# Oxidative coupling of biogas to ethylene over a trilobe-shaped Mn-Na2WO4/α-Al2O3 catalyst in a single-pellet reactor

Valentin L'hospital, Jordan Guillemot, Rémi Beucher, Thomas Michon, Didier Bonnet, Yves Schuurman, David Farrusseng

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Valentin L'hospital, Jordan Guillemot, Rémi Beucher, Thomas Michon, Didier Bonnet, et al.. Oxidative coupling of biogas to ethylene over a trilobe-shaped Mn-Na2WO4/α-Al2O3 catalyst in a single-pellet reactor. Applied Catalysis A: General, 2023, 666, pp.119402. 10.1016/j.apcata.2023.119402. hal-04346322

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# Oxidative coupling of biogas to ethylene over a trilobe-shaped Mn-Na₂WO₄/α-Al₂O₃ catalyst in a single-pellet reactor

Valentin L'hospital<fnref n="1" />, Jordan Guillemot<fnref n="1" />, Rémi Beucher<fnref n="1" />, Thomas Michon<fnref n="2" />, Didier Bonnet<fnref n="2" />, Yves Schuurman<fnref n="1" />, David Farrusseng<fnref n="1" />

**Keywords:** Oxidative coupling of methane, ethylene, C2 products, methane, biogas, CO₂, microreactor, single-pellet reactor

The oxidative coupling of methane (OCM), which offers a direct route for converting methane into higher-value ethylene.

Despite the development of selective catalyst formulations, kinetic and reactor design studies have been hampered by a lack of data for truly isothermal, relevant. Because of exothermicity issues, the temperature of a fixed-bed reactor filled with Mn-Na₂WO₄/SiO₂ catalysts show a sudden temperature run-away accompanied with a drastic loss of selectivity.

The objective of this study is to measure catalyst activity under relevant reaction conditions of pressure (3 bar), biogas composition (no dilution) and contact time, while keeping the process as isothermal as possible. To this aim, we have synthesized and shaped a trilobed Mn-Na₂WO₄/α-Al₂O₃ catalyst, which may be used as is at industrial scale, and measured its performance in a single-pellet reactor.

We found that the presence of CO₂ does not degrade catalyst performance, even on long run, which should enable the use of a CO₂-rich stream such as biogas. Actually, higher methane conversions have been obtained in the presence of CO₂, likely due to a possible contribution of methane dry reforming reaction. We also noticed a slow catalyst activation over the first 100 hours while selectivity was kept constant.

## 1. Introduction

With a view to achieving climate goals, the production of chemicals or liquid fuels from a biogas feedstock is attracting interest as a sustainable alternative solution to fossil resources. It has been determined that biogas production in Europe currently reaches only 2.65% of the theoretical biogas generation potential <fnref n="1" />, which illustrates the value of biogas as an alternative to petroleum-based products. The oxidative coupling of methane (OCM), which offers a direct route for converting methane into higher-value ethylene, appeared more than 40 years ago with the pioneering works of Keller and Bhasin<fnref n="2" /> and Baerns³, and since then, numerous studies have been carried out in both academia and industry to optimize the production of hydrocarbons from natural gas. A wide variety of catalysts have been screened for the OCM reaction ⁴⁻¹². It is established that a Mn-Na₂WO₄ catalyst formulation can achieve promising conversion and C₂ selectivity under relevant conditions of partial pressure and temperature ¹³⁻¹⁵. In the context of biogas valorization, Shi et al. ¹⁴ have studied the impact of CO₂ on the performance and structure of a MnNa₂WO₄/SiO₂ catalyst. They found that both

<footnote n="1">¹ IRCELYON, Institut de Recherches sur la Catalyse et l'Environnement de Lyon, UMR5256 CNRS-Université de Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne, France</footnote>

<footnote n="2">² Axel'One, 69360 Solaize, France</footnote>
CH₄ conversion and C₂ selectivity can be improved in the presence of CO₂ in the feed, even for small amounts of CO₂.

Despite the development of selective catalyst formulations, kinetic and reactor design studies have been hampered by a lack of data for truly isothermal, relevant conditions (i.e., an undiluted inlet feed, and an undiluted, shaped catalyst). Activity measurements performed with a fixed-bed configuration of Mn-Na₂WO₄/SiO₂ pellets show a sudden temperature run-away at a given level of conversion, with a drastic loss of selectivity¹⁶. The critical aspect of hot spot formation is linked to local temperature increases, which cause hydrocarbon oxidation and damage the performance and stability of the catalyst¹⁷.

Micro-reactor technologies that can better manage the excess of heat produced by highly exothermic the reactions has been developed ¹⁸,¹⁹. In the framework of OCM, one noteworthy solution consists in placing a catalyst coating on a metallic heat exchanger²⁰. Yet this catalyst-reactor configuration suffers from metallic migration from the support to the catalyst, which leads to degraded catalytic performance. As a result, the catalytic activity and selectivity at the “true” working temperature of shaped Mn-Na₂WO₄ under relevant process conditions (pressure and reactant composition) remains an open question. In addition, the presence of a large concentration of CO₂ in the feed could induce a deactivation, possibly due to an occurrence of the dry reforming reaction at the outlet of the reactor. Long-time stability tests focusing on the oxidative coupling of biogas under relevant pressure conditions are largely unknown

The objective of this study is to measure catalyst activity under relevant reaction conditions of pressure (3 bar), biogas composition (no dilution) and contact time, while keeping the process as isothermal as possible. To this aim, we have synthesized and shaped a trilobed Mn-Na₂WO₄/α-Al₂O₃ catalyst, which may be used as is at industrial scale, and measured its performance in a single-pellet reactor. The trilobed pellets were evaluated in a representative mixture of undiluted biogas (mixture of CH₄ and CO₂) under pressure, at different temperatures, contact times and CH₄/O₂ ratios. This reactor technology makes it possible to test a shaped catalyst in the smallest reactor volume possible, which prevents thermal runaway.

