Microreactor systems

The benefits of microreactors are many more ideal temperature exchange for gas phase reactions they enable the reaction process to take place in an electric field and the performance of complex catalytic reactions can be simplified [18]. These benefits will also provide access to novel structures. Additionally, the bottleneck caused by limited availability of unusual building blocks and scaffolds will be eased, or altogether eliminated, by microreactor operations and miniaturized screening formats. The advent of microreactor systems for chemical synthesis not only opens the door for direct integration of synthesis and screening, but also provides better access to novel compounds. 

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The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

Lohf, a., Lowe, H., Hessel, V., Ehefeld, W., A standardized modular microreactor system, in Proceedings of the 4th International Conference on Microreaction Technology, IMRET 4, pp. 441 51 (5-9 March 2000), AIChE Topical Conf Proc., Atlanta, USA. 

Garcia-Egido, E., Wong, S. Y. E., A Hantzsch synthesis of 2-aminothiazoles performed in a microreactor system, in Ramsey, J. M., van den Berg, A. (Eds.), Micro Total Analysis Systems,... 

Ehreeld, W., Gebauer, K., Lowe, H., Richter, T., Synthesis of ethylene oxide in a catalytic microreactor system. Stud. 

Srinivasan, R., Schmidt, M.A., Harold, M. P., Lerou, J. J., Ryley, J. F., Reaction engineering for microreactor systems, in Ehreeld, W. (Ed.), Microreaction Technology - Proc. of the 1st International Conference on Microreaction Technology, IMRET 1, pp. 2-9, Springer-VerJag, Berlin (1997). 

Shaw, J., Turner, C., Miller, B., Harper, M., Reaction and transport coupling for liquid and liquid/gas microreactor systems, in Ehreeld, W., Rinaed,... 

JOVANOVIC, G., Sacrittichai, P., Toppinen, S., Microreactors systems for dechlorination of p-chlorophenol on palladium based metal support catalyst theory and experiment, in Proceedings of the 6th International Conference on Microreaction Technology, IMRET 6, 11-14 March 2002, pp. 314-325, AIChE Pub. No. 164, New Orleans (2002). 

The NSR capability of the catalysts was investigated under transient conditions in a flow microreactor system with samples in the powder form. 

In each run, 120 mg of catalyst (75-100 xm) were used and a total flow rate by 200 cm3/min STP was maintained in the different phases. The flow microreactor system... 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

Fig. 9. Pulse microreactor system for use with 13C-labeled hydrocarbons. D, E, and J are microreactors J contains the catalyst to be used for hydrocarbon skeletal reaction D and E are used, when necessary, to generate the required reactant hydrocarbon from a non-hydrocarbon precursor (e.g., alcohol dehydration in D and olefin hydrogenation in E) reactant injected at C. F is a trap which allows the accumulation of products from several reaction pulses before analysis G is a G.P.C. column, K a katharometer. Traps H collect fractions separated on G for subsequent mass spectrometric study. When generating reactant hydrocarbon in D and E, a two-step process is preferable in which, with J below reaction temperature, the purified reactant hydrocarbon is collected in H, and this is recycled as reactant with D and E below reaction temperature but with J at reaction temperature. After C. Corolleur, S. Corolleur, and F. G. Gault, J. Catal. 24, 385 (1972).

Fig. la-e. Selected microreactors, a Stainless steel microreactor system designed by Ehrfeld Mikrotechnik. b Glass microreactor (Watts and Haswell 2005). c Stainless steel microreactor of the CYTOS Lab system (http //www.cpc-net.com/cytosls.shtml). d Silicon-based microreactor designed by Jensen (Ratner et al. 2005). e Glass microreactor of the AFRICA System... 

Stainless steel is the material of choice for process chemistry. Consequently, stainless steel microreactors have been developed that include complete reactor process plants and modular systems. Reactor configurations have been tailored from a set of micromixers, heat exchangers, and tube reactors. The dimensions of these reactor systems are generally larger than those of glass and silicon reactors. These meso-scale reactors are primarily of interest for pilot-plant and fine-chemical applications, but are rather large for synthetic laboratories interested in reaction screening. The commercially available CYTOS Lab system (CPC 2007), offers reactor sizes with an internal volume of 1.1 ml and 0.1 ml, and modular microreactor systems (internal reactor volumes 0.5 ml to... 

