Accelerated Chemical Reactions and Organic Synthesis in Leidenfrost Droplets

Interfacial Behavior and Dechlorination Reaction of Water Droplet Impact on a Heated Extracted Titanium Tailing Surface

The interfacial behaviors of the droplet impact on a heated extracted titanium tailing surface are studied experimentally. The effects of surface temperatures and Weber numbers on the droplet spreading characteristics are examined. The factors affecting the mass fraction and dechlorination ratio of extracted titanium tailings under the action of interfacial behavior have been researched by thermogravimetric analysis. The compositions and microstructures of extracted titanium tailings are characterized using X-ray fluorescence spectroscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The interfacial behaviors on the extracted titanium tailing surface are classified into four regimes, i.e., boiling-induced break-up, advancing recoiling, splash with a continuous liquid film, and splash with a broken film. The maximum spreading factors increase with the surface temperature and the Weber number. It is found that the surface temperature has a dominant influence on the spreading factors and interfacial effect, further affecting its chlorination reaction. SEM-EDS analysis revealed that the extracted titanium tailing particles are irregular shaped. There are some fine pores on the surface after the reaction. The main concentrations are Si, Al, and Ca oxides with a certain amount of C elements. The findings of this research provide a new pathway to utilize the extracted titanium tailings comprehensively.

Reaction Acceleration Promoted by Partial Solvation at the Gas/Solution Interface

The kinetics of organic reactions of different types in microvolumes (droplets, thin films, and sealed tubes) show effects of gas/solution interfacial area, reaction molecularity and solvent polarity. Partial solvation at the gas/solution interface is a major contributor to the 10⁴ -fold reaction acceleration seen in bimolecular but not unimolecular reactions in microdroplets. Reaction acceleration can be used to manipulate selectivity by solvent choice.

Accelerating Electrochemical Reactions in a Voltage-Controlled Interfacial Microreactor

Microdroplet chemistry is attracting increasing attention for accelerated reactions at the solution-air interface. We report herein a voltage-controlled interfacial microreactor that enables acceleration of electrochemical reactions which are not observed in bulk or conventional electrochemical cells. The microreactor is formed at the interface of the Taylor cone in an electrospray emitter with a large orifice, thus allowing continuous contact of the electrode and the reactants at/near the interface. As a proof-of-concept, electrooxidative C-H/N-H coupling and electrooxidation of benzyl alcohol were shown to be accelerated by more than an order of magnitude as compared to the corresponding bulk reactions. The new electrochemical microreactor has unique features that allow i) voltage-controlled acceleration of electrochemical reactions by voltage-dependent formation of the interfacial microreactor; ii) "reversible" electrochemical derivatization; and iii) in situ mechanistic study and capture of key radical intermediates when coupled with mass spectrometry.

The nanoscale Leidenfrost effect

Nanoscale evaporation of liquids plays a key role in several applications including cooling, drag reduction and liquid transport. This research investigates the Leidenfrost effect at the nanoscale as a function of substrate material, droplet size and temperature using molecular dynamics models. Water droplets ranging from 4 nm to 20 nm were simulated over gold and silicon substrates at 293 K, 373 K, 473 K, and 573 K. A significant increase in the kinetic energy (>5000 kcal mol^-1) was observed for molecules in the vicinity of the substrates, indicating the presence of a vapor barrier layer between substrate and liquid. Higher droplet velocities were tracked for hydrophobic gold substrates as compared to hydrophilic silicon substrates indicating the influence of the surface wettability on the Leidenfrost effect. Droplets over silicon substrates had a higher number of fluctuations (peaks and valleys) as compared to gold due to the cyclic behavior of vapor formation. An increase in the interfacial kinetic energies and translatory velocities (>10 m s^-1) were observed as the droplet sizes reduced confirming the Leidenfrost effect at 373 K. This research provides understanding of the Leidenfrost effect at the nanoscale which can impact several applications in heat transfer and droplet propulsion.

