Expanded Reactor Engineering Calculations for the Oxidative Coupling of Methane

: The catalytic activation of CH 4 by limited amounts of O 2 produces a mixture of synthesis gas (CO, H 2 ) and light hydrocarbons (C 2 H x ), the relative amounts of each depending on catalyst type and process conditions. Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La 2 O 3 /CeO 2 catalyst derived from the literature, this study replaces the activating O 2 with moist H 2 O 2 vapor to reduce synthesis gas production while improving C 2 H x yields and selectivities. As the H 2 O 2 content of the activating oxidant rises, more of the CH 4 conversion occurs in the gas phase instead of with the catalytic surface. In a packed bed reactor (PBR), the use of H 2 O 2 allows the PBR “light-off” to occur using a lower feed temperature. In exchange for a small decline in CH 4 conversion, C 2 H x selectivity increases while synthesis gas production drops. In a continuous stirred tank reactor (CSTR), H 2 O 2 improves C 2 H x over synthesis gas across a wider range of feed temperatures than is possible with the PBR. This suggests the CSTR will likely reduce OCM preheating requirements.


Introduction
The expanded use of hydraulic fracturing has resulted in the venting and flaring of large volumes of hydrocarbons, especially natural gas. A lack of local pipeline capacity results in more flaring [1] and fugitive emissions. Public sentiment is prompting government environmental regulators to force the reduction or outright banning of hydrocarbon flaring and fugitive emissions [2] to reduce climate change by global warming. Petroleum and natural gas companies are now actively promoting their efforts to reduce their methane footprints [3]. However, the engineering challenge of CH 4 conversion is considerable. Catalytic methods offer several conversion approaches.
Catalytic activation of CH 4 is generally classified as indirect or direct. Indirect activation produces synthesis gas (primarily CO and H 2 ) using an oxygen source by reforming (H 2 O-steam; CO 2 -dry) or partial oxidation (O 2 ). Synthesis gas can be catalytically converted to useful products such as alcohol (usually CH 3 OH) or higher hydrocarbons (by Fischer-Tropsch process).
Direct activation of CH 4 uses no oxygen source. It directly breaks the very strong CH 3 -H bond (4.39 × 10 5 J/mol). For example, methane dehydroaromatization (MDA) uses a Mo/HZSM-5 zeolite [4] catalyst to form C 2 H 4 and aromatics at 950-1030 K. Unfortunately, MDA is thermodynamically limited. In addition, catalyst activity drops quickly due to coke deposition.
An intermediate direct approach is the oxidative coupling of methane (OCM) that uses a very small amount of O 2 to activate the CH 4 while limiting the coke formation. The OCM catalysts are transition metal oxides on an oxide support, e.g., La 2 O 3 /CaO [5] and La 2 O 3 /CeO 2 [6]. Feed CH 4 /O 2 molar ratios of 7-11 with temperatures~840-1220 K Methane 2022, 1 59 have been studied. The products include C 2+ H x and synthesis gas, with the distribution depending on the catalyst and temperature. More feed O 2 favors CH 4 conversion, but lowers C 2+ H x selectivity in favor of CO x .
Gambo et al. [7] reviewed recent advances in OCM, including the use of catalyst nanowires, and identified avenues for further research. Using proprietary nanowire catalysts, Siluria Technologies demonstrated OCM in a continuous flow large demonstration plant [8,9]. The primary goal is the production of C 2 H 4 for subsequent conversion to gasoline and chemicals. Siluria envisions a flexible two-stage OCM reactor in which the first stage is a packed bed reactor (PBR) feeding CH 4 , O 2 , and possibly C 2 H 6 . The second stage feeds more C 2 H 6 in an endothermic pyrolysis plug flow reactor that uses the first stage exothermicity.
