Alkali-Added Catalysts Based on LaAlO 3 Perovskite for the Oxidative Coupling of Methane

: In this study, we aimed to enhance the catalytic activity of perovskite catalysts and elucidate their catalytic behavior in the oxidative coupling of methane (OCM), using alkali-added LaAlO 3 perovskite catalysts. We prepared LaAlO 3 _XY (X = Li, Na, K, Y = mol %) catalysts and applied them to the OCM reaction. The results showed that the alkali-added catalysts’ activities were promoted compared to the LaAlO 3 catalyst. In this reaction, ethane was first synthesized through the dimerization of methyl radicals, which were produced from the reaction of methane and oxygen vacancy in the perovskite catalysts. The high ethylene selectivity of the alkali-added catalysts originated from their abundance of electrophilic lattice oxygen species, facilitating the selective formation of C 2 hydrocarbons from ethane. The high CO x (carbon monoxide and carbon dioxide) selectivity of the LaAlO 3 catalyst originated from its abundance of nucleophilic lattice oxygen species, favoring the selective production of CO x from ethane. We concluded that electrophilic lattice oxygen species play a significant role in producing ethylene. We obtained that alkali-adding could be an effective method for improving the catalytic activity of perovskite catalysts in the OCM reaction.


Introduction
Since the industrial revolution of the late 18th century, total oil consumption has been increasing gradually with the industrial development of developing countries.Oil depletion is expected to accelerate under these circumstances.Shale gas has attracted significant attention as an upcoming alternative energy source.It has significant reserves in the world, and about 80% to 90% of its components consist of methane (CH4).As shale gas is drawing attention as an alternative energy source, the study of converting methane, which is the main component of shale gas, into high value-added compounds is being actively conducted [1][2][3][4][5].
CH4 has enormous potential as a feedstock for various valuable chemicals and fuel production.There are two methods of converting CH4 into high value-added compounds: direct conversion and indirect conversion of CH4.The indirect conversion of CH4 requires additional energy because it needs a multi-step process [6][7][8][9].For this reason, many researchers believe that the direct conversion of CH4 is more efficient in economic terms [10][11][12][13].Among the direct CH4 conversion methods, the oxidative coupling of methane (OCM) to form C2 hydrocarbons such as ethylene (C2H4) and ethane (C2H6) has recently attracted attention [14][15][16][17][18][19].It has been studied since the 1980s, but it has not been commercialized for various reasons until now.Representative reasons are that a high reaction temperature is required and that a complete oxidation reaction of CH4 occurs competitively.Therefore, it is essential to develop a catalyst capable of lowering the reaction temperature or of promoting partial oxidation of CH4 [20][21][22][23].
Many researchers have conducted studies to identify the active site of the catalyst with high catalytic activities in the OCM to overcome this problem.According to previous reports on the OCM, the Na2/WO4/Mn/SiO2 catalyst is considered the best catalyst for the OCM reaction [24][25][26][27][28]. Nevertheless, there remains a great deal of ambiguity concerning the nature and properties of the active sites because of their complex composition and structure [29].
There are many studies on catalysts for the OCM with a simpler structure than Na/W/Mn metal oxide catalysts [21,22].The most classically used catalyst is the MgO catalyst, which is an alkaline earth oxide catalyst.However, this catalyst is reported to show a low C2 yield of about 5% to 7% at 800 °C [30].Research on catalysts in which alkali metals such as Li are doped into alkaline earth metal catalysts to overcome the drawbacks of low reaction activity has been reported [31][32][33].For the MgO catalyst to which Li was added, the C2 yield increased by about 10% compared to the MgO catalyst.Therefore, many researchers have tried to improve the OCM catalytic activity using various additives.Among them, the most typical additive is alkali metal, and many studies have reported that it was added to promote the OCM catalytic activity [34][35][36][37].Rane et al. [38] reported that the C2 selectivity of CaO catalyst increased by adding alkali metals to the catalyst.Malekzadeh et al. [39] reported that the alkali metals were added to the (Mn + W)/SiO2 catalyst to enhance the catalytic activity.However, it is still relatively low to commercialize the OCM.
Meanwhile, the LaAlO3 perovskite catalyst is reported to have a simple structure and high thermal stability.Besides, it shows considerable catalytic activities in the OCM reaction and can be easily produced by various methods [40,41].Thus, the LaAlO3 perovskite catalyst can be considered as a good model catalyst to investigate the active site in the OCM.We aimed to improve the catalyst activities by adding alkali metals to the LaAlO3 perovskite catalysts and to demonstrate their catalytic behavior in the OCM reaction.

