Article Cite This: J. Phys. Chem. C 2018, 122, 4532−4539
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Adsorptive Removal of Indole and Quinoline from Model Fuel over Various UiO-66s: Quantitative Contributions of H‑Bonding and Acid− Base Interactions to Adsorption Mithun Sarker, Hyung Jun An, and Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea S Supporting Information *
ABSTRACT: Nitrogen-containing compounds (NCCs) such as indole (IND) and quinolone (QUI) in a model fuel were adsorbed over pristine and variously functionalized metal−organic frameworks (MOFs) (here, UiO-66 and −NH2, −NH3+, −COOH, −COONa, −OH, −SO3H functionalized UiO-66s) to quantitatively understand the interactions between the adsorbates (IND and QUI) and UiO-66s. The adsorbed quantity of IND and QUI increased linearly with increasing number of H-acceptors and Hdonors (for H-bond), respectively, on UiO-66s (excluding one MOF for each adsorption), confirming the importance of Hbonding in the adsorption. UiO-66-NH3+ and UiO-66-NH2 showed a deviated trend in the IND and QUI adsorption, respectively; this might be explained by cation−π interactions and base−base repulsion, respectively. Moreover, the QUI adsorption increased linearly with increasing number of acidic sites on the MOFs (excluding basic ones), also suggesting the importance of acid−base interactions. Finally, UiO-66-NH3+ showed the highest adsorption for both IND and QUI among the studied MOFs, suggesting that introducing an ammonium group on MOFs can be one way to develop a competitive adsorbent for the adsorptive denitrogenation of fuels.
1. INTRODUCTION Energy and environment are the most crucial issues in the current decade. The consumption of global energy is increasing everyday with the worldwide developments. With increasing world population and improving living standards, the demand for fossil fuels has gradually increased. Currently, fossil fuels are widely used as energy sources in the world,1 as about 85% of the world’s commercial energy comes from these fuels.2 To satisfy the increasing demand for fossil fuels, new and unconventional sources are being explored, which often contain a huge quantity of environmentally harmful pollutants.3 Moreover, the consumption of fossil fuels without proper pretreatment is responsible for environmental pollution, especially the emission of NOx and SOx, which are discharged by the combustion of nitrogen- and sulfur-containing compounds (NCCs and SCCs, respectively) in fossil fuels.4,5 Both NOx and SOx are considered as air pollutants and are responsible for acid rain,6 which has adverse effects on the environment and manmade structures. Therefore, removing NCCs and SCCs from fuels is very important7,8 and several research groups worldwide have been actively working on this aspect in the recent decades.9 SCCs in fuels have been effectively and widely removed by the hydrodesulfurization (HDS) process for a long time.10,11 During the HDS process, sulfur present in SCCs is removed as H2S by hydrogenation in the presence of a suitable catalyst at © 2018 American Chemical Society
elevated temperatures. However, before the removal of SCCs, NCCs present in fuels should be eliminated since they reduce the activity of HDS catalysts and hinder the catalytic process itself.12 Indole (IND) and quinoline (QUI), and their derivatives are the most common NCCs that exist naturally in fossil fuels.13 NCCs can be classified into two groups: basic and neutral or nonbasic. IND and its derivatives are neutral, whereas QUIs are basic in nature. NCCs from fossil fuels are conventionally removed by the hydrodenitrogenation (HDN) process; however, this technique has some drawbacks.14 For example, HDN requires higher temperatures/pressures, is a kinetically slow multistep process, and consumes more H2 gas compared with HDS, since HDN can be carried out only after the destruction (via hydrogenation) of the cyclic rings of NCCs.15,16 Therefore, new/alternative approaches such as extractive denitrogenation, oxidative denitrogenation, and adsorptive denitrogenation (ADN) have been investigated in parallel with HDN. Among the various methods, ADN might be considered as a promising technique17 because of its operational simplicity and cost effectiveness, as well as the fact that it can be operated without H2 gas or a catalyst under mild conditions. Several adsorbents such as activated carbon,18 Received: January 23, 2018 Revised: February 14, 2018 Published: February 14, 2018 4532
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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Figure 1. (a) XRD patterns and (b) nitrogen adsorption isotherms of studied UiO-66s.
