Ind. Eng. Chem. Res. 2007, 46, 1153-1160
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Microfabricated Multiphase Reactors for the Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen Tomoya Inoue,†,‡ Martin A. Schmidt,§ and Klavs F. Jensen*,‡ Department of Chemical Engineering and Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Hydrogen peroxide synthesis is demonstrated by direct combination of hydrogen and oxygen over supported palladium catalysts in a microchemical reactor. The direct combination process is chemically simple and environmentally benign for producing hydrogen peroxide, but the risk of handling the explosive gas mixture of hydrogen and oxygen over an active palladium catalyst limits implementation. By using a multichannel microchemical reactor with packed-bed catalyst, we realize the direct reaction of hydrogen and oxygen at hydrogen/oxygen ratios in the explosive regime at pressures of 2-3 MPa. As long as millimeter-sized void spaces are avoided, the microchannel structure and catalyst packaging effectively promote the heterogeneous reaction over the homogeneous free radical branching reactions that otherwise would lead to an explosion. Among the Pd/Al2O3, Pd/SiO2, and Pd/C catalysts investigated, Pd/C selectively yields hydrogen peroxide. The decomposition of peroxide is shown to be suppressed by the addition of bromide. Analysis of the microreactor data reveals significantly enhanced mass transfer relative to conventional reactors, consistent with previous multiphase microreactor studies. 1. Introduction Hydrogen peroxide has a wide range of applications, including organic oxidation, bleaching, wastewater treatment, and semiconductor wafer cleaning.1 It has environmental advantages over other agents, e.g., chloride based processes, since the byproduct is water.2 Hydrogen peroxide is primarily produced by the anthraquinone process. At first, 2-alkyl-9,10-anthraquinones are hydrogenated with palladium catalyst, and the product is subsequently oxidized by air to produce hydrogen peroxide. Produced hydrogen peroxide is extracted from the working solution with water, and this aqueous solution is purified and hydrogen peroxide is concentrated by distillation. This process is safe, and easily scaled-up, but anthraquinone decomposes gradually during the redox cycle. The extraction of the resulting byproducts complicates the process and increases energy costs.1 Alternative processes for hydrogen peroxide include electrochemical synthesis and direct combination of hydrogen and oxygen.1,3,4 The latter process has been extensively studied because it is chemically simple and environmentally friendly.4-6 The catalyzed reaction is typically conducted over supported palladium or palladium modified with a small amount of platinum in a liquid mixture of sulfuric acid, phosphoric acid, and bromide.7 The composition of the reaction mixture is important in stabilizing the hydrogen peroxide.7,8 Recently, catalysts based on supported gold catalysts have been proposed as efficient catalysts for this reaction.9-11 Widespread application of the direct synthesis has been limited by the risks inherent in handling an explosive mixture of hydrogen and oxygen. Moreover, high pressures from 2 to 10 MPa are typically needed to promote mass transfer from * To whom correspondence should be addressed. E-mail: kfjensen@ mit.edu. Fax: (617) 258-8224. † Current Address: Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Sendai, Miyagi 983-8551, Japan. ‡ Department of Chemical Engineering. § Microsystems Technology Laboratories.
the gas to liquid phase. Several approaches have been explored to implement the process safely. For example, in a batch reactor process, gas mixing was modified in order to maintain welldispersed gas-bubble distribution.12,13 Membrane reactors have also been considered as a means to safely deliver hydrogen into the reaction system.14,15 In addition, Beckman and co-workers synthesized hydrogen peroxide in supercritical carbon dioxide to enhance mass transfer from gas to aqueous phase.16-18 Recent efforts in process miniaturization and microreaction technology demonstrated safety advantages in conducting highly reactive chemical processes, such as direct fluorination and oxidation.19-28 The safety characteristics of a microreactor derive from the increased heat dissipation, typically by a factor of ∼100, compared with conventional reactors, owing to higher surface-to-volume ratios.29 Furthermore, the microstructure inside the reactors helps to generate high interfacial area between gas and liquid, which produces a corresponding approximately 100-fold enhancement in mass transfer.30,31 These characteristics of microreactors have generated considerable interest in the use of microreactors for direct synthesis of hydrogen peroxide,32-34 including the possible scale-up of microreactor technology to production units.34 Microreactors can be realized in a wide range of materials, including stainless steel,35-38 glass,39-41 ceramics,42 silicon,43 and polymers.44-47 We use silicon because it has a high mechanical strength-to-density ratio, excellent temperature characteristics, and good chemical compatibility. Oxidization of silicon forms a glass layer on the surfaces of the microreactor channels so that the reaction environment becomes the same as in a glass vessel. The fabrication infrastructure developed for microelectromechanical systems (MEMS)48 provides a series of integrated fabrication platforms for producing complex, threedimensional microchannel networks. Well-established wet and dry etching procedures49,50 enable fabrication of high-aspectratio microchannels with controlled sidewall shape and channel dimensions from micrometer to millimeter scales, which becomes particularly useful in creating microstructures for contacting gas-liquid mixtures with large interfacial areas for the direct synthesis of hydrogen peroxide.
