Improved control of aqueous microbubble chemical reactions by applying an ac electrical potential

Abstract

Improved control of reaction products in water-based microbubble chemistry is provided by application of an alternating electrical potential. One example considered in detail is the partial oxidation of methane, where the frequency and amplitude of the alternating electrical potential can be adjusted to selectively enhance methanol production relative to other reaction products.

Description

[0001] Improved control of aqueous microbubble chemical reactions by applying an AC electrical potential by

[0002] Xiaowei Song

[0003] Chanbasha Basheer

[0004] Richard N. Zare

[0005] FIELD OF THE INVENTION

[0006] This invention relates to aqueous microbubble chemistry .

[0007] BACKGROUND

[0008] Recently, enhanced reaction rates for various chemical reactions have been demonstrated in reaction configurations for water microdroplets sprayed into a gas and for gas microbubbles in contact with water . Here microdroplets and microbubbles have diameters of 100 pm or less . In both cases, strong electric fields at the curved interfaces in these configurations are believed to contribute to the observed enhancement of reaction rates . More specifically, there are two factors driving chemistry at the interface of water and gas . One is the strong electric field, that causes the hydroxide anion to be converted to the hydroxyl radical . The other is the electron transfer from hydroxide anion to the H+ cation causing the formation of OH radical and H atom, both of which are highly reactive . This electron transfer is aided by the fact that OH- and H+ cannot be surrounded by water molecules (solvated) in three dimensions at the interface . However, work to date forthese kinds of reactions has not demonstrated the product selectivity that is often needed in practice . Thus it would be an advance in the art to provide improved control of aqueous microbubble chemical reactions .

[0009] SUMMARY

[0010] In one example of this work, we have unexpectedly found that fine control of methane oxidation in sea water can be accomplished using air-methane microbubbles in salt water to which an alternating electric potential is applied. Both frequency and amplitude of this alternating potential are relevant for tuning the reaction to preferentially generate methanol vs . the undesired byproducts dichloromethane and acetic acid. The alternating voltage (e . g. , 100 mV) generates two synergistic partial oxidation of methane (POM) processes dominated by Cl“ Cl* + e“ and O2 + e“ — O2~* under positive and negative potentials, respectively. In one experiment, the selectivity of methanol generation vs . these byproducts is over 90%, and the methane to methanol conversion yield was 57% at a rate of approximately

[0011] 887 pM h-1. FIG. 1A schematically shows this reaction concept where microbubble 102, electrode 104 and saltwater ambient 106 interact . Also shown on FIG. 1A, without being bound by theory, are some proposed reaction paths for these two synergistic partial oxidation of methane processes .

[0012] Although the detailed example considered herein is partial oxidation of methane, this approach is expected to be widely applicable to other aqueous microbubble reactions . For example, it is expected that many other such reactions will have different reaction kinetic time scales for different reaction products, suggesting that appropriate tuning of at least the frequency of the applied electricalpotential can help select products, as in the example of partial oxidation of methane .

[0013] An important application of this work is removing the potent greenhouse gas methane from the atmosphere . It is noteworthy that this reaction is a green chemistry reaction, in contrast to methane removal methods that require electrochemistry or photochemistry.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A schematically shows operating principles of an embodiment of the invention relating to partial oxidation of methane in microbubbles with an applied electrical input .

[0015] FIG. IB shows an exemplary reaction system.

[0016] FIGs . 2A-F show mass spectrometry data relating to partial oxidation of methane in microbubbles with an applied electrical input .

[0017] FIGs . 3A-C show the effect of varying electrical frequency and electrical potential on yields of CH3OH, CH2CI2, and CH3COOH in partial oxidation of methane in microbubbles with an applied electrical input .

[0018] FIG. 4A shows an exemplary plot of methanol concentration vs . time for partial oxidation of methane in microbubbles with an applied electrical input .

[0019] FIG. 4B shows amounts and conversion rates of CH3OH, CH2CI2, and CH3COOH vs . time for partial oxidation of methane in microbubbles with an applied electrical input .

[0020] FIG. 5A shows a characterization setup for reaction mechanics of partial oxidation of methane in microbubbles with an applied electrical input .FIGs . 5B-F show reaction mechanics characterization results .

[0021] DETAILED DESCRIPTION

[0022] Section A describes general principles relating to embodiments of the invention and section B describes a detailed experimental example .

[0023] A) General principles

[0024] An exemplary embodiment of the invention is a method of performing a chemical reaction, the method comprising:

[0025] (i) feeding a gaseous reactant mixture into a bubble generator to form bubbles in an aqueous medium; and

[0026] (ii) applying an alternating electrical input to the aqueous medium to selectively control product production by the chemical reaction.

