Computational Study on the Adsorption of

Jan 16, 2014 - Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education & School of Chemistry and. Chemical ...
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A Combined Experimental/Computational Study on the Adsorption of Organosulfur Compounds over Metal−Organic Frameworks from Fuels Luoming Wu, Jing Xiao,* Ying Wu, Shikai Xian, Guang Miao, Haihui Wang, and Zhong Li* Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education & School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China 510640 S Supporting Information *

ABSTRACT: This work investigates the adsorption of organosulfur compounds in model fuels over metal−organic frameworks (MOFs) using a combined experimental/ computational approach. Adsorption isotherms of three MOFs, MIL-101(Cr), MIL-100(Fe), and Cu-BTC, follow the Langmuir isotherm models, and Cu-BTC shows the highest adsorption capacity for both dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), ascribing to the highest density of adsorption sites and fairly strong adsorption sites on Cu-BTC. Experimental results show adsorption selectivity of various compounds in model fuels follows the order of quinoline (Qu) > indole (In) > DBT > 4,6-DMDBT > naphthalene (Nap), which is consistent with the order of calculated binding energies. Adsorption capacities of thiophenic compounds decrease significantly with the introduction of Qu, In, or water due to their strong competitive adsorptions over the coordinatively unsaturated Cu sites on Cu-BTC. The binding energies of Qu, In, H2O, and DBT are calculated as −56.04, −41.01, −50.27, and −27.52 kJ/mol, respectively. The experimental and computational results together suggest that the adsorption strength of thiophenic compounds over Cu-BTC is dominated by the interaction of both the conjugated π system (π-M) and the lone pair of electrons on sulfur atom (σ-M) of thiophenes, with the coordinatively unsaturated sites (CUS) on Cu-BTC. Alkyl groups on 4- and/or 6-positions of thiophenic compounds function as both eletron donor to increase π-M interaction and steric inhibitor to decrease σ-M interaction. MOFs with strong and highly dense CUS can be promising materials for ADS of fuels.

1. INTRODUCTION Sulfur impurities in diesel fuel convert to SOX after combustion, which not only cause acid rain but also poison the catalysts in the exhaust gas converter for abating CO, NOX, and particulate matter.1−3 Therefore, sulfur content in diesel has been regulated by the governments worldwide. Moreover, as a preferable feedstock for some on-site and on-board fuel-cell applications,4 ultraclean diesel fuel is required since sulfur concentration higher than 1 ppmw in diesel leads to the poisoning of reforming catalysts, water-gas-shift catalysts, and electrodes of fuel cells.5 Therefore, deep desulfurization of diesel fuel has attracted great attention from both industrial and academic communities.6 Adsorptive desulfurization (ADS), on the basis of the ability of a solid adsorbent to selectively adsorb organosulfur compounds,7 is considered to be a promising approach for producing ultraclean fuels. To date, various ADS adsorbents have been designed with unique adsorption mechanisms and further studied for desulfurization of fuels. Yang5,8 studied Cu+ and Ag+ zeolites as π-complexation adsorbents for desulfurization of diesel fuel. The adsorption mechanism states that a π orbital of thiophene donates electron density to the vacant s © 2014 American Chemical Society

orbital of metals, and the d orbital of metals back-donates electron charges to the π* orbital of thiophene. Milenkovic9 and Sevignon10 studied π-acceptor adsorbents for desulfurization, which form charge-transfer complexes with polyaromatic thiophenic compounds. Xiao et al.11,12 studied soft acid metal ion-based adsorbents for the effective removal of thiophenic compounds calculated as soft bases. Sentorun-Shalaby13 investigated mesoporous molecular sieve supported Ni0 sorbents for desulfurization via direct Ni−S chemisorption. Zhou14,15 studied oxygen-functional carbon adsorbents for desulfurization and suggested that the adsorption of the sulfur compounds over activated carbon from the liquid hydrocarbon fuel may involve an interaction of the acidic oxygen functional groups on carbon with the sulfur compounds. Bandosz16 prepared polymer-derived carbons for desulfurization and reported that, in addition to adsorption, oxidation or reactive adsorption of dibenzothiophenes may also occur with the presence of the acidic groups on carbons. Xiao et al.3,17 studied Received: November 27, 2013 Revised: January 12, 2014 Published: January 16, 2014 1080

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computational and experimental results are further correlated to illustrate the adsorption mechanistics.

