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New layered double hydroxides intercalated with substituted pyrroles. 1. In situ polymerization of 4-(1H-pyrrol-1-yl)benzoate
Journal of Physics and Chemistry of Solids 67 (2006) 968–972 www.elsevier.com/locate/jpcs
New layered double hydroxides intercalated with substituted pyrroles. 1. In situ polymerization of 4-(1H-pyrrol-1-yl)benzoate Jairo Tronto a, Fabrice Leroux b, Eduardo Luis Crepaldi a, Zeki Naal c, Stanlei Ivair Klein d, Joa˜o Barros Valim a,* a
Departamento de Quı´mica, Faculdade de Filosofia Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Av. dos Bandeirantes 3900, 14040-901 Ribeira˜o Preto, SP, Brazil b Laboratoire des Mate´riaux Inorganiques, Universite´ Blaise Pascal, UMR-CNRS no. 6002, 63177 Aubie`re cedex, France c Departamento. de Fı´sica e Quı´mica, Faculdade de Cieˆncias Farmaceˆuticas de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Av. dos Bandeirantes 3900, 14040-903 Ribeira˜o Preto, SP, Brazil d Departamento de Quı´mica Inorgaˆnica, Instituto de Quı´mica de Araraquara, Universidade Estadual Paulista, R. Francisco Degni, s/n, 14800-090 Araraquara, SP Brazil
Abstract The two-dimensional hybrid organic–inorganic materials Zn2–Cr and Zn2–Al-LDHs (Layered Double Hydroxides) containing 4-(1H-pyrrol1yl)benzoate anions as the interlayer anions were synthesized by the co-precipitation method at constant pH followed by subsequent hydrothermal treatment for 72 h. The materials were characterized by PXRD, 13C CP-MAS NMR, ESR, TGA, and TEM. The basal spacing found by the X-ray diffraction technique is coincident with the formation of bilayers of the intercalated anions. Solid-state 13C NMR and ESR data strongly suggest the partial in situ polymerization of the 4-(1H-pyrrol-1yl)benzoate anions during coprecipitation. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; Polymers; C. X-ray diffraction; D. Nuclear magnetic resonance (NMR); Electron paramagnetic resonance (EPR)
1. Introduction The combination of (conducting) polymers and inorganic materials is a very promising research field [1–5]. Particularly, Layered Double Hydroxides (LDHs) are intercalation-type materials whose lamellar architecture provides the opportunity of separating, periodically and in the nanoscale (1–2 nm), the inorganic and organic counterparts of the hybrid composite [6–10]. The structure of LDHs can be described considering the brucite-like structure, Mg(OH)2, where M(II) cations are in the centre of edge-sharing octahedra, which results in an overall planar structure. A part of the divalent cations is isomorphously replaced by trivalent cations, positively charged layers are formed and the excess of positive charge is compensated by interleaved anions. A large variety of anions, such as, heteropolyacids, metallo-organic complexes, and also polymers, can be intercalated into LDH layers [6–13]. Some of the resulting materials are extensively studied for their potential * Corresponding author. Tel.: C55 16 602 3766; fax: C55 16 633 8151. E-mail address: [email protected] (J.B. Valim).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.012
applications in the fields of catalysis and catalyst supports, matrices for the controlled release of drugs, plastic additives, adsorbents, flame retardants, ion exchangers, batteries, and (bio)sensors [14–22]. Moreover, synergistic interaction between the organic and inorganic parts may happen, reinforcing the interest of such organic inorganic assemblies. In the wide branch of the hybrid polymer LDH materials, the incorporation of organic moiety is performed using functionalized monomers taking into account the anionic exchange capacity of the LDH host structure. Although to the best of our knowledge, pyrrole derivatives have not been incorporated into LDH gap so far. We now report the synthesis and characterization of Zn2–Al- and Zn2–Cr-LDH intercalated with 4-(1H-pyrrol-1-yl)benzoate and its polymer obtained in situ, which belongs to the ever-growing class of twodimensional inorganic organic hybrid materials. 2. Experimental methods The LDHs hybrid materials with a metal ratio Zn:Cr or Zn:Al of 2:1, were synthesised by coprecipitation at constant pH followed by a hydrothermal treatment. A solution containing 1.92!10K3 mol of Zn(NO3)2$6H2O and 9.60!10K4 mol of Cr(NO3)3$9H2O or Al(NO3)3$9H2O in 18 mL of water was
J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
added into a solution containing 1.92!10K3 mol of 4-(1Hpyrrol-1yl)benzoic acid, noted as PyB, in 70 mL of water. During the addition, 0.5 mol LK1 NaOH solution was added in order to keep the pH constant (G0.2 units) at 7.5 and 8.5 for Zn2–Cr-LDHs and Zn2–Al-LDHs, respectively. After a contact time of 30 min at room temperature, the solid material was separated by centrifugation, washed with water and ethanol. A part of the resulting material was suspended in a solution containing 1.92!10K3 mol of PyB in 70 mL of water, maintained at pH 7.0, and then transferred into a reactor where the pressure was adjusted to 3.0 bar with nitrogen gas and the temperature was set to 75 8C for 72 h. After hydrothermal treatment, the materials were separated, washed, and dried as before. The unsupported polymer used for the TG analysis was prepared according to the literature methods [23]. The LDHs hybrid materials are noted as Zn2Al/PyB and Zn2Cr/ PyB. 2.1. Characterization Powder X-ray diffraction (PXRD) in q–2q mode was conducted using a Siemens D5005 diffractometer, equipped with a graphite monochromator selecting the Cu Ka1 radiation (lZ0.15406 nm). The step used was 0.028/s in the angular domain 2–708. The morphology of the powder LDHs was analyzed by TEM using a PHILIPS EM 208 microscope. Samples were embedded in epoxy resin, ultramicrotomed and supported in copper grids. Thermogravimetric analysis (TGA) were performed with a Shimadzu TGA-50 equipment, under air, from room temperature to 850 8C at a heating rate of 10 8C minK1. Solid state NMR 13C spectra in CP-MAS condition were recorded in a Varian INOVA 300 spectrometer operating at 75.42 MHz. 1H and 13C, 1- and 2-D NMR spectra of the solution were performed in Varian INOVA 500 spectrometer, operating at 500 and 125.65 MHz, respectively. Electron Spin Resonance spectra were recorded at room temperature using a X Band Bruker EMX spectrometer operating at 9.658 GHz. Diphenylpicrylhydrazyl (DPPH) was used to determine the resonance frequency (gZ2.0036G 0.0002). The sweep width was of 200 G and the receiver gain 100,000.
