Robust Two-Dimensional Porphyrinic Copper Metal–Organic Framework for Rapid and Efficient Cr(VI) Removal from Water

Abstract

Hexavalent chromium (Cr­(VI)) is a highly toxic and mobile water contaminant, and its efficient removal under environmentally relevant conditions remains a significant challenge. Herein, we report the synthesis of a two-dimensional porphyrinic copper metal–organic framework (2D-CuTpyP_MOF) containing mixed-valence Cu­(I)/Cu­(II) sites and evaluate its performance for rapid Cr­(VI) adsorption from aqueous solution. The framework was synthesized through the coordination-driven assembly of 5,10,15,20-tetra­(4-pyridyl)­porphyrin (TpyP) with Cu­(II) acetate and characterized using FT-IR, powder X-ray diffraction, SEM-EDS, X-ray photoelectron spectroscopy (XPS), and N₂ sorption analysis. Comparison of the experimental and simulated p-XRD patterns confirmed the successful formation of the targeted crystalline framework, while XPS revealed the coexistence of Cu­(I) and Cu­(II) species. Despite its relatively low BET surface area (12 m² g^–1), 2D-CuTpyP_MOF exhibited a maximum Cr­(VI) adsorption capacity of 96.51 mg g^–1 and achieved 95.5% removal within 10 min at pH 7.0. The adsorption process followed the Langmuir isotherm and pseudo-second-order kinetic model and was found to be spontaneous and endothermic. Structural characterization after adsorption demonstrated excellent framework stability, whereas the combination of adsorption performance and physicochemical characterization suggests that Cr­(VI) uptake is governed predominantly by chemically active Cu-porphyrinic sites rather than by extensive internal porosity. These findings demonstrate that layered porphyrinic MOFs with mixed-valence metal sites are promising adsorbents for rapid oxyanion removal and provide new insights into the design of low-porosity, chemically active frameworks for water purification.

1. Introduction

Hexavalent chromium (Cr­(VI)) is one of the most hazardous inorganic pollutants owing to its high toxicity, carcinogenicity, and mobility in aquatic environments. It is released into water through a variety of industrial activities, including electroplating, leather tanning, textile manufacturing, metallurgy, pigment production, and mining. , Once discharged, Cr­(VI) predominantly exists as soluble oxyanions (HCrO₄ ^– CrO₄ ^2– and Cr₂O₇ ^2–), which readily migrate through aquatic systems and can accumulate in living organisms, posing serious risks to both human health and the environment. Consequently, increasingly stringent regulations on chromium concentrations in drinking water have intensified the demand for efficient and sustainable treatment technologies. −

Among the available remediation technologies, adsorption has emerged as one of the most attractive approaches because of its operational simplicity, low energy requirements, high removal efficiency, and ease of implementation. − Conventional adsorbents, including activated carbons, clays, polymeric materials, and carbon-based nanomaterials, have demonstrated promising Cr­(VI) removal efficiencies; however, many suffer from limited selectivity, slow adsorption kinetics, poor regeneration, or reduced performance under complex water matrices. − These limitations have stimulated the development of advanced adsorbent materials with improved stability, accessibility of active sites, and higher affinity toward chromate species.

Metal-organic frameworks (MOFs) have attracted considerable attention as next-generation adsorbents because of their tunable structures, chemical versatility and the possibility of incorporating specific functional groups and metal centers into well-defined crystalline architectures. − Numerous MOFs have been investigated for Cr­(VI) removal, including Zr-, Fe-, , and Cu-based frameworks, , many of which exhibit high adsorption capacities and rapid uptake. Nevertheless, adsorption performance is frequently associated with extremely high surface areas and permanent porosity, although increasing evidence suggests that chemically accessible surface sites may play a more important role than total pore volume in controlling adsorption behavior.

Porphyrin-based MOFs (PMOFs) − constitute a particularly attractive subclass of MOFs because the conjugated porphyrin macrocycle provides multiple nitrogen coordination sites, excellent chemical stability, and rich electronic properties that facilitate interactions with metal ions and oxyanions. − In addition to their widespread application in photocatalysis, adsorption, and pollutant degradation, − porphyrinic frameworks have recently emerged as promising adsorbents for environmental remediation. − Furthermore, two-dimensional (2D) MOFs offer additional advantages over their three-dimensional (3D) counterparts , by exposing a larger fraction of chemically active sites, reducing diffusion limitations, and promoting rapid mass transfer during adsorption. ,

Despite these advances, reports describing standalone two-dimensional Cu-porphyrin MOFs for rapid Cr­(VI) adsorption under environmentally relevant conditions remain limited. Most porphyrinic Cu-MOFs have been developed as photocatalysts for Cr­(VI) reduction, which generally requires light irradiation, sacrificial reagents, or additional treatment steps. Consequently, the development of chemically robust two-dimensional porphyrinic Cu-MOFs capable of rapidly removing Cr­(VI) through adsorption alone remains a significant challenge. ,

In this study, we report the synthesis and application of a two-dimensional porphyrinic copper metal-organic framework (2D-CuTpyP_MOF) assembled from 5,10,15,20-tetra­(4-pyridyl)­porphyrin (TpyP) and Cu­(II) acetate. The material was comprehensively characterized using FT-IR, p-XRD, SEM-EDS, XPS, and nitrogen sorption analysis, and its adsorption performance toward Cr­(VI) was systematically evaluated through equilibrium, kinetic, thermodynamic, and pH-dependent studies. Particular attention was devoted to elucidating the relationship between the framework structure and adsorption behavior. The results demonstrate that efficient Cr­(VI) removal can be achieved through chemically active mixed-valence Cu­(I)/Cu­(II) sites within a layered porphyrinic framework, even in the absence of a high accessible surface area. These findings provide new insights into the design of chemically active, low-porosity MOFs for water purification.

2. Materials and Methods

2.1. Materials

All chemicals were purchased from commercial suppliers and used without further purification. Propionic acid, pyrrole, 4-pyridinecarboxaldehyde, dichloromethane, methanol, chloroform, copper­(II) acetate, potassium dichromate (K₂Cr₂O₇), hydrochloric acid, sodium hydroxide, sulfuric acid, and 1,5-diphenylcarbazide (DPC) were obtained from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany), unless otherwise stated. Hydrochloric acid was supplied by PanReac AppliChem (Barcelona, Spain). Deionized water was used in all experiments.

2.2. Synthesis of TpyP and 2D-CuTpyP_MOF

2.2.1. Synthesis of TpyP

5,10,15,20-Tetra­(4-pyridyl)­porphyrin (TpyP) was synthesized according to the Adler-Longo procedure with minor modifications (Figure ). , Propionic acid (50 mL) was heated to reflux, followed by the addition of pyridine-4-carboxaldehyde (3.77 mL, 40 mmol). After 10 min, pyrrole (2.77 mL, 40 mmol) was added dropwise and the reaction mixture was refluxed for 2 h. After cooling to room temperature, the mixture was neutralized to pH 8 using aqueous ammonia.

