Remarkable Adsorbent for the Removal of ... - ACS Publications

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Protonated MIL-125-NH2: Remarkable Adsorbent for the Removal of Quinoline and Indole from Liquid Fuel Imteaz Ahmed,† Nazmul Abedin Khan,† Ji Woong Yoon,‡ Jong-San Chang,‡,§ and Sung Hwa Jhung*,† †

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea ‡ Research Group for Nanocatalysts, Division of Green Chemistry & Engineering Research, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34144, Republic of Korea § Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea S Supporting Information *

ABSTRACT: The removal of nitrogen-containing compounds (NCCs) from fossil fuels prior to combustion is currently of particular importance, and so we investigated an adsorptive method using metal−organic frameworks (MOFs) for the removal of indole (IND) and quinoline (QUI), which are two of the main NCCs present in fossil fuels. We herein employed an amino (−NH2)functionalized MIL-125 (MIL-125-NH2) MOF, which was further modified by protonation (P-MIL-125-NH2). These modified MOFs exhibited extraordinary performance in the adsorption of both IND (as representative neutral NCC) and QUI (as representative basic NCC). These MOFs were one of the most efficient adsorbents for the removal of NCCs. For example, P-MIL-125-NH2 showed the highest adsorption capacity for QUI among ever reported adsorbent. The improved adsorption of IND was explained by Hbonding and cation−π interactions for MIL-125-NH2 and P-MIL-125-NH2, respectively, while the mechanisms for QUI were Hbonding and acid−base interactions, respectively. This is a rare phenomenon for a single material (especially not with very high porosity) to exhibit such remarkable performances in the adsorption of both basic QUI and neutral IND. The adsorption results obtained using regenerated MIL-125-NH2 and P-MIL-125-NH2 also showed that these materials can be used several times without any severe degradation. KEYWORDS: adsorption, denitrogenation, mechanism, metal−organic framework, nitrogen-containing compounds, protonation

1. INTRODUCTION

structures. Thus, to mitigate the severe effects of SCCs and NCCs, scientists are investigating potential means for the removal of these materials from fossil fuels. Among these two types of contaminants, SCCs are effectively and widely removed by the hydrodesulfurization (HDS) process. In this process, H2 gas reacts with SSC molecules, and sulfur is removed in the form of H2S gas. However, prior to the removal of SSCs, the fuel should be free of NCCs, as NCCs create adverse effects on the HDS catalysts and also hamper the process itself. In this context, NCCs are conventionally removed via a similar hydrodenitrogenation (HDN) process, although this technique has a number of drawbacks. For example, significantly more H2 gas is required compared with the HDS process, and the former is a slow process that occurs at high temperatures and pressures, and so it is comparatively energy intensive.4,5 As such, a more viable NCC removal

Fossil fuels are currently the most important and widely used energy sources worldwide. Because of improved technological advancements and the increasing population, the demand for and exploration of fossil fuels also increase steadily. As such, new and unconventional sources of fossil fuels containing sulfur, nitrogen, and metal-based contaminants will be employed in subsequent decades. On the other hand, concern for the environmental conservation is also growing rapidly.1−3 In this context, the purification of fossil fuels prior to combustion and exposure to the environment is of growing importance. In recent years, various efforts have been devoted to the removal of sulfur- and nitrogen-containing compounds (SSCs and NCCs, respectively) from the environment.1−5 These two types of compound are reported to cause severe environmental and health issues. For example, the burning of these compounds creates SOx and NOx, which cause several problems in the ecosystem and for human health. Moreover, they are directly responsible for causing acid rain, which has negative consequences on the environment and on man-made © 2017 American Chemical Society

Received: February 8, 2017 Accepted: June 1, 2017 Published: June 1, 2017 20938

DOI: 10.1021/acsami.7b01899 ACS Appl. Mater. Interfaces 2017, 9, 20938−20946

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns and (b) N2 adsorption isotherms of MIL-125s.

