Preparation and vapor adsorption properties of quaternary diammonium-montmorillonites

Microporous and Mesoporous Materials 124 (2009) 30–35

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Preparation and vapor adsorption properties of quaternary diammonium-montmorillonites Yoko Seki, Tomohiko Okada 1, Makoto Ogawa * Graduate School of Science and Engineering, Department of Earth Sciences, Waseda University, Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan

a r t i c l e

i n f o

Article history: Received 26 September 2008 Received in revised form 15 April 2009 Accepted 17 April 2009 Available online 24 April 2009 Keywords: Organo-clay Montmorillonite Diquarternaryammonium cation Adsorption Charge density

a b s t r a c t Adsorption behavior of water, o-xylene and p-xylene vapor to a series of N,N0 -hexamethylalkyldiammonium ions [(CH3)3N+(CnH2n+1)(CH3)3N+; where n = 2,3,6,10] exchanged montmorillonites (Kunipia F from Kunimine Ind. Co., Japan; cation exchange capacity of 119 meq (100 g clay)1 and BENGEL BLIGHT 11 from Hojun, Japan; cation exchange capacity of 78 meq (100 g clay)1) was investigated. All the organo-clays examined in the present study adsorbed these gases to show their microporous nature. The adsorbed amounts of water, o-xylene and p-xylene increased as the size of N,N0 -hexamethylalkyldiammonium was smaller and as the cation exchange capacity of host was larger, showing possible pore volume engineering. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Nanoporous inorganic–organic hybrid materials, such as organic moieties attached zeolites and mesoporous silicas, and porous coordination compounds (metal organic frame work) have been received much attention from a wide range of scientific and practical viewpoints [1–4]. Due to the versatility of the nanostructures, which were controlled by the molecular structures of the organic moieties, the applications of these materials in adsorption and separation, gas storage, catalysis, and optical materials have been proposed so far. In order to optimize the materials performance, the preparation of inorganic–organic hybrid materials with various nanoporous structures is a topic of current interest. Organically modified clay (especially smectites, which is a layered clay mineral consisting of negatively charged silicate layer and the charge compensating interlayer cation, exchanged with organic ions) is a class of inorganic–organic hybrid materials for adsorption, separation and as catalysts and photofunctional materials. Two types of organic cation exchanged smectites are known; one is called organophilic clay which is smectite modified with cationic surfactant. The organophilic clays have been investigated extensively as rheology controlling agents [5], adsorbents for nonionic organic pollutants from environment [6], slow release formulations of pesticides [7], optical materials [8] and plastics fillers [9].

Organically pillared clay is another class of organically modified clays; where a relatively small organoammonium cation, such as tetramethylammonium ion, holds silicate layers like pillars and makes nanospaces surrounded with organic cations and silicate layer. Organically pillared clays have been studied as adsorbents having molecular sieving effect for nonionic organic compounds and as supports for photoactive molecular species to lead photofunctional materials [10–14]. It is known that the adsorptive property of the pillared clay depends on the interlayer nanostructure which was determined by the structure of the interlayer cation and the charge density of the clay [15,16]. In this study, we prepared organically pillared montmorillonites using a series of N,N0 -hexamethylalkyldiammonium ions with the formula of [(CH3)3N+(CnH2n+1)(CH3)3N+; where n = 2,3,6,10] and the adsorption of o-xylene and p-xylene vapors onto the organically modified montmorillonites was investigated to show how the molecular structure (or size) of the interlayer cation affects the adsorptive properties of organoammonium montmorillonites, especially, on the adsorption capacities. Two clays with different cation exchange capacity were used to see the effects of the charge density of host materials on the adsorption. 2. Materials and methods 2.1. Materials

* Corresponding author. Tel.: +81 3 5286 1511; fax: +81 3 3207 4950. E-mail address: [email protected] (M. Ogawa). 1 Present address: Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato, Nagano-shi, Nagano 380-8553, Japan. 1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2009.04.023

Natural Na-montmorillnite (Kunipia F obtained from Kunimine Ind. Co., Japan) and natural Ca-montmorillonite (BENGEL BLIGHT11 from Hojun, Japan; abbreviated as BB11) were used as the host

31

Y. Seki et al. / Microporous and Mesoporous Materials 124 (2009) 30–35

d(001)=1.52 nm

(b) d(020, 110) =0.51 nm d(005) =0.31 nm

Intensity / a.u.

