Reaction Mechanisms of Aqueous Methane Reforming by Continuous Flow Two-Phase Plasma Discharge

Abstract

This study explores nonthermal plasma reactions of methane and water in a two-phase system to produce methanol, examining reaction pathways, kinetics, and product distribution over time. The results show that methanol is the dominant liquid phase product among other oxygenates, including ethanol and acetic acid, with hydrogen as the largest fraction among gas-phase products comprising carbon monoxide, carbon dioxide, ethylene, and acetylene. Conductivity and pH trends of reactant water and their influence on reaction products were also analyzed. Methanol was found to be formed principally from the reactive coupling of methyl and hydroxyl radicals, as well as from methoxy and hydrogen radical combinations. Hydrogen was produced from three pathways: stepwise dehydrogenation of methane through electron-mediated hydrogen abstraction, sequential hydrogenation of ethane to acetylene, and water splitting. The methanol-yielding reactions proceeded at different rates in the liquid and gas phases, with gas-phase reactions occurring approximately nine times faster than the liquid-phase reactions. This work provides valuable insights into reaction pathways for direct methane conversion to oxygenates and value-added gas products under mild conditions using water as an environmentally friendly oxidant.

1. Introduction

As global energy demands continue to rise and concerns about greenhouse gas emissions intensify, developing processes that can convert the potent greenhouse gas, methane, into useful products under mild conditions has become increasingly important [1,2]. The simultaneous production of valuable liquid oxygenates together with value-added gases, such as hydrogen from methane, represents a significant opportunity in the field of sustainable chemistry and energy systems [3,4,5]. Methane’s abundance as a natural gas component and its increasing availability from renewable sources such as biogas also makes it an attractive feedstock for these chemical transformations [6].

Methane is a highly stable gaseous chemical due to its strong C-H bonds and the absence of any functional groups. The thermochemical processes to convert methane to other products require elevated temperatures and pressures, which consequently increase the process costs [7,8]. To overcome this problem, nonthermal plasma (NTP) technologies have emerged as promising approaches for methane activation under ambient conditions, offering an alternative to conventional thermochemical processes [9,10]. NTP systems generate reactive species capable of breaking methane’s strong C-H bonds at room temperature through electron impact reactions rather than thermal energy and make the reaction environment more selective [11,12]. When combined with water as an environmentally benign oxidant, NTP systems can facilitate partial oxidation pathways to produce liquid oxygenates while concurrently producing hydrogen, a clean energy carrier with significant potential for decarbonizing various sectors [13,14,15,16].

While previous studies on plasma-assisted methane conversion have primarily focused on either methanol production or hydrogen generation as separate objectives, some do report the simultaneous production of these different-phase products in a single reaction system [16,17,18]. However, there are limited investigations regarding the mechanistic pathways and kinetics governing the synchronous formation of these products in a two-phase system. Understanding these aspects is crucial for developing more efficient and selective processes for methane conversion.

This study employed a two-phase (gas–liquid) plasma reactor to investigate the non-thermal plasma reaction of methane with water, focusing on both liquid oxygenates and gas-phase products. The distribution and selectivity of products were analyzed over time, reaction kinetics in both phases were examined, and detailed reaction pathways for the formation of key products are proposed. By monitoring pH and conductivity trends throughout the reaction, additional insights were also gained into the evolving chemical environment and its influence on product formation. This comprehensive analysis revealed that methanol formation occurs primarily through radical coupling mechanisms, with gas-phase reactions proceeding approximately nine times faster than liquid-phase reactions, and provides new insights into optimizing dual-phase plasma systems for enhanced methane conversion efficiency while offering valuable insights for the development of more efficient and selective processes for the simultaneous production of hydrogen and valuable oxygenates under mild conditions.

2. Results and Discussion

2.1. Characterization of Liquid Products

The NMR analysis revealed that the liquid product stream contained a variety of oxygenates including methanol, ethanol, methyl formate, acetaldehyde, acetic acid, acetone, 1-propanol, and 2-propanol. Figure 1 shows the concentration trends of these products in the condensate and liquid remnant streams.

Methanol was the dominant product in both streams, comprising the largest fraction of the total product distribution across all conditions with a progressively increasing concentration over the reaction period. Methanol production in the condensate stream was seen to increase steadily from 90 mg/L after 10 min of reaction to about 142 mg/L at the 35th minute mark, illustrating a strong stability in methanol generation [19]. The selectivity in the liquid remnant, however, had a more complex interaction of products [20], with a fluctuating trend from as low as 63% in the first 10 min of reaction, oscillating appreciably over the course of reaction till it reached 72% after 35 min.

