Article pubs.acs.org/cm
Selective Adsorption of Volatile Hydrocarbons and Gases in High Surface Area Chalcogels Containing [ES3]3− Anions (E = As, Sb) Ejaz Ahmed,† Jayaprakash Khanderi,† Dalaver H. Anjum,‡ and Alexander Rothenberger*,† †
Physical Sciences and Engineering Division and ‡Imaging and Characterization Core Lab, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia S Supporting Information *
ABSTRACT: We describe the sol−gel synthesis of the two new chalcogels KFeSbS3 and NaFeAsS3, which demonstrate excellent adsorption selectivity for volatile hydrocarbons and gases. These predominantly mesoporous materials have been synthesized by reacting Fe(OAc)2 with K3SbS3 or Na3AsS3 in a formamide/water mixture at room temperature. Aerogels obtained after supercritical drying have BET surface areas of 636 m2/g and 505 m2/g for KFeSbS3 and NaFeAsS3, respectively, with pore sizes in the micro- (below 2 nm), meso- (2−50 nm), and macro- (above 50 nm) regions.
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INTRODUCTION Owing to the potential applications of aromatic and aliphatic hydrocarbons in chemical and petrochemical industries, their separation has attracted considerable attention.1−3 A close relationship in their boiling points (especially from C4−C10 hydrocarbons) highlights the need for an efficient separation technique.4 Different methods have been reported in the literature for their effective separation, e.g., liquid extraction and extractive distillation;5,6 the second method requires another high boiling-point solvent like sulfolane, N-methyl pyrrolidone, N-formyl morpholine, ethylene glycol, etc. However, an additional distillation process is required to remove solvent from the extract and raffinate phase (the phase remaining after extraction of some specified solute). Also, ionic liquids were applied as promising alternatives to the above-mentioned organic solvents.7 Though ionic liquids have improved the separation process, some drawbacks are also associated with the use of ionic liquids; for example, they are relatively expensive and ionic liquids with halide ions, which are highly corrosive.4 The use of polymeric membranes for the separations of aromatic/aliphatic hydrocarbons has also been practiced due to its energy saving potential and a relatively high separation factor,8,9 but the low flux and harsh operating conditions are the major barriers in this method.4 Consequently, there is still a need for an economic and efficient method for separating aromatic and aliphatic hydrocarbons with a material with high selective capacity and chemical as well as thermal stability. Improvements © 2014 American Chemical Society
of present separation techniques and the search for new methods stimulate investigations of hydrocarbon adsorption with porous materials. Adsorption using porous materials is well-known as an energyefficient, highly selective, and relatively inexpensive procedure. Porous materials with large surface area and pore volume are the most commonly used adsorbents, including polymeric resins,10 zeolites,11 metal organic frameworks,1 silica gel, and activated carbons.12 Recently we have started investigating the adsorption of volatile hydrocarbons using aerogels made from metal chalcogenides (chalcogels).13a Such chalcogels are porous inorganic materials in which nanosized building blocks are interconnected to yield a polymeric network comprising high surface area, large pore size distribution, surface polarizability, and chemical selectivity.13−19 Owing to their remarkable features, these chalcogels show potential applications in catalysis,14 photoluminescence,15 selective gas adsorption,16,19 and heavy metal aqueous waste remediation.17 Metal chalcogels are conveniently prepared by the sol−gel approach. On the internal surface of the material electron-rich chalcogenide ions [Qx]2− (Q = S, Se, Te; x = 1−5) are located that enhance the surface polarizability. It has been observed that the interaction of polarizable adsorptives with the softer surface Received: August 10, 2014 Revised: October 20, 2014 Published: October 28, 2014 6454
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of chalcogenide-based aerogels is stronger than interactions with conventional aerogels like metal oxides, carbons, and organic polymers.18,19 Kanatzidis and co-workers successfully synthesized a variety of chalcogenide aerogels using primary building blocks of main group elements such as tetrahedral clusters ([MQ4]4− and [M2Q6]4−, M = Sn, Ge/Q = S, Se)13b,16,20 or adamantane clusters ([M4Q10]4−, M = Sn, Ge/Q = S, Se)17 and ditopic linear polychalcogenido anions (Sx2−; x = 3−6).21,22 Moreover, transition metal sulfides like tetrahedral anions ([M′S4]2−, M′ = Mo, W) as well as the triangular molybdenum sulfide cluster [Mo3S13]2− have already been employed as precursors.18a,19 It has been recognized that the choice of different chalcogenide clusters, complexes, and salts can give rise to a wide variety of functional porous materials (e.g., selective adsorption of gases, ions, or molecules and ion-exchange) that can surpass traditional metal−oxide aerogels.13,16−19,21−24 As a contribution to the field we expand the synthetic toolbox and report the first chalcogel examples where main group trigonal pyramidal anions ([ES3]3−, E = As and Sb) are the spacers between transition metal ions, forming an anionic porous network with additional alkali metal ions for charge compensation. In the following, the synthesis, characterization, and possible applications, including adsorption of volatile hydrocarbons and gases of two new iron-chalcogenide aerogels KFeSbS3 and NaFeAsS3, are described.