Polymer-based microreactor systems [e.g., made of poly(dimethyl-siloxane) (PDMS)], with inner volumes in the nanoliter to microliter range (Hansen et al. 2006), are relatively inexpensive and easy to produce. Many solvents used for organic transformations are not compatible with the polymers that show limited mechanical stability and low thermal conductivity. Thus the application of these reactors is mostly restricted to aqueous chemistry at atmospheric pressure and temperatures for biochemical applications (Hansen et al. 2006 Wang et al. 2006 Duan et al. 2006). 

Metal-catalyzed cross-couplings are key transformations for carbon-carbon bond formation. The applicability of continuous-flow systems to this important reaction type has been shown by a Heck reaction carried out in a stainless steel microreactor system (Snyder et al. 2005). A solution of phenyliodide 5 and ethyl acrylate 6 was passed through a solid-phase cartridge reactor loaded with 10% palladium on charcoal (Scheme 2). The process was conducted with a residence time of 30 min at 130°C, giving the desired ethyl cinnamate 7 in 95% isolated yield. The batch process resulted in 100% conversion after 30 min at 140°C using a preconditioned catalyst. 

The microreactor system used was the commercial CYTOS College System [18]. The reactor is made of stainless steel, has 100 ptm channels and 2 ml volume. It has two inlets operated by two piston pumps. An additional 45 ml residence time unit (RTU) is coimected to the system after the reactor itself to increase the reaction time. The parts of the device are comiected by polytetrafluoroethylene (PTFE) tubings. 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

The microreactor system consists of a pumping module (R2+) and a four-channel heated component (R4). Two independently conducted flow streams are mixed in a T-piece and driven through a convection-flow coil (CFC, volume 10 ml) made of poly(fluoroacetate) (PFA). After the CFC, the flow is guided through Omnifit glass columns [41] packed with immobilized scavengers. 

The sequence includes several synthetic steps over polymer-supported catalysts in directly coupled commercially available Omnifit glass reaction columns [41] using a Syrris Africa microreactor system [14], Thales H-Cube flow hydrogenator [32] and a microfluidic chip. The process affords the alkaloid in 90% purity after solvent evaporation, but in a moderate 40% yield. After a closer investigation it was concluded that this is due to the poor yield of 50% in the phenolic oxidation step. On condition that this is resolved with the use of a more effective supported agent, the route would provide satisfactory yields and purities of the product. 

Because of the very small fluid layer thickness of the microchannels, the specific interfacial areas (i.e., the interfacial areas per unit volume of the microreactor systems) are much larger than those of conventional systems. 

Because of the very small fluid channels (Re is very small), the flows in microreactor systems are always laminar. Thus, mass and heat transfers occur solely by molecular diffusion and conduction, respectively. However, due to the very small transfer distances, the coefficients of mass and heat transfer are large. Usually, film coefficients of heat and mass transfer can be estimated using Equations 5.9b and 6.26b, respectively. 

As a result of the first two points, microreactor systems are much more compact than conventional systems of equal production capacity. 

There are no scaling-up problems with microreactor systems. The production capacity can be increased simply by increasing the number of microreactor units used in parallel. 

Specimens of catalysts (0.125 gram) were deactivated at 360° C for desorption experiments by using continuous (rather than pulsed) operation. Purified liquid benzene or cumene was pumped to the injection port of the microreactor system with a syringe pump at the rate of 0.00241 moles/hour. Propylene was fed from a gas lecture bottle through a rotameter at a rate of 0.00245 moles/hour. Parent H-mordenite catalyst samples were de-... 

The catalytic activity of SBA and AISBA samples toward cumene cracking were tested in a continuous flow fixed-bed microreactor system with helium (25 mL min 1) as carrier gas. The catalyst load for the tests was 100 mg and the catalyst was preheated at 573 K under helium flow for 3 h. For the reaction, a stream of cumene vapor in helium was generated using a saturator at room temperature. The reaction products were analyzed by gas chromatography. 

Lee, D.S. and Gloyna, E.E, Supercritical water oxidation microreactor system, paper presented at Water Poll. Control Fed. Specialty Conf. New Orleans, LA, April 17-19, 1988. 

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