d(002)= 0.51 nm

d(310, 240, 150) =1.24 nm

Scheme 1. Molecular formula for N,N0 -hexamethylalkyldiammoniums.

d(001)=1.24 nm

Table 1 Names and abbreviations of N,N0 -hexamethylalkyldiammoniums used in the present study.

d(004)=0.31 nm

Name of cation

(a)

0

N,N -hexamethylpropylenediamine

d(11-, 02-) =0.62 nm d(002)= 0.62 nm

0

10

N

N,N0 -hexamethylethylenediamine

20 30 40 2θ θ (Cu K α ) / o

50

60

Fig. 1. XRD patterns of (a) Kunipia F and (b) BB11. Triangles (.) and circles (s) designate montmorillonite and cristobalite, respectively.

materials. The cation exchange capacities (CEC) from each selling company are 119 and 78 meq (100 g clay)1 for Kunipia F and BB11, respectively. While Kunipia F contains little non-expandable impurities, some of the impurities as cristobalite (sum of ca. 10– 15 wt.%) are in Bengel Bright 11 as shown in the XRD patterns (Fig. 1) N,N0 -hexamethylethylenediammonium dichloride and N,N0 -hexamethylpropylenediammonium dichloride were kindly donated by Daiichi Kogyo Seiyaku Co., Ltd. N,N0 -hexamethylhexanediammonium dibromide and N,N0 -hexamethyldecanediammonium diiodide were purchased from Tokyo Kasei Ind. Co. The molecular structures of N,N0 -hexamethylalkyldiammonium ions (abbreviated as C 2þ n ) are shown in Scheme 1. The molecular sizes of these hexamethylalkyldiammonium ions are listed in Table 1, together with the abbreviations of these cations. o-Xylene and p-xylene, from Kanto Chemical Co., Ltd., were used as received.

Abbreviated asr.

0.9

C 2þ 2

3

1.0

C 2þ 3

N,N -hexamethylhexanediamine

6

1.4

C 2þ 6

N,N0 -hexamethyldecanediamine

10

1.8

C 2þ 10

0

d(13-, 20-) =0.26 nm

L/nm

2

halide solution (15 mL) for 1 day, where the added amount of C 2þ n ion was 1.1 times in excess of the CEC. After the aqueous phase was separated by centrifugation, the solid product was washed with deionized water until negative AgNO3 test was obtained and subsequently dried under reduced pressure at room temperature. 2.3. Characterizations XRD patterns were recorded on a Rigaku RAD IIB powder diffractometer equipped with monochromatic CuKa radiation, operated at 20 mA, 40 kV. CHN analysis was performed on a Perkin Elmer 2004 ll instrument. FT-IR spectra were recorded on a Shimadzu FT-IR8200 by using Nujol mull method. 2.4. Adsorption of water, o-xylene and p-xylene vapors The water, o-xylene and p-xylene adsorption isotherms were obtained at 298 K on a BELSORP 18 instrument (Bell Japan). Before 2þ the adsorption experiments, C 2þ 6 -clays and C 10 -clays were dried at 2þ 343 K under reduced pressure for 24 h. For C 2þ 2 - and C 3 -clays, the pretreatment was carried out at a lower temperature (298 K) in or2þ der to avoid the decomposition of C 2þ 2 and C 3 . 3. Results and discussion

2.2. Preparation of pillared clays 3.1. Sample preparation C 2þ n -clays were prepared from Kunipia F and BB11 by the cation exchange reactions with aqueous C 2þ n halides. An aqueous suspension (50 mL) of the clay (1.0 g) was mixed with an aqueous C 2þ n

The basal spacings of the C 2þ n -clays were in a range of 1.34– 1.40 nm (Tables 2A and 2B). The gallery heights of the C 2þ n -clays

Table 2A Structural and compositional results of N,N0 -hexamethylalkyldiammonium-BENGEL BLIGHT11(C 2þ n -BB11).

d value/nm Gallery height/nm Carbon content/mass% Nitrogen content/mass% Amount of organic cation exchanged/meq. (100 g clay)1

