Reactions In Microreactors Research Articles

Rationally engineered smart automotive upholstery leather based on gradient feeding in situ one-pot reaction in microreactors of natural skin

By combining common eco-friendly materials with a "gradient feeding in situ one-step strategy", the limitations of traditional multi-component, complex processes are overcome, enabling the development of ACG leather as a novel smart material.

Assessing mixing uniformity in microreactors via in-line spectroscopy

Mixing behavior is critical for enhancing the selectivity of fast chemical reactions in microreactors. A high Reynolds number (Re) improves the mixing rate and selectivity of the reactions, but some exceptions of increasing side product yield with the higher Re have been reported. This study investigated the mixing uniformity in microreactors with in-line UV–vis spectroscopy to clarify the relationship between reaction selectivity and chaotic mixing with the higher Re. A colorization experiment of thymolphthalein in an acidic solution was conducted with an excess acid amount to the base to indicate a non-uniformly mixed region. Non-uniformity significantly increased with Re. At the same time, the degree of mixing, which was measured by a usual decolorization experiment, showed that the mixing rate increased with Re. The in-line analysis of the Villermaux–Dushman reaction during the mixing clarified that side product yield significantly increased with Re at around 300 and then decreased at around 1100. These results suggest the compensation effect between the mixing uniformity and mixing rate on the selectivity of the mixing-sensitive reactions. Faster mixing, characterized by a larger Re, can disturb mixing uniformity and, in some cases, decrease reaction selectivity.

High-efficiency and safe synthesis of tonalid via two Friedel-Crafts reactions in continuous-flow microreactors

Tonalid, an important fragrance ingredient with widespread application, was synthesized via two Friedel-Crafts reactions, which were catalyzed by AlCl 3 . The traditional tonalid production was conducted in batch stirring tank reactors, suffering from low production capacity and the safety hazard of temperature runaway. To solve these problems, the continuous-flow technologies were developed for the high-efficiency and intrinsically safe synthesis of tonalid in microreactors. Catalyst AlCl 3 was neatly homogenized in proper solvents by forming complex with reactant, which was a necessary step for the continuous synthesis in microreactors. Several reaction conditions, including reactant molar ratio, catalyst concentration, temperature, and microchannel hydrodynamic diameter, were investigated for the two Friedel-Crafts reactions in microreactors. At optimized conditions, the yields of the two Friedel-Crafts reactions were 44.15% and 97.55%, respectively. In comparison with the batch reactors, the reaction times of these two reactions could both be reduced by nearly two thirds in microreactors at the similar yield.

Towards Heterogeneous Catalysis: A Review on Recent Advances of Depositing Nanocatalysts in Continuous-Flow Microreactors.

As a promising technology, microreactors have been regarded as a potential candidate for heterogeneous catalytic reactions as they inherently allow the superior advantages of precise flow control, efficient reactant transfer, flexible operation, etc. However, the wide market penetration of microreactors is still facing severe challenges. One of the most important reasons is the preparation of a high-performance catalytic layer in the microreactor because it can directly influence the catalytic activity and stability the reactor and thus the deployment the microreactor technology. Hence, significant progress in depositing nanocatalysts in microreactors has been made in the past decades. Herein, the methods, principles, recent advances, and challenges in the preparation of the catalyst layer in microreactors were presented. A general description of the physicochemical processes of heterogeneous catalytic reactions in microreactors were first introduced. Then, recent advances in catalyst layer preparation in microreactors were systematically summarized. Particular attention was focused on the most common sol-gel method and its latest developments. Some new strategies proposed recently, including bio-inspired electroless deposition and layer-by-layer self-assembly, were also comprehensively discussed. The remaining challenges and future directions of preparing the catalytic layer in microreactors with high performance and low cost were highlighted.