969
Table 1 X-ray powder diffraction data and formula* for LDHs. Sample
Formula
Basal spacing (nm)
Domain sizea (nm)
Zn2Al/PyB
[Zn0.75Al0.25(OH)2] PyB0.25$0.95H2O [Zn0.74Al0.26(OH)2] PyB0.26$0.94H2O [Zn0.74Cr0.26(OH)2] PyB0.26$0.95H2O [Zn0.72Cr0.28(OH)2] PyB0.28$0.93H2O
1.90
28.3
1.91
51.6
1.93
16.5
1.90
21.8
Zn2Al/PyB after HT Zn2Cr/PyB Zn2Cr/PyB after HT *
Based on TG analyses; a The size of the scattering domain along the c-axis was determined from the width of the second and third 00l X-ray harmonic of oriented samples.
arrangement, the PyB anions should adopt an orientation with its long axis perpendicular to the inorganic sheets, the carboxylate groups of the anions are close to the hydroxide layers and the five-membered heterocyclic rings are in the midplane of interlayer space. Similar arrangement as previously observed for monovalent anions, such as benzoate [24]. After the hydrothermal treatment some structural changes take place, including the growth of the crystallites. The number of stacked platelets estimated from the Scherrer equation also increased, and it was more pronounced for the Zn:Al framework than for the Zn:Cr counterpart. An increase of the diffraction line (110) also indicated an increase in the cation ordering inside the inorganic sheets, as well as a superior stacking sequence, resulting in hybrid materials with overall higher structural organization.
3. Results and discussion From the TG analysis and considering the general formula 3C mK for LDH, ½M2C 1Kx Mx ðOHÞ2 Ax=m $nH2 O, the composition of the hybrid materials was calculated assuming a complete for3C mation into an oxide of ideal stochiometry ‘M2C 1Kx Mx OxC2=2 ’ at 850 8C. The results are show in Table 1. The X-ray powder diffraction data and the formula for the hybrid LDH materials are included in Table 1. PXRD patterns for the hybrid materials are shown in Fig. 1. They exhibit several orders of basal reflections (00l) indicating well ordered layered structures. Taking into account the size of the molecule, the basal spacing in the range of 1.90–1.93 nm, calculated from (00l) peaks, are consistent with those expected for the incorporation of bilayers of PyB anions between LDH sheets. In this interlayer
Fig. 1. PXRD of (a) Zn2Al/PyB, (b) Zn2Al/PyB with hydrothermal treatment, (c) Zn2Cr/PyB, and (d) Zn2Cr/PyB with hydrothermal treatment. * Diffraction peak not identified.
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J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
Fig. 2. TEM images of (a) Zn2Al/PyB with hydrothermal treatment, (b) Zn2Cr/PyB with hydrothermal treatment.
