Flow Distribution and Ozonolysis in Gas−Liquid Multichannel

In all cases, both conversion and selectivity are high, up to 100% at short contact times as low as 1 s. Supporting Information Available ..... measur...
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Flow Distribution and Ozonolysis in Gas-Liquid Multichannel Microreactors Yasuhiro Wada,†,‡ Martin A. Schmidt,§ and Klavs F. Jensen*,† Department of Chemical Engineering and Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Multichannel microreactors for gas-liquid reactions are designed and fabricated using silicon and Pyrex wafers. We perform oxidation of organic reagents by ozone (ozonolysis) in these microreactors motivated by the difficulty of handling ozone, its high reactivity, the potentially high selectivity for oxygenated products, and the reaction often being mass transfer limited. Pressure drop zones are included in the microreactors to achieve a uniform gas-liquid flow regime across the multiple channels. Flow visualization experiments show less than 3% variation in mass flow across the channel for a range of operating conditions. Microfabricated silicon posts are included in the channels as a means to increase mass transfer, and their effectiveness is evaluated by comparing conversion and selectivity to those obtained for microreactors without posts. Oxidations with ozone of phosphite, amine, and olefin as model compounds serve as case studies. In all cases, both conversion and selectivity are high, up to 100% at short contact times as low as 1 s. 1. Introduction Oxidation reactions form an important class of chemical reactions in laboratory investigations and industrial production.1 The combination of oxidizer and solvents often implies a potential for significant thermal effects, including thermal runaway. These risks can be mitigated by the small reactor volumes and high heat transfer rates characteristic of multichannel microreactors.2-4 The high mass transfer rates in gasliquid microreactors offset the reduction in size and enable small scale production through continuous operation and parallel operation of multiple units. Oxidation with ozone (ozonolysis) is a particularly interesting oxidation example for microreactor applications since ozone is difficult to handle, a strong oxidant, and provides opportunities for selective formation of oxidation products that are difficult to synthesize by common oxidation sources. Ozone is toxic, but it has the advantage of generating only oxygen as a side product. Reactions of olefins with ozone have not only high heats of reaction but they also form highly reactive side products such as peroxi-polymers.5 Consequently, ozonolysis requires significant precautions in laboratory use6,7 and raises challenges for industrial applications.7-9 The fast reaction combined with the low solubility of ozone (30 ppm in water at 20 °C10) further implies that the reaction is typically limited by transport of ozone into the liquid. The rate of this transport will be enhanced by the large surface-to-volume ratio (S/V) and high mass transfer rates inherent in microreactors. Reactions with ozone in small channels have been reported for analysis applications,11,12 but to our knowledge, this represents the first successful ozonolysis of organic compounds in a microreactor. Microreactors have been successfully demonstrated for other gas-liquid reactions, including fluorination13-15 and hydrogenation.3,16,17 Microreactor productivity increases by scaling from a single to multiple channels per unit.3,16,18 The number of channels depends on reactor design, reactor chip size, and * To whom correspondence should be addressed. Tel.: (617) 2534589 or -4561. Fax: (617) 258 8224. E-mail: [email protected]. † Department of Chemical Engineering. ‡ Present address: Mitsubishi Chemical Corp., Yokohama Research Center, Yokohama, Japan. § Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA 02139.

