Micro-reactors fabrication

Utilizing a commercially available micro reactor, fabricated from Foturan glass, Ryu and co-workers [86] evaluated a series of [2 + 2] cycloadditions as a means of reducing the reaction times conventionally associated with the synthetic transformation. Using a high-pressure mercury lamp (300 W), the reaction of cyclohex-2-enone (155) with vinyl acetate (14-7) (Scheme 6.40), to afford the cydoadduct 156, was used to compare photochemical efficiency within the micro reactor [1000 pm (width) x 500 pm (depth)] and a conventional batch reactor (10 ml). 

To realize micro reactors with sensing and surface-control structures, the underlying fabrication methods have to go beyond MEMS and classical micro machining [75]. Plastic and glass manufacture and also novel innovative methods such as soft lithography have to be developed further and applied. 

Before showing some examples of press releases and their content, we shall briefly shortly sum up all the information given. Most frequently the relationship of microreaction technology to the development of microelectronics is cited, suggesting a similar success story. Expectations are created that some day micro reactors will be mass fabricated at low cost in a similar way. In addition, it is believed that compactness can be achieved as for the integration of functions in the microelectronics world. In this context, often the vision of a shoebox-sized plant or a plant on a desk is given. 

Daumengrofes Labor aus Aluminium-Folie, Blick durch die Wirtschafi, June 1997 Heterogeneous gas-phase micro reactor micro-fabrication of this device anodic oxidation of aluminum to porous catalyst support vision of complete small laboratory numbering-up development of new silicon device [225]. 

Sichere Chemie in Mikroreaktoren, Frankjurter Allgemeine Zeitung, December 1995 Plant cells as model for micro-reactor development micro-fabrication techniques DuPont s investigations DECHEMA s initiation of micro-reactor platform BASF s investigations general advantages of micro flow [238]. 

The design and fabrication of some gas-phase micro reactors are oriented on those developed for chip manufacture in the framework of microelectronics, relying deeply on silicon micromachining. There are obvious arguments in favor the infrastructure exists at many sites world-wide, the processes are reliable, have excellent standards (e.g. regarding precision) and have proven mass-manufacturing capability. In addition, sensing and control elements as well as the connections for the whole data transfer (e.g. electric buses) can be made in this way. 

Figure 3.18 Schematic drawing of fabrication steps for Si-based micro reactors.

This carbon-carbon bond-generating reaction can be used extensively over a wide range of chemistries [11]. As the reaction is an equilibrium process, needing the removal of water to obtain high yields, chemical means have to be used to accomplish this task. 1,3-Dicyclohexylcarbodiimide (DCC) is a commonly used reagent for this purpose. Alternatively, molecular sieves find use for conventional processing, but are not so favorable for micro-reactor processing, because the sieve needs to be inserted into the micro channel (additional fabrication expenditure) and may disrupt the liquid transport if EOF is applied. 

The micro reactor properties concern process control in the time domain and process refinement in the space domain [65]. As a result, uniform electrical fields are generated and efficiency is thought to be high. Furthermore, electrical potential and currents can be directly measured without needing transducer elements. The reactor fabrication methods for electrical connectors employ the same methods as used for microelectronics which have proven to satisfy mass-fabrication demands. 

Later, Pattekar and Kothare [21] presented a silicon reactor fabricated by deep reactive ion etching (DRIE). It carried seven parallel micro channels of 400 pm depth and 1 000 pm width filled with commercial Cu/ZnO catalyst particles (from Siid-Chemie) trapped by a 20 pm filter, which also was made by DRIE, in the reactor. The reactor was covered by a Pyrex wafer applying anodic bonding. Details of the reactor are shown in Figure 2.3. 

Wang, X., Zhu, J., Bau, H., Gorte, R. J., Fabrication of micro-reactors using tapecasting methods, Catal. Lett. 2001, 77, 173-177. 

Micro structured wells (2 mm x 2 mm x 0.2 mm) on the catalyst quartz wafer were manufactured by sandblasting with alumina powder through steel masks [7]. Each well was filled with mg catalyst. This 16 x 16 array of micro reactors was supplied with reagents by a micro fabricated gas distribution wafer, which also acted as a pressure restriction. The products were trapped on an absorbent plate by chemical reaction, condensation or absorption. The absorbent array was removed from the reactor and sprayed with dye solution to obtain a color reaction, which was then used for the detection of active catalysts by a CCD camera. Alternatively, the analysis was also carried out with a scanning mass spectrometer. The above-described reactor configuration was used for the primary screening of the oxidative dehydrogenation of ethane to ethylene, the selective oxidation of ethane to acetic acid, and the selective ammonoxidation of propane to acrylonitrile. 

An example of a photochemical cyclization was reported by Takei et al. [92], who demonstrated the synthesis of L-pipecolinic acid (160) from an aqueous solution of L-lysine (161), as illustrated in Scheme 6.42. To achieve this photocatalytic transformation, the authors fabricated a Pyrex micro reactor in which the channel cover plate was coated with a 300nm layer of anatase Ti02 (100 nm particles), to afford a titania-coated micro reactor (TCM) the titania film was subsequently loaded with platinum (0.2 wt%), by photodeposition, to enable the TCM to be used... 

This paper summarizes the recent developments in micro-fabrication and its applications in PI and tissue engineering which is complementary to phenomenon-based BI. In such intensified processes, miniaturization is essential and therefore micro-reactors... 

We have also provided evidence that the behavior of microorganisms in confined micro-environment is substantially different and that their desired metabolic activities can be maximized through the modification of the surface characteristics as well as the size of the pores. These characteristics can therefore be utilized in BI as well as in the enhancement of cell penetration and cell proliferation in tissue engineering and when such polymers are grafted. Micro-fabrication technique has also been used in the development of highly porous catalysts with arterial channels feeding nano-pores which provide an extended surface area. Such materials can be used as micro-reactors as well as catalysts. 

A third way to build up pFCs based on MEMS-polymers such as poly-dimethylsiloxane (PDMS) or polymethyl methacrylate (PMMA) or PCB-materials such as polyimid (PI) or FR4. These polymers can be micro-machined by molding or by laser ablation. Shah et al. [22,23] have developed a complete PEMFC system consisting of a PDMS substrate with micro-flow channels upon which the MEA was vertically stacked. PDMS micro-reactors were fabricated by employing micro-molding with a dry etched silicon master. The PDMS spin coated on micro-machined Si was then cured and peeled off from the master. The MEA employed consisted in a Nafion - 12 membrane where they have sputtered Pt through a Mylar mask. Despite an interesting method, this FC gave poor results, a power density of 0.8 mW cm was achieved. 

A same approach has been used by Karnik and co-workers who successfully produced Pd-based membrane via micro-fabrication and used it for hydrogen separation. The authors were able to design and fabricate a micro-reactor that was used for hydrogen separation however, the support (or a part of it) for the micro-membrane is made of copper, which is also active catalyst for WGS reaction. The reactor can be thus used for on-board hydrogen production for micro-fuel cell applications. 

A micro-reactor is generally defined as a device consisting of a number of interconnecting micro-channels in which small quantities of reagents are manipulated, mixed and allowed to react for a specified period of time (Ehrfeld et al., 2000 Wirth, 2008). The movement of fluids within such a device can be achieved in a number of ways with the most common being mechanical micro-pumping and electro-osmotic flow, which may include electrophoresis separations. The typical cross-sectional dimensions of such micro-channels are in fhe range of 10-500 pm and are normally fabricated on the planer surface of substrates... 

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