1.

Synthetic route for the preparation of TpyP.

The product was extracted using dichloromethane/methanol (1:1, v/v), purified by silica gel column chromatography, and precipitated by the addition of dichloromethane and cold methanol. The resulting purple solid was collected by filtration, washed with methanol, and dried under vacuum to afford TpyP in 5.2% isolated yield (0.321 g). The product was characterized using FT-IR, UV-Vis, MALDI-TOF MS, ¹H NMR, and p-XRD (Figures S2–S6).

2.2.2. Synthesis of 2D-CuTpyP_MOF

The two-dimensional porphyrinic copper metal-organic framework (2D-CuTpyP_MOF) was synthesized following a previously reported procedure with minor modifications (Figure ). TpyP (60 mg) and Cu­(OAc)₂ (85.7 mg) were dissolved in a methanol/chloroform mixture (1:1, v/v) and stirred for 35 min at room temperature. The reaction mixture was then left undisturbed for 24 h to allow framework formation.

2.

Schematic representation of the synthesis of 2D-CuTpyP_MOF.

The resulting red precipitate was collected by vacuum filtration, washed with dichloromethane, methanol, and deionized water, and dried at 100 °C for 3 h to yield 50.8 mg of 2D-CuTpyP_MOF. The material was subsequently characterized using FT-IR, p-XRD, SEM-EDS, XPS, and nitrogen sorption measurements.

2.3. Adsorption Experiments

Batch adsorption experiments were performed by adding a predetermined amount of 2D-CuTpyP_MOF to 20 mL of Cr­(VI) solution in 50 mL polypropylene tubes. The suspensions were agitated at 80 rpm using an overhead shaker and rotator. The effects of solution pH (3–9), contact time (5–120 min and 24 h for equilibrium), initial Cr­(VI) concentration (2–100 mg L^–1), adsorbent dosage (0.3–1.0 g L^–1), and temperature (293–323 K) were systematically investigated.

After adsorption, the suspensions were filtered through 0.45 μm nylon membranes, and the residual Cr­(VI) concentration was determined spectrophotometrically. The removal efficiency (R, %) and adsorption capacity (Q _e) were calculated using eqs and .

$$R(\%)=\left(\frac{C_0-C_{\mathrm{f}}}{C_0}\right)\times 100\%$$ 1

$$Q_{\mathrm{e}}=\frac{(C_0-C_{\mathrm{e}})\times V}{m}$$ 2

where C ₀ denotes the initial Cr­(VI) concentration, while C _f refers to the final (residual) concentration in mg/L. Depending on the parameter being evaluated, C _f may correspond either to the equilibrium concentration (C _e) or the concentration at a specific time (C _t ), In addition, V represents the volume of the solution (L), and m indicates the mass of the adsorbent (g).

2.3.1. Adsorption Isotherms

The Langmuir and Freundlich models were used to describe the equilibrium adsorption behavior. The Langmuir model assumes monolayer adsorption on a homogeneous surface, whereas the Freundlich model describes adsorption on heterogeneous surfaces. The corresponding equations are presented below. −

$$Q_{\mathrm{e}}=\frac{Q_m{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$ 3

$$Q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{1/n}$$ 4

2.3.2. Kinetics

Pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were employed to describe the adsorption process. The equations are given below.

$$Q_t=Q_{\mathrm{e}}(1-{\mathrm{e}}^{-k_{1}t})$$ 5

$$Q_t=\frac{k_2{Q}_{\mathrm{e}}^{2}t}{1+k_2{Q}_{\mathrm{e}}t}$$ 6

where Q _e represents the amount of Cr­(VI) adsorbed at equilibrium (mg/g), whereas Q _t denotes the amount of Cr­(VI) adsorbed at a given time t (mg g^–1). The parameter k ₁ corresponds to the rate constant of the pseudo-first-order (PFO) model (min^–1), with t indicating contact time (min). Similarly, k ₂ refers to the rate constant of the pseudo-second-order (PSO) model (g mg^–1 min^–1).

2.4. Determination of Residual Cr­(VI) Concentration

The residual Cr­(VI) concentration was determined using the standard 1,5-diphenylcarbazide (DPC) colorimetric method according to APHA. After reaction with acidified DPC for 15 min, the absorbance was measured at 540 nm using a UV-Vis spectrophotometer. Cr­(VI) concentrations were determined from a calibration curve. ,

2.5. Thermodynamics

The thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were determined from adsorption experiments conducted at 293–323 K using eqs –.

$$K_{\mathrm{c}}=\frac{C_{\mathrm{s}}}{C_{\mathrm{e}}}$$ 7

$${\mathrm{\Delta }G}^{°}={-\mathit{RT}\ln (K}_{\mathrm{c}})$$ 8

$${{\mathrm{\Delta }G}^{°}=\mathrm{\Delta }H}^{°}-T{\mathrm{\Delta }S}^{°}$$ 9

$${\ln (K}_{\mathrm{c}})=(-\frac{{\mathrm{\Delta }H}^{°}}{\mathit{RT}})+\frac{{\mathrm{\Delta }S}^{°}}{R}$$ 10

2.6. Characterization Techniques for TpyP and 2D-CuTpyP_MOF

The synthesized materials were characterized by UV-Vis spectroscopy, FT-IR, p-XRD, MALDI-TOF MS, and ¹H NMR. Morphology and elemental composition were examined by SEM-EDS. Surface chemical composition and oxidation states were investigated using XPS (Kratos Axis SUPRA, monochromated Al Kα source). Nitrogen adsorption-desorption measurements at 77 K were used to determine the BET surface area and pore-size distribution. The point of zero charge (pH_PZC) and chemical stability under different pH conditions were also evaluated.

3. Results and Discussion

3.1. Characterization of TpyP and 2D-CuTpyP_MOF

3.1.1. Characterization of TpyP

The successful synthesis of TpyP was confirmed by spectroscopic and structural characterization using UV-Vis, MALDI-TOF MS, ¹H NMR, FT-IR, and powder X-ray diffraction (p-XRD) analyses (Figures S2–S6). The UV-Vis spectrum (Figure S2) displays the characteristic Soret band at 416 nm, arising from the S₀ → S₂ (π → π*) transition of the conjugated porphyrin macrocycle, together with four Q bands at 510, 543, 585, and 647 nm corresponding to S₀ → S₁ transitions. The presence of four Q bands is characteristic of free-base porphyrins and reflects the reduced molecular symmetry resulting from the two inner N–H protons, in agreement with previous reports.

MALDI-TOF mass spectrometry (Figure S4) exhibits a molecular ion peak at m/z 618.62, together with the expected isotopic distribution, in excellent agreement with the calculated molecular mass of TpyP and literature data.

The ¹H-NMR spectrum (Figure S3) further confirms the formation of the porphyrin macrocycle, displaying resonances at δ = 8.89 ppm assigned to the β-pyrrolic protons doublets at δ = 8.18 and 9.09 ppm corresponding to the pyridyl protons and characteristic inner N–H peak at δ = −2.90 ppm, which is typical of free-base porphyrins.

