Water Microdroplets-Initiated Methane Oxidation

Microdroplet Cascade Catalysis for Highly Selective Production of Propylene Glycol under Ambient Conditions

Conventional propylene glycol (PG) production relies on an energy-intensive two-step thermocatalytic process, contributing significantly to CO2 emissions. A sustainable alternative under ambient conditions remains elusive, hindered by challenges in selectivity and energy efficiency. Here, we present a cascade catalysis strategy for efficient and selective PG production within water microdroplets under ambient conditions. Propylene (CH3CH═CH2) is converted to PG (CH3CH(OH)CH2OH) at the microdroplet/titanium silicalite-1 (TS-1) interface, driven by in situ generated hydrogen peroxide (H2O2) via methyl viologen catalysis. This approach harnesses the water microdroplet interface to confine the reaction, enhancing catalytic activity and increasing selectivity. Our system achieves a PG production efficiency of 680 μM and a selectivity of 88%, while minimizing unwanted side products and energy demands. This innovative method offers a sustainable pathway for PG synthesis and highlights the transformative potential of water microdroplet technology in advancing green chemistry and industrial applications.

Sprayed Microdroplets Architect a Polyoxometalate Framework

Although many past attempts have utilized micron-sized droplets for breaking and forming organic bonds, their potential in promoting inorganic bond formation reactions remains largely unexplored. We report a promising approach to synthesizing a tungsten-based Lindqvist-type polyoxometalate (POM) in various organic and aqueous microdroplets under ambient conditions, eliminating the traditional need for hazardous or corrosive chemicals and high-boiling solvents. When aerosolized, a simple tungstate (WO4 2-) solution spontaneously produces a metal-oxo cluster (W6O19 2-), a valuable POM with broad applications, achieving yields up to 99% in less than a millisecond. Mass spectrometric detection of reactive intermediates unraveled the nucleation mechanism in microdroplets, leading to the formation of polyoxotungstate, which was then further characterized by X-ray crystallography. Empirical observations collectively suggest that rapid solvent evaporation and subsequent enrichment of reactants in the confined volume of microdroplets likely facilitate the growth of the POM through partial solvation at the air-liquid interface.

Charged Water Microdroplets Enable Dissociation of Surrounding Dioxygen

The cleavage of dioxygen (O2) into its atomic constituents typically requires harsh conditions and metal catalysts. We present a remarkable discovery demonstrating that dioxygen can be activated, dissociated, and subsequently transformed into the ozone anion (O3-) without any catalyst at the air-water interface in charged microdroplet sprays. Using online mass spectrometry, we directly detected the dioxygen splitting products O3- and H2O·O3- in microdroplets. The high electric field at the air-water interface, along with microlightning between oppositely charged water microdroplets, induces an electrical discharge responsible for the O-O bond cleavage, leading to the formation of reactive oxygen species (ROS). Isotope labeling experiments further reveal that various ROS, i.e., ·OH, CO3-, and HCO4-, can be generated through the reaction of dioxygen splitting products with water or CO2. This study introduces a sustainable pathway for molecular oxygen utilization and offers new insights into ROS generation in microdroplets.

Highly efficient oxidation of methane into methanol over Ni-promoted Cu/ZSM-5

Direct functionalization of methane in natural gas is of paramount importance but faces tremendous challenges. We reported a nickel-modified copper zeolite catalyst for the selective oxidation of methane into methanol. Using H2O2 as an oxidant in the liquid phase at 80 °C, Cu1Ni0.75/ZSM-5 catalyst presented a relatively high methanol yield of 82 162 μmol gcat -1 h-1 (with a methanol selectivity of ∼74%). Combining series of designed experiments and thorough characterization analysis, including electron microscopy, X-ray photoelectric spectroscopy, Fourier transform infrared reflection as well as in situ diffuse reflectance infrared Fourier transform spectroscopy, abundant CuI active sites were found on Ni-Promoted Cu/ZSM-5, differing from the dominating CuII active sites over Cu/ZSM-5. CuI active sites had an excellent ability to promote CH4 adsorption, CH4 activation and CH3OH generation compared to CuII active sites. This work elucidates a constellation of insightful and potent perspectives for further improvement of metal-zeolite catalysts for the direct oxidation of methane to methanol.