The TEM images of the materials are shown in Fig. 2. They showed cross sectional sessions parallel to the (00l) planes, which evidenced the expanded lamellar structure and confirmed the basal spacings obtained by PXRD. In order to have information on the interlayered monomer, NMR study was carried out. The numbering scheme and the assignments of the 13C chemical shifts for the organic monomer in solution and in the solid state are presented in Table 2. Three quaternary carbons are observed: the acid group at 166.6 ppm (C7), the quaternary in a position from it at 127.0 ppm (C6), and the carbon a to the pyrrole group at 143.0 ppm (C3). However, the correct attributions for the pairs C1, C2 and C4, C5 demanded a proton–carbon correlation spectrum, since the resonances at 118.9 and 118.5 ppm were too close. The heteronuclear multiple bond coherence spectrum (1JCH) gHMBC, is unambiguous to assign the signals at 118.5 and 130.9 ppm to the carbons C4, C5, respectively. Concerning the six membered aromatic ring, a complementary long distance 3JCH correlation spectrum clearly showed that the proton attached to carbon C5 at 130.9 ppm correlates with the quaternaries C7 and C3. In this way, the signal at 118.9 is definitely attributed to the C2 carbon of the pyrrole ring. The 13 C CP-MAS NMR spectra for the free monomer and the organic anions incorporated into the gap of the Zn2Al-LDH is shown in Fig. 3. The resonance peak for the carbon nuclei C7 is located at ca. 175.1, C5 and C3 at ca. 131.6 and 140.8 ppm, respectively. The wide signals centered at 117.3 and 113.7 are therefore attributed to the resonances of the remaining carbons of both rings. In particular, the upfield shift of the C2 carbon Table 2 NMR and
from its original position at 120.1 ppm is a rather strong evidence of its quaternization, which must involve an in situ polymerization process through that particularly reactive organic site close to the nitrogen atom. The similarity of the spectra before and after hydrothermal treatment (Fig. 3b and c) seems to indicate that the polymerization is then occurring during the coprecipitation process. Direct polymerization has also been observed in the case of other polymer/2D host framework systems [25–27]. It is likely that the particular disposition of the anions in bilayers, with the pyrrole groups very close to each another, facilitates the a–a polymerization process, as idealized in Fig. 4. Indeed, PyB polymerization occurs (at least partially) at room temperature, probably initiated by oxygen molecules acting as oxidizing agent, as previously proposed by some authors for the in situ polymerization of aniline derivatives [11,13]. To reinforce our assumption of a polymerization process, ESR spectrum of the composite was analyzed. A narrow ESR
13
C1 C2 N C3 C4
C CP-MAS for PyB, values in ppm. C
DMSO d-6
CP-MAS
C1 C2 C3 C4 C5 C6 C7
111.3 118.9 143.0 118.5 130.9 127.0 166.6
111.2 120.1 142.3 115.6 132.9 122.8 174.0
C5 C6 C7 O
OH
Fig. 3. 13C CP-MAS NMR spectra of (a) PyB, (b) Zn2Al/PyB, and (c) Zn2Al/PyB after hydrothermal treatment.
J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
Fig. 4. Schematic representation of Zn2Al/PyB after in situ polymerization.
Fig. 5. ESR signal of the hybrid material Zn2Al/PyB after hydrothermal treatment.
signal from the Zn2Al/PyB is depicted in Fig. 5. It is expected that upon polymerization, a large array of conjugated double bonds should be formed, to such an extent as to generate lowlying molecular orbitals suitable to accommodate conduction electrons. The signal was composed of a single gaussian line 100
Zn2Al/PyB
90
Zn2Al/PyB
(a)
with a peak to peak line width DHpp of 7.8G1 G at a g-value of 2.0050G0.0004, characteristic of organic radicals. Similar ESR spectra of spin carriers localized along polymer chains had already been observed for polyanilines interleaved into layered compounds [28–30]. Regarding Zn2Cr/PyB framework, the intense signal close to gZ2.000 due to the paramagnetic Cr3C nuclei precludes any conclusions, however, the close similarity of the PXRD data between both hybrid materials may indicate a similar molecular arrangement. One has to note that due to broad overlapping IR spectroscopy was unable to unravel if the polymerization is occurring or not. The results of TG analysis for the unsupported PyB polymer, Zn2Al/PyB and Zn2Cr/PyB with and without hydrothermal treatment are presented in Fig. 6. The unsupported PyB polymer decomposes at 280 8C in two steps. Concerning the hybrid phases, a first weight loss occurs from room temperature up to 100 8C, corresponding to the release of adsorbed water molecules (Fig. 6a). A second step of decomposition occurs between 100 and 210 8C, which is associated to the loss of interlamellar water molecules. A final step of decomposition is attributed to the loss of hydroxyl ions and intercalated anions (polymers) with the subsequent formation of mixed oxides. For the Zn2Cr/PyB, the profiles of the curves of thermal decomposition, displayed in Fig. 6b, are very similar to Zn2Al/PyB. The decomposition of the organic component is largely delayed in temperature when sequestrated between LDH gap. This trend is usually observed in the case of hybrid LDH systems, such as poly(aniline) LDH assembly [11–13]. 4. Conclusions The combined PXRD, 13C NMR, and ESR data obtained for the two-dimensional hybrid materials Zn2–Cr/PyB and Zn2– Al–PyB present a strong evidence of the polymerization of the monomers during the co-precipitation process. The basal spacings values for the new hybrid materials suggest 100 Zn2Cr/PyB
80
70
70
60
Mass (%)
Mass (%)
(b) Zn2Cr/PyB
90
PyB unsupported polymer
80
971
50 40
60 50 40
30
30
20
20
10
10
0
0 100
200
300
400
500
Temperature (˚C)
600
700
800
100
200
300
400 500 Temperature (˚C)
600
700
800
Fig. 6. TG analysis of (a) PyB unsupported polymer, Zn2Al/PyB without and with hydrothermal treatment (HT), (b) Zn2Cr/PyB without and with hydrothermal treatment (HT).
