Jiao-Jiao Caoab,
Jin-Long Geab,
Yan Gaoab and
Bo-Kai Liao
*c
aSchool of Material and Chemical Engineering, Bengbu University, Bengbu, 233030, China
bEngineering Technology Research Center of Silicon-based Materials, Bengbu, 233030, China
cSchool of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, 510006, China
First published on 17th June 2025
This research systematically investigates a library of UiO-66(Zr) dicarboxylate metal–organic frameworks (MOFs), functionalized with polar and nonpolar substituents (–H, –CH3, –NH2), to evaluate their potential as carriers for inhibitor encapsulation, using 8-hydroxyquinoline (8-HQ) as a model compound. The encapsulation process was conducted under vacuum, and 8-HQ uptake was quantified using UV-visible absorption spectroscopy, allowing for precise control over host–guest interactions. Quantitative assessment of 8-HQ loading within each functionalized UiO-66 variant revealed significant variations in uptake efficiency, ranging from 14.66% to 19.45% across the modified frameworks. Furthermore, a density functional theory-based quantitative structure–activity relationship (QSAR) approach was employed to identify key chemical and structural attributes of UiO-66 that significantly influence inhibitor uptake. Multivariate analysis identified two critical physicochemical parameters governing encapsulation efficiency: (1) the dipole moment of the functionalized linkers, which dictates framework-polarity-mediated adsorption, and (2) the hydrogen-bond-donating capacity, which modulates specific interactions with 8-HQ's phenolic oxygen. These descriptors exhibited strong linear correlations with experimental loading data, demonstrating that strategic enhancement of linker polarity and H-bonding capability can amplify inhibitor uptake by 10–20% compared to pristine UiO-66. The QSAR model deciphers MOF performance trends and predicts optimal functional groups via descriptor interpolation, streamlining MOF carrier design for corrosion and drug applications through targeted linker engineering.
Recently, metal–organic frameworks (MOFs), intricate structures formed by metal ions linked with organic ligands, have garnered considerable interest as drug-delivery nanocarriers. This is primarily due to their high loading capacity, exceptional biocompatibility, and impressive thermal stability, making them promising candidates for advanced pharmaceutical applications.9–11 Among MOFs, zirconium-based MOFs, particularly UiO-66, are attractive for controlled inhibitor delivery due to their abundant active sites, outstanding chemical stability, and ultra-high thermal stability. For instance, Zhao et al.12 synthesized graphene oxide/UiO-66 nanocontainers encapsulating benzotriazole (BTA), which were successfully applied to protect copper and its alloys. Moreover, UiO-66 can be customized by incorporating functional metal nodes or groups with varying polarities, hydrophilicities, and acidities to achieve high loading efficiency.13,14 Chen et al.15 used epichlorohydrin to graft UiO-66 onto chitosan, creating a porous chitosan/Zr-MOF composite foam. Their results showed that the hierarchical porosity and abundant functional groups of the composite foam effectively facilitated the removal of ketoprofen from water, achieving a maximum adsorption capacity of 209.7 mg g−1 (at pH = 4), surpassing most reported adsorbents. Modifying organic linkers within the MOF structure is another common and effective approach, with various functional groups demonstrating efficacy.13 Sun et al.16 investigated the adsorption of two common anti-inflammatory drugs, ibuprofen (IBP) and naproxen (NAP), on UiO-66 and UiO-66-NH2. Their results indicated that the presence of additional binding site types and fewer adsorption sites on UiO-66-NH2, compared to UiO-66, led to weaker competitive adsorption of IBP and NAP. Gaudin et al.17 explored the impact of grafting various organic linkers onto the MOF structure for caffeine encapsulation, using QSAR models to demonstrate that grafting polar and H-donor groups onto the MOF's organic linkers increased drug uptake.
Although batch experiments are frequently employed to investigate the encapsulation or loading of guest molecules by various materials, discerning the underlying adsorption mechanisms can be complex. To overcome this limitation, researchers are increasingly integrating spectroscopic analysis with density functional theory (DFT) calculations to elucidate the molecular-level details of adsorption processes.18–20 DFT calculations offer valuable insights into molecular structures, binding sites, and binding energies associated with the adsorption of metals and organic chemicals onto diverse material.21,22 For instance, Jiang et al.23 investigated eco-friendly corrosion inhibitors (chitosan, ferulic acid, cysteine, methionine) derived from natural bio-based materials for steel protection in simulated concrete pore solutions. Quantum chemical (QC) calculations revealed electron-donating capacities and reactive sites, while molecular dynamics (MD) simulations visualized stable inhibitor adsorption on steel surfaces, corroborating experimental data. These inhibitors also enhanced mortar strength (up to an 11.18% compressive strength increase) without compromising workability, presenting a sustainable strategy for improving reinforced concrete durability. Similarly, Zhou et al.24 explored the corrosion inhibition effect of 5-methoxy-indazole (MIA) and methyl 1H-indazole-5-carboxylate (MIC) on copper in H2SO4 through theoretical calculations, employing DFT and MD simulations to further elucidate the corrosion inhibition mechanism of these imidazole derivatives and highlighting their potential application in acidic media. Moreover, Keshmiri et al.25 successfully grew a highly porous and stable covalent organic framework (COF) structure on a multi-walled carbon nanotube (CNT), using it as a nanocontainer for zinc as an inhibitor to produce a coating with superior active/passive anti-corrosion and thermo-mechanical properties.
