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Adsorption of gaseous aromatic compounds by linear quaternary ammonium-modified γ-zirconium phosphate
Applied Clay Science 187 (2020) 105480
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Research Paper
Adsorption of gaseous aromatic compounds by linear quaternary ammonium-modified γ-zirconium phosphate
T
⁎
A. Hayashi, H. Fukui, H. Nakayama , M. Tsuhako Department of Functional Molecular Chemistry, Kobe Pharmaceutical University, Kobe, Hyogo 658-0003, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Layered zirconium phosphate Quaternary ammonium ion Gaseous aromatic hydrocarbons Intercalation Adsorption
In this work, the intercalation of alkyl trimethylammonium ion (CnN+) and dialkyl dimethylammonium ion (2CnN+) were attempted for α‑zirconium phosphate (α-ZrP) and γ-ZrP. And then a pure intercalation compounds were obtained only for γ-ZrP. It was expected that the intercalation of linear quaternary ammonium ion would produce not only a hydrophobic environment in the interlayer space but also form an appropriate void for the adsorption of hydrophobic air pollutants. The basal spacing of 2CnN+-intercalated γ-ZrP was greater than that of CnN+-intercalated γ-ZrP because CnN+ was arranged in a mono-molecular structure in the interlayer space contrary to the bi-molecular structure of 2CnN+. Furthermore, the alkyl chain in the interlayer space of γZrP was in a gauche form for CnN+, for a smaller carbon number (n), and changed to a trans structure for a larger n. γ-ZrP with a modified interlayer space could adsorb gaseous aromatic hydrocarbons contrary to the nonmodified γ-ZrP. Maximum adsorption was attained for C18N+-intercalated γ-ZrP with a mono-molecular structure and trans structure alkyl chain, although the basal spacing and uptake of alkyl ammonium reached a maximum for 2C18N+-intercalated γ-ZrP. Hence, not only the basal spacing and carbon number but also the void space in the interlayer region affected adsorption. It was interesting to note that the conformation of the long alkyl chain was reversibly changed through adsorption and desorption.
1. Introduction Inorganic layered compounds have a two-dimensional layered space and higher thermal stability, compared to organic compounds, and are used as inorganic ion-exchangers, catalysts, dispersing agents for ink, and adsorbents (Yamanaka and Hattori, 1985). The thickness of a layer and the interlayer space constitute the basal space (d). The binding force between the upper and lower layers is the Van der Waals force, which is not considerable. Therefore, the basal spacing can be changed by the incorporation of an external molecule, in a process called intercalation. The intercalation compound resulting from an intercalation reaction is also known as an inorganic-organic hybrid with an alternating laminate of the host (inorganic) and guest (organic) layers; its characteristics differ from those of the host inorganic and guest organic materials. Since the synthesis of crystalline zirconium phosphate in the 1960's, it has been used as a stable ion-exchanger in aqueous solution. Its crystal structure and physical properties have been investigated (Clearfield, 1982; Hasegawa and Tomita, 1991); it includes eight forms with different crystallization-water numbers (Clearfield and Smith, 1969). In particular, the α- and γ-form zirconium phosphates, depicted
in Fig. 1, have been extensively examined (Poojary et al., 1995; Yamanaka, 1998). α-Zirconium bis(monohydrogenphosphate) monohydrate (α-Zr(HPO4)2·H2O; abbreviated as α-ZrP) with a basal spacing of 0.76 nm includes the HPO42− group and one hydration water in the interlayer space (Fig. 1-a). Each layer consists of a plane of ZrO6 octahedra bridged by the monohydrogenphosphate (HPO4) tetrahedra located on the upper and lower sides of the zirconium plane. The HPO42− group can act not only as an ion-exchanger with cations such as Li+, Na+, K+, Rb+ and Cs+, but also as an active Brönsted acid site (Clearfield, 1982; Alberti and Costantino, 1982). On the other hand, γ‑zirconium phosphate dihydrogenphosphate dihydrate (γ-Zr(PO4) (H2PO4)·2H2O; abbreviated as γ-ZrP) consists of two different ZrO6 octahedra in different layers, joined to each other by the PO43− tetrahedra inside and H2PO4− outside these planes (Fig. 1-b) (Clayden, 1987; Poojary et al., 1995). Its basal spacing is 1.22 nm and two waters exist in the interlayer space. The two protons of dihydrogenphosphate (H2PO4−) behave as an ion-exchangeable cation and active Brönsted acid site, as well as a HPO42− group in α-ZrP. However, only one in two protons of H2PO4− in γ-ZrP can be exchanged with another cation, because the third proton of H3PO4 is difficult to deprotonate. Therefore,
⁎ Corresponding author at. Department of Functional Molecular Chemistry, Kobe Pharmaceutical University, 4-19-1 Motoyamakita-machi, Higashinada-ku, Kobe 658-0003, Japan. E-mail address: [email protected] (H. Nakayama).
https://doi.org/10.1016/j.clay.2020.105480 Received 16 July 2019; Received in revised form 22 November 2019; Accepted 26 January 2020 0169-1317/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Zirconium phosphate structures (a) α-Zr(HPO4)2·H2O and (b) γ-Zr(H2PO4)(PO4)·2H2O
the cation-exchange capacities are 3.1 mmol per gram of γ-ZrP that is about half of 6.7 mmol per gram of α-ZrP. Because basic organic compounds can be directly incorporated into the interlayer space of their zirconium phosphates through an acid-base reaction, they have been used as host compounds in intercalation chemistry. Thereby, the modification of the interlayer space by various guest compounds realizes a new function and property for the layered compound (Yamanaka, 1998; Bellezza et al., 2002; Pan et al., 2007). n-Alkylamine was intercalated to form mono- and bi-molecular structures, depending on the uptake of amine, for both α- and γ-ZrP (Tindwa et al., 1985; MacLachlan and Morgan, 1992; Alberti et al., 2000). α,ω-Alkanediamine and polyamine were intercalated to form a mono-molecular structure inclined at 60° with respect to the phosphate layer of α-ZrP (Casciola et al., 1988; Danjo et al., 1999). Phenylethylamine (Hasegawa et al., 1991), pyridine (Yamanaka et al., 1976; Danjo et al., 1993; Hasegawa et al., 1994), quinoline (Hasegawa et al., 1994), N,N-dimethyl-1-phenylethylamine (Hasegawa et al., 2002), and heterocyclic compounds (Danjo et al., 1995a), classified as cyclic amines, have also been investigated for their intercalation behavior into layered phosphates. These cyclic amines were intercalated to form a monomolecular structure in the interlayer space. Furthermore, amino sugar glucosamine was directly intercalated into γ-ZrP, and the chitosan of their polysaccharide was intercalated by exchanging with the interlayer alkylamine of alkylamine-intercalated γ-ZrP (Hayashi et al., 2006). The intercalation behavior of melamine (1,3,5-triazine-2,4,6-triamine) was different from α-ZrP and γ-ZrP. It was easily intercalated into γ-ZrP and two intercalation compounds were obtained by pH or concentration of melamine aqueous solution (Hayashi et al., 2009). The degree of intercalation depends on the pKb values of the amines, for the γ-type in particular. Although solid acid layered zirconium phosphate easily incorporates a basic compound into the interlayer space, neutral and acidic compounds cannot be directly intercalated into zirconium phosphate. However, our group has reported their successful incorporation into zirconium phosphate modified with basic amine or compounds with both amino and other functional groups (Nakayama et al., 2002a, 2002b; Hayashi et al., 2003; Hayashi et al., 2004; Hayashi et al., 2005). Basic amines or compounds with functional groups in the interlayer space expand the basal spacing, form void space, and supply a new functional field. Thus, intercalation compounds can be applied as adsorbents and catalysts. So far, polar molecule has been examined as an adsorbate, that is a chemical adsorption. In this work, we would like to develop adsorbent for hydrocarbon of nonpolar molecule that is difficult to be adsorbed. Linear quaternary ammonium ion is a cationic surface-active agent, and used as an antibacterial agent, fabric softener, and hair conditioner (Muto, 1999; Tezel et al., 2012). Quaternary ammonium ion with small molecular weight is soluble in water. On increasing the alkyl carbon number and molecular weight, its solubility in water decreases and it is soluble in nonpolar solvents. In this work, alkyl trimethylammonium ion (CnN+) and dialkyl dimethylammonium ion (2CnN+) with long alkyl chains are intercalated into α-ZrP and γ-ZrP. It is expected that the
