Attachment of the Sulfonic Acid Group in the Interlayer Space of a

Article pubs.acs.org/Langmuir

Attachment of the Sulfonic Acid Group in the Interlayer Space of a Layered Alkali Silicate, Octosilicate Takanori Nakamura† and Makoto Ogawa*,†,‡ †

Graduate School of Creative Science and Engineering and ‡Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan S Supporting Information *

ABSTRACT: The surface modification of a layered alkali silicate, octosilicate (Na2Si8O17·nH2O), with a sulfonic acid group was conducted. The sulfonic acid group was attached to the silicate layer by the reaction of octosilicate with phenethyl(dichloro)methylsilane and the subsequent sulfonation of the attached phenethyl groups with chlorosulfonic acid. The modified octosilicate is a solid acid as indicated by the intercalation of dodecylamine. A systematic expansion of the interlayer space was observed by the ion exchange with a series of alkyltrimethylammonium ions to show the variation of the layer charge density.

1. INTRODUCTION Layered materials and their intercalates have been extensively investigated for materials applications in a wide range of fields, including environmental, energy, and life sciences.1 The interlayer surface possess chemical reactivity such as ion exchange and hydrogen bonding to accommodate various guest species. The kind and density of the surface functionality play dominant roles in determining properties such as the ion exchange capacity and adsorption capacity and in controlling the nanostructure of the intercalcated compounds. It is apparent that 2:1 type of layered silicates, mica, vermiculite, and smectite shows different reactivity for cation exchange.2 The density of pillar as well as organic cations is known to affect the adsorptive properties of modified smectites such as pillared clays in achieving molecular sieving and in increasing the adsorption capacity.3−5 In this article, we repot the attachment of an acidic group, a phenethylsulfonic acid group, to a layered silicate, octosilicate (Na2Si8O17·nH2O), and the cation exchange and acid−base reactions of the sulfonated octosilicate were investigated. Among available layered solid oxides, the well-defined shape of octosilicate particle is quite unique and interesting for investigations in nanosheet dispersions and nanosheet assembly. Layered alkali silicates are characterized by interlayer hydroxyl groups, which play important roles in ion exchange and grafting.6,7 The cation exchange capacity of layered alkali silicates is determined by the density of hydroxyl group on the layer surface and directly correlates the nanostructure of the intercalcated compounds prepared by the cation-exchange reaction.7−9 The organic derivatives of layered alkali silicates have been prepared through the grafting of organic functionality by the reaction with silane coupling reagents.10−12 © 2012 American Chemical Society

It has been shown that the surface coverage of the organosilyl group affected their adsorptive13−16 and swelling17 properties. In this study, the immobilization of a controlled number of sulfonic acid groups on a layered sodium silicate, octosilicate, was examined. The attachment of sulfonic acid groups on various porous materials has been actively carried out for possible applications in catalysis18−22 and proton conduction23−25 whereas that on layered solids has scarcely been reported.17,26−28 The number (or the density) of sulfonic acid groups on the surface may affect the performance in applications. A two-dimensional nanospace modified with phenethylsulfonic acid groups may be used for substrate- and product-selective adsorption and catalytic reactions. Once the modified layered silicate was exfoliated to nanosheets, they could be regarded as platy nanoparticles covered with sulfonic acid groups. Here, octosilicate was modified with a controlled number of phenethyl groups, and the immobilized phenethyl group was subsequently sulfonated by the reaction with chlorosulfonic acid. The possible design of the density of ionexchangeable acid groups in the interlayer space is examined.

2. EXPERIMENTAL SECTION Materials. Silica gel (special grade) was purchased from Wako Pure Chemicals Industries Co. n-Dodecylamine, hexyltrimethylammonium bromide (abbreviated as C6TMABr), dodecyltrimethylammonium chloride (C12TMACl), and hexadecyltrimethylammonium chloride (C16TMACl) were purchased from Tokyo Kasei Co. Behenyltrimetylammonium chloride (C22TMACl) was donated from Daiichi Kougyo Seiyaku Co. Phenethyl(dichloro)methylsilane was obtained Received: January 27, 2012 Revised: April 19, 2012 Published: April 24, 2012 7505

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Scheme 1. Surface Modification of Octosilicate with the Sulfonated Phenethyl Group

