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Intercalation of Gaseous Thiols and Sulfides into Ag+ Ion-Exchanged Aluminum Dihydrogen Triphosphate Aki Hayashi, Hiroki Saimen, Nobuaki Watanabe, Hitomi Kimura, Ayumi Kobayashi, Hirokazu Nakayama,* and Mitsutomo Tsuhako Department of Functional Molecular Chemistry, Kobe Pharmaceutical University, Kobe, Hyogo 658-8558 Japan Received February 28, 2005. In Final Form: May 19, 2005 Ag+ ion-exchanged layered aluminum dihydrogen triphosphate (AlP) with the interlayer distance of 0.85 nm was synthesized by the ion-exchange of proton in triphosphate with Ag+ ion. The amount of exchanged Ag+ ion depended on the concentration of AgNO3 aqueous solution. Ag+ ion-exchanged AlP adsorbed gaseous thiols and sulfides into the interlayer region. The adsorption amounts of thiols were more than those of sulfides, thiols with one mercapto group > thiol with two mercapto groups > sulfides, and depended on the amount of exchanged Ag+ ion in the interlayer region. The thiols with one mercapto group were intercalated to expand the interlayer distance of Ag+ ion-exchanged AlP, whereas there was no expansion in the adsorption of sulfide. In the case of thiol with two mercapto groups, there was observed contraction of the interlayer distance through the bridging with Ag+ ions of the upper and lower sides of the interlayer region.
1. Introduction Layered phosphates are known as inorganic ionexchangers and host compounds for intercalation chemistry.1,2 There are three types of layered phosphates, that is, R-form (R-M(HPO4)2‚H2O), γ-form (γ-M(PO4)(H2PO4)‚ 2H2O, M ) Zr, Ti, Hf, Pb, Sn), and AlH2P3O10‚2H2O (abbreviated as AlP).1-3 AlH2P3O10 has two kinds of crystal polymorphism (types I and II).3 Type I of AlH2P3O10 has a layered structure and easily changes to AlH2P3O10‚2H2O (AlP) by the absorption of water in the air. In contrast to this, type II of AlH2P3O10 has a nonlayered structure and is nonhygroscopic. AlP is expected to be the host compound of new functional materials commercially because it can be industrially synthesized by the reaction of R-Al2O3 or Al(OH)3 with phosphoric acid.4 According to a previous paper,5 the solubility, density, and surface area of AlP were 0.010 g dm-3 H2O, 2.31 g cm-3, and 5.2 m2 g-1, respectively. The proposed model structure is shown in Figure 1.6 Its interlayer distance is 0.79 nm and protons of end phosphate groups in triphosphate can be simply exchanged with monovalent cations such as Na+, K+, and NH4+.3 Furthermore, it could intercalate basic organic compounds such as n-alkylamine, trimethylamine, aniline, and R,ω-alkanediamine5-7 because AlP is a solid acid with the acid strength of +1.5.5 Previously, we reported that ammonium ion-exchanged AlP could adsorb harmful formaldehyde and carboxylic acid gases.8 Ammonium ion-exchanged AlP was synthe* Author to whom correspondence should be addressed. Phone: +81-78-441-7552; fax: +81-78-441-7553; e-mail: hiro@ kobepharma-u.ac.jp. (1) Clearfield, A., Ed. Inorganic Ion Exchange Materials; CRC: Boca Raton, FL, 1982; pp 1-48. (2) Intercalation Chemistry; Alberti, G., Costantino, U., Eds.; Academic Press: New York, 1982; pp 147-180. (3) Tsuhako, M. Kagaku no Ryoiki 1974, 28, 31-40. (4) Tsuhako, M.; Hasegawa, K.; Matsuo, T.; Motooka, I.; Kobayashi, M. Bull. Chem. Soc. Jpn. 1975, 48, 1830-1835. (5) Tsuhako, M.; Nariai, H.; Motooka, I.; Kobayashi, M. Nippon Kagaku Kaishi 1982, 4, 590-594. (6) Hayashi, A.; Nakayama, H.; Tsuhako, M.; Eguchi, T.; Nakamura, N. J. Inclusion Phenom. Macrocyclic Chem. 1999, 34, 401-412. (7) Tsuhako, M.; Kawamoto, K.; Danjo, M.; Baba, Y.; Murakami, M.; Nariai, H.; Motooka, I. Nippon Kagaku Kaishi 1992, 9, 944-950.