TiCeO-based adsorbents for desulfurization under ambient conditions and reported significant promotion effects of air and light-irradiation of fuel on ADS, which can be attributed to oxidation or chemisorption of thiophenic compounds in situ. ConocoPhillips developed the S-Zorb process for gasoline desulfurization. The sulfur atom of the thiophenic compounds adsorbs and reacts with the sorbent (reduced metal-based) to become metal sulfide, and the regenerated sorbent can be recycled by burning off the sulfur to SO2 and then reducing the sorbent by hydrogen.18 Clariant Süd-Chemie developed a desulfurization process combining a reaction using a pretreatment catalyst, i.e. hydrogenation to hydrogenate sulfur to H2S, followed by adsorption with the ActiSorb (ZnO-based) adsorbent.19 Metal−organic frameworks (MOFs) are a new class of porous, crystalline materials that have attracted much attention.20 Possessing a large surface area, tunable pore size and shape, adjustable composition, and functional pore surface, MOFs show great promise for use in adsorption and catalysis.21 With respect to desulfurization, Matzger22 reported UMCM150 with a high capacity of 41 mg S/g sorb from a high sulfur fuel of 1500 ppmw S and suggested that pore size and shape played an important role for ADS. Zhang23 reported Cu-BTC with a high adsorption capacity of thiophenic compounds of fuel and suggested the adsorption mechanism could be attributed to a combined effect of many factors, including framework structure, suitable pore size and shape, and exposed Lewis acid sites. Jhung24 reported MIL-47 with a remarkable ADS performance and suggested the type of metal ion in the adsorbent has a dominant effect on the adsorption of benzothiophene. Experimental studies suggested that MOFs can be promising materials for liquid phase desulfurization. However, some mechanistic questions remain unclear: How do various MOFs differ in desulfurization performance? What are the strengths and numbers of adsorption sites on MOFs for ADS? What is the adsorption mechanism of thiophenic compounds over MOFs? How do methyl groups on thiophenic compounds affect ADS on MOFs? How do other types of compounds, i.e. aromatics, nitrogen compounds, moisture in fuel, affect ADS capacity and selectivity? Answering these questions is crucial for improved design of ideal MOF materials for ADS from real fuel. Herein, a combined experimental/computational approach was used to gain further understanding of the adsorption of thiophenic compounds over MOFs. Three MOFs, MIL101(Cr), MIL-100(Fe), and Cu-BTC were synthesized by hydrothermal methods. The crystalline structures and textural properties of MOFs are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and N 2 adsorption test. The adsorption isotherms of thiophenic compounds over MOFs are studied in a batch adsorption system, from which the strength and number of adsorption sites on MOFs are derived. Adsorption selectivities of various thiophenic compounds as well as other types of compounds (naphthalene, nitrogen compounds, moisture) were studied in both single-compound and mixed-compounds systems. Computationally, adsorption strengths on hypothesized adsorption sites of Cu-BTC for the adsorption of thiophenic compounds were calculated and compared. Adsorption configurations, coordination geometries, and binding energies of various types of thiophenic compounds/aromatics/nitrogen compounds/H2O over Cu-BTC are calculated using DFT. The

2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. MOF Syntheses and Characterization. Three MOF materialsMIL-101(Cr), MIL-100(Fe), and Cu-BTCare synthesized by hydrothermal methods. The detailed synthesis methods of MIL-101(Cr), MIL-100(Fe), and Cu-BTC are reported elsewhere.25−27 The MOFs were activated at 180 °C under vacuum to remove the coordinated water molecules/solvents at the metal sites but not destruct the frame structures of MOFs (referred to the TG plots25,28,29) and sealed in a desiccator before use. Characterizations of MOFs by N2 adsorption, SEM, and XRD are described in the Supporting Information. For H2O preadsorbed on Cu-BTC, 1 g of vacuum-dried Cu-BTC samples were placed in a sealed container for 0.5 h with relative humidity (RH) of 32.8% controlled by a MgCl2 saturated solution.30 After that, the treated Cu-BTC samples were dried at 180 °C under vacuum and weighted every 0.5 h. The Cu-BTC samples with 18 and 4 wt % of preadsorbed H2O were prepared when the sample weights reached 1.18 and 1.04 g, respectively, and then stored in a dried desiccator for use. 2.2. Model Fuels. The model fuels for the adsorption isotherm study were prepared by adding given amounts (100−400 ppmw) of sulfur compounds (DBT, 98%, or 4,6-DMDBT, 97%) into octane (99%). The single-compound model fuels for adsorption selectivity study were prepared by adding each single sulfur compound into octane as one model fuel, where the same molar concentration (3.12 μmol/g) is maintained for comparison. The mixed-compound model fuel for adsorption selectivity study was prepared by adding the same molar concentration (3.12 μmol/g) of each compound, including benzothiophene (BT, 99%), DBT, 4,6-DMDBT, quinoline (Qu, 98%), indole (In, 98%), and naphthalene (Nap, 99%) into octane solvent. All the chemicals were purchased from Sigma-Aldrich and were used as such without any purification. 2.3. Adsorption Experiments. For adsorption in a stirred batch system, about 5 mL of model fuel (MDF) and 0.05 g of adsorbent were added into a glass tube. Three kinds of MDFs were prepared: MDF1 - 100 ppmw single model compound model fuel; MDF2 - a mixed model fuel with 100 ppmw of BT, DBT, and 4,6-DMDBT; MDF3 - a mixed model fuel with 100 ppmw S/N of BT, DBT, 4,6DMDBT, In, Qu, and Nap. The tube was capped and placed in a batch system using a temperature-controlled shaker at room temperature. After the desired time was reached, 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 sulfur concentrations in the treated model fuel samples were analyzed by a high-performance liquid chromatogram (HPLC) equipped with a UV−vis detector and an ODS-C18 column, and the flow rate was 1.0 cm3 min−1. The concentrations of other compounds in the treated model fuel samples were analyzed by a gas chromatograph (Varian CP3800) equipped with a FID. To facilitate the quantitative analysis and discussion of the adsorptive selectivity of MOF materials for each compound, a selectivity factor14 was used in the present study and defined as

