==== Front
J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

37338458
10.1021/jacs.3c02019
Article
Biomimetic Photodegradation of Glyphosate in Carborane-Functionalized Nanoconfined Spaces
https://orcid.org/0000-0002-4230-3662
Gan Lei †‡
https://orcid.org/0000-0003-1903-7543
Nord Makenzie T. §
Lessard Jacob M. §
Tufts Noah Q. §
Chidambaram Arunraj ∥⊥
Light Mark E. #
https://orcid.org/0000-0001-9690-9259
Huang Hongliang ¶
https://orcid.org/0000-0002-2348-2271
Solano Eduardo ∇
https://orcid.org/0000-0003-2961-7920
Fraile Julio †
https://orcid.org/0000-0002-1970-293X
Suárez-García Fabián ○
https://orcid.org/0000-0001-5000-0277
Viñas Clara †
https://orcid.org/0000-0002-3010-2417
Teixidor Francesc †
https://orcid.org/0000-0003-1670-0020
Stylianou Kyriakos C. *§
https://orcid.org/0000-0002-1648-2169
Planas José G. *†
† Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Bellaterra 08193, Spain
‡ Institute of Physical and Theoretical Chemistry, Graz University of Technology, Graz 8010, Austria
§ Materials Discovery Laboratory (MaD Lab), Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon OR 97331, United States
∥ Institute of Chemical Sciences and Engineering, École Polytechnique Fedérale de Lausanne (EPFL Valais), Rue de l’Industrie 17, Sion 1951, Switzerland
⊥ Chemspeed Technologies AG, Wölferstrasse 8, Füllinsdorf 4414, Switzerland
# Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, U.K.
¶ State Key Laboratory of Separation Membranes and Membrane Processes, School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China
∇ NCD-SWEET Beamline, ALBA Synchrotron Light Source, Cerdanyola del Vallès 08290, Spain
○ Departament of Material Chemistry, Instituto de Ciencia y Tecnología del Carbono, INCAR-CSIC, Oviedo 33011, Spain
* Email: kyriakos.stylianou@oregonstate.edu.
* Email: jginerplanas@icmab.es.
20 06 2023
28 06 2023
145 25 1373013741
26 02 2023
© 2023 The Authors. Published by American Chemical Society
2023
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

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.

Generalitat de Catalunya 10.13039/501100002809 2017/SGR/1720 Department of Chemistry, Oregon State University NA NA China Scholarship Council 10.13039/501100004543 201609110106 Ministerio de Economía y Competitividad 10.13039/501100003329 SEV-2015-0496 Ministerio de Economía y Competitividad 10.13039/501100003329 PID2019-106832RB-I00 Ministerio de Economía y Competitividad 10.13039/501100003329 CTQ20 document-id-old-9ja3c02019
document-id-new-14ja3c02019
ccc-price
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pmcIntroduction

Organophosphorus compounds (OPs) are highly toxic synthetic compounds that include chemical warfare agents, pesticides, and herbicides.1 The high toxicity of OP-based chemical warfare agents and pesticides is attributed to their binding affinity to acetylcholinesterase, resulting in neuromuscular paralysis and death.1−3 Agriculturally, the less lethal but still toxic OP herbicides glyphosate (GP, Round-Up) and glufosinate (GF, Rely and Cheetah) work by inhibiting the shikimic acid pathway present in plants, preventing the biosynthesis of amino acids needed for growth.4−6 GP and GF herbicides are standard solutions used control destructive and invasive crop life, with GP being the most widely used herbicide in the U.S.7,8 However, their frequent use leads to undesirable amounts of residues in groundwater and food, posing a potentially serious risk to public health.7 Detection of GP in human urine samples is evidence of increased public exposure to OP-based herbicides, leading the Environmental Protection Agency (EPA) to set a maximum level for GP in wastewater at 0.7 g/L.9,10 Current water treatment standards employ various techniques to remove GP and its primary degradation product, aminomethylphosphonic acid (AMPA). AMPA has similar toxicity and longer half-life than its parent molecule and it can accumulate in the soil.11−13 AMPA is also problematic because it has been shown to interfere with DNA synthesis and repair in fish and amphibians and can have adverse effects on human blood cells.14,15 Therefore, developing effective technologies to remove toxic OP herbicides such as GP and GF and their metabolites (e.g., AMPA) from water is highly desirable.

To date, various methods for OP removal have been studied, including electrooxidation, adsorption, enzymatic biodegradation, and photocatalytic degradation.16−28 These methods possess several disadvantages, including their inability to capture OPs efficiently, high energy consumption, logistical difficulty, and except for enzymatic biodegradation, lack of control in the resultant products of OP degradation.29 Adsorption is a common strategy for GP removal due to its simple operation and low energy consumption, and adsorbents such as activated carbon (AC) can efficiently remove GP.30 On the other hand, research about GP degradation has focused on chemical oxidation via O3 or Cl2 or the traditional photocatalyst titanium dioxide (TiO2).27 Oxidative processes such as ozonolysis can degrade GP and AMPA, but residual O3 and Cl2 are often observed as byproducts.28 TiO2 can selectively degrade GP, but the major product often depends on the pH of the reaction solution, with sarcosine being favored at low pH.31 However, most studies report that both AMPA and sarcosine are produced with various catalysts, including TiO2, manganese dioxide (MnO2), and MnO2 minerals such as birnessite and ferrioxalate.11,12,32−34 Previously, work with TiO2 has shown that it can degrade up to 92% of GP,33 but the removal efficiency depends on many factors, including the reactor setup, lamp intensity, catalyst loading, pH, and GP concentration.31,35 Goethite or magnetite photocatalysts can also selectively degrade GP to sarcosine, but again the reaction must be buffered to pH 7.36 The above methods are reported as effective treatments for GP-contaminated water. Still, they lack selective interactions with the herbicide or require constant control over the pH of the reaction solution.36,37 Thus, it would be beneficial to develop a catalyst that can selectively degrade GP to avoid producing toxic AMPA without needing to control the pH.