Controlling gas–liquid segment length in microchannels using a high-speed valve

Segmented flow, one of the most important flow patterns in microchannels, has been widely employed in gas–liquid microreactor systems because it enhances mass transfer through the biphasic interface. The segment length is determined by the channel parameters (e.g., size, material type), fluid flow rate, and fluid properties. Thus, it is difficult to manipulate the gas–liquid segment length without affecting the residence time of the fluid. In this work we successfully achieved regularly segment length control by introducing a high-speed valve into a gas–liquid microchannel system while varying the frequency of the valve installed in the gas-phase tube to allow gas and liquid segment lengths to be controlled within a large range. In contrast, the segment lengths were almost constant under the same conditions without the high-speed valve. Furthermore, a gas/liquid flow rate ratio in the range of 0.7–1.2 was deemed necessary for steady segmented flow. Finally, a correlation was proposed to predict the segment length using parameters such as valve frequency and fluid flow rate, and it matched the experimental data well. This segment length control strategy will improve the performance of gas–liquid reactions in microreactors and advance the design of microreactor systems.

Advances in Microfluidic Synthesis of Solid Catalysts

Heterogeneous catalysis plays a central role in the chemical and energy fields, owing to the high and tunable activities of solid catalysts that are essential to achieve the favorable reaction process efficiency, and their ease of recycle and reuse. Numerous research efforts have been focused on the synthesis of solid catalysts towards obtaining the desired structure, property and catalytic performance. The emergence and development of microfluidic reactor technology provide a new and attractive platform for the controllable synthesis of solid catalysts, primarily because of its superior mixing performance and high heat/mass transfer efficiency. In this review, the recent research progress on the synthesis of solid catalysts based on microfluidic reactor technology is summarized. The first section deals with the synthesis strategies for solid catalysts, including conventional methods in batch reactors and microfluidic alternatives (based on single- and two-phase flow processing). Then, different kinds of solid catalysts synthesized in microflow are discussed, especially with regard to the catalyst type, synthetic process, structure and property, and catalytic performance. Finally, challenges in the microreactor operation and scale-up, as well as future perspectives in terms of the synthesis of more types of catalysts, catalyst performance improvement, and the combination of catalyst synthesis process and catalytic reaction in microreactors, are provided.

Continuous Flow Technology as an Enabler for Innovative Transformations Exploiting Carbenes, Nitrenes, and Benzynes.

Miniaturization offered by microreactors provides for superb reaction control as well as excellent heat and mass transfer. By performing chemical reactions in microreactors or tubular systems under continuous flow conditions, increased safety can be harnessed which allows exploitation of these technologies for the generation and immediate consumption of high-energy intermediates. This Synopsis demonstrates the use of flow technology to effectively exploit benzynes, carbenes, and nitrenes in synthetic chemistry programs.

A numerical study of wave characteristics in axisymmetric gas-liquid annular flow in microchannels

Microreactors have emerged as a viable option to perform mass transfer limited gas-liquid reactions due to the advantage of high gas-liquid interfacial area density. Slug and annular flow regimes are the two most commonly preferred flow regimes to perform reactions in microreactors. In the annular flow regime, characterized by a continuous gas core and an annular liquid film on the wall, waves are observed at the interface between the two phases. The characteristics of these waves depend on the properties of the fluids and their flow rates. In this work, the hydrodynamics of axisymmetric gas-liquid annular flow in millimeter-size channels is investigated computationally employing volume of fluid method in OpenFOAM solver. CFD simulations have been performed for nitrogen-water and nitrogen-water and ethylene glycol mixture over a range of gas and liquid flow rates. Depending on the gas and liquid flow rates and liquid viscosity, coalescing and non-coalescing waves are observed at the interface. The properties of the interfacial waves such as frequency, wave spacing and wave velocity are determined using spatiotemporal analysis and the effect of gas and liquid flow rates on the wave characteristics is investigated. Further, the time-averaged pressure gradient and interfacial shear stress are reported.

