorod La2O3 catalyst. Different from general expectation, CO2, resulting from full methane oxidation, dominates all the products in the lower temperature region, whereas the C2 species are only formed at much higher temperatures. Further analysis indicates that the selectivity and apparent activation energy for both COx and C2 products are strongly influenced by the oxygen concentration and temperature. In combination with density functional theory calculations and additional experimental measurements, significant insights are brought to this high-temperature reaction of wide interest. KEYWORDS: oxidative coupling of methane, La2O3 nanorod catalyst, online mass spectrometry, high-temperature product analysis, density functional theory study

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usage alone.6 Ethylene is widely used by the chemical industry, primarily as a chemical building block toward materials such as polyethylene.7 Oxidative coupling of methane (OCM) is one of the most promising processes for natural gas and shale gas valorization to ethylene.8−10 However, many catalysts investigated for the OCM reaction do not produce a yield and conversion that meet the broadly accepted techno-economic targets of ∼35% C2 product yield and 90% C2 selectivity, at a reasonably high rate using undiluted air and methane feeds.11,12

INTRODUCTION The projected future global increase in diversification of industrial chemicals will hold an important role for natural gas utilization in value-added chemical products.1,2 Shale gas and methane hydrate provide enormous potential resources of natural gas as a feedstock. Thus, more efficient technologies for the utilization of natural gas will be vital.3−5 Natural gas production has considerable potential for sustainment of future global energy demands and with prospective areas of industrial applications such as conversion to value-added petrochemicals such as ethylene. Since the 1980s, global companies had already begun making plans to develop an economical process allowing for the upgrading of methane gas obtained from conventional gas directly into ethylene, an important petrochemical with >137 million metric tons of in global © XXXX American Chemical Society

Received: August 3, 2018 Revised: October 19, 2018 Published: October 30, 2018 11761

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effective, such catalysts have yet to demonstrate viability for commercial implementation due to lower C2 yields.31,32 Nonetheless, the low operating temperature associated with the catalytic properties of La2O3 catalysts gives it a huge potential for commercialization. In this work, we focus on studying the catalytic activity over La2O3 nanorod catalysts, previously synthesized by Zhu and co-workers,29 which had demonstrated low light-off temperature of methane and long-term stability under an industrial favored condition in comparison to commercial La2 O3 materials. By applying a custom designed microreactor, which allows for optimal heat exchange and temperature control, we are able to obtain consistent furnace and bed temperatures and reveal the related kinetics for the major OCM carbon products. The results clearly distinguish the different onset temperatures of the OCM products during the course of the reaction. With real-time online analysis under simulated industrial conditions, the apparent activation energy of each product above their onset temperatures are also obtained through the low-temperature end of the Arrhenius plot. Combining both experimental and computational studies, we also propose an explanation for the onset temperature difference.

A generally accepted pathway for the OCM reaction involves the catalytic formation of methyl groups,13,14 which desorb as free radicals (CH3•) that ultimately react via homogeneous pathways. Recombination of CH3 radicals in the gas phase forms C2H6, which subsequently dehydrogenates into C2H4.15 Scheme 1 highlights the possible elementary reaction pathways Scheme 1. Key Elementary Reaction Pathways from OCM, Involving Both Gas-Phase Reactions (Solid Lines) and Catalytic-Phase Reactions (Dashed Lines)

from OCM involving both the gas phase and catalytic phase reactions, derived from work by Lunsford et al.9 COx (CO and CO2), as the major unwanted byproducts, forms either via primary (direct oxidation from CH4 to COx) or secondary pathways (oxidation of C2 products to COx). Methane activation over metal oxides is already an extensively studied field through experimental9,16 and computational studies.17−19 However, driving the selectivity toward C2 products and relating the methane activation to intrinsic properties of the catalyst active sites remain a great challenge. Through DFT modeling, Janik et al. investigated the correlation between oxide catalyst reducibility with C2 product selectivity in OCM.20 This study highlights linear correlations between C−H activation energy, CH3 radical adsorption energy, and the oxygen vacancy formation energy for pure and doped metal-oxides. Janik et al. proposes that an extremely reduced surface results in a slower methane activation while a catalyst surface which is extremely reducible results in overoxidation of methane. Thus, there is an optimal amount of surface reducibility that results in a balance between methane activation and C2 selectivity; though a nonreducible oxide surface allows higher selectivity toward C2 product formation for OCM. On the experimental side, to our knowledge, no kinetic analysis has been conducted in detail for C2 selectivity in OCM. The challenge is the uncontrolled catalyst bed temperature increased by the heat generated from the water and COx formation. A significant temperature difference measured from the furnace and throughout the catalyst bed is normally observed in reactors with simulation of industrial OCM conditions due to the large exothermicity of the reaction. Base catalysts are favored for the OCM reaction due to their unique oxygen anion species (i.e., O−, O22−, O2−, etc.).21,22 Such species are typically formed by the alkali-metal dopant in the catalyst. A whole library of metal oxides have been investigated thoroughly for this reaction, many of which have proven to be effective catalysts. Such catalysts include lithium magnesia catalysts,15,23,24 manganese-doped catalysts,25,26 and in particular, alkaline-earth lanthanum oxide catalysts.27−30 Among these prospective OCM catalysts, La2O3 has attracted wide interest due to its catalytic performance in OCM. Though