Methylene Group Transfer in Carbonyl Compounds Discovered in silico and Detected Experimentally

A previously unknown transformation of aldehydes, ketones, and carboxylic acid derivatives leads to the formation of substituted oxiranes, aziridines, and azirines as shown by DFT and MP2 computations. Formations of 2,2-dimethyloxirane-d₈ from acetone-d₆ , phenylazirine-d₂ from benzonitrile and 2-methyl-2-(4-hydroxyphenyl)-oxirane from 4-hydroxyacetophenone were detected experimentally by electrospray ionization mass-spectrometry with a heated desolvating capillary. This reaction is a truly concerted process characterized by high activation barriers (activation enthalpies 320-480 kJ mol^-1 ).

Selective Synthesis in Microdroplets of 2-Phenyl-2,3-dihydrophthalazine-1,4-dione from Phenyl Hydrazine with Phthalic Anhydride or Phthalic Acid

Pyridazine derivatives are privileged structures because of their potential biological and optical properties. Traditional synthetic methods usually require acid or base as a catalyst under reflux conditions with reaction times ranging from hours to a few days or require microwave assistance to induce the reaction. Herein, this work presents the accelerated synthesis of a pyridazine derivative, 2-phenyl-2,3-dihydrophthalazine-1,4-dione (PDHP), in electrosprayed microdroplets containing an equimolar mixture of phenyl hydrazine and phthalic anhydride or phthalic acid. This reaction occurred on the submillisecond timescale with good yield (over 90 % with the choice of solvent) without using an external catalyst at room temperature. In sharp contrast to the bulk reaction of obtaining a mixture of two products, the reaction in confined microdroplets yields only the important six-membered heterocyclic product PDHP. Results indicated that surface reactions in microdroplets with low pH values cause selectivity, acceleration, and high yields.

Reactive Laser Ablation Electrospray Ionization Time-Resolved Mass Spectrometry of Click Reactions

Reactions in confined compartments like charged microdroplets are of increasing interest, notably because of their substantially increased reaction rates. When combined with ambient ionization mass spectrometry (MS), reactions in charged microdroplets can be used to improve the detection of analytes or to study the molecular details of the reactions in real time. Here, we introduce a reactive laser ablation electrospray ionization (reactive LAESI) time-resolved mass spectrometry (TRMS) method to perform and study reactions in charged microdroplets. We demonstrate this approach with a class of reactions new to reactive ambient ionization MS: so-called click chemistry reactions. Click reactions are high-yielding reactions with a high atom efficiency, and are currently drawing significant attention from fields ranging from bioconjugation to polymer modification. Although click reactions are typically at least moderately fast (time scale of minutes to a few hours), in a reactive LAESI approach a substantial increase of reaction time is required for these reactions to occur. This increase was achieved using microdroplet chemistry and followed by MS using the insertion of a reaction tube-up to 1 m in length-between the LAESI source and the MS inlet, leading to near complete conversions due to significantly extended microdroplet lifetime. This novel approach allowed for the collection of kinetic data for a model (strain-promoted) click reaction between a substituted tetrazine and a strained alkyne and showed in addition excellent instrument stability, improved sensitivity, and applicability to other click reactions. Finally, the methodology was also demonstrated in a mass spectrometry imaging setting to show its feasibility in future imaging experiments.

Reaction Acceleration in Thin Films with Continuous Product Deposition for Organic Synthesis

Thin film formats are used to study the Claisen-Schmidt base-catalyzed condensation of 6-hydroxy-1-indanone with substituted benzaldehydes and to compare the reaction acceleration relative to the bulk. Relative acceleration factors initially exceeded 10³ and were on the order of 10² at steady state, although the confined volume reaction was not electrostatically driven. Substituent effects were muted compared to those in the corresponding bulk and microdroplet reactions and it is concluded that the rate-limiting step at steady state is reagent transport to the interface. Conditions were found that allowed product deposition from the thin film to occur continuously as the reaction mixture was added and as the solvent evaporated. Yields of 74 % and production rates of 98 mg h^-1 were reached in a very simple experimental system that could be multiplexed to greater scales.