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The use of gases as reagents in organic synthesis can be very challenging, particularly at a laboratory scale. This Concept takes into account recent studies to make the case that gases can indeed be efficiently and safely formed from relatively inexpensive commercially available reagents for use in a wide range of organic transformations. In particular, we argue that the exploitation of continuous flow membrane reactors enables the effective separation of the chemistry necessary for gas formation from the chemistry for gas consumption, with these two stages often containing incompatible chemistry. The approach outlined eliminates the need to store and transport excessive amounts of potentially toxic, reactive or explosive gases. The on‐demand generation, separation and reaction of a number of gases, including carbon monoxide, diazomethane, trifluoromethyl diazomethane, hydrogen cyanide, ammonia and formaldehyde, is discussed., Go with the flow: Gas–liquid transformations are very important for organic synthesis. However, gases can be very challenging to use at laboratory scale. Continuous flow membrane reactors enable the safe on‐demand laboratory‐scale generation, separation, and reaction of gases without the need for distillation or gas cylinders.

The hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) was performed in perfluoroalkoxy alkane capillary microreactors packed with a carbon-supported ruthenium (Ru/C) catalyst with an average particle diameter of 0.3 or 0.45 mm. The reaction was executed under an upstream gas–liquid slug flow with 1,4-dioxane as the solvent and H2 as the hydrogen donor in the gas phase. Operating conditions (i.e., flow rate and gas to liquid flow ratio, pressure, temperature and catalyst particle size) were varied in the microreactor to determine the influence of mass transfer and kinetic characteristics on the reaction performance. At 130 °C, 12 bar H2 and a weight hourly space velocity of the liquid feed (WHSV) of 3.0 gfeed/(gcat·h), 100% LA conversion and 84% GVL yield were obtained. Under the conditions tested (70–130 °C and 9–15 bar) the reaction rate was affected by mass transfer, given the notable effect of the mixture flow rate and catalyst particle size on the LA conversion and GVL yield at a certain WHSV. A microreactor model was developed by considering gas–liquid–solid mass transfer therein and the reaction kinetics estimated from the literature correlations and data. This model well describes the measured LA conversion for varying operating conditions, provided that the internal diffusion and kinetic rates were not considered rate limiting. Liquid–solid mass transfer of hydrogen towards the external catalyst surface was thus found dominant in most experiments. The developed model can aid in the further optimization of the Ru/C catalyzed levulinic acid hydrogenation in packed bed microreactors.

Homogeneous Co/Mn/Br catalyzed aerobic oxidation of benzyl alcohol in acetic acid to benzaldehyde was performed in polytetrafluoroethylene microreactors operated under slug flow at temperatures up to 150°C and pressures up to 5 bar. Depending on the bubble velocity and length, a wetted or dewetted slug flow was observed, characterized typically by a complete or partially wetting liquid film around the bubble body. The latter flow suffered from a limited interfacial area for mass transfer. Experiments at temperatures up to ca. 90°C were under kinetic control given no product yield difference under wetted and dewetted slug flows and were used to establish a simplified kinetic expression (first order in benzyl alcohol and zero order in oxygen). This allows to develop a mass transfer model combined with an instantaneous reaction regime that well described the experimental results at higher temperatures where mass transfer was limiting in the dewetted slug flow.

A catalyst preparation method, consisting of slurry washcoating with γ-Al2O3 followed by impregnating platinum on the microreactor walls, has been investigated. The effect of various factors in the preparation procedures on the adhesion of the washcoated γ-Al2O3 was studied, including the slurry property (i.e., the binder type, concentration and molecular weight, γ-Al2O3 concentration and particle size, pH), and the (micro)reactor substrate and channel shape. The results show that the adhesion of γ-Al2O3 washcoat strongly depended on the slurry rheological characteristics. A good adhesion on FeCrAlloy substrates was obtained using the slurry with polyvinyl alcohol as the binder (typical concentration at 3–5 wt% and molecular weight of 57,000–186,000), 20 wt% γ-Al2O3 (particle size being around 3 µm) and pH = 3.5. FeCrAlloy as the substrate exhibited an excellent coating adhesion in rectangular or round channels, primarily due to the formation of alumina film over the surface during thermal pretreatment. The aluminum-free stainless steel as the substrate only showed a good adhesion in a round channel. Well-adhered Pt/γ-Al2O3 catalysts were then applied in a microreactor comprising parallelized microchannels made of FeCrAlloy under the optimized coating procedures, and investigated in terms of its performance in the catalytic methane combustion. It is shown that the reaction temperature has a greater influence on the methane conversion than the flow rate, and a favorable coverage of methane and oxygen on the catalyst surface is essential to obtain a good catalytic performance. These reaction results are in line with the temperature behavior measured along the microreactor.

