Amine-modified MOF-14 as a tunable platform for CO₂ adsorption and supercapacitor energy storage

Abstract

Amine-functionalized copper(II)-based MOF-14 materials were prepared via post-synthetic modification using ethylenediamine (en), diethylenetriamine (deta), and 1,2-bis(3-aminopropylamino)ethane (bape) at varying incorporation levels. Comprehensive structural, morphological, textural, and thermal analyses verified the successful grafting of amine functionalities while maintaining the integrity of the parent framework. CO₂ adsorption studies at 0 °C demonstrated that en- and deta-modified MOF-14 exhibited optimal uptake at loadings of 10–15 wt%, achieving a balance between enhanced chemisorption sites and accessible pore volume, whereas the larger bape ligand led to substantial pore obstruction. Additional adsorption measurements at 25 °C confirmed measurable CO₂ uptake under conditions closer to practical gas separation processes. Furthermore, evaluation of CO₂/N₂ separation performance, including IAST calculations for a 15% CO₂/85% N₂ mixture, revealed substantially enhanced selectivity in the amine-modified frameworks compared to pristine MOF-14. Electrochemical evaluation revealed the excellent energy storage performance of MOF-14 (en) 20%, delivering a high specific capacitance of 972 F g⁻¹, markedly outperforming the pristine material. This study highlights the dual capability of amine-modified MOF-14, presenting efficient low-pressure CO₂ capture alongside outstanding promise as an electrode material for high-performance supercapacitors.

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1. Introduction

Porous coordination polymers (PCPs), commonly referred to as metal–organic frameworks (MOFs), represent one of the most extensively explored classes of porous materials discovered over the past four decades.^1,2 These crystalline architectures are composed of metal ions or clusters interconnected by organic linkers, forming extended polymeric frameworks with exceptionally high surface areas and tunable pore structures. The ability to tailor their topology, pore size, and chemical functionality makes MOFs suitable for a broad range of applications, including gas storage and separation,^3–5 heterogeneous catalysis,^6,7 drug delivery,^8–10 sensing,^11,12 ion exchange,^13,14 environmental remediation,^15–18 and energy storage.^19–21

Among the mentioned applications, gas adsorption, particularly carbon dioxide capture, remains a primary focus due to its environmental significance.²² Anthropogenic CO₂ emissions from fossil fuel combustion, industrial processes, and transportation are a major driver of global warming and climate change.¹⁹ The development of efficient CO₂ capture and storage technologies is therefore essential for mitigating greenhouse gas emissions. MOFs offer significant potential in this regard, owing to their tunable porosity, high density of adsorption sites, and structural versatility. For example, MOF-74 (Mg₂(dobpdc)) exhibits low-pressure CO₂ uptake (up to 30.1 wt%), while NU-111 demonstrates high-pressure storage capacities of up to 61.8 wt%.²³

A widely employed strategy to enhance CO₂ affinity involves incorporating nitrogen-containing or amine functionalities into the MOF structure, either through direct synthesis or post-synthetic modification. Such functionalization promotes chemisorptive interactions with CO₂ at low temperatures, facilitating efficient adsorption–desorption cycles.^24–27 Notably, tetraamine-appended MOF-74 has achieved CO₂ storage capacities of up to 90 wt%.^28,29 The choice of organic linker is also critical, as its geometry, length, and functional groups dictate framework topology, pore dimensions, and chemical environment.^30,31 Polar functional groups and open metal sites enhance CO₂ binding, while extended aromatic linkers can modulate framework flexibility and adsorption selectivity.^32–35

For practical CO₂ capture, several key parameters must be optimized: adsorption capacity, thermal and chemical stability, regeneration efficiency, and cost-effective synthesis.^36,37 High thermal stability is essential for repeated adsorption–desorption cycles, and structural robustness ensures long-term operational durability.^38,39 Recent research efforts have increasingly focused on multifunctional MOFs that combine high gas adsorption performance with other advanced properties, such as electrochemical activity.^40,41 For instance, Bhosale et al. developed a porous nanorod-based Ni-MOF which delivered a specific capacitance of 1956.3 F g⁻¹ at a current density of 5 mA cm⁻².⁴² Krishnan et al. synthesized Cu-MOF and decorated it with reduced graphene oxide to achieve a specific capacitance of 867.09 F g⁻¹.⁴³ Similarly, Khan et al. fabricated amine-functionalized Cd-based MOF and utilized it for energy storage applications and obtained a specific capacitance of 9.8 mF cm⁻². The performance was further enhanced by synthesizing its composite with reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs).⁴⁴ These studies clearly demonstrate that MOFs, owing to their tunable structure and high specific surface area, with some modifications, offer promising potential for energy storage applications.

MOF-14, with the chemical formula [Cu₃(BTB)₂(H₂O)(DMF)₅]·4DMF·2H₂O (BTB = 1,3,5-benzenetribenzoate), is a copper(II)-based coordination framework constructed from dinuclear Cu₂ paddle-wheel secondary building units (SBUs) (Fig. 1a) connected by trigonal BTB linkers coordinated in syn–syn fashion (Fig. 1b).⁴⁵ Each BTB linker coordinates to three Cu₂ clusters, while each cluster is connected to four BTB ligands, generating an augmented (3,4)-connected net with Pt₃O₄ topology (Fig. 1c). The resulting structure consists of two identical, interwoven but non-intersecting frameworks. This interpenetrated architecture forms a three-dimensional network of large, accessible cavities (∼16.4 Å in diameter) interconnected by apertures of approximately 7.7 × 14.0 Å. In the as-synthesized material, these cavities host axially bound water molecules, DMF ligands coordinated to Cu(II) sites, and additional solvent guests, which collectively occupy about two-thirds of the unit cell volume. Removal of these guests exposes coordinatively unsaturated Cu(II) sites (CUSs, Fig. 1d), which act as strong Lewis acid centres. These sites readily interact with Lewis bases such as amines, enabling targeted post-synthetic modification. Such functionalization can enhance CO₂ capture via acid–base interactions and introduce additional redox-active sites, improving electrochemical performance in supercapacitor applications.

Fig. 1 Structural representation of MOF-14: (a) secondary building unit (SBU) consisting of a Cu₂ paddle-wheel cluster; (b) 1,3,5-benzenetribenzoate (BTB) organic linker showing the coordination mode of the carboxylate groups; (c) extended three-dimensional framework of MOF-14 with Pt₃O₄ topology viewed along the a-axis. (d) Paddle-wheel cluster in MOF-14 with coordinatively unsaturated sites (CUSs, red arrows) for binding of amines, and (e) molecular structure of amines used in the post-synthetic modification.

In our previous study, we investigated the post-synthetic modification of HKUST-1 with diamine and triamine linkers, demonstrating that such functionalization significantly enhanced its performance in hydrogen and CO₂ storage applications.⁴⁶ Building on these findings, the present work aimed to expand the linker structure of HKUST-1 by replacing 1,3,5-benzenetricarboxylate (BTC) with the elongated 1,3,5-benzenetribenzoate (BTB) linker, thereby constructing the MOF-14 framework. Both compounds share the characteristic binuclear Cu₂ paddle-wheel secondary building unit and a tricarboxylate-based linker. However, in MOF-14, the BTC linker is expanded by additional phenyl rings, resulting in a larger pore system and enhanced structural complexity. In addition to diamine and triamine modifiers, a tetraamine was also employed, and the resulting materials were evaluated for CO₂ adsorption performance as well as electrochemical properties relevant to supercapacitor applications.