Figure 1. The proposed structure of AlH2P3O10‚2H2O (AlP).6
sized by the ion-exchange of ammonium ion with proton in the interlayer region of AlP. The functionalization or modification of the interlayer region enables the use of layered phosphates as a new adsorbent,8-14 pillared compound for the reaction field and catalyst.15-17 For (8) Hayashi, A.; Yamamoto, Y.; Kouzuma, K.; Nakayama, H.; Tsuhako, M. Phosphorus, Sulfur Silicon Relat. Elem. 2002, 177, 19131914. (9) Danjo, M.; Hayashi, A.; Nakayama, H.; Kimira, Y.; Shimizu, T.; Mizuguchi, Y.; Yagita, Y.; Tsuhako, M.; Nariai, H.; Motooka, I. Bull. Chem. Soc. Jpn. 1999, 72, 2079-2084. (10) Hayashi, A.; Nakayama, H.; Eguchi, T.; Nakamura, N.; Tsuhako, M. Mol. Cryst. Liq. Cryst. 2000, 341, 573-578. (11) Nakayama, H.; Hayashi, A.; Eguchi, T.; Nakamura, N.; Tsuhako, M. Solid State Sci. 2002, 4, 1067-1070. (12) Nakayama, H.; Hayashi, A.; Eguchi, T.; Nakamura, N.; Tsuhako, M. J. Mater. Chem. 2002, 12, 3093-3099. (13) Hayashi, A.; Nakayama, H.; Tsuhako, M. Bull. Chem. Soc. Jpn. 2002, 75, 1991-1996. (14) Hayashi, A.; Nakayama, H.; Tsuhako, M. Bull. Chem. Soc. Jpn. 2003, 76, 2315-2319.
10.1021/la050527v CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005
Intercalation of Gaseous Thiols and Sulfides
example, diethylenetriamine-intercalated and ammoniumexchanged R-ZrPs could be proved to be good adsorbents of formaldehyde and carboxylic acid gases,9-12 and 2-aminoethanethiol-intercalated γ-TiP could adsorb heavy metal ions such as Cd2+, Hg2+, Pb2+, and Cr3+ in wastewater.13 More recently, n-alkylamine-intercalated R-ZrP was verified to adsorb phenols by the hydrophobic interaction between the pillared alkylamine and the aromatic ring of phenols.14 Many thiols are the volatile liquids with low boiling points, and sulfide has a characteristic smell. Also, thiols and sulfides are contained in specific bad smell substances. On the other hand, it is known that Ag+ ion has a strong affinity for the mercapto group to form silver sulfide. In the present work, we synthesized Ag+ ion-exchanged AlP by the ion-exchange with the proton of triphosphate, aiming for the development of good adsorbent of gaseous thiols and sulfides. The adsorption property and its mechanism were investigated by X-ray diffraction, solidstate NMR, and elemental analysis. 2. Experimental Section Chemicals. Aluminum dihydrogen triphosphate dihydrate (AlH2P3O10‚2H2O; abbreviated as AlP) was prepared according to a previous paper.4 Silver nitrate, ethanethiol (HSC2H5), 1,2ethanedithiol (HSC2H4SH), 2-mercaptoethanol (HSC2H4OH), dimethyl sulfide ((CH3)2S), dimethyl disulfide (CH3SSCH3), and allyl isothiocyanate (CH2dCHCH2NCS) were guaranteed reagents from Wako Pure Chemical Industries Ltd. 3-Mercaptopropionic acid (HSC2H4COOH) was purchased from Dojin Chemical Ltd. Alliin and allicin were obtained from LKT Labs, Inc. and were used without further purification. Nitrogen gases with hydrogen sulfide (H2S) (concentration of 97 ppm) and methanethiol (CH3SH) (concentration