intercalation of linear quaternary ammonium ion (CnN+ and 2CnN+) would produce not only a hydrophobic environment in the interlayer space but also form an appropriate void for the adsorption of hydrophobic air pollutants. The hydrophobic air pollutants examined in this work include aromatic hydrocarbons, such as benzene, toluene, ethyl benzene, and xylene, which are volatile harmful substances at ordinary temperature. These aromatic hydrocarbons, which are present in paint, are the causative agent of the sick house syndrome. 2. Materials and methods 2.1. Chemicals α-ZrP was prepared by the direct reaction of zirconium(IV) hydroxide (ZrO2·nH2O) or zirconium(IV) oxide (ZrO2) with orthophosphoric acid in an autoclave at 200 °C, as per the specified reference (Tsuhako et al., 1987); these chemicals were of reagent grade from Wako Pure Chemical Industries Ltd. γ-ZrP was supplied by Daiichi Kigenso Chemical Industries Ltd. The linear quaternary ammonium chlorides or bromides used include alkyl trimethylammonium ion (R(CH3)3N+, R = CnH2n+1, CnN+, n = 6–18) and dialkyl dimethylammonium ion (R2(CH3)2N+, R = 2CnN+, n = 8–18). C10N+, C12N+, C18N+, and 2C12N+ were purchased from Aldrich; whereas, CnN+ and 2CnN+ were from Tokyo Kasei. Benzene, toluene, ethyl benzene, and xylene were purchased from Wako Pure Chemical Industries Ltd. 2.2. Intercalation procedure for quaternary ammonium ion Intercalation was performed by the batch method for α- and γ-ZrP. α- or γ-ZrP (0.2 g) was suspended in 0.1 dm3 of 5–100 mmol dm−3 quaternary ammonium ion in a mixed-solution of water and ethanol (1:1) or only ethanol, and stirred at 23 ± 1 °C or 40 °C for 1 d. Further, these suspensions were filtered and washed thrice with ethanol, and dried in air for 1 d. 2.3. Adsorption of aromatic hydrocarbon gas The adsorption of aromatic hydrocarbon gas was performed at 40 °C using a home-made glass-device (Nakayama et al., 2002a). The used aromatic hydrocarbons included benzene, toluene, ethyl benzene, and xylene. 0.2 g quaternary ammonium ion-intercalated γ-ZrPs were used as the adsorbent. The rate of increase in weight (%) was measured on the time-course. After adsorption, these compounds were degassed in vacuum for the removal of the aromatic hydrocarbons on the sample surface. 2.4. Analytical procedure The basal spacing of the intercalation compound was measured by X-ray diffractometry using a Rigaku Denki Rint 2000 diffractometer (Rigaku Co., Ltd., Tokyo, Japan) with Ni-filtered CuKα radiation and 2
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Fig. 2. XRD patterns of CnN+-intercalated zirconium phosphates (A):(a) α-ZrP, (b) C8N+/α-ZrP, and (c) C18N+/α-ZrP (B):(a) γ-ZrP, (b) C8N+/γ-ZrP, and (c) C18N+/ γ-ZrP
number, the basal spacing expanded, enhancing the reflection intensity. The increase in concentration and reaction temperature increased the uptake and basal spacing. The same behaviors were also observed for the intercalation of 2CnN+, as follows: (1) A mixed solution of ethanol and water (1:1) was a suitable solvent for intercalation, (2) quaternary ammonium-ion was easily intercalated into γ-ZrP rather than α-ZrP, (3) the intercalation compound with γ-ZrP contained negligible quantities of the original γZrP, (4) the basal spacing of the intercalation compound and its uptake increased, depending on the carbon number of the alkyl chain. Thus, the intercalation compound with γ-ZrP is the most suitable for the adsorption of gaseous aromatic hydrocarbons. Hereafter, γ-ZrP is described in detail. The basal spacing (d) of the intercalation compounds are plotted as a function of the carbon number (n) of the alkyl chain of CnN+ or 2CnN+, in Fig. 3A. Two phases were obtained for C6N+ and C10N+, depending on the reaction condition. In contrast, for 2CnN+, a single phase was obtained for all the carbon numbers. The basal spacing increased with the increase in carbon number for both ammonium ions; those of the 2CnN+-intercalated γ-ZrP were greater than those of the CnN+-intercalated γ-ZrP. In the case of CnN+, there were two types of correlation lines between d and n, and jumps in d at n = 10. The irregular increase in basal spacing can be related to several factors, such as the conformation and inclined angle of the alkyl chain to the phosphate layer. For 2CnN+, the slope of the line for the basal spacing changed at 2C12N+; it was assumed that the arrangement of 2CnN+ or the conformation of the alkyl chain in the interlayer space changes at 2C12N+. The maximum uptake is plotted against the carbon number, in Fig. 3B; the uptake of CnN+ increased on increasing the carbon number and reached 1.44 mmol/g for C18N+. This value corresponds to nearly half the theoretical cation-exchange capacity (CEC) (3.1 mmol/g) for γZrP. The plot for the intercalation compound with 2CnN+ is a straight line and reaches 1.39 mmol/g for 2C18N+. It was suggested that an irregular increase in basal spacing is related not only to the increase in carbon number but also to the conformation of the alkyl chain. Therefore, the 13C CP/MAS NMR spectra, which reflect the state of the alkyl chain, were measured for these compounds. Fig. 4 shows the 13C CP/MAS NMR spectra of CnN+-intercalated γ-ZrP (n = 8, 14, 18). The chemical shifts of the carbon in CnN+ were
scan speed of 1°/min. The C and N contents of the intercalation compounds were determined by elemental analysis using a Sumigraph NC80 analyzer (Sumika Chem. Anal. Service, Ltd., Osaka, Japan). The 13C CP/MAS NMR spectra were measured using a Varian INOVA-500 spectrometer operating at 125 MHz (Agilent Technologies Inc., Santa Clara, LA, U.S.A.). The measurement condition included a recycle time of 3–5 s, pulse width of 5 μs, contact time of 2 ms, accumulation up to 40,000 scans, and a magic angle spinning (MAS) rate of 6 kHz. 3. Results and discussion 3.1. Intercalation of quaternary ammonium ion into α- and γ-zirconium phosphate CnN+ with a longer alkyl chain has low solubility in water. Therefore, ethanol or a mixed solvent including ethanol and water (1:1) was used as a solvent for the reaction with α-ZrP or γ-ZrP. After the reaction, the X-ray diffraction patterns (XRD patterns) exhibited an obvious increase in the basal spacing for the compound, which reacted in the mixed solvent. Hence, a mixed solvent is suitable for the intercalation of CnN+ into both α-ZrP and γ-ZrP. Fig. 2 shows the XRD patterns of CnN+-intercalated α- and γ-ZrP at different carbon numbers (n = 8, 18). The reflection of the original α-ZrP at 0.76 nm remained after the reaction with α-ZrP (Fig. 2A), indicating the slight intercalation of CnN+ into α-ZrP. In particular, CnN+ with small carbon numbers (n = 6, 8, 10) were not intercalated into α-ZrP. On the other hand, CnN+ with carbon numbers above 12 were slightly intercalated, demonstrating a lower angle shift in their XRD patterns. On increasing the carbon number of CnN+, its reflection intensity and the basal spacing of the intercalation compound increased. Increasing the reaction temperature and concentration of CnN+ was not effective in intercalating CnN+ into α-ZrP, and an intercalation compound without unreacted αZrP was not obtained at any condition. However, the reflection of the original γ-ZrP (1.2 nm) disappeared and shifted to a lower angle (1.7 nm and 2.8 nm) in the XRD patterns after the reaction with γ-ZrP (Fig. 2B). It indicated that CnN+ was completely intercalated into γ-ZrP, expanding its basal spacing. For all carbon numbers, CnN+ reacted with γ-ZrP in 10 mM CnN+ solutions. This suggests that the CnN+ were intercalated into γ-ZrP rather than α-ZrP. On increasing the carbon 3
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Fig.3. (A) Basal spacing (d) of quaternary ammonium ion-intercalated γ-ZrP and (B) maximum uptake of quaternary ammonium ion ●; CnN+/γ-ZrP, ▼; 2CnN+/γ-ZrP
of the methylene group moved to 33.8 ppm from 30.3 ppm, with the increase in the carbon number of the alkyl chain of CnN+. These results confirm the conversion of the alkyl chain from a bent form (trans/ gauche structure) into a straight form (all-trans structure), with the increase in carbon number, as shown in Fig. 5. The volume occupied by a molecule of CnN+ in the interlayer space was reduced due to the conversion to an all-trans structure, and subsequently, the packing of CnN+ occurred. This result confirms that the uptake increases on increasing the carbon number, as previously mentioned. To investigate the condition of the phosphate group contained in the layered framework, the 31P MAS NMR spectra were measured for the intercalation compounds (Fig. 6). For example, a peak at −9 ppm assigned to the H2PO4− group in γ-ZrP shifted to −13 ppm by the intercalation of C10N+. This high-field shift indicates the formation of a weak hydrogen bond between the C10N+ and H2PO4− group (Nakayama et al., 2002a). This result was also observed for the other carbon numbers. On the other hand, the peak due to the PO43− group moved to −26 ppm from −27 ppm by the intercalation of CnN+ for n = 6–14, and split into two peaks at −26 and − 24 ppm by the intercalation of CnN+ at n = 16 and 18. In general, the peak at −27 ppm does not shift because the PO43− group exists in the center of the layered framework and does not involve protons that can interact with the guest compound. Therefore, it was suggested that the three methyl groups around nitrogen, which interact with the H2PO4− group, affect the layered framework. Based on the basal spacing and size of the guest molecule, it is possible to determine the arrangement of the quaternary ammonium
