(497b) Triphasic Millireactors for Gas-Liquid Reactions with in Situ Catalyst Synthesis
Metal-catalyzed gas-liquid reactions such as hydrogenation, carbonylation and oxygenation are ubiquitous in the pharmaceutical and fine chemical industries, and are conventionally carried out in batch reactors that typically face severe heat and mass transport limitations. The enhanced heat and mass transfer properties characteristic of micro/millireactors offer a potential solution to overcome these challenges. In addition to the challenges associated with heat and mass transfer, the use of cheap and abundantly available metals, such as iron, nickel and cobalt, as catalysts is of great interest. However, such metal catalysts are seldom used due to their instability, which results in rapid agglomeration and oxidation, especially when used as nanoparticles, despite their high reactivity, selectivity and ease of separation. Consequently, the lifespan of these catalysts is tremendously short, of the order of minutes at times, and in situ synthesis followed by usage in stirred batch reactors for extended residence times is extremely challenging. Here, we present a novel method for the in situ synthesis of nanoparticle catalysts for hydrogenations within flow reactors. We present a triphasic millireactor that consists of aqueous solution of metallic salt, organic substrate and hydrogen gas at atmospheric pressure. The nucleation and growth of the nanoparticles is initiated upon the injection of sodium borohydride, a strong reducing agent, and the nanoparticle-catalyzed hydrogenation reaction then commences upon the formation of the nanoparticle catalyst. The rapid mixing and controlled addition of reagents into the flow reactor enables the synthesis of well-defined ultra-small nanoparticles with high catalytic activity. Using Ni-catalyzed hydrogenation of 1-hexene as a model reaction in our reactor system, we are able to achieve a substrate conversion of 87% within a total reactor residence time (nanoparticle synthesis and hydrogenation reaction) of 5 min, at ambient conditions.
 N. Yan, C. Xiao and Y. Kou; Coordination Chemistry Review, 2010, 254, pp. 1179-1218