Adsorption of Transition Metal Cations onto a Lamellar Poly (3

Nov 9, 2010 - Yu Gonda and Hideaki Yoshitake*. DiVision of Materials Science and Chemical Engineering, Graduate School of Engineering, Yokohama ...
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Adsorption of Transition Metal Cations onto a Lamellar Poly(3-aminopropyl)silsesquioxane: Cation-Cation Interaction and Transition of Adsorption Phase Yu Gonda and Hideaki Yoshitake* DiVision of Materials Science and Chemical Engineering, Graduate School of Engineering, Yokohama National UniVersity, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ReceiVed: July 18, 2010; ReVised Manuscript ReceiVed: October 7, 2010

The adsorption of Cu2+ and Co2+ onto poly(3-aminopropylsilsesquioxane) has been carried out to elucidate the effects of the adsorbate-adsorbate interaction on aminopropyl-functionalized silica surfaces. By fitting to the Langmuir equation, N/Cu ) 2 and N/Co ) 1, respectively, have been elucidated at full coverage. The ESR signals of Cu2+ gradually change with increasing adsorption, although the structures are assigned to diamino-Cu(II) complexes on the basis of g values and hyperfine coupling constants. A resonance due to Cu2+-Cu2+ spin-spin coupling is observed in the low field for the saturated adsorption, even at room temperature. This is probably the first spectroscopic observation of such interactions in organic functionalized silica surfaces and implies a highly dense Cu2+ layer. On the other hand, the adsorption isotherm of Co2+ is composed of two stages: the adsorption is once saturated at N/Co ) 2 and then increases again until N/Co ) 1. This characteristic adsorption feature implies a sudden gross change of the adsorption layer when N/Co ) 2 is achieved, which generates a large number of empty sites. The pre-edge peak intensities of the Co K-edge XANES spectra are plotted in accordance with the adsorption isotherm, where a sudden increase is found at the start of the second stage of adsorption. These characteristic adsorption phenomena are discussed with the structure of poly(3-aminopropylsilsesquioxane), where the aminopropyl group is located twodimensionally with a spacing of Si-O-Si. Introduction Ordered organic-inorganic materials have attracted much attention due to their various mesostructural properties in addition to their potential applications for adsorption, separations, and catalysis.1-16 One of the most important properties required for these materials is a large density of functional groups that are accessible to the guest molecules or ions. This is why silica with a large surface area has been extensively used as a substrate material in studies on functionalization. Since the discovery of synthetic routes to fabricate mesoporous silicas,17,18 the surface areas and the thicknesses of pore walls have become much more concrete concepts than before for describing the structure of silica with a large surface area. The properties of functional groups as well as their structural variations after grafting are dependent on the diameters of mesopores.19-23 The typical density of organosilanes grafted onto the surface of mesoporous silica is a few millimoles per gram,20 and the specific surface area can be varied from several hundreds to ∼1400 m2/g.24-28 Consequently, the average surface density of a functional group is normally 1-2 functions/nm2. When organic functional groups are fixed with a high density onto the surface, some of them are positioned very close to their neighbors. In these cases, mutual interference or cooperative effects should be considered in the adsorption of guest ions and molecules. The effect of vicinal sites can be a major problem in the development of functionalized organic-inorganic materials with ordered mesostructures, although the spacing between functional groups is not uniform when they are grafted or introduced by co-condensation onto the silica surface.29 The distributions of such spacings obstruct the elucidation of the influence of * Corresponding author. Tel: +81 45 339 4359. Fax: +81 45 339 4378. E-mail: [email protected].