αi − r =

qi /qr Ce , i/Ce , r

(1)

where qi and qr are the adsorptive capacities of adsorbent for compound i and reference compound r (Nap); Ce,i and Ce,r are the concentrations of compound i and reference compound r, respectively, in liquid phase at equilibrium. 2.4. Computational Methods. 2.4.1. Model Constructions. Three types of MOF crystal structures, Cu-BTC, MIL-101(Cr), and MIL-100(Fe) were taken from the Cambridge Crystallographic Data Centre (CCDC).25,29,31 These MOFs were modeled by all-atom rigid frameworks of single unit cell, as illustrated in Supporting Information Figure S4. Cu-BTC possessed a face-centered-cubic crystalline 1081

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Figure 1. Adsorption isotherms of (a) DBT and (b) 4,6-DMDBT over MIL-101(Cr), MIL-100(Fe), and Cu-BTC. The dashed lines are fitted to Langmuir isotherms. The standard deviation in some data is smaller than the marker sizes. structure, and the main structural characteristic was the paddle-wheel unit assembled from a copper dimer with Cu−Cu distance of 2.63 Å and four BTC linkers.31 Two types of cages, the tetrahedron side pockets (roughly 5 Å diameter with 3.5 Å windows) and the large square-shaped channels (9 × 9 Å),32 are present. MIL-101(Cr) and MIL-100(Fe) were built up from the trimeric Cr(III)/Fe(III) oxide octahedral clusters and the terephthalic/trimesic acid linkers. Composed of supertetrahedra (ST) building blocks, these two MILs contained two types of quasi-spherical mesoporous cages. MIL101(Cr) contained small cages with 29 Å diameter connected through 12 Å pentagonal microporous windows and large cages with 34 Å diameter accessible through both hexagonal and pentagonal windows of 14.7 × 16 Å.33 MIL-100(Fe) is formed by the small cages of 24 Å diameter through 5.6 Å pentagonal microporous windows and the large cages of 29 Å diameter delimited by a 8.6 Å hexagonal aperture.29 Similarly, the metal sites of these three MOFs were covered by a coordinated water molecule from solvent and could be removed by an activation procedure (usually heating in the vacuum) to make the coordinatively unsaturated sites (CUS) available for adsorption. 2.4.2. Binding Energy (BE) Calculations. The MOF structures were optimized prior to the BE calculations. Representative clusters of the MOFs studied were taken from the all-atom frameworks to simulate the local environment of the CUS and to reduce the computational demand. The dangling bonds on these fragmented clusters were terminated by methyl (−CH3) groups to maintain the correct hybridization. All of the calculations in this work were carried out using the Dmol3 module in the Accelrys software package Materials Studio.34 To optimize these clusters, the density functional theory (DFT) method was employed using the gradient corrected (GGA) correlation functional of Perdew and Wang (PW91) with the double numerical plus (DNP) polarization basis set. The convergence threshold parameters for the optimization are set as 2.7 × 10−4 eV (energy), 0.05 eV/Å (gradient), and 0.005 Å (displacement), as reported in the literature.35 The BEs are usually used to estimate the binding strength and the adsorbate−MOF interactions, which were also computed using the GGA/PW91 function with DNP basis sets.36−38 The calculation method used also minimized the basis set superposition error effects (BSSE), which could otherwise strongly decrease the calculation accuracy of BEs obtained from MP2 method with analytical basis set.35 The value of BEs is calculated as the energy difference between the products and the reactants in the adsorption process, as defined in eq 2.

