Ind. Eng. Chem. Res. 2005, 44, 2351-2358
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Silicon Micromixers with Infrared Detection for Studies of Liquid-Phase Reactions Tamara M. Floyd,†,‡ Martin A. Schmidt,§ and Klavs F. Jensen*,† Department of Chemical Engineering and Microsystems Technology Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
We present an integrated microchemical system that combines micromixing, a reaction channel, an IR detection region, and temperature control for monitoring and kinetic studies of liquidphase reactions. The microdevices exploit the transparency of silicon to infrared radiation in most of the wavelength region of interest (4000-800 cm-1), the precise definition of microfluidic channels by deep reactive ion-etching, the high thermal conductivity of silicon, and the fusion bonding of silicon for fixed-path-length transmission cells. Two devices are considered, a simple T-shaped mixer and an efficient mixer with interleaving channels for rapid mixing. The first device is used to characterize IR transmission characteristics in silicon-based microreactors and to demonstrate the feasibility of monitoring exothermic reactions, the hydrolysis of propionyl chloride under isothermal conditions. The mixing characteristics of the second microreactor are evaluated experimentally by an acid-base reaction and predicted by computational fluid dynamics simulations. Typical mixing times are 25 ms. The alkaline hydrolysis of methyl formate, a reaction following second-order kinetics with a half-life of 70 ms, exemplifies the use of the microreactor in determining rate constants. The results demonstrate the main advantages of the integrated microchemical systems in reaction monitoring: faster mixing times, temperature control, in situ detection, and elimination of sample postprocessing. Introduction Continuous-flow reaction methods are popular tools for kinetics studies of reactions in solution.1 After initiation by a mixing step, the reaction proceeds as the liquid flows down a reaction conduit. Detectors placed downstream from the mixing point measure the concentrations of the reactants/products, and kinetic parameters are determined by manipulation of the data. After an initial phase during which the reaction mixture reaches the detection region, the concentrations reach steady state. Consequently, the detector need not have a fast response, but a slower detection method requires a larger volume of reagents. Fused silica systems fabricated using conventional methods have typically been used in such studies2-5 because fused silica is optically transparent in the wavelength regions needed for incorporation of ultraviolet-visible (UV-vis) absorption spectroscopy measurements. Microfabricated chemical systems, also known as micrototal analysis systems, are emerging as the nextgeneration tools for studying kinetics of fast liquid-phase reactions.6-8 The precisely controlled micrometer (and submicrometer) scale mixing lengths realized through microfabrication techniques9-12 correspond to fast mixing times, making it feasible to determine the kinetics of faster reactions. Several techniques have been explored for monitoring species concentration, including fluorescence measurements, electrochemical, optical * To whom correspondence should be addressed. Tel.: (617) 253-4589. Fax: (617) 258-8224. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Current address: Department of Chemical Engineering, Tuskegee University, 522A Luther H. Foster Hall, Tuskegee, AL 36088. § Microsystems Technology Laboratory.
absorption [UV-visible and infrared (IR) wavelengths], and nuclear magnetic resonance (NMR) methods.6-8 Fourier transform infrared spectroscopy (FTIR) methods are efficient, broadly applicable techniques for determining chemical structure and quantifying species concentrations. As a result, FTIR has been implemented in microsystems by a variety of approaches. Examples include capping microchannel structures with AgCl and CaF2 disks,13-15 using silicon wafers as an IR window in a poly(tetrafluoroethylene) (PTFE) flow cell,16 and integrating microreactors17 and micromixers18 with offchip FTIR detection. Here, we present an all-silicon, integrated microchemical system combining micromixing, reaction, IR detection, and temperature control. Silicon is transparent to IR radiation in most of the wavelength region of interest (4000-800 cm-1), and microfluidic channels can be precisely defined in silicon by deep reactive ion-etching (DRIE).19 Moreover, fusion bonding20 of silicon wafers enables the formation of hermetic seals around fixed-path-length transmission cells. The relatively high thermal conductivity of silicon combined with microchannels ensures good thermal uniformity. Silicon also has broad chemical compatibility. In cases where wall reactions are possible, a thin SiO2 layer can be formed by oxidation to increase chemical compatibility, but this layer is still sufficiently thin (∼500 nm) to not adversely impact the IR transmission characteristics of the device. In fact, for transmission studies, dielectric coatings of silicon oxide and nitride can be used to reduce reflections and improve transmission through the flow cell.21 Over the past decade, a number of silicon-based microchemical systems have been demonstrated for homogeneous and heterogeneous reactions.22-26 Combining these microreactor concepts with IR spectroscopy provides an opportunity for extracting chemical kinetics as well as
10.1021/ie049348j CCC: $30.25 © 2005 American Chemical Society Published on Web 12/08/2004
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Figure 1. T-shaped microreactor for IR monitoring of liquid-phase reactions and gasket (IR shutter). Picture shows buried channel structure.
