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Clay-catalyzed phenomena of cationic-dye aggregation and hydroxo-chromium oligomerization
Microporous and Mesoporous Materials 122 (2009) 13–19
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Clay-catalyzed phenomena of cationic-dye aggregation and hydroxo-chromium oligomerization Maria Roulia a,*, Alexandros A. Vassiliadis b a b
Inorganic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 157 71 Athens, Greece Dyeing and Finishing Laboratory, Department of Textiles, Technological Education Institute of Piraeus, 250 Thivon St., 122 41 Athens, Greece
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 19 December 2008 Accepted 7 January 2009 Available online 13 January 2009 Keywords: Montmorillonite Benzothiazole dye Nanocomposites Multilayers Catalysis
a b s t r a c t Higher dye aggregates, organized in layers, were catalytically obtained by intercalating C.I. Basic Blue 41 into montmorillonite and bentonite at very high dye:clay ratios, i.e., larger than 200% of the cation exchange capacity (CEC). Further oligomerization of the chromium oligomers, inserted into montmorillonite intercalated with hydroxo-chromium complexes, was also recognized as clay-promoted. Formation of higher agglomerates proceeding through interactions with the host aluminosilicate suggests that the clays act as aggregation catalysts equally with organic and inorganic compounds. Scanning electron microscopy (SEM) was employed to examine the microstructure of the hybrid materials constructed; both high-order assemblies of the monoazo-cationic dye and higher Cr(III) oligomers characteristically modify the surface morphology of the intercalated clays. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Thanks to their prominent characteristic of adsorbing both organic [1] and inorganic [2] species, clays can produce layered clay–organic and clay–inorganic hybrid intercalation nanomaterials studied as catalysts and their supports, shape selective adsorbents, ion exchangers, ionic conductors and electrodes [3]. Clay– epoxy [4] and clay–polymer [5] nanocomposites exhibit interesting mechanical properties. Dye-filled zeolites [6] and clays intercalated with cationic dyes are associated to solid-state photochemical and photocatalytic reactivity, energy storage and non-linear optics [7,8]. Furthermore, pillaring increases the catalytic effectiveness of montmorillonite in achieving higher conversions, e.g., for cyclohexane dehydrogenation [9]. Chromium pillared clays have been successfully used as catalysts in a variety of reactions including hydrocracking [10] and aromatic ring nitration [11]. Interactions between the parent clay and guest molecules play a vital role in two processes, the organization of elementary clay sheets and the self-association of guest compound [12], e.g., a cationic dyestuff [13], into the aluminosilicate two-dimensional interlayer. Aggregation phenomena are affected by the structure of dyes and clays, the dye content, the exchangeable cations, the clay particle size [14], the layer charge [15] and the coadsorbed water [16]. Pillared metal-containing clays may exhibit attractive properties depending on the development of higher oligomers. * Corresponding author. Tel.: +30 210 7274780; fax: +30 210 7274435. E-mail address: [email protected] (M. Roulia). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.01.004
The synthesis of chromium oligomers intercalated between the aluminosilicate sheets of montmorillonite, the adsorption of C.I. Basic Blue 41 (Fig. 1a) on bentonite and montmorillonite, the spectral properties and the arrangements of C.I. Basic Blue 41 between the clay lamellae, and the characterization of the dye aggregates formed have been reported [1,2]. However, the role of host material in the guest agglomerization has not been investigated. In this study the catalytic action of montmorillonite and bentonite in the formation of high-order chromium oligomers and layered C.I. Basic Blue 41 aggregates is discussed in an attempt to build up a sizeselective method for guest self-assembly based on the unique property of montmorillonite and, to a lesser extent, bentonite [17] to promote the construction of uniformly oriented tactoids. Clay-stimulated oligomerization of the hydroxo-chromium complexes and aggregation of the dye are examined by visible spectrophotometry and X-ray diffraction. The mechanism behind the development of specific agglomerated colored species on dye-treated clays can be established by diffuse reflectance spectroscopy. Both microstructure and surface modification of the guest–clay composites are explored by scanning electron microscopy, ideally suited [18] for determining the configuration, texture and fabric of clays. 2. Experimental The parent Ca2+STx-1 montmorillonite, Texas, was purchased from Clay Minerals Society Repository, University of Missouri – Columbia. After sedimentation to collect the <2 lm fraction, the
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Fig. 1. Structural formulae of unprotonated (a) and protonated (b), (c) and (d) C.I. Basic Blue 41 cation.
clay was buffered at pH = 5 and washed free of Ca2+ via ion exchange to produce Na+-montmorillonite with a cation exchange capacity value of 80 mEq/100 g montmorillonite. Bentonite (Heliopoulos S.A., Greece) was used as received (CEC = 85 mEq/100 g clay). The compositions, chemical for montmorillonite and mineralogical for bentonite, have been reported previously [1]. C.I. Basic Blue 41 (Sigma-Aldrich and Chromatourgia Tripoleos S.A.) is a monoazo-cationic dye with absorption maximum at 608.5 nm in aqueous solution. By adding 0.5 wt% clay suspensions to C.I. Basic Blue 41 solutions in a programmable Atlas Linitest plus laboratory dyeing machine, dye–clay mixtures with dye loadings between 2% and 500% of the clay CEC were prepared at pH = 3. Chromium oligomers were obtained according to the Base–Acid (BA) method [2]; sodium hydroxide and then perchlorate acid were added to an hexa-aqua Cr(III) solution (absorption peaks at 407 and 574 nm). Next, a suspension of Na+-montmorillonite (0.5 wt%) was introduced into the hydroxo-chromium intercalation solution (pH = 2.8). As the intercalated organic dye molecules would eventually undergo thermal decomposition in the case of high-temperature pillaring, all guest–clay mixtures were treated for 1 h at 291 K, a temperature advantageous to reveal the aggregation-related catalytic properties of the clay, and the danger of dye destruction was avoided. A Martin Christ Osterode/Harz centrifuge was employed for separation of the intercalated clay. Absorbances in the visible range were measured on a Varian Cary 300 3E UV–vis spectrophotometer. A Philips PW 1361/01 diffractometer with a Ni-filtered Cu Ka radiation source operated at I = 20 mA and V = 40 kV was used to examine oriented thin films of intercalated clays smeared onto glass slides by scanning at 1o (2h) min1 between 2o and 10o (2h). Films of the dyed clays cast on glass slides were studied by a GretagMacbeth ColorEye XTH diffuse reflectance spectrophotometer from 400 to 700 nm, against a white NIST 2020c reference material, and K/S values were calculated by the Kubelka–Munk equation,
K=S ¼
ð1 RÞ2 2R
ð1Þ
where K, S are the absorption and scattering coefficients, respectively, and R is the diffuse reflectance at a given wavelength. Micrographs of the gold-coated intercalated samples were acquired using a Jeol JSM-5600 scanning electron microscope. 3. Results and discussion 3.1. Aggregation of the dye Wavelength shifts in the absorption spectra of the dye and the dye-treated clay, both in suspension and in solid state, suggest considerable host–guest interactions; as the potential functions of the dye p-electron system in the ground and excited states can be modified both the bond distances and the conjugation system in the dye molecule are affected [19]. The small bathochromic shifts observed have been attributed [20] to the increased polar environment of the clay interlayer compared with water. Consequently, the diffuse reflectance spectra of intercalated C.I. Basic Blue 41 differ from the visible spectrum of the dye in solution, e.g., the 615- and 585-nm peaks (observable in Fig. 2a) assigned to the dye monomers and dimers, respectively, are red-shifted compared with those (608.5 and 577 nm) obtained from aqueous solutions of the dye. Diffuse reflectance spectra of C.I. Basic Blue 41 intercalated into bentonite at several dye loadings are presented in Fig. 2. As aggregation of the dye on clay takes place J-aggregates (680 nm) and sandwich-type molecular assemblies (dimers near 580 nm and H-aggregates at about 530 nm) are observed [1]. Absorption spectra of dye aggregates show significant differences compared with those of dye monomers due to exciton coupling between the transition dipole moments of the individual dye molecules. If the interchromophore electron overlap is small, so that the chromophore units preserve their individuality in the aggregate, the molecular exciton model will be applicable and, then, solutions for the aggregate in terms of the wave-functions and energies for the electronic states of the components may be sought [21]. However, the higher stability of azobenzene H-trimers (giving a large blue shift) over the H-dimers (exhibiting insignificant shift
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Fig. 2. Dye-aggregation-induced blue shift from 620 nm (a) to 460 nm (e), (f) in diffuse reflectance spectra of bentonite intercalated at pH = 3 for 1 h with C.I. Basic Blue 41: (a) 2%, (e) 200%, (f) 400% of the clay CEC.
