Microreactor technologies developed at LLNLuse micromachining techniques to miniaturizethe reactor design. Applications include fuelprocessors for generating hydrogen, chemicalsynthesis, and bioreaction studies.

MicroreactorFrom Wikipedia, the free encyclopedia

A microreactor or microstructured reactor or microchannelreactor is a device in which chemical reactions take place in aconfinement with typical lateral dimensions below 1 mm; themost typical form of such confinement are microchannels.[1]Microreactors are studied in the field of micro processengineering, together with other devices (such as micro heatexchangers) in which physical processes occur. Themicroreactor is usually a continuous flow reactor (contrastwith/to a batch reactor). Microreactors offer many advantagesover conventional scale reactors, including vast improvements inenergy efficiency, reaction speed and yield, safety, reliability,scalability, on-site/on-demand production, and a much finerdegree of process control.

Contents1 History2 Benefits3 Problems4 T reactors5 Applications

5.1 Synthesis5.2 Analysis

5.2.1 NMR5.2.2 Infrared Spectroscopy

6 Academic research7 Market structure8 References

History

Gas-phase microreactors have a long history but those involving liquids started to appear in the late 1990s.[1] Oneof the first microreactors with embedded high performance heat exchangers were made in the early 1990s by theCentral Experimentation Department (Hauptabteilung Versuchstechnik, HVT) of ForschungszentrumKarlsruhe[2] in Germany, using mechanical micromachining techniques that were a spinoff from the manufacture ofseparation nozzles for uranium enrichment.[2] As research on nuclear technology was drastically reduced inGermany, microstructured heat exchangers were investigated for their application in handling highly exothermic anddangerous chemical reactions. This new concept, known by names as microreaction technology or micro processengineering, was further developed by various research institutions. An early example from 1997 involved that ofazo couplings in a pyrex reactor with channel dimensions 90 micrometres deep and 190 micrometres wide.[1]

BenefitsUsing microreactors is somewhat different from using a glass vessel. These reactors may be a valuable tool in thehands of an experienced chemist or reaction engineer:

Microreactors typically have heat exchange coefficients of at least 1 megawatt per cubic meter per kelvin, upto 500 MW m3 K1 vs. a few kilowatts in conventional glassware (1 l flask ~10 kW m3 K1)). Thus,microreactors can remove heat much more efficiently than vessels and even critical reactions such asnitrations can be performed safely at high temperatures.[3] Hot spot temperatures as well as the duration ofhigh temperature exposition due to exothermicity decreases remarkably. Thus, microreactors may allowbetter kinetic investigations, because local temperature gradients affecting reaction rates are much smallerthan in any batch vessel. Heating and cooling a microreactor is also much quicker and operatingtemperatures can be as low as 100 C. As a result of the superior heat transfer, reaction temperatures maybe much higher than in conventional batch-reactors. Many low temperature reactions as organo-metalchemistry can be performed in microreactors at temperatures of 10 C rather than 50 C to 78 C as inlaboratory glassware equipment.

Microreactors are normally operated continuously. This allows the subsequent processing of unstableintermediates and avoids typical batch workup delays. Especially low temperature chemistry with reactiontimes in the millisecond to second range are no longer stored for hours until dosing of reagents is finished andthe next reaction step may be performed. This rapid work up avoids decay of precious intermediates andoften allows better selectivities.[4]Continuous operation and mixing causes a very different concentration profile when compared with a batchprocess. In a batch, reagent A is filled in and reagent B is slowly added. Thus, B encounters initially a highexcess of A. In a microreactor, A and B are mixed nearly instantly and B won't be exposed to a large excessof A. This may be an advantage or disadvantage depending on the reaction mechanism - it is important to beaware of such different concentration profiles.Although a bench-top microreactor can synthesize chemicals only in small quantities, scale-up to industrialvolumes is simply a process of multiplying the number of microchannels. In contrast, batch processes toooften perform well on R&D bench-top level but fail at batch pilot plant level.[5]Pressurisation of materials within microreactors (and associated components) is generally easier than withtraditional batch reactors. This allows reactions to be increased in rate by raising the temperature beyond theboiling point of the solvent. This, although typical Arrhenius behaviour, is more easily facilitated inmicroreactors and should be considered a key advantage. Pressurisation may also allow dissolution ofreactant gasses within the flow stream.

