Microreactor technology and process miniaturization for catalytic reactions—A perspective on recent developments and emerging technologies

Generation of Controlled Liquid–Liquid Slug Flow by Interlocking Two Diaphragm Pumps

From the viewpoint of resource and energy-saving, the high extraction rate of alternating liquid–liquid flow (slug flow) is important given that it enables its novel use in extraction. Additionally, a specific extraction rate must be maintained for the practical application of slug flow to chemical extraction. Although slug flow is easily generated, controlling the slug length is difficult. In this study, two diaphragm pumps were interlocked to generate a slug flow. By linking the movement of the diaphragms of the two pumps, we could successfully and efficiently control the slug length, and the interlocking diaphragms could easily control the length of the aqueous and oil phase segments of the slug flow. The lengths of the aqueous and oil phases of the slug flow, which could not be quantitatively controlled, could be expressed in terms of the linear velocity of the liquid, the kinematic viscosity, and the tube diameter using the Reynolds number. This relation aids the extraction equipment design using slug flow. Furthermore, the mass transfer coefficient of extraction obtained using the slug flow generated by the developed device was similar to that obtained by the conventional method of a syringe pump. These results indicate that slug flow can be successfully applied to extraction processes.

Toward continuous production of high-quality nanomaterials using microfluidics: nanoengineering the shape, structure and chemical composition

Over the last decade, a multitude of synthesis strategies has been reported for the production of high-quality nanoparticles. Wet-chemical methods are generally the most efficient synthesis procedures since high control of crystallinity and physicochemical properties can be achieved. However, a number of challenges remain from inadequate reaction control during the nanocrystallization process; specifically variability, selectivity, scalability and safety. These shortcomings complicate the synthesis, make it difficult to obtain a uniform product with desired properties, and present serious limitations for scaling the production of colloidal nanocrystals from academic studies to industrial applications. Continuous flow reactors based on microfluidic principles offer potential solutions and advantages. The reproducibility of reaction conditions in microfluidics and therefore product quality have proved to exceed those obtained by batch processing. Considering that in nanoparticles' production not only is it crucial to control the particle size distribution, but also the shape and chemical composition, this review presents an overview of the current state-of-the-art in synthesis of anisotropic and faceted nanostructures by using microfluidics techniques. The review surveys the available tools that enable shape and chemical control, including secondary growth methods, active segmented flow, and photoinduced shape conversion. In addition, emphasis is placed on the available approaches developed to tune the structure and chemical composition of nanomaterials in order to produce complex heterostructures in a continuous and reproducible fashion.

Continuous flow technology-a tool for safer oxidation chemistry

The advantages and benefits of continuous flow technology for oxidation chemistry have been illustrated in tube reactors, micro-channel reactors, tube-in-tube reactors and micro-packed bed reactors in the presence of various oxidants.

Reactor Selection for Upgrading Hemicelluloses: Conventional and Miniaturised Reactors for Hydrogenations

This work presents an advanced reactor selection strategy that combines elements of a knowledge-based expert system to reduce the number of feasible reactor configurations with elaborated and automatised process simulations to identify reactor performance parameters. Special focus was given to identify optimal catalyst loadings and favourable conditions for each configuration to enable a fair comparison. The workflow was exemplarily illustrated for the Ru/C-catalysed hydrogenation of arabinose and galactose to the corresponding sugar alcohols. The simulations were performed by using pseudo-2D reactor models implemented in Aspen Custom Modeler® and automatised by using the MS-Excel interface and VBA. The minichannel packings, namely wall-coated minichannel reactor (MCWR), minichannel reactor packed with catalytic particles (MCPR), and minichannel reactor packed with a catalytic open-celled foam (MCFR), outperform the conventional and miniaturised trickle-bed reactors (TBR and MTBR) in terms of space-time yield and catalyst use. However, longer reactor lengths are required to achieve 99% conversion of the sugars in MCWR and MCPR. Considering further technical challenges such as liquid distribution, packing the reactor, as well as the robustness and manufacture of catalysts in a biorefinery environment, miniaturised trickle beds are the most favourable design for a production scenario of 5000 t/a galactitol. However, the minichannel configurations will be more advantageous for reaction systems involving consecutive and parallel reactions and highly exothermic systems.