50 mL of a 20 mmol L−1 alkyltrimethylammonium salt aqueous solution, and the mixture was allowed to react at room temperature for 1 day. For the ion exchange with C22TMA, SPhS-octosilicate was dispersed in aqueous ethanol (1:1 by volume) containing C22TMA chloride. After the ion exchange, the solid products were separated by vacuum filtration and washed with deionized water. Finally, the powder was dried at 60 °C. Intercalation of Dodecylamine into Modified Octosilicates. For the intercalation of C12NH2, SPhS-octosilicate (30 mg) was dispersed in 50 mL of a 20 mmol L−1 C12NH2 aqueous ethanol (1:1 by volume) containing C12NH2, and the mixture was allowed to react at room temperature for 1 day. Then, the solid product was separated by vacuum filtration and washed with deionized water. Finally, the powder was dried at 60 °C. Characterization. X-ray diffraction patterns of the products were recorded on a Rigaku RAD IB powder diffractometer equipped with monochromatic Cu Kα radiation and operated at 20 mA and 40 kV. Solid-state 13C CP/MAS NMR spectra were recorded on a JEOL ECX-400 spectrometer at a resonance frequency of 100.5 MHz with a 45° pulse and a recycle delay of 5 s with a 6-mm-diameter rotor spun at 10 kHz. Solid-state 29Si MAS NMR spectra were also recorded on a same spectrometer at a resonance frequency of 79.4 MHz with a 90° pulse and a recycle delay of 200 s by using a 6-mm-diameter rotor spun at 6 kHz. The 13C and 29Si chemical shifts were referenced to hexamethylbenzene at 17.8 ppm and polydimethylsilane at −32.9 ppm, respectively. Thermogravimetric and differential thermal analysis (TGDTA) curves were recorded on a Rigaku TG8120 at a heating rate of 10 °C min−1 under air using α-Al2O3 as the standard material.

from Azumax Co. Sodium hydroxide (NaOH), chloroform, and chlorosulfonic acid were obtained from Kanto Chemical Co. Chloroform was distilled before use; other chemicals were used without further purification. Octosilicate (Na2Si8O17·nH2O) was synthesized by a reported method.29 Silica gel, NaOH, and deionized water were mixed in a molar ratio of SiO2/NaOH/H2O 4:1:25.8. The mixture was sealed in a Teflon-lined autoclave and treated hydrothermally at 100 °C for 9 days. The product was separated by centrifugation (3500 rpm, 10 min), washed with a dilute aqueous solution of NaOH (pH 10.0), and dried at 40 °C for 2 days. C16TMA-ocotosilicate was prepared as follows. Octosilicate (2.4 g) powder was dispersed in 480 mL of a 0.1 mol L−1 C16TMACl aqueous solution, and the mixture was allowed to react at room temperature for 1 week. After the ion exchange, the solid products were separated by centrifugation (3500 rpm, 10 min) and washed with methanol. Finally, the powder was dried at 40 °C.21 Silylation with Phenethyl(dichloro)methylsilane. The silylation of octosilicate with phenethyl(dichloro)methylsilane was conducted in a similar way to that developed for the silylation of magadiite and octosilicate with organosilanes.10−16,30−34 C16TMAoctosilicate (1.6 g) was dispersed in a toluene (50 mL) solution of phenethyl(dichloro)methylsilane (0.30, 0.43, and 0.90 mL corresponding to 0.70, 1.0, and 2.0 groups per Si8O17, respectively), and the mixture was concentrated for 2 h at 90 Pa and 60 °C to evaporate the solvent. Before evaporation, the mixture was allowed to react at 60 °C for 4 days. The products were washed with a mixture of a 0.1 mol L−1 HCl aqueous ethanol solution (1:1 by volume). The silylated derivatives thus obtained were abbreviated as PhSx-octosilicate (Scheme 1), where x denotes the number of attached phenethylsilyl groups (groups per Si8O17). Sulfonation of the Attached Phenethyl Group. The sulfonation was conducted according to the procedure for the sulfonation of a phenethyl group immobilized on mesoporous silica as follows.35,36 PhSx-octosilicate was dried under reduced pressure at 110 °C for 3 h, dispersed in chloroform with the slow addition of chlorosulfonic acid under a flow of nitrogen, and refluxed for 20 h at 60 °C. The weight ratio was PhSx-octosilicate/chloroform/chlorosulfonic acid 1:150:26.2. The products were washed with chloroform, acetone, and deionized water and dried at 100 °C for 1 day. The products were designated as SPhSx-octosilicate (scheme 1), where x denotes the number of attached sulfonated phenethylsilyl group (groups per Si8O17). Ion Exchange with Alkyltrimethylamonium in Modified Octosilicates. SPhS-octosilicate (30 mg) powder was dispersed in