BB11

C 2þ 2 -BB11

C 2þ 3 -BB11

C 2þ 6 -BB11

C 2þ 10 -BB11

1.52 0.56 – – –

1.40 0.44 3.99 1.28 93

1.40 0.44 4.45 1.28 93

1.41 0.45 5.64 1.28 89

1.41 0.45 6.68 1.11 79

Table 2B Structural and compositional results of N,N0 -hexamethylalkyldiammonium-Kunipia F (C 2þ n -Kunipia F).

d value/nm Gallery height/nm Carbon content/mass% Nitrogen content/mass% Amount of organic cation exchanged/meq. (100 g clay)1

Kunipia F

C 2þ 2 -Kunipia F

C 2þ 3 -Kunipia F

C 2þ 6 -Kunipia F

C 2þ 10 -Kunipia F

1.16 0.20 – – –

1.40 0.44 4.96 1.52 119

1.40 0.44 5.56 1.64 119

1.34 0.38 7.51 1.27 121

1.37 0.41 9.12 1.12 113

Y. Seki et al. / Microporous and Mesoporous Materials 124 (2009) 30–35

were determined by subtracting the thickness of the silicate layer (0.96 nm) from the observed basal spacing to be 0.38 nm–0.45 nm, which corresponded to the size of the trimethylammonium group. The adsorbed C 2þ n ions are thought to be arranged as a monomolecular layer with their alkyl chains parallel to the silicate layers [17]. The carbon and nitrogen contents of C 2þ n -clays are summarized in ions exchanged into the Tables 2A and 2B. The amounts of C 2þ n clays were determined by the carbon contents listed in Table 2. The amounts were 93, 93, 89 and 79 meq. (100 g clay)1 for C 2þ 2 -, 2þ 2þ C 2þ 3 -, C 6 - and C 10 -BB11, respectively, and 119, 119, 121 and 2þ 2þ 2þ 113 meq (100 g clay)1 for C 2þ 2 -, C 3 - and C 6 -, C 10 -Kunipia F, respectively (Table 2A and 2B). Taking the CEC of the host materials (78 meq. (100 g clay)1 and 119 meq (100 g clay)1 for BB11 and Kunipia F, respectively) into consideration, the quantitative ion exchange of BB11 and Kunipia F with C 2þ n ions was confirmed.

140

Amount adsorbed / mg.(g of sample)-1

32

120 100 80 60 40 20 0 0

0.2

3.2. Adsorption of water, o-xylene and p-xylene vapors The adsorption isotherms of water, o-xylene and p-xylene for 2þ C 2þ n -Kunipia F and C n -BB11 are shown in Figs. 2A–4B. According to the IUPAC classification [18], all the isotherms followed type-

0.6

0.8

1

Fig. 3A. Adsorption isotherms of o-xylene (298 K). (s) adsorption isotherm for C 2þ 2 Kunipia F, (}) adsorption isotherm for C 2þ 3 -Kunipia F, (4) adsorption isotherm for 2þ 2þ C 6 -Kunipia F, (h) adsorption isotherm for C 10 -Kunipia F.

150

140

Amount adsorbed / mg.(g of sample)-1

Amount adsorbed / mg.(g of sample)-1

0.4

Relative Pressure

100

50

120 100 80 60 40 20

0 0

0.2

0.4

0.6

0.8

1

Relative Pressure

0

0.2

0.4

0.6

0.8

1

Relative Pressure

Fig. 2A. Adsorption isotherms of water (298 K), (s) adsorption isotherm for C 2þ 2 Kunipia F, (}) adsorption isotherm for C 2þ 3 -Kunipia F, (4) adsorption isotherm for 2þ 2þ C 6 -Kunipia F, and (h) adsorption isotherm for C 10 -Kunipia F.

Amount adsorbed / mg.(g of sample)-1

0

Fig. 3B. Adsorption isotherm for o-xylene (298 K). (s) adsorption isotherm for C 2þ 2 2þ BB11, (}) adsorption isotherm for C 2þ 3 -BB11, (4) adsorption isotherm for C 6 -BB11, 2þ (h) adsorption isotherm for C 6 -BB11.

150

ll, showing the weak affinity between adsorbate and adsorbents 2þ (C 2þ n -Kunipia F and C n -BB11). The amounts of adsorbates for the 2þ 2þ C n -Kunipia F and C n -BB11 as monolayer were obtained by BET plot (Eq. 1) of the isotherms (Figs. 2A–4B).