Fig. 4. 13C CP/MAS NMR spectra of CnN+-intercalated γ-ZrP (a) C8N+/γ-ZrP, (b) C14N+/γ-ZrP, and (c) C18N+/ γ-ZrP
respectively assigned, as indicated in the alkyl chain, in Fig. 4. In these spectra, two types of peaks were observed for the methylene group (C5), at 30.3 ppm for C8N+, 30.3 and 32.7 ppm for C14N+, and 33.8 ppm for C18N+, respectively. Generally, the 13C chemical shift for the methylene group occurs at 30 ppm for a trans/gauche structure and 33 ppm for an all-trans structure (Osman et al., 2004). It was determined that the peak
Fig. 5. Schematic models of CnN+-intercalated γ-ZrP (a) C8N+/γ-ZrP and (b) C18N+/γ-ZrP. 4
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the interlayer space. Contrary to this, for 2C18N+-intercalated γ-ZrP, there was a high intensity peak at 33 ppm, indicating an all-trans structure. On increasing the carbon number, the intercalated 2CnN+ changed into an all-trans structure and the uptake increased, as in CnN+-intercalated γ-ZrP. Because the increment in the basal spacing of 2C18N+ was greater than the size of the 2C18N+ molecule, 2C18N+ may be arranged as a bi-molecular structure in the interlayer space. The 2C18N+ molecule tilts by 35° relative to the phosphate layer, as per the following equation, θ = sin−1[(d-0.94)/(0.127 × 2)n]. It was found that the most stable 2CnN+ has an angle of about 109° between two alkyl chains. If the most stable 2C18N+ was intercalated into γ-ZrP through the interaction of nitrogen with the phosphate group, the average angle between the alkyl chain and phosphate layer was calculated to be 35.5°, which was almost equal to 35°, as mentioned above. Therefore, it was determined that 2C18N+ with the most stable conformation has an alkyl chain with an all-trans structure and intercalates as a bi-molecular structure. It covered several H2PO4− groups, rendering interaction with all the H2PO4− groups impossible; the maximum uptake was half the CEC. 3.2. Adsorption of gaseous aromatic hydrocarbons by CnN+- and 2CnN+intercalated γ-ZrP CnN+- and 2CnN+-intercalated γ-ZrP were used as adsorbents for gaseous aromatic hydrocarbons at 40 °C. In Fig. 7, the rate of increase in weight (%) of toluene by the intercalation compounds is plotted as a function of the adsorption time. The original γ-ZrP did not adsorb toluene gas in contrast to the γ-ZrP modified by CnN+ or 2CnN+. This suggests that an alkyl chain in the interlayer space is required for the adsorption of gaseous aromatic hydrocarbons. The rate of increase in weight increased gradually with the adsorption time, and reached an equilibrium after 15 d. The same behavior was observed for the other aromatic hydrocarbons. Fig. 8 shows the maximum adsorption amount of gaseous aromatic hydrocarbons by CnN+- and 2CnN+-intercalated γZrP (n = 8, 18). For all the intercalation compounds, the rate of increase in weight was as per the following sequence: benzene > toluene > ethylbenzene > xylene. This order is related to the molecular size and vapor pressure of the aromatic hydrocarbons at 40 °C; i.e., 188 mmHg > 59 mmHg > 21 mmHg > 18 mmHg, respectively. It was easy to adsorb under the condition of the smaller molecular size and the higher vapor pressure. The basal spacing after adsorption remained unchanged, irrespective of the increase in adsorption. For the intercalation compounds, the rate of increase in weight was as per the
Fig. 6. 31P MAS NMR spectra of γ-ZrP and CnN+-intercalated γ-ZrP (a) γ-ZrP, (b) C10N+/γ-ZrP, and (c) C16N+/γ-ZrP * shows the side-band peak.
ion in the interlayer space (Danjo et al., 1993). On increasing the carbon number, the trans/gauche structure is converted into an alltrans structure, for C18N+, as described previously. Because the increase in basal spacing was lesser than the size of the C18N+ molecule, C18N+ might build-up as a mono-molecular structure in the interlayer space (Kanzaki and Abe, 1991; Danjo et al., 1995b). The increase in the length of the alkyl chain is 0.127 nm per atom of additional carbon, for an all-trans structure. Generally, the increment of the basal spacing for γ-ZrP is calculated by the subtraction of 0.94 nm of anhydrous γ-ZrP from the basal spacing of the intercalation compounds (d). Thereby, the inclination angle (θ) of the guest molecule in the interlayer space can be calculated by θ = sin−1[(d-0.94)/0.127n] for a mono-molecular structure. It was found that C18N+ molecules were present in γ-ZrP as a mono-molecular structure, with the axial inclined at an angle of θ = sin−1[(2.65–0.94)/0.127 × 18] = 48° with respect to the phosphate layer. On the other hand, in the fundamental structure of γ-ZrP, the adjacent phosphate groups (H2PO4−) are separated by approximately 0.54 nm along the a-axis and 0.66 nm along the b-axis (Poojary et al., 1995). The phosphate group in the upper layer locates in the center of the phosphate group networks in the lower phosphate layer. For C18N+, in which the long alkyl chain is inclined at 48° with respect to the phosphate layer of γ-ZrP, several H2PO4− groups were covered, making interaction with all the H2PO4− groups impossible. Consequently, it was assumed that the maximum uptake was half the CEC. For the other CnN+, which do not have all-trans structure alkyl chains, several factors, such as the conformation of the alkyl chain and the inclination of the angle (θ), cause changes in the basal spacing. Similarly, in the 31P MAS NMR spectra of 2CnN+-intercalated γ-ZrP, the peak at −9 ppm (H2PO4− group) shifted to −13 ppm, indicating weak hydrogen bonding between the quaternary amine of 2CnN+ and the H2PO4− group (Fig. S1A). In Fig. S1B, two peaks at 30 and 33 ppm appeared in the 13C CP/MAS NMR spectra of 2C8N+-intercalated γ-ZrP, suggesting that trans/gauche as well as all-trans structures coexisted in
Fig. 7. Adsorption amount of gaseous toluene by γ-ZrP and quaternary ammonium ion-intercalated γ-ZrP at 40 °C ○; γ-ZrP, ▼; C8N+/γ-ZrP, ●; C18N+/γ-ZrP, ▲; 2C8N+/γ-ZrP, □; 2C18N+/γZrP 5
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transformed into a trans/gauche structure from an all-trans structure. After the evacuation of this sample, the peak of the methylene group returned to 33.7 ppm and the benzene peak disappeared. The evacuation completely removed the adsorbed benzene and transformed the adsorbent into the initial state. These behaviors were also observed for the adsorption of the other aromatic hydrocarbons and by 2C18N+-intercalated γ-ZrP. However, in the case of toluene, ethylbenzene, and xylene adsorption by 2C18N+-intercalated γ-ZrP, an original peak at 33 ppm and a shifted peak at 31 ppm appeared in the spectra. This indicates the existence of an alkyl chain without interaction with the aromatic hydrocarbons due to the small adsorption quantities. On the other hand, in the case of C8N+-intercalated γ-ZrP, a small peak of benzene was observed in the spectrum after adsorption, as shown in Fig. 9B. However, there was no change in the chemical shifts of the alkyl chain of C8N+. In this case, due to the less uptake of C8N+ and the trans/gauche mono-molecular structure, an open space existed in the interlayer space and aromatic hydrocarbons were adsorbed in this space by slight interaction with the alkyl chain. As a result, the basal spacing remained the same, before and after adsorption. Differences in the adsorption occurred because the open space size in the interlayer space was different for C18N+ with a mono-molecular structure and 2C18N+ with a bi-molecular structure. In the case of diethylenetriamine-intercalated α-ZrP, both bent-form and straight-form could adsorb formaldehyde gas regardless of an open space. It was because they have a number of interaction sites with formaldehyde, that is amino or imino group, and formaldehyde occurred self oxidation-reduction in the interlayer space (Nakayama et al., 2002b).
