We describe a microchemical reactor built by silicon processing and metal deposition techniques that enables efficient and safe direct fluorination of toluene, a highly exothermic process difficult to implement conventionally on a macroscopic scale. Gas and liquid reagents were contacted cocurrently at room temperature in the microfabricated reactor, and gas−liquid distribution patterns were characterized. A flow regime map, containing slug and annular-dry flows, was obtained for liquid velocities relevant to gas−liquid reactions in microchemical systems. During annular-dry flow operation, the substrate conversion and product distribution were studied as a function of the operating conditions:  toluene concentration, fluorine-to-toluene molar ratio, solvent type, and quenching conditions. Among the solvents tested, including acetonitrile, methanol, 1,1,2-trichloro-1,2,2-trifluoroethane, and octafluorotoluene, the highest selectivities toward ring fluorination were obtained in acetonitrile. At toluene conversions of 58%, a combined selectivity of ortho-, meta-, and para-fluorotoluenes of up to 24% was obtained.

We describe the design and flow distribution of a scaled-out gas-liquid silicon-based microreactor that consists of three vertically stacked reaction layers each containing 20 reaction channels. The reaction channels are operated in parallel from single feeds of gas and liquid with a liquid throughput of 80 mL/h. Gas and liquid are introduced to the device through single inlet ports, flow vertically to the reaction layers, and are controllably distributed to the horizontal reaction channels through gas and liquid auxiliary channels that present pressure drops significantly larger than that across the reaction channels. The product mixture flows out of the device through a single outlet port. Flow visualization by pulsed-laser fluorescence imaging reveals that the flow regime is uniform across the top layer. By integrating waveguided optical multiphase flow sensors, slug-flow properties are measured across the reaction channels on the top and middle layers; comparable slug frequencies and slug velocities are obtained.

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/Al 2 O 3 , Pd/SiO 2 , 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.

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.