2. EXPERIMENTAL SECTION

technique such as adsorptive denitrogenation (ADN) must be considered, as this process can be operated without the requirement for hydrogen and can be carried out under mild conditions at low expense. Several porous materials, including activated carbon, zeolites, Ti-hexagonal mesoporous materials (HMSs), meso-silica, activated alumina, ion exchange resins, Ni-based adsorbents, and NiMOs, have been applied as adsorbents for ADN.6,7 Recently, significant progress has been made into the discovery and use of porous materials,8−12 such as metal−organic frameworks (MOFs).13,14 MOFs composed of both inorganic (metallic) and organic moieties have a large number of potential applications13−19 and are suitable for adsorptive applications when they contain special functionalities and high surface areas. In particular, the adsorptive removal of hazardous materials is one of the key potential applications18−23 of MOFs due to their different functionalities and remarkable performances. MOFs have also been used for the removal of hazardous materials from fuels through both adsorptive desulfurization24 and ADN.25 The two most common NCCs that occur naturally in fossil fuels are indole (IND), quinoline (QUI), and their derivatives. QUI and its derivatives are basic in nature, while IND and its derivatives are neutral. To date, the successful adsorption of IND and QUI over several MOFs has been reported;25,26 however, their adsorption could be further improved through the appropriate modification of MOFs to obtain the required functionality. MIL-125 (Ti 8 O 8 (OH) 4 (BDC) 6 ) (BDC: benzenedicarboxylate) is a relatively new MOF that contains a Ti8O8 metal cluster27 linked by terepthalate, which originates from terephthalic acid (TPA). In addition, the preparation of MIL125-NH2 (containing NH2 functional moieties)28 using aminofunctionalized TPA (2-amino-TPA) would be expected to lead to additional advantages in adsorption and catalysis.29 For example, MIL-125-NH 2 has been applied in selective adsorptions (including water sorption)30−34 and photocatalysis.35 Thus, we herein report, for the first time, the establishment of an adsorptive method for the remarkable removal of both IND and QUI from a model fuel through interactions between the NH2 groups (and NH3+ groups) in MOFs and the NCCs. A plausible explanation for these interactions is also suggested based on the concepts of H-bonding, acid−base interactions, and cation−π interactions.

