Visible light photoreduction of Cr(VI) in aqueous solution using iron-containing zeolite tubes

In this study, a novel visible light photocatalyst consisting of iron-loaded zeolite tubes was designed to reduce Cr(VI) to Cr(III) in aqueous solution. Iron was incorporated into hollow, porous ZSM-5 tubes using two different methods. In the first method, the iron-encapsulated ZSM-5 tubes were prepared by introducing iron into the mesoporous silica template prior to the formation of the hollow ZSM-5 tubes resulting in an encapsulated iron species. In the second method, iron was ion-exchanged into the ZSM-5 shell during a post-synthesis treatment. The iron-loaded ZSM-5 structures were extensively characterized by powder X-ray diffraction (XRD), elemental analysis, solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), electron paramagnetic resonance (EPR), electron microscopy with energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy (XPS). The iron in the iron-exchanged ZSM-5 tubes tended to aggregate compared to the iron in the iron-encapsulated ZSM-5 tubes. Both the iron encapsulated and iron-exchanged ZSM-5 hollow tubes were active for the visible light photoreduction of Cr(VI) to Cr(III) in aqueous solution. The iron-exchanged ZSM-5 tubes were very active initially but suffered from substantial iron leaching after the first use in aqueous Cr(VI) photoreduction relative to the iron-encapsulated ZSM-5 tubes which showed very little leaching of the iron.