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Figure 1. Photographic illustration of a typical sol−gel process: (a) yellow solution of iron acetate in water, (b) light yellow solution of K3SbS3 (inorganic building blocks) in formamide, (c) mixture of precursor solutions (a and b) at room temperature undergoing gelation, and (d) completed metathesis reaction yielding a black chalcogel KFeSbS3.
Supercritical Drying. Drying of the chalcogels was carried out with an Autosamdri-815B instrument. During supercritical drying, the sample was soaked with liquid CO2 and flushed 15−18 times over a period of 8 h (for 10 cm3 of KFeSbS3 sample) and 6 h (for 8 cm3 of NaFeAsS3 sample) at 10 °C to completely remove the ethanol from the wet gel. Finally, aerogels were obtained after supercritical drying at 35 °C. Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS). SEM images and EDS of the aerogel samples were taken with Quanta 3D FEG. Powdered aerogel samples were placed on carbon tape and taken into the instrument chamber for image capture. Infrared Spectroscopy. Infrared spectra of solid samples were obtained on a Thermo Nicolet 6700 FT-IR system. The spectra of the chalcogels were obtained on fine powders with a resolution of 2 cm−1. Transmission Electron Microscopy (TEM). TEM samples were prepared by suspending the powder aerogel samples in ether and then casting on carbon-coated Cu grid. High-resolution TEM images were obtained using a FEI Company’s Titan G2 80-300 cryo-Twin operated at the accelerating voltage of 300 kV. The electron micrographs were recorded on a charge-coupled devices (CCD) of model US4000 from Gatan, Inc. and this camera was attached below the plate-camera of the microscope. Thermogravimetric Analysis (TGA). The TGA measurements were performed on a NETZSCH STA 449 F3 thermogravimetric analyzer using a heating rate of 20 K/min under N2 flow (20 mL/min). Adsorption Measurements. All physisorption measurements were carried out on a Micromeritics ASAP 2020 HD instrument with Kalrez modification. Nitrogen. Nitrogen adsorption and desorption isotherms were performed at 77 K. About 100−200 mg of sample was taken for each analysis. Before the analysis, samples were degassed at 348 K under vacuum ( 0.5) where meso- (2 < d < 50 nm) and macropores (d > 50 nm) are the key contributors. The adsorption of toluene was also studied by FTIR spectroscopy, and some of the characteristic vibrations have been observed (Figure 5); e.g., the FTIR spectrum of toluene
NaFeAsS3
surface area (m2/g)
toluene adsorption (mg/g)
temperature (K)
2970 3980 740 704 1507 990
585 1096 267 150 234 109
298 298 298 293 298 298
35a 35b 35b 35b 28 12
440 535 558 461 573 636
16 143 276 119 313 277
298 298 298 298 298 298
505
211
298
12 12 13a 13a 13a this work this work
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removal of toluene vapors, and it has been observed that the same chalcogel sample can be utilized for further adsorption studies. The synthesized chalcogels have also been investigated for the adsorption of different gases like CO2, CH4, and H2, where CO2 exhibits the highest adsorption in the aerogels (Figure 6). As CO2 is more polarizable than CH4 and H2 (polarizability α in units of 10−24 cm3: α (CO2) = 2.9 > α (CH4) = 2.6 > α (H2) = 0.8) and their adsorption can be explained by the interaction of CO2 with the sulfidic surface through dispersion forces.29 These results also indicate the potential suitability of chalcogels for gas separation (especially separation of CO2 from H2 during the water gas shift reaction). It has also been determined that the adsorption of gases is lower at 273 K as compared to the adsorption at 263 K (Figure 6).