2. METHODOLOGY 2.1. Catalyst Preparation. The synthesis of the La2O3 nanorod catalysts follows the recipe by Zhu and co-workers.29 Briefly, 25% NH3·H2O was added to an aqueous solution of 0.1 M La(NO3)3 and was vigorously stirred for over 1 h until a white precipitate was observed. The white precipitate was separated and collected. This white solid was washed with nanopure water and ethanol. Afterward, the white solid was suspended in nanopure water and left in the oven overnight at 105 °C. After heating, the separation, washing, and drying procedures were repeated and the product was finally dried in air overnight at 80 °C. 2.2. Catalyst Characterization. 2.2.1. SEM. Scanning electron microscopy (SEM) measurements were conducted using a JSM 7800F Prime SEM (JEOL Ltd.). 2.2.2. N2 Adsorption/Desorption Isotherm. The total surface area of the catalysts was measured by nitrogen adsorption at −196 °C using a Quantachrome Instruments Autosorb iQ2 instrument, using the BET surface area model. The results yield a BET surface area of 16 m2/g with a type I isotherm. Assuming a surface lattice density of 1015/cm2, this surface area yields about 260 μmol/g of surface sites. 2.2.3. XPS. Surface analysis of the catalysts was performed by XPS using a ThermoFischer ESCALAB 250X photoelectron spectrometer. The spectra were recorded using monochromated X-ray irradiation Al Kα (hν = 1486.7 eV) and a 180° double-focusing hemispherical analyzer with a six-channel detector. Sample calcination was performed using a UHVconnected high-pressure gas cell (Model HPGC 300, Fermi Instruments), which applies external heating to a sample sealed in a quartz tube. The available treatment temperature range is from room temperature to 900 °C, and the pressure range is from 1 to 10 bar. In the XPS measurements, the binding energies of the photoemission spectra were calibrated against the C 1s peak of adventitious carbon at 284.8 eV. 2.3. Reaction Kinetics Measurements. 2.3.1. Apparatus Setup. Two reactors are used in this paper. The first is our custom-designed microreactor loaded with a 1 in. quartz tube. The detailed setup was given in our previous publications.33 It 11762

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experiment, only CO2 is observed with a light-off temperature at ∼550 °C and a maximum yield of less than 5%. 2.3.3. Online Mass Spectrometer Measurements and Calibration. Gas analysis was achieved with a Pfeiffer Prisma quadrupole mass spectrometer. As mass spectrometry is used for gas detection, fragmentation correction is required, as the key reaction products have overlapping mass fragments. A fragment estimation assuming an exhaust with gas components of 35% CH4, 10% CO2, 10% CO, 5% C2H4, and 5% C2H6 is shown in Figure S2 in the Supporting Information, adapted from the NIST webbook of the MS library.34 For the analysis, the signals for CH4 (m/z 14), CO2 (m/z 44), and C2H6 (m/z 29, 30) do not require further subtraction from other elements, as the specified fragments are unique. For C2H4, the signals (m/z 26, 27) are always overlapping with C2H6; thus, a fragmentation correction is required. Both NIST webbook and our online MS measurement indicate that C2H6 fragmentation of m/z 26 and 30 has about a 1:1 ratio (Figure S2a). As a result, in the data processing of this work, when C2H6 was the only C2 product, m/z 26 and 30 curves almost overlapped. When C2H4 was also produced, m/e 26 increases much more strongly, as shown in Figure S2b. Afterward, a deconvolution of m/z 26 and 27 with precise fragmentation parameters was performed. After the above processes of the MS measurements, the small signal flow rates were calibrated into real flow rates in μmol/s with the 36Ar signal as the internal reference, which was used as the internal standard for fragmentation correction. To further confirm the calibrated results of MS signals, online micro GC for exhaust gas analysis was applied (Figure S3a). In the calibration process, the sample was exposed to the same reaction flow, and online GC measurements were performed at fixed temperatures around every 40 °C. This final calibration step also enabled CO signal (m/z 28) fragmentation correction from MS measurements. As the CO mass channel was overlapped with many other major products such as CO2, C2 H4, and C2H6 , it makes the direct fragmentation correction from MS raw data not as reliable as the other mass channels mentioned above. 2.3.4. Cofeed Reaction. Before testing, the sample was calcined ex situ at 800 °C and then transferred to the reactor followed by in situ heating to 760 °C for 40 min under a total flow of ∼20 sccm with the gas inlet consisting of Ar, CH4, and O2 (70%:23%:7% (14.4:4.8:1.4 sccm)). The controlled OCM reaction temperature was from 150 to 760 °C with a heating rate of 10 °C/min, if not specified otherwise. Gas detection was conducted using mass spectrometry, as outlined above. For determination of the activation energy, the Arrhenius equation was applied. After calibration, the conversion was estimated by