## 2. Experimental section

### 2.1. Catalyst synthesis

An MnNaW/α-Al₂O₃ catalyst was synthesized by stepwise impregnation with a spraying device. The impregnation device was developed to maximize the dispersion of the impregnation solution on the surface of the supports. This system guarantees a highly homogeneous active phase distribution. The catalytic activity of the α-Al₂O₃ support alone was also measured.

Before the first impregnation, the support (commercial alumina trilobe support) was dried under vacuum (0.1 mbar) at 150°C overnight. The concentration of the solution was adjusted to achieve the desired concentration of metal on the substrate at a water recovery volume (WRV) fixed at 90% to prevent the stagnation of solution in the flask. After the first Mn impregnation sequence (Mn(NO₃)₂·4H₂O, 98.5% pure, Acros Organis), the obtained material was impregnated with an aqueous solution of Na₂WO₄·2H₂O (>99% pure, Sigma Aldrich) and dried under the same conditions. Afterward, the catalyst was calcined at 900°C for 4 h.
## 2.2. Catalyst testing

A single trilobed catalyst (7.5 mg +/- 0.5 mg) was maintained in the isothermal zone by means of quartz wool supported by a titanium sheath, which also reduced the post-catalytic volume. The reactor was a tube of 2 mm ID made of dense α-Al₂O₃. It was chosen because of its chemical inertness and its capacity to withstand pressure. At the considered reaction temperature (>1000 K), a stainless-steel reactor could be catalytically active²¹. The oven temperature was controlled using a thermocouple inserted into the titanium sheath. The catalyst was run under the reaction condition flow of 10 mL min⁻¹ during the heating of the reactor (300 °C h⁻¹).

A methane-to-oxygen ratio (CH₄/O₂) of between 2.4 and 10 was set for the inlet stream, with a constant CH₄ inlet flow of 60%. In the flux, 15 vol% of N₂ was used as an internal standard. The carbon mass balances were calculated for each experiment and were all above 98%.

The reaction was studied at a pressure of 3 bar. The effect of the contact time (W/Fₚ₄) was studied by changing the total flow rate. The effect of reaction temperature was investigated from 650 to 850°C. A mixture of CH₄/O₂/N₂/CO₂ of 60/15/15/10 was typically used for the catalytic tests. For conditions without CO₂, the CO₂ was replaced by He in order to maintain constant methane and O₂ partial pressures. The effects of contact time (W/Fₚ₄) and CH₄/O₂ ratio were studied by increasing the ratio from 2.4 to 10. The Mears criterion was respected, and the appearance of a radial thermal gradient was prevented.

Gas mixtures were prepared from pressurized cylinders containing oxygen, methane, nitrogen or helium (gas purity >99.995% or 99.999%). Flow rates were managed using Brooks® thermal mass flow controllers. Water, as a reaction product, was removed from the reactor outlet by a water trap. The product gases were then sent for analysis to a compact gas chromatograph (Compact GC40®) equipped with two capillary columns: a Carboxen1010 followed by a TCD for the analysis of O₂, N₂, CH₄ and CO, and an RT-Ubond followed by a FID for the analysis of CH₄, CO₂, C₂H₄ and C₂H₆. A methanizer was used to convert the CO₂ into CH₄ for the flame ionization detector.

Catalytic performance indicators to quantify the OCM output are:

Methane/oxygen conversion:

$$X_{CH4} = \frac{F_{CH4}^{in} - F_{CH4}^{out}}{F_{CH4}^{in}} \quad (1) \qquad X_{O2} = \frac{F_{O2}^{in} - F_{O2}^{out}}{F_{O2}^{in}} \quad (2)$$

C₂ selectivity is described as a function of the carbon yield:

$$S_{C2} = \frac{2*(F_{C2H4}^{out} + F_{C2H6}^{out})}{2*(F_{C2H4}^{out} + F_{C2H6}^{out}) + F_{CO2}^{out} + F_{CO}^{out} - F_{CO2}^{in}} \quad (3)$$

CO selectivity is also defined as a function of the carbon yield:

$$S_{CO} = \frac{F_{CO}^{out}}{2*(F_{C2H4}^{out} + F_{C2H6}^{out}) + F_{CO2}^{out} + F_{CO}^{out} - F_{CO2}^{in}} \quad (4)$$

CO₂ selectivity is calculated by taking into account the inlet feed:

$$S_{CO2} = \frac{F_{CO2}^{out} - F_{CO2}^{in}}{2*(F_{C2H4}^{out} + F_{C2H6}^{out}) + F_{CO2}^{out} + F_{CO}^{out} - F_{CO2}^{in}} \quad (5)$$
Contact time is given by the following equations:

$$ \text{Contact time (g s}^{-1} \text{ mol}^{-1}) = \frac{m_{\text{catalyst}}}{F_{\text{CH4}}^{\text{inlet}}} \quad (7) $$

Productivity of a product *i* (µmol g⁻¹ s⁻¹) is the amount of product per gram of catalyst:

$$ P_i = \frac{F_i^{\text{outlet}}}{m_{\text{catalyst}}} \quad (8) $$

with *i* representing a product formed during the OCM reaction, such as CO, CO₂, C₂H₄ or C₂H₆.

