Langmuir 1997, 13, 6473-6479
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Acid-Activated Organoclays: Preparation, Characterization and Catalytic Activity of Acid-Treated Tetraalkylammonium-Exchanged Smectites Christopher Breen,*,† Ruth Watson,† Jana Madejova´,‡ Peter Komadel,‡ and Zenon Klapyta§ Materials Research Institute, Sheffield Hallam University, Sheffield S1 1WB, U.K., Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava, Slovakia, and Faculty of Geology, Geophysics and Environmental Protection, University of Mining and Metallurgy, 30-059 Krako´ w, Poland Received May 19, 1997. In Final Form: August 29, 1997X The catalytic activity of acid-treated clays for reactions involving polar substrates is optimized when the acidity of the surface and the swelling ability of the catalyst are at a maximum. In contrast the activity for reactions using nonpolar substrates is best when the surface area is maximized and the catalyst presents an essentially hydrophobic surface which serves to attract the nonpolar reagents. We have investigated the catalytic ability of acid-activated organoclays (AAOCs) which should provide reasonable levels of acidity, hydrophobicity, and swelling ability for use with nonpolar reagents. A range of organoclays containing tetramethylammonium, dodecyltrimethylammonium, or octadecyltrimethylammonium cations at the 25, 50, or 100% exchange level were prepared and subjected to selected acid-leaching procedures at either 20 or 95 °C. The activity of these AAOCs for the conversion of R-pinene to camphene was investigated. The conditions used for acid leaching seldom removed extensive amounts of organocation, and the yields (40% conversion to camphene) compared favorably with those reported for pillared clays. Acid-leached tetramethylammonium clays were the most active with yields four times higher than those for the corresponding parent clay. Acid-leached dodecyltrimethylammonium and octadecyltrimethylammonium clays were only active when the organocations occupied 25% of the exchange sites.
Introduction Acid-treated clays have been used as an effective source of protons for a considerable period, finding use in industrial processes such as the alkylation of phenols1 and the dimerization and polymerization of unsaturated hydrocarbons.2 Acid-activated clays have recently enjoyed renewed interest in their role as high-surface-area supports for environmentally benign catalysts in Friedel Crafts alkylation and acylation reactions.3,4 Commercial acid-treated clays are normally prepared using a fixed quantity of acid chosen to remove a fixed proportion of cations from the octahedral sheet,5 and few investigations have addressed how the catalytic activity varies with octahedral sheet depletion.6,7 Recently, however, Rhodes and Brown have shown that the extent of acid dissolution can significantly influence the activity of the catalyst for a particular reaction whether it is employed as a support8 or as a catalyst in its own right.9 Using the formation of tetrahydropyranyl ether from dihydropyran and methanol as a test reaction, they established that the catalytic * To whom correspondence should be addressed. Phone: 0114253-3008. Fax: 0114-253-3501. E-mail:
[email protected]. † Sheffield Hallam University. ‡ Slovak Academy of Sciences. § University of Mining and Metallurgy. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Kaplan, H. U.S. Patent 3287422, 1966. (2) Hojabri, F. J. Appl. Chem. Biotechnol. 1971, 21, 87. (3) Brown, D. R. Geol. Carpathica-Clays 1994, 45, 45. (4) Clark, J. H.; Cullen, S. R.; Barlow, S. J.; Bastok, T. W. J. Chem. Soc., Perkin Trans. 2 1994, 1117. (5) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1995, 91, 1031. (6) Breen, C.; Madejova´, J.; Komadel, P. J. Mater. Chem. 1995, 5, 469. (7) Breen, C.; Madejova´, J.; Komadel, P. Appl. Clay Sci. 1995, 10, 219. (8) Rhodes, C. N.; Brown, D. R. J. Chem. Soc., Faraday Trans. 1992, 88, 2269. (9) Rhodes, C. N.; Brown, D. R. Catal. Lett. 1994, 24, 285.
S0743-7463(97)00507-6 CCC: $14.00
activity of an acid-activated clay for reactions in polar media was optimized when the acidity and swelling ability of the catalyst was at a maximum. This occurs at short acid treatment times. In contrast the yield for the acidcatalyzed isomerization of R-pinene to camphene, which represents a reaction in a nonpolar medium, was optimized when the external surface area reached a maximum, which occurs when leaching is extensive. They concluded that the reaction in polar media was optimized at short acid treatment times because the hydrophilic character of the clay attracts the polar reagents to the surface where the catalytic protons reside. This contrasts with the behavior in nonpolar media, where the reaction was optimized when the clay was substantially leached, because at this stage the catalyst was essentially hydrophobic silica, which served to attract the nonpolar reagents. One potential disadvantage of using extensively acid-leached clays is that depletion of the octahedral sheet causes a reduction in the number of cation-exchange sites, which is where the protons reside,7 with a concomitant loss in catalytic activity. It is long established that the hydrophilic nature of swelling clays can be rendered hydrophobic by exchanging the naturally occurring inorganic cations (Ca2+, Na+, K+) with organocations such as monoalkylammonium,10-12 tetramethylammonium,13 octadecyltrimethylammonium,14 and phenyltrimethylammonium15 to select but a few from a very extensive list. This knowledge coupled (10) Bujda´k, J.; Slosiarikova´, H.; C ˇ ´ıcˇel, B. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 13, 321. (11) Bujda´k, J.; Slosiarikova´, H. Appl. Clay Sci. 1992, 7, 263. (12) Bujda´k, J.; Slosiarikova´, H.; C ˇ ´ıcˇel, B. Chem. Pap. 1993, 47, 85. (13) Lee, J. F.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Clays Clay Miner. 1990, 38, 113. (14) (a) Mortland, M. M.; Shaobai, S.; Boyd S. A. Clays Clay Miner. 1986, 34, 581. (b) Boyd, S. A.; Mortland, M. M.; Chiou, C. T. Soil Sci. Soc. Am. J. 1988, 52, 652. (c) Jaynes, W. F.; Boyd, S. A. Soil Sci. Soc. Am. J. 1991, 55, 43.
