The Application of Photoionisation Mass Spectrometry in Coupled Methane Oxidation Reactions

Applications: Methane conversion and utilisation; catalytic oxidation; low-carbon chemical industry; ethylene production

Introduction

Chlorine doping is an effective means of enhancing the ethylene yield of Li-MgO catalysts in the methane oxidation-coupling (OCM) reaction; however, its mechanism of action remains unclear.

The core objective of this study is to precisely analyse the composition of gaseous products and key intermediates in the OCM reaction, and to clarify the influence of chlorine doping on the selectivity of the catalytic reaction. In this study, vacuum ultraviolet photoionisation mass spectrometry (SVUV-PIMS) was employed to analyse the gas-phase components of the reaction, successfully detecting the methyl radical as a gas-phase intermediate, whilst also clarifying the differences in signal intensity among products such as C₂Hₓ, COₓ and formaldehyde. Furthermore, using online gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a hydrogen flame ionisation detector (FID), the separation and quantitative analysis of products such as methane, ethane, ethylene, CO and CO₂ were achieved, providing precise data for the calculation of the C₂H₄/C₂H₆ ratio, methane conversion rate and C₂ product selectivity.

Experimental Materials and Methods

1. Catalyst preparation: Li₂CO₃-MgO and LiCl-MgO catalysts were prepared via the precipitation method using magnesium acetate–lithium carbonate and magnesium chloride–lithium carbonate as precursors, respectively; these were then dried and calcined for subsequent use.

2. Key characterisation methods: SVUV-PIMS was employed to analyse gaseous reaction intermediates and products; GC-TCD/FID was used for the quantitative detection of products such as methane, ethane and ethylene.

‌Results and Conclusions

1. Mechanism by which the catalyst regulates the reaction pathway

SVUV-PIMS experiments were used to detect gaseous intermediates and products of the OCM reaction, demonstrating that LiCl-MgO inhibits the deep oxidation of C₂H₆ (HCHO accumulation) and that ethylene accounts for a higher proportion of the C₂ products. Combining the results from Figures 1 and 2, it can be seen that molecular ion peaks of CH₃*, C₂H₄, C₂H₆, CO, CO₂ and HCHO were successfully detected at different photon energies (10 eV, 11 eV, 11.8 eV, 14.2 eV, etc.). The signal intensities for C₂Hₓ (C₂H₄, C₂H₆) and COₓ (CO, CO₂) were higher for Li₂CO₃-MgO, whilst the HCHO signal was stronger for LiCl-MgO.

The signal for the methyl radical (CH₃*) was stronger on Li₂CO₃-MgO than on LiCl-MgO, indicating that Li₂CO₃-MgO exhibits stronger activation of CH₄ (generating more CH₃*).

Although the total signal intensity of the C₂Hₓ products (C₂H₄: m/z = 28; C₂H₆: m/z = 30) generated under Li₂CO₃-MgO catalysis exhibit a higher overall signal intensity; however, the C₂ products consist predominantly of ethane, with ethylene accounting for an extremely low proportion, whereas ethylene is the main component of the C₂ products generated under LiCl-MgO catalysis.

The COₓ products (CO: m/z = 28; CO₂: m/z = 44). The CO₂ peak is extremely intense, whilst the CO peak is very weak; in the case of LiCl-MgO, the intensity of the CO peak is significantly enhanced, whilst that of the CO₂ peak is reduced (CO/CO₂ ≈ 1.15). This indicates that Li₂CO₃-MgO tends towards deep oxidation to form CO₂, whereas LiCl-MgO tends towards partial oxidation to form CO.

The molecular ion peak intensity of the LiCl-MgO intermediate HCHO (m/z = 30) is significantly higher than that of Li₂CO₃-MgO. As HCHO is a key intermediate in the CH₄ → CO reaction, its accumulation in LiCl-MgO indicates that it inhibits deep oxidation, thereby preserving a carbon source for the dehydrogenation of C₂H₆ to form C₂H₄.

Figure 1: SVUV-PIMS spectra of the OCM reaction catalysed by Li₂CO₃-MgO and LiCl-MgO at different photon energies

Figure 2 SVUV-PIMS spectra of the OCM reaction catalysed by Li₂CO₃-MgO and LiCl-MgO at a photon energy of 11 eV

2. Evaluation of catalyst performance

GC-TCD/FID experiments were conducted to evaluate the catalytic performance for the OCM reaction, directly confirming that both the absolute yield and relative proportion of ethylene from LiCl-MgO were significantly higher than those from Li₂CO₃-MgO.

2.1 CH₄ Conversion Rate and C₂ Selectivity

Li₂CO₃-MgO: C₂ selectivity was slightly higher (Figure 3A), but the proportion of ethylene in the C₂ products was extremely low (Figure 3B). LiCl-MgO: C₂ selectivity was lower than that of Li₂CO₃-MgO, but the proportion of ethylene in the C₂ products was extremely high; consequently, the absolute yield of ethylene was higher.

2.2 C₂H₄/C₂H₆ and CO/CO₂ Ratios

Li₂CO₃-MgO: C₂H₆ accounts for approximately 67 per cent of the C₂ products, whilst C₂H₄ accounts for only about 33 per cent; LiCl-MgO: C₂H₄ accounts for ≥67 per cent of the C₂ products, which is significantly higher than that of Li₂CO₃-MgO.

Li₂CO₃-MgO: The CO/CO₂ ratio was almost 0 (only CO₂ was produced); LiCl-MgO: The CO/CO₂ ratio gradually increased with reaction time, rising from an initial value of 0.81 to 1.15 at the steady state, indicating the production of a large amount of CO. This corresponds with the strong HCHO signal observed in the SVUV-PIMS data, confirming that deep oxidation was suppressed and that ethylene was not further oxidised.

Figure 3 CH₄ conversion and C₂ selectivity (A) and C₂H₄/C₂H₆ and CO/CO₂ ratios (B) for the OCM reaction catalysed by Li₂CO₃-MgO and LiCl-MgO.

Mass spectrometry-related techniques (SVUV-PIMS and GC) played a pivotal role in this study; not only did they accurately capture information on reaction intermediates and products and quantify the performance differences between various catalysts, but they also provided reliable data to elucidate the mechanism by which chlorine doping regulates the reaction pathway. By combining mass spectrometry with other characterisation techniques, this study has deepened our fundamental understanding of the Li-MgO-catalysed OCM reaction and provided an important reference for catalyst design and performance optimisation in the field of low-carbon chemical engineering for the efficient conversion of methane to ethylene.

References

[1] Wu Hangfei;  Sun Xiao;  Yuan Zhenxuan;  Xiao Tianci; Pan Yang;  Huang Weixin*, Promotion Effect of Chlorine Additive on Li-MgO Catalysts in the Oxidative Coupling of Methane, The Journal of Physical Chemistry C, 2025, doi.org/10.1021/acs.jpcc.5c01902