Catalyst Preparation
LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts were prepared using a citrate sol-gel method.Lanthanum nitrate (La(NO3)3•6H2O, Sigma Aldrich, St. Louis, MO, USA), aluminum nitrate (Al(NO3)3•9H2O, Sigma Aldrich), and alkali metals (lithium nitrate (LiNO3), sodium nitrate (NaNO3), potassium nitrate (KNO3) (Sigma Aldrich), were employed as metal precursors.0.00625 mol of La(NO3)3 precursor was dissolved in 25 mL of deionized water (solution A). 0.00625 mol of Al(NO3)3 was dissolved in 25 mL of deionized water (solution B). 5 mol % of alkali metals and 0.02632 mol of citric acid were dissolved in 100 mL deionized water (solution C).Solutions A and B were added dropwise into solution C upon stirring at 600 rpm.After stirring the mixed solution at room temperature for 1 h, the water was evaporated at 80 °C until a gel was obtained, which was then dried overnight in a convection oven at 200 °C.After grinding the dried gel using a mortar, LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts were finally obtained by calcination in a muffle furnace at 950 °C for 5 h.

Catalyst Characterization
The crystalline structures of the prepared LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts were confirmed by powder X-ray diffraction (XRD) measurements using X'pert-Pro PAN-analytical diffractometer with Cu-Kα radiation (λ = 1.54056Å).The diffraction patterns were recorded within the 2θ range: 10° to 90°.To confirm the specific surface area of the prepared catalysts, nitrogen adsorption-desorption isotherms were determined using a constant-volume adsorption apparatus (TriStar II 3020, Micromeritics) at -196 °C.The specific surface area was determined using the Brunauer-Emmett-Teller (BET) equation.Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was conducted using an ICPS-8100 spectrometer (Shimadzu) to confirm the chemical composition of the catalysts.The field emission scanning electron microscope (FE-SEM) images were obtained on an EM-30AX (COXEM).The prepared catalysts were pretreated by conducting resin and coating with platinum.X-ray photoelectron spectroscopy (XPS) analysis was conducted using a K-alpha instrument (Thermo Fisher) equipped with a monochromatic Al Kα X-ray source to determine the O 1s binding energies of the catalysts, which were calibrated based on the C 1s peak at 284.5 eV.CO2-temperature-programmed desorption (CO2-TPD) measurement was conducted using a BELCAT B (MicrotracBEL, Osaka, Japan) and each catalyst (0.05 g) was loaded into the quartz reactor for the TPD apparatus.It was pretreated at 400 °C for 1 h under a flow of He (30 mL/min).Once cooled down to room temperature with helium, 5% CO2 in He (30 mL/min) was pulsed into the reactor at room temperature for 40 min to adsorb CO2.Finally, it was purged at room temperature with He for 30 min to desorb physically adsorbed CO2.The CO2-TPD profiles of the catalysts were recorded from room temperature to 950 °C at a heating rate of 10 °C/min and the desorbed CO2 was detected by a thermal conductivity detector (TCD).

Catalytic Reaction
The OCM was conducted via a continuous flow quartz reactor with an internal diameter of 6 mm.The catalyst was loaded on the reactor by closing the upper and lower ends of the catalysts with quartz wool.The empty space of the reactor was filled with zirconia-silica ceramic beads.The reactor was heated to 775 °C under nitrogen flow, and the feed was then changed to the reactant feed (CH4:O2:N2 = 3:1:1, v/v/v).The total reactant volume flow rate was fixed at 20 mL/min with a gas hourly space velocity of 10,000 h −1 .A cold trap was employed to remove the water vapor produced during the reaction.An online gas chromatography system (YL-6500, Younglin, Korea) with a flame ionization detector (FID) and a thermal conductivity detector (TCD) was conducted to periodically analyze gas products.A carboxen-1000 column was used to separate the reaction products.CH4 and C2 hydrocarbons were detected by the FID, while CO and CO2 were detected by the TCD.The catalytic performance of CH4 conversion and C2 and COx selectivity were calculated using the following equations.The yield of C2 hydrocarbons was obtained by multiplying the value obtained for methane conversion with that of C2 selectivity.