identifying optimal reaction conditions. In the following sections, we describe the design, microfabrication, and characterization of silicon-based microchemical systems combining micromixing, a reaction channel, an IR detection region, and temperature control. Experimental Section Microfabrication. Two all-silicon microreactors were designed and fabricated for the present study. The microreactors were fabricated in a class 100 clean room environment by using standard silicon microfabrication tools. The microreactor chips were designed as disks (25 mm diameter) to fit into a standard liquid-phase sampling setup used for FTIR spectroscopy. The first reactor was a simple T-shaped contactor (Figure 1) with 50 µm deep channels. This simple geometry was chosen to explore issues in implementing FTIR monitoring in silicon microreactors. The device was fabricated using two double-sided polished 100 mm silicon wafers. The channel pattern was defined on the first wafer with photoresist and etched by deep reactive etching (DRIE).19 Next, the patterned silicon wafer and a blank silicon wafer were cleaned and contacted. The wafer pair was then annealed at 1100 °C for 1 h to complete a fusion bond20 between the two wafers. Characterization of the bond revealed a successful bond with no visible bond line (Figure 1). After bonding, the microreactor inlet and outlet ports were patterned and etched by DRIE. A thin ring was patterned and etched in the same process steps to release the individual microreactor chipssfour per wafer. A thin elastomer (Viton, the DuPont Co., Wilmington, DE) gasket served the dual purpose of sealing the device in the sample setup (see below) and limiting the transmitted IR radiation to the probe region. The gasket material shuttered the IR beam and isolated the transmission measurement to the region of interest. The second device design (Figure 2) consisted of three regions, a section of interleaving channels (∼400 µm deep) for rapid liquid mixing based on an earlier design,27 a parallel flow heat exchanger, and a wide, shallow depth (50 µm) IR monitoring region. IR detection for the conditions of this study required a probing depth 4 mL/min) would have required larger back pressures than possible with the current syringe pump setup. Alternatively, the probe point could be moved upstream. Neither of these options was pursued because the flow rate range 0.001-4.0 mL/ min corresponding to residence times of 3.2-12700 ms was more than sufficient for the FTIR studies. The CFD simulations provide further insight into the details of the mixing behavior in the device (Figure 8). Cross sections of the channel at different locations show progress in mixing as the reactant streams are interleaved and focused. The no-slip condition at the solid walls implies that the fluid moves slower adjacent to the walls, and therefore diffusion leads to a larger spread in concentration as compared to the center of the flow channel. This behavior is most pronounced immediately after the inlet section (Figure 8b), and it causes the concentration distribution to “fan-out” at the top and bottom of the channel. Consistent with observations by Branjeberg et al.,33 the outer layer widths mix slower that the inner layers because the outermost layers effectively have to diffuse across the entire
reactor width, whereas the inner layers only have to diffuse into the adjacent layer. This effect can be addressed by making the flows in the two outside channels approximately half the flows in the inner layers. IR Transmission and Measurement of FTIR Spectra. Silicon wafer thickness, concentration and nature of dopants, and reflections from the silicon surface adversely inpact IR transmission and reduce the signal-to-noise ratio for liquid-phase, FTIR detection. Undoped float zone prepared silicon has the least absorption but is expensive. Generally, resistivities (F) should be greater than 10 Ω‚cm for boron-doped samples and greater than l Ω‚cm for phosphorus-doped samples. Although the transmission (∼40%) obtained using uncoated samples is sufficient for most applications, using an antireflective (AR) coating increases transmission through silicon. These AR coatings, however, need to be chemically compatible and should not complicate microfabrication of the device. Lightly doped (F ) 1050 Ω‚cm silicon with boron; p type) was chosen for the present study because it has a good IR transmission (41.7%) and is commercially readily available. Dielectric coatings are generally used to minimize reflections and thus increase transmission. A single, homogeneous antireflection coating with a coating representing a quarter wavelength in thickness (δ ) λ/4n, λ is wavelength and n is the index of refraction of the coating) is the simplest method for reducing reflections. Reflectance cannot be completely eliminated for a scanning spectrometer, but zero reflectance at a single wavelength can be achieved by selecting the index of refraction according to
n ) xn1n2
(4)
where n1 and n2 are the refractive indices of silicon and the fluid.21 It is not always possible to find a coating
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Figure 9. FTIR data of the progress of hydrolysis of propionyl chloride: (1) 2.4 s, (2) 4.9 s, (3) 48.6 s, (4) 81 s, and (5) 243 s, 0.4 M propionyl chloride at 23 °C.
material with the exact optical properties, but using a material with an index of refraction approximating that of eq 4 significantly reduces reflections. In that case, the residual reflectance at the desired wavelength is given by
R)
(n1n2 - n2)2 (n1n2 + n2)2
(5)
We chose to use 0.1 µm silicon dioxide coatings because they are readily produced by oxidation of silicon, integrate well with the fabrication process, and are chemically compatible with any reaction that can be performed in glassware. For this choice of coating, the ratio of the reflection of the coated sample to the uncoated sample is 0.64 for a single reflection. Multiple reflections within the silicon microreactor further reduce transmission, but the silicon microreactor system provides IRthroughput comparable to commercial sampling systems, with the added advantage of an exact path length controlled by microfabrication and not by O-ring seals to fragile IR window materials (e.g., CaF2 and KBr). The hydrolysis of propionyl chloride in tetrahydrofuran (Figure 9) serves as a model reaction for FTIR monitoring of the T-microreactor. The reaction is sufficiently fast that appreciable conversion is achieved for typical contact times for microreactors (