The p-XRD pattern (Figure S5) showed distinct reflections at 2θ = 8.5 and 9.4°, associated with large interplanar spacings due to the extended porphyrin structure. Additional intense peaks at 2θ = 20 and 21.5° indicated high crystallinity, while reflections at 2θ = 25.4, 26.3, and 28.8° are consistent with intermolecular π–π interactions. The observed reflections are in good agreement with previously reported diffraction patterns of TpyP and are characteristic of the ordered packing of porphyrin molecules stabilized by π–π interactions.

The FT-IR spectrum (Figure S6) shows the characteristic vibrational features of TpyP, including N–H stretching at 3310 cm^–1, aromatic C–H stretching between 3023 and 3093 cm^–1, CN stretching at 1593 cm^–1, aromatic CC vibrations in the 1542–1468 cm^–1 region, C–N stretching bands at 1100–1000 cm^–1 and pyrrolic C–H bending at 784 cm^–1, consistent with previously reported spectra.

Collectively, the spectroscopic and structural characterization confirms the successful synthesis of highly crystalline TpyP, which served as the precursor for the preparation of the 2D-CuTpyP_MOF.

3.1.2. Characterization of 2D-CuTpyP_MOF

3.1.2.1. FT-IR Analysis of 2D-CuTpyP_MOF before and after Cr­(VI) Adsorption

The FT-IR spectra of TpyP, Cu­(II) acetate, and the synthesized 2D-CuTpyP_MOF are presented in Figure . The spectrum of Cu­(II) acetate exhibits the characteristic asymmetric and symmetric stretching vibrations of the acetate groups at 1600 and 1444 cm^–1, respectively, together with Cu–O vibrations at 686 and 625 cm^–1, in agreement with previous reports.

3.

FT-IR spectra of (a) TpyP, (b) copper acetate and (c) 2D-CuTpyP_MOF.

Following coordination, the FT-IR spectrum of 2D-CuTpyP_MOF remains the characteristic vibrational features of the porphyrinic linker while exhibiting subtle changes associated with framework formation. The broad band centered at approximately 1590 cm^–1, assigned to the CN stretching vibration of the pyridyl groups, becomes broader and slightly shifted compared with the free ligand, indicating coordination of Cu­(II) ions to the pyridyl nitrogen atoms. The aromatic CC stretching bands (1540–1468 cm^–1), C–N stretching vibrations (1401–1351 cm^–1) and the characteristic aromatic C–H out-of-plane bending band at 784 cm^–1 remain clearly visible, demonstrating that the porphyrin macrocycle is preserved during framework assembly. In addition, the Cu–O vibrations observed at approximately 686 and 625 cm^–1 further support the successful incorporation of copper within the coordination framework. ,,

To evaluate the structural stability of the material, the FT-IR spectra were recorded before and after Cr­(VI) adsorption (Figure ). The two spectra are nearly identical, with the characteristic framework vibrations remaining unchanged after adsorption. In particular, no significant peak shifts, disappearance of characteristic bands, or new absorption features are observed following Cr­(VI) uptake. The preservation of the CN, aromatic CC, C–N, and Cu–O vibrations indicates that the Cu-porphyrinic framework remains structurally intact throughout the adsorption process. These results demonstrate good structural stability of 2D-CuTpyP_MOF and negligible linker leaching under the investigated adsorption conditions, supporting its potential for further evaluation in water treatment applications.

4.

FT-IR spectra of 2D-CuTpyP_MOF (a) after adsorption of Cr­(VI) and (b) before adsorption.

3.1.2.2. p-XRD Analysis of 2D-CuTpyP_MOF

The powder X-ray diffraction (p-XRD) patterns of TpyP, Cu­(II) acetate, and the synthesized 2D-CuTpyP_MOF are shown in Figure . The diffraction pattern of hydrated copper acetate exhibits numerous sharp reflections, confirming the high crystallinity of the precursor and agreeing well with previously reported data. ,

5.

p-XRD spectrum of (a) simulated single crystal p-XRD of 2D-CuTpyP_MOF, (b) 2D-CuTpyP_MOF, (c) copper acetate and (d) TpyP.

Following coordination-driven self-assembly, the p-XRD pattern of 2D-CuTpyP_MOF differed markedly from those of both the free TpyP ligand and Cu­(II) acetate, indicating the formation of a new crystalline phase. The appearance of intense reflections in the low-angle region, particularly the prominent diffraction peak at 2θ = 6.1°, is characteristic of layered porphyrinic coordination frameworks possessing large periodic repeat distances. Additional reflections at 7.2, 8.9, 10.9, 13.6, 14.3, 14.6, 15.1, 15.8, 17.6, 20.2, 22.8, 23.5, 24.2, 27.9, 28.6, and 33.9° further confirm the formation of a well-defined crystalline structure, in agreement with previously reported Cu-TpyP frameworks.

Compared with the diffraction pattern of the free ligand, the appearance of new reflections together with significant changes in peak positions and relative intensities demonstrates that coordination of Cu­(II) ions induces a substantial reorganization of the molecular packing, resulting in the formation of an extended two-dimensional coordination network. The intense low-angle reflections are consistent with the layered architecture of the framework, whereas the higher-angle peaks originate from lattice planes with shorter interplanar spacings, reflecting the long-range structural order of the crystalline material.

To further verify the structural assignment, the experimental p-XRD pattern was compared with the simulated diffraction pattern calculated from the reported single-crystal structure of 2D-CuTpyP_MOF (Figure ). Although some differences in relative peak intensities are observed between the experimental and simulated p-XRD patterns, the experimental pattern obtained for 2D-CuTpyP_MOF is almost identical to the experimental p-XRD patterns reported for the same Cu-TPyP-based 2D framework , as well as for closely related Cu-porphyrin MOF systems, particularly in the low-angle region, where the characteristic reflections of the layered framework were observed. The differences from the simulated pattern may be attributed to preferred orientation and different exposure of crystallographic planes, as well as crystallite size, crystallinity, solvent-related effects and guest molecules. , Therefore, the similarity of the main experimental reflections with those reported for real bulk Cu-TPyP-based materials supports the formation of the proposed 2D-CuTpyP_MOF structure.

Overall, the p-XRD results demonstrate the successful synthesis of a highly crystalline two-dimensional Cu-porphyrin framework. The close similarity between the diffraction pattern of 2D-CuTpyP_MOF and those reported for closely related Cu-TPyP-based two-dimensional frameworks, together with the absence of reflections attributable to the starting materials, provides strong evidence for the formation of the desired MOF.