is designed with a special purpose for optimal heat exchange and temperature control with a special cooling spiral around the reactor tube, and the assembly is encased in a lightweight metallic heating block. A blank 3 mm bypass tube is also embedded in the same temperature-controlled region. Gas detection was performed by online mass spectroscopy (Pffeifer PrismaPlus), and the exhaust was connected to an online micro GC instrument (3000 Micro GC; Inficon) for MS signal calibration. Another reactor is closer to an industrial standard evaluation system, which was also used in our previous publications.29 OCM performed in this setup was evaluated in a fixed-bed 0.5 in. quartz tubular reactor with a cold trap placed at the outlet of the quartz tube in order to separate the condensed water vapor from the reaction products. A 100 mg portion of the sample diluted with 900 mg of quartz sand (sieve fraction 20− 30) was loaded for evaluation, which occupied about 10 mm of the internal volume. Gas detection was performed using an online micro GC instrument (3000 Micro GC; Inficon) equipped with two columns (molecular sieve 5A and a Plot U column) and two thermal conductivity detectors (TCD). 2.3.2. Reaction Flow and Temperature Control. A significant temperature difference measured between the furnace and the catalyst bed is normally observed for OCM in larger scale reactors due to the large heat of formation of up to ∼800 kJ/mol CH4 (complete oxidation of methane to water and CO2). In this study, the microreactor apparatus outlined above was used to evaluate the catalytic activity for OCM at atmospheric pressure. The apparatus is capable of managing a precise real-time temperature control of the catalyst under industrial OCM conditions, thus allowing intrinsic kinetic measurements. This is due to two factors. First, the reaction scale is much smaller in the microreactor with similar gas hourly space velocity (GHSV), which generates much less heat. Second, the microreactor was optimized with an internal structure for high heat conductivity and improved temperature control. Around the reactor tube, there is no heat insulation material like that in a conventional tube furnace structure. The heating block is also compact with a higher wall to volume ratio. Thus, the overall heat capacitance of the whole reactor is reduced to a minimum. In addition, this heat exchange is further enhanced by the embedded gas cooling spiral between the reactor tube and the heating block, which immediately carries away the excess heat if the temperature is over the thermocouple set point. In each of the experiments in the microreactor, the reactant gas mixture was prepared in flow-controlled manifolds. All gases are of UHP quality. The total sample loading is 20 mg of a calcined nanorod La2O3 sample physically mixed with silica gel (Sigma-Aldrich, high-purity grade, 200−425 mesh) with a total weight of 100 mg. In all experiments reported here, 20.6 sccm of total gas flow was generally used, if not otherwise specified, with a total pressure of 1.0 bar. For Arrhenius plots with constant gas feed, a GHSV was maintained at about 60000 h−1 g−1, with a CH4:O2:Ar gas input of 70%:23%:7% (14.4:4.8:1.4 sccm). Since no pressure change was observed over the course of the reaction, the overall GHSV was consistent throughout. This GHSV was close to the industrial laboratory standard of 72,000 cm3 h−1 g−1, which we applied for the 1/2 in. tube reactor. The temperature ramping was programmed from 50 to 760 °C with a heating rate of 10 °C/ min. A blank reaction was performed by observing the stream coming through the bypass (Figure S5). In the blank control

conversion (%) =

[CH4 feed] − [CH4 outlet] × 100 [CH4 feed]

Note that the conversion was not corrected for dilution effects. The reaction product yield was estimated by yield (%) =

n[product] × 100 [CH4 feed]

where n denotes the molar quantity of carbon in the product molecule. The molar selectivity was calculated by 11763

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n[product] × 100 CH4 converted

where in the data analysis, the CH4 converted is the total moles of carbon from the products of COx (CO, CO2) and C2 (C2H4, C2H6). A carbon balance analysis between input CH4 and output carbon components (COx, C2, and CH4 outlet) is plotted in Figure S3. 2.3.5. Temperature-Programmed Desorption (TPD). CO2TPD was performed under a constant Ar flow (20 sccm) to calibrate surface carbonates. The sample was pretreated in Ar at 760 °C for 1 h. CO2 exposed was performed with 1% CO2 flow (20 sccm, balanced with Ar) at 600 °C, held for 1 min. The sample was cooled to 130 °C under the same gas f low. The sample was held at 130 °C for 10 min with pure Ar flushing before TPD was performed. The TPD heating control was similar to the Arrhenius plot at the same ramping rate of 10 °C/min. For the CO2-TPD experiments, the amount of CO2 product was calibrated with 36Ar (0.337% nature abundance) as the internal standard and converted to the flow rate (nmol/s). 2.4. Computational Details. Periodic DFT calculations were carried out with the Perdew−Burke−Ernzerhof (PBE)35 exchange-correlation functional and the projector-augmented wave (PAW) method36,37 to describe the core and valence electrons as implemented in the Vienna ab initio program (VASP).38,39 The energy cutoff for the plane wave basis set was set to 520 eV. Bulk La2O3 and its surfaces studied in this work all have significant band gaps; therefore, Gaussian smearing with a width of 0.05 eV was used. The electronic energy was converged to 10−5 eV for the supercell, whereas the force on each relaxed atom was converged to 0.02 eV/Å. Symmetric and stoichiometric slab models of the La2O3 (001) and (011) surfaces were built from the optimized bulk structure. Two repeating units for the (001) surface and four repeating units for the (011) surface along the surface normal were included in the slab models with the bottom half fixed at their bulk positions, similar to the models used in our previous work.40 A vacuum layer of 15 Å was inserted between adjacent slabs, and reactions were allowed only on the relaxed side of the slab. A p(2 × 2) supercell for the (001) surface and a p(2 × 1) supercell for the (011) surface were used with a Γcentered Monkhorst−Pack41 k-point mesh of (3 × 3 × 1), which converges the total energy of the supercell to ∼10−4 eV. Transition states were located with the climbing image nudged elastic band (CI-NEB) approach.42,43

