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. 2026 Sep 15;11(38):57925–57934. doi: 10.1021/acsomega.6c07336

Melamine-Modified MIL-101(Cr) with Enhanced CO2 Affinity for Low-Pressure Capture

Andrés Lancheros †,*, Claudio Contreras-Díaz †, Esteban Camú †, Thiago C dos Santos ‡, Célia M Ronconi §, Néstor Escalona †, Élodie Blanco †,∥,*
PMCID: PMC13629403  PMID: 42825151

Abstract

MIL-101­(Cr) was post-synthetically modified with melamine to investigate the effect of nitrogen-rich functionalities on low-pressure CO2 adsorption behavior. Melamine incorporation partially reduced the accessible surface area and pore volume of the framework while preserving its crystallinity and structural stability. Despite the decrease in porosity, the modified MIL-101­(Cr)-MEL exhibited enhanced CO2 adsorption performance at low pressures and elevated temperatures compared with pristine MIL-101­(Cr). At 60 °C and 1 bar, the CO2 uptake of MIL-101­(Cr)-MEL was more than twice that of the pristine material, while the working capacity in the 0.08–0.14 bar range increased fourfold. IAST calculations revealed significantly enhanced CO2/N2 selectivity at low pressures, reaching selectivity values close to 90 below 0.05 bar. Dynamic breakthrough experiments under both dry and humid CO2/N2 gas streams further confirmed the preferential CO2 adsorption of MIL-101­(Cr)-MEL, with only a 13.4% reduction in CO2 adsorption capacity under humid conditions. The modified material also exhibited higher isosteric enthalpies of adsorption at low CO2 coverage, indicating enhanced CO2 affinity relative to pristine MIL-101­(Cr). However, the moderate −ΔHads values suggest that adsorption is governed by reversible non-covalent CO2-melamine interactions rather than by the formation of strongly bound CO2-amine adducts, providing an effective balance between CO2 affinity and regenerability. DRIFTS analyses confirmed reversible CO2 adsorption without evidence of carbonate or carbamate formation, supporting a mechanism dominated by moderate-strength non-covalent interactions. Overall, these results demonstrate that melamine functionalization provides a simple and effective strategy to enhance low-pressure CO2 capture performance under thermally demanding conditions.

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Introduction

The continuous increase in atmospheric CO2 concentration caused by fossil fuel combustion remains one of the major environmental challenges associated with climate change and global warming. , Among the different mitigation strategies, adsorption-based carbon capture technologies have attracted considerable attention because solid adsorbents can provide lower regeneration energy penalties and greater cyclic stability than conventional aqueous amine absorption systems. , In particular, the capture of diluted CO2 streams under post-combustion and direct-air-capture-relevant conditions requires adsorbents capable of maintaining strong CO2 affinity at low partial pressures and elevated temperatures. ,

Metal-organic frameworks (MOFs) have emerged as promising adsorbents for CO2 capture because of their exceptionally high surface areas, tunable pore structures, and versatile surface chemistry. − Their structural diversity enables the rational design of porous environments with tailored adsorption properties, making them attractive materials for selective gas separation processes. − However, under diluted CO2 conditions, adsorption performance becomes increasingly governed by the strength and density of specific adsorption sites rather than solely by the total accessible surface area or pore volume. Consequently, the incorporation of high-affinity adsorption sites capable of selectively interacting with CO2 molecules at low pressures has become an important strategy for improving the performance of porous adsorbents. ,

Among the different MOFs investigated for CO2 capture, MIL-101­(Cr) has attracted particular interest because of its exceptional thermal and chemical stability, large pore volume, and high surface area, which facilitate gas diffusion and adsorption. − Recent studies have further expanded the potential of MIL-101­(Cr) for acid gas capture through advanced structural engineering. Strategies including the incorporation of nitrogen-doped carbon quantum dots and graphene oxide nanocomposites, , as well as the development of core-shell MIL-101­(Cr)@UiO-66­(Zr) heterostructures, have demonstrated enhanced CO2 and H2S adsorption and improved gas separation performance by tailoring pore architecture and increasing the availability of high-affinity adsorption sites.