Heterogeneous Microfluidic Reactors: A Review and an Insight of Enzymatic Reactions

AbstractHeterogeneous reactions are considered the heart of chemical synthesis with numerous industrial applications due to their high conversion rates and low reaction time. On the other hand, the performance of heterogeneous reactions suffers from several drawbacks such as lower product selectivity and high mass transfer resistance that, in many cases, reduce the reaction rate. The efficiency of various conventional mixing techniques in heterogeneous reactors is believed to play a critical role in controlling the product quality and mass transfer rates besides other essential factors. Microfluidics technology provides a unique opportunity to revisit many established heterogeneous reaction processes to optimize and understand the reaction mechanisms. The unique flow conditions in microflow systems provide an excellent platform for exploring the effect of high‐precision micromixing techniques on reaction rates and productivities. Recent advances in microreactor technology in general and specifically heterogeneous reactions in microflow systems with an insight on the enzymatic reactions in microreactors are reviewed.

Synthesis of arenediazonium salts and Suzuki- Miyaura cross-coupling reaction in microreactors

The process characteristics of two main steps in the preparation of 4-nitrobiphenyl (4-NBP) from 4-nitroaniline (PNA) were clearly revealed by applying two continuous-flow microreactors. The first step was the synthesis of 4-nitrobenzene diazonium tetrafluoroborate (4-NBDT) through the diazotization reaction. The reaction temperature, the residence time, and the molar ratio of different reactants were investigated to achieve optimal reaction conditions with the highest yield of 4-NBDT. Notably, the yield of 4-NBDT reached 97% at 25 °C only in 2.3 s with the optimal molar ratio of HBF4, NaNO2 and PNA (i.e., 2.72:1.05:1). The second step was the Suzuki-Miyaura cross-coupling reaction, in which HNO3 was used to transform the reaction system into a homogeneous state. The effects of mixing performance, the molar percentage of Pd(OAc)2 to 4-NBDT, the reaction temperature and the residence time on the Suzuki-Miyaura cross-coupling reaction were investigated systematically. With the molar percentage of Pd(OAc)2 to 4-NBDT of 1 mol%, the yield of 4-nitrobiphenyl (4-NBP) was optimized to 99% in just 13 min at 25 °C. This study demonstrated the excellent potential of utilizing the continuous-flow microreactor for the diazotization and the following Suzuki-Miyaura cross-coupling reaction.

Transesterification of Sunflower Oil over Waste Chicken Eggshell-Based Catalyst in a Microreactor: An Optimization Study.

The statistical experimental design (DoE) and optimization (Response Surface Methodology combined with Box–Behnken design) of sunflower oil transesterification catalyzed by waste chicken eggshell-based catalyst were conducted in a custom-made microreactor at 60 °C. The catalyst was synthesized by the hydration–dehydration method and subsequent calcination at 600 °C. Comprehensive characterization of the obtained catalyst was conducted using: X-ray powder diffractometry (XRD), X-ray fluorescence (XRF), Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), N2 physisorption, and Hg-porosimetry. Structural, morphological, and textural results showed that the obtained catalyst exhibited high porosity and regular dispersity of plate-like CaO as an active species. The obtained optimal residence time, catalyst concentration, and methanol/oil volume ratio for the continuous reaction in microreactor were 10 min, 0.1 g g−1, and 3:1, respectively. The analysis of variance (ANOVA) showed that the obtained reduced quadratic model was adequate for experimental results fitting. The reaction in the microreactor was significantly intensified compared to a conventional batch reactor, as seen through the fatty acid methyl esters (FAMEs) content after 10 min, which was 51.2% and 18.6%, respectively.

Effect of Steam to Carbon Ratio (S:C) on Steam Methane Reforming’s yield over Coated Nickel Aluminide (Ni<sub>3</sub>Al) Catalyst in Micro Reactor

