Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.


Introduction
Methane, the main component of natural gas, is burnt for heating purposes but is also an important chemical feedstock for the production of syngas by steam reforming, dry reforming, and partial oxidation [1]. Furthermore, direct conversion of methane into methanol and higher hydrocarbons under oxidative conditions is studied as well [2].
Oxidative coupling of methane (OCM) was first introduced in 1982 by Keller and Bhasin [3] for the production of C2 products-namely, C 2 H 4 and C 2 H 6 -in the presence of metal catalysts at atmospheric pressure and temperatures in the range of 700-1300 K. The main challenge of the process is the achievement of high C2 product selectivity at high methane conversion. Since OCM product yields depend considerably on the catalyst used, various researchers have worked with different catalyst formulations in order to maximize yield of C2 species. Most of the catalysts investigated for OCM are oxides of pure or modified transition metals [4], IA or IIA group elements [5], Mn/Na 2 WO 4 /SiO 2 catalysts [6], or Li/MgO catalysts [7] in the temperature range 1000-1100 K. Other wellknown catalysts for OCM are La-doped catalysts such as LaAlO 3 [8], La 2 O 3 /CeO 2 [9], and Sr/La 2 O 3 [10]. However, no particular catalyst has yet met the criteria for a commercial realization of the process.
Alumina has been widely used as a support material in industrial catalysis applications due to its advantageous surface and bulk properties and its ability to promote high metal catalyst stability [11,12]. For instance, Pt/Al 2 O 3 catalysts have been investigated for catalytic partial oxidation of methane for the formation of CO and H 2 at temperatures above 1273 K [13]. In this context, unsaturated alumina atoms are considered to play an important role in stabilizing Pt atoms and maintain a high Pt dispersion in Pt/Al 2 O 3 catalysts [11,12]. Hohn et al. [14] reported for the first time the functionality of Pt/Al 2 O 3 catalysts in OCM at a very high temperature of about 1600 K for short contact times of 20 ms. Thereby, by using a Pt-coated monolith at low CH 4 /O 2 ratios, high space velocities, and low N 2 dilution, a selectivity of about 20% of C2 products was achieved. The Pt catalyst was primarily used for the activation of methane via total oxidation at the position near the entrance of the monolithic structure, leading to a rise in the temperature, which is high enough for coupling products to form downstream.
The mechanism of OCM is not yet well understood. Both surface reactions as well as reactions between gaseous species seem to be a prerequisite for achieving significant yields of C2 products. Methane likely interacts with the available oxygen species both in the gas phase and over the catalytic surface [15]. The C-H bond is activated by the oxygen species on the catalyst surface, forming CH 3 radicals desorbing into the gas phase [9,10,[15][16][17]. The combination of CH 3 radicals results in ethane formation in the gas phase and its further dehydrogenation leads to formation of ethylene. The limited yields of desired C2 products is due to consecutive oxidation of C2 products to CO and CO 2 aside from the thermodynamically favored path to syngas [18].
Quiceno et al. [19] studied the partial oxidation of methane over a Pt wire gauze at very short contact times to form CO and H 2 , along with the formation of C2 species in the gas phase at elevated pressures. By exploiting molecular beam spectroscopy for their in situ study on Pt catalysts, Geske et al. [20] also observed C2 products, above 1350 K. Furthermore, a rise in CH 4 conversion along with C2 products formation was ascertained at a CH 4 /O 2 ratio of 0.6, and a maximum selectivity of C 2 H 2 of about 17% was observed at approximately 1600 K along with traces of benzene and coke. C2 products formed were attributed to the CH 3 radicals formed in the gas phase at high temperatures above 1400 K among CH 3 , CH 2 , and CH radicals. In that way, CH 2 and CH radicals were excluded for being the source of C 2 H 2 formed.
In the present work, a Pt/Al 2 O 3 -coated catalytic monolith is used for OCM at short contact times and high temperatures. The objective of this study is a better understanding of the role of catalytic and homogeneous gas phase reactions on the formation of C2 hydrocarbons by using multiscale numerical simulations.