For:

3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms of Thiophenic Compounds over MOFs. To understand the adsorption performance of MOFs for thiophenic compounds in the MDF, three MOFs, MIL-101(Cr), MIL-100(Fe), and Cu-BTC with varied textural properties (Supporting Information Table S1), chemical properties, and significantly different metal centers are chosen to be studied. As alkylated DBTs with alkyl groups at the 4and/or 6-positions are difficult to be removed by conventional hydrodesulfurization (HDS) due to the steric hindrance of alkyl groups.39 Two thiophenic compounds, DBT and 4,6-DMDBT with and without the methyl groups are compared. Figure 1 shows the adsorption isotherms of DBT and 4,6-DMDBT over the three MOFs. The isotherms obey the Langmuir models and can be represented by the equation qe = (KqmCe)/(1 + KCe), where Ce is the concentration of DBT/4,6-DMDBT in the liquid phase at equilibrium and qe is the equilibrium adsorption capacity of DBT/4,6-DMDBT on the MOF materials. The adsorption equilibrium constant is K, while qm represents the maximum adsorption capacity of DBT/4,6-DMDBT. The plots of Ce/qe versus Ce for the MOFs are shown in Supporting Information Figure S5. For all the MOF samples, good linear relationships were obtained between Ce/qe versus Ce (standard deviation R2 > 90%). The experimental data fit well to the Langmuir adsorption isotherm models, indicating that the adsorption of DBT/4,6-DMDBT can be expressed by the Langmuir adsorption isotherms. Table 1 also lists adsorption parameters based on Langmuir isotherms for the adsorption of DBT and 4,6-DMDBT over Table 1. Adsorption Parameters Based on Langmuir Isotherms for Adsorption of DBT and 4,6-DMDBT over MIL-101(Cr), MIL-100(Fe), and Cu-BTC MOFs MIL101(Cr)

MOFs + adsorbates → MOFs(adsorbates)

⇒BE = EMOFs(adsorbates) − EMOFs − Eadsorbates

MIL100(Fe)

(2)

where EMOFs(adsorbates) is the total energy of the MOFs/adsorbates sorption system in equilibrium state, while EMOFs and Eadsorbates are the total energy of the adsorbate-free MOF structures and the adsorbates, respectively. A negative value of BE suggests an exothermic adsorption of the adsorbate molecule over MOFs. A higher absolute value of BEs indicates a higher adsorption strength.

Cu-BTC

a

1082

K (g/μg)

qm per unit mass (mg/g)

qm per unit area (μmol/m2)

qe at 100 ppmwa (mg/g)

DBT

0.0082

6.57

0.074

2.96

4,6DMDBT DBT

0.0078

5.49

0.062

2.41

0.0033

6.50

0.107

1.62

0.0042

7.94

0.130

2.35

0.0099 0.0092

18.38 9.15

0.561 0.279

9.12 4.38

types of sulfur adsorbate

4,6DMDBT DBT 4,6DMDBT

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Table 2. Adsorptive Capacity and Selectivity for Each Compound in MDF1a adsorbent MIL-101

MIL-100

Cu-BTC

a

Q Ce α Q Ce α Q Ce α

Nap

BT

DBT

DMDBT

In

Qu

26.48 2.75 1.00 27.52 2.74 1.00 30.96 2.69 1.00

25.15 2.77 0.94 17.52 2.88 0.61 25.22 2.77 0.79

78.17 2.02 4.02 45.37 2.48 1.82 143.45 1.10 11.33

67.03 2.18 3.20 61.48 2.26 2.71 105.95 1.63 5.65

202.43 0.27 79.12 158.25 0.89 17.68 194.69 0.38 45.06

214.51 0.10 234.08 200.80 0.29 69.11 218.41 0.04 473.06

Q in μmol-S/g-sorb and Ce in μmol/g.

donate electron to the conjugated π system of DBT and contribute to improved ADS capacity, when the adsorption occurs through conjugated π system of thiophenic compounds to adsorbents, such as activated carbon sorbents.14,15 The varied trends of adsorption capacity of DBT versus 4,6DMDBT over different MOFs may indicate varied dominant adsorption configurations of thiophenic compounds. 3.2. Adsorption Selectivity of MOFs for Different Compounds in Model Fuel. Diesel fuel is a mixture of saturates, aromatics, trace amount of S/N compounds, etc.7 Desulfurization performance of adsorbents is not only determined by adsorption capacity of thiophenic compounds, but adsorption selectivity of these thiophenic compounds relative to the other components. The adsorption selectivities of three thiophenic compounds, including BT, DBT, and 4,6DMDBT, two organonitrogen compounds, Qu and In, and Nap as the reference from single-model compound model fuels (MDF1) were examined and are listed in Table 2. The comparison of adsorption selectivities of various S and N compounds over MIL-101(Cr), MIL-100(Fe), and Cu-BTC is shown in Figure S7. For each of the thiophenic compounds, selectivity follows the order of Cu-BTC > MIL-101(Cr) > MIL100(Fe). Cu-BTC shows the highest ADS capacity among the three MOFs as mentioned in section 3.1 and also demonstrates the highest ADS selectivity. For each MOF material, the adsorption selectivity follows the order of DBTs > Nap > BT, which is consistent with the order of their π electron numbers of the adsorbates (13, 10, and 9, respectively), suggesting the conjugated π electrons on thiophenic compounds contribute to its adsorption over MOF materials. This could be due to the π electron−adsorbent interaction, similar as ADS over carbon materials.7 Two featuresconjugated π electrons and lone pair of electrons on S atom on thiophenic compounds can contribute to its adsorption. Alkyl groups on thiophenic compounds cause variation in π interaction and steric hindrance effect. For MIL101(Cr) and Cu-BTC, the adsorption selectivity follows the order of DBT > 4,6-DMDBT, indicating the methyl groups on S compound have a negative effect on its adsorption, which further suggests that the electron transfer from the S atom plays a predominant role in the adsorption. For MIL-100(Fe), the adsorption selectivity follows the order of 4,6-DMDBT > DBT, suggesting π-interaction plays a more significant role. Therefore, both π electrons and lone pair of electrons contribute to its interaction with MOF materials for the adsorption of thiophenic compounds over MOFs. The dominant interaction force depends on the types of MOFs used. When the adsorption of organonitrogen compounds is compared to thiophenic compounds, the selectivity follows

