What Matters to the Adsorptive Desulfurization Performance of Metal

Sep 9, 2015 - Khan et al.(36) investigated the adsorption capacity of three isotypic MOFs and found that MIL-47 exhibited the highest BT adsorption ca...
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What Matters to the Adsorptive Desulfurization Performance of Metal-Organic Frameworks? Yu-Xia Li, Wen-Juan Jiang, Peng Tan, Xiao-Qin Liu,* Dong-Yuan Zhang, and Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) show high potential in adsorptive removal of aromatic sulfur compounds; however, the crucial factors affecting the adsorption performances are scarcely clarified. In the present study, three classic aromatic sulfur compounds (i.e., thiophene, benzothiophene, and 4,6-dimethyldibenzothiophene) as well as five typical MOFs (i.e., MOF-5, HKUST-1, MIL-53(Fe), MIL-53(Cr), and MIL101(Cr)) were selected for study. The adsorptive desulfurization performances of MOFs were investigated by using a fixed-bed adsorption system. In the case of thiophene, the adsorption capacity of MOFs decreases in the order MIL-53(Cr) > HKUST-1 > MOF-5 > MIL-53(Fe) > MIL-101(Cr). For the first time, the adsorbate−adsorbent interaction was examined in detail by using infrared spectra and temperature-programmed desorption. Such an interaction was demonstrated to be the most important factor affecting adsorption performance. When the molecular size of aromatic sulfur compounds is comparable to or smaller than the window diameter of MOFs, the influence of window diameter becomes apparent. It is surprising to find that the adsorbate−adsorbent interaction plays a major role, which is responsible for the poor adsorption performance of MIL-101(Cr) with quite high porosity. Therefore, metal sites and structure that contribute to the adsorbate−adsorbent interaction should be considered to be the most significant factor aiming to develop new MOFs for adsorptive desulfurization.

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INTRODUCTION

The development of metal−organic frameworks (MOFs) spurs the study of nanoporous materials.18−21 MOFs are assembled from organic linkers and metal ion or cluster nodes.22−24 The structure of MOF can be tailored by the judicious choice of metal-based building blocks and organic linkers with almost endless geometrical and chemical variations. Their high porosity and large surface areas offer a range of promising applications in gas storage,25−27 separation,21,28,29 catalysis,30,31 and drug delivery.32 Recently, MOFs have also been applied to the adsorptive removal of aromatic sulfur compounds from fuels. Cychosz et al.33,34 evaluated the adsorption capacity of MOFs and found that MOFs performed better as compared with the benchmark zeolite NaY. The MOF material UMCM-150 was capable of removing 40 g S/kg adsorbent from the BT model fuel (1500 ppmw), which is much higher than that over NaY (8 g S/kg adsorbent). Peralta et al.35 studied the selective adsorption performance of MOFs and zeolites. Their results showed that HKUST-1 could adsorb thiophene selectively from the mixture containing toluene, while there was no selectivity over NaY. Khan et al.36 investigated the adsorption capacity of three isotypic MOFs and found that MIL-47 exhibited the highest BT adsorption

Nowadays deep desulfurization from transportation fuels has attracted increasing attention because of adverse environmental impacts of sulfur oxides presented in engine exhaust emissions and fuel-cell catalysts poisoning issues.1−5 Traditionally, transportation fuels are subjected to catalytic hydrogenation for sulfur removal;6,7 however, the hydrodesulfurization (HDS) technology requires harsh reaction conditions such as elevated temperatures (>300 °C) and high hydrogen pressures (>4 MPa).8 In addition, HDS is ineffective for deep desulfurization to ppm levels because in the hydrogenation reactions the aromatic hydrocarbons contained in fuels are more reactive than those of aromatic sulfur compounds such as thiophene, benzothiophene (BT), and 4,6-dimethyldibenzothiophene (DMDBT).9−11 Hence, the efficient removal of aromatic sulfur compounds in fuels becomes the central issue for the demand of increasingly stringent environmental regulations. Adsorptive desulfurization, based on the ability of a solid adsorbent that can selectively adsorb aromatic sulfur compounds, is considered to be one of the most promising methods.3,12 In the past decade, various porous materials such as zeolites,13 silicas,14−16 and activated carbons17 have been employed for adsorptive removal of aromatic sulfur compounds. It is found that the efficiency of adsorptive desulfurization is strongly dependent on the adsorbents used. © 2015 American Chemical Society

Received: May 26, 2015 Revised: September 9, 2015 Published: September 9, 2015 21969

