Design of Microporous Material HUS-10 with Tunable Hydrophilicity

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Design of Microporous Material HUS-10 with Tunable Hydrophilicity, Molecular Sieving, and CO2 Adsorption Ability Derived from Interlayer Silylation of Layered Silicate HUS‑2 Nao Tsunoji,*,† Sota Yuki,† Yasunori Oumi,§ Miyuki Sekikawa,‡ Yukichi Sasaki,‡ Masahiro Sadakane,† and Tsuneji Sano*,† †

Graduate School of Engineering, Department of Applied Chemistry, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan ‡ Japan Fine Ceramics Center, Atsuta-ku, Nagoya 456-8587, Japan § Division of Instrument Analysis, Life Science Research Center, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan S Supporting Information *

ABSTRACT: The attractive properties of zeolites, which make them suitable for numerous applications for the energy and chemical industries and for life sciences, are derived from their crystalline framework structures. Herein, we describe the rational synthesis of a microporous material, HUS-10, utilizing a layered silicate precursor, HUS-2, as a structural building unit. For the ordered micropores to be formed, interlayer pillars that supported the original silicate layer of HUS-2 were immobilized through the interlayer silylation of silanol groups with trichloromethylsilane and a subsequent dehydration−condensation reaction of the hydroxyl groups on the preintroduced tetrahedral units. An actual molecular sieving ability, enabling the adsorption of molecules smaller than ethane, was confirmed in the ordered micropores of HUS-10. The hydrophilic adsorption could also be controlled by changing the number of methyl and hydroxyl groups in the immobilized interlayer pillars. In addition, when the adsorption behaviors of CO2, CH4, and N2 on HUS-10 were compared to those on siliceous MFI and CDO zeolites with approximately the same pore diameter, the CO2 adsorption capacity of HUS-10 was comparable. Conversely, because of the adsorption inhibition of CH4 and N2, HUS-10 exhibited larger CO2/CH4 and CO2/N2 adsorption ratios relative to those of MFI and CDO zeolites. These results reveal that the unique microporous framework structure presented by the rational structural design using the layered silicate precursor HUS-2 has the potential to separate CO2 from gas mixtures. KEYWORDS: layered silicate, zeolite, interlayer silylation, CO2 adsorption, molecular sieve

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INTRODUCTION

structures to obtain desired physicochemical properties is still a critical issue. For more rational zeolite synthesis to be accomplished, a different synthesis approach using two-dimensional silicate precursors as a “structural building unit” has attracted considerable attention.7−11 Layered precursors, such as zeolitic materials and silicates, possess reactive neighboring silanol (SiOH and SiO−) groups regularly located on the layered silicate surface. Because the interlayer silanol groups are easily functionalized by various modifications, such as cation exchange, silylation, and pillaring, layered silicates have attracted interest in many applications, including catalysis and adsorption.8,12 In addition, because the silicate layer framework exhibits structural similarity with the zeolite framework, transformation of the layered precursor into a zeolite can be

Zeolites, which have solid acidity, high internal surface area, and molecular sieving and ion-exchange abilities, are important materials in the chemical industry. Owing to their attractive structural features in terms of selective separation, storage, and conversion of suitable chemicals, they have been utilized in environmentally friendly and economically beneficial applications, such as hydrocarbon conversion in the petroleum industry and separation and refining processes in chemical industries.1−3 To date, significant effort has been made to synthesize novel zeolites having new framework structures and beneficial physicochemical features using complex structuredirecting agents, isomorphous substitution of framework atoms, and other novel synthesis strategies.4−6 However, in general, the development of traditional hydrothermal zeolite synthesis is based on continuing zeolite preparation and research of their fundamental properties, which require step-by-step operations and are very labor-intensive. Therefore, the design of zeolite © XXXX American Chemical Society