Figure 1. XPS La 3d5/2 spectra of sample (a) as-prepared, (b) after external 800 °C calcination during 4 h under atmospheric conditions, and (c) after 800 °C calcination in a high-pressure cell directly attached to the XPS spectrometer.

of spectra deconvolution is described in detail in work by Sunding et al.45 The spectra were fitted with three components: the main component, labeled c4f0, and two shakeup satellite components, labeled c4f1L bonding and c4f1L antibonding, as marked in Figure 1.c. The splitting of lanthanum 3d5/2 between c4f0 and c4f1L bonding peaks is sensitive depending on its surface electron structure. The La 3d5/2 spectrum of the as-prepared sample demonstrated a splitting of 3.9 eV (Figure 1.a), which is associated with the La(OH)3 compound.44,45 For the sample calcined at 810 °C in a muffle furnace, the spectrum indicated component splitting (Figure 1.b) identical with that of the as-prepared sample. However, after the sample was calcined in situ in the ultrahighvacuum system, the La 3d5/2 spectrum yielded a splitting of 4.7 eV (Figure 1.c), which is related to La2O3.44 Furthermore, C 1s spectra of the as-prepared sample and the sample calcined in the muffle furnace also demonstrates the presence of some carbonate (∼289.5 eV). After calcination under UHV, this carbonate-related peak is completely removed. This demonstrates that the lanthanum oxide surface components are very critical for exposure to the atmosphere. XPS does not provide the only evidence of this. For example, Vannice et al.,46 from their FT-IR studies, also noted that an uncalcined catalyst possesses a surface of only La(OH)3. For a more accurate and adequate analysis of the La2O3 surface, in situ treatments without any contact with air are crucial. Thus, all results below were obtained after in situ treatments. 3.2. Cofeed Reaction. The exothermicity-induced temperature difference between the bed and furnace were measured for both the 1/2 in. tube reactor and the micro reactor. The 1/ 2 in. tube reactor is the same reactor used in previous work by Zhu et al. Heaters of both the micro reactor and 1/2 in. tube reactor are imbedded with a thermocouple close to the tube

3. RESULTS AND DISCUSSION 3.1. SEM and XPS Characterization. In order to verify its structural stability after high-temperature treatment, the La2O3 sample was calcined at 810 and 1150 °C in a muffle furnace, followed by SEM (Figure S4). Multiple BET measurements were also performed and yield results of around 16 m2/g for both temperatures. Note that this method has a relatively high error due to the low specific surface area of the sample. When the BET and SEM results are combined, it is concluded that the sample shows no significant sintering in the temperature region above. Figure 1 presents XPS La 3d5/2 region spectra of the considered samples. The La 3d5/2 core level spectra were monitored and deconvoluted after Shirley background subtraction. Lanthanum compounds are known to show strong satellite peaks in photoelectron spectra.44 The method utilized 11764

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Figure 2. (A) Temperature measurements of the furnace temperature and the catalyst bed temperature from an industrial purpose test. (B) Comparison of temperatures of the micro reactor furnace and micro reactor bed. Points in (A) show the raw temperature data (the blue line represents the furnace temperature and red line the bed temperature). Points in (B) show the temperature difference between the furnace temperature and bed temperature.

conditions and ramping to 760 °C (Figure 4). At this temperature, we observe a CO2 yield of 11%, a CO yield of 3%, a C2H6 yield of 6.3%, a C2H4 yield of 4.6%, and a total of about ∼25% of CH4 being converted. Among all of the carbon products, CO has the smallest yield at 760 °C. It is generally accepted that the effective OCM reaction on oxides requires temperatures around or greater than 700 °C.47,48 Our results here confirms prior work by Zhu et al. that using this La2O3 nanorod catalyst allows for a lower light-off temperature.29 We can also confirm that no C3+ products were observed, which is consistent with prior work. Further, it can be seen that, in addition to the lower light-off temperature, there is a different temperature dependence for COx and C2 products that is clearly observed when a fast-time resolution online detection system is used. Figure 3 shows the initial CO2, CO, C2H6, and C2H4 product yield as a function of temperature. The curves clearly identify the temperature regions for each product onset at 0.1%. As shown, CO2 is formed at the lowest temperature, already beginning at ∼400 °C. Both CO and hydrogen are detected in the same temperature region; their low-temperature behavior will be separately discussed in a following publication. Both C2 products begin at significantly higher temperatures with the onset for C2H6 products at 520 °C and that for C2H4 at around 585 °C. Early methane combustion studies on oxides reported a CO2 appearance which is in line with other studies which have used mass spectrometry for gas detection.30,49 To the best of our knowledge, this is the first time the subsequent main carbon product components of OCM appearance have been observed at distinct temperatures. Carbonates were always observed from similar catalyst systems in previous studies, after either a simple exposure of CO2 or annealing in the ambient reactant after the OCM reaction. In this study, to figure out if the possible preadsorbed carbonate species will affect the CO2 initial product rate, we performed a CO2 TPD experiment using a calibrated mass spectrometer. The sample CO2 exposure preparation is described in section 2.3.5.