zeolites,19 Si−Zr Cogel,20 silica−alumina adsorbents,21 and ionexchange resin22 have been utilized for ADN of fuels. Recently, remarkable developments have been made in the field of porous materials,23−26 including the invention and successful utilization of metal−organic frameworks (MOFs). MOFs27−29 are an advanced class of porous materials that have attracted much attention because of their facile synthesis, excellent porosity, and improved functionalities as well as several potential applications,30−32 especially in liquid-phase adsorption for water and fuel purification.33−35 Currently, MOFs have also been used for the removal of SCCs and NCCs via adsorptive desulfurization (ADS) and ADN, respectively.36,37 Moreover, the adsorption efficiency of MOFs for the removal of NCCs can be further enhanced by suitably modifying them with different functionalities,38,39 since the functional groups present on the MOFs can induce the desired properties and enhance their adsorptive performance.40,41 Among various MOFs, UiO-66 has attracted much attention because of its high chemical and thermal stability, facile synthesis, easy functionalization (via direct synthesis and postsynthetic modification), and potential applications.42−44 Moreover, UiO-66s with various functional groups were applied in ADN to investigate the effect of different functional sites on the adsorption. In this study, as much as eight UiO-66s with various functional groups were applied in ADN (especially, IND and QUI removal) in order to understand the adsorption mechanism in a quantitative way. The adsorption could be interpreted quantitatively by H-bonding and acid−base interactions, and some other mechanisms such as cation-π interactions were also discussed.
(HCOOH, > 85%), ethanol (C2H5OH, 94%), and methanol (CH3OH, 99%) were sourced from OCI Co., Ltd. UiO-66s were synthesized following previously reported procedures45−49 and named as UiO-66, UiO-66-NH2, UiO-66COOH(30%), UiO-66-OH, UiO-66-(OH)2, and UiO-66SO3H(18%) depending on the applied linkers. UiO-66-NH3+ and UiO-66-COONa(30%) were prepared by protonation and deprotonation of UiO-66-NH2 and UiO-66-COOH(30%), respectively. The detailed procedures for the synthesis of UiO-66s, protonation of UiO-66-NH2, and deprotonation of UiO-66-COOH(30%) are described in the Supporting Information (SI). 2.2. Characterization of MOFs. An X-ray powder diffractometer (D2 Phaser, Bruker, Germany) with Cu−Kα radiation was applied to analyze the structural characteristics of all the studied UiO-66s. The textural properties of the MOFs were measured by nitrogen adsorption at −196 °C using a surface area and porosity analyzer (Tristar II 3020, Micromeritics, USA) after evacuation at 150 °C for 12 h. The surface areas of the studied UiO-66s were determined by using the Brunauer−Emmett−Teller (BET) equation. Field-emission scanning electron microscopy (FESEM) was performed using a Hitachi model S-4300 microscope. Elemental analyses of the MOFs were executed to determine the hydrogen, carbon, nitrogen, oxygen, and sulfur contents using an elemental analyzer (Flash 2000, Thermo Scientific) fitted with a TCD detector. Fourier transform infrared (FTIR) spectra of the UiO-66s were obtained using a Jasco FTIR-4100 spectrometer equipped with an attenuated total reflectance module at a maximum resolution of 4.0 cm−1, to determine the functional groups of the studied UiO-66s. UV/vis diffuse reflectance spectra of pristine UiO-66, UiO-66-NH2, and UiO-66-NH3+ were recorded using a SCINCO S-2100 spectrophotometer, where BaSO4 was used as a reference. 2.3. Adsorption Experiments. Stock solutions of model fuel containing IND and QUI separately (each 10,000 mg·L−1) were prepared by dissolving each compound in n-octane. Solutions of a fixed concentration of IND or QUI (1000 mg· L−1 each, prepared by further dilution of the stock solution) were used to determine the adsorbed amounts (qt) at various times. To determine the maximum adsorption capacities of the adsorbents, solutions of IND and QUI at various concentrations (from 500 to 2000 mg·L−1) were prepared by successive dilution of the stock solutions. Prior to the adsorption experiments, the MOFs were evacuated at 150 °C for 12 h in a vacuum oven to remove moisture. For each adsorption experiment, an exact amount of adsorbent (∼5.0 mg) was added to an IND or QUI solution (5.0 mL), and the mixture was shaken in an incubated shaker for 1 to 12 h at 20 °C. After the adsorption experiments, the solutions were