Ethanol emerged as the second most abundant product in both streams but with unclear variations in the liquid remnant, oscillating over the observation period. Other reaction products included methyl formate, 1-propanol, 2-propanol, acetic acid, and acetaldehyde in the condensate, or acetone in the liquid remnant. This phase-specific product distribution suggests different reaction pathways dominating in each phase, with the gas-phase environment favoring acetaldehyde formation and the liquid-phase conditions promoting acetone production.

In the condensate stream, minor product concentrations were seen to be almost constant over the reaction period while there was an observable decrease in their selectivity as methanol and ethanol production increased. The liquid remnant stream showed similar trends but with slight increases in methyl formate, acetic acid, and 1-propanol over time. The absence of formic acid or formaldehyde in both phases could be attributed to the reaction conditions. While the plasma environment limits extensive overoxidation of methanol, any formic acid formed would likely be rapidly consumed either through esterification with methanol to produce methyl formate [21,22,23] or through oxidation to carbon dioxide [24]. The presence of methyl formate in both streams supports this explanation, indicating that some formic acid intermediates were likely formed but efficiently converted through secondary reactions. Any formaldehyde produced was also probably quickly condensed with methanol to yield methyl formate along with the release of hydrogen gas, with the methyl formate undergoing subsequent decarbonylation to produce carbon monoxide [25].

2.2. Conductivity and pH Trends

Observations from Figure 2 were that the conductivity of the liquid remnant increased progressively over the course of the reaction from 1.98 μS/cm to about 53 μS/cm, implying the generation of conductive reactive species upon the discharge of plasma. The condensate on the other hand was seen to reduce. The most plausible ascription to this would be the continuous transition of water from the liquid phase into the gas phase which is finally collected as condensate. This transition led to a concentration effect of produced conductive yet non-volatile species in the liquid remnant solution, and concomitantly a dilution effect in the condensate solution. The conductivity trend in the liquid remnant solution showed a positive correlation with the concentration of formed products (primarily methanol) over time, and that of the condensate was inversely related. As such, a definite claim cannot be made that the species responsible for solution conductivity increase are the key ones leading to the formation of methanol.

The pH in the liquid remnant stream showed an initial drop from 6.82 to about 6.58 within the first three minutes of reaction, signaling the possible generation of slightly acidic species. Afterwards, the pH was almost stable and only decreased minimally over the rest of the reaction period, reaching a final value of 6.28. The condensate portion was also within a narrow pH range of 4.9 to 5.6 over the 35 min period. The trend was that the pH decreased gradually from 5.6 to 4.9 within 25 min, possibly due to the dissolution of gas products to form acidic products (such as CO₂ to carbonic acid) before increasing to its final value of 5.28. This increase could be attributed to the dilution effect caused by the increasing volume of condensed water over time and the subsequent reduction in the concentration of the acidic products including acetic acid initially formed.

2.3. Methanol Reaction Kinetics

There have been several reports of the kinetics of this partial oxidation reaction, but many were based on the use of oxygen as an oxidant. The first order reaction kinetics was reported for methane concentration, while ${\displaystyle \frac{2}{3}}$-order was for oxygen concentration by Webley et al. [26]. Li et al. [27] also reported a 0.7 order in methane concentration and −0.9 in water concentration.

This work was however only able to estimate the kinetics based on the rate of methanol formation. Since there were two different streams of products from the reaction, i.e., the liquid remnant and condensate, the kinetics were analyzed in both segments. For the two scenarios, the production of methanol over the course of the reaction was seen to follow zero-order kinetics, as illustrated in Figure 3.

The integrated rate law equations for zero-order, first-order, and second-order reactions were used to plot the varying concentrations of methanol over a 35 min reaction period. The zero-order model gave the best fit with R-squared values of 0.9783 and 0.8032 for the condensate and liquid streams, respectively, but the concentration profiles warranted closer examination. The liquid remnant data points showed relatively rapid initial methanol accumulation (3–10 min), followed by continued steady increase (10–25 min), before decelerating in the later time points (25–35 min) where the rate of accumulation appears to taper. While the apparent zero-order kinetics could imply that the rate of methanol production in the liquid phase was independent of the concentration of reactants within the range of concentrations studied, the temporal pattern could also be an indicator of a more complex concentration profile where the reaction operates under conditions of methane saturation and power limitations before approaching equilibrium [28,29]. By contrast, the condensate stream demonstrated a more consistent linear behavior throughout the reaction period. This behavior could be reasonably attributed to the continuous removal of volatile products from the reaction environment, which alleviated saturation effects observed in the liquid stream.