Unique catalytic role of intermolecular electric fields that emanate from Lewis acids in a ring closing carbonyl olefin metathesis reaction

Electric field (EF) catalysis has evolved as an effective tool for controlling reactivity and selectivity of reactions. While EF catalysis brings precise control over reactivity, it also challenges the concept's practical realization due to the difficulties in juxtaposing reactants with directional EF. The present density functional theory (DFT) studies demonstrate the catalytic role of the inherent intermolecular EFs that originate from Lewis acids (LA) during a ring-closing carbonyl-olefin metathesis (RCCOM) reaction. The specificity of LA coordination to reactants generates specifically oriented intermolecular EF components along the reaction axis which is defined parallel to the direction of flow of electrons wherein the influence of the EF would be at maximum. By examining the thermal [2+2] cycloaddition and carbonyl-ene reaction steps in a RCCOM reaction as model systems, the results revealed the pivotal role of intermolecular EF in mixing some of the dormant ionic structures into normal covalent structures and facilitating a partial rotation of the nonbonding orbitals at the carbonyl oxygen to enhance an ionic pseudo-pericyclic pathway. The unique role of intermolecular EF is further verified by modelling the pristine reaction, in the absence of LAs, under oriented external EFs. The conspicuous intermolecular EF component adds to other modes of catalysis, such as conventional Lewis acidity, to result in the gross catalytic effect. The findings offer insights into the practical realization of EF catalysis by harnessing the intermolecular EFs and point out the need to include intermolecular EF as an inevitable factor for a holistic explanation of any catalytic mechanism.

Methane Bubbled Through Seawater Can be Converted to Methanol With High Efficiency

Partial oxidation of methane (POM) is achieved by forming air-methane microbubbles in saltwater to which an alternating electric field is applied using a copper oxide foam electrode. The solubility of methane is increased by putting it in contact with water containing dissolved KCl or NaCl (3%). Being fully dispersed as microbubbles (20-40 µm in diameter), methane reacts more fully with hydroxyl radicals (OH·) at the gas-water interface. The alternating voltage (100 mV) generates two synergistic POM processes dominated by Cl- → Cl· + e- and O2 + e- → O2 -• under positive and negative potentials, respectively. By tuning the frequency and amplitude, the extent and path of the POM process can be precisely controlled so that more than 90% methanol is selectively formed compared to the two byproducts, dichloromethane, and acetic acid. The methane to methanol conversion yield is estimated to be 57% at a rate of approximately 887 µM h-1. This method appears to have potential for removing methane from air using seawater or for converting higher-concentration methane sources into value-added methanol.

Spontaneous Generation of Alkoxide Radical from Alcohols on Microdroplets Surface

Water microdroplets have been demonstrated to exhibit extraordinary chemical behaviors, including the abilities to accelerate chemical reactions by several orders of magnitude and to trigger reactions that cannot occur in bulk water. One of the most striking examples is the spontaneous generation of hydroxyl radical from hydroxide ions. Alcohols and alkoxide ions, being structurally similar to water and hydroxide ions, might exhibit similar behavior on microdroplets. Here, we report the spontaneous generation of alkoxide radicals from alcohols (RCH2OH) in aqueous microdroplets through quantum chemical calculations, quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations, ab initio MD simulations, and mass spectrometry. Our results show that an electric field (EF) on the order of 10-1 V/Å and partial solvation at the air-water interface jointly promote the dissociation of RCH2OH into RCH2O- and H3O+ ions. QM/MM MD simulations indicate that RCH2O- can be ionized to produce RCH2O⋅ radicals on the microdroplet surface. Furthermore, partial solvation and the EF collaboratively catalyze the isomerization of the RCH2O⋅ radical into a more stable tautomer, R⋅CHOH. This study highlights the molecular mechanisms underlying the widespread generation of radicals at the microdroplet surface and provides insights into the importance of fundamental alcohol chemistry in the atmosphere.