Taylor flow is a strategy-aimed flow to transfer conventional single-phase into a more efficient two-phase flow resulting in an enhanced momentum/heat/mass transfer rate, as well as a multitude of other advantages. To date, Taylor flow has focused on the processes involving gas–liquid and liquid–liquid two-phase systems in microchannels over a wide range of applications in biomedical, pharmaceutical, industrial, and commercial sectors. Appropriately micro-structured design is, therefore, a key consideration for equipment dealing with transport phenomena. This review paper highlights the hydrodynamic aspects of gas–liquid and liquid–liquid two-phase flows in microchannels. It covers state-of-the-art experimental and numerical methods in the literature for analyzing and simulating slug flows in circular and non-circular microchannels. The review’s main objective is to identify the considerable opportunity for further development of microflows and provide suggestions for researchers in the field. Available correlations proposed for the transition of flow patterns are presented. A review of the literature of flow regime, slug length, and pressure drop is also carried out.

A practical homogeneous and gas‐liquid palladium‐catalyzed Catellani‐type reaction using a continuous‐flow platform is described. The implementation of continuous‐flow technology allowed the acceleration of the transformation and, for the first time, expansion of the chemical space to gaseous olefins (i.e., ethylene, propylene and 3,3,3‐trifluoropropene), thus providing a safe and practical approach to sterically hindered ortho‐disubstituted styrenes and vinyl arenes. The complete control over the stoichiometry of gaseous reagents through flow technology proved essential for directing the selectivity of the Catellani reaction to the desired products.

International audience; The hydrogenation of carbon dioxide offers a large range of possible reactions for converting hydrogen to chemical compounds that can be easily stored, transported and used as fuels or platform molecules. In this study, CO2 hydrogenation was biocatalysed by multispecies microbial communities to produce formate, butyrate and acetate. A hybrid metal/microbial catalysis was pointed out in the presence of iron. Addition of FeCl3 10 mM increased the production of acetate by 265% and butyrate by 73%, to 5.26 and 14.19 g/L, respectively. A stable acetate production rate of 830 mg/L/d was thus sustained for more than 20 days. The presence of iron promoted the selection of Firmicutes and the best performances were linked to the growth of a restricted number of dominant species of two genera: Clostridium and Megasphaera. Various possible catalysis mechanisms are discussed and guidelines are proposed for further development and scale-up of the process.

Hydrodynamics of gas-liquid systems has been studied by high-speed camera. The flow patterns of CO2/water-based fluids were investigated in three glass microreactors having different geometries. Four main regimes were observed: bubbly, slug, slug-annular and annular flow. The forces being responsible for the pattern formation were quantified using dimensionless numbers: Bond (Bo), Reynolds (Re), Weber (We) and Capillary (Ca) numbers. The study showed that the interfacial tension and the gas inertia are the main forces that drive the interface.

International audience; The aim of this paper is to investigate the squeezing-to-dripping transition for bubble formation in a microfluidic T-junction by cross-flowing rupture technique using a high-speed digital camera. Experiments were conducted in a glass microfluidic T-junction with the cross-section of the microchannel of 120 mu m wide and 40 mu m deep. N(2) bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Three different regimes were identified for generating different kinds of bubbles: squeezing, dripping and transition regimes. Various forces exerted on the gaseous thread in different regimes were analyzed. Long slug bubbles were formed in the squeezing regime, while dispersed bubbles in the dripping regime. The transition regime formed short slug bubbles. The bubble sizes in various regimes could be correlated with several dimensionless numbers such as the ratio of gas/liquid flow rates and capillary number. The two-step model for droplets (Steegmans et al., 2009) was extended to describe the bubble formation.