3. RESULTS AND DISCUSSION Preparation of Phenetylsulfonated Octosilicates. Figure 1 shows the XRD patterns of PhSx-octosilicates together with those of octosilicate and C16TMA-octosilicate. The basal spacing of C16TMA-octosilicate (2.78 nm, which is consistent with the reported value8) decreased upon reaction with phenethyl(dichloro)methylsilane, suggesting the desorption of the C16TMA ion. The basal spacing varied depending on the loaded amounts of phenethyl(dichloro)methylsilane. The TGDTA curves of the PhSx-octosilicate prepared with the largest number (2 groups per Si8O17) of phenethyl(dichloro)methylsilane are shown in Figure 2 together with those of pristine octosilicate and C16TMA-octosilicate. The C16TMA 7506

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Table 1. Composition of Products number of attached group/groups per Si8O17 ignition lossa

ash (SiO2)b

PhSc

SPhSd

Mass % PhS1.4‑oct. SPhS0.9‑oct. PhS1.0‑oct. SPhS0.8‑oct. PhS0.7‑oct. SPhS0.6‑oct.

20 23 17 21 13 17

72 66 77 72 77 74

1.4 0.9 1.0 0.8 0.7 0.6

0.9 0.7 0.6

Determined from TG (mass loss from 400 to 800 °C). bDetermined from TG (ash). cCalculated from TG (mass %) by assuming that all of the attached silyl groups are C6H5(CH2)2Si(CH3) for PhSxoctosilicate. dCalculated from titration with NaOH by assuming that SPhSx is SO3H−C6H5(CH2)2Si(CH3). a

Figure 1. XRD patterns of (a) octosilicate, (b) C16TMA-octosilicate, (c) PhS0.7-octosilicate, (d) PhS1.0-octosilicate, and (e) PhS1.4octosilicate.

on the composition. Similar variations in the basal spacings depending on the number of attached organosilyl groups have been observed for mercaptopropylsilane (MPS)-modified octosilicate17 and for an analogous layered silicate, magadiite, modified with alkylsilanes.13−16,30,31 The controlled surface coverage with organosilyl groups has been reported for other silylated derivatives of layered silicates and titanates, and the surface coverage was reported to affect their adsorptive properties13−16 and swelling in organic solvents.38,39 When octosilicate was ion exchanged with organoammonium ions quantitatively (2 groups per Si8O17), the interlayer distance varied depending on the molecular weight of the organoammonium cations.8 This reflects that the intercalated guest species are densely packed to expand the interlayer space. For the organosilylated derivatives, the interlayer distance also depends on the molecular size of attached organosilyl groups when grafting occurs quantitatively (2 groups per Si8O17). The number of organosilyl groups also affects the interlayer distance, indicating that the intercalated functional silyl group tends to take densely packed states. If the sulfonation of the attached phenethyl group occurs quantitatively, then it is possible to control the surface coverage with sulfonic acid groups on the silicate sheets. Accordingly, the sulfonation of the phenethyl group was conducted using chlorosulfonic acid. The XRD patterns of the sulfonated products (SPhSx-octosilicates) are shown in the Supporting Information, where the basal spacing slightly increased from that of the starting materials (PhSx-octosilicates). The increase in the basal spacing after sulfonation was ascribed to the balance of the increase due to the attached sulfonic acid group and the decrease due to the partial decomposition of the phenethyl group by the oxidation via chlorosufonic acid during the sulfonation. To avoid decomposition, sulfonation was conducted under conditions as mild as possible in the present study.35,36 Consequently, the sulfonation of part of the phenethyl group was achieved while another part of the phenethyl group was removed from the silicate sheet during sulfonation. The decomposition of the anchoring organic group during sulfonation has been observed for the immobilization of sulfonic acid groups on mesoporous silicas.35,36 The 13C CP/MAS NMR spectra of Phs1.4-octosilicate and SPhS0.9-octosilicate are shown in Figure 3. PhS1.4-ocotosilicate shows signals at 19.1 and 29.5 ppm corresponding to the methylene C5 and C4 carbons and a signal at −1.5 ppm from the methyl C6 carbon. The signals occurring at 128.8 ppm were