972
J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
a polymerization proceeding via a ‘zig-zag’ fashion, similarly to syndiotactic polymers such as poly(styrene sulfonate) into relative LDH host structures [31,32]. Acknowledgements Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o PauloFAPESP (process 02/00130-7), Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior-CAPES (process BEX3391/04-3) and Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-CNPq. Fabrice Leroux thanks FAPESP for the financial support during his visit at Universidade de Sa˜o Paulo (process 03/13585-5). References [1] M.S. Wang, T.J. Pinnavaia, Clay–polymer nanocomposites formed from acidic derivatives of montmorillonite and an epoxy resin, Chem. Mater. 6 (1994) 468–474. [2] E.P. Giannelis, Polymer layered silicate nanocomposites, Adv. Mater. 8 (1996) 29–35. [3] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng., R 28 (2000) 1–63. [4] M.J. MacLachlan, I. Manners, G.A. Ozin, New (inter)faces: polymers and inorganic materials, Adv. Mater. 12 (2000) 675–681. [5] R. Gangopadhyay, A. De, Conducting polymer nanocomposites: a brief overview, Chem. Mater. 12 (2000) 608–622. [6] F. Leroux, J.P. Besse, Polymer interleaved layered double hydroxide: a new emerging class of nanocomposites, Chem. Mater. 13 (2001) 3507–3515. [7] F. Cavani, F. Triffiro´, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catal. Today 11 (1991) 173–301. [8] E.L. Crepaldi, J.B. Valim, Layered double hydroxides: structure, synthesis, properties and applications, Quim. Nova 21 (1998) 300–311. [9] A. Vaccari, Preparation and catalytic properties of cationic and anionic clays, Catal. Today 41 (1998) 53–71. [10] S.P. Newman, W. Jones, Synthesis, characterization and applications of layered double hydroxides containing organic guests, New J. Chem. 2 (1998) 105–115. [11] E.M. Moujahid, M. Dubois, J.P. Besse, F. Leroux, Role of atmospheric oxygen for the polymerization of interleaved aniline sulfonic acid in LDH, Chem. Mater. 14 (2002) 3799–3807. [12] E.M. Moujahid, J.P. Besse, F. Leroux, Synthesis and characterization of a polystyrene sulfonate layered double hydroxide nanocomposite. In-situ polymerization vs. polymer incorporation, J. Mater. Chem. 12 (2002) 3324–3330. [13] V.P. Isupov, L.E. Chupakhina, M.A. Ozerova, V.G. Kostrovsky, V.A. Poluboyarov, Polymerization of m-NH2C6H4COO anions in the intercalation compounds of aluminium hydroxide [LiAl2(OH)6][mNH2C6H4COO]$nH2O, Solid State Ionics 141 (2001) 231–236. [14] A. Vaccari, Clays and catalysis: a promising future, Appl. Clay Sci. 14 (1999) 161–198.
[15] M.J. Climent, A. Corma, S. Iborra, J. Primo, Base catalysis for fine chemicals production—claisen-schmidt condensation on zeolites and hydrotalcites for the production of chalcones and flavanones of pharmaceutical interest, J. Catal. 151 (1995) 60–66. [16] J.H. Choy, S.Y. Kwak, Y.J. Jeong, J.S. Park, Inorganic layered double hydroxides as nonviral vectors, Angew. Chem. Int. Ed. 39 (2000) 4042–4045. [17] J. Tronto, M.J. dos Reis, F. Silve´rio, V.R. Balbo, J.M. Marchetti, J.B. Valim, In vitro release of citrate anions intercalated in magnesium aluminium layered double hydroxides, J. Phys. Chem. Solids 65 (2004) 475–480. [18] E.L. Crepaldi, J. Tronto, L.P. Cardoso, J.B. Valim, Sorption of terephthalate anions by calcined and uncalcined hydrotalcite-like compounds, Colloids Surf. A: Physicochem. Eng. Aspects 211 (2002) 103–114. [19] X.D. Wang, Q. Zhang, Effect of hydrotalcite on the thermal stability, mechanical properties, rheology and flame retardance of poly(vinyl chloride), Polym. Int. 53 (2004) 698–707. [20] C.S. Liao, W.B Ye, Structure and conductive properties of poly(ethylene oxide)/layered double hydroxide nanocomposite polymer electrolytes, Electrochim. Acta 49 (2004) 4993–4998. [21] J.V. de Melo, S. Cosnier, C. Mousty, C. Martelet, N. Jaffrezic-Renault, Urea biosensors based on immobilization of urease into two oppositely charged clays (laponite and Zn–Al layered double hydroxides), Anal. Chem. 74 (2002) 4037–4043. [22] J. Tronto, K.C. Sanchez, E.L. Crepaldi, Z. Naal, S.I. Klein, J.B. Valim, Synthesis, characterization and electrochemical study of layered double hydroxides intercalated with 2-thiophenecarboxylate anions, J. Phys. Chem. Solids 65 (2004) 493–498. [23] S.P. Armes, Optimum reaction conditions for the polymerization of pyrrole by iron(III) chloride in aqueous solution, Synth. Met. 20 (1987) 365–371. [24] F. Kooli, I.A. Chisem, M. Vucelic, W. Jones, Synthesis and properties of terephthalate and benzoate intercalates of Mg–Al layered double hydroxides possessing varying layer charge, Chem. Mater. 