ProblemsAlthough there have been reactors made for handling particles, microreactors generally do not tolerateparticulates well, often clogging. Clogging has been identified by a number of researchers as the biggesthurdle for microreactors being widely accepted as a beneficial alternative to batch reactors. So far, the so-called microjetreactor[6] is free of clogging by precipitating products. Gas evolved may also shorten theresidence time of reagents as volume is not constant during the reaction. This may be prevented byapplication of pressure.Mechanical pumping may generate a pulsating flow which can be disadvantageous. Much work has been

Glass Microreactors involvemicrofabricated structures toallow flow chemistry to beperformed at a microscale.Applications include CompoundLibrary Generation, ProcessDevelopment and CompoundSynthesis

devoted to development of pumps with low pulsation. A continuous flow solution is electroosmotic flow(EOF).Typically, reactions performing very well in a microreactor encounter many problems in vessels, especiallywhen scaling up. Often, the high area to volume ratio and the uniform residence time cannot easily be scaled.Corrosion imposes a bigger issue in microreactors because area to volume ratio is high. Degradation of fewm may go unnoticed in conventional vessels. As typical inner dimensions of channels are in the same orderof magnitude, characteristics may be altered significantly.

T reactorsOne of the simplest forms of a microreactor is a 'T' reactor. A 'T' shape is etched into a plate with a depth that maybe 40 micrometres and a width of 100 micrometres: the etched path is turned into a tube by sealing a flat plate overthe top of the etched groove. The cover plate has three holes that align to the top-left, top-right, and bottom of the'T' so that fluids can be added and removed. A solution of reagent 'A' is pumped into the top left of the 'T' andsolution 'B' is pumped into the top right of the 'T'. If the pumping rate is the same, the components meet at the topof the vertical part of the 'T' and begin to mix and react as they go down the trunk of the 'T'. A solution of productis removed at the base of the 'T'.

Applications

Synthesis

Microreactors can be used to synthesise material more effectively thancurrent batch techniques allow. The benefits here are primarily enabled bythe mass transfer, thermodynamics, and high surface area to volume ratioenvironment as well as engineering advantages in handling unstableintermediates. Microreactors are applied in combination withphotochemistry, electrosynthesis, multicomponent reactions andpolymerization (for example that of butyl acrylate). It can involve liquid-liquidsystems but also solid-liquid systems with for example the channel wallscoated with a heterogeneous catalyst. Synthesis is also combined with onlinepurification of the product.[1] Following Green Chemistry principles,microreactors can be used to synthesize and purify extremely reactiveOrganometallic Compounds for ALD and CVD applications, with improvedsafety in operations and higher purity products.[7][8]

In one microreactor study a Knoevenagel condensation[9] was performedwith the channel coated with a zeolite catalyst layer which also serves toremove water generated in the reaction:

A Suzuki reaction was examined in another study[10] with a palladium catalyst confined in a polymer network ofpolyacrylamide and a triarylphosphine formed by interfacial polymerization:

The combustion of propane was demonstrated to occur at temperatures as low as 300C in a microchannel setupfilled up with an aluminum oxide lattice coated with a platinum / molybdenum catalyst:[11]

Analysis

Microreactors can also enable experiments to be performed at a far lower scale and far higher experimental ratesthan currently possible in batch production, while not collecting the physical experimental output. The benefits hereare primarily derived from the low operating scale, and the integration of the required sensor technologies to allowhigh quality understanding of an experiment. The integration of the required synthesis, purification and analyticalcapabilities is impractical when operating outside of a microfluidic context.

NMR

Researchers at the Radboud University Nijmegen and Twente University, the Netherlands, have developed amicrofluidic high-resolution NMR flow probe. They have shown a model reaction being followed in real-time. Thecombination of the uncompromised (sub-Hz) resolution and a low sample volume can prove to be a valuable toolfor flow chemistry.[12]

Infrared Spectroscopy

Mettler Toledo and Bruker Optics offer dedicated equipment for monitoring with attenuated total reflectancespectrometry (ATR spectrometry) in microreaction setups. The former has been demonstrated for reactionmonitoring.[13] The latter has been successfully used for reaction monitoring[14] and determing dispersioncharacteristics[15] of a microreactor.

Academic research

Glass Microreactor. The channels ofthe chip in the picture are 150 mwide and 150 m deep.