2. EXPERIMENTAL SECTION 2.1. Chemicals and Synthesis of Adsorbents. All the chemicals used were commercially available and applied without further purification. Zirconium(IV) chloride (ZrCl4, 99.5%), 2-aminoterephthalic acid (NH2-TPA, C8H7NO4, 99.0%), 2,5-dihydroxyterephthalic acid ((HO)2-TPA, C8H6O6, 97.0%), and isophthalic acid (IPA, C8H6O4, 99.0%) were obtained from Alfa Aesar. 2-Hydroxyterephthalic acid ((HO)TPA, C8H6O5, > 98%) and monosodium 2-sulfoterephthalate (2-NaSO3H−TPA, C8H5NaO7S, > 98%) were collected from Tokyo Chemical Industry Co, Ltd. IND (C8H7N, 98%), QUI (C9H7N, 99%), and terephthalic acid (TPA, C6H4-1,4(CO2H)2, 99%) were purchased from Sigma-Aldrich. n-Octane (C8H14, 97%) and sodium hydroxide (NaOH, 99%) were acquired from Junsei Chemical Company and Merck KGaA, respectively. N,N-Dimethylformamide (DMF, 99%), N,Ndiethylacetamide (DMA, 99%), hydrochloric acid (HCl, 37%), acetic acid (CH 3 COOH, 99.5%), formic acid 4533
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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The Journal of Physical Chemistry C filtered using a hydrophobic polytetrafluoroethylene syringe filter (0.5 μm), and the residual concentrations of the adsorbates were estimated using UV spectrometric analysis (UV-1800, Shimadzu, Japan). The UV absorbances at 287 and 313 nm were used to determine the concentrations of IND and QUI, respectively.
confirmed the presence of CO in the carboxyl groups, which suggested the existence of −COOH and −COONa groups in UiO-66-COOH(30%) and UiO-66-COONa(30%), respectively.46,47 The C−OH stretching band51 of UiO-66-OH and UiO-66-(OH)2 was identified at 1227 cm−1, and the broad band at 3220 cm−1 (shown in Figure 2b) was attributed to the stretching vibration of −OH groups.52 Moreover, the stretching band at 1180 cm−1 could be attributed to the asymmetric stretching modes of OSO, whereas the band at 1076 cm−1 was due to the presence of S−O vibrational stretching in UiO66-SO3H(18%).49 Elemental analysis results, shown in Table S1, confirmed the presence of nitrogen in UiO-66-NH2 and UiO-66-NH3+, and the existence of sulfur in UiO-66-SO3H(18%). The protonation of UiO-66-NH2 to UiO-66-NH3+ was confirmed by UV/vis spectroscopy. The UV/vis spectra shown in Figure S2 indicated the successful protonation of UiO-66NH2 by HCl treatment. A bathochromic shift was observed in the UV/vis absorption spectrum of UiO-66-NH2 upon protonation by HCl treatment, similar to the previously reported result.53−55 3.2. Adsorption Results. Figure 3 shows the adsorbed quantities (qt) of IND and QUI over all the studied UiO-66s for various adsorption times. The qt values based on surface area were also compared, as shown in Figure S3. From the figures, it could be clearly observed that UiO-66-NH 3+ exhibited the maximum adsorption, based on the weight and surface area of the adsorbent, for both IND and QUI. Moreover, UiO-66-(OH)2 was very competitive in the adsorption of IND and QUI from fuel. As shown in Figure S4, adsorption isotherms over UiO-66s, for IND and QUI, over a broad range of adsorbate concentrations (500−2000 mg·L−1), were obtained. The experiments were conducted for 12 h to ensure complete equilibration, even though adsorption equilibria were reached within 6 h, as shown in Figure 3. The maximum adsorption capacities (Q0), b-values, and correlation parameters (R2) for both IND and QUI were calculated from Langmuir plots56 (shown in Figure S5), and are presented in Table S2. From Table S2, it was observed that the Q0-values for IND and QUI adsorption over UiO-66-NH3+ were the highest among those of the MOFs with a UiO-66 structure, in agreement with the results shown in Figure 3. In addition, UiO-66-NH3+ was quite competitive against the reported adsorbents (as represented in Table S3) for IND and QUI adsorption, even though less effective than protonated MIL-12554 and graphene oxide/MIL101.57 Moreover, the reusability of UiO-66-NH3+ for IND and QUI adsorption was evaluated, after simple ethanol washing. As
