Low-temperature loading of Cu+ species over porous metal-organic frameworks (MOFs) and adsorptive desulfurization with Cu+-loaded MOFs

Journal of Hazardous Materials 237–238 (2012) 180–185

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Low-temperature loading of Cu+ species over porous metal-organic frameworks (MOFs) and adsorptive desulfurization with Cu+ -loaded MOFs Nazmul Abedin Khan, Sung Hwa Jhung ∗ Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Cu+ can be introduced in one step over metal-organic frameworks (MOFs).  Cu+ -loaded MOFs show high uptake of benzothiophene.  Cu+ and MOFs show a synergetic effect in the adsorption.  Cu+ -loaded MOFs are very stable and lead to ␲-complexation to benzothiophene.

a r t i c l e

i n f o

Article history: Received 12 June 2012 Received in revised form 10 August 2012 Accepted 11 August 2012 Available online 19 August 2012 Keywords: Supported cuprous ion Facile synthesis Metal-organic framework Desulfurization Adsorption

a b s t r a c t Porous metal-organic frameworks (MOFs, MIL-100-Fe, iron-benzenetricarboxylate) supported with Cu+ species were obtained for the first time under mild condition without high temperature calcinations. The Cu+ -loaded MOFs were evaluated as efficient adsorbents for the liquid-phase adsorption of benzothiophene (BT). The effect of Cu+ loading on the adsorption kinetics and maximum adsorption capacity (Q0 ) for the adsorption of BT was also studied. Q0 increased with increasing copper loading up to a Cu/Fe (wt./wt.) ratio of 0.07 in Cu+ -loaded-MIL-100-Fe, resulting in an increase in the Q0 by 14% compared with the virgin MIL-100-Fe without Cu+ ions. Since the surface area and pore volume decrease with the loading of copper, the increased Q0 over the Cu+ -loaded MIL-100-Fe adsorbents suggests specific favorable interactions (probably by ␲-complexation) between Cu+ and BT. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Recently, there has been considerable demand to reduce the content of sulfur-containing compounds such as thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT) and dimethyldibenzothiophene (DMDBT) in commercial diesel or gasoline to very low levels in order to prevent air pollution from SOx and deactivation of catalysts. For example, based on EU and US guidelines, the sulfur level in fuels should be less than 10 and 15 ␮g/g respectively [1–5]. So far, various methods such as hydrodesulfurization, oxidation and adsorption have been investigated for sulfur removal [1–5],

∗ Corresponding author. Tel.: +82 53 950 5341; fax: +82 53 950 6330. E-mail address: [email protected] (S.H. Jhung). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.025

and adsorption has been regarded as one of the most competitive methods especially for ultralow sulfur content [5]. Various porous adsorbents such as activated carbons [6–15], zeolites [16–19] and mesoporous materials [20] have been studied for adsorptive removal of sulfur compounds. For efficient removal, not only adequate porosity/pore size but also specific adsorption sites are required [6,12]. Introduction of an acidic component to porous materials results in a specific adsorption of slightly basic sulfur compounds through acid–base interactions [13–17,21]. Moreover, it has been reported that metal ions like Cu+ , Ag+ , Pd2+ and Pt2+ have an adsorption capability for thiophenic compounds through ␲-complex formation [18,19,22,23]. Yang et al. have developed ␲-complex-based adsorbents for desulfurization [18]. Generally, ␲-complexation adsorbents have been prepared by high temperature calcination of metal-ion-exchanged zeolitic materials [18].