Boosting Reactive Oxygen Species Generation via Contact-Electro-Catalysis with FeIII-Initiated Self-cycled Fenton System

Contact Electro-Catalysis (CEC) using commercial dielectric materials in contact-separation cycles with water can trigger interfacial electron transfer and induce the generation of reactive oxygen species (ROS). However, the inherent hydrophobicity of commercial dielectric materials limits the effective reaction sites, and the generated ROS inevitably undergo self-combination to form hydrogen peroxide (H2O2). In typical CEC systems, H2O2 does not further decompose into ROS, leading to suboptimal reaction rates. Addressing the generation and activation of H2O2 is therefore crucial for advancing CEC. Here, we synthesized a catalyst by loading the dielectric material polytetrafluoroethylene (PTFE) onto ZSM-5 (PTFE/ZSM-5, PZ for short), achieving uniform dispersion of the catalyst in water for the first time. The introduction of an FeIII-initiated self-cycling Fenton system (SF-CEC), with the synergistic effects of O2 activation and FeIII-activated H2O2, further enhanced ROS generation. In the FeIII-initiated SF-CEC system, the synergistic effects of ROS and protonated azo dyes enabled nearly 99 % degradation of azo dyes within 10 minutes, a sixfold improvement compared to the CEC system. This represents the fastest degradation rate of methyl orange dye induced by ultrasound to date. Without extra oxidants, this system enabled stable dissolution of precious metals in weakly acidic solutions at room temperature, achieving 80 % gold dissolution within 2 hours, 2.5 times faster than similar CEC systems. This study also corrects the unfavorable perception of CEC applications under acidic conditions, providing new insights for the fields of dye degradation and precious metal recovery.

Polyethylene Recycling via Water Activation by Ball Milling

Polyethylene (PE) is the most prevalent type of plastic waste and also the most challenging to depolymerize because of its inert carbon-carbon (C-C) bonds.[1] High temperature and noble metals are usually required for depolymerization.[2] To avoid using noble metals, costly reagents and harsh reaction conditions, it is worthwhile but challenging to explore new reaction pathways.[3] We report an unprecedented mechanochemical reaction of PE and water to result in shorter-chain alkanes, alkenes, alcohols, and ketones (Cn, where n≲50), with above 80 % of starting carbon converted into these products, which could be a valuable feedstock for re-entering chemical value chains. This reaction is driven solely by ball milling, without heating and pressurizing. No costly catalysts are used. Instead, only earth-abundant Al2O3 was milled with reactants.

Stabilizing Highly Reactive Aryl Carbanions in Water Microdroplets: Electrophilic Ipso-Substitution at the Air-Water Interface

The fleeting existence of aryl carbanion intermediates in the bulk phase prevents their direct observation and spectroscopic measurement. In sharp contrast, we report the direct interception of such unstable species at the air-water interface of microdroplets. We observed the transformation of three types of aryl acids (benzoic, phenylsulfinic, and phenylboronic acids) into phenyl carbanion (Ph-) in water microdroplets, as examined by mass spectrometry. Experimental and theoretical evidence suggests that the high intrinsic electric field at the microdroplet surface is likely responsible for cleaving the respective acid functional groups of these substrates, generating Ph-, which can subsequently be trapped by an electrophile, including a proton, to yield the corresponding ipso-substitution product. While catalyst-free decarboxylation at ambient temperature is challenging in the bulk phase, we report over 30% instantaneous conversion of benzoic acid to Ph- in sprayed aqueous microdroplets in less than a millisecond. Thus, this study lays the foundation of a green chemical pathway for the aromatic electrophilic ipso-substitution reaction by spraying an aqueous solution of aryl acids, eliminating the need for any catalyst or reagent.