In this work, a rapid, high-yield synthesis of MOF-14 under mild conditions, followed by post-synthetic modification with three polyamines: ethylenediamine (en), diethylenetriamine (deta), and 1,2-bis(3-aminopropylamino)ethane (bape) (Fig. 1e), at various loadings was investigated. The structural, thermal, morphological, textural, and electrochemical properties of pristine and amine-functionalized MOF-14 were systematically investigated. Despite extensive research on MOF based materials, most studies focus either on gas adsorption or electrochemical energy storage independently, often relying on carbonization or heavy composite formation that compromises structural integrity. Herein, our particular emphasis was placed on correlating amine loading with CO₂ adsorption performance and supercapacitor behaviour, with the aim of identifying optimal modification conditions for multifunctional applications.

2. Experimental details

2.1 Chemicals

All chemicals used for the synthesis, solvent exchange, and post-synthetic modification of MOF-14 were purchased from Sigma-Aldrich (analytical grade) and used without further purification. The reagents included 1,3,5-benzenetribenzoic acid (H₃BTB, ≥98%), copper(II) nitrate hemipentahydrate (Cu(NO₃)₂·2.5H₂O, ≥99%), N,N′-dimethylformamide (DMF, ≥99.8%), pyridine (≥99%), ethanol (EtOH, ≥99%), methanol (MeOH, ≥99.8%), ethylenediamine (en, ≥99%), diethylenetriamine (deta, 99%), and 1,2-bis(3-aminopropylamino)ethane (bape, 96%).

2.2 Synthesis of MOF-14 and solvent exchange

The as-synthesized MOF-14 (AS), with the formula [Cu₃(BTB)₂(H₂O)(DMF)₅]·4DMF·2H₂O, was prepared according to the following procedure.⁴⁶ 1,3,5-Benzenetribenzoic acid (H₃BTB, 0.023 g, 0.052 mmol) and copper(II) nitrate hemipentahydrate (Cu(NO₃)₂·2.5H₂O, 0.065 g, 0.28 mmol) were dissolved in a mixed solvent of ethanol (3 mL), N,N′-dimethylformamide (DMF, 3 mL), and deionized water (2 mL), in the presence of excess pyridine (0.62 mmol). The reaction mixture was maintained at 65 °C for 24 h, affording light-blue, needle-shaped crystals of MOF-14. The product was air-stable and insoluble in water as well as in common organic solvents. The synthesis was repeated in multiple batches until approximately 4 g of MOF-14 (AS) was obtained, sufficient for solvent exchange, post-synthetic modification, and characterization.

For activation, coordinated and pore-occluded DMF molecules (b.p. 153 °C) in MOF-14 (AS) were replaced with methanol (b.p. 65 °C) to facilitate solvent removal at lower temperatures. The solvent exchange was carried out in a Soxhlet apparatus for 48 h, using methanol as the extracting solvent, yielding the exchanged form, MOF-14 (EX). Subsequent activation afforded MOF-14 (AC).

2.3 Post-synthetic amine modification

Activated MOF-14 (AC) (100 mg; activation conditions: 100 °C, 30 min, oven-dried) was dispersed in 5 mL of anhydrous methanol under a nitrogen atmosphere. The desired amine, ethylenediamine (en), diethylenetriamine (deta), or 1,2-bis(3-aminopropylamino)ethane (bape) (Fig. 1e), was added in the required amount to achieve target mass fractions of 5, 10, 15, 20, or 30 wt% relative to MOF-14. Anhydrous methanol was then added to adjust the total volume of the suspension to 10 mL. The reaction mixtures were stirred for 24 h under an inert atmosphere. The resulting amine-modified MOF-14 materials were collected by filtration, washed repeatedly with methanol, and dried before further characterization. For ethylenediamine, all intended loadings (5–30 wt%) were successfully obtained. In contrast, for deta and bape, structural degradation of MOF-14 occurred at loadings of 20 and 30 wt%, and therefore only samples containing 5, 10, and 15 wt% amine were successfully prepared (see Fig. S1 in SI).

2.4 Characterization

Fourier-transform infrared (FTIR) spectra were recorded at ambient laboratory temperature on a Thermo Scientific Avatar FT-IR 6700 spectrometer equipped with an ATR accessory, in the wavenumber range of 4000–400 cm⁻¹, and a resolution of 2 cm⁻¹ with 128 repetitions.

Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D2 diffractometer using Cu Kα radiation (λ = 1.5406 Å) in Bragg–Brentano geometry over the 2θ range of 2–40° at a scan rate of 0.5° min⁻¹ to confirm the crystalline structure and phase purity of the samples.

Thermal stability, dehydration behaviour, and the amount of grafted amine were determined by thermogravimetric analysis (TGA) using a Hitachi DSC7020 analyzer. The samples were heated from 25 °C to 850 °C under a continuous air flow.

The morphology and elemental composition were examined using a Nova NanoSEM 450 field-emission scanning electron microscope (FEI) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector, operated at an accelerating voltage of 15 kV.

Nitrogen (N₂) and carbon dioxide (CO₂) adsorption/desorption isotherms were measured using a NovaTouch LX4 surface area and pore size analyzer (Quantachrome, Anton Paar, 2021) after degassing the samples at 100 °C for 24 h.

Electrochemical properties were evaluated using a BioLogic VSP-300 electrochemical workstation by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements to assess the energy storage capabilities of the synthesized materials.

3. Results and discussion

3.1 Synthesis, postsynthetic modification and powder X-ray diffraction

The copper(II)-based MOF-14 with the formula [Cu₃(BTB)₂(H₂O)(DMF)₅]·4DMF·2H₂O was synthesized by solvothermal reaction of 1,3,5-benzenetribenzoic acid (H₃BTB) and Cu(NO₃)₂·2.5H₂O in a mixture of DMF, ethanol, and water in the presence of excess pyridine. The as-synthesized product (MOF-14 (AS)) was further subjected to solvent exchange using methanol to afford the methanol-exchanged sample (MOF-14 (EX)), and subsequent thermal treatment yielded the activated material (MOF-14 (AC)).

The phase purity and crystallinity of MOF-14 were evaluated by powder X-ray diffraction (PXRD) for the AS, EX, and AC samples. The experimental PXRD patterns closely match the simulated powder diffraction pattern generated from the reported single-crystal crystallographic data (see Fig. S2a in SI),⁴⁷ confirming successful formation of the target MOF-14 structure. Importantly, all major reflections remain at identical 2θ positions after solvent exchange and thermal activation, demonstrating that the framework topology is preserved throughout the treatment steps.

Upon thermal activation, the coordinated water molecules at the axial positions of the Cu₂ paddle-wheel clusters were removed, generating coordinatively unsaturated Cu(II) sites (CUSs, Fig. 1d). These open metal sites (OMSs) act as strong Lewis acid centres capable of binding Lewis basic amine molecules through metal–nitrogen coordination. Owing to the relatively large pore apertures (7.7 × 14.0 Å) and spacious internal cavities (16.4 × 16.4 Å) of MOF-14, all three employed polyamines: ethylenediamine (en), diethylenetriamine (deta), and 1,2-bis(3-aminopropylamino)ethane (bape) (Fig. 1e), can access the framework interior and coordinate directly to the CUSs, enabling uniform functionalization throughout the material.