microfabrication challenges. Uniform distribution of gas and liquid flows to each reaction channel is a critical issue for gasliquid multichannel microreactors. Special distributor designs are typically needed to eliminate poor distribution resulting in different flow regimes within the device and adversely influencing reactor performance.18 Here, we describe a flow distribution scheme based pressure drop sections at the inlet of reaction channels to ensure uniform distribution of gas and liquids to 16 parallel channels formed in silicon substrates by deep reactive ion etching (DRIE) processes. As in macroscopic reactors, packing the microreactor with particles or a building structured packing increases the mass and heat transfer.3,16,19 The same DRIE process that forms the reaction channels can also create arrays of posts in each channel. The performance of microreactors with and without posts is evaluated with ozonolysis of model compounds. Although the high surface area-to-volume ratios in microreactors improve mass and heat transfer, they also introduce the potential of wall-catalyzed reactions. The effect can be exploited by coating the walls with a catalyst.16,17,20 Here we oxidize the silicon to silicon oxide so that the reaction mixture is exposed to a glass surface inert for liquid-phase ozonolysis as in bench scale experiments. 2. Experimental Section 2.1. Microreactor Design. The multichannel microreactor for multiphase reactions (Figures1 and 2) consisted of inlets, manifolds for liquid and gas (100-300 µm wide × 300 µm deep), 48 pressure drop channels (40 µm wide × 25 µm deep, 2.5 mm long for a gas channel, 2.2 mm long for a liquid channel), 16 individual reaction channels (600 µm wide × 300 µm deep, 22.7 mm long), an outlet manifold, and an outlet. The 16 reaction channels were designed to investigate the distribution in multichannel reactor. The number of channels and sizes were selected on the basis of experiences with previous 10 channel microreactors with posts, pressure drop, and chip size. The number of chips and size were selected to optimize the yield from a 100 mm wafer (8 microreactors of 40 mm × 16 mm). All channels were fabricated in silicon, and fluid connections were made on the back side. Gas and liquid were supplied through separate inlets and distributed by manifolds to pressure drop channels. For each reaction channel, one gas channel was placed on each side of the liquid channel to provide

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Figure 1. Schematic of 16-channel microreactor: plane view (top) and cross sections (bottom) identified by their corresponding letter in the plane view drawing. The dashed line indicates the boundary between the two Si substrates. No bond line is visible after fusion bonding.

symmetric inlet flow conditions (Figure 2). To ensure uniform gas and liquid distribution among channels, the pressure drop channels were designed to produce a 40 times larger pressure drop due to small cross section than the downstream reaction channels and outlet regions (Figure 2). If unbalanced, the strong surface interfacial forces in multichannel microreactors easily lead to poor fluid distribution and channeling so that liquids flow through some channels and gas passes through others. In an early version of the multichannel microreactor without pressure drop zones (details not shown), channeling was observed at all flow rates with liquid and gas separating in different channels as well as liquid backing into in gas lines. To avoid these problems and ensure uniform distribution across the reactor channels, a pressure drop zone was designed at the start of each channel. This pressure drop zone was dimensioned to have a 40-fold higher pressure drop than the rest of the channel. Each reaction channel had approximately 1000 50 µm diameter posts spaced on 110 µm centers to increase the gas/liquid interface area and, correspondingly, mass transfer rates. The number of posts and their dimensions were based on liquid hold-up on the wall and posts as well as pressure drop considerations as in previous investigations.2-4 The posts had the additional advantage of increasing heat transfer from the reaction zone, which reduced

the chance for formation of higher temperature regions that could otherwise adversely influence selectivity and safety. 2.2. Design of Manifold and Pressure Drop Channels. Reaction channels and fluid distribution systems were realized by patterning and etching two wafers, which were subsequently bonded to form enclosed feed channels and manifolds. The gas manifold, pressure drop zone consisting of 48 pressure drop channels, and 16 reaction channels were etched in the top surface of the first silicon wafer according to the design in Figure 1. Every reaction channel was connected to two shallow-pressure drop channels for gas and one channel for liquid. Gas was supplied to the 32 pressure drop channels via a manifold placed on the topside of the top wafer and connected to the gas inlet formed by a through hole etched in the second wafer. A total of 16 individual holes (50 µm diameter) were etched from the back side of the first wafer to connect the liquid pressure drop channels to the liquid-phase manifold (fabricated in the second wafer) upon bonding of the two silicon wafers. Manifolds for the liquid feeds and the reactor channel effluent were fabricated in the front side of the second wafer and then connected to the main liquid feed inlet port and reactor exit port, respectively, by through-holes etched from the back. The total pressure drop was observed to ∼0.07 atm with a liquid (ethyl acetate)