3.1.2.3. SEM and EDS Analysis of 2D-CuTpyP_MOF before and after Cr­(VI) Adsorption

The morphology of 2D-CuTpyP_MOF was investigated by scanning electron microscopy (SEM) (Figure ). The material consists of irregularly shaped, plate-like particles that aggregate into larger secondary structures. with rough and heterogeneous surfaces. This morphology is characteristic of layered two-dimensional porphyrinic metal-organic frameworks and is consistent with previously reported Cu-porphyrin MOFs. ,− Although the intrinsic porosity of the framework cannot be directly visualized because of particle stacking and the bulk nature of the sample, the observed morphology is in good agreement with the expected layered architecture. Energy-dispersive X-ray spectroscopy (EDS) was employed to verify the elemental composition of the synthesized framework (Figure S7). The spectra reveal the presence of Cu, C, N, and O, corresponding to the expected constituent elements of 2D-CuTpyP_MOF, while no additional signals associated with impurities were detected within the instrumental detection limits. Furthermore, elemental mapping demonstrates a homogeneous distribution of all constituent elements throughout the material, supporting the uniform formation of the coordination framework.

6.

SEM images of 2D-CuTpyP_MOF before and after adsorption.

Following Cr­(VI) adsorption, the SEM images reveal that the morphology of 2D-CuTpyP_MOF remains essentially unchanged, with the aggregated plate-like particles and rough surface features being well preserved (Figure ). These observations indicate that the adsorption process does not induce noticeable structural degradation of the framework. Consistent with this, EDS spectra of the Cr­(VI)-loaded material display the characteristic Cu, C, N, and O signals together with the appearance of a distinct chromium peak (Figures S8 and S9). Elemental mapping further confirms the homogeneous distribution of chromium over the MOF particles (Figure ), providing direct evidence for successful chromium uptake. Overall, the SEM and EDS analyses demonstrate that 2D-CuTpyP_MOF possesses the layered morphology expected for two-dimensional porphyrinic coordination frameworks and retains its structural integrity after chromium adsorption. The preservation of the morphology together with the detection of chromium on the material surface provides strong evidence for the excellent stability of the framework during the adsorption process.

7.

SEM-EDS elemental mapping of Cr-loaded 2D-CuTpyP_MOF after adsorption, confirming the presence and spatial distribution of chromium on the material.

3.1.2.4. XPS Analysis of 2D-CuTpyP_MOF

The survey spectrum revealed the presence of C, O, N, and Cu at the surface of the sample. Detailed scans of the most prominent peak for each element were collected, however, in this analysis, only N and Cu are included as the C and O signal is greatly influenced by the side carbon chains of the MOFs.

The detailed spectra of the N 1s and Cu 2p_3/2 regions are presented in Figure a,b, respectively. Firstly, the atomic ratio between N and Cu was calculated and was found to be 3.65, revealing an excess of Cu compared to the porphyrinic structures, which have 8 N atoms. The Cu 2p_3/2 region reveals two distinct peaks at 932.5 and 934.5 eV, along with satellite features around 942 eV. It is well established that these features are present only on Cu­(II) and thus fitting was performed to distinguish between Cu­(II) and Cu­(I) as demonstrated in Figure b. The ratio between the two oxidation states was 1. It has been suggested that the Cu­(I) is present at the MOF edge sites, while the Cu­(II) is at the centre of the porphyrinic structure. Revisiting the N/Cu ratio, if only the Cu­(II) is taken into consideration, the ratio is 7.3, much closer to the nominal 8. However, a closer inspection of the N 1s spectrum, reveals that only 62% of the porphyrinic structures are metalated, after fitting for N–M, N–C, and N–H components and restricting their ratios, as it was described for similar Cu and H₂ phthalocyanine molecules. This seems to be in contrast with the N/Cu­(II) ratio, which suggested complete metalation. To get more information on this, the satellite structure of known oxides, Cu phthalocyanine, and our sample was compared and while our 2D-CuTpyP_MOF resembled that of the Cu phthalocyanine, it failed to match it. These lead us to the conclusion that 62% of the porphyrinic structures are metalated, Cu­(I) is present at the edges of the MOF structure and while the majority of the Cu²⁺ sits at the porphyrin centres, other species are present, most likely in the form of hydroxides. Due to the complexity of the system no further fitting was performed, as the error would be significant.

8.

(a) XPS spectrum of the N 1s region, and (b) XPS spectrum of the Cu 2p_3/2 region.

3.1.2.5. Nitrogen Porosimetry Isotherms and Literature Comparison

Nitrogen adsorption-desorption measurements (Table S1) at 77 K assessed the textural properties of 2D-CuTpyP_MOF. The isotherm indicated a Type II profile, , suggesting primary adsorption on the external surface with limited microporosity (low nitrogen uptake at P/P ₀ <0.1) (Figure a). A slight hysteresis at intermediate pressures implies minor mesoporosity, while a surge at P/P ₀ >0.9 relates to capillary condensation. The BET surface area was about 12 m² g^–1, indicating low surface area, with a total pore volume of 0.09 cm³ g^–1 (Figure b). Pore size analysis yielded 19 m² g^–1 (BJH) and 36 m² g^–1 (DFT) surface areas, with consistent pore volumes (Figure c). Micropore volume was 0.0107 cm³ g^–1, highlighting minimal microporosity. The DFT pore size distribution showed a maximum near 6.2 Å and broader features for mesoporous voids, with BJH indicating pore sizes around 20.7 Å, confirming predominant porosity from larger voids (Figure d).

9.

Nitrogen adsorption-desorption isotherm and related textural analysis of the 2D-CuTpyP_MOF: (a) adsorption-desorption isotherm at 77 K, (b) BET plot, (c) DFT pore size distribution, and (d) cumulative pore volume plot.

Compared to porous CuTPyP analogues reported in Table for gas storage applications, 2D-CuTpyP_MOF possesses a substantially lower surface area and porosity. However, its good chemical stability and high Cr­(VI) adsorption capacity demonstrate that adsorption is governed predominantly by chemically active sites associated with the porphyrinic copper framework rather than by extensive internal porosity.

1. Comparison of Structural Features, Nitrogen Sorption Properties, and Applications of Representative TpyP-Based Cu Frameworks Reported in the Literature and the 2D-CuTpyP_MOF Synthesized in This Work.

material	metal node	BET (m2 g–1)	isotherm type	application	ref.
CuTPyP-Cu(OAc)2	Cu paddlewheel	812	I	CO2 storage
CuTPyP-Cu(naphthoate)	Cu paddlewheel	low	I/II	CO2/N2 separation
SnXTPyP-CuOAc	Cu paddlewheel	16-20	II	CO adsorption
2D-CuTpyP_MOF	Cu paddlewheel	12	II	Cr(VI) adsorption	this work

Overall, the low surface area and limited microporosity are consistent with the structural characteristics of layered two-dimensional porphyrin-based MOFs. The low N₂ uptake may arise from strong interlayer interactions, partial pore blocking, incomplete activation, and/or aggregation of the layered crystallites, which reduce the accessibility of the intrinsic porosity. The nitrogen sorption results indicate that 2D-CuTpyP_MOF possesses a relatively low accessible surface area and limited microporosity compared with permanently porous CuTPyP analogues reported in the literature. The type II adsorption isotherm and the small BET surface area suggest that adsorption is dominated by external surfaces and interparticle voids rather than by a fully accessible intrinsic microporous network. Such behavior may arise from strong interlayer interactions between the porphyrinic sheets, aggregation of the layered crystallites, partial pore blocking, or incomplete accessibility of the framework pores to N₂ molecules at 77 K.