Post-synthetic amine modification of MOF-14 (AC) was carried out using polyamine molecules: en, deta, and bape, at various weight loadings (5, 10, 15, 20, and 30 wt%). While the modification with ethylenediamine was successful across the full mass fraction range, the use of deta and bape resulted in structural degradation at higher amine loadings (20 and 30 wt%), likely due to excessive interaction of the bulky amines with the framework. Therefore, only samples with 5, 10, and 15 wt% content of deta and bape were retained for further analysis.

PXRD patterns of all amine-functionalized samples, en-, deta-, and bape-modified series at the investigated loadings, are provided in Fig. S2b, c and d, respectively. The characteristic diffraction peaks of MOF-14 remain at identical 2θ positions after post-synthetic amine treatment, confirming preservation of the crystalline framework. Minor variations in relative peak intensities are attributed to partial pore occupation, changes in electron density upon amine coordination to the open Cu(II) sites, and possible preferred orientation of anisotropic crystallites during PXRD sample preparation. In contrast, samples prepared with higher loadings of deta and bape (20–30 wt%) exhibit complete disappearance of characteristic Bragg reflections and display a broad halo-type diffraction profile, indicating full amorphization of the MOF-14 framework.

The synthesized MOF-14 and amine-functionalized derivatives were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TG), scanning electron microscopy (SEM), and nitrogen adsorption measurements to evaluate their thermal stability, structural integrity, and textural properties.

3.2 Infrared spectroscopy

Infrared (IR) spectroscopy was employed to verify the formation of the MOF-14 framework and to monitor structural changes during solvent exchange and thermal activation. The spectra of MOF-14 (AS), MOF-14 (EX), and MOF-14 (AC) depicted in Fig. S3 in SI were recorded in the range of 4000–400 cm⁻¹, and selected absorption bands are summarized in Table 1. All three materials exhibit prominent bands characteristic of metal–carboxylate coordination. The asymmetric (ν(COO⁻)_as) and symmetric (ν(COO⁻)_s) stretching vibrations of the coordinated carboxylate groups are observed in the regions 1605–1598 cm⁻¹ and 1385–1398 cm⁻¹, respectively. The position of these bands confirms successful coordination between Cu(II) ions and the carboxylate moieties of the 1,3,5-benzenetribenzoate (BTB) linker. Bands observed in the 1546–1525 cm⁻¹ region are assigned to aromatic C=C skeletal vibrations (ν(C=C)_ar), which support the incorporation of the benzene-based linker into the MOF framework. Additionally, a medium-intensity band appearing around 776–779 cm⁻¹ is attributed to the bending vibration of the carboxylate group (δ(COO⁻)), commonly arising from out-of-plane deformation modes. The presence of all these framework-related bands across all samples confirms the integrity of the MOF-14 throughout the solvent exchange and activation processing. In the as-synthesized material (MOF-14 (AS)), a broad absorption band at 3441 cm⁻¹ is observed, corresponding to O–H stretching vibrations from both coordinated water molecules and physisorbed moisture within the pores. Two bands at 2926 and 2860 cm⁻¹ are attributed to aliphatic C–H stretching vibrations of DMF (N,N′-dimethylformamide), while the intense band at 1660 cm⁻¹ is assigned to the C=O stretching vibration of DMF molecules. Following solvent exchange with methanol (MOF-14 (EX)), the O–H stretching band shifts to 3404 cm⁻¹ and becomes broader, indicating the presence of hydrogen-bonded methanol within the pore system. Multiple C–H aliphatic stretching bands at 2977, 2940, and 2831 cm⁻¹ are observed, which are characteristic of methanol. Importantly, the DMF-related C=O stretching band at 1660 cm⁻¹ disappears, confirming its successful removal and replacement by methanol molecules. In the activated sample (MOF-14 (AC)), no O–H or aliphatic C–H stretching bands are observed, indicating complete removal of residual solvent molecules. The framework-specific bands remain unchanged, with only minor shifts in the carboxylate vibrational modes, suggesting a slight change in the coordination environment but preservation of the MOF structure.

Table 1 Selected infrared absorption bands (in cm⁻¹) observed for MOF-14 materials in different stages (as-synthesized, solvent-exchanged, activated) and after post-synthetic amine modification with ethylenediamine (en), diethylenetriamine (deta), and 1,2-bis(3-aminopropylamino)ethane (bape)

	ν(OH)	ν(NH)	ν(CH)aliph	ν(C=O)	δ(NH)	νas(COO^−)	νs(COO^−)	δ(COO^−)
MOF-14(AS)	3441	—	2926 2860	1660	—	1605	1385	779
MOF-14 (EX)	3404	—	2977 2940 2831	—	—	1601	1389	779
MOF-14 (AC)	—	—	—	—	—	1598	1398	776
MOF-14 (en) 5%	—	3243 3142	2948 2891	—	1581	1529	1374	772
MOF-14 (en) 10%	—	3243 3143	2948 2889	—	1580	1531	1373	774
MOF-14 (en) 15%	—	3243 3142	2947 2889	—	1580	1531	1369	777
MOF-14 (en) 20%	—	3241 3142	2946 2891	—	1579	1530	1366	777
MOF-14 (en) 30%	—	3242 3140	2947 2890	—	1579	1530	1365	778
MOF-14 (deta) 5%	—	3320 3234 3145	2933 2880	—	1580	1532	1372	773
MOF-14 (deta) 10%	—	3320 3235 3145	2933 2879	—	1581	1537	1370	779
MOF-14 (deta) 15%	—	3322 3233 3147	2935 2881	—	1579	1536	1368	778
MOF-14 (bape) 5%	—	3367 3289 3235 3143	2952 2928 2878	—	1586	1544	1369	781
MOF-14 (bape) 10%	—	3365 3286 3238 3142	2955 2928 2878	—	1585	1543	1365	780
MOF-14 (bape) 15%	—	3369 3289 3237 3142	2953 2928 2878	—	1585	1543	1366	780

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After activation, MOF-14 was post-synthetically modified with various polyamine ligands, ethylenediamine (en), diethylenetriamine (deta), and 1,2-bis(3-aminopropylamino)ethane (bape), in mass fractions ranging from 5 to 30 wt%. The incorporation of these amines was confirmed by IR spectroscopy through the emergence of new vibrational bands not present in the unmodified framework (see Fig. 2 and Table 1). All amine-modified materials exhibited intense and well-defined bands in the region 3300–3100 cm⁻¹, corresponding to N–H stretching vibrations. In the case of MOF-14 (en) samples, two distinct bands at ∼3243 and 3142 cm⁻¹ are consistent with symmetric and asymmetric stretching modes of primary amine groups. For MOF-14 (deta) materials, three separate N–H stretching bands appear at ∼3320, 3234, and 3145 cm⁻¹, reflecting the presence of both primary and secondary amine functionalities in the molecule. The IR spectra of bape-modified MOF-14 show four N–H stretching bands between 3367 and 3143 cm⁻¹, which can be attributed to its extended structure with multiple terminal and internal amino groups. The increased complexity and intensity of N–H bands in the bape series correlate with the higher number of donor sites per molecule. The presence of aliphatic C–H stretching vibrations in the range 2955–2878 cm⁻¹ further supports the successful encapsulation or grafting of alkylamine chains within the MOF-14. Additionally, all amine-functionalized samples display a band in the range 1580–1586 cm⁻¹, assigned to N–H bending vibrations (δ(NH)). At the same time, small but consistent shifts are observed in the carboxylate vibrational modes when compared to the activated MOF-14. Specifically, the ν_as(COO⁻) and ν_s(COO⁻) bands are shifted to 1579–1586 cm⁻¹ and 1365–1374 cm⁻¹, respectively, while the bending mode δ(COO⁻) appears at 772–781 cm⁻¹. These shifts are attributed to weak interactions between the amine groups and the MOF structure, most likely via hydrogen bonding or electrostatic effects involving the carboxylate functionalities or the open Cu(II) coordination sites. The extent of these shifts correlates modestly with the number of amino groups and the molecular structure of the grafted amine. Importantly, no new bands indicative of degradation or structural decomposition were observed, confirming that the framework remained intact during post-synthetic modification.