Fig. 8. Maximum adsorption amount of gaseous aromatic hydrocarbons by quaternary ammonium ion-intercalated γ-ZrP at 40 °C ; C8N+/γ-ZrP, ; C18N+/γ-ZrP, ; 2C8N+/γ-ZrP, □; 2C18N+/γ-ZrP
following sequence, for any aromatic hydrocarbon: C8N+ < 2C8N+ < 2C18N+ < C18N+. The adsorption of aromatic hydrocarbons was relatively independent of the uptake of quaternary ammonium ion and the basal spacing. The driving force for adsorption is the hydrophobic interaction with the alkyl chain. Therefore, among the guest molecules examined in this study, it was expected that the maximum adsorption would be obtained for 2C18N+-intercalated γ-ZrP with a large carbon number, basal spacing, and uptake of 2C18N+. However, C18N+-intercalated γ-ZrP exhibited the maximum adsorption, unexpectedly. Table 1 summarizes the basal spacing (d) of the intercalation compounds, uptake of CnN+ or 2CnN+ in the interlayer space, conformation of the alkyl chain, arrangement of CnN+ or 2CnN+, and the adsorption amount of benzene. The adsorption amount of benzene was demonstrated as the increase in weight (%) and adsorbed mol against one mol of intercalation compound. 2C18N+-intercalated γZrP has a bi-molecular structure and was densely packed with a long alkyl chain, contrary to the mono-molecular structure of C18N+. It was clear that the adsorption was affected not only by the hydrophobicity but also by the open space size in the interlayer space of the modified γZrP. The open space size produced by different arrangements of the quaternary ammonium ion is related to the adsorption amount. Their adsorption of benzene was calculated to be about 7 and 5 molecules against one molecule of C18N+ and 2C18N+, respectively. To investigate the adsorption behavior, the solid-state 13C CP/MAS NMR was measured for the samples. For example, Fig. 9A shows the spectra of C18N+-intercalated γ-ZrP before and after the adsorption of gaseous benzene, in comparison with the parent γ-ZrP after adsorption. After the adsorption of gaseous benzene, a benzene peak was observed in the spectrum at 129.1 ppm, in addition to the C18N+ peaks. Contrary to this, there was no peak in the parent γ-ZrP after adsorption. This supports that benzene can be adsorbed by the intercalation of C18N+ into the interlayer space of γ-ZrP. A peak at 33.8 ppm in the alkyl chain of C18N+ shifted to 30.8 ppm after adsorption. It was confirmed that the adsorbed benzene does not exist on the surface but in the interlayer space through the interaction with C18N+, and the methylene group
4. Conclusions Linear quaternary ammonium ion was easily intercalated into γ-ZrP, contrary to incomplete intercalation compound with α-ZrP. For γ-ZrP, the basal spacing of the intercalation compound and the uptake of quaternary ammonium ion was dependent on the carbon number of the alkyl chain. The arrangement of CnN+ and 2CnN+ in the interlayer space involved mono-molecular and bi-molecular structures, respectively. Furthermore, 13C CP/MAS NMR measurements revealed that the alkyl chain converted from a trans/gauche structure into an all-trans structure, on increasing the carbon number of the alkyl chain. CnN+- and 2CnN+-intercalated γ-ZrP could adsorb gaseous aromatic hydrocarbons at 40 °C contrary to the parent γ-ZrP. The most adsorbed hydrocarbon was benzene. From the 13C CP/MAS NMR spectra, it was determined that the adsorbed aromatic hydrocarbons do not exist on the surface but are present in the interlayer space through the interaction with the alkyl chain. Maximum adsorption was observed for C18N+-intercalated γ-ZrP rather than 2C18N+-intercalated γ-ZrP. It was established that not only hydrophobic interaction, but also open space was required in the interlayer space. If the open space is regulated by controlling the uptake of CnN+ and 2CnN+, adsorption may be increased. The intercalation compound after adsorption can be transformed to the initial state by drying in a vacuum, wherein the adsorbed aromatic hydrocarbons are removed and the alkyl chain conformation returns. It is interesting to note that the conformation of the alkyl chain for C18N+-intercalated γ-ZrP reversibly changed, before and after adsorption. These intercalation compounds can be reused for adsorption.
Table 1 Summary of the adsorption of gaseous benzene by the intercalation compounds of γ-ZrP at 40 °C. Guest
d (nm)
Uptake amount of guest (mmol/g·γZrP)
Conformation, Arrangement
Adsorption amount of benzene (%)
Adsorption amount of benzene (mol/mol·ICa)
C8N+ C18N+ 2C8N+ 2C18N+
1.6 2.8 2.4 3.5
0.44 1.1 0.78 1.4
trans/gauche, mono-molecular all-trans, mono-molecular trans/gauche, bi-molecular all-trans, bi-molecular
17 44 27 30
1.1 3.6 2.0 3.3
a
Intercalation compound 6
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Fig. 9. 13C CP/MAS NMR spectra of γ-ZrP and C18N+-intercalated γ-ZrP beforeand after the adsorption of gaseous benzene (A):(a) γ-ZrP after adsorption, (b) C18N+/γ-ZrP before adsorption, (c) C18N+/γ-ZrP after adsorption, and (d) compound (c) after evacuation (B):(a) γ-ZrP after adsorption, (b) C8N+/γ-ZrP before adsorption, (c) C8N+/γ-ZrP after adsorption, and (d) compound (c) after evacuation
Declaration of Competing Interest
Hasegawa, Y., Matsuda, R., Kisa, M., Iso, M., 2002. Intercalation of N,N-dimethyl-1phenylethylamine into α-zirconium phosphate. J. Incl. Phenom. Macrocycl. Chem. 42 (1–2), 33–38. Hayashi, A., Nakayama, H., Tsuhako, M., 2003. Adsorption of phenols by alkylamineintercalated α-zirconium phosphate. Bull. Chem. Soc. Jpn. 76 (12), 2315–2319. Hayashi, A., Katsuta, H., Tsuruta, A., Nakayama, H., Tsuhako, M., 2004. Adsorption of fragrant gases with aldehyde group by linear amine-intercalated α-zirconium phosphate. Phosphorus Res. Bull. 17, 107–112. Hayashi, A., Fujimoto, Y., Ogawa, Y., Nakayama, H., Tsuhako, M., 2005. Adsorption of gaseous formaldehyde and carboxylic acids by ammonium-ion-exchanged α-zirconium phosphate. J. Colloid Interface Sci. 283 (1), 57–63. Hayashi, A., Nakabayashi, Y., Yasutomi, J., Nakayama, H., Tsuhako, M., 2006. Intercalation of glucosamine and chitosan into layered zirconium phosphates. Bull. Chem. Soc. Jpn. 79 (2), 262–269. Hayashi, A., Nakayama, H., Tsuhako, M., 2009. Intercalation of melamine into layered zirconium phosphates and their adsorption properties of formaldehyde in gas and solution phase. Solid State Sci. 11 (5), 1007–1015. Kanzaki, Y., Abe, M., 1991. The intercalation of n-alkylamines into layered transition metal phosphates-solvent effect. Bull. Chem. Soc. Jpn. 64 (6), 1846–1853. MacLachlan, D.J., Morgan, K.R., 1992. Phosphorus-31 solid-state NMR studies of the structure of amine-intercalated α-zirconium phosphate. 2. Titration of α-zirconium phosphate with n-propylamine and n-butylamine. J. Phys. Chem. 96 (8), 3458–3464. Muto, H., 1999. Study of recent softening finishing agents and rinses for washing soaps. Senzai, Kankyo Kagaku Kenkyukaishi 23 (1), 54–55. Nakayama, H., Hayashi, A., Eguchi, T., Nakamura, N., Tsuhako, M., 2002a. Unusual adsorption mechanism for carboxylic acid gases by polyamine-intercalated α-zirconium phosphate. J. Mater. Chem. 12 (10), 3093–3099. Nakayama, H., Hayashi, A., Eguchi, T., Nakamura, N., Tsuhako, M., 2002b. Adsorption of formaldehyde by polyamine-intercalated α-zirconium phosphate. Solid State Sci. 4 (8), 1067–1070. Osman, M.A., Ploetze, M., Skrabal, P., 2004. Structure and properties of alkylammonium monolayers self-assembled on montmorillonite platelets. J. Phys. Chem. B 108 (8), 2580–2588. Pan, B.C., Zhang, Q.R., Zhang, W.M., Pan, B.J., Du, W., Ly, L., Zhang, Q.J., Xu, Z.W., Zhang, Q.X., 2007. Highly effective removal of heavy metals by polymer-based zirconium phosphates: a case study of lead ion. J. Colloid Interface Sci. 310 (1), 99–105. Poojary, D.M., Shpeizer, B., Clearfield, A., 1995. X-ray powder structure and Rietveld refinement of γ-zirconium phosphate, Zr(PO4)(H2PO4)·2H2O. J. Chem. Soc. Dalton Trans. (1), 111–113. Tezel, U., Pavlostathis, S.G., Keen, P.L., Montforts, M.H.M.M., 2012. Role of quaternary ammonium compounds on antimicrobial resistance in the environment. WileyBlackwell, pp. 349–387. Tindwa, R.M., Ellis, D.K., Peng, G.Z., Clearfield, A., 1985. Intercalation of n-alkylamines by α-zirconium phosphate. J. Chem. Soc. Faraday Trans. 