of 94 ppm) were purchased from Sumitomo Seika. Ion-Exchange Procedure. AlP (1.0 g) was suspended in 0.10 dm3 of 10-100 mmol dm-3 AgNO3 to exchange the proton of the triphosphate group with the Ag+ ion, and the suspension was stirred at room temperature for 5 h under shading the light. The obtained compound (Ag+ ion-exchanged AlP) was filtered, washed with distilled water, and then dried in air under shading the light. The amount of exchanged Ag+ ion was determined by using the supernatant solution after equilibrium. Adsorption of Gaseous Thiols and Sulfides. Two methods were adopted for the adsorption of gaseous thiols and sulfides according to their vapor pressures. In the first method, ppm order of hydrogen sulfide and methanethiol in nitrogen gas were adsorbed by the use of a gas bag for 0.010-0.030 g of Ag+ ionexchanged AlP because of their high vapor pressure (their boiling points were -60.3 and 6.0 °C, respectively). The adsorption amount was determined by using a detector tube. In the second method, the adsorption in a closed glass device was employed for the other thiols and sulfides.12 Their adsorptions using 0.30 g of Ag+ ion-exchanged AlP were carried out at 40 °C for thiols and dimethyl disulfide, except for ethanethiol and dimethyl sulfide at room temperature. The adsorption was monitored by the increase in weight (%) of Ag+ ion-exchanged AlP with time, and the final adsorption amounts (mmol) were estimated from elemental analysis of carbon. Before analysis, adhesional adsorbates (thiols and sulfides) were removed by the evacuation for 5 h. Analytical Procedure. Powder X-ray diffraction patterns were recorded on a Rigaku Denki Rint 2000 diffractometer with Ni-filtered Cu KR radiation and 2θ angle ranging from 2 to 20°. Chemical analyses of Ag+ ion and carbon were performed using a gravimetric analysis of silver chloride of supernatant solution and elemental analysis of the compound after the adsorption by (15) Kanzaki, Y.; Abe, M. Bull. Chem. Soc. Jpn. 1991, 64, 22922294. (16) Bellezza, F.; Cipiciani, A.; Costantino, U.; Negozio, M. E. Langmuir 2002, 18, 8737-8742. (17) Geng, L.; Li, N.; Xiang, M.; Wen, X.; Xu, D.; Zhao, F.; Li K. Colloids Surf., B 2003, 30, 99-109.
Langmuir, Vol. 21, No. 16, 2005 7239 Table 1. Characteristics of Ag+ Ion-Exchanged AlP amount of exchanged substitutional d (nm) ratio of (relative sample Ag+ ion no. (mmol/g‚AlP) Ag+ ion (%) intensity) AlP 1 2
0.0 0.7 2.6
0 10 38
3
5.0
73
4
6.0
87
0.79 (100) 0.79 (100) 0.79 (100) 0.85 (26) 0.79 (16) 0.85 (100) 0.85 (100)
δend (31P) (ppm) -14.9 -14.8
-20.9 -20.5 -20.4
-14.7
-20.4
-22.7 -22.4 -22.2
-14.9
Sumigraph NC-80, respectively. 31P MAS NMR measurement was carried out using JEOL GX-270W spectrometer with a recycle delay of 20 s, accumulation of 8 FID signals, and magic-angle spinning rate of 4.00 kHz. 13C CP/MAS NMR spectrum was measured using a recycle time of 5 s, pulse width of 2 ms, and magic-angle spinning rate of 6.00 kHz at 125.7 MHz with a Varian Inova UNITY-500.