in absorption) [22] supports the possibility that other interaction, i.e., bonding, besides hydrophobic and nonpolar attractions is involved. Fig. 2a shows that at very low dye loadings (2% CEC) monomers, in the range of 620 nm, and dimers are the dominating dye forms. At 3% CEC (Fig. 2b), H- and J-aggregates are also present. As the dye:clay ratio increases (Fig. 2c and d) up to 40% CEC, H-type aggregates are the most absorbing species. From X-ray diffraction patterns depicted in Fig. 3a and b basal spacings of 2.50 and 2.40 nm, respectively, were estimated for montmorillonite and bentonite at dye loadings higher than 200% CEC; such galleries can be explained on the assumption that high-order dye aggregates are formed. In Fig. 2e and f (200% and 400% CEC, respectively) the presence of these higher agglomerates that did not initially exist in the intercalating dye solution is verified by the peak position at 460 nm. The peaks in Fig. 2 provide no indication of saturation; deconvolution of the spectra confirmed that wide peaks correspond to several different absorbing dye species. Furthermore, the spectral shape showed no dependence on chromophore dilution, e.g., by barium sulfate. Thus, it is concluded that the dye molecules increasingly aggregate at high dye loadings and form self-assembled oriented
Fig. 3. X-ray diffraction patterns of dye-intercalated (a) montmorillonite and (b) bentonite, at dye:clay ratios larger than 200% CEC (pH = 3).
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multilayers between the clay lamellae. The absorption H-band of dye aggregates is blue-shifted as the dye loading is increased. Dye aggregation is illustrated in Fig. 2 as a dye-loading-induced hypsochromic effect; owing to these layered dye aggregates, the adsorption capacity of the clays is enhanced. In fact, both clays retained about 80 wt% C.I. Basic Blue 41 at a dye loading of 200% CEC and amounts of dye almost equal to their weight at 400% CEC, as is evident from Table 1. In contrast, at dye loadings lower than 20% CEC the dye-treated samples contained <8 wt% dye. Self-association tendency of the dye is increased with amount of dyestuff retained by the clays; therefore, the absorption wavelengths (Fig. 2) can be tuned systematically by proper dye:clay ratio adjustment (Table 1) and subtle dye-arrangement modification. It becomes tenable, from X-ray diffraction measurements, that the basal spacing is steadily increased with dye loading. For example, at a dye loading up to 10% CEC a basal spacing of 1.48 nm was estimated for bentonite, indicating that single-molecule dye layers and slipped H-dimers are the prevailing species in the interlamellar space, while, as dye loadings were increased from 50% to 125% CEC the basal spacing reached 1.65 nm due to a sandwich-type dye bilayer and, probably, H-aggregates at a glide arrangement. Moreover, basal spacings of about 2.50 nm (Fig. 3a) correspond to higher multilayers of the dye intercalated with the aromatic rings either parallel or tilted with respect to the silicate sheets. The clay ability to promote high-order dye transformation at larger dye:clay ratios confirms the selectivity of the process in terms of both the size of dye aggregates and the height of clay galleries. Apparently, stronger dye–clay interactions are expected in the case of parallel arrangement of the dye species. In conclusion, dye orientation could hardly be judged only from basal spacing determination; though, it seems that even the highest interlayer obtained is exceedingly narrow for any dye species formed to perpendicularly arrange in a longitudinal orientation and, also, that the dye adsorbed in large amounts, e.g., at a dye loading of 200% CEC (Table 1), is densely packed into the 1.5-nm interlayer available. A simulation of the dye aggregation between the clay lamellae was achieved, as presented in Fig. 4, in acid solutions of C.I. Basic Blue 41 containing 0.27–2.7 M hydrochloric acid; in all cases the [H+] is at least 300 higher compared with that in the intercalation solution, as the pH of the latter does not fall below 3. Deconvolution of the spectra reveals that the 550- to 650-nm envelope comprises dye monomers (608.5 nm) and dimers (577 nm) with their relative absorbance varying with pH. The isosbestic point at 504 nm suggests that these species are gradually transformed to higher dye aggregates (460 nm) as acidity increases. In the strongly acidified solution the dye cations are protonated (Fig. 1c) at the nitrogen atom of the diazene group [23] rather than (Fig. 1b) at the tertiary amino nitrogen atom [24], and contribute as agglomeration initiators to generate sandwich-type H-aggregates. Molecular aggregation of azo dyes is favored under acidic conditions [25] and high-order H-aggregates producing strong blue shifts are formed via p–p stacking [26]. Dye conformations that facilitate such intermolecular stacking stabilize H-aggregates promoting interactions between the dye monomers, while bulky conformations obviously impose a steric hindrance that limits the encapsulation of the monomers within the aggregate cluster. At reduced pH values the protonated C.I. Basic Blue 41 may appear as a resonance hybrid between quinone diimide (Fig. 1d) and phenylenediamine (Fig. 1c) azonium structures that promote the delocalization of positive charges [27] influencing the electron density of the double bond of the diazene group. This bond acts as an effective barrier to changes in the molecular conformation [25,28]; thus, protonation at the azo b-nitrogen atom, i.e., that more remote from the amino group, leads to a significant reduction of the double-bond character of the –N@N– group and the energetic barrier is lessened. Then, the molecule can adopt a planar conformation [29] facilitating the for-
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Table 1 C.I. Basic Blue 41 retained (±8%) by the clays at 291 K (t = 1 h, pH = 3). Dye loading
Dye content
(% CEC)
(g dye/g clay)
b
(% CEC)
(nm)
Montmorillonitea
Bentoniteb
Montmorillonite
Bentonite
0.00528 0.00811 0.0183 0.0408 0.0628 0.0845 0.128 0.171 0.214 0.415 0.575 0.790 0.987 1.01 1.02 1.02
0.00234 0.00741 0.0160 0.0373 0.0585 0.0788 0.119 0.159 0.199 0.398 0.598 0.790 0.891 0.909 0.907 0.960
1.4 2.1 4.7 10 15 20 30 40 50 100 149 200 250 261 264 264
0.57 1.8 3.9 9.1 14 19 29 39 48 97 146 192 217 221 221 234
620
dye aggregation
2 3 5 10 15 20 30 40 50 100 150 200 250 300 350 400 a
Wavelength of K/S maximum
460
1
CEC = 0.80 mEq g . CEC = 0.85 mEq g1.