Another factor to be considered is the role of mass transfer limitations and reactions at the gas–liquid interface where plasma is most likely generated. Methane is only sparingly soluble in water (22 mg/L at 25 °C) [30], and thus its reaction in the bulk liquid is restricted, possibly accounting for the slow buildup of methanol in the liquid stream. Additionally, plasma generated radicals such as •OH [31] and H• [32] are very short-lived and most reactive at or near the gas–liquid boundary [33], implying that the majority of reactions in the liquid stream is governed by interfacial events rather than bulk reactions. This could explain why methanol formation in the liquid remnant proceeded rapidly at first and then slowed as the interfacial reaction zone became limiting.

Judging from the kinetics in the two streams, it can be concluded that the reaction took place in two distinct phases, i.e., liquid and gas. The liquid phase reaction was estimated to proceed at a rate of 0.43 mgL⁻¹min⁻¹. Prior to analysis, it was hypothesized that the bulk of methanol was formed in the liquid phase and then subsequently evolved into the gas phase as the temperature of the system rose well above 65 °C (methanol boiling point). This, however, did not seem to be the case but rather the attainment of a two-phase reaction system as the temperature rose gradually, concomitantly yielding a good amount of water vapor which interacted with methane in the gas phase with higher frequency of particle collisions and without solubility impediments, therefore producing more methanol. Condensates started collecting into the receiving flask of the condensation setup at about the eighth-minute mark when the reactor temperature was almost at the peak of 96 °C. The gas phase reaction was seen to proceed at a rate of 3.89 mgL⁻¹min⁻¹, which was over 9 times faster than that in the liquid phase. Ultimately, the apparent zero-order behavior of both phases could be ascribed to a combination of methane saturation, plasma power input, mass transfer constraints, gas–liquid interfacial reactivity, and product removal dynamics.

2.4. Characterization of Gas Products

The analysis of gas-phase products was conducted using a GC-TCD system with three different columns (Hayesep DB, Shincarbon ST, and Porapak N), and the reaction products included molecules spanning hydrogen, carbon monoxide, carbon dioxide, ethane, ethylene, and acetylene in addition to residual oxygen. Quantification of unconverted methane, carbon monoxide, carbon dioxide, and hydrogen, which formed the bulk of the effluent gas mixture, were carried out using the Shincarbon ST column.

Because the introduced methane gas underwent plasma treatment in just one pass through the discharge zones, the composition of the effluent gas was virtually uniform over the course of the experiment. At any given sampling time, the gas mixture comprised 31.4 ± 1.2% methane, 12.7 ± 0.4% carbon monoxide, 5.7 ± 0.5% carbon dioxide, and 44.3 ± 0.7% hydrogen. The high hydrogen content indicates efficient C-H bond activation and water splitting, while the CO:CO₂ of approximately 2.2:1 suggests predominantly partial oxidation conditions with limited completed combustion. The presence of C₂ hydrocarbons also confirms C-C coupling reactions occurring through radical intermediates.

2.5. Identification of Reactive Species in Plasma

The presence of an amalgam of reactive species was confirmed in the methane-water plasma by optical emission spectroscopy as shown in Figure S1.

Some peaks in the emission spectrum fitted strongly with the wavelengths of the reference peaks, whereas others were slightly off and did not have an exact coincidence of wavelengths. The species that did not have a very strong correlation with references included Balmer delta hydrogen (H_δ) at 405.09 nm [34,35], CH radical at 426.97 nm [34,36], Balmer gamma hydrogen (H_γ) at 439.77 nm [34,35], and C₂ band peak [34,37] or carbon monoxide (CO) [38] at 520.86 nm. Carbon monoxide cation (CO⁺) at 589.58 nm [39], Balmer alpha hydrogen (H_α) at 656.07 nm [34,35], as well as oxygen atoms (O) at 777.04 nm and 843.94 nm [40,41] all had very close matches to the reference wavelengths. These observations indicated that the plasma generated was very effective in dissociating the introduced reactants to yield the needed reactive species that resulted in product formation. Similarly, Jun-Feng et al. [37] and Shi et al. [42] also detected the reactive radicals in methane plasma by OES as CH, C₂, H_α, H_β, and H_γ. Their work, as well as other reports [43,44], was also unable to capture any CH₃ or CH₂ radical lines for the possible reasons that the former had no light emission capabilities under the discharge conditions and the latter was very short-lived, being converted immediately into the CH radical. Feng et al. [45] attributed the inability to capture these species to their excited states being in the infrared region, which is beyond the detection range of most OES instruments. While these uncaptured species are generally known to have very little to no emission intensities, the polycarbonate body of the reactor combined with the moderately sensitive probe could have also hindered the detection of these species in plasma.