2.1. Chemicals and Synthesis of the Adsorbents. QUI (C9H7N, 99%) and IND (C8H7N, 98%) were purchased from Sigma-Aldrich, and aminoterephthalic acid (NH2-TPA, 98%) was acquired from Alfa Aesar. n-Octane (C8H14, 97%) and terephthalic acid (TPA, 99%) were purchased from Junsei Chemical Company. N,N-Dimethylformamide (DMF, 99%), isopropyl alcohol (C3H7OH, 97%), and hydrochloric acid (HCl, 35%) were obtained from OCI Company Ltd. Titanium isopropoxide (C12H28O4Ti, 97%) was purchased from Alfa Aesar. Protonated QUI (PQUI) was prepared by stirring QUI (1 mL) in 1 M HCl (20 mL) followed by distillation. This material was then purified by stirring in water, removal of the aqueous phase, and distillation. All chemicals utilized in this study were of analytical grade and were used without any further purification. 2.2. Preparation of the MOFs. MIL-125 and MIL-125-NH2 were prepared by following previously reported methods.36 In the case of MIL-125, a solution of titanium isopropoxide (9 mmol) and TPA (15 mmol) in a mixture of DMF and methanol (9:1, v/v, 50 mL) was prepared under stirring. For MIL-125-NH2, a solution of titanium isopropoxide (3 mmol) and TPA (6 mmol) and/or NH2-TPA (with a predetermined ratio) in a mixture of DMF and methanol (1:1, v/v, 50 mL) was prepared. The mixtures were transferred to 100 mL Teflonlined autoclaves and heated at 150 °C for 16 h in a convection oven. After cooling to room temperature, a white-yellow powdered product was recovered by filtration. The materials were purified by thorough washing with DMF and methanol and dried in an oven at 100 °C. The aminated MIL-125 containing different contents of NH2 groups was named MIL-125-NH2(x), where x indicates the molar percentage of NH2-TPA (based on the total moles of TPA and NH2-TPA) in the applied linker materials for the MOF synthesis. For example, MIL-125NH2(50) indicates that the MOF was obtained from an equimolar mixture of TPA and NH2-TPA. Subsequently, MIL-125-NH2(100) was protonated by stirring a sample of the MOF (0.1 g) in an aqueous HCl (0.01M) solution (10 mL) for 24 h. The resulting mixture was then filtered, washed with water several times, and dried. The protonated form of the MOF was named P-MIL-125-NH2(100). 2.3. Characterization. Powder X-ray diffraction (XRD) analysis was conducted using a D2 PHASER X-ray diffractometer (Bruker) with Cu Kα radiation. The N2 adsorption of each MOF was measured at −196 °C using a surface area and porosity analyzer (Tristar II 3020, Micromeritics) after evacuating the samples at 170 °C for 12 h. Elemental analyses to determine the N and O contents of the adsorbents were conducted using an elemental analyzer (Flash 2000, Thermo Scientific) equipped with a thermal conductivity detector. Fourier transform infrared (FTIR) spectroscopy was performed using a JASCO FT/IR-4100 in attenuated total reflection mode (resolution = 4 cm−1). Thermogravimetric analyses (TGA) of the MOFs were carried out using a PerkinElmer TGA 4000 system (ramping rate of 10 °C/min up to 600 °C, under air flow (20 cm3/min)). UV−vis diffuse reflectance spectra of MOFs were obtained with a SCINCO S-2100 20939


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ACS Applied Materials & Interfaces spectrophotometer (BaSO4 was applied as a reference). The concentration of counteranion in P-MIL-125-NH2 was checked by using an ion chromatograph (IC, Dionex ICS-5000) after dissolving the MOF with aqueous NaOH solution (pH: 11.5). 2.4. General Procedures for the Adsorption Experiments. Stock solutions (each at a concentration of 10 000 ppm) of the model fuel were prepared separately for the two adsorbates (QUI and IND) by dissolving each in n-octane. For PQUI adsorption, a mixture of 95% n-octane and 5% isopropyl alcohol was used as the solvent to easily dissolve the PQUI. Solutions of varying concentrations were then prepared by successive dilutions of the stock solutions. A solution containing a fixed concentration of IND and QUI (1000 ppm each) was also used to determine the adsorption capacities at various adsorption times. To obtain the maximum adsorption capacities (Q0) of the adsorbents, several solutions with different concentrations (300−1800 ppm) of IND or QUI were prepared using the same solvent. Prior to adsorption, the adsorbents were dried in a vacuum oven for 12 h at 150 °C and were stored in a desiccator. For each adsorption experiment, a sample of the adsorbent (∼5.0 mg) was added to the model fuel (∼5.0 mL) and stirred magnetically for a predetermined time (15−240 min) while maintaining the adsorption temperature at 25 °C. After complete adsorption, the solution was separated from the solid using a syringe filter (PTFE, hydrophobic, 0.5 μm) and analyzed using a gas chromatograph (GC, DS Science, IGC 7200) equipped with a flame ionization detector (FID). An adsorption was repeated for three times, and the average value (relative error was generally less than 5%) was reported in this paper. To regenerate the adsorbents, a sample (0.1 g) of the material was added to ethanol (20 mL) and sonicated for 2 h in a sonication bath. After this time, the mixture was filtered and sonicated once again as above. Finally, the material was recovered by filtration, washing with ethanol and water, and drying. Detailed adsorption-related calculations are provided elsewhere.37

Table 1. Textural Properties of the MIL-125s and Maximum Adsorption Capacities and b-Values for IND and QUI SBET (m2/g)