neighboring sites on surfaces that are densely covered with functional groups. Mesostructured polysilsesquioxane, which is prepared by the condensation of monoorganotrialkoxysilane, RSi(OR′)3, has been studied recently from a variety of standpoints.30-47 This solid has been synthesized mostly through self-assembly mechanisms by the aid of a long alkyl chain (as R) or a surfactant such alkanoic acid, as shown in Scheme 1, which usually provides a good lamellar pattern in the X-ray diffraction analysis. Some of these layered materials contain almost exclusively T3 species as Si atom,33,35,43-45,47 and consequently, they form solids with the stoichiometric chemical formula [RSiO1.5]n. This structural property makes a strong contrast to the chemical formulas of functionalized silica materials, which cannot be defined any more accurately than [RxSiOy(OH)z]- (x + y + z ) 4, x < 1). The component R in [RSiO1.5]n can be a long hydrocarbon chain,30,31 whereas polysilsesquioxane can work as a functionalized silica material when a functional group such as -NH2 and -SH is contained in R.32-47 The interlayer species, organic ions, inorganic ions, surfactants, etc. can be exchanged for each other.39-41,43-45,47 Under some conditions,44,45 these reactions result in delamination by the formation of coordination bonds and removal of the surfactant from the lamellar solid. These solids can adsorb environmentally hazardous ions42,45,47 and molecules39,40 without any significant loss of surfactants or degradation of the lamellar structure.43,47 Considering these characteristics that have been elucidated in previous studies, polysilsesquioxanes could possibly form a new category of mesostructured materials. The structure of [RSiO1.5]n can be considered as an “end of thinning” silica solid with a simultaneous increased density of functional groups.2 The structure of polysilsesquioxane is much more rigid than polysiloxane, [RR’SiO]n, which is basically a pendant polymer,

10.1021/jp106680h  2010 American Chemical Society Published on Web 11/09/2010

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SCHEME 1: Synthesis of Polysilsesquioxane from 3-Aminopropyltriethoxysilane by the Aid of a Carboxylate

although this polymer has two functions per Si atom. In the structure of polysilsesquioxane, each function is positioned with the spacing of Si-O-Si. Although the lateral periodicity in the polysilsesquioxane layer has not been well elucidated yet,32,38 the interactions between the organic sites may appear explicitly in the adsorption of cations due to their extremely short distances.2 The immobilization of transition metal cations has been studied frequently over the past few decades, not only to search new and practical materials for environmental remediation but also to reveal the characteristics of the new synthesized materials, which usually have remarkable mesostructures.1-16,19-23,49-57 The capture of transition metal cations has been widely applied as a benchmark test in the materials chemistry of polymers and solids. Although structural analysis based on the local structures of cation trapped by surface ligands has been undertaken in several studies,1-16,50,51,54,57 the factors that influence adsorption due to the mesostructure of the frameworks have been largely ignored. This may be partly because the majority of the physicochemical analyses developed so far are mainly effective only for the elucidation of local structures (bond lengths, angles, etc.). The effect of interference or cooperation between the adsorption sites or between the substrates and the adsorption sites have been studied much less than the local structures. Considering that a large adsorption capacity is usually required for the development of mesostructured materials, these interferences are an important subject that should be clarified. We have studied two kinds of polyaminoalkylsilsesquioxane, [NH2C3H6SiO1.5]n44,45 and [NH2C2H4NHC3H6SiO1.5]n,46,47 to demonstrate the formation of long-range lamellar structures when they are prepared using carboxylates with a long alkyl chain and high capacities toward the adsorption of transition metal cations and hazardous oxyanions. The latter solid provides chelate-type adsorption sites with two NH2C2H4NHC3H6- functions for Cu2+ adsorption,47 suggesting a vicinal positioning of organic functions with a short length. In this paper, we unveil the adsorption characteristics of transition metal cations on [NH2C3H6SiO1.5]n prepared from 3-aminopropyltriethoxylsilane by the aid of spectroscopies. Our main concern is influence of the interaction between adsorbate cations captured at such condensed sites (Scheme 2). We chose the 3-aminopropyl group to elucidate the “neighboring” effect of cation sites, not only because it can form complexes with a variety of cations with various coordination numbers but also because the mobilities of metal cations can increase more when they are captured by 3-aminopropyl functions than by strong ligands that provide