DOI: 10.1021/acs.jpcc.5b07546 J. Phys. Chem. C 2015, 119, 21969−21977

Article

The Journal of Physical Chemistry C

Teflon-lined steel autoclave and kept at 150 °C for 15 h. The obtained solids were then filtered, washed with DMF, and exchanged with dichloromethane. MIL-53(Cr) was synthesized from a mixture of Cr(NO3)3· 9H2O (5 mmol), H2BDC (5 mmol), HF (5 mmol), and deionized water (25.2 mL). The mixture was kept under stirring conditions for ∼2 h at room temperature to homogenize the synthetic system. Subsequently, the mixture was transferred to a Teflon-lined steel autoclave and kept at 220 °C for 72 h. The obtained solids were then filtered, washed by DMF, and exchanged with dichloromethane. MIL-101(Cr) was synthesized from the same mixture as MIL-53(Cr). The mixture was kept under stirring conditions for ∼2 h at room temperature to homogenize the synthetic system. Subsequently the mixture was transferred to a Teflonlined steel autoclave and kept at 220 °C for 8 h. The obtained solids were then filtered, washed by DMF, and exchanged with dichloromethane. Characterization. Powder X-ray diffraction (XRD) patterns of the materials were recorded using a Bruker D8 Advance diffractometer with Cu Kα radiation in the 2θ ranges from 2 to 50° at 40 kV and 30 mA. The N2 adsorption− desorption isotherms were measured by using ASAP 2020 at −196 °C. MOFs were degassed at 150 °C for 12 h under vacuum prior to analysis. The Brunauer−Emmett−Teller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. IR spectra of the samples diluted with KBr were recorded on a Nicolet Nexus 470 spectrometer. For the measurement of IR spectra after adsorption with thiophene, thiophene was first adsorbed by the MOFs in the liquid phase. After the MOFs were evacuated to remove physically adsorbed thiophene, the IR spectra were recorded. TPD experiments were conducted on a BELSORP BEL-CAT-A apparatus. About 100 mg of samples was pretreated at 150 °C under He for 12 h. After cooling to room temperature in a He atmosphere, the gas was switched to thiophene vapor with He. After physically adsorbed thiophene was purged by a He flow at room temperature, the sample was heated to 280 °C and the thiophene liberated was monitored continuously by a mass spectrometer (MS). Adsorptive Desulfurization Test. The model fuel used for experiments was prepared by mixing thiophene, BT, and DMDBT with isooctane, and the sulfur content was ∼550 ppmw (parts per million by weight). Adsorptive experiments were performed in a vertical quartz column at room temperature. The model fuel was allowed to contact the adsorbent pumped up with a mini creep pump at the rate of 3 mL·h−1. MOFs were pretreated in flowing He at 150 °C for 12 h. Effluent solutions were collected periodically until saturation was reached. The sulfur content of effluent solutions was analyzed using a Varian 3800 gas chromatograph (GC) equipped with a pulsed-flame photometric detector (PFPD). A calibration curve was prepared to verify the GC results. Breakthrough curves were generated by plotting the normalized sulfur concentration versus the cumulative fuel volume, which was normalized by the adsorbent weight. The normalized sulfur concentration (c/c0) was obtained by measuring the ratio of the detected sulfur content (c) to that of the initial sulfur content (c0). The adsorption capacity was obtained from integral calculus.

capacity. On the basis of these results, it is conclusive that MOFs are highly promising for the adsorptive removal of aromatic sulfur compounds. In contrast with the increasing use of MOFs as adsorbents for sulfur removal, however, less attention has been paid to the clarification of crucial factors affecting the adsorption capacity of MOFs. The relationship between properties of MOFs and their adsorption performances has never been explored systematically, although it is extremely desirable for developing new, efficient MOFs for adsorptive desulfurization. Herein, five typical MOFs, namely, MOF-5, HKUST-1, MIL53(Fe), MIL-53(Cr), and MIL-101(Cr), were selected for investigation. They are assembled from four different metal ions (Zn2+, Cu2+, Fe3+, and Cr3+) and two different organic ligands (1,4-dicarboxybenzene, H2BDC and 1,3,5-benzenetricarboxylic acid, H3BTC) that possess four different crystalline structures. Three representative aromatic sulfur compounds with one to three rings, namely, thiophene, BT, and DMDBT, were employed for adsorption. Dynamic breakthrough curves were measured systematically with different adsorbates and adsorbents. In addition to the properties of MOFs and aromatic sulfur compounds, the adsorbate−adsorbent interaction was examined in detail by using infrared (IR) spectra and temperatureprogrammed desorption (TPD) of thiophene for the first time. On the basis of experimental results, the crucial factors affecting the adsorption capacity of MOFs were disclosed, and the relationship between properties of MOFs and their adsorption performances was proposed.