Vmic. T‑plot (cm3/g)

MIL-125

1378

0.534

MIL-125NH2(100)

1561

P-MIL-125NH2(100)

1413

material

adsorbate

Q0 (mg/g)

0.604

IND QUI IND

264 103 502

0.571

QUI IND

460 583 (0.496a) 546 (0.500a)

QUI

b-value ×1000 (L/mg) 2.29 2.27 4.19 1.22 19.5 19.0

a

Figures in parentheses show the maximum adsorption capacities (Q0) in adsorbed volumes (cm3) of adsorbates per unit gram of the adsorbent P-MIL-125-NH2(100).

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Materials. Figure 1a shows the XRD patterns of the MIL-125, MIL-125-NH2(100), and PMIL-125-NH2(100) adsorbents employed in this study. XRD patterns of MIL-125-NH2(33) and MIL-125-NH2(67) were very similar to that of the pristine MIL-125 (data not shown). The XRD patterns of the obtained MIL-125s were comparable with the simulated patterns, thereby confirming the successful syntheses of MIL-125 and MIL-125-NH2(100). In addition, the similarity of the three XRD patterns indicates similar crystal structures in all materials and also suggests no reduction in material quality during the protonation of MIL-125-NH2(100) to give P-MIL-125-NH2(100). Additionally, the nitrogen adsorption isotherms shown in Figure 1b and the BET surface areas and micropore volumes (summarized in Table 1, obtained from the isotherms) indicate no significant differences in porosity between the three materials. Figure 2 shows the FTIR spectra of MIL-125 and of the MIL-125-NH2 samples bearing different quantities of −NH2 groups. From these spectra, the successful introduction of the NH2 groups in the MOF structure can be confirmed. Corresponding transmission troughs of the −NH2 groups were observed at wavelengths of 1619, 1340, 1257, and 764 cm−1, and the absorption peaks corresponding to C−N stretching were located at 1257 and 1340 cm−1.37,38 In addition, N−H wagging and bending bands were observed at 763 and 1619 cm−1, respectively.37,38 Moreover, the differences in trough intensities correspond to the various concentrations of −NH2 groups within the MIL-125s. The FTIR spectra of the MOFs in 3800−2800 cm−1 are shown in Figure S1 (Supporting Information). In harmony with the result on C−N stretching, it can be confirmed again that the −NH2 content of MOFs

Figure 2. FTIR spectra of MIL-125s having different −NH2 content.

increases monotonously with increasing the content of NH2TPA in the MOF synthesis, based on the stretching band of −NH2 at 3376 cm−1 (and at 3487 cm−1, even though not very clear).39 The absorbance at ∼3670 cm−1 is because of μ2-OH40 groups on MIL-125 (irrespective of the presence or absence of −NH2). The poor resolution of these FTIR bands is partly because of hydration of MOF samples under the experimental condition. The thermal stability of MOFs was analyzed with TGA. As shown in Figure S2 (highlighted with dotted circle), the stability generally decreased with increasing the content of −NH2 in the MOFs, which is similar to the results observed in UiO-66(Zr)-NH2,41 MIL-53(Al)-NH2,42 and MOF-5(Zn)NH2.43 As illustrated in Figure S3, the color of MIL-125s changed from white into deep yellow with increasing the content of −NH2 in the MOFs, which is in harmony with the yellow color of MIL-125-NH2.35 Elemental analysis of the materials showed that MIL-125, MIL-125-NH2(100), and P-MIL-125-NH2(100) contained 0.3, 4.6, and 3.8% nitrogen, respectively, and the stoichiometric nitrogen contents for these materials were calculated as 0, 5.1, and 4.5%, respectively. This confirms the successful loading of −NH2 groups on the functionalized MOFs, despite the experimental nitrogen contents of MIL-125-NH2(100) and PMIL-125-NH2(100) being slightly lower than the calculated values. Furthermore, the trace of nitrogen (0.3%) observed in the pristine MIL-125 could be a remnant of the DMF employed for washing the material. 20940


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Figure 3. Adsorbed quantities over MIL-125s in different times for (a) IND and (b) QUI.