© 1997 American Chemical Society
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with the recent enhancement of the catalytic activity of pillared clays, via pillaring acid-activated clays,16-18 where it has been shown that the activity and selectivity of titanium pillared clay catalysts can be significantly influenced by the nature of the source clay used, led us to assess the catalytic efficiency of mildly acid-activated organoclays for reactions in nonpolar media. This approach initiates an exploration of the resistance of organoclays to octahedral ion leaching and has the potential to reduce the concentration of Al, Mg, and Fe in effluent flows from the leaching process. The conversion of R-pinene to camphene, our chosen test reaction, has industrial significance because camphene is an intermediate in the synthesis of camphor, which has value due to its aroma and pharmaceutical properties.19 Acidified titanium oxide is usually employed in the transformation of R-pinene to camphene,20 although recent studies have considered the use of zeolites,21,22 pillared interlayer clays (PILCs),22 and (both crystalline and amorphous) zirconium and tin phosphates.23 Here we report the use of acidactivated tetraalkylammonium-exchanged smectites for the conversion of R-pinene to camphene, via ring expansion, and limonene, via ring opening.24 In particular we are interested in producing cost-effective, hydrophobic catalysts of relatively high acidity in a short time using simple exchange procedures. Moreover, the ability of organoclays to swell in nonpolar solvents should provide the opportunity to access a considerable portion of the potential surface area, which has been estimated to be near 800 m2 g-1. Being cognizant of the fact that clays containing a high proportion of octahedral magnesium or iron are leached more readily than their aluminum-rich counterparts6,7,25-27 we have selected three source clays of different octahedral composition, which have been the focus of acid-leaching studies previously,7,28,29 to ascertain whether the elemental composition of the parent smectite influences the properties of the resulting catalysts. Experimental Section Materials. Three dioctahedral smectites were used in this study. JP is a hydrothermal, aluminum-rich montmorillonite from the Kremnica mountains in central Slovakia;30,31 SWa-1, a ferruginous smectite, was obtained from The Source Clays Repository of the Clay Minerals Society, Columbia, MO, and Stebno (ST) is an iron-rich beidellite from the Czech Republic, (15) (a) Jaynes, W. J.; Boyd, S. A. J. Air Waste Manage. Assoc. 1990, 40, 1649. (b) Jaynes, W. J.; Boyd, S. A. Clays Clay Miner. 1991, 39, 428. (16) Mokaya, R.; Jones, W. J. Chem. Soc., Chem. Commun. 1994, 929. (17) Bovey, J.; Kooli, F.; Jones, W. Clay Miner. 1996, 31, 501. (18) Kooli, F.; Bovey, J.; Jones W. J. Mater. Chem. 1997, 7, 153. (19) Albert, R. M.; Traynor, S. G.; Webb, R. L. In Naval Storess Production, Chemistry, Utilisation; Zinkel, D. F., Russell, J., Eds.; Pulp Chemicals Association: New York, 1989. (20) Severino, A.; Vital, J.; Lobo, L. S. In Heterogeneous Catalysis and Fine Chemicals III; Guisnet, M., et al., Eds.; Elsevier: Amsterdam, 1993; pp 685-692. (21) Yu, J.-Q.; Zhou, P.; Xiao, S.-D. Chin. J. Chem. 1995, 13, 280. (22) De Stefanis, A.; Perez, G.; Ursini, O.; Tomlinson, A. A. G. Appl. Catal., A 1995, 132, 353. (23) Cruz-Costa, M. C.; Johnstone, R. A. W.; Whittaker, D. J. Mol. Catal. A.: Chem. 1996, 104, 251. (24) Williams, C. M.; Whittaker, D. J. Chem. Soc. B 1971, 668. (25) Osthaus, B. B. Clays Clay Miner. 1956, 4, 301. (26) C ˇ ´ıcˇel, B.; Nova´k, I. Proceedings of the 7th Conference on Clay Mineralogy and Petrology, Karlovy Vary Czechoslovakia; Kenta, J., Ed.; Univerzita Karbova: 1976; pp 163-171. (27) Nova´k, I.; C ˇ ´ıcˇel, B. Clays Clay Miner. 1978, 26, 341. (28) Tka´cˇ, I.; Komadel, P.; Muller, D. Clay Miner. 1994, 29, 11. (29) Breen, C.; Zahoor, F. D.; Madejova´, J.; Komadel, P. J. Phys. Chem. 1997, 101, 5324. (30) Sˇ amajova´, E.; Kraus, I.; Lajcˇa´kova´, A. Geol. CarpathicasClays 1992, 43, 21. (31) Sˇ ucha, V.; Kraus, I.; Mosser, C.; Hroncova´, Z.; Soboleva, K. A.; Sˇ ira´nova´, V. Geol. CarpathicasClays 1992, 43, 13.
Breen et al. Table 1. Structural Formulas of Smectites in the Fine Fractions of the Samples Used sample
M+ a
Sib
Alb
Alc
Fec
Mgc
JP19 ST26 SWa-130
0.91 0.95 0.81
7.71 7.22 7.33
0.29 0.78 0.67
3.00 1.96 0.91
0.38 1.60 2.86
0.63 0.58 0.28
a
Interlayer cations. b Tetrahedral cations. c Octahedral cations.
which contains about 21% of its total iron bound in goethite.32 Coarse samples of JP, SWa-1, and ST were suspended in deionized water, treated five times with 1.0 M aqueous calcium chloride, washed until free of chloride, and centrifuged, and the nominally