Received: August 27, 2015 Accepted: October 19, 2015

A

DOI: 10.1021/acsami.5b07996 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

transport and eliminating diffusional limitations into the micropores. Equally important is that new types of microporous materials with attractive structural features can be obtained by successful silylation of layered silicate precursors with novel framework structures. Roth et al. reported the formation of novel zeolites obtained from the structural transformation of the layered precursor IPC-1P with a silicate layer framework that was prepared by removing framework germanium from UTL zeolite. Two types of crystalline microporous materials, IPC-2 and IPC-4, were prepared through the interlayer condensation and silylation reaction.22 Furthermore, the potential of the silylated derivative of layered silicate RUB-51 as a separation medium between CO2 and CH4 was also reported by Asakura et al.37 Thus, a new methodology to attain beneficial structural features of zeolite derived from novel layered silicate precursors is key for more rational design of innovative microporous materials. Recently, we reported the successful synthesis and structural analysis of several novel layered silicates, Hiroshima University Silicates (HUSs), which were synthesized using alkylammonium cations with different structures.38−45 We have already evaluated their adsorption40−42,44 and potential as precursors for catalysts39,45,46 and porous materials.43,46 Among them, HUS-2 has a unique and novel framework structure that consists of 4-, 5-, and 6-membered rings and is somewhat similar to HEU zeolites including bre(10T)-type composite building units (Figure 2).39 The reactive neighboring silanol

achieved by post-treatment of the ordered interlayer silanol groups. The constructed porous framework structure thus directly reflects the silicate layer precursor framework. That is, if the crystal structures of the layered precursors are determined, then we can easily speculate about the framework structures of zeolites that are obtained. The structural transformation from the layered precursor to zeolite includes dehydration and condensation (topotactic conversion)13−24 and interlayer silylation.25−37 The topotactic conversion proceeds by the dehydration−condensation of the interlayer silanol groups on both sides of the silicate layers (Figure 1, top). Several novel

Figure 1. Formation of interlayer micropore in the layered silicate PLS-3. The circular dotted line represents neighboring silanol (SiOH and SiO−) groups in the interlayer.26

zeolites, such as CDO (the three characters indicate the framework type),15 NSI,16 RWR,17 and RRO,18 were obtained by only the topotactic conversion of layered silicate precursors. Additionally, Moteki et al. first reported the synthesis of siliceous SOD zeolite without occluded organic matter by topotactic conversion of layered silicate RUB-15 and its physical adsorption of hydrogen molecules.19 In contrast, interlayer silylation, the incorporation of new Si−O−Si pillars into the interlayer to form connections between stacked silicate layers, gives more designable micropore construction and the formation of interlayer pores larger than those resulting from the topotactic dehydration−condensation process (Figure 1, bottom). Silicate layer precursors, including PLS-1,25 PLS-3,26 PLS-4,26 MCM-47,27 PREFER,26,28 MWW(P),28 RUB-36,32−35 RUB-39,31,32 RUB-51,37 and Nu-6(1),36 have been silylated with various silylating agents, such as chlorosilane or alkoxysilane. Inagaki et al. prepared interlayer expanded zeolite (IEZ)-1 by interlayer silylation of the layered silicate PLS-1 with dichlorodimethylsilane. The prepared organic−inorganic hybrid microporous material exhibited specific adsorption for benzene because of the presence of organic (methyl) groups retained in the interlayer micropores.25 The crystal structures of the interlayer expanded porous materials were determined by using several analysis methods, such as high-resolution transmission electron microscopy29 and powder X-ray diffraction.26,30 Furthermore, several groups have performed interlayer silylation of aluminosilicate-layered precursors and evaluated the catalytic performance of the obtained porous materials as solid acids.28,34,35 In the studies, the catalytic activity of the IEZ-like porous material was higher than those of the unexpanded materials as a result of improving the molecular

Figure 2. (A) Crystal structure of HUS-2. The silicate framework structures along the (B) c and (C) b axes. The silicate layer consisted of 4-, 5-, and 6-membered rings and (D) bre-type composite building units. The circular dotted lines represent neighboring silanol (SiOH and SiO−) groups in the interlayer.

(SiOH and SiO−) groups are located along the a axis in a linear fashion. The linear arrangements consisting of the neighboring silanol are located along the c axis with a relatively long periodic distance of 0.96 nm. Herein, we achieved the designed synthesis of microporous HUS-10 derived from the interlayer silylation and subsequent calcination of the layered silicate HUS-2. We also evaluated the molecular sieving ability, tunable hydroB


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Figure 3. Formation of ordered microporous material HUS-10 from the layered silicate HUS-2.