CH conversion % =
moles of CH consumed moles of CH in the feed × 100 C Selectivity % = 2 × moles of C hydrocarbons in the output stream moles of CH in the feed × 100 CO Selectivity % = moles of CO in the output stream moles of CH in the feed × 100

Formation of LaAlO3_XY (X = Li, Na, K, Y = mol %) Catalysts
LaAlO3 catalysts are generally reported to have excellent thermal stability and considerable activity in the OCM.In this study, we prepared LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts by adding alkali metals to the LaAlO3 catalyst to increase the catalytic activity.The LaAlO3 catalyst was prepared in the same method for comparison.
We performed powder XRD measurements to confirm the structural properties of LaAlO3_XY and LaAlO3 catalysts.A JCPDS card (LaAlO3: #85-0848) was used to confirm all the XRD peaks [40].As shown in Figure 1, only XRD peaks for the LaAlO3 perovskite were found in LaAlO3_X5, LaAlO3_X10, and LaAlO3_X20 catalysts.It showed that LaAlO3_X5, LaAlO3_X10, and LaAlO3_X20 catalysts have been successfully prepared.However, LaAlO3_X30 catalyst showed other peaks with the LaAlO3 perovskite peak.Thus, we conducted an OCM reaction using LaAlO3_X5, LaAlO3_X10, LaAlO3_20 catalysts, excluding the LaAlO3_X30 catalyst.

Catalyst Performance of LaAlO3_XY (X = Li, Na, K, Y = mol %) Catalysts in the OCM
The prepared LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts were applied to the OCM reaction.Figure 2 shows the obtained results of the catalyst activities.For comparison, the catalytic activity of the LaAlO3 catalyst was shown in Figure 2. The reaction was conducted three times to show the average value of the catalytic activities.The standard deviation of the experimental values was shown by the red line.Interestingly, C2 yield and selectivity increased as the content on the added alkali metal increased.All LaAlO3_XY catalysts presented higher C2 yield and selectivity than the LaAlO3 catalyst.Among them, LaAlO3_Li20 catalyst showed the highest C2 yield.In particular, the ratio of CO and CO2 varied greatly depending on the prepared catalysts.Alkali metal-added LaAlO3_XY catalyst showed higher CO2 selectivity than the LaAlO3 catalyst.From these results, it was confirmed that LaAlO3_XY and LaAlO3 catalysts have the same perovskite structure but that they showed different catalytic activities.Additionally, it was confirmed that the catalytic activity can be promoted by adding alkali metals to the LaAlO3 catalyst.We obtained that, among LaAlO3_XY catalysts with increased activity, LaAlO3_X5, which had the effect of increasing activity with the smallest amount of alkali metal added, was the most meaningful.We tried to investigate the active site of the catalyst for the OCM using LaAlO3_X5 catalysts.

Catalyst Performance of LaAlO3_X5 (X = Li, Na, K) Catalysts in the OCM
Figure 3 shows the results of the catalytic activities for the OCM reaction using LaAlO3_X5 (X = Li, Na, K) catalysts.Figure 3 shows the LaAlO3 catalyst's results for comparison.The reaction was performed three times, as in the previous experiment, showing an average value of the activity.The standard deviation of the experimental values is shown by the red line.In the catalytic activities' results, the CH4 conversion of LaAlO3_X5 and LaAlO3 catalysts was about 30% for all four catalysts.However, the C2 selectivity, Cox selectivity, and C2 yield of prepared catalysts were considerably different.For C2 selectivity, LaAlO3_Li5, LaAlO3_K5, LaAlO3_Na5, and LaAlO3 were higher (in that order).LaAlO3_X5 catalysts increased by about 8% compared to the LaAlO3 catalyst.In C2 yield's results, LaAlO3_Li5, LaAlO3_Na5, LaAlO3_K5, and LaAlO3 were the highest (in that order).In general, LaAlO3_X5 catalysts increased by about 2% compared to the LaAlO3 catalyst.The CO selectivity and the CO2 selectivity of the four catalysts were also considerably different.The CO selectivity increased in the order of LaAlO3, LaAlO3_Li5, LaAlO3_Na5, and LaAlO3_K5.The CO2 selectivity was higher in the order of LaAlO3_K5, LaAlO3_Na5, LaAlO3_Li5, and LaAlO3.The prepared catalysts showed considerably different catalytic activities.Therefore, we analyzed the causes of these differences in activities and investigated the active sites of the catalysts for the OCM in this study.