Nevertheless, despite its highly porous structure, pristine MIL-101­(Cr) exhibits relatively low affinity toward CO2 at low pressures because its adsorption behavior is mainly dominated by weak physisorptive interactions. , Therefore, several post-synthetic modification strategies have been explored to introduce nitrogen-containing functionalities capable of strengthening CO2-framework interactions and enhancing adsorption selectivity under diluted CO2 conditions. ,

Amine functionalization is one of the most widely explored approaches to improve CO2 adsorption performance in porous materials due to the strong interactions that nitrogen-containing groups can establish with CO2 molecules. − In many cases, these modifications increase CO2 uptake and selectivity by introducing stronger adsorption sites within the porous structure.

However, conventional amine-functionalized systems also present several limitations that may compromise their long-term practical application. High amine loadings often lead to a significant reduction in the accessible surface area and pore volume of the porous support, resulting in diffusion limitations, slower adsorption-desorption kinetics, and higher regeneration energy requirements. , In addition, amines may undergo oxidative degradation, volatilization, or leaching under operating conditions, while prolonged exposure to CO2 can promote irreversible urea formation, contributing to adsorbent deactivation. Furthermore, the presence of humidity and acid gases such as SO2 and NO x can adversely affect adsorption performance through the formation of strongly bound sulfate and nitrate species that block active sites. Therefore, the development of adsorbents capable of providing moderate-strength, reversible CO2 interactions while preserving fast adsorption behavior and structural stability remains highly desirable for cyclic carbon capture applications.

In this context, melamine (2,4,6-triamino-1,3,5-triazine) emerges as an attractive modifier because of its high nitrogen content, multiple amino functionalities, and ability to establish moderate-strength non-covalent interactions with CO2 molecules through hydrogen bonding and electrostatic interactions. , The incorporation of melamine within porous frameworks may therefore enhance low-pressure CO2 affinity even at the expense of accessible porosity. Such behavior is particularly relevant for highly diluted CO2 streams, where adsorption affinity becomes more critical than maximum adsorption capacity at high pressure. ,

In this work, MIL-101­(Cr) was modified through melamine incorporation to investigate the effect of nitrogen-rich adsorption sites on low-pressure CO2 capture behavior. Despite the partial reduction in accessible porosity after melamine incorporation, the modified material exhibited enhanced low-pressure CO2 uptake, improved CO2/N2 selectivity, and superior working capacity under elevated-temperature conditions. Furthermore, isosteric enthalpy calculations and DRIFTS analyses suggest that improved adsorption performance originates from moderate-strength non-covalent CO2-melamine interactions rather than from irreversible chemisorptive binding. These results demonstrate that adsorption affinity can play a more critical role than total porosity in determining CO2 capture performance under low-pressure conditions.

Materials and Methods

Reagents

Cr­(NO3)3·9H2O, terephthalic acid, HF, and melamine were purchased from Sigma-Aldrich and used as received without further purification.

Porous Material Preparation

Synthesis of MIL-101­(Cr)

MIL-101­(Cr) was synthesized through a hydrothermal procedure adapted from a previously reported method. Briefly, 1.67 g of terephthalic acid (C6H4(COOH)2) and 4.00 g of chromium­(III) nitrate nonahydrate (Cr­(NO3)3·9H2O) were dissolved in 42 mL of deionized water, followed by the addition of 0.5 mL of hydrofluoric acid (HF). The resulting mixture was transferred into a Teflon-lined stainless-steel autoclave and heated at 220 °C for 8 h. After cooling to room temperature, the obtained green solid was recovered, repeatedly washed with ethanol (95%), and dried at 80 °C for 24 h.

Melamine Impregnation on MIL-101­(Cr)

Melamine (Figure ) was incorporated into the activated MIL-101­(Cr) through a wet impregnation procedure. Briefly, 0.050 g of melamine was dissolved in 100 mL of anhydrous ethanol, followed by the addition of 0.200 g of MIL-101­(Cr), corresponding to a nominal melamine loading of 25 wt % relative to the MOF mass. The suspension was stirred at 500 rpm for 4 h at room temperature. Subsequently, the solvent was removed by evaporation at approximately 80 °C. The obtained solid was further dried at 80 °C overnight and stored in a desiccator prior to characterization.

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Chemical structure of melamine.

