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

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.

Biomimetic Photodegradation of Glyphosate in Carborane-Functionalized Nanoconfined Spaces.

The removal of organophosphorus (OP) herbicides from water has been studied using adsorptive removal, chemical oxidation, electrooxidation, enzymatic degradation, and photodegradation. The OP herbicide glyphosate (GP) is one of the most used herbicides worldwide, leading to excess GP in wastewater and soil. GP is commonly broken down in environmental conditions to compounds such as aminomethylphosphonic acid (AMPA) or sarcosine, with AMPA having a longer half-life and similar toxicity to GP. Metal-organic frameworks (MOFs) are excellent materials for purifying OP herbicides from water due to their ability to combine adsorption and photoactivity within one material. Herein, we report the use of a robust Zr-based MOF with a meta-carborane carboxylate ligand (mCB-MOF-2) to examine the adsorption and photodegradation of GP. The maximum adsorption capacity of mCB-MOF-2 for GP was determined to be 11.4 mmol/g. Non-covalent intermolecular forces between the carborane-based ligand and GP within the micropores of mCB-MOF-2 are thought to be responsible for strong binding affinity and capture of GP. After 24 h of irradiation with ultraviolet-visible (UV-vis) light, mCB-MOF-2 selectively converts 69% of GP to sarcosine and orthophosphate, following the C-P lyase enzymatic pathway and biomimetically photodegrading GP. Circumventing the production of AMPA is desirable, as it has a longer half-life and similar toxicity to GP. The exceptional adsorption capacity of GP by mCB-MOF-2 and its biomimetic photodegradation to non-toxic sarcosine make it a promising material for removing OP herbicides from water.

Degradation of G-Type Nerve Agent Simulant with Phase-Inverted Spherical Polymeric-MOF Catalysts.

For the neutralization of chemical warfare agents (CWAs), the generation of an effective catalyst that can be handled safely and applied in personal protective equipment is required. Recently, zirconium-based metal-organic frameworks (Zr-MOFs: UiO-66 and UiO-67) have shown great promise in the degradation of CWAs, including nerve agents. Their catalytic activity is owed to the interplay of both Zr(IV) Lewis acids and Lewis basic groups in the MOF structure. The latter act as proximal bases that can interact with CWAs and improve the catalytic activity of Zr-MOFs. The powder form of MOFs, though, makes them impractical catalysts, as it is challenging to handle, regenerate, and reuse them. To address this challenge, we have synthesized three Zr-MOFs with Lewis basic amino and pyridine functionalities and shaped them in spherical polymeric beads using the phase inversion method. Using this method, we can generate beads with many polymer and MOF combinations (MOF@polymer). We controlled the MOF loading in these beads, and scanning electron microscopy images revealed that the MOF crystals are evenly distributed in the polymeric matrix, ensuring effective catalytic activity. We used these beads to degrade dimethyl p-nitrophenyl phosphate (DMNP), a simulant for the G-type nerve agent. Using 31P NMR, we showed that UiO-66-NH2@PES and UiO-67-(NH2)2@PES PES: poly(ether sulfone) beads destruct DMNP to dimethyl phosphate (DMP) with a half-life (t1/2) of 5.09 and 4.34 min, respectively. Beads made of hydrophobic polymers such as poly(vinylidene fluoride) (PVDF), polystyrene (PS), and Zr-MOFs with pyridine functionalities show that the quantitative hydrolysis of DMNP requires more time compared to that seen with the UiO-66-NH2@PES beads. Our work highlights the facile shaping of MOF powders into beads that can be easily regenerated with their catalytic activity to be maintained for at least three cycles of use.