Currently, two known enzymes—C–P lyase and GP oxidoreductase (GOX)—can biodegrade GP (Figure 1). The enzymatic complex C–P lyase catalyzes the cleavage of the C–P bond, metabolizing GP to sarcosine (2-(methylamino)-acetic acid) and orthophosphate.38 Sarcosine produced through the C–P lyase pathway possesses no known toxicity and is metabolized to glycine by sarcosine oxidase.37 The GOX enzyme cleaves the C–N bond in GP, producing AMPA and glyoxylate or acetic acid.32,33,37,38 Unfortunately, the GOX biodegradation pathway often results in secondary contamination through the release of AMPA, which cannot be metabolized intracellularly.37 Therefore, the C–P lyase pathway is preferable as no toxic AMPA is produced.

Figure 1 Schematic illustration of the photooxidation of GP under UV light in the presence of a catalyst. GP can be degraded to (i) sarcosine and orthophosphate or (ii) AMPA and short-chain organic acids depending on the enzymatic pathway (C–P lyase and oxidoreductase, respectively).

Enzymatic biodegradation is highly selective, with selectivity attributable to the presence of a specific reaction center. Localization and capture of GP within a confined space could mimic enzyme selectivity, and the nanoconfined spaces within a porous material could allow for a greater degree of product selectivity. GP treatment technology requires improvement, as adsorption and degradation alone have yet to be demonstrated as efficient strategies for GP removal. Thus, developing superior methods to control GP pollution is central to public and environmental health. We envision that an ideal material for GP removal combines high adsorption and photodegradation capacities. This material should be water stable, photoactive, and porous, with localized and nanoconfined binding sites that allow for enhanced adsorption and selective GP photodegradation.

Metal–organic frameworks (MOFs) are ideal materials for combining adsorption and photoactivity. MOFs are a highly versatile class of porous materials made from metal clusters linked by organic ligands. To date, MOFs have shown great promise in many research areas,39−46 including in the efficient capture of herbicides25,39,41,45,46 and the photodegradation of organic pollutants such as dyes.47 Among these, zirconium (Zr)-carboxylate MOFs have received primary interest due to their chemical, thermal, and hydrolytic stabilities, mild synthetic conditions, and versatile connectivity.44,48−52 Zhu et al. first reported the adsorptive removal of OP herbicides GP and GF using UiO-67.26 They discovered that the Zr–O(H) and bridging Zr–O groups in the nodes of UiO-67 served as natural binding sites for the phosphonic group in the herbicides GP and GF.26 Zr-MOFs can also act as potent catalysts for the degradation of OP compounds due to the high concentration of Lewis acidic-Zr(IV) sites in the Zr6 clusters.25 Additionally, the steric and electronic microenvironments created in MOF pore spaces allow for reaction control.53 In this regard, the functionalized pore spaces of MOFs can mimic enzyme pockets, leading to increased reactivity and product selectivity.54−56 Thus, we envision that the high stability, adsorption capacity, and photocatalytic activity within the nanoconfined pore spaces of Zr(IV)-based MOFs make them excellent candidates for the adsorption and photodegradation of the OP herbicides GP and GF.

Icosahedral carboranes {1,n-C2B10H12 [n = 2 (ortho-), 7 (meta-) or 12 (para-)]} are a class of commercially available and exceptionally stable 3-dimensional-aromatic boron-rich clusters that possess material-favorable properties such as thermal and chemical stability and high hydrophobicity.57−61 The more electronegative carbon atoms contribute more electrons to cluster bonding than the boron atoms, which results in the carbon atoms effectively having an electron-withdrawing character.62,63para-Carborane has been used as a scaffold to grow MOF structures, resulting in the generation of highly stable and porous materials.64−70 We later developed a series of ortho-71 and meta-carborane72−76 based coordination polymers and MOFs, some with outstanding water stabilities. Herein, we describe the synthesis of a water-stable and porous Zr(IV)-carborane-based MOF with a tetracarboxylate meta-carborane ligand, denoted as mCB-MOF-2, for the capture and photodegradation of GP. We found that mCB-MOF-2 has a larger adsorption capacity for both GP and GF compared to other adsorptive materials used for their removal to date. Density functional theory (DFT) calculations suggest that non-covalent intermolecular forces between the carborane-based linker and GP in the micropores of mCB-MOF-2 are responsible for their enhanced interactions. Due to a higher observed uptake capacity of GP relative to GF by mCB-MOF-2, we investigated this material for the photodegradation of GP. We found that GP photodegradation by mCB-MOF-2 is biomimetic, following the C–P lyase enzymatic pathway, forming only non-toxic sarcosine as a product without the requirement of pH control.