3. RESULTS 3.1. Characterization of Adsorbents. The XRD patterns of the pristine and functionalized UiO-66s, shown in Figure 1a, completely matched with the simulated pattern, confirming the successful synthesis of UiO-66s. The nitrogen adsorption isotherms of the functionalized UiO-66s along with that of pristine UiO-66 are represented in Figure 1b, and their BET surface areas with the micropore volumes, obtained from the isotherms, are summarized in Table 1. The functionalized UiOTable 1. Textural Properties of Different UiO-66s Used in This Study material UiO-66 UiO-66-NH2 UiO-66-NH3+ UiO-66COOH(30%) UiO-66COONa(30%) UiO-66-OH UiO-66-(OH)2 UiO-66-SO3H(18%)
BET surface area (m2· g−1) micropore volume (cm3·g−1) 1012 806 742 791
0.44 0.40 0.30 0.36
671
0.20
804 726 892
0.29 0.26 0.31
66s showed slightly reduced surface area than pristine UiO-66, which might be due to the presence of bulky functional groups attached to the pristine MOF.50 The SEM images of the UiO66s, displayed in Figure S1, demonstrated that the physical appearance of the functionalized UiO-66s (except UiO-66-OH and UiO-66-(OH)2) had a nearly similar morphology to that of the pristine UiO-66. The particle size of UiO-66-OH and UiO66-(OH)2 was smaller than that of the other UiO-66s. Moreover, the homogeneous crystals of the prepared UiO66s suggested the absence of other phases, which is in agreement with the XRD results. The presence of different functional groups was identified by FTIR analysis, as shown in Figure 2a. The stretching bands at 1257 and 1340 cm−1 (corresponding to C−N bonds) in UiO66-NH2 and UiO-66-NH3+ indicated the presence of an amino and ammonium group in their structure, respectively.39 The existence of a broad band from ∼1700 to ∼1750 cm−1
Figure 2. FTIR of (a) studied UiO-66s and (b) UiO-66, UiO-66-OH and UiO-66-(OH)2 in the wavenumber from 4000 to 2800 cm−1. 4534
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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Figure 3. Effect of contact time on (a) IND and (b) QUI adsorptions over various UiO-66s. The initial concentration of IND or QUI was 1000 mg· L−1 in n-octane.
Figure 4. Effect of number of H-bond acceptor site of MOF on the adsorbed amounts (q12h, based on unit surface area) of IND over studied UiO66s: (a) Raw data showing all studied UiO-66s. The red circle shows the q12h of UiO-66-NH3+. (b) Selected data excluding UiO-66-NH3+.
Figure 5. Effect of number of H-bond donor site of MOF on the adsorbed amounts (q12h, based on unit surface area) of QUI over studied UiO-66s: (a) Raw data showing all studied UiO-66s. The red circle shows the q12h of UiO-66-NH2. (b) Selected data excluding UiO-66-NH2.
H-bond donor.45,47,66 In this study, H-bonding was considered to interpret (especially, in a quantitative way) the adsorption of IND (which has hydrogen that is effective as a H-bond donor) over UiO-66s. Figure 4a shows the effect of the number of Hbond acceptor moieties in the MOFs on the q12h values (based on the surface area) of all UiO-66s. Even though the correlation coefficient (R2), based on all the studied UiO-66s, was relatively low (R2 = 0.202), it was highly increased and acceptable (R2 = 0.890) when the result with UiO-66-NH3+ was excluded from the plot (Figure 4b). As represented in Figure S7, similar tendencies were also observed for the qt values (with or without UiO-66-NH3+) based on the unit weight of the MOFs. The relatively low R2 in this plot suggested that the tendency is more reliable if the results are reported/interpreted based on the surface area rather than the weight of the adsorbents (which is expected based on the general contribution of porosity to adsorption59). We also investigated the possibility of contribution of IND, considering the presence of N as a H-bond acceptor in the adsorption. As shown in Figure S8, the R2 value was quite low (R2= 0.515). Moreover, R2 could not be increased much without deleting several data points. Therefore, the results from
shown in Figure S6, the performance of the regenerated UiO66-NH3+ does not diminish severely with increasing number of recycles, suggesting that UiO-66-NH3+ can be recycled several times for IND and QUI adsorption.
