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Specific uptake of aromatic compounds from aqueous solution by montmorillonite modified with tetraphenylphosphonium
Journal of Physics and Chemistry of Solids 73 (2012) 120–123
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Specific uptake of aromatic compounds from aqueous solution by montmorillonite modified with tetraphenylphosphonium Tomohito Kameda n, Shuko Shimamori, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980–8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2011 Received in revised form 11 October 2011 Accepted 14 October 2011 Available online 20 October 2011
Montmorillonite (MT) modified with tetraphenylphosphonium (TPP MT) had specific uptake behavior for aromatics in aqueous solution. This is attributed to the extent of p–p stacking interactions between the benzene rings of intercalated TPP þ and the benzene rings of aromatics with different electronic states. The uptake order of aromatics by TPP MT was in contrast to that by layered double hydroxide (LDH) intercalated with 2,7-naphthalene disulfonate. The selective uptake of target aromatic compounds from aqueous solution can be achieved by combining appropriate inorganic layered compounds and modified aromatic ions. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis
1. Introduction Montmorillonite (MT) is a cation-exchange-layered compound expressed by the general formula M0.33(Al1.67Mg0.33)Si4O10(OH)2 nH2O, where M represents the interlayer cation [1,2]. MT modified with an organic cation (Organo-MT) has been used to take up nonionic organic compounds (NOCs) from aqueous solution [3–6]. For example, MT modified with hexadecyltrimethylammonium is superior to natural MT for removing perfluorinated compounds from aqueous solution. MTs modified with polydiallyldimethylammonium and cetyltrimethylammonium have high affinities for phenol. Organo-MT is also able to take up nitroaromatic compounds and organochloride pesticide. However, Organo-MT has not been shown to take up NOCs specifically from an aqueous solution using the characteristics of modified organic cation. The novel aspect of this study is that the Organo-MT utilizes changes in the electron richness of aromatic rings in the intercalated organic cations and NOCs in the solution to achieve specific uptake. Aromatic rings are activated and deactivated, with respect to electrophiles, by electron-donating and withdrawing substituents, respectively [7]. Activated and deactivated systems interact strongly, generating a p–p stacked geometry [7]. Based on this theory, MT modified with aromatic cations is expected to specifically take up nonionic aromatic compounds from aqueous solution due to strong electronic attractions between the aromatic rings. In this study, we examined the uptake of six aromatic compounds containing different functional groups from an aqueous solution using MT intercalated with tetraphenylphosphonium (TPP þ ) (Fig.1(a)) containing aromatic hydrocarbons. To demonstrate
n
Corresponding author. Tel./fax: þ 81 22 795 7212. E-mail address: [email protected] (T. Kameda).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.10.018
the effect of the interlayer cation, MT intercalated with an aliphatic hydrocarbon, dodecyltrimethylammonium (DTMA þ ) (Fig.1(b)), was also examined as a reference material.
2. Experimental 2.1. Preparation MT (Product name: Kunipia F) with a cation-exchange capacity (CEC) of 115 cmol/kg was supplied by Kunimine Industries Co., Ltd., Japan. TPP þ -intercalated MT (TPP MT) and DTMA þ -intercalated MT (DTMA MT) were prepared by stirring MT in TPP and DTMA solutions, respectively. 17.4 g MT was added to 200 mL of TPP and DTMA solutions (the concentration: 0.05 M, 0.1 M) at 60 1C with gentle agitation. In this case, the molar ratios of TPP þ or DTMA þ to CEC in MT are 0.5 and 1.0 for 0.05 M and 0.1 M concentrations, respectively. The resulting suspensions were allowed to settle at 60 1C for 2 h. The TPP MT and DTMA MT particles were recovered by filtration, repeatedly washed with deionized water, and then dried under reduced pressure (133 Pa) at 40 1C for 40 h. It is hereafter named as TPP0.05 M MT, TPP0.1 M MT, DTMA0.05 M MT, and DTMA0.1 M MT, which were derived from the preparation concentration. 2.2. Uptake of aromatic compound from aqueous solution The six aromatic compounds were 1,2-dimethoxybenzene (DMB), N,N-dimethylaniline (DMA), anisole (AS), benzaldehyde (BA), nitrobenzene (NB), and 1,3-dinitrobenzene (DNB). Singlecompound solutions were prepared at 0.1 mM for each aromatic compound. Mixed solution I contained 0.1 mM DNB, BA, and
T. Kameda et al. / Journal of Physics and Chemistry of Solids 73 (2012) 120–123
121
CH3 +
130, 200
006
(a)
d001 = 18.7 Å
CH3
N
CH3
(b)
005
002
H3C
Montmorillonite
110, 020
003
P+
001
5000 cps
d001 = 12.7 Å
Fig. 1. Chemical structures of (a) TPP þ and (b) DTMA þ .
