Electric Field and Mobile Oxygen Promote Low-Temperature Oxidative Coupling of Methane over La1-x Ca x AlO3-δ Perovskite Catalysts.

Oxidative coupling of methane (OCM) over La1-x M x AlO3-δ (M = Ca, Sr, Ba; x = 0, 0.1, 0.2, 0.3) in an electric field at low temperature (423 K) was investigated. Among the tested catalysts, the La0.7Ca0.3AlO3-δ catalyst showed the highest performance in terms of C2H6 + C2H4 yield (11.1%). Surface mobile oxygen species (O2 2- or O-), which were considered as active oxygen species for the OCM reaction, increased with increasing Ca doping amount, and thereby the La0.7Ca0.3AlO3-δ catalyst showed the best catalytic activity.

Introduction

Oxidative coupling of methane (OCM) is a reaction to produce C2 hydrocarbons from methane and oxygen directly. It has been attracting lots of attention and has been investigated from the 1980s because of the need for technical development to convert natural gas to added value chemicals.[1−5] Even though the OCM reaction is exothermic thermodynamically, it requires a high reaction temperature over 900 K because of high energy required for dissociation of the C–H bond in methane.[6] At such high temperature, C2 hydrocarbons tend to be oxidized to CO and CO2, resulting in a drastically decreased C2 selectivity. To prevent the undesirable nonselective reactions, we conducted the OCM reaction in a low-temperature region by applying an electric field. Under constant current to the catalyst-bed, the OCM reaction proceeded at the low reaction temperature of 423 K over Ce2(WO4)3/CeO2 catalyst.[7−9]

Many researchers have reported that the active oxygen species on the catalyst is an active site of the OCM reaction.[1,10−13] These oxygen species (e.g., O2–, O–, O22–, and/or O2–) abstract hydrogen atom from methane.[6] Perovskite-type oxide materials are known to generate such oxygen species with high mobility in low-temperature regions relatively.[14] Especially, among many kinds of perovskite-type oxides, LaAlO3 shows high OCM activity.[6,10,15,16] By doping alkali or alkali-earth metal cations, which have strong basic nature, high C2 selectivity can be obtained.[17,18] In addition, substitution of La3+ cation by such cations resulted in increase of oxygen deficiency in the lattice structure and this, in turn, contributed to production of O– and O22– type oxygen species.[19,20]

Herein, we investigated catalytic activity of OCM in the electric field at the low temperature of 423 K over alkali-earth metal cations (Ca, Sr, and Ba)-substituted LaAlO3 catalysts. The reaction mechanism and effects of substituted alkali-earth metal cations on its structure, electronic state, and OCM activity were also investigated.

Results and Discussion

First, we investigated the catalytic OCM activity of La0.7M0.3AlO3−δ (M = Ca, Sr, and Ba) at 423 K in the electric field, and the results are presented in Figure and Table S1. The results show that La0.7Ca0.3AlO3−δ, La0.7Sr0.3AlO3−δ, and La0.7Ba0.3AlO3−δ showed OCM activity at the low temperature of 423 K in the electric field (imposed current 3.0 mA). Among the tested catalysts, La0.7Ca0.3AlO3−δ showed the highest C2H6 + C2H4 yield and selectivity. It is noteworthy that the La0.7Ca0.3AlO3−δ catalyst showed higher C2H6 + C2H4 yield than that of the previously reported catalysts,[7,8] thanks to its redox property (details are discussed in following section). Moreover, the La0.7Ca0.3AlO3−δ catalyst showed stable activity for at least 180 min (Figure S1) without significant structural deformation and coke formation (Figures S2 and S3). From these results, La0.7Ca0.3AlO3−δ was the most suitable catalyst among the La–M–Al–O perovskite catalysts in the OCM reaction under electric field.

Figure 1

Results of OCM activity tests over La0.7M0.3AlO3−δ (M = Ca, Sr, Ba) perovskite catalysts at 423 K in an electric field (3 mA).

