Environmentally persistent free radicals
(EPFRs) have been recognized
as one of the important emerging contaminants with biological toxicity,
environmental persistence, and global mobility. Previous studies have
identified the catalytic role of surface metal oxides in EPFRs formation
and illustrated the metal-dependence of EPFRs by studying on various
metal oxide nanoparticles and single crystals. However, there is still
lack of an understanding on the formation of EPFRs from the point
of view of metal sites. Various factors (e.g., crystalline phases
and surface species) of metal oxides are regarded to contribute to
the generation of EPFRs, which present profound difficulties for scientists
to tease apart the impact of metal type. Herein, a laboratory investigation,
in terms of the acidity and oxidation strength of metal cations, was
conducted by selecting metal-variable isostructural metal–organic
frameworks as material platforms. Specifically, we evaluated EPFRs
generation on MIL-100(M) (M = Al, Cr, Fe) from chlorine-substituted
phenol vapor and catechol under thermal conditions. It is found that
high Lewis acidity of metal sites is crucial for capturing the above
two phenolic precursors, activating the O–H bond and promoting
EPFRs formation. Radical species with half-life as long as 70 days
were generated on MIL-100 rich in 5-fold coordinated Al3+ sites. The unpaired electron spin density donation was further confirmed
by using 27Al solid-state nuclear magnetic resonance spectroscopy.
Despite their higher oxidation power than Al3+, the exposed
Cr3+ and Fe3+ sites show undetectable catalytic
activity for the formation of EPFRs, because of their insufficient
Lewis acidity. Our results suggest that the surface species rather
than Lewis acid sites may be a major contributor to the formation
of EPFRs on metal oxides like Fe2O3.