## 2.3. Characterization

X-ray diffraction analysis was carried out using on powders (crushed pellets) a Bruker D8 Advance A25 diffractometer in Bragg-Brentano configuration equipped with a Cu anode (Cu Kα radiation: 0.154184 nm) and a LynxEye 1-D detector.

Specific surface areas were measured by nitrogen adsorption of α-Al₂O₃ and Mn-Na₂WO₄/α-Al₂O₃ catalysts at 77 K using a Belsorp mini X instrument from BEL MicrotracBel Corp. Prior to measurements, the material was degassed at 250°C for 4 h. The specific surface areas were calculated according to the method of Brunauer, Emmet and Teller (BET). Analyses were carried out on 400 mg of trilobed pellets (i.e., approx. 50 pellets).

ICP-OES measurements were performed with an Agilent 5800 spectrometer. The Mn-Na₂WO₄/α-Al₂O₃ sample was previously dissolved in an acidic mixture of H₂SO₄ + HNO₃ + HF to perform the analyses of Mn, Na and Al. For W compounds, the process involved dissolution in H₂SO₄ + HNO₃ + HF, then evaporation and dissolution in NaOH.

SEM analyses were carried out with a Zeiss Sigma Field Emission SEM with an Oxford INCA PentaFETx3 EDS system (model 8100). Analyses were carried out on an uncrushed trilobed catalyst.

# 3. Results and discussion

## N₂ physisorption and ICP-OES

The composition of the final supported catalyst (Fig. 1) is very close to the desired one based on the optimized chemical composition in the work of Serres *et al.*²². It is important to note that the ICP analysis was carried out on 500 mg of sample, which corresponds to an average of 50 to 100 catalyst trilobes.

The surface area of the catalyst was measured before and after a 12-hour reaction at 800°C under the reactive mixture. The surface area decreased from 13 to 6 m² g⁻¹ (Table 1). Hence, it is obvious that major catalyst restructuration occurs upon reaction. ICP analysis was not carried out on the spent catalyst, because one single pellet is not enough for the analysis.

<table><caption>Table 1 Specific surface areas of Mn-Na₂WO₄/α-Al₂O₃ and α-Al₂O₃ measured via N₂ adsorption at 77 K and the ICP-OES of Mn-Na₂WO₄/α-Al₂O₃ before reaction</caption><thead><tr><th></th><th>[Mn]</th><th>[Na]</th><th>[W]</th><th>[Al]</th><th>S<sub>BET</sub><sup>c</sup><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>V<sub>pores</sub><sup>c</sup><br>(cm<sup>3</sup> g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Composition target<sup>a</sup></td><td>5.85</td><td>4.01</td><td>16.02</td><td>51.51</td><td></td><td></td></tr></tbody></table>
<table><tr><td>MnNaW/α-Al<sub>2</sub>O<sub>3</sub><br>fresh catalyst <sup>b</sup></td><td>6.54</td><td>4.64</td><td>17.67</td><td>48.29</td><td>13</td><td>4.5 x10<sup>-2</sup></td></tr><tr><td>MnNaW/α-Al<sub>2</sub>O<sub>3</sub><br>spent catalyst</td><td>/</td><td>/</td><td>/</td><td>/</td><td>6</td><td>1.6 x 10<sup>-2</sup></td></tr><tr><td>α-Al<sub>2</sub>O<sub>3</sub> before<br>reaction</td><td>/</td><td>/</td><td>/</td><td>/</td><td>8</td><td>1.3 x 10<sup>-2</sup></td></tr></table>

<sup>a</sup> Theoretical value in fresh sample (in wt%)

<sup>b</sup> ICP-OES measurement after calcination at 900°C, 4 hours (in wt%)

<sup>c</sup> N<sub>2</sub> adsorption at 77 K

<figure><img src="image_2.png" /><figcaption>Figure 1 Fresh trilobed pellets of Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub></figcaption></figure>

## XRD

The XRD patterns of the Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> catalyst before and after reaction and the α-Al<sub>2</sub>O<sub>3</sub> support before reaction are presented in Fig. 2. Three different crystal phases can be observed on fresh catalysts: Na<sub>2</sub>WO<sub>4</sub>, α-Al<sub>2</sub>O<sub>3</sub> (in large majority) and traces of Mn<sub>2</sub>O<sub>3</sub>. After 12 hours under reaction, the Mn<sub>2</sub>O<sub>3</sub> diffraction peak at 33.3° disappears and the presence of an additional phase of galaxite, (Mn<sub>0.89</sub>Al<sub>0.11</sub>)Al<sub>2</sub>O<sub>4</sub>, on the spent catalysts is detected in a minor amount. This suggests that Mn<sub>2</sub>O<sub>3</sub> present in the fresh catalyst may have reacted with the alumina support during the reaction.

