Effect of N-Types and Adsorption Mechanisms - American Chemical

Article pubs.acs.org/JPCC

Adsorptive Denitrogenation of Fuel over Metal Organic Frameworks: Effect of N‑Types and Adsorption Mechanisms Ying Wu, Jing Xiao,* Luoming Wu, Ma Chen, Hongxia Xi, Zhong Li, and Haihui Wang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education and School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China 510640 S Supporting Information *

ABSTRACT: This work investigates adsorptive denitrogenation (ADN) of fuels over metal organic frameworks using a combined experimental/ computational approach. MIL-101(Cr) shows high ADN capacities at low concentrations, ascribing to the sites on MIL-101(Cr) offering the strongest adsorption. Adsorption capacity of MIL-101(Cr) is higher for basic quinoline than that for nonbasic indole due to a greater adsorption strength of quinoline (−61.31 kJ/mol) than indole (−38.33 kJ/mol). Adsorption selectivity of various types of compounds in fuels follows the order of organonitrogen ≫ organosulfur > naphthalene, in good agreement with the order of adsorption strength as BEN (−62 ∼ −34 kJ/mol) < BES (−32 ∼ −24 kJ/mol) < BENap (−21.65 kJ/mol), suggesting MIL-101(Cr) is a highly selective adsorbent for ADN. ADN is negligibly affected by polyaromatic hydrocarbons, but suppressed by oxygenate cosolvent, that is, tetrahydrofuran to varied extents, depending on the varied adsorption mechanisms affected by N-types, including N-basicity, positive charge on H bound to N, and H-substitution.

1. INTRODUCTION Ultradeep desulfurization1 of diesel fuel has attracted widespread attention due to the environmental concerns.2 Sulfur contents in fuels have been regulated to be lower and lower. Since 2006, U.S. EPA regulations have required the sulfur content in diesel fuel to be less than 15 ppm by weight (ppmw).3 Removing organosulfur compounds is achieved by catalytic hydrotreating processes in refinery. However, the presence of organonitrogen compounds in oil feedstock strongly inhibits ultradeep hydrodesulfurization (HDS), especially HDS of the refractory sulfur compounds, such as 4,6-dimethyldibenzothiophene(4,6-DMDBT) in diesel.4 Moreover, organonitrogen compounds poison the catalysts in subsequent refining processes, such as isomerization, reforming, catalytic cracking, hydrocracking, and so on.5 In addition, the presence of nitrogen compounds reduces the thermal and oxidative stability of fuels.6 Deep denitrogenation is further urged by the demand toward zero-sulfur fuel for on-board fuel cell applications.2a Therefore, deep denitrogenation of diesel fuel has attracted great attentions from both industrial and academic communities. Adsorptive denitrogenation (ADN),7 on the basis of the ability of a solid adsorbent to selectively adsorb nitrogen compounds under ambient conditions, is considered to be a promising approach for the removal of organonitrogen compounds from diesel fuel. Adsorbent is the key for ADN. Adsorbents studied include activated carbons,4a,8 silica− alumina,9 and so on. Wen et al.10 showed a greater adsorption affinity of nitrogen compounds than sulfur ones on activated © 2014 American Chemical Society

carbon in which the adsorption is controlled by the intraparticle diffusion rather than the external diffusion. Sano et al.8b reported a dramatically improved adsorption capacity for organonitrogen compounds by introducing specific types of oxygen functional groups on activated carbon by HNO3, H2SO4, or H2O2 oxidation. Almarri et al.4a studied selective ADN over carbon- and alumina-based adsorbents, and reported that the types of oxygen functionalities may be crucial in determining the selectivity for various nitrogen compounds. Li et al.8a further suggested that the strong carboxyls, weak carboxyls, and anhydrides on activated carbons play important roles in determining ADN performance from liquid hydrocarbons. Kim et al.11 reported that basic nitrogen compounds can be effectively removed by activated alumina through acid− base interaction. Rajagopal et al.9 compared alumina, silica and silica−aluminas for ADN, and reported that the denitrogenation selectivity depends only on the concentration of Bronsted acid sites. As a new family of porous adsorbent materials, metal organic frameworks (MOFs)12 have attracted enormous attention. Possessing a large surface area, tunable pore size and shape, adjustable composition and functionalizable pore surface,13 MOFs show great promises for use in adsorption and separation-based technologies, such as high capacity and selectivity CO2 capture,14 reversible H2 storage,15 highReceived: May 9, 2014 Revised: September 2, 2014 Published: September 7, 2014 22533

dx.doi.org/10.1021/jp5045817 | J. Phys. Chem. C 2014, 118, 22533−22543

The Journal of Physical Chemistry C

Article

Table 1. Textural Properties of MIL-101(Cr), MIL-100(Fe), and Cu-BTC

a

MOFs

chemical formulaa

SBET (m2/g)