Currently, 8-hydroxyquinoline (8-HQ) and its derivatives are recognized as effective corrosion inhibitors for steel, aluminum, copper, and magnesium alloys, demonstrating superior inhibition performance in acidic, saline, and other corrosive environments.26 To the best of my knowledge, systematic research examining the impact of carrier structure on the loading capacity of these inhibitors is limited. Therefore, in this study, UiO-66 was modified with 2-amino-terephthalic acid and 2-methyl terephthalic acid via solvothermal synthesis, resulting in UiO-66-NH2 and UiO-66-CH3, respectively. Subsequently, the adsorption of 8-HQ inhibitors by these modified MOFs was investigated. The UiO-66 derivatives and UiO-66-X@8-HQ composites were characterized using transmission electron microscopy (TEM), powder X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and UV-vis spectroscopy. The corrosion inhibiting effectiveness of the UiO-66-X@8-HQ composites was evaluated for steel in CO2-saturated 0.5 mol per L NaCl solution using FE-SEM, XPS, and electrochemical methods. Finally, the adsorption behavior of the degradation products released from the UiO-66-X particles on the steel surface was investigated through theoretical calculations. Comprehensive spectroscopic characterization confirms the successful incorporation of 8-hydroxyquinoline (8-HQ) into UiO-66-X (X= –H, –NH2, –CH3) frameworks while preserving their crystalline integrity, as evidenced by XRD and FT-IR analyses. Thermogravimetric analysis (TGA) quantified distinct loading capacities of 16.01%, 14.66%, and 19.45% for UiO-66, UiO-66-NH2, and UiO-66-CH3, respectively, with methyl-functionalized variants demonstrating superior loading performance. UV-vis kinetic studies revealed time-dependent 8-HQ release profiles adhering to first-order kinetics (R2 > 0.98), achieving equilibrium after sustained release. The release rates followed the hierarchy: UiO-66 > UiO-66-CH3 > UiO-66-NH2, suggesting that amino groups enhance host–guest interactions to retard molecular diffusion. Systematic QSAR modeling decoupled structure–property relationships, identifying key molecular descriptors governing MOF performance. This integrated experimental-computational strategy establishes a robust framework for rational MOF carrier design, enabling precise linker engineering to advance corrosion-inhibitive and controlled drug delivery systems.
The reactants, including terephthalic acid (H2BDC), 2-aminoterephthalic acid (H2BDC-NH2), 2-methylterephthalic acid (H2BDC-CH3), and 8-hydroxyquinoline (8-HQ), were procured from Aladdin. Zirconium chloride (ZrCl4) was sourced from Merck Chemicals Co. Solvents including N,N-dimethylformamide (DMF), methanol (CH3OH), and ethanol (C2H5OH) were sourced from Tianjin Guangfu Chemicals. The solvents utilized in our experiments were of analytical grade and employed without the need for additional purification. A CO2-saturated 0.5 mol per L NaCl solution served as the testing medium, and the solution was continuously sparged with CO2 to maintain saturation throughout the duration of the tests.
To impregnate the 8-HQ inhibitor within UiO-66, 0.2 g of pre-dried UiO-66 (dry at 70 °C for 24 h) was introduced into 250 mL of an ethanolic solution containing 0.4 g of 8-HQ. The mixture was stirred continuously under vacuum for 24 hours to facilitate loading, followed by centrifugation and thorough washing with ethanol. The resulting UiO-66@8-HQ particles were then dried in an oven. Following the same procedure, UiO-66 was replaced with UiO-66-NH2 and UiO-66-CH3 to synthesize UiO-66-NH2@8-HQ and UiO-66-CH3@8-HQ, respectively.