Simulation Study of Literature Experimental Results
Lunsford et al. [16] reported that gas-phase reactions predominate over the heterogeneous reactions under oxidative coupling conditions at higher temperatures and facilitate the formation of coupling products. In order to investigate the role of the surface and gas-phase chemistry under oxidative coupling over Pt, simulation studies of experiments were conducted by a two-dimensional flow field description of a single channel of the catalytic monolith using DETCHEM CHANNEL [21,22]. The operating conditions were taken from the pioneering work of Hohn et al. [14]. A detailed gas phase reaction mechanism, PolyMech [23], and/or a multistep surface reaction mechanism, Kahle-Deutschmann [24], were utilized for the numerical simulations.
The simulations were conducted under isothermal conditions, with CH 4 /O 2 ratios in the range of 1. 4-2.4, at the corresponding, experimentally observed temperature of 1673-1763 K. Two-dimensional numerical simulations were first conducted along the monolithic structure with the total length of 0.01 m.
As shown in Figure 1, the simulation using homogeneous reactions only predict C 2 H 2 as the major C2 product and a steady rise in the C2 selectivity with increasing CH 4 /O 2 ratios. In contrast with numerically predicted results, the experimentally measured selectivity of C2 products describe an opposite trend, and a gradual decline in C2 selectivity with the rise in CH 4 /O 2 ratios is obtained. Thus, it seems very unlikely that only homogeneous chemistry is taking place. As shown in Figure 1, the simulation using homogeneous reactions only predict C2H2 as the major C2 product and a steady rise in the C2 selectivity with increasing CH4/O2 ratios. In contrast with numerically predicted results, the experimentally measured selectivity of C2 products describe an opposite trend, and a gradual decline in C2 selectivity with the rise in CH4/O2 ratios is obtained. Thus, it seems very unlikely that only homogeneous chemistry is taking place. In case of considering the surface kinetics only over the length of the catalyst, the simulations predict that CO2, H2O, CO, and H2 are formed as a combination of first total oxidation and partial oxidation downstream where all the oxygen is already gone. The selectivity and conversion of the gas phase species along the monolith for CH4/O2 ratios of 1.4 and 2.4 are shown in Figure 2. The simulations considering only the surface reactions cannot predict the formation of C2 products, since no coupling reactions are included in the surface mechanism.
Next, we assume that the reaction zone can be divided in two broad zones for further simulations. In the first zone at the front face of the catalyst channel, heterogeneous surface chemistry leads to total oxidation (CO2, H2O) and partial oxidation (CO, H2) products. Initially, a high selectivity of CO2 and H2O is observed at the entrance of the catalytic zone with significant O2 conversion, followed by formation of partial oxidation products along with a rise in CH4 conversion. At experimental conditions chosen by Hohn et al. [14], this zone is an extremely small area of ca. 0.002 m, where oxygen is available for the oxidation of methane. Since most of the methane is converted over the surface sites, only a small part leads to formation of coupling products in the gas phase in the second zone. In case of considering the surface kinetics only over the length of the catalyst, the simulations predict that CO 2 , H 2 O, CO, and H 2 are formed as a combination of first total oxidation and partial oxidation downstream where all the oxygen is already gone. The selectivity and conversion of the gas phase species along the monolith for CH 4 /O 2 ratios of 1.4 and 2.4 are shown in Figure 2.
As shown in Figure 1, the simulation using homogeneous reactions only predict C2H2 as the major C2 product and a steady rise in the C2 selectivity with increasing CH4/O2 ratios. In contrast with numerically predicted results, the experimentally measured selectivity of C2 products describe an opposite trend, and a gradual decline in C2 selectivity with the rise in CH4/O2 ratios is obtained. Thus, it seems very unlikely that only homogeneous chemistry is taking place. In case of considering the surface kinetics only over the length of the catalyst, the simulations predict that CO2, H2O, CO, and H2 are formed as a combination of first total oxidation and partial oxidation downstream where all the oxygen is already gone. The selectivity and conversion of the gas phase species along the monolith for CH4/O2 ratios of 1.4 and 2.4 are shown in Figure 2. The simulations considering only the surface reactions cannot predict the formation of C2 products, since no coupling reactions are included in the surface mechanism.
Next, we assume that the reaction zone can be divided in two broad zones for further simulations. In the first zone at the front face of the catalyst channel, heterogeneous surface chemistry leads to total oxidation (CO2, H2O) and partial oxidation (CO, H2) products. Initially, a high selectivity of CO2 and H2O is observed at the entrance of the catalytic zone with significant O2 conversion, followed by formation of partial oxidation products along with a rise in CH4 conversion. At experimental conditions chosen by Hohn et al. [14], this zone is an extremely small area of ca. 0.002 m, where oxygen is available for the oxidation of methane. Since most of the methane is converted over the surface sites, only a small part leads to formation of coupling products in the gas phase in the second zone. The simulations considering only the surface reactions cannot predict the formation of C2 products, since no coupling reactions are included in the surface mechanism.
Next, we assume that the reaction zone can be divided in two broad zones for further simulations. In the first zone at the front face of the catalyst channel, heterogeneous surface chemistry leads to total oxidation (CO 2 , H 2 O) and partial oxidation (CO, H 2 ) products. Initially, a high selectivity of CO 2 and H 2 O is observed at the entrance of the catalytic zone with significant O 2 conversion, followed by formation of partial oxidation products along with a rise in CH 4 conversion. At experimental conditions chosen by Hohn et al. [14], this zone is an extremely small area of ca. 0.002 m, where oxygen is available for the oxidation of methane. Since most of the methane is converted over the surface sites, only a small part leads to formation of coupling products in the gas phase in the second zone.
In the second zone, as the oxygen is almost fully consumed, gas phase chemistry becomes significant. At experimental conditions adopted by Hohn et al. [14], at varying CH 4 /O 2 ratios, oxygen is fully consumed within the first few millimeters of the catalyst. Further downstream lies the second reaction zone where the gas phase chemistry takes place and ultimately results in the formation of coupling products.
Hence, simulations over the complete length of the catalyst (0.01 m) were conducted at different CH 4 /O 2 ratios, with both surface reactions and gas phase reactions, at adiabatic conditions with an inlet reactor temperature of 773 K. For the first 0.002 m of the catalyst (zone 1), only the surface kinetic model was considered and, subsequently, only the gas phase kinetic model was used to simulate the remaining 0.008 m length of the catalyst monolith (zone 2). The simulation results according to this configuration are shown in Figure 3. It is observed that the kinetic simulation follows the experimentally observed trend, where the selectivity of C2s is reduced with increasing CH 4 /O 2 ratios. The experimental data suggest C 2 H 2 as the major product at CH 4 /O 2 ratios < 2, while C 2 H 4 is seen as the major product at CH 4 /O 2 ratios ≥ 2. However, there are deviations in the product distribution predicted by the model, C 2 H 2 has been predicted as the major C2 product at all variable CH 4 /O 2 ratio conditions. Therefore, further experimental and modeling studies were conducted as a part of this research to understand the underlying rationale.
In the second zone, as the oxygen is almost fully consumed, gas phase chemistry becomes significant. At experimental conditions adopted by Hohn et al. [14], at varying CH4/O2 ratios, oxygen is fully consumed within the first few millimeters of the catalyst. Further downstream lies the second reaction zone where the gas phase chemistry takes place and ultimately results in the formation of coupling products.
Hence, simulations over the complete length of the catalyst (0.01 m) were conducted at different CH4/O2 ratios, with both surface reactions and gas phase reactions, at adiabatic conditions with an inlet reactor temperature of 773 K. For the first 0.002 m of the catalyst (zone 1), only the surface kinetic model was considered and, subsequently, only the gas phase kinetic model was used to simulate the remaining 0.008 m length of the catalyst monolith (zone 2). The simulation results according to this configuration are shown in Figure 3. It is observed that the kinetic simulation follows the experimentally observed trend, where the selectivity of C2s is reduced with increasing CH4/O2 ratios. The experimental data suggest C2H2 as the major product at CH4/O2 ratios < 2, while C2H4 is seen as the major product at CH4/O2 ratios ≥ 2. However, there are deviations in the product distribution predicted by the model, C2H2 has been predicted as the major C2 product at all variable CH4/O2 ratio conditions. Therefore, further experimental and modeling studies were conducted as a part of this research to understand the underlying rationale.

Simulation Studies with Novel Inhouse Experimental Results
It is also noteworthy that the residence time during the Hohn et al. [14] experiment is of a few milliseconds due to the small channel diameter and the high volumetric flow rate (Table 1). However, at higher space velocities, the length of the first zone is expected to become longer and the second zone with the homogeneous gas phase chemistry is expected to be relatively shifted along the reactor length or even downstream of the monolith, in the empty reactor tube, where the temperature is still very high.