81 (2), 545–552. Tsuhako, M., Horii, Y., Nariai, H., Motooka, I., 1987. The synthesis of zirconium(IV) bis (hydrogen phosphate) monohydrate Zr(HPO4)2·H2O. Nippon Kagaku Kaishi 8, 1541–1544. Yamanaka, S., 1998. Intercalation. Materials chemistry using spaces. Nyu Seramikkusu 11 (4), 1–6. Yamanaka, S., Hattori, M., 1985. In: Kato, C., Kuroda, K. (Eds.), Development and application of layered compound. CMC, Tokyo, pp. 92–98. Yamanaka, S., Horibe, Y., Tanaka, M., 1976. Uptake of pyridine and n-butylamine by crystalline zirconium phosphate. J. Inorg. Nucl. Chem. 38 (2), 323–326.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2020.105480. References Alberti, G., Costantino, U., 1982. In: Whittingham, M.S., Jacobson, A.J. (Eds.), Intercalation Chemistry. Academic Press, Inc, New York, pp. 147–180. Alberti, G., Marmottini, F., Cavalaglio, S., Severi, D., 2000. Intercalation processes of nalkyl monoamines in γ-zirconium phosphate. Langmuir 16 (9), 4165–4170. Bellezza, F., Cipiciani, A., Costantino, U., Negozio, E.M., 2002. Zirconium phosphate and modified zirconium phosphates as supports of lipase. Preparation of the composites and activity of the supported enzyme. Langmuir 18 (23), 8737–8742. Casciola, M., Costantino, U., Di Croce, L., Marmottini, F., 1988. Intercalation of α,ωalkyldiamines in layered α-zirconium phosphate and the inclusion behavior of some of the intercalates obtained. J. Incl. Phenom. 6 (3), 291–306. Clayden, J.N., 1987. Solid-state nuclear magnetic resonance spectroscopic study of γzirconium phosphate. J. Chem. Soc. Dalton Trans. 1877–1881. Clearfield, A., 1982. In: Clearfield, A. (Ed.), Inorganic Ion Exchange Materials. CRC Press, Boca Raton and Florida, pp. 1–74. Clearfield, A., Smith, G.D., 1969. Crystallography and structure of α-zirconium bis (monohydrogen orthophosphate) monohydrate. Inorg. Chem. 8 (3), 431–436. Danjo, M., Baba, Y., Tsuhako, M., Nariai, H., Motooka, I., 1993. Intercalation behavior of pyridine and related compounds into layered zirconium phosphate. Phosphorus Res. Bull. 3, 25–30. Danjo, M., Kakiguchi, K., Yanagida, T., Baba, Y., Tsuhako, M., Yamaguchi, S., Nariai, H., Motooka, I., 1995a. Effect of pKa value upon intercalation of heterocyclic compounds into layered phosphate. Phosphorus Res. Bull. 5, 131–136. Danjo, M., Baba, Y., Tsuhako, M., Yamaguchi, S., Hayama, M., Nariai, H., Motooka, I., 1995b. Intercalation of quaternary ammonium ions into γ-titanium phosphate. Bull. Chem. Soc. Jpn. 68 (6), 1607–1612. Danjo, M., Hayashi, A., Nakayama, H., Kimira, Y., Shimizu, T., Mizuguchi, Y., Yagita, Y., Tsuhako, M., Nariai, H., Motooka, I., 1999. Structure and organic gas-adsorption properties of some polyamine intercalated α-zirconium phosphates. Bull. Chem. Soc. Jpn. 72 (9), 2079–2084. Hasegawa, Y., Tomita, I., 1991. Intercalation chemistry of zirconium phosphates. Trends Inorganic Chem. 2, 171–187. Hasegawa, Y., Seki, H., Tomita, I., 1991. Intercalation of phenylethylamines into α-zirconium phosphate and characterization of intercalates. J. Incl. Phenom. Mol. Recognit. Chem. 10 (3), 313–328. Hasegawa, Y., Akimoto, T., Kojima, D., 1994. Intercalation of pyridine and quinoline into α-zirconium phosphate. J. Incl. Phenom. Mol. Recognit. Chem. 20 (1), 1–12.
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Research Paper
Adsorption of gaseous aromatic compounds by linear quaternary ammonium-modified γ-zirconium phosphate
T
⁎
A. Hayashi, H. Fukui, H. Nakayama , M. Tsuhako Department of Functional Molecular Chemistry, Kobe Pharmaceutical University, Kobe, Hyogo 658-0003, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Layered zirconium phosphate Quaternary ammonium ion Gaseous aromatic hydrocarbons Intercalation Adsorption
In this work, the intercalation of alkyl trimethylammonium ion (CnN+) and dialkyl dimethylammonium ion (2CnN+) were attempted for α‑zirconium phosphate (α-ZrP) and γ-ZrP. And then a pure intercalation compounds were obtained only for γ-ZrP. It was expected that the intercalation of linear quaternary ammonium ion would produce not only a hydrophobic environment in the interlayer space but also form an appropriate void for the adsorption of hydrophobic air pollutants. The basal spacing of 2CnN+-intercalated γ-ZrP was greater than that of CnN+-intercalated γ-ZrP because CnN+ was arranged in a mono-molecular structure in the interlayer space contrary to the bi-molecular structure of 2CnN+. Furthermore, the alkyl chain in the interlayer space of γZrP was in a gauche form for CnN+, for a smaller carbon number (n), and changed to a trans structure for a larger n. γ-ZrP with a modified interlayer space could adsorb gaseous aromatic hydrocarbons contrary to the nonmodified γ-ZrP. Maximum adsorption was attained for C18N+-intercalated γ-ZrP with a mono-molecular structure and trans structure alkyl chain, although the basal spacing and uptake of alkyl ammonium reached a maximum for 2C18N+-intercalated γ-ZrP. Hence, not only the basal spacing and carbon number but also the void space in the interlayer region affected adsorption. It was interesting to note that the conformation of the long alkyl chain was reversibly changed through adsorption and desorption.
1. Introduction Inorganic layered compounds have a two-dimensional layered space and higher thermal stability, compared to organic compounds, and are used as inorganic ion-exchangers, catalysts, dispersing agents for ink, and adsorbents (Yamanaka and Hattori, 1985). The thickness of a layer and the interlayer space constitute the basal space (d). The binding force between the upper and lower layers is the Van der Waals force, which is not considerable. Therefore, the basal spacing can be changed by the incorporation of an external molecule, in a process called intercalation. The intercalation compound resulting from an intercalation reaction is also known as an inorganic-organic hybrid with an alternating laminate of the host (inorganic) and guest (organic) layers; its characteristics differ from those of the host inorganic and guest organic materials. Since the synthesis of crystalline zirconium phosphate in the 1960's, it has been used as a stable ion-exchanger in aqueous solution. Its crystal structure and physical properties have been investigated (Clearfield, 1982; Hasegawa and Tomita, 1991); it includes eight forms with different crystallization-water numbers (Clearfield and Smith, 1969). In particular, the α- and γ-form zirconium phosphates, depicted
in Fig. 1, have been extensively examined (Poojary et al., 1995; Yamanaka, 1998). α-Zirconium bis(monohydrogenphosphate) monohydrate (α-Zr(HPO4)2·H2O; abbreviated as α-ZrP) with a basal spacing of 0.76 nm includes the HPO42− group and one hydration water in the interlayer space (Fig. 1-a). Each layer consists of a plane of ZrO6 octahedra bridged by the monohydrogenphosphate (HPO4) tetrahedra located on the upper and lower sides of the zirconium plane. The HPO42− group can act not only as an ion-exchanger with cations such as Li+, Na+, K+, Rb+ and Cs+, but also as an active Brönsted acid site (Clearfield, 1982; Alberti and Costantino, 1982). On the other hand, γ‑zirconium phosphate dihydrogenphosphate dihydrate (γ-Zr(PO4) (H2PO4)·2H2O; abbreviated as γ-ZrP) consists of two different ZrO6 octahedra in different layers, joined to each other by the PO43− tetrahedra inside and H2PO4− outside these planes (Fig. 1-b) (Clayden, 1987; Poojary et al., 1995). Its basal spacing is 1.22 nm and two waters exist in the interlayer space. The two protons of dihydrogenphosphate (H2PO4−) behave as an ion-exchangeable cation and active Brönsted acid site, as well as a HPO42− group in α-ZrP. However, only one in two protons of H2PO4− in γ-ZrP can be exchanged with another cation, because the third proton of H3PO4 is difficult to deprotonate. Therefore,
⁎ Corresponding author at. Department of Functional Molecular Chemistry, Kobe Pharmaceutical University, 4-19-1 Motoyamakita-machi, Higashinada-ku, Kobe 658-0003, Japan. E-mail address: [email protected] (H. Nakayama).
