Adsorption Behavior of Metal–Organic Frameworks for Thiophenic

Article pubs.acs.org/IECR

Adsorption Behavior of Metal−Organic Frameworks for Thiophenic Sulfur from Diesel Oil Hong-Xing Zhang,†,§ Hong-Liang Huang,‡,§ Chun-Xi Li,*,†,§ Hong Meng,§ Ying-Zhou Lu,§ Chong-Li Zhong,‡,§ Da-Huan Liu,‡,§ and Qing-Yuan Yang‡,§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, China § College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡

ABSTRACT: Four metal−organic frameworks (MOFs) are synthesized using two different metal centers and two different organic ligands, viz., Cu3[C6H3(CO2)3]2 (Cu-BTC), Cu[O2C−C6H4−CO2] (Cu-BDC), Cr(OH)[O2C−C6H4−CO2] (CrBDC), and Cr3F(H2O)3O[C6H3(CO2)3]2 (Cr-BTC). Their adsorption behaviors for thiophenic sulfurs in model diesel oils are systematically investigated at mild temperatures and follow the order Cu-BTC > Cr-BDC > Cr-BTC ≫ Cu-BDC. Meanwhile, the adsorption capacity of different sulfur compounds follows the order dibenzothiophene (DBT) > benzothiophene (BT) > 3methylthiophene (3-MT). The MOFs adsorption mechanism is regarded as a combined effect of many factors involving appropriate framework structure, suitable pore size and shape, and exposed Lewis acid site matching the S-compound to be adsorbed. The difference in adsorptive activity among the organosulfurs is mainly ascribed to their π-electron number and the electron density on the S-atom. Finally, the used MOF can be easily regenerated by solvent washing and recycled at least five times.

1. INTRODUCTION Sulfur (S) compounds in fuel oils as environmental pollutants are a major concern due to their exhaust gases containing SOx, which not only contribute to acid rain but are also harmful to human health.1 As a result, more stringent environmental regulations to lower the S-content in fuel oils are continuously being introduced by governments.2 The traditional industrial process is hydrodesulfurization (HDS), which is effective for aliphatic and acyclic S-compounds but less effective for benzothiophene (BT), dibenzothiophene (DBT), and their derivatives.3,4 To achieve ultradeep desulfurization, some nonHDS alternative technologies such as adsorption desulfurization (ADS),5,6 extractive desulfurization (EDS),7,8 oxidative desulfurization (ODS), 9,10 biodesulfurization (BDS), 11 and others12,13 have been proposed. Among the non-HDS alternative technologies, ADS is considered as one of the most promising processes and has attracted much attention due to some advantages, such as mild operation conditions and no need for hydrogen or oxygen.14 Many studies on ADS of transportation fuels have been undertaken to develop adsorbents using activated carbons,15 zeolites,16 mixed metal oxides,17 and clays.18 The adsorption mechanisms have also been explored by various researchers. Yang et al. investigated the adsorption of transition metals (such as Ag+ and Cu+) on exchanged Y zeolite adsorbents under ambient conditions and attributed their high Sadsorption capacity to the π-complexation between the thiophenic S-compounds and metal ions.19 Song and coworkers studied the performance of various transition metalbased adsorbents for selective adsorption desulfurization and proposed that the S-compounds are adsorbed through direct sulfur−metal (S-M) interaction.20 © 2012 American Chemical Society

Metal−organic frameworks (MOFs) are an emerging class of highly porous materials, built from organic linkers and inorganic metal (or metal-containing cluster) nodes, which can exhibit extremely high surface areas, as well as tunable pore size and functionality, and can act as hosts for a variety of guest molecules.21 To date, MOFs have exhibited high adsorption capacities for some gases, such as H2, N2, O2, CO2, and CH4, greatly exceeding those of activated carbons and zeolites.22 Recently, MOFs have been reported to adsorb significant amounts of sulfur. In 2008, Cychosz et al.23 studied the adsorption characteristics of five different MOFs for BT, DBT, and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in model oil and identified some MOFs with S-adsorption capacities exceeding those of zeolites. They suggested that the adsorption capacity of MOF is determined by its pore size and shape, where the interaction between the S-compound and the framework plays a key role. This view was shared by Achmann and co-workers,24 who investigated the adsorptivity of four different MOFs for thiophene in real fuel. However, Khan and co-workers25 argued that the different rates of BT adsorption over MOFs cannot be explained by the pore size because all of the MOFs employed in their research have isotypic structure, but that the specific interaction, such as the acid−base one, is considered as the real determinant. In another study,26 the authors also considered that there is no correlation between the adsorption capacity and pore size, and the real cause was ascribed to the interaction between the S-atom and the surface cations through both S-M bonding and π-complexation. Received: Revised: Accepted: Published: 12449