Porous Material Characterization

Characterization Techniques

Powder X-ray diffraction (PXRD) patterns were recorded using a Rigaku MiniFlex II diffractometer (Tokyo, Japan) equipped with Cu Kα radiation (λ = 1.5418 Å) over a 2θ range of 2–40°.

Thermogravimetric analysis (TGA) was carried out using an SDTQ-600 TGA/DSC instrument (TA Instruments) under a nitrogen atmosphere (20 mL min–1). Approximately 10 mg of sample was heated from 30 to 1000 °C at a heating rate of 5 °C min–1.

Nitrogen adsorption-desorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 system (Norcross, USA). Prior to the measurements, the samples were degassed under vacuum at 120 °C for 18 h. The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) method following the Rouquerol consistency criteria. Pore size distributions were obtained using the nonlocal density functional theory (NLDFT) method with a nitrogen kernel, while the total pore volume was estimated from the amount adsorbed at a relative pressure (P/P0) of 0.99.

CO2 Sorption Measurements

CO2 adsorption isotherms were measured in the low-pressure region (0–1 bar) at 0, 30, 45, and 60 °C using a Micromeritics ASAP 2020 system (Norcross, USA). Prior to the measurements, the samples were degassed under vacuum at 120 °C for 18 h to remove pre-adsorbed species from the porous structure.

The isosteric enthalpy of adsorption (−ΔHads) was determined from the CO2 adsorption isotherms collected at different temperatures using the Clausius–Clapeyron eq (eq ).

ln⁡P=−ΔHR1T+c 1

where p is the equilibrium pressure, T is the absolute temperature, and R is the universal gas constant (8.314 J mol–1 K–1). The isosteric enthalpy of adsorption (−ΔHads) was obtained from the slope of the linear plot of ln­(p) versus 1/T, providing information about the strength of the interactions between CO2 molecules and the adsorption sites within the porous structure.

The CO2/N2 selectivity was predicted using the Ideal Adsorbed Solution Theory (IAST) from the experimentally obtained single-component adsorption isotherms (eq ).

Sads=qCO2/qN2pCO2/pN2 2

where qCO2 and qN2 represent the adsorption capacities of CO2 and N2 at their respective partial pressures in the binary gas mixture, with pCO2 = 0.15 bar and pN2 = 0.85 bar. Single-component adsorption isotherms for CO2 and N2 were collected at 30 °C in the low-pressure region (0–1 bar).

Pressure swing adsorption (PSA) cyclic stability was evaluated using a Micromeritics 3Flex analyzer (Micromeritics Instrument Corp., USA). Five consecutive CO2 adsorption-desorption cycles were performed by alternating the pressure between 0.015 and 1.0 bar.

Kinetic CO2 Adsorption Measurements

Kinetic CO2 adsorption experiments were carried out using a Shimadzu DTG-60/60-H thermogravimetric analyzer (Japan) following the procedure reported by Hisse et al. (2024). Approximately 5 mg of sample was placed in an alumina pan and pretreated at 120 °C under a N2 flow of 50 mL min–1 for 1 h, or until a constant mass was reached. Subsequently, the temperature was reduced to 30 °C, and a CO2 flow of 50 mL min–1 was introduced while maintaining a N2 flow of 50 mL min–1 through the balance side. The CO2 uptake was continuously monitored until adsorption equilibrium was achieved. Desorption experiments were initiated by interrupting the CO2 flow, and the mass variation was recorded until the sample recovered its initial mass.

Breakthrough Experiments

Dynamic breakthrough experiments were performed using a Micromeritics 3Flex system coupled to a U-shaped quartz reactor. Approximately 300 mg of MIL-101­(Cr)-MEL was activated under vacuum at 120 °C for 18 h using a Smart VacPrep system. The activated sample was then transferred to the quartz reactor and purged with Ar prior to the breakthrough experiments. The experiments were conducted at 30 °C. He (10 sccm) was used as an inert tracer gas, after which the feed was switched to a binary mixture of CO2 (10 sccm) and N2 (10 sccm). The outlet gas composition was continuously monitored using an MKS Cirrus 2 mass spectrometer to obtain breakthrough curves. For experiments under humid conditions, the CO2/N2 gas mixture was passed through a water saturator at room temperature before entering the adsorption bed.