Smeso (m2/g)

Smicro (m2/g)

Vtotal (cm3/g)

Vmeso (cm3/g)

Vmicro (cm3/g)

pore size (Å)

MIL-101(Cr) Cu-BTC MIL-100(Fe)

Cr3O[C6H4-(CO2)2]3 Cu3[C6H3-(CO2)3]2 Fe3O[C6H3-(CO2)3]2

2789 1024 1894

1710 124 932

1145 900 964

1.32 0.51 0.84

0.99 0.41 0.39

0.42 0.10 0.36

18.9 20.2 18.0

Taken from the Cambridge Crystallographic Data Centre (CCDC).24

selectivity propene/propane separation,16 adsorptive removal of hazardous compounds,17 and so on. In terms of denitrogenation, Maes et al.18 carried out an experimental study using Lewis acidic MOFs for the removal of nitrogen compounds in fuel and suggested that MOFs containing trimers of metal octahedral with Lewis acid sites are suitable adsorbents for selective denitrogenation. Voorde et al.19 studied the role of the metal ion on mesoporous MOF MIL-100 for the adsorption of N/S-heterocyclic contaminants experimentally and suggested that indole adsorb strongly over MIL-100 through coordination on coordinately unsaturated sites. Ahmed et al.20 explored ADN over MIL-101 modified with acidic and basic sites experimentally and suggested that acidic sites improved the adsorption uptake of basic quinoline significantly due to an acid−base interaction. Regeneration studies also suggested that MOFs can be recycled after solvent washing. However, the information on underlying interaction mechanism as well as the selectivity of various compounds in fuel is still insufficient due to the limitations of experimental approach. As is well-known, basic and nonbasic organonitrogen compounds, including quinolines, indoles, carbazoles, and so on, are present in diesel fuel, which could behave differently over MOFs. In addition, the composition of fuels could cause variation in adsorption selectivity. Moreover, mechanistic questions like interaction strengths and adsorption configurations of various organonitrogen compounds as well as other compositions in fuel over MOFs are not clarified, which can be elucidated involving a complementary tool: computational simulation. Answering these questions is crucial for the rational design of MOF materials with ideal ADN performance from real diesel fuel. In this work, we carried out a combined experimental/ computational study to gain fundamental understanding of the adsorption of organonitrogen compounds over MOFs. Three MOFs, MIL-101, MIL-100, and Cu-BTC are synthesized and characterized. The adsorption isotherms of basic quinoline and nonbasic indole over the three MOFs are studied in a batch adsorption system. Adsorption selectivities of four organonitrogen compounds, including quinoline (QUI), indole (IND), carbazole (CAR), and methyl carbazole (MCAR, the H atom bound to N atom in CAR is replaced by a −CH3 group), as well as other types of compounds (naphthalene (NAP), organosulfur compounds of dibenzothiophene (DBT and 4,6-DMDBT)) in diesel are studied in model fuels. The effects of fuel composition, including an aromatic compound (NAP) and an oxygenate cosolvent tetrahydrofuran (THF) on ADN are studied. Computationally, potential adsorption sites on MIL-101 for the adsorption of basic and nonbasic organonitrogen compounds are calculated and compared. Adsorption configurations and binding energies (BEs) of various types of organonitrogen compounds/organosulfur compounds/aromatics over MIL-101(Cr) are calculated. Effects of additional THF on ADN are investigated. The adsorption mechanisms are discussed based on the combined computational/experimental results.