Hierarchically structured porous organic polymer microspheres with built-in Fe3O4 supraparticles: construction of dual-level pores for Pt-catalyzed enantioselective hydrogenation

We present the preparation of nanoscale porous organic polymer (POP) composite microspheres consisting of an Fe3O4 supraparticle as the core and micro-/mesoporous POP as the shell. The core/shell-structured Fe3O4@PS microspheres were synthesized, and then subjected to a seeded swelling polymerization in the PS shell using a mixture of divinyl benzene (DVB) and vinylbenzene chloride (VBC). The formed poly(VBC-co-DVB) networks were blended with PS in the shell, resulting in a distinct phase segregation. Upon a Friedel-Craft-type hyper-crosslinking treatment, the POP structure was obtained showing the specific porosity. During this process, the non-crosslinking PS chains were extracted out of the shell and large-sized mesopores were achieved with the micropores obtained by the hyper-crosslinking networks. By varying the feed amount of VBC and DVB, the phase segregation was changed and the prevailing mesopores could be tuned from 4 nm to 30 nm. The contribution of micropores to the entire porosity was accordingly altered. In light of the characteristics of dual-level pores and magnetic recyclability, the Pt nanoparticles were locally reduced within the mesopores, and the reactive substrates were then accessible to microporous zones in the POP shell. The enantioselective hydrogenation of ethyl pyruvate was applied to evaluate the catalytic activity of the Fe3O4@POP/Pt composites. Under optimized conditions (20 bar H-2 pressure, 20 h, R.T. and molar ratio of Pt/CD: 6/1), the Fe3O4@POP/Pt composite catalysts containing 10 wt% Pt nanoparticles were validated to have excellent catalytic activity, outstanding reusability, and a high product yield and ee value (80.7%) for the enantioselective conversion of ethyl pyruvate to ethyl lactate.