Catalytic Microreactor Research Articles

Synthesis of highly porous polymer microspheres with interconnected open pores for catalytic microreactors

• A synthetic method for producing highly porous PS microspheres was developed. • The porous PS microspheres had interconnective open pores. • PS seed particles were dispersed in different types of oil-in-water emulsions. • Catalytic microreactors were fabricated by using the porous PS microspheres. • The microreactors exhibited extremely high catalytic activity. Porous polymer microspheres with interconnective open pores have attracted significant interest, owing to their unique properties as compared to those of traditional microspheres. However, developing a facile synthetic method to produce these unique particles remains a challenge. Herein, a method is proposed to produce highly porous polystyrene (PS) microspheres with interconnective open pores and a uniform size distribution, which were prepared by simply dispersing PS seed particles in an oil-in-water emulsion system. The morphological evolutions of the PS microspheres exposed to different types of oil-in-water emulsions were systematically investigated, and the relationship between the colloidal stability of the emulsions and the final structure of the PS microspheres was studied. Based on these results, the best oil-in-water system for synthesizing highly porous PS microspheres with interconnected open pores was found. The proposed synthetic method was demonstrated to offer significant advantages over previously developed techniques; it was simple, fast, and easy to process, and did not require special additives such as surfactants, complex techniques, etching processes to generate pores, and sophisticated equipment. Catalytic microreactors were fabricated by synthesizing mono- and bimetallic nanocrystals on the surface of the porous microspheres via in situ chemical reduction. When the microreactors were used as a heterogenous catalyst for the 4-nitrophenol (4-NP) reduction reaction by sodium borohydride (NaBH 4 ), they exhibited extremely high catalytic activity and excellent recyclability. These results demonstrated that the highly porous microspheres have very high porosity, huge surface area per unit volume, and good chemical stability, indicating their great potentials as a supporting material for practical applications in catalysis.

Modeling of a membrane integrated catalytic microreactor for efficient DME production from syngas with CO2

Conversion of syngas (CO + CO2 + H2) to dimethyl ether (DME) with in–situ steam separation is modeled in a membrane integrated, isothermal catalytic microchannel reactor. Reaction channel, involving washcoated form of physically mixed Cu–ZnO/Al2O3 and HZSM–5 catalysts, is separated from the permeate channel by a supported sodalite (SOD) membrane layer. Conservation of momentum and mass within the porous washcoat and the channels, and cross–membrane material transport are modeled in two dimensions at steady–state to elucidate the effects of temperature, pressure, syngas composition and permeate flow properties. Dosing syngas to both channels at 523 K, 50 bar, CO2/COx (COx: CO + CO2) = 0.5 and H2/COx = 2.0 gives CO and CO2 conversions, and DME yield of 33.2, 12.4 and 15.3 %, respectively, which are 29.9, 7.2 and 12.7 % without membrane. These findings are coherent with relaxed thermodynamic limitations on methanol synthesis and dehydration upon selective H2O removal. Higher permeate syngas flow rates promote steam efflux and H2 influx across the membrane and further elevate CO2 conversion and DME yield up to 15.8 and 17.4 %, respectively. These metrics can be improved up to 23.4 and 22.9 %, respectively upon dosing pure H2 as the permeate fluid. However, a positive pressure gradient from reaction to permeate channel does not improve reactor performance. Coherent with its higher dehydration activity, HZSM–5 responds to membrane assistance stronger than γ–Al2O3.

Numerical investigations on influence factors in topology optimization for catalytic microreactors

This article is mainly to study the reaction performance of microreactor by topology optimization method. The influence of the shape of reaction region and reaction parameters on the catalytic microreactors is studied by numerical simulation. It can be found that the circular reaction region is more conducive to catalytic reaction, the square reaction region is next best reaction region shape, and the ellipse reaction region has the worst catalytic effect. When D = 1e−10, P = 0.25, K a=1, D a=1e−4, the catalytic reaction performance is better, and the change of Ka has a great influence on conversion rate. The catalyst distribution obtained by topology optimization method will be helpful to the study of catalytic microreactor in the future. Researchers can use similar catalyst distributions to improve microreactor performance.