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MD simulations, can provide insights into the most probable inhibitor adsorption orientations on the target metal surface. Initially, an iron crystal was imported, cleaved along the Fe(110) plane, and truncated into three layers. A supercell (6 × 6) was then constructed with dimensions of (24.82 Å × 24.82 Å × 8.11 Å) to provide an ample surface for interaction with the tested inhibitor. Subsequently, a vacuum slab of zero thickness was created. An amorphous cell containing 500 H2O molecules, 5 Cl− ions, 5 H3O+ ions, and one molecule of the inhibitor of interest was assembled. The simulation was conducted at 298 K under the NVT ensemble with a time step of 1 fs and a simulation duration of 500 ps, utilizing the COMPASS force field. Ultimately, the interaction force between the corrosion inhibitor and the carbon steel is calculated using the subsequent equation:29
Eint = Etot − (Esur+sol + Einh+sol) + Esol | (4) |
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Fig. 2 SEM images of (a) UiO-66, (b) UiO-66-CH3, (c) UiO-66-NH2, (d) UiO-66@8-HQ, (e) UiO-66-CH3@8-HQ and (f) UiO-66-NH2@8-HQ. |
Fig. 3 presented TEM images and EDX dot mappings, detailing the elemental composition and distribution within the selected samples. As shown in Fig. 3(a–c), the pristine UiO-66 particles exhibited a consistent octahedral morphology with diameters exceeding 100 nm. The octahedral geometry was largely maintained following linker modification.32 Similarly, minimal morphological alterations are observed between UiO-66@8-HQ, UiO-66-CH3@8-HQ, and UiO-66-NH2@8-HQ. However, the elemental dot mappings for these modified species clearly demonstrate the uniform distribution of carbon, nitrogen, oxygen, and zirconium throughout the UiO-66-X framework, which suggested incomplete or non-ideal encapsulation of the 8-HQ inhibitor within UiO-66, UiO-66-NH2, and UiO-66-CH3.27
Fig. 4a presented the X-ray diffraction (XRD) patterns of the UiO-66 derivatives and the UiO-66-X@8-HQ composites. Diffraction peaks observed at 2θ values of 7.38° and 8.50°, corresponding to the (1 1 1) and (2 0 0) planes, respectively, align with previously reported results,27,33,34 confirming the successful synthesis of UiO-66. Furthermore, the near-identical XRD patterns of UiO-66 and its derivatives, UiO-66-X, indicate that these materials maintain the same three-dimensional porous crystalline lattice structure as the parent UiO-66, characterized by hexanuclear zirconium oxyhydroxide clusters and octahedral cages assembled with modified terephthalate linkers.13 Following the synthesis of UiO-66-X@8-HQ, the characteristic peaks of the UiO-66 derivatives remain, demonstrating that the composites retain their crystalline structures without significant loss of crystallographic integrity. However, a subtle shift of the peaks toward lower angles was observed in the UiO-66-X@8-HQ patterns. These variations in the XRD patterns are attributed to the successful incorporation of 8-HQ molecules within the UiO-66-X@8-HQ framework.
Fig. 4b presented a comparative analysis of the FTIR spectra obtained for UiO-66 derivatives and UiO-66-X@8-HQ composites. In the UiO-66 spectrum, characteristic vibrations of Zr–O bonds were identified at 667 cm−1 and 558 cm−1.35,36 Peaks observed at 2920 cm−1, 1662 cm−1, 1581 cm−1, and 1396 cm−1 corresponded to the stretching vibrations of aromatic C–H, CO, and C
C bonds, respectively, originating from the organic linkers. The peak at 3450 cm−1 was attributed to hydroxyl groups and/or adsorbed water molecules present on the sample surface.37 For UiO-66 containing an amino group, the peak at 3328 cm−1 signified the characteristic absorption of the amino group (–NH2), the peak at 1439 cm−1 corresponded to the stretching vibration of the C–N bond, and the peak at 581 cm−1 indicates the Zr–O bond, collectively confirming the successful synthesis of UiO-66-NH2.38 The peak at 1379 cm−1 was identified as the characteristic absorption of the methyl group,39 while the peak at 590 cm−1 was attributed to the Zr–O bond, confirming the successful preparation of UiO-66-CH3. Moreover, in the spectra of UiO-66-NH2@8-HQ and UiO-66-CH3@8-HQ, peaks observed at 1326.00 cm−1, 1468.55 cm−1, and 1501.56 cm−1 were indicative of the C–C and C–N stretching vibrations within the aromatic ring structure of 8-HQ,40 thereby demonstrating the successful synthesis of the UiO-66-X@8-HQ hybrids.