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. MOF Syntheses and Characterization. MIL101(Cr), MIL-100(Fe), and Cu-BTC were synthesized by hydrothermal methods.21 The precursors of inorganic and organic units of the MOFs were dissolved and mixed with solvents, and then loaded in a Teflon-lined autoclave. The autoclave sits under a desired temperature for a given period of time, and then cooled down to room temperature. The precipitates were filtered, washed with water or purified with organic solvents, and the crystalline materials of MOFs were obtained. The MOFs were further activated at 180 °C under vacuum to remove the coordinated water molecules/solvents at the metal sites, and sealed in a desiccator before use. The labsynthesized MOFs were characterized by N2 adsorption test,22 XRD, and SEM. With the textural properties, XRD patterns, and SEM images shown in Table 1 and Supporting Information, Figures S2 and S3. The crystalline structures of MOFs are confirmed. Reference adsorbents, including Na−Y zeolite (Beijing Xianfeng Chemicals, SBET of 719 m2/g23), ACF (activated carbon fiber, HN-1, Huaneng China, SBET of 1256 m2/g22c), and AC (activated carbon, maxsorb2, Kaisai Japan, SBET of 2069 m2/g22c) were dried under vacuum at 110 °C overnight before adsorption tests. 2.2. Model Fuels. For adsorption isotherm study, the model fuels (MDF-1s) were prepared by adding given amounts (100−1000 ppm-N) of an organonitrogen compound into octane (99%). For adsorption selectivity study, MDF-2s, MDF3s, and MDF-4s were prepared by dissolving given molar concentration (7.14 μmol/g) of each single compound (QUI 97%, IND 98%, CAR 99%, MCAR 99%, DBT 98%, 4,6DMDBT 97%, NAP 99%) into three types of solvents, including octane, 2 wt % of THF as cosolvent with 98 wt % of octane, and 2 wt % of NAP cosolvent with 98 wt % of octane, respectively. All the chemicals were purchased from Sigma-Aldrich and were used as such without any purification. 2.3. Adsorption Experiments. For static adsorption experiments in a batch reactor, about 5 mL of MDF and 0.05 g of adsorbent were mixed in a sealed glass tube at room temperature until the adsorption equilibrium is reached in 2 h. After that, the mixture was filtered, and the treated MDFs were analyzed to estimate the adsorption uptake and selectivity of the adsorbents for various compounds in the fuel. The nitrogen/sulfur concentrations in the treated MDF samples were analyzed by a high-performance liquid chromatogram (HPLC) equipped with a UV−vis detector and an ODS-C18 column at the flow rate of 1.0 cm3·min−1. The measurement were carried out at 280 nm on UV−vis, in which range the peak of additive THF (absorption wavelength 90% (Supporting Information, Figure S4), suggesting a good coincidence of the experimental data with the Langmuir adsorption isotherms. The results indicate that the adsorption of QUI and IND obeys the Langmuir adsorption models. It should be noted that the adsorption can also be fitted to Freudlich model (Supporting Information, Figure S5 and Table S3). However, the adsorption isotherm data were obtained at ppm concentration range, single-layer adsorption represented by a Langmuir model was assumed, and thus, the parameters from a fitted Langmuir model were used for further comparison and discussion. Two types of organonitrogen compounds are present in fuels, basic compounds, that is, QUI with a lone pair of electrons on the nitrogen atom responsible for the basicity, and nonbasic compounds, that is, IND with the delocalized lone pair of electrons on nitrogen atom conjugated to the aromatic π system. For basic QUI, equilibrium adsorption capacities (Qequil) at lower concentrations (below 50 ppm-N) follow the