This study outlines a volatile organic compounds (VOCs) removal spray tower consisting of an empty cylindrical vessel and nozzles spraying an oil/water emulsion into the vessel. Spraying an emulsion into a spray tower absorbs both lipophilic and hydrophilic compounds, facilitates oil transport, improves the interfacial area (oil coats water droplets) and increases the turbulent flow regime enhancing mass transfer. Toluene was chosen as the synthetic lipophilic pollutant. The choice of the organic absorbent, silicone oil (47v350 Rhordorsil ® ), was made by considering several properties: thermal and mechanical stability, non-biodegradable nature, insolubility in water and low value of Henry's constant. Gas–liquid mass transfer and the hydrodynamics of the gas–liquid contactor were thoroughly investigated. It was observed that emulsion spraying allowed the dual absorption of hydro- and lipophylic VOCs (efficiency around 90% for both) and strongly enhanced the liquid mass transfer coefficient. Finally, a model describing the efficiency of the process as a function of time was developed. The predicted values are in good agreement with the experimental results.

This study deals with CO 2 mass transfers and biomass conversion in an industrial horizontal tubular photobioreactor. An analytical approach is used to determine an expression modeling the influence of CO 2 mass transfers on the overall biomass conversion efficiency for a given culture broth, heat and light conditions. Fluid mechanics and mass transfer are predicted with a classical two-phase flow approach ( Taitel and Dukler, 1976 ) combined with a dissolution correlation developed and tested in the laboratory ( Valiorgue et al., 2011 ). The influence of the stripping gas, removing the excess of oxygen in the liquid, on the conversion to biomass efficiency is shown to be not negligible. The expression is used to evaluate how the photobioreactor's design and process parameters can be tuned in order to improve biomass conversion efficiency. The biomass conversion efficiency evolution with the photobioreactor's length was found to behave asymptotically and it was explained by the relative orders of magnitude of gas dissolution and gas stripping. It has been shown that the gas flow rate for stripping and therefore the oxygen removal will be limited when further increasing the industrial photobioreactor's length for a given objective of CO 2 conversion to biomass efficiency.

International audience; The hydrodynamics of liquid slugs in gas-liquid Taylor flow in straight and meandering microchannels have been studied using micro Particle Image Velocimetry. The results confirm a recirculation motion in the liquid slug, which is symmetrical about the center line of the channel for the straight geometry and more complex and three-dimensional in the meandering channel. An attempt has also been made to quantify and characterize this recirculation motion in these short liquid slugs (Ls/w

International audience; The effect of fluid properties and operating conditions on the generation of gas-liquid Taylor flow in microchannels has been investigated experimentally and numerically. Visualisation experiments and 2D numerical simulations have been performed to study bubble and slug lengths, liquid film hold-up and bubble velocities. The results show that the bubble and slug lengths increase as a function of the gas and liquid flow rate ratios. The bubble and slug lengths follow the model developed by Garstecki et al. (Lab chip 6:437-446, 2006) and van Steijn et al. (Chem Eng Sci 62:7505-7514, 2007), however, the model coefficients appear to be dependent on the liquid properties and flow conditions in some cases. The ratio of the bubble velocity to superficial two-phase velocity is close to unity, which confirms a thin liquid film under the assumption of a stagnant liquid film. Numerical simulations confirm the hypothesis of a stagnant liquid film and provide information on the thickness of the liquid film.

The design and operation of liquid–liquid and gas–liquid immiscible systems depend mainly on the method of dispersion/phase contacting, whilst gas–liquid–solid and gas–gas–solid systems depend not only on the phase contacting, but also on the solid integration principles employed. The microreactor design ideas employable, the methods of phase contacting and the consequent effects on the reaction yields are reviewed.

In this paper a new experimental method for determining the kinetics of fast precipitation reactions is introduced. Use is made of a laminar jet reactor, which is also frequently applied to determine the kinetics of homogeneous gas-liquid reactions. The liquid containing one or more of the precipitating reactants passes a gas-filled reactor as a stagnant jet in which no mixing occurs. The remaining reactant needed for precipitation is supplied in gaseous form and causes the precipitation reaction to occur while it is diffusing into the jet. Hydrodynamics as well as transport phenomena are precisely known for this system, whereas agglomeration can be minimized by adjustment of the concentration of the solute supplied by the gas. The kinetics of the different crystallization steps can be determined by analyzing the size distribution of the produced particles. This new method is experimentally demonstrated for the precipitation of CuS using H2S gas. The obtained data were successfully used to simulate a packed bed absorber in which H 2S is absorbed by a CuSO4 solution.