Figure 2. TG-DTA curves of (a) octosilicate, (b) C16TMAoctosilicate, (c) PhS1.4-octosilicate, and (d) PhS1.4-octosilicate after sulfonation. TG curves () and DTA curves (···).

ions decompose at around 300 °C as seen in Figure 2b, which shows an exothermic reaction in the corresponding DTA curve. The thermal behavior of C16TMA-octosilicate is consistent with the previous report.8 The exothermic reaction was not seen in the DTA curves of the present silylated products to confirm the removal of C16TMA from the products during the silylation. The decomposition of PhSx-octosilicates appeared at higher temperature. The TG curves of PhSx-octosilicates showed the weight loss due to the oxidative decomposition of the organic group at around 300−500 °C. The numbers of attached phenethylsilyl groups were estimated from the TG curves to be 1.4, 1.0, and 0.70 groups per Si8O17 for the products prepared when the added amounts of silane coupling reagents were 2.0, 1.0, and 0.7 (groups per Si8O17), respectively (Table 1). Thus, the attachment of a controlled number of phenethyl groups was achieved by simply changing the added amount of silane coupling reagent in the reaction mixture. As reported previously for the silylation of magadiite,10,30,31,37 it is thought that the immobilization of a larger number of organosilyl groups (as large as 2 groups per Si8O17) will be possible when a large excess of silane coupling reagents was used. There is a linear relationship between the basal spacing of PhSx-octosilicates and the number of attached phenethyl groups (Table 1), indicating that the phenethyl group is distributed on the silicate sheet homogeneously in the three silylated derivatives and the distance between adjacent phenethyl groups is thought to vary systematically depending 7507

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parameters of the silicate), the distances between the adjacent sulfonic acid groups were calculated to be 0.73, 0.82, and 0.97 nm (equivalent to the occupied area of each group as 1.7, 1.3, and 1.1 groups/nm2) in SPhS0.9-octosilicate, SPhS0.8-octosilicate, and SPhS0.6-octosilicate, respectively. The silylation and subsequent sulfonation of octosilicate were also followed by 29Si NMR spectroscopy. The 29Si NMR spectra of octosilicate and SPhS0.9-octiosilicate are shown in Figure 4. 29Si-MAS NMR spectra of the octosilicate and

Figure 3. 13C CP/MAS NMR spectra of (a) PhS1.4-octosilicate, and (b) after sulfonation.

attributed to the aromatic carbons. SPhS0.9-octosilicate shows new signals at 143.1 and 137.5 ppm, which were attributed to the aromatic hydrocarbon attached to the sulfonic acid group. These results are consistent with those reported for the silica gel system as well as surfactant-templated mesoporous silica systems, where sulfonic acid groups were attached to the phenethyl groups on the surface, confirming the successful immobilization of the sulfonated phenethyl groups on the surface.35,36 The number of remaining phenethyl groups (or immobilized sulfonated phenethyl groups) was determined from the TG curves. The TG-DTA curves of the sulfonated product prepared from PhS1.4-octosilicate are shown in Figure 2d as an example, where an exothermic reaction (in the DTA curve) accompanying weight losses (in the TG curve) was seen in the temperature range of 400−600 °C, which is higher than that observed for PhSx-octosilicates. From the TG curves, the number of sulfonated phenethyl groups was estimated to be 0.9, 0.8, and 0.6 for the products prepared from PhS1.4-, PhS1.0-, PhS0.7-octosilicates, respectively. These values were consistent with the population of acidic sites determined by titration with an aqueous solution of NaOH (Table 1). The basal spacings of SPhSx-octosilicates increase depending on the composition (Supporting Information), indicating that sulfonic acid groups are distributed on the silicate layer homogenously, and the particle-level heterogeneity is less plausible. From the XRD results and the adsorption probe study, the possible controlled, homogeneous distribution of organic functionalities on layered solids has been suggested.13−17,30,31,37 Depending on the materials and synthesis methods, the organic functionality may heterogeneously distribute between particles to give split (intercalated and nonintercalated phase) XRD patterns. In the present case, all of the samples gave single-phase XRD patterns to exclude the possibility of particle-level heterogeneity. The structures of the products were discussed with respect to the chemical composition. On the basis of the size of the sulfonated phenethyl groups and the composition of the products, the expected values of the interlayer expansion were calculated to be 0.66 and 0.65 nm for PhS1.4-octosilicate and SPhS0.9-octosilicate, respectively. These values are consistent with the experimentally determined values, indicating that the organic functionalities are densely packed in the interlayer spaces. On the basis of the composition (Table 1) and the available surface area of octosilicate, 0.53 nm2/Si8O17unit cell (ab = 0.73 nm × 0.73 nm, where a and b are the lattice