This work looks into the effect of Steam to Carbon ratio (S:C) on methane (CH4) conversion and hydrogen (H2) yield over coated Nickel Aluminide (Ni3Al) catalyst in micro reactor. The Ni3Al is an intermetallic alloy which known to have good catalytic activity and selectivity. The Ni3Al catalyst precursor was prepared through dip coating technique at 10wt% on top of substrate plate and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope-Energy Dispersive X-Ray Spectroscopy (SEM-EDX), Temperature Programming Reduction (TPR), activated by H2 reduction, and catalytic activity test in steam methane reforming (SMR) reaction in micro reactor at S:C 2, S:C 3 and S:C 4 with 650°C reaction temperature and 300 minutes reaction time. The characterization showed the presence of Ni3Al on top of the coating surface and successfully been activated at 500°C and 46 minutes. The CH4 conversion and H2 yield in the product of the reaction were quantified using the Gas Chromatograph technique. From the series of experiments, it was found that S:C 4 produced the highest methane conversion of 65.56% and S:C 3 produced the highest hydrogen yield of 41.34%. The S:C 2, showed faster and smoother stability trend conversion as early as 180 minutes from the start of the reaction. However, S:C 3 showed the most optimum methane conversion and hydrogen yield and achieved stability trend conversion within the defined reaction time range of 300 minutes. It is inferred that the S:C 3 is the best steam to carbon ratio for the developed catalyst in these settings.

Isothermal reaction calorimetry using Peltier elements for exothermic reactions in microreactors

Isothermal reaction calorimetry using Peltier elements for exothermic reactions in microreactors

Use of metal mesh to assist the multi-phase reaction in a microreactor

Glycerol is a by-product from biodiesel production. Glycerol can be further converted to glycerol carbonate by several reaction routes. One of the feasible routes that have been generally reported is the reaction between glycerol and urea, while ammonia gas is generated as one of byproducts. The reaction generally requires a heterogeneous catalyst, which causes mass transfer problem. Most researchers have applied this reaction in batch, continuous-stirred tank, or fixed bed reactors, in which the external mass transfer resistance is still dominant. Due to many advantages of microreactor including high surface area-to-volume ratio, short diffusion distance, small mass and heat transfer resistances, applying the microreactor can solve the problems and enhance the reaction, Therefore, in this work, microreactor was utilized in the production of glycerol carbonate from glycerol and urea, which one side of microchannel wall was coated by commercial catalyst. In addition, Metal mesh sheet with pores is used to separate ammonia gas from the reaction. This research studies the effect of residence time, microchannel thickness, shear stress of reactants and a quantity of generated gas on a catalyst slipping, and also investigates the effect of metal mesh size on yield of glycerol carbonate from reaction in microreactor.

Metallorganic reactions in the polytropic microreactors

High exothermicity, unstable intermediates, high reaction rates are the features that make metallorganic reactions very challenging, especially in commercial operation. No wonder that there is a large interest for the alternative production technology. This paper reviews a research program by Bayer on metallorganic reactions in microreactors. Selected aspects of use of micro-reaction technology for this reaction class are discussed. Two operational issues, i.e. temperature control and clogging are highlighted. Furthermore, the design concept of a MRT based commercial unit and its economics are discussed.

Nano-architectured CeO2 supported Rh with remarkably enhanced catalytic activity for propylene glycol reforming reaction in microreactors

The influence of morphology and crystalline facets of nanoshaped ceria oxide support in Rh/CeO2 and Rh/CeO2/Al2O3 catalysts have been investigated by oxidative steam reforming of propylene glycol in microreactors at 630 °C at S/C = 3.5 and O/C = 0.15. First, well−defined ceria nanorods (NRs) and nanocubes (NCs) have been successfully synthesized by a facile hydrothermal method and then they are deposited on to α−Al2O3. The result of transmission electron microscopy indicates that the ceria nanorods exposed (110) and (100) planes but nanocubes possess only (100) planes. The developed shape controlled ceria nanocrystals and they are mixed with alumina have been used as supports for the preparation of highly dispersed and size controlled Rh nanoparticles of Rh/CeO2 and Rh/CeO2/Al2O3 catalysts. Their performance was tested for the propylene glycol reforming reaction in microreactors, which provide high surface of the catalyst coatings, heat and mass transfer rate and controllability. The XPS and TEM results indicate that the CeO2 morphology shows the selective exposure of different crystal planes which has a significant impact on the dispersion of Rh nanoparticles that influenced in the reforming reaction. The steam reforming reaction experiments show that the ceria nanorods supported Rh catalyst was achieved significantly better activity and stability for 100 h of time on stream than for the all other catalysts. This could be related to the better dispersion of Rh species and the higher surface oxygen vacancies associated with the exposed (100) and (110) crystal planes favor the stabilization of Rh nanoparticles against sintering and elimination of carbon deposition on active Rh nanoparticles during reforming reaction.