[0027] The bubbles preferably have an average diameter in a range from 10 nm to 100 pm, and more preferably have an average diameter in a range from 10 nm to 1 pm. Other possible bubble average diameter ranges include : from 1 pm to about 500 pm, from about 10 pm to about 500 pm, from about 10 pm to about 300 pm, from about 10 pm to about 200 pm, from about 10 pm to about 100 pm, from about 20 pm to about 500 pm, from about 20 pm to about 250 pm, from about 20 pm to about 100 pm, from 1 pm to about 200 pm, from 1 pm to about 100 pm, from 1 pm to about 50 pm, from about 10 pm to about 50 pm, from about 20 pm to about 40 pm, from about 10 nm to less than 1 pm, from about 10 nm to 0.99 pm, from about 10 nm to about 800 nm, from about 10 nm to about 600 nm, from about 10 nm to about 500 nm, from about 10 nm to about 300 nm, from about 10 nm to about 200 nm, from about10 nm to about 100 nm, from about 50 nm to about 500 nm, from about 50 nm to about 400 nm, from about 20 nm to about 300 nm, from about 20 nm to about 200 nm, or from about 20 nm to about 100 nm.

[0028] The alternating electric input preferably has a frequency in a range from 10 Hz to 1 MHz, more preferably from 10 Hz to 10 KhZ . In cases where the electrical input is characterized by voltage, the electric potential is preferably in a range from 10 mV to 500 mV. In cases where the electrical input is characterized by electric field, the electric field strength is preferably in a range from

[0029] 1 mV / cm to 100 mV / cm.

[0030] The alternating electric input can be applied using two or more electrodes immersed in the aqueous medium. In practice, the reaction vessel can serve as one of the electrodes, with the other electrode (e . g. , 104 on FIG. IB) being immersed in the aqueous medium without touching the reaction vessel .

[0031] At least one of the two or more electrodes preferably has a porous surface with a pore size in a range from 5 nm to 50 pm, and more preferably has a pore size in a range from 5 nm to 500 nm. Other possible electrode pore size ranges include : from about 10 nm to about 500 pm, from about 10 nm to about 200 pm, from about 10 nm to about 100 pm, from about 50 nm to about 500 pm, from about 50 nm to about 200 pm, from about 50 nm to about 100 pm, from about 100 nm to about 500 pm, from about 200 nm to about 500 pm, from about 500 nm to about 500 pm, from about 100 nm to about 1 pm, from about 200 nm to about 1 pm, from about 300 nm to about 1 pm, from about 1 pm to about 500 pm, from about 10 pm to about 500 pm, from about 50 pm to about 500 pm, from about 100 pm to about 500 pm, or from about 100 pm to about 400 pm, such as about 200 pm.Practice of the invention does not depend critically on the electrode materials . Naturally, the electrode materials need to be electrically conductive enough to serve as an electrode, but that ' s the only essential requirement . In practice, it is expected that different electrode materials may be convenient for different reactions . In the partial oxidation of methane example considered herein, copper oxide was a convenient electrode material .

[0032] The aqueous medium is preferably at least 1% by volume water . Suitable compositions of the aqueous medium include, but are not limited to : water, salt water, sea water, mixtures of water and one or more organic solvents, and mixtures of salt water and one or more organic solvents . For example the aqueous medium can be salt water having a halide salt composition in a range from 1% to 5% by weight .

[0033] In the example of partial oxidation of methane, the gaseous reactant mixture includes molecular oxygen, the halide salt composition includes a chloride salt, and the alternating electric input generates chlorine radicals from chloride ions in a first polarity, and wherein the alternating electric input generates 02“ radicals from molecular oxygen in a second polarity opposite the first polarity. Here the gaseous reactant mixture includes methane and methanol can be a desired product of the chemical reaction. The molar selectivity of methanol vs . all products of the chemical reaction is preferably at least 80% (more preferably 90% or more) . The chemical reaction is preferably performed at a temperature in a range from 0 °C to 100 °C and at a pressure in a range from 0.8 atmospheres to 1.2 atmospheres . More preferably, the temperature range is from 20 °C to 30 °C . Such a chemical reaction can be used for removing methane from air or for converting amethane source into methanol . The methane source can have a concentration from about 2ppm methane up to 100% methane .

[0034] B) Experimental Proof of Concept and Examples

[0035] Bl ) Introduction

[0036] As the major component of natural gas, methane (CH4) is emitted in huge amounts annually from various sources, including wetlands, coal mining, agriculture, and oil and gas systems . It is a greenhouse gas that has a 25-fold higher thermal potency than carbon dioxide (CO2) and accounts for approximately 30% of global temperature rise since the Industrial Revolution. Partial oxidation of methane-to-methanol (POM) conversion offers a viable way to cut greenhouse gas leaks and emissions while producing more value-added products . The production and composition of syngas impose constraints on the two-step process used in traditional conversion methods, which results in poorer methanol yields and higher carbon emissions . This method is also inappropriate for green and economic methane usage because it operates under high-pressure and high-temperature conditions .