MIL-101(Cr), MIL-100(Fe), and Cu-BTC, where adsorption parameters K and qm were estimated from a linear regression of Ce/q versus Ce. Among the three MOF materials, MIL-100(Fe) shows the lowest K value (0.0033 g/μg for DBT and 0.0042 g/ μg for 4,6-DMDBT), suggesting MIL-100(Fe) may have sites which offer the weakest adsorption for thiophenic compounds, while Cu-BTC shows the highest qm values on both weight basis and surface area basis, suggesting Cu-BTC may have higher density of adsorption sites. Compared to the values reported, Cu-BTC has a slightly higher qm value of 18.38 mg/g than the best carbon aerogel adsorbent (with the qm value of 15.1 mg/g) for DBT reported by Haji et al.40 Meanwhile, both Cu-BTC and MIL-101(Cr) have high K values over 0.008 g/μg for DBT, which is over 4 times higher than that of the activated carbon (0.001 74 g/μg) which had the highest total sulfur uptake of mixed thiophenic compounds reported by Zhou et al.14 Additionally, five regeneration cycles of Cu-BTC adsorbent were carried out by solvent (methanol) washing, followed with vacuum drying/activation at 180 °C. The adsorption performance of the regenerated Cu-BTC (Supporting Information Figure 6) shows that the adsorption capacity of Cu-BTC can be recovered by solvent regeneration, implying a reversible adsorption/desorption characteristic of organosulfur compounds over Cu-BTC. The results indicate that Cu-BTC could be a promising adsorbent for the adsorption of thiophenic compounds. As shown in Table 1, for the adsorption of both DBT and 4,6-DMDBT, Cu-BTC shows higher adsorption capacity compared to MIL-101(Cr) and MIL-100(Fe) at low sulfur concentration (100 ppm), suggesting Cu-BTC is also more suitable for deep desulfurization of fuel. The BET surface area of MIL-101(Cr), MIL100(Fe), and Cu-BTC are 2789, 1907, and 1024 m2/g, respectively, as listed in Table S1. Cu-BTC with the lowest surface area shows the highest adsorption capacity for both DBT and 4,6-DMDBT, suggesting the textural properties may not govern the adsorption of thiophenic compounds. It should be noted in Figure S1, MIL-101(Cr) and MIL-100(Fe) have both micropores (pore size of 1−2 nm) and mesopores (pore size of 2−3 nm), while Cu-BTC is microporous (pore size of 0.5−1 nm), which are accessible to the refractory organosulfur molecules, i.e. DBT (calculated molecule dimension of 3.7 Å × 4.7 Å × 9.1 Å) and 4,6-DMDBT (calculated molecule dimension of 4.2 Å × 5.6 Å × 9.1 Å). It is known that alkyl groups on DBT affect its adsorption capacity over the desulfurization adsorbent in different fashions. The alkyl groups on DBT may hinder ADS sterically and result in reduced ADS capacity, when the adsorption occurs via S atom of thiophenic compounds to ADS adsorbents, such as Nibased sorbents.13 On the other hand, alkyl groups on DBT can 1083

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Figure 2. (a) Adsorption capacity and (b) selectivity of three thiophenic compounds over Cu-BTC from various model fuels: MDF1, MDF2, and MDF3.