Physical Properties and Chemical Composition of LaAlO3_X5 (X = Li, Na, K) Catalysts
Although LaAlO3_X5 (X = Li, Na, K) catalysts and the LaAlO3 catalyst had the same LaAlO3 perovskite structure, the catalytic activities were considerably different for the OCM.We performed BET, ICP-AES, and FE-SEM analysis to confirm the physical and chemical properties of the prepared catalysts prior to investigating the active site of the catalysts.
We performed BET analysis to compare the specific surface area of the catalysts used in the reaction.Table 1 contains the analysis results.From the BET analysis results, it was confirmed that LaAlO3_X5 catalysts had reduced specific surface area compared to the LaAlO3 catalyst.The specific surface area of the existing LaAlO3 catalyst was 3.5 m 2 /g, and the Li-added LaAlO3_Li5 catalyst had the smallest specific surface area of 1.4 m 2 /g.LaAlO3_Na5 and LaAlO3_K5 catalysts showed 2.9 and 3.1 m 2 /g of specific surface area, respectively.These results were consistent with the results of many studies that the addi-tion of alkali metals to conventional catalysts reduces the specific surface area of the catalyst.We confirmed that the catalysts' properties were influenced by adding alkali metals to the LaAlO3 catalyst through these results.Although the specific surface areas of the catalysts were reduced by adding alkali metals, the CH4 conversions of LaAlO3_X5 catalysts were similar to that of the LaAlO3 catalyst.These results are not common in catalysis; according to the studies, it is expected that the methyl radical binding occurs in the weather after CH4 is converted from the catalyst surface to methyl radicals [36].
Table 2 shows the ICP-AES analysis results for LaAlO3_X5 and LaAlO3 catalysts.For the LaAlO3_Li5 catalyst with the addition of Li, it was confirmed that all the Li added was volatilized during the calcination of the catalyst manufacturing process and did not remain.Li species evaporation is consistent with previous literature reports [42].According to previous studies, Li species in Li-doped MgO catalysts were vaporized at about 760-860 K through the reaction of Li species with oxygen.Therefore, it was expected that Li species were volatilized during the calcination process.For LaAlO3_Na5 and LaAlO3_K5 catalysts with the addition of Na and K, it was confirmed that about 1 mol % of Na and K remained.Studies have confirmed that when alkali metals, such as Na and K, are added in perovskite catalysts, the amount remains less than the actual amount added since the ionic radii do not match [43].We conducted an FE-SEM analysis on the alkali metal-added LaAlO3_X5 and LaAlO3 catalysts to confirm the surface changes of the catalysts.Figure 4 shows the results.The FE-SEM analysis result confirmed that the surface of the LaAlO3_X5 catalysts with the alkali metal added and the existing LaAlO3 catalyst were considerably different.For the LaAlO3 catalyst, it was confirmed that the surface of the catalyst was relatively smooth.However, LaAlO3_X5 catalysts showed relatively uneven surfaces.The catalyst surface of the LaAlO3_Li5 catalyst was considerably different compared to the LaAlO3 catalyst, even though there was no Li left in the catalyst.It was confirmed that Li was volatilized and that the structural change of the catalyst occurred.For LaAlO3_Na5 and LaAlO3_K5 catalysts, the addition of the added alkali metals caused a structural change in the LaAlO3 catalyst, which was expected.The three LaAlO3_X5 (X = Li, Na, K) and LaAlO3 catalysts showed quite different catalytic activities for the OCM.From the difference in catalytic activities, it was confirmed that the correlation between the active site of the catalyst, particularly the oxygen species on the surface of the catalyst and the activity, can be properly investigated.The active site of the catalyst used in the OCM has not been understood.Heterogeneous and homogeneous reactions occur simultaneously in the OCM reaction.The activity of the catalytic reaction is quite related to the surface oxygen species properties of the catalysts, according to many studies on the OCM [44][45][46][47][48].We conducted an XPS analysis for the prepared four catalysts to investigate the surface oxygen species properties of the catalysts.
XPS analysis for prepared the catalysts was performed, and the results are shown in Figure 5.The red line in the graph showed the nucleophilic lattice oxygen species (Olat(n)), and the blue line showed the electrophilic lattice oxygen species (Olat(e)).From these results, the relative areas and area ratios corresponding to the nucleophilic oxygen species (Olat(n)) and the electrophilic oxygen species (Olat(e)) of the catalytic lattice oxygen were calculated in Table 3.The LaAlO3_X5 catalysts to which the alkali metal was added possessed a relatively abundant electrophilic lattice oxygen species compared to the LaAlO3 catalyst.The LaAlO3_Li5 catalyst with the highest C2 selectivity had a relatively large amount of electrophilic lattice oxygen species (Olat(e)).However, the LaAlO3 catalyst with the lowest C2 selectivity possessed the least electrophilic lattice oxygen species (Olat(e)).For the nucleophilic oxygen species, it was confirmed that the oxygen species were abundant in the order of LaAlO3, LaAlO3_K5, LaAlO3_Li5, and LaAlO3_Na5 catalysts.The LaAlO3 catalyst with the highest COx selectivity had the relatively highest nucleophilic oxygen species, and the LaAlO3_Na5 catalyst with the lowest COx selectivity had the relatively lowest nucleophilic oxygen species.When these XPS analysis results were compared with the catalytic activities for the OCM, it was expected that the CH4 adsorbed by the adsorbed oxygen species on the catalyst surface would be converted to the methyl radical from the catalyst surface [49,50].After that, C2H6 was formed by the combination of two methyl radicals.Thus, we concluded that the electrophilic lattice oxygen species (Olat(e)) on the catalyst surface were responsible for converting C2H6 into C2H4, and the nucleophilic lattice oxygen species (Olat(e)) were responsible for converting C2H6 into CO or CO2 according to the strength of the nucleophilic lattice oxygen species.Many studies analyzed the strength of oxygen species on the catalyst surface using XPS and TPD analysis in the OCM.However, it is difficult to compare the strength of nucleophilic lattice oxygen species because of XPS.Thus, we performed CO2-TPD analysis to compare the strength of the nucleophilic lattice oxygen species (Olat(n)) [51].CO2-TPD analysis is an analytical method that analyzes the base point century of the catalyst surface by comparing the temperature at which CO2 is desorbed.In this study, CO2-TPD analysis was performed on prepared LaAlO3_X5 (X = Li, Na, K) and LaAlO3 catalysts, and the results are shown in Figure 6.Table 4 shows the temperature and amount of CO2 desorption for the prepared catalysts.From the CO2-TPD results, peaks displayed between about 500-700 °C mean nucleophilic lattice oxygen species (Olat(n)) according to the studies.Therefore, we obtained that the higher temperature at which CO2 is desorbed indicates the possession of a strong nucleophilicity of lattice oxygen by the catalysts.In the CO2-TPD analysis results, the temperature at which CO2 was desorbed increases in the order of LaAlO3_K5, LaAlO3_Na5, LaAlO3_Li5, and LaAlO3.These indicate that the LaAlO3_K5 catalyst had a strong nucleophilicity of lattice oxygen, and the LaAlO3 catalyst had a weak nucleophilicity of lattice oxygen.These results suggested that the catalysts possessing a strong nucleophilicity of lattice oxygen prefer CO2 in COx and those retaining a weak nucleophilicity of lattice oxygen produce CO more easily.

