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Aminated Phyllosilicates Synthesized via a Sol-Gel Process Maria G. da Fonseca, Ce´sar R. Silva, and Claudio Airoldi* Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, Sa˜ o Paulo, Brazil Received December 31, 1998. In Final Form: April 13, 1999 Three analogous phyllosilicates were synthesized via a sol-gel process from the reaction of magnesium ions with aminated trialkoxysilanes in an aqueous basic medium at 373 K. The inorganic-organic hybrids obtained, designated as SILMgx (x ) 1-3) are related to [3-aminopropyl]-, [N-(2-aminoethyl)-3-aminopropyl], and [(10-amino)-4,7-diazanonyl]trimethoxysilane, respectively. The elemental analysis of these hybrids gave the following C/N ratios: 3.1, 2.5, and 2.4. Thermal analysis data showed 50.7, 56.2, and 58.0% weight losses for the same sequence of hybrids. However, the thermal stability decreased in the order SILMg1 > SILMg2 > SILMg3. The surface areas of these matrixes are SILMg2 > SILMg3. Surface Area. The surface areas for the inorganicorganic hybrids, determined by the BET technique, were 0.62 ( 0.02, 0.18 ( 0.01, and 1.66 ( 0.14 m2 g-1 for SILMgx (x ) 1-3), respectively. The low area can be attributed to the high level of organofuntionalization of the hybrid material, since the pendant groups block the access of the gaseous nitrogen to the surface. X-ray Diffraction. The crystallinity of these hybrid materials analyzed through X-ray diffractometry data
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presented quite suggestive results. In agreement with Figure 1, the peaks of the phyllosilicates correspond to lamellar distances of 1745, 2063, and 2323 pm for SILMgx (x ) 1-3). However, natural phyllosilicate talc, whose structure is identical to those obtained here, has a lamellar distance of 934 pm.11-13 The spectra of these hybrids showed any signal which could be related to the presence of Mg(OH)2. In that case, well-characterized peaks should be displayed at 477 pm (2θ ) 18.6°) and 237 pm (2θ ) 38.0°) due to 001 and 101 reflections.11 This fact is in agreement with the excess of magnesium not being deposited as the mineral brucite from a new phase on the inorganic backbone. This result corroborated the unequivocal fact that the excess of magnesium should be complexed by the basic nitrogen centers of the pendant groups, attached to the inorganic structure of these lamellar phyllosilicate hybrids. The larger values observed for the lamellar distances in synthetic hybrids are, therefore, related to the presence of the organic groups in these new structures. The increasing length of the organic chain attached to the inorganic matrix caused an increase in the interlamellar distance from SILMg1 to SILMg3. Although an increase of the distance is observed with an increase of the length of the organic chain, then, this enhancement from SILMg1 to SILMg2 is 318 pm, while from SILMg2 to SILMg3 this value is only 260 pm, as is shown in Figure 2. This fact indicated that the free space in the inorganic layers is not enough to accommodate attached organic chains, when two of these molecules are simultaneously bonded opposite inorganic layers of the same free interlamellar space. Due to this restraint, the molecules are alternatively disposed along the free space by superimposing part of them in a lamella, or the set of molecules are disposed in an inclined parallel disposition within the lamellae, giving an additional disorder of these phyllosilicates. This same conclusion was also obtained by relating the interlamellar distance of these hybrids with that found for talc, which is 934 pm; then, the free distance d can be given by d ) d(001)SILMgx - d(001)talc. From this expression the values 797, 1119, and 1452 pm, for SILMg1, SILMg2, and SILMg3, respectively, were obtained. The lengths of the pendant organic chains in SILMgx were estimated using the bond distances and assuming a zigzag conformation of the chains. Thus, the length of the free chain in the gallery space was calculated as 543, 939, and 1205 pm, for the following radicals attached to the inorganic backbone: (CH2)3NH2, (CH2)3NH(CH2)2NH2, and (CH2)3NH(CH2)2NH(CH2)2NH2. These results are in agreement with the distribution of the organic chain groups into the same lamellar cavity by alternating a sequence of molecules or the maintenance in inclined distributions, as proposed before and illustrated in Figure 2. This behavior is completely evident with SILMg3. This proposition is based on the fact that the size of the two chains will give a distance greater than that of the free lamellar space to accommodate both groups, which should be 2410 pm, but the lamellar distance value found for SILMg3 was only 2323 pm. These two inclined aminated groups in the free gallery space in each lamella are in agreement with the suggested 2:1 phyllosilicate structure. Another charac(11) Rayner, J. H.; Brown, G. Clays Clay Miner. 1973, 21, 103. (12) Moore, D. M.; Reynolds, R. C. X-ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford University Press: London, 1997. (13) Brindely, G. W.; Brown, G. Crystal Structures of Clay Minerals and their X-ray Identification, 1th ed.; Mineralogical Society: London, 1980.