[0037] The C— H activation of CH4 is one of the "holy grails" in the catalysis field because this molecule has a nonpolar tetrahedral shape and strong C— H bonding energy of 439 kJ mol-1. From another aspect, methane oxidation can also be easily overdone to form other oxygenates such as methyl peroxide, formic acid, acetic acid, and CO2 because its target POM product, methanol, is much more easily oxidized than CH4 . This fact poses another challenge for sophisticated control over the POM path.

[0038] A wide range of catalysis strategies have been extensively developed including thermal catalysis,photocatalysis, and electrochemical oxidation. Thermal catalysis employs transition metals like palladium, gold, and copper dispersed on porous supporting material like alumina, zeolite, or a metal organic framework to increase surface area and activity. The high temperature and catalyst poisoning with the operation time remain the major issue of this strategy. Photocatalysis is a promising alternative that uses photosensitive semiconductors loaded with various metal alloys to promote C— H activation at room temperature . However, it also faces practical challenges in low solar energy conversion efficiency, high recombination rates of photogenerated carriers, complex catalyst engineering strategies, and expensive oxidants like hydrogen peroxide (H2O2) . In contrast, electrocatalytic oxidation could be another promising resort because the interfacial charge transfer-based redox process can be more precisely and digitally controlled. The charge separation of electron (e~) and hole (h+) by contact electrification across water-air-solid interfaces have been proved to be an effective way to generate reactive oxygen species such as hydroxyl radicals (OH • ) and hydrogen peroxide (H2O2) .

[0039] The presence of OH • and H2O2 at the water microdroplet interface has been increasingly supported by both experimental observations and theoretical studies . In our previous work, we demonstrated that spraying water microdroplets can facilitate the oxidation of methane to methanol in the presence of atmospheric oxygen and ultrasound. Various reactive oxygen species at the gas-water interface (GWI ) are crucial in the partial oxidation of methane (POM) . However, the spraying process occurs rapidly, on the millisecond time scale, allowing only a small fraction of methane molecules to interact with the reactive GWI region, while a significant portion of methane remainsunreacted. Furthermore, achieving precise control over the POM to avoid overoxidation remains a significant challenge . This underscores the need for improvements in methanol selectivity and conversion efficiency.

[0040] As the inverse of water microdroplets, micron-sized bubbles are likely to exhibit similar GWI characteristics . Microbubbles have been employed in water treatment processes due to their high concentrations of reactive oxygen species at the GWI . This insight led us to consider replacing the spraying microdroplet approach with a microbubbling system to enhance the overall methane conversion yield.

[0041] Microbubbles are a promising alternative because they tend to persist in water longer before shrinking, coalescing, or bursting. Consequently, the extended lifespan of gas substrates within microbubbles offers a prolonged reaction time, potentially improving the conversion of methane into oxygenated products . Our most recent works not only proved the existence of H2O2 in the electrogenerated microbubble interface but also showed its application in C— H activation and methane oxidation.

[0042] Inspired by these rationales, we are motivated to develop an environment-friendly and economical microbubble interfacial electrochemical oxidation method to scale up the methane removal from air and its conversion to methanol in a highly selective way. Micro-bubbling integrated with electrocatalysis under alternating potential strategy is presented. The use of potassium chloride and sodium chloride in the developed system are explored with the expectation that seawater, as a widely available natural resource, can be used in this processing method.

[0043] B2 ) Results and DiscussionB2.1 ) Construction of POM Setup

[0044] FIG. IB is a diagram of the scaleup setup for the methane removal . Components include compression gas cylinders 114 and 116 for air and methane (including regulators and check valves) , a closed chamber 120 for gas storage and feeding, a water circulation pump 122, a microbubble generator 112, a reaction beaker 108 filled with saltwater 106 to a liquid level 110, an AC function generator 124, a CuO mesh electrode 104, and Teflon tubing throughout for gases and liquids . Further features shown on FIG. IB are microbubbles 102, passages 112a, 112b, 112c in microbubble generator 112, and gas mixing junction 118.