The effect of electric field on the activity of La0.7Ca0.3AlO3−δ was investigated by comparing the activity with or without the electric field. Results are shown in Figure and Table S2. As shown in Figure , with the aid of electric field, CH4 conversion, O2 conversion, and C2H6 + C2H4 yield for the reaction in the electric field were extremely high at low temperature, whereas no OCM activity was found without the electric field. On the other hand, in the reaction without the electric field, C2H6 + C2H4 yield steeply increased with temperature above 800 K. It is well known that perovskite oxide releases its lattice oxygen at high temperature (i.e., above 800 K for La0.7Sr0.3AlO3−δ), and it contributed to hydrocarbon conversion by the Mars van Krevelen mechanism.[21−24] The isotopic oxygen exchange tests revealed that the lattice oxygen was exchanged by gas phase molecular oxygen at 473 K with the electric field and at above 800 K without the electric field (see the Supporting Information, Figures S4 and S5). The temperature of the catalyst was measured by a thermocouple, and the effect of Joule heat by the electric field on the catalytic activity was negligible. From these results, it is considered that applying electric field to La0.7Ca0.3AlO3−δ promotes the release of its lattice oxygen even at low temperature below 700 K, and thereby the OCM reaction takes place.

Figure 2

Temperature dependencies of OCM activity with or without electric field over the La0.7Ca0.3AlO3−δ catalyst.

In addition, as shown in Table S1, field intensity of the applied electric field was ca. 150 V mm–1, which is much lower than that of a plasma-catalyst hybrid system.[25] Moreover, the present electrocatalytic system can proceed the OCM reaction at lower temperature (<700 K) than that of a high-temperature electrochemical reaction using a SOFC (solid oxide fuel cell) system (>873 K),[14,26] although the field intensity was higher than that of the SOFC system (<30 V mm–1). Therefore, the present reaction system, catalytic reaction in an electric field, is a mild and efficient low-temperature electrocatalytic OCM.

To clarify effects of Ca doping amount in LaAlO3 on its structure and OCM activity, La1–CaAlO3−δ catalysts with various Ca doping amount (x = 0, 0.1, 0.2, and 0.3) were prepared and evaluated. Details of the structural characterizations are described in the Supporting Information (Figures S6, S7, and Table S3). Figure and Table S4 show results of activity tests over La1–CaAlO3−δ catalysts in the electric field. O2 partial pressure decreased from 15% to 5% for comparison of these activities and selectivity precisely. In the case of LaAlO3 catalyst, spark discharge was observed due to its low electron, hole, and/or ion conductivity,[27] and no products were formed. As the Ca doping amount increased, CH4 and O2 conversions and C2H6 + C2H4 yield increased. Reportedly, the amount of lattice oxygen defect increases by substituting cations with different oxidation numbers in perovskite structures, which promotes redox reaction using lattice oxygen through the Mars van Krevelen mechanism.[21−24] In the present catalytic system, the lattice oxygen contributes to methane activation and C2 product formation. Therefore, it is considered that Ca doping to LaAlO3 contributed to increase the amount of lattice oxygen defects, and thereby the conversions and C2H6 + C2H4 yield increased as Ca doping amount increased.

Figure 3

Effects of the amount of Ca doping (x) in La1–CaAlO3−δ on conversions and C2H6 + C2H4 yield.

X-ray photoelectron spectroscopy (XPS) measurements were conducted to investigate the difference in electronic states of lattice or surface oxygen over La1–CaAlO3−δ catalysts. Figure S8 shows O 1s spectra of La1–CaAlO3−δ catalysts. Each O 1s spectrum could be deconvoluted to three peaks: the peaks at around 529, 531, and 532–534 eV were assignable to Olat (lattice oxygen; O2–), Os (surface or mobile oxygen; O22– and O–), and H2O(ad) (adsorbed H2O), respectively.[10,16,28−30] The mobile oxygen (Os) ratio was calculated by the following eq .