Nitrogen adsorption–desorption isotherms and pore size distribution curves for the UiO-66 derivatives and UiO-66-X@8-HQ composites were presented in Fig. 4c and d, respectively. As shown in Fig. 4c, all samples exhibited type I isotherms, consistent with IUPAC classification,41 indicative of microporous materials. Specific surface areas, determined using the Brunauer–Emmett–Teller (BET) method, were 1271.22 m2 g−1 for UiO-66, 771.10 m2 g−1 for UiO-66-CH3, and 846.42 m2 g−1 for UiO-66-NH2 (Table 1). Following the incorporation of 8-HQ, the specific surface areas decreased to 803.17 m2 g−1, 637.30 m2 g−1, and 284.06 m2 g−1 for UiO-66@8-HQ, UiO-66-CH3@8-HQ, and UiO-66-NH2@8-HQ, respectively. Non-local density functional theory (NLDFT) analysis revealed pore sizes primarily within the 0–3 nm range (Fig. 4d), further confirming the presence of micropores.42 The average pore diameters for UiO-66, UiO-66-CH3, and UiO-66-NH2 were 1.59 nm, 1.77 nm, and 1.62 nm, respectively. Correspondingly, the total pore volumes were 0.51 cm3 g−1 for UiO-66, 0.34 cm3 g−1 for UiO-66-CH3, and 0.45 cm3 g−1 for UiO-66-NH2. Notably, the pore diameters and volumes of the UiO-66-X@8-HQ composites were lower than those of the pristine UiO-66 and its derivatives, suggesting that the introduction of 8-HQ molecules into the pores of the UiO-66 derivatives leads to a partial pore blockage and consequently, a decrease in specific surface area.15,43,44
Samples | BET surface area (m2 g−1) | Pore volume (cm3 g−1) | Pore width (nm) |
---|---|---|---|
UiO-66 | 1271.22 | 0.51 | 1.59 |
UiO-66@8-HQ | 803.17 | 0.35 | 1.56 |
UiO-66-CH3 | 771.10 | 0.45 | 1.77 |
UiO-66-CH3@8-HQ | 637.30 | 0.27 | 10.25 |
UiO-66-NH2 | 846.42 | 0.34 | 1.62 |
UiO-66-NH2@8-HQ | 284.06 | 0.11 | 1.53 |
XPS analysis, as depicted in Fig. 4e, furnishes compelling evidence of the successful loading of 8-HQ onto functionalized UiO-66 carriers. The appearance of characteristic 8-HQ peaks in the XPS spectra confirms the effective incorporation of the inhibitor. Specifically, the observed chemical shift in the N 1s spectrum, for instance, from 399.8 eV to 400.5 eV, is attributable to the presence of amino groups, thereby unambiguously verifying 8-HQ loading on the UiO-66-NH2 variant. Concurrently, the C 1s spectrum reveals discernible alterations in bonding states, including the emergence of a new component at 285.2 eV corresponding to methyl groups, consistent with the structural characteristics of the UiO-66-CH3 carrier. These collective spectral shifts strongly suggest that hydrogen-bonding interactions between the functional groups (–NH2 and –CH3) and 8-HQ are the predominant mechanism governing the loading process, an observation that aligns favorably with our density functional theory (DFT) simulations.45,46 Fig. 4f displayed the zeta potential profiles of UiO-66-X and UiO-66-X@8-HQ (X = –H, –CH3, –NH2).28 Prior to loading, the pristine 8-HQ exhibits a negative zeta potential (−2.3 ± 0.5 mV), while the UiO-66-X series showed positive surface charges (+15.2 to +18.6 mV), consistent with their respective intrinsic physicochemical properties. Upon encapsulation, a significant reduction in zeta potential was observed for all UiO-66-X@8-HQ composites, with values decreasing to +12.1 ± 0.3 mV (UiO-66-NH2@8-HQ), +8.7 ± 0.4 mV (UiO-66-CH3@8-HQ), and +10.2 ± 0.2 mV (UiO-66@8-HQ). This systematic charge neutralization phenomenon provided compelling evidence for the successful incorporation of 8-HQ into the MOF carriers, likely mediated by electrostatic interactions between the negatively charged inhibitor and the positively functionalized UiO-66 frameworks.
To investigate the mechanism of UiO-66 and its derivatives loading the 8-HQ, kinetic models such as pseudo first-order was applied to analyze the adsorption data. Eqn (5) describes the pseudo-first-order:16
Ct/Ce = 1 − Ae(−kt) | (5) |
Ct and Ce represent the concentrations of 8-HQ at time t and at equilibrium, respectively. A is a constant, t denotes the time of release, and k signifies the release rate constant. The release profile initially exhibited a rapid rate, which subsequently decelerated as the concentration increased over time. To investigate the 8-HQ release mechanism and the influence of the UiO-66 series framework, the experimental data were analyzed using the pseudo-first-order kinetic model (eqn (5)).45 This model suggests an initial phase of rapid release, followed by a gradual slowdown as the concentration approaches equilibrium. Fig. 6a presented the standard curve, while Fig. 6b illustrated the time-dependent release behavior of UiO-66-X@8-HQ. The observed rapid release within the first two hours, followed by a sustained, slower release until equilibrium, can be attributed to the materials' large specific surface area and ordered pore structure. Consequently, the release rates for the modified materials followed the order UiO-66@8-HQ < UiO-66-CH3@8-HQ < UiO-66-NH2@8-HQ, with UiO-66-NH2 exhibiting a higher equilibrium release than both UiO-66 and UiO-66-CH3. Table 2 summarized the kinetic parameters for 8-HQ adsorption by UiO-66-X@8-HQ. The high fitting determination coefficients (R2) of 0.97, 0.99, and 0.94 for UiO-66@8-HQ, UiO-66-NH2@8-HQ, and UiO-66-CH3@8-HQ, respectively, indicated that their release profiles are well-described by the pseudo-first-order kinetic model. The release rate constant (k) values for UiO-66 and UiO-66-CH3 were higher than that for UiO-66-NH2, suggesting faster release kinetics for the former two.50 This may be related to the enhanced transfer of hydrophilic 8-HQ to the carrier surface in solution. The higher k values observed for UiO-66-NH2 and UiO-66-CH3 compared to UiO-66 can potentially be attributed to the smaller BET surface areas and comparatively larger pore sizes of the modified UiO-66-NH2 and UiO-66-CH3 materials.