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EXPERIMENTAL SECTION Chemicals. The chemicals Cr(NO3)2·9H2O (>99.5%), Cu(NO 3) 2·3H2 O (>99.5%), Zn(NO 3) 2·6H2 O (>99.5%), FeCl3·6H2O (>99.5%), H2BDC (>98%), H3BTC (>98%), and hydrofluoric acid (40%) were purchased from Sinopharm Chemical Reagent. Methanol (>99.5%), ethanol (>99.7%), N,N-dimethylformamide (DMF) (>99.5%), and dichloromethane (>99.5%) were purchased from Wuxi Yasheng Chemical. Triethylamine (TEA, 99%), thiophene (99%), BT (97%), DMDBT (>99%), and isooctane (>99%) were provided by Aladdin Chemical Reagent. Deionized water was used for all of the experiments. Materials Synthesis. The adsorbents used in the present study, namely, MOF-5,25 HKUST-1,24 MIL-53(Fe),32 MIL53(Cr),32 and MIL-101(Cr),30 were synthesized according to the reported methods. MOF-5 was prepared as follows. Zn(NO3)2·6H2O (4 mmol) and H2BDC (2 mmol) were dissolved in DMF (40 mL) with mild stirring. Then TEA (2.2 mL) was added dropwise to the above solution. The resultant solution was stirred for 3 h at room temperature. The precipitate was collected by centrifugation, washed with DMF three times, and exchanged with dichloromethane. HKUST-1 was prepared as follows. Cu(NO3)2·3H2O (3.6 mmol) and H3BTC (2 mmol) were dissolved in the mixed solvent consisting of ethanol (12 mL) and deionized water (12 mL). The mixture was heated at 120 °C for 12 h. The obtained solids were filtered, washed with DMF and methanol, and exchanged with dichloromethane. MIL-53(Fe) was prepared from a solution dissolving FeCl3· 6H2O (1.3 mmol) and H2BDC (1.3 mmol) in DMF (27.1 mL). The mixture was kept under stirring conditions for ∼2 h at room temperature to homogenize the synthetic system. Subsequently the resulting mixture was transferred into a 21970

DOI: 10.1021/acs.jpcc.5b07546 J. Phys. Chem. C 2015, 119, 21969−21977

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Figure 1. Schematic structure of metal and organic linker in MOFs. Zn oxide cluster, lime; Cu oxide cluster, sky blue; Fe oxide cluster, light orange; Cr oxide cluster, teal; carbon, gray.

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MIL-53(Fe) only shows a low surface area of 26 m2·g−1 because of the closed structure.32,37 MIL-101(Cr) shows a quite different shape of isotherm. A sharp enhancement in uptake could be observed at low relative pressures; with the increase in relative pressure the uptake increases gradually. Additional uptakes near the relative pressure of 0.15 and 0.23 reflect the presence of mesopores with the size of 29 and 34 Å.38 MIL101(Cr) exhibits a surface area of 2322 m2·g−1 and a pore volume of 1.225 cm3·g−1, which is markedly larger than that of MIL-53(Cr). It is known that the surface areas of MOFs are strongly dependent on the methods for their synthesis and post-treatment, and the surface area of MIL-101(Cr) is reported to vary from 2160 to 4230 m2·g−1.20,27,30,38 For the purification by the two-step treatment using hot DMF and ethanol that were employed in the present study, the surface area of MIL-101(Cr) was reported to be 2220 m2·g−1,27 which is quite close to the value of the present sample (2322 m2·g−1). The difference in the pore volume of HKUST-1 and MIL53(Cr) is very slight, which is apparently lower than that of MIL-101(Cr), higher than that of MOF-5 and MIL-53(Fe). Further information on the MOFs is provided by IR spectra presented in Figure S3. The IR spectrum of MOF-5 shows strong characteristic absorptions for the symmetric and asymmetric vibrations of H2BDC (1610−1550 and 1420− 1335 cm−1) and adsorbed water (3500−3200 cm−1).39 Various bands are observed in the 900−650 and 1250−1000 cm−1 regions, and they can be attributed to the C−H out-of-plane vibrations of aromatic ring.39,40 For HKUST-1, the vibration of CC of benzene groups is observed at 1566 cm−1. The bands at 725 and 769 cm−1 attributed to metal Cu substitution on benzene groups are regarded as the characteristic bands of HKUST-1, whereas the bands at 1042 and 1110 cm−1 originate from stretching vibrations of C−O−Cu.41,42 The IR spectrum of MIL-53(Cr) gives a strong band at 3420 cm−1, which is caused by the stretching vibration of O−H in MIL-53(Cr).32 The bands at 1555 and 1404 cm−1 correspond to symmetric and asymmetric vibrations of H2BDC, respectively. The band at 580 cm−1 results from the bending vibration of Cr−O.43 The characteristic peaks of MIL-53(Fe) are similar to that of MIL53(Cr). The sample MIL-101(Cr) reveals two bands at 1510 and 1393 cm−1 that can be attributed to the vibrations characteristic of framework −(O−C−O)− groups, confirming the presence of dicarboxylate within MIL-101(Cr).44 Large bands at 3500 and 1630 cm−1 indicate the presence of water molecules within MIL-101(Cr).45 The XRD results, in combination with N2 adsorption and IR data, indicate that different types of MOFs are successfully synthesized. These MOFs are assembled from four metals and two ligands, forming five MOFs with four different crystalline structures. Their surface areas range from 26 to 2322 m2·g−1, whereas pore volumes range from 0.062 to 1.225 cm3·g−1. An obvious difference also exists in the window diameter, which varies between 7.5 and 12.0 Å. The differences as well as

