Adsorptive removal of indole and quinoline from model fuel using adenine-grafted metal-organic frameworks

Accepted Manuscript Title: Adsorptive removal of indole and quinoline from model fuel using adenine-grafted metal-organic frameworks Authors: Mithun Sarker, Ji Yoon Song, Ah Rim Jeong, Kil Sik Min, Sung Hwa Jhung PII: DOI: Reference:

S0304-3894(17)30797-5 https://doi.org/10.1016/j.jhazmat.2017.10.041 HAZMAT 18946

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

20-7-2017 7-10-2017 20-10-2017

Please cite this article as: Mithun Sarker, Ji Yoon Song, Ah Rim Jeong, Kil Sik Min, Sung Hwa Jhung, Adsorptive removal of indole and quinoline from model fuel using adenine-grafted metal-organic frameworks, Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.10.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Adsorptive removal of indole and quinoline from model fuel using adenine-grafted metal-organic frameworks Mithun Sarker,a Ji Yoon Song,a Ah Rim Jeong,a Kil Sik Min,b and Sung Hwa Jhung*,a a

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National

University, Daegu 41566, Republic of Korea b

*

Department of Chemistry Education, Kyungpook National University, Daegu 41566, Republic of Korea

Corresponding author. E-mail: [email protected], Fax: 82-53-950-6330

Graphical Abstract

Research Highlight

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MIL-101 MOF was firstly grafted with adenine and further protonated.

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Modified MOFs were applied in adsorptive denitrogenation of model fuel.

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The new adsorbents showed remarkable adsorption capacities for indole and quinoline.

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The new adsorbents can be regenerated by simple ethanol washing.

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Adsorption mechanisms (H-bonding, cation-π, acid-base interaction) were suggested.

A highly porous metal-organic framework (MOF), MIL-101, was modified for the first time with the nucleobase adenine (Ade) by grafting onto the MOF. The Ade-grafted MOF, Ade-MIL-101, was further protonated to obtain P-Ade-MIL-101, and these MOFs were utilized to remove nitrogen-containing compounds (NCCs) (such as indole (IND) and quinolone (QUI)) from a model fuel by adsorption. These functionalized MOFs exhibited remarkable adsorption performance for NCCs compared with that shown by commercial activated carbon (AC) and pristine MIL-101, even though the porosities of the functionalized-MOFs were lower than that of pristine MIL-101. P-Ade-MIL-101 has 12.0 and 10.8 times capacity to that of AC for IND and QUI adsorption, respectively; its adsorption performance was competitive with that of other reported adsorbents. The remarkable adsorption of IND and QUI by Ade-MIL-101 was attributed to H-bonding. H-bonding combined with cation-π interactions was proposed as the mechanism for the removal of IND by P-Ade-MIL-101, whereas acid-base interactions were thought to be responsible for QUI adsorption by P-Ade-MIL-101. Moreover, P-Ade-MIL-101 can be regenerated without any severe degradation and used for successive adsorptions. Therefore, PAde-MIL-101 was recommended as an effective adsorbent for fuel purification by adsorptive removal of NCCs. Keywords: adenine grafting; adsorption; denitrogenation; metal-organic framework

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1. Introduction Energy is one of the most vital issues in the current century. The consumption of energy is increasing day by day with the increasing population and worldwide developments. Currently, fossil fuels are the most important and widely used energy sources in the world [1], with about 85% of the world's commercial energy coming from fossil fuels [2]. However, the utilization of fossil fuels without adequate pretreatment is responsible for environmental pollution including the emission of NOx and SOx, which are released from the combustion of nitrogen- and sulfur-containing compounds (NCCs and SCCs, respectively) in fuels [3-5]. Both NOx and SOx are harmful to the environment and can be easily converted into acid rain [6], which has a negative impact on the environment and manmade structures. Currently, the utilization of clean fuels [7, 8] is a global issue for minimizing the environmental pollution. Therefore, removing SCCs and NCCs from fuels is very important [9, 10]. SCCs in fuels are effectively and widely removed by the hydrodesulfurization (HDS) process [11, 12]. During HDS, sulfur in SCCs is removed in the form of H2S gas by hydrogenation of the SCCs at elevated temperatures in the presence of a suitable catalyst. However, NCCs present in fuels reduce the catalytic activity of HDS catalysts [13] and also hamper the HDS process itself. Indole (IND), quinoline (QUI), and their derivatives are the most common NCCs that exist naturally in fossil fuels [14]. Therefore, prior to HDS, fuels should be free from NCCs. NCCs from fossil fuels are conventionally removed by the hydrodenitrogenation (HDN) process [15]. However, new alternative approaches such as oxidative denitrogenation, extractive denitrogenation, and adsorptive denitrogenation (ADN) have been investigated. Among the various methods, ADN can be considered as a promising technique [16,17] since it is simple, relatively cheap, and can be operated under mild conditions without the need for expensive hydrogen or an oxidizing agent. Several adsorbents such as activated carbon [1820], silica-alumina adsorbents [21], zeolites [22], Li-modified mesoporous silica [23], Si-Zr cogel [24], and ion-exchange resins [25] have been used for ADN of fuels.

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Recently, significant progress has been made in the field of porous materials [26-30] including metal-organic frameworks (MOFs). MOFs [31-34] are an advanced class of porous materials that have received much attention due to their simple synthesis, excellent porosity, and facile functionalization. There are several potential applications of MOFs, particularly in liquid-phase adsorption for water and fuel purification [35-39]. A few MOFs have also been applied in ADN (such as adsorptive removal of IND and QUI from fuel) [40, 41]. Various attempts, including functionalization [42, 43], have been made to increase the adsorptive efficiency of MOFs. Functionalizing MOFs has attracted much attention since different functional groups introduced onto MOFs can induce desired properties and enhance their adsorptive performance [44-46]. One of the most effective methods of functionalizing MOFs is grafting suitable materials on the coordinatively unsaturated sites (CUSs) of the metals present in MOFs [47]. Grafting on the surface of MOFs has been extensively studied using chromium (III) terephthalate (MIL-101) MOF and several interesting materials [48, 49]. Moreover, functionalization of MIL-101 (via grafting) with nucleobases such as adenine (Ade), thymine and guanine might also be possible given the nonbonding electron pairs on nucleobases. Very recently, thymine-functionalized MIL-101 [50] was actually prepared and utilized for removing Hg2+ from water. Therefore, the possible functionalization of MOFs using other nucleobases and utilizing them as adsorbents is very interesting and needs more research. Herein, we report for the first time Ade-functionalized MIL-101(Cr) (via Ade grafting on pristine MIL-101) and the possible application of Ade-modified MOFs for the adsorptive removal of IND and QUI from a model fuel. Ade-grafted MIL-101 (Ade-MIL-101), especially protonated Ade-MIL-101 (PAde-MIL-101), showed remarkable adsorption capacities for both IND and QUI compared with that of pristine MIL-101 and commercial activated carbon, even though the porosities of Ade-MIL-101 and PAde-MIL-101 were lower than that of pristine MIL-101. Moreover, plausible adsorption mechanisms are suggested based on H-bonding, acid-base interactions, and cation-π interactions.

