Benign fabrication process of hierarchal porous polyurea microspheres with tunable pores and porosity: Their Pd immobilization and use for hexavalent chromium reduction

Developing new strategies for fabrication of porous materials with tunable morphology which have fewer environmental impacts is highly desired. Herein, conventional precipitation polymerization of polyurea (PU) synthesis is converted to interfacial polymerization via simple microfluidics and hierarchically porous and uniform PU microspheres (PPM) are prepared by the reaction of toluene diisocyanate and water, eliminating the use of toxic amines and organic solvents. The morphology and porosity of PPM are probed. The size of PPM is controlled by changing the diameter of flow channels of microfluidic device and their pore size and porosity are tuned by optimizing the polymerization conditions. Moreover, palladium (Pd) is immobilized on PPM by impregnation and subsequent reduction approach to get Pd@PPM. The hybrid composite, Pd@PPM, is used for catalytic reduction of toxic Cr 6+ to benign Cr 3+ by formic acid as a reductant along with sodium format as a promoter in aqueous system. Pd@PPM exhibited high catalytic activity and good stability for Cr 6+ reduction and their large size with high uniformity eased their recovery to reuse them in subsequent Cr 6+ reduction. Porous PU with controllable shape and morphology are rarely available. This work therefore provides new strategy to synthesis PPM with controllable porous morphology and their use as an effective support for catalysis.