Considering the potential and the constraints of OCM, the reactor type becomes important. Conventional packed beds are not economically viable for OCM [10]. Other configurations such as membrane reactors and fluidized beds should be considered. A recent study [11] compared the packed bed reactor (PBR) and continuous stirred tank reactor (CSTR) for OCM based on calculations with a detailed reaction mechanism [6]. Higher feed temperatures were required to achieve a "light off" of the PBR, while the CSTR required considerably lower feed temperatures to reach nearly comparable conversions. The CSTR-a fluidized bed in likely practice-favored synthesis gas production over C 2+ H x as compared to the PBR.
In a difficult experimental study, Liu et al. [12] considered the rapid PBR "light off". Using careful temperature control and real-time product measurement in a micro-reactor during OCM over La 2 O 3 nanorod catalysts with a feed CH 4 /O 2 = 3, Liu et al. observed that CO 2 is the dominant product at the lower temperatures (<853 K) when relative O 2 concentrations are high. The system transitions through a window (~873 K) to higher temperatures (>913 K) that favor C 2+ H x at the lower O 2 levels. In effect, a competition between CO x and C 2+ H x formation occurs in this window. This suggests that reactor configuration and temperature will be critical for OCM reactor design. These observations also show that, in OCM, the production of byproduct syngas is unavoidable if C 2+ H x is the goal.
In typical OCM, some of the limited O 2 dissociates on the catalyst surface [13]. An adsorbed •O s then abstracts an H• from CH 4 to form the key •CH 3 gas phase radical. Oxygen atoms on the metal oxide lattice surface might directly abstract the H• from CH 4 . In this case, the gas phase O 2 replenishes the resulting surface vacancy, leaving behind an adsorbed •O s [7]. In either case, the •CH 3 radicals can combine to form C 2 H 6 . Further reactions form the desired C 2+ H x products, and the undesired CO x and coke. An alternate CH 3 -H bond activator would reduce CO x while possibly enhancing C 2+ H x formation. Unfortunately, the desired C 2+ H x products are more susceptible to oxidation than the reactant CH 4 . This can occur via gas phase O 2 or surface oxygen species [7,13].
An alternate or supplementary oxidant that might be less aggressive toward C 2+ H x by reducing surface oxygen species while still activating the CH 4 is the •OH gas phase radical. Gas phase hydroxyl radicals can be formed from the gas phase decomposition of co-fed H 2 O 2 vapor: H 2 O 2 + M = 2 •OH + M. Section 3 below summarizes experimental literature on the use of H 2 O 2 for OCM that motivates this paper.
This paper is also a sequel to the Rivera et al. [11] study on OCM. It uses the same elementary reaction mechanism as that developed by Karakaya et al. [6]. The paper considers vapor phase H 2 O 2 as a supplemental or alternative activating oxidizer to reduce syngas production in favor of C 2+ H x . Both CSTR and PBR are considered.

Kinetic Mechanism and Computational Tool
The detailed reaction mechanism used in this study was developed by Karakaya et al. [6]. It is composed of series and parallel elementary reactions [13] in both the gas and surface phases. The gas phase portion is taken from Chen et al. [14]. The surface portion is inspired by the work of Alexiadis et al. [15].
In the current study, the OCM mechanism was employed in reactor simulations by Detchem ® [16]. This program achieves material and energy balances using the mechanism, based on required reactor input data and parameters. In this study, separate adiabatic PBR (modeled as plug flow) and CSTR (modeled as perfectly mixed) runs were conducted with the Detchem ® PBED and CSTR applications, respectively. See [11] for a listing of the governing balance equations used in each reactor simulation.
Comparative results were prepared in terms of conversions X CH4 , selectivities S j of useful products (C 2 H x , CO, H 2 ) or byproducts (H 2 O, CO 2 ), and yields Y j : where F j = molar flow rates, F j,in = molar rate at the reactor inlet, and n j is the number of CH 4 moles needed to make one mole of product (byproduct). For example, for C 2 H 4 , n j = 2; for H 2 , n j = 0.5.