DMB. Mixed solution II contained 0.1 mM DNB, NB, BA, AS, DMA, and DMB. TPP MT or DTMA MT was added to 50-mL Erlenmeyer flasks containing 20 mL of each single-compound solution and mixed compound solutions I and II. For the single-compound solutions, the molar ratio (A) of intercalated TPP þ and DTMA þ to each aromatic compound was 50 or 100. For the mixed compound solutions I and II, the molar ratio (B) of intercalated TPP þ to total aromatic compound was 50. The flasks were shaken at 25 1C for 4 h, which was attained to equilibrium. The resulting suspensions were filtered, and the filtrate was analyzed for aromatic compounds.
d001 = 18.8 Å
(c)
d001 = 13.9 Å
2.3. Characterization methods X-ray diffraction (XRD) data for TPP MT and DTMA MT were acquired using a diffractometer (RINT 2200; Rigaku, Tokyo, Japan) (CuKa radiation, 40 kV, 20 mA, 21 min 1 scan rate). In order to know the intercalation degree of TPP þ in MT, the concentration of TPP þ in the filtrate obtained by the preparation of TPP MT was determined by high-performance liquid chromatography (HPLC) at a wavelength of 200 nm, with an error of 0.6%. In order to know the intercalation degree of DTMA þ in MT, the concentration of DTMA þ in the filtrate obtained by the preparation of DTMA MT was determined by total organic carbon (TOC), with an error of 0.02%. For each adsorption experiment, the concentrations of the aromatic compounds in the filtrate were determined by HPLC at a wavelength of 200 nm for DMB, DMA, and AS, 204 nm for BA, 265 nm for NB, and 235 nm for DNB with an error of 0.6%.
(d)
d001 = 17.6 Å
2.4. Theoretical calculations The molecular geometry of isolated TPP þ , DTMA þ , DMB, DMA, AS, BA, NB, and DNB in the ground state was calculated using the ab initio Hartree–Fock method with an STO-3G basis set in Gaussian 03 [8].
3. Results and discussion 3.1. Preparation Fig. 2 shows the XRD patterns for (a) MT, (b) TPP0.05 M MT, (c) TPP0.1 M MT, (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT. The basal spacing (d001) of MT increased from 12.7 A˚ to 18.2 A˚ and 18.8 A˚ for (b) TPP0.05 M MT and (c) TPP0.1 M MT, respectively, confirming that TPP þ was intercalated in the interlayer of MT. In ˚ Thus, the this case, the thickness of MT host layer is 9.6 A. interlayer spacing is calculated to be 8.6 and 9.2 A˚ for TPP0.05 M MT and TPP0.1 M MT, respectively. The molecular
(e)
0
10
20 2θ/deg. (CuKα)
30
40
Fig. 2. XRD patterns for (a) MT, (b) TPP0.05 M MT, (c) TPP0.1 M MT, (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT.