Fig. 2 IR spectra of amine-modified MOF-14 with (a) ethylenediamine (en), (b) diethylenetriamine (deta), and (c) 1,2-bis(3-aminopropylamino)ethane (bape) with different mass fractions.

3.3 Thermogravimetric analysis

All materials were characterized by thermogravimetric analysis (TG) combined with differential thermal analysis (DTA). The measurements were performed in an air atmosphere over the temperature range of 25–850 °C, and the obtained TG and DTA curves for MOF-14 are presented in Fig. S4 in SI, and for amine-modified materials in Fig. 3. Based on these measurements, the thermal stability of the samples was evaluated, and the amount of grafted amines was determined. To eliminate the contribution of adsorbed and weakly bound solvents, all TG curves were normalized to the sample mass at 150 °C.^48,49 The corresponding weight losses were then distributed between the mass change of the organic framework and the residual mass after complete combustion. This normalization allowed for a more precise calculation of the amount of grafted amines.

Fig. 3 Thermogravimetric (TG), differential thermal analysis (DTA), and normalized TG curves of MOF-14 samples functionalized with different amounts of (a), (d) and (g) ethylenediamine (en), (b), (e) and (h) diethylenetriamine (deta), and (c), (f) and (i) 1,2-bis(3-aminopropylamino)ethane (bape).

For the methanol-exchanged MOF-14 sample, the TG curve (see Fig. S4 in SI) shows an initial weight loss of 13.8% in the temperature range of 25–150 °C, which is attributed to the removal of solvents (water and methanol). This desolvation step is accompanied by an endothermic effect on the DTA curve with a peak at 68 °C. The material remains stable up to 280 °C, after which a major weight loss occurs between 280 and 500 °C due to the decomposition of the BTB linker (organic part of the framework). This process is associated with two distinct exothermic peaks on the DTA curve at 318 and 352 °C.

The TG, normalized TG, and DTA curves for all amine-modified MOF-14 samples are presented in Fig. 3. Based on the residual masses, the amount of grafted amines was calculated in both mg g⁻¹ and mmol g⁻¹, and the results are summarized in Table 2. On the TG curves of the amine-modified samples, the first step corresponds to the removal of solvent molecules, with weight losses ranging from 9.5–17.9% for en-modified, 9.0–12.1% for deta-modified, and 11.8–12.9% for bape-modified materials, occurring in the temperature range of 25–150 °C. The dehydration/desolvation step was accompanied by endothermic effects on the DTA curves, observed at 54–60 °C for en, 50–62 °C for deta, and 52–60 °C for bape-modified materials. In the second step of thermal decomposition, the release of grafted amines was observed, manifested by endothermic effects on the DTA curves. For ethylenediamine-modified samples, three endothermic peaks were detected in the temperature range of 211–316 °C, while diethylenetriamine-modified samples exhibited a single endothermic effect within 252–293 °C, and 1,2-bis(3-aminopropylamino)ethane-modified samples displayed two endothermic peaks in the range of 253–264 °C. Further increase in temperature leads to the decomposition of the polymeric framework of MOF-14, accompanied by major weight loss, characteristic of the oxidative degradation of the BTB linker. From the normalized TG curves, the amounts of grafted amines were calculated based on the difference in residual masses between the pristine MOF-14 and the amine-modified materials.

Table 2 Calculated amount of grafted amines (mg g⁻¹ and mmol g⁻¹) determined from normalized TG curves

Sample	Amount of amine
	mg g^−1	mmol g^−1
MOF-14 (en) 5%	82.3	1.371
MOF-14 (en) 10%	178.1	2.968
MOF-14 (en) 15%	178.7	2.978
MOF-14 (en) 20%	177.0	2.950
MOF-14 (en) 30%	200.8	3.346
MOF-14 (deta) 5%	165.4	1.603
MOF-14 (deta) 10%	188.0	1.822
MOF-14 (deta) 15%	224.8	2.179
MOF-14 (bape) 5%	329.8	1.892
MOF-14 (bape) 10%	337.8	1.938
MOF-14 (bape) 15%	341.5	1.959

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The calculated amounts of grafted amines (see Table 2) reflect the influence of both the type of amine and its nominal loading during the modification process. For the en-modified samples, the amine content increases significantly from 82.3 mg g⁻¹ (1.371 mmol g⁻¹) at 5% loading to 178.1–178.7 mg g⁻¹ (≈2.97–2.98 mmol g⁻¹) at 10–15% loading, followed by a slight decrease at 20% loading (177.0 mg g⁻¹; 2.950 mmol g⁻¹) and a subsequent increase to 200.8 mg g⁻¹ (3.346 mmol g⁻¹) at 30% loading. This trend suggests that, above a certain concentration, the grafting efficiency approaches saturation, with additional amine incorporation being limited by steric hindrance or pore blocking effects. In the case of deta-modified materials, the grafted amounts are consistently higher compared to the en-series at the same nominal loadings, starting from 165.4 mg g⁻¹ (1.603 mmol g⁻¹) at 5% loading and reaching 224.8 mg g⁻¹ (2.179 mmol g⁻¹) at 15% loading. The higher mass values for deta compared to en at low loadings can be attributed to the larger molecular weight of deta, even though the molar amounts remain lower. The bape-modified samples exhibit the highest grafted amounts in terms of mass, ranging from 329.8 to 341.5 mg g⁻¹, which is expected given the much higher molecular weight of bape. However, the molar quantities remain relatively low (1.892–1.959 mmol g⁻¹), indicating that, although more mass is incorporated, the number of grafted molecules is smaller due to the bulky structure of bape, which can also lead to partial pore blocking.

3.4 SEM with EDX

The morphology of pristine and amine-modified MOF-14 samples was examined by field-emission SEM equipped with EDX. The methanol-exchanged MOF-14 shows elongated prismatic/rod-like crystals with smooth faces and well-defined edges, typical for MOF-14, and a relatively narrow size dispersion (see Fig. 4). After ethylenediamine (en) modification, progressive surface roughening and fragmentation are observed: at 5 wt% the parent morphology is still discernible but the crystal surfaces become textured; at 10–15 wt% the particles lose their regular prismatic shape and form irregular agglomerates; at 20–30 wt% dense aggregates with poorly defined boundaries dominate, consistent with extensive surface coverage and interparticle adhesion via hydrogen-bonding among amine groups. The deta series preserves more of the parent shape at 5 wt% but shows marked roughening and plate-like fragments by 10–15 wt%, indicating partial collapse of outer layers under bulkier triamine grafting. The bape series exhibits the strongest distortion already at 5 wt% (rounded edges, fused aggregates), and at 10–15 wt%, the micrographs are dominated by irregularly coalesced particles suggestive of pore blocking and extensive surface coating by the bulky tetraamine.