reactor region for temperature control feedback. To measure the bed and heater temperatures simultaneously, an additional thermocouple was placed through the inlet of the reactor tube and inserted directly into the catalyst bed. In the long-running and high-temperature experiments performed in the micro reactor in this paper, the thermocouple was only set in the heater, as contacts between the catalyst and the metallic thermocouple will gradually induce contamination to the small amount of catalyst loaded. In the case of the industrial reactor, two thermocouples were always used. We also performed the OCM reaction using a GHSV of 60000 cm3 h−1 g−1, which is comparable to the GHSV used in the industrial reactor (72000 cm3 h−1 g−1). To the best of our knowledge, such measurements are seldom performed using MS due to the complex calibration procedures required for it, particularly as applied to OCM on La2O3. It should be noted that our operating conditions are more in line with the industrial conditions used. Thus, the flow rates and observed conversions are higher than one would typically use for more fundamental kinetics studies. From the 1/2 in. tube reactor the bed and furnace temperature difference was very small at 450 °C. After that, the temperature difference immediately increased to 300 °C (Figure 2A). This is the light-off temperature of OCM on La2O3.29 On the other hand, in the 1 in. micro reactor, simultaneous measurement of the bed temperature and the furnace temperature indicated a minimum temperature difference from each other, as shown in Figure 2B. A closer inspection of the two temperature curves indicates that starting from 450 °C there is also a small temperature difference built up, but the maximum temperature difference is less than 20 °C as the bed temperature reaches almost 800 °C (Figure 2B). With the temperature difference between the catalyst bed and gas phase being minimized in our customized designed micro reactor, a temperature-dependent intrinsic kinetics study for the main OCM carbon products CO2, C2H6, and C2H4 was performed. After in situ annealing under an Ar flow, the La2O3 nanorods were cooled to 50 °C followed by exposure to OCM 11765

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a maximum; meanwhile, both C2 products rates increase sharply (Figure 4A). In this narrow temperature zone, there is a clear trend in this OCM reaction that the products transfer from 4+ and 2+ valences to 2− and 3− valences. Over this zone, this reaction becomes real partial oxidation. Further details regarding these coinciding behaviors will be elaborated further on in this section. An Arrhenius style plot was obtained for analyzing apparent activation energy, as shown in Figure 5. This is primarily to further analyze the temperature dependence of product formation, as the online MS is able to track the gas signals in real time. However, as industrially favored conditions are applied, maximum conversion of CH4 already exceeds the conventional upper limit, where significant C2 product can be observed. Thus, there is some sacrifice to the accuracy at the high-temperature range of energy barrier analysis. The Arrhenius plot emphasizes the temperature zone mentioned earlier. On application of the constant methane input flow rate of 14.4 sccm to the conversion ratio in Figure 4A, the actual product flow rates were obtained in mol/s. On normalization of the rate to the total sample loading (20 mg) or the corresponding surface area, the Arrhenius plot is presented as either activity (10−4 mol s−1 g−1, left scale) or TOF (1015 molecules cm−2 s−1, right scale). The processing results clearly elucidate that the turnover zone separates two kinetic regimes for the CO2 and C2 products. The CO2 product is always at the highest rate, but it levels off after the turnover zone. Meanwhile the C2 products continue to increase after this turnover zone with a much lower apparent activation energy barrier and finally the combined C2 selectivity becomes close to that of CO2. This would suggest that, as the reaction parameters pass the temperature turnover zone, a competition between COx and C2 formations begins. On combination of the O2 concentration information from Figure 4, after the turnover zone at about 700 °C, as pointed out in Figure 4A, oxygen is almost depleted in the reactor and the CO2 rate reaches a maximum, but both C2 products are still increasing steadily (Figure 4B). The above results in Figure 4 and the related discussion indicate that the oxygen concentration in OCM is reversely correlated with the C2 formation rate as the temperature increases. The production selectivity of COx and C2 as a function of the average oxygen concentration at the outlet during OCM (by varying the reactor temperature) is plotted in Figure 6. The selectivity of all carbon products is plotted as solid lines. The operating temperature is also plotted as a dashed line (right scale). In Figure 6, C2 selectivity should be correlated with both O2 concentration and temperature in the system. The red box marks the reaction conditions in the turnover zone, as the COx products are close to maximum. The blue box marks the reaction conditions at high temperature and near zero oxygen. The plot shows very clearly that, at lower temperature, the relatively rich O2 environment strongly favors COx formation. Once COx formation reaches a maximum after the turnover zone and consumes around twothirds of O2, the remaining O2 in the system is only used for C2 formation as the temperature increases. At temperatures higher than 700 °C, as oxygen almost depleted to the minimum, the C2 selectivity is nearly controlled by a single parameter, only responding to temperature increases (Figure 6B). It is not generally expected that CO2, as the fully oxidized product with carbon +4 valence, dominates the low-temperature carbon product in OCM, a partial oxidation reaction. To

Figure 3. Yield vs temperature of the OCM reaction over La2O3 catalyst by cofeeding 1:3 O2 and CH4. The figure highlights the appearance of CO, CO2, C2H4, and C2H6 “capped at 1% yield”.