Sustained Regeneration of Hydrogen Peroxide at the Water-Gas Interface of Electrogenerated Microbubbles on an Electrode Surface

Microbubbles, inside-out microdroplets, act as extraordinary microreactors to facilitate thermodynamically unfavorable reactions in bulk solutions of water. We explored the formation of hydrogen peroxide (H2O2) and its sustained regeneration at the interface of water-gas microbubbles. For this purpose, the chemiluminescence of luminol was recorded by a digital camera to map the intensity of blue light emission over the time of about 20 min. The formation and regeneration of hydrogen peroxide were also monitored by fluorescence microscopic imaging of a hydrogen peroxide probe. The microscopic images consistently show a stable glow around the microbubbles over time during which the formed hydrogen peroxide diffuses into the bulk solution. This observation confirms that the concentration of H2O2 at the interface is 30 times higher than that in the water solution bulk after several minutes, which can be attributed to its regeneration at the water-gas interface. These findings increase our understanding of why the chemistries of gas microbubbles in water and water microdroplets surrounded by gas are so distinct from those of bulk-phase water.

Direct Conversion of N2 and Air to Nitric Acid in Gas-Water Microbubbles

We report a simple, direct, and green conversion of air/N2 to nitric acid by bubbling the gas through an aqueous solution containing 50 μM Fe2+ ions. Air stone, along with ultrasonication, was employed to generate gas microbubbles. H2O2 produced at the water-gas interface undergoes Fenton's reaction with Fe2+ ions to produce OH• that efficiently activates N2, yielding nitric acid as the final product. Nitrate (NO3-) formation occurs without the use of any external electric potential or radiation. The concentration of NO3- increased linearly with time over a period of 132 h. The average NO3- production rate is found to be 12.9 ± 0.05 μM h-1. We envision that this nitrogen fixation strategy that produces nitric acid in an eco-friendly way might open the possibility for the energy-efficient and green production of nitric acid.

Catalyst-Free Oxidation of Styrene to Styrene Oxide Using Circulating Microdroplets in an Oxygen Atmosphere

Water microdroplets possess unique interfacial properties that enable chemical reactions to occur spontaneously and increase the reaction rate by orders of magnitude. In this study, water containing styrene (SY) was cyclically sprayed into the air to form microdroplets with an average diameter of 6.7 μm. These microdroplets allowed SY to be oxidized into styrene oxide (SO) without catalysts. No oxidation products of SY were observed in the bulk solution under the same conditions, while in microdroplet reactions 4.2% conversion of SY with approximately 3.1 mM SO was detected. Compared with the traditional spraying microdroplet method, the oxidation product concentration was enhanced by 1000 times. Experiments proved that an aerobic environment boosts SY oxidation, leading to a proposed dual-path hydrogen peroxide (H2O2) oxidation mechanism at the droplet interface. This was confirmed by density functional theory calculations (DFT). Furthermore, in the presence of additional ultrasound, the SY oxidation process initiated by water droplets can be further enhanced, and 7.0% conversion of SY with approximately 5.2 mM SO was detected. The cyclic spraying method greatly enhanced the oxidation product concentration, showing the potential for large scale chemical production using microdroplets.

Methane C(sp3)-H bond activation by water microbubbles

Microbubble-induced oxidation offers an effective approach for activating the C(sp3)-H bond of methane under mild conditions, achieving a methane activation rate of up to 6.7% per hour under optimized parameters. In this study, microbubbles provided an extensive gas-liquid interface that promoted the formation of hydroxyl (OH˙) and hydrogen radicals (H˙), which facilitated the activation of methane, leading to the generation of methyl radicals (CH3˙). These species further participated in free-radical reactions at the interface, resulting in the production of ethane and formic acid. The microbubble system was optimized by adjusting gas-liquid interaction time, water temperature, and bubble size, with the optimal conditions (150 s of water-gas interaction, 15 °C, 50 μm bubble size) yielding a methane conversion rate of 171.5 ppm h-1, an ethane production rate of 23.5 ppm h-1, and a formic acid production rate of 2.3 nM h-1 during 8 h of continuous operation. The stability and efficiency of this process, confirmed through electron spin resonance, high-resolution mass spectrometry, and gas chromatography, suggest that microbubble-based methane activation offers a scalable and energy-efficient pathway for methane utilization.