<figure><img src="image_3.png" /><figcaption>Figure 2 XRD analyses of fresh and spent Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> and α-Al<sub>2</sub>O<sub>3</sub>; (♦) Na<sub>2</sub>WO<sub>4</sub>, (■) (Mn<sub>0.89</sub>Al<sub>0.11</sub>)Al<sub>2</sub>O<sub>4</sub>, (●) α-Al<sub>2</sub>O<sub>3</sub>, (※)Mn<sub>2</sub>O<sub>3</sub></figcaption></figure>

## Scattering electron microscopy
A whole pellet was also analyzed by scanning electron microscopy (Fig. S1). It can be seen that the material is compact and not very porous, which is consistent with the analyses obtained during N₂ physisorption. EDX analysis was performed on the same trilobed pellet, as shown in Fig. S2. The distribution of the different active species (Mn, Na and W) is clearly visible, although in some places, phases composed primarily of Na and W can be observed, while other phases are composed primarily of Mn. The alumina, which forms the support of the trilobed pellet, is clearly visible throughout the picture. We may suppose a phase change to be possible at these reaction temperatures, as indicated by XRD analysis.

## Thermal analysis

The thermogravimetric analyses of the fresh Mn-Na₂WO₄/α-Al₂O₃ and spent Mn-Na₂WO₄/α-Al₂O₃ show a very good thermal stability up to 900°C (Fig. S3). The weight loss of the fresh Mn-Na₂WO₄/α-Al₂O₃ does not exceed 0.4 wt%. For the spent Mn-Na₂WO₄/α-Al₂O₃, we can observe a constant mass loss between 100 and 500°C, which should indicate the presence of coke (0.5 wt%).

## Temperature

Radical reactions can occur, especially at temperatures above 650°C²³⁻²⁶, at which radical concentrations in the gas phase start to be significant. In our case, gas phase reactions are likely to occur, because the reaction proceeds under a pressure of 3 bar and with undiluted reactants. Hence, the catalytic activity of the catalyst must be compared with the alumina support as a reference. Catalytic results for the Mn-Na₂WO₄/α-Al₂O₃ trilobed catalyst and an α-Al₂O₃ trilobe are compared in Table 2.

<table><caption>Table 2 Catalytic tests with Mn-Na₂WO₄/α-Al₂O₃ and α-Al₂O₃, with and without CO₂ in the inlet feed: CH₄/O₂/N₂/CO₂ or He: 60/15/15/10, 650-850°C, 3 bar, contact time: 1,800 g s⁻¹ mol⁻¹</caption><thead><tr><th rowspan="2">Temp (°C)</th><th colspan="2">Conv. (%)</th><th colspan="4">Selectivity (%)</th><th colspan="4">Productivity (µmol g<sub>cat</sub><sup>-1</sup> s<sup>-1</sup>)</th></tr><tr><th>O<sub>2</sub></th><th>CH<sub>4</sub></th><th>CO</th><th>CO<sub>2</sub></th><th>C<sub>2</sub>H<sub>4</sub></th><th>C<sub>2</sub>H<sub>6</sub></th><th>CO</th><th>CO<sub>2</sub></th><th>C<sub>2</sub>H<sub>4</sub></th><th>C<sub>2</sub>H<sub>6</sub></th></tr></thead><tbody><tr><td colspan="11" style="text-align: center; font-weight: bold;">α-Al<sub>2</sub>O<sub>3</sub> (biogas)</td></tr><tr><td>700</td><td>31</td><td>6</td><td>60</td><td>34</td><td>1</td><td>5</td><td>12</td><td>7</td><td>0</td><td>1</td></tr><tr><td>725</td><td>41</td><td>9</td><td>54</td><td>35</td><td>3</td><td>8</td><td>16</td><td>10</td><td>0</td><td>1</td></tr><tr><td>750</td><td>47</td><td>10</td><td>50</td><td>32</td><td>5</td><td>12</td><td>19</td><td>12</td><td>1</td><td>2</td></tr><tr><td>775</td><td>54</td><td>12</td><td>47</td><td>28</td><td>9</td><td>16</td><td>21</td><td>12</td><td>2</td><td>4</td></tr><tr><td>800</td><td>60</td><td>15</td><td>43</td><td>26</td><td>14</td><td>18</td><td>23</td><td>14</td><td>4</td><td>5</td></tr><tr><td>825</td><td>67</td><td>16</td><td>41</td><td>21</td><td>20</td><td>18</td><td>25</td><td>13</td><td>6</td><td>5</td></tr><tr><td>850</td><td>74</td><td>18</td><td>39</td><td>20</td><td>26</td><td>16</td><td>28</td><td>14</td><td>9</td><td>6</td></tr><tr><td colspan="11" style="text-align: center; font-weight: bold;">Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> (biogas)</td></tr><tr><td>700</td><td>24</td><td>4</td><td>14</td><td>69</td><td>2</td><td>15</td><td>2</td><td>8</td><td>0</td><td>1</td></tr><tr><td>725</td><td>48</td><td>8</td><td>9</td><td>65</td><td>6</td><td>20</td><td>3</td><td>23</td><td>1</td><td>4</td></tr><tr><td>750</td><td>75</td><td>15</td><td>6</td><td>53</td><td>15</td><td>26</td><td>5</td><td>38</td><td>6</td><td>9</td></tr><tr><td>775</td><td>94</td><td>22</td><td>5</td><td>43</td><td>26</td><td>26</td><td>6</td><td>46</td><td>14</td><td>14</td></tr><tr><td>800</td><td>99</td><td>25</td><td>5</td><td>38</td><td>32</td><td>25</td><td>6</td><td>45</td><td>19</td><td>14</td></tr><tr><td>825</td><td>100</td><td>26</td><td>6</td><td>36</td><td>36</td><td>23</td><td>7</td><td>44</td><td>22</td><td>14</td></tr><tr><td>850</td><td>100</td><td>26</td><td>8</td><td>34</td><td>38</td><td>20</td><td>10</td><td>42</td><td>24</td><td>13</td></tr><tr><td colspan="11" style="text-align: center; font-weight: bold;">Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> (methane)</td></tr><tr><td>700</td><td>31</td><td>3</td><td>6</td><td>85</td><td>1</td><td>8</td><td>2</td><td>20</td><td>0</td><td>1</td></tr><tr><td>725</td><td>51</td><td>6</td><td>6</td><td>73</td><td>4</td><td>17</td><td>2</td><td>30</td><td>1</td><td>4</td></tr><tr><td>750</td><td>72</td><td>10</td><td>5</td><td>59</td><td>12</td><td>24</td><td>3</td><td>40</td><td>4</td><td>8</td></tr><tr><td>775</td><td>91</td><td>16</td><td>4</td><td>49</td><td>21</td><td>26</td><td>4</td><td>47</td><td>10</td><td>13</td></tr><tr><td>800</td><td>99</td><td>19</td><td>3</td><td>43</td><td>28</td><td>25</td><td>4</td><td>47</td><td>16</td><td>14</td></tr><tr><td>825</td><td>100</td><td>20</td><td>4</td><td>39</td><td>33</td><td>24</td><td>4</td><td>42</td><td>17</td><td>13</td></tr><tr><td>850</td><td>100</td><td>20</td><td>6</td><td>37</td><td>36</td><td>21</td><td>6</td><td>40</td><td>19</td><td>12</td></tr></tbody></table>
Relatively high conversions of CH₄ and CO₂ are obtained on an α-Al₂O₃ trilobe, even at moderate temperature (Fig. 3a). At 700°C the conversion of O₂ already reaches 31%, yet O₂ is not fully converted at 850°C. For the entire temperature range, the product distribution consists predominantly of CO, with a surprisingly moderate selectivity in C2 products, up to 42% at 850°C (Fig 3b and 3c). This indicates that the α-Al₂O₃ trilobe allows oxidation of methane and favors (dry) reforming reactions, yet with OCM occurring in parallel.