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2. Materials and methods 2.1 Chemicals Chromium nitrate nonahydrate (Cr(NO3)3∙9H2O, 99%) and Ade (98%) were obtained from Samchun Pure Chemical Co., Ltd. and Alfa Aesar, respectively. QUI (C9H7N, 99%), IND (C8H7N, 98%) and terephthalic acid (TPA, C6H4-1,4-(CO2H)2, 99%) were purchased from Sigma-Aldrich. Granular AC (2–3 mm, practical grade) and n-octane (C8H14, 97%) were acquired from Daejung Chemicals & Metal Co. and Junsei Chemical Company, respectively. Sodium hydroxide (NaOH, 99%) was purchased from Merck KGaA. N,N-dimethylformamide (DMF, 99%), hydrochloric acid (HCl, 37%), ethanol (C2H5OH, 94%), and methanol (CH3OH, 99%) were sourced from OCI Co., Ltd. All chemicals were used without further purification. 2.2 Synthesis of adsorbents MIL-101 was synthesized following a previously reported procedure [51], using Cr(NO3)3∙9H2O, TPA, and deionized water. Ade-MIL-101 was obtained using previously reported procedures [52-54] by grafting Ade on the CUSs of pristine MIL-101. Prior to functionalization, the pristine MIL-101 was dehydrated at 150 °C overnight to remove moisture and generate CUSs. Ade solutions were prepared by adding varying amounts (0.5 mmol to 2.5 mmol) of Ade to anhydrous DMF (30 mL) using an ultrasound reactor (Sonics, model: VCX750). The dehydrated MIL-101 (0.3 g) was suspended in the Ade solution in a round-bottom flask equipped with a reflux condenser and a magnetic stirrer. The slurry was refluxed for 12 h with continuous stirring and then cooled to room temperature. The solid product was filtered, washed three times with DMF and finally methanol, and then dried at 100 °C for 24 h. Ade-MIL-101 samples containing different amounts of Ade were named Ade(x)-MIL-101, where x indicates the quantity of Ade (in mmol) added to 0.3 g of pristine MIL-101. As shown in Scheme 1, Ade grafting on MIL-101 might proceed through the N1 site, because of the basicity of nitrogen in Ade [55].

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As shown in Scheme 1, Ade(1)-MIL-101 was subsequently protonated by stirring Ade-MIL-101 (0.1 g) in an aqueous HCl (0.01 M) solution (20 mL) for 24 h. The resulting mixture was then filtered, washed several times with water to remove unreacted HCl and dried at 100 °C. The protonated form of the MOF was named P-Ade(1)-MIL-101. 2.3 Characterization of adsorbents X-ray powder diffraction (D2 Phaser, Bruker, Germany) with Cu-Kα radiation was used to evaluate the structural characteristics of the adsorbents. The textural properties of the adsorbents were measured, after evacuation at 150 °C for 12 h, via nitrogen adsorption at -196 °C using a surface-area and porosity analyzer (Tristar II 3020, Micromeritics, USA). The surface areas of the adsorbents were calculated using the Brunauer-Emmett-Teller (BET) equation. Moreover, Barrett−Joyner−Halenda (BJH) equation was applied to get the pore-size distributions from the desorption isotherms. The total pore volume of the adsorbents was determined from the amount of nitrogen adsorbed at P/P0 = 0.99. Elemental analyses were carried out to determine the N content of the adsorbents using an elemental analyzer (Flash 2000, Thermo Scientific) equipped with a thermal conductivity detector. An FTIR spectrometer (Jasco FTIR-4100) equipped with an attenuated total reflectance module (ATR, maximum resolution: 4.0 cm-1) was used not only to confirm the grafting of Ade but also to confirm the adsorption/desorption of adsorbates on the functionalized MOFs. Thermogravimetric analysis (TGA) was conducted under air flow (20 cm3·min-1) from 30 °C to 800 °C at a heating rate of 10 °C min1

using a PerkinElmer TGA 4000 system. UV/Vis diffuse reflectance spectra of the MOFs were acquired

using a SCINCO S-2100 spectrophotometer (BaSO4 was used as a reference). The counteranion (Cl-) concentration in P-Ade(1)-MIL-101 was measured using ion chromatography (IC, Dionex ICS-5000), after dissolving the MOF in an aqueous NaOH (0.01 M) solution. The solution was separated using an adequate amount of eluent (aqueous KOH solution in 10–58 mM, flow rate: 1.0 mL/min), and the Clpeak was identified using a standard solution. The Cl- content in the MOF was estimated using standard solutions.

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2.4 Adsorption experiments Stock solutions of both IND and QUI (each 10,000 mg·L-1) were prepared by dissolving each in noctane. Solutions of IND and QUI at desired concentrations (500 mg·L-1 to 2000 mg·L-1) were prepared from the stock solutions by successive dilutions. Before conducting the adsorption experiments, the adsorbents were evacuated at 100 C for 12 h in a vacuum oven to remove moisture. For each adsorption experiment, an adsorbent (~5.0 mg) was added to an IND or QUI solution (5 mL) and the mixture was shaken for 1 h to 12 h at 20 °C. After finishing the adsorption experiments, the solutions were filtered using a polytetrafluoroethylene syringe filter (hydrophobic, 0.5 μm), and the residual concentrations of IND and QUI were determined using UV spectrometric analysis (UV-1800, Shimadzu, Japan). The UV absorbances at 287 nm and 313 nm were used to calculate the concentrations of IND and QUI, respectively. To regenerate the adsorbents, a specific amount (50 mg, considering loss of adsorbent during adsorption and regeneration processes) of the relevant MOF was added to ethanol (25 mL) and sonicated for 2 h in a sonication bath (Bransonic, model 1210R). The MOF regeneration process was repeated three times to completely remove the adsorbed IND. Finally, the material was collected by filtration, followed by washing with an ethanol/water mixture (1:1, v/v) and drying. XRD analysis was carried out to confirm the structural stability of the MOFs after regeneration. FTIR was also used to confirm the successful regeneration and ready desorption of adsorbates from the MOFs. 3. Results and discussion 3.1 Characterization of adsorbents The XRD patterns of pristine MIL-101, Ade(1)-MIL-101, and P-Ade(1)-MIL-101 shown in Fig. 1(a) entirely match with the simulated MIL-101 pattern, confirming that the MIL-101 MOFs were successfully synthesized and their structures remain unchanged after the grafting and protonation procedures. The XRD patterns of the synthesized Ade(x)-MIL-101 MOFs (with different Ade contents) shown in Fig. S1(a) also affirm their successful syntheses. The nitrogen adsorption isotherms of Ade(1)-