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CONCLUSIONS We have applied the metathesis strategy established by Kanatzidis and co-workers13b,16−19,21−24 and described the synthesis, characterization, and possible applications of two new quaternary chalcogels. KFeSbS3 and NaFeAsS3 indicate further potential applications for chalcogels in the separation of aromatics from aliphatic hydrocarbons at relatively low temperatures, i.e., room temperature. The adsorption capacities of the three volatile hydrocarbons are found in the following order: toluene > cyclohexane > cyclopentane. It has been established that the adsorption capacity for aromatic hydrocarbon (toluene) in our synthesized chalcogels is relatively higher than in activated carbon, silica gel, and 13X zeolite (having comparable surface area)12 and in some MOFs like Cu3(BTC)2 (BET = 1507 m2/g) as well as some polyoxometalate incorporated MOFs.28 The hydrocarbon adsorption capacity depends on the specific surface areas of the materials, which can be customized by changing the composition. Because of the higher adsorption capacity of toluene, such porous materials could also be useful for catalytic oxidation reactions. Toluene is a typical aromatic hydrocarbon that possesses three primary C−H bonds and can be oxidized to several oxygenates like benzyl alcohol, benzaldehyde, benzoic acid, benzoate, etc.30 In addition, the relatively higher adsorption capacities and affinity for volatile hydrocarbons, especially toluene, can make chalcogels very promising for application in the adsorption or removal of volatile organic compounds (VOCs) which are the most common air pollutants.31 Toluene
Figure 5. Infrared spectra of KFeSbS3 after toluene adsorption at room temperature and desorption after heating at 100 °C for 2 h.
adsorbed on the KFeSbS3 chalcogel showed only little differences in the CC stretching vibrations (1454, 1502, and 1592 cm−1), but the wavenumbers of the CH stretching vibration bands of the aromatic ring (3016 cm−1) and methyl group were shifted (2919 and 2861 cm−1)27a to a bit lower values maybe because of some interaction with the chalcogel framework. The lower vibrational modes are found in the region from 688 to 725 cm−1 which could be assigned to ring-bending and out-of-plane CH bending bands.27d It has been observed that the adsorbed quantity of the toluene vapors over the iron chalcogels is relatively higher than the adsorbed amount reported for activated carbon, silica gel, and 13X zeolite (having comparable surface area)12 and some metal organic frameworks (MOFs) like Cu3(BTC)2 (BET = 1507 m2/g) as well as some polyoxometalate MOFs (Table 2).28 In order to recover the adsorbed toluene, the chalcogel samples were heated at 100 °C for 2 h under vacuum. After the drying treatment, FTIR spectra have been recorded which indicate almost complete desorption/recovery of toluene; e.g, the FTIR spectrum for KFeSbS3 shows the absence of the characteristic vibration bands for toluene after drying (Figure 5). The adsorption phenomena have also been studied after the 6458
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Figure 6. Adsorption isotherms of CO2, CH4, and H2 observed in (a) KFeSbS3 at 263 K, (b) KFeSbS3 at 273 K, (c) NaFeAsS3 at 263 K, and (d) NaFeAsS3 at 273 K.
effective use in hydrocarbon adsorption/separation or purification and selective gas separation (especially separation of CO2 from H2 during the water gas shift reaction). A comparison of the adsorption capacity of toluene (Table 2) and a comparison of selectivity of gases (CO2/H2, CO2/CH4) in different porous materials (Table 3) have been provided. From the adsorption isotherms of CO2, CH4, and H2, the selectivities of adsorption of equimolar gas mixtures in the chalcogels have been calculated according to the ideal adsorbed solution theory (IAST; Supporting Information, Figure S5-9, Table S1-2).16,19,34 It has been observed that the selectivities of the gas pair CO2/H2 is close to 200 in both chalcogels while the selectivities for the gas pair CO2/CH4, 100 and 60, have been observed in NaFeAsS3 and KFeSbS3, respectively. This indicates a large difference in the adsorbed amounts of carbon dioxide in comparison to hydrogen and methane (Supporting Information, Figure S5-9).