Simulation Studies with Novel Inhouse Experimental Results
It is also noteworthy that the residence time during the Hohn et al. [14] experiment is of a few milliseconds due to the small channel diameter and the high volumetric flow rate (Table 1). However, at higher space velocities, the length of the first zone is expected to become longer and the second zone with the homogeneous gas phase chemistry is expected to be relatively shifted along the reactor length or even downstream of the monolith, in the empty reactor tube, where the temperature is still very high. The two-zone kinetic model proposed above was further evaluated against additional experiments conducted as part of this study with a 1% Pt/Al 2 O 3 catalyst. Simulations were conducted over the maximum length of 0.4 m including not only the monolith, but also the empty reactor tube downstream. DETCHEM MONOLITH [21] was used to model the 0.01 m catalyst and DETCHEM CHANNEL [, 21,22] was employed to model the 0.39 m quartz glass tubular reactor downstream of the monolith. Certain experimental parameters were varied over the range of conditions to investigate the optimization of C2s selectivity.

Study of N 2 Dilution
Experiments were conducted for variable N 2 dilution at a CH 4 /O 2 ratio of 2 with residence time of 10 ms over the in-house prepared 1% Pt/Al 2 O 3 catalyst. Experimentally, the selectivity of C2 species increases to about 6% with the decrement in N 2 dilution from 80% to 35%. As shown in Figure 4, the kinetic model predicts the N 2 selectivity of total C2 species in good agreement with the experimental values and reproduces the shape of the plot. However, at around 50% dilution, the kinetic model overpredicts the C2 selectivity to almost double the experimental values. The rise in C2 selectivity correlates to the rise in CH 4 conversion from 50% to 80% as the N 2 dilution was reduced from 80% dilution to 35% dilution. Additionally, C 2 H 2 is observed as major product in the simulation results. Although the model overestimates the temperature, the predicted temperature follows the same trend as observed in the experiments, namely, an increase in temperature with decreasing dilution.
Simulations were conducted over the maximum length of 0.4 m including not only the monolith, but also the empty reactor tube downstream. DETCHEM MONOLITH [21] was used to model the 0.01 m catalyst and DETCHEM CHANNEL [, 21,22] was employed to model the 0.39 m quartz glass tubular reactor downstream of the monolith. Certain experimental parameters were varied over the range of conditions to investigate the optimization of C2s selectivity.

Study of N2 Dilution
Experiments were conducted for variable N2 dilution at a CH4/O2 ratio of 2 with residence time of 10 ms over the in-house prepared 1% Pt/Al2O3 catalyst. Experimentally, the selectivity of C2 species increases to about 6% with the decrement in N2 dilution from 80% to 35%. As shown in Figure 4, the kinetic model predicts the N2 selectivity of total C2 species in good agreement with the experimental values and reproduces the shape of the plot. However, at around 50% dilution, the kinetic model overpredicts the C2 selectivity to almost double the experimental values. The rise in C2 selectivity correlates to the rise in CH4 conversion from 50% to 80% as the N2 dilution was reduced from 80% dilution to 35% dilution. Additionally, C2H2 is observed as major product in the simulation results. Although the model overestimates the temperature, the predicted temperature follows the same trend as observed in the experiments, namely, an increase in temperature with decreasing dilution.

Variation of Flow Rate
The flow rate, i.e., the gas hourly space velocity (GHSV), is known to be a crucial parameter affecting the C2 selectivity. Several authors working with different catalysts ascertained a distinct influence of space velocity over different catalysts on the C2 selectivity by OCM. Witt et al. [25] studied a Rh catalyst and established that 99% of O 2 conversion is desirable for achieving a stable C2 selectivity. Increasing GHSV beyond 4.4 × 10 5 h −1 results in a drop in O 2 conversion and leads to a decline of C2 species. Sarsani et al. [26] discovered that thermal effects and heat loss affect the C2 selectivity apart from the threshold O 2 conversion required for C2 products formation. It was examined that heat loss was dominant at lower GHSV, thereby preventing the autothermal operation of the reactor system. For a La-Ce powder catalyst system, a residence time of 10 ms or less enabled an adiabatic rise in temperature for significant C2 production. Kooh et al. [27] found that higher GHSV enables higher C2 yields by establishing a large difference between furnace temperature and catalyst hotspot temperature. Takanabe et al. [28] employed a Mn/Na 2 WO 4 /SiO 2 catalyst and established the direct association between CH 4 conversion and C2 yields with GHSV. Low GHSV or higher contact time facilitated higher CH 4 conversion and, thereby, higher C2 yields.
During the experiments conducted over the 1 wt.% Pt/Al 2 O 3 catalyst at 35% N 2 dilution, a CH 4 /O 2 ratio of 2 with varying GHSV conditions and C2 selectivity plateaus at 7% selectivity with 94% CH 4 conversion at space velocity of 4.5 × 10 5 h −1 and declines at higher GHSV. The temperature measured after the catalyst remains almost constant at variable space velocities; the decline in experimentally determined C2 selectivity can be attributed to the decline in CH 4 conversion due to decreasing contact time. Although the numerically predicted kinetic model matches the experimentally determined results well at lower space velocities, at higher space velocities a rise in C2 selectivity is predicted, which differs from the experimentally determined values ( Figure 5). Despite the low contact times, the hotspot temperature and the overall CH 4 conversion remain constant. This indicates that even though less CH 4 is expected to be converted in the catalytic zone along the length of monolith, as an effect of increase in GHSV, the model predicts that the overall CH 4 conversion is compensated in the gas phase zone behind the catalyst (Figure 5b), leading to CH 3 radicals and, subsequently, C2 species. However, such an effect cannot be validated in the present experimental campaign, and further work is required to understand the impact of GHSV.
enabled an adiabatic rise in temperature for significant C2 production. Kooh et al. [27] found that higher GHSV enables higher C2 yields by establishing a large difference between furnace temperature and catalyst hotspot temperature. Takanabe et al. [28] employed a Mn/Na2WO4/SiO2 catalyst and established the direct association between CH4 conversion and C2 yields with GHSV. Low GHSV or higher contact time facilitated higher CH4 conversion and, thereby, higher C2 yields.
During the experiments conducted over the 1 wt.% Pt/Al2O3 catalyst at 35% N2 dilution, a CH4/O2 ratio of 2 with varying GHSV conditions and C2 selectivity plateaus at 7% selectivity with 94% CH4 conversion at space velocity of 4.5 × 10 5 h -1 and declines at higher GHSV. The temperature measured after the catalyst remains almost constant at variable space velocities; the decline in experimentally determined C2 selectivity can be attributed to the decline in CH4 conversion due to decreasing contact time. Although the numerically predicted kinetic model matches the experimentally determined results well at lower space velocities, at higher space velocities a rise in C2 selectivity is predicted, which differs from the experimentally determined values ( Figure 5). Despite the low contact times, the hotspot temperature and the overall CH4 conversion remain constant. This indicates that even though less CH4 is expected to be converted in the catalytic zone along the length of monolith, as an effect of increase in GHSV, the model predicts that the overall CH4 conversion is compensated in the gas phase zone behind the catalyst ( Figure  5b), leading to CH3 radicals and, subsequently, C2 species. However, such an effect cannot be validated in the present experimental campaign, and further work is required to understand the impact of GHSV.