2.6. Reaction Pathways

Based on the reactive species, intermediates, and products obtained from this process, certain pathways were proposed for the reactions. In the reaction where there were no metal catalysts associated with the C-H bond activation in methane, in addition to activation by plasma, a Fenton-type mechanism that relied on free radicals for methane C-H bond activation also governed the reaction. During the reaction, reactive species generated by the impact of plasma on water were capable of abstracting hydrogen from methane. The activation of methane by the •OH radical in this mechanism was known to be highly exothermic (ΔH ≅ −60 kJ mol⁻¹) and proceeded with a very low barrier of only 15 kJ mol⁻¹ to yield a methyl radical and water [7]. The detachment of hydrogen radicals and gas from methane resulted in the formation of methyl (CH₃•), methylene (CH₂•), methylidyne (CH•), and carbon radicals, which were key intermediates for the production of various gas and liquid products [46,47]. Two molecules of CH₃•, CH₂•, and CH• species each combined to form ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂), respectively. These gases then underwent dehydrogenation, resulting in the formation of the next hydrogen-deficient gas in the sequence [48]. Carbon monoxide (CO) was anticipated to be formed by the combination of carbon and atomic oxygen radicals, and carbon dioxide could be formed when there was extra oxygen to react with [47].

The mechanism for methanol formation from methane and water has been studied in several publications [15,48,49]. Isotope tracing experiments revealed in these studies that the predominant pathway for methanol formation was the direct combination of methyl (CH₃•) and hydroxyl radicals (•OH), with the minor pathway being the methoxy radical (CH₃O•) and hydrogen atom (H) combination process. Their findings suggested that optimizing the reaction to promote the CH₃• + •OH pathway was critical for enhancing methanol selectivity and yield, as CH₃O• intermediates appeared to be less favorable [15]. Ethanol, the principal byproduct observed in the reaction, was anticipated to be formed from the coupling of •OH radicals and ethyl radicals (C₂H₅•) [50]. The formed ethanol then underwent dehydrogenation to spawn acetaldehyde (CH₃CHO) [51], which subsequently reacted with •O to produce acetic acid (CH₃COOH) [52,53]. Methyl formate (HCOOCH₃) was possibly formed from the rapid coupling of CH₃• and formic acid, which could be generated at any point in time. It could also be produced by the esterification of formic acid with methanol [21,22,23] or the slight overoxidation of formed methanol. There was also the possibility that methyl formate yielded acetic acid by isomerization [54,55], or gave rise to more methanol together with CO as decomposition products. Ethyl radical was predicted to react with produced methanol to form 1-propanol (C₃H₇OH) whereas 2-propanol (CH₃CHOHCH₃) arose from CH₃• coupling with α-hydroxyethyl radical (CH₃CHOH•). Acetone was then formed by the successive dehydrogenation of CH₃CHOH• or the double CH₃• coupling with CO.

In the increasing order of oxidation degree, methanol oxidation yielded HCOOCH₃ > HCHO > HCOOH > CO > CO₂ [56]. The absence of methanol’s common oxidation products, formaldehyde and formic acid, led to the speculation that methanol formation occurred in a stable environment that limited overoxidation. Lack of formaldehyde and formic acid detection also suggested that the CO₂ observed in this system was from the overoxidation of other products but methanol, implying a suitable system for the partial oxidation of methane to methanol.

The reactions involving water and methane to form methanol, as well as other reported products in this work, are enclosed in

Table 1. Plasma reaction equations.