DRIFTS Analysis of CO2 Adsorption

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed using a Bruker Vertex 70 spectrophotometer (Billerica, USA) equipped with a Harrick Praying Mantis accessory and an HVC-DRP-4 reaction chamber fitted with ZnSe windows. Prior to the measurements, the samples were pretreated in situ at 115 °C under a helium flow for 15 min and subsequently cooled to 30 °C. A background interferogram was recorded under continuous helium flow before CO2 exposure. The samples were then exposed to a CO2 stream for 20 min, after which the gas flow was switched back to helium and the first spectrum was immediately collected. Additional spectra were subsequently acquired at 2 min intervals over a total monitoring period of 20 min. All interferograms were collected by averaging 256 scans with a spectral resolution of 16 cm–1.

Results and Discussion

The successful synthesis of MIL-101­(Cr) was confirmed by PXRD, N2 adsorption at 77 K, and thermogravimetric analyses. The PXRD pattern (Figure S1a) agrees well with previously reported MIL-101­(Cr) structures, confirming the formation of a highly crystalline framework. , The pristine MIL-101­(Cr) exhibited a BET surface area of 3303 m2 g–1 (Figure S1b), consistent with literature values reported for highly porous MIL-101­(Cr) materials. Furthermore, the TGA and DTG profiles (Figure S1c) displayed three main weight-loss regions centered around 100, 250, and 400–450 °C, associated with the removal of physisorbed water, desorption of confined species, and framework decomposition into Cr2O3, respectively.

Figure a presents the N2 adsorption-desorption isotherms of MIL-101­(Cr) and MIL-101­(Cr)-MEL measured at 77 K. Both materials exhibit type I isotherms according to the IUPAC classification, characteristic of predominantly microporous solids, while the small hysteresis loop observed at intermediate relative pressures suggests the presence of secondary mesoporosity associated with the MIL-101­(Cr) framework. The sharp N2 uptake at low relative pressures confirms the highly porous nature of both materials.

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(a) N2 adsorption-desorption isotherms at 77 K and (b) pore size distribution profiles of MIL-101­(Cr) and MIL-101­(Cr)-MEL.

The pristine MIL-101­(Cr) exhibited a significantly higher N2 uptake and BET surface area than MIL-101­(Cr)-MEL, whose surface area decreased to 2090 m2 g–1 after melamine incorporation. Similarly, the pore size distribution profiles (Figure b) reveal a reduction of approximately 36% in accessible pore volume after modification, while preserving the characteristic pore size distribution of MIL-101­(Cr). These results indicate that melamine incorporation partially occupies the porous structure without causing significant framework collapse or loss of structural integrity. Importantly, although melamine incorporation reduces the accessible porosity of the material, the preservation of the pore structure suggests that adsorption sites remain accessible for gas adsorption under low-pressure conditions.

The marked decrease in accessible pore volume (∼36%) while preserving the characteristic pore size distribution and crystallinity of MIL-101­(Cr) indicates that melamine is primarily confined within the porous network rather than merely deposited on the external surface. Such confinement is expected to be favored by weak host-guest interactions, including possible hydrogen bonding between melamine amino groups and oxygen atoms of the framework.

Water adsorption isotherms (Figure ) reveal a significant decrease in H2O uptake after melamine incorporation into MIL-101­(Cr). While the pristine material exhibits a pronounced increase in water adsorption at moderate-to-high relative pressures, MIL-101­(Cr)-MEL shows substantially lower uptake over the entire pressure range, reaching a maximum adsorption capacity of approximately 52 mmol g–1. The high-water uptake of pristine MIL-101­(Cr) is associated with its large accessible pore volume and hydrophilic adsorption sites within the framework. After melamine incorporation, the reduced H2O adsorption is mainly attributed to partial pore occupation and decreased accessibility of water molecules to the porous structure. This lower water uptake may be beneficial for low-pressure CO2 capture applications, where competitive adsorption between H2O and CO2 can negatively affect adsorption performance.

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Water adsorption isotherms of MIL-101­(Cr) and MIL-101­(Cr)-MEL.