Results and Discussion

Synthesis and Characterization of mCB-MOF-2

A new V-type (bent) bis-phenyl tetracarboxylic acid ligand derived from the m-carborane cluster [mCB-H4L2: 1,7-di(3,5-dicarboxyphenyl)-1,7-dicarba-closo-dodecaborane], was synthesized via Cu(I) coupling and oxidation.77 This new ligand, mCB-H4L2, was characterized by spectroscopic and analytical techniques (see the Supporting Information). Heating mCB-H4L2 with ZrCl4 in the presence of formic acid and N,N-dimethylformamide (DMF) at 120 °C for 48 h yielded single, colorless prism-shaped crystals of mCB-MOF-2 (Figure S1). Single-crystal X-ray diffraction (SCXRD) studies revealed that mCB-MOF-2 crystallizes in the space group P6/mmm and possesses csq topology (Figure 2 and Table S1). Such a topology is commonly observed in Zr-MOF structures with planar tetracarboxylic ligands such as NU-1000, PCN-222, MOF-545, or MMMPF-6.52 The framework of mCB-MOF-2 consists of octahedral Zr6-clusters linked by bent mCB-L2 ligands (Figure 2). Each octahedral Zr6-unit is capped by μ3-O/OH groups providing a Zr6(μ3-O)4(μ3-OH)4 core. Eight octahedral edges of each Zr6O4(OH)4 core are connected to eight mCB-MOF-2 units, with the remaining four Zr(IV) edges occupied by terminal OH/OH2 ligands.78 This gives rise to mCB-MOF-2 with a molecular formula of [Zr6(μ3-O)4(μ3-OH)4(OH)4(H2O)4(mCB-L2)2] guest molecules, and it possesses hexagonal and triangular channels with pore diameters of 1.2 and 0.8 nm, respectively (Figure 2). The V-shape of our ligand imposes a shrink of the resultant network compared to ordinary planar tetracarboxylic linkers. The latter usually provides mesoporous materials,51 whereas our new material is microporous (Figures 2 and S2). After removing the guest solvent molecules, the total solvent-accessible volume of mCB-MOF-2 was calculated to be 52.5% by PLATON.79 Combining the accessible volume and the density of the static structure of mCB-MOF-2 gave a pore volume of 0.57 cm3/g.

Figure 2 Crystal structure of mCB-MOF-2. (a) View of the 8-connected Zr6-cluster and (b) the carboxylate mCB-L2 linker. (c) Axial cross-section view of the extended structures showing the 2-dimensional 44 networks. (d) 3-Dimensional framework with hexagonal and triangular 1-dimensional channels, view of the “stacking” of (c). Structures to compose the porous network; Zr6 clusters are shown as violet polyhedra. Except in (b), H atoms are omitted for clarity. Color code: B, orange; C, gray; O, red; and Zr, green.

The bulk phase and analytical purity of mCB-MOF-2 were confirmed by powder X-ray diffraction (PXRD, Figure S3), elemental analysis, and infrared spectroscopy (IR). IR spectra showed characteristic (Zr)O–H/O–H2 stretching bands (in the range 3600–3700 cm–1),51,78,80 and a B–H stretching band for the carborane fragments at 2606 cm–1 (Figure S4). Thermogravimetric analysis (TGA) of mCB-MOF-2 after its immersion in acetone revealed that it is stable up to 270 °C (Figure S5). Variable temperature synchrotron wide angle X-ray scattering (WAXS) measurements showed that mCB-MOF-2 retains its structural architecture up to 270 °C under dynamic vacuum, consistent with the TGA data (Figure S6). PXRD patterns indicated that the structure of the activated MOF, mCB-MOF-2′, is intact upon removal of the guest molecules from its cavities (Figure S3). Type I N2 isotherms collected at 77 K and 1 bar confirmed the microporous nature of mCB-MOF-2′, giving a Brunauer–Emmett–Teller (BET) surface area of 1095 m2/g (Figure 3b), and an experimental pore volume of 0.44 cm3/g, which is slightly lower than the calculated pore volume of the static structure (0.57 cm3/g). mCB-MOF-2′ was also found to be porous to CO2 (273–313 K), CH4, and H2O vapors at 298 K, and H2 at 77 K and 1 bar (Figure S7).

Figure 3 Comparison of (a) PXRD patterns and (b) N2 adsorption isotherms collected at 77 K and 1 bar for activated mCB-MOF-2′ and after being treated under different conditions.