Figure 4. (A) XRD patterns, (B) 29Si MAS NMR spectra, and (C) 13C CP MAS NMR spectra of (a) HUS-2, (b) HUS-2(C12TMA), and (c) HUS2S. Ind. Co. Ltd., Japan, 0.33 mol). The mixture was stirred at 60 °C for 24 h and then centrifuged to remove the supernatant. This procedure was repeated three times. The obtained solid was separated by centrifugation, washed with a water/ethanol (50/50 vol %) solution, and then dried at 70 °C overnight. Interlayer Silylation and Calcination of HUS-2 [HUS-2S, HUS10(200−550)]. HUS-2(C12TMA) (2.0 g) was dried in a vacuum at 70 °C for 3 h and then dispersed in dehydrated toluene (50 mL). An excess amount (1.5 g, 5 equiv of 4 SiO−/SiOH groups) of trichloromethylsilane (Tokyo Chemical Ind. Co. Ltd.) and 2,2′bipyridyl (3.3 g, Tokyo Chemical Ind. Co. Ltd.) were added to the mixture and stirred at 100 °C for 3 d under a N2 atmosphere. The resulting solid product was centrifuged and washed with toluene and then dried at 70 °C overnight. For the HUS-2S to be transformed into a porous material through the interlayer dehydration−condensation reaction, HUS-2S was calcined at 200, 400, 500, and 550 °C for 6 h. The resulting samples are denoted as HUS-10(200), HUS-10(400), HUS-10(500), and HUS-10(550), respectively. Characterization. Powder X-ray diffraction (XRD) patterns of the solid products were collected using a powder X-ray diffractometer (Rigaku MiniFlex) with graphite-monochromatized Cu Kα radiation at 30 kV and 15 mA. The crystal morphology was observed using a Hitachi S-4800 scanning electron microscope (SEM) coupled with an energy-dispersive X-ray (EDX) analyzer. 29Si magic-angle spinning (MAS) NMR spectra were recorded at 119.17 MHz on a Varian 600PS solid NMR spectrometer using a 6-mm diameter zirconia rotor. The rotor was spun at 7 kHz. The spectra were acquired using 6.7 μs pulses, a 100 s recycle delay, and 1000 scans. 3-(Trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt was used as a chemical shift reference. 1H−13C cross-polarized (CP)-MAS NMR spectra were measured with a spinning frequency of 7 kHz, a 90° pulse length of 2.2 μs, and a cycle delay time of 100 s. The 13C chemical shifts were referenced to hexamethylbenzene. Thermal analyses were carried out using a thermogravimetric/differential thermal analysis (TG/DTA) apparatus (SSC/5200, Seiko Instruments). A sample (∼3 mg) was

philicity, and potential application for CO2 adsorption of the obtained porous material.

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EXPERIMENTAL SECTION

Preparation of Crystalline Porous Silicas. Figure 3 exhibits the formation scheme of the ordered microporous material HUS-10 from the layered silicate HUS-2. First, to keep the stacked silicate layers in a state suitable for the formation of the porous structure during several post treatments, choline cations in the interlayer of HUS-2 were ion exchanged with dodecyltrimethylammonium cations, yielding ionexchanged HUS-2 [HUS-2(C 12TMA)]. Subsequently, HUS-2(C12TMA) was silylated with trichloromethylsilane. The introduced tetrahedral unit derived from the silylation reacted with two silanol groups on the silicate layer surface. As a result of hydrolysis after the silylation, one methyl group remained and one hydroxyl group formed on the immobilized tetrahedral unit (HUS-2S). Finally, by calcination, the dehydration−condensation of the hydroxyl groups proceeded between two immobilized tetrahedral units on both sides of the silicate layers, affording the successful formation of micropores in HUS-10. Synthesis of Layered Silicate HUS-2. HUS-2 (Si20O40(OH)4· 4[C5H14NO])39 was synthesized by hydrothermal treatment of starting gels containing fumed silica (Cab-O-Sil M5, Cabot Co.), choline hydroxide (48−50 wt %, Aldrich), sodium hydroxide (>99%, High Purity Chemicals Inc., Japan), and distilled water. The starting gel, with the chemical composition SiO2:0.4 choline hydroxide:0.1 NaOH:5.5 H2O, was transferred into a Teflon-lined stainless steel vessel and heated under static conditions at 150 °C for 2 d. The solid product obtained was separated by centrifugation, washed with distilled water, and dried at 70 °C overnight. Ion Exchange of HUS-2 with Dodecyltrimethylammonium Cations [HUS-2(C12TMA)]. HUS-2 ions exchanged with dodecyltrimethylammonium cations [HUS-2(C12TMA)] were prepared as follows. HUS-2 (10 g) was dispersed in an aqueous solution (500 mL) of dodecyltrimethylammonium chloride (C12TMACl, Tokyo Chemical C


