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Partial Exchange of Fe(III) Montmorillonite with Hexadecyltrimethylammonium Cation Increases Catalytic Activity for Hydrophobic Substrates Philip J. Wallis,*,† Alan L. Chaffee,‡ Will P. Gates,§ Antonio F. Patti,†, and Janet L. Scott‡ )
† Centre for Green Chemistry, ‡Department of Chemistry, and §Department of Civil Engineering, Monash University, Clayton, Victoria 3800, Australia, and School of Applied Science and Engineering, Monash University, Churchill, Victoria 3842, Australia
Received September 3, 2009 Fe(III) montmorillonite clay that was partially exchanged with hexadecyltrimethylammonium (HDTMAþ) cations achieved increased catalytic activity for the oxidative coupling of hydrophobic organic substrates. A series of mixedcation organoclays were produced, where the organic cation content ranged from 6 to 50% relative to the cationexchange capacity (CEC) of the clay, and were tested for catalytic activity using different Fe(III)-mediated oxidative coupling reactions. Enhanced catalytic activity by Fe3þ/HDTMAþ montmorillonite for coupling hydrophobic substrates was observed, with maximum catalytic activity in the oxidative coupling of 2-naphthol observed at 6% HDTMAþ coverage. However, maximum catalytic activity with a more hydrophobic substrate, anthrone, was achieved with 50% HDTMAþ coverage, indicating that matching levels of organic modification to substrate hydrophobicity improves catalytic activity. The organization of the organic cations at the clay surfaces proved to be heterogeneous, as determined by scanning transmission X-ray microscopy (STXM) and powder X-ray diffraction. Results from molecular dynamics simulations supported the heterogeneous nature of the catalysts but also pointed toward large regions within the interlayers that may be filled with nonreactive hydrated Fe oxides resulting from the organic cation treatment. The exchangeable Fe content of the organic treated clays, as determined by AAS and ICP measurements, was observed to be higher than expected relative to that of Fe-saturated clay, substantiating this hypothesis. These findings have implications for the development of substrate-specific clay catalysts, where the composition and configuration of exchangeable cations can be matched to a particular substrate or reaction.
Introduction Factors that control the activity of modified clay minerals as catalysts include diffusion limitations and the availability of active sites.1 The diffusion of reactants to active sites within the clay (or of reaction products away from active sites) can often be limited by the availability of pores, the accessibility of diffusion channels (tortuosity), and/or steric hindrance (e.g., physical blocking of interlayer sites). Because most exchange sites of clays are located deep within the interlayer, large organic molecules may not readily penetrate the narrow interlayer space if it is not expanded or is otherwise inaccessible.2,3 Diffusion limitations can be overcome by a number of strategies, including acid activation and pillaring. Mild acid activation of the clay, achieved by treatment with strong mineral acids, can cause delamination of the clay platelets, thereby increasing the exposed surface area of the clay.4,5 One disadvantage of acid activation is that dissolution of the clay occurs, resulting in a loss of cation-exchange capacity (CEC) and thus a loss of potential catalytically active sites.6,7 *To whom correspondence should be addressed. E-mail: phil.wallis@msi. monash.edu.au. Tel: þ61 3 9905 8709. Fax: þ61 3 9905 9348. Present address: Monash Sustainability Institute, Monash University, Clayton, Victoria 3800, Australia. (1) Breen, C.; Watson, R. Appl. Clay Sci. 1998, 12, 479–494. (2) Boyd, S. A.; Shaobai, S.; Lee, J. F.; Mortland, M. M. Clays Clay Miner. 1988, 36, 125–130. (3) Churchman, G. J.; Gates, W. P.; Theng, B. K. G.; Yuan, G. Clays and Clay Minerals for Pollution Control. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006; Vol. 1, p 625. (4) Wallis, P. J.; Gates, W. P.; Patti, A. F.; Scott, J. L. Green Chem. 2007, 9, 980-986. (5) Rhodes, C. N.; Brown, D. R. Catal. Lett. 1994, 24, 285–291. (6) Kooli, F. Langmuir 2008, 25, 724–730. (7) Bovey, J.; Jones, W. J. Mater. Chem. 1995, 5, 2027–35.