100

p=½vðp0  pÞ ¼ ð1=vm cÞ þ ½ð1  cÞ=vmc ðp=p0 Þ

50

0 0

0.2

0.4

0.6

0.8

1

Relative Pressure Fig. 2B. Adsorption isotherms of water (298 K). (s) adsorption isotherm for C 2þ 2 2þ BB11, (}) adsorption isotherm for C 2þ 3 -BB11, (4) adsorption isotherm for C 6 -BB11, 2þ (h) adsorption isotherm for C 10 -BB11.

ð1Þ

where p, p0, v, vm and c represent the pressure, saturated vapor pressure, maximum adsorbed amount, adsorbed amount of water as monolayer and constant, respectively. The adsorbed amounts of water, o- and p-xylenes as monolayer thus determined are shown in Table 3. It should be noted here that the organically pillared montmorillonites examined in the present study do not swell in water, o-xylene and p-xylene. This means that the adsorption occurs in the nanopore of the pillared montmorillonites. The adsorbed amounts of water were 54, 47, 37 and 23 mg g1 2þ 2þ 2þ for C 2þ 2 -Kunipia F, C 3 -Kunipia F, C 6 -Kunipia F and C 10 -Kunipia F, respectively. The larger amounts of water adsorbed when the size of C 2þ n ions were smaller, suggesting that the nanospace formed in the interlayer space for the water adsorption decreased as the size of C 2þ n increases.

Y. Seki et al. / Microporous and Mesoporous Materials 124 (2009) 30–35

Amount adsorbed / mg.(g of sample)-1

150

100

50

0 0

0.2

0.4

0.6

0.8

1

Relative Pressure Fig. 4A. Adsorption isotherms of p-xylene (298 K). (s) adsorption isotherm for C 2þ 2 Kunipia F, (}) adsorption isotherm for C 2þ 3 -Kunipia F, (4) adsorption isotherm for 2þ 2þ C 6 -Kunipia F, (h) adsorption isotherm for C 10 -Kunipia F.

Amount adsorbed / mg.(g of sample)-1

150

100

50

0

0

0.2

0.4

0.6

0.8

1

Relative Pressure Fig. 4B. Adsorption isotherms of p-xylene (298 K). (s) adsorption isotherm for C 2þ 2 2þ BB11, (}) adsorption isotherm for C 2þ 3 -BB11, (4) adsorption isotherm for C 6 -BB11, 2þ (h) adsorption isotherm for C 10 -BB11.

Table 3 Monolayer amount of water and nonionic organic compounds adsorbed. Amount adsorbed/mg (g of sample)1

C 2þ 2 -BB11 C 2þ 3 -BB11 C 2þ 6 -BB11 C 2þ 10 -BB11 C 2þ 2 -Kunipia C 2þ 3 -Kunipia C 2þ 6 -Kunipia C 2þ 10 -Kunipia

F F F F

Water

o-Xylene

p-Xylene

36 36 27 19 54 47 37 23

43 33 24 19 57 38 33 28

57 47 28 14 71 62 38 33

The adsorption isotherms of o-xylene and p-xylene for C 2þ n Kunipia F are shown in Figs. 3A and 3B, respectively. As mentioned above, according to the IUPAC classification [18], all the isotherms followed type-ll, showing the weak affinity between adsorbate molecules and C 2þ n -Kunipia F. It was reported that the adsorption isotherms of o-xylene for smectites modified with tetramethylammonium (abbreviated as TMA) also followed type-ll [19]. The amounts of o-xylene adsorbed were determined by BET plot of isotherms to be 57, 38, 34 and 28 mg g1 and the amounts of p-xylene