RESULTS AND DISCUSSION Structural Properties of Different MOFs. Figure 1 presents the structure of five MOFs, namely, MOF-5, HKUST-1, MIL-53(Fe), MIL-53(Cr), and MIL-101(Cr). MOF-5 is constructed from inorganic [OZn4]6+ groups joined to an octahedral array of [O2C−C6H4−CO2]2− groups, which leads to the formation of a porous cubic framework. HKUST-1 is a Cu-based MOF and can be characterized by a 3D system of square-shaped pores. MIL-53(Fe) and MIL-53(Cr) are built up from infinite chains of corner-sharing MO4(OH)2 (M = Fe3+ or Cr3+) octahedra interconnected by the dicarboxylate groups. MIL-53(Fe) and MIL-53(Cr) represent a class of dynamic hybrid solids whose pore sizes can be modulated upon adsorption of guest molecules. Despite the same starting materials, MIL-101(Cr) has a quite different structure from MIL-53(Cr). MIL-101(Cr) is made of trimers of chromium octahedra linked with dicarboxylate groups, forming supertetrahedral motifs that further assemble to produce a crystallized mesoporous hybrid solid. Figure S1 shows the XRD patterns of different MOFs. The XRD pattern of MOF-5 suggests a 3D cubic porous system.26 The XRD pattern of HKUST-1 indicates that HKUST-1 is a highly ordered 3D octahedral material.24 MIL-53(Fe) and MIL53(Cr) show a very close pattern, which mirrors 1D rhombicshaped tunnels in a 3D structure.32 In the case of MIL-101(Cr), the cubic symmetry can be reflected by the XRD pattern.28 The XRD results give evidence of the successful synthesis of MOFs. Figure S2 gives N2 adsorption isotherms of different MOFs. The isotherms of MOF-5 and HKUST-1 are of type I, indicative of microporous character.25,28 Corresponding textual parameters were listed in Table 1. The networks of MOF-5 are Table 1. Textual Properties of Different MOFs Used in the Study adsorbent MOF-5 HKUST-1 MIL53(Fe) MIL53(Cr) MIL101(Cr)

SBET (m2· g−1)

Vp (cm ·g )

window diameter (Å)

Zn4O(BDC)3 Cu3(BTC)2 Fe(OH) (BDC)

819 1436 26

0.342 0.534 0.062

7.5 8.0 8.5

Cr(OH) (BDC)

1356

0.511

8.5

Cr3O(H2O)2F(BDC)3

2322

1.225

12.0

formula

3

−1

made of 7.5 Å channels with a surface area of 819 m2·g−1. HKUST-1 possesses 8.0 Å square channels and a surface area of 1436 m2·g−1, which is obviously higher than MOF-5. N2 adsorption characterization of MIL-53(Cr) reveals a type I isotherm with a surface area of 1356 m2·g−1, and the window diameter of 8.5 Å can be obtained from the crystallographic structure. Despite the same crystalline type with MIL-53(Cr), 21971