8 (1996) 1969–1977. [25] V. Mehrotra, E.M. Giannelis, Nanometer scale multilayers of electroactive polymers: intercalation of polypyrrole in mica-type silicates, Solid State Ionics 51 (1992) 115–122. [26] C. Vaysse, L. Guerlou-Demourgues, E. Duguet, C. Delmas, Acrylate intercalation and in situ polymerization in iron-, cobalt-, or manganesesubstituted nickel hydroxides, Inorg. Chem. 42 (2003) 4559–4567. [27] C. Vaysse, L. Guerlou-Demourgues, C. Delmas, E. Duguet, Tentative mechanisms for acrylate intercalation and in situ polymerization in nickelbased layered double hydroxides, Macromolecules 37 (2004) 45–51. [28] S. Uma, J. Gopalakrishnan, Polymerization of aniline in layered perovskites, Mater. Sci. Eng., B 34 (1995) 175–179. [29] N.S.P. Bhuvanesh, J. Gopalakrishnan, Polymerization of aniline in layered HMMoO6$H2O (MZNb, Ta), Mater. Sci. Eng., B 53 (1998) 267–271. [30] N. Kinomura, T. Toyama, N. Kumada, Intercalative polymerization of aniline in VOPO4$2H2O, Solid State Ionics 78 (1995) 281–286. [31] L. Vieille, C. Taviot-Gue´ho, J.P. Besse, F. Leroux, Hydrocalumite and its polymer derivatives. 2. Polymer incorporation versus in situ polymerization of styrene-4-sulfonate, Chem. Mater. 15 (2003) 4369–4376. [32] L. Vieille, E.M. Moujahid, C. Taviot-Gue´ho, J.P. Besse, F. Leroux, In situ polymerization of interleaved monomers: a comparative study between hydrotalcite and hydrocalumite host structures, J. Phys. Chem. Solids 65 (2004) 385–393.
New layered double hydroxides intercalated with substituted pyrroles. 1. In situ polymerization of 4-(1H-pyrrol-1-yl)benzoate Jairo Tronto a, Fabrice Leroux b, Eduardo Luis Crepaldi a, Zeki Naal c, Stanlei Ivair Klein d, Joa˜o Barros Valim a,* a
Departamento de Quı´mica, Faculdade de Filosofia Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Av. dos Bandeirantes 3900, 14040-901 Ribeira˜o Preto, SP, Brazil b Laboratoire des Mate´riaux Inorganiques, Universite´ Blaise Pascal, UMR-CNRS no. 6002, 63177 Aubie`re cedex, France c Departamento. de Fı´sica e Quı´mica, Faculdade de Cieˆncias Farmaceˆuticas de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Av. dos Bandeirantes 3900, 14040-903 Ribeira˜o Preto, SP, Brazil d Departamento de Quı´mica Inorgaˆnica, Instituto de Quı´mica de Araraquara, Universidade Estadual Paulista, R. Francisco Degni, s/n, 14800-090 Araraquara, SP Brazil
Abstract The two-dimensional hybrid organic–inorganic materials Zn2–Cr and Zn2–Al-LDHs (Layered Double Hydroxides) containing 4-(1H-pyrrol1yl)benzoate anions as the interlayer anions were synthesized by the co-precipitation method at constant pH followed by subsequent hydrothermal treatment for 72 h. The materials were characterized by PXRD, 13C CP-MAS NMR, ESR, TGA, and TEM. The basal spacing found by the X-ray diffraction technique is coincident with the formation of bilayers of the intercalated anions. Solid-state 13C NMR and ESR data strongly suggest the partial in situ polymerization of the 4-(1H-pyrrol-1yl)benzoate anions during coprecipitation. q 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; Polymers; C. X-ray diffraction; D. Nuclear magnetic resonance (NMR); Electron paramagnetic resonance (EPR)
1. Introduction The combination of (conducting) polymers and inorganic materials is a very promising research field [1–5]. Particularly, Layered Double Hydroxides (LDHs) are intercalation-type materials whose lamellar architecture provides the opportunity of separating, periodically and in the nanoscale (1–2 nm), the inorganic and organic counterparts of the hybrid composite [6–10]. The structure of LDHs can be described considering the brucite-like structure, Mg(OH)2, where M(II) cations are in the centre of edge-sharing octahedra, which results in an overall planar structure. A part of the divalent cations is isomorphously replaced by trivalent cations, positively charged layers are formed and the excess of positive charge is compensated by interleaved anions. A large variety of anions, such as, heteropolyacids, metallo-organic complexes, and also polymers, can be intercalated into LDH layers [6–13]. Some of the resulting materials are extensively studied for their potential * Corresponding author. Tel.: C55 16 602 3766; fax: C55 16 633 8151. E-mail address: [email protected] (J.B. Valim).