In Figure S6, a major CO2 desorption peak is observed around 440 °C, which is in line with observations from Zhu et al.29,50 as well as observations in the literature.51,52 As shown in the results, it is a peak that ascends at 270 °C and has an apex at 440 °C and then descends with a broad shoulder around 600 °C. On applicataion of calibration with an internal Ar carrier with a fixed flow rate, integration of the total broad peak gives 1.4 μmol, which yields 70 μmol/g or ∼27% of the total surface coverage (on the basis of BET results and with the assumption of 1015 sites/cm2). No higher temperature features were observed between 600 and 760 °C. The maximum desorption rate at 440 °C is only 1.4 nmol/s. In comparison with the CO2 product rate at the same temperature in Figure 3, on applying the 14.4 sccm CH4 flow input rate to the yield curve, at the same temperature the CO2 flow rate in reaction is around 60 nmol/s. Indeed, before the OCM reaction process the surface was preheated in Ar at 760 °C and protected in the same inert gas afterward, so that the actual CO2 pre-exposure is very limited. Thus, the contribution of surface carbonate to CO2 is negligible in the reaction rate measurements. Figure 4 shows the results highlighting all observed components and their respective product yields and CH4 conversion in full scale. This result was confirmed with an online GC measurement of the product temperature dependence, as shown in Figure S2A. The main carbon product yields including CO2, CO, C2H6, and C2H4 output are plotted as a function of temperature in real time. The CO yield curve is also given in Figure 3, as was confirmed by online GC (Figure S2B). In Figure 4, the CH4 signal is plotted as net conversion by taking the ratio of CH4 converted and the CH4 total feed, as shown in the methodology. The oxygen output signal (right scale) was normalized to the inlet concentration. All carbon product yield curves demonstrated a distinct change in behavior at a temperature window of around 600 °C, which we will refer as a “turnover zone”. At temperatures below this zone, the overall reaction is mainly governed by total oxidation as CO2 (and CO) dominates the products. As the temperature increases into the turnover zone, the increase in CO2 formation rate decreases and the CO formation rate reaches 11766

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Figure 4. (A) Conversion/yield vs temperature of the OCM reaction over La2O3 catalyst by cofeeding 1:3 O2 and CH4. The panel showcases all the components: (a) normalized O2 outlet (right scale) and percentages of (b) CH4 conversion, (c) CO2 yield, (d) CO yield, (e) C2H6 yield, and (f) C2H4 yield. (B) Enlarged view (in the 580−640 °C temperature range) of the carbon products signals in the turnover zone, (C) Enlarged view around 700 °C of the carbon product signals.

further understand this, we illustrate an energy diagram of the OCM reaction in Figure S7, combining the apparent activation energy barriers obtained from the Arrhenius plot and the free energy data obtained from Fierro.53 CO is not included in this discussion, since both its formation rate and reaction heat of formation (−543 kJ/mol) are much less than those of CO2 over the whole process and thus will not affect the main conclusion below. In the low-temperature region, CH4 conversion is quite low and the fitted results are sufficient with the Arrhenius plot convention. The diagram shows that the apparent formation barrier of CO2 is only around 130 kJ/ mol, far less than those for both C2 species (190 and 342 kJ/ mol). As a result, it is expected that, at lower temperature with sufficient oxygen, thermodynamics favor the total oxidation of CH4 and yield only CO2 product. It is only after reaction conditions changed to lower oxygen concentration and much higher temperatures that C2 products are favored. At higher temperatures, after the turnover zone, the apparent energy barrier of ethane formation shows a sharp decrease. However, since CH4 conversion already exceeds the Arrhenius plot limit, the direct fitting result of the apparent energy barrier of ethane at 114 kJ/mol is not as reliable, as the results from

Figure 5. Arrhenius plot of the OCM reaction, showing OCM reaction activity (left scale) and turnover frequency (right scale), vs 1000/T and observing (a) CO2, (b) C2H4, and (c) C2H6.

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Figure 6. (A) Selectivity (%) vs relative O2 concentration at the outlet during the reaction (%) during the OCM reaction over La2O3 catalyst by cofeeding 1:3 O2 and CH4, showing (a) CO2 selectivity, (b) total C2 selectivity, and (c) the temperature (°C) during the course of the reaction vs the relative O2 concentration in rthe eaction (%). (B) Enlarged view (in the 0−5% relative O2 concentration range).