SCHEME 2: Adsorption and Accumulation of Transition Metal Cations on Poly(3-aminopropyl)silsesquioxane

more stable complexes, such as an N-(2-aminoethyl)aminopropyl group. The latter condition will facilitate the observation of phenomena due to cooperative effects such as phase transitions. Experimental Section Synthesis of Poly(3-aminopropyl)silsesquioxane. Lameller poly(3-aminopropyl)silsesquioxane was prepared by following the procedure detailed in a previous study.45 In brief, sodium laurate (LAS, Tokyo Kasei, Co. Ltd.) was dissolved in a mixture of water (29.3 g)-ethanol (10.5 g) at 333 K, after which 2.00 g of 3-aminopropyltriethoxysilane (APTES, Tokyo Kasei, Co. Ltd.) was added dropwise into the solution. The molar ratio of the reactants was 1 sodium laurate/1 APTES/180 H2O/20 EtOH. This mixture was stirred for 1 h at room temperature before the addition of HCl to adjust the pH of the solution to 10. The mixture was then heated in a Teflon bottle at 373 K for 2 d.

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The precipitate was washed with a small amount of water-ethanol mixture by stirring at 333 K. After collection by filtration, the product was dried in air at 373 K. This organosilica composite is hereafter denoted as LAS-NH2C3H6SiO1.5. Adsorption of Cu2+ and Co2+. A 0.05 g portion of LASNH2C3H6SiO1.5 was dispersed in 10 mL of copper(II) chloride hexahydrate (CuCl2 · 6H2O, Wako) dissolved in an equivolume mixture of 2-propanol (99.5%, Wako) and water. The adsorption was carried out for 4 h at room temperature with continuous stirring. The concentrations of Cu before and after the adsorption in the solution was quantified by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Shimadzu ICP-8000E. The same procedure was applied using cobalt(II) chloride hexahydrate (CoCl2 6H2O, Wako) for the adsorption of Co2+. Analyses of Solids. The periodic structure was confirmed by X-ray diffraction (XRD) using a RINT 2200 diffractometer (Rigaku Co.) with Cu KR radiation (40 kV and 20 mA). For the X-band (9440 MHz) ESR measurements, the solids after Cu2+ adsorption were loaded into 3 mm o.d. × 2 mm i.d. quartz tubes. The spectra were recorded at room temperature on a JEOL-JES-FA200 resonance spectrometer (JEOL) calibrated with a DPPH standard (g ) 2.0036). Typical microwave powers of 4 mW and a modulation magnetic field of ∼100 kHz were commonly applied. The X-ray absorptions of the Co K edge were recorded in the beamline BL-7C at the Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan (Proposal No. 2009G054), with a ring energy of 2.5 GeV and a stored current of 300-450 mA. A Si(111) double-crystal monochromator was used. The incident X-rays were focused, and the higher harmonics were removed by total reflection on a Rh-Ni composite mirror. The solid powders after adsorption were collected by filtration and dried at 333 K before the spectrum was measured by a conventional transmission mode using gas ion chambers. The data were processed using a REX 2000 program assembly (version 2.3.3, Rigaku Co.).

Gonda and Yoshitake

Figure 1. X-ray diffraction patterns of LAS-NH2C3H6SiO1.5 (a) as prepared and after the adsorption of Cu2+ or Co2+ for (b) 2, (c) 5, (d)10, (e) 15, (f) 30, (g) 60, and (h) 240 min.

Results and Discussion Periodic Structure and Adsorption Isotherms. Figure 1 depicts the diffraction patterns of LAS-NH2C3H6SiO1.5, both “as prepared” and after the adsorption of Cu2+ and Co2+. As reported previously, the pattern for a single lamellar phase, where d ) 2.94, 1.53, and 1.02 nm due to 100, 200, and 300 diffractions, appears before adsorption. This is attributed to a layer-by-layer structure built up with disk-like micelles of LAS and extremely thin [NH2C3H6SiO1.5]n layers.44,45 The peaks from 200 and 300 diffractions are smeared gradually in the course of adsorption at room temperature, although they are still observable after 4 h. A slight shift is found in the diffuse peak, around 2θ ) 20-21°, which is equivalent to the basal plane distance d from 0.431 to 0.441 nm. Since this broad diffraction is attributed to the chain-chain spacing of the LAS assembly,58-61 a slight enlargement of average spacing possibly occurs after the adsorption. Nevertheless, the shift does not imply any significant recombination of molecules in the interlayer micelles when compared with the variation of basal plane distance reported in the literature for carboxylate micelles in “all-anti” conformation from 0.41 to 0.58 nm.44,58-61 The value of d, in fact, depends sensitively on the substrate of the micelle film. The observed differences in Figure 1 are much smaller than those found in the literature58-61 and can be explained by the slight change in the chemical environment around headgroup of carboxylate. The decrease of 100, 200, and 300 diffraction peaks is more remarkable in the adsorption of Cu2+ than in that of Co2+. This