Microreactors, and more generally, micro process engineering, are the subject of worldwide academic research. Aprominent recurring conference is IMRET, the International Conference on Microreaction Technology.Microreactors and micro process engineering have also been featured in dedicated sessions of other conferences,such as the Annual Meeting of the American Institute of Chemical Engineers (AIChE), or the InternationalSymposia on Chemical Reaction Engineering (ISCRE). Research is now also conducted at various academicinstitutions around the world, e.g. at the Massachusetts Institute of Technology (MIT) in Cambridge/MA, Universityof Illinois Urbana-Champaign, Oregon State University in Corvallis/OR, at University of California, Berkeley inBerkeley/CA in the United States, at the EPFL in Lausanne, Switzerland, at Eindhoven University of Technology inEindhoven, at Radboud University Nijmegen in Nijmegen, Netherlands and at the LIPHT [1] (http://www-lipht.u-strasbg.fr/Interface/index.php) of Universit de Strasbourg in Strasbourg and [2](http://www.lgpc.fr/Objets%7CLGPC) of the University of Lyon, CPE Lyon, France.

Market structureDepending on the application focus, there are various hardware suppliersand commercial development entities to service the evolving market. Oneview to technically segment market, offering and market clearing stemsfrom the scientific and technological objective of market agents:

a. Ready to Run (turnkey) systems are being used where theapplication environment stands to benefit from new chemicalsynthesis schemes, enhanced investigational throughput of up toapproximately 10 - 100 experiments per day (depends on reactiontime) and reaction subsystem, and actual synthesis conduct atscales ranging from 10 milligrams per experiment to triple digit tonsper year (continuous operation of a reactor battery).

b. Modular (open) systems are serving the niche for investigations oncontinuous process engineering lay-outs, where a measurableprocess advantage over the use of standardized equipment is anticipated by chemical engineers. Multipleprocess lay-outs can be rapidly assembled and chemical process results obtained on a scale ranging fromseveral grams per experiment up to approximately 100 kg at a moderate number of experiments per day (3-15). A secondary transfer of engineering findings in the context of a plant engineering exercise (scale-out)then provides target capacity of typically single product dedicated plants. This mimics the success ofengineering contractors for the petro-chemical process industry.

c. Dedicated developments. Manufacturer of microstructured components are mostly commercial developmentpartners to scientists in search of novel synthesis technologies. Such development partners typically excel inthe set-up of comprehensive investigation and supply schemes, to model a desired contacting pattern orspatial arrangement of matter. To do so they predominantly offer information from proprietary integratedmodeling systems that combine computational fluid dynamics with thermokinetic modelling. Moreover, as arule, such development partners establish the overall application analytics to the point where the critical initialhypothesis can be validated and further confined.

References1. ^a b c d Recent advances in synthetic micro reaction technology Paul Watts and Charlotte Wiles Chem. Commun.,

2007, 443 - 467, doi:10.1039/b609428g (http://dx.doi.org/10.1039%2Fb609428g)

Example of a flow reactor system.

2. ^a b Schubert, K.; Brandner, J.; Fichtner, M.; Linder, G.; Schygulla, U.; Wenka, A. (January 2001)."Microstructure Devices for applications in thermal and chemicalprocess engineering". Microscale Thermophysical Engineering (Taylor& Francis) 5 (1): 1739. doi:10.1080/108939501300005358(http://dx.doi.org/10.1080%2F108939501300005358). ISSN 1556-7265 (//www.worldcat.org/issn/1556-7265).

3. ^ D.Roberge, L.Ducry, N.Bieler, P.Cretton, B.Zimmermann, Chem.Eng. Tech. 28 (2005) No. 3, online available

(http://www.lonza.com/group/en/company/news/publications_of_lonza.-ParSys-0002-ParSysdownloadlist-0001-DownloadFile.pdf/1_050510_Microreactor%20Technology%20A%20Revolution%20for%20the%20Fine%20Chemical%20and%20Pharmaceutical%20Industries.pdf)

4. ^ T.Schwalbe, V.Autze, G.Wille: Chimica 2002, 56, p.636, see also Microflow Synthesis(http://www.mrsp.net/MRSP_Chimica_Oggi.pdf)

5. ^ T.Schwalbe, V.Autze, M. Hohmann, W. Stirner: Org.Proc.Res.Dev 8 (2004) p. 440ff, see also Continuousprocess research and implementation from laboratory to manufacture (http://www.mrsp.net/MRSP_lo-res.pdf)