Figure 4. 29Si MAS NMR spectra of (a) octosilicate and (b) PhS1.4octosilicate and (c) after sulfonation.

C16TMA-octosilicate showed similar signals due to the Q3 and Q4 environments of silicon, showing that the structural regularity did not change during the ion-exchange process.29 29 Si-MAS NMR spectrum of PhS1.4-octosilicate (Figure 4b) showed the new signals due to the D environment of silicon (R2-Si(OSi)2) at around −15 ppm and a decrease in the relative intensity of the Q3 signal at −100 ppm compared to the Q4 signal at around −110 ppm (Q3/Q4 = 0.13 and 1.33 for Naoctosilicate, respectively), which indicated that ca. 90% of the silanol group was functionalized with the phenethyl silyl group. After sulfonation, the Q3 (Q3/Q4 = 0.39) signal increased compared to that of PhS1.4-octosilicate and that of the D environment of silicon decreased. From the change in the Q3/ Q4 ratio, ca. 72% of the silanol group was functionalized with phenethyl sulfonic acid groups. This value is not consistent with the amount of acid (0.9 group per Si8O17) determined by titration or the organic content (0.9 group per Si8O17) determined by TG. Considering the broadening of the signals, structural modification during the silylation and subsequent sulfonation occurred. After the silylation, the (400) reflection was not clearly seen in the XRD pattern, supporting the structural modification in the crystalline silicate framework. Acid−Base Reaction and Cation Exchange. The SPhSoctosilicate prepared in the present study can possible be a solid acid for a variety of applications. One of the characteristics of the present materials is their controlled acidic strength. The present samples contain an acidic functionality in the interlayer space, which shows limited substrate access if compared to those of large pore materials such as silica gels and mesoporous silicas. Accordingly, we have to conduct the characterization of acidity under carefully determined conditions, and comparing with other solid acid catalysts is not simple. To evaluate the acidic characteristics of SPhS-octosilicates, where the density of sulfonic acid on the silicate layer varies, the cation exchange 7508

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NMR) and the number of attached phenethyl sulfonic acid groups (0.9 groups per Si8O17 by TG or titration, Table 1). Thus, the expected amounts of interlayer expansion for SPhS0.9octosilicate after reaction with CnTMA are 73% of that of the octosilicate and are correlated to the experimentally determined values (Figure 5). These observations confirmed that the layer charge was successfully controlled by the number of attached sulfonated phenethyl groups. This idea was supported by the fact that the ion exchange of SPhS-octosilicates with C16TMA resulted in products with basal spacings corresponding to the number of attached sulfonated phenethyl groups (2.62, 2.28, and 1.88 nm for SPhS0.9-, SPhS0.8-, and SPhS0.6-octosilicates, respectively). The Supporting Information shows the XRD patterns of SPhS0.9-octosilicate after the reaction with dodecylamine, wherein the basal spacing increased from 1.65 to 2.14 nm. However, H+-octosilicate did not expand the interlayer space by the reaction with dodecylamine under the same reaction conditions (Supporting Information). The stronger acidity of sulfonic group and the organophilic interactions between dodecylamine and the phenethyl group are through to concern the difference in the reactivity toward dodecylamine. Thus, in addition to the controllable amount of acidic functionality on the layer surface, the immobilization of sulfonic acid groups resulted in the products with base-exchange abilities different from those of original silicic acid (H+-octosilicate). The presently prepared layer-charge-controllable organosilicates may find applications as solid electrolytes, catalysts, and scaffolds for the guest organization of functional host−guest materials. Accordingly, further characterization of the present products and the preparation of sulfonated octosilicates with different composition are being carried out in our laboratory, and the results will be reported at a later time.