Figure 2. Adsorption isotherm of Cu2+ by LAS-NH2C3H6SiO1.5.

is probably due to the interaction between cation and ligand being stronger in the former adsorption than in the latter, which weakens the COOH-NH2 bond. Figure 2 shows adsorption isotherm of Cu2+ by LASNH2C3H6SiO1.5. The adsorption increases nearly vertically until 1 mmol/g and is almost saturated at 1.46 mmol/g when the equilibrium concentration is 12 mmol/L. These characteristics are attributed to IUPAC type-I adsorption with the mechanism where the adsorption is stronger at low coverage than at high coverage. In contrast, the adsorption of Co2+ initially saturated at 1.86 mmol/g, when the equilibrium concentration is 4 mmol/L, before it starts to increase again with a large slope to be fully saturated at 2.37 mmol/g, as shown in Figure 3. The isotherm obviously consists of more than a single adsorption mode; the completion

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Figure 3. Adsorption isotherm of Co2+ by LAS-NH2C3H6SiO1.5. The data numbers correspond to those of Co K-edge spectra.

of the adsorption phase by the first mode initiates the second mode. The occurrence of another rise in adsorption after the initial saturation suggests that the interaction of a very small number of additional cations with the completed adsorption phase induces a vast structural change in the adsorption layer. This is a process accompanied by cooperative effects that arise from the mass number of atoms and has rarely been reported in studies of the adsorption on organo-functionalized silica materials. Since most of the organic functions are distributed on the surface of organic-inorganic solids, the interactions between the adsorbates are also nonuniform, and cooperative phenomena rarely appear through adsorption. In contrast, successive adsorption has often been observed for small molecules on the flat surfaces of single crystals,62 which can be clearly detected from changes in the two-dimensional diffraction patterns from the surface. On these well-defined model surfaces, the first phase completed by an adsorbate layer is condensed by a phase transition into the second phase. The lateral interaction between the adsorbates is clearly important for these phenomena. Considering the structure of LASNH2C3H6SiO1.5,35,36,44 it is likely that the two-dimensional densely packed layer of the 3-aminopropyl functional group mediates the lateral interactions among the adsorbates and induces a cooperative effect, causing the successive adsorption. Spectroscopic Analyses of Local Structures of Adsorption Sites. The electron spin resonance of Cu2+ adsorbed on LASNH2C3H6SiO1.5 is plotted in Figures 4 and 5. The spectra are described by an axial g tensor and an axial hyperfine interaction. Hyperfine coupling in the perpendicular spectral region could not be resolved. The calculated axial g tensor in Figure 4 remains almost constant at g⊥ ) 2.07 with increasing amount of adsorbed Cu2+. On the other hand, the parallel component decreases with increasing adsorption: g| ) 2.373, 2.351, and 2.341 for Cu/N ) 0.05, 0.25 and 0.5, respectively. The hyperfine coupling constant also varies according to the adsorption of Cu2+: A| ) 0.0184, 0.0171, and 0.0137 cm-1 for Cu/N ) 0.05, 0.25 and 0.5, respectively. The g| of Cu2+ complexes with amine coordinations have been reported to be between 2.20 and 2.2163-65 For square planar complexes with a chloride, CuCl42-, and water, Cu(H2O)42+, g| increases to 2.23366,67 and 2.421,64 respectively. Cu2+ coordinated with both amine and water provides intermediate values for g; for example, g| ) 2.281 for Cu(en)(H2O)22+, where “en” is ethylenediamine ligand.64 The same shift is also found for the chloride-amine complexes EtNH2CuCl4 and MeNH2CuCl4, which exhibit g| values of 2.276 and 2.281, respectively.68 Thus, the coordinations of Cl- and

Figure 4. Electron spin resonance of Cu2+ adsorbed on LASNH2C3H6SiO1.5 at room temperature. The adsorption is indicated with the molar ratio of amine to copper, and N/Cu ) 2 means the saturated adsorption.