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Besides, Shi et al.27 found that the adsorption capacity of MOFs can be enhanced through the impregnation of transition metal carbides. In this work, four different MOFs consisting of two different central metals (Cu2+ and Cr3+) and two different organic ligands were prepared to systematically study their adsorption performance for DBT, BT, and 3-methylthiophene (3-MT) from model oil. These two metals are chosen because Cr3+ is a hard Lewis acid site which can be a contrast to the intermediate Lewis acid site of Cu2+. Besides, the synthetic method for these MOFs is well established and reported in the literature.28−31 Although some data for BT or DBT have been given in various literature reports, it is also necessary to investigate the adsorption behavior of the four MOFs in the same batches under the same experimental conditions, because the property of each MOF, such as surface area, pore size, and pore volume, will be different under different synthetic conditions, batches, and raw materials, and the experimental results will also lose their comparability because the experimental conditions are not the same in these literature reports. In addition, research on the regeneration of MOF adsorbents is still scarce. As one of the elemental parts of the ADS process, the regeneration performance of adsorbents should not be neglected because it not only influences the S-removal but also affects its durability. In view of this, the regeneration ability of MOF is also investigated in the present work. The aim of this paper is to investigate the relationship between the composition and structure of MOFs and their adsorptivity, to explore the possibility of using MOFs as efficient adsorbents for desulfurization, and to gain a better insight for the adsorptive desulfurization mechanism of the MOFs.

the measurement of nitrogen isotherms, all samples were previously degassed under vacuum at 150 °C for about 24 h. The specific areas of the samples were determined according to standard BET procedure using nitrogen adsorption data taken in the relative equilibrium pressure (P/P0) range between 0.05 and 0.2 and with a value of 0.162 nm2 for the cross-section of adsorbed nitrogen molecule. 2.4. Adsorption Test. Model diesel oils were prepared by dissolving DBT, BT, or 3-MT in n-octane, respectively, with their initial S-content all being 1000 ppm. All these stock solutions were used directly in the following adsorption experiments. Prior to adsorption experiments, each MOF was degassed under vacuum at 150 °C overnight to remove water and other contaminants. Their desulfurization performances were tested in a glass batch reactor which has a heating jacket (the experimental setup is shown in Scheme 1), at atmospheric Scheme 1. Adsorption Setup: (1) Homemade Glass Batch Reactor; (2) Magnetic Stirring Apparatus; (3) Cooler

pressure under stirring. The model oil (10 g) was mixed with the adsorbent (0.04−0.20 g) in the glass reactor under stirring for 1 h. The reaction temperature was controlled by jacket heating. The liquid phase was then separated from the adsorbents by filtration, and the S-content of the treated oil was determined by a gas chromatography-flame ionization detector (GC-FID) (Shimadzu, GC-2010). The adsorption capacity was calculated by the following formula: W (C0 − C i) × 10−3 Qi = M where Qi is the adsorption capacity of sulfur adsorbed on the adsorbent (mg S·g−1 MOF), W is the mass of model oil (g), M is the mass of the MOF used (g), and C0 and Ci are the initial and final S-concentrations in the model oil (μg/g), respectively. This experiment was repeated for different MOFs, temperatures, and S-compounds. 2.5. Regeneration Experiments. Regeneration of spent MOF was performed by solvent washing. The spent MOF was washed with methanol overnight using a Soxhlet extractor to remove adsorbed S-compounds and then dried under vacuum at 150 °C overnight. The performance of the regenerated MOF was tested as the fresh adsorbent. 2.6. Analytical Methods. Gas chromatography (GC) was used for the quantitative assay of organosulfur in the oil phase. The GC instrument (Shimadzu, GC-2010) was equipped with a SE-54 capillary column (5% phenyl polydimethylsiloxane as stationary phase; 30 m × 0.25 mm i.d. × 0.25 μm film thickness; Lanzhou Institute of Chemical Physics, China) for DBT and BT, and an AT.FFAP capillary column (polyethylene glycol modified with nitroterephthalic acid as stationary phase; 30 m × 0.53 mm i.d. × 1.0 μm film thickness; Lanzhou Institute