3.1.2.6. Point of Zero Charge (pH_PZC) Analysis

The surface charge properties of 2D-CuTpyP_MOF were investigated by pH_PZC measurements, determined using the pH substance method and the resulting curve is shown in Figure . , Interestingly, the material exhibits two apparent charge-neutrality regions at approximately 3.67 and 6.25, reflecting the presence of multiple protonation and deprotonation equilibria associated with the pyridyl nitrogen atoms and Cu coordination sites within the porphyrinic framework. Similar behavior has previously been reported for porphyrin- and pyridyl-containing coordination materials possessing chemically distinct surface functionalities. − Below pH 3.67, the surface is expected to be predominantly positively charged owing to protonation of the nitrogen-containing groups, whereas above pH 6.25 progressive deprotonation results in an increasingly negative surface charge. Between these two regions, the surface charge is governed by the coexistence of different protonation states, giving rise to the observed dual pH_PZC behavior.

10.

Diagram of the pH_PZC of 2D-CuTpyP_MOF.

The pH_PZC is particularly relevant for Cr­(VI) adsorption because chromium speciation varies with solution pH, with HCrO₄ ^– predominating under acidic conditions and CrO₄ ^2– becoming the main species at neutral and alkaline pH values. Despite these changes in both surface charge and chromium, 2D-CuTpyP_MOF maintains high adsorption efficiency across a broad pH range. This observation suggests that electrostatic attraction alone cannot account for the adsorption behavior. Instead, Cr­(VI) uptake is likely governed by the combined contribution of electrostatic interactions, surface complexation, and coordination with chemically active Cu-porphyrinic sites, as discussed in the proposed adsorption mechanism.

Overall, the pH_PZC results demonstrate that the adsorption performance of 2D-CuTpyP_MOF is only weakly influenced by surface charge alone, highlighting the important role of chemically active adsorption sites in achieving efficient Cr­(VI) removal over a wide pH range.

3.1.2.7. Chemical Stability of 2D-CuTpyP_MOF

The chemical stability of 2D-CuTpyP_MOF was evaluated under acidic, neutral, and alkaline conditions by dispersing the material in aqueous solutions at pH 3, 7, and 11 for 24 h. Throughout the stability tests, the suspensions retained their characteristic orange-red color, while the corresponding supernatants remained clear and colorless after centrifugation and filtration (Figure S10), indicating the absence of visible framework degradation. UV-Vis analysis of the filtrates revealed no detectable porphyrin absorption bands, demonstrating negligible leaching of the organic linker under all investigated conditions. These results indicate that the Cu-porphyrinic framework remains chemically stable over a broad pH range relevant to water treatment applications. The excellent aqueous stability is further supported by the preservation of the FT-IR, p-XRD, and SEM-EDS characteristics after Cr­(VI) adsorption collectively demonstrating that the framework retains both its chemical composition and crystalline structure during the adsorption process. The high stability of 2D-CuTpyP_MOF under acidic, neutral, and alkaline conditions highlights its suitability for practical water purification applications, where adsorbents are frequently exposed to variable water chemistries.

3.2. Evaluation of Adsorption Process

3.2.1. Effect of Initial Solution pH on the Adsorption of Cr­(VI)

Solution pH is one of the most important parameters influencing Cr­(VI) adsorption, as it simultaneously affects both the distribution of chromium and the surface charge of the adsorbent. The effect of pH on the adsorption performance of 2D-CuTpyP_MOF was therefore investigated over the pH range of 3–9, and the results are presented in Figure . As shown in Figure , the adsorption efficiency remained high throughout the investigated pH range, with the maximum Cr­(VI) removal observed under mildly acidic conditions. A gradual decrease in adsorption was observed with increasing pH however, the material maintained excellent adsorption performance even under near-neutral conditions. The ability of 2D-CuTpyP_MOF to efficiently remove Cr­(VI) at pH 7 is particularly significant from a practical perspective, as treatment of industrial wastewater under neutral conditions minimizes the need for extensive pH adjustment. ,,

11.

Effect of initial solution pH for the adsorption of Cr­(VI) on 2D-CuTpyP_MOF. C ₀ 5 mg/L, dose 0.5 g/L, 293 K, 24 h contact time.

The observed pH dependence can be explained by the combined influence of chromium distribution and the surface charge of the adsorbent. Under acidic conditions (pH < 6), Cr­(VI) exists predominantly as HCrO₄ ^–, whereas CrO₄ ^2– becomes the dominant species at neutral and alkaline pH values. At the same time, the pH_PZC measurements (Section ) indicate that the surface of 2D-CuTpyP_MOF is predominantly positively charged under acidic conditions owing to protonation of the pyridyl nitrogen atoms and Cu coordination sites. These conditions promote favorable electrostatic attraction between the positively charged surface and negatively charged chromate species, resulting in enhanced adsorption.

As the solution pH increases, progressive deprotonation of the framework leads to a less positively charged and eventually negatively charged surface, reducing the electrostatic contribution to adsorption. Nevertheless, the relatively small decrease in Cr­(VI) removal observed at neutral and alkaline pH values indicates that electrostatic attraction is not the sole driving force governing adsorption. Instead, the adsorption behavior suggests the additional contribution of chemically active Cu-porphyrinic sites capable of interacting with chromate species through surface complexation and coordination interactions. This interpretation is consistent with the XPS analysis, which revealed the coexistence of mixed-valence Cu­(I)/Cu­(II) species, as well as with the proposed adsorption mechanism discussed in Section .

The high adsorption efficiency maintained over a broad pH range distinguishes 2D-CuTpyP_MOF from many previously reported MOF-based adsorbents that exhibit optimal performance only under strongly acidic conditions. This behavior demonstrates that the layered porphyrinic framework provides chemically accessible adsorption sites capable of efficiently binding Cr­(VI) even when electrostatic interactions become less favorable, highlighting the practical applicability of the material for water purification under environmentally relevant conditions.

3.2.2. Effect of Contact Time and Kinetic Modeling

The effect of contact time on Cr­(VI) adsorption by 2D-CuTpyP_MOF was investigated to evaluate the adsorption rate and determine the equilibrium time. As shown in Figure a, Cr­(VI) uptake proceeded rapidly during the initial stage of adsorption, with approximately 95.5% removal achieved within the first 10 min, followed by a gradual approach to equilibrium. The rapid initial adsorption can be attributed to the abundance of readily accessible active sites on the external surface of the layered framework. As these sites become progressively occupied, the adsorption rate decreases until equilibrium is established.

12.