Fig. 4. Acidity-dependent transformation of C.I. Basic Blue 41 monomers and dimers to higher aggregates in hydrochloric acid: (a) UV–vis spectrum at 2.7 M HCl; the absorption at 460 nm (inset) corresponds to the high-order dye aggregates.
mation of high-stability H-aggregates [22]. Overall, protonation could affect the stacking of the dye molecules due to a change in the conjugation system [30]. Contrarily, it was observed that as pH increases the higher dye aggregates of Fig. 4 are reconverted to the initial dye species, namely dimers and monomers. Another important experimental observation is that C.I. Basic Blue 41 does not decompose in acidic environments [17,31]; on this basis, it is unlikely that any degradation products could be included among the dye species involved in Fig. 4. Hence, equilibrium is expected to establish between protonated and deprotonated forms of the azo dye [28] as can be seen in Fig. 1. In fact, the 735-nm peak assigned to protonated dye has been observed in the visible spectrum of C.I. Basic Blue 41 from solution [1]. Once formed, under the experimental conditions of Fig. 4, the protonated molecules may either exist in solution or act as an intermediate being transformed to stable H-aggregates; the energetically favorable route is the latter. As an extra positive charge that the dye cation has to accommodate is added by proton-
ation to the molecule, a less stable protonated dye compared with the unprotonated one will be expected. Therefore, unless stabilized by the clay, as is discussed in the following, protonated dye molecules are not probable to appear in the absorption spectra. Molecular dye aggregation in the presence of electrolytes is a common feature. The influence of inorganic salts on the aggregation of a cationic dye in aqueous solutions is related to electrostatic repulsion between the dye and the inorganic cations, thus forcing the dye cations to ‘‘concentrate” in water and undergo self-association. As a rule, bulkier positive ions in the solution result in a greater degree of dye cation dimerization [32]. In our case, a significant concentration of sodium chloride (3 M) in aqueous C.I. Basic Blue 41 solutions led to an increased dimerization of the dye (Fig. 5), but no formation of higher aggregates was observed. Thus, we conclude that the transformation to high-order dye aggregates can only be obtained by acidification of the solution. However, it was realized that in the weakly acid intercalating solution the presence of clay promotes the aggregation process via dye–clay complex formation. Guest–host interactions [33] involve attachment of the dye primarily to the negative sites on the clay surface, that serve for electron donors to the cationic dye, the positive charge of which is decreased facilitating the protonation of the diazene group. The protonated dye molecules stabilized by the clay are easily detected; diffuse reflectance spectra at increased dye loadings shown in Fig. 2d–f suggest strong absorptions above 700 nm due to protonated dye [1,23,34] existing in the dyed-clay environment. Owing to the electrodonating character of clays and the electrostatic dye–dye intermolecular attraction, the H-aggregates formed are of high stability as already mentioned. Specifically, it is obvious from Fig. 2c that the clays catalyze the dye protonation and, in turn, initiate the H-type aggregation even at relatively low dye loadings. This protonation-induced rise in dye aggregates (Fig. 2d) results in energy stabilization and, as the dye loading is increased, high-order assemblies are favorably developed, further reducing the wavelength of K/S maximum (Fig. 2e and f). Coexistent protonated and agglomerated dye species are important for clay-catalyzed high-order aggregation, while, in the absence of clay, raised concentrations of hydrochloric acid are required. The active centers on the clay stimulate the dye protonation and function as aggregation sites creating regions of
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Fig. 5. Visible spectrum of C.I. Basic Blue 41 in the presence of 3 M NaCl.
high dye concentration in an acidic aluminosilicate environment, imitating that of the strongly acid solution of the dyestuff; thus, we conclude that both clays may act as catalysts in the aggregation of C.I. Basic Blue 41 at very high dye loadings. Apparently, the formation of higher aggregates is due to the fact that at increased dye:clay ratios the dye adsorption occurs more rapidly compared with the dye diffusion inside the substrate. It was found that the catalytic action of bentonite, natural clay containing small amounts of calcite [1], is identical compared with that of montmorillonite, i.e., a clay mineral, in the formation of layered high-order dye aggregates. It was proved experimentally that bentonite, the main component of which is montmorillonite, retains almost a hundredfold amount of C.I. Basic Blue 41, e.g., 0.96 g dye/g bentonite at a dye loading of 400% CEC (Table 1), compared with pure calcite demonstrating an adsorption capacity of 0.01 g dye/g calcite. It is well-known that the thermodynamically stable trans-isomers of cationic azobenzene dyes [35] reversibly isomerize to cis-azobenzenes which show no evidence of aggregation [22]. Commonly, trans-to-cis isomerization does not occur unless the dye is UV-irradiated [36] particularly in solution and, to a lesser extent, in the adsorbed state [35,37]. Furthermore, it has been found that the cis-isomers are spontaneously reconverted to trans either with time [35] or by thermal paths [22,37]. In all experiments carried out both in this work and previously [17] no spectral change with time and temperature was observed, indicating the absence of isomerization. Thus, the spectra obtained should be attributed to trans-C.I. Basic Blue 41, as isomerization of the dominantly present trans-isomers would require breaking down the dye aggregates formed, which might be energetically disadvantageous [35]. 3.2. Oligomerization of the hydroxo-Cr(III) complexes On the initial assumption that montmorillonite and bentonite may exert a marked catalytic action in the formation of highorder dye aggregates, it needs particular consideration that the dye-treated clays show an analogy with chromium-intercalated montmorillonite. Cationic forms of several Cr(III) oligomers (peaking at 421 and 583 nm), prepared via chromite cleavage during acidification of the Cr(III) solution, were obtained by the BA method. Undesirably, these oligomers were gradually converted to monomeric, dimeric and trimeric species (as hexamers and higher oligomers are consumed rapidly at pH = 2.8) and the UV–vis peaks were shifted towards lower wavelengths (419 and 581 nm, respectively) within
Fig. 6. X-ray diffraction patterns of montmorillonite intercalated with hydroxochromium complexes at pH = 2.8 for (a) 1 h and (b) 24 h.
a few hours. Accordingly, in acid solutions oligomerization is quenched and the protonated Cr(III)-aqua ions produced undergo cleavage and intramolecular rearrangements. On the contrary, for hydroxo-Cr(III) complexes intercalated in montmorillonite the chromium oligomers were inserted into the interlayer and a basal spacing of 1.96 nm (Fig. 6a) was obtained; as the intercalation time was increased from 1 to 24 h (Fig. 6b) higher galleries (d001 = 2.10 nm) were achieved, indicating further oligomerization of chromium assemblies. In general, intercalation solutions containing substantial amounts of large polyoxo-chromium oligomers are prerequisites for the preparation of chromia pillared clays with high basal spacings. Gallery heights are of significance as hydroxo-Cr(III)–clay intercalates, converted upon heating to pillared products, may function as catalysts. For instance, chromium pillared clays with high galleries are effective as catalysts for benzylic oxidation and selective deprotection of benzyl ethers and benzylamines [38], while chromium-interlayered clays with a lower spacing of 1.44–1.53 nm exhibit increased efficiency for deep oxidation of dichloromethane [39]. Unlike large interlayer heights, galleries of less than 0.4 nm may end up with low catalytic reactivity because of the inaccessibility to the gallery chromia by the reactants [9]. It has been shown [40] that during the chromium hydrolytic oligomerization, deprotonation of Cr(III) complexes, occurring at pH close to 5.0, leads to a rapid formation of conjugate bases and hydrogen-bonded aggregates. These coordinated hydroxides [41], being better nucleophiles than water towards adjacent Cr(III) centers, increase the lability of chromium coordination sphere and facilitate fast condensation to polynuclear hydrolyzed compounds. We suggest that montmorillonite may also act catalytically in the construction of higher hydroxo-chromium oligomers, most probably by promoting deprotonation. The clay does function as a proton acceptor and increases the concentration of the reacting species by bringing them close to each other through hydrogen-bond formation. Analogous activity of montmorillonite during pillaring with chromium complexes has been reported [9,42]. Hence, we presume that the clay equally promotes aggregation phenomena of C.I. Basic Blue 41 and Cr(III) hydroxo-complexes, allowing for a generalization of its catalytic properties towards both organic and inorganic compounds. Apparently, montmorillonite and bentonite exclusively accelerate guest–guest agglomeration reactions. The intercalation of dye
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Fig. 7. Scanning electron micrographs of dye-treated (a) montmorillonite and (c) bentonite, at very high (>200% CEC) C.I. Basic Blue 41 loadings, host bentonite (b) and intercalated with higher hydroxo-Cr(III) oligomers montmorillonite (d).