Additively Manufactured Robust Microfluidics via Silver Clay Extrusion

We report novel, low-cost, high-temperature compatible, high-pressure compatible, and chemically resistant additively manufactured microfluidics. The devices were monolithically fabricated by extruding silver clay with a fused filament fabrication 3D printer frame fitted with a syringe extruder, followed by annealing at 885 °C in air. Analysis of the printable feedstock shows that the green material is an alloy composed of silver and copper microparticles blended with an organic binder matrix, while analysis of printed and annealed samples shows that the material is completely free of binder and compatible with at least 800 °C operation. Characterization of the thermal, electrical, and mechanical properties of printed and annealed structures yields values close to those of bulk sterling silver, except for a significantly smaller Young's modulus. Metrology of test structures evidences linearity between printed dimensions and computer-aided design values. Layers as thin as 150 μm and working, watertight closed channels as narrow as 200 μm were consistently resolved. A proof-of-concept microfluidic that catalytically decomposes hydrogen peroxide was designed, fabricated, and characterized; the experimental performance of the catalytic microreactor is in agreement with reduced-order modeling.

Fructose dehydration to hydroxyl-methylfurfural in an immobilized catalytic microreactor

In this paper we report a microfluidic platform that allows for high temperature, high pressure conversion with inline spectroscopic measurement for a fast and accurate determination of both reaction rate constant and activation energy. The dehydration of fructose to hydroxyl-methylfurfural has been performed in this immobilized microreactor with both dense zirconia and porous titania layers, as a starting point to probe the potential of abundant metal oxide catalysts.

Patchy Templated Synthesis of Macroporous Colloidal Hollow Spheres and Their Application as Catalytic Microreactors.

Porous colloidal hollow spheres have been applied to diversified fields over the past few decades. However, developing simple and efficient methods to prepare such porous hollow spheres with macro pores remains a challenge. To address this problem, we present a patchy templated synthesis route, which can be used to prepare such colloidal hollow spheres that have macro pores through the shells. This was achieved by using patchy poly(styrene-co-sodium styrenesulfonate) spheres as the template and poly(allylamine hydrochloride) as binding molecules. SiO2 can site-selectively only grow on one kind of patch, resulting in the formation of porous hollow spheres. The pore sizes can be tuned from ∼50 to 400 nm. The resulting porous hollow spheres have a Janus character so that Au nanoparticles can only be attached to the interior surfaces in situ, which can be used as catalytic microreactors and show the catalytic performance of pore size dependence.

Microfluidics-enabled acceleration of Fenton oxidation for degradation of organic dyes with rod-like zero-valent iron nanoassemblies

The advent of microfluidic technology brings new tools and insights to a wide range of applications across chemical and biomedical engineering. In this study, we first demonstrate the development of rod-like zero-valent iron (rZVI) multistack nanoassemblies and examine their superior catalytic capability with microfluidic on-chip platform. rZVI having an average dimension of 27 nm in diameter and 98 nm in length is easily synthesized during the reduction of ferric chloride by sodium borohydride with ethanol as the solvent. The effect of a series of parameters (including precursor type, solvent type, reducing agent concentration, and reaction time) on structural changes is investigated. Miniaturized five-loop spiral-shaped microfluidic device is employed, as a proof of concept, to evaluate the Fenton-like catalytic degradation capability of organic dyes (methylene blue, Rhodamine B, trypan blue, doxorubicin, and methyl orange). In comparison to conventional batch catalysis system, such microfluidic on-chip system could significantly reduce the runtime from a timescale of hours to only seconds. In addition, on-chip catalysis performance can be well regulated by resident time (the longer the resident time, the higher the degradation efficiency), and rZVI shows superior reusability even after eight cycles. This study not only highlights the rational design of nanoparticulate system toward efficient organic dyes removal but also sheds new lights on the development of on-chip catalytic microreactors.

Hydrous titania nanosheets constructed hierarchical hollow microspheres as a highly efficient dual-use decontaminant for elimination of heavy metal ions and organic pollutants

The development of high-performance dual-use decontaminants is of practical significance to alleviate the serious environmental problems led by both heavy metal ions and organic pollutants in wastewater. Herein, novel hierarchical hollow microspheres well-organized by hydrous titania nanosheets (hhHTO) were synthesized via a simple one-pot template-free hydrothermal route. The formation process of hhHTO was investigated and a possible growth mechanism was also proposed. Owing to the integrated features (i.e., large surface area (156.4 m2 g−1), abundant surface Ti–OH groups, and highly open structure), the as-synthesized hhHTO exhibited a remarkable adsorption capacity (up to 459.04 mg g−1) and a very fast adsorption kinetics (within 10 min) for Pb2+ removal, which can be hardly achieved simultaneously in previous investigations. Moreover, this hierarchical hollow adsorbent showed almost unchanged removal efficiency after six-time reuse. In H2O2-assisted photocatalytic processes, the hhHTO could act as catalytic microreactors which was characterized by strong enrichment capacity for H2O2 and high shell permeability, thereby exhibiting significantly higher photocatalytic efficiency than commercial titania (P25) on the degradation of rhodamine B. Besides, these microreactors were also demonstrated to be highly efficient for catalytic degradation of other recalcitrant organic pollutants and exhibited an excellent reusability during cyclic operation.