3. Results and Discussion I. Synthesis of Ag+ Ion-Exchanged AlP. Ag+ ionexchanged AlP was obtained by the cation-exchange reaction of proton in triphosphate group with Ag+ ion. In practice, it was synthesized by suspending AlP in silver nitrate (AgNO3) aqueous solution with various concentrations for 5 h. Table 1 summarizes the amount of exchanged Ag+ ion, the substitutional ratio of Ag+ ion, the interlayer distance (relative intensity) of the XRD pattern, and the 31 P chemical shift of AlP for four samples with different Ag+ ion loading. The crystallinity of obtained Ag+ ionexchanged AlP was slightly inferior to the host AlP, but it was found from the XRD pattern that the layered structure with the interlayer distance of 0.85 nm was held. The amount of exchanged Ag+ ion increased with the concentration of Ag+ ion in aqueous solution and then reached a maximum amount of 6.0 mmol g-1. Because the theoretical cation-exchange capacity of AlP is 6.87 mmol g-1, its substitutional ratio of Ag+ ion corresponds to 87%. In this work, four samples with different amounts of exchanged Ag+ ion were obtained as shown in Table 1 and were abbreviated as samples 1-4. The relative intensity of Ag+ ion-exchanged AlP with the interlayer distance of 0.85 nm grew with increasing the substitutional ratio of Ag+ ion, indicating that Ag+ ion was uptaken into the interlayer region of AlP. 31P MAS NMR spectrum was measured to investigate the triphosphate in AlP, which is composed of two end phosphate groups (δend) and a middle phosphate group (δmiddle) as shown in Figure 1.6 The peaks of two end phosphate groups (δend) are observed at -20.9 and -22.7 ppm, and a peak of middle phosphate group (δmiddle) is at -32.5 ppm in the spectrum of host AlP (Figure 2a).6 A peak with a dotted line around -13.5 ppm is due to impurities occurring in synthesis. It means that AlP has two crystallographically inequivalent end phosphate groups. These two peaks of AlP (δend) shifted downfield (-14.9 ppm) by the substitution of Ag+ ion, contrary to a peak (δmiddle) with no shift (Figure 2e). In general, the downfield shift represents the deprotonation of the proton in the end phosphate group.6 Therefore, it was confirmed that the proton of the end phosphate group in triphosphate exchanged with the Ag+ ion. The peaks of host AlP disappeared at a maximum substitutional ratio of the Ag+ ion (sample 4), suggesting that the ion-exchange reaction is almost complete (Table 1). XRD and 31P MAS NMR data showed that samples 1-3 were the mixture of host AlP and Ag+ ion-exchanged AlP. These samples (samples 1-4) were used as the adsorbent of gaseous thiols and sulfides.
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Figure 4. Adsorption of thiols and sulfides by Ag+ ionexchanged AlP (sample 4). O, HSC2H5; 2, HSC2H4OH; [, HSC2H4SH; 0, CH3SSCH3. Table 2. Maximum Adsorption Amounts of Thiols and Sulfides by Sample 4 adsorption amount (mmol/g‚AlP) 31
+
Figure 2. P MAS NMR spectra of AlP and Ag ion-exchanged AlP (sample 4) before and after the adsorption of thiols. * shows spinning sideband peak. (a) AlP, (b) HSC2H4SH, (c) HSC2H4OH, (d) HSC2H5, (e) before (sample 4).
Figure 3. Adsorption of (A) methanethiol and (B) hydrogen sulfide by Ag+ ion-exchanged AlP. [, AlP; 4, sample 1; 0, sample 2; 1, sample 3; O, sample 4.
II. Adsorption of Methanethiol and Hydrogen Sulfide. Methanethiol and hydrogen sulfide gases exist as a putrefied smell in our usual life and evolve a very strong smell. Figure 3 shows the adsorption ratio (%) of methanethiol (concentration of 94 ppm) and hydrogen sulfide (concentration of 97 ppm) as a function of adsorption time by the gas bag method. The Ag+ ion-exchanged AlP adsorbed both gases rapidly, whereas the host AlP did not adsorb them at all. 0.030 g of samples 2-4 could completely adsorb methanethiol within 1 min (Figure 3A). Comparing the adsorption ratio at 10 s, it increased with the amount of exchanged Ag+ ion. Also, hydrogen sulfide could be completely adsorbed by only 0.010 g of samples 2-4 for 10 s, although it took 10 min for sample 1 (Figure 3B). Consequently, for the adsorption of these gases with low concentration, the substitutional ratio of Ag+ ion over 38% is better. After the adsorption experiment, the color of samples changed to light yellow for methanethiol and to gray for hydrogen sulfide. It may suggest that it is not
HSC2H5 HSC2H4OH HSC2H4SH (CH3)2S CH3SSCH3
5.8 5.7 1.8 0.2 0.3