0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2006.01.012
applications in the fields of catalysis and catalyst supports, matrices for the controlled release of drugs, plastic additives, adsorbents, flame retardants, ion exchangers, batteries, and (bio)sensors [14–22]. Moreover, synergistic interaction between the organic and inorganic parts may happen, reinforcing the interest of such organic inorganic assemblies. In the wide branch of the hybrid polymer LDH materials, the incorporation of organic moiety is performed using functionalized monomers taking into account the anionic exchange capacity of the LDH host structure. Although to the best of our knowledge, pyrrole derivatives have not been incorporated into LDH gap so far. We now report the synthesis and characterization of Zn2–Al- and Zn2–Cr-LDH intercalated with 4-(1H-pyrrol-1-yl)benzoate and its polymer obtained in situ, which belongs to the ever-growing class of twodimensional inorganic organic hybrid materials. 2. Experimental methods The LDHs hybrid materials with a metal ratio Zn:Cr or Zn:Al of 2:1, were synthesised by coprecipitation at constant pH followed by a hydrothermal treatment. A solution containing 1.92!10K3 mol of Zn(NO3)2$6H2O and 9.60!10K4 mol of Cr(NO3)3$9H2O or Al(NO3)3$9H2O in 18 mL of water was
J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
added into a solution containing 1.92!10K3 mol of 4-(1Hpyrrol-1yl)benzoic acid, noted as PyB, in 70 mL of water. During the addition, 0.5 mol LK1 NaOH solution was added in order to keep the pH constant (G0.2 units) at 7.5 and 8.5 for Zn2–Cr-LDHs and Zn2–Al-LDHs, respectively. After a contact time of 30 min at room temperature, the solid material was separated by centrifugation, washed with water and ethanol. A part of the resulting material was suspended in a solution containing 1.92!10K3 mol of PyB in 70 mL of water, maintained at pH 7.0, and then transferred into a reactor where the pressure was adjusted to 3.0 bar with nitrogen gas and the temperature was set to 75 8C for 72 h. After hydrothermal treatment, the materials were separated, washed, and dried as before. The unsupported polymer used for the TG analysis was prepared according to the literature methods [23]. The LDHs hybrid materials are noted as Zn2Al/PyB and Zn2Cr/ PyB. 2.1. Characterization Powder X-ray diffraction (PXRD) in q–2q mode was conducted using a Siemens D5005 diffractometer, equipped with a graphite monochromator selecting the Cu Ka1 radiation (lZ0.15406 nm). The step used was 0.028/s in the angular domain 2–708. The morphology of the powder LDHs was analyzed by TEM using a PHILIPS EM 208 microscope. Samples were embedded in epoxy resin, ultramicrotomed and supported in copper grids. Thermogravimetric analysis (TGA) were performed with a Shimadzu TGA-50 equipment, under air, from room temperature to 850 8C at a heating rate of 10 8C minK1. Solid state NMR 13C spectra in CP-MAS condition were recorded in a Varian INOVA 300 spectrometer operating at 75.42 MHz. 1H and 13C, 1- and 2-D NMR spectra of the solution were performed in Varian INOVA 500 spectrometer, operating at 500 and 125.65 MHz, respectively. Electron Spin Resonance spectra were recorded at room temperature using a X Band Bruker EMX spectrometer operating at 9.658 GHz. Diphenylpicrylhydrazyl (DPPH) was used to determine the resonance frequency (gZ2.0036G 0.0002). The sweep width was of 200 G and the receiver gain 100,000.
969
Table 1 X-ray powder diffraction data and formula* for LDHs. Sample
Formula
Basal spacing (nm)
Domain sizea (nm)
Zn2Al/PyB
[Zn0.75Al0.25(OH)2] PyB0.25$0.95H2O [Zn0.74Al0.26(OH)2] PyB0.26$0.94H2O [Zn0.74Cr0.26(OH)2] PyB0.26$0.95H2O [Zn0.72Cr0.28(OH)2] PyB0.28$0.93H2O
1.90
28.3
1.91
51.6
1.93
16.5
1.90
21.8
Zn2Al/PyB after HT Zn2Cr/PyB Zn2Cr/PyB after HT *
Based on TG analyses; a The size of the scattering domain along the c-axis was determined from the width of the second and third 00l X-ray harmonic of oriented samples.
arrangement, the PyB anions should adopt an orientation with its long axis perpendicular to the inorganic sheets, the carboxylate groups of the anions are close to the hydroxide layers and the five-membered heterocyclic rings are in the midplane of interlayer space. Similar arrangement as previously observed for monovalent anions, such as benzoate [24]. After the hydrothermal treatment some structural changes take place, including the growth of the crystallites. The number of stacked platelets estimated from the Scherrer equation also increased, and it was more pronounced for the Zn:Al framework than for the Zn:Cr counterpart. An increase of the diffraction line (110) also indicated an increase in the cation ordering inside the inorganic sheets, as well as a superior stacking sequence, resulting in hybrid materials with overall higher structural organization.