Figure 1. Unit cell structure, Cr3O cluster and terephthalic acid linker of MIL-101(Cr) (color code: C, gray; O, red; H, white; Cr, purple).

2.4.2. Computational Details. To accelerate the calculation and simplify the model, the Cr3O cluster and terephthalic acid linker from the unit cell were extracted for BE calculations. The Cr3O cluster retained the chemical environment of the coordinatively unsaturated site (CUS), the similar cluster size of Cu-BTC have been proved to be effective in the energy calculation.27 While the cleaved bonds of the cluster was saturated by methyl (−CH3) groups to maintain the original hybridization.28 The adsorption configuration of the adsorbed guest molecules over the clusters was optimized by the density functional theory (DFT) in Dmol3 code,29 which provided fast convergent three-dimensional numerical integration to compute the matrix elements. The gradient corrected (GGA) correlation functional with Perdew and Wang (PW91) were used. The double-ξ numerical plus polarization (DNP) basis set was employed, which can achieve comparable calculation results as that from the 6-31G(d, p) basis set.26 The parameters for the optimization were 1 × 10−5 kcal mol−1 (energy), 0.0005 kcal mol−1 Å−1 (forces), and 5.0 × 10−6 Å (displacement), respectively. 22535



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concentration ranges (50−300 ppm-N), the Qequil order turns to be MIL-101(Cr) > MIL-100(Fe) > Cu-BTC, consistent with the order of the BET surface area and pore volume of these MOFs (as listed in Table 1), indicating the textural properties may dominate QUI adsorption capacity at high concentrations. In the case of nonbasic IND, a similar trend of MIL-101(Cr) ∼ Cu-BTC > MIL-100(Fe) is found for Qequil at lower equilibrium concentrations (0−50 ppm-N). The highest Qequil on MIL101(Cr) could be due to the highest K value of 2.52 × 107 mg/ g. While at higher equilibrium concentrations (50−300 ppmN), the Qequil order changes to Cu-BTC > MIL-101(Cr) > MIL-100(Fe), which is different from the order of the BET surface area and pore volume. The inconsistency suggests that textural properties of the MOFs may not govern IND adsorption capacity at high concentration. The highest Qequil on Cu-BTC is resulted from the larger number of available adsorption sites on Cu-BTC for IND (qm of 43.96 mg/g as listed in Table 2). It is noticeable that adsorption capacities for basic QUI are higher than nonbasic IND over three MOFs at low equilibrium concentrations, that is, 20 ppm, as listed in Table 2, consistent with the capacity order of QUI > IND on activated carbon.10 Comparing with literatures, K values for both QUI and IND over these MOFs (1 × 107 to 4 × 107 mg/g) are much greater (a magnitude higher) than that for organosulfur compounds over other MOFs (3 × 106 to 1 × 107 mg/g33) and carbon materials (1 × 106 to 6 × 106 mg/g32), suggesting a strong adsorption of organonitrogen compounds in fuels over the MOFs studied. From the perspective of industrial applications, adsorption is specifically suitable for the selective removal of contaminants (i.e., denitrogenation of fuel) at low concentrations. Therefore, MIL-101(Cr) shown high capacities for both QUI and IND at low concentrations are chosen to be further studied. Additionally, compared to some commercial adsorbents, MIL-101(Cr) also shows higher capacities (6.9 and 6.5 mg/g) than Na−Y (0.9 and 3.2 mg/g), ACF (3.1 and 2.9 mg/g), and AC (6.1 and 5.6 mg/g) at initial QUI and IND concentrations of 100 ppm-N. 3.2. Adsorption Selectivity of MIL-101(Cr) for Each Compound in Various MDFs. For an effective adsorbent, adsorption selectivity for the target adsorbate is as important as adsorption capacity. In real fuels, a mixture of saturates, aromatics, and trace amounts of organosulfur and organonitrogen compounds, as well as fuel additives with varied chemical properties, and so on, are present.2c Therefore, adsorption selectivity of adsorbent for organonitrogen compounds relative to others compositions in fuel are critical to determine its ADN performance. The adsorption capacities and selectivities of MIL-101(Cr) for three organonitrogen compounds, including QUI, IND, and MCAR (CAR was not compared as it cannot dissolve in pure

Figure 2. Adsorption isotherms of (a) QUI and (b) IND from MDF1s over MIL-101(Cr), MIL-100(Fe), and Cu-BTC. The solid lines are fitted to Langmuir isotherms.

order of MIL-101(Cr) > Cu-BTC > MIL-100(Fe), as shown in Figure 2a. As listed in Table 2, the adsorption equilibrium constants (K) for QUI adsorption over MIL-101(Cr), Cu-BTC, and MIL-100(Fe) are 3.91 × 107, 2.88 × 107, and 1.02 × 107 mg/g, respectively, following the order of MIL-101(Cr) > CuBTC > MIL-100(Fe). The results suggest that the highest Qequil on MIL-101(Cr) could be due to its strongest adsorption site for QUI, indicated by its highest K value. Qequil at 20 ppm-N in the unit of mg-N/mmol-cus also follows the order of MIL101(Cr) > Cu-BTC > MIL-100(Fe), suggesting coordinatively unsaturated sites (CUS) may play a critical role for the adsorption of QUI over MOFs at low concentrations, which will be further elucidated in Computational Methods. At higher

Table 2. Adsorption Parameters Based on Langmuir Isotherms for Adsorption of IND and QUI over MIL101, MIL100, and CuBTC MOFs

adsorbate

MIL-101(Cr)

QUI IND QUI IND QUI IND

MIL-100(Fe) Cu-BTC

K (mg/g)

qm per unit mass (mg/g)

qm per unit area (umol/ m2)

Qequil at 20 ppm (mg/g)

Qequil at 20 ppm (mg-N/mmol -cus)

× × × × × ×

32.57 25.61 34.48 13.08 19.88 43.96

0.365 0.287 0.565 0.214 0.607 1.341

14.28 8.59 5.86 2.87 7.26 7.71

3.16 1.90 1.17 0.57 1.47 1.56

3.91 2.52 1.02 1.40 2.88 1.06

107 107 107 107 107 107

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Table 3. Adsorptive Capacity and Selectivity for Each Compound over MIL-101(Cr) from Various MDFs MDFs

