Heterogeneously Catalyzed Continuous-Flow Hydrogenation Using Segmented Flow in Capillary Columns

Ni@C Catalyzed Hydrogenation of Acetophenone to Phenylethanol under Industrial Mild Conditions in Flow Reactor

The catalytic hydrogenation of organic substrates containing plenty of unsaturated functional groups is an important step in the industrial preparation of fine chemicals and has always been a hot spot...

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Enabling intensification of multiphase chemical processes with additive manufacturing

Fixed bed supports of various materials (metal, ceramic, polymer) and geometries are used to enhance the performance of many unit operations in chemical processes. Consider first metal and ceramic monolith support structures, which are typically extruded. Extruded monoliths contain regular, parallel channels enabling high throughput because of the low pressure drop accompanying high flow rate. However, extruded channels have a low surface-area-to-volume ratio resulting in low contact between the fluid phase and the support. Additive manufacturing, also referred to as three dimensional printing (3DP), can be used to overcome these disadvantages by offering precise control over key design parameters of the fixed bed including material-of-construction and total bed surface area, as well as accommodating system integration features compatible with continuous flow chemistry. These design parameters together with optimized extrinsic process conditions can be tuned to prepare customizable separation and reaction systems based on objectives for chemical process and/or the desired product. We discuss key elements of leveraging the flexibility of additive manufacturing to intensification with a focus on applications in continuous flow processes and disperse, multiphase systems enabling a range of scalable chemistry spanning discovery to manufacturing operations.

Catalytic exchange of hydrogen isotopes intensified by two-phase stratified flow in wettability designable microchannels

This work develops a reliable method to achieve stratified flow in squared cross-section microchannels by endowing the four channel walls with very different wettabilities. The gas-liquid flows are studied experimentally in six kinds of microchannels with different wetting partitions on the channel walls, where occurrence of the stratified flow is found to strongly depend on the wetting partition and the individual flow rate of gas or liquid. Among them, the microchannel possessing three hydrophobic and one hydrophilic walls (3S1H) shows great potential in facilitating liquid phase catalytic exchange (LPCE) of hydrogen isotopes. Subsequently, a 3S1H microchannel reactor coated with hydrophobic Pt/activated carbon/polydimethylsiloxane (Pt/AC/PDMS) is fabricated to perform the LPCE at 30-80 °C and in a wide range of flow rates of hydrogen and water. The experimental outcomes not only unveil the relationship of the isotopic exchange performance with the temperature and flow pattern but also exhibit an outstanding isotopic exchange performance under the stratified flow.

Visualization of two-phase reacting flow behavior in a gas–liquid–solid microreactor

The hydrodynamic characteristics of gas–liquid two-phase flow can significantly affect the performance of gas–liquid–solid microreactors.

Mesoscale triphasic flow reactors for metal catalyzed gas–liquid reactions

Design and operation of a mesoscale triphasic reactor for flow hydrogenations, capable of delivering kg per day productivity from a single channel.

Continuous-flow processes for the catalytic partial hydrogenation reaction of alkynes

The catalytic partial hydrogenation of substituted alkynes to alkenes is a process of high importance in the manufacture of several market chemicals. The present paper shortly reviews the heterogeneous catalytic systems engineered for this reaction under continuous flow and in the liquid phase. The main contributions appeared in the literature from 1997 up to August 2016 are discussed in terms of reactor design. A comparison with batch and industrial processes is provided whenever possible.

Selective hydrogenation of alkynes over ppm-level Pd/boehmite/Al2O3 beads in a continuous-flow reactor

High activity and selectivity to the semi-hydrogenated products for alkyne hydrogenation over ppm-level Pd/boehmite/Al₂O₃ beads in a safe continuous-flow reactor.

Nanocatalysis in Flow

Nanocatalysis in flow is catalysis by metallic nanoparticles (NPs; 1-50 nm) performed in microstructured reactors. These catalytic processes make use of the enhanced catalytic activity and selectivity of NPs and fulfill the requirements of green chemistry. Anchoring catalytically active metal NPs within a microfluidic reactor enhances the reagent/catalyst interaction, while avoiding diffusion limitations experienced in classical approaches. Different strategies for supporting NPs are reviewed herein, namely, packed-bed reactors, monolithic flow-through reactors, wall catalysts, and a selection of novel approaches (NPs embedded on nanotubes, nanowires, catalytic membranes, and magnetic NPs). Through a number of catalytic reactions, such as hydrogenations, oxidations, and cross-coupling reactions, the advantages and possible drawbacks of each approach are illustrated.

