Accelerated photochemical reactions at oil-water interface exploiting melting point depression

Real-time probing of fast chemical reactions at the oil/water interface during microdroplet generation

HYPOTHESIS

Probing the interfacial reactions during microdroplet generation is experimentally challenging, limiting the understanding of the reaction dynamics at an early stage. Real-time monitoring of the temporal evolution of interfacial tension offers a quantitative approach for probing interfacial reactions and reveals the reaction rate-control mechanisms governing the interfacial reaction process.

EXPERIMENTS

An automated microfluidic platform is developed to monitor the dynamic interfacial tension during microdroplet generation within a second, using oleic acid (HOA) and sodium hydroxide (NaOH) as a representative reaction system. The evolution of interfacial concentration of the reaction product is quantified by experiments using various reactant concentrations and flow rates, allowing for identifying the rate-control mechanisms.

FINDINGS

The reactions are identified to be mass transfer-controlled or adsorption-controlled at different HOA concentrations with high NaOH/HOA concentration ratios, enabling the determinations of mass transfer boundary layer thickness and adsorption rate constant of HOA. A modified Damköhler number is introduced to characterize the transition between the rate-control mechanisms. Up to one-third of HOA in microdroplets is found to be consumed during the generation stage within one second, and smaller droplets exhibit higher consumption proportions.

Stefan-Boltzmann Water Catalyzed Propane Dehydrogenation

The traditional wisdom of utilizing water in practical olefin synthesis has been viewed as a means to reduce alkane partial pressure and assist in coke removal. However, under such harsh reaction conditions, the potential catalytic role of the water molecule remains unclear. This study explores the intriguing concept that the water molecule, through the selective excitation of molecular vibrations and collisions induced by thermal energy and thermal radiation, could act as a catalyst in homogeneous gaseous propane dehydrogenation. This occurs via the generation of the OH⋅ radicals, resulting in an olefin yield of 37.93 %, a single pass propane conversion of 51.14 %, and an excellent stability of more than 2000 hours.

On-water accelerated sulfenylation of indole derivatives under visible light irradiation

A visible-light promoted sulfenylation of N-carboxyindoles with thiols showed substantially higher rate and selectivity when conducted "on water". An EDA complex was proposed to form at the water-oil interface, generating thiyl radicals and thus initiating a chain reaction.

Acceleration of Enzyme-Catalyzed Reactions at Aqueous Interfaces through Enhanced Reaction Kinetics of Microdroplets

Enzyme-catalyzed reactions have the advantages of excellent selectivity, low cost, and mild reaction conditions, but the slow reaction kinetics limit their practical applications. Herein, a microdroplet generator that can continuously and rapidly generate water microdroplets with tunable size was designed and used for the study of an enzyme-catalyzed reaction in microdroplets. Using glucose oxidase as a model and resazurin as a fluorescence probe, the fluorescence intensity of the collected microdroplets sprayed into the gas phase was 35 times higher than that in the bulk system, demonstrating obvious reaction acceleration in the microdroplets. Mechanistic studies demonstrated that local concentration enrichment and enzyme reorientation at the gas-water interfaces play key roles in the acceleration of enzymatic reactions in microdroplets. Further, the potential application of the reaction system in glucose sensing was investigated. Finally, we also studied the reaction acceleration of enzymic catalysis at the oil-water interfaces. Online measurement of the fluorescence signal of microdroplets sprayed into the mineral oil revealed a reaction acceleration factor of 6.2. It was demonstrated that aqueous microdroplets provided a green, efficient, and convenient methodology for enzyme-catalyzed reactions.

Photomechanochemistry: harnessing mechanical forces to enhance photochemical reactions

Photomechanochemistry, i.e., the merger of light energy and mechanical forces, is emerging as a new trend in organic synthesis, enabling unique reactivities of fleeting excited states under solvent-minimized conditions. Despite its transformative potential, the field faces significant technological challenges that must be addressed to unlock its full capabilities. In this Perspective, we analyze selected examples to showcase the available technologies to combine light and mechanical forces, including manual grinding, vortex and shaker mixing, rod milling, and ball milling. By examining the advantages and limitations of each approach, we aim to provide an overview of the current state of synthetic photomechanochemistry to identify opportunities for future advancements in this rapidly evolving area of research.

Accelerated Synergistic Photo-Fenton/Photocatalysis Reactions at Aqueous Interfaces

Photocatalysis and photo-Fenton oxidation are promising advanced oxidation technologies for water treatment. Nevertheless, their relatively slow kinetics largely limited their practical applications. Herein, we performed synergistic photocatalysis and photo-Fenton reactions in water microdroplets for the degradation of organic dyes. The efficiency of the microdroplet-based photoreactions was significantly improved with a degradation rate of 98.96% in microdroplets, while it was only 38.14% in the bulk solution. The enhanced degradation efficiency was due to the synergistic effect of the photocatalysis and photo-Fenton reactions in the microdroplets. First, the enrichment of both the dye (rhodamine B) and the catalyst (g-C3N4 nanosheets) at the aqueous interfaces enlarged the local surface concentration, playing a role in the reaction acceleration. Second, the spontaneously generated hydrogen peroxide (17.13 μM) at the aqueous interfaces triggered the photo-Fenton cycle and thus largely promoted the charge separation of g-C3N4 as well as the effective utilization of the photogenerated electrons and holes, leading to a significantly improved degradation efficiency of organic dyes. Further, we quantified the reaction kinetics of individual microdroplets in a real-time manner. The reaction constant in 10 μm microdroplets was 4.86 × 10-3 s-1, which was 22 times higher than that in the bulk phase (0.22 × 10-3 s-1). This study provided a better understanding of accelerated photoreactions at aqueous interfaces and a strategy for addressing the low efficiency of organic dye degradation.