In the prior study [11] of OCM with O 2 (no H 2 O 2 ) as the activator, twenty cases were considered in separate CSTR and PBR calculations. The process parameters considered are summarized in Table 1. The parametric study included the molar feed CH 4 /O 2 ratio ("low" = 7, "high" = 11; LR and HR, respectively), molar feed rate ("low" = 8.984 × 10 −5 , "high" = 1.412 × 10 −4 mole/s; called LF and HF, respectively), and feed temperature (843-1243). At each of the five feed temperatures, four cases were considered: LR_LF, LR_HF, HR_LF, and HR_HF. These conditions were inspired by published laboratory data [6]. Each reactor simulation assumed the same catalytic site density and total catalytic surface area, resulting in the same 2.42 × 10 −6 total moles of sites. A processing rate can be defined as the ratio of total molar feed rate to the total number of catalyst sites. The processing rate range is 37.1-58.3 s −1 . These parameters motivate the present study. Larger reactors can be scaled from these conditions.  [18,19] observed that H 2 O 2 increased CH 4 conversion, while enhancing C 2+ hydrocarbon yields even up to benzene. They speculated that H 2 O 2 decomposition resulted in more •OH radicals for gas phase activation of the CH 4 . They even observed OCM in the presence of H 2 O 2 vapor without catalyst at temperatures as low as 673 K, with considerable selectivity for C 2 H x . These studies motivate this paper.
Considering the importance of making any OCM process as "green" as possible, a potentially sustainable source of H 2 O 2 uses a photo-activated TiO 2 -Au-Si catalyst while feeding O 2

Incremental H 2 O 2 Replacing O 2 to PBR at Fixed Feed Temperature
Rivera et al. [11] showed that, for the parametric range considered (Table 1), the highest CH 4 conversions for the PBR were nearly 40% for the LR cases, with little impact of flow rate, for a 1243 K feed temperature. The LR_LF case showed the highest sum_C 2 H x (sum of C 2 H 6 , C 2 H 4 , and C 2 H 2 ) selectivities and yields. First, for the same 1243 K feed temperature, H 2 O 2 was incrementally substituted for O 2 . It was assumed that H 2 O 2 vapor will be available at 20 mole percent with the balance of H 2 O vapor. The starting point was the LR_LF (feed CH 4 /O 2 = 7, feed processing rate 37.1/s) case [11] in the PBR. The PBR bed length was that used in the experiments described elsewhere [6]. This high feed temperature did ignore the H 2 O 2 stability issue. The insights gained, however, will help identify the utility of H 2 O 2 as a potential CH 4 activator. Figure 1 shows that the final PBR bed temperature drops with increasing H 2 O 2 content. This was attributed to the reduced overall reaction exothermicity suggested by Equations (1) and (2). However, the bed temperature peaked much earlier when H 2 O 2 was used in the feed oxidant. A closer examination of the post-entrance region is revealed in Figure 2 below. The H 2 O 2 -containing cases showed a near-immediate rise from the 1243 K feed temperature. The very early single peak for the R = 1 (no feed O 2 ) case roughly corresponded to the exhaustion of the H 2 O 2 , which was consistent with the extremely rapid H 2 O 2 decomposition described earlier at this temperature. The presence of the feed O 2 caused a second local temperature peak further downstream. These peaks (R = 0.33, 0.67 cases) and the much later single peak (R = 0) corresponded approximately to where the O 2 ran out, with no further adsorbed •O s .
The R = 1 case (no feed O 2 ) showed almost no adsorbed species, suggesting that all the CH 4 conversion effectively occurred in the gas phase (i.e., non-catalytic). On the contrary, for the R = 0.33 and 0.67 cases, while H 2 O 2 was still present, there was a complex parallel/series scheme ongoing with both catalytic and gas-phase reactions occurring.