Figure 4. Adsorption isotherms for (a) IND and (b) QUI over MIL-125s.

different concentrations of the model fuels. From these isotherms, Langmuir plots were created as shown in Figure S8, which allowed the maximum adsorption capacities (Q0) of both NCCs to be calculated. The Q0 values for IND and QUI over the three adsorbents are listed in Table 1. In both cases, Q0 increased in the order MIL-125 < MIL-125-NH2(100) < PMIL-125-NH2(100). Moreover, the Q0 values for IND and QUI over P-MIL-125-NH2(100) are quite similar (∼87%) to the micropore volume of the adsorbent (Table 1), suggesting the adsorptions are highly efficient. The observed highest Langmuir constant (b-value) for P-MIL-125-NH2(100) also indicates preferred adsorption over this material for both of the adsorbates. The maximum adsorption capacities for IND over MIL-125-NH2(100) and P-MIL-125-NH2(100) were 1.9 and 2.2 times that of the pristine MOF, respectively. In addition, in the case of QUI, the capacities over MIL-125-NH2(100) and PMIL-125-NH2(100) were 4.5 and 5.3 times that of the pristine MOF, respectively. Indeed, the adsorption capacities of P-MIL125-NH2 observed toward both IND and QUI in this study were remarkable compared with previous reports. As shown in Table S1, the adsorption of IND and QUI over P-MIL-125NH2(100) is comparable to the highest reported results37,46 even though the BET surface area of P-MIL-125-NH2(100) is only 23.8% that of the previously reported GnO/MIL-101. More importantly, P-MIL-125-NH2(100) showed the highest adsorption of QUI from n-octane among the reported results so far. This remarkable adsorption over P-MIL-125-NH2(100) can

The protonation of MIL-125-NH2 to have P-MIL-125-NH2 was confirmed with FTIR, UV/vis, and IC. As presented in Figure S4, the FTIR band corresponding to −NH2 (amine deformation band) of MOF was shifted to high wavenumber by ∼10 cm−1, which suggest the protonation of −NH2 to form −NH3+.44 UV/vis spectra in Figure S5 show not only the existence of −NH2 in MIL-125-NH2(100)35 but also the successful protonation in P-MIL-125-NH2(100) by HCl treatment. The bathochromic shift of UV/vis absorption upon protonation of a polymer (with nonbonding electron pair on N) was reported earlier.45 The presence of Cl− in PMIL-125-NH2(100) as counteranion was confirmed with IC, and the degree of protonation was estimated to be 95.1%. 3.2. Adsorption Results. Figures 3a and 3b show the adsorbed quantities (qt) of IND and QUI, respectively, over MIL-125, MIL-125-NH2(100), and P-MIL-125-NH2(100) with respect to adsorption time. Here, the qt followed the order MIL-125 < MIL-125-NH2(100) < P-MIL-125-NH2(100) for both IND and QUI. This tendency of increasing qt with the content of −NH2 groups in the MOFs can also be observed in Figures S6 and S7. Indeed, it was found that the adsorbed quantities of both IND and QUI increased monotonously upon increasing the content of −NH2 group in the MIL-125s, which corresponds to the results outlined in Figure 3. Figures 4a and 4b show the adsorption isotherms of IND and QUI, respectively, over the three adsorbents. These isotherms were obtained following the adsorption of IND and QUI from 20941