Combined heat and mass transfer analyses in catalytic microreactors partially filled with porous material - The influences of nanofluid and different porous-fluid interface models

This paper reports a theoretical analysis of heat and mass transfer in the microchannels partially filled with porous materials and used in thermos-chemical microreactors. A first order catalytic chemical reaction is considered on the internal surfaces of a parallel-plates microchannel. The local thermal non-equilibrium approach along with two well-established porous-fluid interface models is employed to investigate the heat transfer within the porous section of the microreactor. The analysis further accounts for the finite thickness of the surrounding solid walls of the microchannel. The dispersion equations in both porous and clear sections of the microchannel are coupled with the fluid temperature through considering the thermal diffusion of mass. In addition, to enhance heat transfer in the partially-filled microchannel, the base fluid is replaced by a nanofluid. The results show that inclusion of the finite thickness of the walls in the thermal analysis can majorly affect the temperature fields and Nusselt number (Nu). In particular, the optimal thickness of the porous insert for achieving the maximum Nu is found to be strongly influenced by the wall thickness. It is also shown that the specific porous-fluid interface model, thicknesses of the porous section and that of the walls, and the volumetric concentration of the nanoparticles can all impart significant effects upon the concentration of chemical species and their distribution across the microchannel. More specifically, the concentration field within the porous region is found to be considerably dependent on the implemented porous-fluid interface model.

Towards coupling direct activation of methane with in situ generation of H2O2

This study aims to shed light on the one-step oxidation of methane to methanol with in situ generated H2O2 by means of a wall-coated catalytic microreactor.

Improved oxidation of hydrogen off-gas by hydrophobic surface modification: A multiscale density functional theory study

A catalytic micro-reactor for converting hydrogen off-gas into water was recently developed, through which the conversion efficiency of hydrogen gas was greatly improved by hydrophobic modification of the catalytic substrate. Herein, a hybrid theoretical method is reported that combines density functional theory (DFT) on both the quantum and molecular scales. This method allows the microscopic study of the mechanism by which the surface catalytic reaction can be manipulated. Specifically, quantum DFT calculations are performed to quantify the molecular interaction between the catalytic substrate and reagent or product. Classical DFT investigations are subsequently carried out to determine the local concentrations of reagents near catalytic sites subject to different surface coating conditions. Finally, the reaction efficiency is determined from the local concentrations based on collision theory. This multiscale method provides molecular insight for quantifying the effect of catalytic surface modification on the reaction efficiency. The method reveals that an optimal surface hydrophobic modification can promote the densities of reagents near the substrate, while depleting the produced water. These two factors promote the conversion efficiency. The exclusion of produced water from the catalytic substrate is affected more by the degree of polymer grafting than by the chain length of hydrophobic polymer moieties.

High density gold nanoparticles immobilized on surface via plasma deposited APTES film for decomposing organic compounds in microchannels

Immobilization of colloidal particles (e.g. gold nanoparticles (AuNps)) on the inner surface of micro-/nano- channels has received a great interest for catalysis. A novel catalytic ozonation setup using a gold-immobilized microchannel reactor was developed in this work. To anchor AuNps, (3-aminopropyl) triethoxysilane (APTES) with functional amine groups was deposited using plasma enhanced chemical vapor deposition (PECVD) process. The results clearly evidenced that PECVD processing exhibited relatively high efficiency for grafting amine groups and further immobilizing AuNPs. The catalytic activity of gold immobilized microchannel was evaluated by pyruvic acid ozonation. The decomposition rate calculated from High Performance Liquid Chromatography (HPLC) indicated a much better catalytic performance of gold in microchannel than that in batch. The results confirmed immobilizing gold nanoparticles on plasma deposited APTES for preparing catalytic microreactors is promising for the wastewater treatment in the future.

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.