Amphiphilic palladium NHC-complexes with chelating bis-NHC ligands based on imidazole-4,5-dicarboxylic acid: synthesis and catalysis in water

Efficient catalytic systems for various organic transformations in green solvents, especially water, are in great demand. Catalytically active bis-NHC complexes of palladium(II) based on imidazole-4,5-dicarboxylic acid with different lipophilicities were obtained. The synthesis of imidazolium salts was complicated by the formation of side products of nucleophilic substitution by iodide ions in the Menshutkin reaction involving alkyl iodides, which was successfully resolved by using alkyl tosylates. The synthesis of bis-NHC complexes of palladium(II) was carried out in situ using Pd(OAc)2 and KI from imidazolium tosylate salts. The structures of all compounds were well-characterized by a complex of modern physical methods. Typical for gemini surfactants, imidazolium salts aggregate in water to form submicron 200 nm (in the case of di-butyl salt) or compact 6 nm (in the case of di-tetradecyl salt) particles. The catalytic activity of the complexes and systems in situ with Pd(OAc)2 in the hydrogenation reaction of nitroaromatics has been studied. The complex with butyl substituents was found to be superior to known catalytic systems in the reduction of p-nitrophenol (kapp = 1.53 min-1). According to microscopy data, after reduction palladium nanoparticles remained uniformly distributed in the butyl complex, while a lipophilic shell in the tetradecyl complex prevented the access of water-soluble regents to Pd centers. However, the lipophilic tetradecyl complex is more active in the reduction of water-insoluble p-ethylnitrobenzene and in cross-coupling reactions using water-insoluble lipophilic aryl halides due to the combination of micellar and metal complex catalysis. Our results provide insight into amphiphilic NHC palladium complexes as promising catalytic systems for aqueous media.

Visible-Light-Induced Divergent Oxygenation of Methylbenzene Utilizing Aryl Halides

The selective oxidation of methylbenzene to value-added products is of indisputable importance in organic synthesis. Although photocatalytic oxidation reactions of toluene have achieved great success for the preparation of its oxidative products, such as carboxylic acids, benzaldehyde, and benzoate, there remains a lack of a unified photocatalytic system for the selective preparation of these oxidation products. Herein, we report a metal- and additive-free photocatalytic protocol enabled by aryl halides using O2 as a green oxidant for the selective synthesis of the above-mentioned three oxidation products by adjusting the reaction solvent. This strategy features many advantages, including environmentally friendly and mild reaction conditions, broad substrate applicability and functional group tolerance, and potential practical application for the synthesis of aromatic carboxylic drugs and polymer materials and degradation of polystyrene waste. The continuous-flow system was utilized for the oxidation of toluene, which resulted in a reduced reaction time and increased production efficiency. Detailed mechanistic investigation revealed that the hydrogen atom transfer process was facilitated by the bromine radical from aryl halides for further oxidation, and an electron donor-acceptor complex of methylbenzene and aryl halides may exist.

Mimicking on-water surface synthesis through micellar interfaces

The chemistry of the on-water surface, characterized by enhanced reactivity, distinct selectivity, and confined reaction geometry, offers significant potential for chemical and materials syntheses. However, the utilization of on-water surface synthesis is currently limited by the requirement for a stable air-water interface, which restricts its broader synthetic applications. In this work, we present a approach that mimics on-water surface chemistry using micelles. This method involves the self-assembly of charged surfactant molecules beyond their critical micelle concentration (CMC), forming micellar structures that simulate the air-water interface. This creates an environment conducive to chemical reactions, featuring a hydrophobic core and surrounding water layer. Utilizing such mimicking on-water surface with the assembly of porphyrin-based monomers featuring distinct confined geometry and preferential orientations, we achieve reactivity and selectivity (≥99%) in fourteen different reversible and irreversible chemical reactions. Extending the versatility of this approach, we further demonstrate its applicability to two-dimensional (2D) polymerization on micellar interfaces, successfully achieving the aqueous synthesis of crystalline 2D polymer thin layers. This strategy significantly broadens the accessibility of on-water surface chemistry for a wide range of chemical syntheses.