Thin-Film Processing of Polypropylene and Polystyrene Sheets by a Continuous Wave CO2 Laser with the Cu Cooling Base

Carbon dioxide (CO₂) laser is widely used in commercial and industrial fields to process various materials including polymers, most of which have high absorptivity in infrared spectrum. Thin-film processing by the continuous wave (CW) laser is difficult since polymers are deformed and damaged by the residual heat. We developed the new method to make polypropylene (PP) and polystyrene (PS) sheets thin. The sheets are pressed to a Cu base by extracting air between the sheets and the base during laser processing. It realizes to cut the sheets to around 50 µm thick with less heat effects on the backside which are inevitable for thermal processing using the CW laser. It is considered that the boundary between the sheets and the base is in thermal equilibrium and the base prevents the sheets from deforming to support the backside. The method is applicable to practical use since it does not need any complex controls and is easy to install to an existing equipment with a minor change of the stage.

Upgrading of Bio-Syngas via Steam-CO2 Reforming Using Rh/Alumina Monolith Catalysts

Steam-CO2 reforming of biomass derived synthesis gas (bio-syngas) was investigated with regard to the steam concentration in the feed using Rh-loaded alumina foam monolith catalysts, which was also accompanied by thermodynamic equilibrium calculation. With 40 vol % steam addition, steam methane reforming and water gas shift reaction were prevailed at the temperature below 640 °C, above which methane dry reforming and reverse-water gas shift reaction were intensified. Substantial change of activation energy based on the methane conversion was observed at 640 °C, where the reaction seemed to be shifted from the kinetic controlled region to the mass transfer controlled region. At the reduced steam of 20 vol %, the increase in the gas velocity led to the increase in the contribution of steam reforming. Comparing to the absence of steam, the addition of steam (40 vol %) resulted in the increase in the production of H2 and CO2, which in turn increased the H2/CO ratio by 95% and decreased the CO/CO2 ratio by 60%. Rh-loaded alumina monolith was revealed to have a good stability in upgrading of the raw bio-syngas.

Solid Circulating Velocity Measurement in a Liquid–Solid Micro-Circulating Fluidised Bed

Liquid–solid circulating fluidised beds (CFB) possess many qualities which makes them useful for industrial operations where particle–liquid contact is vital, e.g., improved heat transfer performance, and consequent uniform temperature, limited back mixing, exceptional solid–liquid contact. Despite this, circulating fluidised beds have seen no application in the micro-technology context. Liquid–solid micro circulating fluidised bed (µCFBs), which basically involves micro-particles fluidisation in fluidised beds within the bed of cross-section or inner diameter at the millimetre scale, could find potential applications in the area of micro-process and microfluidics technology. From an engineering standpoint, it is vital to know the solid circulating velocity, since that dictates the bed capability and operability as processing equipment. Albeit there are several studies on solid circulating velocity measurement in CFBs, this article is introducing the first experimental study on solid circulating velocity measurement in a CFB at micro-scale. The experimental studies were done in a novel micro-CFB which was fabricated by micro milling machining 1 mm2 cross-section channels in Perspex and in a 4 mm2 cross-section micro-CFB made by additive manufacturing technology. Soda-lime glass and polymethyl methacrylate (PMMA) micro-particles were employed as solid materials and tap water as the liquid medium. The digital particle image velocimetry (PIV) method was used as a measurement technique to determine the particle velocity in the micro-CFB system and validated by the valve accumulation technique using a novel magnetic micro-valve. The measured critical transition velocity, Ucr, is comparable to the particle terminal velocity, i.e., the normalised transition velocity is approximately 1 in line with macroscopic systems results and our previous study using simple visual observation. As in macroscopic CFB systems, Ucr decreased with solid inventory (1–9%) and finally becomes stable when the solid inventory is high enough (10–25%) and it increases with a reduction in particle size and density.

Liquid Marbles as Miniature Reactors for Chemical and Biological Applications

The need for miniaturised reaction systems has led to the development of various microreactor platforms, such as droplet-based microreactors. However, these microreactors possess inherent drawbacks, such as rapid evaporation and difficult handling, that limit their use in practical applications. Liquid marbles are droplets covered with hydrophobic particles and are a potential platform that can overcome the weaknesses of bare droplets. The coating particles completely isolate the interior liquids from the surrounding environment, thus conveniently encapsulating the reactions. Great efforts have been made over the past decade to demonstrate the feasibility of liquid marble-based microreactors for chemical and biological applications. This review systemically summarises state-of-the-art implementations of liquid marbles as microreactors. This paper also discusses the various aspects of liquid marble-based microreactors, such as the formation, manipulation, and future perspectives.