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MIL-101, P-Ade(1)-MIL-101, and pristine MIL-101 are shown in Fig. 1(b) and their BET surface areas and total pore volumes, determined from the isotherms, are summarized in Table 1. The porosity of pristine MIL-101 decreased after grafting because of either the bulky Ade attached to the pristine MOF structure or partial degradation of the MOF under harsh conditions [48]. However, interestingly, as shown in Fig. S2(a), the pore size distributions did not change noticeably with modifications. Moreover, as shown in Fig. S1(a), the crystallinity of the Ade(x)-MIL-101 MOFs decreased with increasing Ade content (especially when applied Ade is equal to or more than 1.5 mmol). The BET surface area and pore volume, determined from the isotherms in Fig. S1(b), also decreased steadily with increasing Ade content (Table S1). FTIR spectra of Ade(1)-MIL-101 and P-Ade(1)-MIL-101 are shown in Fig. 1(c). The presence of the C–N stretching bands at 1216 and 1054 cm-1 [56, 57], along with the N–H stretching band at 1604 cm1

indicate the successful grafting of Ade on MIL-101 [58]. Moreover, the –NH2 stretching band of the

Ade-grafted MOFs was identified at 1672 cm-1 [59]. As shown in Fig. S1(c), the intensity of the C–N stretching band in Ade(x)-MIL-101 increased steadily with increasing Ade content. The thermal stability of the MOFs was assessed using TGA, as shown in Fig. 1(d). The thermal stability of MIL-101 changed little upon functionalization. However, the amount of water desorbed (~100 °C) increased upon functionalization, and the chromium-oxide content remaining after heat treatment at 400 °C or higher decreased, which is probably because of the functionalization. Based on the N content (7.9 wt%, determined from elemental analysis) in Ade(1)-MIL-101, the grafting yield of Ade was found to be 38.5%. This grafting yield is comparable to previously reported yields, as shown in Table S2. Moreover, the protonation of Ade(1)-MIL-101 to P-Ade(1)-MIL-101 was confirmed by IC and UV/Vis spectroscopy. The existence of Cl- as a counteranion in P-Ade(1)-MIL-101 was affirmed by IC measurements, and the degree of protonation was estimated to be 94.5%. In addition, UV/Vis spectra shown in Fig. S2(b) indicate the successful protonation of Ade(1)-MIL-101 by HCl treatment. A bathochromic shift in the UV/Vis absorption of Ade(1)-MIL-101 was observed

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because of protonation, which is similar to acid-loaded Al-PMOF and protonated MIL-125-NH2 [60, 61]. 3.2 Adsorption results Figs. 2(a) and 2(b) show the adsorbed quantities (qt) of IND and QUI (1000 mg·L-1 for each), respectively, as a function of the Ade content introduced in the grafting procedure (see Experimental section). From the figures, it is clear that the adsorbed amounts of IND and QUI increased with increasing Ade content up to a certain value. Ade(1)-MIL-101 exhibited the maximum adsorption for both IND and QUI. Therefore, further detailed investigations on IND and QUI adsorption were performed using Ade(1)-MIL-101. Moreover, the Ade(x)-MIL-101 MOFs showed higher increments in IND adsorption, compared with QUI adsorption, representing a comparatively favorable effect for IND adsorption. The efficiency of IND and QUI adsorption over pristine MIL-101, Ade(1)-MIL-101, P-Ade(1)-MIL101, and commercial AC, were investigated for contact times between 1 and 12 h, with the results shown in Fig. 3. The adsorption efficiency for both IND and QUI decreased in the order P-Ade(1)-MIL101 > Ade(1)-MIL-101 > MIL-101 > AC. Interestingly, the amounts adsorbed increased after protonation of Ade(1)-MIL-101 even though the porosity decreased. P-Ade(1)-MIL-101 and Ade(1)MIL-101 adsorbed 1.4 and 1.2 times IND, respectively, compared to pristine MIL-101 after 12 h (q12 h). Similarly, P-Ade(1)-MIL-101 and Ade(1)-MIL-101 had 1.3 and 1.1 times capacity, respectively for QUI adsorption to that of pristine MIL-101. Moreover, IND adsorptions over P-Ade(1)-MIL-101 and Ade(1)MIL-101 were 12.0 and 10.8 times, respectively, to that of commercial AC. Similarly, P-Ade(1)-MIL-101 and Ade(1)-MIL-101 adsorbed 10.8 and 9.5 times QUI, respectively, to that of AC. Adsorption isotherms for IND and QUI, obtained using a broad range of adsorbate concentrations (500–2000 mg·L-1), are shown in Figs. 4(a) and 4(b), respectively. Even though adsorption equilibriums were reached within 6 h, the experiments were conducted for 12 h to ensure complete equilibration. For both IND and QUI, the amount adsorbed at equilibrium decreased in the order P-Ade(1)-MIL-101

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> Ade(1)-MIL-101 > MIL-101, which is very similar to the corresponding results in Figs. 3(a) and 3(b). The maximum adsorption capacities (Q0), b-values, and correlation parameters (R2) for the adsorbents were obtained from Langmuir plots [62], and are presented in Table 1. The Q0-value for IND adsorption over Ade(1)-MIL-101 and P-Ade(1)-MIL-101 was 491 and 532 mg·g-1, respectively; the Q0 value for QUI adsorption was 466 and 511 mg·g-1 for Ade(1)-MIL-101 and P-Ade(1)-MIL-101, respectively, which is competitive with other studied adsorbents, as shown in Table 2.

Moreover, the separation factors (RL) for IND and QUI shown in Figs. S3(a) and S3(b), respectively, also indicated that P-Ade(1)-MIL-101 is the most favorable adsorbent, given its minimum RL values [63]. Therefore, P-Ade(1)-MIL-101 is one of the most prominent adsorbents among those studied for removing IND and QUI from a model fuel, based on its remarkably high Q0 and b-values and relatively low RL values. 3.3 Adsorption mechanism It is essential to investigate plausible adsorption mechanisms to not only identify the interactions between an adsorbent and an adsorbate but also for possible commercial applications of an adsorbent. Various adsorption mechanisms involving van der Waals forces [64-67], acid-base interactions [68, 69], π-complexation [70, 71], and H-bonding [72-74] interactions have been employed to explain IND and QUI adsorption from fuels. The surface area and porosity of the adsorbent are considered as one of the main criteria for adsorption via van der Waals interactions [64, 65]. However, the surface areas of Ade(1)-MIL-101 and P-Ade(1)-MIL-101 are lower than that of pristine MIL-101. Therefore, the enhanced adsorption of IND and QUI by Ade(1)-MIL-101 and PAde(1)-MIL-101 cannot be explained by van der Waals interactions. Additionally, π-complexation [70], which involves only a few metal ions, such as Cu(I) and Ag(I), is unlikely to be significant in Adefunctionalized MIL-101s, considering the absence of such metal ions. Moreover, acid-base interactions are not applicable to IND adsorption by Ade(1)-MIL-101 and P-Ade(1)-MIL-101, due to its neutral