Chemical stability was investigated by immersing mCB-MOF-2′ in water under various conditions (RT, 90 °C, acidic or basic) for 24–48 h, followed by PXRD and BET (Figure 3a,b and Table S2). PXRD patterns of mCB-MOF-2′ before and after incubation in a closed vial of water for 48 h at RT or 90 °C perfectly matched the simulated pattern derived from the single crystal structure of mCB-MOF-2 (Figure 3a). Samples showed little change in their PXRD patterns after treatment under acidic (HCl, pH 1) or basic (NaOH, pH 11) conditions, in highly concentrated HCl (12 M) for 24 h, and even after being in water at 90 °C for 2 months (Figure 3a). While PXRD analysis showed no change in crystallinity, porosity investigations revealed minor changes in the N2 uptake (77 K and 1 bar) and BET surface areas under those harsh conditions (Figure 3b and Table S2). We also evaluated the possible influence of the carborane units on the hydrophobic properties of mCB-MOF-2′. Contact angle (Θc ∼ 0°) measurements indicated that the surface of mCB-MOF-2′ is hydrophilic, and water adsorption isotherms of mCB-MOF-2′ collected at 298 K (Figure S7d) showed a two-step process, which can be correlated to the filling of the two cavities present in the structure. The overall data demonstrate the excellent chemical and hydrolytic stability of mCB-MOF-2, in line with the presence of Zr6-clusters and carborane units in the MOF structure.44,71,72,81

Adsorption of Glyphosate and Glufosinate

Due to the stability of mCB-MOF-2 in aqueous, acidic, and basic solutions, we explored the capture of two of the most frequently used OPs in agriculture, GP and GF. We first tested the capture of GP and GF with mCB-MOF-2′ at room temperature (295 K) for 48 h (Figure 4a). The isotherms revealed the relationship between the system’s equilibrium concentration (Ce) and the amount of herbicide adsorbed (qe) on the MOF. As, to the best of our knowledge, only a few reports have been published on the adsorption of these herbicides on MOFs, we evaluated the experimental isotherms using the Langmuir and Freundlich isothermal models to understand the adsorption behavior thoroughly.25,26,82

Figure 4 (a) Solution adsorption isotherms of GP and GF using mCB-MOF-2′ at 295 K and (b) Freundlich model plots.

Analysis of each model showed that the empirical Freundlich model (Figure 4b) has a better fit than the Langmuir model (Figure S8 and Table S3) in both cases, contrary to the adsorption on UiO-67 or NU-1000.25,26,82 Other isothermal models, such as Temkin and Dubinin–Radushkevich models, do not provide a better fit to the experimental isotherms (Figures S9 and S10 and Table S4). Our data suggest that the adsorption of GP and GF on mCB-MOF-2′ follows the Freundlich isothermal model, indicating the energetical equivalency and heterogeneity of the adsorption sites. Both n values were less than 1.0 (Table S3), demonstrating favorable adsorption of GP and GF on mCB-MOF-2′. The maximum GP and GF adsorption capacities (qmax) on mCB-MOF-2′ were 11.4 and 7.2 mmol g–1, respectively. The higher adsorption value for GP indicates that mCB-MOF-2′ presents distinctive affinities for OPs with differing molecular structures. Previous work has shown that the Lewis acidic Zr6-clusters have a high affinity for the Lewis basic phosphonate functional groups of GP and GF, and therefore, we expect the Zr metal nodes in the framework to be the primary adsorption sites for the herbicides.25,83 Thus, the presence of a methyl group on the phosphonate moiety of GF (not present in GP) might diminish its bonding with the Zr–OH groups in mCB-MOF-2′, as observed in the case of UiO-67.26 Nevertheless, the adsorption capacities of GP and GF on mCB-MOF-2′ are higher than NU-1000 (8.9725 or 10.1 mmol g–1 in the present work), UiO-67 (7.90 mmol g–1 for GP),25 and other reported materials to date (Table S5). We further evaluated the stability and recyclability of mCB-MOF-2′ following the adsorption of GP and GF, with PXRD patterns showing that mCB-MOF-2′ retains its structure after adsorption in 0.05 mmol L–1 GP or GF solution for 48 h (Figures S11 and S12). mCB-MOF-2 was then washed with acidified water three times to remove any GP or GF from the pores, activated at 60 °C for 4 h, and the regenerated material was reused for the GP and GF uptake experiments. No significant decrease in the BET surface area of the material (Figures S13 and S14) nor in the adsorption capacity for GP and GF after at least three adsorption cycles (Figure 5) was observed, demonstrating that mCB-MOF-2′ could be recycled and reused for GP and GF capture. Comparing the adsorption capacities of GP and GF on different porous materials (Table S5) reveals that mCB-MOF-2′ is an excellent candidate for the adsorptive removal of OPs for environmental pollution management.

Figure 5 Regeneration of mCB-MOF-2′ and cycling of GP and GF adsorption after 48 h. For comparison, the adsorption of GP on NU-1000 was used (1 h, after which the uptake was saturated).