In contrast, O₂ conversions are higher for the Mn-Na₂WO₄/α-Al₂O₃ catalyst, from 24% at 700°C up to 100% from 850°C. As expected, we can observe similar tendencies for CH₄ conversion, with a greater increase for the catalyst, passing from 4 to 26% of CH₄ conversion against 6 to 18% for the support on its own (Fig. 3a). A clear shift in the selectivity can be observed. The Mn-Na₂WO₄ catalyst allows a major gain in C2 products at the expense of CO. Indeed, the difference in selectivity is to the benefit of C2 (C₂H₄ + C₂H₆) selectivity for the catalyst, since this value passes from 17% at 700°C to 59% at 850°C for the catalyst, versus 6% to 42% in the same temperature range for the support (Fig 3b and 3c). When the O₂ conversion exceeds 99%, we observe a major change in the C2 product distribution. Whereas ethylene is the minor product below 99% conversion, ethylene is produced in majority after 99%, with an ethylene/ethane ratio close to 2. This corresponds to the endothermic reaction of ethane dehydrogenation at the end of the reactor in the absence of O₂. The C₂ productivity reaches 31 µmol g_cata⁻¹ s⁻¹ at 850°C.

<figure><img src="image_4.png" /><figcaption>Figure 3 a) CH₄ and O₂ conversion (%), b) CO selectivity (%) and c) C2 selectivity as a function of the reaction temperature (°C) with and without CO₂ in the inlet feed: CH₄/O₂/N₂/CO₂: 60/15/15/10, 700-850°C, 3 bar, contact time: 1,800 g s⁻¹ mol⁻¹</figcaption></figure>

The impact of CO₂ on catalyst performance is positive yet minor. Methane conversion is 30% higher at equal O₂ conversion. The selectivity toward C2 is enhanced from 17% to 58%. We can observe a higher CO selectivity (10% compared with 6% at 850°C).

The ambition of this work is to use biogas as a raw material for high-value chemicals. To investigate the effect of inlet CO₂ concentration, the feed gas composition in CO₂ was tuned using helium to maintain similar partial pressures of inlet CH₄, O₂ and N₂ tracer. The addition of CO₂ does not seem to affect O₂ conversion. However, looking at the CH₄ conversion, we can see that tests performed in the
presence of CO₂ show a slightly higher CH₄ conversion, going from 20% without CO₂ to 26% with CO₂ at higher temperature, which may be due to a parallel occurrence of the dry reforming reaction. For C₂ selectivity, particularly for C₂H₄ selectivity when CO₂ is added to the stream, the selectivity increases slightly. These results appear to be in agreement with the work of Shi *et al.*¹⁴, who showed that both CH₄ conversion and C₂ selectivity can be improved with CO₂ in the feed. An experiment was also carried out by exposing the Mn-Na₂WO₄/α-Al₂O₃ to CH₄, CO₂ and N₂ (without O₂). At 800°C, low CH₄ and CO₂ conversions were observed, with some CO and C₂H₆ produced.