(a) Effect of contact time on Cr­(VI) adsorption by 2D-CuTpyP_MOF and (b) nonlinear fitting of the pseudo-first-order and pseudo-second-order kinetic models. Conditions: C ₀ = 5 mg L^–1, adsorbent dosage = 0.5 g L^–1, pH 7.0. and T = 293 K.

The remarkably fast adsorption observed for 2D-CuTpyP_MOF is particularly advantageous for practical water treatment applications, where short contact times are essential for efficient continuous-flow or batch processes. The rapid uptake further suggests that diffusion limitations are minimal, which is consistent with the layered morphology observed by SEM and the high accessibility of the chemically active Cu-porphyrinic sites.

To further elucidate the adsorption kinetics the experimental data were fitted using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models (Figure b). The kinetic parameters obtained from both models are summarized in Table . The PSO model provided a significantly better fit to the experimental data, as evidenced by its higher correlation coefficient (R ² = 0.9994) and the close agreement between the experimental (Q _exp = 6.07 mg g^–1) and calculated equilibrium adsorption capacities (Q _cal = 6.05 mg g^–1). In contrast, the PFO model showed a lower correlation coefficient (R ² = 0.9954) and slightly underestimated the equilibrium adsorption capacity (Q _cal = 5.93 mg g^–1).

2. Kinetic Parameters Obtained from the Pseudo-First-Order and Pseudo-Second-Order Models for Cr­(VI) Adsorption Onto 2D-CuTpyP_MOF.

		PFO			PSO
adsorbent	Q exp (mg g–1)	K 1 (min–1)	Q cal (mg g–1)	R 2	K 2 (L (mg min)−1)	Q cal (mg g–1)	R 2
2D-CuTpyP_MOF	6.07	0.4670	5.93	0.9954	0.2601	6.05	0.9994

The superior agreement of the PSO model suggests that the adsorption rate is governed predominantly by strong interactions between Cr­(VI) species and the chemically active surface sites of the adsorbent rather than by simple physical diffusion alone. This interpretation is consistent with the coexistence of mixed-valence Cu­(I)/Cu­(II) sites revealed by XPS analysis and the proposed adsorption mechanism involving surface complexation and coordination interactions. Similar kinetic behavior has been reported for Cu-based MOFs and porphyrinic coordination frameworks used for heavy metal and oxyanion adsorption.

Overall, the kinetic analysis demonstrates that the combination of a layered architecture and chemically accessible Cu-porphyrinic active sites enables exceptionally rapid Cr­(VI) uptake under near-neutral conditions. This behavior distinguishes 2D-CuTpyP_MOF from many previously reported MOF adsorbents that require substantially longer equilibrium times, highlighting its potential for practical water treatment applications.

3.2.3. Isotherm and Thermodynamic Studies

The equilibrium adsorption behavior of Cr­(VI) onto 2D-CuTpyP_MOF was evaluated using the Langmuir and Freundlich isotherm models to elucidate the interaction between Cr­(VI) species and the adsorbent surface. The experimental isotherms and the corresponding fitting parameters are presented in Figure and Table , respectively.

13.

Langmuir and Freundlich isotherm fittings for Cr­(VI) adsorption onto 2D-CuTpyP_MOF: dose 0.5 g/L, pH 7.0, 10 min, at different temperatures (303-323 K).

3. Constants of Langmuir and Freundlich Isotherm Models for the Adsorption of Cr­(VI) Onto 2D-CuTpyP_MOF: Dose 0.5 g/L, pH 7.0, 10 Min, at Different Temperatures (303–323 K).

		Freundlich isotherm model			Langmuir isotherm model
adsorbent	T (K)	1/n	K F (mg g–1) (L mg–1)1/n	R 2	Q m (mg g–1)	K L (L mg–1)	R 2
2D-CuTpyP_MOF	303	0.3762	20.10	0.8946	91.87	0.2154	0.9901
	313	0.3653	21.25	0.9020	91.01	0.2551	0.9943
	323	0.3598	23.29	0.8944	96.51	0.2642	0.9920

Among the two models, the Langmuir isotherm provided the best fit to the experimental data, exhibiting consistently higher correlation (R ² > 0.99) than the Freundlich model. This result indicates that Cr­(VI) adsorption occurs predominantly through monolayer adsorption on a relatively homogeneous surface containing energetically equivalent adsorption sites. The excellent agreement with the Langmuir model further suggests that lateral interactions between adsorbed species are minimal under the investigated experimental conditions.

The maximum Langmuir adsorption capacity (Q _m) generally increased with increasing temperature, from 91.87 mg g^–1 at 303 K to 96.51 mg g^–1 at 323 K, indicating that elevated temperatures enhance the affinity of the adsorbent toward Cr­(VI). ,, The relatively high adsorption capacities obtained for 2D-CuTpyP_MOF compare favorably with many previously reported MOF-based adsorbents for Cr­(VI) removal. − Considering the relatively low BET surface area of the material (12 m² g^–1), the observed adsorption performance highlights the dominant role of chemically accessible Cu-porphyrinic active sites rather than extensive internal porosity.

The Freundlich parameter 1/n remained between 0.35–0.38 over the investigated temperature range. Since 1/n < 1, the adsorption process can be considered highly favorable indicating strong affinity between Cr­(VI) ions and the adsorption sites of 2D-CuTpyP_MOF.

To further evaluate the nature of the adsorption process, the thermodynamic parameters Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), were calculated, and the results are summarized in Table .

4. Thermodynamic Parameters for Cr­(VI) Adsorption on 2D-CuTpyP_MOF.

material	T (K)	ΔG° (kJ mol–1)	ΔH° (kJ mol–1)	ΔS° (kJ mol–1 K–1)	R 2
2D-CuTpyP_MOF	303	–5.212	11.666	0.0557	0.9835
	313	–5.769
	323	–6.326

The negative values of ΔG° at all examined temperatures (−5.212 to −6.326 kJ mol^–1), demonstrate that Cr­(VI) adsorption occurs spontaneously. Furthermore, the increasingly negative ΔG° values with increasing temperature indicate that adsorption becomes progressively more favorable at higher temperatures, in good agreement with the experimental adsorption data shown in Figure .

14.

Effect of temperature on the adsorption of Cr­(VI) onto 2D-CuTpyP_MOF: C ₀ 5 mg L^–1, dose 0.5 g L^–1, pH 7.0, 10 min, at different temperatures (303–323 K).

The positive enthalpy change (ΔH° = 11.666 kJ mol^–1) indicates that the adsorption process is endothermic, suggesting that increased temperature promotes the interaction between Cr­(VI) species and the active adsorption sites of the framework. The relatively low magnitude of ΔH° (<40 kJ/mol) further suggests that adsorption is not dominated by strong irreversible chemical bonding but is more likely governed by the combined contribution of electrostatic interactions, surface complexation, and coordination interactions, consistent with the proposed adsorption mechanism.

