Heterogeneously Catalyzed Continuous-Flow Hydrogenation Using Segmented Flow in Capillary Columns

This paper explores how the visible features of segmentedflow, under reaction conditions, can be used in lab-scale multiphase heterogeneous catalysis. Continuous-flow microreactors are now routinely used in bench-scale synthesis and optimization applications, owing to their small reactant inventory, negligible heat effects at small scales, and fast mixing. However, miniaturizing multiphase heterogeneous catalysis on chips is considerably more difficult than homogeneous liquid phase chemistry. Several applications of gas–liquid and gas–liquid–solid reactions have been reported. In these continuous-flow devices, the catalyst was immobilized on the wall of the channel or incorporated as powder. A powder packed-bed gas–liquid microreactor may appear ideal for offthe-shelf catalysts, but in practice such reactors are cumbersome: critical packing parameters vary from one instance to the next and channeling and flow hysteresis abound, as we have recently visualized. For immobilized catalysts, Kobayashi et al. have advocated creating a thin film of liquid on the walls, sheared along by a fast-flowing gas stream. A drawback of this system is that it is hard to control or visualize how long the reactants are in contact with the catalyst, because both phases each move at their own velocity. Especially for more complex pathways, the spread in residence time reduces yields. Here, we overcome monitoring and yield problems by using wall-catalyzed segmented flow (Figure 1). Flow segmentation has found widespread use in liquid–liquid and gas–liquid applications, motivated by good contact between fluid phases. In this work, we highlight that the flow pattern also enhances contact with the catalyst on the wall, inspired by our work in pilot-plant studies for monolith-based reactors. We focus on using standard capillary columns, readily available to the bench chemist. The uncomplicated construction and, equally important, simple visual monitoring will be a powerful tool in the hands of synthetic chemists. We used commercially available fused-silica capillaries coated with a 6 mm thick layer of high surface area g-Al2O3 that we pretreated and impregnated with a [Pd(OAc)2] solution (Figure 2 and see the Supporting Information for details). This resulted in nanosized Pd particles evenly dispersed on the gAl2O3 coating layer (Figure 2c). We tested their activity with the well-studied hydrogenation of cyclohexene. In a 17 cm capillary (residence time 4 s) the conversion was 43% at Figure 1. Continuous capillary microreactor with a Pd catalyst immobilized on the inner wall operated in segmented gas–liquid flow.