[0045] We developed an initial prototype setup designed for methane removal from air, with its critical components illustrated in FIG. IB . Methane and air are supplied through two compressed gas cylinders, with the mixing ratio adjustable to simulate realistic atmospheric methane concentrations by regulating the partial pressures and flow rates . The primary component is a microbubble generator 112 submerged in saltwater 106 that simulates seawater . A circulation pump 122 ( 1 L min-1, 87 psi) inj ects water into a narrow vertical channel 112a within the microbubble generator probe 112. This action creates a localized low-pressure zone within the probe chamber, which draws the airmethane mixture into a horizontal channel 112b . At the junction spacer 112c, the gas and water fully mix before being expelled through the probe outlet, generating microbubbles . The clear saltwater assumes a white, emulsionlike appearance during operation, with microbubbles ranging narrowly in size from 20 to 40 pm. Unreacted gas within the microbubbles rises to the water surface after traveling from the bottom of the reaction container ( 100 mm in height) and is recaptured by the probe for subsequent oxidation cycles .The second core part of the experimental setup is the electrocatalysis system, including an AC waveform function generator 124 connected to the copper oxide (CuO) mesh electrode 104 by a conducting wire . The CuO mesh has an average pore size of 200 pm to ensure sufficient gas diffusion and mass / electron transfer between the gas-liquid-solid tri-phases . Also shown is the electrical connection between AC function generator and reaction vessel 108 to complete the electrical circuit .

[0046] B2.2 ) POM Products Detection

[0047] FIGs . 2A-F show representative mass spectra of water samples that were bubbled with methane gas under different conditions . FIG. 2A relates to methane gas pumped into deionized water to generate macrobubbles . FIG. 2B relates to dispersed methane gas into DI water to form micron-size bubbles . FIG. 2C relates to dispersed methane gas in saltwater (KC1, 3%) to form micron-size bubbles . FIG. 2D relates to application of a +100 mV direct current (DC) potential onto the electrode . FIG. 2E relates to application of an alternating current (AC) potential (amplitude ±100 mV, frequency 50 Hz) onto the electrode . FIG. 2F is the same as FIG. 2E except the AC potential parameters are amplitude ±250 mV and frequency 50 Hz .

[0048] The experiment commenced by directly inj ecting the methane-air gas mixture into deionized water as a negative control . Methane gas was continuously pumped into the water, forming large bubbles with diameters exceeding 1 mm. After one hour of sparging, the sample was analyzed using nanoelectrospray ionization mass spectrometry (nESI-MS) . No ions associated with methanol were detected, with the exception that the bicarbonate ion (HCOsA m / z 60.9920,FIG. 2A) appeared, which likely originated from CO2 naturally dissolved in the water . We then used the microbubble generator to pump the methane into the water . Surprisingly, peaks at m / z 66.9549 and m / z 68.9516 with approximately a 3 : 1 ratio appeared, and these peaks matched the methanol molecule adducted with a chloride ion

[0049] ( [CH3OH + CID based on the mass shift and the abundance of chlorine isotopes . Apart from methanol, an acetic acid peak (HAc, CH3COO-, m / z 59.0128 ) was detected with an equivalent intensity (FIG. 2B) . Technically, this result indicated that the trace level of background chloride ions found in the water container and tubing can assist in the ionization of methanol by H-Cl- hydrogen bonding. More importantly, it agrees with our hypothesis that water microbubbles do have the ability to oxidize methane because of the existence of HO • and H2O2 across the GWI region, like what we previously discovered at the microdroplet interface .

[0050] Recognizing the role of chloride in successfully detecting methanol, we intentionally added potassium chloride to the water, creating a 3% saltwater solution for the microbubbling test . The relative abundance of the methanol peak increased by 20% compared to the unsalted condition. Additionally, a series of weak peaks appeared at m / z 82.9450, 83.9430, 84.9420, 85.9454, and 86.9493

[0051] (FIG. 2C) , with distributions matching deprotonated dichloromethane (DCM) , namely, CHCI2A This result suggests that chloride not only aids in ionization but also participates in the methane oxidation process . Next, we applied a +100 mV DC potential to the CuO mesh electrode (50 mm x 50 mm, 5 mm thickness) immersed in 3% saltwater while microbubbling methane for one hour . The intensity of the methanol ion became comparable to that of the HCO3- base peak, while the CHC12~ ion intensity increased to 40%, andCHsCOCc reached 15% (FIG. 2D) . These results indicate that the application of positive DC potential significantly enhances methane oxidation and chlorination.

[0052] Finally, when we replaced the DC potential with a sine waveform AC potential at a frequency of 100 Hz and an amplitude of ±100 mV, the methanol ion intensity surpassed that of HCO3-, becoming the predominant peak in the mass spectrum. In contrast, the CHC12~ and CHsCOO-peaks did not show significant increases (FIG. 2E) . This suggests that an AC potential improves methanol selectivity compared to a DC potential . However, when the amplitude was increased from ±100 mV to ±250 mV, CHC12~ became the base peak instead of methanol (FIG. 2F) , indicating that the selectivity between methanol and DCM can be reversed by adjusting the potential .