Variation in CH 4 /O 2 Ratio
Experiments were conducted for variable CH 4 /O 2 ratios from 1.8-2.4, under conditions with 35% N 2 dilution, space velocity of 4.5 × 10 5 h −1 , and corresponding residence time of 8 ms over 1% Pt/Al 2 O 3 catalyst. Simulations conducted including only surface kinetics along the catalyst depict that due to the very low residence time, the oxygen is not fully consumed in the catalytic zone up to the end of the monolith. Unlike the Hohn et al. [14] experiments, oxygen is rather fully converted in the gas phase zone after the monolith. Hence, the two-zone model simulations were configured so that the surface chemistry was considered along the catalyst's overall length (0.01 m) and the gas phase chemistry was considered along the empty tube reactor downstream of the monolithic catalyst (0.39 m). In Figure 6, the kinetic simulations envision the experimentally measured C2 selectivity for a CH 4 /O 2 ratio ≥ 2 quite closely. However, at a CH 4 /O 2 ratio < 2, the kinetic model overpredicts the C2 selectivity and predicts the maximum C2 selectivity at a CH 4 /O 2 ratio of 1.6 with 14% C2 selectivity and 92% CH 4 conversion. Experimentally, the maximum C2 selectivity of 7% with 94% CH 4 conversion is observed at a CH 4 /O 2 ratio of 2.0. Kinetic study at a CH 4 /O 2 ratio of 1.4 is estimated to explain the reduction in C2 selectivity observed at a CH 4 /O 2 ratio of 1.8 experimentally. Thus, to have a better understanding, the axial profiles for conversion of reactants and formation of products are discussed in more detail in the next section. residence time of 8 ms over 1% Pt/Al2O3 catalyst. Simulations conducted including only surface kinetics along the catalyst depict that due to the very low residence time, the oxygen is not fully consumed in the catalytic zone up to the end of the monolith. Unlike the Hohn et al. [14] experiments, oxygen is rather fully converted in the gas phase zone after the monolith. Hence, the two-zone model simulations were configured so that the surface chemistry was considered along the catalyst's overall length (0.01 m) and the gas phase chemistry was considered along the empty tube reactor downstream of the monolithic catalyst (0.39 m). In Figure 6, the kinetic simulations envision the experimentally measured C2 selectivity for a CH4/O2 ratio ≥ 2 quite closely. However, at a CH4/O2 ratio < 2, the kinetic model overpredicts the C2 selectivity and predicts the maximum C2 selectivity at a CH4/O2 ratio of 1.6 with 14% C2 selectivity and 92% CH4 conversion. Experimentally, the maximum C2 selectivity of 7% with 94% CH4 conversion is observed at a CH4/O2 ratio of 2.0. Kinetic study at a CH4/O2 ratio of 1.4 is estimated to explain the reduction in C2 selectivity observed at a CH4/O2 ratio of 1.8 experimentally.
Thus, to have a better understanding, the axial profiles for conversion of reactants and formation of products are discussed in more detail in the next section.

Catalytic Reaction Kinetics over the Monolith
During the experiments, the reactor's final product composition was measured by analyzing the effluent gas stream's composition. While these results allow for optimizing the process parameters with respect to maximum CH4 conversion and C2 selectivity,

Catalytic Reaction Kinetics over the Monolith
During the experiments, the reactor's final product composition was measured by analyzing the effluent gas stream's composition. While these results allow for optimizing the process parameters with respect to maximum CH 4 conversion and C2 selectivity, spatial information on selectivity and conversion along the length of the reactor are of major relevance in order to better understand the reaction hierarchy. Simulations of the reactor model coupled with detailed kinetic models were used to determine the molar fractions of the products as the reactant gases are converted inside the reactor, with an emphasis on the catalytic zone.
In Figure 7, the two-dimensional temperature profile across the monolith, obtained using DETCHEM MONOLITH , describes a rise in temperature, which is observed at the onset of the catalyst surface. However, for lower CH 4 /O 2 ratios, the maximum temperature reported across the monolith is substantially higher. Furthermore, axial profiles of reactants and products over the Pt catalyst, obtained using the Kahle-Deutschmann kinetic model, are shown in   A sensitivity analysis of the Kahle-Deutschmann mechanism [24] (supplemented in Appendix A) allows us to analyze the major pathways for CH 4 conversion and formation of products over the Pt catalytic surface under adiabatic conditions at a CH 4 /O 2 of 1.4. The following are the primary reactions on the catalyst's surface in the total oxidation zone: i.
Methane adsorption with hydrogen atom abstraction ( Figure A2a): ii. The process of water formation ( Figure A2b): iii. CO 2 desorption ( Figure A2c): CPOX (catalytic partial oxidation) zone: In this zone, H 2 is generated on the catalytic surface by steam reforming and partial oxidation of methane to syngas near the exit of the monolith under low fuel input conditions (CH 4 /O 2 = 1.4). Under high fuel conditions (CH 4 /O 2 = 2.4), the total oxidation zone is shorter and the CPOX zone is prominent, which results in an increase in CO and H 2 mole fractions throughout the catalyst length. In the case of high fuel circumstances (CH 4 /O 2 = 2.4), the dominant CPOX and steam reforming results in a relatively lower hotspot temperature near the exit of the monolith (1500 K). Under both low (CH 4 /O 2 = 2.4) and high (CH 4 /O 2 = 2.4) fuel conditions, the available oxygen is not completely utilized on the catalyst surface.
The following are the primary reactions on the catalyst's surface in the CPOX zone: i. Carbon monoxide is formed on the surface ( Figure A2d): ii. The desorption of hydrogen atoms after recombination of hydrogen atoms (formed in reaction (1)) on the catalyst's surface ( Figure A2e): In the simulations shown in this section, only the surface reactions are considered and the formation of C2 products is not predicted, because coupling reactions are not considered on the surface. However, when both the surface and gas phase kinetics are considered in the kinetic model, C2s are formed at a thermally insulated empty reactor tube downstream of the monolith. Additionally, the presence of O 2 at the exit of the monolith is of importance in all cases, as O 2 is not fully consumed along the monolithic channel ( Figure 8). The remaining CH 4 and O 2 at the exit of the catalyst play a crucial role on the coupling of methane in the gas phase, which is discussed in detail in the next section.