˚ Therefore, TPP þ is length of TPP þ is calculated to be 10.8 A. considered to be inclined at the angle of 531 and 581 to the host layers of MT for TPP0.05 M MT and TPP0.1 M MT, respectively. Similarly, the basal spacing (d001) of MT increased from 12.7 A˚ to 13.9 A˚ and 17.6 A˚ for (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT, respectively, confirming that DTMA þ was also intercalated in
122
T. Kameda et al. / Journal of Physics and Chemistry of Solids 73 (2012) 120–123
the interlayer of MT. The interlayer spacing is calculated to be 4.3 and 8.0 A˚ for DTMA0.05 M MT and DTMA0.1 M MT, respec˚ tively. The molecular length of DTMA þ is calculated to be 22.7 A. Therefore, DTMA þ is considered to be inclined at the angle of 111 and 211 to the host layers of MT for DTMA0.05 M MT and DTMA0.1 M MT, respectively. Table 1 shows the intercalation degree of TPP þ and DTMA þ in the interlayer of MT. The TPP0.05 M MT and TPP0.1 M MT contained 50.0 and 89.3% of TPP þ for CEC of MT, respectively. The DTMA0.05 M MT and DTMA0.1 M MT contained 49.8 and 93.2% of DTMA þ for CEC of MT, respectively. These results indicate that the expected amounts of TPP þ and DTMA þ were intercalated in the interlayer of MT. 3.2. Uptake of aromatic compound from aqueous solution Fig. 3 shows the uptake of DMB and DNB from singlecompound solution by (a) TPP0.05 M MT, (b) TPP0.1 M MT, (c) DTMA0.05 M MT, and (d) DTMA0.1 M MT at A¼ 50. The uptake of DMB and DNB for TPP0.1 M MT was lower than that for TPP0.05 M MT, respectively. These results suggest that the larger content of TPP þ in the interlayer of MT results in the decrease of interlayer space, leading to less accommodation of DMB and DNB. For DTMA MTs, the uptake of DMB and DNB was almost similar, and low. In addition, the uptake of DMB was larger than that of DNB for TPP0.05 M MT. These results will be examined and discussed in the following paragraph. Table 2 shows the uptake of each aromatic compound from single-compound solutions by TPP0.05 M MT, DTMA0.05 M MT, and NDS LDH at A ¼50 or 100. In this case, LDH means layered double hydroxides, which have anion exchange properties and are able to intercalate a variety of anions in the interlayer [9–11]. The result for NDS LDH, which had 2,7-naphthalene disulfonate (NDS2 ) intercalated in the interlayer of LDH, was cited from Refs. [12,13]. Recently, we showed that uptake by NDS LDH was high in the following order: DNB4NB4BA 4AS4 DMA4DMB [12,13]. The benzene rings of aromatics are electron rich in the Table 1 Intercalation degree of TPP þ and DTMA þ in the interlayer of MT. TPP0.05 M MT TPP0.1 M MT DTMA0.05 M MT DTMS0.1 M MT Intercalation degree/%
50.0
89.3
49.8
93.2
100 DMB
Uptake / %
80
DNB
60
40
20
0 a
b
c
d
Fig. 3. Uptake of DMB and DNB from single-compound solution by (a) TPP0.05 M MT, (b) TPP0.1 M MT, (c) DTMA0.05 M MT, and (d) DTMA0.1 M MT at A* ¼50. *A means the molar ratio of intercalated ion in MT to DMB and DNB.
Table 2 Uptake of each aromatic compound from single-compound solutions by TPP0.05 M MT, DTMA0.05 M MT, and NDS LDH at Aa ¼ 50 or 100. Material
TPP0.05 M MT(A ¼50) TPP0.05 M MT(A ¼100) DTMA0.05 M MT(A¼ 50) NDS LDHb(A ¼ 50)
Uptake/% DNB
NB
BA
AS
DMA
DMB
43.1 63.0 17.3 84.7
70.2 81.1 32.9 56.5
92.0 95.0 27.1 46.4
80.6 95.0 2.5 28.3
94.8 93.6 60.0 20.4
91.8 97.4 5.2 14.2
a A means the molar ratio of intercalated ion in MT or LDH to each aromatic compound. b Cited from Refs. [12,13].