6. ^ Wille, Ch; Gabski, H.-P; Haller, Th; Kim, H; Unverdorben, L; Winter, R (2003). "Synthesis of pigments in athree-stage microreactor pilot plantan experimental technical report". Chemical Engineering Journal 101 (1-3):179185. doi:10.1016/j.cej.2003.11.007 (http://dx.doi.org/10.1016%2Fj.cej.2003.11.007). and literature citedtherein

7. ^ Method of Preparing Organometallic Compounds Using Microchannel Devices, 2009, Francis Joseph Lipiecki,Stephen G. Maroldo, Deodatta Vinayak Shenai-Khatkhate, and Robert A. Ware, US 20090023940(http://www.freepatentsonline.com/y2009/0023940.html)

8. ^ Purification Process Using Microchannel Devices, 2009, Francis Joseph Lipiecki, Stephen G. Maroldo, DeodattaVinayak Shenai-Khatkhate, and Robert A. Ware, US 20090020010(http://www.freepatentsonline.com/y2009/0020010.html)

9. ^ Knoevenagel condensation reaction in a membrane microreactor Sau Man Lai, Rosa Martin-Aranda and KingLun Yeung Chem. Commun., 2003, 218 - 219, doi:10.1039/b209297b (http://dx.doi.org/10.1039%2Fb209297b)

10. ^ Instantaneous Carbon-Carbon Bond Formation Using a Microchannel Reactor with a Catalytic MembraneYasuhiro Uozumi, Yoichi M. A. Yamada, Tomohiko Beppu, Naoshi Fukuyama, Masaharu Ueno, and TakehikoKitamori J. Am. Chem. Soc.; 2006; 128(50) pp 15994 - 15995; (Communication) doi:10.1021/ja066697r(http://dx.doi.org/10.1021%2Fja066697r)

11. ^ Low temperature catalytic combustion of propane over Pt-based catalyst with inverse opal microstructure in amicrochannel reactor Guoqing Guan, Ralf Zapf, Gunther Kolb, Yong Men, Volker Hessel, Holger Loewe, JianhuiYe and Rudolf Zentel Chem. Commun., 2007, 260 - 262, doi:10.1039/b609599b(http://dx.doi.org/10.1039%2Fb609599b)

12. ^ A Microfluidic High-Resolution NMR Flow Probe Jacob Bart, Ard J. Kolkman, Anna Jo Oosthoek-de Vries,Kaspar Koch, Pieter J. Nieuwland, Hans (J. W. G.) Janssen, Jan (P. J. M.) van Bentum, Kirsten A. M. Ampt,Floris P. J. T. Rutjes, Sybren S. Wijmenga, Han (J. G. E.) Gardeniers and Arno P. M. KentgensJ. Am. Chem.Soc.; 2009; 131(14) pp 5014 - 5015; doi:10.1021/ja900389x (http://dx.doi.org/10.1021%2Fja900389x)

13. ^ Carter, Catherine F.; Lange, Heiko; Ley, Steven V.; Baxendale, Ian R.; Wittkamp, Brian; Goode, Jon G.; Gaunt,Nigel L. (19 March 2010). "ReactIR Flow Cell: A New Analytical Tool for Continuous Flow Chemical Processing".Organic Process Research & Development 14 (2): 393404. doi:10.1021/op900305v(http://dx.doi.org/10.1021%2Fop900305v).

14. ^ Minnich, Clemens B.; Kpper, Lukas; Liauw, Marcel A.; Greiner, Lasse (2007). "Combining reaction calorimetryand ATR-IR spectroscopy for the operando monitoring of ionic liquids synthesis". Catalysis Today 126 (1-2): 191195. doi:10.1016/j.cattod.2006.12.007 (http://dx.doi.org/10.1016%2Fj.cattod.2006.12.007).

15. ^ Minnich, Clemens B.; Sipeer, Frank; Greiner, Lasse; Liauw, Marcel A. (16 June 2010). "Determination of theDispersion Characteristics of Miniaturized Coiled Reactors with Fiber-Optic Fourier Transform Mid-infrared

Spectroscopy". Industrial & Engineering Chemistry Research 49 (12): 55305535. doi:10.1021/ie901094q(http://dx.doi.org/10.1021%2Fie901094q).

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