Figure 5. Electron spin resonance of Cu2+ adsorbed on LASNH2C3H6SiO1.5 in the low-field region at room temperature. The adsorption is indicated with the molar ratio of amine to copper, and N/Cu ) 2 means the saturated adsorption.

H2O are likely incorporated into the formation of the adsorption site. These mixed ligand coordination structures provide hyperfine coupling constants from 0.0125 to 0.0185 cm-1,63-67 which is consistent with the experimental results in Figure 4. The gradual change of the ESR structural parameters in the course of the adsorption corresponds to no discontinuous change in the adsorption isotherm (Figure 2). On the other hand, at the saturation, a resonance attributable to the ∆ms ) (2 transitions appears at 159 mT. This is due to the spin-spin coupling due to Cu2+-Cu2+ pairs.69-72 The appearance of this peak, even at room temperature, implies a uniform and short Cu-Cu distance and, consequently, a dense Cu2+ layer in the solid. The ESR spectra of Cu2+ adsorbed onto [NH2CH2CH2NHC3H6SiO1.5]n, which is a polysilsesquioxane with an ethylenediamine group, contain no feature at low field wherever they are measured at the adsorption saturation.48 In this case, Cu2+ forms a Cu(en)2 type coordination structure (i.e., Cu/N ≈ 0.25), and the Cu2+

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2+

Figure 6. Cu K-edge XANES spectra of Cu adsorbed on LASNH2C3H6SiO1.5. The adsorption is indicated with the molar ratio against amino group, and N/Cu ) 2 means the saturated adsorption.

ions are separated from each other by the chelating ligands. This comparison suggests that the interaction between Cu2+ ions becomes significant when the amino groups of polysilsesquioxane [NH2C3H6SiO1.5]n are occupied by Cu2+ and the adsorbed cations are positioned with the spacing of aminopropyl group in [NH2C3H6SiO1.5]n. Figure 6 shows the K-edge spectra of Cu2+ adsorbed on LASNH2C3H6SiO1.5 in the course of adsorption. The spectra are normalized with the peak intensity at 8994.6-8993.0 eV, which is attributed to 1s-4p transitions.73 In these plots, the pre-edge adsorption often observed at 8975 eV, which is attributed to inhibited 1s-3d transition, is almost absent. This peak has often been used to evaluate the degree of 2p-3d hybridizations to bring about a distortion of coordination structure. The negligible intensity of this absorption suggests octahedral coordination without significant distortion.74,75 The shoulder just before the edge position at 8982 eV is clearly decreased with increasing amount of Cu2+. This peak has been attributed to 1s-4p transition accompanied by a “shake-down process”. The intensity of the peak at 8989 eV, often assigned to the 1s-4p, 1s-4p with a “shake-down process” and 1s-5p with a “shake-down process”, increases with Cu2+ loading. The peak top of the absorption bands due to the 1s-4p transitions shifts to lower energy gradually with increasing Cu2+ uptake. It changes from 8994.6 to 8993.0 eV for the Cu/N from 0.05 to 0.50. The change is not as large as the difference of edge energy between Cu2+ and Cu+, and the spectral feature does not show any evidence of reduction into Cu+. In addition, the shoulder around 8997 eV, attributable to 1s-5p transition,73 is not strong enough to be compared. The monotonic changes of the peak intensities are consistent with the mode of adsorption in the isotherm, although the peaks in question are mostly related with the transitions from the inner shell to unoccupied states. Figure 7 shows the K-edge XANES spectra of Co2+ adsorbed on LAS-NH2C3H6SiO1.5 and a magnification of the pre-edge region. The pre-edge peak, which is also attributed to the 1s-3d transition, is clearly observed in all samples after adsorption. The energy gap between this pre-edge absorption and the edge position is about 11.5 eV, which agrees with that of the spectra of Co2+.76,77 These spectra, therefore, indicate that the adsorption does not change the oxidation state of Co2+. The intensity of the pre-edge peak varies slightly according to the amount of adsorption. The integrations of the peak are plotted in Figure 8 against the data number of the adsorption isotherm. The intensity

Gonda and Yoshitake

Figure 7. Co K-edge XANES spectra of Co2+ adsorbed on LASNH2C3H6SiO1.5. The magnification of the pre-edge region is also shown in the inset. The 12 spectra from the bottom to the top were measured for the samples with the data numbers in Figure 3. The spectra were plotted by dislocating from the preceding point with the same offset.