The positive entropy change (ΔS° = 0.0557 kJ mol^–1 K^–1) indicates an increase in disorder at the solid-solution interface during the adsorption process. This behavior is commonly attributed to the release of hydration water molecules accompanying chromate adsorption and further supports the favorable interaction between Cr­(VI) species and the chemically active surface of the adsorbent.

Overall, the equilibrium and thermodynamic analyses demonstrate that Cr­(VI) adsorption onto 2D-CuTpyP_MOF is a spontaneous, endothermic, and highly favorable process that is well described by the Langmuir isotherm. The combination of relatively high adsorption capacity, rapid equilibrium, and efficient adsorption despite the material’s low surface area further emphasizes that adsorption is governed primarily by chemically active Cu-porphyrinic sites rather than by extensive porosity. These findings reinforce the potential of 2D-CuTpyP_MOF as an efficient adsorbent for Cr­(VI) remediation under environmentally relevant conditions.

3.2.4. Effect of Adsorbent Dosage

The effect of adsorbent dosage on Cr­(VI) removal by 2D-CuTpyP_MOF was investigated over the concentration range of 0.3–1.0 g L^–1, and the results are presented in Figure . Increasing the adsorbent dosage resulted in a substantial improvement in Cr­(VI) removal efficiency, which increased from approximately 70% at 0.3 g L^–1 to nearly 99% at 1.0 g L^–1. This behavior is attributed to the greater availability of accessible adsorption sites and the increased number of chemically active Cu-porphyrinic centers available for interaction with Cr­(VI) oxyanions.

15.

Effect of dosage of 2D-CuTpyP_MOF for adsorption of Cr­(VI): C ₀ 5 mg L^–1, pH 7.0, 10 min, at 293 K.

In contrast, the equilibrium adsorption capacity (Q _e) decreased from approximately 7.5 to 3.0 mg g^–1 with increasing adsorbent dosage. This inverse relationship is commonly observed in adsorption systems and arises because, at a fixed initial Cr­(VI) concentration, the available adsorption sites increasingly exceed the number of adsorbate molecules. Consequently, a significant fraction of the adsorption sites remains unoccupied, resulting in a lower adsorption capacity expressed on a mass basis. In addition, partial aggregation of MOF particles at higher dosages may reduce the effective surface accessibility and further limit the utilization of available adsorption sites.

A pronounced increase in removal efficiency was observed between 0.4 and 0.5 g L^–1, whereas only marginal improvements were achieved at higher dosages. This behavior indicates that an adsorbent dosage of 0.5 g L^–1 provides sufficient number of active adsorption sites to achieve efficient Cr­(VI) removal under the investigated experimental conditions. Accordingly, this dosage was selected for all subsequent adsorption experiments.

Overall, the dosage study demonstrates that 2D-CuTpyP_MOF achieves high Cr­(VI) removal using a relatively low adsorbent dosage while maintaining rapid adsorption kinetics. The efficient utilization of chemically accessible adsorption sites further supports the suitability of the layered porphyrinic framework for practical water treatment applications.

4. Proposed Adsorption Mechanism

Based on the combined results of the adsorption experiments, pH_PZC measurements, kinetic and thermodynamic analyses, and structural characterization, a plausible mechanism for Cr­(VI) adsorption onto 2D-CuTpyP_MOF is proposed (Figure ). The adsorption process appears to arise from the cooperative contribution of electrostatic interactions, surface complexation, and coordination with chemically active Cu-porphyrinic sites rather than from a single dominant mechanism. ,,−

16.

Proposed mechanism of Cr­(VI) adsorption onto 2D-CuTpyP_MOF.

Initially, Cr­(VI) oxyanions diffuse from the solution to the external surface and interlayer regions of the two-dimensional framework. The layered morphology observed by SEM, together with the rapid adsorption kinetics, suggests that the highly accessible surface of the material facilitates efficient mass transfer and minimizes diffusion limitations. This structural feature is consistent with the rapid Cr­(VI) uptake observed experimentally.

The pH_PZC measurements demonstrate that the surface charge of 2D-CuTpyP_MOF varies with solution pH owing to the protonation and deprotonation of the pyridyl nitrogen atoms and Cu coordination sites. Under acidic conditions, electrostatic attraction between the positively charged surface and HCrO₄ ^– is expected to contribute to adsorption, whereas at neutral and alkaline pH values, where CrO₄ ^2– predominates, electrostatic interactions become less favorable. Nevertheless, the consistently high adsorption efficiency across a broad pH range indicates that electrostatic attraction alone cannot account for the observed adsorption behavior. ,,−

XPS analysis revealed the coexistence of Cu­(I) and Cu­(II) species within the framework, suggesting the presence of mixed-valence copper sites. Such sites have previously been associated with coordinatively unsaturated or defect-related environments in Cu-based MOFs and may provide chemically active centers for interaction with chromate oxyanions through weak coordination or surface complexation. ,,− In addition, the nitrogen-containing pyridyl and porphyrinic moieties may contribute to adsorption through secondary coordination and electrostatic interactions, further enhancing the affinity of the framework toward Cr­(VI).

Although mixed-valence Cu centers may participate in redox processes, the present study does not provide direct spectroscopic evidence for the reduction of Cr­(VI) during adsorption. Consequently, the proposed mechanism is dominated by adsorption and surface complexation rather than by a redox-mediated pathway.

The adsorption behavior is well described by the Langmuir isotherm and the pseudo-second-order kinetic model, indicating predominantly monolayer adsorption involving strong interactions between Cr­(VI) species and chemically active surface sites. ,,,,, Furthermore, the spontaneous and endothermic nature of the process suggests that increasing temperature enhances the interaction between chromate species and the adsorption sites without evidence of irreversible chemical transformation.

The proposed mechanism is further supported by the excellent structural stability of the material after adsorption. FT-IR, p-XRD, and SEM-EDS analyses reveal that the Cu-porphyrinic framework remains chemically, crystallographically, and morphologically intact, while EDS confirms the presence of chromium on the surface of the adsorbent. Collectively, these findings indicate the rapid Cr­(VI) removal achieved by 2D-CuTpyP_MOF originates from the synergistic contribution of its layered architecture, chemically accessible mixed-valence Cu­(I)/Cu­(II) sites, and porphyrinic coordination environment, which together provide an efficient and robust platform for aqueous chromate adsorption. ,,−

5. Comparison with Literature

The adsorption performance of 2D-CuTpyP_MOF was compared with recently reported MOF-based adsorbents for Cr­(VI) removal (Table ). Although several MOFs exhibit higher maximum adsorption capacities, adsorption performance should be evaluated by considering not only the equilibrium capacity but also the operating conditions, including solution pH, contact time, and adsorbent dosage. From a practical perspective, rapid adsorption under environmentally relevant conditions is often more important than achieving the highest possible equilibrium capacity.