Droplet Coalescence Phenomena during Liquid–Liquid Heterogeneous Reactions in Microreactors

The sulfonation of naphthalene with sulfuric acid to produce naphthalene sulfonic acid was carried out in capillary microreactors as a model liquid–liquid heterogeneous reaction to study droplet coalescence phenomena. The effects of various factors associated with droplet coalescence or the reaction kinetics, such as capillary length (residence time), reaction temperature, molar ratio of reactants, and capillary wall wetting properties, on the reaction performance were investigated. In particular, the influence of droplet coalescence on the specific interfacial area, the mass transfer rate, and the reaction performance was evaluated in detail. Surprisingly, the specific interfacial area could decrease up to 93% along the flow direction in the capillary microreactor with the droplet coalescence. Interestingly, the microreactor wall surface properties were found to affect the reaction performance significantly. Moreover, the Hatta number was evaluated, which reflected the competition between the mass transfer and the reaction during the sulfonation in the capillary microreactors with droplet coalescence.

Local Mass Transfer Phenomena and Chemical Selectivity of Gas‐Liquid Reactions in Capillaries

AbstractGas‐liquid reactions in microreactors play an important role in scientific research and industry. Enhancing heat and mass transfer allows overcoming mass transfer limitations in gas‐liquid reactions. Mass transfer can be further increased by employing helically coiled capillaries, which induce Dean vortices and improve radial mixing. A colorimetric technique is proposed for gas‐liquid reactions in straight and helically coiled capillaries in order to visualize local mass transfer phenomena and concentration distributions. This method is based on the consecutive oxidation of leuco‐indigo carmine and enables noninvasive investigation of mass transfer and chemical selectivity in microchannels with high temporal and spatial resolution.

Synthesis of tin monosulfide (SnS) nanoparticles using surfactant free microemulsion (SFME) with the single microemulsion scheme

Synthesis of monomorphic, SnS nanoparticles without using a capping agent is a difficult task with chemical route of synthesis. This paper reports on synthesis of tin monosulfide (SnS) nanopartilces with dimension in the quantum-dot regime using surfactant free microemulsion with single microemulsion scheme. This has been achieved by reaction in microreactors in the CME (C: chlorobenzene, M: methanol and E: ethylene glycol) microemulsion system. This is an easy and controllable chemical route for synthesis of SnS nanoparticles. Nanoparticle diameter showed prominent dependence on microemulsion concentration and marginal dependence on microemulsion temperature in the temperature range studied. The SnS nanoparticles formed with this method form stable dispersion in Tolune.

Performance and cold spot effect of methanol steam reforming for hydrogen production in micro-reactor

Catalytic hydrogen production by methanol steam reforming is considered to be an attractive hydrogen source option. However, performance of conventional packed beds for methanol steam reforming may be limited by heat transfer, which results in a low effective factor of the catalyst. In this work, CuO/ZnO/Al2O3 catalyst particles with diameter of 0.5 mm were filled in the micro-reactor for hydrogen production by methanol steam reforming. At steam to methanol ratio of 1.3, effects of inlet temperature, space velocity on reactor performance were investigated. The temperature distribution in the catalyst bed has also been studied. Results showed that methanol conversion decreased with space velocity and it increased with inlet temperature. Hydrogen production yield increased firstly and then decreased with space velocity. And it increased with the increasing of temperature. However, CO content in the product also increased with temperature, and it decreased with the increasing of inlet space velocity, especially at low temperature condition. Compared to conventional tubular fixed bed reactor, the micro-reactor presented a slightly better performance due to the fixed bed nature of catalyst, which is inefficiency in tubular reactor. Cold spot was observed at inlet of micro-reactor reaction chamber, and the highest temperature difference at cold spot area reached about 6 K. The maximum temperature difference throughout the catalyst bed reached 10 K in the micro-reactor.