reaction of SPhS-octosilicate with alkyltrimethylammonium was conducted. The basal spacings of the products after the ion exchange of SPhS0.9-octosilicateare with CnTMA are summarized in Figure 5 to show that the basal spacing increased

Figure 5. Variation of the basal spacing of CnTMA exchanged forms as a function of the number of carbon atoms in the alkyl chain for (○) SPhS0.9-octosilicate and (×) octosilicate.7

linearly as a function of the alkyl chain length of the intercalated CnTMA ions. The XRD patterns of the products prepared by the cation exchange of CnTMA with SPhS0.9-octosilicate are shown in the Supporting Information. The variation of the basal spacing of alkyltrimethylammonium-octosilicates has already been reported.8 The inclination of the relationship reflects the layer charge density and is often used to estimate the layer charge density of various layered materials.40 Judging from the inclination seen in Figure 5, the surface layer charge decreased after the modification. This was caused by the incomplete sulfonation of the attached phenethyl group. On the basis of the number of intercalated alkyltrimethylammoniums and the number of grafted sulfonated phenethyl groups, the packing of the organic functionality can be discussed. The surface area of the interlayer space of the octosilicate was estimated to be ca. 0.53 nm2 (a × b), and the interlayer volume was calculated from the following equation.13−16

4. CONCLUSIONS The surface modification of a layered alkali silicate, octosilicate (Na2Si8O17·nH2O), with sulfonic acid groups was conducted to obtain solid acids with different amounts of surface acidic functionality. A controlled number of phenethyl groups was attached to the silicate layer by the reaction of octosilicate with phenethyl(dichloro)methylsilane using hexadecyltrimethylammonium-exchanged octosilicate as the intermediate. The attached phenethyl group was sulfonated by its reaction with chlorosulfonic acid. The cation exchange reactions of octosilicate were modified with the sulfonated phenethyl group toward a series of quaternary alkylammonium ions to show the variation in the layer charge density. The products also accommodate dodecylamine, which has not been intercalated into the protonated octosilicate, probably as a result of hydrophobic interactions and acid−base reactions with a strongly acidic site (the sulfonated phenethyl group).

interlayer volume (nm 3/Si8O17 ) = interlayer spacing Δd(nm) × interlayer surface area(nm 2/Si8O17 )

The size of sulfonated phenethyl groups was calculated on the assumption that the molecule is a rectangular parallelepiped. From these calculations, the degrees of packing are ca. 103% for SPhS0.9-oct. After the reaction with CnTMA, the packing densities are ca. 111, 103, 97, and 82% for SPhS0.9octosilicate/C22TMA, SPhS0.9-octosilicate/C16TMA, SPhS0.9octosilicate/C12TMA, and SPhS0.9-octosilicate/C6TMA, respectively. Consequently, the intercalated alkyltrimethylammonium ions are densely packed in the interlayer space to give paraffin-type structures. The negative charge of octosilicate comes from the hydroxyl group, and there are thought to be 2 groups per Si8O17 unit. However, the surface charge density of SPhS0.9-octosilicate caused by the remaining OH groups and phenethyl sulfonic acid groups was calculated to be 73% of that of Na-octosilicate, from the Q3 to Q4 ratio (Q3/Q4) of Na-octosilicate and SPhS0.9-octosilicate (1.33 and 0.39, calculated from 29Si MAS

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ASSOCIATED CONTENT

S Supporting Information *

Relationship between the basal spacing and the number of attached groups for functionalized octosilicates. XRD patterns of functionalized octosilicates. Products after the ion exchange with alkylammonium ions or C12NH2. This material is available free of charge via the Internet at http://pubs.acs.org. 7509

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Tel +81-3-5286-1511. Fax +81-33207-4950. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Global COE Program of MEXT and a Waseda University grant for special research projects (2011A-604 and 2010B-061).

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