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Additionally, Cu2 O-loaded porous materials like Al2 O3 , MCM-41, and SBA-15 have also been reported for adsorptive removal of sulfur compounds through ␲-complex formation between Cu+ and sulfur compounds [24,25]. The loading of Cu2 O has been usually done through the following complex steps: (i) impregnation of copper salts onto a porous support; (ii) formation of CuO through calcination in the presence of air and finally (iii) reduction of CuO to Cu2 O with high temperature (around 700 ◦ C) calcination at inert condition. Even though the ␲-complex-based adsorbents are very selective toward thiophenics, a porous material is needed for high dispersion. Importantly, porous materials with low thermal stability such as metal-organic frameworks (MOFs, see below) and carbons cannot be used as support materials for active metal components because of the high-temperature treatment. Therefore, it is important to find some convenient methods to load the active component Cu+ onto thermally unstable and highly porous supports without high temperature calcination. MOFs are fascinating materials as supports for catalysts and adsorbents because of the designable crystalline structure, huge porosity and easy tunability of their pore size and shape from a microporous to mesoporous scale [26–29]. Recently, MOF-type materials have also been investigated for the adsorptive removal of hazardous materials such as benzene [30,31] and dyes [32,33]. Sulfur-containing compounds have also been adsorbed/removed with MOFs both in liquid [34–40] and gas phases [41,42]. A few important factors, such as coordinatively unsaturated sites (CUS or open metal sites) [34], pore functionalities [42], and acid–base interactions [38,39] have been suggested for the efficient adsorptive removal of sulfurcontaining compounds with MOFs. Interestingly, CuCl2 -loaded MIL-47 (vanadium-benzenedicarboxylate) showed remarkable adsorptive performance for BT adsorption which involved reducing Cu2+ to Cu+ (by V3+ of the MIL-47) at ambient condition for efficient ␲-complex formation with sulfur compounds [43]. However, it is difficult to prepare ␲-complexation adsorbents such as Cu2 O-loaded MOFs through high temperature calcination because of the low thermal stability of the MOFs [44,45]. Among the numerous MOFs reported so far, one of the most popular solid is the porous iron-benzenetricarboxylate (Fe-BTC named MIL-100-Fe) [46–48], which has been studied for its potential applications. MIL-100-Fe, with a chemical formula of Fe(III) 3 O(H2 O)2 (F){C6 H3 (CO2 )3 }2 ·nH2 O (n ∼ 14.5), has been investigated for its catalysis [47] and adsorption [48,49] properties. Herein, we report the loading of Cu+ on the porous MIL-100-Fe material through low-temperature synthesis of Cu2 O particles for the first time. Moreover, Cu+ -supported MIL-100-Fe was applied in adsorptive desulfurization of benzothiophene to take advantage of the Cu+ sites for adsorption.

2. Experimental

181

2.2. Preparation of adsorbents MIL-100-Fe was synthesized from metallic iron, H3 BTC, HF, HNO3 and deionized water following the reported method [46] under an autogenous pressure at 180 ◦ C. The reactant composition for the syntheses was 1.0 Fe0 :0.66 H3 BTC:2.0 HF:1.2 HNO3 :280 H2 O. A reaction mixture of 30 g was loaded into a 100 mL Teflon autoclave, which was sealed and placed in a microwave oven (MARS-5, CEM, maximum power of 1200 W) in order to take advantage of the rapid synthesis under microwave irradiation [50–52]. The autoclave in the microwave oven was heated to the reaction temperature in 2 min and maintained for 2 h. The microwave power was 400 W throughout all the synthesis steps including the heating-up stage. After the crystallization for 2 h, the autoclave was cooled to room temperature and a light orange solid product was recovered by filtration. The purification of the as-synthesized MIL100-Fe was performed in two steps using hot water (stirring 1.0 g of MIL-100-Fe into 350 mL of water at 80 ◦ C for 5 h) and hot ethanol (stirring 1.0 g of MIL-100-Fe into 200 mL of ethanol at 60 ◦ C for 3 h). The purified MIL-100-Fe, after filtration, was dried overnight in air and stored in a desiccator. The reaction mixture for cuprous oxide (Cu2 O) was prepared similar to previously reported methods [53,54]. In a typical procedure, a solution mixture was prepared by mixing 1.0 mL of 0.68 M copper sulfate, 17 mL of deionized water, and 0.30 g of 0.50 mM PVP in a 30 mL glass vial. The solution mixture was stirred for 10 min, and then a mixture of 1.0 mL of 1.2 M sodium carbonate and 1.0 mL of 0.74 M sodium citrate was added dropwise into the above solution. A dark blue color soon appeared without any precipitation. After about 10 min, 1 mL of 1.4 M glucose solution was slowly dropped into it. MIL-100-Fe (0.3 g) was added into 5, 10 and 15 mL of Cu2 O precursor solutions and stirred magnetically for 5 h at room temperature. Finally the reaction mixture was loaded in a Teflon lined autoclave (30 mL) and put in a preheated electric oven at 80 ◦ C for 2 h. After the reaction, the autoclave was cooled to room temperature and the solid products were recovered by filtration and washing several times (with water and absolute ethanol) similar to the previously described method [53]. The obtained solids were dried overnight at 80 ◦ C and named as Cu+ (L), Cu+ (M) and Cu+ (H) for the 5, 10 and 15 mL Cu2 O precursor, respectively; additionally, the virgin MIL-100-Fe without cuprous ion was designated as Cu+ (0). The phase of the samples was determined with an X-ray diffractometer (D2 Phaser, Bruker, CuK␣ radiation). The crystal morphology and composition (Cu/Fe) of the samples were examined with a FE-SEM and EDS (field emission scanning electron microscope and energy-dispersive X-ray spectroscopy, Hitachi, S-4300), respectively. The nitrogen adsorption isotherms were obtained at −196 ◦ C with a surface area and porosity analyzer (Micromeritics, Tristar II 3020) after evacuation of the adsorbents at 150 ◦ C for 12 h. The surface area was calculated with the BET equation.