Spontaneous formation of reactive redox radical species at the interface of gas diffusion electrode

The aqueous interface-rich system has been proposed to act as a trigger and a reservoir for reactive radicals, playing a crucial role in chemical reactions. Although much is known about the redox reactivity of water microdroplets at "droplets-in-gas" interfaces, it remains poorly understood for "bubbles-in-water" interfaces that are created by feeding gas through the porous membrane of the gas diffusion electrode. Here we reveal the spontaneous generation of highly reactive redox radical species detected by using electron paramagnetic resonance under such conditions without applying any bias and loading any catalysts. In combination with ultraviolet-visible spectroscopy, the redox feature has been further verified through several probe molecules. Unexpectedly, introducing crown ether allows to isolate and stabilize both water radical cations and hydrated electrons thus substantially increasing redox reactivity. Our finding suggests a reactive microenvironment at the interface of the gas diffusion electrode owing to the coexistence of oxidative and reductive species.

Oxidation of Ammonia in Water Microdroplets Produces Nitrate and Molecular Hydrogen

Water microdroplets containing dissolved ammonia (30-300 μM) are sprayed through a copper oxide mesh with a 200 μm average pore size, resulting in the formation of nitrate (NO3-) and the release of molecular hydrogen (H2). The products result from a redox process that takes place at the liquid-solid interface through contact electrification, where no external potential is applied. Oxidation is initiated by superoxide radical anions (O2-) that originate from the oxygen in the air surrounding the microdroplets and from the hydroxyl radicals (OH•) originating from the water-air interface. Two spin traps (TEMPO and DMPO) capture these radicals as well as NH2OH+•, HNO, NO•, NO2•, and NOOH, which are detected by mass spectrometry. We also directly observed N2O2-• by the same means. We found that the hydrogen atom from the ammonia molecule can be set free not only in the form of H• but also as H2, which is detected using a residue gas analyzer. The oxidation process can be significantly enhanced by a factor of 3 when the sprayed microdroplets are irradiated with ultraviolet light (265 nm, 5 W). 35% of 300 μM ammonia can be degraded within 20 μs, and the nitrate conversion rate is estimated to be 15 nmol·mg-1·h-1.

Catalyst-free selective oxidation of C(sp3)-H bonds in toluene on water

The anisotropic water interfaces provide an environment to drive various chemical reactions not seen in bulk solutions. However, catalytic reactions by the aqueous interfaces are still in their infancy, with the emphasis being on the reaction rate acceleration on water. Here, we report that the oil-water interface activates and oxidizes C(sp3)-H bonds in toluene, yielding benzaldehyde with high selectivity (>99%) and conversion (>99%) under mild, catalyst-free conditions. Collision at the interface between oil-dissolved toluene and hydroxyl radicals spontaneously generated near the water-side interfaces is responsible for the unexpectedly high selectivity. Protrusion of free OH groups from interfacial water destabilizes the transition state of the OH-addition by forming π-hydrogen bonds with toluene, while the H-abstraction remains unchanged to effectively activate C(sp3)-H bonds. Moreover, the exposed free OH groups form hydrogen bonds with the produced benzaldehyde, suppressing it from being overoxidized. Our investigation shows that the oil-water interface has considerable promise for chemoselective redox reactions on water without any catalysts.

Heavy Water Microdroplet Surface Enriches the Lighter Isotopologue Impurities

Water microdroplets promote unusual chemical reactions at the air-water interface. However, the interfacial structure of water microdroplets and its potential influence on chemical processes are still enigmatic. Here, we present evidence of in-droplet fractionation of water isotopologues. Employing a sonic spray, we atomized the heavy water (D2O, 99.9 atom % D) solution of three classes of organic compounds (basic, acidic, and neutral). The analytes were predominantly desorbed from the resulting droplet surface in protonated form rather than deuterated form, as detected by mass spectrometry. This result remained unaltered upon adding formic acid-d2 (DCOOD) to the droplet. Monitoring Dakin oxidation of benzaldehyde at the surface of binary microdroplets composed of 1:1 (v/v) D2O/H218O revealed the preferred formation of phenolate-16O over phenolate-18O. Atmospheric pressure chemical ionization mass spectrometric analysis of the vapor composition in the sprayed aerosol revealed the preferential evaporation of lighter water isotopologue impurities from the surface of heavy water microdroplets. These results indicate the enrichment of lighter water isotopologue impurities (HOD/H2O) on the surface of heavy water microdroplets, implying possible future developments for water isotopologue fractionation using microdroplets.