following order: DMB4 DMA4AS 4BA4NB 4DNB. The electron richness of the NDS2 naphthalene ring is due to the two-SO3 groups, which are weak electron-withdrawing groups. It may be that the negatively-charged SO3 donates electrons to the naphthalene ring. Thus, the specific uptake was caused by the p–p stacking interactions between the electron-rich naphthalene ring of NDS2 intercalated into the interlayer of NDS LDH and the electron-poor benzene rings of aromatics. That is, the electron-poorer benzene rings of aromatics generate stronger interactions with the electron-richer naphthalene ring of intercalated NDS2 , resulting in greater uptake of aromatics by NDS LDH. The data in Table 2 show that the uptake of BA, AS, DMA, and DMB by TPP0.05 M MT at A¼50 was greater than 80%. The uptake for NB was lower, 70.2%, and the lowest uptake was 43.1% for DNB. The increase of A from 50 to 100 resulted in an increase in the uptake of aromatic compounds. The uptake of BA, AS, DMA, and DMB by TPP0.05 M MT at A ¼100 was greater than 90% but that of DNB and NB did not reach 90%. These results indicate that TPP0.05 M MT took up aromatic compounds preferentially in the following order: DMBEDMA EAS EBA4NB 4DNB. In contrast, the uptake by DTMA0.05 M MT was random, and uptake was independent of the type of aromatic compound. This random uptake was due to the hydrophobic interaction between the alkyl groups of DTMA þ intercalated in the interlayer of MT and the benzene ring of the compound. Therefore, the specific uptake by TPP0.05 M MT is attributable to the p–p stacking interactions between the benzene ring of the compound and the benzene rings of TPP þ intercalated in the interlayer of MT. The electronic state of the benzene rings of intercalated TPP þ is considered as follows. TPP þ has positively charged phosphorus bonded to four benzene rings in the center of the molecular structure. It is likely that the positively charged phosphorus withdraws electrons from the benzene rings, leaving the benzene rings of TPP þ electron poor. Accordingly, preferential uptake by TPP0.05 M MT occurs in the following order: DMBEDMAEAS EBA4NB4DNB. The uptake is caused by the p–p stacking interactions between the electron-poor benzene rings of TPP þ intercalated into the interlayer of MT and the benzene rings of aromatics that are electron rich in the following order: DMB4DMA4AS 4BA 4NB4DNB. The electron-rich benzene rings of aromatics generate stronger interactions with the electron-poor benzene rings of intercalated TPP þ , resulting in greater uptake of aromatics by TPP0.05 M MT. For the uptake of aromatics by TPP0.05 M MT, the basal spacing observed by XRD did not change in the course of reactions. The molecular length of DMB, DMA, AS, BA, NB, and DNB is calculated ˚ respectively. On the other to be 8.1, 7.8, 7.8, 8.1, 8.3, and 8.5 A, hand, the interlayer spacing for TPP0.05 M MT is calculated to be ˚ Therefore, TPP0.05 M MT has sufficient accommodation 8.6 A. space for the aromatics. It can be stated that the effects of bulkiness of aromatics on the insertion reaction was little.
T. Kameda et al. / Journal of Physics and Chemistry of Solids 73 (2012) 120–123
Table 3 Uptake of multiple aromatic compounds from mixtures in aqueous solution by TPP0.05 M MT at Ba ¼ 50. Uptake/%
Mixed I Mixed II
123
solution can be achieved by combining appropriate inorganic layered compounds and modified aromatic ions.
References
DNB
NB
BA
AS
DMA
DMB
51.1 31.1
– 57.5
80.0 56.2
– 96.1
– 96.2
90.7 90.7
a B means the molar ratio of intercalated TPP þ in MT to total aromatic compound.
As shown in Fig. 3, the uptake of DMB was similar to that of DNB for TPP0.1 M MT. These results suggest that the larger content of TPP þ in the interlayer of MT results in the decrease of function of aromatic rings such as p–p stacking interaction. Due to the decrease of interlayer space, aromatic rings of TPP þ cannot work efficiently. Table 3 shows the uptake of multiple aromatic compounds from mixtures in aqueous solution by TPP0.05 M MT at B¼ 50. TPP0.05 M MT showed good uptake of aromatic compounds from mixed solution I, in the following order: DMB 4BA4DNB. The uptake of DMB was around 90%, whereas the uptake of DNB was around 50%. TPP0.05 M MT also exhibited good uptake of aromatic compounds from mixed solution II, in the following order: DMBEDMA EAS 4BAENB4DNB. These orders were almost same as that for the uptake of aromatics from the singlecompound solutions (Table 2). Thus, TPP0.05 M MT in aqueous solution containing a mixture of aromatic compounds exhibited selective uptake of the aromatic compounds with electron-rich benzene rings.
