ABSTRACT
Abstract This research relates to developing of immobilized homogeneous Co(II) catalyst and a demonstration of its performance in hydrocarbon oxidations. Although the catalyst material is normally a solid, it performs as if it were a homogeneous catalyst when immobilized by dissolution in an ionic liquid and deposited on a catalyst support. Evidence shows that after immobilization, the typical homogeneous catalytic properties of Co(II) catalyst are still observed. Surprisingly, the immobilized catalyst is also thermally stable up to 360oC and essentially no metal leaching has been observed under oxidation reaction conditions up to 250oC, and 220 psig. Because the obvious mass transport limitation between organic phase and catalyst phase, integration of such a catalyst system into microchannel reactor becomes extremely attractive. Microchannel reaction technology is an advanced chemical process technology which has not been previously explored for biphasic catalytic oxidation, yet it appears to have the essential features required for step changes in this emerging chemical process area. One significant characteristic of such an immobilized homogeneous catalyst in a microchannel reactor is the high mass transport efficiencies, enhancing reaction rates and space time yields while minimizing side reactions that can be enhanced by mass transfer limitations. The typical high ratio of geometric surface area of the reactor walls to total reactor volume in microchannel devices should be a strong benefit to the concept. In the cyclohexane oxidation reaction, preliminary experimental results have shown 100% selectivity towards desired products. Compared to conventional homogeneous catalyst, turnover frequency of the immobilized Co(II) catalyst is almost 7 times higher than reported for conventional hydrocarbon oxidation reaction schemes. The uniqueness of such a catalyst-reactor integration allows homogeneous oxidation reactions to be operated in heterogeneous mode therefore enhancing productivity and improving overall process economy while at the same time reducing the environmental impact of operating the process in the conventional fashion with a water based reaction media.
Introduction Although it has been long recognized that homogeneous reactions are often not commercially viable due to catalyst recovery difficulties, today some chemical processes are still operated in homogeneous mode. Hydroformylation catalysis, for example, is one of the largest volume process in the chemical industry (1). Significant efforts have been made on immobilizing organometallic species responsible for catalysis (2). One problem encountered in using conventional immobilized catalyst has been the poor product selectivity. Over the last 15 years, biphasic catalysis based on using ionic liquids has attracted significant attention in the scientific community as an alternative reaction medium for homogeneous catalysis (3). On the basis of their highly charged nature, ionic liquids are well suited for biphasic reactions with organic substrates. Chauvin, et. al., utilized water soluble phosphine ligands to retain active rhodium complex in ionic liquid phase and used them successfully in biphasic hydrformylation reactions (4). Although liquidliquid biphasic catalysis has been successfully demonstrated, heterogeneous catalysts are still preferred by industry because of the ease of product separation and catalyst recovery. This research was directed towards immobilization of ionic liquid phases containing catalytic specie onto high surface solid supports (5,6). The active species dissolved in ionic liquids performs as a homogeneous catalyst, therefore, making it is possible to operate a homogeneous reaction in a heterogeneous mode. This research is related to exploring application of biphasic catalysis in hydrocarbon oxidation reactions which are commercially operated using homogeneous catalysts. Particularly, the oxidation reactions we are interested include cyclohxane oxidation to cyclohexanol and cyclohexanone, intermediates for adipic acid production (7). Also interested in our research is oxidation of aromatics. Essentially, hydrocarbon oxidation using biphasic catalysis is a new approach in this field. Biphasic catalysis is known to occur at interfaces between ionic liquid and organic reactant phases. For hydrocarbon oxidation reactions, oxygen penetration and dissolution in ionic liquids may be limiting steps. Solving such a mass transport limitation is critical for commercial applications. In our research, we use a thin film of ionic liquid to immobilize homogeneous Co(II) catalyst onto high surface area supports, and integrate such a catalyst system into in a microchannel reactor. The advantage of such an integration is high mass transport efficiencies, enhancing reaction rates and space time yields while minimizing side reactions associated with mass transfer limitations. The typical high ratio of geometric surface area of the reactor walls to total reactor volume in microchannel devices should be a strong benefit to the concept. Additional features related to utilizing a microchannel reactor is the possibility of operating oxidation reaction near or inside a flammable regime (8).