Instead, at a gas inlet feed temperatures of 475 °C the maximum activity was observed immediately (showing 98% O2 conversion), with both C2 and CO2 products being produced at the same time.29 It is only in the micro reactor with optimal heat exchange design which minimizes the temperature runaway effect that such intrinsic changes can be clearly distinguished. There is no essential difference between the OCM reactions in these two reactors, but the micro reactor is able to bring a more detailed insight into this reaction. This two-stage reaction model is further supported by analyzing the reaction heat distribution inside the 1/2 in. industrial standard evaluation reactor used in previous studies.29 As shown in Figure 7, at a fixed furnace temperature of 800 °C, the reactor internal temperature was measured at different points along the catalyst bed. As shown in the figure, in zone B, which is the catalyst bed, the average temperature is about 100 °C higher than the furnace temperature. However, within zone B, the temperature distribution is neither uniform nor symmetric. A peak temperature is observed only 2 mm from the upper flow and keeps decreasing from this point in the next 8 mm to the end of the catalyst bed. This indicates that most of the heat is generated in the beginning section of the catalyst region. Removing half of the catalyst in this setup resulted in losing most of the C2 products, while CO2 conversion remains at a similar level. This result is perfectly in line with the two-step model. In the beginning of the bed, the highest oxygen partial pressure and temperature conditions favor the exothermic formation of CO2 and generate the excess heat. This sharply increases the bed temperature to the maximum and consumes most of the oxygen input, which also pushes the reaction conditions into the C2-favored window. Part of the heat generation is thus reduced from 800 kJ/mol for CO2 toward 100 kJ/mol for C2 species. This explains why in the second half of the bed the temperature decreases but most C2 products come from this region. DFT calculations were performed to investigate CH4 activation and CH3 radical generation on the stoichiometric

lower temperature end. However, since a strong competition starts between the COx and C2 products, we expect that the apparent energy barrier of ethane is close to that at CO2 at low temperature, which is 130 kJ/mol. The temperature-dependence study indicates that this turnover zone between 580 and 640 °C covers several sharp changes in the OCM reaction, including oxygen concentration, C2 product selectivity, COx product maximum, and C2 formation barrier change. This suggests that further studies of the catalytic system within this zone are necessary, since these may not be simply attributed to coincidences, but rather to kinetics considerations (i.e., O2 partial pressure). Figure 6 also indicates that at temperatures higher than the turnover zone, where higher C2 selectivity is favored, the actual methane oxygen ratio is far higher than the initial ratio fed. On the other hand, the initial methane oxygen input ratio at around 3:1 is an optimized rate obtained from most standard industrial evaluation systems (for instance, the 1/2 in. tube reactor applied in this paper for temperature runaway comparison). The energy diagram in Figure 6 also helps describe the process and explains whether there is a conflict between these two results from different reactor designs. First, it is worth noting that the free energy change for CO2 formation is as large as 800 kJ/mol. As discussed above, at low temperature CH4 will exclusively form CO2, consuming most of the oxygen, and thus the reaction system will immediately release large amounts of heat. For a general designed tube reactor with much higher loading and lower thermal conductivity, this reaction heat of formation will be accumulated around the catalyst region and steeply boost the catalyst bed to a much higher temperature. Such a boost is what is shown in Figure 2A as the temperature runaway, which immediately raises the reactor temperature from 475 to 750 °C. The reaction temperature/oxygen conditions are thus pushed into the blue window in Figure 5, favoring higher C2 selectivity. In the 1/2 in. tube reactor, this process is so fast that such reaction condition transfer cannot be observed. 11768

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Figure 7. Measurement of reactor bed temperature across different points of the reactor, with a fixed gas inlet feed at 800 °C. (A) denotes the zone between the tube inlet and the catalyst bed, (B) the catalyst bed zone, and (C) the zone between the catalyst bed and the tube outlet.

the CH3 radical, followed by the coupling of two CH3 radicals in the gas phase. Hydrogen abstraction from CH4 by a surface oxygen species to form the CH3 radical is usually considered the rate-determining step in the OCM reaction,8 which is consistent with our predicted high endothermicity of this process on both La2O3 surfaces. Our calculations further show that the above hydrogen abstraction process consists of two elementary steps: CH4 activation at the lattice oxygen site on the La2O3 surface and desorption of the resulting CH3 species from the catalyst surface, both of which were predicted to be endothermic. Thus, the sum of the endothermicity in the dissociative adsorption of CH4 and the CH3 desorption energy yields the apparent activation energy for C2H6 formation, which is the energy required for hydrogen abstraction from CH4 by the La2O3 surface: 387.9 kJ/mol for La2O3 (001) and 354.1 kJ/mol for La2O3 (011). The La2O3 (011) surface is favored for this reaction pathway, and its calculated value is rather close to our experimentally estimated apparent energy barrier for C2H6 formation in the low-temperature region of 342 kJ/mol, as shown in Figure S7, suggesting that this surface may be the active surface for the OCM reaction in this temperature range. Thus, in the low-temperature region, the pathway for CH3 radical formation over the La2O3 catalyst

La2O3 (001) and (011) surfaces, with their potential energy surfaces shown in Figure 8. These two La2O3 surfaces were previously shown to be the most stable among the low-index La2O3 surfaces.40 The formation of C2H6 is generally accepted to occur via CH4 dissociation on the catalyst surface to form

Figure 8. Comparison of potential energy surfaces in kJ/mol for CH4 activation on La2O3 (001) (black) and (011) surfaces (green). 11769