molecules and hydroxo-Cr(III) oligomers is not accompanied by a chemical attachment to the aluminosilicate sheets; the clay, simultaneously being the catalyst [9,42] and the host material, may be recovered. However, where the strategy of choice is to tailor integrated systems (composed of the inorganic matrix and immobilized aggregates) for specific applications, it is essential that the clay be permanently incorporated into the host–guest nanocomposites prepared. 3.3. Surface modification Examination of scanning electron micrographs indicates substantial changes in microstructure of the high-dye-loading intercalated clays compared with the parent material. Obviously, scanning electron microscopy alone cannot provide direct experimental evidence for the catalytic action of clays. On the microphase scale, however, it can successfully correlate the fine details of the guest-treated host surface with the stacking effect caused by the catalytically induced guest aggregation. The micrograph in Fig. 7a reveals that the distinctively lamellar structure of untreated montmorillonite is significantly modified, as an increased amount of dye is incorporated between the clay cohesive thin sheets. The aggregation of dye molecules is facilitated, as they approach a large number of exposed negative sites. The tight stacking of dye layers placed within the interlayer fills the voids, resulting in a condensed morphology of dye-treated montmorillonite (Fig. 7a); a characteristic wavy surface can be clearly seen. On the other hand, the chromium-intercalated clay of Fig. 7d is rugged and anomalous compared with the dyed sample, displays irregular flower-like
edges, and shows an open structure due to the limited amount (4 wt%) of chromium present in the interlayer region. A lamellar construction, somewhat diverse compared with host montmorillonite, is presented in Fig. 7b for undyed bentonite. A dyed-montmorillonite-resembling microstructure appears as the dye molecules are introduced into bentonite, but the overall particle arrangements are now different (Fig. 7c); the dye-treated samples exhibit a unique crinkly, ridged, less condensed texture that partly accounts for lower dye retention and reduced basal spacing of bentonite. Patterns of both dyed clays shown in Fig. 7a and c resemble desiccation mud cracks and other dewatering features, are characteristic of the smectite group, and develop from shrinkage of the expandable clay minerals as they dry [18]. 4. Conclusions The catalytic action of montmorillonite and bentonite in the formation of layered C.I. Basic Blue 41 agglomerates was investigated. Both X-ray diffraction and diffuse reflectance results indicated the presence of spectrally distinct extended dye aggregates between the clay lamellae at dye loadings higher than 200% CEC and pH = 3. These assemblies found in the interlayer, but not in the intercalating solution, suggest that the clays act as aggregation catalysts; domains with increased guest concentration in the acidic host environment are responsible, as the anionic surfaces of clays template the formation of stable H-aggregates stimulating the protonation of the dye. Absorption wavelengths for dye aggregates were steadily reduced with increasing dye:clay ratio confirming the clay ability to selectively promote high-order dye aggregation.
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On the contrary, in the absence of clay the dye transformation to higher H-aggregates can only be achieved in strongly acid aqueous C.I. Basic Blue 41 solutions. Similar aggregation phenomena were observed during intercalation of montmorillonite with Cr(III) hydroxo-complexes. Besides bringing the reactants into close proximity, the clay seems to promote agglomeration of the intercalant species through proton transfer reactions occurring at the negative aluminosilicate surface. Both dye-treated and chromium-intercalated clays exhibited remarkable alterations in morphology and microstructure. Acknowledgment Triacryl Blue RL 330% brand (C.I. Basic Blue 41) was supplied by Chromatourgia Tripoleos S.A., Greece. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M. Roulia, A.A. Vassiliadis, J. Colloid Interf. Sci. 291 (2005) 37. M. Roulia, Mater. Chem. Phys. 91 (2005) 281. M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. A. Yasmin, J.L. Abot, I.M. Daniel, Scr. Mater. 49 (2003) 81. B. Chen, J.R.G. Evans, Scr. Mater. 54 (2006) 1581. G. Calzaferri, S. Huber, H. Maas, C. Minkowski, Angew. Chem. Int. Ed. 42 (2003) 3732. J.K. Thomas, Chem. Rev. 93 (1993) 301. T. Shichi, K. Takagi, J. Photochem. Photobiol. C 1 (2000) 113. M.S. Tzou, T.J. Pinnavaia, Catal. Today 2 (1988) 243. S.D. Bodman, W.R. McWhinnie, V. Begon, I. Suelves, M.J. Lazaro, T.J. Morgan, A.A. Herod, R. Kandiyoti, Fuel 81 (2002) 449. T. Mishra, K.M. Parida, J. Mol. Catal. A 121 (1997) 91. G.W. Beall, M. Goss, Appl. Clay Sci. 27 (2004) 179.
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[13] J. Bujdák, N. Iyi, J. Phys. Chem. B 110 (2006) 2180. [14] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, J. Colloid Interf. Sci. 255 (2002) 254. [15] J. Bujdák, Appl. Clay Sci. 34 (2006) 58. [16] V. Ramamurthy, D.R. Sanderson, D.F. Eaton, J. Am. Chem. Soc. 115 (1993) 10438. [17] M. Roulia, A.A. Vassiliadis, Micropor. Mesopor. Mater. 116 (2008) 732. [18] B.F. Bohor, R.E. Hughes, Clays Clay Miner. 19 (1971) 49. [19] G. Schulz-Ekloff, D. Wöhrle, B. van Duffel, R.A. Schoonheydt, Micropor. Mesopor. Mater. 51 (2002) 91. [20] M. Ogawa, R. Kawai, K. Kuroda, J. Phys. Chem. 100 (1996) 16218. [21] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. [22] X. Song, J. Perlstein, D.G. Whitten, J. Am. Chem. Soc. 119 (1997) 9144. [23] H. Kashida, H. Asanuma, M. Komiyama, Supramol. Chem. 16 (2004) 459. [24] T. Stoyanova, S. Stoyanov, L. Antonov, V. Petrova, Dyes Pigm. 31 (1996) 1. [25] J. Del Nero, R.E. de Araujo, A.S.L. Gomes, C.P. de Melo, J. Chem. Phys. 122 (2005) 104506. [26] K.K. Karukstis, L.A. Perelman, W.K. Wong, Langmuir 18 (2002) 10363. [27] H.P. de Oliveira, Microchem. J. 88 (2008) 32. [28] H.P. de Oliveira, E.G.L. Oliveira, C.P. de Melo, J. Colloid Interf. Sci. 303 (2006) 444. [29] A. Navarro, F. Sanz, Dyes Pigm. 40 (1999) 131. [30] G. Zhang, M. Liu, J. Phys. Chem. B 112 (2008) 7430. [31] A.P. D’Rozario, B. Parton, A. Williams, Gazz. Chim. Ital. 117 (1987) 395. [32] H. Bose, Indian J. Chem. A 26 (1987) 652. [33] K. Takagi, T. Kurematsu, Y. Sawaki, J. Chem. Soc. Perkin Trans. 2 (1991) 1517. [34] Z. Yermiyahu, I. Lapides, S. Yariv, Clay Miner. 38 (2003) 483. [35] J. Bujdák, N. Iyi, T. Fujita, J. Colloid Interf. Sci. 262 (2003) 282. [36] A. Jafari, H. Tajalli, M. Abedi, Opt. Commun. 281 (2008) 2959. [37] M. Ogawa, Chem. Mater. 8 (1996) 1347. [38] B.M. Choudary, A.D. Prasad, V. Bhuma, V. Swapna, J. Org. Chem. 57 (1992) 5841. [39] L. Storaro, R. Ganzerla, M. Lenarda, R. Zanoni, A. Jiménez López, P. OliveraPastor, E. Rodríguez Castellón, J. Mol. Catal. A 115 (1997) 329. [40] A. Drljaca, L. Spiccia, Polyhedron 15 (1996) 2875. [41] S.J. Crimp, L. Spiccia, H.R. Krouse, T.W. Swaddle, Inorg. Chem. 33 (1994) 465. [42] R.M. Carr, Clays Clay Miner. 33 (1985) 357.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Clay-catalyzed phenomena of cationic-dye aggregation and hydroxo-chromium oligomerization Maria Roulia a,*, Alexandros A. Vassiliadis b a b
Inorganic Chemistry Laboratory, Department of Chemistry, University of Athens, Panepistimiopolis, 157 71 Athens, Greece Dyeing and Finishing Laboratory, Department of Textiles, Technological Education Institute of Piraeus, 250 Thivon St., 122 41 Athens, Greece
a r t i c l e
i n f o
Article history: Received 9 May 2008 Received in revised form 19 December 2008 Accepted 7 January 2009 Available online 13 January 2009 Keywords: Montmorillonite Benzothiazole dye Nanocomposites Multilayers Catalysis