[0053] B2.3) Alternating Potential Optimization

[0054] FIGs . 3A-C are heatmaps showing the influences of the AC frequency and potential on switching the methane removal products among methanol (FIG. 3A) , dichloromethane

[0055] (FIG. 3B) , and acetic acid (FIG. 3C) .

[0056] We systematically optimized the alternating frequency and amplitude to elucidate their roles in influencing methane oxidation pathways and product selectivity. The production of CH3OH, CH2CI2, and CH3COOH was compared under various combinations of frequency, ranging from 10 Hz to 10 kHz, and absolute amplitude, spanning from 10 mV to 500 mV. The selectivity under these conditions is presented as a heatmap (FIG. 3A) . As shown, the selectivity for methane-to-methanol conversion remained consistently high, ranging from 63% to 97%, compared to methane-to-dichloromethane and methane-to-acetic acid, across multiple frequency and amplitude combinations . Notably, methanolselectivity exceeded 90% under several specific alternating parameter sets, including 10 Hz / ±10 mV ( 94.7%) , 10 Hz / ±100 mV ( 96. 6%) , 250 Hz / ±25 mV ( 90.8%) , 250 Hz / ±250 mV ( 94.2%) , 1 kHz / ±50 mV ( 91.4%) , 5 kHz / ±100 mV ( 91.9%) , and 5 kHz / ±500 mV ( 96.2%) (FIG. 3A) . These findings indicate the existence of multiple local optima for frequencies at different potential values . Additionally, a clear pattern emerged where the optimal potential generally increased as the alternating frequency was increased.

[0057] Regarding CH2CI2, its predominant production only showed up in several specific condition sets such as

[0058] 10 Hz / ±25 mV (58.8%) , 10 Hz / ±250 mV ( 68.9%) and 1 kHz / ±500 mV ( 64.5%) . Even under these conditions, the CH2CI2 selectivity remains relatively lower than what can be achieved for the methane-to-methanol conversion (FIG. 3B) . For CH3COOH, we found that it is mainly produced on even fewer occasions at 10 Hz / ±500 mV ( 67.8%) and 5 kHz / ±25 mV ( 85%) (FIG. 3C) . These results clearly demonstrate that the methane-to-methanol conversion can be selectively and efficiently achieved by precisely controlling the alternating potential amplitude and frequency applied to the CuO mesh electrode immersed in saltwater through which methane is microbubbled.

[0059] In addition to characterization via nano-electrospray ionization mass spectrometry (nESI-MS) , the POM products were further validated through nuclear magnetic resonance (NMR) spectroscopy. Three representative samples, prepared under optimal conditions favoring the production of CH3OH ( 10 Hz / ±100 mV) , CH2CI2 ( 10 Hz / ±250 mV) , and CH3COOH ( 10 Hz / ±500 mV) , were analyzed using proton NMR (XH-NMR) . The integration of peaks corresponding to chemical shifts at 5.3 ppm (CH2CI2) , 3 . 3ppm (CH3OH) , and 2.0 ppm (CH3COOH) demonstrated an abundance ratio highly consistent with thatpredicted by ion intensities obtained from mass spectrometry .

[0060] B2.4 ) Quantitative Evaluation of the POM Process

[0061] FIGs . 4A-B show quantitative estimation of the methane removal performance . FIG. 4A shows the concentration of generated methanol with processing time (n = 3) , and FIG. 4B shows the total amount (left vertical axis) of methane removed from the air and the conversion yield (right vertical axis) .

[0062] Following the investigation of frequency and potential, the combination of 10 Hz and ±100 mV was selected as the optimal condition due to its highest yield and relatively low energy input . The precise concentrations of methanol, dichloromethane (DCM) , and acetic acid (HAc) were determined using a set of quantification curves derived from a dilution series of standard solutions for each compound. A linear response was observed between the normalized ion intensity and concentration within the ranges of 100-2000 pM for methanol, 50-1000 pM for DCM, and 10-200 pM for HAc, respectively .

[0063] The methane-to-methanol conversion process was monitored for over 6 hours . As shown in FIG. 4A, the methanol concentration gradually increased for the first 3 hours . Thereafter, the methanol concentration reached a relatively constant level of 2948 ± 137 pM. This might be attributed to the depletion of available oxygen in the air and water for the partial methane oxidation process . In contrast, the concentrations of generated DCM and HAc remained at a constant level of 60.5 ± 12.5 pM and

[0064] 45.4 ± 3.8 pM, respectively.Regarding conversion efficiency, approximately 480 pmol of total carbon was converted by microbubbling methane gas into 150 mL of saltwater over a 6-hour period (FIG. 4B) . Considering a 10% methane composition in the total volume of 200mL of gas consumed (36mLh-1) , the total amount of methane molecules was estimated to be approximately 838 pmol, based on the ideal gas law at 101 kPa and 288 K. Consequently, the overall methane conversion yield was determined to be 57.3%, with a methane-to-methanol conversion yield of 55. 6% and a selectivity as high as 97% . This performance, no matter what is the selectivity, yield, rate, or operation condition, shows highly competitive advantages compared to many known reports to the best of our knowledge .

