Post-synthesis functionalization of a zeolitic imidazolate structure ZIF-90: a study on removal of Hg(ii) from water and epoxidation of alkenes

Abstract

Thiol-functionalized ZIF-90 (ZIF-90-SH) was prepared via condensation of the free aldehyde group of ZIF-90 with 2-mercaptoethylamine to an imine through a one-step post-synthesis route. A subsequent reaction with manganese(II) acetate resulted in the formation of a new Mn(II)-immobilized hybrid material (ZIF-90-S[Mn]). This material was characterized by chemical analysis, X-ray diffraction, N₂ adsorption–desorption, thermogravimetric analysis and Fourier transform infrared spectroscopy, which confirmed the successful functionalization of the as-synthesized ZIF-90. ZIF-90-SH employed as an adsorbent for the removal of Hg(II) ions from an aqueous solution at room temperature exhibited an adsorption capacity of ca. 22.4 mg g⁻¹. The material showed 96–98% removal of Hg(II) ions in a low concentration range (0.1 to 10.0 mg L⁻¹). ZIF-90-S[Mn] showed good catalytic activity and selectivity in the epoxidation of several important alkenes using molecular oxygen under atmospheric pressure. Recycling and hot filtration experiments coupled with a blank test showed that epoxidation proceeded over ZIF-90-S[Mn] on the external surface of the ZIF-90 matrices, where the active Mn(II) sites were located. The catalyst could be recycled up to four times without significant loss of efficiency.