4258 DOI: 10.1021/la9033047
Pillaring, where clay layers are propped apart by metal oxides or polymeric compounds arrayed in loosely random matrices that prop apart the interlayer space, can also be used to overcome diffusion limitations.8-11 The pillaring agents themselves may also display catalytic activity.11 Modified clays containing both reactive inorganic cations and hydrophobic organic cations, here termed mixed-cation organoclays, could have an enhanced affinity for hydrophobic reagents and reaction media and thus improve substrate access to active catalytic sites. Partial exchange of long-chain organic cations with Fe(III) montmorillonite might improve the interlayer diffusion of substrates, as the inclusion of hydrophobic organic cations both increases the interlayer spacing and enhances the affinity of the catalyst surface for hydrophobic substrates. Organic solvents are known to cause swelling of organoclays, thus reactions performed in organic solvents may further enhance substrate access to active sites compared to more hydrophilic cation-exchanged clays.12,13 Depending on the organic cation used and the charge density of the clay, an interlayer spacing greater than 20 A˚ can be achieved,14 even for partially intercalated material. Such partially loaded (8) Yamanaka, S.; Hattori, M. Catal. Today 1988, 2, 261–270. (9) Kou, M. R. S.; Mendioroz, S.; Salerno, P.; Munoz, V. Appl. Catal., A 2003, 240, 273–285. (10) Akcay, M. Appl. Catal., A 2004, 269, 157–160. (11) Kloprogge, J. T.; Duong, L. V.; Frost, R. L. Environ. Geol. 2005, 47, 967– 981. (12) Nzengung, V. A.; NkediKizza, P.; Jessup, R. E.; Voudrias, E. A. Environ. Sci. Technol. 1997, 31, 1470–1475. (13) Nzengung, V. A.; Voudrias, E. A.; NkediKizza, P.; Wampler, J. M.; Weaver, C. E. Environ. Sci. Technol. 1996, 30, 89–96. (14) Slade, P. G.; Gates, W. P. Appl. Clay Sci. 2004, 25, 93–101.
Published on Web 11/16/2009
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organoclays might retain a considerable fraction of the active form of exchangeable inorganic cations and have the potential to be very tunable because a variety of both inorganic and organic cations can be simultaneously intercalated, the latter in part related to the recent interest in ionic liquids.15 Few published accounts describe mixed-cation organoclays exchanged with both reactive inorganic cations and surface-modifying organic cations and rarely for catalytic applications.16-20 The relationships between the structure and function of organoclays have been probed by the investigation of bulk structural properties using analytical techniques such as powder X-ray diffraction (p-XRD), along with assumptions about charge distribution and cation conformation, in order to draw conclusions about ordering within clay particles.21,22 Clay aggregates can be analyzed on the micrometer to nanometer scale by a number of techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX).23,24 Nanometer-scale structural features of clays, including d-space heterogeneity, can be directly measured using high-resolution transmission electron microscopy (HRTEM).25,26 Synchrotronbased techniques, such as scanning transmission X-ray microscopy (STXM), can be used to analyze the distribution of elements within clay aggregates on submicrometer (tens of nanometers) scales.27,28 In the area of minerals, STXM has previously been applied to studies of heterogeneity in colloidal soil particles29 and the sorption of europium onto smectite clay30 and to identify different mineral colloids, including montmorillonite, from multiphase mixtures.31 STXM has the potential to be a useful analytical technique for probing the bulk distribution of organic molecules and certain inorganic cations on microscopic aggregates of montmorillonite clay because it does not require possibly destructive sample-preparation steps, such as required, for example, by HRTEM. Molecular dynamics (MD) simulations may be applied to model clay nanostructure, complementing microscopy methods. A number of studies have been published on the application of molecular dynamics to clay surfaces, many dealing with the determination of suitable force fields for (15) Welton, T. Coord. Chem. Rev. 2004, 248, 2459–2477. _ (16) Zyza, M.; Kzapyta, Z. Mineral. Pol. 1977, 7, 15–26. (17) Dyrek, K.; Kzapyta, Z.; Sojka, Z. Clays Clay Miner. 