These observations were consistent with the claim that the CH 4 conversion is accelerated by •OH gas phase radicals produced from the H 2 O 2 dissociation [18,19]. The R = 1 case (no feed O2) showed almost no adsorbed species, suggesting that all the CH4 conversion effectively occurred in the gas phase (i.e., non-catalytic). On the contrary, for the R = 0.33 and 0.67 cases, while H2O2 was still present, there was a complex parallel/series scheme ongoing with both catalytic and gas-phase reactions occurring.
These observations were consistent with the claim that the CH4 conversion is accelerated by •OH gas phase radicals produced from the H2O2 dissociation [18,19].   The R = 1 case (no feed O2) showed almost no adsorbed species, suggesting that all the CH4 conversion effectively occurred in the gas phase (i.e., non-catalytic). On the contrary, for the R = 0.33 and 0.67 cases, while H2O2 was still present, there was a complex parallel/series scheme ongoing with both catalytic and gas-phase reactions occurring.
These observations were consistent with the claim that the CH4 conversion is accelerated by •OH gas phase radicals produced from the H2O2 dissociation [18,19].   Figure 3 shows that increasing the H 2 O 2 content improved the selectivity of sum_C 2 H x , while lowering both the selectivity and yield for syngas (H 2 + CO). There was a negligible impact on sum_C 2 H x yields. The reduction in syngas was due almost entirely to a reduction in CO. Finally, for these four cases from R = 0 to 1, the CH 4 conversions were: 39.3, 36.6, 35.1, and 33.8%, respectively. In all cases, the final O 2 and H 2 O 2 conversions were 100%. a negligible impact on sum_C2Hx yields. The reduction in syngas was due almost entirely to a reduction in CO. Finally, for these four cases from R = 0 to 1, the CH4 conversions were: 39.3, 36.6, 35.1, and 33.8%, respectively. In all cases, the final O2 and H2O2 conversions were 100%. An expanded look at the long-post-entrance region provides more insight into the dramatic impact of substituting some of the feed O2 with H2O2 vapor. Figure 4 shows the selectivities for the CO, C2H6, C2H4, and CH4 conversions for the LR_LF case, at 1243 K feed temperature, for feed ratio cases R = 0 and R = 0.33. The curves were almost unchanged after the 0.006 m bed length was reached. Some key points can be made here. In the R = 0 case, CO selectivity exceeded C2H4 before the temperature peaked (see Figure 1), but was lower than C2H6. After the peak temperature, consistent with experimental observations by Liu et al. [12], C2H4 exceeded CO. In the R = 0.33 case, the H2O2 (not shown) dissociated immediately upon entry. The resulting •OH radicals abstracted •H atoms from CH4, causing a spike in C2H6 formation, An expanded look at the long-post-entrance region provides more insight into the dramatic impact of substituting some of the feed O 2 with H 2 O 2 vapor. Figure 4 shows the selectivities for the CO, C 2 H 6 , C 2 H 4 , and CH 4 conversions for the LR_LF case, at 1243 K feed temperature, for feed ratio cases R = 0 and R = 0.33. The curves were almost unchanged after the 0.006 m bed length was reached.