Homogeneous Reaction Kinetics after the Monolith
In the previous section, we described that along the monolith channel, zones of dominating reactions are predicted by simulations. At the empty reactor tube after the monolith exit, the coupling zone develops, where some of the residual methane is transformed into coupling products. Figure 9 depicts the predicted mole fractions of primary products and additional C2 species generated as a function of axial position up to the exit of the insulated empty tube reactor at z = 0.4 m. From the profiles obtained, we can also establish different reaction zones based on dominant products over the reactor length in the gas phase zone. Different reactions are dominant in the gas phase depending on the inlet CH 4 /O 2 ratios.
Coupling zone: Under low input fuel conditions (CH 4 /O 2 = 1.4, CH 4 /O 2 = 1.6), the coupling zone follows the partial oxidation at the end of the catalyst surface due to very fast kinetics of available O 2 at high temperature. At the catalyst outlet, the gas phase temperature is high as 2300 K (CH 4 /O 2 = 1.4) and 2100 K (CH 4 /O 2 = 1.6) and unreacted methane and oxygen interact with the active radicals in the gas phase and form CH 3 radicals (Figure 9b,d). For all fuel conditions, C2 formation is attributed to the following reactions: i.
The OH radicals desorbed from the catalytic surface react with methane to generate water: ii. On complete consumption of available oxygen, methane releases methyl radicals by H-atom abstraction (reaction (8)) or initiation/thermal decomposition (reaction (9)) Catalysts 2022, 12, 189 10 of 21 CH 4 (+M) CH · 3 + H · (+M) (9) In the previous section, we described that along the monolith channel, zones of dominating reactions are predicted by simulations. At the empty reactor tube after the monolith exit, the coupling zone develops, where some of the residual methane is transformed into coupling products. Figure 9 depicts the predicted mole fractions of primary products and additional C2 species generated as a function of axial position up to the exit of the insulated empty tube reactor at z = 0.4 m. From the profiles obtained, we can also establish different reaction zones based on dominant products over the reactor length in the gas phase zone. Different reactions are dominant in the gas phase depending on the inlet CH4/O2 ratios. Further, a reaction flow analysis (RFA) is performed for the PolyMech mechanism at 1673 K, CH 4 /O 2 of 1.4, 35% N 2 dilution and is supplemented in Appendix A. The RFA ( Figure A1) suggests that due to the fast kinetics and low activation energy of reaction (7) at high temperature, reaction (7) predominates over reaction (8) and (9). Thereby, CH 3 radicals are formed in the gas phase, which is the source initiator for the coupling products formation. CH 3 radicals combine together at high temperature to form C 2 H 6 . The concentration of C 2 H 6 is negligible, as it is quickly consumed at high temperature to form C 2 H 4 ( Figure A1). By interacting with CH 3 and H radicals, C 2 H 4 is rapidly transformed to C 2 H 2 , which is a more thermodynamically stable under these conditions due to its Gibbs free energy [29,30].
At high fuel conditions (CH 4 /O 2 = 2.4), a shift in the coupling zone is observed along the length of the reactor. Initially, C 2 H 6 is formed, which is subsequently transformed into C 2 H 4 and C 2 H 2 at high temperatures. A stable concentration of C 2 H 2 is visualized at the end of the reactor (Figure 9e). Here, the available O 2 combines with a H radical to form metastable HO 2 radicals (reaction (10)), which act as a reducing agent [31] and inhibit the further conversion of CH 4 (Figure 9f).
Methane reforming zone: Methane reforming is also dominant in the gas phase zone in the case of low input fuel conditions (CH 4 /O 2 = 1.4, CH 4 /O 2 = 1.6) but does not take place in a discrete zone (Figure 9a,c). Along the axial position of the reactor, both steam reforming (SR) and dry reforming (DR) of methane is contemplated along with the pyrolytic coupling zone of methane in a short zone in the gas phase [29,30]. Reforming is the endothermic zone that occurs after the initial formation of CH 3 radicals under the presence of O-containing species. In Figure 10, the two-dimensional concentration profiles at CH 4 /O 2 of 1.4 and CH 4 /O 2 of 1.6 over the homogeneous reforming zone are shown. Figure 10 describes that decline in temperature as well as CO 2 and H 2 O concentrations along with the rise in CO and H 2 can be attributed to the onset of the endothermic reforming reactions. In homogeneous steam reforming of methane, H2 and CO are the main products, but small fractions of C2 are produced as byproducts [32,33]. From a mechanistic point of view, the available H2O generates OH radicals that react with methane (reaction (7)) to generate the desired CH3 radicals for C2 formation [34]. Simultaneously, the H2O concentration decreases, while the concentration of CO and H2 increases. The decline in H2O mole fraction along the axial length of reactor downstream of the monolith, along In homogeneous steam reforming of methane, H 2 and CO are the main products, but small fractions of C2 are produced as byproducts [32,33]. From a mechanistic point of view, the available H 2 O generates OH radicals that react with methane (reaction (7)) to generate the desired CH 3 radicals for C2 formation [34]. Simultaneously, the H 2 O concentration decreases, while the concentration of CO and H 2 increases. The decline in H 2 O mole fraction along the axial length of reactor downstream of the monolith, along with the rise in C 2 H 2 , CO, and H 2 mole fractions over the reforming zone, can also be visualized in Figure 10.
In the context of homogeneous dry reforming of methane, Angeli et al. [35] observed that C2 products are formed as byproducts along with CO and H 2 at a temperature of 1623 K. Thus, dry reforming enhances the C2 products formation at high temperature. CO 2 is converted at a high pace, with the major reaction pathway being its interaction with hydrogen atoms to generate CO and an OH radical.
As a consequence, CO 2 promotes the formation of OH radicals in the dry reforming of methane, thereby, increasing the reactivity of the primary reaction pathway (reaction (7)) for C2 formation [36]. The decline in CO 2 mole fraction along with rise in C 2 H 2 is more profound in the case of a CH 4 /O 2 of 1.4, as compared with CH 4 /O 2 of 1.6 ( Figure 10). Thus, it can be postulated that reforming has a stronger influence at lower CH 4 /O 2 ratios.
Acetylene reforming zone: At substantially higher temperatures achieved at low fuel conditions (CH 4 /O 2 = 1.4), the interaction of C 2 H 2 with OH radicals causes the observed decrease in C 2 H 2 and an increase in CO mole fractions (Figure 9a). Once the proportion of OH radicals generated as a result of the reaction (11) is elevated, acetylene interacts with the OH radicals to form CH 2 CO, which then decomposes into CO and methyl radicals (reaction (12)): The C 2 H 2 conversion rate is affected by the CO 2 /CH 4 ratio. After notable methane conversion, high CO 2 concentrations promote the reforming of C 2 H 2 . Moreover, Savchenko et al. [35] studied homogeneous dry reforming of methane and found that with higher initial CO 2 /CH 4 ratio conditions, C 2 H 2 formed was faster converted to CO, leading to its decline (Figure 11b). In our experiments, due to the long total oxidation zone, the maximum CO 2 concentration at the outlet of the monolith is obtained for an initial CH 4 /O 2 ratio of 1.4. Furthermore, the higher CH 4 conversion achieved at a CH 4 /O 2 ratio of 1.4 causes the rise in CO 2 /CH 4 ratio along the length of the reactor downstream of the monolith, as shown in Figure 11a, thereby declining C 2 H 2 mole fraction.  Figure 10). Thus, it can be postulated that reforming has a stronger influence at lower CH4/O2 ratios. Acetylene reforming zone: At substantially higher temperatures achieved at low fuel conditions (CH4/O2 = 1.4), the interaction of C2H2 with OH radicals causes the observed decrease in C2H2 and an increase in CO mole fractions (Figure 9a). Once the proportion of OH radicals generated as a result of the reaction (11) is elevated, acetylene interacts with the OH radicals to form CH2CO, which then decomposes into CO and methyl radicals (reaction (12)): CH CO + H • → CO + CH • The C2H2 conversion rate is affected by the CO2/CH4 ratio. After notable methane conversion, high CO2 concentrations promote the reforming of C2H2. Moreover, Savchenko et al. [35] studied homogeneous dry reforming of methane and found that with higher initial CO2/CH4 ratio conditions, C2H2 formed was faster converted to CO, leading to its decline (Figure 11b). In our experiments, due to the long total oxidation zone, the maximum CO2 concentration at the outlet of the monolith is obtained for an initial CH4/O2 ratio of 1.4. Furthermore, the higher CH4 conversion achieved at a CH4/O2 ratio of 1.4 causes the rise in CO2/CH4 ratio along the length of the reactor downstream of the monolith, as shown in Figure 11a, thereby declining C2H2 mole fraction. The relatively lower endothermicity of C2H2 reforming reaction and the exothermic water gas shift reaction are responsible for the fact that the temperature profile is not strongly affected in the gas phase. The relatively lower endothermicity of C 2 H 2 reforming reaction and the exothermic water gas shift reaction are responsible for the fact that the temperature profile is not strongly affected in the gas phase.