Catalyst-Free Visible Light-Driven Hydrosulfonylation of Alkenes and Alkynes with Sulfonyl Chlorides in Water

A convenient and sustainable method for synthesizing sulfonyl-containing compounds through a catalyst-free aqueous-phase hydrosulfonylation of alkenes and alkynes with sulfonyl chlorides under visible light irradiation is presented. Unactivated alkenes, electron-deficient alkenes, alkyl and aryl alkynes can be hydrosulfonylated with various sulfonyl chlorides at room temperature with excellent yields and geometric selectivities by using tris(trimethylsilyl)silane as a hydrogen atom donor and silyl radical precursor to activate sulfonyl chlorides. Mechanistic studies revealed that the photolysis of tris(trimethylsilyl)silane in aqueous solution to produce silyl radical is crucial for the success of this reaction.

Dynamic covalent and noncovalent assembly of o-nitrosocumene in organic solvents and water

ortho-Nitrosocumene (o-NC) exhibits dynamic N,N bonding, interchanging monomer and E/Z-azodioxide dimer structures, the extent of which depends on the environment. As a solid, o-NC is a Z-dimer; in organic solvent, the monomer is favored; and in water, dimers are favored. A supramolecular assembly of o-NC is observed as a separate species by NMR in water, shown to be a novel nanometer-sized aggregate containing ∼2000 molecules.

Light-induced selectivity in an exemplary photodimerization reaction of varied azaanthracenes

Currently, there is intense interest in light-driven chemical reactions, including photocatalytic processes, photopolymerization and photodimerization. The need for regiocontrol in such reactions is obvious, especially in cases where many products can potentially be formed. Here, the photodimerization involving various azaanthracenes is presented for the first time. Specifically, 2-azaanthracene (A) and N-methyl-2-azaanthracene (M) are considered. Photoreactions of A, M and the A + M mixture under two irradiation wavelengths (365 and 420 nm) and in two solvents (methanol, dichloromethane) were carried out. In the case of A, four regiomers were obtained, in contrast to the available literature data, where only two products were reported. The relative ratio of these products is a function of the irradiation wavelength, the solvent used, and the irradiation time. In the case of M, we have identified two main products and a small amount of a third one, again contradicting the literature data. Irradiation of an equimolar A and M mixture at 365 nm led to a mixture of several products, where the yield of the AM dimers was about 40%. Importantly, the change of the irradiation wavelength to 420 nm significantly increased the AM yield (to about 80%). We demonstrated that only two AM dimers were formed (out of a possible four). The products were comprehensively characterized by NMR spectroscopy. We have determined the photophysical parameters of A and M and measured the quantum yield of photodimerization using UV-vis spectroscopy. The quantum-chemical calculations in the excited state allowed us to propose a plausible explanation for why only two AM dimers are formed upon irradiation. The presented results indicated that photodimerization among various molecules can have advantages and, in particular, does not need to give a complex mixture of multiple products. Importantly, it has been observed that the wavelength shift can significantly improve the photoreaction selectivity.

Accelerated Zymonic Acid Formation from Pyruvic Acid at the Interface of Aqueous Nanodroplets

To explore the role of the liquid interface in mediating reactivity in small compartments, the formation kinetics of zymonic acid (ZA) is measured in submicron aerosols (average radius = 240 nm) using mass spectrometry. The formation of ZA, from a condensation reaction of two pyruvic acid (PA) molecules, proceeds over days in bulk solutions, while in submicron aerosols, it occurs in minutes. The experimental results are replicated in a kinetic model using an apparent interfacial reaction rate coefficient of krxn = (0.9 ± 0.2) × 10-3 M-1 s-1. The simulation reveals that surface activity of PA coupled with an enhanced interfacial reaction rate drives accelerated ZA formation in aerosols. Experimental and simulated results provide compelling evidence that the condensation reaction of PA occurs exclusively at the aerosol interface with a reaction rate coefficient that is enhanced by 4 orders of magnitude (∼104) relative to what is estimated for macroscale solutions.

Vigorously stirred La2O3 suspensions for Michael additions in water

This work demonstrates the effectiveness of vigorously stirred lanthanum oxide (La2O3) suspensions in catalyzing Michael additions in water. These surfactant-free suspensions offer a counterintuitive yet highly efficient approach compared to traditional methods. Notably, the reactions are ineffective in the absence of water, suggesting a crucial role for the aqueous environment.

Improving reproducibility through condition-based sensitivity assessments: application, advancement and prospect

The fluctuating reproducibility of scientific reports presents a well-recognised issue, frequently stemming from insufficient standardisation, transparency and a lack of information in scientific publications. Consequently, the incorporation of newly developed synthetic methods into practical applications often occurs at a slow rate. In recent years, various efforts have been made to analyse the sensitivity of chemical methodologies and the variation in quantitative outcome observed across different laboratory environments. For today's chemists, determining the key factors that really matter for a reaction's outcome from all the different aspects of chemical methodology can be a challenging task. In response, we provide a detailed examination and customised recommendations surrounding the sensitivity screen, offering a comprehensive assessment of various strategies and exploring their diverse applications by research groups to improve the practicality of their methodologies.

Challenges and Future Perspectives in Photocatalysis: Conclusions from an Interdisciplinary Workshop

Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.