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ACS Applied Materials & Interfaces also be highlighted by calculating the adsorption capacity per unit surface area. In this context, the adsorption capacities of IND and QUI over P-MIL-125-NH2(100) were 0.412 and 0.384 mg/m2, respectively, while the corresponding values for GnO/MIL-101 were 0.176 and 0.144 mg/m2, respectively.46 As such, the adsorption capacities of P-MIL-125-NH2(100) toward IND and QUI (per unit surface area) were 2.7 and 2.8 times those of GnO/MIL-101, respectively. These remarkable improvements in adsorption can be attributed to the −NH2 groups and protonated −NH2 groups of the MOFs (see below). 3.3. Adsorption Mechanism. A number of different mechanisms for the adsorption of NCCs over various MOFs have been reported in the literature.6,47−50 To date, the adsorption of IND has been explained mainly in the context of van der Waals forces,51 π-complexation,48,52 and H-bonding37,50 mechanisms. On the other hand, QUI adsorption has been explained in the context of van der Waals forces,51 πcomplexation,52 and acid−base interaction53,54 mechanisms. Adsorption due to van der Waals interactions is dependent mainly on the surface area of the adsorbent. However, the surface areas of the MIL-125s employed in this study remained relatively constant following modification, even though the adsorption capacity was improved significantly. Therefore, the improved adsorptions of IND and QUI by MIL-125-NH2(100) and P-MIL-125-NH2(100) cannot be explained using either porosity or van der Waals interactions. In addition, only a handful metals with specific oxidation states (such as Cu(I) and Ag(I)) exhibit π-complexation,48 and Ti ions (such as those present in our MOFs) are not reported to display this phenomenon. Moreover, the oxidation state of Ti in the MIL125s does not change upon the introduction of −NH2 groups or further protonation, and so this mechanism is also unlikely to explain the differences in adsorption observed herein. Furthermore, one may consider the μ2-OH of MIL-125s may have an important role in the adsorptions. However, the contribution of this μ2-OH can be neglected because the content of such group does not dependent on the presence or absence of −NH2 in the MIL-125s.35 Finally, because of the differences in chemical properties of IND and QUI, the adsorption mechanisms of the two species may also be different. The adsorption mechanisms for IND and QUI will therefore be discussed separately in the following sections. Adsorption of IND. Recently, the H-bonding mechanism has been applied to explain the adsorption of IND over MOFs functionalized with −NH2, −COOH, −OH, and −SO3H groups, where IND was an H-bond donor.54−56 Therefore, the improved adsorption of IND over MIL-125-NH2(100) can be explained similarly. As shown on the left side of Scheme 1a, the N atoms of the −NH2 groups on the MOFs can act as H-bond acceptors, while the H atoms attached to the N atoms of IND can act as H-bond donors. However, the remarkably enhanced adsorption of IND over P-MIL-125-NH2(100) cannot be simply explained with Hbonding. As such, two other potential mechanisms could perhaps explain the high qt observed over P-MIL-125NH2(100). The first mechanism is the cation−π interaction, which occurs between a cation and the π-electrons of a benzene ring of an aromatic molecule.57,58 Indeed, many biological substances, such as IND, have been reported to exhibit cation−π interactions (for example, between −NH3+ or other cationic groups and benzene, toluene, phenol, IND, etc.).57−59 It was reported that the interaction between −NH3+ and IND

Scheme 1. Plausible Adsorption Mechanisms for (a) IND and (b) QUI over MIL-125-NH2(100) and P-MIL-125NH2(100)a

a

The dipole moment of IND was shown to highlight the increased density of π-electrons on benzene rings of IND.