Tables and S5 present the result of O 1s XPS measurements. From Table , the Os ratio increases by increasing the amount of Ca doping. Therefore, we deduced the relationship between C2H6 + C2H4 yield and Os ratio as shown in Figure . A linear correlation was found between the Os ratio and C2H6 + C2H4 yield (and CH4 conversion), suggesting that the Os ratio over La1–CaAlO3−δ catalyst plays an important role in the OCM reaction. It was reported that electrophilic oxygen species (e.g., O–), which was formed by reaction of O2 molecule with metal cations or oxygen vacancies over the surface of metal oxide catalysts,[4,31,32] contribute homolytic C–H bond dissociation to form methyl radicals.[33−36] Also, it was reported that Os species (O– and O22–) participate in the redox reaction.[21,28] From the obtained results and literature, it is considered that Os on La1–CaAlO3−δ is the active oxygen species that participates in the OCM reaction as a redox active site via the Mars van Krevelen mechanism. Therefore, the OCM activity improved by Ca doping thanks to increase in the Os ratio as increasing the substituting amount of Ca.

Table 1

Surface Oxygen (Os) Ratio over La1–CaO3−δ (x = 0, 0.1, 0.2, 0.3) Catalysts Obtained by O 1s XPS Analysis

	oxygen distributions at surface/%
catalyst	H2O(ad)	Os	Olat	Os ratioa/%
LaAlO3	16.3	16.9	66.8	20.2
La0.9Ca0.1AlO3−δ	16.8	21.4	61.9	25.7
La0.8Ca0.2AlO3−δ	16.2	25.3	58.3	30.3
La0.7Ca0.3AlO3−δ	13.7	29.0	57.3	33.6

Os ratio (%) = {Os/(Os + Olat)} × 100.

Figure 4

Relationship between Os ratio and OCM activity of La1–CaAlO3−δ catalysts.

To elucidate the reaction pathway on the La0.7Ca0.3AlO3−δ catalyst in the electric field, the effect of contact time (W/FCH) was investigated. O2 partial pressure was set to 5% to evaluate the product selectivity at the initial stage. Figure and Table S6 present the influence of W/FCH on CH4 conversion, O2 conversion, and selectivity of each product. As shown in Figure a, CH4 conversion and O2 conversion increase as the contact time increases. As shown in Figure b, C2H6 selectivity decreased gradually as the W/FCH increased, and C2H4 selectivity was almost constant in low W/FCH region, and it decreased with further increase in W/FCH. On the other hand, C2H2 selectivity was almost zero under these conditions, suggesting that C2H2 was hardly produced by successive dehydrogenation of C2H4. CO and CO2 were formed even at low W/FCH, and CO selectivity increased as the contact time increased, which implies that CO and CO2 were produced from methane directly (parallel reaction) in low W/FCH region and by oxidation of C2 products (successive reaction) in high W/FCH region. From these results, the main reaction pathway is suggested as follows: C2H6 was formed by OCM reaction over the La0.7Ca0.3AlO3−δ catalyst at first, and then C2H4 was produced by oxidative dehydrogenation of C2H6, and finally they were oxidized to CO and CO2.

Figure 5

Effect of W/FCH on activity and selectivity of La0.7Ca0.3AlO3−δ catalyst: (a) conversions and C2H6 + C2H4 selectivity, (b) product selectivities.

In order to investigate the reactivity of C2 products, the reactant was changed from CH4 to C2H6 or C2H4. The results of each catalytic activity test are shown in Table S7. In each case, C2 hydrocarbons, CH4, CO, and CO2 were produced; however C3+ hydrocarbons were not detected. From Table S6, C2H6 conversion was lower than CH4 conversion, and C2H6 was mainly converted to C2H4 by oxidative dehydrogenation. Similar to the CH4 reaction, C2H2 selectivity in C2H6 or C2H4 reaction was also very low. Although main products from C2H4 were CO and CO2, C2H4 conversion was low despite the presence of unreacted O2. Considering the obtained results, the C–H bond of C2H6 was selectively dissociated rather than C–C bond dissociation. Also, both dehydrogenation and oxidation of C2H4 hardly proceeded in the present system. Scheme shows a possible reaction pathway. Therefore, the high C2H6 + C2H4 yield was obtained over the La0.7Ca0.3AlO3−δ catalyst with the electric field.