the physical adsorption but the adsorption through the interaction between the mercapto group and the Ag+ ion in the interlayer region. III. Adsorption of the Other Thiols and Sulfides. The adsorption of the other thiols and sulfides, that is, ethanethiol, 1,2-ethanedithiol, 2-mercaptoethanol, 3-mercaptoproionic acid, dimethyl sulfide, and dimethyl disulfide, were performed by using a closed glass device.12 Ethanethiol and dimethyl sulfide were adsorbed at room temperature because their boiling point is ca. 35 °C, and the other thiols and dimethyl disulfide were adsorbed at 40 °C. The increase in weight (%) of sample 4 by the adsorption of these gases was plotted as a function of adsorption time in Figure 4. The increase in weight (%) reached the equilibrium for 2 weeks, except for 2-mercaptoethanol and 3-meraptopropionic acid. The colors of the samples after the adsorption changed to light brown for ethanethiol, yellow for 1,2-ethanedithiol and dimethyl disulfide, and gray for dimethyl sulfide. For comparison, the adsorption experiment of ethanol and propionic acid gases with no mercapto group were carried out in a similar manner. Their gases could not be adsorbed to the Ag+ ion-exchanged AlP, suggesting that the adsorption of thiols and sulfides are attributable to the mercapto group and the sulfide. According to HSAB (hard and soft acid and base), Ag+ ion and thiol or sulfide are classified into soft acid and base, respectively. On the other hand, alcohol and carboxylic acid are assumed as hard base. Therefore, it is suggested that Ag+ ion of soft acid has greater interaction on thiol or sulfide of soft base than alcohol and carboxylic acid of hard base. After the adsorption, the sample was evacuated to remove excess thiols and sulfides on the surface of Ag+ ion-exchanged AlP and then was characterized by XRD, solid-state NMR, and elemental analysis to investigate the adsorption mechanism. Table 2 shows the maximum adsorption amounts (mmol) of thiols and sulfides for sample 4 calculated from elemental analysis of carbon (%). The maximum adsorption amounts increased as follows: sulfides < thiol with two mercapto groups < thiols with one mercapto group (Table 2). Although the vapor pressures of thiols and sulfides examined are similar, Ag+ ion-exchanged AlP adsorbed thiols preferentially. Figure 5 shows the relationship between the adsorption amounts of thiols (mmol g-1 of AlP) (w) and the amount of exchanged
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Figure 5. The relationship between the amount of exchanged Ag+ ion in Ag+ ion-exchanged AlP (n) and adsorption amounts of thiols (w). O, HSC2H5; 2, HSC2H4OH; [, HSC2H4SH. Table 3. Interlayer Distance of Sample 4 before and after the Maximum Adsorption of Thiols and Sulfides d (nm) (relative intensity) before HSC2H5 HSC2H4OH HSC2H4COOHa HSC2H4SH (CH3)2S CH3SSCH3 a
0.85 (100) 1.07 (100) 1.07 (100) 1.30 (48) 0.85 (100) 0.77 (100) 0.85 (100) 0.85 (100)
Figure 6. Time dependence of XRD patterns of Ag+ ionexchanged AlP (sample 4) after the adsorption of 2-mercaptoethanol. (a) Before (sample 4), (b) 2, (c) 4, (d) 6, (e) 9, (f) 15, (g) 19, (h) 22 d.
No maximum adsorption.
Ag+ ion (mmol g-1 of AlP) (n). The adsorption amounts of thiols increased almost linearly with the amount of exchanged Ag+ ion, suggesting that thiols were adsorbed by the interaction with Ag+ ion. In the case of 2-mercaptoethanol and 1,2-ethanedithiol, the adsorption amount reached to equilibrium at 5 mmol of exchanged Ag+ ion per gram of AlP. The adsorption amount of ethanethiol and 2-mercaptoethanol with one mercapto group agreed with the amount of exchanged Ag+ ion; that is, the interaction of ethanethiol or 2-mercaptoethanol molecules with Ag+ ion is 1 by 1. On the other hand, in the case of 1,2-ethanedithiol with two mercapto groups, the adsorption amount was half of the other thiols. This suggests that one 1,2-ethanedithiol molecule interacted with two Ag+ ions in the interlayer region; mercapto groups at both ends interacted with Ag+ ions. The XRD measurement was performed to determine the arrangement of thiols and sulfides in the interlayer region. The interlayer distance of sample 4 expanded from 0.85 to 1.07 nm after the adsorption of ethanethiol and 2-mercaptoethanol as shown in Table 3. Figure 6 shows the adsorption time dependence of XRD patterns of sample 4 for 2-mercaptoethanol. The relative peak area of 1.07 nm phase increased with the adsorption time of 2-mercaptoethanol, and the peak of 0.85 nm disappeared after 22 d. This indicates that the expansion of interlayer distance resulted from the adsorption of 2-mercaptoethanol and that the adsorbed molecule was co-intercalated into the interlayer region of Ag+ ion-exchanged AlP. For 3-mercaptopropionic acid, the interlayer distance (1.30 nm) expanded more than those of the others because of the size effect of this molecule (Table 3). The expansion of interlayer distance after the adsorption of thiols with one mercapto group suggests that the adsorbed molecule arranges similarly to the monolayer structure of nalkylamine-intercalated AlP (Figure 7).7,18 In contrast to this, the interlayer distance decreased to 0.77 nm after the adsorption of 1,2-ethanedithiol (Table 3). This inter(18) MacLachlan, D. J.; Morgan, K. R. J. Phys. Chem. 1992, 96, 34583464.
