On-Chip Catalytic Microreactors for Modern Catalysis Research

Direct integration of gold-carbon nanotube hybrids in continuous-flow microfluidic chips: A versatile approach for nanocatalysis

A carbon nanotube-based packed-bed microreactor was developed for the on-chip oxidation of silanes. The process is catalyzed by a heterogeneous gold-carbon nanotube hybrid that was embedded in the device using a micrometric restriction zone. Integration of the nanohybrid permitted efficient flow aerobic oxidation of the substrates into the corresponding silanols with high selectivity and under sustainable conditions.

Silicone Nanofilament Support Layers in an Open-Channel System for the Fast Reduction of Para-Nitrophenol

Chemical vapor phase deposition was used to create hydrophobic nanostructured surfaces on glass slides. Subsequently, hydrophilic channels were created by sputtering a metal catalyst on the channels while masking the outside. The surface tension gradient between the hydrophilic surface in the channels and the outside hydrophobicity formed the open-channel system. The reduction of para-nitrophenol (PNP) was studied on these devices. When compared to nanostructure-free reference systems, the created nanostructures, namely, silicone nanofilaments (SNFs) and nano-bagels, had superior catalytic performance (73% and 66% conversion to 55% at 0.5 µL/s flow rate using 20 nm platinum) and wall integrity; therefore, they could be readily used multiple times. The created nanostructures were stable under the reaction conditions, as observed with scanning electron microscopy. Transition electron microscopy studies of platinum-modified SNFs revealed that the catalyst is present as nanoparticles ranging up to 13 nm in size. By changing the target in the sputter coating unit, molybdenum, gold, nickel and copper were evaluated for their catalytic efficiency. The relative order was platinum < gold = molybdenum < nickel < copper. The decomposition of sodium borohydride (NaBH₄) by platinum as a concurrent reaction to the para-nitrophenol reduction terminates the reaction before completion, despite a large excess of reducing agent. Gold had the same catalytic rate as molybdenum, while nickel was two times and copper about four times faster than gold. In all cases, there was a clear improvement in catalysis of silicone nanofilaments compared to a flat reference system.

The Complexity of Porous Media Flow Characterized in a Microfluidic Model Based on Confocal Laser Scanning Microscopy and Micro-PIV

Abstract

In this study, the complexity of a steady-state flow through porous media is revealed using confocal laser scanning microscopy (CLSM). Micro-particle image velocimetry (micro-PIV) is applied to construct movies of colloidal particles. The calculated velocity vector fields from images are further utilized to obtain laminar flow streamlines. Fluid flow through a single straight channel is used to confirm that quantitative CLSM measurements can be conducted. Next, the coupling between the flow in a channel and the movement within an intersecting dead-end region is studied. Quantitative CLSM measurements confirm the numerically determined coupling parameter from earlier work of the authors. The fluid flow complexity is demonstrated using a porous medium consisting of a regular grid of pores in contact with a flowing fluid channel. The porous media structure was further used as the simulation domain for numerical modeling. Both the simulation, based on solving Stokes equations, and the experimental data show presence of non-trivial streamline trajectories across the pore structures. In view of the results, we argue that the hydrodynamic mixing is a combination of non-trivial streamline routing and Brownian motion by pore-scale diffusion. The results provide insight into challenges in upscaling hydrodynamic dispersion from pore scale to representative elementary volume (REV) scale. Furthermore, the successful quantitative validation of CLSM-based data from a microfluidic model fed by an electrical syringe pump provided a valuable benchmark for qualitative validation of computer simulation results.

Graphic Abstract

A Polyoxometalate-Based Microfluidic Device for Liquid-Phase Oxidation of Glycerol

Peroxidation of glycerol has been carried out in a polyoxometalate (POM)-based microfluidic reactor, which was fabricated on a capillary by using a layer-by-layer strategy. Lactic acid (LA) is produced selectively in high yield with a TOF as high as 20 000 h^-1 , compared to a TOF of 200 h^-1 in batch mode. This POM microfluidic reactor is readily prepared, scalable, highly stable, reusable, and also potentially applicable to selective oxidation of other bio-wastes.

Cucurbit[7]uril-based high-performance catalytic microreactors

Catalytic microreactors manufactured using microfluidic devices have received significant research interest in recent years. However, little attention has been paid to immobilising metallic nanoparticles (NPs) onto microchannel walls for high efficiency catalytic reactions. We demonstrate a facile preparation of cucurbit[7]uril-based catalytic microreactors, where metallic NPs are immobilised onto microchannels via supramolecular complexation with methyl viologen@cucurbit[7]uril (CB[7]). These microreactors exhibit a remarkable catalytic activity owing to the substantially high surface area to volume ratio of the microchannels and metallic NPs. Superior to most conventional heterogeneous catalytic reactions, separation post reaction and complicated recycling steps of the catalysts are not required. Moreover, CB[7] can complex a variety of metallic NPs to its portal, providing a multifunctional high-performance in situ catalytic platform.