Catalyst Preparation
The 1% Pt/Al 2 O 3 catalyst powder was prepared via incipient wetness impregnation (IWI) [37]. For this, the commercial Al 2 O 3 support (Puralox, Sasol, Sandton, South Africa) was calcined for 5 h at 700 • C and the crystalline (NH 3 ) 4 Pt(NO 3 ) 2 , which served as Pt precursor (Alfa Aesar, Haverhill, MA, USA) was dissolved in purified water equal to the pore volume of the calcined Al 2 O 3 before impregnation. After 2 h of drying at 70 • C and calcination of the received powder at 823 K for 5 h under static air, the powder catalyst was wash-coated onto cordierite monolithic honeycombs using the methodology previously described by Karinshak et al. [38]. The resulting honeycomb catalyst samples with a cell density of 400 cpsi, a length of 0.01 m, and a diameter of 0.01 m exhibited a noble metal loading of 30 g/ft 3 . The noble metal dispersion of 47% was measured by means of CO chemisorption, as described in detail by Chan et al. [39].

Reactor Setup and Experimental Procedure
The experiments were performed in a quartz glass tubular reactor, which was placed in a furnace (HST 12/400, Carbolite, Sheffield, UK), enabling heating, and the reactor's reaction zone was additionally wrapped with thermal insulation to ensure adiabatic operation. Before flowing into the reactor, the feed gas mixture consisting of methane, oxygen, and nitrogen was premixed and preheated to 463 K, while the furnace temperature itself was kept constant at 773 K. Experiments were performed at atmospheric pressure and at flowrates of minimum 5 slpm (standard liters per minute), resulting in residence times of milliseconds in the monolithic catalyst samples. The quartz glass tubular reactor, which is schematically depicted in Figure 12, is about 0.6 m long. Two thermocouples were placed-one inside the monolith and one after the monolith-to measure the temperature at the catalyst surface and the temperature after the catalyst in the reactive zone, respectively. was wash-coated onto cordierite monolithic honeycombs using the methodology previously described by Karinshak et al. [38]. The resulting honeycomb catalyst samples with a cell density of 400 cpsi, a length of 0.01 m, and a diameter of 0.01 m exhibited a noble metal loading of 30 g/ft 3 . The noble metal dispersion of 47% was measured by means of CO chemisorption, as described in detail by Chan et al. [39].