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used here were purchased from different companies, specifically, cupric nitrate trihydrate (Cu(NO3)2·3H2O, AR grade) and n-octane (AR grade) from Tianjin Guangfu Technology Development Co. Ltd.; chromium nitrate nonahydrate (Cr(NO3)3·9H2O, AR grade) from Xilong Chemical Co. Ltd.; metallic chromium (Cr, 99.90%) from Sinopharm Chemical Reagent Co. Ltd.; 1,3,5-benzenetricarboxylic acid (H3BTC, 98%) and terephthalic acid (H2BDC, 99%) from TCI Co. Ltd.; hydrofluoric acid (HF, 40%), methanol (AR grade), ethanol (AR grade), and N,Ndimethylformamide (DMF, AR grade) from Beijing Chemical Works; dibenzothiophene (DBT, 99%), benzothiophene (BT, 97%), and 3-methylthiophene (3-MT, 99%) from Acros Organics, USA. All reagents were used as received without further purification. 2.2. Adsorbent Preparation. The four MOFs, i.e., Cu3[C6H3(CO2)3]2 (Cu-BTC, also known as HKUST-1), Cu[O2C−C6H4−CO2] (Cu-BDC), Cr(OH)[O2C−C6H4− CO2] (Cr-BDC, also known as MIL-53), and Cr3F(H2O)3O[C6H3(CO2)3]2 (Cr-BTC, also known as MIL-100), were synthesized and activated according to previous literature reports.28−31 2.3. Characterization. The powder X-ray diffraction pattern (XRD) of the synthesized MOFs were recorded on a Rigaku 2500 VBZ+/PC diffractometer using monochromatized Cu Kα radiation under 40 kV and 200 mA in the scan range of 2θ from 5° to 90° with a scan step of 0.02°. The textural properties of the synthesized MOFs were determined from the nitrogen adsorption−desorption isotherms recorded at 77 K with a Thermo Finnigan Sorptomatic 1990 apparatus. Prior to 12450

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30 °C, respectively. The results are displayed in Figure 2. The adsorption capacity for DBT at the two temperatures increases

of Chemical Physics, China) for 3-MT. The GC operating conditions were as follows: the temperatures for injector, detector, and oven were 340 °C, 250 °C, and 250 °C, respectively, for DBT, and 250 °C, 250 °C, and 200 °C, respectively, for BT. For 3-MT, the temperatures for injector and detector were 180 °C and 200 °C, and the oven temperature was programmed from 60 °C (hold for 3 min) to 80 °C (hold for 2 min) at 2.5 °C min−1. The injection volume was 0.4 μL for all samples.

3. RESULTS AND DISCUSSION 3.1. Characterization of the MOFs. The powder XRD patterns of the four MOFs are exhibited in Figure 1. These

Figure 2. Effect of adsorption time on sulfur adsorption capacity to DBT over Cu-BTC. Conditions: DBT oil = 10 g; Cu-BTC = 0.1 g.

rapidly in the initial 10 min, i.e., 41.3 mg S/g MOF at 20 °C and 44.2 mg S/g MOF at 30 °C, respectively. Then the adsorption capacity increases steadily and almost reaches a plateau in 60 min, being 50.0 and 52.5 mg S/g MOF, respectively. As the adsorption time is prolonged to 120 min, the adsorption capacity at 20 °C and 30 °C only increases by 0.2 and 0.1 mg S/g MOF, respectively. Thus, 60 min may be deemed an appropriate adsorption time and is used in the following study. The equilibrium time is about 120 min, and the adsorption capacity of Cu-BTC for DBT at 20 °C and 30 °C are 50.2 and 52.6 mg S/g MOF, respectively. 3.3. Effect of MOFs on S-Adsorption. In an attempt to determine the adsorption capacity of the four MOFs for sulfur, some experiments are conducted in DBT model oil at 30 °C with the mass ratio of oil to MOF, m(oil)/m(MOF), ranging from 50 to 250. Under these conditions, the adsorption capacity of MOFs for sulfur with an increasing m(oil)/ m(MOF) value, expressed as mg of sulfur per g MOF, are summarized in Figure 3. As shown in Figure 3, Cu-BTC shows

