Hydrocarbon hydrogenation and dehydrogenation reactions in microfabricated catalytic reactors

Heterogeneous catalytic hydrogenation reactions in continuous-flow reactors

Microreactor technology and continuous flow processing in general are key features in making organic synthesis both more economical and environmentally friendly. Heterogeneous catalytic hydrogenation reactions under continuous flow conditions offer significant benefits compared to batch processes which are related to the unique gas-liquid-solid triphasic reaction conditions present in these transformations. In this review article recent developments in continuous flow heterogeneous catalytic hydrogenation reactions using molecular hydrogen are summarized. Available flow hydrogenation techniques, reactors, commonly used catalysts and examples of synthetic applications with an emphasis on laboratory-scale flow hydrogenation reactions are presented.

In situ immobilization of palladium nanoparticles in microfluidic reactors and assessment of their catalytic activity

We report on the synthesis and characterization of catalytic palladium nanoparticles (Pd NPs) and their immobilization in microfluidic reactors fabricated from polydimethylsiloxane (PDMS). The Pd NPs were stabilized with D-biotin or 3-aminopropyltrimethoxysilane (APTMS) to promote immobilization inside the microfluidic reactors. The NPs were homogeneous with narrow size distributions between 2 and 4 nm, and were characterized by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and x-ray diffraction (XRD). Biotinylated Pd NPs were immobilized on APTMS-modified PDMS and glass surfaces through the formation of covalent amide bonds between activated biotin and surface amino groups. By contrast, APTMS-stabilized Pd NPs were immobilized directly onto PDMS and glass surfaces rich in hydroxyl groups. Fourier transform infrared spectroscopy (FT-IR) and x-ray photoelectron spectroscopy (XPS) results showed successful attachment of both types of Pd NPs on glass and PDMS surfaces. Both types of Pd NPs were then immobilized in situ in sealed PDMS microfluidic reactors after similar surface modification. The effectiveness of immobilization in the microfluidic reactors was evaluated by hydrogenation of 6-bromo-1-hexene at room temperature and one atmosphere of hydrogen pressure. An average first-run conversion of 85% and selectivity of 100% were achieved in approximately 18 min of reaction time. Control experiments showed that no hydrogenation occurred in the absence of the nanocatalysts. This system has the potential to provide a reliable tool for efficient and high throughput evaluation of catalytic NPs, along with assessment of intrinsic kinetics.

An electronic Venturi-based pressure microregulator

Microfluidic systems often use pressure-driven flow to induce fluidic motion, but control of pumps and valves can necessitate numerous external connections or an extensive external control infrastructure. Here, we describe an electronically controlled pressure microregulator that can output pressures both greater and less than atmospheric pressure over a range of 2 kPa from a single pressurized air input of 110 kPa. Multiple independently controlled microregulators integrated in one device can potentially share the same air input. The microregulator operates by using embedded resistive heaters to vary the temperature of a gas flowing through a converging-diverging Venturi nozzle between 25 degrees C and 85 degrees C with a resolution of 33 Pa degrees C(-1). We established the switching speed of the microregulator by accurately moving 1 microL droplets of water in a microchannel via pneumatic propulsion. Droplet deceleration from approximately 1 cm s(-1) to zero velocity required less than 0.8 s. The component is readily integrable into most device designs containing fluidic channels and electronics without introducing additional fabrication complexity.

Multiphase microfluidics: from flow characteristics to chemical and materials synthesis

We review transport characteristics of pressure-driven, multiphase flows through microchannel networks tens of nanometres to several hundred of micrometres wide with emphasis on conditions resulting in enhanced mixing and reduced axial dispersion. Dimensionless scaling parameters useful in characterizing multiphase flows are summarized along with experimental flow visualization techniques. Static and dynamic stability considerations are also included along with methods for stabilizing multiphase flows through surface modifications. Observed gas-liquid and immiscible liquid-liquid flows are summarized in terms of flow regime diagrams and the different flows are related to applications in chemistry and materials synthesis. Means to completely separate multiphase flows on the microscale and guidelines for design of scalable multiphase systems are also discussed.

Control and detection of chemical reactions in microfluidic systems

Recent years have seen considerable progress in the development of microfabricated systems for use in the chemical and biological sciences. Much development has been driven by a need to perform rapid measurements on small sample volumes. However, at a more primary level, interest in miniaturized analytical systems has been stimulated by the fact that physical processes can be more easily controlled and harnessed when instrumental dimensions are reduced to the micrometre scale. Such systems define new operational paradigms and provide predictions about how molecular synthesis might be revolutionized in the fields of high-throughput synthesis and chemical production.

A Microfluidic Device for Conducting Gas-Liquid-Solid Hydrogenation Reactions

We have developed an efficient system for triphase reactions using a microchannel reactor. Using this system, we conducted hydrogenation reactions that proceeded smoothly to afford the desired products quantitatively within 2 minutes for a variety of substrates. The system could also be applied to deprotection reactions. We could achieve an effective interaction between hydrogen, substrates, and a palladium catalyst using extremely large interfacial areas and the short path required for molecular diffusion in the very narrow channel space. This concept could be extended to other multiphase reactions that use gas-phase reagents such as oxygen and carbon dioxide.