2.1. Materials 2.3. Adsorption experiments Metallic iron, glucose (C6 H12 O6 ) and hydrofluoric acid (HF, 40%) were purchased from DC Chemical, Korea. 1,3,5Benzenetricarboxylic acid or trimesic acid (H3 BTC, C9 O6 H6 , 98%), n-octane (C8 H18 , 99%) and benzothiophene (C8 H6 S, 98%) were purchased from Sigma–Aldrich Co. Ethanol (C2 H5 OH, 94%), nitric acid (HNO3 , 60%), sodium citrate (C6 H5 O7 Na3 ·2H2 O, 99%) and copper sulfate (CuSO4 ·5H2 O, 99%) were purchased from OCI company Ltd., Korea. PVP (polyvinylpyrrolidone, K-30, MW : 40,000) and anhydrous sodium carbonate (Na2 CO3 , 99%) were purchased from Junsei Chemicals Co. Ltd. (Japan) and Daejung Chemicals Co. (Korea), respectively. All the chemicals used in this study were analytical grade and used without further purification.

A stock solution of BT (10,000 ␮g/g) was prepared by dissolving BT in n-octane. BT solutions with different concentrations of BT (5000–50 ␮g/g) were prepared by successive dilutions of the stock solution with n-octane. The BT concentrations were determined using a gas chromatography (DS 6200, DS Science Inc.) equipped with a flame ionization detector. Before adsorption, the adsorbents were dried overnight under vacuum at 100 ◦ C and were kept in a desiccator. The adsorbents were put into BT solutions with fixed BT concentrations. The BT solutions containing the adsorbents were mixed well with magnetic stirring and maintained for a fixed period of time (1–24 h)

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(a)

+

Cu (0) + Cu (L) + Cu (M) + Cu (H)

Cu2O

(b)

+

*

Cu (L) + Cu (M) + Cu (H)

Intensity (a.u)

Intensity (a.u)

*

5

10

15

20

25

30

20

30

40

2 theta (deg)

50

*

60

2 theta (deg)

Fig. 1. XRD patterns of (a) virgin and various Cu+ -supported adsorbents; (b) Cu2 O and various Cu+ -supported adsorbents in high 2Â region. ‘*’ represents the diffractions from Cu2 O.

Fig. 2. FE-SEM images of (a) Cu+ (0) and (b) Cu+ (M).

at a constant temperature of 25 ◦ C. After adsorption for a predetermined time, the solution was separated from the adsorbents with a syringe filter (PTFE, hydrophobic, 0.5 ␮m), and the BT concentration was measured. The maximum adsorption capacity was calculated with the Langmuir adsorption isotherm (see supporting information) after adsorption for 24 h in various conditions. The adsorption kinetic constants were obtained with adsorption data for various times using the pseudo-second-order rate equation that were obtained with a non-linear model (see supporting information).

high and medium Cu2 O content, respectively) at 2Â of 36.3, 42.3 and 60.9, suggesting the synthesis of octahedral Cu2 O (JCPDS-05-0667) [53] when the content of copper is higher than a certain value. Even though the EDS and XPS analyses confirm the presence of the copper species, no XRD peaks of Cu2 O were observed for the Cu+ (L) adsorbent. This may be due to a well-dispersed and low content of the Cu2 O particles supported on the porous MIL-100-Fe frameworks. However, it may be concluded that the synthesis procedure of Cu+ or Cu2 O is feasible even in the presence of a porous MOF MIL-100-Fe or Cu+ (0).