The PXRD patterns of theoretical MIL-101­(Cr), as-synthesized MIL-101­(Cr), and MIL-101­(Cr)-MEL are presented in Figure . The diffraction pattern of the synthesized material agrees well with the theoretical MIL-101­(Cr) structure, confirming the successful formation of the crystalline framework. After melamine incorporation, the main characteristic diffraction peaks of MIL-101­(Cr) remain largely preserved, indicating that the overall framework structure and crystallinity are maintained after modification. Minor changes in peak intensity and slight peak broadening are observed for MIL-101­(Cr)-MEL, which may be associated with partial pore occupation and local structural modifications within the porous framework.

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PXRD patterns of theoretical MIL-101­(Cr), as-synthesized MIL-101­(Cr), and MIL-101­(Cr)-MEL.

Figure presents the CO2 adsorption isotherms of MIL-101­(Cr) and MIL-101­(Cr)-MEL measured between 0 and 60 °C in the low-pressure region (0–1 bar). At 0 °C, pristine MIL-101­(Cr) exhibits the highest CO2 uptake at pressures approaching 1 bar, reaching approximately 4.0 mmol g–1, whereas MIL-101­(Cr)-MEL reaches approximately 3.3 mmol g–1. This behavior is consistent with the larger accessible surface area and pore volume of pristine MIL-101­(Cr). However, despite its lower total uptake, MIL-101­(Cr)-MEL displays a steeper adsorption profile in the low-pressure region, indicating the presence of higher-affinity adsorption sites after melamine incorporation.

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CO2 adsorption isotherms of MIL-101­(Cr) and MIL-101­(Cr)-MEL measured at (a) 0 °C, (b) 30 °C, (c) 45 °C, and (d) 60 °C.

As temperature increases, the adsorption capacity of pristine MIL-101­(Cr) decreases sharply, reflecting the strong temperature dependence of physisorption-dominated adsorption. In contrast, MIL-101­(Cr)-MEL retains substantially higher CO2 uptake, particularly at low pressures and elevated temperatures. At 60 °C, MIL-101­(Cr)-MEL reaches a CO2 uptake of 1.06 mmol g–1 at 1 bar, whereas pristine MIL-101­(Cr) adsorbs only 0.45 mmol g–1. These results indicate that melamine incorporation modifies the dominant adsorption behavior of MIL-101­(Cr), shifting the adsorption process from porosity-dominated uptake toward affinity-controlled adsorption under diluted CO2 conditions.

The working capacities (WC) calculated within the 0.08–0.14 bar pressure range are presented in Figure , as this interval is relevant for low-pressure CO2 capture applications. While pristine MIL-101­(Cr) exhibits a slightly higher WC at 0 °C, MIL-101­(Cr)-MEL progressively outperforms the pristine material as temperature increases. At 60 °C, MIL-101­(Cr)-MEL retains a WC of 0.20 mmol g–1, compared with only 0.05 mmol g–1 for pristine MIL-101­(Cr), corresponding to a fourfold enhancement after melamine incorporation. This improved low-pressure adsorption performance at elevated temperature highlights the importance of introducing high-affinity adsorption sites for efficient CO2 capture under thermally demanding conditions.

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Low-pressure CO2 working capacities of MIL-101­(Cr) and MIL-101­(Cr)-MEL within the 0.08–0.14 bar pressure range at 0, 30, 45, and 60 °C.

Importantly, the enhanced low-pressure CO2 affinity of MIL-101­(Cr)-MEL suggests that adsorption performance becomes increasingly governed by adsorption affinity rather than total porosity under diluted CO2 conditions. The steeper initial uptake and reduced temperature sensitivity observed for the modified material further indicate that melamine incorporation promotes more effective CO2 capture under low-pressure conditions.

The CO2/N2 selectivity was evaluated using IAST based on single-component adsorption isotherms fitted with Langmuir, Freundlich, and dual-site Langmuir–Freundlich (DSLF) models (Figures S2 and S3). The DSLF model provided the best agreement with experimental data and was therefore used for selectivity calculations.

As shown in Figure , MIL-101­(Cr)-MEL exhibits a remarkably high CO2/N2 selectivity at very low pressures, reaching values close to 90 below 0.05 bar, which is substantially higher than pristine MIL-101­(Cr) (∼30). This behavior indicates that melamine incorporation introduces high-affinity adsorption sites that strongly favor CO2 adsorption under diluted CO2 conditions. As pressure increases, the selectivity of MIL-101­(Cr)-MEL decreases progressively and stabilizes near ∼19, suggesting progressive saturation of the high-affinity adsorption sites. In contrast, pristine MIL-101­(Cr) exhibits a weaker pressure dependence, consistent with adsorption dominated mainly by weak physisorptive interactions. Overall, these results further support that melamine incorporation shifts the adsorption behavior of MIL-101­(Cr) from porosity-dominated adsorption toward affinity-controlled CO2 capture at low pressures.