Figure 8. The intensity of pre-edge peaks of Co K-edge XANES spectra of Co2+ adsorbed on LAS-NH2C3H6SiO1.5. The data number is the same as in the adsorption isotherm in Figure 3.

decreases monotonically from no. 1 to no. 6, whereas a sudden increase is observed in no. 7 and decreases gradually again from no. 8 to no. 12. This plot clearly indicates a discontinuous change of the coordination environment of Co2+ between no. 6 and no. 7, where the second rise of adsorption isotherm is observed. The declining of the peak according to the progress of adsorption can be attributed to the dehybridization and enhancement of the coordination symmetry of Co2+. Considering that the aminopropyl functions are positioned on a pseudo-twodimensional plane in the polysilsesquioxane, the adsorbate in the first mode of adsorption is likely to be trapped by NH2sites, causing a stereochemical distortion, and the increase of adsorbent ions can result in the lowering the coordination number of amino functions and a relaxation of distortion. The jump at the data no. 7 in Figure 8 implies a sudden increase in the number of available ligands that can coordinate with Co2+. Thus, this discontinuous change in the pre-edge intensity supports the occurrence of overall reconstruction of the adsorption layer. On the other hand, as shown in Figure S1 of the Supporting Information, we could not obtain any evidence for the Co-Co bonds in Co K-edge Extended X-ray Absorption Fine Structure (EXAFS) at room temperature. The radial distribution function

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is basically identical for all samples. The curve-fitting result (Table S1 of the Supporting Information) shows no meaningful differences. However, since oxygen and nitrogen in the first shell are not distinguished by EXAFS analysis, we cannot exclude the occurrence of ligand exchange reactions between NH2- and H2O. Adsorption Layer of NH2C3H6SiO1.5. The Langmuir equilibrium constant K (in mol/mol), which represents the distribution coefficient between 1 g of solid and 1 g of solution, deduced by fitting of the following equation,

C 1 C + ) N N∞ KN∞

(1)

to the data of Figure 3, are 1.8 × 106 and 3.1 × 104 for the first and second modes of adsorption, respectively. The plots are shown in Figure S2 of the Supporting Information. N and N∞ in the equation are adsorption amount of Co2+ and adsorption capacity of Co2+, respectively. Good linearities are found in the plots of this equation in Figure S2, suggesting uniform adsorption occurs in each adsorption mode. From this Langmuir fitting, the values of N∞ in the first and second adsorption modes are 1.9 and 3.2 mmol/g, respectively, which correspond to N/Co ) 1.7 and 1.0, respectively. The amount of amino group is 3.2 mmol/g. These two N/Co ratios agree with 2 and 1, respectively, within an error of 15%. From the stoichiometric relationships, two NH2C3H6- ligands are coordinated to one Co2+ in the first mode, and one NH2C3H6- ligand is bound to one Co2+ ion in the second mode. The reduction of the equilibrium constant by 1/58 is attributed partly to the decrease in the coordination number of the amine ligands on the surface. The absence of a notable Cl shell in the EXAFS spectra suggests that the other ligands are water. As stated above, an entire rearrangement of the adsorption layer at the completion of the first adsorption mode is induced by the capture of new adsorbate approaching to this layer. From the spectroscopic analyses, this is related to changes in the local structure of adsorbates: the symmetry change of the adsorbed cations. It is likely that multiple ligands coordinate to a cation in low coverage, inducing a stereochemical distortion. The gradual decrease between nos. 1 and 6 implies that the repulsion between adsorbed cations, which increases with the coverage, reduces this distortion. Thus, the sudden rise of the pre-edge peak at the transition of adsorption mode (at no. 7) can be explained by a major formation of free ligands with lowering the symmetry of the adsorbed Co2+ cations. On the other hand, the Langmuir constants for Cu2+ adsorption are K ) 4.1 × 105 and N∞ ) 1.5 mmol/g, which are calculated from the plot in Figure S3 of the Supporting Information. From the latter parameter, the stoichiometric relation between amino group and Cu2+ is calculated to be N/Cu ) 2.1. The equilibrium constant is smaller than the first mode of Co2+ adsorption but larger than the second mode. The N/Cu ratio suggests the formation of diamino copper(II) complexes. This structure is consistent with the observation of Cu2+-Cu2+ spin-spin coupling at room temperature, which is allowed only in a condensed adsorption layer with a strong interaction between Cu2+ ions, in addition to that between the ligands and Cu2+. Since the amino functions of polyaminopropylsilsesquioxane are positioned nearly in a plane, even though the undulation would be large due to the thin layer structure, some transition of the structure of the adsorption layer is possible under conditions that the lateral interactions between the adsorbates