Scheme 1

Possible Reaction Pathway

Conclusions

In conclusion, OCM has been conducted over various perovskite oxide catalysts in the electric field at low temperature. Applying the electric field facilitated the OCM reaction under low reaction temperature (423 K). The La0.7Ca0.3AlO3−δ catalyst showed the highest C2H6 + C2H4 yield (11.1%) than ever. Surface mobile oxygen species (Os: O22– or O–) on La1–CaAlO3−δ was considered as active oxygen species for the OCM reaction via the Mars van Krevelen mechanism. The Os ratio increased with increasing the amount of Ca doping, and thereby the OCM activity improved. In this catalytic system, oxidative dehydrogenation of C2H6 easily proceeds, whereas successive C2H4 conversion hardly does, resulting in high C2H6 + C2H4 yield. La0.7Ca0.3AlO3−δ generated effective oxygen species in the electric field, which brought high C2H6 + C2H4 yield at low reaction temperature of 423 K.

Experimental Section

Catalyst Preparation

Perovskite oxide catalysts (La1–MAlO3−δ; M = Ca, Sr, Ba; x = 0, 0.1, 0.2, 0.3) were prepared by a citric acid complex method according to the reported procedure.[21,22,27] First, metal nitrate precursors were dissolved with distilled water in a Teflon beaker and stirred. Next, ethylene glycol and citric acid were added to this solution. The molar ratio of metal/citric acid/ethylene glycol was 1:3:3. This solution was heated to 353 K for 15 h, and the gel was heated and stirred to dry up the water completely. Precalcination was conducted at 673 K for 2 h. Thereafter, the obtained powder was calcined at 1123 K for 10 h.

Activity Tests

Catalytic activity tests were conducted in a fixed-bed flow type reactor with a quartz tube (6 mm o.d., 4 mm i.d.).[37]Figure S9 shows schematic diagram of the reactor. The catalysts were sieved to 355–500 μm, and 100 mg of it was inserted into the reactor. To impose electric field to the catalyst, two stainless-steel electrodes were inserted to the upper-side and the bottom-side of the catalyst-bed. The constant current of 3.0 mA was imposed to the catalyst bed using a dc power supply. The reactant feed gases were methane, oxygen, and argon in the ratio of CH4/O2/Ar = 25:15:60 (total flow rate 100 SCCM) for catalyst screening test and stability test and CH4/O2/Ar = 25:5:70 (total flow rate 100 SCCM) for investigating the effects of W/FCH and Ca-doping amounts. For determining the reactivity of ethane and ethylene, C2H6 and C2H4 were fed into the reactor instead of methane. The ratio of the feed gas was C2H6 or C2H4/O2/Ar = 25:15:60 (total flow rate 100 SCCM). The product gases were analyzed with a GC-FID (GC-2014s; Shimadzu Corp.) with Porapak N packed column and a handmade methanizer (Ru/Al2O3) and a GC-TCD (GC-2014s; Shimadzu Corp.) with molecular sieve 5A packed column. CH4 conversion, O2 conversion, C2 selectivity, C2 yield, C2H6 conversion, and C2H4 conversion were calculated by the following formulas. The carbon balance was almost 100%.

Characterization

The catalyst structure was characterized by powder X-ray diffraction (SmartLab III; Rigaku Corp.) at 40 kV and 40 mA with Cu Kα radiation and Raman spectroscopy (NRS-4500; Jasco Corp.) with green laser (λ = 538 nm). Transmission electron microscopy (TEM) observations were performed using field emission-transmission electron microscope (JEM2100-F; JEOL Ltd.) operated at 200 kV. The specific surface area of each catalyst was measured by N2 physisorption at 77 K with the Brunauer–Emmett–Teller method (GeminiVII; Micromeritics Instrument Corp.) after pretreatment at 573 K for 2 h in an Ar atmosphere (Table S8). XPS (VersaProbe2; ULVAC-PHI Inc.) measurements were conducted with an Al Kα X-ray source. The binding energies were referenced to C 1s peak at 284.8 eV. The 16O2/18O2 isotopic oxygen exchange tests in the temperature-programmed reaction or isothermal transient reaction were conducted using the fixed-bed continuous flow-type reactor equipped with a quadrupole mass spectrometer (QGA; Hiden Analytical Ltd.). Details are described in the Supporting Information.