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character. Therefore, another mechanism is required to explain the adsorption of IND and QUI from a model fuel by Ade(1)-MIL-101 and P-Ade(1)-MIL-101. H-bonding [72] is one of the primary interactions utilized to explain liquid-phase adsorption, especially in the purification of fuel and water. Recently, IND adsorption by MOFs functionalized with –NH2, –OH, –COOH, and –SO3H groups have been explained by H-bonding, where IND was a H-bond donor [75-78]. Moreover, MOFs having specific functional groups can act as H-bond acceptors and/or donors [72]. Therefore, the enhanced adsorption of IND by Ade(1)-MIL-101 and P-Ade(1)-MIL-101 can be explained by H-bonding. According to Scheme 2(a) (left side), the pyridinic nitrogen (N3) of Ade(1)MIL-101 can act as a H-bond acceptor given the basicity order of nitrogen in Ade [55], while IND can act as a H-bond donor through the H atoms attached to its N atoms. However, the improved adsorption of IND by P-Ade(1)-MIL-101 cannot be explained simply by Hbonding. As indicated in Scheme 2(b) (left side), two possible interactions might be responsible for the high qt observed for IND adsorption by P-Ade(1)-MIL-101. First, a cation-π interaction can occur between a cation (due to protonation of the N3 nitrogen of Ade(1)-MIL-101) and the π-electrons of aromatics [79, 80]. It has been reported that IND exhibits this type of cation-π interaction [61, 81] and the binding energy of interaction between organic cations and IND is quite similar to or even stronger than H-bonding [79]. Another plausible scenario is the existence of weak H-bonding between the H atom of protonated nitrogen (NH+) in P-Ade(1)-MIL-101 and the π-electrons of the benzene ring in IND. Very recently, this type of H-bond between IND and protonated MIL-125-NH2 was reported by us [61]. Therefore, the improved IND adsorption by P-Ade(1)-MIL-101 could be explained by cation-π interactions with additional H-bonding. On the other hand, acid-base interactions are commonly found in adsorption mechanisms for basic QUI [69, 82]. These interactions occur because of an attraction between the basic QUI and an acidic adsorbent. One plausible reason for the remarkable performance of P-Ade(1)-MIL-101 for QUI adsorption is also an acid-base interaction between acidic NH+ groups on the MOF and the basic QUI.

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However, it is difficult to explain the improved adsorption of QUI by Ade(1)-MIL-101 in terms of acidbase interactions since both the adsorbate and adsorbent are basic materials. Therefore, another adsorption mechanism is necessary to clarify the enhanced adsorption of QUI by Ade(1)-MIL-101. It was recently reported that QUI could be adsorbed efficiently by –NH2 functionalized MOFs via Hbonding [61, 83]. Therefore, Ade(1)-MIL-101 may interact with QUI through H-bonding due to the presence of –NH2 in the MOF (–NH2 group: H-donor; QUI: H-acceptor). The proposed mechanisms for the favorable adsorption of QUI by Ade(1)-MIL-101 and P-Ade(1)-MIL-101 are illustrated in Scheme 2(a) and 2(b), respectively, with H-bonding and acid-base interactions being the major interactions in the adsorption of QUI by Ade(1)-MIL-101 and P-Ade(1)-MIL-101, respectively. Based on the above discussion, was can summarize that H-bonding is very effective for both IND and QUI adsorption by Ade(1)-MIL-101. Moreover, we propose that cation-π interactions also play a role in IND adsorption by P-Ade(1)-MIL-101. Finally, acid-base interactions are thought to be the sole interaction for the adsorption of QUI by P-Ade(1)-MIL-101. 3.4 Reusability of P-Ade(1)-MIL-101 and Ade(1)-MIL-101 Reusability is the most important criterion for commercial applications of adsorbents since it is directly related to the cost effectiveness of the adsorption process. Therefore, the reusability of PAde(1)-MIL-101 and Ade(1)-MIL-101 for IND adsorption was investigated via ethanol washing, as shown in Figs. 5(a) and Fig. S4(a), respectively. The performance of the regenerated MOFs does not diminish severely with increasing number of recycles, indicating that these materials can be recycled several times without any serious loss in adsorption capacity. The slight decrease of adsorption capacity after regeneration might be due to (i) partial pore filling with adsorbates and (ii) partial decrease in crystallinity of MOFs during regeneration. Importantly, even after recycling, the MOFs showed very competitive adsorption compared with commercial AC. IND adsorption and subsequent successful regeneration of P-Ade(1)-MIL-101 and Ade(1)-MIL-101 were also confirmed by FTIR spectroscopy, as shown in Figs. 5(b) and S4(b), respectively. Several characteristic IND bands were

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observed (after adsorption) in the spectra for both P-Ade(1)-MIL-101 and Ade(1)-MIL-101, indicating the presence of adsorbed IND on the MOFs. The disappearance of these bands in the spectra of the recycled MOFs also confirmed the success of the facile regeneration using ethanol treatment. Moreover, the integrity of the crystal structure of these materials after regeneration was confirmed by XRD analysis (shown in Fig. S5), even though the intensity of the XRD peaks decreased slightly. Therefore, considering their remarkable adsorption efficiency and adequate reusability, we propose P-Ade(1)-MIL-101 and Ade(1)-MIL-101 as potential adsorbents for removing NCCs from fuels. 4. Conclusion In this study, Ade was grafted on MIL-101 (and subsequently protonated) for the first time and applied successfully in the adsorptive removal of NCCs (such as IND and QUI) from a model fuel. The adsorption of IND and QUI by P-Ade(1)-MIL-101 was 1.4 and 1.3 times, respectively, to that of pristine MIL-101. Importantly, P-Ade(1)-MIL-101 has 12.0 and 10.8 times capacity for IND and QUI adsorption, respectively, to that of AC and its adsorption performance was competitive with that of other reported adsorbents. The maximum adsorption capacities of P-Ade-MIL-101 for IND and QUI were 532 and 511 mg·g-1, respectively. The remarkable adsorption of IND and QUI by Ade(1)-MIL-101 was attributed to H-bonding (for IND, MOF: H-acceptor and for QUI, MOF: H-donor), while a combination of H-bonding and cation-π interactions was proposed for IND adsorption by P-Ade(1)-MIL-101. Additionally, acidbase interactions were suggested as the primary interactions in QUI adsorption by P-Ade(1)-MIL-101. Finally, Ade(1)-MIL-101 and P-Ade(1)-MIL-101 could be regenerated without severe degradation by an ethanol treatment. Therefore, P-Ade(1)-MIL-101 is proposed as a promising adsorbent for removing NCCs from fuels based on its facile synthesis, remarkable adsorption capacity, and excellent reusability. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (grant number: 2017R1A2B2008774).

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References: [1]

N.L. Panwar, S.C. Kaushi, S. Kothari, Role of renewable energy sources in environmental protection: a review, Renewable Sustainable Energy Rev. 15 (2011) 1513–1524.

[2]

S. Kumar, H.-T. Kwon, K.-H. Choi, W. Lim, J.H. Cho, K. Tak, I. Moon, LNG: An ecofriendly cryogenic fuel for sustainable development, Appl. Energy 88 (2011) 4264–4273.