parameters

NAP

QUI

IND

MDF-2s

Q μmol-N/S/g-sorb Ce μmol/g α Q μmol-N/S/g-sorb Ce μmol/g α

60.52 6.29 1.00 20.52 6.85 1.00

490.31 0.22 234.08 271.52 3.31 27.42

462.69 0.61 79.12 423.47 1.16 121.75

MDF-3s

CAR

MCAR

DBT

DMDBT

400.88 1.48 90.42

158.48 4.90 3.36 105.97 5.65 6.27

178.67 4.62 4.02 123.8 5.39 7.67

153.22 4.98 3.20 80.62 6.01 4.48

IND follow the order of MDF-2 ∼ MDF-4 > MDF-3, suggesting the presence of 2 wt % of NAP cosolvent has negligible impact on ADN capacity, while the presence of THF cosolvent inhibits the adsorption of organonitrogen compounds over MIL-101(Cr), and more strongly for quinoline than indole. The underlying mechanistics will be further elucidated in the following computational section. 3.3. Molecular Simulation of Adsorption Thermodynamics on MIL-101(Cr). 3.3.1. Adsorption of QUI and IND. Adsorption configurations and BEs of QUI and IND on the three potential adsorption sites (M-, O-, and L-) of the MIL101(Cr) are calculated, with the computational results, as shown in Figure 4. Both QUI and IND possess conjugated aromatic rings with π electron numbers of 10, which could cause π−π stacking interaction with the L-site (the organic phenyl linker) on MIL-101(Cr). However, the adsorption strengths of the L-site for both QUI and IND are much weaker compared with those of M-site and O-site; thus, the adsorption of QUI and IND over L-site of MIL-101(Cr) can be neglected when compared to that of M-site and O-site. The result agrees well with the reported calculated radial distribution functions (RDF) results,34 the dominant adsorption site of Tryptophan (a kind of amino acid with similar structure of QUI and IND) on MIL-101(Cr) is through M-site rather than other sites. For the adsorption of QUI, the adsorption strength follows the order of M-site (BE of −63.31 kJ/mol) ≫ O-site (BE of −12.40 kJ/mol), suggesting M-site is the major adsorption site for QUI, and M-site here refers to the coordinatively unsaturated site (CUS) of Cr3+, as illustrated in Figure 4. The driving force of the adsorption is likely through Bronsted base (lone pair of electrons on N atom of QUI) and acid (CUS of Cr3+ in MIL-101(Cr)) interaction, and the O-site was less important. From the calculation of atomic charges reported by Kim et al.,35 the Cr3+ of dehydrated MIL-101(Cr) possesses more positive charge after the coordinated water molecules are removed, and thus, the CUS of Cr3+ is more acidic to adsorb a Bronsted base like QUI. Different from QUI, the optimized position of IND over Msite is not right above the M-site, but shifts to a position inbetween M- and O-sites forming an inclined angle (68.4°), with the most exothermic adsorption of −38.33 kJ/mol, as shown in Figure 4b, IND-M, suggesting both M- and O-sites contributes to the adsorption of IND. The IND-O site interaction is likely to occur between the H atom bond to N in IND and O site (bond distance of 2.02 Å, N−H−O angle close to 0°) for Hbonding interaction. H-bonding conformation is satisfied as the bond distance less than 3.5 Å, and the hydrogen donor− acceptor angle less than 30°).36 The large inclined angle of 68.4° of the optimized adsorption configuration suggested that the planar π−M interaction between a conjugated π system in IND and CUS of Cr3+ on MIL-101(Cr) may not be a major driving force for the adsorption, unlike other adsorbate molecules with aromatic phenyl ring structures adsorbed over

octane), two organosulfur compounds, DBT and DMDBT, and aromatic NAP as the reference compound from MDF-2s using octane as the solvent were examined, and the results are listed in Table 3. The adsorption selectivity follows the order of organonitrogen compounds (234.08 and 79.12 for QUI and IND, respectively) ≫ organosulfur compounds (4.02 for DBT and 3.20 for DMDBT) > NAP (1.00), suggesting MIL-101(Cr) is a highly selective adsorbent for organonitrogen compounds in fuels. As an exceptional case, MCAR has a similar adsorption selectivity (3.36) as organosulfur compounds. Adsorption selectivity of different nitrogen compounds was noted to follow the order of QUI > IND > MCAR. To dissolve CAR into model fuel, 2 wt % of THF was added as cosolvent as MDF-3s, with the corresponding adsorption capacities and selectivities as shown in Table 3. Consistent with the trend observed in MDF-2s, the adsorption selectivity follows the order of organonitrogen compounds (121.75, 90.42, and 27.42 for IND, CAR, and QUI, respectively) ≫ organosulfur compounds (7.67 for DBT and 4.48 for DMDBT) > NAP (1.00). Different from MDF-2s, adsorption selectivity of organonitrogen compounds in MDF-3s follows the order of IND > CAR > QUI > MCAR, suggesting the additional 2 wt % THF cosolvent had a strong impact on ADN selectivity. It is surprising that, after adding THF as cosolvent in MDFs, adsorption selectivity for QUI over MIL-101(Cr) drops dramatically from 234.08 to 27.42; in sharp contrast, the selectivity for IND increases from 79.12 to 121.75, which is even higher than QUI. The reversed selectivity order of QUI and IND suggest the effects of THF on adsorption selectivity vary with the types of organonitrogen compounds, which may ascribe to the varied adsorption configurations due to the varied chemical properties of adsorbate and/or reference compounds. It should be noted that cosolvent of THF cause no deterioration to the MOF structure as it remains similar textural properties after regeneration, as shown in Supporting Information, Table S2. Figure 3 shows ADN capacities of MIL-101(Cr) for QUI and IND from various MDFs. ADN capacities for both QUI and

Figure 3. Adsorption capacities of QUI and IND over MIL-101(Cr) from MDFs. 22537



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Figure 4. Adsorption configurations and the corresponding BEs of (a) QUI, (b) IND at the metal (M-), oxygen (O-), and linker (L-) sites of MIL101(Cr) (color code: C, gray; O, red; H, white; N, blue; Cr, purple).