Methane 2022, 1, FOR PEER REVIEW 6 a negligible impact on sum_C2Hx yields. The reduction in syngas was due almost entirely to a reduction in CO. Finally, for these four cases from R = 0 to 1, the CH4 conversions were: 39.3, 36.6, 35.1, and 33.8%, respectively. In all cases, the final O2 and H2O2 conversions were 100%. An expanded look at the long-post-entrance region provides more insight into the dramatic impact of substituting some of the feed O2 with H2O2 vapor. Figure 4 shows the selectivities for the CO, C2H6, C2H4, and CH4 conversions for the LR_LF case, at 1243 K feed temperature, for feed ratio cases R = 0 and R = 0.33. The curves were almost unchanged after the 0.006 m bed length was reached. Some key points can be made here. In the R = 0 case, CO selectivity exceeded C2H4 before the temperature peaked (see Figure 1), but was lower than C2H6. After the peak temperature, consistent with experimental observations by Liu et al. [12], C2H4 exceeded CO. In the R = 0.33 case, the H2O2 (not shown) dissociated immediately upon entry. The resulting •OH radicals abstracted •H atoms from CH4, causing a spike in C2H6 formation,  Some key points can be made here. In the R = 0 case, CO selectivity exceeded C 2 H 4 before the temperature peaked (see Figure 1), but was lower than C 2 H 6 . After the peak temperature, consistent with experimental observations by Liu et al. [12], C 2 H 4 exceeded CO. In the R = 0.33 case, the H 2 O 2 (not shown) dissociated immediately upon entry. The resulting •OH radicals abstracted •H atoms from CH 4 , causing a spike in C 2 H 6 formation, and a quickly rising CH 4 conversion. The C 2 H 6 rapidly dehydrogenated to C 2 H 4 . The CO peaked at approximately where the temperature peaked. Unlike the R = 0 case, both C 2 species exceeded CO prior to the temperature peak. This all occurred much faster than for the R = 0 case. The ultimate CH 4 conversion for the R = 0.33 case was only slightly lower than for the R = 0 case, while showing a higher C 2 H 4 selectivity and lower CO. Liu et al. [12], for the R = 0 case, concluded that the selectivities of COx and C 2 depended on local O 2 concentration and temperature. Using H 2 O 2 added the further complexity of gas phase chemistry to the surface reactions.

Use of H 2 O 2 to Decrease Feed Temperature to PBR
We now discuss whether the replacement of O 2 by H 2 O 2 allows for a lowering of the overall PBR feed temperature, which would be an energy and cost saving. This analysis used the R = 0 and R = 0.33 feeds with the LR_LF case, with the results shown in Table 2. The partial H 2 O 2 substitution for O 2 produced a respectable CH 4 conversion at the lower feed temperatures where O 2 feed alone showed no OCM activity (R = 0 for 1043 and 943 K feeds). These results at lower feed temperatures were consistent with those observed experimentally [17][18][19]. At the 843 K feed temperature, even the R = 0.33 case was poor.

Incremental H 2 O 2 Replacing O 2 to CSTR
As mentioned above, the CH 4 /O 2 and H 2 O/H 2 O 2 vapor streams would likely be fed separately into the CSTR due to safety concerns about preheating a vapor stream containing H 2 O 2 . For example, for the R = 0.33 and HR_HF case, to achieve an effective (hypothetical) 843 K feed temperature while holding the H 2 O/H 2 O 2 stream at 373 K, the CH 4 /O 2 stream would be preheated to about 883 K. The HR_HF case was chosen for this CSTR analysis because it showed the best yield and selectivity of sum_C 2 H x at the lowest feed temperatures in the earlier study [11].
For the CSTR calculations, the volume was the same as the open (gas) volume of the packed bed, with the same catalyst surface area. The CSTR might be a single-phase ideal fluidized bed, or a perfectly mixed (e.g., jet-stirred) reactor with catalyst on the walls.
Substitution of all or some of the feed O 2 content with H 2 O 2 had a marked impact on the CSTR performance. Figure 5 (left) shows that substituting H 2 O 2 for O 2 reduced the exit temperature somewhat (~65-110 K), as might be expected from the lower exothermicity (see Equations (2) and (3)). However, Figure 5 Figure 5. Impact of H2O2 content in feed oxidant on CSTR exit temperature (left) and CH4 conversion (right) for HR_HF case where molar R = H2O2/(O2 + H2O2). Figure 6 shows that the yield of sum_C2Hx was notably higher than the CO + H2 yield for all R cases, while increasing H2O2 had a greater impact on yields at the higher feed temperatures. These results were consistent with those revealed in Figure 3 for the PBR. Selectivities of sum_C2Hx remained higher than CO + H2, especially at the lower feed temperatures, as seen in Figure 7.   Figure 6 shows that the yield of sum_C 2 H x was notably higher than the CO + H 2 yield for all R cases, while increasing H 2 O 2 had a greater impact on yields at the higher feed temperatures. These results were consistent with those revealed in Figure 3 for the PBR. Selectivities of sum_C 2 H x remained higher than CO + H 2 , especially at the lower feed temperatures, as seen in Figure 7.
