Elucidation of the role of metals in the adsorption and photodegradation of herbicides by metal-organic frameworks

Abstract

Here, four MOFs, namely Sc-TBAPy, Al-TBAPy, Y-TBAPy, and Fe-TBAPy (TBAPy: 1,3,6,8-tetrakis(p-benzoic acid)pyrene), were characterized and evaluated for their ability to remediate glyphosate (GP) from water. Among these materials, Sc-TBAPy demonstrates superior performance in both the adsorption and degradation of GP. Upon light irradiation for 5 min, Sc-TBAPy completely degrades 100% of GP in a 1.5 mM aqueous solution. Femtosecond transient absorption spectroscopy reveals that Sc-TBAPy exhibits enhanced charge transfer character compared to the other MOFs, as well as suppressed formation of emissive excimers that could impede photocatalysis. This finding was further supported by hydrogen evolution half-reaction (HER) experiments, which demonstrated Sc-TBAPy's superior catalytic activity for water splitting. In addition to its faster adsorption and more efficient photodegradation of GP, Sc-TBAPy also followed a selective pathway towards the oxidation of GP, avoiding the formation of toxic aminomethylphosphonic acid observed with the other M³⁺-TBAPy MOFs. To investigate the selectivity observed with Sc-TBAPy, electron spin resonance, depleted oxygen conditions, and solvent exchange with D₂O were employed to elucidate the role of different reactive oxygen species on GP photodegradation. The findings indicate that singlet oxygen (¹O₂) plays a critical role in the selective photodegradation pathway achieved by Sc-TBAPy.

Fig. 1. Structure representation of the M³⁺-TBAPy, [M₂(OH)₂(TBAPy)], family of MOFs, including Sc-TBAPy, Al-TBAPy, Y-TBAPy, and Fe-TBAPy.

a View of the TBAPy ligand coordinated to eight neighboring M³⁺ atoms. b, c Packing of the structure viewed along a and b directions, respectively. The orientation of the TBAPy ligand around the M³⁺-chains leads to the generation of M³⁺-TBAPy with three-dimensional pores: pore A (pink), pore B (yellow), and pore C (green). Atom color code: green, M³⁺; gray, C; red, O; white, H.

Fig. 2. Solid-state characterization of the four M³⁺-TBAPy MOFs.

a Powder X-Ray diffraction (PXRD) patterns. All PXRD patterns of MOFs match the simulated M³⁺-TBAPy PXRD pattern (black), indicating that they are isostructural and can be synthesized as phase pure. b Type I N₂ isotherms collected at 77 K and 1 bar reveal that all MOFs are microporous. Filled symbols represent adsorption and empty symbols represent desorption. c EIS Nyquist plots reveal that Sc-TBAPy (blue) has the largest semicircle radius, representing the highest charge transfer resistance. d Kubelka–Munk transformation diffuse reflectance spectra collected from 200 to 800 nm. Black line corresponds to the absorption spectrum of the protonated ligand, H₄TBAPy.

Fig. 3. Transient electronic spectra of MOFs after 400 nm excitation.

Selective time points from the fs-TA spectra for (a) Sc-TBAPy, (b) Al-TBAPy, (c) Fe-TBAPy, and (d) Y-TBAPy, suspended in DMF, are plotted with the spectral evolution denoted by the color-gradient arrows representing early-to-late time points. The prominent stimulated emission (SE) bands from the aggregated ligand state are denoted in (b–d).

Fig. 4. Photocatalytic activity of MOFs.

a Hydrogen evolution rates measured using Sc-TBAPy (blue), Y-TBAPy (yellow), Al-TBAPy (green), and Fe-TBAPy (red) with triethylamine (TEA) as a sacrificial reagent and Ni₂P as co-catalysts. b Oxidation of GP studied under oxic conditions in H₂O.

Fig. 5. Key product formation upon GP degradation by.

a Sc-TBAPy, (b) Al-TBAPy, (c) Fe-TBAPy, and (d) Y-TBAPy, under oxic conditions in H₂O. All MOFs degrade GP, with Sc-TBAPy exhibiting superior and selective catalytic activity in the degradation of GP to glycine and formic acid.

Fig. 6. Comparison of ROS formation by Sc-TBAPy (blue) and Al-TBAPy (green) using ESR spectroscopy.

The intensity vs. time plots of O₂^•–, •OH, and ¹O₂ reveal that Sc-TBAPy exhibits higher ROS formation compared to Al-TBAPy upon light irradiation.

Fig. 7. Product formation by the M³⁺-TBAPy MOFs under different conditions.

The branched pathways highlight the impact of ROS on GP degradation.