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Figure 2. Multichannel microreactor and design of the pressure drop zone. (a) Relationship between structure and normalized pressure drop. The depth was measured from the surface of the Si wafer. The depths of manifold and reaction channels were 300 µm. The pressure drop across the shallow channels (25 µm deep) dominates the total pressure drop. (b) Fabricated structure measured by a WYKO NT3300 Optical Profiler (Veeco, Woodbury, NY). (c) Picture of microfabricated multichannel microreactor. Details of four sets of reaction channels and pressure drop channels imaged by a WYKO NT3300 Optical Profiler.

volumetric flow rate (QL) of 0.10 mL/min and a gas flow rate (QG) of 10.0 cm3 (at standard conditions) (sccm). 2.3. Fabrication Process. The microreactors (Figures 1 and 2) were fabricated in two silicon wafers (100 mm diameter and 525 µm thick, Silicon Quest) and one Pyrex wafer (7740, 100 mm diameter 1 mm thick, Bullen Ultrasonics). All silicon wafers were double-side polished, and the Pyrex wafer was sufficiently smooth to enable anodic bonding. The silicon wafers were patterned by photolithography and etched with anisotropic deep reactive ion etching (DRIE)21 on both sides. DRIE was chosen because of its ability to realize structures on the micron scale with high aspect ratio (depth to width). As a first step in the fabrication, ∼0.5 µm of silicon oxide was grown on the both sides of the wafers with wet and dry oxidation. The oxide had the dual role of protecting the bonding surfaces and serving as a mask during DRIE. Next the pressure drop and reactor channels were patterned into the oxide on the front side of the wafer. The front side was then coated with photoresist (AZ9260, 10 µm, Clariant), and the reactor channels but not the pressure drop channels were defined and aligned to the underlying oxide mask. This nested mask arrangement enabled realization of deep reactor channels (∼300 µm) and shallow-pressure drop regions (∼25 µm).22 DRIE with the photoresist in place produced ∼275 µm deep reactor channels. The photoresist was stripped, and a second DRIE now using the oxide mask etched the pressure drop channels to a depth of 25 µm while the depth of the reaction channels increased to the design value, 300 µm. The gas manifold, as well as gas, liquid, and exit through-holes, were

defined by photolithography and formed by DRIE. Similarly, the liquid feed and effluent manifolds were defined on the front side of the second wafer. On the back of the second wafer, we use similar lithography and DRIE procedures to fabricate through-holes to the main gas and liquid feed ports as well as the reactor exit port. The dimensions of the fabricated structure were measured by WYKO NT3300 (Veeco Instruments Inc.) (see Figure 2). The oxide on the two wafers was removed by buffered oxide etchant (BOE). The wafers were then aligned and thermally bonded at 1100 °C. This step was followed by oxidization to grow a 0.5 µm silicon oxide, which meant that all surfaces in contact with reacting fluids had characteristics similar to those of glassware used in previous bench scale investigations of ozonolysis. The front side of the oxidized two wafer stack was capped by anodic bonding (at ∼400 °C) to the Pyrex wafer, which produced the final reaction. The quality of the bond was inspected visually by absence of interference patterns present in case of defective bonding. Eight microreactors (see Figure 2c) were diced from the final wafer assembly. Multichannel microreactors without posts were fabricated by the same process and with the same masks except for the lack of posts. To quantify the uniformity of gas and liquid flow distribution in the multichannel reactor, we also fabricated a set of multichannel microreactors with the exit manifold replaced by individual outlets for each of the 16 reaction channels. This was done by through-holes in the second wafer. The rest of the fabrication scheme remained the same. In this case, 18 PEEK

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Figure 3. (a) Schematic of holder and microreactor. (b) Schematic of the experimental setup for conducting ozonolysis (c) Photograph of microreactor on the holder with Plexiglas covercompression seal.