Two-dimensional analytical investigation of coupled heat and mass transfer and entropy generation in a porous, catalytic microreactor

Influences of the solid body of microreactors (or the microstructure) upon the transfer processes and hence on the performance of microreactors have been recently emphasised. Nonetheless, the subtle connections between microstructure design and micro-transport phenomena are still largely unknown. To resolve this, the current paper presents an analytical study of the advective-diffusive transport phenomena in a microreactor filled with porous media and with catalytic surfaces. The system under investigation includes the fluid and porous solid phases inside a microchannel with thick walls and subject to uneven thermal loads. The thermal diffusion of mass, viscous dissipation of the flow momentum and local thermal non-equilibrium in the porous medium are considered. The axial variations of heat and mass transfer processes are also taken into account and two-dimensional solutions of the temperature and concentration fields are provided. The local and total entropy generation within the system are further calculated. The results clearly demonstrate the major influences of thick walls on the thermal behaviour and subsequently on the mass transfer and entropy generation of the microreactor. In particular, the Nusselt number is shown to be strongly dependent upon the configuration of microstructure such that it decreases significantly by thickening the walls. The results also demonstrate that for finite Soret numbers the total irreversibility of the system is dominated by the Soret effect. The analytical results of this work can be further used for the validation of future numerical analyses of microreactors.

Non-Equilibrium Thermodynamic Analysis of Double Diffusive, Nanofluid Forced Convection in Catalytic Microreactors with Radiation Effects

This paper presents a theoretical investigation of the second law performance of double diffusive forced convection in microreactors with the inclusion of nanofluid and radiation effects. The investigated microreactors consist of a single microchannel, fully filled by a porous medium. The transport of heat and mass are analysed by including the thick walls and a first order, catalytic chemical reaction on the internal surfaces of the microchannel. Two sets of thermal boundary conditions are considered on the external surfaces of the microchannel; (1) constant temperature and (2) constant heat flux boundary condition on the lower wall and convective boundary condition on the upper wall. The local thermal non-equilibrium approach is taken to thermally analyse the porous section of the system. The mass dispersion equation is coupled with the transport of heat in the nanofluid flow through consideration of Soret effect. The problem is analytically solved and illustrations of the temperature fields, Nusselt number, total entropy generation rate and performance evaluation criterion (PEC) are provided. It is shown that the radiation effect tends to modify the thermal behaviour within the porous section of the system. The radiation parameter also reduces the overall temperature of the system. It is further demonstrated that, expectedly, the nanoparticles reduce the temperature of the system and increase the Nusselt number. The total entropy generation rate and consequently PEC shows a strong relation with radiation parameter and volumetric concentration of nanoparticles.

Catalytic microreactor with electrodeposited hierarchically nanostructured nickel coatings for gas-phase fluorination reactions

The fabrication of a catalytic microreactor for the reaction of undiluted carbonyl difluoride and elemental fluorine to synthesize trifluoromethyl hypofluorite, CF3OF, on CsF catalyst supported on F2-passivated nanosctructured Ni coating was studied. The nanosctructured Ni support for catalyst immobilization was electrodeposited by a two-step procedure, consisting of a low current density step followed by a brief high current density one, for a hierarchical differentiation of structural features. An aqueous solution of NiCl2 with diethanolamine, as crystal modifier, and sodium lauryl sulphate, as anti-pitting agent, was used as electrolyte. Constant-pH Ni electrocrystallization was performed on H2SO4-etched Cu substrates in a range of pH from 1 to 4 via a HCl/H3BO3 based buffer. Passivation was carried out under up to 300mbar of undiluted F2. XRD, XPS, SEM, AFM, and static contact angle measurements were performed. Ni coatings obtained from pH 3 electrolytes were selected for microreactor fabrication on the basis of characterization data, due to the reproducibility and homogeneity of the structured Ni layers. The catalytic microreactor allowed the quantitative production of CF3OF from pure reactants, on demand, and removing any criticality relative to thermal and safety control of the synthesis. The CF3O-group selective transfer ability of the synthetized hypofluorite has raised interest in pharmaceutical and agrochemical industries in recent years.

Facile Synthesis of Dodecamine Organic Cage‐Based Monolithic Microreactor via Ring‐Opening Polymerization Following Spontaneous Reduction of Gold Ions for Continuous Flow Catalysis

AbstractThe reduction product of imine‐based porous organic cage CC1, namely dodecamine organic cage RCC1, was introduced as a novel crosslinker in fabrication of polymeric monolith by ring‐opening polymerization with ethylene glycol diglycidyl ether (EGDGE). Due to the presence of amines in polymeric matrix, poly(RCC1‐co‐EGDGE) monoliths could be employed as dual functional reductant and stabilizer in the formation of gold nanoparticles (AuNPs) on their surface by spontaneous reduction of Au3+ to Au0. The immobilized AuNPs, together with micrometer‐scale macropores of the monolith, provided the feasibility of continuous flow catalysis. Then AuNPs‐immobilized poly(RCC1‐co‐EGDGE) monolith (150 μm i.d. × 28 cm in length) was used as a catalytic microreactor in reduction of 8‐nitroquinoline to 8‐aminoquinoline. The reactant in the outflow completely disappeared, and high‐yield product could be conveniently collected, indicating the promising of RCC1‐based monolithic microreactor in catalysis.