Fig. 4 SEM images of (a) pristine MOF-14 and MOF-14 modified with (b) ethylenediamine (en), (c) diethylenetriamine (deta), and (d) 1,2-bis(3-aminopropylamino)ethane (bape) at the indicated mass fractions.

Elemental analysis by EDX on a representative highly loaded sample (MOF-14 (en) 30%) confirms the presence of C, Cu, O and N; a standardless quantification yielded approximately C ∼ 42 wt%, Cu ∼ 20 wt%, O ∼ 3.5 wt% and N ∼ 2.4 wt% (see inset table in Fig. S5 in SI). The detection of nitrogen is consistent with successful ethylendiamine incorporation, while copper from the paddle-wheel clusters and oxygen from carboxylates remain prominent, as expected for the MOF-14 framework.

3.5 Nitrogen adsorption/desorption measurements

All samples, including MOF-14 and its amine-functionalized derivatives, were characterized by nitrogen adsorption/desorption measurements at −196 °C. These analyses enabled the determination of Brunauer–Emmett–Teller surface areas (S_BET), the results of which are summarized in Table 3 and Fig. 5. All materials were activated under vacuum at 100 °C prior to measurement. For MOF-14, a preceding methanol exchange step (MOF-14 (EX)) facilitated the removal of high-boiling DMF from the pores, allowing complete activation at this relatively low temperature. The choice of a mild activation temperature was deliberate, aiming to prevent framework degradation and to avoid the release of coordinated amines from the pores in functionalized samples.

Table 3 Textural properties (S_BET) and CO₂ adsorption capacities@0 °C and 101 325 Pa of pristine and amine-functionalized MOF-14 samples

Sample	SBET (m^2 g^−1)	V(CO2) (cm^3 g^−1)	n(CO2) (mmol g^−1)	w(CO2) (wt%)	V(CO2) (cm^3 m^−2)
MOF-14 (AC)	1815.0	129.7	5.79	25.47	0.07
MOF-14 (en) 5%	10.8	31.8	1.42	6.24	2.94
MOF-14 (en) 10%	8.9	61.2	2.73	12.02	6.88
MOF-14 (en) 15%	5.9	53.7	2.40	10.54	9.10
MOF-14 (en) 20%	5.1	45.6	2.03	8.95	8.94
MOF-14 (en) 30%	4.1	43.1	1.92	8.46	10.51
MOF-14 (deta) 5%	14.3	57.5	2.57	11.29	4.02
MOF-14 (deta) 10%	6.1	57.2	2.55	11.23	9.38
MOF-14 (deta) 15%	8.3	58.6	2.61	11.51	7.06
MOF-14 (bape) 5%	6.1	50.9	2.27	9.99	8.34
MOF-14 (bape) 10%	4.2	43.4	1.94	8.52	10.33
MOF-14 (bape) 15%	3.7	35.1	1.57	6.89	9.49

---

Fig. 5 The comparison of S_BET areas for amine-modified materials.

The N₂ adsorption/desorption isotherm of activated MOF-14 (AC) (Fig. S6 in SI) exhibits a steep uptake at very low relative pressures (p/p₀ < 0.05) followed by an early plateau, characteristic of a microporous material. According to the IUPAC classification,⁵⁰ the profile corresponds to type I(b), indicative of networks with predominantly narrow-to-wide micropores (≈0.7–2 nm), consistent with the ∼1.1 nm apertures and ∼1.6 nm cavities of MOF-14. The loop between adsorption and desorption branches at higher p/p₀ is minimal. A slight deviation at p/p₀ ≳ 0.5 can be ascribed to inter-particle condensation/voids (H4-like artefact) rather than intrinsic mesoporosity, in line with the rigid framework. The BET surface area determined under the BET consistency criteria is 1815 m² g⁻¹, confirming efficient activation.

Upon post-synthetic modification with polyamines, a pronounced decrease in accessible surface area was observed for all functionalized samples compared to MOF-14 (AC), consistent with partial pore filling and blockage by amine molecules. The extent of S_BET reduction depended on both the type and loading of the amine (Fig. 5 and Table 3). For ethylenediamine, the surface area decreased progressively from 10.8 m² g⁻¹ at 5 wt% loading to 4.1 m² g⁻¹ at 30 wt%, reflecting the efficient pore penetration of this small, bidentate amine. In contrast, diethylenetriamine at 5 wt% retained the highest relative surface area among all modified samples (14.3 m² g⁻¹), while higher loadings (10 and 15 wt%) led to a pronounced drop (6.1 and 8.3 m² g⁻¹, respectively), likely due to steric hindrance and pore blocking by the bulkier tridentate ligand. For 1,2-bis(3-aminopropylamino)ethane, which is the largest tetraamine tested, even low loadings resulted in a marked reduction in surface area (6.1 m² g⁻¹ at 5 wt%), with further decreases at 10 and 15 wt% (4.2 and 3.7 m² g⁻¹, respectively). This trend is attributed to the larger molecular size and greater number of donor atoms, which promote strong coordination to open Cu(II) sites but also substantially reduce accessible pore volume.

The observed decrease in S_BET with increasing amine loading reflects the balance between framework functionalization and retention of porosity. While amine grafting introduces polar sites beneficial for CO₂ adsorption and electrochemical activity, excessive loading can compromise gas-accessible surface area, highlighting the need for optimization depending on the intended application.

3.6 Carbon dioxide adsorption measurements

Carbon dioxide adsorption measurements were performed at 0 °C up to 101 325 Pa on pristine MOF-14 and amine-modified materials. Uptakes are reported as V(CO₂) (cm³ g⁻¹, STP), n(CO₂) (mmol g⁻¹), and w(CO₂) (wt%). The complete results are summarized in Table 3, and representative isotherms are shown in Fig. 6.

Fig. 6 Comparison of the CO₂ adsorption isotherm of activated MOF-14 and (a) en-, (b) deta- and (c) bape-modified materials with different mass fractions.

The activated MOF-14 displays the highest total CO₂ capacity within the series, reaching 129.7 cm³ g⁻¹ (5.79 mmol g⁻¹; 25.47 wt%) at 0 °C and 101 325 Pa (Fig. 6). This performance is consistent with its very high specific surface area (1815 m² g⁻¹) and the presence of accessible coordinatively unsaturated Cu(II) sites that provide strong adsorption centres for CO₂. Incorporation of en decreases the accessible porosity but introduces basic sites that promote CO₂ binding (Fig. 6a). The uptake evolves with loading as 31.8 → 61.2 → 53.7 → 45.6 → 43.1 cm³ g⁻¹ for 5, 10, 15, 20, 30 wt% en, respectively (i.e., 1.42, 2.73, 2.40, 2.03, 1.92 mmol g⁻¹). The maximum around 10–15 wt% reflects a balance between increasing amine site density (favouring chemisorption) and progressive pore blocking at higher loadings. Deta-functionalized samples deliver robust and nearly composition-independent capacities, with 57.5, 57.2, 58.6 cm³ g⁻¹ for 5, 10, 15 wt%, respectively (≈2.55–2.61 mmol g⁻¹) (Fig. 6b). Despite the lower S_BET than pristine MOF-14, the higher number of amine functions per molecule sustains strong CO₂ affinity while avoiding excessive pore occlusion within this loading window. The bulkier tetraamine bape causes a monotonic decrease with loading: 50.9, 43.4, 35.1 cm³ g⁻¹ at 5, 10, 15 wt% (i.e., 2.27, 1.94, 1.57 mmol g⁻¹) (Fig. 6c). These trends are consistent with steric hindrance and partial pore blocking, which limit the accessible volume and reduce the net benefit of the higher nominal number of amine groups.