a b s t r a c t Higher dye aggregates, organized in layers, were catalytically obtained by intercalating C.I. Basic Blue 41 into montmorillonite and bentonite at very high dye:clay ratios, i.e., larger than 200% of the cation exchange capacity (CEC). Further oligomerization of the chromium oligomers, inserted into montmorillonite intercalated with hydroxo-chromium complexes, was also recognized as clay-promoted. Formation of higher agglomerates proceeding through interactions with the host aluminosilicate suggests that the clays act as aggregation catalysts equally with organic and inorganic compounds. Scanning electron microscopy (SEM) was employed to examine the microstructure of the hybrid materials constructed; both high-order assemblies of the monoazo-cationic dye and higher Cr(III) oligomers characteristically modify the surface morphology of the intercalated clays. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction Thanks to their prominent characteristic of adsorbing both organic [1] and inorganic [2] species, clays can produce layered clay–organic and clay–inorganic hybrid intercalation nanomaterials studied as catalysts and their supports, shape selective adsorbents, ion exchangers, ionic conductors and electrodes [3]. Clay– epoxy [4] and clay–polymer [5] nanocomposites exhibit interesting mechanical properties. Dye-filled zeolites [6] and clays intercalated with cationic dyes are associated to solid-state photochemical and photocatalytic reactivity, energy storage and non-linear optics [7,8]. Furthermore, pillaring increases the catalytic effectiveness of montmorillonite in achieving higher conversions, e.g., for cyclohexane dehydrogenation [9]. Chromium pillared clays have been successfully used as catalysts in a variety of reactions including hydrocracking [10] and aromatic ring nitration [11]. Interactions between the parent clay and guest molecules play a vital role in two processes, the organization of elementary clay sheets and the self-association of guest compound [12], e.g., a cationic dyestuff [13], into the aluminosilicate two-dimensional interlayer. Aggregation phenomena are affected by the structure of dyes and clays, the dye content, the exchangeable cations, the clay particle size [14], the layer charge [15] and the coadsorbed water [16]. Pillared metal-containing clays may exhibit attractive properties depending on the development of higher oligomers. * Corresponding author. Tel.: +30 210 7274780; fax: +30 210 7274435. E-mail address: [email protected] (M. Roulia). 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.01.004
The synthesis of chromium oligomers intercalated between the aluminosilicate sheets of montmorillonite, the adsorption of C.I. Basic Blue 41 (Fig. 1a) on bentonite and montmorillonite, the spectral properties and the arrangements of C.I. Basic Blue 41 between the clay lamellae, and the characterization of the dye aggregates formed have been reported [1,2]. However, the role of host material in the guest agglomerization has not been investigated. In this study the catalytic action of montmorillonite and bentonite in the formation of high-order chromium oligomers and layered C.I. Basic Blue 41 aggregates is discussed in an attempt to build up a sizeselective method for guest self-assembly based on the unique property of montmorillonite and, to a lesser extent, bentonite [17] to promote the construction of uniformly oriented tactoids. Clay-stimulated oligomerization of the hydroxo-chromium complexes and aggregation of the dye are examined by visible spectrophotometry and X-ray diffraction. The mechanism behind the development of specific agglomerated colored species on dye-treated clays can be established by diffuse reflectance spectroscopy. Both microstructure and surface modification of the guest–clay composites are explored by scanning electron microscopy, ideally suited [18] for determining the configuration, texture and fabric of clays. 2. Experimental The parent Ca2+STx-1 montmorillonite, Texas, was purchased from Clay Minerals Society Repository, University of Missouri – Columbia. After sedimentation to collect the <2 lm fraction, the
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Fig. 1. Structural formulae of unprotonated (a) and protonated (b), (c) and (d) C.I. Basic Blue 41 cation.
clay was buffered at pH = 5 and washed free of Ca2+ via ion exchange to produce Na+-montmorillonite with a cation exchange capacity value of 80 mEq/100 g montmorillonite. Bentonite (Heliopoulos S.A., Greece) was used as received (CEC = 85 mEq/100 g clay). The compositions, chemical for montmorillonite and mineralogical for bentonite, have been reported previously [1]. C.I. Basic Blue 41 (Sigma-Aldrich and Chromatourgia Tripoleos S.A.) is a monoazo-cationic dye with absorption maximum at 608.5 nm in aqueous solution. By adding 0.5 wt% clay suspensions to C.I. Basic Blue 41 solutions in a programmable Atlas Linitest plus laboratory dyeing machine, dye–clay mixtures with dye loadings between 2% and 500% of the clay CEC were prepared at pH = 3. Chromium oligomers were obtained according to the Base–Acid (BA) method [2]; sodium hydroxide and then perchlorate acid were added to an hexa-aqua Cr(III) solution (absorption peaks at 407 and 574 nm). Next, a suspension of Na+-montmorillonite (0.5 wt%) was introduced into the hydroxo-chromium intercalation solution (pH = 2.8). As the intercalated organic dye molecules would eventually undergo thermal decomposition in the case of high-temperature pillaring, all guest–clay mixtures were treated for 1 h at 291 K, a temperature advantageous to reveal the aggregation-related catalytic properties of the clay, and the danger of dye destruction was avoided. A Martin Christ Osterode/Harz centrifuge was employed for separation of the intercalated clay. Absorbances in the visible range were measured on a Varian Cary 300 3E UV–vis spectrophotometer. A Philips PW 1361/01 diffractometer with a Ni-filtered Cu Ka radiation source operated at I = 20 mA and V = 40 kV was used to examine oriented thin films of intercalated clays smeared onto glass slides by scanning at 1o (2h) min1 between 2o and 10o (2h). Films of the dyed clays cast on glass slides were studied by a GretagMacbeth ColorEye XTH diffuse reflectance spectrophotometer from 400 to 700 nm, against a white NIST 2020c reference material, and K/S values were calculated by the Kubelka–Munk equation,
K=S ¼
ð1 RÞ2 2R
ð1Þ
where K, S are the absorption and scattering coefficients, respectively, and R is the diffuse reflectance at a given wavelength. Micrographs of the gold-coated intercalated samples were acquired using a Jeol JSM-5600 scanning electron microscope. 3. Results and discussion 3.1. Aggregation of the dye Wavelength shifts in the absorption spectra of the dye and the dye-treated clay, both in suspension and in solid state, suggest considerable host–guest interactions; as the potential functions of the dye p-electron system in the ground and excited states can be modified both the bond distances and the conjugation system in the dye molecule are affected [19]. The small bathochromic shifts observed have been attributed [20] to the increased polar environment of the clay interlayer compared with water. Consequently, the diffuse reflectance spectra of intercalated C.I. Basic Blue 41 differ from the visible spectrum of the dye in solution, e.g., the 615- and 585-nm peaks (observable in Fig. 2a) assigned to the dye monomers and dimers, respectively, are red-shifted compared with those (608.5 and 577 nm) obtained from aqueous solutions of the dye. Diffuse reflectance spectra of C.I. Basic Blue 41 intercalated into bentonite at several dye loadings are presented in Fig. 2. As aggregation of the dye on clay takes place J-aggregates (680 nm) and sandwich-type molecular assemblies (dimers near 580 nm and H-aggregates at about 530 nm) are observed [1]. Absorption spectra of dye aggregates show significant differences compared with those of dye monomers due to exciton coupling between the transition dipole moments of the individual dye molecules. If the interchromophore electron overlap is small, so that the chromophore units preserve their individuality in the aggregate, the molecular exciton model will be applicable and, then, solutions for the aggregate in terms of the wave-functions and energies for the electronic states of the components may be sought [21]. However, the higher stability of azobenzene H-trimers (giving a large blue shift) over the H-dimers (exhibiting insignificant shift
M. Roulia, A.A. Vassiliadis / Microporous and Mesoporous Materials 122 (2009) 13–19
Fig. 2. Dye-aggregation-induced blue shift from 620 nm (a) to 460 nm (e), (f) in diffuse reflectance spectra of bentonite intercalated at pH = 3 for 1 h with C.I. Basic Blue 41: (a) 2%, (e) 200%, (f) 400% of the clay CEC.