10.1021/ie061277w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/17/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 4, 2007
Figure 1. Schematic of the 10-channel microreactor reactor. Top: plane view showing gas distribution (A), gas-liquid contacting area (B), catalyst loading channel (C), reaction channels, and exit section (D). The lines labeled A, B, C, and D correspond to the four cross sections below the microreactor drawing.
Connecting the microreactor to fluid feed and exit lines creates additional challenges in safely applying microreactors to high-pressure hydrogen and oxygen reactions. Compression sealing a device with O-rings to a machined holder that has macroscopic fluid connections adds additional thermal mass, and the mismatch between the coefficients of thermal expansion (CTE) of the chuck material and the silicon device can lead to reactor chip fracture. Herein, we introduce a connection (“packaging”) scheme that enables operation at elevated pressures (2-3 MPa) without the complication of a macroscopic holder. Details of microreactor fabrication and the overall reaction system are described in the following section along with the experimental procedures. Subsequent sections summarize the results of the direct synthesis of hydrogen peroxide from hydrogen and oxygen over different supported palladium catalysts. A model of the reactor system allows extraction of kinetic and transport parameters, which are shown to be in good agreement with macroscopic kinetic studies and related investigations of mass transfer in microreactors. 2. Experimental Details Reactor Design and Fabrication. The reactor design was based on a 10-channel microreactor fabricated by Losey and co-workers (Figure 1).30,31 The reactor was constructed following the procedures of Losey and co-workers as a three 100 mm wafer stack of a top Pyrex wafer (0.5 mm thick), middle silicon wafer (0.5 mm), and bottom silicon wafer (1 mm). The thick bottom wafer served to increase mechanical strength for the high-pressure reactions. Hydrogen or deuterium is supplied from the bottom inlet (Figure 1A) and guided into the reactor through channels fabricated at the top layer of silicon. Oxygen is fed through the bottom inlet (B) and distributed among the channels through the silicon structure in B. Hydrogen and oxygen flows are each split among 10 channels and then further divided so that the gases enter 25 µm wide slits creating large interfacial areas for rapid mixing. The reaction solution enters through the bottom holes shown in C and is then distributed to each reactor channel. Perpendicular to these inlet channels is a 400 µm wide channel for bringing the catalyst (typically as a slurry) to the 10 reaction channels. At the outlet of each of the 20 mm long and 625 µm wide reaction channels, a series of posts (25 µm spacing) etched in the silicon retain the catalyst particles. The depth of the channels was controlled by deep reactive ion etching (DRIE) to 350 µm. The fabrication processes involved multiple photolithographic and etch steps, a silicon fusion bond, and an anodic bond analogous to the procedure originally developed by Losey et al.31 Catalyst. We formulated palladium catalysts supported on activated carbon, alumina, and silica based on information in
the patent literature.5,7,8,12,13 Activated carbon supported catalyst (Pd/C) was purchased from Aldrich (palladium 5 wt %, 276707), and alumina supported catalyst (Pd/Al2O3) was purchased from Alfa Aesar (palladium 5 wt %). Silica supported catalyst was prepared in our laboratory by the following impregnation method. Palladium chloride (II) (Alfa Aesar, 99.9% metal basis) and platinum chloride (IV) (Alfa Aesar, 99.9% (metals basis)) were dissolved into 1.0 N hydrochloric acid completely (without crystalline residue), and silica gel (Alfa Aesar, silica gel, 60 angstroms, 400-600 mesh powder, surface area ) 500-600 m2/g) was then added to this solution. The resultant slurry was stirred overnight, slightly filtered, dried at 343 K, and calcined under a gas flow mixture of hydrogen and nitrogen at 573 K.12,13 Prior to loading the catalyst particles into the microreactor, the catalyst was sieved to collect the fraction between 50 and 75 µm. This catalyst fraction was suspended in deionized water (slurry content ≈ 0.5 wt %) and then pushed into the microreactor by a microsyringe. The microfabricated filter at the end of each reaction channel meant that the channel was packed with catalyst particles from the filter end toward the inlet. In order to minimize void space and avoid explosions in the inlet section, catalyst loading was followed by loading inert silica particles of the same size fraction. Catalyst-loaded microreactors were dried in an oven, and then inlet and exit tubes were attached to the chip as described in the subsequent section. Reactor-Macro Interface and Reactants Delivery. Since the direct hydrogen peroxide synthesis reaction requires high pressure (2.0 MPa or higher), we used the scheme shown in Figure 2 to interface with feed and exit lines. PEEK tubing (Upchurch, 1/16 in. o.d., 0.030 in. i.d.) was attached to the silicon microreactor reactor by epoxy (Devcon, 2 ton black) with 24 h curing at room temperature. In order to increase the adhesive area, 1/16 in. stainless steel ferrules (Valco Instruments Co. Inc., ZF1S6) were first placed on the PEEK tubing and the flat side of the ferrules was glued to the microreactor. The assembled reactor was placed on an aluminum plate (6 mm thick) (Figure 3a) with holes corresponding to inlet and exit tubes and then covered with a poly(methylmethacrylate) (PMMA) plate (0.6 mm thick), which also served as a shield in the case of an explosion fracturing the reactor. The reaction effluent was collected into a stainless steel accumulator that maintained pressure and separated gas and liquid effluents (Figure 3b). We purged the accumulator with nitrogen to dilute the hydrogen content to