tubes (1.6 mm o.d.) were connected to the 2 inlets and 16 outlets by using epoxy. For all other experiments, the microreactor was mounted on a machined stainless steel holder (Figure 3) equipped with three inlet ports (reactant liquid, ozone/oxygen gas, and nitrogen/ solvent) and one outlet port for the reactor effluent. The reacted fluid was quenched by a quenching reagent and diluted with nitrogen gas immediately after entering the stainless steel holder to ensure that the measured reaction conversions and yields were those produced in the microreactor and not an artifact of reactions continuing in the exit channels. Inlets and outlet in the holder were connected to gas and liquid sources by 1.6 mm tubing (o.d.) tubing and Swagelok connectors. Stainless steel and PTFE tubes were used because of their resistance to ozone and low activity in ozone decomposition. A Kalrez (DuPont, 0.8 mm thickness) sheet with punched holes corresponding to the microreactor inlet and outlet ports provided sealing between microreactor and holder. A Plexiglas cover with screw holes pressed the microreactor onto the gasket and enabled flow visualization of the multiphase flow with a stereo microscope (MZ12, Leica Microsystems Inc). 2.4. Oxidation Reactions with Ozone. Chemicals were purchased from Sigma- Aldrich and used as received without further purification, and gases (oxygen and nitrogen) were purchased from BOC Gas. Ozone was produced by an electric discharge in a commercial ozonizer (OT-5, Ozonetechnology AB) cooled by a recirculating water cooler (CFT-33, NESLAB). Gas flow rates were controlled by mass flow controllers (Unit Instruments, UFC-1101 for O3/O2 and UFC-1100 and UFC-

1100A for N2 depending on the flow rate range). Ozone/oxygen gas mixtures were generated from 95% O2 gas balanced with 5% N2. The concentration of ozone in oxygen was monitored immediately before the microreactor by calibrated UV absorption at 254 nm. Concentrations ranged from 5.6 to 7.4% depending on flow rates, pressures of gas, and ozonizer temperature. Reaction solutions were prepared off-line and then delivered to the microreactor by using a syringe pump (Harvard Apparatus, PHD2000), typically at a volumetric flow ratio (liquid reagent: ozone/oxygen gas) of 1:100. Tridecane was added to the reaction solution as an internal standard since the oxidation of this alkane without any functional groups is much slower than those for the compounds investigated, phosphite, amine, and alkene. Typically, reagent feed rates were triethyl phosphite in ethyl acetate (0.247 M, liquid volumetric flow rate (QL) 0.05 mL/min, and superficial liquid velocity (JL) 0.35 mm/s) and ozone/oxygen gas mixtures (5.9% in O2, gas flow rate (QG) 4.7 cm3 (at standard conditions) (sccm), and gas superficial velocity (JG) 33 mm/s). Flows of 200 cm3/min (at standard conditions) (sccm) of nitrogen and 0.2 mL/min of ethyl acetate were mixed in the tubing and then passed through the nitrogen/quencher inlet. Oxidation with ozone was performed at room temperature. The liquid reactor effluent was collected in a glass vial at 0 °C along with the outlet tubing to reduce evaporation. Any remaining ozone in the exit gas was scrubbed by bubbling the gas through an aqueous potassium iodide solution (5 wt %). The collected liquid product was analyzed by gas chromatog-

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Figure 4. Gas-liquid flow regimes observed in the multichannel microreactor with posts: (a) slug flow (gas as dark area); (b) churn flow (the interface fluctuates rapidly and liquid periodically spans the entire channel); (c) annular flow (gas flows at the center of reactor). The conditions used in the oxidation experiments fall within the rectangle in dashed lines, The crosshatched lines in images a and c represent scratches on the plastic cover.