in absorption) [22] supports the possibility that other interaction, i.e., bonding, besides hydrophobic and nonpolar attractions is involved. Fig. 2a shows that at very low dye loadings (2% CEC) monomers, in the range of 620 nm, and dimers are the dominating dye forms. At 3% CEC (Fig. 2b), H- and J-aggregates are also present. As the dye:clay ratio increases (Fig. 2c and d) up to 40% CEC, H-type aggregates are the most absorbing species. From X-ray diffraction patterns depicted in Fig. 3a and b basal spacings of 2.50 and 2.40 nm, respectively, were estimated for montmorillonite and bentonite at dye loadings higher than 200% CEC; such galleries can be explained on the assumption that high-order dye aggregates are formed. In Fig. 2e and f (200% and 400% CEC, respectively) the presence of these higher agglomerates that did not initially exist in the intercalating dye solution is verified by the peak position at 460 nm. The peaks in Fig. 2 provide no indication of saturation; deconvolution of the spectra confirmed that wide peaks correspond to several different absorbing dye species. Furthermore, the spectral shape showed no dependence on chromophore dilution, e.g., by barium sulfate. Thus, it is concluded that the dye molecules increasingly aggregate at high dye loadings and form self-assembled oriented
Fig. 3. X-ray diffraction patterns of dye-intercalated (a) montmorillonite and (b) bentonite, at dye:clay ratios larger than 200% CEC (pH = 3).
15
multilayers between the clay lamellae. The absorption H-band of dye aggregates is blue-shifted as the dye loading is increased. Dye aggregation is illustrated in Fig. 2 as a dye-loading-induced hypsochromic effect; owing to these layered dye aggregates, the adsorption capacity of the clays is enhanced. In fact, both clays retained about 80 wt% C.I. Basic Blue 41 at a dye loading of 200% CEC and amounts of dye almost equal to their weight at 400% CEC, as is evident from Table 1. In contrast, at dye loadings lower than 20% CEC the dye-treated samples contained <8 wt% dye. Self-association tendency of the dye is increased with amount of dyestuff retained by the clays; therefore, the absorption wavelengths (Fig. 2) can be tuned systematically by proper dye:clay ratio adjustment (Table 1) and subtle dye-arrangement modification. It becomes tenable, from X-ray diffraction measurements, that the basal spacing is steadily increased with dye loading. For example, at a dye loading up to 10% CEC a basal spacing of 1.48 nm was estimated for bentonite, indicating that single-molecule dye layers and slipped H-dimers are the prevailing species in the interlamellar space, while, as dye loadings were increased from 50% to 125% CEC the basal spacing reached 1.65 nm due to a sandwich-type dye bilayer and, probably, H-aggregates at a glide arrangement. Moreover, basal spacings of about 2.50 nm (Fig. 3a) correspond to higher multilayers of the dye intercalated with the aromatic rings either parallel or tilted with respect to the silicate sheets. The clay ability to promote high-order dye transformation at larger dye:clay ratios confirms the selectivity of the process in terms of both the size of dye aggregates and the height of clay galleries. Apparently, stronger dye–clay interactions are expected in the case of parallel arrangement of the dye species. In conclusion, dye orientation could hardly be judged only from basal spacing determination; though, it seems that even the highest interlayer obtained is exceedingly narrow for any dye species formed to perpendicularly arrange in a longitudinal orientation and, also, that the dye adsorbed in large amounts, e.g., at a dye loading of 200% CEC (Table 1), is densely packed into the 1.5-nm interlayer available. A simulation of the dye aggregation between the clay lamellae was achieved, as presented in Fig. 4, in acid solutions of C.I. Basic Blue 41 containing 0.27–2.7 M hydrochloric acid; in all cases the [H+] is at least 300 higher compared with that in the intercalation solution, as the pH of the latter does not fall below 3. Deconvolution of the spectra reveals that the 550- to 650-nm envelope comprises dye monomers (608.5 nm) and dimers (577 nm) with their relative absorbance varying with pH. The isosbestic point at 504 nm suggests that these species are gradually transformed to higher dye aggregates (460 nm) as acidity increases. In the strongly acidified solution the dye cations are protonated (Fig. 1c) at the nitrogen atom of the diazene group [23] rather than (Fig. 1b) at the tertiary amino nitrogen atom [24], and contribute as agglomeration initiators to generate sandwich-type H-aggregates. Molecular aggregation of azo dyes is favored under acidic conditions [25] and high-order H-aggregates producing strong blue shifts are formed via p–p stacking [26]. Dye conformations that facilitate such intermolecular stacking stabilize H-aggregates promoting interactions between the dye monomers, while bulky conformations obviously impose a steric hindrance that limits the encapsulation of the monomers within the aggregate cluster. At reduced pH values the protonated C.I. Basic Blue 41 may appear as a resonance hybrid between quinone diimide (Fig. 1d) and phenylenediamine (Fig. 1c) azonium structures that promote the delocalization of positive charges [27] influencing the electron density of the double bond of the diazene group. This bond acts as an effective barrier to changes in the molecular conformation [25,28]; thus, protonation at the azo b-nitrogen atom, i.e., that more remote from the amino group, leads to a significant reduction of the double-bond character of the –N@N– group and the energetic barrier is lessened. Then, the molecule can adopt a planar conformation [29] facilitating the for-
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Table 1 C.I. Basic Blue 41 retained (±8%) by the clays at 291 K (t = 1 h, pH = 3). Dye loading
Dye content
(% CEC)
(g dye/g clay)
b
(% CEC)
(nm)
Montmorillonitea
Bentoniteb
Montmorillonite
Bentonite
0.00528 0.00811 0.0183 0.0408 0.0628 0.0845 0.128 0.171 0.214 0.415 0.575 0.790 0.987 1.01 1.02 1.02
0.00234 0.00741 0.0160 0.0373 0.0585 0.0788 0.119 0.159 0.199 0.398 0.598 0.790 0.891 0.909 0.907 0.960
1.4 2.1 4.7 10 15 20 30 40 50 100 149 200 250 261 264 264
0.57 1.8 3.9 9.1 14 19 29 39 48 97 146 192 217 221 221 234
620
dye aggregation
2 3 5 10 15 20 30 40 50 100 150 200 250 300 350 400 a
Wavelength of K/S maximum
460
1
CEC = 0.80 mEq g . CEC = 0.85 mEq g1.