33

2þ 2þ adsorbed were 71, 62, 38 and 33 mg g1 for C 2þ 2 -, C 3 -, C 6 - and 2þ C 10 -Kunipia F, respectively (Table 3) by BET plot of the isotherms. The amounts of o- and p-xylenes increased as the size of C 2þ n ions were smaller, which is same as the results of water adsorption as mentioned above. It was reported that the amount of o-xylene adsorbed was 52 mg g1 for Arizona montmorillonite (CEC = 120 meq (100 g clay)1), which was close to the CEC of Kunipia F used in the present study) modified with TMA [19]. The adsorbed amounts (57 and 52 mg g1) of o-xylene were almost same for C 2þ 2 -Kunipia F and TMA-montmorillonite. The size of C 2þ 2 (0.47 nm  0.47 nm  0.90 nm) is twice larger than that of TMA (0.47 nm  0.47 nm  0.47 nm) and the amount of C 2þ 2 intercalated into the interlayer space of the host was half as many as that of TMA since C 2þ 2 is bidentate. That means the remaining nanospace in the interlayers is similar in volume in TMA-Arizona montmorillonite and C 2þ 2 -Kunipia F. It is thought that the remaining nanospace in the interlayer is effective adsorption space for organic compounds [19,20] and the adsorption capacity of pillared clay is proportional to the remaining nanospace in the interlayers [21]. This coincides with the present experimental results on the adsorption of water and xylenes, suggesting that, in point of the 2þ adsorption capacity, C 2þ n -Kunipia F having small size C n cation is better adsorbent for o- and p-xylenes, if compared with those having larger size C 2þ n cation. The shape of the water adsorption isotherm for C 2þ 2 -Kunipia F was similar to that of C 2þ 3 -Kunipia F (Fig. 2A), showing in little difference in their adsorbed amounts (Table 3). This similarity was 2þ also seen in the adsorption of p-xylene on C 2þ 2 - and C 3 -Kunipia F (Fig. 2A and Table 3). On the contrary, the strength of o-xyleneadsorbent interactions was shown to be different between C 2þ 2 2þ and C 2þ 3 -Kunipia F; the initial slope of the isotherm for C 2 -Kunipia F was larger than that for C 2þ 3 -Kunipia F (Fig. 3A), leading to the relatively larger adsorbed amount of o-xylene for C 2þ 2 -Kunipia F (57 mg g1) than that (38 mg g1) for C 2þ 3 -Kunipia F (Table 3). oXylene has adjoining two methyl groups, so breadth of o-xylene is wider than that of p-xylene having two methyl groups facing each other. Since the gallery height of C 2þ 2 -Kunipia F (0.44 nm) was same to the value of C 2þ 3 -Kunipia F (0.44 nm), the difference 2þ in the distance between the adjacent C 2þ 2 (or C 3 ) has been thought ions is to give the different pore size. The distance between C 2þ 2 thought to be wide enough for o-xylene to access, while the ions. adsorption of o-xylene is slightly restricted by bulkier C 2þ 3 The distances between C 2þ n ions (x in Fig. 5) were estimated from the adsorbed amounts of C 2þ n ions for Kunipia F, ideal surface areas ions as of montmorillonite (704 m2.g1) [22] and the size of C 2þ n ions are summarized in Table 1. The distances between C 2þ n 2þ 0.5 nm and 0.4 nm for C 2þ 2 -Kunipia F and C 3 -Kunipia F, respectively, which explain the difference in the adsorption of o- and pxylenes. The possible interlayer cation size effects on the adsorption of vapors discussed above were examined using montmorillonite with different layer charge density. The adsorption isotherms of water for C 2þ n -BB11 are shown in Fig. 2B. The variation of the adsorbed amounts of water for C 2þ n -BB11 showed similar trend to that observed for C 2þ n -Kunipia F; the larger adsorption capacity was obtained for C 2þ n -clays with smaller size. The amounts of water 2þ adsorbed were 36, 36, 27 and 19 mg g1 for C 2þ 2 -BB11, C 3 -BB11, 2þ 2þ C 6 -BB11 and C 10 -BB11, respectively. The amounts of o-xylene adsorbed were 43, 33, 25 and 19 mg g1, and the amount of p-xylene 2þ adsorbed were 57, 47, 28 and 14 mg g1 for C 2þ 2 -BB11, C 3 -BB11, 2þ 2þ C 6 -BB11 and C 10 -BB11, respectively (Table 3). It is thought that in C 2þ n -BB11, water, o-xylene and p-xylene were adsorbed to the remaining nanospace. It was reported that the amount of p-xylene adsorbed was 55 mg g1 for Wyoming montmorillonite (reference clay SWy-1, CEC = 76.4 meq (100 g clay)1 [21], which was close to the CEC of

34

Y. Seki et al. / Microporous and Mesoporous Materials 124 (2009) 30–35

A: C32+-BB11

B: C32+-Kunipia F (b)

(a)

3800

3400

3000

Wavenumber / cm-1

Transmittance /a.u.

Transmittance /a.u.