Microdroplets initiate organic-inorganic interactions and mass transfer in thermal hydrous geosystems

Organic-inorganic interactions regulate the dynamics of hydrocarbons, water, minerals, CO2, and H2 in thermal rocks, yet their initiation remains debated. To address this, we conducted isotope-tagged and in-situ visual thermal experiments. Isotope-tagged studies revealed extensive H/O transfers in hydrous n-C20H42-H2O-feldspar systems. Visual experiments observed water microdroplets forming at 150-165 °C in oil phases near the water-oil interface without surfactants, persisting until complete miscibility above 350 °C. Electron paramagnetic resonance (EPR) detected hydroxyl free radicals concurrent with microdroplet formation. Here we propose a two-fold mechanism: water-derived and n-C20H42-derived free radicals drive interactions with organic species, while water-derived and mineral-derived ions trigger mineral interactions. These processes, facilitated by microdroplets and bulk water, blur boundaries between organic and inorganic species, enabling extensive interactions and mass transfer. Our findings redefine microscopic interplays between organic and inorganic components, offering insights into diagenetic and hydrous-metamorphic processes, and mass transfer cycles in deep basins and subduction zones.

Molecular Mechanism for Converting Carbon Dioxide Surrounding Water Microdroplets Containing 1,2,3-Triazole to Formic Acid

Spraying water microdroplets containing 1,2,3-triazole (Tz) has been found to effectively convert gas-phase carbon dioxide (CO2), but not predissolved CO2, into formic acid (FA). Herein, we elucidate the reaction mechanism at the molecular level through quantum chemistry calculations and ab initio molecular dynamics (AIMD) simulations. Computations suggest a multistep reaction mechanism that initiates from the adsorption of CO2 by Tz to form a CO2-Tz complex (named reactant complex (RC)). Then, the RC either is reduced by electrons that were generated at the air-liquid interface of the water microdroplet and then undergoes intramolecular proton transfer (PT) or switches the reduction and PT steps to form a [HCO2-(Tz-H)]- complex (named PC-). Subsequently, PC- undergoes reduction and the C-N bond dissociates to generate COOH- and [Tz-H]- (m/z = 69). COOH- easily converts to HCOOH and is captured at m/z = 45 in mass spectroscopy. Notably, the intramolecular PT step can be significantly lowered by the oriented electric field at the interface and a water-bridge mechanism. The mechanism is further confirmed by testing multiple azoles. The AIMD simulations reveal a novel proton transfer mechanism where water serves as a transporter and is shown to play an important role dynamically. Moreover, the transient •COOH captured by the experiment is proposed to be partly formed by the reaction with H•, pointing again to the importance of the air-water interface. This work provides valuable insight into the important mechanistic, kinetic, and dynamic features of converting gas-phase CO2 to valuable products by azoles or amines dissolved in water microdroplets.

Fenton-mediated thermocatalytic conversion of CO2 to acetic acid by industrial waste-derived magnetite nanoparticles

Iron oxide dust, discarded as industrial waste, has been used here to fabricate magnetic iron oxide nanoparticles (Fe3O4-NPs). We have proposed the thermo-catalytic reduction of carbon dioxide (CO2) using Fe3O4-NPs in the presence of H2O2 to get acetic acid (AcOH) at near ambient conditions (100 °C, 10 bar) with a maximum yield of ∼0.4 M in a batch-reactor. The importance of H2O2 can be described as it facilitates the production of higher concentrations of OH˙ and H+/˙, which consequently supports the synthesis of AcOH.