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Development of Novel Pillared Clays for the Encapsulation of Inorganic Complexes Joa˜o Pires,*,† Joana Francisco,† Ana Carvalho,† M. Brotas de Carvalho,† Ana Rosa Silva,‡ Cristina Freire,‡ and Baltazar de Castro‡ Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆ ncias, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal, and REQUIMTE, Departamento de Quı´mica, Faculdade de Cieˆ ncias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal Received September 15, 2003. In Final Form: January 29, 2004 Several pillared clays were prepared by using a polyalcohol (ethylene glycol or poly(vinyl alcohol)) or a poly(ethylene oxide) surfactant as an interlayer gallery template and an aluminum oligomer species as the pillaring agent. The use of polyalcohols or nonionic surfactants, such as Tergitol, gave materials which, in general, presented larger basal spacing than those found for the solids prepared by a similar procedure but without additives. The initial positive effect in the expansion of the clay interlayers was not totally retained after calcination of the materials; most probably, at the end, the basal spacing is still ruled by the intercalating aluminum species. The pillared clay with the largest basal spacing and specific surface area was used to encapsulate copper(II) complexes with pentadentate N3O2 Schiff base ligands derived from copper(II) acetylacetonate by in situ synthesis. The characterization made (X-ray diffraction, X-ray photoelectron spectroscopy, FTIR spectroscopy, chemical analysis, and low-temperature N2 adsorption) provided evidence that copper(II) complexes with pentadentate N3O2 Schiff base ligands were efficiently entrapped within the lower dimension pores of the pillared clay and that they interact strongly with the pillared clay matrix.
1. Introduction The search for inorganic porous materials with pore widths larger than those found in the traditional zeolites has been strongly pursued in the past decades. Several routes were explored, and one of them is related with the preparation of porous materials from the direct intercalation of clays by cationic oligomers, which have led to the development of pillared clays (PILCs). Their preparation methods and catalytic or adsorption properties have been reviewed by several authors.1-6 More recently, new families of porous clays were obtained, namely, the group of porous clay heterostructures1,7,8 and that of composite clay nanostructures.9 In the latter materials, the aqueous suspension of the clay is mixed with a nonionic surfactant, for instance, a poly(ethylene oxide) or other substances such as poly(vinyl alcohol)s,10-12 before the intercalation of cationic oligomers. The main objective of this procedure is to increase the interlayer spacing, allowing the synthesis of materials with larger pore widths than those obtained with the more usual methodology of PILC preparation. Traditionally, clay-based porous materials, mainly PILCs, were used as adsorbents and catalysts in several † ‡
Universidade de Lisboa. Universidade do Porto.
(1) Ding, Z.; Kloprogge, J. T.; Frost, R. L.; Lu, G. Q.; Zhu, H. Y. J. Porous Mater. 2001, 8, 273. (2) Gil, A.; Gandı´a, L. M.; Vicente, M. A. Catal. Rev.sSci. Eng. 2000, 42, 145. (3) Pinnavaia, T. J. Science 1983, 220, 365. (4) Burch, R., Ed. Catal. Today 1988, 2, 1. (5) Figueras, F. Catal. Rev.sSci. Eng. 1988, 30, 457. (6) Delmon, B.; Grange, P. Petrochemie 1991, 107, 376. (7) Pichowicz, M.; Mokaya, R. Chem. Commun. 2001, 2100. (8) Polverejan, M.; Pauly, T. R.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 2698. (9) Zhu, H. Y.; Lu, G. Q. Langmuir 2001, 17, 588. (10) Suzuki, K.; Mori, T. Appl. Catal. 1990, 63, 181. (11) Suzuki, K.; Horio, M.; Masuda, H.; Mori, T. J. Chem. Soc., Chem. Commun. 1991, 873.
reactions,1,2,13-15 but recently their use as supports for the encapsulation of inorganic complexes, providing an alternative method to prepare heterogeneous catalysts based on transition metal complexes that usually show high chemical selectivity in homogeneous phase catalysts, was also considered.16-18 We have been interested in the preparation and characterization of pillared clays based on natural Portuguese smectites by direct intercalation of cationic oligomer species;13-15 lately we have started to exploit their porous properties for the heterogenization of transition metal complexes.17,18 The objective of this work is twofold: (i) to prepare pillared clays by using a polyalcohol (ethylene glycol or poly(vinyl alcohol)) or a poly(ethylene oxide) surfactant as an interlayer gallery template and an aluminum oligomer species ([Al13O4(OH)24+x(H2O)12-x](7-x)+) as the pillaring agent and (ii) to use these novel PILCs to encapsulate transition metal complexes. To our knowledge, the use of poly(ethylene oxide) surfactants to obtain clays pillared with aluminum oxide was only attempted with a synthetic clay.9 The complexes chosen for the encapsulation studies were those already tested in previous works:17,18 bis(acetylacetonate)copper(II), [Cu(acac)2], and the pentadentate N3O2 Schiff base derivates, bis(acetylacetonate)-3-amino-bis(propyldiimine) and bis(acetylacetonate)-3-methylamino-bis(propyldi(12) Zhu, H. Y.; Vansant, E. F.; Lu, G. Q. J. Colloid Interface Sci. 1999, 210, 352. (13) Pereira, P.; Pires, J.; Carvalho, M. B. Langmuir 1998, 14, 4584. (14) Pires, J.; Carvalho, A.; Carvalho, M. B. Microporous Mesoporous Mater. 2001, 43, 277. (15) Carvalho, A. P.; Martins, A.; Silva, J. M.; Pires, J.; Vasques, H.; Carvalho, M. B. Clays Clay Miner. 2003, 51, 340. (16) Ramaswamy, V.; Krishnan, M.; Ramaswamy, A. V. J. Mol. Catal. A: Chem. 2002, 181, 81. (17) Ferreira, R.; Freire, C.; Castro, B.; Carvalho, A.; Pires, J.; Carvalho, M. B. Eur. J. Inorg. Chem. 2002, 3032. (18) Carvalho, A. P.; Castanheira, C.; Cardoso, B.; Pires, J.; Silva, A. R.; Freire, C.; Castro, B.; Carvalho, M. B. J. Mater. Chem. 2004, 14, 374.
10.1021/la0357209 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/06/2004
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Pires et al. Scheme 1
imine), abbreviated by [Cu(acac)2trien)] and [Cu(acac)2Metrien)], respectively. Catalyst applications of the latter complexes have not been reported, but copper(II) acetylacetonate has been found to be an efficient catalyst in the aziridination of olefins19 and has been also used as a catalyst in the production of urethanes from the reaction between hexamethylenediisocyanate and ethanol.20 Nevertheless, these complexes are easy to prepare and characterize and thus can be used as molecular probes for the characterization of matrix porosity and host-guest interactions within the PILC. 2. Experimental Section 2.1. Preparation of the Materials. The preparation of materials has involved two main phases: (i) preparation of a series of the pillared clays and (ii) encapsulation of copper Schiff base complexes in the PILC which has exhibited the most suitable porous system. The different sequential main steps involved in the two phases are depicted in Scheme 1. The clay used in this work is from a Portuguese soil deposit in Benavila and was characterized in a previous study.21 The analysis of the fraction
