J. Phys. Chem. C 2010, 114, 539–545
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Nanospace Engineering of Methylviologen Modified Hectorite-Like Layered Silicates with Varied Layer Charge Density for the Adsorbents Design Tomohiko Okada,† Takayuki Matsutomo,‡ and Makoto Ogawa*,‡ Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu UniVersity, Wakasato 4-17-1, Nagano 380-8553, Japan, and Department of Earth Sciences and Graduate School of Science and Engineering, Waseda UniVersity, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan ReceiVed: September 17, 2009; ReVised Manuscript ReceiVed: NoVember 9, 2009
Nanospace engineering of porous organic-clay intercalation compounds has been conducted by ion exchange reactions of 1,1′-dimethyl-4,4′-bipyridinium chloride (methylviologen) with different layer charged hectoritelike layered silicates (cation exchange capacity derived from the amount of the cation exchanged: 42, 74, and 85 mequivalent/100 g clay) prepared by the reaction of LiF, Mg(OH)2, and colloidal silica. The nanospace created with silicate layers and 1,1′-dimethyl-4,4′-bipyridinium cations has been used for the adsorption of organic compounds (N,N-dimethylaniline and 2,4-dichlorophenol) in the interlayer spaces of a series of the 1,1′-dimethyl-4,4′-bipyridinium-layered silicate intercalation compounds. All of the intercalation compounds adsorbed N,N-dimethylaniline from aqueous solution. The basal spacing of the intercalates did not change by the adsorption, indicating that the adsorbed N,N-dimethylaniline existed in the interlayer nanospace. The intercalated N,N-dimethylaniline polymerized to develop a purple color when the layered silicates with the cation exchange capacities of 74 and 42 meq/100 g were used. On the other hand, the intercalation compound with a larger content of 1,1′-dimethyl-4,4′-bipyridinium (cation exchange capacity: 85 meq/100 g) suppressed the polymerization (to dimer or trimer) of N,N-dimethylaniline due to the smaller pore. The adsorption capacity of 2,4-dichlorophenol was larger when the 1,1′-dimethyl-4,4′-bipyridinium content was smaller. It is found that the volume of the nanospace formed with 1,1′-dimethyl-4,4′-bipyridinium and silicate layers is controllable by using the layered silicate with varied layer charge density. Introduction Controlling spatial distribution (homogeneous distribution or clustering) of organic species in a confined nanospace is an important subject to bring out novel functions of the adsorbed molecules and to produce a specific reaction container with nanometer-scale. Confined spaces of microporous and mesoporous inorganic solids are possible to address the separation of organic species spatially, reflecting their regulated structures.1 Porous coordination compounds (metal organic framework)2 and organosilicas with a crystal-like periodicity3 are a class of materials where organic moieties are spatially arranged in a regular manner. On the other hand, layered inorganic solids are useful scaffolds to create nanospace by the intercalation of organic species into the two-dimensional expandable interlayer spaces as so-called “pillar”.4,5 In such a hybrid system, engineering of nanospace in the interlayers is realized by varying the amount and the size (molecular structure) of organic moieties intercalated.4 Due to the versatility of the nanostructures, the applications of these materials in adsorption and separation, catalysis, and photochemistry have been proposed so far.5e,6,7 Among possible layered materials capable of attaching organic moieties, a smectite group of layered clay minerals has most widely been investigated due to their stability, large surface area, natural abundance, and rich host-guest chemistry.8 Smectites are comprised of ultrathin (ca. 1.0 nm) crystalline aluminosilicate layers superimposed by hydrated interlayers.9 * To whom all correspondence should be addressed. Phone: +81-3-52861511. Fax: +81-3-3207-4950. E-mail:
[email protected]. † Shinshu University. ‡ Waseda University.