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CO2/N2IAST selectivity of MIL-101­(Cr) and MIL-101­(Cr)-MEL as a function of pressure at 30 °C.

The isosteric enthalpy of CO2 adsorption (−ΔHads) was calculated using the Clausius–Clapeyron equation from adsorption isotherms collected at 0, 30, and 45 °C. The corresponding isostere plots used for the calculations are presented in Figures S4 and S5, while the resulting −ΔHads profiles are shown in Figure .

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Isosteric enthalpy of CO2 adsorption (−ΔHads) of MIL-101­(Cr) and MIL-101­(Cr)-MEL as a function of CO2 uptake.

Pristine MIL-101­(Cr) exhibits nearly constant −ΔHads values of approximately 23–24 kJ mol–1 over the entire loading range, which is characteristic of adsorption dominated mainly by weak physisorptive interactions. In contrast, MIL-101­(Cr)-MEL displays significantly higher −ΔHads values at low CO2 loadings, reaching ∼37 kJ mol–1, followed by a progressive decrease toward ∼23 kJ mol–1 as the CO2 uptake increases.

The higher initial enthalpy observed for MIL-101­(Cr)-MEL indicates the presence of stronger adsorption sites introduced after melamine incorporation, which preferentially interact with CO2 molecules at low coverage. The progressive decrease in −ΔHads with increasing CO2 uptake suggests gradual saturation of these high-affinity adsorption sites, after which adsorption becomes increasingly dominated by weaker interactions similar to those observed for pristine MIL-101­(Cr). This behavior is consistent with the enhanced low-pressure CO2 uptake and high CO2/N2 selectivity observed for MIL-101­(Cr)-MEL, further supporting that melamine incorporation promotes affinity-controlled CO2 adsorption under diluted CO2 conditions.

Figure a shows the dynamic CO2 uptake profiles of MIL-101­(Cr) and MIL-101­(Cr)-MEL obtained under thermogravimetric adsorption conditions. Both materials exhibit rapid CO2 uptake upon exposure to the gas stream, followed by a gradual approach to equilibrium. Pristine MIL-101­(Cr) reaches a slightly higher equilibrium adsorption capacity (∼1.0 mmol g–1) than MIL-101­(Cr)-MEL (∼0.95 mmol g–1), which is consistent with its larger accessible pore volume.

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(a) Dynamic CO2 uptake profiles and (b) derivative adsorption rate curves of MIL-101­(Cr) and MIL-101­(Cr)-MEL obtained from thermogravimetric adsorption measurements.

The corresponding derivative adsorption rate curves (Figure b) reveal comparable adsorption kinetics for both materials, with maximum adsorption rates occurring between approximately 140 and 170 s. A slight reduction in the maximum adsorption rate is observed for MIL-101­(Cr)-MEL; however, both materials exhibit rapid CO2 uptake and reach equilibrium within a similar time interval. These results indicate that melamine incorporation enhances low-pressure CO2 affinity while preserving the fast adsorption kinetics and accessibility of the porous structure.

Figure a shows five consecutive PSA cycles for CO2 on MIL-101­(Cr)-MEL. The adsorption capacity remained nearly constant throughout the repeated adsorption-desorption cycles, with no noticeable loss in CO2 uptake. The complete regeneration achieved after each desorption step indicates that the adsorbed CO2 can be readily released under the applied PSA conditions, demonstrating the excellent cyclic stability and regenerability of the adsorbent.

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(a) PSA cyclic CO2 adsorption-desorption performance of MIL-101­(Cr)-MEL, (b) binary CO2/N2 breakthrough curves, and (c) CO2/N2 breakthrough under humid conditions.

To experimentally validate the CO2/N2 selectivity predicted by IAST (Figure ), dynamic breakthrough experiments were performed using a binary CO2/N2 gas mixture. As shown in Figure b, N2 broke through the adsorption bed before CO2, whereas the delayed CO2 breakthrough indicates its stronger affinity for the adsorption sites of MIL-101­(Cr)-MEL. This breakthrough behavior is consistent with the equilibrium adsorption results and experimentally confirms the high CO2/N2 selectivity predicted by IAST.