the order of Qu > In ≫ S on all the studied three MOFs at low concentrations (100 ppmw). As shown in Figure S7a,b, the adsorption selectivity for N compounds is an order of magnitude higher than thiophenic compounds, suggesting N compounds adsorb much more strongly over MOFs. The adsorption mechanisms of different compounds in model fuel over MOFs are further analyzed using DFT in section 3.4. 3.3. Effect of Organonitrogen (N-) Compounds and Moisture on the Adsorption of Thiophenic Compounds over Cu-BTC. Among the three MOF materials studied, CuBTC shows the highest adsorption capacity and selectivity to thiophenic compounds. To understand the effect of Ncompounds on the adsorption of thiophenic compounds over Cu-BTC, adsorption capacity and selectivity of three thiophenic compounds, including BT, DBT, and 4,6-DMDBT, are examined in various mixed model fuels with and without additional N-compounds. Adsorptive capacity and selectivity for each compound in MDF3 are listed in Table S2. Similar as MDF2, similar selectivity order (Qu > In ≫ S) is observed in the mixed MDF (MDF3). The comparison of adsorption capacity and selectivity of thiophenic compounds over Cu-BTC of various MDFs are shown in Figure 2. It can be noted in Figure 2a that the adsorption capacity follows the order of MDF1 ∼ MDF2 > MDF3 over Cu-BTC. Slightly decreased adsorption capacity in MDF2 compared to that in MDF1 can be due to the mild competitive adsorption from Nap in MDF2. Moreover, comparable adsorption capacity in MDF1 and MDF2 suggests similar adsorption behaviors of various sulfur compounds (BT, DBT, and 4,6-DMDBT) in MDF1 and MDF2 over Cu-BTC. In contrast, a much lower capacity of thiophenic compounds (DBT and 4,6-DMDBT) is noted in MDF3, suggesting the strong inhibition from N-compounds, indole, and quinoline to the adsorption of thiophenic compounds. In Figure 2b, adsorption selectivity of thiophenic compounds (DBT and 4,6-DMDBT) in MDF3 drops dramatically compared to that in MDF2, which further suggests a strong inhibition from N-compounds to the adsorption of thiophenic compounds, which may be ascribed to the competition between N- and S-compounds for shared adsorption sites on Cu-BTC. Besides N-compounds, trace amounts of O-containing additives and moisture are present in real fuels.41 H2O is reported to be strongly adsorbed on Cu sites of Cu-BTC,42,43 which may strongly affect the adsorption of thiophenic compounds. Figure 3 shows adsorption isotherms of DBT over Cu-BTC with 0, 4, and 18 wt % of preadsorbed H2O contents. After the adsorption of moisture on Cu-BTC, the color of Cu-BTC changes from dark blue to hydrated light blue,

Figure 3. Adsorption isotherms of DBT over Cu-BTC and H2O-pre adsorbed Cu-BTC with H2O content of 0, 4, and 18 wt %.

as illustrated in Figure 3. With 4 wt % of adsorbed H2O, CuBTC had a dramatically reduced ADS capacity to almost ineffectiveness. The ADS capacity was observed to be further reduced with 18 wt % of preadsorbed H2O on Cu-BTC, but the reduction was not significant. The effects of N-compounds and H2O on DBT adsorption are further understood in section 3.4. 3.4. Molecular Calculations of Adsorption Thermodynamics. 3.4.1. Adsorption of Various Compounds over CuBTC. To understand the adsorption mechanism, adsorption selectivity, and competitive adsorptions of various compounds in fuel over Cu-BTC, DFT was used to examine the adsorption sites, configurations, and adsorption energies of different compounds over Cu-BTC. Three adsorption sites on CuBTC, the Cu metal site (M-site), the O atom site (O-site), and the phenyl linker site (L-site), as demonstrated in Figure 4a, are considered. The adsorption configuration and BEs of DBT on these adsorption sites of Cu-BTC are shown in Figure 4. The BEs on M-, O-, and L-sites are −27.52, −8.86, and −21.71 kJ/ mol, respectively, following the order of M-site > L-site > Osite. The L-site is directly linked to three M-sites on Cu-BTC, making it difficult to be accessed by a DBT molecule when the neighboring adsorption sites are occupied. This is possibly due to the bulky size of the DBT, which is bigger than the distance of neighboring adsorption sites (distance of neighboring Msites of 8.00 Å), as illustrated in Figure S8. Therefore, only the M-sites on Cu-BTC are considered for the adsorption in the following discussion. It can be also noted that the adsorption mainly occurs on the coordinatively unsaturated metal sites (CUS) specifically, as shown in Figure 4b, similar as the dominant adsorption sites for CO244 and H2O42 adsorption. It should be mentioned that since the size of cluster for DFT calculation is small, the effects of structural properties and 1084

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Figure 4. (a) Three adsorption sites (M-, O-, L-) on Cu-BTC. Adsorption configuration and BEs of DBT on (b) M-, (c) O-, and (d) L-sites of CuBTC.

Figure 5. Adsorption configuration and BEs of various compounds on Cu CUS site of Cu-BTC: (a) DBT, (b) 4,6-DMDBT, (c) Qu, (d) In, (e) Nap, and (f) H2O.