Strategic Application of Residence-Time Control in Continuous-Flow Reactors

As a sustainable alternative for conventional batch-based synthetic techniques, the concept of continuous-flow processing has emerged in the synthesis of fine chemicals. Systematic tuning of the residence time, a key parameter of continuous-reaction technology, can govern the outcome of a chemical reaction by determining the reaction rate and the conversion and by influencing the product selectivity. This review furnishes a brief insight into flow reactions in which high chemo- and/or stereoselectivity can be attained by strategic residence-time control and illustrates the importance of the residence time as a crucial parameter in sustainable method development. Such a fine reaction control cannot be performed in conventional batch reaction set-ups.

Continuous-flow technology—a tool for the safe manufacturing of active pharmaceutical ingredients

In the past few years, continuous-flow reactors with channel dimensions in the micro- or millimeter region have found widespread application in organic synthesis. The characteristic properties of these reactors are their exceptionally fast heat and mass transfer. In microstructured devices of this type, virtually instantaneous mixing can be achieved for all but the fastest reactions. Similarly, the accumulation of heat, formation of hot spots, and dangers of thermal runaways can be prevented. As a result of the small reactor volumes, the overall safety of the process is significantly improved, even when harsh reaction conditions are used. Thus, microreactor technology offers a unique way to perform ultrafast, exothermic reactions, and allows the execution of reactions which proceed via highly unstable or even explosive intermediates. This Review discusses recent literature examples of continuous-flow organic synthesis where hazardous reactions or extreme process windows have been employed, with a focus on applications of relevance to the preparation of pharmaceuticals.

Highly Selective Continuous-Flow Synthesis of Potentially Bioactive Deuterated Chalcone Derivatives

The selective synthesis of various dideuterochalcones as potentially bioactive deuterium-labeled products is presented, by means of the highly controlled partial deuteration of antidiabetic chalcone derivatives. The benefits of continuous-flow processing in combination with on-demand electrolytic D₂ gas generation has been exploited to avoid over-reaction to undesired side products and to achieve selective deuterium addition to the carbon-carbon double bond of the starting enones without the need for unconventional catalysts or expensive special reagents. The roles of pressure, temperature, and residence time proved crucial for the fine-tuning of the sensitive balance between the product selectivity and the reaction rate. The presented flow-chemistry-based deuteration technique lacks most of the drawbacks of the classical batch methods, and is convenient, time- and cost-efficient, and safe.

A catalytic chiral gel microfluidic reactor assembled via dynamic covalent chemistry

A novel dynamic covalent gel strategy is reported to immobilize an asymmetric catalyst within the channels of a microfluidic flow reactor. A layer of a catalytically active Mn-salen dynamic covalent imine gel matrix was coated onto a functionalized capillary. Mn-salen active moiety was incorporated into dynamic covalent imine gel matrix via the reaction of a chiral Mn-salen dialdehyde unit with a tetraamine linker. The catalytic activity of the capillary reactor has been demonstrated in enantioselective kinetic resolution of secondary alcohols.

Structuring catalyst and reactor – an inviting avenue to process intensification

Multiphase catalytic processes involve the combination of scale-dependent and scale-independent phenomena, often resulting in a compromised, sub-optimal performance.

The expanding utility of continuous flow hydrogenation

There has been an increasing body of evidence that flow hydrogenation enhances reduction outcomes across a wide range of synthetic transformations.

Characterization of Dispersion Effects on Reaction Optimization and Scale-Up for a Packed Bed Flow Hydrogenation Reactor

A well known advantage of flow chemistry reactors in chemical synthesis is the ability to screen multiple catalysts and reaction parameters with optimal conditions scaled accordingly. This approach, however, consumes significant quantities of material as the reactor must be equilibrated with the reactants in a continuous, steady-state mode before the start of the reaction. In this work we explore a screening and reaction approach using bolus injections, which is more conducive to the lower material consumption that may be required in a drug discovery setting. A commercially available ThalesNano H-Cube® was evaluated to determine the practicality of this approach for heterogeneous hydrogenations. When working with boluses in flow systems, one of the biggest limitations can be the inherent dispersion of the reactant stream caused by the reactor. The dispersion on the H-Cube® was characterized to determine the minimum volume for the reactor to reach a steady-state. The H-Cube® fluidics and heating coil were found to generate significantly more dispersion than the reaction cartridge (CatCart®) itself, increasing the minimum volume of injection required to achieve steady-state. A 2 mL injection was found as a good compromise between maximizing material conservation and sufficient volume of reaction at steady-state condition. Conditions optimized at 2 mL screening scale were successfully scaled five-fold, while lower volume bolus injections were shown to be less predictable. A stacked injection protocol using lower volume boluses was found to be a reliable alternative to scale reactions while efficiently conserving material. This application of small bolus injections to flow reaction screening and scale-up provides a desirable alternative to traditional continuous flow approaches in the material-limited discovery setting.