Catalysis of Organic..

92Oxidation with Microchannel Immobilized HomogeneousIntroductionAlthough it has been long recognized that homogeneous reactions are often notcommercially viable due to catalyst recovery difficulties, today some chemicalprocesses are still operated in homogeneous mode. Hydroformylation catalysis, forexample, is one of the largest volume process in the chemical industry (1).Significant efforts have been made on immobilizing organometallic speciesresponsible for catalysis (2). One problem encountered in using conventionalimmobilized catalyst has been the poor product selectivity. Over the last 15 years,biphasic catalysis based on using ionic liquids has attracted significant attention inthe scientific community as an alternative reaction medium for homogeneouscatalysis (3). On the basis of their highly charged nature, ionic liquids are wellsuited for biphasic reactions with organic substrates. Chauvin, et. al., utilized watersoluble phosphine ligands to retain active rhodium complex in ionic liquid phase andused them successfully in biphasic hydrformylation reactions (4). Although liquidliquidbiphasic catalysis has been successfully demonstrated, heterogeneous catalystsare still preferred by industry because of the ease of product separation and catalystrecovery. This research was directed towards immobilization of ionic liquid phasescontaining catalytic specie onto high surface solid supports (5,6). The active speciesdissolved in ionic liquids performs as a homogeneous catalyst, therefore, making it ispossible to operate a homogeneous reaction in a heterogeneous mode.This research is related to exploring application of biphasic catalysis inhydrocarbon oxidation reactions which are commercially operated usinghomogeneous catalysts. Particularly, the oxidation reactions we are interestedinclude cyclohxane oxidation to cyclohexanol and cyclohexanone, intermediates foradipic acid production (7). Also interested in our research is oxidation of aromatics.Essentially, hydrocarbon oxidation using biphasic catalysis is a new approach in thisfield. Biphasic catalysis is known to occur at interfaces between ionic liquid andorganic reactant phases. For hydrocarbon oxidation reactions, oxygen penetrationand dissolution in ionic liquids may be limiting steps. Solving such a mass transportlimitation is critical for commercial applications. In our research, we use a thin filmof ionic liquid to immobilize homogeneous Co(II) catalyst onto high surface areasupports, and integrate such a catalyst system into in a microchannel reactor. Theadvantage of such an integration is high mass transport efficiencies, enhancingreaction rates and space time yields while minimizing side reactions associated withmass transfer limitations. The typical high ratio of geometric surface area of thereactor walls to total reactor volume in microchannel devices should be a strongbenefit to the concept. Additional features related to utilizing a microchannel reactoris the possibility of operating oxidation reaction near or inside a flammable regime(8).