Figure 5. Adsorption configurations and the corresponding BEs of (a) QUI; (b) IND; (c) CAR; (d) MCAR; (e) DBT; (f) DMDBT; and (g) NAP on MIL-101(Cr) (color code: C, gray; O, red; H, white; N, blue; S, yellow; Cr, purple).

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Figure 6. Electrostatic potential maps (ESP) and the ESP charges on N/S/H atoms of QUI, IND, CAR, MCAR, and DBT.

MOFs,37 suggested by the electron-withdrawing CUS of Cr3+ on MIL-101(Cr), which may interact with electron-donating N in IND, to complement the H-bonding. It is noticeable that the adsorption strength of IND on O-site is just slightly weaker (−37.80 kJ/mol) than that of M-site, with an almost perpendicular configuration (83.8°), as shown in Figure 4b, IND-O, suggesting the IND-O site interaction is thermodynamically favorable and contributes greatly to the overall adsorption energy of IND over MIL-101(Cr). 3.3.2. Adsorption of Different Compounds in Fuel. To understand the adsorption selectivity of different compounds in fuel over MIL-101(Cr), the adsorption of various organonitrgen compounds, including QUI, IND, CAR, and MCAR; organosulfur compounds, including DBT and DMDBT; and reference aromatic compound NAP, on MIL-101(Cr) are examined. Figure 5 shows adsorption configurations, coordination geometries, and BEs of these molecules on MIL-101(Cr). The calculated BEs follow the order of organonitrogen compounds > organosulfur compounds > NAP, which is consistent with the experimental results of adsorption selectivity order in Table 3. The calculation results further consolidate MIL-101(Cr) is a highly selective adsorbent for ADN of fuels. NAP adsorbs almost parallel (4.6°) to the metal−oxygen plane when adsorbed over MIL-101(Cr), as the interaction goes through its conjugated π system. The adsorption of NAP is the least exothermic with BE of −21.65 kJ/mol, thus, its impact on ADN is little, consistent with the similar adsorption capacities for QUI and IND from MDF-2s and MDF-3s, as shown in Figure 3. Both DBT and 4,6-DMDBT adsorb on the CUS of Cr3+ forming inclined angles (39.6° for DBT in Figure 5e and 12.7° for DMDBT in Figure 5f), suggesting both conjugated π system and lone pair of electrons on sulfur atom contribute to the adsorption of organosulfur compounds on MIL-101(Cr), similar as the adsorption of organosulfur compounds adsorbed over carbon adsorbents.32,38 In this case, methyl groups on 4,6-positions of DMDBT act as both electron donor to enhance π-adsorbent interaction and steric hinder to reduce S-adsorbent interaction. The BE of DMDBT (−24.02 kJ/mol) is less exothermic than DBT (−30.52 kJ/ mol), which is consistent with a lower adsorption selectivity of DMDBT than DBT obtained from the experimental results in Table 3, suggesting the negative steric hindrance effect is slightly stronger than the positive electron donating effect of two methyl groups in DMDBT adsorbed over MIL-101(Cr),

consistent with the computational results of strong steric hindrance effects of aromatic compounds when adsorbed over Cu-BTC and CPO-27-Ni reported by Peralta et al.39 The adsorption affinity of various organonitrogen compounds over MIL-101(Cr) follows the order of QUI > CAR > IND > MCAR, with the BEs of −61.31, −42.59, −38.33, −34.02 kJ/mol, respectively as listed in Figure 5. Figure 6 shows the ESP and ESP charges on N/S/H atoms of QUI, IND, CAR, MCAR, and DBT. The highest adsorption strength of QUI should be due to its highest electronegativity (−0.688) on N atom, as shown in Figure 6b. For nonbasic organonitrogen compounds, the higher adsorption strength of CAR can be ascribed to the higher positive charge on H atom in CAR (0.413) than IND (0.364), as shown in Figure 6c and a, resulting in a stronger H-bonding interaction. When the H atom bound to the N atom in CAR is replaced by a −CH3 group (MCAR), the adsorption is the least exothermic among the studied four organonitrogen compounds with the BE of −34.02 kJ/mol. The most stable adsorption configuration of MCAR is through the conjugated π system of MCAR for the π−M interaction. Meanwhile, by replacing the H atom with −CH3 group in MCAR, the hydrogen bonding interaction with O-site of MIL-101(Cr) disappears apparently. It should also be mentioned that the BE of MCAR is more exothermic than that of CAR adsorbed on MIL-101(Cr) through π−M interaction (−23.55 kJ/mol; Supporting Information, Figure S6), suggesting −CH3 group acts as π electron donor to the conjugated π system of MCAR. As shown in Figure 6, the electronegativity on N atom in all the studied organonitrogen compounds is slightly higher than S atom in DBT (−0.028), resulting in higher adsorption affinities of organonitrogen compounds rather than DBTs. Interestingly, the order of adsorption affinity is consistent with the selectivity order of QUI > IND > MCAR in MDF-2s, and QUI > IND in MDF-4s, but contradictory with the selectivity order of IND > CAR > QUI > MCAR from MDF-3s in Table 3. The different trends suggest that a higher adsorption affinity of adsorbate does not necessarily result in a higher adsorption selectivity, which could be strongly influenced by the composition of the solvent used, that is, THF addition in the MDF-3s. It should be mentioned that, in real fuels, oxygenate fuel additives are commonly added to improve fuel properties,40 thus, the impact on ADN over MOFs should be taken into consideration in post-ADN process for on-board fuel cell applications. 22539