Reactor Setup and Experimental Procedure
The experiments were performed in a quartz glass tubular reactor, which was placed in a furnace (HST 12/400, Carbolite, Sheffield, UK), enabling heating, and the reactor's reaction zone was additionally wrapped with thermal insulation to ensure adiabatic operation. Before flowing into the reactor, the feed gas mixture consisting of methane, oxygen, and nitrogen was premixed and preheated to 463 K, while the furnace temperature itself was kept constant at 773 K. Experiments were performed at atmospheric pressure and at flowrates of minimum 5 slpm (standard liters per minute), resulting in residence times of milliseconds in the monolithic catalyst samples. The quartz glass tubular reactor, which is schematically depicted in Figure 12, is about 0.6 m long. Two thermocouples were placed-one inside the monolith and one after the monolithto measure the temperature at the catalyst surface and the temperature after the catalyst in the reactive zone, respectively. The coated monolithic catalyst was placed 0.2 m downstream of the reactor inlet. An uncoated cordierite honeycomb (diameter, 0.01 m; length, 0.01 m), placed upstream of the coated catalyst, served as a heat shield and ensured optimal heat transfer. A small space of approximately 0.005 m was kept between the heat shield and the coated monolith catalyst to avoid flow disturbance at the front entrance of the catalytic monolith.
Experimental results by Hohn et al. [14] were used as a preliminary investigation to evaluate and understand the surface and gas phase chemistry involved in the catalytic coupling of methane under oxidative conditions. Similar to the present study, the authors used a quartz glass tubular reactor with a diameter of 0.018 m and length of 0.4 m. The study employed alumina foam impregnated with Pt, resulting in a Pt loading of 2-3% on the structured substrate. To prevent radiation losses, an uncoated monolith was placed in front of the catalyst and the reactor was insulated from outside in order to allow an operation under adiabatic conditions. The coated monolithic catalyst was placed 0.2 m downstream of the reactor inlet. An uncoated cordierite honeycomb (diameter, 0.01 m; length, 0.01 m), placed upstream of the coated catalyst, served as a heat shield and ensured optimal heat transfer. A small space of approximately 0.005 m was kept between the heat shield and the coated monolith catalyst to avoid flow disturbance at the front entrance of the catalytic monolith.
Experimental results by Hohn et al. [14] were used as a preliminary investigation to evaluate and understand the surface and gas phase chemistry involved in the catalytic coupling of methane under oxidative conditions. Similar to the present study, the authors used a quartz glass tubular reactor with a diameter of 0.018 m and length of 0.4 m. The study employed alumina foam impregnated with Pt, resulting in a Pt loading of 2-3% on the structured substrate. To prevent radiation losses, an uncoated monolith was placed in front of the catalyst and the reactor was insulated from outside in order to allow an operation under adiabatic conditions.
The literature data of the study by Hohn et al. [14] and the experiments of the present study conducted with a 1% Pt/Al 2 O 3 catalyst were utilized for the validation of the kinetic model. Table 1 summarizes the experimental conditions used for the comprehension and validation of the model. Experiments over a wide range of input parameters including catalyst metal loadings, space velocities, residence times, N 2 dilution, and catalyst dimensions were selected.

Model Description and Numerical Simulation
In multiscale modelling of heterogeneous catalytic reactors, the steps involved in the process can be predicted starting with the reaction mechanism at atomic scale, reaction rates and adsorption and diffusion processes over the catalyst surface, and reaction rates in the gas-phase [40]. The numerical simulations in this work include the catalytic performance simulations of the chemical system under reaction conditions by coupling the 2D DETCHEM CHANNEL [21,22] reactor model with the elementary step-based kinetic models of gas phase and surface reactions. Further, DETCHEM MONOLITH [21] simulations are conducted over honeycomb catalyst to include transient conditions. DETCHEM MONOLITH models the transient temperature field, by a thorough simulation of representative channels using DETCHEM CHANNEL code.

Mathematical Model
In the 2D reactor model (DETCHEM CHANNEL ), the flow field of a cylindrical channel of the monolith is calculated under steady-state conditions by solving parabolized Navier-Stokes equations in the boundary layer approximation: Continuity Equation Species continuity equation where r is the radial coordinate, z is the axial coordinate, ρ is the density, u is the axial component of velocity, v is the radial component of velocity, λ is the thermal conductivity, p is the pressure, Y s is mass fraction of the species s, µ is the viscosity, j s is the radial flux of species s, . ω s is the gas-phase production rate of species s, and h s is the enthalpy density of the species s.
DETCHEM MONOLITH solves the unsteady-state two-dimensional heat conduction equations: where t is the time, T temperature, ρ density, c p is the heat capacity, λ ij tensor of heat conductivity, and q is heat source term for interaction within the channels. (20) where σ is the channel density (in channels per cross-sectional area). Neumann boundary conditions are applied: where σ SB is the Stefan-Boltzmann constant, c is the heat transfer coefficient, is the emissivity, and T surr the surrounding temperature.
The state of the reactive catalytic surface is described by the mean-field model, i.e., in terms of coverages. The coverage, θ s , can be defined as the fraction of surface sites covered with surface species or adsorbed gas phase species where c s is the concentration of species s, σ s is the number of sites occupied by one molecule of the species s, and Γ is the surface site density. For Pt, the surface density was taken as 2.72 × 10 −5 mol m −2 [24]. A surface reaction can be represented as where N g is the number of gas phase species, N s is the number of surface species, and v ik =v ik − v ik is the stoichiometric coefficients. The total molar production rate . s i of surface species i on the catalyst is calculated by The kinetics of the elementary surface reactions are represented by a modified Arrhenius equation: where k fk is the reaction rate coefficient, A k is the preexponential factor, β k is a temperature exponent, E a,k is the activation energy, ik is a coverage-dependent activation energy, and R is the gas constant [21].

Reaction Flow Analysis
A reaction flow analysis (RFA) identifies and describes the major successive pathways for the formation of products and utilization of reactants in a chemical reaction mechanism based on the rate of species production. In this work, the methodology was adopted from Gossler et al. [41]. The integral reaction flow analysis identifies the contribution of the most important reactions throughout the time interval considered.

Sensitivity Analysis
A sensitivity analysis identifies rate-determining steps as well as the essential parameters of the process. The approach adopted by Herrera Delgado et al. [42] is utilized for sensitivity analysis studies over the surface mechanism. In its most primitive form, a completely mixed reactor with only surface reactions operating at a constant temperature is examined.
The change in the amount n i of species is given by where A cat is the catalytic surface area. A time-dependent sensitivity coefficient E i,k (t) is defined as the change in the number of species i in relation to a relative change in the rate coefficient k fk , Thus, the time development of the sensitivity coefficient is solved by inserting Equations (25) and (27) into Equation (28): As a result, the sensitivity coefficient describes the contribution of the k th reaction to the generation of species i.