To provide additional insight into adsorption efficiency, the CO₂ uptake was normalized to the BET surface area (expressed as cm³ CO₂ per m², Table 3). Although pristine MOF-14 (AC) exhibits the highest total CO₂ uptake due to its large accessible surface area, its normalized uptake is only 0.07 cm³ m⁻². In contrast, the amine-functionalized samples display substantially higher CO₂ adsorption density per unit surface area, with values ranging from 2.94 to 10.51 cm³ m⁻². In the en series, the normalized uptake increases systematically with amine loading (2.94 → 6.88 → 9.10 cm³ m⁻² for 5–15 wt%), reaching a maximum of 10.51 cm³ m⁻² at 30 wt% loading. Similar enhancements are observed for the deta and bape series, where values up to ∼9–10 cm³ m⁻² are obtained. Overall, this corresponds to an increase of approximately one to two orders of magnitude compared to pristine MOF-14, indicating a transition from predominantly surface-area-driven physisorption in MOF-14 (AC) to functionality-driven adsorption in the amine-modified materials. The markedly higher CO₂ uptake per unit N₂-accessible surface area highlights the enhanced adsorption efficiency associated with the introduced amine functionalities.

Fig. 7 shows the relationship between w(CO₂) (wt%) and the real amine content calculated from TG curves (mmol g⁻¹) for MOF-14 modified with en, deta, and bape. For en, a slightly increasing trend is observed with a local maximum at approximately 3.0 mmol g⁻¹ (sample MOF-14 (en) 10%, ∼12 wt% CO₂), followed by a decline at higher loadings, consistent with pore blocking. The deta series exhibits a nearly constant but slightly positive slope (∼11.2–11.5 wt% over 1.6–2.2 mmol g⁻¹), suggesting that multiple –NH groups enhance CO₂ affinity without significantly compromising pore accessibility in this range. In contrast, bape shows a pronounced negative trend, decreasing from ∼10 wt% at ∼1.9 mmol g⁻¹ to ∼6.9 wt% at ∼2.0 mmol g⁻¹, reflecting steric hindrance and early pore blocking even at moderate loadings. A comparison at similar real amine contents (∼1.9–2.0 mmol g⁻¹) indicates the performance order deta > bape > en, with inter-series differences exceeding the variability within each series. Overall, the plot confirms that actual grafted amine content, rather than nominal dosing, is the key predictor of CO₂ uptake, while the amine type critically determines the slope and direction of the trend (positive for en/deta, negative for bape).

Fig. 7 Correlation between CO₂ uptake (wt%) and real amine content (mmol g⁻¹) for MOF-14 modified with en, deta, and bape.

While the total capacities at 1 atm for amine-modified samples are lower than for pristine MOF-14, their isotherms exhibit a markedly steeper initial slope (see Fig. 6), evidencing enhanced low-pressure affinity due to CO₂–amine chemisorption superimposed on physisorption. This profile is advantageous in processes operating at sub-atmospheric or modest CO₂ partial pressures, such as post-combustion polishing (VSA/TVSA cycles), indoor/cabin air scrubbing, and personal protective filters, where rapid capture at low p(CO₂) governs performance.^51–53

Overall, optima near 10–15 wt% (en or deta) maximize low-pressure uptake while limiting pore blocking, offering a complementary alternative to pristine MOF-14, lower total capacity at atmospheric pressure, but superior efficiency in the practically relevant low-pressure regime.

To further evaluate the CO₂ separation performance of the prepared materials under conditions relevant to gas separation processes, additional adsorption measurements were performed for selected representative samples. Four materials were chosen to cover the key structural regimes observed in this study: pristine activated MOF-14 (AC) as the reference material with the highest accessible porosity, MOF-14 (en) 10% representing the composition with the highest CO₂ uptake among the modified samples, MOF-14 (en) 20% corresponding to the material exhibiting the best electrochemical performance (see Section 3.6 Electrochemical measurements below), and MOF-14 (en) 30% representing the high amine loading regime where pore blocking becomes significant. Adsorption isotherms of CO₂ and N₂ at 0 °C were measured for these samples, and the corresponding CO₂/N₂ selectivity values were subsequently calculated. The obtained adsorption capacities and selectivity values are summarized in Table 4.

Table 4 CO₂ and N₂ adsorption capacities@0 °C and 25 °C with calculated CO₂/N₂ selectivity for pristine MOF-14 and selected en-modified samples

Sample	n(CO2)@25 °C (mmol g^−1)	n(CO2)@0 °C (mmol g^−1)	n(N2)@0 °C (mmol g^−1)	Selectivity n(CO2)/n(N2)^a	SelectivityIAST^b
a CO2/N2 selectivity at 0 °C was determined from single-component adsorption isotherms as the ratio of the CO2 uptake to the corresponding N2 uptake.b CO2/N2 selectivity calculated for a binary gas mixture containing 15% CO2 and 85% N2 using ideal adsorbed solution theory (IAST) based on the fitted single-component adsorption isotherms measured at 0 °C.
MOF-14 (AC)	3.24	5.79	0.323	18	105
MOF-14 (en) 10%	1.38	2.73	0.017	161	600
MOF-14 (en) 20%	1.05	2.03	0.011	185	830
MOF-14 (en) 30%	0.97	1.92	0.008	240	880

---

The adsorption isotherms of CO₂ and N₂ measured at 0 °C are presented in Fig. S7 in SI. The obtained results show that nitrogen adsorption is significantly suppressed in the amine-modified materials. This behaviour can be attributed to partial pore blocking caused by the incorporated amine molecules, which significantly reduces the accessibility of the porous structure for N₂ molecules. In addition, since adsorption is an exothermic process, the adsorbed amount of N₂ further decreases with increasing temperature. Consequently, the selected amine-modified samples adsorb only very small amounts of nitrogen at 0 °C, indicating a strong preference of the framework for CO₂ adsorption.

To quantify this behaviour, CO₂/N₂ selectivity at 0 °C was evaluated using two different approaches. In the first approach, the apparent selectivity was estimated directly from single-component adsorption isotherms as the ratio of the CO₂ uptake to the corresponding N₂ uptake:

where n_CO2 and n_N2 represent the adsorbed amounts of CO₂ and N₂ (mmol g⁻¹) at the same pressure (101 325 Pa) and temperature (0 °C). Based on this simple approach, pristine MOF-14 (AC) exhibits a selectivity of 18, while the amine-modified samples show substantially higher values ranging from 161 to 240. The increase in selectivity is primarily associated with the drastic suppression of N₂ adsorption upon amine incorporation.