A mathematical model has been developed that describes the precipitation of metal sulffides in an aqueous solution containing two different heavy metal ions. The solution is assumed to consist of a well-mixed bulk and a boundary layer that is contacted with hydrogen sulphide gas. The model makes use of Higbie's penetration model to calculate the transfer of gaseous hydrogen sulphide to this boundary layer. The conditions that have been used in the simulations resemble those of industrial wastewater from a zinc factory. The model predicts the rate of H2S absorption, the size distribution of the metal sulphide crystals and the selectivity of precipitation. Higher precipitation rates are predicted at higher pH values and higher H2S concentrations. In all cases considered, the rate of precipitation is fully controlled by mass transfer of H2S, higher H2S concentrations and higher specific surface areas yielding higher precipitation rates. The size of the obtained crystals is predicted to increase with H2S concentration, but to decrease with specific surface area and liquid side mass transfer. These results illustrate the importance of reactor layout and operating conditions on the process of gas–liquid precipitation.

The ammonia chemical ionization mass spectra of 28 methyl ester acetate derivatives of bile acids and related compounds have been determined by gas-liquid chromatography-mass spectrometry. Advantages of ammonia ionization over the previously studied isobutane ionization include a 130-270% enhancement in the sensitivity of base peak monitoring, and direct determination of molecular weight from the base peak (M + NH4+) in the mass spectrum of any of the derivatives. Minor ions in the ammonia spectra also allow selective detection of 3-keto compounds and can indicate unsaturation or double bond conjugation in the molecule. The significance of these studies for the detection and quantitation of bile acids is discussed.

The well-known bile acid analysis technique used by us and others (Grundy, Ahrens, and Miettinen. 1965. J. Lipid Res. 6:397-410) does not allow for the detection of hyodeoxycholic acid, a product of quantitative importance in rodent feces. Using updated methodology, it was established that hyodeoxycholic acid and omega-muricholic acid, both apparent conversion products of beta-muricholic acid, occur in apppreciable amounts in intestinal contents and feces of conventional Wistar type Lobund rats. In conventional rats, these bile acids comprise about 50% of fecal bile acids; they are not found in intestinal contents or feces of germfree rats. Others have demonstrated that hyodeoxycholic acid if formed by combined action of gut flora and liver. A new method for the separation of conjugated and free bile acids in biological samples was developed. Results with this method confirmed the total conjugation of bile acids in the germfree rat, and the almost total deconjugation that takes place in the cecum of the conventional rat.

17alpha-Methyltestosterone-4-(14)C was fed to two dogs in an experiment to determine tissue localization and metabolic disposition of this hypocholesterolemic steroid. No accumulation of the drug was found in any tissue, although a small amount of radioactivity was detected in the liver and the ileal mucosa of one animal. Most of the administered radio-activity was excreted in urine and feces. The urinary metabolites consisted largely of highly polar compounds which appeared resistant to glucuronidase treatment or solvolysis procedures. Analysis of the fecal metabolites showed the presence of unchanged methyltestosterone, of four isomeric methylandrostanediols, and of labeled unidentified polar compounds. Of the four identified methylandrostane-diols, the predominating fecal diols were 17alpha-methyl-5alpha-androstane-3beta,17beta-diol (45-62%) and 17alpha-methyl-5-androstane-3alpha,17-diol (12-28%); 17alpha-methyl-5alpha-androstane-3alpha,-17-diol and the 5beta:3beta isomer were found in very small amounts only.

Cholesteryl alkyl ethers have been isolated from bovine cardiac muscle and characterized by thin-layer and gas-liquid chromatography. The fraction contained at least three homologues. Cholesteryl hexadecyl ether, which accounted for over 90% of the total components observed on gas chromatography, was identified by mass spectrometry.

A method is described for the synthesis of cyclic thionocarbonates of 1-O-alkyl glycerols in quantitative yield. These derivatives of glycerol ethers can be quantitated by UV absorbance, analyzed by gas-liquid chromatography, or quantitatively converted to allyl alkyl ethers for gas-chromatographic analysis.