Scheme 1. Oxidation of Triethyl Phosphite

raphy and mass spectroscopy (GC-MS, Hewlett-Packard 6890 GC, HP5973 mass-selective detector, and HP-INNOWAX column). In the case of oxidation of amine, typical conditions were octylamine in ethyl acetate (0.105 M, liquid volumetric flow rate (QL) 0.088 mL/min, and superficial liquid velocity (JL) 0.62 mm/s) and ozone/oxygen gas mixtures (6.2% in O2, gas flow rate (QG) 10.0 cm3 (at standard conditions) (sccm), and gas superficial velocity (JG) 70 mm/s). Typical conditions for alkene oxidation were 1-decene in ethyl acetate (0.280 M, liquid volumetric flow rate (QL) 0.059 mL/min, and superficial liquid velocity (JL) 0.41 mm/s) and ozone/oxygen gas mixtures (7.4% in O2, gas flow rate (QG) 10.0 cm3 (at standard conditions 273.15 K and 1 atm) (sccm), and gas superficial velocity (JG) 70 mm/ s). Flows of 200 cm3/min (at standard conditions) (sccm) of nitrogen and 0.2 mL/min of quenching and reduction reagent in solvent (triethyl phosphite in ethyl acetate, 0.561 M) were mixed in the tubing and then passed through the nitrogen/ quencher inlet. 3. Results and Discussion 3.1. Flow Distribution and Regimes. Flow visualizations were performed with oxygen and ethyl acetate to investigate the multiphase flow regimes expected under reaction conditions. Different regimes were observed depending on the gas and liquid flow rates (Figure 4), but for each set of conditions, each channel showed the same flow pattern (Figure 5). At low liquid to gas flow rates and all gas flow rates, an “annular flow” (Figure 4) was observed in which the liquid was preferential distributed along the wall while the gas passed through the center of the channel. A “slug flow” with channel spanning gas plugs passing through a continuous liquid phase existed at high liquid to gas flow ratios. At high gas and liquid flow rates, the flow became complex with rapidly undulating gas liquid interface shapes, here denoted as “churn flow.” Movies of the observed flows are provided in the Supporting Information. These movies further demonstrate the uniformity of flow regimes across the 16 reactor channels.

The microreactor with individual outlets for each channel was used to quantify the flow distribution across the channels. Flows of 5 sccm nitrogen and 0.05 mL/min ethanol were introduced into the reactor for 180 min, and the effluent was collected from each channel. The reactor channel effluents varied by less than 3.0% between channels. This small variation can be attributed by minor variations in etch depth of the pressure drop channels, measured to range between 24.9 and 25.1 µm. 3.2. Oxidation with Ozone. To demonstrate ozonolysis in the multiphase microreactor, we explored three model reactions: ozonolysis of triethyl phosphite, octylamine, and 1-decene. Annular flow was observed for all reaction conditions in the microreactor with posts, whereas, in the microreactor without posts, slug flow was present in all cases. Prior to the oxidation with ozone, oxygen gas was used instead of ozone/oxygen gas, and it was confirmed that oxidation did not proceed at all. 3.3. Oxidation of Phosphite. Triethyl phosphite serves as the first example as it is easily oxidized by ozone (Scheme 1). Selectivity is high in all cases, and for equivalence ratios of unity or greater the conversion approaches 100% (Table 1). To explore the effect of posts in the channel on mass transfer, oxidation was also performed in microreactors fabricated without posts and in a PTFE tube with the same inner volume as the microreactor with posts. Annular flow was observed in the microreactors with posts whereas slug flow was present in the nonpost microreactors. The results (Table 1) demonstrate that both conversion and selectivity are higher in the multireactor with post than in the reactor without posts. Moreover, the microreactors have better performance than a simple PTFE tube. The fast reaction rate of ozone (0.76 × 105, 0.76 × 105, and 1.7 × 106 L/(mol/s) for 1-pentene, 1-hexene, and 2-hexene, respectively in CCl423) and low solubility of ozone (220 ppm in methyl acetate with 6% concentration of ozone in the gas phase24) imply that the reaction is mass transfer limited. Moreover, the Peclet numbers (Pe ) VL/D) for the multichannel reactor are sufficient large (Pe ∼ 104), that we can use a simple plug flow analysis to extract the overall mass transfer coefficient (kLa):