4. DISCUSSION So far, various adsorption mechanisms such as van der Waals force,57−59 acid−base interaction,60,61 π-complexation,62,63and H-bonding64,65 interactions have been utilized to explain IND and QUI adsorption over MOFs from fuels. However, van der Waals interaction and π-complexation were not applicable for the adsorption in this study (over UiO-66s) because of the lower porosity of the functionalized UiO-66s than that of the pristine one and the absence of specific metal ions (such as Cu(I) and Ag(I)), which are important criteria for van der Waals interaction and π-complexation, respectively. Recently, H-bonding64 was utilized to explain liquid-phase adsorption, especially in the removal of NCCs from fuel by using MOFs. For example, IND adsorption over MOFs functionalized with −NH2, −OH, −COOH, and −SO3H groups has been described by H-bonding, where IND acts as a 4535
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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Table 2. Number of H-Donor and Acceptor Present Per Benzene Ring of Linker in the Studied UiO-66s and q12h (Based on Unit Weight and Surface Area of UiO-66s) for IND and QUI, Respectively, over Different UiO-66s IND
QUI
material
no. of H-donora
no. of H-acceptorb
q12h (mg/g)
q12h (mg/m2)
q12h (mg/g)
q12h (mg/m2)
UiO-66 UiO-66-NH2 UiO-66-NH3+ UiO-66-COOH(30%) UiO-66-COONa(30%) UiO-66-OH UiO-66-(OH)2 UiO-66-SO3H(18%)
0 2.0 3.0 0.3 0 1.0 2.0 0.18
0 1.0 0 0.6 0.6 1.0 2.0 0.54
127 182 230 183 136 174 224 170
0.125 0.226 0.310 0.232 0.203 0.217 0.308 0.191
142 122 218 138 99 156 198 128
0.140 0.151 0.293 0.175 0.147 0.194 0.272 0.143
a
Number of H-donor (per benzene ring of linker) for H-bonding with H-accepting adsorbates. bNumber of H-acceptor (per benzene ring of linker) for H-bonding with H-donating adsorbates.
Figure 6. Effect of number of acid site of MOF on the adsorbed amounts (q12h, based on unit surface area) of QUI over studied UiO-66s: (a) Raw data considering all acidic UiO-66s and pristine UiO-66. The red circle shows the q12h of UiO-66-NH3+. (b) Selected data excluding UiO-66-NH3+.
UiO-66-NH2) could also be explained by H-bonding, where QUI was the H-bond acceptor and the MOFs acted as a Hbond donor. The exceptionally low qt of UiO-66-NH2 for QUI might be due to base−base repulsion between the basic QUI and the basic-NH2 group of the MOF. However, it should be noted that H-bonding between −NH2 and QUI is working, since q12h of UiO-66-NH2 for QUI might be very low if there is no Hbonding (with only the described repulsion). The q12h value (based on surface area) was higher than that of pristine UiO-66, as shown in Table 2. Therefore, not only base−base repulsion but also H-bonding was responsible for QUI adsorption over UiO-66-NH2, similar to the adsorption of pyridine over UiO66-NH2.70 We also investigated any possibility of the contribution of QUI as a H-bond donor. Because of the very low R2 values (as shown in Figure S10), based on both the unit surface area and the weight of the MOFs, it can be said that QUI cannot act as a H-bond donor. This is easily understood because no hydrogen in QUI is useful as a H-donor. On the other hand, acid−base interactions have also been applied to explain the adsorption of the basic QUI over an acidic adsorbent, especially acidic MOFs.60,61,71 To investigate the contribution of acid−base interactions (between the basic QUI and the acidic MOFs) in the QUI adsorption, the effect of the number of acid sites (per linker; shown in Table S4) of UiO-66s (acidic and pristine) on q12h (based on the unit surface area) was plotted, as shown in Figure 6a (here, UiO-66-NH2 and UiO-66-COONa were excluded because of their basicity). The R2 value was not very high (R2 = 0.680) if all the acidic UiO-66s were considered; however, R2 could be increased up to an acceptable value (R2 = 0.962) when UiO-66-NH3+ was excluded (as shown in Figure 6b), similar to IND adsorption shown in Figure 4. Therefore, acid−base interactions may also