Table 3. Comparison of CO2/H2 and CO2/CH4 Selectivities in Different Porous Materials adsorbent FAU-type zeolite Cu-BTC MOF-5 (IRMOF-1) SIFSIX-2-Cu-i SIFSIX-3-Zn Mg2(dobdc) Chalcogel (Co, Ni-MoS4) Chalcogel (CoMo3S13) Chalcogel-Pt-1 (PtGeS) Chalcogel-PtSb-1 (PtSbGeSe) KFeSbS3 NaFeAsS3
surface area (m2/g)
temperature (K)
selectivity of CO2/H2, CO2/ CH4
3213 1600 2304
298 298 296
18, NA 60, 10 25, 3
36 37 37
735 250 1800 340
298 298 313 273
33, 240 >1800, 231 800, NA 16, NA
38 38 39 18a
570
273
120, NA
19
323
273
6, NA
16
226
273
12, NA
16
636
273
217, 60
505
273
188, 100
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ASSOCIATED CONTENT
* Supporting Information S
EDS/TEM results, TGA, powder diffraction patterns, gases selectivity data, and IAST calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
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is known as one of the most representative VOC compounds because of its high POCP (photochemical ozone creativity potential) and adverse effect on the human body, particularly on the nerve system.32,33 Therefore, our synthesized chalcogels might be valuable for the design of the adsorption-based process for the disposal of wastes containing VOCs. These initial results open up new prospects for chalcogenide-based materials for
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 6459
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2001. (c) Huang, C. Y.; Song, M.; Gu, Z. Y.; Wang, H. F.; Yan, X. P. Environ. Sci. Technol. 2011, 45, 4490. (d) Smith, B. C. Infrared Spectral Interpretation: A Systematic Approach, 1st ed.; CRC Press; New York, 1998. (28) Ma, F. J.; Liu, S. X.; Liang, D. D.; Ren, G. J.; Wei, F.; Chen, Y. G.; Su, Z. M. J. Solid State Chem. 2011, 184, 3034. (29) Lide, D. R. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: New York, 2000. (30) Acharyya, S. S.; Ghosh, S.; Tiwari, R.; Sarkar, B.; Singha, R. K.; Pendem, C.; Sasaki, T.; Bal, R. Green Chem. 2014, 16, 2500. (31) Rahmani, F.; Haghighi, M.; Estifaee, P. Microporous Mesoporous Mater. 2014, 185, 213. (32) Ousmane, M.; Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Venezia, A. M.; Deganello, G.; Retailleau, L.; Boreave, A.; Giroir-Fendler, A. Appl. Catal., B 2011, 101, 629. (33) Sivaramakrishnan, R.; Tranter, R. S.; Brezinsky, K. Combust. Flame 2004, 139, 340. (34) Myers, A. L. Adsorption-J. Int. Adsorpt. Soc. 2003, 9, 9. (35) (a) Yang, K.; Xue, F.; Sun, Q.; Yue, R.; Lin, D. J. Environ. Chem. Eng. 2013, 1, 713. (b) Yang, K.; Xue, F.; Sun, Q.; Lin, D. J. Hazard. Mater. 2011, 195, 124. (36) Ducrot-Boisgontier, C.; Parmentier, J.; Faour, A.; Patarin, J.; Pirngruber, G. D. Energy Fuels 2010, 24, 3595. (37) (a) Yang, Q.; Zhong, C. J. Phys. Chem. B 2006, 110, 17776. (b) Zhao, Z.; Li, Z.; Lin, Y. S. Ind. Eng. Chem. Res. 2009, 48, 10015. (38) Nugent, P.; Belmabkhout, Y.; Burd, S. D.; Cairns, A. J.; Luebke, R.; Forrest, K.; Pham, T.; Ma, S.; Space, B.; Wojtas, L.; Eddaoudi, M.; Zaworotko, M. J. Nature 2013, 495, 80. (39) Herm, Z. R.; Swisher, J. A.; Smit, B.; Krishna, R.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 5664.
ACKNOWLEDGMENTS Electron microscope imaging work (TEM, SEM) was performed in the Advanced Nanofabrication, Imaging and Characterization Core Lab at KAUST. This research was supported by King Abdullah University of Science and Technology (KAUST) baseline funding.