4. Conclusions TPP MT took up large amounts of aromatics from aqueous solution in the following order: DMBEDMA EAS EBA4NB4DNB. This is attributed to the extent of the p–p stacking interactions between the benzene rings of intercalated TPP þ and the benzene rings of aromatics with different electronic states. The uptake order of aromatics by TPP MT was in contrast to that by NDS LDH. The intercalation of aromatic cations in the interlayer of MT resulted in an electron-poor aromatic ring, which interacted more readily with the electron-rich rings of aromatic compounds. On the contrary, the intercalation of aromatic anions in the interlayer of LDH resulted in an electron-rich aromatic ring, which interacted more readily with the electron-poor rings of aromatic compounds [12–15]. These studies suggest that the selective uptake of target aromatic compounds from aqueous
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Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Specific uptake of aromatic compounds from aqueous solution by montmorillonite modified with tetraphenylphosphonium Tomohito Kameda n, Shuko Shimamori, Toshiaki Yoshioka Graduate School of Environmental Studies, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980–8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 September 2011 Received in revised form 11 October 2011 Accepted 14 October 2011 Available online 20 October 2011
Montmorillonite (MT) modified with tetraphenylphosphonium (TPP MT) had specific uptake behavior for aromatics in aqueous solution. This is attributed to the extent of p–p stacking interactions between the benzene rings of intercalated TPP þ and the benzene rings of aromatics with different electronic states. The uptake order of aromatics by TPP MT was in contrast to that by layered double hydroxide (LDH) intercalated with 2,7-naphthalene disulfonate. The selective uptake of target aromatic compounds from aqueous solution can be achieved by combining appropriate inorganic layered compounds and modified aromatic ions. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis
1. Introduction Montmorillonite (MT) is a cation-exchange-layered compound expressed by the general formula M0.33(Al1.67Mg0.33)Si4O10(OH)2 nH2O, where M represents the interlayer cation [1,2]. MT modified with an organic cation (Organo-MT) has been used to take up nonionic organic compounds (NOCs) from aqueous solution [3–6]. For example, MT modified with hexadecyltrimethylammonium is superior to natural MT for removing perfluorinated compounds from aqueous solution. MTs modified with polydiallyldimethylammonium and cetyltrimethylammonium have high affinities for phenol. Organo-MT is also able to take up nitroaromatic compounds and organochloride pesticide. However, Organo-MT has not been shown to take up NOCs specifically from an aqueous solution using the characteristics of modified organic cation. The novel aspect of this study is that the Organo-MT utilizes changes in the electron richness of aromatic rings in the intercalated organic cations and NOCs in the solution to achieve specific uptake. Aromatic rings are activated and deactivated, with respect to electrophiles, by electron-donating and withdrawing substituents, respectively [7]. Activated and deactivated systems interact strongly, generating a p–p stacked geometry [7]. Based on this theory, MT modified with aromatic cations is expected to specifically take up nonionic aromatic compounds from aqueous solution due to strong electronic attractions between the aromatic rings. In this study, we examined the uptake of six aromatic compounds containing different functional groups from an aqueous solution using MT intercalated with tetraphenylphosphonium (TPP þ ) (Fig.1(a)) containing aromatic hydrocarbons. To demonstrate
n
Corresponding author. Tel./fax: þ 81 22 795 7212. E-mail address: [email protected] (T. Kameda).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.10.018
the effect of the interlayer cation, MT intercalated with an aliphatic hydrocarbon, dodecyltrimethylammonium (DTMA þ ) (Fig.1(b)), was also examined as a reference material.
2. Experimental 2.1. Preparation MT (Product name: Kunipia F) with a cation-exchange capacity (CEC) of 115 cmol/kg was supplied by Kunimine Industries Co., Ltd., Japan. TPP þ -intercalated MT (TPP MT) and DTMA þ -intercalated MT (DTMA MT) were prepared by stirring MT in TPP and DTMA solutions, respectively. 17.4 g MT was added to 200 mL of TPP and DTMA solutions (the concentration: 0.05 M, 0.1 M) at 60 1C with gentle agitation. In this case, the molar ratios of TPP þ or DTMA þ to CEC in MT are 0.5 and 1.0 for 0.05 M and 0.1 M concentrations, respectively. The resulting suspensions were allowed to settle at 60 1C for 2 h. The TPP MT and DTMA MT particles were recovered by filtration, repeatedly washed with deionized water, and then dried under reduced pressure (133 Pa) at 40 1C for 40 h. It is hereafter named as TPP0.05 M MT, TPP0.1 M MT, DTMA0.05 M MT, and DTMA0.1 M MT, which were derived from the preparation concentration. 2.2. Uptake of aromatic compound from aqueous solution The six aromatic compounds were 1,2-dimethoxybenzene (DMB), N,N-dimethylaniline (DMA), anisole (AS), benzaldehyde (BA), nitrobenzene (NB), and 1,3-dinitrobenzene (DNB). Singlecompound solutions were prepared at 0.1 mM for each aromatic compound. Mixed solution I contained 0.1 mM DNB, BA, and
T. Kameda et al. / Journal of Physics and Chemistry of Solids 73 (2012) 120–123
121
CH3 +
130, 200
006
(a)
d001 = 18.7 Å
CH3
N
CH3
(b)
005
002
H3C
Montmorillonite
110, 020
003
P+
001
5000 cps
d001 = 12.7 Å
Fig. 1. Chemical structures of (a) TPP þ and (b) DTMA þ .