[0065] B2.5) Capture of Reactive Radicals and Crucial Intermediates FIGs . 5A-F relate to various factors that influence the generation of radicals and products during the methane removal process . FIG. 5A is schematic diagram of the on-line mass spectrometer setup for capturing radicals and intermediates generated by the AC potential during the methane removal process . FIG. 5B shows the influence of the polarity of the AC potential on different radicals and intermediates . FIG. 5C shows the influence of the salt type and concentration on the methanol generation. FIG. 5D shows abundance changes of three critical radicals as the absolute potential amplitude increases . FIG. 5E shows the influence of the wave shape on the three major products . FIG. 5F shows the impact of AC frequency on tuning the switching of methane removal products between methanol and dichloromethane .

[0066] We developed an alternative process monitoring setup to gain insights into the POM (partial oxidation of methane)mechanism (FIG. 5A) . The system utilizes a tee-union with coaxial capillary sprayers to mix methane and water, generating microdroplets . To mitigate the risk of ignition, methane was premixed with argon (partial pressure ratio of 5 : 95) rather than air . The water phase contained two spin traps, TEMPO and DMPO, employed to capture reactive radicals and key intermediates .

[0067] A CuO mesh electrode, pre-soaked in 3% KC1 solution and thoroughly dried, was connected to an AC function generator and positioned in front of an Orbitrap mass spectrometer (MS) inlet . As the methane-water microdroplets passed through the mesh electrode, the dissolved salt became involved in the POM reaction at the gas-liquid-solid triinterface, closely simulating the conditions in the microbubble system.

[0068] Given the known concentrations of TEMPO ( 6.4 mM) and DMPO ( 8.9mM) , the concentrations of various radicals were estimated by comparing the ion intensities of reacted species to the unreacted TEMPO or DMPO ions . As a result, TEMPO and DMPO effectively captured several major reactive species : hydrogen radical (H • 1.89 ± 0.21 mM) , hydroxyl radical (OH - , 2. 60 ± 0.32 pM) , and methyl radical (CHa - , 9.46 ± 2.87 pM) . Additionally, small amounts of chlorine radical (Cl - 0.29 ± 0.22 pM) and hydroperoxyl radical (OOH • , 0.13 ± 0.05 pM) were also detected by the MS . These species were observed under applied potentials ranging from 10 to 500 mV and frequencies from 10 Hz to 10 kHz .

[0069] B2. 6) Influence of Salt and Alternating Potential Parameters We continued exploring other critical factors that could influence radical generation and reaction pathways to further elucidate the mechanism. First, we observed thatmore H • and Cl • radicals were detected only in the working mode with a positive alternating potential . In contrast, higher concentrations of CH3 • and OOH • radicals were observed under a negative alternating potential-only mode . The amount of OH • remained stable between positive and negative potential modes (FIG. 5B) . These findings suggest that 01 • and OOH • are the predominant species responsible for activating the 0— H bond of CH4 during the positive and negative phases, respectively. We speculate that 01 • originates from the single-electron oxidation of Cl~ under positive potential (with CuO serving as the anode) , while OOH • likely arises from the reduction of O2 dissolved in water or air, facilitated by H - (or H+ from HC1) and electrons donated by the CuO cathode .

[0070] The addition of salt, such as sodium chloride or potassium chloride, is important for providing Cl- to generate C1 - , which initiates the POM reaction. This is evidenced by a 12-fold increase in methanol production when the salt concentration exceeded 50 mM (FIG. 5C) . The presence of chloride ions also enhances methane' s solubility in water caused by weak interactions with the methane molecules .

[0071] Upon tuning the applied alternating potential, the abundance of CH3 • reached its maximum at +100 mV, whereas Cl • peaked at +250 mV. This trend persisted across varying frequencies ( 10 Hz, 100 Hz, 1 kHz, and 1 MHz) . These potentials are consistent with prior findings where maximum CH3OH production occurred at 10 Hz / ±100 mV and CH2CI2 production at 10 Hz / ±250 mV. Interestingly, the relative abundances of CH3 • and Cl • appeared competitive; as one increased, the other decreased. Nevertheless, both radicals maintained a stable level of OH • , essential for methane oxidation (FIG. 5D) . This indicates that two distinct,radical-driven processes occur independently yet synergistically during the alternating potential cycles . This hypothesis is supported by the observation that a sine waveform alternating potential was more effective than a square waveform (FIG. 5E) . The smooth transition of a sine wave better coordinated the two synergistic methane oxidation processes, whereas a square wave favored only one oxidation pathway.