Experimental and Numerical Investigation of Fuel-Lean H2/CO/Air and H2/CH4/Air Catalytic Microreactors

ABSTRACTThe catalytic combustion of fuel-lean H2/CO/air and H2/CH4/air mixtures (equivalence ratios φ = 0.3–0.5) was investigated experimentally and numerically in a 30 × 30 × 4 mm3 microreactor made of SiC and equipped with six 1.5-mm internal diameter platinum tubes. The goal was to demonstrate high surface temperatures (>1200 K) with good spatial uniformity, for power generation applications in conjunction with thermophotovoltaic devices. Surface temperatures were measured with an infrared camera while exhaust gas compositions were assessed with a micro gas chromatograph. Three-dimensional simulations with detailed hetero-/homogeneous chemistry, conjugate heat transfer in the solid, and external heat losses complemented the measurements. The diverse transport (Lewis number), kinetic (catalytic reactivity), and thermodynamic (volumetric heat release rate) properties of the H2, CO, and CH4 fuels gave rise to rich combustion phenomena. Optimization of the channel flow directions mitigated the high spatial non-uniformities of temperature, which were induced by the low Lewis number of H2. Measured surface temperature distributions had mean values as high as 1261 K, with standard deviations as low as 10.6 K. Syngas or biogas (H2/CO mixtures) yielded lower wall temperatures compared to undiluted H2, even for small volumetric CO:H2 ratios (1:9 and 2:8). Although CO had a high catalytic reactivity when combusting in H2/CO mixtures, its larger than unity Lewis number did not allow for the attainment of high surface temperatures. Mixtures of H2/CH4 (such as fuels produced by natural gas decarbonization) were the least attractive due to the substantially lower catalytic reactivity of CH4.

Impact of Gaseous Chemistry in H2–O2–N2 Combustion over Platinum at Fuel-Lean Stoichiometries and Pressures of 1.0–3.5 bar

The catalytic and gas-phase combustion of fuel-lean H2–O2–N2 premixtures was investigated in a channel coated with platinum at pressures 1.0–3.5 bar, a range encompassing microreactors in portable power generation systems and passive hydrogen recombiners in nuclear power plants. One-dimensional Raman spectroscopy assessed the progress of catalytic hydrogen combustion, while planar laser-induced fluorescence (LIF) of the hydroxyl radical monitored gaseous combustion. Simulations were performed using a 2-D code with detailed catalytic and gas-phase reaction mechanisms and realistic transport. Both LIF measurements and simulations revealed that homogeneous combustion was vigorously sustained below ∼2.5 bar, while it was effectively suppressed at higher pressures. This was due to the intricate pressure dependency of the hydrogen gaseous ignition chemistry and the competition between catalytic and gaseous chemical reactions for hydrogen consumption. Parametric simulations determined the smallest critical channel heights allowing for appreciable gaseous combustion and their dependency on pressure, wall temperature, and inlet velocity. It was shown that for the narrower channels of catalytic microreactors homogeneous combustion was relevant only for wall temperatures above 1300 K, whereas for the wider hydrogen recombiner channels it could be relevant for wall temperatures down to 1100 K. Furthermore, at 1100 K the critical channel heights were nonmonotonic functions of pressure, reaching their peaks at 2.0–2.5 bar.

Facile Synthesis of Catalytic AuPd Nanoparticles within Capillary Microreactors Using Polyelectrolyte Multilayers for the Direct Synthesis of H2O2.

Microreactors present innovative solutions for problems pertaining to conventional reactors and therefore have seen successful application in several industrial processes. Yet, its application in heterogeneously catalyzed gas–liquid reactions has been challenging, mainly due to the lack of an easy and flexible methodology for catalyst incorporation inside these reactors. Herein, we report a facile technique for obtaining small (<2 nm) and well-distributed catalytic nanoparticles on the walls of silica-coated capillaries, that act as micro(channel) reactors. These particles are formed in situ on the reactor walls using polyelectrolyte multilayers (PEMs), built by layer-by-layer self-assembly. Manipulating the PEMs’ synthesis condition gives easy control over metal loading, without compromising on particle size. Both monometallic (Au and Pd) and bimetallic (AuPd) nanoparticles were successfully obtained using this technique. Finally, these catalytic microreactors were found to exhibit exceptional activity for the direct synthesis of hydrogen peroxide from H2 and O2.