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Figure 7. Adsorption configurations and the corresponding BEs of THF at the metal (M-), oxygen (O-), and linker (L-) sites of MIL-101(Cr).

Figure 8. Interaction mechanisms of (a) basic and (b) nonbasic organonitrogen compounds over MIL-101(Cr).

and thus the inhibition to CAR is stronger than IND. In addition, the negative steric effect of adsorbed THF on the neighboring M-sites is stronger for the adsorption of bulky CAR molecule than smaller IND over the O-sites of MIL101(Cr). Therefore, the inhibition of THF to organonitrogen compounds follows the order of QUI > CAR > IND, resulting in the observed selectivity order of IND > CAR > QUI from MDF-3s in Table 3; (d) as both MCAR and THF share the same (and the only type of) adsorption site (M-site) on MIL101(Cr), and the adsorption strength of MCAR (−34.02 kJ/ mol) is much weaker than THF (−60.62 kJ/mol), the adsorption of MCAR is strongly suppressed with the presence of THF in fuel, resulting in a much lower adsorption selectivity of MCAR, consistent with the experimental result (6.27) in Table 3. 3.3.3. Adsorption Mechanisms of Basic and Nonbasic Organonitrogen Compounds. As illustrated in Figure 8a, the interaction between basic organonitrogen compounds, that is, QUI, and MIL-101(Cr) is dominated by the Bronsted base (through lone pair of electrons on N atom in QUI) and acid (CUS of Cr3+ in MIL-101(Cr)) interaction. For nonbasic organonitrogen compounds, (i) IND and CAR with unsubstituted H bond to N atom, Both M-site (CUS of Cr3+) and Osite on MIL-101(Cr) play a role on the adsorption of nonbasic organonitrogen compounds to different extents, likely through hydrogen bonding H−O and N−M electron donor−acceptor interactions, respectively, as illustrated in Figure 8b, Hunsubstituted. If the H atom is bond to N in nonbasic organonitrogen compounds, H−O interaction dominates (α > 45°); (ii) Once the H atom bond to N is replaced by alkyl group, H−O interaction disappears, N−M interaction is

Figure 7 compares adsorption configurations and the corresponding BEs of THF at the metal (M-), oxygen (O-), and linker (L-) sites of MIL-101(Cr). Both O- and L-sites adsorb THF weakly with BEs of −5.63 and −6.67 kJ/mol, respectively, and thus are negligible. At the M-site, the adsorption of THF is most exothermic, with BE of −60.62 kJ/mol, close to that of QUI (−61.31 kJ/mol). The calculation results suggest that the adsorption of THF occurs through Msite, more specifically the CUS of Cr3+ on MIL-101(Cr). As the adsorption of organonitrogen compounds could go through Msites on MIL-101(Cr) majorly (for basic QUI) or partially (for nonbasic IND), the presence of THF addition in the solvent could hinder ADN to different extents, as discussed in the following: (a) as the basic QUI and THF share the same (and the only) adsorption site of CUS of Cr3+ with similar adsorption affinities, the presence of THF strongly inhibits the adsorption of QUI, resulting in a much lower adsorption selectivity of QUI in Table 3; (b) for nonbasic IND, the adsorption can be determined by both H-bonding interaction and N−M interactions, and dominant by H-bonding interaction through the O-site on MIL-101(Cr). That is to say, once the M-sites are occupied by the strongly adsorbed THF molecule, the neighboring O-sites on MIL-101(Cr) are still available to adsorb IND with a high BE of −37.80 kJ/mol, as illustrated in Figure 4, IND-O. Therefore, the inhibition of THF to the adsorption of IND is much weaker than that to QUI, in good agreement with the experimental results; (c) similar explanation can be applied for nonbasic CAR as IND. On the other hand, CAR has a stronger N−M interaction than IND suggested by a higher electronegativity (−0.702 vs −0.452), which is suppressed by the additional THF in fuel, 22540



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weakened with a low electronegativity of N, and π−M interaction dominates (α < 45°), as illustrated in Figure 8b, H-substituted. The presence of alkyl group acts as π electron donor to enhance the π−M interaction. Therefore, the overall adsorption configuration of nonbasic organonitrogen compounds on MIL-101(Cr) is formed in an inclined angle, depending on the positive charges on H atom, electronegativity on N atom, conjugated π electrons, and so on.