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Fig. 6 (a) Standard curve of 8-HQ and (b) release concentrations of UiO-66-X@8-HQ of UiO-66-X@8-HQ (X = –H/–CH3/–NH2). |
Samples | Ce (mg L−1) | A | k (h−1) | (R2) |
---|---|---|---|---|
UiO-66@8-HQ | 2.15 | −1.13 | 0.13 | 0.97 |
UiO-66-CH3@8-HQ | 5.24 | −3.47 | 0.58 | 0.94 |
UiO-66-NH2@8-HQ | 3.74 | −3.36 | 1.23 | 0.99 |
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Fig. 7 Polarization curves of carbon steel in CO2-saturated 0.5 mol per L NaCl solution with 100 mg per L UiO-66-X@8-HQ (X = –H/–CH3/–NH2) at 25 °C. |
Samples | Ecorr (mV) | ba (mV dec−1) | bc (mV dec−1) | icorr (A cm−2) | IE (%) |
---|---|---|---|---|---|
Blank | −770 | 43 | −96 | 2.36 × 10−5 | |
8-HQ | −695 | 37 | −65 | 3.88 × 10−6 | 83.58 |
UiO-66@8-HQ | −696 | 52 | −88 | 6.99 × 10−6 | 70.42 |
UiO-66-CH3@8-HQ | −760 | 71 | −125 | 6.44 × 10−6 | 72.14 |
UiO-66-NH2@8-HQ | −712 | 36 | −96 | 5.86 × 10−6 | 75.24 |
To elucidate the corrosion mechanism of carbon steel in solutions with and without various UiO-66-X@8-HQ additives, electrochemical impedance spectroscopy (EIS) was performed. Fig. 8 presented the impedance spectra obtained for carbon steel immersed in 0.5 mol per L NaCl solution saturated with CO2, both with and without the UiO-66-X@8-HQ compounds. A summary diagram of the AC impedance spectra of carbon steel in the presence of UiO-66-X@8-HQ was shown in Fig. 9. Equivalent circuits (Fig. 10) were employed to fit the corresponding EIS data, where n represents the dispersion effect index, Rs is the solution resistance, Rf is the resistance of the corrosion product film, Rct is the charge transfer resistance of the corrosion reaction, and W is the Warburg impedance, which reflects the diffusion process of electroactive species. CPEf and CPEdl are constant phase elements (CPEs) representing the capacitive response of the corrosion product film and the double-layer capacitance, respectively.51,52 Specifically, Fig. 8a displayed the fitted data for the blank solution and the solution containing UiO-66@8-HQ, while Fig. 8b showed the fitted data for the blank solution and the solutions containing 8-HQ, UiO-66-NH2@8-HQ, and UiO-66@8-HQ. The fitting parameters derived from these analyses are detailed in Table 4.
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Fig. 8 (a) Nyquist and (b) Bode plots of carbon steel in CO2-saturated 0.5 mol per L NaCl solution containing 100 mg per L UiO-66-X@8-HQ (X = –H/–CH3/–NH2) at 25 °C. |
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Fig. 9 Time dependence of Rct + Rf for carbon steel in CO2-saturated 0.5 mol per L NaCl solution containing 100 mg per L UiO-66-X@8-HQ (X = –H/–CH3/–NH2) at 25 °C. |
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Fig. 10 Electrochemical equivalent circuits for fitting the EIS (a) blank (b) UiO-66-X@8-HQ (X = –H/–CH3/–NH2). |
Rs (Ω cm2) | CPEdl-T (Sn1 Ω−1 cm−2) | n1 | Rct (Ω cm2) | CPEf-T (Sn2 Ω−1 cm−2) | n2 | Rf (Ω cm2) | |
---|---|---|---|---|---|---|---|
Blank | 3.8 | 2.51 × 10−5 | 0.92 | 41.5 | 1.01 × 10−4 | 0.60 | 269 |
8-HQ | 3.1 | 1.60 × 10−5 | 0.95 | 97.9 | 1.11 × 10−5 | 0.60 | 1841 |
UiO-66@8-HQ | 6.9 | 2.92 × 10−5 | 0.87 | 67.1 | 1.46 × 10−4 | 0.65 | 861 |
UiO-66-NH2@8-HQ | 6.2 | 1.96 × 10−5 | 0.89 | 89.0 | 5.71 × 10−5 | 0.57 | 1453 |
UiO-66-CH3@8-HQ | 3.3 | 1.06 × 10−5 | 0.99 | 83.8 | 7.31 × 10−5 | 0.60 | 1058 |
EIS analysis of carbon steel in the blank solution, as depicted in the Nyquist and Bode plots, revealed distinct features attributable to different processes. The high-frequency region exhibited an arc corresponding to the capacitive behavior of the corrosion product film formed on the electrode surface. Conversely, the low-frequency region displayed an arc related to the charge transfer resistance. Furthermore, the observed tail in the response suggested a diffusion-controlled process, likely associated with the transport of corrosion products. As illustrated in Fig. 8a, the addition of MOF matrix composites, such as UiO-66-X@8-HQ, to the solution results in the disappearance of the Warburg impedance in the low-frequency region and a significant increase in the capacitive-reactance arc diameter, particularly evident with the addition of UiO-66-X@8-HQ and 8-HQ. The Bode plots in Fig. 8b demonstrated that the impedance modulus and phase angle values increase successively upon the introduction of UiO-66-X@8-HQ compared to the blank solution. This observation indicated that the presence of these MOF matrix composites impedes the corrosion reaction on the carbon steel surface, thus inhibiting its corrosion.53 Consistent with these findings, Table 4 showed a substantially higher impedance value for carbon steel in the solution containing UiO-66-X@8-HQ compared to the blank solution, suggesting that the incorporation of UiO-66-X@8-HQ effectively reduces the corrosion rate of carbon steel. The impedance values follow the order UiO-66-NH2@8-HQ > UiO-66-CH3@8-HQ > UiO-66@8-HQ, suggesting that the released 8-HQ contributed to the protective layer formed on the carbon steel surface.54 As observed in Table 4, the impedance value of carbon steel in the solution containing UiO-66-X@8-HQ was significantly higher compared to that in the blank solution. This suggested that the incorporation of UiO-66-X@8-HQ effectively diminished the corrosion rate of carbon steel.