3. Result and discussion +

3.1. Physical properties of the adsorbents

Intensity (a.u)

The crystal structure of the synthesized MOFs was confirmed to be the MIL-100-Fe as determined by the XRD patterns in Fig. 1a [46,49]. As shown in Figs. 1a and 2, there is no change in the crystal structure or morphology after the loading of the Cu+ species. The presence of the copper in the Cu+ -loaded adsorbents was confirmed with an EDS analysis. The Cu/Fe compositions (based on wt./wt.) of the prepared adsorbents were found to be 0.05, 0.07 and 0.1 for the Cu+ (L), Cu+ (M) and Cu+ (H), respectively (Table 1). The oxidation state of the loaded copper (before adsorption of BT) was determined with an X-ray photoelectron spectroscopy (XPS). As shown in Fig. 3, the oxidation state of the loaded copper species was +1. Fig. 1b shows the XRD patterns of the adsorbents in the high 2Â region to confirm the presence of Cu+ (in the form of Cu2 O). As shown in the figure, the characteristic peaks of Cu2 O at very low intensity were observed for Cu+ (H) and Cu+ (M) (adsorbents having

Cu (H) +

Cu (M) +

Cu (L) CuCl CuCl2

970

960

950

940

930

Binding energy (eV) Fig. 3. XPS spectra of Cu+ -supported MIL-100-Fe adsorbents in copper region. The XPS spectra of CuCl2 and CuCl are also included as reference materials.

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183

Table 1 Physiochemical properties and the results for the BT adsorption of virgin and Cu+ -supported adsorbents. Adsorbent +

Cu (0) Cu+ (L) Cu+ (M) Cu+ (H) a b c

Cu/Fe (wt./wt.)a

BET surface area (m2 /g)

Total pore volume (cm3 /g)

k2 b (non linear) (g/mg h)

Q0 (mg/g)

0.0 0.05 0.07 0.1

2163 2076 1964 1674

0.99 0.97 0.92 0.80

0.0184 0.0176 0.0165 0.0163

135 NDc 154 NDc

Based on EDS analysis. At 1000 ␮g/g of BT. Not determined.

600

3

Adsorbed amount (cm /g)

700

500 +

Cu (0) + Cu (L) + Cu (M) + Cu (H)

400

300

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po) Fig. 4. Nitrogen adsorption isotherms of the virgin and Cu+ -supported MIL-100-Fe adsorbents.

The nitrogen adsorption isotherms and summarized textural properties of all the adsorbents including the Cu+ (0) are shown in Fig. 4 and Table 1, respectively. The surface area and total pore volume decreased with increasing loading of the Cu+ species, which may indicate the presence of some guest materials (or probably Cu2 O) supported on the Cu+ (0) framework. However, the decrease in the porosity and surface area were not severe excluding the Cu+ (H). The adsorbed nitrogen over Cu+ (H) was comparatively low probably due to the partial filling of the pores of the Cu+ (0) with the excess amount of Cu+ species or Cu2 O. 3.2. Adsorption results

the adsorption times; whereas, negligible adsorption was observed with Cu2 O (or Cu+ ) itself without any support, confirming the necessity of supports for the adsorption of BT. The adsorption kinetic constants (Table 1) of the pseudo-second-order rate equation showed that the adsorption rate was slightly decreased with the Cu+ loading probably because of pore-blocking by the loaded Cu+ . The adsorbed quantity increased with an increase of Cu+ content up to a certain value and the highest adsorption of BT was obtained by Cu+ (M). Even though the Cu+ (H) had the highest Cu+ content, it showed the lowest adsorptive performance among the Cu+ -supported adsorbents, and this may be due to a too high degree of pore filling with the Cu+ or Cu2 O. Therefore, the amount of Cu+ on the support could not be increased much and further studies including BT adsorption were done only for the virgin MIL-100-Fe and Cu+ (M). The adsorption isotherms of the Cu+ (0) and Cu+ (M) were obtained after adsorption for 24 h, which is considered to be a sufficient time for adsorption equilibrium, and the results are compared in Fig. 6. The adsorption isotherms have been plotted to follow the Langmuir equation (supporting Fig. 2), and the maximum adsorption capacities Q0 are summarized in Table 1. The Q0 values for the Cu+ (0) and Cu+ (M) were 135 and 154 mg/g, respectively, assuring around a 14% increase in the maximum adsorption capacity with the Cu+ -loaded MIL-100-Fe compared to the virgin MIL-100Fe or Cu+ (0). The increased Q0 over Cu+ (M), even with a decreased porosity, suggests the presence of a specific favorable interaction between the Cu+ of the adsorbent and BT. Adsorbents with the capability of ␲-complexation have been studied widely for efficient adsorptive removal of sulfur compounds [18,19,22–25,43]. It has been reported that, loaded Cu2 O particles can also be used as a source of active Cu+ species in ␲-complex formation between adsorbents and sulfur compounds [24,25]. Cu2 O particles, however, were obtained only under high temperature treatment for the conversion of copper salts to CuO