[3]

G.H.C. Prado, Y. Rao, A. de Klerk, Nitrogen removal from oil: a review, Energy Fuels 31 (2017) 14–36.

[4]

G.C. Laredo, P.M. Vega-Merino, F. Trejo-Zárraga, J. Castillo, Denitrogenation of middle distillates using adsorbent materials towards ULSD production: a review, Fuel Process. Technol. 106 (2013) 21–32.

[5]

V.C. Srivastava, An evaluation of desulfurization technologies for sulfur removal from liquid fuels, RSC Adv. 2 (2012) 759–783.

[6]

Y. Jin, L. Lu, X. Ma, H. Liu, Y. Chi, K. Yoshikawa, Effects of blending hydrothermally treated municipal solid waste with coal on co-combustion characteristics in a lab-scale fluidized bed reactor, Appl. Energy 102 (2013) 563–570.

[7]

I. Dincer, C. Acar, A review on clean energy solutions for better sustainability, Int. J. Energy Res. 39 (2015) 585–606.

[8]

A.M. Liaquat, M.A. Kalam, H.H. Masjuki, M.H. Jayed, Potential emissions reduction in road transport sector using biofuel in developing countries, Atmos. Environ. 44 (2010) 3869-3877.

[9]

M. Yu, N. Zhang, L. Fan, C. Zhang, X. He, M. Zheng, Z. Li, Removal of organic sulfur compounds from diesel by adsorption on carbon materials, Rev. Chem. Eng. 31 (2015) 27– 43.

15

[10]

H. Zhang, G. Li, Y. Jia, H. Liu, Adsorptive removal of nitrogen-containing compounds from fuel, J. Chem. Eng. Data 55 (2010) 173–177.

[11]

A. Stanislaus, A. Marafi, M.S. Rana, Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production, Catal. Today 153 (2010) 1–68.

[12]

A. Samokhvalov, B.J. Tatarchuk, Review of experimental characterization of active sites and determination of molecular mechanisms of adsorption, desorption and regeneration of the deep and ultra deep desulfurization sorbents for liquid fuels, Catal. Rev. Sci. Eng. 52 (2010) 381–410.

[13]

N.A. Khan, S.H. Jhung, Scandium-Triflate/Metal−Organic Frameworks: Remarkable adsorbents for desulfurization and denitrogenation, Inorg. Chem. 54 (2015) 11498−11504.

[14]

R. Epping, S. Kerkering, J.T. Andersson, Influence of different compound classes on the formation of sediments in fossil fuels during aging, Energy Fuels 28 (2014) 5649–5656.

[15]

H. Song, J.-A. You, B. Li, C. Chen, J. Huang, J. Zhang, Synthesis, characterization and adsorptive denitrogenation performance of bimodal mesoporous Ti-HMS/KIL-2 composite: a comparative study on synthetic methodology, Chem. Eng. J. 327 (2017) 406– 417.

[16]

N.A. Khan, N. Uddin, C.H. Choi, S.H. Jhung, Adsorptive denitrogenation of model fuel with CuCl-loaded adsorbents: contribution of π-complexation and direct interaction between adsorbates and cuprous ions, J. Phys. Chem. C 121 (2017) 11601−11608.

[17]

H. Zhang, H. Song, Study of adsorptive denitrogenation of diesel fuel over mesoporous molecular sieves based on breakthrough curves, Ind. Eng. Chem. Res. 51 (2012) 16059−16065.

[18]

M. Almarri, X. Ma, C. Song, Role of surface oxygen-containing functional groups in liquid-phase adsorption of nitrogen compounds on carbon-based adsorbents, Energy Fuels 23 (2009) 3940–3947. 16

[19]

J. Wen, X. Han, H. Lin, Y. Zheng, W. Chu, A critical study on the adsorption of heterocyclic sulfur and nitrogen compounds by activated carbon: equilibrium, kinetics and thermodynamics, Chem. Eng. J. 164 (2010) 29–36.

[20]

I. Ahmed, S.H. Jhung, Remarkable improvement in adsorptive denitrogenation of model fossil fuels with CuCl/activated carbon, prepared under ambient condition, Chem. Eng. J. 279 (2015) 327–334.

[21]

J.M.P.F. Silva, E.B. Silveira, A.L.H. Costa, C.O. Veloso, C.A. Henriques, F.M.Z. Zotin, M.L.L. Paredes, R.A. Reis, S.S.X. Chiaro, Influence of the chemical composition of silicaalumina adsorbents in sulfur and nitrogen compounds removal from hydrotreated diesel, Ind. Eng. Chem. Res. 53 (2014) 16000– 16014.

[22]

D. Liu, J. Gui, Z. Sun, Adsorption structures of heterocyclic nitrogen compounds over Cu(I)Y zeolite: a first principle study on mechanism of the denitrogenation and the effect of nitrogen compounds on adsorptive desulfurization, J. Mol. Catal. A: Chem. 291 (2008) 17–21.

[23]

A. Koriakin, K.M. Ponvel, C.-H. Lee, Denitrogenation of raw diesel fuel by lithiummodified mesoporous silica, Chem. Eng. J. 162 (2010) 649–655.

[24]

Y.-S. Bae, M.-B. Kim, H.-J. Lee, C.-H. Lee, J.W. Ryu, Adsorptive denitrogenation of light gas oil by silica–zirconia cogel, AIChE J. 52 (2006) 510–521.

[25]

L.-L. Xie, A. Favre-Reguillon, X.-X. Wang, X. Fu, M. Lemaire, Selective adsorption of neutral nitrogen compounds from fuel using ion-exchange resins, J. Chem. Eng. Data 55 (2010) 4849–4853.

[26]

K. Ariga, J. Li, J. Fei, Q. Ji, J.P. Hill, Nanoarchitectonics for dynamic functional materials from atomic-/molecular-level manipulation to macroscopic action, Adv. Mater. 28 (2016) 1251–1286.

17

[27]

K.S. Lakhi, D.-H. Park, K. Al-Bahily, W. Cha, B. Viswanathan, J.-H. Choy, A. Vinu, Mesoporous carbon nitrides: synthesis, functionalization, and applications, Chem. Soc. Rev. 46 (2017) 72–101.

[28]

Y. Belmabkhout, V. Guillerm, M. Eddaoudi, Low concentration CO2 capture using physical adsorbents: are metal–organic frameworks becoming the new benchmark materials? Chem Eng. J. 296 (2016) 386–397.

[29]

T. Ennaert, J.V. Aelst, J. Dijkmans, R.D. Clercq, W. Schutyser, M. Dusselier, D. Verboekend, B.F. Sels, Potential and challenges of zeolite chemistry in the catalytic conversion of biomass, Chem. Soc. Rev. 45 (2016) 584–611.

[30]

S.H. Jhung, N.A. Khan, Z. Hasan, Analogous porous metal–organic frameworks: synthesis, stability and application in adsorption, CrystEngComm 14 (2012) 7099–7109.

[31]

G. Maurin, C. Serre, A. Cooper, G. Férey, The new age of MOFs and of their porousrelated solids, Chem. Soc. Rev. 46 (2017) 3104-3107.