The dynamic breakthrough curves obtained under humid CO2/N2 conditions are presented in Figure c. Compared with the dry condition, the presence of water vapor results in a slight decrease in CO2 adsorption capacity (13.4%), which can be attributed to competitive adsorption between H2O and CO2 molecules. However, MIL-101­(Cr)-MEL maintains preferential CO2 adsorption, as evidenced by the delayed CO2 breakthrough relative to N2. This behavior is consistent with the lower H2O uptake observed after melamine incorporation (Figure ), which results from partial pore occupation and reduced accessibility of hydrophilic adsorption sites, thereby limiting water adsorption and improving the humidity tolerance of the MIL-101­(Cr) modified with melamine.

The DRIFTS spectra of MIL-101­(Cr) and MIL-101­(Cr)-MEL, including the characteristic bands associated with melamine incorporation, are provided in Figure S6. The spectra of both materials recorded before and after CO2 exposure are presented in Figure S7, while the corresponding difference spectra highlighting the physisorption and chemisorption regions are shown in Figure S8.

Figure presents the most relevant spectral regions associated with CO2 adsorption, corresponding to the physisorption (2400–2300 cm–1) and chemisorption-related/framework-perturbation (1700–1100 cm–1) domains. In the 2400–2300 cm–1 region (Figure a,c,e), both MIL-101­(Cr) and MIL-101­(Cr)-MEL exhibit the characteristic splitting of the ν 3 asymmetric stretching mode of adsorbed CO2, which is commonly associated with physisorbed CO2 confined within porous frameworks. The preservation of these bands indicates that CO2 adsorption remains predominantly reversible and dominated by confinement effects within the porous structure.

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DRIFTS difference spectra of MIL-101­(Cr) and MIL-101­(Cr)-MEL after CO2 adsorption at different temperatures: (a) MIL-101­(Cr) at 30 °C in the physisorption region; (b) MIL-101­(Cr) at 30 °C in the chemisorption-related/framework-perturbation region; (c) MIL-101­(Cr)-MEL at 30 °C in the physisorption region; (d) MIL-101­(Cr)-MEL at 30 °C in the chemisorption-related/framework-perturbation region; (e) MIL-101­(Cr)-MEL at 60 °C in the physisorption region; and (f) MIL-101­(Cr)-MEL at 60 °C in the chemisorption region.

More significant differences between both materials are observed in the 1700–1400 cm–1 region (Figure b, d, f). For pristine MIL-101­(Cr), the bands centered near 1646 and 1456 cm–1 exhibit slight shifts toward lower wavenumbers after CO2 exposure, indicating weak perturbations of the framework environment caused by physisorbed CO2 molecules. In contrast, MIL-101­(Cr)-MEL exhibits distinct spectral behavior, particularly for the band near 1456 cm–1, which shifts toward higher wavenumbers after CO2 adsorption at both 30 and 60 °C. These differences suggest that melamine incorporation modifies the local electrostatic environment of the porous framework and promotes stronger interactions between CO2 molecules and the adsorption sites.

Importantly, no new bands associated with carbonate, bicarbonate, or carbamate species were observed after CO2 exposure, even at 60 °C. This indicates that CO2 adsorption in MIL-101­(Cr)-MEL is not dominated by irreversible chemisorption, but rather by moderate-strength interactions capable of enhancing low-pressure CO2 affinity while preserving reversible adsorption behavior. These observations are fully consistent with the enhanced low-pressure CO2 uptake, high CO2/N2 selectivity, and increased −ΔHads values observed for MIL-101­(Cr)-MEL.

Although no spectroscopic evidence of carbonate or carbamate formation was observed, the enhanced low-pressure CO2 adsorption behavior of MIL-101­(Cr)-MEL suggests the presence of specific interactions between melamine functionalities and CO2 molecules.