DOI: 10.1021/acs.jpcc.5b07546 J. Phys. Chem. C 2015, 119, 21969−21977

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101(Cr) are accessible for aromatic sulfur compounds. For the five MOFs investigated, the adsorption capacity for thiophene follows the order MIL-53(Cr) > HKUST-1 > MOF-5 > MIL53(Fe) > MIL-101(Cr). To date, various adsorbents have been employed for the removal of aromatic sulfur compounds from fuels. The classic AgY zeolite was reported to capture 0.90 mmol·g−1 of thiophene from the model fuel with a sulfur concentration of 2000 ppmw.47 The combination of Cu(I)Y zeolite with activated carbon was used as adsorbent as well.48 This adsorbent can capture 0.59 mmol·g −1 of sulfur from commercial diesel with a sulfur content of 430 ppmw. For CuCl and PdCl2 supported on the MCM-41 and SBA-15 mesoporous materials, PdCl2/SBA-15 showed the highest capacity that is able to capture 1.20 mmol·g−1 of sulfur from JP-5 light fraction.49 In this case, the JP-5 light fraction with a sulfur content of 841 ppmw was used for adsorption. Although the adsorption capacity of an adsorbent is strongly dependent on the conditions such as sulfur type and concentration, the above results indicate that some MOFs are promising candidates for applications in adsorptive desulfurization. It is also interesting to note that different MOFs show quite different adsorption performances on diverse aromatic sulfur compounds. MOF-5 is capable of removing 0.23 mmol·g−1 of thiophene and 0.08 mmol·g−1 of BT, while it cannot capture DMDBT. Similar trends are also observed for HKUST-1 and MIL-53(Fe), in which no DMDBT is adsorbed on HKUST-1, whereas a tiny amount of DMDBT (0.02 mmol·g−1) is adsorbed on MIL-53(Fe). In the case of MIL-53(Cr), however, the capacity for BT is higher than that for thiophene and can reach 1.38 mmol·g−1. This is obviously different from its counterpart MIL-53(Fe) with different metal sites; however, for DMDBT with three rings, the adsorption capacity decreases to 0.08 mmol·g−1. Unlike other MOFs, the adsorption capacity of MIL-101(Cr) increases with growing molecular size of aromatic sulfur compounds. The capacity for BT and DMDBT is 0.21 and 0.22 mmol·g−1, respectively and is apparently higher than that for thiophene (0.12 mmol·g−1). In short, for different aromatic sulfur compounds, the adsorption capacity of MOF-5, HKUST-1, and MIL-53(Fe) follows the order thiophene > BT > DMDBT, which contrary to that of MIL-101(Cr). In the case of MIL-53(Cr), the adsorption capacity for different sulfur compounds decreases in the order BT > thiophene > DMDBT. Adsorbate−Adsorbent Interaction. As previously described, different MOFs show quite different adsorption performances on desulfurization. Nonetheless, it is quite difficult to correlate these performances with any properties of MOFs reported up to now (e.g., surface area, window diameter, and metal site). To disclose the essential factors influencing the adsorption performance, we examined the adsorbate−adsorbent interaction by using IR and TPD techniques for the materials adsorbed with thiophene. The IR spectrum of pure thiophene was first recorded (Figure S5), and corresponding vibrational mode assignments are shown in Table S1.50,51 The intense characteristic band at 701 cm−1 is ascribed to the aromatic S−C stretching vibration. IR spectra of MOFs before and after adsorption of thiophene are displayed in Figure 3 for comparison. The band of S−C stretching vibration is observable on all MOFs adsorbed with thiophene. It is noticeable that the band shifts to a high wavelength after adsorbed on MOFs, while the change is dependent on MOFs. For MOF-5 and HKUST-1, the band of S−C stretching vibration appears at 712 cm−1, which is 11 cm−1

similarities in metal sites, crystalline structures, and textual parameters make these MOFs suitable for revealing the crucial factors that affect the adsorption capacity. Adsorptive Desulfurization Performance. Three representive aromatic sulfur compounds containing one to three rings, that is, thiophene, BT, and DMDBT, are selected for investigation. Their molecular structures and properties are listed in Table 2. The π electron number of thiophene, BT, and Table 2. Structure and Properties of Aromatic Sulfur Compounds Used in the Study

DMDBT is 6, 9, and 13, respectively. Similarly, the electron density on the sulfur atom of aromatic sulfur compounds increases in the order thiophene < BT < DMDBT. The presence of additional aromatic rings in BT and DMDBT increases the π electron number and subsequently enhances the interaction of aromatic sulfur compounds with metal sites. The molecular size of thiophene, BT, and DMDBT is 5.6 × 7.7, 6.5 × 8.9, and 7.8 × 12.3 Å, respectively. Figure 2 illustrates the breakthrough curves of three aromatic sulfur compounds in a fixed-bed adsorber with different MOFs. The saturated adsorption capacity of different adsorbents is calculated by integral calculus from breakthrough curves and listed in Table 3. It is noticeable that different adsorbents have significantly different adsorption capacity for aromatic sulfur compounds. Because the window diameter of each MOF is larger than the molecular size of thiophene, thiophene molecules can enter the pores of all MOFs freely. The adsorption capacity of MOF-5 for thiophene is 0.23 mmol·g−1. Nevertheless, the adsorption capacity of HKUST-1 is 0.49 mmol·g−1, which is over two times higher than that of MOF-5. For the two isotypic MOFs with different metal ions, MIL53(Fe) and MIL-53(Cr) exhibit a pronounced difference in thiophene capture. MIL-53(Fe) can remove 0.20 mmol·g−1 of thiophene, while MIL-53(Cr) exhibits the best performance and is capable of removing 1.23 mmol·g−1 of thiophene. It is surprising that only 0.12 mmol·g−1 of thiophene is captured over MIL-101(Cr), despite the fact that MIL-101(Cr) has the identical metal and ligand to MIL-53(Cr) as well as the largest pore size and surface area. The adsorption of CO on MIL101(Cr) was also conducted and the result is shown in Figure S4. The adsorption amount is 1.24 mmol·g−1, which is comparable to that reported in literature (1.13 mmol·g−1) and indicates the liberation of coordinatively unsaturated sites on the material.46 Obviously, the adsorption capacity of MIL101(Cr) on CO is higher than that on thiophene (0.12 mmol· g−1). This can be ascribed that only part of active sites on MIL21972