Fig. 4. Acidity-dependent transformation of C.I. Basic Blue 41 monomers and dimers to higher aggregates in hydrochloric acid: (a) UV–vis spectrum at 2.7 M HCl; the absorption at 460 nm (inset) corresponds to the high-order dye aggregates.
mation of high-stability H-aggregates [22]. Overall, protonation could affect the stacking of the dye molecules due to a change in the conjugation system [30]. Contrarily, it was observed that as pH increases the higher dye aggregates of Fig. 4 are reconverted to the initial dye species, namely dimers and monomers. Another important experimental observation is that C.I. Basic Blue 41 does not decompose in acidic environments [17,31]; on this basis, it is unlikely that any degradation products could be included among the dye species involved in Fig. 4. Hence, equilibrium is expected to establish between protonated and deprotonated forms of the azo dye [28] as can be seen in Fig. 1. In fact, the 735-nm peak assigned to protonated dye has been observed in the visible spectrum of C.I. Basic Blue 41 from solution [1]. Once formed, under the experimental conditions of Fig. 4, the protonated molecules may either exist in solution or act as an intermediate being transformed to stable H-aggregates; the energetically favorable route is the latter. As an extra positive charge that the dye cation has to accommodate is added by proton-
ation to the molecule, a less stable protonated dye compared with the unprotonated one will be expected. Therefore, unless stabilized by the clay, as is discussed in the following, protonated dye molecules are not probable to appear in the absorption spectra. Molecular dye aggregation in the presence of electrolytes is a common feature. The influence of inorganic salts on the aggregation of a cationic dye in aqueous solutions is related to electrostatic repulsion between the dye and the inorganic cations, thus forcing the dye cations to ‘‘concentrate” in water and undergo self-association. As a rule, bulkier positive ions in the solution result in a greater degree of dye cation dimerization [32]. In our case, a significant concentration of sodium chloride (3 M) in aqueous C.I. Basic Blue 41 solutions led to an increased dimerization of the dye (Fig. 5), but no formation of higher aggregates was observed. Thus, we conclude that the transformation to high-order dye aggregates can only be obtained by acidification of the solution. However, it was realized that in the weakly acid intercalating solution the presence of clay promotes the aggregation process via dye–clay complex formation. Guest–host interactions [33] involve attachment of the dye primarily to the negative sites on the clay surface, that serve for electron donors to the cationic dye, the positive charge of which is decreased facilitating the protonation of the diazene group. The protonated dye molecules stabilized by the clay are easily detected; diffuse reflectance spectra at increased dye loadings shown in Fig. 2d–f suggest strong absorptions above 700 nm due to protonated dye [1,23,34] existing in the dyed-clay environment. Owing to the electrodonating character of clays and the electrostatic dye–dye intermolecular attraction, the H-aggregates formed are of high stability as already mentioned. Specifically, it is obvious from Fig. 2c that the clays catalyze the dye protonation and, in turn, initiate the H-type aggregation even at relatively low dye loadings. This protonation-induced rise in dye aggregates (Fig. 2d) results in energy stabilization and, as the dye loading is increased, high-order assemblies are favorably developed, further reducing the wavelength of K/S maximum (Fig. 2e and f). Coexistent protonated and agglomerated dye species are important for clay-catalyzed high-order aggregation, while, in the absence of clay, raised concentrations of hydrochloric acid are required. The active centers on the clay stimulate the dye protonation and function as aggregation sites creating regions of
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17
Fig. 5. Visible spectrum of C.I. Basic Blue 41 in the presence of 3 M NaCl.
high dye concentration in an acidic aluminosilicate environment, imitating that of the strongly acid solution of the dyestuff; thus, we conclude that both clays may act as catalysts in the aggregation of C.I. Basic Blue 41 at very high dye loadings. Apparently, the formation of higher aggregates is due to the fact that at increased dye:clay ratios the dye adsorption occurs more rapidly compared with the dye diffusion inside the substrate. It was found that the catalytic action of bentonite, natural clay containing small amounts of calcite [1], is identical compared with that of montmorillonite, i.e., a clay mineral, in the formation of layered high-order dye aggregates. It was proved experimentally that bentonite, the main component of which is montmorillonite, retains almost a hundredfold amount of C.I. Basic Blue 41, e.g., 0.96 g dye/g bentonite at a dye loading of 400% CEC (Table 1), compared with pure calcite demonstrating an adsorption capacity of 0.01 g dye/g calcite. It is well-known that the thermodynamically stable trans-isomers of cationic azobenzene dyes [35] reversibly isomerize to cis-azobenzenes which show no evidence of aggregation [22]. Commonly, trans-to-cis isomerization does not occur unless the dye is UV-irradiated [36] particularly in solution and, to a lesser extent, in the adsorbed state [35,37]. Furthermore, it has been found that the cis-isomers are spontaneously reconverted to trans either with time [35] or by thermal paths [22,37]. In all experiments carried out both in this work and previously [17] no spectral change with time and temperature was observed, indicating the absence of isomerization. Thus, the spectra obtained should be attributed to trans-C.I. Basic Blue 41, as isomerization of the dominantly present trans-isomers would require breaking down the dye aggregates formed, which might be energetically disadvantageous [35]. 3.2. Oligomerization of the hydroxo-Cr(III) complexes On the initial assumption that montmorillonite and bentonite may exert a marked catalytic action in the formation of highorder dye aggregates, it needs particular consideration that the dye-treated clays show an analogy with chromium-intercalated montmorillonite. Cationic forms of several Cr(III) oligomers (peaking at 421 and 583 nm), prepared via chromite cleavage during acidification of the Cr(III) solution, were obtained by the BA method. Undesirably, these oligomers were gradually converted to monomeric, dimeric and trimeric species (as hexamers and higher oligomers are consumed rapidly at pH = 2.8) and the UV–vis peaks were shifted towards lower wavelengths (419 and 581 nm, respectively) within
Fig. 6. X-ray diffraction patterns of montmorillonite intercalated with hydroxochromium complexes at pH = 2.8 for (a) 1 h and (b) 24 h.
a few hours. Accordingly, in acid solutions oligomerization is quenched and the protonated Cr(III)-aqua ions produced undergo cleavage and intramolecular rearrangements. On the contrary, for hydroxo-Cr(III) complexes intercalated in montmorillonite the chromium oligomers were inserted into the interlayer and a basal spacing of 1.96 nm (Fig. 6a) was obtained; as the intercalation time was increased from 1 to 24 h (Fig. 6b) higher galleries (d001 = 2.10 nm) were achieved, indicating further oligomerization of chromium assemblies. In general, intercalation solutions containing substantial amounts of large polyoxo-chromium oligomers are prerequisites for the preparation of chromia pillared clays with high basal spacings. Gallery heights are of significance as hydroxo-Cr(III)–clay intercalates, converted upon heating to pillared products, may function as catalysts. For instance, chromium pillared clays with high galleries are effective as catalysts for benzylic oxidation and selective deprotection of benzyl ethers and benzylamines [38], while chromium-interlayered clays with a lower spacing of 1.44–1.53 nm exhibit increased efficiency for deep oxidation of dichloromethane [39]. Unlike large interlayer heights, galleries of less than 0.4 nm may end up with low catalytic reactivity because of the inaccessibility to the gallery chromia by the reactants [9]. It has been shown [40] that during the chromium hydrolytic oligomerization, deprotonation of Cr(III) complexes, occurring at pH close to 5.0, leads to a rapid formation of conjugate bases and hydrogen-bonded aggregates. These coordinated hydroxides [41], being better nucleophiles than water towards adjacent Cr(III) centers, increase the lability of chromium coordination sphere and facilitate fast condensation to polynuclear hydrolyzed compounds. We suggest that montmorillonite may also act catalytically in the construction of higher hydroxo-chromium oligomers, most probably by promoting deprotonation. The clay does function as a proton acceptor and increases the concentration of the reacting species by bringing them close to each other through hydrogen-bond formation. Analogous activity of montmorillonite during pillaring with chromium complexes has been reported [9,42]. Hence, we presume that the clay equally promotes aggregation phenomena of C.I. Basic Blue 41 and Cr(III) hydroxo-complexes, allowing for a generalization of its catalytic properties towards both organic and inorganic compounds. Apparently, montmorillonite and bentonite exclusively accelerate guest–guest agglomeration reactions. The intercalation of dye
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Fig. 7. Scanning electron micrographs of dye-treated (a) montmorillonite and (c) bentonite, at very high (>200% CEC) C.I. Basic Blue 41 loadings, host bentonite (b) and intercalated with higher hydroxo-Cr(III) oligomers montmorillonite (d).