has a binding energy of 25.7 kcal/mol, which is similar to or even stronger than a strong H-bond.50,57 This mechanism may therefore account for the increase in the adsorption capacity of P-MIL-125-NH2(100) toward IND. Another possibility is the presence of a weak H-bond between the π-electrons of the phenyl ring of IND and the H atoms of the −NH3+ groups. This type of H-bond between aromatics and cationic amino groups has been previously reported59,60 and can further increase the capability of the protonated MOF to adsorb IND. The possible mechanisms for IND adsorption over P-MIL-125-NH2(100) are illustrated in Scheme 1a (right side), and we concluded that the enhanced IND adsorption over P-MIL-125-NH2(100) might be explained through a combination of both cation−π interactions and Hbonds. Adsorption of QUI. The most common and important adsorption mechanism for basic NCCs such as QUI is based on acid−base interactions.61−63 In earlier studies employing acidic MOFs for the removal of mixed NCCs in fuels, basic NCCs were highly and selectively adsorbed compared with neutral NCCs (such as IND) or other adsorbates, and this was attributed to acid−base interactions.62,63 In the current study, one plausible reason for the remarkable performance of P-MIL125-NH2(100) toward QUI adsorption is the acid−base interaction between acidic −NH3+ groups on the MOF and the basic QUI. This contribution of acid−base interactions in the ADN (especially basic NCCs such as QUI) is further confirmed by the adsorption of PQUI over MIL-125NH2(100). As shown in Figure 5, the adsorbed amount of 20942


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understandable. Therefore, the mechanisms accounting for the favorable adsorption of QUI over MIL-125-NH2(100) and PMIL-125-NH2(100) are illustrated in Scheme 1b, which shows that H-bond and acid−base interactions are the major mechanisms involved in QUI adsorption over MIL-125NH2(100) and P-MIL-125-NH2(100), respectively. Interestingly, H-bonds are very effective in the adsorption of not only IND (over both MIL-125-NH2(100) and P-MIL-125NH2(100)) but also QUI (over MIL-125-NH2(100)). Indeed, H-bonding has recently been suggested as a potential adsorption mechanism not only for fuels55,66−68 but also for water purification.55,69,70 Considering the above observations, P-MIL-125-NH2(100) was found to be one of the most efficient adsorbents for both IND and QUI, as it exhibits adsorption either highest or similar to the highest values reported for both QUI and IND, respectively (Table S1). Moreover, previously no other modification techniques for adsorbents showed such enhanced adsorption for either of these NCC materials. 3.4. Reusability of P-MIL-125-NH2(100). The commercial application of materials depends mainly on their reusability in adsorption, separation, and similar processes. Therefore, the reusability of MIL-125-NH2(100) and P-MIL-125-NH2(100) was investigated through regeneration by washing with ethanol. The regenerated material was then employed as an adsorbent for both IND and QUI adsorption over five runs. The results of the adsorption tests using the regenerated materials are shown in Figures 6a and 6b for QUI adsorption over MIL-125NH2(100) and P-MIL-125-NH2(100), respectively. As observed in these figures, the degree of adsorption remained relatively constant up to the fifth run, which indicates that these materials can be recycled a number of times without any significant loss in the adsorption capacity. Moreover, the MOFs exhibited competitive adsorption even after each recycle. For example, the qt of the recycled P-MIL-125-NH2(100) was ∼8 times that of conventional activated carbon adsorbents. Successful regeneration of the MOF was also confirmed by FTIR and XRD, as shown in Figure S9. The regenerated PMIL-125-NH2(100) exhibited similar FTIR and XRD (even with slightly broadened peaks) results to those of the fresh material, which indicates the presence of intact functional groups (or bonding) and structures after the regeneration procedure. Moreover, there was only a very slight decrease in BET surface area of P-MIL-125-NH2(100) upon adsorption/ regeneration (from 1413 to 1388 m2/g).

Figure 5. Amounts of adsorbed PQUI over MIL-125 and MIL-125NH2(100) in different times.