DMB. Mixed solution II contained 0.1 mM DNB, NB, BA, AS, DMA, and DMB. TPP MT or DTMA MT was added to 50-mL Erlenmeyer flasks containing 20 mL of each single-compound solution and mixed compound solutions I and II. For the single-compound solutions, the molar ratio (A) of intercalated TPP þ and DTMA þ to each aromatic compound was 50 or 100. For the mixed compound solutions I and II, the molar ratio (B) of intercalated TPP þ to total aromatic compound was 50. The flasks were shaken at 25 1C for 4 h, which was attained to equilibrium. The resulting suspensions were filtered, and the filtrate was analyzed for aromatic compounds.
d001 = 18.8 Å
(c)
d001 = 13.9 Å
2.3. Characterization methods X-ray diffraction (XRD) data for TPP MT and DTMA MT were acquired using a diffractometer (RINT 2200; Rigaku, Tokyo, Japan) (CuKa radiation, 40 kV, 20 mA, 21 min 1 scan rate). In order to know the intercalation degree of TPP þ in MT, the concentration of TPP þ in the filtrate obtained by the preparation of TPP MT was determined by high-performance liquid chromatography (HPLC) at a wavelength of 200 nm, with an error of 0.6%. In order to know the intercalation degree of DTMA þ in MT, the concentration of DTMA þ in the filtrate obtained by the preparation of DTMA MT was determined by total organic carbon (TOC), with an error of 0.02%. For each adsorption experiment, the concentrations of the aromatic compounds in the filtrate were determined by HPLC at a wavelength of 200 nm for DMB, DMA, and AS, 204 nm for BA, 265 nm for NB, and 235 nm for DNB with an error of 0.6%.
(d)
d001 = 17.6 Å
2.4. Theoretical calculations The molecular geometry of isolated TPP þ , DTMA þ , DMB, DMA, AS, BA, NB, and DNB in the ground state was calculated using the ab initio Hartree–Fock method with an STO-3G basis set in Gaussian 03 [8].
3. Results and discussion 3.1. Preparation Fig. 2 shows the XRD patterns for (a) MT, (b) TPP0.05 M MT, (c) TPP0.1 M MT, (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT. The basal spacing (d001) of MT increased from 12.7 A˚ to 18.2 A˚ and 18.8 A˚ for (b) TPP0.05 M MT and (c) TPP0.1 M MT, respectively, confirming that TPP þ was intercalated in the interlayer of MT. In ˚ Thus, the this case, the thickness of MT host layer is 9.6 A. interlayer spacing is calculated to be 8.6 and 9.2 A˚ for TPP0.05 M MT and TPP0.1 M MT, respectively. The molecular
(e)
0
10
20 2θ/deg. (CuKα)
30
40
Fig. 2. XRD patterns for (a) MT, (b) TPP0.05 M MT, (c) TPP0.1 M MT, (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT.