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Figure 7. Scanning electron micrograph for SILMg1. The bar represents 10 µm.
Figure 8. Scanning electron micrograph for SILMg2. The bar represents 10 µm.
teristic of the 2:1 trioctahedral structure was obtained by the peak at d ) 156 pm, which corresponds to the 060 plane.11-13 IR Spectroscopy. The infrared spectra are illustrated in Figure 3. These inorganic-organic hybrids presented identical bands for these three new materials that were attributed as follows: 2940 cm-1 [δ(C-H)], 1570 cm-1 [ν(N-H)], 1450 cm-1 [ν(CH2)], 1320 cm-1 [δ(N-C)], 1180 cm-1 [δ(Si-C)], 1010 cm-1 [ν(Si2O5)], 800 cm-1 [ν(N-H)]
out of the plane, and a540 cm-1 [δ(Mg-O)].14-16 This last band is also present in natural silicates, as observed for (14) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley and Sons: New York, 1986. (15) Silverstein, R. M.; Bassler, G. C.; Morrel, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley and Sons: New York, 1991. (16) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to Spectroscopy: A Guide for Students of Organic Chemistry, 2th ed.; Saunders College Publishing: Orlando, FL, 1996.
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Figure 9. Scanning electron micrograph for SILMg3. The bar represents 10 µm.
Figure 10. Distribution map of magnesium atoms in the SILMg3 matrix shown in Figure 9.
talc. This band is another support of the existence of a phyllosilicate structure in this synthetic hybrid route for this kind of hybrid. Solid-State 13C and 29Si. Solid-state 13C NMR spectroscopy is a useful tool to characterize the structure of the silylating agent attached to the hybrid silicate. The 13C CP/MAS spectra for the solid hybrids SILMgx are shown in Figure 4. The signals assigned are in a good agreement with the previous reports on corresponding
silica hybrids, which were also synthesized by using a sol-gel process.17-21 The presence of unhydrolyzed meth(17) Albert, K.; Pfleiderer, B.; Bayer, E. In Chemically Modified Surfaces in Science and Industry; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach Science Publishers: New York, 1988; p 233. (18) Albert, K.; Pfleiderer, B.; Bayer, E. In Chemically Modified Surfaces in Science and Industry; Leyden, D. E., Collins, W. T., Eds.; Gordon and Breach Science Publishers: New York, 1988; p 287. (19) Lishko, T. P.; Glushchenko, L. V.; Kholin, Y. V.; Zaitev, Z. N.; Bugaevskii, A.; Donskaya, N. D. Russ. J. Phys. Chem. 1991, 65, 1584.