Reaction	Products
$\mathrm{C}{\mathrm{H}}_4+{\mathrm{e}}^{-}$	$\mathrm{C}{\mathrm{H}}_3\bullet +\bullet \mathrm{H}+{\mathrm{e}}^{-}$
	$\mathrm{C}{\mathrm{H}}_2\bullet +{\mathrm{H}}_2+{\mathrm{e}}^{-}$
	$\mathrm{C}\mathrm{H}\bullet +{\mathrm{H}}_2+\bullet \mathrm{H}+{\mathrm{e}}^{-}$
	$\mathrm{C}\bullet +2{\mathrm{H}}_2+{\mathrm{e}}^{-}$
${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$	$\bullet \mathrm{O}\mathrm{H}+\bullet \mathrm{H}+{\mathrm{e}}^{-}$
	${\mathrm{H}}_2+\bullet \mathrm{O}+{\mathrm{e}}^{-}$
$\bullet \mathrm{O}\mathrm{H}+{\mathrm{e}}^{-}$	$\bullet \mathrm{O}+\bullet \mathrm{H}+{\mathrm{e}}^{-}$
$\bullet \mathrm{O}+\bullet \mathrm{O}$	${\mathrm{O}}_2$
$\mathrm{C}{\mathrm{H}}_3\bullet +\bullet \mathrm{O}\mathrm{H}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$
$\mathrm{C}{\mathrm{H}}_3\bullet +\bullet \mathrm{O}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\bullet$
$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\bullet +\bullet \mathrm{H}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$
$\mathrm{C}\bullet +\bullet \mathrm{O}$	$\mathrm{C}\mathrm{O}$
$\mathrm{C}\mathrm{O}+\bullet \mathrm{O}$	$\mathrm{C}{\mathrm{O}}_2$
$\mathrm{C}\mathrm{O}+2\bullet \mathrm{C}{\mathrm{H}}_3$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}{\mathrm{H}}_3$
$\mathrm{C}\mathrm{O}+\bullet \mathrm{H}$	$\mathrm{H}\mathrm{C}\mathrm{O}\bullet$
$\mathrm{C}\mathrm{H}\bullet +\bullet \mathrm{O}$	$\mathrm{H}\mathrm{C}\mathrm{O}\bullet$
$\mathrm{H}\mathrm{C}\mathrm{O}\bullet +\bullet \mathrm{O}\mathrm{H}$	$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$
$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+\mathrm{C}{\mathrm{H}}_3\bullet$	$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}{\mathrm{H}}_3+\bullet \mathrm{H}$
$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{C}{\mathrm{H}}_3$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$
	$\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}+\mathrm{C}\mathrm{O}$
$2\mathrm{C}{\mathrm{H}}_3\bullet$	${\mathrm{C}}_2{\mathrm{H}}_6$
${\mathrm{C}}_2{\mathrm{H}}_6+{\mathrm{e}}^{-}$	${\mathrm{C}}_2{\mathrm{H}}_5\bullet +\bullet \mathrm{H}+{\mathrm{e}}^{-}$
${\mathrm{C}}_2{\mathrm{H}}_5\bullet +\bullet \mathrm{O}\mathrm{H}$	${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$
${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+{\mathrm{H}}_2$
$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+\bullet \mathrm{O}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$
$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+\bullet \mathrm{H}$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}\mathrm{H}$
$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}\mathrm{H}\mathrm{C}{\mathrm{H}}_3$	${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}{\mathrm{H}}_3+{\mathrm{H}}_2$
$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+\mathrm{C}{\mathrm{H}}_3$	$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}\mathrm{H}\mathrm{C}{\mathrm{H}}_3$
${\mathrm{C}}_2{\mathrm{H}}_5\bullet +\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$	${\mathrm{C}}_3{\mathrm{H}}_7\mathrm{O}\mathrm{H}+\bullet \mathrm{H}$
${\mathrm{C}}_2{\mathrm{H}}_6$	${\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{H}}_2$
$2\mathrm{C}{\mathrm{H}}_2\bullet$	${\mathrm{C}}_2{\mathrm{H}}_4$
$\mathrm{C}{\mathrm{H}}_2\bullet +\bullet \mathrm{O}$	$\mathrm{C}{\mathrm{H}}_2\mathrm{O}$
$\mathrm{C}{\mathrm{H}}_2\mathrm{O}+\bullet \mathrm{O}$	$\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$
$2\mathrm{C}\mathrm{H}\bullet$	${\mathrm{C}}_2{\mathrm{H}}_2$

[46,47,48,57,58,59] as well as pictorially summarized in Figure 4.

3. Materials and Methods

3.1. Materials

Great Value brand distilled water distributed by Walmart was acquired at WinCo Foods in Moscow, ID, USA. Additionally, 99.5% purity methane gas was sourced from Oxarc Inc. in Lewiston, ID, USA.