PQUI over MIL-125-NH2(100) was ∼7 times that observed over MIL-125, which contained no basic −NH2 groups. As such, the huge increase in qt in the presence of −NH2 groups confirms the favorable interactions between the acidic PQUI and the basic MIL-125-NH2(100). Furthermore, although MIL-125-NH2(100) is not as efficient an adsorbent as P-MIL-125-NH2(100), MIL-125-NH2(100) is more efficient than the pristine MOF in the adsorption of QUI, as shown in Figure 3b. However, in this case, acid−base interactions cannot explain the improved adsorption since both the adsorbate (QUI) and adsorbent are basic materials. Therefore, a different adsorption mechanism, such as Hbonding, is required to explain the improved adsorption of QUI over MIL-125-NH2(100). Previously, Hasan et al.38 suggested H-bond to explain favorable adsorption of basic pyridine over UiO-66-NH2, even though a repulsive interaction was originally expected. Moreover, a recent study showed that QUI could be adsorbed effectively over MOF (MIL-101) functionalized with −NH2 groups, with the increased adsorption also being explained in the context of H-bonding.64 Additionally, an earlier study demonstrated H-bonding interactions between QUI and the −OH moieties of carboxylic acids.65 Considering the possible role of the −NH2 group (similar to the −OH group) as an H-donor to QUI, the −NH2 moieties of MIL-125NH2(100) may therefore interact with QUI via H-bonds. Based on the relative strength of interactions (in general, acid−base > H-bond > van der Waals), the qt of QUI shown in Figure 3b is

Figure 6. Reusability of MIL-125s (by ethanol washing) for the adsorption of QUI over (a) MIL-125-NH2(100) and (b) P-MIL-125-NH2(100). The lower and upper horizontal lines show the qt of QUI over activated carbon and pristine MIL-125, respectively. 20943



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ABBREVIATIONS ADN, adsorptive denitrogenation; IC, ion chromatography; IND, indole; MOF, metal−organic framework; NCC, nitrogencontaining compound; PQUI, protonated QUI; QUI, quinoline; SCC, sulfur-containing compound; TGA, thermogravimetric analysis.

The results of the reusability experiments for IND adsorption are shown in Figure S10. In the case of MIL-125-NH2, very encouraging reusability results were obtained. However, as shown in Figure S10b, the reusability of P-MIL-125-NH2 toward the adsorption of IND was relatively poor compared with that of QUI under the same conditions. This may be explained by the relatively strong interactions between IND and P-MIL-125(NH2) (cation−π interactions), as explained previously. Although P-MIL-125-NH2 showed poor reusability for IND, the adsorption was steady following the third run, and no further degradation was observed between the third and fifth runs. Moreover, this result is still remarkable compared to the adsorption of IND over activated carbon or pristine MIL-125 (Figure S10b).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01899. Figures S1−S10 and Table S1 (PDF)

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REFERENCES

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4. CONCLUSION The modification of MIL-125 by −NH2 functionalization and subsequent protonation produced a remarkable improvement in the adsorption of the NCCs IND and QUI. The adsorptions of IND over MIL-125-NH2 and P-MIL-125-NH2 were 1.9 and 2.2 times that observed over pristine MIL-125, respectively, while in the case of QUI, the corresponding adsorptions were 4.5 and 5.3 times, even though the porosities of the three MOFs were comparable. These extraordinary results were particularly interesting, as such enhancements in adsorption for both the neutral IND and the acidic QUI had not been reported in previous studies. Moreover, the maximum adsorption capacities of P-MIL-125-NH2 for IND and QUI were comparable to or even higher than the best reported results. Furthermore, we proposed that the mechanisms for IND adsorption over MIL-125-NH2 and P-MIL-125-NH2 may involve H-bonding and cation−π interactions, respectively, while the key mechanisms involved in QUI adsorption were likely H-bonding and acid−base interactions, respectively.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*Phone +82-10-2818-5341; Fax +82-53-950-6330; e-mail [email protected]. ORCID

Sung Hwa Jhung: 0000-0002-6941-1583 Author Contributions

I.A. and N.A.K. contributed equally to this work. Funding

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (Grant 2015R1A2A1A15055291). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express sincere thanks to Prof. Kil Sik Min and Miss Ah Rim Jeong for UV/vis analyses. Helpful discussions with Mr. Biswa Nath Bhadra are also highly acknowledged. 20944


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