1 kLa ) - ln(1 - XO3) t Here t is the contact time and XO3 is the conversion of ozone, computed from the conversion of liquid-phase reactant (XA) and the number of ozone equivalent (ΦO3) as XO3 ) XA/ΦO3. Analysis of the data in Table 1 gives kLa = 2.5 s-1 for microreactors with posts, kLa = 0.5 s-1 for microreactors without posts, and kLa = 0.4 s-1 for the simple tube reactor. The high mass transfer coefficient for the microreactors with posts reflects the larger conversions observed with this reactor configuration, and the magnitude of kLa is consistent with earlier work on packed bed microreactors for hydrogenation.3 The increased mass transfer in microreactors with posts can be attributed to the larger interfacial area created by the presence of the posts (Figure 6), analogous to the use of inert packing material in conventional macroscopic gas-liquid contactors. 3.4. Oxidation of Amine. Amines are known to be oxidized to nitro compounds by ozone25,26 In fact, ozone is effective for preparation of polynitro compounds because multiple nitro groups can be introduced safely. As a demonstration, octylamine was oxidized into nitrooctane. The reaction required 3 equiv of ozone as it proceeded stepwise through two intermediates (Scheme 2). At 0.32 s contact time, amine was converted into nitrooctane in 98.9% conversion and 79.7% in selectivity with

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Figure 5. Pictures of multiphase flow at JG ) 35 mm/s (5.0 sccm) and JL ) 0.35 mm/s (0.05 mL/min): (a) gas manifold, pressure drop channels, and reactors; (b) starting point of the reactor; (c) middle part of the reactors. The faint white artifact at the bottom right in (a) is a reflection on the cover of the microreactor holder. Note the similar flow appearance across the 16 channels. Table 1. Ozonolysis of Triethyl Phosphite in Multichannel Reactors with and without Microfabricated Posts microreactor [P(OEt)3] (mol/L) O3/O2 (%) QG (sccm)a QL (mL/min) no. of ozone equiv contact time (s) conversn of reactant (%) selectivity (%) posts posts posts posts posts no posts no posts no posts PTFEb

0.247 0.247 0.247 0.247 0.247 0.244 0.244 0.244 0.247

5.60 5.76 5.87 6.06 5.96 6.02 6.16 6.56 5.53

2.5 3.8 4.7 9.1 9.3 1.3 5.0 10.0 5.0

0.05 0.05 0.05 0.05 0.10 0.0138 0.055 0.11 0.05

0.51 0.79 1.00 1.99 1.00 1.04 1.02 1.09 1.00

1.25 0.83 0.67 0.35 0.34 2.42 0.63 0.31 0.63

48.1 81.0 97.0 100.0 100 51.4 47.6 45.7 23.6

95.1 94.9 98.1 98.2 98.7 84.1 87.6 89.1 86.3

a sccm ) cm3/min at standard conditions (273.15 K and 1 atm). b PTFE tubing 0.787 mm i.d. and 109 mm length ()inner volume of microreactor with posts).

Scheme 2. Ozonolysis of Octylamine

Figure 6. Schematic cross section of the microreactor under multiphase flow: comparison of reactor with posts and reactor without posts.

the generation of octyl hydroxylamine and nitrosooctane as intermediates (14.0% and 5.2%, respectively) (Table 2). 3.5. Oxidation of Olefin. The oxidation of alkenes is also an important example of use of ozone in organic synthesis.27 Typically, the olefin is oxidized with ozone to produce an ozonide that by reduction with sulfide, phosphorus compounds, metal, or hydrogen produces an aldehyde or a ketone (Scheme

3). Alternatively, oxidative treatment of the ozonide yields a carboxylic acid. As an example, 1-decene was also oxidized with ozone and the product was reduced to nonanal after reductive treatment with triethyl phosphite. The triethyl phosphite in ethyl acetate solution was supplied in the quench inlet. The results are summarized in Table 2. The oxidation is slower than that for the amine, but 2 equiv of ozone is sufficient for complete conversion. Moreover, the selectivity is 100% in all case, even though the reaction was performed under room