Density Functional Theory of GP Adsorption by mCB-MOF-2′

DFT calculations suggest that the adsorption conformations of a GP molecule in mCB-MOF-2′ involve two different adsorption sites: (i) in the triangular channels (Figure 6a) and (ii) between two adjacent Zr nodes in orthogonal windows to the hexagonal and triangular channels (Figure 6b), named c-pores as in NU-1000.84 According to the DFT calculations, GP coordinates to mCB-MOF-2′ through the phosphonic acid end of the molecule and also shows hydrogen bonding between GP and the surrounding framework in both pores (triangular and c-pores; Figures 6 and S15). The related DFT calculations for the mesoporous NU-1000 showed the GP molecules adsorb in the c-pores and mesoporous hexagonal channels through the phosphonic acid, but hydrogen bonding with the surrounding framework is only observed in the microporous c-pores.84 A binding energy of −241 kJ/mol was found for the c-pores of mCB-MOF-2′ (Figure 6b) and −201 kJ/mol for NU-1000. It is thought that the smaller c-pores’ size in mCB-MOF-2′ compared to NU-1000 may explain the higher binding energy for our MOF. For the other adsorption site, the narrow triangular channels in mCB-MOF-2′ (Figure 6a) provide the possibility of non-covalent intermolecular forces between the carborane-based linker and GP molecule, which is not observed in the mesoporous hexagonal channels in NU-1000. Thus, a much higher binding energy was calculated for these adsorption sites in mCB-MOF-2′ (−231.0 kJ/mol) than for the mesopores in NU-1000 (−192.9 kJ/mol). The DFT calculations demonstrate the strong interaction between GP and our microporous carborane-based framework, combined with higher Zr6 cluster density (0.57 mmol/g in mCB-MOF-2′ and 0.46 mmol/g in NU-1000), explaining the higher GP uptake compared to mesoporous NU-1000.

Figure 6 DFT calculated conformations of GP binding in mCB-MOF-2′. (a) GP in the triangular pores of mCB-MOF-2′ shows a non-covalent interaction with the surrounding framework [blue dotted line (C–H···O, (i): H···O 2.855 Å, CHO 134°)] and (b) in the c-pores of mCB-MOF-2′ showing non-covalent interactions with the surrounding framework {blue dotted lines [C–H···O, (ii): H···O 2.196 Å; CHO 162°; O–H···C, and (iii): H···C 2.884 Å, OHC 126°]}.

Optoelectronic Properties of mCB-MOF-2′ and NU-1000

To investigate the optoelectronic properties of mCB-MOF-2′ and, for comparison, NU-1000, we studied the UV–vis diffuse reflectance spectra (DRS) of these two MOFs (Figure S12a). mCB-MOF-2 and NU-1000 samples exhibit an absorption band edge at approximately 330 and 470 nm (Figure S16a), corresponding to band gaps (Eg) of 3.67 and 2.75 eV, respectively, according to the Tauc plots (Figure S16b). Compared to mCB-MOF-2, a wider spectral absorption band and narrower band gap were observed in NU-1000. The conduction band (CB) and valence band (VB) positions of mCB-MOF-2 and NU-1000 were determined based on Mott–Schottky analyses (Figure S16c,d). The positive slopes of Mott–Schottky plots indicated the typical n-type semiconductors for both MOFs. Notably, the smaller slope for mCB-MOF-2 showed higher donor density than for NU-1000. The flat potentials determined from the intersection value were calculated to be −1.55 and −1.45 eV versus Ag/AgCl for mCB-MOF-2 and NU-1000, respectively. Usually, the bottom of the CB is approximated by the flat band potential; thus, the CB edges are −1.35 eV for mCB-MOF-2 and −1.25 eV for NU-1000. The more negative CB edge of mCB-MOF-2 suggests a stronger photo-reducing ability than NU-1000. From the band gap energies and CB positions of mCB-MOF-2 and NU-1000, the VB edges were calculated to be 2.32 eV for mCB-MOF-2 and 1.5 eV for NU-1000. The more positive VB for mCB-MOF-2 indicates a stronger GP oxidative ability than NU-1000 (Figure 7). As the highest occupied molecular orbital (HOMO) of GP lies at −1.25 V versus the normal hydrogen electrode (NHE),85mCB-MOF-2 has a stronger thermodynamic driving force for GP oxidative degradation compared to NU-1000.

Figure 7 Schematic energy level diagrams of NU-1000 (orange) and mCB-MOF-2 (cyan).

Photodegradation of Glyphosate

Considering the optoelectronic properties, exceptional stability, large GP adsorption capacity, and potential photoactivity (Figure S16) of mCB-MOF-2, the photodegradation of GP was investigated. The photodegradation efficiency of mCB-MOF-2 was evaluated using 0.02 and 0.01 M GP solutions (Figure 8a). Interestingly, characterization of the resulting products with 1H and 31P NMR indicated that mCB-MOF-2 selectively decomposes GP to sarcosine and orthophosphate by following the C–P lyase enzymatic pathway (Figures 2, S17a, and S18a). At the same time, irradiation of the 0.02 M GP solution without a photocatalyst showed no GP degradation (Figure S19). After irradiation for 24 h, mCB-MOF-2 degraded 69 and 87% of GP when 0.02 and 0.01 M solutions were used, respectively, with sarcosine and orthophosphate being the only products after photodegradation (Figures 8a, S17a, and S18a). Gas chromatography–mass spectrometry (GCMS) also confirmed the selective degradation of GP by mCB-MOF-2 (Figures S20–S25 and Table S7). PXRD collected post-irradiation verified that mCB-MOF-2 is stable under UV light (Figure S26), and N2 isotherms showed a slight decrease in the BET surface area from 1095 to 995 m2/g (Table S2).

Figure 8 (a) Photodegradation results show that in 24 h, mCB-MOF-2 degrades 69 and 87% of 0.02 and 0.01 M GP to sarcosine, respectively; and (b) conversion of 0.02 M GP solution to sarcosine and AMPA using mCB-MOF-2 or TiO2. In 24 h, TiO2 degrades 63% of 0.02 M GP, non-selectively, into a mixture of sarcosine, AMPA, acetic, and formic acid.

