(171c) High Pressure Catalytic Hydrogenation of Acetone in a PDMS Based Recirculating Microreactor System

Mass transfer resistances often play a limiting role in gas-liquid-solid reactions. Catalytic hydrogenations are such a class of reactions where high operating pressures and mechanical stirring are used to overcome these resistances. Microreactors¹ offer the opportunity to overcome such limitations using large interfacial areas. Their low holdup volumes are advantageous when dealing with hazardous reagents². Consequently, a number of researchers have looked at conducting hydrogenations in microfluidic setups^3,4.

PDMS based microreactors offer the advantages of low cost and ease of manufacture as compared to silica based microreactors⁵. Yet PDMS based microreactors lack mechanical strength and thus there have been no reports of high pressure (>100 psi) hydrogenations using such a setup.

We report the high pressure (725 psi) catalytic hydrogenation of acetone to isopropanol over bulk Pd catalyst in a PDMS based microreactor. To overcome limitations of the mechanical stability of the PDMS based microreactor to high pressures, a novel arrangement was used wherein the microreactor was placed in a high pressure Parr reactor. Reaction mixture was recirculated from a 7ml reservoir (also within the Parr reactor) to the microreactor and back by means of an external HPLC pump. This setup allowed subjecting the reaction mixture and catalyst to high hydrogen pressures while keeping the pressure within and outside the microreactor equal, preventing breakup of the microreactor. By passing water saturated with hydrogen at high pressure through a narrow channel with immobilized catalyst, we alleviate resistance to mass transfer between gas and liquid phases. This enables study of the intrinsic kinetics of a reaction.

Starting with 5.5 ml of 8.8 mM of acetone in water, a yield of 6.25% to isopropanol was obtained at 55°C and 725 psi hydrogen pressure in 22 hours. The setup was tested for durations of upto 92 hours, with corresponding increase in the yield of isopropanol. Tests at higher temperatures and approaches to achieve higher catalyst loadings are underway.

1 DeMello, A., Control and detection of chemical reactions in microfluidic systems, Nature, 442, pp.394 (2006)

2 Chambers, R. D. & Spink, R. C. H. Microreactors for elemental fluorine. Chem. Commun. 883?884 (1999)

3 Besser, R. S., Ouyang, X. & Surangalikar, H. Hydrocarbon hydrogenation and dehydrogenation reactions in microfabricated catalytic reactors. Chem. Eng. Sci. 58, 19?26 (2003)

4 Kobayashi, J. et al. A microfluidic device for conducting gas?liquid?solid hydrogenation reactions. Science 304, 1305?1308 (2004)

5 McDonald J., Whitesides G., Accounts of Chemical Research, vol 35, no 7, pg 491 (2002)