https://doi.org/10.1016/j.clay.2020.105480 Received 16 July 2019; Received in revised form 22 November 2019; Accepted 26 January 2020 0169-1317/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Zirconium phosphate structures (a) α-Zr(HPO4)2·H2O and (b) γ-Zr(H2PO4)(PO4)·2H2O
the cation-exchange capacities are 3.1 mmol per gram of γ-ZrP that is about half of 6.7 mmol per gram of α-ZrP. Because basic organic compounds can be directly incorporated into the interlayer space of their zirconium phosphates through an acid-base reaction, they have been used as host compounds in intercalation chemistry. Thereby, the modification of the interlayer space by various guest compounds realizes a new function and property for the layered compound (Yamanaka, 1998; Bellezza et al., 2002; Pan et al., 2007). n-Alkylamine was intercalated to form mono- and bi-molecular structures, depending on the uptake of amine, for both α- and γ-ZrP (Tindwa et al., 1985; MacLachlan and Morgan, 1992; Alberti et al., 2000). α,ω-Alkanediamine and polyamine were intercalated to form a mono-molecular structure inclined at 60° with respect to the phosphate layer of α-ZrP (Casciola et al., 1988; Danjo et al., 1999). Phenylethylamine (Hasegawa et al., 1991), pyridine (Yamanaka et al., 1976; Danjo et al., 1993; Hasegawa et al., 1994), quinoline (Hasegawa et al., 1994), N,N-dimethyl-1-phenylethylamine (Hasegawa et al., 2002), and heterocyclic compounds (Danjo et al., 1995a), classified as cyclic amines, have also been investigated for their intercalation behavior into layered phosphates. These cyclic amines were intercalated to form a monomolecular structure in the interlayer space. Furthermore, amino sugar glucosamine was directly intercalated into γ-ZrP, and the chitosan of their polysaccharide was intercalated by exchanging with the interlayer alkylamine of alkylamine-intercalated γ-ZrP (Hayashi et al., 2006). The intercalation behavior of melamine (1,3,5-triazine-2,4,6-triamine) was different from α-ZrP and γ-ZrP. It was easily intercalated into γ-ZrP and two intercalation compounds were obtained by pH or concentration of melamine aqueous solution (Hayashi et al., 2009). The degree of intercalation depends on the pKb values of the amines, for the γ-type in particular. Although solid acid layered zirconium phosphate easily incorporates a basic compound into the interlayer space, neutral and acidic compounds cannot be directly intercalated into zirconium phosphate. However, our group has reported their successful incorporation into zirconium phosphate modified with basic amine or compounds with both amino and other functional groups (Nakayama et al., 2002a, 2002b; Hayashi et al., 2003; Hayashi et al., 2004; Hayashi et al., 2005). Basic amines or compounds with functional groups in the interlayer space expand the basal spacing, form void space, and supply a new functional field. Thus, intercalation compounds can be applied as adsorbents and catalysts. So far, polar molecule has been examined as an adsorbate, that is a chemical adsorption. In this work, we would like to develop adsorbent for hydrocarbon of nonpolar molecule that is difficult to be adsorbed. Linear quaternary ammonium ion is a cationic surface-active agent, and used as an antibacterial agent, fabric softener, and hair conditioner (Muto, 1999; Tezel et al., 2012). Quaternary ammonium ion with small molecular weight is soluble in water. On increasing the alkyl carbon number and molecular weight, its solubility in water decreases and it is soluble in nonpolar solvents. In this work, alkyl trimethylammonium ion (CnN+) and dialkyl dimethylammonium ion (2CnN+) with long alkyl chains are intercalated into α-ZrP and γ-ZrP. It is expected that the
intercalation of linear quaternary ammonium ion (CnN+ and 2CnN+) would produce not only a hydrophobic environment in the interlayer space but also form an appropriate void for the adsorption of hydrophobic air pollutants. The hydrophobic air pollutants examined in this work include aromatic hydrocarbons, such as benzene, toluene, ethyl benzene, and xylene, which are volatile harmful substances at ordinary temperature. These aromatic hydrocarbons, which are present in paint, are the causative agent of the sick house syndrome. 2. Materials and methods 2.1. Chemicals α-ZrP was prepared by the direct reaction of zirconium(IV) hydroxide (ZrO2·nH2O) or zirconium(IV) oxide (ZrO2) with orthophosphoric acid in an autoclave at 200 °C, as per the specified reference (Tsuhako et al., 1987); these chemicals were of reagent grade from Wako Pure Chemical Industries Ltd. γ-ZrP was supplied by Daiichi Kigenso Chemical Industries Ltd. The linear quaternary ammonium chlorides or bromides used include alkyl trimethylammonium ion (R(CH3)3N+, R = CnH2n+1, CnN+, n = 6–18) and dialkyl dimethylammonium ion (R2(CH3)2N+, R = 2CnN+, n = 8–18). C10N+, C12N+, C18N+, and 2C12N+ were purchased from Aldrich; whereas, CnN+ and 2CnN+ were from Tokyo Kasei. Benzene, toluene, ethyl benzene, and xylene were purchased from Wako Pure Chemical Industries Ltd. 2.2. Intercalation procedure for quaternary ammonium ion Intercalation was performed by the batch method for α- and γ-ZrP. α- or γ-ZrP (0.2 g) was suspended in 0.1 dm3 of 5–100 mmol dm−3 quaternary ammonium ion in a mixed-solution of water and ethanol (1:1) or only ethanol, and stirred at 23 ± 1 °C or 40 °C for 1 d. Further, these suspensions were filtered and washed thrice with ethanol, and dried in air for 1 d. 2.3. Adsorption of aromatic hydrocarbon gas The adsorption of aromatic hydrocarbon gas was performed at 40 °C using a home-made glass-device (Nakayama et al., 2002a). The used aromatic hydrocarbons included benzene, toluene, ethyl benzene, and xylene. 0.2 g quaternary ammonium ion-intercalated γ-ZrPs were used as the adsorbent. The rate of increase in weight (%) was measured on the time-course. After adsorption, these compounds were degassed in vacuum for the removal of the aromatic hydrocarbons on the sample surface. 2.4. Analytical procedure The basal spacing of the intercalation compound was measured by X-ray diffractometry using a Rigaku Denki Rint 2000 diffractometer (Rigaku Co., Ltd., Tokyo, Japan) with Ni-filtered CuKα radiation and 2
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Fig. 2. XRD patterns of CnN+-intercalated zirconium phosphates (A):(a) α-ZrP, (b) C8N+/α-ZrP, and (c) C18N+/α-ZrP (B):(a) γ-ZrP, (b) C8N+/γ-ZrP, and (c) C18N+/ γ-ZrP
number, the basal spacing expanded, enhancing the reflection intensity. The increase in concentration and reaction temperature increased the uptake and basal spacing. The same behaviors were also observed for the intercalation of 2CnN+, as follows: (1) A mixed solution of ethanol and water (1:1) was a suitable solvent for intercalation, (2) quaternary ammonium-ion was easily intercalated into γ-ZrP rather than α-ZrP, (3) the intercalation compound with γ-ZrP contained negligible quantities of the original γZrP, (4) the basal spacing of the intercalation compound and its uptake increased, depending on the carbon number of the alkyl chain. Thus, the intercalation compound with γ-ZrP is the most suitable for the adsorption of gaseous aromatic hydrocarbons. Hereafter, γ-ZrP is described in detail. The basal spacing (d) of the intercalation compounds are plotted as a function of the carbon number (n) of the alkyl chain of CnN+ or 2CnN+, in Fig. 3A. Two phases were obtained for C6N+ and C10N+, depending on the reaction condition. In contrast, for 2CnN+, a single phase was obtained for all the carbon numbers. The basal spacing increased with the increase in carbon number for both ammonium ions; those of the 2CnN+-intercalated γ-ZrP were greater than those of the CnN+-intercalated γ-ZrP. In the case of CnN+, there were two types of correlation lines between d and n, and jumps in d at n = 10. The irregular increase in basal spacing can be related to several factors, such as the conformation and inclined angle of the alkyl chain to the phosphate layer. For 2CnN+, the slope of the line for the basal spacing changed at 2C12N+; it was assumed that the arrangement of 2CnN+ or the conformation of the alkyl chain in the interlayer space changes at 2C12N+. The maximum uptake is plotted against the carbon number, in Fig. 3B; the uptake of CnN+ increased on increasing the carbon number and reached 1.44 mmol/g for C18N+. This value corresponds to nearly half the theoretical cation-exchange capacity (CEC) (3.1 mmol/g) for γZrP. The plot for the intercalation compound with 2CnN+ is a straight line and reaches 1.39 mmol/g for 2C18N+. It was suggested that an irregular increase in basal spacing is related not only to the increase in carbon number but also to the conformation of the alkyl chain. Therefore, the 13C CP/MAS NMR spectra, which reflect the state of the alkyl chain, were measured for these compounds. Fig. 4 shows the 13C CP/MAS NMR spectra of CnN+-intercalated γ-ZrP (n = 8, 14, 18). The chemical shifts of the carbon in CnN+ were