[32]

A.J. Howarth, A.W. Peters, N.A. Vermeulen, T.C. Wang, J.T. Hupp, O.K. Farha, Best practices for the synthesis, activation, and characterization of metal−organic frameworks, Chem. Mater. 29 (2017) 26−39.

[33]

Y. Cui, B. Li, H. He, W. Zhou, B. Chen, G. Qian, Metal–organic frameworks as platforms for functional materials, Acc. Chem. Res. 49 (2016) 483–493.

[34]

P. Silva, S.M.F. Vilela, J.P.C. Tome, F.A.A. Paz, Multifunctional metal–organic frameworks: from academia to industrial applications, Chem. Soc. Rev. 44 (2015) 6774– 6803.

[35]

H. Furukawa, K.-E. Cordova, M. O’Keeffe, O.M. Yaghi, The chemistry and applications of metal-organic frameworks, Science 341(2013) 1230444.

18

[36]

E.M. Dias, C. Petit, Towards the use of metal–organic frameworks for water reuse: a review of the recent advances in the field of organic pollutants removal and degradation and the next steps in the field, J. Mater. Chem. A 3 (2015) 22484–22506.

[37]

B.V. de Voorde, B. Bueken, J. Denayer, D. de Vos, Adsorptive separation on metal– organic frameworks in the liquid phase, Chem. Soc. Rev. 43 (2014) 5766–5788.

[38]

I. Ahmed, S.H. Jhung, Adsorptive desulfurization and denitrogenation using metal-organic frameworks, J. Hazard. Mater. 301 (2016) 259–276.

[39]

N.A. Khan, Z. Hasan, S.H. Jhung, Adsorptive removal of hazardous materials using metal– organic frameworks, (MOFs): a review, J. Hazard. Mater. 244 (2013) 444–456.

[40]

Y. Wu, J. Xiao, L. Wu, M. Chen, H. Xi, Z. Li, H. Wang, Adsorptive denitrogenation of fuel over metal organic frameworks: effect of N-types and adsorption mechanisms, J. Phys. Chem. C 118 (2014) 22533−22543.

[41]

P.W. Seo, I. Ahmed, S. H. Jhung, Adsorptive removal of nitrogen-containing compounds from a model fuel using a metal–organic framework having a free carboxylic acid group, Chem Eng. J. 299 (2016) 236–243.

[42]

Z. Wang, S.M. Cohen, Postsynthetic modification of metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1315–1329.

[43]

S.M. Cohen, Postsynthetic methods for the functionalization of metal-organic frameworks, Chem. Rev. 112 (2012) 970–1000.

[44]

A. Torrisi, R.G. Bell, C. Mellot-Draznieks, Functionalized MOFs for Enhanced CO2 Capture, Cryst. Growth Des.10 (2010) 2839-2841.

[45]

I. Ahmed, S.H. Jhung, Effective adsorptive removal of indole from model fuel using a metal-organic framework functionalized with amino groups, J. Hazard. Mater. 283 (2015) 544–550.

19

[46]

J.-X. Qin, P. Tan, Y. Jiang, X.-Q. Liu, Q.-X. He, L.-B. Sun, Functionalization of metal– organic frameworks with cuprous sites using vapor-induced selective reduction: efficient adsorbents for deep desulfurization, Green Chem. 18 (2016) 3210–3215.

[47]

Y.K. Hwang, D.-Y. Hong, J.-S. Chang, S.H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M. Daturi, C. Serre, G Férey, Amine grafting on coordinatively unsaturated metal centers of MOFs: consequences for catalysis and metal encapsulation, Angew. Chem. Int. Ed. 47 (2008) 4144-4148.

[48]

E. Haque, J.E. Lee, I.T. Jang, Y.K. Hwang, J.-S. Chang, J. Jegal, S.H. Jhung, Adsorptive removal of methyl orange from aqueous solution with metal-organic frameworks, porous chromium-benzenedicarboxylates, J. Hazard. Mater. 181 (2010) 535–542.

[49]

Z. Hasan, J.W. Jun, S.H. Jhung, Sulfonic acid-functionalized MIL-101(Cr): an efficient catalyst for esterification of oleic acid and vapor-phase dehydration of butanol, Chem. Eng. J. 278 (2015) 265–271.

[50]

X. Luo, T. Shena, L. Ding, W. Zhong, J. Luo, S. Luo, Novel thymine-functionalized MIL101 prepared by post-synthesis and enhanced removal of Hg2+ from water, J. Hazard. Mater. 306 (2016) 313–322.

[51]

N.A. Khan, S.H Jhung, Phase-transition and phase-selective synthesis of porous chromium-benzenedicarboxylates, Cryst. Growth Des. 10 (2010) 1860-1865.

[52]

D.Y Hong, Y.K Hwang, C. Serre, G Férey, J.S Chang, Porous chromium terephthalate MIL-101 with coordinatively unsaturated sites: surface functionalization, encapsulation, sorption and catalysis, Adv. Funct. Mater. 19 (2009) 1537−1552.

[53]

S.-N. Kim, S.-T. Yang, J. Kim, J.-E. Park, W.-S. Ahn, Post-synthesis functionalization of MIL-101 using diethylenetriamine: a study on adsorption and catalysis, CrystEngComm 14 (2012) 4142–4147.

20

[54]

M. Sarker, J.Y. Song, S.H. Jhung, Adsorption of organic arsenic acids from water over functionalized metal-organic frameworks, J. Hazard. Mater. 335 (2017)162–169.

[55]

F Tureček, X. Chen, Protonated adenine: tautomers, solvated clusters, and dissociation mechanisms, J. Am. Soc. Mass Spectrom.16 (2005) 1713–1726.

[56]

J.Y. Song, S.H. Jhung, Adsorption of pharmaceuticals and personal care products over metal-organic frameworks functionalized with hydroxyl groups: Quantitative analyses of H-bonding in adsorption, Chem. Eng. J. 322 (2017) 366–374.

[57]

Z. Hasan, E.-J. Choi, S.H. Jhung, Adsorption of naproxen and clofibric acid over a metal– organic framework MIL-101 functionalized with acidic and basic groups, Chem. Eng. J. 219 (2013) 537-544.

[58]

P.W. Seo, N.A. Khan, Z. Hasan, S.H. Jhung, Adsorptive removal of artificial sweeteners from water using metal−organic frameworks functionalized with urea or melamine, ACS Appl. Mater. Interfaces 8 (2016) 29799–29807.

[59]

C.L. Angel, An infrared spectroscopic investigation of nucleic acid constituents, J. Chem. Soc. (1961) 504-515.

[60]

O.T. Wilcox, A. Fateeva, A.P. Katsoulidis, M.W. Smith, C.A. Stone, M.J. Rosseinsky, Acid loaded porphyrin-based metal–organic framework for ammonia uptake, Chem. Commun. 51 (2015) 14989--14991.