It is not straightforward to compare different catalysts exposed to a variety of conditions in other works, however we can compare them according to certain criteria such as temperature, contact time and reaction mixture composition (Table 3). The Mn-Na₂WO₄/α-Al₂O₃ catalyst developed by our team shows competitive methane conversion compared to the works of Shi *et al.* and Uzunoglu *et al.* As for the C₂ selectivity, it is similar to the results of other works, especially for the Na₂WO₄/Mn/SiO₂ and 1.9%Mn-5%Na₂WO₄/SiO₂ catalysts. However, we can note that the catalyst from the Palermo *et al.* study has a high CH₄ conversion as well as an impressive C₂ selectivity. However, in terms of C₂H₄ productivity (µmol gcat⁻¹ s⁻¹), our catalyst shows higher performances.

<table><caption>Table 3 OCM performance of Mn-Na-W based catalysts</caption><thead><tr><th>Catalyst</th><th>T (°C)</th><th>Contact time (g s mol<sup>-1</sup>)</th><th>CH<sub>4</sub>/O<sub>2</sub>/N<sub>2</sub>/CO<sub>2</sub></th><th>X<sub>CH4</sub> (%)</th><th>C2 sel (%)</th><th>C<sub>2</sub>H<sub>4</sub> prod (µmol gcat<sup>-1</sup> s<sup>-1</sup>)</th><th>Ref</th></tr></thead><tbody><tr><td>Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub></td><td>850</td><td>1,800</td><td>6/1.5/1.5/1</td><td>26</td><td>58</td><td>24</td><td>Our work</td></tr><tr><td>Mn-Na<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub></td><td>725</td><td>4,400</td><td>5/1/-/-</td><td>22.3</td><td>82.8</td><td>/</td><td>27</td></tr><tr><td>19.5wt%Mn-Na-W/SiC</td><td>800</td><td>2,600</td><td>16/4/1/-</td><td>10</td><td>60-65</td><td>18</td><td>22</td></tr><tr><td>Na<sub>2</sub>WO<sub>4</sub>/Mn/SiO<sub>2</sub></td><td>800</td><td>9,100</td><td>3/1/1/-</td><td>29.5</td><td>55</td><td>/</td><td>14</td></tr><tr><td>1.9%Mn-5%Na<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub></td><td>800</td><td>10,200</td><td>61/8/31/-</td><td>12.3</td><td>63.5</td><td>/</td><td>28</td></tr><tr><td>Mn-Na<sub>2</sub>WO<sub>4</sub>/SiO<sub>2</sub></td><td>850</td><td>39,200</td><td>4.5/1/-/-</td><td>33</td><td>80</td><td>6.7</td><td>29</td></tr></tbody></table>

## Long-time testing

Co-feeding with CO₂ may eventually result in coke deposition and catalyst deactivation. The fact that the CO selectivity is increased in the biogas model feed suggests the occurrence of dry reforming, a reaction that is very often accompanied by degraded catalytic performance. To study the stability of the Mn-Na₂WO₄/α-Al₂O₃ catalyst over time in the OCM reaction, a 100 h catalytic test at 850°C was performed, at 3 bar and at a contact time of 2,000 g s⁻¹ mol⁻¹ (Fig. 4).

On the contrary, we observe continuous activation of the catalyst over the 100-hour run time. The conversions of CH₄ and O₂ increase from 18% to 22% and from 76% to 99%, respectively. The steady state appears to be reached at approximately 90 hours of time on stream. It is important to note that selectivities remain almost unchanged despite this significant increase in conversion. It is possible that in certain published works, the time on stream was not long enough for this phenomenon to be observed. However, in some other works, catalytic tests were performed over a longer period of time ranging from 20 to 500 hours, and in these cases it was possible to observe a slight increase in activity at the beginning of the catalytic test ³⁰⁻³². This increase was certainly weaker than the one we observed, but it is still noteworthy. The activation phenomenon could be explained by a catalytic restructuration enabled by the high temperature and the reduction conditions of the feed mixture. At these temperatures, one of the active phases of the catalyst can migrate closer to the surface, thereby becoming more accessible for the OCM reaction and improving the conversions. We can also suggest a different hypothesis. As supported by the work of Shi *et al.*, it is possible that the manganese phase could migrate to the surface of the material in this temperature range¹⁴. Pushing this rationale further, it is also conceivable that phases may have partially melted, leading to a major catalytic
restructuration. This suggestion is supported by the appearance of crystallized (Mn<sub>0.89</sub>Al<sub>0.11</sub>)Al<sub>2</sub>O<sub>4</sub> and Mn<sub>2</sub>O<sub>3</sub> and by the lower surface area after reaction. We can also hypothesize that the stepwise impregnation protocol (Mn and then W) that was used here did not achieve a very stable phase despite the heat treatment at 900°C for 4 hours.

<figure><img src="image_5.png" /><figcaption>Figure 4 Conversion (%) and selectivity (%) over 100 hours at 850°C, 3 bar and 2,000 g s<sup>-1</sup> mol<sup>-1</sup> for the Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> catalyst</figcaption></figure>

## Role of alumina support

In order to assess whether or not the alumina support might play a catalytic role, in particular to produce CO<sub>2</sub>, we have studied the effect of the CH<sub>4</sub>/O<sub>2</sub> ratio on the α-Al<sub>2</sub>O<sub>3</sub> support and the Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub>. The CH<sub>4</sub>/O<sub>2</sub> ratio was varied from 2.4 to 10 by changing the O<sub>2</sub> partial pressure in the reaction stream, while the CH<sub>4</sub> partial pressure was kept constant by adding He as a diluent. Catalytic results are tabulated in Table 3 for the lowest contact time and in (Table S1 to S5) for all the other contact times.