˚ Therefore, TPP þ is length of TPP þ is calculated to be 10.8 A. considered to be inclined at the angle of 531 and 581 to the host layers of MT for TPP0.05 M MT and TPP0.1 M MT, respectively. Similarly, the basal spacing (d001) of MT increased from 12.7 A˚ to 13.9 A˚ and 17.6 A˚ for (d) DTMA0.05 M MT, and (e) DTMA0.1 M MT, respectively, confirming that DTMA þ was also intercalated in
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the interlayer of MT. The interlayer spacing is calculated to be 4.3 and 8.0 A˚ for DTMA0.05 M MT and DTMA0.1 M MT, respec˚ tively. The molecular length of DTMA þ is calculated to be 22.7 A. Therefore, DTMA þ is considered to be inclined at the angle of 111 and 211 to the host layers of MT for DTMA0.05 M MT and DTMA0.1 M MT, respectively. Table 1 shows the intercalation degree of TPP þ and DTMA þ in the interlayer of MT. The TPP0.05 M MT and TPP0.1 M MT contained 50.0 and 89.3% of TPP þ for CEC of MT, respectively. The DTMA0.05 M MT and DTMA0.1 M MT contained 49.8 and 93.2% of DTMA þ for CEC of MT, respectively. These results indicate that the expected amounts of TPP þ and DTMA þ were intercalated in the interlayer of MT. 3.2. Uptake of aromatic compound from aqueous solution Fig. 3 shows the uptake of DMB and DNB from singlecompound solution by (a) TPP0.05 M MT, (b) TPP0.1 M MT, (c) DTMA0.05 M MT, and (d) DTMA0.1 M MT at A¼ 50. The uptake of DMB and DNB for TPP0.1 M MT was lower than that for TPP0.05 M MT, respectively. These results suggest that the larger content of TPP þ in the interlayer of MT results in the decrease of interlayer space, leading to less accommodation of DMB and DNB. For DTMA MTs, the uptake of DMB and DNB was almost similar, and low. In addition, the uptake of DMB was larger than that of DNB for TPP0.05 M MT. These results will be examined and discussed in the following paragraph. Table 2 shows the uptake of each aromatic compound from single-compound solutions by TPP0.05 M MT, DTMA0.05 M MT, and NDS LDH at A ¼50 or 100. In this case, LDH means layered double hydroxides, which have anion exchange properties and are able to intercalate a variety of anions in the interlayer [9–11]. The result for NDS LDH, which had 2,7-naphthalene disulfonate (NDS2 ) intercalated in the interlayer of LDH, was cited from Refs. [12,13]. Recently, we showed that uptake by NDS LDH was high in the following order: DNB4NB4BA 4AS4 DMA4DMB [12,13]. The benzene rings of aromatics are electron rich in the Table 1 Intercalation degree of TPP þ and DTMA þ in the interlayer of MT. TPP0.05 M MT TPP0.1 M MT DTMA0.05 M MT DTMS0.1 M MT Intercalation degree/%
50.0
89.3
49.8
93.2
100 DMB
Uptake / %
80
DNB
60
40
20
0 a
b
c
d
Fig. 3. Uptake of DMB and DNB from single-compound solution by (a) TPP0.05 M MT, (b) TPP0.1 M MT, (c) DTMA0.05 M MT, and (d) DTMA0.1 M MT at A* ¼50. *A means the molar ratio of intercalated ion in MT to DMB and DNB.
Table 2 Uptake of each aromatic compound from single-compound solutions by TPP0.05 M MT, DTMA0.05 M MT, and NDS LDH at Aa ¼ 50 or 100. Material
TPP0.05 M MT(A ¼50) TPP0.05 M MT(A ¼100) DTMA0.05 M MT(A¼ 50) NDS LDHb(A ¼ 50)
Uptake/% DNB
NB
BA
AS
DMA
DMB
43.1 63.0 17.3 84.7
70.2 81.1 32.9 56.5
92.0 95.0 27.1 46.4
80.6 95.0 2.5 28.3
94.8 93.6 60.0 20.4
91.8 97.4 5.2 14.2
a A means the molar ratio of intercalated ion in MT or LDH to each aromatic compound. b Cited from Refs. [12,13].