[61]

I. Ahmed, N.A. Khan, J.W. Yoon, J.-S. Chang, S.H. Jhung, Protonated MIL-125-NH2: remarkable adsorbent for the removal of quinoline and indole from liquid fuel, Appl. Mater. Interfaces 9 (2017) 20938–20946.

[62]

K.-Y.A. Lin, W.-D. Lee, Self-assembled magnetic graphene supported ZIF-67 as a recoverable and efficient adsorbent for benzotriazole, Chem. Eng. J. 284 (2016) 1017– 1027.

21

[63]

Y.S. Seo, N.A. Khan, S.H. Jhung, Adsorptive removal of methylchlorophenoxypropionic acid from water with a metal-organic framework, Chem. Eng. J. 270 (2015) 22–27.

[64] B.V. de Voorde, M. Boulhout, F. Vermoortele, P. Horcajada, D. Cunha, J.S. Lee, J.-S. Chang, E.

Gibson, M. Daturi, J.-C. Lavalley, A. Vimont, I. Beurroies, D.E. de Vos, N/S-heterocyclic contaminant removal from fuels by the mesoporous metal−organic framework MIL-100: the role of the metal ion, J. Am. Chem. Soc. 135 (2013) 9849−9856. [65]

I. Ahmed, S.H. Jhung, Remarkable adsorptive removal of nitrogen-containing compounds from a model fuel by a graphene oxide/MIL-101 composite through a combined effect of improved porosity and hydrogen bonding, J. Hazard. Mater. 314 (2016) 318–325.

[66]

I. Ahmed, N.A. Khan, S.H. Jhung, Graphite oxide/metal−organic framework (MIL-101): remarkable performance in the adsorptive denitrogenation of model fuels, Inorg. Chem. 52 (2013) 14155−14161.

[67]

M. Maes, M. Trekels, M. Boulhout, S. Schouteden, F. Vermoortele, L. Alaerts, D. Heurtaux, Y.-K. Seo, Y.K. Hwang, J.-S. Chang, I. Beurroies, R. Denoyel, K. Temst, A. Vantomme, P. Horcajada, C. Serre, D.E. De Vos, Selective removal of N-heterocyclic aromatic contaminants from fuels by Lewis acidic metal-organic frameworks, Angew. Chem. Int. Ed. 50 (2011) 4210–4214.

[68]

I. Ahmed, Z. Hasan, N.A. Khan, S.H. Jhung, Adsorptive denitrogenation of model fuels with porous metal-organic frameworks (MOFs): effect of acidity and basicity of MOFs, Appl. Catal. B: Environ. 129 (2013) 123–129.

[69]

I. Ahmed, N.A. Khan, Z. Hasan, S.H. Jhung, Adsorptive denitrogenation of model fuels with porous metal-organic framework (MOF) MIL-101 impregnated with phosphotungstic acid: effect of acid site inclusion, J. Hazard. Mater. 250–251 (2013) 37–44.

22

[70]

N.A. Khan, S.H. Jhung, Adsorptive removal and separation of chemicals with metalorganic frameworks: contribution of π-complexation, J. Hazard. Mater. 325 (2017) 198– 213.

[71]

I. Ahmed, S.H. Jhung, Adsorptive denitrogenation of model fuel with CuCl-loaded metalorganic frameworks (MOFs), Chem. Eng. J. 251 (2014) 35–42.

[72]

I. Ahmed, S.H. Jhung, Applications of metal-organic frameworks in adsorption/separation processes via hydrogen bonding interactions. Chem. Eng. J. 310 (2017) 197–215.

[73]

P. Tan, X.-Y. Xie, X.-Q. Liu, T. Pan, C. Gu, P.-F. Chen, J.-Y. Zhou, Y. Pan, L.-B. Sun, Fabrication of magnetically responsive HKUST-1/Fe3O4 composites by dry gel conversion for deep desulfurization and denitrogenation, J. Hazard. Mater. 321 (2017) 344-352.

[74]

I. Ahmed, N.A. Khan, S.H. Jhung, Adsorptive denitrogenation of model fuel by functionalized UiO-66 with acidic and basic moieties, Chem. Eng. J. 321 (2017) 40–47.

[75]

P.W. Seo, I. Ahmed, S.H. Jhung, Adsorption of indole and quinoline from a model fuel on functionalized MIL-101: effects of H-bonding and coordination, Phys. Chem. Chem. Phys. 18 (2016) 14787-14794.

[76]

I. Ahmed, M. Tong, J.W. Jun, C. Zhong, S.H. Jhung, Adsorption of nitrogen-containing compounds from model fuel over sulfonated metal–organic framework: contribution of hydrogen-bonding and acid–base interactions in adsorption. J. Phys. Chem. C 120 (2016) 407–415.

[77]

J.Y. Song, I. Ahmed, P.W. Seo, S.H. Jhung, UiO-66-type metal-organic framework with free carboxylic acid: versatile adsorbents via H-bond for both aqueous and non-aqueous phases, ACS Appl. Mater. Interfaces 8 (2016) 27394–27402.

[78]

M. Sarker, J.Y. Song, S.H. Jhung, Carboxylic-acid-functionalized UiO-66-NH2: A promising adsorbent for both aqueous- and non-aqueous-phase adsorptions, Chem. Eng. J. 331 (2018) 124–131. 23

[79]

J.C. Ma, D.A. Dougherty, The cation-π interaction. Chem. Rev. 97 (1997) 1303−1324.

[80]

J.Y. Lee, S.J. Lee, H.S. Choi, S.J. Cho, K.S. Kim, T.-K. Ha, Ab initio study of the complexation of benzene with ammonium cations, Chem. Phys. Lett. 232(1995) 67–71.

[81]

.H. Basch, W.J. Stevens, Hydrogen bonding between aromatics and cationic amino groups, J. Mol. Struct. (Theochem) 338 (1995) 303–315.

[82]

I. Ahmed, J.W. Jun, B.K. Jung, S.H. Jhung, Adsorptive denitrogenation of model fossil fuels with lewis acid-loaded metal–organic frameworks (MOFs), Chem. Eng. J. 255 (2014) 623−629.

[83]

B. Liu, Y. Peng, Q. Chen, Adsorption of N/S-heteroaromatic compounds from fuels by functionalized MIL-101 (Cr) metal–organic frameworks: the impact of surface functional groups, Energy Fuels 30 (2016) 5593–5600.

Table Legends: 1. Table 1. Textural properties of MIL-101s and Langmuir parameters for adsorption of IND and QUI over the MIL-101s. 2. Table 2. Maximum adsorption capacity (Q0) for IND and QUI over various adsorbents including MIL-101s.

24

Scheme Legends: Scheme 1. Scheme to show Ade grafting on MIL-101, and additional protonation of Ade-MIl-101. Scheme 2. Plausible mechanisms of IND and QUI adsorption over a) Ade-MIL-101 and b) P-Ade-MIL101.