To better understand the efficiency of mCB-MOF-2 in the photodegradation of GP, the same experiments were performed using the common photocatalyst TiO2. The latter produces five times less sarcosine than mCB-MOF-2 after a 30 min irradiation (Figure 8b). Contrary to mCB-MOF-2, 1H and 31P NMR analysis indicated that both AMPA and sarcosine are produced with TiO2 in addition to acetic and formic acid (Figures S17b and S18b). While TiO2 can still photodegrade GP, it is non-selective and produces the toxic and longer-lived product AMPA. mCB-MOF-2 demonstrates a greater efficiency for converting GP to sarcosine than the TiO2 standard. We suspect that the carborane-decorated, nanoconfined pores and high adsorption capacity of mCB-MOF-2′ increase its photodegradation efficiency and selectivity, as GP can access additional active sites within the MOF pore, and reactions are not limited to the surface.

Intrigued by the selectivity of mCB-MOF-2 in GP photodegradation, we performed a 9 h photodegradation experiment using non-porous TiO2 or zirconium dioxide (ZrO2) and porous MOFs that have previously been studied for GP adsorption (UiO-66 and NU-1000) (Figure S27). Consistent with the above results, TiO2 was non-selective and less efficient for the degradation of GP (Figure 9) compared to the MOF photocatalysts. ZrO2 was investigated to probe the impact of the Zr–O(H) sites during photodegradation. ZrO2 also performed relatively poorly for GP degradation and produced both AMPA and sarcosine (Figure 9). The poor performances of TiO2 and ZrO2 could be due to their reduced porosity, as any photodegradation reactions would be limited to the surface.

Figure 9 Comparing photocatalysts in the degradation of 0.02 M GP solution after 9 h. mCB-MOF-2 is the only material among them that is selective, degrading GP into sarcosine only. All other materials degrade GP into sarcosine (blue) and AMPA (green). GP photodegradation to sarcosine with mCB-MOF-2 increases using a lower (0.01 M) concentration.

On the other hand, GP photodegradation with porous UiO-66 was slightly more efficient than mCB-MOF-2 (Figures 9 and S27). However, it also lacked selectivity as AMPA was the primary product (64.7%), compared to sarcosine (18.1%). Among all the tested photocatalysts, mesoporous NU-1000 showed the best performance for GP photodegradation (Figure 9). While NU-1000 was the most efficient, it was also non-selective, producing both AMPA (38.1%) and sarcosine (57.5%). The wider spectral absorption band and narrower band gap observed in NU-1000, compared with those for mCB-MOF-2 (Figure S16), signify its larger light utilization. The latter agrees with the higher GP degradation for NU-1000. However, the more positive VB for mCB-MOF-2 (Figure 7) indicates a stronger thermodynamic driving force for GP oxidation than NU-1000. The stronger photo-redox ability of mCB-MOF-2, in addition to the nanoconfinement of GP in the micropores of our MOF, could explain the preferential C–P bond activation by mCB-MOF-2.38

We analyzed each photocatalyst’s 9 h sample by GCMS to ensure selectivity and confirm the degradation products. Due to the low volatility and high polarity of GP and AMPA, derivatization was necessary for GCMS analysis of the samples and standards (see the Supporting Information).86 The GCMS total ion chromatogram (TIC) (Figure 10) shows the retention time (tR) for the standards and samples. As shown in Figure 10, AMPA (tR 5.79 min) co-elutes with dimethyl glutarate (DMG, tR 5.82 min, 91% match from MS database), likely a byproduct of the derivatization reaction, as it was also present in the derivatized standards. Notably, the TIC shows that the AMPA and DMG peaks are unresolved for the AMPA standard and all photocatalysts except for mCB-MOF-2, where no peak for AMPA is observed (Figures 10, S20, and S21). Additionally, MS results for mCB-MOF-2 around the retention time of AMPA showed no major ions indicative of derivatized AMPA. In contrast, the other photocatalysts had distinct ions (m/z) for AMPA in their mass spectra (Figures S21–S25). Thus, the GCMS results support our conclusion from the 1H and 31P NMR, as no AMPA was observed in the mCB-MOF-2 sample for either technique and provides additional evidence for the selective GP degradation route.

Figure 10 GCMS TIC for standards and tested photocatalysts after 9 h GP irradiation. AMPA (tR 5.79 min) co-elutes with a byproduct of the derivatization process, most likely dimethyl glutarate (tR 5.82 min). GCMS detected no AMPA for mCB-MOF-2, but it has a distinctive peak and mass spectrum for the other photocatalysts.

We tested the uptake of AMPA by mCB-MOF-2 (following methods from the GP and GF uptake experiments) and found that after 1 h, mCB-MOF-2 captured 86.8% of the AMPA (200 ppm) present in the solution, compared to 93.2 and 14.6% for NU-1000 and TiO2, respectively (Table S8 and Figure S28). This result was not surprising, as AMPA is structurally similar to GP, and the low uptake for TiO2 can be attributed to its lack of porosity. Upon capture, the AMPA-loaded mCB-MOF-2 was irradiated with light. The BET surface area of mCB-MOF-2 after photocatalysis was found to be 916 m2/g, slightly higher than the surface area of the AMPA-loaded mCB-MOF-2 before irradiation (807 m2/g, Figure S29). No degradation products were observed by 1H NMR after photocatalysis; however, traces of orthophosphate could be detected by 31P NMR (Figure S30). These data suggest that although photodegradation of AMPA is possible, it seems negligible under the present reaction conditions.