<table><caption>Table 4 Catalytic tests with Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub> and α-Al<sub>2</sub>O<sub>3</sub>, CH<sub>4</sub>/O<sub>2</sub> from 2.4 to 10 (CH<sub>4</sub> constant), 800-850°C, 3 bar, contact time: 1,800 g s mol<sup>-1</sup></caption><thead><tr><th rowspan="3">Temp (°C)</th><th colspan="7">Mn-Na<sub>2</sub>WO<sub>4</sub>/α-Al<sub>2</sub>O<sub>3</sub></th><th colspan="4">Productivity (µmol g<sub>cat</sub><sup>-1</sup> s<sup>-1</sup>)</th></tr><tr><th rowspan="2">CH<sub>4</sub>/O<sub>2</sub></th><th colspan="3">Conv. (%)</th><th colspan="4">Selectivity (%)</th><th rowspan="2">CO</th><th rowspan="2">CO<sub>2</sub></th><th rowspan="2">C<sub>2</sub>H<sub>4</sub></th><th rowspan="2">C<sub>2</sub>H<sub>6</sub></th></tr><tr><th>O<sub>2</sub></th><th>CH<sub>4</sub></th><th>CO</th><th>CO<sub>2</sub></th><th>C<sub>2</sub>H<sub>4</sub></th><th>C<sub>2</sub>H<sub>6</sub></th></tr></thead><tbody><tr><td rowspan="5">800</td><td>2.4</td><td>73</td><td>24</td><td>11</td><td>46</td><td>27</td><td>16</td><td>14</td><td>59</td><td>17</td><td>10</td></tr><tr><td>3</td><td>66</td><td>19</td><td>11</td><td>42</td><td>28</td><td>19</td><td>11</td><td>43</td><td>14</td><td>10</td></tr><tr><td>3.5</td><td>56</td><td>14</td><td>12</td><td>39</td><td>27</td><td>22</td><td>9</td><td>28</td><td>10</td><td>8</td></tr><tr><td>5</td><td>58</td><td>11</td><td>10</td><td>36</td><td>28</td><td>26</td><td>6</td><td>23</td><td>9</td><td>8</td></tr><tr><td>10</td><td>66</td><td>7</td><td>7</td><td>32</td><td>27</td><td>34</td><td>3</td><td>13</td><td>5</td><td>7</td></tr><tr><td rowspan="6">850</td><td>2.4</td><td>66</td><td>21</td><td>15</td><td>45</td><td>28</td><td>12</td><td>17</td><td>46</td><td>16</td><td>7</td></tr><tr><td>3</td><td>83</td><td>24</td><td>10</td><td>45</td><td>31</td><td>15</td><td>12</td><td>56</td><td>19</td><td>9</td></tr><tr><td>3.5</td><td>84</td><td>22</td><td>10</td><td>41</td><td>32</td><td>17</td><td>11</td><td>28</td><td>18</td><td>9</td></tr><tr><td>4</td><td>81</td><td>19</td><td>11</td><td>39</td><td>32</td><td>18</td><td>11</td><td>37</td><td>16</td><td>9</td></tr><tr><td>5</td><td>83</td><td>16</td><td>10</td><td>37</td><td>33</td><td>20</td><td>9</td><td>31</td><td>14</td><td>9</td></tr><tr><td>10</td><td>81</td><td>9</td><td>8</td><td>32</td><td>31</td><td>29</td><td>4</td><td>15</td><td>7</td><td>7</td></tr></tbody><tfoot><tr><td colspan="12" style="text-align: center;">α-Al<sub>2</sub>O<sub>3</sub></td></tr></tfoot></table>
<table><tbody><tr><td rowspan="5">800</td><td>2.4</td><td>42</td><td>13</td><td>45</td><td>37</td><td>8</td><td>10</td><td>33</td><td>28</td><td>3</td><td>4</td></tr><tr><td>3</td><td>39</td><td>10</td><td>45</td><td>36</td><td>7</td><td>11</td><td>26</td><td>21</td><td>2</td><td>3</td></tr><tr><td>3.5</td><td>38</td><td>8</td><td>46</td><td>36</td><td>7</td><td>12</td><td>22</td><td>17</td><td>2</td><td>3</td></tr><tr><td>4</td><td>38</td><td>7</td><td>46</td><td>35</td><td>6</td><td>12</td><td>20</td><td>15</td><td>1</td><td>3</td></tr><tr><td>5</td><td>38</td><td>7</td><td>46</td><td>35</td><td>6</td><td>13</td><td>17</td><td>13</td><td>1</td><td>2</td></tr><tr><td rowspan="5">850</td><td>10</td><td>44</td><td>4</td><td>49</td><td>31</td><td>5</td><td>15</td><td>11</td><td>7</td><td>1</td><td>2</td></tr><tr><td>2.4</td><td>77</td><td>25</td><td>41</td><td>36</td><td>16</td><td>8</td><td>58</td><td>50</td><td>11</td><td>5</td></tr><tr><td>3</td><td>67</td><td>18</td><td>44</td><td>31</td><td>15</td><td>10</td><td>44</td><td>31</td><td>8</td><td>5</td></tr><tr><td>3.5</td><td>64</td><td>16</td><td>46</td><td>29</td><td>15</td><td>11</td><td>38</td><td>24</td><td>6</td><td>4</td></tr><tr><td>4</td><td>62</td><td>14</td><td>44</td><td>29</td><td>15</td><td>12</td><td>33</td><td>22</td><td>6</td><td>5</td></tr><tr><td rowspan="2">850</td><td>5</td><td>61</td><td>11</td><td>45</td><td>28</td><td>14</td><td>13</td><td>28</td><td>18</td><td>4</td><td>4</td></tr><tr><td>10</td><td>69</td><td>7</td><td>47</td><td>25</td><td>11</td><td>17</td><td>18</td><td>9</td><td>2</td><td>3</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Figure 5 Selectivity of a) CO, b) CO₂, c) C₂H₄ and d) C₂H₆ as a function of the CH₄/O₂ ratio at 800°C, 3 bar and 1,800 g s mol⁻¹</figcaption></figure>