scan speed of 1°/min. The C and N contents of the intercalation compounds were determined by elemental analysis using a Sumigraph NC80 analyzer (Sumika Chem. Anal. Service, Ltd., Osaka, Japan). The 13C CP/MAS NMR spectra were measured using a Varian INOVA-500 spectrometer operating at 125 MHz (Agilent Technologies Inc., Santa Clara, LA, U.S.A.). The measurement condition included a recycle time of 3–5 s, pulse width of 5 μs, contact time of 2 ms, accumulation up to 40,000 scans, and a magic angle spinning (MAS) rate of 6 kHz. 3. Results and discussion 3.1. Intercalation of quaternary ammonium ion into α- and γ-zirconium phosphate CnN+ with a longer alkyl chain has low solubility in water. Therefore, ethanol or a mixed solvent including ethanol and water (1:1) was used as a solvent for the reaction with α-ZrP or γ-ZrP. After the reaction, the X-ray diffraction patterns (XRD patterns) exhibited an obvious increase in the basal spacing for the compound, which reacted in the mixed solvent. Hence, a mixed solvent is suitable for the intercalation of CnN+ into both α-ZrP and γ-ZrP. Fig. 2 shows the XRD patterns of CnN+-intercalated α- and γ-ZrP at different carbon numbers (n = 8, 18). The reflection of the original α-ZrP at 0.76 nm remained after the reaction with α-ZrP (Fig. 2A), indicating the slight intercalation of CnN+ into α-ZrP. In particular, CnN+ with small carbon numbers (n = 6, 8, 10) were not intercalated into α-ZrP. On the other hand, CnN+ with carbon numbers above 12 were slightly intercalated, demonstrating a lower angle shift in their XRD patterns. On increasing the carbon number of CnN+, its reflection intensity and the basal spacing of the intercalation compound increased. Increasing the reaction temperature and concentration of CnN+ was not effective in intercalating CnN+ into α-ZrP, and an intercalation compound without unreacted αZrP was not obtained at any condition. However, the reflection of the original γ-ZrP (1.2 nm) disappeared and shifted to a lower angle (1.7 nm and 2.8 nm) in the XRD patterns after the reaction with γ-ZrP (Fig. 2B). It indicated that CnN+ was completely intercalated into γ-ZrP, expanding its basal spacing. For all carbon numbers, CnN+ reacted with γ-ZrP in 10 mM CnN+ solutions. This suggests that the CnN+ were intercalated into γ-ZrP rather than α-ZrP. On increasing the carbon 3
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Fig.3. (A) Basal spacing (d) of quaternary ammonium ion-intercalated γ-ZrP and (B) maximum uptake of quaternary ammonium ion ●; CnN+/γ-ZrP, ▼; 2CnN+/γ-ZrP
of the methylene group moved to 33.8 ppm from 30.3 ppm, with the increase in the carbon number of the alkyl chain of CnN+. These results confirm the conversion of the alkyl chain from a bent form (trans/ gauche structure) into a straight form (all-trans structure), with the increase in carbon number, as shown in Fig. 5. The volume occupied by a molecule of CnN+ in the interlayer space was reduced due to the conversion to an all-trans structure, and subsequently, the packing of CnN+ occurred. This result confirms that the uptake increases on increasing the carbon number, as previously mentioned. To investigate the condition of the phosphate group contained in the layered framework, the 31P MAS NMR spectra were measured for the intercalation compounds (Fig. 6). For example, a peak at −9 ppm assigned to the H2PO4− group in γ-ZrP shifted to −13 ppm by the intercalation of C10N+. This high-field shift indicates the formation of a weak hydrogen bond between the C10N+ and H2PO4− group (Nakayama et al., 2002a). This result was also observed for the other carbon numbers. On the other hand, the peak due to the PO43− group moved to −26 ppm from −27 ppm by the intercalation of CnN+ for n = 6–14, and split into two peaks at −26 and − 24 ppm by the intercalation of CnN+ at n = 16 and 18. In general, the peak at −27 ppm does not shift because the PO43− group exists in the center of the layered framework and does not involve protons that can interact with the guest compound. Therefore, it was suggested that the three methyl groups around nitrogen, which interact with the H2PO4− group, affect the layered framework. Based on the basal spacing and size of the guest molecule, it is possible to determine the arrangement of the quaternary ammonium
Fig. 4. 13C CP/MAS NMR spectra of CnN+-intercalated γ-ZrP (a) C8N+/γ-ZrP, (b) C14N+/γ-ZrP, and (c) C18N+/ γ-ZrP
respectively assigned, as indicated in the alkyl chain, in Fig. 4. In these spectra, two types of peaks were observed for the methylene group (C5), at 30.3 ppm for C8N+, 30.3 and 32.7 ppm for C14N+, and 33.8 ppm for C18N+, respectively. Generally, the 13C chemical shift for the methylene group occurs at 30 ppm for a trans/gauche structure and 33 ppm for an all-trans structure (Osman et al., 2004). It was determined that the peak
Fig. 5. Schematic models of CnN+-intercalated γ-ZrP (a) C8N+/γ-ZrP and (b) C18N+/γ-ZrP. 4
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the interlayer space. Contrary to this, for 2C18N+-intercalated γ-ZrP, there was a high intensity peak at 33 ppm, indicating an all-trans structure. On increasing the carbon number, the intercalated 2CnN+ changed into an all-trans structure and the uptake increased, as in CnN+-intercalated γ-ZrP. Because the increment in the basal spacing of 2C18N+ was greater than the size of the 2C18N+ molecule, 2C18N+ may be arranged as a bi-molecular structure in the interlayer space. The 2C18N+ molecule tilts by 35° relative to the phosphate layer, as per the following equation, θ = sin−1[(d-0.94)/(0.127 × 2)n]. It was found that the most stable 2CnN+ has an angle of about 109° between two alkyl chains. If the most stable 2C18N+ was intercalated into γ-ZrP through the interaction of nitrogen with the phosphate group, the average angle between the alkyl chain and phosphate layer was calculated to be 35.5°, which was almost equal to 35°, as mentioned above. Therefore, it was determined that 2C18N+ with the most stable conformation has an alkyl chain with an all-trans structure and intercalates as a bi-molecular structure. It covered several H2PO4− groups, rendering interaction with all the H2PO4− groups impossible; the maximum uptake was half the CEC. 3.2. Adsorption of gaseous aromatic hydrocarbons by CnN+- and 2CnN+intercalated γ-ZrP CnN+- and 2CnN+-intercalated γ-ZrP were used as adsorbents for gaseous aromatic hydrocarbons at 40 °C. In Fig. 7, the rate of increase in weight (%) of toluene by the intercalation compounds is plotted as a function of the adsorption time. The original γ-ZrP did not adsorb toluene gas in contrast to the γ-ZrP modified by CnN+ or 2CnN+. This suggests that an alkyl chain in the interlayer space is required for the adsorption of gaseous aromatic hydrocarbons. The rate of increase in weight increased gradually with the adsorption time, and reached an equilibrium after 15 d. The same behavior was observed for the other aromatic hydrocarbons. Fig. 8 shows the maximum adsorption amount of gaseous aromatic hydrocarbons by CnN+- and 2CnN+-intercalated γZrP (n = 8, 18). For all the intercalation compounds, the rate of increase in weight was as per the following sequence: benzene > toluene > ethylbenzene > xylene. This order is related to the molecular size and vapor pressure of the aromatic hydrocarbons at 40 °C; i.e., 188 mmHg > 59 mmHg > 21 mmHg > 18 mmHg, respectively. It was easy to adsorb under the condition of the smaller molecular size and the higher vapor pressure. The basal spacing after adsorption remained unchanged, irrespective of the increase in adsorption. For the intercalation compounds, the rate of increase in weight was as per the
Fig. 6. 31P MAS NMR spectra of γ-ZrP and CnN+-intercalated γ-ZrP (a) γ-ZrP, (b) C10N+/γ-ZrP, and (c) C16N+/γ-ZrP * shows the side-band peak.