Total Internal Reflection Transient Absorption Microscopy: An Online Detection Method for Microfluidics

Microreactors have garnered widespread attention for their tunability and precise control of synthetic parameters to efficiently produce target species. Despite associated advances, a lack of online detection and optimization methods has stalled the progression of microfluidic reactors. Here we employ and characterize a total internal reflection transient absorption microscopy (TIRTAM) instrument to image excited state dynamics on a continuous flow device. The experiments presented demonstrate the capability to discriminate between different chromophores as well as in differentiating the effects of local chemical environments that a chromophore experiences. This work presents the first such online transient absorption measurements and provides a new direction for the advancement and optimization of chemical reactions in microfluidic devices.

On-chip organic synthesis enabled using an engine-and-cargo system in an electrowetting-on-dielectric digital microfluidic device

This paper presents a microfluidic chemical reaction using an electrowetting-on-dielectric (EWOD) digital microfluidic device. Despite a number of chemical/biological applications using EWOD digital microfluidic devices, their applications to organic reactions have been seriously limited because most of the common solvents used in synthetic organic chemistry are not compatible with EWOD devices. To address this unsolved issue, we first introduce a novel technique using an "engine-and-cargo" system that enables the use of non-movable fluids (e.g., organic solvents) on an EWOD device. With esterification as the model reaction, on-chip chemical reactions were successfully demonstrated. Conversion data obtained from on-chip reactions were used to characterize and optimize the reaction with regard to reaction kinetics, solvent screening, and catalyst loading. As the first step toward on-chip combinatorial synthesis, parallel esterification of three different alcohols was demonstrated. Results from this study clearly show that an EWOD digital microfluidic platform is a promising candidate for microscale chemical reactions.

Improving Productivity of Multiphase Flow Aerobic Oxidation Using a Tube-in-Tube Membrane Contactor

The application of flow reactors in multiphase catalytic reactions represents a promising approach for enhancing the efficiency of this important class of chemical reactions. We developed a simple approach to improve the reactor productivity of multiphase catalytic reactions performed using a flow chemistry unit with a packed bed reactor. Specifically, a tube-in-tube membrane contactor (sparger) integrated in-line with the flow reactor has been successfully applied to the aerobic oxidation of benzyl alcohol to benzaldehyde utilizing a heterogeneous palladium catalyst in the packed bed. We examined the effect of sparger hydrodynamics on reactor productivity quantified by space time yield (STY). Implementation of the sparger, versus segmented flow achieved with the built in gas dosing module (1) increased reactor productivity 4-fold quantified by space time yield while maintaining high selectivity and (2) improved process safety as demonstrated by lower effective operating pressures.

High-precision digital droplet pipetting enabled by a plug-and-play microfluidic pipetting chip

Emerging demands for handling minute liquid samples and reagents have been constantly growing in a wide variety of medical and biological areas. This calls for low-volume and high-precision liquid handling solutions with ease-of-use and portability. In this article, a new digital droplet pipetting method is introduced for the first time, derived from the microfluidic impact printing principle. Configured as a conventional handheld pipette, the prototype device consists of a plug-and-play and disposable microfluidic pipetting chip, driven by a programmable electromagnetic actuator for on-demand dispensing of nanoliter droplets. In particular, the impact-driven microfluidic pipetting chip, in place of the traditional disposable pipette tips, offers both liquid loading and droplet generation. The printing nozzle has been micro-fabricated using a femtosecond laser with a super-hydrophobic structure, in order to minimize the dispensing residues. As a result of the high-precision droplet dispensing principle, the variations of the dispensed volume have been successfully reduced from 49.5% to 0.6% at 0.1 μL, as compared to its commercial counterparts. A proof-of-concept study for concentration dilution and quantitative analysis of cell drug resistance has been carried out by using the digital droplet pipetting system, demonstrating its potential in a broad range of biomedical applications which require both high precision and low-volume processing.

Liquid fuel synthesis in microreactors

This review paper provides an in-depth review of microreactors for the intensification of the liquid fuel production processes.