[0072] Further investigation into alternating frequencies revealed that CH3OH and CH2CI2 production exhibited complementary, competitive trends . CH3OH production dominated at frequencies of 10 Hz ( 100 ms-1) , 100 Hz

[0073] ( 10 ms-1) , 1 kHz ( 1 ms-1) , and 10 kHz ( 0.1 ms-1) . In contrast, CH2CI2 production was favored at 250 Hz (4 ms-1) and 5000 Hz ( 0.2 ms-1) (FIG. 5F) . These findings suggest that the CH3OH and CH2CI2 pathways have distinct reaction kinetics, with CH3OH formation occurring on a timescale of approximately 100 ps, whereas CH2CI2 formation is slower, around 200 ps . Although Cl • is important for initiating C— H activation in CH4 to form CH3 • and HC1, chlorination appears to be slower than methane oxidation, where CH3OH is formed from CH • and OH • . This explains why alternating electrocatalysis at the right frequency achieves high selectivity for CH3OH over its byproduct, CH2CI2.

[0074] B2.7 ) Possible Reaction Mechanisms

[0075] The conversion of methane to methane has been achieved through a complex series of radical-initiated chemical reactions, and it would seem foolhardy to claim that a full understanding of the mechanism for this process has been achieved. Nevertheless, the experimental findings described above do suggest a broad outline of what happens in the POMprocess . The alternating redox potential leads to synergistic methane oxidation, composed of two working phases . Under a positive potential, Cl - is the major activation radical, and the CuO mesh electrode serves as an anode to remove an electron from Cl~ to generate reactive Cl - (Equation 1 ) . Cl - reacts with CH4 to generate CH3 • (Equation 2 ) and liberate in solution H+and Cl~ . At the same time, at the GWI, 0H loses an electron to form the OH -radical (Equation 3) , which recombines with CH3 • to form methanol (Equation 4 ) . Overall, the sum of these steps is shown in Equation 5.

[0076] Cl- - Cl- +e- ( 1 ) Cl- + CH4CH3- + HC1 (H++ C1-) (2 ) OH- OH- +e- (3) CH3- + OH- CH3OH (4 ) CH4+ OH- CH3OH + H++ 2e- (5) Under a negative potential, O2 is the major source of activation radicals, and the CuO mesh electrode serves as a cathode to donate one electron to generate reactive 02 (Equation 6) , which can further attach the H+generated from HC1 (Equation 2 ) in the positive potential phase to form the hydroperoxide radical OOH - (Equation 7 ) . The OOH - radical can react with CH4to generate either H2O2 and CH3 • (Equation 8 ) or CH3OH and OH - (Equation 9) . The formed H2O2 can also be decomposed to 2 OH • , which is catalyzed by CuO in a Fenton-like reaction (Equation 10) . Once again, OH -recombines with CH3 • to yield methanol (Equation 11 ) . The overall reaction is shown in Equation 12.

[0077] O2 + e O2 ( 6) O2+ H+OOH- (7 )OOH- + CH4CH3- + HOOH ( 8 ) OOH- +CH4CH3OH + OH- ( 9) HOOH^ 2 OH- ( 10) CH3- + OH- CH3OH ( 11 ) CH4+ O2+ H2O CH3OH + 2OH- ( 12 ) The amount of acetic acid suggested that methane tends to be overoxidized in microbubbles as well . Acetic acid can be formed through multiple radical reaction pathways . First, the generated methyl radical (CHa-) can react with the CO generated from the methane partial oxidation and OH • across the water microbubble interface . In addition, the methyl radical can also react with the CO2either from the air or the methane overoxidation to form CHaCOO • , followed by the hydrogenation to form CH3COOH or single electron transfer to form CHaCOO-. The third way is that two methyl radicals will form ethane (C2He) first and then further be oxidized into acetic acid. The gas chromatography (GC) and GC-MS tests confirmed the existence of approximately 0.3% CO, 0.1% CO2, and 0.001% C2He in the gas sample above the microbubbling reaction solution, supporting the three possible paths mentioned above . In addition, formic acid was also detected in the form of the sodiated dimer [HCOONa + HCOO]-at m / z = 112.9845. However, its intensity was only at an ignorable level that was much lower than the level of acetic acid, methanol, and dichloromethane .

[0078] During this POM process, the chloride ion can increase the rate of methane conversion into methanol by serving as a catalyst . This ion changes the electrical properties of the metal catalyst and stabilizes reactive intermediates, stopping methanol from being overoxidized and making it easier to form methanol . Additionally, chloride ions can aidin producing active species like copper-chloride complexes, which are helpful for the efficient oxidation of methane . Chloride ions make electrolytes more conductive, keep active catalytic sites stable in electrochemical systems, and improve the efficiency of the conversion process .