Figure 1. XRD spectrum of the four MOFs: (A) Cu-BTC; (B) CuBDC; (C) Cr-BDC; (D) Cr-BTC.

diffraction patterns are in accordance with the reported ones,29,30,32,33 which indicate that the crystal structures of the synthesized MOFs are basically consistent with those in the reported literature. The BET surface area and pore volume were obtained from nitrogen physisorption isotherms at 77 K, and the results are given in Table 1. As shown in Table 1, the BET surface area of Table 1. Surface area and Pore Volumes for the Four MOFs MOF

BET surface areaa (m2/g)

pore volumea (cm3/g)

pore sizeb (Å)

Cr-BDC Cr-BTC Cu-BDC Cu-BTC

1511.281 2116.886 507.621 1601.385

0.744 0.715 0.342 0.786

8.6/9.430 25/2948 6.049 6.126

a Obtained from the N2 adsorption experiment. bCollected from the literature.

Figure 3. Sulfur adsorption capacity to DBT over different MOFs. Conditions: stirring time = 60 min; T = 30 °C.

the best adsorptivity, followed by Cr-BDC and Cr-BTC. In contrast, Cu-BDC shows a negligible adsorptivity. Accordingly, Cu-BDC is not considered for further studies. Figure 3 also shows that the adsorption capacities of Cu-BTC, Cr-BDC, and Cr-BTC all increase with the increasing m(oil)/m(MOF) ratio and almost reach a plateau at m(oil)/m(MOF) = 200:1, being 56.1, 41.0, and 30.7 mg S/g MOF, respectively. An m(oil)/

the four MOFs follows the order Cr-BTC > Cu-BTC > CrBDC > Cu-BDC. The differences between the specific pore volumes of Cr-BDC, Cr-BTC, and Cu-BTC are very slight, all being much higher than that of Cu-BDC. 3.2. Effect of Time on S-Adsorption. The experiments to decide an appropriate adsorption time were conducted in DBT model oil (10 g) with Cu-BTC (0.1 g) adsorbent at 20 °C and 12451

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m(MOF) value higher than 200:1 does not lead to an impressive increase in the adsorption capacity of MOFs. The desulfurization performance of the MOFs used in this manuscript is compared with some conventional zeolites or activated carbon adsorbents,34−36 and the MOFs show a higher adsorptivity. With respect to the adsorptivity of Cu-BTC for DBT, our result is comparable with that reported by Cychosz23 and Blanco-Brieva.26 However, the adsorptivity of Cu-BTC, CrBDC, and Cr-BTC for DBT is a slightly inferior to UMCM-150 but superior to most of other MOFs reported by Cychosz23 and Blanco-Brieva,26 e.g., MOF-5, MOF-177, and F300. Usually the adsorptivity of an adsorbent is related to its surface area or pore volume. However, according to the above results, neither of these correlations holds in this study. This phenomenon was also discovered in other reported studies with MOFs for desulfurization from fuel oil,23,26 which indicates that it is the chemical interaction rather than the physical interaction that plays a key role for the S-adsorption of the MOFs materials, and thus the influence of the surface area is not observed. The adsorptive differences between the four MOFs, i.e., Cu-BTC, Cr-BDC, Cr-BTC, and Cu-BDC, is interpreted as follows. First, the poor adsorptivity of Cu-BDC for DBT may be ascribed to its particular structure unlike the other three MOFs. According to the research of Carson and co-workers,29 Cu-BDC appears to have a lamellar geometry that forms twodimensional tunnels. In contrast, the other three MOFs are coordinated in three dimensions to create empty cubes with large square pores.31,32,37 Second, the adsorption of thiophenic compounds over metal oxides is mainly realized through a Lewis acid−base interaction.38 Besides, Song and co-workers20 proposed that thiophenic S-compounds can be removed from the transportation fuels either by the direct sulfur−metal (S-M) interaction or by the π-complexation. Thus, the reason for the higher adsorptivity of Cu-BTC compared with Cr-BDC and CrBTC may lie in the different metal centers. According to Pearson’s hard−soft acid−base principle,39 S-compounds tend to be intermediate to soft bases, and the soft S-compounds prefer to interact with intermediate or soft Lewis acid sites, such as Cu2+, Zn2+, or Ag+, but show comparatively weak interaction with hard Lewis acid sites such as Cr3+, Al3+, and Fe3+. This fits well with the experimental results as displayed in Figure 3 and other data for some MOFs reported in the literature.25,26 However, as previously reported in the literature,23,27 MOF-5 with a Zn2+ metal center shows a lower adsorptivity for thiophenic sulfurs, possibly because it has no exposed metal sites, leading to inaccessible metal sites.40 Finally, because CrBDC and Cr-BTC have the same metal center, the difference between their adsorptivity is probably due to their different porosity. From the literature,23 the adsorption capacity of MOFs for a given organosulfur compound is also determined by their pore size and shape. This is because, compared with the pore size of MOFs, smaller adsorbates can go through the pore channels while larger ones are blocked.41 That is, the adsorbent with pore size similar to that of the adsorbed thiophenic sulfur can enhance its adsorption potential.42 In this regard, Cr-BDC exhibits adsorptivity higher than that of CrBTC because the pore size of Cr-BTC is much larger than that of the reported molecular size of DBT of 0.8 nm.43 Generally speaking, the adsorptivity of MOFs for organosulfur compounds results from the comprehensive effect of manifold factors involving (1) appropriate framework structure, (2) exposed Lewis acid sites matching the given organosulfur