3. Results and discussion From the TG analysis and considering the general formula 3C mK for LDH, ½M2C 1Kx Mx ðOHÞ2 Ax=m $nH2 O, the composition of the hybrid materials was calculated assuming a complete for3C mation into an oxide of ideal stochiometry ‘M2C 1Kx Mx OxC2=2 ’ at 850 8C. The results are show in Table 1. The X-ray powder diffraction data and the formula for the hybrid LDH materials are included in Table 1. PXRD patterns for the hybrid materials are shown in Fig. 1. They exhibit several orders of basal reflections (00l) indicating well ordered layered structures. Taking into account the size of the molecule, the basal spacing in the range of 1.90–1.93 nm, calculated from (00l) peaks, are consistent with those expected for the incorporation of bilayers of PyB anions between LDH sheets. In this interlayer
Fig. 1. PXRD of (a) Zn2Al/PyB, (b) Zn2Al/PyB with hydrothermal treatment, (c) Zn2Cr/PyB, and (d) Zn2Cr/PyB with hydrothermal treatment. * Diffraction peak not identified.
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Fig. 2. TEM images of (a) Zn2Al/PyB with hydrothermal treatment, (b) Zn2Cr/PyB with hydrothermal treatment.
The TEM images of the materials are shown in Fig. 2. They showed cross sectional sessions parallel to the (00l) planes, which evidenced the expanded lamellar structure and confirmed the basal spacings obtained by PXRD. In order to have information on the interlayered monomer, NMR study was carried out. The numbering scheme and the assignments of the 13C chemical shifts for the organic monomer in solution and in the solid state are presented in Table 2. Three quaternary carbons are observed: the acid group at 166.6 ppm (C7), the quaternary in a position from it at 127.0 ppm (C6), and the carbon a to the pyrrole group at 143.0 ppm (C3). However, the correct attributions for the pairs C1, C2 and C4, C5 demanded a proton–carbon correlation spectrum, since the resonances at 118.9 and 118.5 ppm were too close. The heteronuclear multiple bond coherence spectrum (1JCH) gHMBC, is unambiguous to assign the signals at 118.5 and 130.9 ppm to the carbons C4, C5, respectively. Concerning the six membered aromatic ring, a complementary long distance 3JCH correlation spectrum clearly showed that the proton attached to carbon C5 at 130.9 ppm correlates with the quaternaries C7 and C3. In this way, the signal at 118.9 is definitely attributed to the C2 carbon of the pyrrole ring. The 13 C CP-MAS NMR spectra for the free monomer and the organic anions incorporated into the gap of the Zn2Al-LDH is shown in Fig. 3. The resonance peak for the carbon nuclei C7 is located at ca. 175.1, C5 and C3 at ca. 131.6 and 140.8 ppm, respectively. The wide signals centered at 117.3 and 113.7 are therefore attributed to the resonances of the remaining carbons of both rings. In particular, the upfield shift of the C2 carbon Table 2 NMR and
from its original position at 120.1 ppm is a rather strong evidence of its quaternization, which must involve an in situ polymerization process through that particularly reactive organic site close to the nitrogen atom. The similarity of the spectra before and after hydrothermal treatment (Fig. 3b and c) seems to indicate that the polymerization is then occurring during the coprecipitation process. Direct polymerization has also been observed in the case of other polymer/2D host framework systems [25–27]. It is likely that the particular disposition of the anions in bilayers, with the pyrrole groups very close to each another, facilitates the a–a polymerization process, as idealized in Fig. 4. Indeed, PyB polymerization occurs (at least partially) at room temperature, probably initiated by oxygen molecules acting as oxidizing agent, as previously proposed by some authors for the in situ polymerization of aniline derivatives [11,13]. To reinforce our assumption of a polymerization process, ESR spectrum of the composite was analyzed. A narrow ESR
13
C1 C2 N C3 C4
C CP-MAS for PyB, values in ppm. C
DMSO d-6
CP-MAS
C1 C2 C3 C4 C5 C6 C7
111.3 118.9 143.0 118.5 130.9 127.0 166.6
111.2 120.1 142.3 115.6 132.9 122.8 174.0
C5 C6 C7 O
OH
Fig. 3. 13C CP-MAS NMR spectra of (a) PyB, (b) Zn2Al/PyB, and (c) Zn2Al/PyB after hydrothermal treatment.
J. Tronto et al. / Journal of Physics and Chemistry of Solids 67 (2006) 968–972
Fig. 4. Schematic representation of Zn2Al/PyB after in situ polymerization.
Fig. 5. ESR signal of the hybrid material Zn2Al/PyB after hydrothermal treatment.