5. Comparison of the Cr­(VI) Adsorption Performance of 2D-CuTpyP_MOF with Recently Reported MOF-Based Adsorbents.

adsorbent	C 0 (mg/L)	pH	dosage (g/L)	R (%)	contact time (min)	Q m (mg/g)	fitting models	ref.
Ce-MOF-808	50	2.0	0.33		720	42.74	PSO, Langmuir
ZnPor-COF	30	5.0	0.50	99.7	60	273.09	PSO, Freundlich
Fe-BDC@AC	5	5.5	1.00	85.0	50	79.10	PFO, Langmuir
Z-A/Fe3O4/BC/MOF-5	15	5.0	0.30	95.1	120	47.57	PSO, Freundlich
3D-UiO-66-Ce(IV) MOF, Ce-asp	100	1.0	2.00	96.0	300	74.10	PSO, Langmuir
DUT-67	50	4.0		96.1		105.42	PSO, Langmuir
Cu (I)-MOF		6.0	0.45		180	95.92	PFO, Langmuir
Urea-MOF(Fe)	5.0	5.6	0.8	95.2	40	8.1	PSO, Langmuir
MIL-125(Ti)-AO	160	3.0	0.5		400	271.00	PSO, Langmuir
MXene@MIL-53-NH2	20	3.0	0.2	91.0	120	224.46	PSO, Langmuir
AG/Zn-MOF-0.8	10	9.0	1	86.0	300	62.5	PSO, Langmuir
2D-CuTpyP_MOF	5	7.0	0.50	95.5	10	96.51	PSO, Langmuir	this study

The 2D-CuTpyP_MOF developed in this work achieved 95.5% Cr­(VI) removal with only 10 min at neutral pH (pH 7.0) using a relatively low adsorbent dosage (0.5 g L^–1). This performance compares favorably with many previously reported MOF-based adsorbents, which typically require strongly acidic conditions, substantially longer equilibrium times, or higher adsorbent dosages to achieve similar removal efficiencies. For example, Ce-MOF-808 and MIL-125­(Ti)-AO require several hundred minutes to reach equilibrium, whereas 3D-UiO-66-Ce­(IV) operates efficiently only under highly acidic conditions (pH 1.0). In contrast, 2D-CuTpyP_MOF combines rapid adsorption kinetics with efficient operation under near-neutral conditions, making it more attractive for practical water treatment applications.

Although materials such as ZnPor-COF and MXene@MIL-53-NH₂, exhibit higher adsorption capacities, these systems generally require longer contact times and more acidic operating conditions. Moreover, the adsorption capacity of 2D-CuTpyP_MOF (96.51 mg g^–1) is comparable to several recently reported Cu- and Fe-based MOFs while offering significantly faster adsorption. These observations demonstrate that adsorption efficiency is not determined solely by surface area or maximum adsorption capacity, but also by the accessibility and chemical nature of the active adsorption sites.

The excellent adsorption performance of 2D-CuTpyP_MOF is particularly noteworthy considering its relatively low BET surface area (12 m² g^–1). Unlike many highly porous MOFs whose performance relies primarily on extensive internal porosity, the present material demonstrates that chemically accessible mixed-valence Cu­(I)/Cu­(II) sites within a layered porphyrinic framework can efficiently promote Cr­(VI) adsorption even in the absence of exceptionally high surface areas. This structure-property relationship represents one of the principal findings of the present work.

Overall, the comparison with the literature demonstrates that 2D-CuTpyP_MOF offers an attractive balance between adsorption capacity, rapid kinetics, structural stability, and operation under environmentally relevant conditions. These characteristics highlight the potential of chemically active two-dimensional porphyrinic MOFs as practical adsorbents for the removal of toxic oxyanions from water.

6. Conclusions

A two-dimensional porphyrinic copper metal-organic framework (2D-CuTpyP_MOF) was synthesized through the coordination-driven assembly of 5,10,15,20-tetra­(4-pyridyl)­porphyrin (TpyP) with Cu­(II) acetate. Comprehensive structural characterization by FT-IR, p-XRD, SEM-EDS, XPS and nitrogen sorption analysis confirmed the successful formation of a crystalline layered framework containing mixed-valence Cu­(I)/Cu­(II) sites and demonstrated its excellent structural stability before and after Cr­(VI) adsorption.

The synthesized material exhibited rapid and efficient Cr­(VI) removal under environmentally relevant conditions, achieving 95.5% removal within only 10 min at neutral pH and a maximum adsorption capacity of 96.51 mg g^–1. Equilibrium and kinetic analyses indicated that the adsorption process follows the Langmuir isotherm and pseudo-second-order kinetic model, while thermodynamic studies demonstrated that the process is spontaneous and endothermic. The combined adsorption experiments and physicochemical characterization suggest that Cr­(VI) uptake arises primarily from the chemically active Cu-porphyrinic sites together with the accessible layered architecture of the framework rather than by exceptionally high internal surface area. One of the principal findings of this work is that efficient Cr­(VI) adsorption can be achieved using a low-porosity two-dimensional MOF through the rational design of chemically active adsorption sites. This observation highlights the importance of the chemical nature and accessibility of surface coordination environments over conventional strategies based primarily on maximizing BET surface area.

Although the present study demonstrates excellent adsorption performance under controlled laboratory conditions, further evaluation using real water matrices containing competing inorganic ions, dissolved organic matter, and other naturally occurring constituents will be essential to assess the practical applicability of the material under environmentally relevant conditions. −

Overall, the combination of rapid adsorption kinetics, efficient operation under near-neutral conditions, excellent structural stability, and a straightforward synthetic procedure demonstrates the potential of 2D-CuTpyP_MOF as a promising adsorbent for Cr­(VI) remediation. More broadly, this work provides a useful design strategy for developing chemically active layered porphyrinic MOFs in which adsorption performance is governed by the accessibility and chemical functionality of active metal sites rather than by extensive porosity alone. This concept may guide the future development of multifunctional MOFs for the removal of toxic oxyanions and other emerging contaminants from water.

Data will be made available on request.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c07902.

• Details of the X-ray photoelectron spectroscopy (XPS) analysis of 2D-CuTpyP_MOF, photographs of the TpyP synthesis, UV-Vis, ¹H NMR, MALDI-TOF MS, powder X-ray diffraction (p-XRD), and FT-IR characterization data for TpyP, EDS analysis and elemental mapping of 2D-CuTpyP_MOF before and after Cr­(VI) adsorption, photographs from the chemical stability experiments and nitrogen adsorption-desorption measurements; Figures S1–S10 and Table S1 (PDF)

D.R.B.: Methodology, formal analysis, data curation, writingoriginal draft, and investigation. K.-S.K.: Methodology, formal analysis, data curation, writingoriginal draft, and investigation. N.K.: Methodology, formal analysis, and investigation. C.D.: Formal analysis, data curation, and writingoriginal draft. M.A.I.: Formal analysis, data curation, and writingoriginal draft. A.K.T.: Methodology, writingreview and editing, supervision, software, and conceptualization. K.L.: Methodology, writingreview and editing, supervision, software, and conceptualization. All authors have given approval to the final version of the manuscript.

The open access publishing of this article is financially supported by HEAL-Link.

The authors declare no competing financial interest.