1983, 31, 223–229. (18) Breen, C.; Moronta, A. J. Phys. Chem. B 2000, 104, 2702–2708. (19) Breen, C.; Moronta, A. J. Phys. Chem. B 1999, 103, 5675–5680. (20) Nawani, P.; Gelfer, M. Y.; Hsiao, B. S.; Frenkel, A.; Gilman, J. W.; Khalid, S. Langmuir 2007, 23, 9808–9815. (21) Mercier, L.; Detellier, C. Clays Clay Miner. 1994, 42, 71–76. (22) He, H.; Zhou, Q.; Martens, W. N.; Kloprogge, T. J.; Yuan, P.; Xi, Y.; Zhu, J.; Frost, R. L. Clays Clay Miner. 2006, 54, 689–696. (23) Zhou, C. H.; Tong, D. S.; Bao, M.; Du, Z. X.; Ge, Z. H.; Li, X. N. Top. Catal. 2006, 39, 213–219. (24) Guo, H.; Jing, X.; Zhang, L.; Wang, J. J. Mater. Sci. 2007, 42, 6951–6955. (25) Clinard, C.; Mandalia, T.; Tchoubar, D.; Bergaya, F. Clays Clay Miner. 2003, 51, 421–429. (26) Yaron-Marcovich, D.; Chen, Y.; Nir, S.; Prost, R. Environ. Sci. Technol. 2005, 39, 1231–1238. (27) Kilcoyne, D.; Tylisczcak, T.; Steele, W. F.; Fakra, S.; Hitchcock, P.; Franck, K.; Anserson, E.; Harteneck, B.; Rightor, E. G.; Mitchell, G. E.; Hitchcock, A. P.; Yang, L.; Warwick, T.; Ade, H. J. Synchrotron Radiat. 2003, 10, 125– 136. (28) Ildefonse, P.; Calas, G.; Flank, A. M.; Lagarde, P.; Low, Z Nucl. Instrum. Methods Phys. Res., Sect. B 1995, 97, 172–175. (29) Schumacher, M.; Christl, I.; Scheinost, A. C.; Jacobsen, C.; Kretzschmar, R. Environ. Sci. Technol. 2005, 39, 9094–9100. (30) Bauer, A.; Rabung, T.; Claret, F.; Schaefer, T.; Buckau, G.; Fanghaenel, T. Appl. Clay Sci. 2005, 30, 1–10. (31) Yoon Tae, H.; Johnson Stephen, B.; Benzerara, K.; Doyle Colin, S.; Tyliszczak, T.; Shuh David, K.; Brown Gordon, E. Langmuir 2004, 20, 10361– 10366. (32) Chatterjee, A.; Ebina, T.; Onodera, Y.; Mizukami, F. J. Chem. Phys. 2004, 120, 3414–3424.
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modeling the behavior of interlayer cations and other sorbed organic molecules.32-38 The utility of clays as catalysts in organic synthesis is reflected in the vast body of research into synthesis procedures utilizing such catalysts.39-41 A majority of clay catalysis studies have been carried out using inorganic cations: usually either transition metal ions,39,42-45 including Fe3þ, or protons (in the case of acidactivated clays5,46,47) either arrayed on exchange sites or (co)adsorbed to clay surfaces. Exceptions include porphyrinclay intercalates48-51 or where an organic modifier is also the reactive species.52,53 Clays containing hydrated inorganic exchange cations are generally hydrophilic and are incapable of dispersion and swelling under hydrophobic reaction conditions, as, for example, those encountered with organic solvents such as toluene.54,55 Organoclay complexes involving clays loaded with quaternary ammonium cations (e.g., hexadecyltrimethylammonium (HDTMAþ)) are more hydrophobic than Naþ clays and as such have a higher sorptive capacity for hydrophobic substrates such as phenols.1,22,56 The aim of this study was to investigate the effects of treating Fe(III) montmorillonite clay with different quantities of a hydrophobic organic cation: hexadecyltrimethylammonium. Montmorillonite clay, exchanged with a combination of organic cations and Fe3þ cations, is predicted to provide enhanced affinity for hydrophobic organic molecules as well as reactive sites for oxidative coupling catalysis.