are comparable to the difference of two types of stable adsorption structures. Since the stability of the tetrahedral coordination of Co2+ is close to that of octahedral, (in fact, [Co(H2O)4]2+ (Td) exists in the equilibrium with [Co(H2O)6]2+ (Oh) in aqueous solution), the allowance for the distortion due to a small displacement of the ligand position is larger than for other transition metal cations.78,79 These characteristics of the coordination bonds of the Co2+ complexes imply that their local structures are quite sensitive to the interactions with other ions and the functional groups in a condensed phase. These interactions could facilitate alternation of the coordination structure due to changes in the surrounding chemical conditions, such as condensation by the increase in the coverage. On the other hand, the six-coordinated Cu2+ has almost exclusively distorted octahedral structures with four short bonds and two long bonds.79 Although from one to four amine ligands are easily coordinated to Cu2+, their coordination constants are considerably different (4.27, 7.82, 10.72, and 12.90 for β1, β2, β3, and β4, respectively, of aqueous [Co(NH3)n]2+, which make a strong contrast to aqueous [Co(NH3)n]2+ complexes; that is, 2.11, 3.74, 4.79, 5.55, 5.73, and 5.11 for β1, β2, β3, β4, β5, and β6, respectively). These energetic profiles suggest that the Cu2+ adsorbates are possibly accumulated accompanied by an increase in the distortion of the local structure, rather than the accumulation of the adsorbates induces the change in the coordination number of amine. Conclusion The adsorption of cations onto poly(3-aminopropylsilsesquioxane) can be a benchmark test to unveil the effects of adsorbate-adsorbate interactions during the adsorption reactions on the aminopropyl-functionalized silica surfaces. This is due to the extremely dense population of the 3-aminopropyl group, which is located two-dimensionally. The adsorption isotherms are analyzed using the Langmuir equation. This analysis reveals that N/Cu ) 2 (1.5 mmol/g) and N/Co ) 1 (3.2 mmol/g), respectively, are achieved at the full coverage, indicating a large capacity of poly(3-aminopropylsilsesquioxane) for transition metal cation adsorption. The gradual change in ESR signals of Cu2+ implies that the adsorption sites are not isolated, but are condensed. Nevertheless, all structures are assigned to a diamino Cu(II) complex on the basis of the g values and hyperfine coupling constants. A resonance due to the Cu2+-Cu2+ spin-spin coupling is observed for the saturated adsorption, even under room temperature measurements. This observation implies a highly dense adsorption layer and is probably the first spectroscopic detection of the interaction between Cu2+ captured by aminopropyl group on the silica surface. The adsorption isotherm of Co2+ is composed of two modes; the adsorption is initially saturated at Co/N ) 1:2 and then increases again until Co/N ) 1:1. This characteristic feature implies a gross structural change of the adsorption layer at Co/N ) 1:2 to generate a large number of empty sites. The pre-edge peak intensity of the Co K-edge XANES spectra suddenly increases at the start of the second mode in the adsorption isotherm. This spectroscopic observation demonstrates that the strength of the ligands changes substantially at the transition of the adsorption mode. These adsorption phenomena are discussed with the mesostructural characteristics of poly(3-aminopropylsilsesquioxane); the aminopropyl group is located pseudo-two-dimensionally with a spacing of Si-O-Si. The differences in adsorption between Cu2+ and Co2+ are explained with their coordination constants to amine and the stabilities of their symmetry.