Figure S8 showed that IND cannot be a H-bond acceptor, which is easily understood based on the fact that the nonbonding electron pair (on nitrogen) is already applied for the aromaticity of the pyrrole ring. Based on above discussion, it can be concluded that IND adsorption over UiO-66s can be quantitatively explained by H-bonding (IND: H-donor), excluding a special case (here, UiO-66-NH3+). The exceptional adsorption of IND over UiO-66-NH3+ might be explained by cation−π interactions between a cation site (NH3+) and the π-electrons of the aromatics.67,68 This type of cation−π interaction has also been applied to interpret the high IND adsorption over protonated MIL-125 and protonated Ade-MIL-101 (adenine-grafted MIL-101)54,55 Moreover, weak H-bonding can be possible between the H atom of the protonated −NH3+ group (in UiO-66-NH3+) and the πelectrons of the benzene ring of IND, as recently reported by us.54,55 Therefore, the exceptional adsorption of IND by UiO66-NH3+ might be explained by cation−π interactions along with the additional weak H-bonding. Recently, QUI adsorption over MOFs has also been explained a few times by H-bonding, where QUI acts as a Hbond acceptor.54,69 In this study, the QUI adsorption was interpreted similar to the IND adsorption (but the directions of the H-bond are opposite to each other). Figures 5a and S9a show the effect of the number of H-bond donor moieties in the MOFs on the q12h values, based on the surface area and unit weight, respectively, for all UiO-66s. The R2 value (R2 = 0.654), based on all the UiO-66s was relatively low; however, the R2 value could be increased much or to an acceptable level (R2 = 0.963) when the result with UiO-66-NH2 was excluded (as shown in Figure 5b). A similar tendency was observed in Figure S9b, based on the unit weight of the adsorbent. Therefore, the adsorption of QUI over the functionalized UiO-66s (except 4536
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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ABBREVIATIONS ADN, adsorptive denitrogenation; NCC, nitrogen containing compounds; SCC, sulfur containing compounds; HDS, hydrodesulfurization; HDN, hydrodenitrogenation; QUI, quinoline; MOF, metal−organic framework
be applicable in the interpretation of QUI adsorption, excluding UiO-66-NH3+. The exceptionally high adsorption of QUI over UiO-66-NH 3+ might be explained, similar to previous discussion, via cation-π interactions or the high contribution of H-bonding (because of three H-donors). However, the studied MOFs that are useful as a H-donor also have acidity; therefore, further work is required to exactly discriminate the contributions of H-bonding and acid−base interactions to the adsorption of QUI over MOFs (for example, by using MOFs with aliphatic alcohols with high pKa values).
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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/acs.jpcc.8b00761. Detailed synthesis, additional characterization, and adsorption data (PDF)
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5. CONCLUSION In this study, adsorption of IND and QUI over eight UiO-66s (pristine UiO-66 and seven functionalized UiO-66s) has been carried out to quantitatively understand the adsorption mechanism. The following conclusions can be drawn from the adsorption over various UiO-66s. First, the adsorption of IND and QUI over UiO-66s can be explained mainly by Hbonding (IND: as H-donor; QUI: H-acceptor). However, there are a few exceptions such as UiO-66-NH3+ and UiO-66-NH2 that show unexpected adsorption for IND and QUI, respectively. The exceptionally high adsorption of IND over UiO-66-NH3+ may be explained by cation−π interactions and weak H-bonding. Moreover, the relatively low QUI adsorption over UiO-66-NH2 may be due to base−base repulsion, even though H-bonding is still observed. Second, acid−base interactions may also be considered to interpret the adsorption of the basic QUI over the acidic or neutral UiO-66s, excluding UiO-66-NH3+ showing very high adsorption, which may be due to further contribution of cation-π interactions and H-bonding (due to three H-donors). Third, UiO-66-NH3+ shows the highest adsorption capacities for both IND and QUI among the studied UiO-66s, suggesting that introducing an ammonium group on MOFs might be an effective way to develop adsorbents for ADN.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 82-10-28185341; Fax: 82-53-950-6330; E-mail: sung@ knu.ac.kr. ORCID
Sung Hwa Jhung: 0000-0002-6941-1583 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS 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 Number: 2017R1A2B2008774). 4537
DOI: 10.1021/acs.jpcc.8b00761 J. Phys. Chem. C 2018, 122, 4532−4539
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