Chemical Reaction System Gas Phase Reaction Models
Four different gas phase homogeneous models were initially considered in this work for the oxidative coupling of methane at high temperatures. The first mechanism was based on the widely used GRI Mech 3.0 [43], developed for natural gas by the Combustion Laboratory at the University of California, Berkeley. The mechanism consists of 53 species and 325 reactions to describe the combustion of natural gas in the temperature range of 1000-2500 K at 0.1-10 bar and for equivalence ratios of ϕ = 0.1-5.
The second was the detailed mechanism by Healy et al. [44] with 289 species and 3128 reactions. The Healy-Curran model has been validated for oxidation of methane and propane mixtures for the temperature range of 740-1550 K at compressed gas pressures of 10-30 bar and for varying equivalence ratios of 0.3-3.0 in a high-pressure shock tube and in a rapid compression machine.
Another mechanism for ethane oxidation with 180 species and 2069 reactions based on work by Dean's group [45] was applied as well. This mechanism was validated for a wide range of conditions with temperatures varying between 700 K and 1800 K, pressures of approximately 1-10 bar, and equivalence ratios of ϕ = 0.1-1.5. The range of experiments include a high-pressure flow reactor, shock tube studies, and low-pressure flame experiments.
A relatively smaller mechanism with 58 species and 588 reactions, PolyMech by Porras et al. [23], was used for the simulations as well. The mechanism includes both oxidative and pyrolytic reaction pathways for the conversion of methane and the formation of aromatics up to benzene. Fuel-rich methane/dimethyl ether/air mixtures (fuel-air equivalence ratio ϕ = 1-20) were used for its validation in the temperature range of 630-1500 K, with a pressure of 1-20 bar in a shock tube and in a rapid compression machine experiment.
The homogeneous gas phase models were compared for the experimental conditions used by Hohn et al. [14] at a CH 4 /O 2 ratio of 1.7, 20% N 2 , and at a residence time of 20 ms. DETCHEM CHANNEL [21,22] simulations were conducted isothermally at the experimentally observed temperature of 1673 K. These reaction conditions were selected for the evaluation of the gas phase kinetic models because the temperature is very high; therefore, the gasphase reactions are expected to take place in high rates. Hohn et al. [14] compared the experimental data with that of the model presented by Dean et al. [45]. However, the large number of defined species and chemical reactions act as an impediment in its utilization in further studies. Among the four mechanisms studied in this work, the Healy-Curran model [44] and the PolyMech mechanism [23] predict the experimentally measured end-ofpipe C2 selectivity and CH 4 conversion considerably well. However, neither of the two models was able to accurately describe the product distribution of C2 species (C 2 H 6 , C 2 H 4 , C 2 H 2 ). The PolyMech [23] mechanism has the advantage of including a relatively smaller number of species and elementary reactions, thus, it requires less computational time for the numerical simulations. Furthermore, PolyMech has been successfully used for the simulation of gas phase dry reforming experiments at high temperature [35] and, thereby, was selected to be used in this work.

Surface Reaction Models
Platinum-based catalysts over support materials such as CeO 2 or Al 2 O 3 are known to be active in the oxidation of various hydrocarbons. In this study, two heterogeneous detailed kinetic models for platinum catalyst were used to understand and evaluate the role of the catalyst in the oxidative coupling of methane at high temperatures. In the first mechanism, Quiceno et al. [19] described the high activity of platinum for partial oxidation of methane at elevated temperatures. The mechanism, consisting of 21 species and 36 reactions, has been validated by experiments in a platinum wire gauze reactor at 1.3 bar and temperatures ranging from 700-1100 K (CH 4 /O 2 = 2.5, diluted by 80% He, residence time τ = 36 s). Above 1200 K, a significant amount of hydrogen is formed, while steam is observed at lower temperatures. C2 species are not included in this mechanism. In the second mechanism, the kinetics of methane dry reforming at high temperatures over platinum are addressed by the Kahle-Deutschmann model [24]. The mechanism, comprising 22 species and 58 reactions, was validated at a temperature of 1123-1273 K and a pressure of 20 bar, for experiments carried out over a catalytic bed of Pt-containing pellets with a CH 4 /CO 2 ratio of 1.0.
The two heterogeneous models were examined for the experimental conditions used by Hohn et al. [14]. Both models predict equivalent catalytic activity leading to similar composition of reaction products and elevation in temperature due to exothermic reactions. However, among the two kinetic models studied over platinum catalysts, the Kahle-Deutschmann mechanism [24] was selected for further evaluation of experiments in this work due to being thermodynamically consistent, in contrast to the model by Quiceno et al. [19]. Thus, reversibility of forward and backward reactions is ensured microkinetically [42]. It is noteworthy that no catalytic coupling reactions are taken into consideration over the surface. The mechanisms are available electronically (Available online: www.detchem.com/mechanisms (accessed on 6 May 2021).

Conclusions
In this work, a combined experimental and simulation investigation on oxidative coupling of methane over Pt/Al 2 O 3 was conducted employing detailed kinetic models from literature for both surface kinetics over Pt and gas phase kinetics. Varying the reaction conditions during experiments revealed that low N 2 dilution and higher space velocity favors the formation of C2 species. However, when increasing the GHSV beyond 4.5 × 10 5 h −1 , we found a decline in C2 selectivity. Experiments at different inlet CH 4 /O 2 ratios showed a maximum C2 selectivity of 7% C2 with 94% CH 4 conversion at a CH 4 /O 2 ratio of 2.
The simulation investigation of the oxidative coupling over Pt monoliths showed that both heterogeneous and homogeneous chemistry together are required for describing the formation of C2 species. The highly active Pt catalyst facilitates the exothermic oxidation of methane, consuming the majority of the oxygen on the catalyst surface and simultaneously increasing the temperature. The remaining oxygen and desorbed OH radicals facilitate the formation of CH 3 radicals from CH 4 in the gas-phase downstream of the monolith, which can then participate in coupling reactions to form C2 species. Among the C2 species, C 2 H 2 is the main product. However, depending on the CH 4 /O 2 ratio at the inlet and, consequently, at the outlet conditions of the monolith, the ratio of CO 2 /CH 4 in the reaction gas stream plays a key role in the further consumption of C 2 H 2 and can decrease the overall obtained selectivity of the process. Additionally, the simulation results do not agree completely with the experimental ones with respect to the distribution of C2 species but describe the total C2 selectivity well. These discrepancies are likely because the kinetic model was not initially developed for OCM and, thus, some gas phase reactions affecting the formation of C2 species require further tuning. Future studies should, additionally, lay particular focus on considering interacting species and reactions combining the two mechanisms.
It is evident that in order to optimize the selectivity of the desired products, the reaction conditions and reactor parameters should be selected carefully. Understanding the impact of various reaction conditions on the product selectivity is significant for both academia and industry.