The plots of selectivities as a function of the CH₄/O₂ ratio reveal important insights (Fig. 5). The CO and ethylene selectivities do not vary much with the CH₄/O₂ ratio. It appears that the C₂H₄ selectivity remains high (25-28%) even at low CH₄:O₂ (2.4). Possibly, a higher yield of ethylene could be obtained at a lower CH₄/O₂ ratio, because this would allow greater CH₄ conversion. However, it must be mentioned that this operating condition is hardly achievable in a fixed-bed reactor, as this would exceed inflammability limits. In contrast, the selectivity of ethane decreases as the oxygen concentration increases, with an increase of CO₂ selectivity. Hence, we can conclude that the ethane is converted to CO₂ in an O₂-rich atmosphere and that ethane dehydrogenation does not occur.

We note the obvious catalytic role of alumina, which acts as a combustion catalyst, producing mainly CO₂ and CO. Interestingly, the curves of the CO₂ selectivity are essentially superposed for the support and the supported catalyst (Fig 6a), which seems to indicate that the main origin of CO₂ production may be the alumina support and not Mn-Na₂WO₄.
The plot of the C2 yield ($C_2H_4$ and $C_2H_6$) as a function of methane conversion (as $CH_4/O_2$ decreases) further confirms the potential value in operating at a low $CH_4/O_2$ ratio to achieve maximum C2 yield (Fig. 6b).

<figure><img src="image_7.png" /><figcaption>Figure 6a) $CO_2$ yield as a function of $CH_4$ conversion and 6b) C2 yield as a function of $CH_4$ conversion at 800°C, 3 bar and 1,800 g s$^{-1}$ mol$^{-1}$ obtained with the $Mn-Na_2WO_4/α-Al_2O_3$ catalyst and the $α-Al_2O_3$ support.</figcaption></figure>

For new insights into the reaction mechanism occurring in the OCM, the productivities ($\mu mol\ g^{-1} s^{-1}$) of OCM components were plotted as a function of the oxygen consumption rate ($\mu mol\ g^{-1} s^{-1}$) for different contact times and $CH_4/O_2$ inlet ratios (Fig. 7). The dashed colored lines represent the experiments run at a similar inlet ratio, and the dashed black lines the ones conducted at the same contact time (from 18,000 to 1,800 g s$^{-1} mol^{-1}$). (18,000 14,000 10,000 6,000 and 1,800 g s$^{-1} mol^{-1}$)

<figure><img src="image_8.png" /><figcaption>Figure 7 $C_2H_6$ productivity ($\mu mol\ s^{-1} g^{-1}$) at 800°C, 3 bar, with different total inlet flow rates and $CH_4/O_2$ ratios (from 2.4 to 10). The dashed colored lines represent the experiments run at a similar inlet ratio, and the dashed black lines the ones conducted at the same contact time</figcaption></figure>

It appears that $C_2H_6$ productivity increases when the contact time decreases, regardless of the $CH_4/O_2$ ratio. Moreover, irrespective of the contact time, $C_2H_6$ productivity is similar, and forms "plateaus," at each contact time. This observation confirms that $C_2H_6$ is formed first in the OCM reaction and is then converted to $C_2H_4$ and $CO_x$, as often mentioned in the literature$^{33,34}$. Moreover, with the constant $C_2H_6$
productivity occurring at different oxygen consumption, it can be assumed that ethane formation is a fast reaction that takes place in the early stages of the mechanism and that excess C₂H₆ from the catalytic activity is directly converted to C₂H₄ and COₓ. This hypothesis is supported by the results obtained at lower temperature, where we see that C₂H₆ is formed before C₂H₄, and in greater quantity.

## Conclusion

We successfully tested a single catalyst pellet for OCM under relevant process conditions of gas composition, temperature and pressure. The single trilobed Na₂WO₄/α-Al₂O₃ reactor achieve comparable selectivity and productivity than laboratory small scale fixed bed reactors.

We found that the presence of CO₂ does not degrade catalyst performance, even on long run, which should enable the use of a CO₂-rich stream such as biogas. Actually, higher methane conversions have been obtained in the presence of CO₂, likely due to a possible contribution of methane dry reforming reaction. We also noticed a slow catalyst activation over the first 100 hours while selectivity was kept constant. Catalyst restructuration was observed.

We show that a higher C2 yields could be obtained if it were possible to operate at a lower CH₄/O₂ ratio in safe and/or non-explosive conditions at large scale. The single pellet reactor as miniaturized is intrinsically safe and prevent temperature run away.

## Acknowledgment

*This paper reports work undertaken in the context of the project entitled "C123: Methane oxidative conversion and hydroformylation to propylene." C123 has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 814557.*

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