To further elucidate the influence of 8-HQ released from the UiO-66 series on the corrosion behavior of carbon steel, EIS measurements were conducted on carbon steel at 25 °C after varying immersion times in solutions containing UiO-66-X@8-HQ. The EIS fitting results were presented in Fig. 9. As depicted, the impedance values of carbon steel increased progressively with extended immersion time, eventually reaching a plateau, indicating the maximal release of 8-HQ corrosion inhibitor from the UiO-66 series composites. The observed trend in impedance values (Rct + Rf) follows the order: UiO-66-NH2@8-HQ > UiO-66-CH3@8-HQ > UiO-66@8-HQ. These findings suggested that UiO-66-X@8-HQ and other MOF-based composites can be effectively utilized as corrosion inhibitors in corrosive media, providing long-term protection for the carbon steel substrate.
SEM images revealed the corrosive effects on carbon steel surfaces following a 24 hours immersion in a 0.5 mol per L NaCl solution, both with and without the addition of various UiO-66-X@8-HQ composites under saturated CO2 conditions. As depicted in Fig. 11a, the carbon steel surface in the blank solution exhibited a layer of loosely adhered corrosion products and a notably uneven texture. Conversely, the surface treated with UiO-66@8-HQ (Fig. 11b) displayed residual mechanical scratches from the abrasive pretreatment, accompanied by only a limited amount of corrosion products. Furthermore, the addition of UiO-66-CH3@8-HQ (Fig. 11c) resulted in a smoother carbon steel surface with a sparse distribution of corrosion products. Most notably, the presence of UiO-66-NH2@8-HQ (Fig. 11d) yielded a remarkably flat surface with a near absence of corrosion products. These observations suggest that the incorporation of UiO-66-X@8-HQ and related MOF matrix composites into the corrosive medium effectively mitigates corrosion and provides a protective effect for the carbon steel substrate.
XPS spectra were acquired to elucidate the surface chemical composition of carbon steel samples following a 24 hours immersion in a 0.5 mol per L NaCl solution containing 100 mg per L UiO-66-X@8-HQ and saturated with CO2 (Fig. 12). To gain a deeper understanding of the elemental chemical environments and to attribute specific components to their corresponding chemical states, peak fitting analysis was performed on selected high-resolution spectra.
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Fig. 12 Full-survey XPS spectra of carbon steel in CO2-saturated 0.5 mol per L NaCl solution containing 100 mg per L UiO-66-X@8-HQ (X = –H/–CH3/–NH2) for 24 h at 25 °C. |
Fig. 13 illustrates the high-resolution C 1s, N 1s, and Fe 2p spectra.53 The C 1s spectrum (Fig. 13a) obtained from the UiO-66@8-HQ treated carbon steel surface exhibits multiple bands at 289.22 eV, 285.92 eV, and 284.82 eV, corresponding to CN, C–N, and C–C/CH bonds, respectively. On the surface of carbon steel treated with UiO-66-NH2@8-HQ, peaks at 286.89 eV, 285.00 eV, and 283.50 eV were assigned to C–N and C–C/CH bonds, respectively. For the UiO-66-CH3@8-HQ treated carbon steel surface, peaks at 287.52 eV, 286.17 eV, and 284.77 eV were attributed to C
N, C–N, and C–C/CH bonds, respectively.55 The N 1s spectrum (Fig. 13b) of the UiO-66@8-HQ treated carbon steel surface revealed peaks at 401.67 eV, 400.37 eV, and 398.97 eV, corresponding to Fe–N, C–N/C
N, and N–N/N
N bonds, respectively. The energy peak positions of the corresponding bonds on the UiO-66-NH2@8-HQ treated carbon steel surface were shifted to approximately 400.95 eV, 399.55 eV and 398.95 eV, respectively. Similarly, on the surface of the UiO-66-CH3@8-HQ treated carbon steel, these bonds were observed at 401.17 eV, 400.17 eV, and 398.92 eV, respectively.56 The O 1s spectrum (Fig. 13c) exhibited three components with peaks at 529.9 eV, 531.2 eV, and 532.4 eV, corresponding to Fe2O3, C
O, and C–O groups, respectively. The Fe 2p3/2 spectrum (Fig. 13d) displayed fitted peaks at 709.7 eV, 710.8 eV, and 712.1 eV, which are associated with Fe–N, Fe2O3, and FeOOH states, respectively.57,58 The presence of Fe–N peaks indicated the formation of coordination bonds between 8-HQ and the carbon steel surface, suggesting a strong chemisorption of 8-HQ. This process involved the donation of lone pairs of electrons from nitrogen atoms and hydroxyl groups of 8-HQ to unoccupied molecular orbitals of the carbon steel, where the corrosion inhibitor inhibited corrosion ions on the carbon steel surface through chemisorption.59