Table 2. Oxidation of Amine and Alkene with Multichannel Microreactor with Postsa reactant

reactant concn (mol/L)

O3/O2 (%)

QG (sccm)

QL (mL/min)

no. of ozone equiv

contact time (s)

conversn (%)

selectivity (%)

octylamine 1-decene 1-decene 1-decene

0.105 0.280 0.280 0.280

6.16 7.26 7.39 6.63

10 10 10 10

0.088 0.116 0.0589 0.035

2.99 1.00 2.00 2.99

0.32 0.31 0.32 0.32

98.7 63.0 100 100

79.7 100 100 100

a

P(OEt)3/EtOAc was added from the quencher inlet for consumption of remaining ozone and reduction of ozonide to produce aldehyde.

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Scheme 3. Ozonolysis of 1-Decene Followed by Reductive Treatment with Triethyl Phosphite To Form Nonanal

temperature without external cooling. The overall mass transfer coefficients (kLa) for the microreactors with posts in these studies are consistent with those obtained in the triethyl phosphite ozonolysis experiments, i.e., kLa = 2.5 s-1. 4. Conclusion We have demonstrated a silicon-Pyrex multichannel microreactor design with a pressure drop zone for control of gasliquid contact as well as uniform distribution of the two phases. Even at small flow rates, the flow regimes across the channels were similar and the volumes exiting the individual channels were the same. Ozonolysis was performed at room temperature with several model reactants to demonstrate the feasibility of safely performing the highly reactive process in microreactors with high conversion and selectivity. Even at a short contact time of 0.67 s, a stoichiometric amount of ozone was sufficient to achieve larger than 97% conversion in the oxidation of phosphite. The microfabricated posts in the reactor channels enhanced the overall mass transfer coefficient analogous to packing in conventional systems. The posts had the additional benefit of producing a larger heat transfer area from removal of the reaction energy. Examples of ozonolysis of an amine and an olefin relevant to the production of fine chemical intermediates also produced high yields. Thus, this study provides a systematic design for achieving uniform contacting in microstructured multichannel microreactors, and the reaction examples demonstrates the potential for safely performing high reactive systems gas-liquid reactions in microreactors. Acknowledgment The technical assistance from the staff of the Microsystems Technology Laboratories at MIT is gratefully acknowledged along with financial support from the MIT Microchemical Systems Technology and Mitsubishi Chemical Corp. Supporting Information Available: Movies showing the gas and liquid flows. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Hudlicky, M. Oxidations in Organic chemistry; American Chemical Society: Washington, DC, 1990. (2) Ehrfeld, W.; Hessel, V.; Lowe, H. Microreactors: New Technology for Modern Chemistry; Wiley-VCH: Weinheim, Germany, 2000. (3) Losey, M. W.; Schmidt, M. A.; Jensen, K. F. Microfabricated multiphase packed-bed reactors: Characterization of mass transfer and reactions. Ind. Eng. Chem. Res. 2001, 40, 2555.