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Figure 2. Breakthrough curves of three aromatic sulfur compounds in a fixed-bed adsorber with different MOFs.

α, β, γ, and η at approximately 60, 90, 130, and 150 °C, respectively. A higher desorption temperature corresponds to a stronger interaction of thiophene with MOFs. MOF-5 and HKUST-1 possess similar TPD profiles and give rise to a desorption peak at site γ, implying a relatively strong adsorbate−adsorbent interaction. MIL-53(Fe) shows a desorption peak at site α accompanied by a shoulder at site β, mirroring a relatively weak adsorbate−adsorbent interaction. It is worth noting that a desorption peak at site η becomes visible on MIL-53(Cr) together with a peak at site β, whereas only a single desorption peak at site α appears on MIL-101(Cr). This reveals the strongest adsorbate−adsorbent interaction in thiophene adsorbed on MIL-53(Cr) and the weakest adsorbate−adsorbent interaction in thiophene adsorbed on MIL-101(Cr). In terms of the previously mentioned IR and TPD results, the adsorbate−adsorbent interaction should follow the order MIL-53(Cr) > HKUST-1 ≈ MOF-5 > MIL-53(Fe) > MIL101(Cr). The metal sites as well as structures of MOFs are responsible for the different adsorbate−adsorbent interaction. For the metal sites in MOFs, their interactions with aromatic sulfur compounds can be interpreted by using Pearson’s concept of hard and soft acids and bases (HSAB).52 According to the HSAB concept, aromatic sulfur compounds tend to be

Table 3. Adsorption Capacity of Different Adsorbents for Different Sulfur Compounds at Saturation adsorption capacity (mmol S·g−1) adsorbent

thiophene

BT

DMDBT

MOF-5 HKUST-1 MIL-53(Fe) MIL-53(Cr) MIL-101(Cr)

0.23 0.49 0.20 1.23 0.12

0.08 0.12 0.03 1.38 0.21

0 0 0.02 0.08 0.22

higher than that of pure thiophene (701 cm−1). Apparently, thiophene has an interaction with the adsorbents, leading to the shift of S−C stretching vibration. In the case of MIL-53(Fe) and MIL-101(Cr), the band of S−C stretching vibration is detected at 710 cm−1, corresponding to a shift of 9 cm−1. Surprisingly, a high wavelength of 716 cm−1 is observed for the S−C stretching vibration on MIL-53(Cr), which is 15 cm−1 higher than that of pure thiophene. That means, the interaction of thiophene with MIL-53(Cr) is the strongest among the MOFs studied. More detailed information can be obtained from the TPD profiles of thiophene. As shown in Figure 4, the desorption of thiophene can be tentatively divided into four parts denoted as 21973