Insights on the effects of N-type and fuel composition disclosed in the work may provide guidance for ADN processes with MOFs for ultraclean fuels.

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ASSOCIATED CONTENT

S Supporting Information *

The compositions of the four types of model fuels (Table S1); Textural properties of original MIL-101 and MIL-101 after regeneration (Table S2); Adsorption parameters based on fitted Freudlich isotherms (q = K × P(1/n)) for adsorption of IND and QUI over MIL-101(Cr), MIL-100(Fe), and Cu-BTC (Table S3); The HPLC spectra of IND in MDF-3s before and after the adsorption over MIL-101(Cr) (Figure S1); XRD patterns (Figure S2); SEM images (Figure S3) of MIL101(Cr), MIL-100(Fe), and Cu-BTC; plots of Ce/q vs Ce for quinoline and indole adsorption over MIL-101(Cr), MIL100(Fe), and Cu-BTC (Figure S4); Adsorption isotherms of (a) QUI; (b) IND over MIL-101(Cr), MIL-100(Fe), and CuBTC fitted into Freudlich model (Figure S5); Adsorption configuration and the corresponding BE of CAR adsorbed on MIL-101(Cr) through π−M interaction (Figure S6). This material is available free of charge via the Internet at http:// pubs.acs.org.

4. CONCLUSIONS A combined experimental/computational study on the adsorption thermodynamics of organonitrogen compounds over MOFs from fuels is carried out. The following conclusions can be drawn: (a) Adsorption isotherms of both basic quinoline and nonbasic indole follow the Langmuir isotherm models, and MIL-101(Cr) has a high adsorption capacity at low N-concentrations, ascribing to its strong adsorption sites on MIL-101(Cr). The adsorption capacity for quinoline is higher than that for indole on MIL-101(Cr), consistent with the higher calculated adsorption strength of quinoline (BEquinoline of −61.31 kJ/mol) than indole (BEquinoline of −38.33 kJ/mol). (b) Adsorption selectivity of various types of compounds in fuel follows the order of organonitrogen ≫ organosulfur > naphthalene, in good agreement with the order of calculated adsorption strength as BEN (−62 ∼ −34 kJ/ mol) < BES (−32 ∼ −24 kJ/mol) < BENap (−21.65 kJ/ mol), suggesting MIL-101(Cr) is a highly selective adsorbent for ADN. (c) The adsorption of basic organonitrogen compounds, that is, quinoline, over MIL-101(Cr) is dominated by Bronsted base (through lone pair of electrons on N atom in quinoline) and acid (CUS of Cr3+ in MIL101(Cr)) interaction; the adsorption of nonbasic organonitrogen compounds is determined by both H− O hydrogen bonding and N−M electron donor− acceptor interactions, when the H atom bond to N atom is not substituted (IND and CAR); planar π−M interaction governs when the H atom is substituted (MCAR). The interaction strengths of nonbasic organonitrogen compounds depend on the positive charges on the H atom, electronegativity on the N atom, conjugated π electrons, and so on. (d) ADN is strongly inhibited by THF as oxygenate cosolvent in model fuels, due to a strong competitive adsorption of THF over CUS on MIL-101(Cr) with BE THF of −60.62 kJ/mol. Moreover, adsorption selectivity from THF-added fuels follows the order of indole > carbazole > quinoline, reversed to the order of the calculation BEs. This can be ascribed to the discrepancy in adsorption configurations of different types of organonitrogen compounds compared to THF over MIL-101(Cr), resulting in varied inhibition effects of THF on ADN to different extents. On the other hand, the presence of NAP as cosolvent in MDF-4s has negligible impact on ADN, due to a weaker π−M interaction adsorption of NAP over MIL-101(Cr). The result suggests oxygenate fuel additives rather than polyaromatic hydrocarbons may have a strong impact on ADN for on-board fuel cell applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 87113501. Fax: +86 20 87113513. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to acknowledge the research grants provided by Guangdong Natural Science Foundation (S2013040014747), the National Natural Science Foundation of China (21306054, 21225625, and 21436005), Specialized Research Fund for the Doctoral Program of Higher Education (20130172120018), and Fundamental Research Funds for the Central Universities.

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REFERENCES

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