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Fig. 13 High-resolution XPS spectra of carbon steel in CO2-saturated 0.5 mol per L NaCl solution with UiO-66-X@8-HQ (X = –H/–CH3/–NH2) for 24 h at 25 °C (a), C 1s, (b) N 1s, (c) O 1s and (d) Fe 2p. |
Sample | EHOMO (eV) | ELUMO (eV) | γ (eV) | χ (eV) | σ (eV) | ΔE (eV) | ΔN | μ (D) | Ebinding (kJ mol−1) |
---|---|---|---|---|---|---|---|---|---|
8-HQ | −6.16 | −1.68 | 2.24 | −3.92 | 0.45 | 4.48 | 1.88 | 2.99 | — |
UiO-66 | −7.41 | −2.12 | 2.64 | −4.76 | 0.38 | 5.28 | 1.75 | 2.65 | — |
UiO-66-CH3 | −7.09 | −2.06 | 2.51 | −4.58 | 0.40 | 5.03 | 1.80 | 3.02 | — |
UiO-66-NH2 | −5.89 | −1.87 | 2.01 | −3.88 | 0.50 | 4.01 | 2.08 | 4.45 | — |
UiO-66@8-HQ | −6.05 | −1.65 | 2.20 | −3.85 | 0.46 | 4.39 | 1.90 | 5.57 | −3.04 |
UiO-66-CH3@8-HQ | −5.52 | −2.04 | 1.74 | −3.78 | 0.57 | 3.48 | 2.37 | 5.46 | −12.54 |
UiO-66-NH2@8-HQ | −5.57 | −2.78 | 1.40 | −4.18 | 0.72 | 2.79 | 3.10 | 4.67 | −22.33 |
Upon complexation with 8-HQ, significant changes in electronic structure were observed. For UiO-66@8-HQ, EHOMO and ELUMO both increase, and the gap narrows (ΔE = 4.39 eV), while the dipole moment rises to 5.57 D, revealing enhanced charge separation in the assembly. Critically, the binding energy (Ebinding) was calculated as −3.04 kJ mol−1, evidencing a spontaneous but modestly strong interaction between 8-HQ and pristine UiO-66. For the methylated complex (UiO-66-CH3@8-HQ), the EHOMO further increases to −5.52 eV, the ΔE contracts to 3.48 eV, and dipole moment decreases to 5.46 D; notably, the binding energy becomes substantially more negative (−12.54 kJ mol−1), indicating increased thermodynamic favorability. This enhancement was likely driven by the methyl group modifying the local electron density and perhaps facilitating van der Waals or weak π–π interactions. The aminated derivative displayed the most dramatic effects: UiO-66-NH2@8-HQ attains a significantly elevated ELUMO (−2.78 eV), a considerably narrowed gap (ΔE = 2.79 eV), and a reduced dipole moment (μ = 4.67 D) relative to its unbound analogue. Its remarkably negative binding energy (−22.33 kJ mol−1) suggested the interaction is highly spontaneous, which is plausibly attributable to strong hydrogen bonding between the NH2 group and 8-HQ's heteroatoms, as further evidenced by the largest difference in chemical hardness (σ = 0.72 eV) and softness (γ = 1.40 eV).
Overall, these data delineated a clear trend: functionalization of UiO-66, particularly with electron-donating groups such as NH2, drastically enhances the host–guest interaction with 8-HQ, both by promoting electronic compatibility (reduced ΔE, increased charge transfer, and dipole moments) and providing sites for specific non-covalent interactions. The methyl group, while not as effective as NH2, still favorably shifts the binding energy compared to unmodified UiO-66, supporting the hypothesis that modulation of framework functionality offers a powerful lever for tuning guest adsorption properties in metal–organic frameworks. This insight not only deepens our understanding of host–guest chemistry in UiO-66 derivatives but also underscores the critical role of molecular design in optimizing sorbent materials for targeted applications.
Molecular dynamics (MD) simulations were conducted to analyze the adsorption behavior of 8-HQ on the surfaces of the UiO-66 series. Fig. 15 presented simulated side and top views of the 8-HQ inhibitor adsorbed on these surfaces, while Table 6 summarizes the corresponding theoretical calculations. As illustrated in Fig. 15a and d, 8-HQ molecules tended to adsorb onto the UiO-66 surface at an inclined angle. A similar inclined orientation is observed for 8-HQ adsorption on UiO-66-CH3 (Fig. 15b and e) and UiO-66-NH2 (Fig. 15c and f). These MD simulations demonstrated the feasibility of 8-HQ adsorption on all UiO-66 series surfaces. Furthermore, the adsorption energies listed in Table 6 for UiO-66@8-HQ, UiO-66-CH3@8-HQ, and UiO-66-NH2@8-HQ are −240.287 kJ mol−1, −465.807 kJ mol−1, and −617.032 kJ mol−1, respectively. A greater absolute adsorption energy indicates a stronger interaction between the carrier and the guest molecule. Therefore, 8-HQ exhibited a spontaneous binding affinity for the UiO-66 series surfaces, with the following order of binding energies: UiO-66-NH2@8-HQ > UiO-66-CH3@8-HQ > UiO-66@8-HQ.