following order: DMB4 DMA4AS 4BA4NB 4DNB. The electron richness of the NDS2 naphthalene ring is due to the two-SO3 groups, which are weak electron-withdrawing groups. It may be that the negatively-charged SO3 donates electrons to the naphthalene ring. Thus, the specific uptake was caused by the p–p stacking interactions between the electron-rich naphthalene ring of NDS2 intercalated into the interlayer of NDS LDH and the electron-poor benzene rings of aromatics. That is, the electron-poorer benzene rings of aromatics generate stronger interactions with the electron-richer naphthalene ring of intercalated NDS2 , resulting in greater uptake of aromatics by NDS LDH. The data in Table 2 show that the uptake of BA, AS, DMA, and DMB by TPP0.05 M MT at A¼50 was greater than 80%. The uptake for NB was lower, 70.2%, and the lowest uptake was 43.1% for DNB. The increase of A from 50 to 100 resulted in an increase in the uptake of aromatic compounds. The uptake of BA, AS, DMA, and DMB by TPP0.05 M MT at A ¼100 was greater than 90% but that of DNB and NB did not reach 90%. These results indicate that TPP0.05 M MT took up aromatic compounds preferentially in the following order: DMBEDMA EAS EBA4NB 4DNB. In contrast, the uptake by DTMA0.05 M MT was random, and uptake was independent of the type of aromatic compound. This random uptake was due to the hydrophobic interaction between the alkyl groups of DTMA þ intercalated in the interlayer of MT and the benzene ring of the compound. Therefore, the specific uptake by TPP0.05 M MT is attributable to the p–p stacking interactions between the benzene ring of the compound and the benzene rings of TPP þ intercalated in the interlayer of MT. The electronic state of the benzene rings of intercalated TPP þ is considered as follows. TPP þ has positively charged phosphorus bonded to four benzene rings in the center of the molecular structure. It is likely that the positively charged phosphorus withdraws electrons from the benzene rings, leaving the benzene rings of TPP þ electron poor. Accordingly, preferential uptake by TPP0.05 M MT occurs in the following order: DMBEDMAEAS EBA4NB4DNB. The uptake is caused by the p–p stacking interactions between the electron-poor benzene rings of TPP þ intercalated into the interlayer of MT and the benzene rings of aromatics that are electron rich in the following order: DMB4DMA4AS 4BA 4NB4DNB. The electron-rich benzene rings of aromatics generate stronger interactions with the electron-poor benzene rings of intercalated TPP þ , resulting in greater uptake of aromatics by TPP0.05 M MT. For the uptake of aromatics by TPP0.05 M MT, the basal spacing observed by XRD did not change in the course of reactions. The molecular length of DMB, DMA, AS, BA, NB, and DNB is calculated ˚ respectively. On the other to be 8.1, 7.8, 7.8, 8.1, 8.3, and 8.5 A, hand, the interlayer spacing for TPP0.05 M MT is calculated to be ˚ Therefore, TPP0.05 M MT has sufficient accommodation 8.6 A. space for the aromatics. It can be stated that the effects of bulkiness of aromatics on the insertion reaction was little.
T. Kameda et al. / Journal of Physics and Chemistry of Solids 73 (2012) 120–123
Table 3 Uptake of multiple aromatic compounds from mixtures in aqueous solution by TPP0.05 M MT at Ba ¼ 50. Uptake/%
Mixed I Mixed II
123
solution can be achieved by combining appropriate inorganic layered compounds and modified aromatic ions.
References
DNB
NB
BA
AS
DMA
DMB
51.1 31.1
– 57.5
80.0 56.2
– 96.1
– 96.2
90.7 90.7
a B means the molar ratio of intercalated TPP þ in MT to total aromatic compound.
As shown in Fig. 3, the uptake of DMB was similar to that of DNB for TPP0.1 M MT. These results suggest that the larger content of TPP þ in the interlayer of MT results in the decrease of function of aromatic rings such as p–p stacking interaction. Due to the decrease of interlayer space, aromatic rings of TPP þ cannot work efficiently. Table 3 shows the uptake of multiple aromatic compounds from mixtures in aqueous solution by TPP0.05 M MT at B¼ 50. TPP0.05 M MT showed good uptake of aromatic compounds from mixed solution I, in the following order: DMB 4BA4DNB. The uptake of DMB was around 90%, whereas the uptake of DNB was around 50%. TPP0.05 M MT also exhibited good uptake of aromatic compounds from mixed solution II, in the following order: DMBEDMA EAS 4BAENB4DNB. These orders were almost same as that for the uptake of aromatics from the singlecompound solutions (Table 2). Thus, TPP0.05 M MT in aqueous solution containing a mixture of aromatic compounds exhibited selective uptake of the aromatic compounds with electron-rich benzene rings.
4. Conclusions TPP MT took up large amounts of aromatics from aqueous solution in the following order: DMBEDMA EAS EBA4NB4DNB. This is attributed to the extent of the p–p stacking interactions between the benzene rings of intercalated TPP þ and the benzene rings of aromatics with different electronic states. The uptake order of aromatics by TPP MT was in contrast to that by NDS LDH. The intercalation of aromatic cations in the interlayer of MT resulted in an electron-poor aromatic ring, which interacted more readily with the electron-rich rings of aromatic compounds. On the contrary, the intercalation of aromatic anions in the interlayer of LDH resulted in an electron-rich aromatic ring, which interacted more readily with the electron-poor rings of aromatic compounds [12–15]. These studies suggest that the selective uptake of target aromatic compounds from aqueous
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