DOI: 10.1021/acs.jpcc.5b07546 J. Phys. Chem. C 2015, 119, 21969−21977

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enter the pores of MOFs.34 As a result, both the metal sites and the entire pore surface are responsible for the capture of aromatic sulfur compounds. Besides the metal sites, the structure of MOFs also has an effect on the adsorbate−adsorbent interaction. It is known that MIL-53(Cr) shows a special response toward guest molecules, which is caused by the breathing of frameworks.55 The anhydrous MIL-53(Cr) solid exhibits an open structure, and a closure of the pores occurs at adsorbate loading as a consequence of either strong hydrogen bonds or donor− acceptor interactions between the guest molecules and the inorganic chains.56 This leads to a strong adsorbate−adsorbent interaction for thiophene adsorbed on MIL-53(Cr). To examine the breathing of MIL-53(Cr) during adsorption, thiophene adsorption isotherm was measured, and the result is shown in Figure S6. Remarkably, a clear step can be observed in the adsorption isotherm. This suggests a sudden change of the properties of the adsorbent,57−60 thus demonstrating the occurrence of breathing during the adsorption of thiophene over MIL-53 (Cr). In addition, the confinement of thiophene inside pores of individual MOFs is different in each case. As the adsorbed molecules are confined in a closed space, their losses in degrees of freedom will occur.61,62 Differences in rotational freedom in the pores of different MOFs may have a substantial impact on the adsorption of thiophene.63 Therefore, the preferred way to achieve high uptake of thiophene on MIL53(Cr) may also be based on the different confinement of the adsorbate in the pore system of an adsorbent. At low concentration, thiophene probably adsorbs in the narrow pore form. In the narrow pore form, confinement is strong, which can explain the high shift of the IR band and the hightemperature TPD peak. The step in the adsorption isotherm indicates a transformation from the narrow pore to the large pore form, which explains the high adsorption capacity. Unlike the analogue MIL-53(Cr), the dehydrated phase of MIL53(Fe) shows closed pores with scarce porosity.64,65 Also, these pores in MIL-53(Fe) are smaller in contrast with that in MIL53(Cr). As a result, the interaction of thiophene with MIL53(Fe) is much weaker than that of thiophene with MIL53(Cr). Among the MOFs studied, MIL-101(Cr) is the unique one containing mesopore cages with internal free diameters of 29 and 34 Å. The mesoporous structure favors the mass transport of adsorbate molecules but decreases the adsorbate− adsorbent interaction, especially when the metal sites only provide a weak interaction with adsorbate molecules. By combining the analysis of metal sites and structures of MOFs, it is reasonable to deduce that the adsorbate−adsorbent interaction follows in the order MIL-53(Cr) > HKUST-1 ≈ MOF-5 > MIL-53(Fe) > MIL-101(Cr). This fits well with the experimental data from IR and TPD. The adsorbate−adsorbent interaction is of significant importance for adsorptive desulfurization of MOFs. Examination of Crucial Factors Affecting Adsorptive Desulfurization. On the basis of the aforementioned results, it is clear different MOFs exhibit quite different adsorption performances on aromatic sulfur compounds. Thiophene has a molecular size of 5.6 × 7.7 Å and can enter the pores of all MOFs investigated (window diameters vary from 7.5 to 12 Å). It is thus possible to compare the capacities of different MOFs in terms of thiophene adsorption. As previously described, the adsorption capacity for thiophene decreases in the order MIL53(Cr) > HKUST-1 > MOF-5 > MIL-53(Fe) > MIL-101(Cr). To explain the adsorption capacity of different MOFs, we

Figure 3. IR spectra of different MOFs before and after adsorption of thiophene, where “-T” represents the sample adsorbed with thiophene.

Figure 4. TPD profiles of thiophene for different MOFs used in the study.

intermediate or soft bases and prefer to interact with intermediate or soft acid sites but show comparatively weak interactions with hard acid sites.35,52,53 In the light of Pearson’s classification,52 Cu2+ and Zn2+ are intermediate acids, while Fe3+ and Cr3+ belong to hard acids. As a result, the interaction of thiophene with Cu2+ and Zn2+ is stronger than that with Fe3+ and Cr3+. This can thus account for the stronger adsorbate− adsorbent interaction in HKUST-1 and MOF-5 as compared with that in MIL-53(Fe) and MIL-101(Cr). It is worth noting that MOF-5 and HKUST-1 possess similar adsorption strength upon thiophene according to the IR and TPD results. In this case, the metal sites of MOF-5 and HKUST-1 (namely Cu2+ and Zn2+) are intermediate acids according to the HSAB concept, and thus they show comparable interactions with thiophene. Apparently, the adsorptive removal of aromatic sulfur compounds can be realized through the interactions of the metal centers in the MOF frameworks with the delocalized π electrons of the aromatic rings.36,54 Also, the interaction strength depends on the kind of metal centers. In the meanwhile, aromatic sulfur compounds can be physisorbed on the entire pore surface provided that the compounds can 21974

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porosity of MOFs, which can be reflected by surface areas and pore volumes, the adsorbate−adsorbent interaction should be considered. It is thus easy to understand that MIL-101(Cr) possessing quite high porosity exhibits a relatively poor adsorption performance. That means, porosity might not be considered as the most important factor aiming to develop new MOFs for adsorptive desulfurization. In contrast, metal sites and structure that contribute to the adsorbate−adsorbent interaction should be taken into account. Work is in progress to examine the effect of adsorbate−adsorbent interactions on the adsorption performance of more MOFs (e.g., ZIF-8, UiO-66, and MOF-177).68