25

Figure Legends: 1. Fig. 1. (a) XRD patterns, (b) nitrogen adsorption isotherms, (c) FTIR spectra and (d) TGA patterns of the studied MIL-101s. 2. Fig. 2. Effect of amount of applied Ade (in grafting on MIL-101) on the quantities of adsorbed (a) IND and (b) QUI over Ade-MIL-101s. The initial concentration of adsorbates was 1000 mg·L1

and the adsorptions were carried out at 20 oC for 12 h.

3. Fig. 3. Effect of contact time on (a) IND and (b) QUI adsorptions over various adsorbents. The initial concentrations of adsorbates were 1000 mg·L-1 and the experiments were carried out at 20 oC for 12 h. 4. Fig. 4. Adsorption isotherms of (a) IND and (b) QUI over different MIL-101s from model fuel. The adsorptions were carried out at 20 oC for 12 h. 5. Fig. 5. (a) Reusability of P-Ade(1)-MIL-101 for IND (1000 mg·L-1, 12 h) adsorption, after ethanol treatment of used adsorbent. The horizontal line shows the adsorptive performance of fresh activated carbon. (b) FTIR spectra of fresh P-Ade(1)-MIL-101, pure IND, IND-adsorbed PAde(1)-MIL-101 and recycled P-Ade(1)-MIL-101 (from bottom to top of the spectra. Dotted lines highlight the FTIR bands of IND.

26

Table 1. Textural properties of MIL-101s and Langmuir parameters for adsorption of IND and QUI over the MIL -101s.

Material

MIL-101

Ade(1)-MIL-101

P-Ade(1)-MIL-101

SABET

PVtot

(m2·g-1)

(cm3·g-1)

3158

1.77

2563

2422

Qo

b × 10-3

(mg·g-1)

(L·mg-1)

IND

410

8.2

0.998

QUI

422

8.6

0.999

IND

491

14.3

0.999

QUI

466

11.2

0.999

IND

532

19.9

0.999

QUI

511

22.7

0.999

Adsorbate

1.64

1.53

27

R2

Table 2. Maximum adsorption capacity (Q0) for IND and QUI over various adsorbents including MIL-101s.

SABET

Qo (IND)

Qo (QUI)

Adsorbent

Reference (m2·g-1)

(mg·g-1)

(mg·g-1)

AC

2330

118*

145*

[19]

AC

1016

63

64

[20]

UiO-66

982

213

-

[45]

UiO-66-NH2

750

312

-

[45]

MIL-125

1378

264

103

[61]

MIL-125-NH2

1561

502

460

[61]

P-MIL-125-NH2

1413

583

546

[61]

MIL-101

2905

416

446

[65]

GnO/MIL-101

3221

593

484

[65]

MIL-101-ED

2230

336

301

[75]

UiO-66-SO3H (18)

972

239

-

[76]

MIL-101

3158

410

422

this study

Ade(1)-MIL-101

2563

491

466

this study

28

P-Ade(1)-MIL-101

*

2432

532

Converted from mmol·g-1 to mg·g-1.

29

511

this study

H N

Cr

O

O N

Cr

Cr

O

O

N H NH2

N

Cr

Cr

O

O

O

O

N H NH2

N HCl ( 0.01 M)

DMF

+ N

N Cl-

N Cr

N

N

N H

Reflux

NH2 O

O

MIL-101

Adenine

O

O

Ade-MIL-101

P-Ade-MIL-101

Scheme 1. Scheme to show Ade grafting on MIL-101, and additional protonation of Ade-MIl-101.

30

a)

IND

QUI

N N

H

NH N H

N N

NH N

H-bond

H-bond

H

N N

N

H

N

H

Cr

Cr

Ade-MIL-101 surface

b)

IND

QUI

H N Cation-π interaction

N

N H

N N

H

Acid base interaction

NH

NH H

N

H-bond H H

N

N N

N H

Cr

H

N

Cr

P-Ade-MIL-101 surface

Scheme 2. Plausible mechanisms of IND and QUI adsorption over (a) Ade-MIL-101 and (b) P-Ade-MIL101.

31

32

Intensity (a.u.)

P-Ade(1)-MIL-101

Ade(1)-MIL-101

MIL-101 Simulated MIL-101

5

10

1672 1604

15 20 2 theta (deg.)

1216

25

1054

Quantity adsorbed (cm3/g-STP)

(a)

1200

(b) 900 600 300 0 0.00

30

(d) Weight (%)

Intensity (a.u.)

1.00

MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

60 30

MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

1500 1200 -1 Wave number (Cm )

0.25 0.50 0.75 Relative pressure (P/Po)

120

(c)

90

1800

MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

0

900

0

200 400 600 Temperature (oC)

800

Fig. 1. (a) XRD patterns, (b) nitrogen adsorption isotherms, (c) FTIR spectra and (d) TGA patterns of th e studied MIL-101s.

33

500

500

-1

(b) mg.g 12 h,

400

q

q12 h, mg.g

-1

(a)

300

200

0

1 2 3 Adenine content, mmol

4

400

300

200

0

1 2 3 Adenine content, mmol

4

Fig. 2. Effect of amount of applied Ade (in grafting on MIL-101) on the quantities of adsorbed (a) IND and (b) QUI over Ade-MIL-101s. The initial concentration of adsorbates was 1000 mg·L-1 and the adso rptions were carried out at 20 oC for 12 h.

34

600

600

(b)

(a) 450

300

AC MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

150 0

q12 h , mg.g-1

q12 h, mg.g-1

450

0

3

6 9 Time, h

12

300

AC MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

150

15

0

0

3

6 9 Time, h

12

15

Fig. 3. Effect of contact time on (a) IND and (b) QUI adsorptions over various adsorbents. The initial c oncentrations of adsorbates were 1000 mg·L-1 and the experiments were carried out at 20 oC for 12 h .

35

600

600

(a)

(b) 450 -1

qe, mg.g

qe, mg.g

-1

450 300 150 0

150

MIL-101 Ade(1)-MIL-101 P-Ade(1)-MIL-101

0

500

1000 -1 Ce, mg.L

1500

300 MIL-101

Ade(1)-MIL-101 P-Ade(1)-MIL-101

2000

0

0

500

1000 -1 Ce, mg.L

1500

2000

Fig. 4. Adsorption isotherms of (a) IND and (b) QUI over different MIL-101s from model fuel. The ads orptions were carried out at 20 oC for 12 h.

36

600

(b) Intensity (a.u.)

q12 h, mg.g

-1

(a) 450 300 150 0

1456

1800

1st

2nd 3rd 4th Number of cycles

1335 1277

1600 1400 -1 Wave number (Cm )

1200

Fig. 5. (a) Reusability of P-Ade(1)-MIL-101 for IND (1000 mg·L-1, 12 h) adsorption, after ethanol treatm ent of used adsorbent. The horizontal line shows the adsorptive performance of fresh activated carb on. (b) FTIR spectra of fresh P-Ade(1)-MIL-101, pure IND, IND-adsorbed P-Ade(1)-MIL-101 and recycle d P-Ade(1)-MIL-101 (from bottom to top of the spectra). Dotted lines highlight the FTIR bands of IND.

37

38

39