Figure 7. The adsorption mechanism of thiols and sulfides for Ag+ ion-exchanged AlP. Table 4. 13C NMR Chemical Shift Data of Thiols in Aqueous Solution and Adsorbed into Sample 4 HSCRH2CβH3 HSCRH2CβH2OH HSCRH2CRH2SH
aqueous solution adsorbed into sample 4
CR
Cβ
22.3 34.2
21.7 21.8
CR
Cβ
28.7 66.0 31.3, 40.2 66. 7
CR 31.4 40.6
layer distance corresponds to that of the host AlP. However, 13C CP/MAS NMR spectra after the adsorption (Table 4) showed the existence of 1,2-ethanedithiol in the interlayer region. As the adsorption amount of 1,2ethanedithiol was half of the amount of exchanged Ag+ ion as mentioned in Figure 5, both ends of the mercapto groups would bridge to interact with Ag+ ions of the upper and lower sides of the interlayer region similarly to R,ωalkanediamine-intercalated AlP as shown in Figure 7.6 Thereby, the interlayer distance decreased because of the attraction of the upper and lower layers by two mercapto groups. On the other hand, the interlayer distance after the adsorption of sulfides was maintained at 0.85 nm as shown in Table 3. Because sulfur atom in sulfides exists in the center of a molecule, the adsorbed sulfide molecules would arrange parallel against the layer to interact with the Ag+ ion. It was responsible for unaltered interlayer distance and a few adsorption amounts. To examine the adsorbed thiols and sulfides in the interlayer region, 13C CP/MAS NMR was measured for sample 4 after the adsorption. The spectra after the adsorption showed the peaks corresponding to thiols and sulfides, testifying that the adsorbed thiol and sulfide
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molecules existed in the interlayer region. The 13C chemical shifts of thiols adsorbed into sample 4 are summarized in Table 4 together with those in aqueous solution. For ethanethiol and 2-mercaptoethanol, the signal of adjacent carbon CR to mercapto group shifted considerably downfield after the adsorption, whereas there was no shift of Cβ signal. A signal of 1,2-ethanedithiol also showed a downfield shift at 40.6 ppm. This downfield shift means that the mercapto group interacts with Ag+ ion such as the observation for metal-ligand interaction.19 In the case of 2-mercaptoethanol, the peak of CR split into two peaks at 31.3 and 40.2 ppm. The large difference of these chemical shifts implies that there were two types of interactions between the mercapto group and the Ag+ ion. 31P MAS NMR spectrum was measured to investigate the change of the host AlP. Figure 2 shows the 31P MAS NMR spectra of AlP and sample 4 before and after the adsorption of thiols. The peaks of the end phosphate group in the host AlP (-20.9 and -22.7 ppm) shifted to downfield (-14.9 ppm) by the exchange of proton with Ag+ ion as mentioned above (Figure 2e). This peak moved upfield around -22 ppm after the adsorption of any thiols, although a peak of unreacted Ag+ ion site (-14.9 ppm) also appeared in the case of 1,2-ethanedithiol. This fact supports the scheme that Ag+ ion interacted with the mercapto group. Its upfield shift means that the degree of deprotonation of the end phosphate group became weak. These tendencies were observed also for the adsorption of sulfides, although the degree of changes was small. These results lead to a decisive conclusion that gaseous thiols were adsorbed by the interaction with Ag+ ion in the interlayer region, the interaction of soft acid and base. Ag+ ion-exchanged AlP adsorbed gaseous thiols and sulfides into the interlayer region as shown in the schematic models of Figure 7. It was confirmed from the results of solid-state NMR that the driving force of adsorption accompanied with intercalation is the interaction of the mercapto group with the Ag+ ion. The thiols