Previous density functional theory (DFT) studies have shown that melamine interacts with CO2 mainly through non-covalent interactions involving distinct roles for the triazine ring nitrogen atoms and the amino (−NH2) groups. The pyridine-like nitrogen atoms of the triazine ring act as Lewis base sites, donating electron density to the electrophilic carbon atom of CO2 through a tetrel bond with an N···C distance of approximately 2.83 Å. At the same time, the −NH2 groups act as hydrogen-bond donors, forming two nearly symmetrical NH···O hydrogen bonds with the oxygen atoms of CO2, each with H···O distances of approximately 2.24 Å (Figure ).

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Proposed non-covalent CO2-melamine interactions.

These interactions are characterized by moderate stabilization energies and do not involve the formation of covalent CO2-amine adducts, which is consistent with the reversible adsorption behavior, moderate −ΔHads values, and preserved adsorption kinetics observed for MIL-101­(Cr)-MEL.

The adsorption enthalpy comparison presented in Figure and Table S1, including representative amine-modified MIL-100­(Cr) and MIL-101­(Cr) adsorbents from the literature, further supports this interpretation. The moderate −ΔHads values of MIL-101­(Cr)-MEL suggest a balance between CO2 affinity and reversibility, avoiding both the low affinity and strong temperature dependence typical of weakly physisorptive materials and the high regeneration penalties often associated with strong chemisorption. Such a balance represents a highly desirable characteristic for practical low-pressure CO2 capture applications.

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Comparison of isosteric enthalpy of CO2 adsorption for amine-functionalized MIL-101­(Cr)/MIL-100­(Cr)-based adsorbents, using values reported in the literature. −

Conclusions

Melamine incorporation into MIL-101­(Cr) was successfully achieved while preserving the crystallinity and structural stability of the framework. Although melamine functionalization reduced the accessible porosity of the material, the modified MIL-101­(Cr)-MEL exhibited enhanced CO2 adsorption performance under low-pressure and elevated-temperature conditions. In particular, MIL-101­(Cr)-MEL showed significantly improved working capacity and CO2/N2 selectivity in the low-pressure region, together with reduced temperature sensitivity compared with pristine MIL-101­(Cr).

The increased isosteric enthalpy of adsorption and the DRIFTS analyses indicate that melamine incorporation introduces moderate-strength adsorption sites capable of promoting stronger and more selective interactions with CO2 molecules without evidence of irreversible chemisorption. These interactions enhance low-pressure CO2 affinity while preserving reversible adsorption behavior and rapid adsorption kinetics.

Overall, the results demonstrate that improving adsorption affinity can be more effective than maximizing total porosity for CO2 capture under diluted CO2 conditions. Therefore, melamine functionalization represents a simple and effective post-synthetic strategy for tuning the adsorption behavior of MIL-101­(Cr)-based materials toward affinity-controlled CO2 capture applications.

Supplementary Material

ao6c07336_si_001.pdf (676.2KB, pdf)

Acknowledgments

This work was supported by FONDEQUIP Project No. EQM160070 and FONDECYT Regular Project No. 1220763. A.L. acknowledges the financial support provided by the Agencia Nacional de Investigación y Desarrollo (ANID) through FONDECYT Postdoctorado No. 3240504. C.C.D. acknowledges ANID for the Doctoral Fellowship No. 21211077. N.E. and E.B. acknowledge ANID for financial support through the Anillo en Tecnología 2024 Project No. ACT240050. The authors also thank the Multiuser Laboratories of the Universidade Federal Fluminense (UFF) for access to the materials characterization facilities (http://www.uff.br/lamate/).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c07336.

  • PXRD pattern, N2 physisorption isotherm, pore size distribution, and TGA/DTG profiles of pristine MIL-101­(Cr) (Figure S1); adsorption isotherm fitting of CO2 and N2 on MIL-101­(Cr) using Langmuir, Freundlich, and dual-site Langmuir–Freundlich models (Figure S2); adsorption isotherm fitting of CO2 and N2 on MIL-101­(Cr)-MEL using the same models (Figure S3); isostere plots for CO2 adsorption on MIL-101­(Cr) and MIL-101­(Cr)-MEL (Figures S4 and S5); DRIFTS spectra of MIL-101­(Cr) and MIL-101­(Cr)-MEL before and under CO2 atmosphere (Figures S6–S8); comparison of amine-functionalized adsorbents and isosteric heats of CO2 adsorption at zero coverage (Table S1) (PDF)

The authors declare no competing financial interest.

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