molecules and hydroxo-Cr(III) oligomers is not accompanied by a chemical attachment to the aluminosilicate sheets; the clay, simultaneously being the catalyst [9,42] and the host material, may be recovered. However, where the strategy of choice is to tailor integrated systems (composed of the inorganic matrix and immobilized aggregates) for specific applications, it is essential that the clay be permanently incorporated into the host–guest nanocomposites prepared. 3.3. Surface modification Examination of scanning electron micrographs indicates substantial changes in microstructure of the high-dye-loading intercalated clays compared with the parent material. Obviously, scanning electron microscopy alone cannot provide direct experimental evidence for the catalytic action of clays. On the microphase scale, however, it can successfully correlate the fine details of the guest-treated host surface with the stacking effect caused by the catalytically induced guest aggregation. The micrograph in Fig. 7a reveals that the distinctively lamellar structure of untreated montmorillonite is significantly modified, as an increased amount of dye is incorporated between the clay cohesive thin sheets. The aggregation of dye molecules is facilitated, as they approach a large number of exposed negative sites. The tight stacking of dye layers placed within the interlayer fills the voids, resulting in a condensed morphology of dye-treated montmorillonite (Fig. 7a); a characteristic wavy surface can be clearly seen. On the other hand, the chromium-intercalated clay of Fig. 7d is rugged and anomalous compared with the dyed sample, displays irregular flower-like
edges, and shows an open structure due to the limited amount (4 wt%) of chromium present in the interlayer region. A lamellar construction, somewhat diverse compared with host montmorillonite, is presented in Fig. 7b for undyed bentonite. A dyed-montmorillonite-resembling microstructure appears as the dye molecules are introduced into bentonite, but the overall particle arrangements are now different (Fig. 7c); the dye-treated samples exhibit a unique crinkly, ridged, less condensed texture that partly accounts for lower dye retention and reduced basal spacing of bentonite. Patterns of both dyed clays shown in Fig. 7a and c resemble desiccation mud cracks and other dewatering features, are characteristic of the smectite group, and develop from shrinkage of the expandable clay minerals as they dry [18]. 4. Conclusions The catalytic action of montmorillonite and bentonite in the formation of layered C.I. Basic Blue 41 agglomerates was investigated. Both X-ray diffraction and diffuse reflectance results indicated the presence of spectrally distinct extended dye aggregates between the clay lamellae at dye loadings higher than 200% CEC and pH = 3. These assemblies found in the interlayer, but not in the intercalating solution, suggest that the clays act as aggregation catalysts; domains with increased guest concentration in the acidic host environment are responsible, as the anionic surfaces of clays template the formation of stable H-aggregates stimulating the protonation of the dye. Absorption wavelengths for dye aggregates were steadily reduced with increasing dye:clay ratio confirming the clay ability to selectively promote high-order dye aggregation.
M. Roulia, A.A. Vassiliadis / Microporous and Mesoporous Materials 122 (2009) 13–19
On the contrary, in the absence of clay the dye transformation to higher H-aggregates can only be achieved in strongly acid aqueous C.I. Basic Blue 41 solutions. Similar aggregation phenomena were observed during intercalation of montmorillonite with Cr(III) hydroxo-complexes. Besides bringing the reactants into close proximity, the clay seems to promote agglomeration of the intercalant species through proton transfer reactions occurring at the negative aluminosilicate surface. Both dye-treated and chromium-intercalated clays exhibited remarkable alterations in morphology and microstructure. Acknowledgment Triacryl Blue RL 330% brand (C.I. Basic Blue 41) was supplied by Chromatourgia Tripoleos S.A., Greece. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
M. Roulia, A.A. Vassiliadis, J. Colloid Interf. Sci. 291 (2005) 37. M. Roulia, Mater. Chem. Phys. 91 (2005) 281. M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399. A. Yasmin, J.L. Abot, I.M. Daniel, Scr. Mater. 49 (2003) 81. B. Chen, J.R.G. Evans, Scr. Mater. 54 (2006) 1581. G. Calzaferri, S. Huber, H. Maas, C. Minkowski, Angew. Chem. Int. Ed. 42 (2003) 3732. J.K. Thomas, Chem. Rev. 93 (1993) 301. T. Shichi, K. Takagi, J. Photochem. Photobiol. C 1 (2000) 113. M.S. Tzou, T.J. Pinnavaia, Catal. Today 2 (1988) 243. S.D. Bodman, W.R. McWhinnie, V. Begon, I. Suelves, M.J. Lazaro, T.J. Morgan, A.A. Herod, R. Kandiyoti, Fuel 81 (2002) 449. T. Mishra, K.M. Parida, J. Mol. Catal. A 121 (1997) 91. G.W. Beall, M. Goss, Appl. Clay Sci. 27 (2004) 179.
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[13] J. Bujdák, N. Iyi, J. Phys. Chem. B 110 (2006) 2180. [14] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, J. Colloid Interf. Sci. 255 (2002) 254. [15] J. Bujdák, Appl. Clay Sci. 34 (2006) 58. [16] V. Ramamurthy, D.R. Sanderson, D.F. Eaton, J. Am. Chem. Soc. 115 (1993) 10438. [17] M. Roulia, A.A. Vassiliadis, Micropor. Mesopor. Mater. 116 (2008) 732. [18] B.F. Bohor, R.E. Hughes, Clays Clay Miner. 19 (1971) 49. [19] G. Schulz-Ekloff, D. Wöhrle, B. van Duffel, R.A. Schoonheydt, Micropor. Mesopor. Mater. 51 (2002) 91. [20] M. Ogawa, R. Kawai, K. Kuroda, J. Phys. Chem. 100 (1996) 16218. [21] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. [22] X. Song, J. Perlstein, D.G. Whitten, J. Am. Chem. Soc. 119 (1997) 9144. [23] H. Kashida, H. Asanuma, M. Komiyama, Supramol. Chem. 16 (2004) 459. [24] T. Stoyanova, S. Stoyanov, L. Antonov, V. Petrova, Dyes Pigm. 31 (1996) 1. [25] J. Del Nero, R.E. de Araujo, A.S.L. Gomes, C.P. de Melo, J. Chem. Phys. 122 (2005) 104506. [26] K.K. Karukstis, L.A. Perelman, W.K. Wong, Langmuir 18 (2002) 10363. [27] H.P. de Oliveira, Microchem. J. 88 (2008) 32. [28] H.P. de Oliveira, E.G.L. Oliveira, C.P. de Melo, J. Colloid Interf. Sci. 303 (2006) 444. [29] A. Navarro, F. Sanz, Dyes Pigm. 40 (1999) 131. [30] G. Zhang, M. Liu, J. Phys. Chem. B 112 (2008) 7430. [31] A.P. D’Rozario, B. Parton, A. Williams, Gazz. Chim. Ital. 117 (1987) 395. [32] H. Bose, Indian J. Chem. A 26 (1987) 652. [33] K. Takagi, T. Kurematsu, Y. Sawaki, J. Chem. Soc. Perkin Trans. 2 (1991) 1517. [34] Z. Yermiyahu, I. Lapides, S. Yariv, Clay Miner. 38 (2003) 483. [35] J. Bujdák, N. Iyi, T. Fujita, J. Colloid Interf. Sci. 262 (2003) 282. [36] A. Jafari, H. Tajalli, M. Abedi, Opt. Commun. 281 (2008) 2959. [37] M. Ogawa, Chem. Mater. 8 (1996) 1347. [38] B.M. Choudary, A.D. Prasad, V. Bhuma, V. Swapna, J. Org. Chem. 57 (1992) 5841. [39] L. Storaro, R. Ganzerla, M. Lenarda, R. Zanoni, A. Jiménez López, P. OliveraPastor, E. Rodríguez Castellón, J. Mol. Catal. A 115 (1997) 329. [40] A. Drljaca, L. Spiccia, Polyhedron 15 (1996) 2875. [41] S.J. Crimp, L. Spiccia, H.R. Krouse, T.W. Swaddle, Inorg. Chem. 33 (1994) 465. [42] R.M. Carr, Clays Clay Miner. 33 (1985) 357.