A silicate layer is composed of two silicon tetrahedral sheets and one aluminum (or magnesium) octahedral sheet. It is wellknown that the charge compensating interlayer cations in the interlayer space of the negatively charged silicate layer can be readily exchanged with various organic cations. A small amount of organic cations prefer to be immobilized, producing a wider nanospace in the interlayer space for accommodation of a large amount of molecules (larger capacity). On the other hand, in the case of immobilizing photoactive species to prevent deactivation of the dye by aggregation, dispersion at the molecular level in a large amount of photoinactive cationic species (i.e., cationic surfactant) is a possible solution.10 One of the advantages of smectites to control the spatial distribution of organic moiety is the variation of cation exchange capacity (CEC), which directly correlates the layer charge density. The CEC of smectites available commercially is in the range of 60-120 meq/100 g.4b It is possible to reduce the CEC by the thermal treatment of Li exchanged forms (Hofmann-Klemen effect).11 It has been recognized that the CEC is a key factor to determine many important properties of smectites and their intercalation compounds.12 The variation of CEC (or selecting a smectite with appropriate CEC value), which affects the distances of adjacent interlayer cations, has been shown by differences in separation efficiency for optical resolution of a racemic mixture,13 the assemblage of a cationic dye after cation exchange,14,15 and photoprocesses of adsorbed cationic dyes.16–19 Organically pillared smectite, where a relatively small organoammonium cation, such as tetramethylammonium ion, holds silicate layers like pillars to make nanospaces surrounded with organic cations and silicate layers, has been known to be a system to possibly conduct nanospace engineering.20 It is known
10.1021/jp9089886 2010 American Chemical Society Published on Web 12/03/2009
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Okada et al.
TABLE 1: Sample List and the Adsorbed Amounts and the Basal Spacing of the MV2+ Exchanged Samples LiF:Mg(OH)2:SiO2 ratio of the starting mixture (sample name)
adsorbed amount of MV2+ a [meq /100 g clay]
basal spacing (gallery heightb) [nm]
pore volume [cm3/g]
average distance between negative charges [nm]
2.8:4.6:8 (MV2+-hectorite (85)) 1.4:5.3:8 (MV2+-hectorite (74)) 0.14:5.93:8 (MV2+-hectorite (42))
85 74 42
1.31 (0.35) 1.31 (0.35) 1.3 (0.3)
0.055 0.064 0.088
0.84 0.88 1.21
a
size of pore formed by MV2+ and silicate layers [nm] maximum
minimum
ratio of area occupied by MV2+ to surface area of silicate layer [%]
1.05 1.13 1.78
0.34 0.42 1.07
60 54 29
Determined from TG curve. b Gallery height [nm] ) basal spacing [nm] -0.96 [nm].
that the adsorptive properties of the pillared smectite depend on the interlayer nanostructure which has been determined by the structure of the interlayer cation and the charge density of the clay.4a,20 Selecting pillaring agents with different molecular structures and smectites with different CECs results in the controlled molecular sieving functions for nonionic organic compounds20–23 and photofunctions derived from the molecular distribution of photoactive molecules.24–28 However, some characteristics, which depend on the origin of smectites, like the position of isomorphous substitution and particle size make it difficult to discuss the effects of CEC on the physicochemical properties of smectites and their intercalation compounds. Therefore, there is a need to prepare smectites with a series of CECs. We recently reported the preparation of smectites (trioctahedral group, hectorite-like) with CEC ranges from 60 to 90 meq/100 g clay by the reaction of LiF, Mg(OH)2, and colloidal silica.29 The materials are ideal for investigating the possible layer charge effects, because of little effects of the other characteristics as isomorphous substitution and particle size. Here, we report the adsorptive properties of the hectorite-like silicates after cation exchange with 1,1′-dimethyl-4,4′-bipyridinium (abbreviated as MV2+) ions to investigate effects of the spatial distribution of MV2+ on the adsorption of nonionic organics. Cation exchange reactions of commercially available smectites with MV2+ chloride have been conducted,22,30–32 and the adsorption capacity of nonionic organics for MV2+ smectites has been reported to be different depending on the nature of the smectites (natural montmorillonite and a synthetic saponite).22 The preparation of MV2+ exchanged hectorite-like silicates with a series of CECs is expected to give adsorbents to discuss the effects of CEC on the adsorption capacity. In order to elucidate this point, the adsorption behavior of 2,4dichlorophenol and N,N-dimethylaniline from aqueous solutions has been investigated. Experimental Section Materials. LiF and Mg(OH)2 were obtained from Kanto Kagaku Ind. Co. Silica sol (SiO2:20.3 mass %, SiO2/Na2O ) 95) was kindly donated by Nissan Chemical Ind., Co. MV2+ chloride, 2,4-dichlorophenol (abbreviated as DCP), and N,Ndimethylaniline (abbreviated as DMA) were purchased from Tokyo Kasei Ind. Co. All of these chemicals were used without further purification. Sample Preparation. Sample preparation was conducted as reported previously.29 Mg(OH)2 was added to an aqueous solution of LiF and the mixture was vigorously mixed by magnetic stirring for 30 min at room temperature. The molar ratio of LiF:Mg(OH)2:SiO2 in the starting mixture was 1.4:5.3: 8.0. To the suspension was added silica sol, and the suspension was mixed with a homogenizer for another 30 min at room temperature. The slurry was brought to reflux with stirring at 373 K for 2 days. The product was separated by ultracentrifugation (25 krpm for 20 min) and dried under a reduced pressure
at room temperature. In order to vary CEC, the molar ratios of LiF:Mg(OH)2:SiO2 in the starting mixture were changed as listed in Table 1. Ion exchange reactions of the silicates thus obtained were conducted by using MV2+ chloride as follows: the aqueous solution of MV2+ chloride was mixed with an aqueous suspension of the silicate (100 mL: 12 g clay/L) and the mixture was mixed with stirring at room temperature for 1 day. The added amounts of MV2+ chloride are 0.25, 0.43, or 0.50 mmol/g. After the ion exchange, the product was washed until a negative Cltest was obtained. The product was then collected by centrifugation and was dried under a reduced pressure. The adsorbed amounts of MV2+ were determined by the difference in the concentration of MV2+ in aqueous phase by means of UV absorption spectra (λmax at around 250 nm). Adsorption of Nonionic Organic Compounds. The adsorption isotherms of DMA and DCP for the MV2+-smectites were obtained as follows: the adsorbents (25 mg) were allowed to react with 50 mL of aqueous solutions of adsorbates (0.1-2.0 mmol /L, pH ≈ 7 for DMA and DCP) in a glass tubes for 1 day at room temperature. Since the dissociation constant of DCP is 7.89, DCP is existed as phenolates in the aqueous solutions before adsorption. Blank samples containing 50 mL of the aqueous solutions, without adsorbents, were also prepared to estimate vaporization losses and the adsorption of the organic compounds on the glass vessel. After the resulting solids were separated by centrifugation, the concentrations of the remaining DMA and DCP in supernatant were determined by HPLC (λmax ) 254 nm) and UV absorption spectra (λmax ) 284 nm), respectively. Characterization. XRD patterns were obtained by a Rigaku RAD IB diffractometer (Cu KR radiation) operated at 20 mA, 40 kV. UV-vis spectra were recorded on a Shimadzu UV3100PC spectrophotometer. The diffuse reflectance spectra were obtained by using a Shimadzu UV-3100PC spectrometer equipped with an integrated sphere attachment. TG curves were recorded on a Rigaku TG8120 instrument at a heating rate of 10 K/min and using R-alumina as the standard material. HPLC was performed on a Thermoelectron LCQ Deca equipped with a UV-vis detector, using an octadecylsilane column (SunFireTM C18) with acetonitrile flow at 313 K. Results and Discussion Sample Preparation. Preparation of smectites with different CEC and their cation exchange abilities have been confirmed as reported in our previous report.29 As the synthetic smectites, the XRD patterns are broad, indicating the low crystallinity29 (The XRD patterns are shown in the Supporting Information, Figure S1) The amount of the adsorbed MV2+ and the basal spacing of the products are summarized in Table 1. The products are designated as MV2+-hectorite (n) where n denotes the amount of the adsorbed MV2+ [meq/100 g] (Table 1). The basal spacings of the products were 1.3 nm consistent with the value reported for the MV2+-smectite intercalation compounds so
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Figure 1. Adsorption isotherms of DMA on MV2+-hectorite (n) from aqueous solution.