DBT through H-metal site interaction with BE of −41.01 kJ/ mol as shown in Figure 5d, which is weaker than N-metal site interaction in quinoline adsorption over Cu-BTC. The calculation results suggest a strong competitive adsorption between N-compounds (quinoline and indole) and thiophenic compounds over the M-sites on Cu-BTC, which is consistent with the experimental results of strong inhibiting effect of Ncompounds on the adsorption of thiophenic compounds in section 3.3. The adsorption of different types of N-compounds over MOFs will be further discussed in future studies. Figure 5f shows adsorption configuration and BE of H2O on Cu CUS of Cu-BTC. The adsorption of H2O on CUS is much stronger (−50.27 kJ/mol) than DBT. It should be noted that a low BE (−4.74 kJ/mol) is present on H2O-saturated CUS, significantly less exothermic than the CUS site. The computational results are consistent with the experimental results of strong inhibiting effect of preadsorbed H2O on Cu-BTC on ADS in Figure 3, which further consolidates the observation that the interaction between DBT and Cu-BTC predominantly goes through Cu CUS sites on Cu-BTC. BEs of Cu-BTC to Ncompounds and H2O on CUS are as high as −40 to −60 kJ/

neighbor molecules on site are difficult to be examined and can be further studied in the future work. To understand the adsorption selectivity of different compounds over Cu-BTC, the adsorption of DBT, 4,6DMDBT, Qu, In, Nap, and H2O on M-site (Cu CUS site) of Cu-BTC is examined. Figure 5 shows adsorption configurations, coordination geometries, and BEs of these molecules on Cu CUS site of Cu-BTC. The calculated BEs follows the order of Qu > In > DBT > 4,6-DMDBT > Nap, consistent with the adsorption selectivity order of these compounds in Table 2. Naphthalene adsorbs almost parallel to the metal−oxygen plane when adsorbed over Cu-BTC, as the interaction is through its conjugated π system. The adsorption of naphthalene is the least exothermic with BE of −20.85 kJ/ mol. Quinoline and indole are almost completely perpendicular (88.3° for indole and 86.8° for quinoline) to the metal−oxygen plane when adsorbed over Cu-BTC, suggesting N-site or NHsite, rather than the conjugated π system dominates the adsorption. The adsorption of quinoline is the most exothermic with BE of −56.04 kJ/mol, which can be attributed to the strong basicity of N atom compared to S. Indole interacts with 1085

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Figure 6. Adsorption configurations and BEs of DBT on CUS, and density of CUS (NCUS) on (a) Cu-BTC, (b) MIL-101(Cr), and (c) MIL100(Fe).

Figure 6 shows the adsorption configurations and BEs of DBT on CUS and density of CUS on the three MOFs. The CUS density is calculated based on one CUS per metal site of MOF.47 The adsorption of DBT over MIL-100(Fe) is the least exothermic, in good agreement with the lowest K value in Table 1 from the adsorption isotherm study. Meanwhile, CuBTC has the highest density of CUS (4.95 μmol/g) on the weight basis, consistent with its greatest qm value in Table 1. Even though the adsorption of DBT over Cu-BTC is slightly less exothermic than MIL-101(Cr), Cu-BTC has a higher density of CUS for DBT adsorption, resulting in a higher DBT adsorption capacity of Cu-BTC compared to MIL-101(Cr) in Table 1. The computational results further confirm that the adsorption of thiophenic compounds over MOFs can be determined by both the adsorption strength of CUS and the density of available CUS on MOFs. It should be noted that the adsorption strength of the CUS on different MOFs vary with not only the adsorbate molecules, but the properties of metal centers,48 such as Lewis acidity, coordination geometry, etc.