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oxy groups originating from the silylating agent could not be detected because the O-C*H3 signal is expected to appear around 50 ppm. However, this is the same region where all matrixes presented peaks related to carbon 3. Only a small shoulder was detected for the SILMg1 hybrid, but the degree of the organic group population is low, due the high efficiency of the reaction. The structure of the organic chain of the silylating agent bonded to the backbone of silicates did not change during the synthetic route employed. This conclusion is also consistent with the previous elemental analyses. 29Si NMR spectra provide information about the nature of the attachments of pendant groups on hybrid silicates. 29 Si HPDEC NMR spectra for SILMgx (x ) 1-3) are illustrated in Figure 5. These spectra showed the presence of three signals at -67, -55, and -48 ppm. The assignments are based on previous studies of analogous systems, which involved hybrid-containing silicas and other phyllosilicates.22-26 As recommended, all 29Si spectra were acquired under the same experimental conditions. For this reason comparisons could be established. Thus, the amounts of silicon species T1, T2, and T3 were estimated by calculating the peak area of each species, and results are listed in Table 2. In agreement with the obtained percentages for the silicon species, it was observed that SILMg3 has the largest degree of polymerization when compared with the other two hybrids. On the contrary, the lowest degree of condensation was detected for SILMg2, where the T1 species was estimated to be 36%. However, the species T3 was not detected before8 in the aminated phyllosilicates synthesized in the same aqueous basic medium used in this investigation. This fact is clear evidence that the reactions should be performed above room temperature and also that the largest aging time is necessary to obtain materials with a higher degree of condensation and better crystanillity according to the X-ray diffractograms. Structure and Properties. NMR and X-ray results allowed us to propose a schematic representation of the structure of the hybrid of phyllosilicate shown in Figure 6, which is in agreement with the structure first suggested.3 The structure of the synthetic lamellar polymer is composed of an inorganic network linked through organic chains. The schematic structure is very close to (20) Yang, J. J.; El-Nahhal, I. M.; Maciel, G. E. J. Non-Cryst. Solids 1996, 204, 105. (21) Yang, J. J.; El-Nahhal, I. M.; I-Ssuer, C.; Maciel, G. E. J. NonCryst. Solids 1997, 209, 19. (22) Sinford, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 3767. (23) Sanz, J.; Serratosa, J. M. J. Am. Chem. Soc. 1984, 106, 4790. (24) Sherriff, B. L.; Grundy, H. D.; Hartman, J. S. Eur. J. Mineral. 1991, 3, 751. (25) Fyfe, C. A.; Zhang, Y.; Aroca, P. J. Am. Chem. Soc. 1992, 114, 3252. (26) Nakata, S.; Tanaka, Y.; Asaoka, S.; Nakamura, M. J. Mol. Struct. 1998, 441, 267.
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that found in natural smectite.12,13 The organic chains are covalently attached by Si-C bonds in the disposed tetrahedral R-Si-O3 group of the inorganic backbone, as shown in Figure 6a for R ) (CH2)3NH2, (CH2)3NH(CH2)2NH2, and (CH2)3NH(CH2)2NH(CH2)2NH2. Magnesium cations are located inside both silicate tetrahedral sheets, are centered in octahedral sites, and are completely coordinated by oxygen atoms. These aminated materials have a layered structure shown in Figure 6b, whose interlamellar space depends on the type and size of the organic chain of each molecule R incorporated into the phyllosilicate. The aminated phyllosilicate materials were employed to extract metallic cations such as Co2+, Ni2+, Cu2+, and Zn2+ from diluted aqueous solutions due to the Lewis basic properties of the organic aminated pendant groups attached in the inorganic network of the phyllosilicates. The results show that these materials have a great ability to preconcentrate metallic cations from solutions.27 Scanning Electron Microscopy. The photomicrographs obtained by scanning electron microscopy showed, in all cases, well-formed particles of irregular shapes and sizes. The shapes of the particles are due to the method of preparation employed and are shown in figures 7-9. In this procedure, the gel obtained was dried and the final product was presented as a pulverized powder. Figure 10 shows that magnesium is uniformly dispersed in all matrixes, as indicated in the photograph of the phyllosilicate SilMg3. This behavior was also observed for the other two hybrids synthesized. The well-dispersed bright points shown in Figure 10 corresponded to the presence of magnesium atoms in the inorganic network of the matrix. Conclusions A series of layered aminated magnesium organosilicates having organic groups covalently attached to an inorganic network was synthesized via the condensation reaction involving magnesium cations and the desired alkoxysilane in a single-step route. However, the process is more effective and reproducible for aminated compounds using higher temperatures to obtain better crystallinity and a higher degree of cross-linking in comparison with other published results. The synthetic phyllosilicate hybrid materials have a smectite-like structure, whose lamellar space is basically determined by the size of the organic moiety of the trialkoxysilane employed. Acknowledgment. The authors are indebted to FAPESP for financial support. CAPES-PICD, FAPESP, and CNPq are also thanked for fellowships. LA9817866 (27) Fonseca, M. G.; Airoldi, C. Unpublished results.