3.2. Experimental Setup and Operation

Precedence was given to liquid oxygenates production so optimal levels of applied power (367 W), methane gas flow rate (273 mL/min), and liquid flow rate (51 mL/min of water) determined from previous studies for enhanced methanol production were used for this study [60]. Prior to each experimental run, the reactor was purged with methane gas flowing at the set point regulated by a Sierra mass flow controller. Water was then introduced from a three-necked 250 mL round bottomed flask using a Masterflex peristaltic pump (Model No. 07528-10 manufactured by Cole Parmer Instrument Company in Vernon Hills, IL, USA) after which the required power was applied via a variac variable transformer to initiate the reaction.

The 20 min reaction utilized controlled power input, monitored by a Watts meter. A condensing system connected to the flask facilitated the collection of liquid products and the cooling of effluent gases for subsequent analysis. The experimental setup is shown in Figure S2.

3.3. Sample Analysis

For the kinetic study, 5 mL aliquots were taken from the liquid remnant contained in the 250 mL flask at the 3rd and 7th minute marks and then at regular 5 min intervals from both the flask and the container of the condensed liquid products. To identify all the products captured in the liquid phase, a Bruker 1H Nuclear Magnetic Resonance (NMR) spectrometer (Bruker BioSpin GmbH & Co. KG, Ettlingen, Germany) operating at 500 MHz was used for the analysis. The procedure used was consistent with that reported by Du et al. [61]. NMR samples were prepared by adding 630 μL of liquid product sample to 70 μL of deuterium oxide (D₂O) and 30 μL of 205.5 mg/L aqueous dimethyl sulfoxide (DMSO) as an internal standard and then transferring the resultant mixture carefully into the NMR tubes for analysis. Analysis of the NMR spectra was carried out with Advanced Chemistry Development (ACD)/Labs Spectrus Processor software (Version 2023.2.4).

The permanent gases collected from the condensing setup were captured in gas sampling bags after the entire reaction period for subsequent analysis using an Agilent 6890N GC system (Agilent Technologies, Inc., Santa Clara, CA, USA) equipped with a thermal conductivity detector (TCD) connected to a ShinCarbon ST Micropacked Column (Restek Catalog No. 19043, Restek Corporation, Bellefonte, PA, USA). 50 μL of gas samples were injected into the GC in triplicates and the mean values recorded.

3.4. Plasma Characterization

Optical emission spectroscopy (OES), performed with an Ocean Insight HDX-XR spectrometer (Serial No. HDX00577, Ocean Insight, Orlando, FL, USA), was used to reveal the reactive species in the plasma to infer reaction pathways. Complementary measurements, obtained via a Tektronix oscilloscope (Model TBS1052B-EDU, Tektronix, Inc., Beaverton, OR, USA) connected across the reactor electrodes, were employed to determine the plasma discharge power, peak-to-peak voltage, and the discharge current.

4. Conclusions

This study demonstrates the successful conversion of methane into a variety of gas and liquid products in a two-phase plasma system using water as an environmentally friendly oxidant. The nonthermal plasma reaction produced a diverse range of liquid oxygenates with methanol as the dominant product (selectivity reaching 91% in the condensate stream), alongside gas products predominantly comprising hydrogen (44.3%), carbon monoxide (12.7%), and carbon dioxide (5.7%). The primary route for methanol formation revealed by the reaction pathway analysis and corroboration from literature was through the reactive coupling of methyl and hydroxyl radicals, with a secondary pathway involving methoxy and hydrogen radical combinations. The absence of formaldehyde and formic acid in the product mixture suggested that while the plasma environment limited extensive methanol overoxidation, any formaldehyde formed could likely be condensed rapidly with methanol to yield methyl formate along with the release of hydrogen. Similarly, any formic acid intermediate formed were also likely consumed rapidly either through esterification with methanol to produce methyl formate or through oxidation to carbon monoxide and carbon dioxide.

Kinetic analysis revealed that methanol production followed zero-order kinetics in both liquid and gas phases, most probably because of the system being saturated with methane and the reaction being limited by plasma power input rather than reactant concentration. The gas-phase reaction proceeded approximately nine times faster than the liquid-phase reaction, which emphasized the importance of phase considerations in plasma-assisted methane conversion systems. The plasma system also exhibited a good buffering capacity as shown by the minimal pH variation despite ongoing reactions in both phases. The increase in conductivity in the liquid remnant yet decrease in the condensate suggested concentration of conductive species in the remnant and dilution in the condensate, though no direct correlation with methanol formation was established. This thorough investigation into reaction pathways, kinetics, and product distribution offers valuable insights into developing more efficient and selective processes for the simultaneous production of syngas and valuable oxygenates from methane under mild conditions.