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Acknowledgment. The authors are grateful for the support given by Prof. M. Yagi with the ESR measurement carried out in the Instrumental Analysis Center, YNU. This study is financially supported by a Grant-in-Aid for Scientific Research (JPSP). They also thank Prof. M. Nomura (KEK-PF) for his kind consideration for the allocation of beam time to our XAFS measurements. Supporting Information Available: EXAFS spectra and results of curve-fitting analysis of Co2+ captured by (NH2C3H6SiO1.5)n and Langmuir plots of the adsorption of Co2+ and Cu2+. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yoshitake, H. New J. Chem. 2005, 29, 1107. (2) Yoshitake, H. J. Mater. Chem. 2010, 20, 4537. (3) Corriu, R. J. P.; Mehdi, A.; Reye´, C. J. Mater. Chem. 2005, 15, 4285. (4) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559. (5) Sanchez, C.; Boissie`re, C.; Grosso, D.; Laberty, C.; Nicole, L. Chem. Mater. 2008, 20, 682. (6) Hatton, B.; Landskron, K.; Whitnall, W.; Perovic, D.; Ozin, G. A. Acc. Chem. Res. 2005, 38, 305. (7) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20, 891. (8) Liu, J.; Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Gong, M. AdV. Mater. 1998, 10, 161. (9) Fryxell, G. E.; Liu, J. Adsorption on Silica Surfaces; Papirer, E., Ed.; Marcel Dekker: New York, 2000; pp665. (10) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (11) Clark, J. H.; Macquarrie, D. J. Chem. Commun. 1998, 853. (12) Clark, J. H.; Macquarrie, D. J.; Wilson, K. Stud. Surf. Sci. Catal. 2000, 129, 251. (13) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (14) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403. (15) Anwander, R. Chem. Mater. 2001, 13, 4419. (16) Kickelbick, G. Angew. Chem., Int. Ed. 2004, 43, 3102. (17) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (18) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (19) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Bull. Chem. Soc. Jpn. 2003, 76, 847. (20) Yoshitake, H.; Yokoi, T.; Tatsumi, T. Chem. Mater. 2002, 14, 4603. (21) Yoshitake, H.; Koiso, E.; Horie, H.; Yoshimura, H.; Tatsumi, T. Chem. Lett. 2004, 33, 872. (22) Yoshitake, H.; Koiso, E.; Horie, H.; Yoshimura, H.; Tatsumi, T. Microporous Mesoporous Mater. 2005, 85, 183. (23) Yokoi, T.; Yoshitake, H.; Tatsumi, T. J. Mater. Chem. 2004, 14, 951. (24) Kang, K. K.; Ahn, W. S. J. Mol. Catal. A: Chem. 2000, 159, 403. (25) Morey, M.; Davidson, A.; Stucky, G. D. Microporous Mater. 1996, 6, 99. (26) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (27) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. (28) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865. (29) Miyajima, T.; Abry, S.; Zhou, W.; Albela, B.; Bonneviot, L.; Oumi, Y.; Sano, T.; Yoshitake, H. J. Mater. Chem. 2007, 17, 3901. (30) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135. (31) Shimojima, A.; Sugahara, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2847. (32) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150. (33) Moreau, J. J. E.; Pichon, B. P.; Arrachart, G.; Man, M. W. C.; Bied, C. New J. Chem. 2005, 29, 653. (34) Moreau, J. J. E.; Pichon, B. P.; Man, M. W. C.; Bied, C.; Pritzkow, H.; Bantignies, J.-L.; Dieudonne´, P.; Sauvajol, J.-L. Angew. Chem., Int. Ed. 2004, 43, 203. (35) Kaneko, Y.; Iyi, N.; Matsumoto, T.; Fujii, K.; Kurashima, K.; Fujita, T. J. Mater. Chem. 2003, 13, 2058.

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