signal from the Zn2Al/PyB is depicted in Fig. 5. It is expected that upon polymerization, a large array of conjugated double bonds should be formed, to such an extent as to generate lowlying molecular orbitals suitable to accommodate conduction electrons. The signal was composed of a single gaussian line 100
Zn2Al/PyB
90
Zn2Al/PyB
(a)
with a peak to peak line width DHpp of 7.8G1 G at a g-value of 2.0050G0.0004, characteristic of organic radicals. Similar ESR spectra of spin carriers localized along polymer chains had already been observed for polyanilines interleaved into layered compounds [28–30]. Regarding Zn2Cr/PyB framework, the intense signal close to gZ2.000 due to the paramagnetic Cr3C nuclei precludes any conclusions, however, the close similarity of the PXRD data between both hybrid materials may indicate a similar molecular arrangement. One has to note that due to broad overlapping IR spectroscopy was unable to unravel if the polymerization is occurring or not. The results of TG analysis for the unsupported PyB polymer, Zn2Al/PyB and Zn2Cr/PyB with and without hydrothermal treatment are presented in Fig. 6. The unsupported PyB polymer decomposes at 280 8C in two steps. Concerning the hybrid phases, a first weight loss occurs from room temperature up to 100 8C, corresponding to the release of adsorbed water molecules (Fig. 6a). A second step of decomposition occurs between 100 and 210 8C, which is associated to the loss of interlamellar water molecules. A final step of decomposition is attributed to the loss of hydroxyl ions and intercalated anions (polymers) with the subsequent formation of mixed oxides. For the Zn2Cr/PyB, the profiles of the curves of thermal decomposition, displayed in Fig. 6b, are very similar to Zn2Al/PyB. The decomposition of the organic component is largely delayed in temperature when sequestrated between LDH gap. This trend is usually observed in the case of hybrid LDH systems, such as poly(aniline) LDH assembly [11–13]. 4. Conclusions The combined PXRD, 13C NMR, and ESR data obtained for the two-dimensional hybrid materials Zn2–Cr/PyB and Zn2– Al–PyB present a strong evidence of the polymerization of the monomers during the co-precipitation process. The basal spacings values for the new hybrid materials suggest 100 Zn2Cr/PyB
80
70
70
60
Mass (%)
Mass (%)
(b) Zn2Cr/PyB
90
PyB unsupported polymer
80
971
50 40
60 50 40
30
30
20
20
10
10
0
0 100
200
300
400
500
Temperature (˚C)
600
700
800
100
200
300
400 500 Temperature (˚C)
600
700
800
Fig. 6. TG analysis of (a) PyB unsupported polymer, Zn2Al/PyB without and with hydrothermal treatment (HT), (b) Zn2Cr/PyB without and with hydrothermal treatment (HT).
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a polymerization proceeding via a ‘zig-zag’ fashion, similarly to syndiotactic polymers such as poly(styrene sulfonate) into relative LDH host structures [31,32]. Acknowledgements Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o PauloFAPESP (process 02/00130-7), Coordenac¸a˜o de Aperfeic¸oamento de Pessoal de Nı´vel Superior-CAPES (process BEX3391/04-3) and Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico-CNPq. Fabrice Leroux thanks FAPESP for the financial support during his visit at Universidade de Sa˜o Paulo (process 03/13585-5). References [1] M.S. Wang, T.J. Pinnavaia, Clay–polymer nanocomposites formed from acidic derivatives of montmorillonite and an epoxy resin, Chem. Mater. 6 (1994) 468–474. [2] E.P. Giannelis, Polymer layered silicate nanocomposites, Adv. Mater. 8 (1996) 29–35. [3] M. Alexandre, P. Dubois, Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials, Mater. Sci. Eng., R 28 (2000) 1–63. [4] M.J. MacLachlan, I. Manners, G.A. Ozin, New (inter)faces: polymers and inorganic materials, Adv. Mater. 12 (2000) 675–681. [5] R. Gangopadhyay, A. De, Conducting polymer nanocomposites: a brief overview, Chem. Mater. 12 (2000) 608–622. [6] F. Leroux, J.P. Besse, Polymer interleaved layered double hydroxide: a new emerging class of nanocomposites, Chem. Mater. 13 (2001) 3507–3515. [7] F. Cavani, F. Triffiro´, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and applications, Catal. Today 11 (1991) 173–301. [8] E.L. Crepaldi, J.B. Valim, Layered double hydroxides: structure, synthesis, properties and applications, Quim. Nova 21 (1998) 300–311. [9] A. Vaccari, Preparation and catalytic properties of cationic and anionic clays, Catal. Today 41 (1998) 53–71. [10] S.P. Newman, W. Jones, Synthesis, characterization and applications of layered double hydroxides containing organic guests, New J. Chem. 2 (1998) 105–115. [11] E.M. Moujahid, M. Dubois, J.P. Besse, F. Leroux, Role of atmospheric oxygen for the polymerization of interleaved aniline sulfonic acid in LDH, Chem. Mater. 14 (2002) 3799–3807. [12] E.M. Moujahid, J.P. Besse, F. Leroux, Synthesis and characterization of a polystyrene sulfonate layered double hydroxide nanocomposite. In-situ polymerization vs. polymer incorporation, J. Mater. Chem. 12 (2002) 3324–3330. [13] V.P. Isupov, L.E. Chupakhina, M.A. Ozerova, V.G. Kostrovsky, V.A. Poluboyarov, Polymerization of m-NH2C6H4COO anions in the intercalation compounds of aluminium hydroxide [LiAl2(OH)6][mNH2C6H4COO]$nH2O, Solid State Ionics 141 (2001) 231–236. [14] A. Vaccari, Clays and catalysis: a promising future, Appl. Clay Sci. 14 (1999) 161–198.
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