Fig. 5 shows that the quantities of adsorbed BT over the Cu+ supported adsorbents were higher than that over the Cu+ (0) for all

120 80

100

60

80

qe (mg/g)

qt (mg/g)

+

Cu (0) + Cu (M)

+

Cu (0) + Cu (L) + Cu (M) + Cu (H) Cu2O

40

20

60 40 20

0

0 0

5

10

15

20

25

Time (h) Fig. 5. Effect of contact time and Cu+ loading on the adsorption of BT. The adsorption temperature and initial BT concentration were 25 ◦ C and 1000 ␮g/g, respectively.

0

500

1000

1500

2000

Ce (µg/g) Fig. 6. Adsorption isotherms for BT adsorption over the Cu+ (0) and Cu+ (M) at 25 ◦ C.

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and for the reduction of CuO to Cu2 O. In this study, Cu+ or Cu2 O was incorporated into thermally unstable MIL-100-Fe through a simple one-step synthesis of Cu2 O particles at low temperature. Interestingly, the incorporated Cu+ species was stable at ambient condition that does not take part in any redox reactions for days (it is quite well-known that 2Cu+ can be easily disproportionated into Cu0 and Cu2+ in the presence of water [55,56]) probably because of the highly dispersed and well-separated state on the pores of the MIL-100-Fe or Cu+ (0). 4. Conclusion In this work, porous MIL-100-Fe, one of the typical metalorganic frameworks, has been used for the first time as support materials for Cu+ species, which were obtained from one-step synthesis of Cu2 O particles at low temperature. The Cu+ /MIL-100-Fe has been employed in liquid-phase adsorption of benzothiophene (BT). Even though the surface area and pore volume decreased with Cu+ loading, the maximum adsorption capacity (Q0 ) for BT increased with the loading of Cu+ up to certain content probably because of the ␲-complexation between the BT and Cu+ species. Based on these results, it can be concluded that active Cu+ species can be incorporated facilely onto thermally unstable porous materials like metal-organic frameworks, carbons under mild condition (without selective reduction of Cu2+ to Cu+ at about 700 ◦ C). Therefore, supported Cu+ species may be successfully applied in adsorptive removal of chemicals that can form ␲-complexes with Cu+ . Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (grant number 2012004528). This research was also partly supported by Kyungpook National University Research Fund, 2011. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2012.08.025. References [1] V.C. Srivastava, An evaluation of desulfurization technologies for sulfur removal from liquid fuels, RSC. Adv. 2 (2012) 759–783. [2] B. Pawelec, R.M. Navarro, J.M. Campos-Martin, J.L.G. Fierro, Towards near zerosulfur liquid fuels: a perspective review, Catal. Sci. Technol. 1 (2011) 23–42. [3] A. Stanislaus, A. Marafi, M.S. Rana, Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production, Catal. Today 153 (2010) 1–68. [4] I.V. Babich, J.A. Moulijn, Science and technology of novel processes for deep desulfurization of oil refinery streams: a review, Fuel 82 (2003) 607–631. [5] A. Samokhvalov, B.J. Tatarchuk, Review of experimental characterization of active sites and determination of molecular mechanisms of adsorption, desorption and regeneration of the deep and ultradeep desulfurization sorbents for liquid fuels, Catal. Rev. Sci. Eng. 52 (2010) 381–410. [6] M. Seredych, E. Deliyanni, T.J. Bandosz, Role of microporosity and surface chemistry in adsorption of 4,6-dimethyldibenzothiophene on polymer-derived activated carbons, Fuel 89 (2010) 1499–1507. [7] A. Zhou, X. Ma, C.S. Song, Liquid-phase adsorption of multi-ring thiophenic sulfur compounds on carbon materials with different surface properties, J. Phys. Chem. B 110 (2006) 4699–4707. [8] C.O. Ania, T.J. Bandosz, Metal-loaded polystyrene-based activated carbons as dibenzothiophene removal media via reactive adsorption, Carbon 44 (2006) 2404–2412. [9] Y. Sano, K.H. Choi, Y. Korai, I. Mochida, Adsorptive removal of sulfur and nitrogen species from a straight run gas oil over activated carbons for its deep hydrodesulfurization, Appl. Catal. B: Environ. 49 (2004) 219–225.

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