The direct assembly of metalloporphyrin and Mg–Al layered double hydroxides nanosheets: a highly efficient catalyst for the green epoxidation of olefins

Abstract

Co 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (CoTPPS) anions were successfully immobilized into the lamellar space of Mg–Al LDH through the spontaneous flocculation method by using exfoliated LDH nanosheets and guest molecules as building blocks. A variety of techniques including XRD, IR, UV–Vis, SEM, TEM and TG–DSC have been applied to characterize the morphology and microstructure of the obtained hybrids. It was found that the ratio between the metalloporphyrin molecules and LDH nanosheets has an important effect on the structure of restacking products. The interlayer spacing determined from XRD result suggests that a perpendicular orientation of the CoTPPS anions in the lamellar space. The surface charge changes associated with the flocculation process were traced through zeta potential measurements. The immobilized CoTPPS could act as an efficient and reusable heterogeneous catalyst for selective epoxidation of various alkenes including different cycloalkenes, styrene derivatives and long-chain alkenes. Cyclohexene undergoes up to 99% conversion within 2 h and 90% selectivity with molecular oxygen as environmentally friendly oxidant and isobutyraldehyde as co-reductant at room temperature. The CoTPPS/LDH2.0 can be easily recovered and reused for at least 5 times without obvious decrease of activity. This work provides a facile route to synthesize a highly efficient catalyst for the green epoxidation of olefins.