mol, much stronger than to thiophenic compounds (−20 to −30 kJ/mol), which may be attributed to the greater electronegativity of N and O atoms than S. Therefore, for ADS of real fuel with MOF, it can be expected that the presence of trace amounts of O-containing organic additives to enhance fuel quality41 could inhibit ADS strongly due to the strong competitive adsorption. It should be noted that the significant inhibition effects of N-compounds and H2O on the adsorption of organosulfur compounds over carbon7 and πcomplexation28 adsorbents of fuels were reported previously in the literature, which were also suggested to be present when applying MOFs as ADS adsorbents in this study. A guard adsorbent bed (e.g., O-functionalized activated carbon45,46) to preremove N-/O-compounds in fuel may be employed to mitigate their strong inhibitions on ADS over MOFs. Both DBT and 4,6-DMDBT adsorb on the Cu metal site forming inclination angles (35.7° for DBT in Figure 5a and 26.2° for 4,6-DMDBT in Figure 5b), suggesting both conjugated π system and lone pair of electrons on sulfur atom contribute to the adsorption of thiophenic compounds over the metal sites. In this case, methyl groups on 4,6-positions of DBT act as both electron donor to enhance π-adsorbent interaction and steric hinder to reduce S-adsorbent interaction. The BE of 4,6-DMDBT (−25.71 kJ/mol) is slightly less exothermic than DBT (−27.52 kJ/mol), suggesting a stronger adsorption of DBT than 4,6-DMDBT, which coincides with a higher adsorption selectivity of DBT than 4,6-DMDBT experimentally in Figure S7, suggesting the steric hindrance effect is slightly stronger than the positive electron donating effect of two methyl groups in 4,6-DMDBT adsorption. The adsorption strength of different adsorbate molecules on the Msite of Cu-BTC can be governed by the interaction strengths of either π-Cu CUS interaction or σ (lone pair of electrons on S/ N)−Cu CUS interaction, or both interactions, depending on the properties of the adsorbate molecules, such as Lewis basicity, conjugated π electron numbers, steric effects, etc. The adsorption of thiophenic compounds over Cu-BTC is illusrated in Figure S9. Alkyl groups on 4- and/or 6-positions of thiophenic compounds function as both eletron donor to increase π-M interaction and steric inhibitor to decrease σ-M interaction. The comprehensive alkyl effect may vary depending on numbers and chain lengths of the alkyl groups on 4- and 6positions. 3.4.2. Adsorption of DBT on the Metal CUS Sites of MOFs. The adsorption of DBT on the metal sites of Cu-BTC, MIL101(Cr), and MIL-100(Fe) is further studied and compared.

4. CONCLUSIONS A combined experimental/computational study on the adsorption of thiophenic compounds over Cu-BTC, MIL101(Cr), and MIL-100(Fe) from model fuels is carried out. Adsorption isotherms of DBT/4,6-DMDBT over MOFs follow the Langmuir adsorption isotherm models. Among the studied three MOFs, Cu-BTC possesses the highest density of adsorption sites which offer a strong adsorption to thiophenes, resulting in the highest adsorption capacity for thiophenic compounds. Experimental results show adsorption selectivity follows the order of Qu > In > DBT > 4,6-DMDBT > Nap, which is consistent with the order of calculated BEs. Adsorption capacities of thiophenic compounds decrease significantly with the introduction of quinoline, indole, or H2O in MDF. The BEs of quinoline, indole, H2O, and DBT over the coordinatively unsaturated sites on Cu-BTC are calculated as −56.04, −41.01, −50.27, and −27.52 kJ/mol, respectively, corroborating the experimental observation that the strong competition on the adsorption of thiophenic compounds over Cu-BTC. The combined experimental and computational results suggest that the adsorption of thiophenic compounds over Cu-BTC is dominated by the interaction between both conjugated π system (π-M) and lone pair of electrons on sulfur atom (σ-M) of thiophenic compounds and the coordinatively unsaturated sites (CUS) on Cu-BTC. Alkyl groups on 4- and/or 6-positions 1086

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of thiophenic compounds increase the eletron density of the adsorbate to increase π-M interaction and also increase the steric hindrance to decrease the σ-M interaction, which could be affected by the numbers and chain lengths of the alkyl groups. MOFs with strong and highly dense CUS can be promising materials for ADS of fuels, and such MOFs (e.g., MOF-74) can be further developed for ADS based on DFT screening.

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ASSOCIATED CONTENT

S Supporting Information *

Textural properties (Table S1), adsorptive capacity and selectivity for each compound in MDF3 (Table S2), pore size distributions (DFT) (Figure S1), XRD (experimental) and PXRD (simulated) patterns (Figure S2), SEM images (Figure S3) of MIL-101(Cr), MIL-100(Fe), and Cu-BTC; the unit cells of Cu-BTC, MIL-101(Cr), and MIL-100(Fe) for calculations (Figure S4); plots of Ce/q versus Ce for DBT and 4,6-DMDBT adsorption over MIL-101(Cr), MIL-100(Fe), and Cu-BTC (Figure S5); adsorption performance of Cu-BTC in five regeneration cycles (Figure S6); adsorption selectivities of various S/N-compounds over MIL-101, MIL-100, and Cu-BTC using Nap as the reference: (a) thiophenic compounds; (b) organonitrogen compounds (Figure S7); size of DBT molecule compared to the distances of M- and L-sites in Cu-BTC (Figure S8); interaction mechanism between thiophenic compounds and Cu-BTC (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.X.). *E-mail [email protected] (Z.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the research grants provided by the National Natural Science Foundation of China (21306054, 21225625), Guangdong Natural Science Foundation (S2013040014747), Specialized Research Fund for the Doctoral Program of Higher Education (20130172120018), and Fundamental Research Funds for the Central Universities (2013ZM0047).

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