with one mercapto group, such as ethanethiol, 2-mercaptoethanol, and 3-mercaptopropionic acid, were adsorbed to expand the interlayer distance of Ag+ ionexchanged AlP. Contrary to this, 1,2-ethanedithiol caused the decrease of the interlayer distance through the bridge with the upper and lower Ag+ ions of the interlayer region by two mercapto groups. On the other hand, because sulfides were adsorbed as the parallel monolayer in the interlayer space, there were no changes of interlayer distance. IV. Adsorption of Familiar Ill-Smelling Gases. Allyl isothiocyanate (3-isothiocyanatopropene) is known as the pungent condiment ingredient of an onion and has the nature of lachrymation and sterilization (Figure 8A). On the other hand, allicin (allyl 2-propene thiosulfinate) is contained in garlic. The conversion from alliin to allicin is produced by external injury and enzymatic process by alliinase (Figure 8A). An increase in weight (%) by the adsorption of allyl isothiocyanate and allicin for sample 4 was plotted in Figure 8B. It was found that Ag+ ionexchanged AlP could adsorb these ill-smelling gases effectively. The color of sample 4 after the adsorption changed completely from white to black for allyl isothiocyanate at 20 d. It suggests that the interaction of Ag+ ion with allyl isothiocyanate is similar to thiols and sulfides. Thus, it will be expected to adsorb other isothiocyanate (19) Zelakiewicz, B. S.; Tong, Y. Mater. Res. Soc. Symp. Proc. 2003, 738, 215-219.
Hayashi et al.
Figure 8. (A) Chemical structures of allyl isothiocyanate and allicin and (B) adsorption by Ag+ ion-exchanged AlP (sample 4). 2, allyl isothiocyanate; 9, allicin.
compounds, such as 4-methylthio-3-butenyl isothiocyanate (MTBI) contained in Japanese radish and (2-isothiocyanato-ethyl)-benzene in Japanese horseradish. 4. Conclusion Ag+
ion-exchanged AlP was obtained by the exchange of proton in triphosphate group with Ag+ ion. The amounts of exchanged Ag+ ion depended on the concentration of AgNO3 aqueous solution, and the maximum uptake amount of Ag+ ion was 6.0 mmol g-1. The interlayer distance of Ag+ ion-exchanged AlP expanded to 0.85 nm. Ag+ ion-exchanged AlP could effectively adsorb the thiols and sulfides of ill-smelling gases, and the adsorption amount was in the order thiols with one mercapto group > thiol with two mercapto groups > sulfides. The adsorption amounts of thiols increased with the amount of exchanged Ag+ ion and depended on the number of mercapto group in thiols. It was confirmed from the result of solid-state NMR that the thiols and sulfides were intercalated into the interlayer region through the interaction of mercapto group or sulfide with Ag+ ion. The interlayer distance after the adsorption of thiols with one mercapto group expanded to arrange similarly to nalkylamine-intercalated AlP, whereas thiol with two mercapto groups caused the decrease of the interlayer distance because of arranging the bridged monolayer structure such as R,ω-alkanediamine-intercalated AlP. In the case of sulfides, the interlayer distance after the adsorption was the same as that before the adsorption. Furthermore, Ag+ ion-exchanged AlP could adsorb illsmelling gases such as allyl isothiocyanate and allicin. It is known that Ag+ ion exhibits the most bactericidal activity in metal ions, irrespective of gram-negative or positive bacilli.20 Ag+ ion-exchanged AlP will be expectable to use as new functional materials and will open the way for its application in the adsorbents. For example, it can recognize the number of mercapto groups and sulfides and would be used as a thiol sensor. LA050527V (20) Feng, Q. L.; Wu, J.; Chen, G. Q.; Cui, F. Z.; Kim, T. N.; Kim, J. O. J. Biomed. Mater. Res. 2000, 52, 662-668.