compound, and (3) suitable pore size and shape matching the given organosulfur compound. 3.4. Effect of Temperature on S-Adsorption. As an important factor for both kinetics and equilibrium of adsorption, temperature is investigated in the range from 20 °C to 60 °C under atmospheric pressure with Cu-BTC. Figure 4 clearly indicates a strong influence of temperature on the

Figure 4. Effect of temperature on sulfur adsorption capacity to DBT over Cu-BTC. Conditions: stirring time = 60 min.

adsorption capacity of Cu-BTC. This effect is clearer with m(oil)/m(MOF) above 50. Moreover, Figure 4 also shows that higher temperature can result in faster adsorption equilibrium. Differences in the adsorption capacity at a constant m(oil)/ m(MOF) = 100:1 versus the adsorption temperature are given in Figure 5. From Figure 5, for all the three MOFs, their

Figure 5. Effect of temperature on sulfur adsorption capacity to DBT over different MOFs. Conditions: the mass ratio of m(oil)/m(MOF) = 100:1; stirring time = 60 min.

adsorption capacity increases slightly from 20 °C to 30 °C and then decreases slowly as the temperature rises above 30 °C. This indicates that the adsorption of these MOFs is a result of the combined effects of physisorption and chemisorption because high temperature is favorable for chemisorption but unfavorable for physisorption. In detail, at low temperature, the adsorption process is basically governed by weak van der Waals interactions; however, at higher temperature, chemisorption is dominant. Nevertheless, this chemisorption is not yet a true chemical reaction and thus results in some desorption of DBT at temperatures higher than 30 °C. 3.5. Adsorption of Different S-Compounds. The adsorptivity of the three MOFs, i.e., Cu-BTC, Cr-BDC, and Cr-BTC, for different S-compounds are investigated at 30 °C 12452

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additional aromatic rings in BT and DBT can increase the πelectron number, enhancing the probability of π-complexation to the exposed metal sites.44 Moreover, the electron density on the S-atom of thiophenic sulfurs increases in the order 3-MT (5.697) < BT (5.739) < DBT (5.758),45,46 which probably enhances the direct interaction between the metal sites and the S-molecules.20 3.6. Regeneration Performance of MOF. The adsorbents are required to be regenerative for multiple cycles to reduce the adsorbent cost. Generally, there are two techniques for regeneration of the adsorbent: the thermal treatment and the solvent elution.4,47 In this study, the solvent elution is adopted. As a representative procedure, the regenerated Cu-BTC is tested in DBT model oil with m(oil)/m(MOF) = 100:1 at 30 °C. Effects of the regeneration times are shown in Figure 9. In

with m(oil)/m(MOF) ranging from 50:1 to 250:1. The results are shown in Figures 6−8. These figures show that the

Figure 6. Sulfur adsorption capacity for different S-compounds over Cu-BTC. Conditions: stirring time = 60 min; T = 30 °C.