(b)

3800

(a)

3400

3000

Wavenumber / cm-1

2þ Fig. 6. The change in the FT-IR spectra of (A) C 2þ 3 -BB11 and (B) C 3 -Kunipia F (a) before and (b) after drying at r.t. in vacuum.

Fig. 5. Schematic drawing of the interlayer structure of C 2þ n -clays.

BB11 used in the present study) modified with tetramethylphosphonium (abbreviated as TMP). The amounts of p-xylene adsorbed for C 2þ 2 -BB11 and TMP-montmorillonite were nearly identical. Con(bidentate, 0.47 nm  0.47 sidering the size and charge of C 2þ 2 nm  0.90 nm) and TMP (monodentate, 0.60 nm  0.60 nm  have occupied the 0.60 nm), it is thought that TMP and C 2þ 2 interlayer spaces to give approximate volume of the remaining nanospace. This explains the fact that adsorption capacity of organo-clays depends on the size of interlayer cation. It was reported that the amounts of benzene, toluene and p-xylene adsorbed in the interlayer spaces increased as the CEC of host materials was lower because of increasing of the interlayer nanospace [19]. The CEC of Kunipia F (119 meq. (100 g clay)1) is higher than that of BB11 (78 meq. (100 g clay)1), so that the nanospace of C 2þ n -Kunipia F was thought to be smaller than that of the corre2þ sponding C 2þ n -BB11. However, C n -Kunipia F adsorbed larger amounts of water, o-xylene and p-xylene than corresponding C 2þ n -BB11 (Table 3). The possible role of water remained even after 2þ the pre-treatment (degassing at 298 K for C 2þ 2 - and C 3 -clays or at 2þ and C -clays) is more plausible to explain the re343 K for C 2þ 6 10 duced the adsorption capacities of water and two xylenes on 1 due to C 2þ n -BB11. The IR absorption band at around 3400 cm -BB11 (Fig. 6A) did hydrogen-bonded adsorbed water [23] of C 2þ 3 not change after the drying, while the relative absorbance of the band due to the adsorbed water (around 3400 cm1)/structural OH (3620 cm1) decreased (from 0.7 to 0.5) by drying for C 2þ 3 -Kunipia F (Fig. 6B). The organodications are crowded in C 2þ 3 -Kunipia F, preventing the adsorbed water molecules from forming extensive hydrogen bonded networks to allow a more efficient outgassing (dehydration) at lower temperature (298 K) for Kunipia than for BB11. On the contrary, C 2þ 3 cations are further apart on the BB11 surface thus the interactions between water–water and water– siloxane surface may slightly be stronger and the hydrogen-bonding networks may be more extensive to make degassing at 298 K

less efficient than with Kunipia F. Further systematic study is required to explain the adsorption capacity (or pore volume) nanospace of organically pillared clays. Spatial distribution (homogeneous distribution or clustering) of the organodications is not clear at present. As reported in the adsorption of phenols on a dye–clay in water, an example that inhomogeneous distribution of a dye (derived from charge distribution and dye-aggregation) affects the adsorbed amounts of phenols [24] has been reported. In the present system, the adsorbed amounts of water and xylenes decreased with increase in the size of the organodications. Although the cations with longer alkyl chains (for example, C 2þ 10 ) may tend to aggregate in the interlayer space, we could not find any characteristic of the adsorption affected by the inhomogeneous distribution of the cations. Therefore, we think that the cations homogeneously distributed over the clay surfaces comparatively. 4. Conclusions N,N0 -hexamethylalkyldiammonium cations were intercalated into the interlayer of two natural montmorillonites (Kunipia F and BB11) with different cation exchange capacity quantitatively. The organoammonium-montmorillonites adsorbed water, o-xylene and p-xylene from vapors. The adsorbed amounts of water, ions were o-xylene and p-xylene increased as the size of C 2þ n smaller. Acknowledgment This work was financially supported by a Grant-in-Aid for Scientific Research (B) (19350103) from Japan Society for the Promotion of Science. References [1] K. Maeda, J. Akimoto, Y. Kiyozumi, F. Mizukami, Angew. Chem., Int. Ed. Engl. 34 (1995) 1199. [2] K. Yamamoto, Y. Sakata, Y. Nohara, Y. Takahashi, T. Tatsumi, Science 300 (2003) 470. [3] A. Stein, Adv. Mater. 15 (2003) 763. [4] M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 122 (2000) 1391.

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