Experimental Section Materials. SWy-2 montmorillonite (The Clay Minerals Society’s Source Clays Project) was saturated with Naþ by (33) Chatterjee, A.; Iwasaki, T.; Ebina, T. J. Phys. Chem. A 2000, 104, 8216– 8223. (34) Rinnert, E.; Carteret, C.; Humbert, B.; Fragneto-Cusani, G.; Ramsay, J. D. F.; Delville, A.; Robert, J. L.; Bihannic, I.; Pelletier, M.; Michot, L. J. J. Phys. Chem. B 2005, 109, 23745–23759. (35) Tambach, T. J.; Boek, E. S.; Smit, B. Phys. Chem. Chem. Phys. 2006, 8, 2700–2702. (36) Heinz, H.; Suter, U. W. Angew. Chem., Int. Ed. 2004, 43, 2239–2243. (37) Heinz, H.; Koerner, H.; Anderson, K. L.; Vaia, R. A.; Farmer, B. L. Chem. Mater. 2005, 17, 5658–5669. (38) Slade, P. G.; Gates, W. P. Clays Clay Miner. 2004, 52, 204–210. (39) Balogh, M.; Laszlo, P. Organic Chemistry Using Clays.; Springer-Verlag: Berlin, 1993; p 184. (40) Purnell, J. H. Catal. Lett. 1990, 5, 203–10. (41) Varma, R. S. Tetrahedron 2002, 58, 1235–1255. (42) Choudary, B. M.; Sarma, M. R.; Kumar, K. V. J. Mol. Catal. A: Chem. 1994, 87, 33–38. (43) Kantam, M. L.; Kavita, B.; Figueras, F. Catal. Lett. 1998, 51, 113–115. (44) Kantam, M. L.; Santhi, P. L. Synth. Commun. 1996, 26, 3075–3079. (45) Wallis, P. J.; Booth, K. J.; Patti, A. F.; Scott, J. L. Green Chem. 2006, 8, 333– 337. (46) Hart, M. P.; Brown, D. R. J. Mol. Catal. A: Chem. 2004, 212, 315–321. (47) Komadel, P.; Madejova, J. Acid Activation of Clay Minerals. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2006; Vol. 1, p 263. (48) Machado, A. M.; Wypych, F.; Drechsel, S. M.; Nakagaki, S. J. Colloid Interface Sci. 2002, 254, 158–164. (49) Martinez-Lorente, M. A.; Battioni, P.; Kleemiss, W.; Bartoli, J. F.; Mansuy, D. J. Mol. Catal. A: Chem. 1996, 113, 343–353. (50) Barloy, L.; Battioni, P.; Mansuy, D. J. Chem. Soc., Chem. Commun. 1990, 1365-1367. (51) Crestini, C.; Pastorini, A.; Tagliatesta, P. J. Mol. Catal. A: Chem. 2004, 208, 195–202. (52) Georgakilas, V.; Gournis, D.; Bourlinos, A. B.; Karakassides, M. A.; Petridis, D. Chem.;Eur. J. 2003, 9, 3904–3908. (53) Hunter, D. B.; Gates, W. P.; Bertsch, P. M.; Kemner, K. M., Degradation of Tetraphenylboron at Hydrated Smectite Surfaces Studied by Time Resolved IR and X-ray Absorption Spectroscopies. In Mineral Water Interfacial Reactions: Kinetics and Mechanisms; Sparks, D. L., Grundl, T. J., Eds.; Americal Chemical Society: Washington, DC, 1999; p 282. (54) Burgentzle, D.; Duchet, J.; Gerard, J. F.; Jupin, A.; Fillon, B. J. Colloid Interface Sci. 2004, 278, 26–39. (55) Folkers, C. L.; Welch, A. P. J. Am. Ceram. Soc. 1955, 38, 454–461. (56) Mortland, M. M.; Shaobai, S.; Boyd, S. A. Clays Clay Miner. 1986, 34, 581– 585.
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a suspension of 160 g in two successive 1.5 L aliquots of 1 M NaCl(aq) (Merck), each for 2 days. During washing by centrifugation to remove excess salts and sedimentation to remove coarse particles, the clay solids (