Figure 9. Effect of recycle times of Cu-BTC on the sulfur adsorption capacity for DBT. Conditions: the mass ratio m(oil)/m(MOF) = 100:1; stirring time = 60 min; T = 30 °C. Figure 7. Sulfur adsorption capacity for different S-compounds over Cr-BDC. Conditions: stirring time = 60 min; T = 30 °C.

comparison with fresh Cu-BTC, the adsorption capacity decreases by less than 5% at the second recycle time and then remains almost at the same level. Until the fifth regeneration, the adsorption capacity decreases by 11.6% but still remains at a high value. From the IR spectra of the fresh and fifth regenerated Cu-BTC (see Figure 10), there is no

Figure 8. Sulfur adsorption capacity for different S-compounds over Cr-BTC. Conditions: stirring time = 60 min; T = 30 °C.

adsorptivity of each MOF for different S-compounds follows the order DBT > BT > 3-MT at any value of m(oil)/m(MOF). This behavior can be related to the double adsorption mechanism, i.e., physisorption and chemisorption, proposed for the experiments conducted at different temperatures. Taking BT for example, if sole weak physisorption is active in the adsorption process, the adsorption capacity for BT should be higher than for DBT due to more steric restrictions of the DBT molecule. However, when both physisorption and chemisorption occur in the adsorption process, and even chemisorption is dominant, the adsorption capacity for BT should be lower than for DBT. In fact, the presence of

Figure 10. The IR spectra of the fresh (A) and fifth regenerated CuBTC (B).

difference between them, which indicates that no change occurs in the groups of Cu-BTC during its regeneration, thus likely resulting in retention of high adsorption capacity. The small decrease in adsorption capacity may result from a minor increase in crystallinity of the used MOF, as shown in Figure 11, especially in the low 2θ° region. The increase in crystallinity of the used MOF may originate from the ripening of the MOF 12453

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(2) Li, H.; Jiang, X.; Zhu, W.; Lu, J.; Shu, H.; Yan, Y. Deep Oxidative Desulfurization of Fuel Oils Catalyzed by Decatungstates in the Ionic Liquid of [Bmim]PF6. Ind. Eng. Chem. Res. 2009, 48, 9034−9039. (3) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211−263. (4) Tang, X.; Qian, W.; Hu, A.; Zhao, Y.; Fei, N.; Shi, L. Adsorption of Thiophene on Pt/Ag-Supported Activated Carbons Prepared by Ultrasonic-Assisted Impregnation. Ind. Eng. Chem. Res. 2011, 50, 9363−9367. (5) Jeevanandam, P.; Klabunde, K. J.; Tetzle, S. H. Adsorption of Thiophenes out of Hydrocarbons Using Metal Impregnated Nanocrystalline Aluminum Oxide. Microporous Mesoporous Mater. 2005, 79, 101−110. (6) Gao, X.; Mao, H.; Lu, M.; Yang, J.; Li, B. Facile synthesis route to NiO−SiO2 intercalated clay with ordered porous structure: Intragallery interfacially controlled functionalization using nickel−ammonia complex for deep desulfurization. Microporous Mesoporous Mater. 2012, 148, 25−33. (7) Nie, Y.; Li, C.; Wang, Z. Extractive Desulfurization of Fuel Oil Using Alkylimidazole and Its Mixture with Dialkylphosphate Ionic Liquids. Ind. Eng. Chem. Res. 2007, 46, 5108−5122. (8) Revelli, A.; Mutelet, F.; Jaubert, J. Extraction of Benzene or Thiophene from n-Heptane Using Ionic Liquids. NMR and Thermodynamic Study. J. Phys. Chem. B 2010, 114, 4600−4608. (9) Morales, D. P.; Taylor, A. S.; Farmer, S. C. Desulfurization of Dibenzothiophene and Oxidized Dibenzothiophene Ring Systems. Molecules 2010, 15, 1265−1269. (10) Kumar, S.; Srivastava, V. C.; Badoni, R. P. Oxidative desulfurization by chromium promoted sulfated zirconia. Fuel Process. Technol. 2012, 93, 18−25. (11) Irani, Z. A.; Mehrnia, M. R.; Yazdian, F.; Soheily, M. Analysis of petroleum biodesulfurization in an airlift bioreactor using response surface methodology. Bioresour. Technol. 2011, 102, 10585−10591. (12) Guo, B.; Wang, R.; Li, Y. Gasoline alkylation desulfurization over Amberlyst 35 resin: Influence of methanol and apparent reaction kinetics. Fuel 2011, 90, 713−718. (13) Pasel, J.; Wang, Y.; Hürter, S.; Dahl, R.; Peters, R.; Schedler, U.; Matuschewski, H. Desulfurization of jet fuel by pervaporation. J. Membr. Sci. 2012, 390−391. (14) Tang, X.; Shi, L. Study of the Adsorption Reactions of Thiophene on Cu(I)/HY−Al2O3 by Fourier Transform Infrared and Temperature-Programmed Desorption: Adsorption, Desorption, and Sorbent Regeneration Mechanisms. Langmuir 2011, 27, 11999− 12007. (15) Bu, J.; Loh, G.; Gwie, C. G.; Dewiyanti, S.; Tasrif, M.; Borgna, A. Desulfurization of diesel fuels by selective adsorption on activated carbons: Competitive adsorption of polycyclic aromatic sulfur heterocycles and polycyclic aromatic hydrocarbons. Chem. Eng. J. 2011, 166, 207−217. (16) Lin, L.; Zhang, Y.; Zhang, H.; Lu, F. Adsorption and solvent desorption behavior of ion-exchanged modified Y zeolites for sulfur removal and for fuel cell applications. J. Colloid Interface Sci. 2011, 360, 753−759. (17) Huang, L.; Wang, G.; Qin, Z.; Dong, M.; Du, M.; Ge, H.; Li, X.; Zhao, Y.; Zhang, J.; Hu, T.; Wang, J. In situ XAS study on the mechanism of reactive adsorption desulfurization of oil product over Ni/ZnO. Appl. Catal., B 2011, 106, 26−38. (18) Tang, X.; Meng, X.; Shi, L. Desulfurization of Model Gasoline on Modified Bentonite. Ind. Eng. Chem. Res. 2011, 50, 7527−7533. (19) Yang, R. T.; Takahashi, A.; Yang, F. H. New Sorbents for Desulfurization of Liquid Fuels by π-Complexation. Ind. Eng. Chem. Res. 2001, 40, 6236−6239. (20) Velu, S.; Ma, X.; Song, C. Selective Adsorption for Removing Sulfur from Jet Fuel over Zeolite-Based Adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293−5304. (21) Meek, S. T.; Greathouse, J. A.; Allendorf, M. D. Metal-Organic Frameworks: A Rapidly Growing Class of Versatile Nanoporous Materials. Adv. Mater. 2011, 23, 249−267.