Methane 2022, 1, FOR PEER REVIEW 8 Figure 5. Impact of H2O2 content in feed oxidant on CSTR exit temperature (left) and CH4 conversion (right) for HR_HF case where molar R = H2O2/(O2 + H2O2). Figure 6 shows that the yield of sum_C2Hx was notably higher than the CO + H2 yield for all R cases, while increasing H2O2 had a greater impact on yields at the higher feed temperatures. These results were consistent with those revealed in Figure 3 for the PBR. Selectivities of sum_C2Hx remained higher than CO + H2, especially at the lower feed temperatures, as seen in Figure 7.  Methane 2022, 1, FOR PEER REVIEW 8 Figure 5. Impact of H2O2 content in feed oxidant on CSTR exit temperature (left) and CH4 conversion (right) for HR_HF case where molar R = H2O2/(O2 + H2O2). Figure 6 shows that the yield of sum_C2Hx was notably higher than the CO + H2 yield for all R cases, while increasing H2O2 had a greater impact on yields at the higher feed temperatures. These results were consistent with those revealed in Figure 3 for the PBR. Selectivities of sum_C2Hx remained higher than CO + H2, especially at the lower feed temperatures, as seen in Figure 7.

Brief Reactor Comparison Summary
A simple comparison between the PBR and CSTR for OCM with and without H 2 O 2 as an activating oxidant is shown in Table 4. This illustration is based on the LR_LF case (see Table 1). The results will vary somewhat for other cases, but the observations will be similar. This summary considers both the current results and those published earlier with just O 2 as an activator [11]. While the claims are based on calculations using the Karakaya et al. [6] mechanism for La 2 O 3 /CeO 2 catalyst, it is anticipated other OCM catalysts would give rise to similar claims.  Table 4 shows several points. The lowest practical feed temperature is the value below which there is no appreciable CH 4 conversion. All remaining values in each column correspond to those temperatures. Replacing a portion of the feed O 2 with H 2 O 2 vapor allows the CSTR to achieve good CH 4 conversions at the lowest feed temperature. It also allows the PBR to run with a reduced feed temperature. Even at this low feed temperature, the CSTR has a sum_C 2 H x selectivity that exceeds the PBR at a much higher temperature. The CSTR also shows reduced syngas (CO + H 2 ) and improved sum_C 2 H x selectivity when using the H 2 O 2 .

Simple Layout of a "Green" OCM Plant
Although Equations (2) and (3) show that OCM via O 2 and H 2 O 2 is exothermic, a future sustainable OCM plant must consider O 2 and H 2 O 2 production and overall plant heat integration. Figure 8 offers a simple schematic. The OCM reactor feeds CH 4 and a combination of O 2 and vapor phase moist H 2 O 2 . The O 2 is produced in a solar powered air separation plant [23] that enjoys the energy and economic savings from chemical looping instead of cryogenic separation [24,25]. Aqueous H 2 O 2 is produced by the solar powered catalyzed reaction of O 2 and acidic liquid H 2 O [20]. Vapor phase H 2 O 2 is stripped out of the liquid by the N 2 or He [21] recovered from the natural gas. Heavier-than-CH 4 saturated hydrocarbons (C 2 H 6 -C 5 H 12 ) are separated out from the natural gas. Post-OCM reactor processing separates the CO and H 2 as synthesis gas and the desirable coupled hydrocarbons (e.g., C 2 H 4 and C 2 H 6 ). Byproduct CO 2 from Separations_2 and Pretreatment can be captured with caustic scrubbing, and subsequently sequestered.
Methane 2022, 1, FOR PEER REVIEW 10 The CSTR also shows reduced syngas (CO + H2) and improved sum_C2Hx selectivity when using the H2O2.