ion in the interlayer space (Danjo et al., 1993). On increasing the carbon number, the trans/gauche structure is converted into an alltrans structure, for C18N+, as described previously. Because the increase in basal spacing was lesser than the size of the C18N+ molecule, C18N+ might build-up as a mono-molecular structure in the interlayer space (Kanzaki and Abe, 1991; Danjo et al., 1995b). The increase in the length of the alkyl chain is 0.127 nm per atom of additional carbon, for an all-trans structure. Generally, the increment of the basal spacing for γ-ZrP is calculated by the subtraction of 0.94 nm of anhydrous γ-ZrP from the basal spacing of the intercalation compounds (d). Thereby, the inclination angle (θ) of the guest molecule in the interlayer space can be calculated by θ = sin−1[(d-0.94)/0.127n] for a mono-molecular structure. It was found that C18N+ molecules were present in γ-ZrP as a mono-molecular structure, with the axial inclined at an angle of θ = sin−1[(2.65–0.94)/0.127 × 18] = 48° with respect to the phosphate layer. On the other hand, in the fundamental structure of γ-ZrP, the adjacent phosphate groups (H2PO4−) are separated by approximately 0.54 nm along the a-axis and 0.66 nm along the b-axis (Poojary et al., 1995). The phosphate group in the upper layer locates in the center of the phosphate group networks in the lower phosphate layer. For C18N+, in which the long alkyl chain is inclined at 48° with respect to the phosphate layer of γ-ZrP, several H2PO4− groups were covered, making interaction with all the H2PO4− groups impossible. Consequently, it was assumed that the maximum uptake was half the CEC. For the other CnN+, which do not have all-trans structure alkyl chains, several factors, such as the conformation of the alkyl chain and the inclination of the angle (θ), cause changes in the basal spacing. Similarly, in the 31P MAS NMR spectra of 2CnN+-intercalated γ-ZrP, the peak at −9 ppm (H2PO4− group) shifted to −13 ppm, indicating weak hydrogen bonding between the quaternary amine of 2CnN+ and the H2PO4− group (Fig. S1A). In Fig. S1B, two peaks at 30 and 33 ppm appeared in the 13C CP/MAS NMR spectra of 2C8N+-intercalated γ-ZrP, suggesting that trans/gauche as well as all-trans structures coexisted in
Fig. 7. Adsorption amount of gaseous toluene by γ-ZrP and quaternary ammonium ion-intercalated γ-ZrP at 40 °C ○; γ-ZrP, ▼; C8N+/γ-ZrP, ●; C18N+/γ-ZrP, ▲; 2C8N+/γ-ZrP, □; 2C18N+/γZrP 5
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transformed into a trans/gauche structure from an all-trans structure. After the evacuation of this sample, the peak of the methylene group returned to 33.7 ppm and the benzene peak disappeared. The evacuation completely removed the adsorbed benzene and transformed the adsorbent into the initial state. These behaviors were also observed for the adsorption of the other aromatic hydrocarbons and by 2C18N+-intercalated γ-ZrP. However, in the case of toluene, ethylbenzene, and xylene adsorption by 2C18N+-intercalated γ-ZrP, an original peak at 33 ppm and a shifted peak at 31 ppm appeared in the spectra. This indicates the existence of an alkyl chain without interaction with the aromatic hydrocarbons due to the small adsorption quantities. On the other hand, in the case of C8N+-intercalated γ-ZrP, a small peak of benzene was observed in the spectrum after adsorption, as shown in Fig. 9B. However, there was no change in the chemical shifts of the alkyl chain of C8N+. In this case, due to the less uptake of C8N+ and the trans/gauche mono-molecular structure, an open space existed in the interlayer space and aromatic hydrocarbons were adsorbed in this space by slight interaction with the alkyl chain. As a result, the basal spacing remained the same, before and after adsorption. Differences in the adsorption occurred because the open space size in the interlayer space was different for C18N+ with a mono-molecular structure and 2C18N+ with a bi-molecular structure. In the case of diethylenetriamine-intercalated α-ZrP, both bent-form and straight-form could adsorb formaldehyde gas regardless of an open space. It was because they have a number of interaction sites with formaldehyde, that is amino or imino group, and formaldehyde occurred self oxidation-reduction in the interlayer space (Nakayama et al., 2002b).
Fig. 8. Maximum adsorption amount of gaseous aromatic hydrocarbons by quaternary ammonium ion-intercalated γ-ZrP at 40 °C ; C8N+/γ-ZrP, ; C18N+/γ-ZrP, ; 2C8N+/γ-ZrP, □; 2C18N+/γ-ZrP
following sequence, for any aromatic hydrocarbon: C8N+ < 2C8N+ < 2C18N+ < C18N+. The adsorption of aromatic hydrocarbons was relatively independent of the uptake of quaternary ammonium ion and the basal spacing. The driving force for adsorption is the hydrophobic interaction with the alkyl chain. Therefore, among the guest molecules examined in this study, it was expected that the maximum adsorption would be obtained for 2C18N+-intercalated γ-ZrP with a large carbon number, basal spacing, and uptake of 2C18N+. However, C18N+-intercalated γ-ZrP exhibited the maximum adsorption, unexpectedly. Table 1 summarizes the basal spacing (d) of the intercalation compounds, uptake of CnN+ or 2CnN+ in the interlayer space, conformation of the alkyl chain, arrangement of CnN+ or 2CnN+, and the adsorption amount of benzene. The adsorption amount of benzene was demonstrated as the increase in weight (%) and adsorbed mol against one mol of intercalation compound. 2C18N+-intercalated γZrP has a bi-molecular structure and was densely packed with a long alkyl chain, contrary to the mono-molecular structure of C18N+. It was clear that the adsorption was affected not only by the hydrophobicity but also by the open space size in the interlayer space of the modified γZrP. The open space size produced by different arrangements of the quaternary ammonium ion is related to the adsorption amount. Their adsorption of benzene was calculated to be about 7 and 5 molecules against one molecule of C18N+ and 2C18N+, respectively. To investigate the adsorption behavior, the solid-state 13C CP/MAS NMR was measured for the samples. For example, Fig. 9A shows the spectra of C18N+-intercalated γ-ZrP before and after the adsorption of gaseous benzene, in comparison with the parent γ-ZrP after adsorption. After the adsorption of gaseous benzene, a benzene peak was observed in the spectrum at 129.1 ppm, in addition to the C18N+ peaks. Contrary to this, there was no peak in the parent γ-ZrP after adsorption. This supports that benzene can be adsorbed by the intercalation of C18N+ into the interlayer space of γ-ZrP. A peak at 33.8 ppm in the alkyl chain of C18N+ shifted to 30.8 ppm after adsorption. It was confirmed that the adsorbed benzene does not exist on the surface but in the interlayer space through the interaction with C18N+, and the methylene group
4. Conclusions Linear quaternary ammonium ion was easily intercalated into γ-ZrP, contrary to incomplete intercalation compound with α-ZrP. For γ-ZrP, the basal spacing of the intercalation compound and the uptake of quaternary ammonium ion was dependent on the carbon number of the alkyl chain. The arrangement of CnN+ and 2CnN+ in the interlayer space involved mono-molecular and bi-molecular structures, respectively. Furthermore, 13C CP/MAS NMR measurements revealed that the alkyl chain converted from a trans/gauche structure into an all-trans structure, on increasing the carbon number of the alkyl chain. CnN+- and 2CnN+-intercalated γ-ZrP could adsorb gaseous aromatic hydrocarbons at 40 °C contrary to the parent γ-ZrP. The most adsorbed hydrocarbon was benzene. From the 13C CP/MAS NMR spectra, it was determined that the adsorbed aromatic hydrocarbons do not exist on the surface but are present in the interlayer space through the interaction with the alkyl chain. Maximum adsorption was observed for C18N+-intercalated γ-ZrP rather than 2C18N+-intercalated γ-ZrP. It was established that not only hydrophobic interaction, but also open space was required in the interlayer space. If the open space is regulated by controlling the uptake of CnN+ and 2CnN+, adsorption may be increased. The intercalation compound after adsorption can be transformed to the initial state by drying in a vacuum, wherein the adsorbed aromatic hydrocarbons are removed and the alkyl chain conformation returns. It is interesting to note that the conformation of the alkyl chain for C18N+-intercalated γ-ZrP reversibly changed, before and after adsorption. These intercalation compounds can be reused for adsorption.
Table 1 Summary of the adsorption of gaseous benzene by the intercalation compounds of γ-ZrP at 40 °C. Guest
d (nm)
Uptake amount of guest (mmol/g·γZrP)
Conformation, Arrangement
Adsorption amount of benzene (%)
Adsorption amount of benzene (mol/mol·ICa)
C8N+ C18N+ 2C8N+ 2C18N+
1.6 2.8 2.4 3.5
0.44 1.1 0.78 1.4
trans/gauche, mono-molecular all-trans, mono-molecular trans/gauche, bi-molecular all-trans, bi-molecular
17 44 27 30
1.1 3.6 2.0 3.3
a
Intercalation compound 6
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Fig. 9. 13C CP/MAS NMR spectra of γ-ZrP and C18N+-intercalated γ-ZrP beforeand after the adsorption of gaseous benzene (A):(a) γ-ZrP after adsorption, (b) C18N+/γ-ZrP before adsorption, (c) C18N+/γ-ZrP after adsorption, and (d) compound (c) after evacuation (B):(a) γ-ZrP after adsorption, (b) C8N+/γ-ZrP before adsorption, (c) C8N+/γ-ZrP after adsorption, and (d) compound (c) after evacuation
Declaration of Competing Interest
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