Conclusions
In this study, we prepared alkali-added perovskite catalysts (LaAlO3_XY, X = Li, Na, K, Y = mol %) by the citrate sol-gel method for the oxidative coupling of methane (OCM).It was expected that the catalytic activity could be further enhanced by adding alkali metals since the LaAlO3 perovskite catalyst has a considerable activity for the OCM reaction.As a result of conducting the OCM reaction using the prepared catalysts, we confirmed that LaAlO3_XY had increased activity compared with the LaAlO3 catalyst.We performed BET, ICP, and FE-SEM analyses to investigate the physical and chemical properties of LaAlO3_X5 catalysts.Through the analysis results of these characteristics, it was confirmed that adding alkali metals to the LaAlO3 catalyst influences the characteristics of the prepared catalysts.We performed XPS and CO2-TPD analyses to investigate the catalytic behavior of the prepared catalysts in this reaction.The XPS analysis result showed that LaAlO3_XY catalysts with high C2H4 selectivity had a relatively abundant electrophilic oxygen species (Olat(e)).The LaAlO3 catalyst with a high COx (carbon monoxide and carbon dioxide) selectivity had a relatively abundant nucleophilic oxygen species (Olat(n)).As a result of CO2-TPD analysis, it was obtained that LaAlO3_X5 catalysts had a strong nucleophilicity of lattice oxygen and preferred CO2 in COx.However, it was obtained that the LaAlO3 catalyst had a weak nucleophilicity of lattice oxygen and produced CO more easily.These results suggested that CH4 firstly converts methyl radicals, which are subsequently dimerized into C2H6 through a homogenous reaction.The nucleophilic oxygen species (Olat(n)) on the surface of the catalysts are responsible for converting C2H6 (produced from the dimerization of methyl radicals) into CO or CO2, and the electrophilic oxygen species (Olat(e)) are responsible for converting C2H6 to C2H4.Thus, we concluded that a perovskite catalyst rich in electrophilic oxygen species should be designed to obtain an efficient OCM catalyst and that alkali-adding could be one of the efficient methods for improving catalytic activity of perovskite catalysts for the OCM reaction.

3. 5 .
Investigation of the Active Site of LaAlO3_X5 (X = Li, Na, K) and LaAlO3 Catalysts on the OCM 3.5.1.The Active Site of LaAlO3_X5 (X = Li, Na, K) and LaAlO3 Catalysts for C2 Selectivity and COx Selectivity
a Peak area calculated from TPD profiles with CO2 adsorption.