The above experiments indicate that AMPA might be sequestered in mCB-MOF-2 and therefore prevented its detection in the supernatant after GP degradation. We note however that if a significant amount of AMPA was produced during GP photodegradation and subsequently absorbed and blocked the active sites of mCB-MOF-2, the GP degradation rate would have greatly decreased. This is not the case and sarcosine production only increased over the 24 h reaction (Figure 8b). These results indicate that even in the event that mCB-MOF-2 adsorbs AMPA and could potentially photodegrade it, the GP degradation process into sarcosine is much faster.

Photodegradation Pathway

The carborane moieties within the porous framework of mCB-MOF-2 are expected to contribute significantly toward the high GP uptake observed. Carboranes are strong electron-withdrawing groups.57,62 Due to this, the carboxylic acid functional groups on the mCB-L2 ligands become more acidic, weakening their conjugate bases. As a result, the carboxylate groups may coordinate less strongly to each Zr(IV), causing Zr to be less electronically satisfied and, therefore, more Lewis acidic. Thus, the Zr(IV) clusters of mCB-MOF-2 may experience an electron deficiency, enhancing their electrophilic behavior.80 This more significant degree of electron-receptivity is proposed to activate the Zr(IV) sites and promote increased capture through coordination with the Lewis basic phosphonic group in GP. It is important to note that the adsorption site (Zr(IV) clusters) and pore environment in mCB-MOF-2 are effectively surrounded by carborane fragments (Figures 6 and S15), so that adsorbed GP molecules are nanoconfined in the narrow pores of the MOF.

Irradiation of a photocatalyst produces both electrons (e–) and holes (h+). The holes can directly oxidize pollutants or oxidize oxygen to produce reactive oxygen species (ROS). ROS are generally accepted as the primary oxidizing agents in photodegradation reactions and can include hydroxyl radicals (•OH), superoxide radicals, and singlet oxygen (1O2).87−90 Electron spin resonance (ESR) spectra were recorded to investigate the generation of ROS (•OH, •O2–, and 1O2) in our photocatalytic experiments. We have confirmed the presence of reactive species 1O2, as well as and •OH, by conducting ESR measurements on our MOF in the presence of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), respectively. No TEMPO–1O2, DMPO–•OH signals were observed in the absence of UV light (Figures S32–S35). Upon irradiation, a 1:1:1 signal corresponding to TEMPO–1O2, a 1:1:1:1 signal corresponding to DMPO–, and a 1:2:2:1 signal corresponding to DMPO–•OH appeared, suggesting the generation of 1O2, •O2–, and •OH, respectively.91 Both •OH and •O2– can act as nucleophiles to attack the electrophilic P-atom of GP (step III in Scheme 1), cleaving the C–P bond. The ESR data confirm the generation of ROS but do not confirm the dominance of a specific ROS in the photodegradation of GP. To evaluate the role of each ROS in the observed photodegradation reaction, we have further investigated the effect of radical scavengers. Sodium azide (NaN3), triethylamine (TEA), triethanolamine (TEOA), and tert-butanol (BuOH) are commonly employed as radical scavengers for 1O2,92•O2–, h+, and •OH, respectively.47,93 To investigate the reactive species responsible for the photodegradation of GP by mCB-MOF-2, the aforementioned ROS scavengers were added to the photocatalytic reaction setup. The results show that NaN3 is the more efficient scavenger, although all scavengers significantly inhibited the degradation of GP (Figure S31). In the presence of NaN3, the degradation of GP at 30 min and 9 h was 0 and 6.21%, respectively. In comparison, without NaN3, 43.96 and 70.46% of GP were degraded in the same time intervals (0.02 M GP solution). Even though the scavenger experiments suggest that 1O2 has a dominant role in the photodegradation of GP, all ROS contribute to this catalytic reaction.

Scheme 1 GP Photodegradation Cycle

Starting from an accessible pore of mCB-MOF-2: (I) GP adsorption within the pore; (II) UV irradiation and photogeneration of reactive oxidation species; and (III) C–P lyase biomimetic photodegradation of GP into sarcosine and phosphoric acid.