For comparison, previously reported MOF materials typically exhibit lower CO₂/N₂ selectivities when estimated from single-component adsorption capacities. For example, the widely studied porous framework MIL-101(Cr) shows a CO₂/N₂ selectivity of approximately 7.4 under comparable conditions.⁵⁴ Amine-functionalized MOF materials generally exhibit higher selectivity due to stronger interactions between CO₂ molecules and basic amine sites. For instance, the ethylenediamine-functionalized framework ED@MOF-520 shows a CO₂/N₂ selectivity of ∼50,⁵⁵ while the amine-decorated CAU-1 framework exhibits a selectivity of about 101.⁵⁶ In contrast, the amine-modified MOF-14 samples investigated in this work display substantially higher values ranging from 161 to 240, demonstrating the strong preference of the functionalized framework for CO₂ adsorption.

To obtain a more realistic estimate of separation performance under mixed-gas conditions, the selectivity was also evaluated using ideal adsorbed solution theory (IAST).⁵⁷ IAST is a thermodynamic model widely used to predict multicomponent adsorption behaviour from single-component adsorption isotherms. The selectivity for a binary gas mixture can be expressed as:

where x_i represents the mole fraction of component i in the adsorbed phase and y_i represents the mole fraction of the corresponding component in the gas phase. In this work, the selectivity was calculated for a typical post-combustion gas mixture containing 15% CO₂ and 85% N₂. The calculated IAST selectivity values increase from 105 for pristine MOF-14 to values between 600 and 880 for the amine-modified samples (see Table 4).

For comparison, the IAST-predicted CO₂/N₂ selectivity for flue-gas mixtures containing 15% CO₂ and 85% N₂ was compared with previously reported amine-functionalized MOF materials. Amine-tethered MIL-101-DETA exhibits CO₂/N₂ selectivity of 346, demonstrating the beneficial effect of grafted amine functionalities on CO₂ affinity.⁵⁸ Comparable values have been reported for diamine-appended frameworks such as dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine; dobpdc = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), which display CO₂/N₂ selectivity of approximately 300–400 due to cooperative CO₂ adsorption within the framework channels.⁵⁹ Even higher separation performance has been reported for polyethylenimine-modified MIL-101(Cr), reaching CO₂/N₂ selectivity values of 600–770 under flue-gas conditions, depending on the PEI loading within the framework.⁶⁰ Among the most extreme examples are diamine-appended frameworks such as mmen-Mg₂(dobpdc) (mmen = N,N′-dimethylethylenediamine), which exhibit exceptionally high CO₂/N₂ selectivity exceeding 49 000, attributed to cooperative chemisorption of CO₂ forming ammonium carbamate chains within the framework pores.⁶¹

In comparison with previously reported systems, the amine-modified MOF-14 materials investigated in this work exhibit high IAST CO₂/N₂ selectivity values ranging from 600 to 880. The incorporation of ethylenediamine into the MOF-14 framework, therefore, strongly enhances the preferential adsorption of CO₂ over N₂, placing these materials among highly competitive MOF-based adsorbents reported for CO₂/N₂ separation under flue-gas conditions. Although amine incorporation partially reduces the total CO₂ adsorption capacity due to pore blocking, it significantly improves separation performance by suppressing N₂ adsorption and promoting strong CO₂–amine interactions within the modified framework. Furthermore, immobilization of amine molecules within a solid MOF matrix provides an additional practical advantage, as free amines are typically liquid, highly reactive, and corrosive,⁶² whereas their incorporation into the porous framework yields a stable solid adsorbent that is easier to handle and integrate into practical adsorption processes.

Up to this point, the discussion has primarily focused on CO₂ adsorption behaviour at 0 °C, which was used as the reference condition for evaluating adsorption capacities and CO₂/N₂ selectivity. In order to assess the adsorption performance under conditions closer to practical gas separation processes, additional adsorption measurements were carried out at 25 °C. The obtained CO₂ capacities at ambient temperature are summarized in Table 4. At 25 °C, the pristine MOF-14 (AC) exhibits a CO₂ uptake of 3.24 mmol g⁻¹, whereas the amine-modified samples display lower adsorption capacities ranging from 0.97 to 1.38 mmol g⁻¹. Among the modified materials, MOF-14 (en) 10% shows the highest uptake (1.38 mmol g⁻¹), indicating that moderate amine loading provides a favourable balance between accessible porosity and the density of CO₂-affinitive amine sites. Further increase in amine content leads to a gradual decrease in CO₂ uptake (1.05 and 0.97 mmol g⁻¹ for MOF-14 (en) 20% and 30%, respectively).

3.7 Electrochemical measurements

Electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) (see Fig. 8 and Table 5) were performed on a 3-electrode cell using 1 M H₂SO₄ as an electrolyte, Ag/AgCl electrode as reference electrode, and utilizing the platinum wire as counter electrode. For the working electrode, a slurry was prepared by adding synthesized material, acetylene black and binder in an 8 : 1 : 1 ratio in DMF solvent. This solution was sonicated for 2 h, and the obtained slurry was pasted on a high-density graphite sheet with thickness 0.5 mm, using the drop casting technique.⁶³ The loaded mass evaluated by subtracting the mass of electrode from the mass of electrode pasted with active material was controlled at 1 mg cm⁻². Since all electrodes were prepared with a fixed content of acetylene black, any change in the electrochemical performance is attributed to framework functionalization rather than variation in conductive additive content.

Fig. 8 (a)–(d) CV at 10 mV s⁻¹, (e)–(h) GCD at 1 A g⁻¹ and (i)–(l) EIS curve for MOF-14 (AS), MOF-14 (en), MOF-14 (deta) and MOF-14 (bape).

Table 5 Value of specific capacitance in three-electrode configuration for as-synthesized and amine-modified MOF-14

Sample	Specific capacitance (F g^−1)
	CCV	CGCD
MOF-14 (AS)	180.13	157.10
MOF-14 (en) 5%	186.41	117.87
MOF-14 (en) 10%	232.12	183.54
MOF-14 (en) 15%	470.95	332.19
MOF-14 (en) 20%	972.22	465.80
MOF-14 (en) 30%	758.30	359.09
MOF-14 (deta) 5%	474.41	313.50
MOF-14 (deta) 10%	706.85	432.82
MOF-14 (deta) 15%	302.88	212.03
MOF-14 (bape) 5%	221.93	135.43
MOF-14 (bape) 10%	540.72	368.53
MOF-14 (bape) 15%	71.45	33.44

---

Fig. 8(a)–(d) demonstrates the CV curve of MOF-14 (AS), MOF-14 (en), MOF-14 (deta) and MOF-14 (bape) at a scan rate of 10 mV s⁻¹ in the potential range from −0.2 V to 0.7 V. The clear appearance of redox peaks in the CV curve shows the presence of redox reactions. The value of peak current increases on modifying the samples with amine, reaching a maximum value of 972.22 F g⁻¹ for MOF-14 (en) 20%. This might be attributed to the introduction of polar functional sites due to the amine group, which enhances the material's ability to adsorb and store charge electrostatically. On further increasing the amine concentration beyond a specific value, the peak current and hence the area under the CV curve decrease, which might be due to the blocking of pores because of excessive amine concentration, which impedes the charge transfer process and thus reduces specific capacitance.