considered various factors such as porosity and window diameter; however, it is difficult to correlate the adsorption capacity with these properties. Interestingly, the adsorption capacity of MOFs is in good agreement with the adsorbate− adsorbent interaction, which follows the order MIL-53(Cr) > HKUST-1 ≈ MOF-5 > MIL-53(Fe) > MIL-101(Cr). This means that the adsorbate−adsorbent interaction plays a major role in adsorptive desulfurization. For HKUST-1 and MOF-5 with a comparable adsorbate−adsorbent interaction, the adsorption capacity of HKUST-1 is higher than that of MOF5. This can be ascribed to the higher porosity, namely, higher surface area and pore volume, of HKUST-1. For different aromatic sulfur compounds, the adsorption amount decreases in the order thiophene > BT > DMDBT over MOF-5, HKUST-1, and MIL-53(Fe). One may contribute this sequence to the molecular size of aromatic sulfur compounds. In other words, aromatic sulfur compounds with a larger molecular size are more difficult to enter the pores and subsequently captured by MOFs. Slow diffusion would lead to a strongly dispersed breakthrough curve, while this is not observed in most cases. In terms of Denayer’s reports, the preferential adsorption of thiophene can be ascribed to entropic effects.62,66 When the difference in adsorption enthalpy is not big, the adsorption of the smaller molecule is favored because it has an entropic advantage. In other words, the smaller molecule has access to a larger number of configurations in the adsorbed state.62 In the case of MIL-53(Cr), the adsorption amount of BT is bigger than that of thiophene. The breathing of frameworks should be responsible for the adsorption behavior, which makes the active sites in pores accessible to BT; however, the adsorption amount of DMDBT declines sharply over MIL53(Cr). This is because the molecular size of DMDBT is too large to enter the pores of MIL-53(Cr). For MIL-101(Cr), the adsorption amount of different sulfur follows the order DMDBT > BT > thiophene, which can be explained by the following factors. First, MIL-101(Cr) shows the largest window diameter among the MOFs investigated, which means that all adsorbate molecules (i.e., thiophene, BT, and DMDBT) can enter the pores freely. This is difficult for other MOFs with a smaller window diameter. Second, different aromatic sulfur compounds possess different interactions with the same MOFs. The conjugated π electrons on aromatic compounds contribute to their adsorption on MOF materials. The presence of additional aromatic rings in BT and DMDBT can increase the π electron number and subsequently enhance the π-complexation interaction with the exposed metal sites.53 Third, the direct interactions between the sulfur atoms and metal sites increase with increasing electron densities on sulfur atoms of aromatic compounds (Table 3).53,67 As a result, the adsorption amount becomes higher for an aromatic sulfur compound with more rings if the adsorbate can enter the pores of MOFs freely. This can thus well interpret the adsorption behavior of different aromatic sulfur compounds over MIL-53(Cr) and MIL101(Cr). According to the previously described analysis, it is safe to say that the adsorbate−adsorbent interaction is the most crucial factor affecting adsorption performance provided that sulfur compounds can enter the pores of MOFs. Metal sites and structure of MOFs as well as types of aromatic sulfur compounds are responsible for the adsorbate−adsorbent interaction. When the molecular size of sulfur compounds is analogous or smaller than the window diameter of MOFs, the effect of window diameter becomes evident. In addition to the

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CONCLUSIONS Five typical MOFs derived from four metal centers and two organic ligands, that is, MOF-5, HKUST-1, MIL-53(Fe), MIL53(Cr), and MIL-101(Cr), were employed for investigation. Their adsorption capacities for three representive aromatic sulfur compounds (i.e., thiophene, BT, and DMDBT) were systematically studied using a fixed-bed adsorption system. The factors including the adsorbate−adsorbent interaction, window diameter, and porosity were considered to correlate with the adsorption performance of MOFs. It is demonstrated for the first time that the adsorbate−adsorbent interaction plays a major role in adsorptive desulfurization. Metal sites and structure of MOFs as well as types of aromatic sulfur compounds is responsible for the adsorbate−adsorbent interaction. The effect of window diameter becomes apparent when the molecular size of aromatic sulfur compounds is comparable or smaller than the window diameter of MOFs. The present study may provide some clues to the selection and synthesis of efficient MOFs for adsorptive desulfurization.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07546. IR spectra, XRD patterns, N2 adsorption isotherms, adsorption isotherm of CO on MIL-101(Cr), and adsorption isotherm of thiophene on MIL-53(Cr). (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*X.-Q.L.: E-mail: [email protected]. *L.-B.S.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support of this work by the National Natural Science Foundation of China (21576137), the Distinguished Youth Foundation of Jiangsu Province (BK20130045), the Fok Ying-Tong Education Foundation (141069), the National High Technology Research and Development Program of China (863 Program, 2013AA032003), the National Basic Research Program of China (973 Program, 2013CB733504), and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions. 21975

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