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REFERENCES
(1) Herm, Z. R.; Bloch, E. D.; Long, J. R. Chem. Mater. 2014, 26, 323. (2) Rolling, P.; Lamers, M.; Staudt, C. J. Membr. Sci. 2010, 362, 154. (3) Deng, Z.; Nicolas, C. H.; Guo, Y.; Giroir-Fendler, A.; Pera-Titus, M. Sep. Purif. Technol. 2011, 80, 323. (4) Yu, H. Y.; Cai, Z. Z.; Liu, X. Y.; Li, M.; Shi, Z. S.; Cui, Z. C. J. Appl. Polym. Sci. 2014, 131, 40156. (5) Aguero, F. N.; Barbero, B. P.; Gambaro, L.; Cadus, L. E. Appl. Catal., B 2009, 91, 108. (6) Hamid, S. H.; Ali, M. A. Energy Sources 1996, 18, 65. (7) Meindersma, G. W.; de Haan, A. B. Chem. Eng. Res. Des. 2008, 86, 745. (8) Ren, J. Z.; Staudt-Bickel, C.; Lichtenthaler, R. N. Sep. Purif. Technol. 2001, 22−3, 31. (9) Matsui, S.; Paul, D. R. J. Membr. Sci. 2002, 195, 229. (10) Simpson, E. J.; Abukhadra, R. K.; Koros, W. J.; Schechter, R. S. Ind. Eng. Chem. Res. 1993, 32, 2269. (11) Ghiaci, M.; Abbaspur, A.; Kia, R.; Seyedeyn-Azad, F. Sep. Purif. Technol. 2004, 40, 217. (12) Wang, C. M.; Chang, K. S.; Chung, T. W.; Wu, H. D. J. Chem. Eng. Data 2004, 49, 527. (13) (a) Ahmed, E.; Rothenberger, A. Microporous Mesoporous Mater. 2014, 199, 74. (b) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.; Kanatzidis, M. G. Science 2007, 317, 490. (c) Irina, R. P.; Indika, U.; Arachchige, I. U.; Georgiev, D. G.; Brock, S. L. Angew. Chem. 2010, 122, 3743. (d) Mohanan, J. L.; Arachchige, I. U.; Brock, S. L. Science 2005, 307, 397. (e) Yao, Q. H.; Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2009, 131, 2800. (14) Yuhas, B. D.; Smeigh, A. L.; Samuel, A. P. S.; Shim, Y.; Bag, S.; Douvalis, A. P.; Wasielewski, M. R.; Kanatzidis, M. G. J. Am. Chem. Soc. 2011, 133, 7252. (15) Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2007, 129, 1840. (16) Bag, S.; Kanatzidis, M. G. J. Am. Chem. Soc. 2010, 132, 14951. (17) Oh, Y.; Bag, S.; Malliakas, C. D.; Kanatzidis, M. G. Chem. Mater. 2011, 23, 2447. (18) (a) Bag, S.; Gaudette, A. F.; Bussell, M. E.; Kanatzidis, M. G. Nat. Chem. 2009, 1, 217. (b) Yao, Q. H.; Brock, S. L. Inorg. Chem. 2011, 50, 9985. (c) Arachchige, I. U.; Brock, S. L. Acc. Chem. Res. 2007, 40, 801. (d) Arachchige, I. U.; Brock, S. L. J. Am. Chem. Soc. 2006, 128, 7964. (19) Shafaei-Fallah, M.; Rothenberger, A.; Katsoulidis, A. P.; He, J. Q.; Malliakas, C. D.; Kanatzidis, M. G. Adv. Mater. 2011, 23, 4857. (20) Bag, S.; Arachchige, I. U.; Kanatzidis, M. G. J. Mater. Chem. 2008, 18, 3628. (21) Shafaei-Fallah, M.; He, J. Q.; Rothenberger, A.; Kanatzidis, M. G. J. Am. Chem. Soc. 2011, 133, 1200. (22) Oh, Y.; Morris, C. D.; Kanatzidis, M. G. J. Am. Chem. Soc. 2012, 134, 14604. (23) Polychronopoulou, K.; Malliakas, C. D.; He, J. Q.; Kanatzidis, M. G. Chem. Mater. 2012, 24, 3380. (24) Riley, B. J.; Chun, J.; Um, W.; Lepry, W. C.; Matyas, J.; Olszta, M. J.; Li, X. H.; Polychronopoulou, K.; Kanatzidis, M. G. Environ. Sci. Technol. 2013, 47, 7540. (25) Sommer, H.; Hoppe, R. Z. Anorg. Allg. Chem. 1977, 430, 199. (26) (a) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids: Principles, Methodology, and Applications; Academic Press: San Diego, CA, 1999. (b) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (27) (a) Palomares, A. E.; EderMirth, G.; Lercher, J. A. J. Catal. 1997, 168, 442. (b) Loconto, P. R. Trace Environmental Quantitative Analysis: Principles, Techniques and Applications, 2nd ed.; CRC Press, New York, 6460
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