Figure 11. The XRD spectra of the fresh (A) and fifth regenerated CuBTC (B).

powder during the course of use, where the tiny particles are preferentially dissolved in the solvent and then recrystallize on the MOF surface, forming larger MOF crystals. In fact, the ripening process is widely used to homogenize the particle size distribution in the manufacture of nanomaterials, and thus the crystallinity and the particle size of the MOFs can reach equilibrium after definite times of use. As a whole, the MOF adsorbent, i.e., Cu-BTC, can be easily regenerated after the adsorption desulfurization process and recycled five times at least.

4. CONCLUSIONS In this work, four MOFs are synthesized by using two different metal centers and two different organic ligands, viz., Cu-BTC, Cr-BDC, Cr-BTC, and Cu-BDC. Their adsorption behaviors for thiophenic S-compounds in model diesel oil are systematically investigated through batch adsorption tests. From the experimental results, all MOFs except Cu-BDC show high adsorption abilities toward DBT at temperatures close to ambient. The adsorption capacity of MOFs for DBT follows the order Cu-BTC > Cr-BDC > Cr-BTC ≫ Cu-BDC. The adsorption of MOFs is considered as a result of the comprehensive effect of many factors, such as appropriate framework structure, exposed Lewis acid site matching the Scompound to be adsorbed, and suitable pore size and shape. The adsorption capacity for different S-compounds decreases in the order DBT > BT > 3-MT, where π-electron number and electron density on the S-atom are regarded as the key factors. Finally, the used MOF can be easily regenerated by solvent washing and reused at least five times.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. and fax: +86 10 64410308. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the support from the Fundamental Research Foundation of Sinopec (grant no. X505015). REFERENCES

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dx.doi.org/10.1021/ie3020395 | Ind. Eng. Chem. Res. 2012, 51, 12449−12455