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Fig. 15 MD simulation diagrams for 8-HQ on the UiO-66 series: top view (a) UiO-66, (b) UiO-66-CH3 and (c) UiO-66-NH2, side view: (d) UiO-66, (e) UiO-66-CH3 and (f) UiO-66-NH2. |
Samples | Etot | EUiO-66-X | Einh | Eint |
---|---|---|---|---|
UiO-66@8-HQ | −285.240 | −176.174 | 131.221 | −240.287 |
UiO-66-CH3@8-HQ | −501.748 | −249.659 | 213.718 | −465.807 |
UiO-66-NH2@8-HQ | −648.402 | −269.920 | 238.550 | −617.032 |
The differential loading of 8-HQ onto the UiO-66 series can be attributed to the influence of the functional groups. The presence of the amino group on UiO-66-NH2 likely promotes preferential interactions between the UiO-66-NH2 carrier and the ethanol solvent, thereby reducing the loading capacity for the 8-HQ inhibitor. Conversely, the methyl group on UiO-66-CH3 enhances hydrophobicity, leading to an improved loading capacity for 8-HQ. The observed differences in the 8-HQ release rates from the UiO-66 series are likely due to the formation of hydrogen bonds between the amino group on UiO-66-NH2 and 8-HQ, strengthening the binding interaction and resulting in the slowest release rate. The methyl group enhances van der Waals forces, leading to a stronger binding interaction between UiO-66-CH3 and 8-HQ compared to UiO-66@8-HQ.
Fig. 16 illustrated the electron density distribution of the optimized 8-HQ molecule. As depicted, electron density was largely delocalized across the conjugated bonds within the nitrogen-containing heterocyclic ring and adjacent hydroxyl groups. Furthermore, the electrostatic potential map revealed that regions of nucleophilic reactivity (red, negative charge) and electrophilic reactivity (green, positive charge) are primarily concentrated around the oxygen and nitrogen atoms. The calculated energy level of 8-HQ (4.48 eV) was below the Fermi level of iron (Fe = 4.5 eV), suggesting the likelihood of electron transfer from the corrosion inhibitor to the metal surface. These observations underscore the significant contribution of the N–N bond and the –OH group to the overall molecular activity of the 8-HQ inhibitor.
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Fig. 16 Molecular charge distribution of 8-HQ after DFT optimization (legend: grey represents carbon; white represents hydrogen, blue represents nitrogen, and magenta represents oxygen). |
MD simulations were employed to investigate the inhibition mechanism of 8-hydroxyquinoline (8-HQ) on carbon steel corrosion. A (6 × 6) supercell configuration of the Fe(110) surface was constructed, and the interactions between this surface and 8-HQ inhibitor molecules were simulated within a simulation box, utilizing periodic boundary conditions to replicate the corrosive environment. The simulated adsorption system incorporated Cl−, H+, CO2, and H2O molecules, alongside the 8-HQ inhibitor molecules, while other simulation parameters remained constant. As illustrated in Fig. 17, 8-HQ exhibited a predominantly parallel adsorption orientation on the carbon steel surface, as observed from both side and top views. Furthermore, the calculated adsorption energy of 8-HQ on the Fe(110) surface was −309.30 kJ mol−1, indicative of a strong interaction. This robust adsorption was attributed to the presence of electron-donating nitrogen atoms within the N–N group and the hydroxyl group in the 8-HQ molecule, which facilitated favorable interactions with the iron surface.
(1) UiO-66, UiO-66-NH2, and UiO-66-CH3 exhibited distinct 8-HQ loading capacities of 16.01%, 14.66%, and 19.45%, respectively, with corresponding release efficiencies of 98.89%, 97.70%, and 98.89%. Competitive solvation analysis indicated that the amino group in UiO-66-NH2 preferentially interacts with ethanol solvent molecules, consequently reducing 8-HQ uptake. Conversely, methyl-functionalized UiO-66-CH3 enhanced hydrophobic affinity for 8-HQ, leading to optimized loading performance. Molecular simulations further revealed that amino-functionalization strengthens 8-HQ binding energy through intensified host–guest interactions, while methyl substitution enhances van der Waals forces at the interface.
(2) The mechanism of corrosion inhibition involves the chemisorption of 8-HQ onto carbon steel surfaces via Fe–N coordination bonds, forming a protective molecular layer that effectively impedes electrolyte penetration. This adsorption-driven barrier effect significantly suppresses both anodic dissolution and cathodic oxygen reduction reactions, thereby achieving robust corrosion protection.
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