Residence time distribution effects on continuous-flow reaction in a polymer gel-based porous monolith: investigation of an asymmetric reaction with supported Hayashi–Jørgensen catalysts

Abstract

Immobilized catalysts are easy to reuse and applicable to continuous-flow reactions, but their catalytic activity decreases due to poor diffusivity of reactants. To mitigate the molecular diffusion resistance, it is important to design support materials that boost the diffusion of reactants toward the catalyst. Gels are polymers consisting of cross-linked networks that swell in aqueous and/or organic solvents. The gels are insoluble, like solids, but create a unique reaction environment within the network, where the molecular diffusivity is quite fast, as in a homogeneous system. In this respect, monolithic porous gels (MPGs) composed of organic polymer have been developed for continuous-flow reactors to mitigate the diffusion resistance of the reactants. However, the effects of the gel properties (e.g. cross-linking structure) on the molecular diffusivity through the MPG under continuous-flow conditions have not been quantitatively evaluated so far. Here, we prepared MPGs supporting Hayashi–Jørgensen catalysts and assessed the diffusivity within the gel on the basis of the residence time distribution, as well as on the catalytic performance in Michael addition reactions. The MPG with low cross-linking density exhibited a high swelling porosity (ε_swollen), which corresponded to fast molecular diffusion of the reactants. The MPG with the lowest cross-linking density demonstrated the highest diffusivity and superior turnover frequency (ε_swollen = 100%, TOF = 2.4 h⁻¹) in the continuous-flow reactions.