The charge-storing ability of MOF-14 samples was further analyzed by performing GCD at a current density of 1 A g⁻¹, as shown in Fig. 8(e)–(h). A non-linear relation between the potential and time in the GCD curve represents the pseudocapacitive charge storage mechanism of material. The shape of modified MOF-14 samples resembles well of the unmodified MOF-14 with an increase in charging and discharging time symbolizing the same charge storage mechanism as MOF-14 (AS). The increase in discharging time in modified MOF-14 can be attributed to additional adsorption sites of the amine group, which contribute to longer charging–discharging time and hence increase the specific capacitance of the material with a maximum value of 465.80 F g⁻¹ for MOF-14 (en) 20%. The specific capacitance of the material from CV and GCD is calculated using the following equations.^64,65

where C_CV and C_GCD represent the specific capacitance from CV and GCD measurements, represents the area under the CV curve, I, dt, m, ν, and ΔV shows the discharging time, current, mass of material deposited on the electrode, scan rate and potential window, respectively. Table 5 summarizes the specific capacitance evaluated for all the samples from CV and GCD measurements. The highest value of specific capacitance was observed for MOF-14 (en) 20% sample.

The effect on conductivity of modified MOF-14 samples was analyzed by obtaining the electrochemical impedance spectroscopy (EIS) in a frequency range from 100 kHz to 0.1 Hz. In the high-frequency region, the absence of a semi-circle in all the samples demonstrates the supercapacitive behavior of the MOF-14 sample. In addition, the beginning of the arc in high frequency region shows the solution resistance, which is minimum for MOF-14 (en) 20%, thus validating the CV and GCD results. In addition, the angled line in a low-frequency region shows the capacitive and diffusive contribution to the specific capacitance. It is concluded that the unmodified or MOF-14 (AS) sample has a capacitive contribution, while its amine modification results in both capacitive and diffusive contributions, resulting in better energy storage performance.

From Table 5, it is evident that among the amine modifications, ethylenediamine (en) modification results in better energy storage properties than deta and bape. This might be due to the optimized balance between pore accessibility and ion diffusion for MOF-14 (en). This ensures the minimum pore blockage and allows the ions to access the high specific surface area of MOF-14, and helps in faster ion transport. For MOF-14 (deta) and MOF-14 (bape), a higher density of amine functional groups results in agglomeration and blocks the pores of MOF-14, thus compromising the electrochemical performance.

These materials were developed as electrode materials in order to accumulate electrochemical energy. For MOF-14 (en) 20%, an excellent specific capacitance value of 972.22 F g⁻¹ (from CV) and 465.80 F g⁻¹ (from GCD) was observed compared to other investigated MOF materials (see Table 6). The lower GCD value confirms that diffusion limitations are present at constant current conditions, which supports that the reported CV value is not artificially inflated but scan-rate dependent. The synthesized and amine-modified MOF-14 nanocomposites exhibit great potential as an electrode material under three electrode configuration. While the present study focuses on elucidating charge storage characteristics rather than full device optimization, the notable CV and GCD responses suggest that controlled amine modification is an effective strategy for improving electrochemical performance in MOF-based systems. These findings provide a mechanistic foundation for further investigation toward practical supercapacitor configurations and long-term cycling evaluation.

Table 6 Comparison of specific capacitance of amine-modified MOF-14 with other MOF materials

Material	Specific capacitance (F g^−1)	Ref.
Cd_AT-BP/rGO	58.5@5 mV s^−1	44
Cd_AT-BP/CNT	17.5@5 mV s^−1	44
Ammonia functionalized ZIF-8	260@2 A g^−1	66
Mn–Ni MOF	711@1 A g^−1	67
Covalent graphene-MOF	651@2 A g^−1	68
ZnCo-MOF/GS	695@1 A g^−1	69
Zn-MOF/PANI	372@0.1 A g^−1	70
Ni-Tyr-NDI-MOF/GF	330@1 A g^−1	71
rGO/Ni-MOF@PANI	204@0.5 A g^−1	72
MOF-14 (en) 20%	972@10 mV s^−1	This work
	466@1 A g^−1

---

4. Conclusion

In summary, this work demonstrates that post-synthetic amine functionalization provides a versatile and effective route to tailor the performance of MOF-14 for CO₂ capture and electrochemical energy storage. Ethylenediamine, diethylenetriamine, and 1,2-bis(3-aminopropylamino)ethane were successfully introduced into the MOF framework without compromising structural stability; however, the balance between available porosity and functional group density was strongly governed by the amine type and incorporation level. CO₂ adsorption studies revealed that moderate loadings of en and deta (10–15 wt%) afforded the highest uptake, leveraging enhanced chemisorptive interactions while maintaining sufficient pore accessibility, whereas bape induced significant steric restriction. Furthermore, evaluation of CO₂/N₂ separation performance, including both single-component selectivity estimates and IAST calculations for a 15% CO₂/85% N₂ mixture, demonstrated that amine incorporation substantially enhances the preferential adsorption of CO₂ over N₂. The amine-modified MOF-14 materials exhibit high selectivity values, placing them among competitive MOF-based adsorbents for CO₂/N₂ separation under flue-gas conditions. Electrochemical analysis identified MOF-14 (en) 20% as a high-performing electrode material, delivering a remarkable specific capacitance of 972 F g⁻¹, significantly exceeding that of the pristine parent material.

Overall, the results highlight the critical role of optimizing functional group incorporation to achieve multifunctionality within a single material system. Importantly, controlled amine loading enables rational tuning of MOF-14 toward either optimized CO₂ capture or enhanced electrochemical energy storage, underscoring its adaptability as a multifunctional materials platform rather than reliance on a single universally optimal composition. By judicious adjustment of amine content, MOF-14 can therefore be selectively tailored depending on the targeted application. This study positions amine-modified MOF-14 as a promising platform for next-generation applications at the intersection of environmental remediation and energy storage technologies.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included in the manuscript itself and as part of the supplementary information (SI). Supplementary information includes additional characterization data such as PXRD, IR spectra, TG/DTA analysis, EDX data, and gas adsorption measurements (N₂ and CO₂), along with images of the prepared samples. See DOI: https://doi.org/10.1039/d5ma01438g.

The raw data will also be made available upon genuine request.

Acknowledgements

P. S. gratefully acknowledges the financial support of the Department of Science and Technology, Ministry of Science and Technology, Govt. of India, for women scientists under WISE-PhD grant (DST/WISE-PhD/PM/2023/110). Manisha gratefully acknowledge the Council of Scientific and Industrial Research (CSIR) for awarding the Senior Research Fellowship (Ref. No. 16/06/2019(i) EU-V). AS is highly thankful to Science and Engineering Board (SERB), Department of Science and Technology (DST), Government of India for providing research funding under the SERB Startup Research Grant (SRG) scheme (File No. SRG/2021/002019) and SERB International Research Experience (SIRE) scheme (File No. SIR/2022/001589). M. A. thanks EU NextGenerationEU through the Recovery and Resilience Plan for Slovakia under the project no. 09I03-03-V03-00034 (MASS-PRAM), no. 09I02-03-V01-00022 (SUNFLOWERS), VEGA project no. 1/0058/25, and the European Union for providing a grant under KA171 – Mobility of higher education students and staff call. The authors thank Klaudia Simanova from Pavol Jozef Safarik University in Kosice, Slovak Republic, for the preparation of the materials.

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