Based on the DFT calculations, ESR experiments, and ROS trapping, the photocatalytic degradation mechanism of GP by mCB-MOF-2′ is proposed in Scheme 1. As previously stated, the electron-withdrawing carborane moieties within the porous framework of mCB-MOF-2 cause Zr(IV) clusters to be more Lewis acidic, and coordination of Zr(IV) sites with the Lewis basic phosphonic group in GP is expected to activate the C–P bond. When mCB-MOF-2 is irradiated by UV–vis light, it generates photoexcited electrons that transition to the CB, leaving holes in the VB. The photogenerated electrons in the CB can reduce dissolved or adsorbed O2 to produce •O2–, which can then be oxidized to 1O2 by photogenerated holes in the VB (step II in Scheme 1). As a powerful oxidant, 1O2 could oxidize GP into non-toxic sarcosine and orthophosphate (step III in Scheme 1). Concurrently, holes in the VB can oxidize water to produce •OH (step II in Scheme 1). Photogenerated •OH and •O2– can participate in nucleophilic attack on the P-atom via an SN2-like substitution. The O-atom of the incoming nucleophile is incorporated into the phosphate to form the observed orthophosphate, and the carbon-centered radical hydrolyzes an H2O molecule to form sarcosine. Thus, our carborane-decorated MOF is highly selective in GP photodegradation compared to the other photocatalysts employed in this work, producing sarcosine and orthophosphate exclusively.

Previous studies on GP photodegradation using manganese oxide have proposed a charge transfer mechanism where the metal site is reduced, either from an electron transfer from GP, creating a phosphate-based radical,94 or electron transfer from water resulting in metal reduction and water oxidation to create radicals that subsequently attack the weakened C–P bond.12 Carbon-centered radical mechanisms have also been proposed in the photodegradation of GP.12,31,94 Nucleophiles present in the reaction pot (H2O, −OH, and •OH) can participate in nucleophilic attack of the P-atom in GP, resulting in the incorporation of an oxygen atom from the nucleophile to form orthophosphate. Oxygen incorporation by the phosphate group has previously been documented through isotope (18O) analysis, and the O-atom likely comes from dissolved O2 or H2O,12,95 which were abundant in our reaction system. Since the reaction pH was between 2 and 4, protons and water were also available. Therefore, the carbon-centered radical could acquire a proton from the solution or hydrolyze an H2O molecule to form sarcosine.12,94

The observed selectivity on GP degradation by our carborane mCB-MOF-2 framework can be explained by a combination of factors, including (i) a more Lewis acid character of the Zr metal centers, as consequence of the weaker coordination with the carborane-based carboxylate linkers and (ii) the nanoconfinement of the absorbed GP in the nanopores of mCB-MOF-2. The coordination of GP through the phosphonic acid to the Zr metal center is expected to further activate the C–P bond and make it more susceptible to nucleophilic attack by •O2– and •OH, as well as to oxidation by 1O2. It is well known that the reactivity and selectivity of reactive species such as ROS can be greatly enhanced when conducted in confined nanospaces.96−98 The GP degradation process by mCB-MOF-2 is taking place in the nanoconfined pores of the MOF, and this is thought to facilitate the observed selectivity.

Conclusions

We report, for the first time, the use of a thermal and hydrolytically stable MOF made of Zr(IV) and a tetracarboxylate carborane ligand, namely, mCB-MOF-2, for the adsorption of GP and GF and biomimetic photodegradation of GP. These herbicides are of great concern owing to their indiscriminate use and negative impacts on human and environmental health. The microporous nature of the carborane-decorated mCB-MOF-2 leads to the efficient capture of both GP and GF. The synergy between the Zr(IV) node and carborane linker of mCB-MOF-2 is vital for the efficient degradation of GP. mCB-MOF-2 selectively degrades GP into sarcosine and orthophosphate, acting in a biomimetic fashion by following the C–P lyase pathway. Our MOFs’ selective degradation to non-toxic sarcosine is significant, as it does not require control over the pH of the reaction solution. This is compared to traditionally accepted TiO2, which produces undesired and toxic AMPA and non-toxic sarcosine. Band structure analysis of mCB-MOF-2 revealed our MOF as an n-type semiconductor with a high donor density and low CB edge, explaining the strong photo-redox ability of our material. The observed selectivity on GP degradation by mCB-MOF-2 is thought to be due to the enhanced Lewis acid character of the Zr metal centers, binding of GP, and its nanoconfinement into the carborane-decorated channels of the MOF. Our findings highlight the potential for mCB-MOF-2 to be used in herbicide capture and degradation to non-hazardous products. We envision the employment of this MOF in a continuous process in agricultural settings. The unique biomimetic photodegradation of GP by mCB-MOF-2 provides an opportunity for future investigations into the mechanisms of GP photodegradation and how this can be expanded to other herbicides.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.3c02019.Syntheses, experimental details, WAXS spectra, PXRD patterns, isotherms, 1H and 31P NMR spectra, and GCMS chromatograms (PDF)

Supplementary Material

ja3c02019_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

L.G., F.T., C.V., and J.G.P. thank MINECO grants CTQ20, PID2019-106832RB-I00, and the Generalitat de Catalunya (2017/SGR/1720) for financial support. ICMAB acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under grant SEV-2015-0496. L.G. acknowledges the China Scholarship Council (CSC) for his Ph.D. grants (201609110106). K.C.S. thanks the Department of Chemistry at Oregon State University for support through the start-up funding and SciRis-II. M.T.N. thanks the Department of Chemistry for support through the Milton Harris Graduate Fellowship (2022). The authors thank Dr. Patrick Reardon and Steve Huhn for helpful discussions associated with the NMR experiments and Dr. Christine Pastorek and the Integrated Lab at OSU for access to GCMS instrumentation. Some experiments were performed at the XALOC and NCD-SWEET beamlines of the ALBA synchrotron with the support of ALBA staff.
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