Simple Layout of a "Green" OCM Plant
Although Equations (2) and (3) show that OCM via O2 and H2O2 is exothermic, a future sustainable OCM plant must consider O2 and H2O2 production and overall plant heat integration. Figure 8 offers a simple schematic. The OCM reactor feeds CH4 and a combination of O2 and vapor phase moist H2O2. The O2 is produced in a solar powered air separation plant [23] that enjoys the energy and economic savings from chemical looping instead of cryogenic separation [24,25]. Aqueous H2O2 is produced by the solar powered catalyzed reaction of O2 and acidic liquid H2O [20]. Vapor phase H2O2 is stripped out of the liquid by the N2 or He [21] recovered from the natural gas. Heavier-than-CH4 saturated hydrocarbons (C2H6-C5H12) are separated out from the natural gas. Post-OCM reactor processing separates the CO and H2 as synthesis gas and the desirable coupled hydrocarbons (e.g., C2H4 and C2H6). Byproduct CO2 from Separations_2 and Pretreatment can be captured with caustic scrubbing, and subsequently sequestered.

Conclusions
Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La2O3/CeO2 catalyst borrowed from the literature, this study considered the incremental replacement of the activating O2 with moist H2O2 vapor. Both packed bed reactor (PBR) and continuous stirred tank reactor (CSTR) configurations were used. As the H2O2 content of the oxidant increased, more of the CH4 conversion occurred in the gas phase with less assistance from the catalytic surface. Hydroxyl (•OH) radicals from rapid H2O2 decomposition abstracted •H atoms from CH4 to produce •CH3 radicals. This occurred in parallel to a similar abstraction by oxygen atoms (•Os) adsorbed on the catalyst surface when O2 was fed. In the PBR, H2O2 allowed the "light-off" temperature jump to occur using a lower feed temperature. Even though there was a slight decline in CH4 conversion, the C2Hx selectivity increased while synthesis gas dropped. Since significant preheat was still needed, process safety considerations might dictate that H2O2 vapor is better suited to the continuous stirred tank reactor (CSTR) configuration where the H2O2/H2O vapor stream can be fed at lower temperatures separately from the preheated CH4/O2 stream. In a CSTR, H2O2 significantly improved C2Hx production compared to synthesis gas over all feed temperatures studied, thus showing that OCM is possible with

Conclusions
Using an elementary reaction mechanism for the oxidative coupling of methane (OCM) on a La 2 O 3 /CeO 2 catalyst borrowed from the literature, this study considered the incremental replacement of the activating O 2 with moist H 2 O 2 vapor. Both packed bed reactor (PBR) and continuous stirred tank reactor (CSTR) configurations were used. As the H 2 O 2 content of the oxidant increased, more of the CH 4 conversion occurred in the gas phase with less assistance from the catalytic surface. Hydroxyl (•OH) radicals from rapid H 2 O 2 decomposition abstracted •H atoms from CH 4 to produce •CH 3 radicals. This occurred in parallel to a similar abstraction by oxygen atoms (•O s ) adsorbed on the catalyst surface when O 2 was fed. In the PBR, H 2 O 2 allowed the "light-off" temperature jump to occur using a lower feed temperature. Even though there was a slight decline in CH 4 conversion, the C 2 H x selectivity increased while synthesis gas dropped. Since significant preheat was still needed, process safety considerations might dictate that H 2 O 2 vapor is better suited to the continuous stirred tank reactor (CSTR) configuration where the H 2 O 2 /H 2 O vapor stream can be fed at lower temperatures separately from the preheated CH 4 /O 2 stream. In a CSTR, H 2 O 2 significantly improved C 2 H x production compared to synthesis gas over all feed temperatures studied, thus showing that OCM is possible with significantly less preheating compared to PBR. A future OCM plant can operate in a more "green" way with the use of solar-activated H 2 O 2 production, and solar-powered O 2 production from chemical-looping air separation.