Mass spectrometric directed system for the continuous-flow synthesis and purification of diphenhydramine

A highly integrated approach to the development of a process for the continuous synthesis and purification of diphenhydramine is reported. Mass spectrometry (MS) is utilized throughout the system for on-line reaction monitoring, off-line yield quantitation, and as a reaction screening module that exploits reaction acceleration in charged microdroplets for high throughput route screening. This effort has enabled the discovery and optimization of multiple routes to diphenhydramine in glass microreactors using MS as a process analytical tool (PAT). The ability to rapidly screen conditions in charged microdroplets was used to guide optimization of the process in a microfluidic reactor. A quantitative MS method was developed and used to measure the reaction kinetics. Integration of the continuous-flow reactor/on-line MS methodology with a miniaturized crystallization platform for continuous reaction monitoring and controlled crystallization of diphenhydramine was also achieved. Our findings suggest a robust approach for the continuous manufacture of pharmaceutical drug products, exemplified in the particular case of diphenhydramine, and optimized for efficiency and crystal size, and guided by real-time analytics to produce the agent in a form that is readily adapted to continuous synthesis.

Continuous flow hydrogenations using novel catalytic static mixers inside a tubular reactor

Continuous flow reactor for the hydrogenation of organic substrates using novel catalytic static mixers.

CFD Modeling and Operation Strategies for Hetero-/Homogeneous Combustion of Methane-Air Mixtures in Catalytic Microreactors Using Detailed Chemical Kinetics

The hetero-/homogeneous combustion of methane-air mixtures in platinum-coated microreactors was investigated by means of two-dimensional CFD (computational fluid dynamics) simulations with detailed chemical reaction schemes, detailed species transport, and heat transfer mechanisms in the solid wall. Detailed homogeneous and heterogeneous chemical kinetic mechanisms are employed to describe the chemistry. The effects of the reactor size, inlet velocity and feed composition were elucidated. Operation strategies for controlling the heterogeneous and homogeneous chemistry in heterogeneous-homogeneous microreactors were developed. Simulations using these mechanisms suggested that homogeneous chemistry can be sustained for gaps well below the quenching distance because of enhanced catalyst-induced heating. This finding has very important ramifications for catalyst safety and lifetime, as well as can be used to produce chemicals, e. g. in oxidative coupling and oxidative dehydrogenation reactions. The proportion of heterogeneous and homogeneous contributions depends strongly upon the reactor operating conditions. Reactor size plays a vital role in the homogeneous chemistry contribution. Smaller reactors result in reduced homogeneous chemistry contribution. Pure heterogeneous chemistry can occur under certain proper conditions, such as heat loss/heat exchange rates, feed compositions, and flow rates. The competition or synergism between homogeneous and heterogeneous chemistry was delineated.

Ultrasound and green chemistry--Further comments

In the light of recent discussions regarding the association of sonochemistry and sustainable methods, as well as the controversial misuse and abuse of the "green" concept through the scientific literature, this manuscript provides further thoughts hoping to be of benefit to the broad readership of this journal and practitioners of sustainable chemistry in general.

An experimental study on the numbering-up of microchannels for liquid mixing

The numbering-up of zigzag-form microchannels for liquid mixing was experimentally investigated in a multichannel micromixer including 8 parallel channels, based on the Villermaux-Dushman reaction system, with an appropriate sulphuric acid concentration. The results showed that the micromixing performance in such micromixers could reach the same quality as in a single microchannel, when flat constructal distributors with bifurcation configurations were used. The mixing performance did not depend on whether a vertical or horizontal micromixer position was selected. Surprisingly, the channel blockage somewhat increased the micromixing performance in the multichannel micromixer due to the fluid redistribution effect of the constructal distributors. This effect could also be confirmed by CFD simulations. However, the channel blockage resulted in a higher pressure drop and thus higher specific energy dissipation in the multichannel micromixer. The local pressure drop caused by fluid splitting and re-combination in the numbering-up technique could be neglected at low Reynolds numbers, but it became larger with increasing flow rates. The operational zone for the mixing process in multichannel micromixers was sub-divided into two parts according to the specific energy dissipation and the mixing mechanisms.

Porous double-layer polymer tubing for the potential use in heterogeneous continuous flow reactions

Functional polymer tubing with an OD of 1/16 or 1/8 in. was fabricated by a simple polymer coextrusion process. The tubing was made of an outer impervious polypropylene layer and an inner layer, consisting of a blend of a functional polymer, polyethylene-co-methacrylic acid, and a sacrificial polymer, polystyrene. After a simple solvent leaching step using common organic solvents, the polystyrene was removed, leaving behind a porous inner layer that contains functional carboxylic acid groups, which could then be used for the immobilization of target molecules. Solution-phase reactions using amines or isocyanates have proven successful for the immobilization of a series of small molecules and polymers. This flexible multilayered functional tubing can be easily cut to the desired length and connected via standard microfluidic fittings.