far.22,30–32 The interlayer expansions of the MV2+-smectites were determined to be 0.3 nm by subtracting the thickness of the silicate layer (ca. 1.0 nm) from the observed basal spacings. Taking the gallery heights and the size of MV2+ (0.63 × 1.34 × 0.3 nm)33 into consideration, the adsorbed MV2+ ions are thought to be arranged as a monomolecular layer with their molecular planes (0.3 nm) parallel to the silicate layers.22,34,35 The difference in the adsorbed amount of MV2+ is expected to give MV2+ intercalation compounds with different spatial distribution of MV2+. Effect of CEC on the Adsorption of DMA. Figure 1 shows the adsorption isotherms of DMA on MV2+-hectorite (n) from aqueous solutions. According to the Giles classification,36 the isotherm on MV2+-hectorite (85) was type-S; only a small amount of DMA was adsorbed on MV2+-hectorite (85) when the equilibrium concentration was low (Figure 1). With the increase in the equilibrium concentration, the amount of the adsorbed DMA increased. On the contrary, the adsorption isotherms on MV2+-hectorite (42) and (74) were type L. To estimate the adsorption capacity, the adsorption isotherms were fitted to the Langmuir equation37 as eq 1
1/Q ) 1/Qmax + (1/aQmax)(1/Ce)
(1)
where Qmax and a are constants related to maximum adsorbed amount and binding energy, respectively. Q and Ce designate the adsorbed amounts of DMA and the equilibrium concentrations, respectively. Langmuir parameters based on the adsorption isotherms are presented in Table 2. Since the correlation coefficients (r2 values) corresponding to the isotherms on MV2+hectorite (42) and (74) (r2 ) 0.96-0.97) are close to 1, the isotherms are empirically consistent with Langmuir model. The adsorption capacities derived from Langmuir plots of the isotherms for MV2+-hectorite (42) and (74) are summarized 0.39 and 0.12 mmol/g, respectively, as listed in Table 2. Negligible change in the basal spacing of the MV2+-hectorite (n) was observed by the adsorption, indicating that DMA was accommodated in the pore created by MV2+ in the interlayer space (Supporting Information, Figure S1). In order to discuss the effects of spatial distribution of MV2+ in hectorite on the DMA adsorption, the size of the pore formed in the interlayer space was estimated from the distance between adjacent negative
charges, which are estimated from the ideal surface area of hectorite,9 the molecular size of MV2+ (CambridgeSoft Chem3D) and the adsorbed amount of MV2. The discussion was based on the reports for tetramethylammonium-saponite,26 transition metal complex-clays38 and quaternary diammonium-montmorillonites,39 and the results are summarized in Table 1 (details have been described in the Supporting Information). Regarding MV2+ as a rectangular shape (0.63 nm × 1.34 nm × 0.3 nm),33 the pore size can be minimum or maximum in the cases that MV2+ ions arrange as a monomolecular layer with the long or short side of MV2+ toward the section of silicate layers, respectively. Judging from the molecular size of DMA (0.94 × 0.65 × 0.43 nm3, see the Supporting Information), pore volume and the ranges of the pore size shown in Table 1, the pore size of MV2+-hectorite (42) is large enough to accommodate DMA. As a result, large maximum adsorbed amount was shown for the DMA adsorption on MV2+-hectorite (42). The color of MV2+-hectorite (n) changed from yellow to purple when the CEC is low (Figure 2). Diffuse reflectance spectra of the products obtained after the adsorption of DMA (initial concentration of 2.0 mmol/L) (Figure 3) show absorption bands in the range of 500 - 700 nm, which are not observed for MV2+-hectorite (n) before the adsorption. DMA was also adsorbed on raw hectorite (initial concentration of 2.0 mmol/ L) to show the color change to purple or purple-green with an absorption bands at 420 and at around 600 nm as shown in the diffuse reflectance spectrum (Figure 4). It has been recognized that DMA is coloring agent of clays through the oxidation to methyl violet (2, Scheme 1)40 with the absorption maximum of ca. 600 nm. The purple quinoid form (3), which is obtained by further oxidation of 1, becomes yellow by protonation (4), giving absorption bands at around 420 nm. The observed redox reactions have been explained considering the fact that a DMA derivative was oxidized by molecular oxygen adsorbed on clay surfaces.41 It has been postulated that a DMA derivative could be oxidized by atmospheric oxygen adsorbed on the surface edges through catalysis of the surface acidity (imposed by octahedral sheets).42 Thus, we deduce that the color change is due to the dye-formation from DMA on the surface of the silicate layer, rather than the reactions between MV2+-hectorite (n) and DMA. More apparent color (larger optical density) was developed when the adsorbent with lower MV2+ content was used (Figures 2 and 3) due to larger adsorbed amounts of DMA. Interestingly, the color development was not observed for MV2+-hectorite (85) even when the adsorbed amount of DMA on MV2+-hectorite (85) was close to that on MV2+-hectorite (74) (ca. 0.08 mmol/g at equilibrium concentration of 1.1 mmol/L, Figure 1). We elucidate that dense packing of MV2+ in MV2+-hectorite (85) prevents dye-formation from DMA. Aggregation and polymerization of photoactive species are not usually preferred because they resulted to deactive the excited processes.4b,c For example, efficient photochemical hole burning25 has been achieved by the incorporation of 1,4-dihydroxyanthraquinone in tetramethylammonium-saponite (CEC ) 71 meq/100 g), where independent micropore (pore size of ca. 0.5 × 0.5 nm2) played an important role to prevent dye aggregation. Effect of CEC on the Adsorption of DCP. The adsorption isotherms of DCP on MV2+-hectorite (74) and MV2+-hectorite (42) from aqueous solution are shown in Figure 5. The isotherm on MV2+-hectorite (74) is type-S where the adsorbed amount of DCP at a low concentration (