(4) Jensen, K. F. Microreaction engineeringsis small better? Chem. Eng. Sci. 2001, 56, 293. (5) Urben, P. G. Handbook of reactiVe Chemical Hazards; Butterworth Heinemann: Oxford, U.K., 1990. (6) Kula, J. Safer Ozonolysis Reactions: A Compilation of Laboratory Experience. Chem. Health Saf. 1999, 21-22. (7) Brandeis, M. A. Chemical Engineering in Support of Development Chemistry. Chem. Ind. 1986, 3, 90. (8) Shober, B. D. Ozonolysis and Reduction in Fine Chemical Industry. Chim. Oggi 1995, 21. (9) Throckmorton, P. E. Pilot Run, Plant Design and Cost Analysis for Reductive Ozonolysis of Methyl Soyate. J. Am. Oil. Chem. Soc 1972, 49, 643. (10) Rice, R. C.; Netzer, A. Handbook of ozone: Ann Arbor Science: Ann Arbor, MI, 1982; p 105. (11) Machado, E.; da Rosa, M. B.; Flores, E. M. M.; Paniz, J. N. G.; Martins, A. F. Spectrophotometric Detection of Ozone in Ozonized Air Currents with Chemical Gas-Liquid Transfer Using a Microreactor. Anal. Chim. Acta 1999, 380, 93. (12) Johnston, A. E.; Dutton, H. J. Reductive Ozonolysis for Monoenoic Fatty Acid Structure Determination in the Microreactor Apparatus. J. Am. Oil Chem. Soc. 1972, 49, 98. (13) de Mas, N.; Gu¨nther, A.; Schmidt, M. A.; Jensen, K. F. Microfabricated multiphase reactors for the selective direct fluorination of aromatics. Ind. Eng. Chem. Res. 2003, 42, 698. (14) Chambers, R. D.; Spink, R. C. H. Microreactors for elemental fluorine. Chem. Commun. 1999, 883-884. (15) Jahnisch, K.; Hessel, V.; Lowe, H.; Baerns, M. Chemistry in microstructured reactors. Angew. Chem., Int. Ed. 2004, 43, 406. (16) Losey, M. W.; Jackman, R. J.; Firebaugh, S. L.; Schmidt, M. A.; Jensen, K. F. Design and fabrication of microfluidic devices for multiphase mixing and reaction. J. Microelectromech. Syst. 2002, 11, 709. (17) Kobayashi, J.; Mori, Y.; Okamoto, K.; Akiyama, R.; Ueno, M.; Kitamori, T.; Kobayashi, S. A microfluidic device for conducting gasliquid-olid hydrogenation reactions. Science 2004, 304, 1305. (18) de Mas, N.; Gunther, A.; Kraust, T.; Schmidt, M. A.; Jensen, K. F. Scaled-out multilayer gas-liquid microreactor with integrated velocimetry sensors. Ind. Eng. Chem. Res. 2005, 44, 8997. (19) Deshmukh, S. R.; Vlachos, D. G. Novel micromixers driven by flow instabilities: Application to postreactors. AIChE J. 2005, 51, 3193. (20) Srinivasan, R.; Hsing, I.-M.; Berger, P. E.; Jensen, K. F.; Firebaugh, S. L.; Schmidt, M. A.; Harold, M. P.; Lerou, J. J.; Ryley, J. F. Micromachined Reactors for Catalytic Partial Oxidation Reactions. AIChE J. 1997, 43, 3059. (21) Ayon, A. A.; Braff, R.; Lin, C. C.; Sawin, H. H.; Schmidt, M. A. Characterization of a Time Multiplexed Inductively Coupled Plasma Etcher. J. Electrochem. Soc. 1999, 146, 339. (22) Ajmera, S. K.; Delattre, C.; Schmidt, M. A.; Jensen, K. F. Microfabricated cross-flow chemical reactor for catalyst testing. Sens. Actuators, B 2002, 82, 297. (23) Williamson, D. G.; Cvetanovic, R. J. Rates of Ozone-Olefin Reactions in Carbon Tetrachloride Solutions. J. Am. Chem. Soc. 1968, 90, 3668. (24) Sugimitsu, H. Base and Application of Ozone; Korin Co., Inc.: Tokyo, 1996. (25) Atkins, R. L. Synthesis of Polynitrobenzenes. Oxidation of Polynitroanilines and Their N-Hydroxy, N-Methosy, and N-Acetyl Derivatives. J. Org. Chem. 1984, 49, 503. (26) Bachman, G. R.; Strawn, K. G. Ozone Oxidation of Primary Amines to Nitroalkanes. J. Org. Chem. 1968, 33, 314. (27) Smith, M. B. Organic Synthesis; McGraw-Hill Science: New York, 2001.

ReceiVed for reView July 11, 2006 ReVised manuscript receiVed September 19, 2006 Accepted September 21, 2006 IE060893P