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increase. This is interesting, since although thermodynamics favor C2 combustion to CO2,55,56 our results show that there is no net increase of COx products after 700 °C. Thus, in the high-temperature region, on the basis of our observations, it is impossible for the C2 produced to be further converted to CO2. In this high-temperature region, O2 has reached near 100% depletion; it is likely that any remaining O2 only contributes to C2 formation rather than combustion. This suggests that at least this part of the C2 products is formed in parallel with the CO2 product in the reaction pathways. Regarding the relation between the two C2 species, since C2H4 has an apparent energy barrier very close to that of C2H6 but comes with a higher formation temperature, this implies that ethylene is a downstream product of ethane, which was also suggested by previous publications.57,58

mainly involves CH4 dissociation on the (011) surface, resulting in the CH3 species adsorbed at the La bridge site followed by its direct desorption from the La site to form the CH3 free radical in the gas phase. It is interesting to note that the above simple pathway may only be related to a very high formation barrier of the CH3 radical. In this pathway, only the stoichiometric surface is involved in the catalytic process and no other surface species such as peroxide or vacancies are involved in the desorption step. However, this pathway does not seem to explain the formation of C2 products in the high-temperature region, since it only agrees with that in the low-temperature region (Figure 6). In the higher temperature zone, the C2 products were found to form at much higher selectivity, with a much lower apparent C2H6 formation barrier (114 kJ/mol). This suggests that, above the turnover zone, the catalytic surface is significantly changed and results in a major change in the C2H6 formation mechanism. The significant change in the surface structure at the turnover zone is further supported by the fact emphasized above that the CO2 production rate also maximizes due to the competition in O2 consumption between COx and C2 product formation. However, on consideration of the complexity of the reaction conditions at this temperature, no further suggestions from the modeling will be attempted in this work, and further experimental and computational studies on the structural change of the catalyst surface at the higher temperature zone under OCM reaction conditions will be reported in our future work. DFT results suggest that at lower temperature the C2 products are formed through an almost noncatalytic pathway. Previously it has also been suggested in the literature that undesired COx products may be formed by two pathways (Scheme 2): (a) in parallel reaction pathways directly from

4. CONCLUSIONS The customized microreactor design minimized the temperature runaway effect of OCM with the La2O3 catalyst system. In combination with online mass spectrometry, our results show very clearly the different temperature dependences of the major carbon products in OCM. The fully oxidized CO2 product dominates the low-temperature region starting at about 400 °C. Only at a much higher temperature of around 600 °C are C2 products from partial oxidation observed. A two-stage reaction model was suggested on the basis of the kinetics diagram. Further analysis correlates this model with the distinctive CO2 and C2 selectivity favored condition windows combining temperature and oxygen concentration parameters. In a large-scale reactor with minimized heat exchange design, these two reaction stages are immediately bridged by the boosting temperature runaway induced by the high reaction heat of CO2 formation dominating the lowtemperature, relatively oxygen rich window. This is further proved by an in situ temperature distribution analysis in a 1/2 in. tube reactor. In the micro reactor with efficient heat exchange, the temperature-dependent kinetics information reveals that, behind the temperature runaway, there is actually a dedicated turnover zone at around 600 °C in this reaction. Throughout this temperature turnover zone, the competition between the initial CO2 and C2 formations is indicated by the simultaneous sharp changes in the reaction rates and the selectivity trend. DFT calculations suggest that less efficient C2 formation at lower temperature is only related to La sites on La2O stoichiometry 3 surfaces without any add-on species. This suggests that, to understand OCM, further investigation must be focused on catalyst structure changes around the temperature turnover zone, which for this reaction has not been well controlled in previous studies.

Scheme 2. Possible Pathways of Forming COx in OCM

methane and oxygen via combustion reactions (Figure S5) and (b) in sequential reactions, indirectly by further oxidation of the C2 products.54 Our results so far still cannot exclude the possibility of either pathway. However, from the product temperature dependence, further insights are provided into this high-temperature process. First, it is clear that the products are produced under very different conditions. CO2 is the main product at lower temperature and under oxygen-rich conditions. This does not exclude the possibility that the CO2 product reaction pathway may still be involved with C2related intermediates at relatively low temperature. However, in the high-temperature region, it should be noted that reaction barrier of forming C2 is very different from those at lower temperature and C2 products are still efficiently produced under oxygen-poor conditions. Although the thermodynamics favor CO2 formation, as shown in Figure 4, it is observed that CO2 formation reaches a maximum at 700 °C while C2 products and CH4 conversion continue to

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b03102. Plot showing accumulated mass fragments of key OCM components and a comparison of mass fragments 26 and 30 for C2H6 and C2H4, micro-GC reaction data (for comparison and confirmation of the MS data presented in the work presented here), a carbon balance analysis of all carbon components during the course of OCM, SEM images of the La2O3 sample after extremely high temperature treatment, tabulation of La 3d5/2 intensities, 11770

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reaction data of a control experiment performing OCM with no catalyst present, a CO2 TPD up to 760 °C after CO2 pre exposed on the sample, and an energy diagram (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.Y.: [email protected]. ORCID

Jerry Pui Ho Li: 0000-0002-4891-3226 Shenggang Li: 0000-0002-5173-0025 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the startup support provided by ShanghaiTech University, the National Natural Science Foundation of China (No. 21573148, No. 91745105, No. 21473233), and Shanghai Pujiang Program (No. 15PJ1405800) for funding their participation in this work. The authors also acknowledge Shell Global Solutions International BV for additional funding provided through The Shell Foundation Grants (No. PT66201, No. PT32281), and in particular Dr. Carl Mesters and Dr. Alexander van der Made, from Shell Global Solutions International BV for helpful discussions. The authors also acknowledge the assistance from Xiaohong Zhou and Cairu Guan. This work was conducted in the facility of the School of Physical Science and Technology at ShanghaiTech University.

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