Performance of NIR-Mediated Antibacterial Continuous Flow Microreactors Prepared by Mussel-Inspired Immobilization of Cs0.33WO3 Photothermal Agents

An antibacterial continuous flow microreactor was successfully prepared by sequential mussel-inspired surface engineering of microchannels by using catechol-grafted poly(N-vinylpyrrolidone) and immobilization of near-infrared active Cs_0.33WO₃ nanoparticles inside the polydimethylsiloxane(PDMS)-based microreactors. Excellent phothothermal antibacterial acitivity over 99.9% was accomplished toward Gram-positive and -negative bacteria upon near-infrared irradiation during continuous operation up to 30 days. This was achieved without releasing Cs_0.33WO₃ nanoparticles from the surface of the microchannels, confirming the robust immobilization of photothermal agents through the mussel-inspired chemistry. The cleaning of used microreactors was easily attainable by simple acid treatment to release immobilized photothermal agents from the surface of the microchannels, enabling efficient recycling of used microreactors.

Porous Au-Ag Alloy Particles Inlaid AgCl Membranes As Versatile Plasmonic Catalytic Interfaces with Simultaneous, in Situ SERS Monitoring

We present a novel porous Au-Ag alloy particles inlaid AgCl membrane as plasmonic catalytic interfaces with real-time, in situ surface-enhanced Raman spectroscopy (SERS) monitoring. The Au-Ag alloy particles inlaid AgCl membranes were obtained via a facile two-step, air-exposed, and room-temperature immersion reaction with appropriate annealing process. Owing to the designed integration of semiconductor component AgCl and noble metal Au-Ag particles, both the catalytic reduction and visible-light-driven photocatalytic activities toward organic contaminants were attained. Specifically, the efficiencies of about 94% of 4-nitrophenol (4-NP, 5 × 10(-5) M) reduction after 8 min of reaction, and degradation of rhodamine 6G (R6G, 10(-5) M) after 12 min of visible light irradiation were demonstrated. Moreover, efficiencies of above 85% of conversion of 4-NP to 4-aminophenol (4-AP) and 90% of R6G degradation were achieved as well after 6 cycles of reactions, by which robust recyclability was confirmed. Further, with distinct SERS signals generated simultaneously from the surfaces of Au-Ag particles under laser excitation, in situ SERS monitoring of the process of catalytic reactions with superior sensitivity and linearity has been realized. Overall, the capability of the Au-Ag particles inlaid AgCl membranes to provide SERS monitored catalytic and visible-light-driven photocatalytic conversion of organic pollutants, along with their mild and cost-effective fabrication method, would make sense for in-depth understanding of the mechanisms of (photo)catalytic reactions, and also future development of potable, multifunctional and integrated catalytic and sensing devices.

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.

Enzymatic Catalysis Combining the Breath Figures and Layer-by-Layer Techniques: Toward the Design of Microreactors

Herein, we report the fabrication of microstructured porous surfaces with controlled enzymatic activity by combining the breath figures and the layer-by-layer techniques. Two different types of porous surfaces were designed based on fluorinated and carboxylated copolymers in combination with PS, using poly(2,3,4,5,6-pentafluorostyrene)-b-polystyrene (PS5F31-b-PS21) and polystyrene-b-poly(acrylic acid) (PS19-b-PAA10) block copolymers, respectively. For comparative purposes, flat surfaces having similar chemistry were obtained by spin-coating. Poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) multilayers incorporating alkaline phosphatase (ALP) were built on these porous surfaces to localize the enzyme both inside and outside of the pores using PS/PS5F31-b-PS21 surfaces and only inside the pores on PS/PS19-b-PAA10 surfaces. A higher catalytic activity of ALP (about three times) was obtained with porous surfaces compared to the flat ones. The catalysis happens specifically inside the holes of PS/PS19-b-PAA10surfaces, where ALP is located. This opens the route for applications in microreactors.

Continuous synthesis of a co-doped TiO2 photocatalyst and its enhanced visible light catalytic activity using a photocatalysis microreactor

Three co-doped TiO₂ samples were synthesized by a continuous precipitation method with a valve assisted micromixer.

Dendrimer-encapsulated Pd nanoparticles as catalysts for C–C cross-couplings in flow microreactors

Dendrimer-encapsulated Pd nanoparticles anchored within continuous flow microreactors are efficient for C–C cross-coupling reactions. This is witnessed by the good catalytic performance for the Heck–Cassar and Suzuki–Miyaura couplings.