[0079] Alternating current (AC) possesses unique advantages over direct current (DC) in multiple aspects . Varied potentials can fine tune the multiplex redox process within one working period avoiding metal catalyst deposition and loss in catalytic activity. It can also stabilize active species near the electrode surface and minimize mass transfer distances . The AC potential quickly switches a single electrode' s working mode between anode and cathode, avoiding metal catalyst deposition and boosting the duration. It provides the necessary energy to overcome activation barriers, coordinates two reaction routes to facilitate them both, and ensures the continuous regeneration of reactive species like C1 - , OH • , and OOH • . It was also worth noting that alternating current can significantly change reaction outcomes by controlling the intensity of oxidation and reduction. Apart from methanol, other value-added products like acetic acid and dichloromethane can also be intentionally produced by controlling the alternating frequency and potential . This shows the largest advantage of AC over conventional DC in controlling the POM process .

[0080] Copper oxide is known to promote the breakdown of C— H bonds in methane and produce active oxygen species through water vapor oxidation. However, CuO' s efficacy as an electrocatalyst may be hampered by its propensity to be reduced to Cu at potentials higher than its functional potential . This issue might be well resolved by changing the polarity on the CuO mesh throughout the microbubblingprocess . Such polarity reversal also makes an important contribution to creating a highly reactive gas-water interface, facilitating methane gas dispersal and extending the methane gas dwell time at the GWI .

[0081] B3) Conclusion

[0082] The trade-off between reactivity and selectivity of direct methane-to-methanol conversion is still challenging to balance, particularly at room temperature . Methanol is readily overoxidized into CO and CO2, reducing methanol selectivity as methane conversion rises . This study successfully developed a highly selective partial oxidation method to convert methane gas to methanol . This is accomplished by bubbling methane through saltwater, to which a low-voltage alternating potential is applied to a copper oxide mesh. The total conversion yield is above 57% at a rate of 887pMh-1. The selectivity of methanol is above 90% versus the other products . The method only uses an alternating potential as low as 10 mV. The treatment can directly use saltwater or even seawater to remove the methane emission from the air and convert it into value-added products .

Claims

CLAIMS1. A method of performing a chemical reaction, the method comprising :(i) feeding a gaseous reactant mixture into a bubble generator to form bubbles in an aqueous medium; and(ii) applying an alternating electrical input to the aqueous medium to selectively control product production by the chemical reaction.

2. The method of claim 1, wherein the bubbles have an average diameter in a range from 10 nm to 100 gm.

3. The method of claim 2, wherein the bubbles have an average diameter in a range from 10 nm to 1 gm.

4. The method of claim 1, wherein the alternating electric input is applied to the aqueous medium at a frequency ranging from 10 Hz to 1 MHz and at an electric field strength ranging from 1 mV / cm to 100 mV / cm.

5. The method of claim 1, wherein the alternating electric input is applied to the aqueous medium at a frequency ranging from 10 Hz to 1 MHz and at an electric potential ranging from 10 mV to 500 mV.

6. The method of claim 1, wherein the alternating electric input is applied using two or more electrodes immersed in the aqueous medium.

7. The method of claim 1, wherein at least one of the two or more electrodes has a porous surface with a pore size in a range from 5 nm to 50 pm.

8. The method of claim 7, wherein at least one of the two or more electrodes has a porous surface with a pore size in a range from 5 nm to 500 nm.

9. The method of claim 1, wherein the aqueous medium is at least 1% by volume water, and wherein a composition of the aqueous medium is selected from the group consisting of : water, salt water, sea water, mixtures of water and one or more organic solvents, and mixtures of salt water and one or more organic solvents .

10. The method of claim 9, wherein the aqueous medium is salt water having a halide salt composition in a range from 1% to 5% by weight .

11. The method of claim 10, wherein the gaseous reactant mixture includes molecular oxygen, wherein the halide salt composition includes a chloride salt, and wherein the alternating electric input generates chlorine radicals from chloride ions in a first polarity, and wherein the alternating electric input generates 02“ radicals from molecular oxygen in a second polarity opposite the first polarity .

12. The method of claim 11, wherein the gaseous reactant mixture comprises methane and wherein methanol is a desired product of the chemical reaction.

13. The method of claim 12, wherein a molar selectivity of methanol vs . all products of the chemical reaction is at least 80% .

14. The method of claim 1, wherein the chemical reaction is performed at a temperature in a range from 0 °C to 100 °C and at a pressure in a range from 0.8 atmospheres to 1.2 atmospheres .

15. The method of claim 13, wherein the chemical reaction is performed at a temperature in a range from 20 °C to 30 °C .

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