Versatile, kinetically controlled, high precision electrohydrodynamic writing of micro/nanofibers

Direct writing of hierarchical micro/nanofibers have recently gained popularity in flexible/stretchable electronics due to its low cost, simple process and high throughput. A kinetically controlled mechanoelectrospinning (MES) is developed to directly write diversified hierarchical micro/nanofibers in a continuous and programmable manner. Unlike conventional near-field electrospinning, our MES method introduces a mechanical drawing force, to simultaneously enhance the positioning accuracy and morphology controllability. The MES is predominantly controlled by the substrate speed, the nozzle-to-substrate distance, and the applied voltage. As a demonstration, smooth straight, serpentine, self-similar, and bead-on-string structures are direct-written on silicon/elastomer substrates with a resolution of 200 nm. It is believed that MES can promote the low-cost, high precision fabrication of flexible/stretchable electronics or enable the direct writing of the sacrificial structures for nanoscale lithography.

Improved catalytic activity and stability using mixed sulfonic acid- and hydroxy-bearing polymer brushes in microreactors

Sulfonic acid-bearing polymer brushes were grown on the inner walls of continuous flow glass microreactors and used in the acid-catalyzed hydrolysis of benzaldehyde dimethyl acetal as a test reaction. Randomly 1:1 mixed polymer brushes of poly-3-sulfopropyl methacrylate (PSPM) and poly-2-hydroxyethyl methacrylate (PHEMA) showed a 6-fold increase of the TOF value compared to the solely PSPM-containing microreactor. This remarkable improvement is attributed to the cooperative stabilizing effect of proximal OH groups on the active sulfonic acid moieties within the brush architecture. In fact, the rational mixing of SPM with methyl methacrylate (MMA) as an OH-free comonomer caused a drop in the activity of the resulting catalytic platform. A 5-fold increase of the TON of the 1:1 PSPM-PHEMA versus the PSPM homopolymer brush systems additionally demonstrates the substantial increase in the stability of the mixed brushes catalytic platform, which could be continuously run over 7 days without significant loss of activity. The 1:1 PSPM-PHEMA mixed brush catalytic system also showed a good activity in the deprotection of 2-benzyl tetrahydropyranyl ether.

Tailoring Au-Ag-S composite microstructures in one-pot for both SERS detection and photocatalytic degradation of plasticizers DEHA and DEHP

We report a facile single-step one-pot solvothermal process for tailoring the Au-Ag-S microstructures as bifunctional substrates for both surface-enhanced Raman scattering (SERS) detection and photocatalytic degradation of plasticizers diethylhexyl phthalate (DEHP) and diethylhexyl adipate (DEHA). Typically, two different microstructures, the Ag2S particles inlaid Au microflowers (Ag2S-Au MFs) and Au particles decorated AgAuS microsheets (Au-AgAuS MSs) were obtained. The Ag2S-Au MF substrates finally turned out to provide 0.9 × 10(-9) and 0.9 × 10(-7) M of the limits of detection (LODs) for DEHP and DEHA in orange juice. And on the other hand, the Au-AgAuS MSs achieved complete degradation of DEHP and DEHA (1 × 10(-5) M) after 20 and 25 min of UV light irradiation, respectively. It is believed that the facile preparation and appreciable SERS and catalytic activities of these Au-Ag-S microstructures would make much sense to develop novel multifunctional sensing and monitoring devices.

Mechanism of ion transfer in supported liquid membrane systems: electrochemical control over membrane distribution

A polarization study carried out on a thin supported liquid membrane separating two aqueous compartments is presented. Transfer of both the ionized and uncharged form of an organic tracer dye, rhodamine B ([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium chloride), across supported liquid membranes composed of one of 1-octanol (octan-1-ol), 1,9-decadiene (deca-1,9-diene), 1,2-dichlorobenzene, or nitrophenyl octyl ether (1-(2-nitrophenoxy)octane) was studied using cyclic voltammetry and UV-vis absorption spectrophotometry. Concentration analysis indicates that the high membrane concentration of rhodamine B determines the ionic transfer observed via voltammetry, which is consistent with the low aqueous ionic concentration and large membrane/aqueous distribution of the molecule. The observed double-transfer voltammogram, although it has been largely neglected in previous literature, is a logical consequence of the presence of two liquid-liquid interfaces and is rationalized in terms of ion transfer across the two interfaces on either side of the membrane and supported by voltammograms obtained for a series of ions of varied lipophilicity. The bipolar nature of the voltammetric response offers an effective way of mass transport control via changing polarity of the applied voltage and finds immediate use in extraction, purification, and separation applications.