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ACS Applied Materials & Interfaces heated in a flow of air (50 mL min−1) at a heating rate of 10 °C min−1 from room temperature to 800 °C. Nitrogen and argon adsorption isotherms were obtained at −196 and −189 °C, respectively, using a conventional volumetric apparatus (BELSORP-max, Bel Japan). Prior to performing the adsorption measurements, the calcined samples (∼50 mg) were heated at 200 °C for 3 h under evacuation. Adsorption measurements of H2O, CO2, CH4, C2H6, n-butane, and isobutene were also conducted. Elemental analysis was carried out using a PerkinElmer 2400 II CHN analyzer at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University. Transmission electron microscopy (TEM) images were taken using a Hitachi H9500 microscope at an acceleration voltage of 300 kV.

surface, and the immobilized unit retained one methyl group and formed one hydroxyl group as a result of hydrolysis (see Figure 3, center). However, we could not rule out the possible presence of internal silanol groups within the silicate framework of HUS-2 as evidenced by 29Si CP MAS NMR. SEM images of HUS-2, HUS-2(C12TMA), and HUS-2S confirmed the successful ion exchange and silylation of HUS-2 without damage to its crystal morphology (square, plate-like) or size (1.0 × 3.0 μm) (Figure 5A−C).

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RESULTS AND DISCUSSION Ion Exchange and Interlayer Silylation of HUS-2. Lowangle X-ray diffraction (XRD) patterns show diffraction peaks attributed to basal spacing of HUS-2, HUS-2(C12TMA), and HUS-2S at 7.5° (d-spacing = 1.2 nm), 4.6° (d-spacing = 1.96 nm), and 5.6° (d-spacing = 1.62 nm), respectively (Figure 4A). The 29Si magic-angle spinning (MAS) NMR spectra of HUS2(C12TMA) (Figure 4B) exhibited two resonance peaks between −90 and −105 ppm, which were assigned to a Q3 [HOSi(OSi)3 or −OSi(OSi)3] structure with silanol or silanolate groups on the silicate layer surface, whereas the three resonance peaks between −105 and −120 ppm were assigned to a Q4 (Si(OSi)4) structure within the silicate framework. These five resonance peaks are attributed to five independent T-sites within the HUS-2 framework. Because there was no change in the peak intensity ratios of Q3/Q4 (40/ 60) before or after ion exchange, we confirmed no collapse of the original silicate layer of HUS-2 during the ion-exchange treatment. The 1H−13C cross-polarized (CP) MAS NMR spectrum of HUS-2 (Figure 4C) revealed the presence of interlayer choline cations, as evidenced by three resonance peaks at approximately 55, 57, and 70 ppm. After ion-exchange treatment of HUS-2 [HUS-2(C12TMA)], the resonance peaks attributed to the choline cations disappeared and new resonance peaks derived from the C12TMA molecules appeared at 10−80 ppm. Also, CHN elemental analysis revealed that the carbon and nitrogen molar ratios (C/N ratios) of HUS-2 and HUS-2(C12TMA) were 5.0 and 14.8, respectively (Table S1). These results indicate the complete removal and ion exchange of the interlayer choline cations with dodecyltrimethylammonium cations and interlayer expansion. Upon the subsequent silylation of HUS-2(C12TMA) with trichloromethylsilane (HUS-2S), the Q3 peak completely disappeared, and a new resonance peak was observed at −54 ppm in the 29Si MAS NMR spectrum. Because the peak intensity at −54 ppm was relatively enhanced as compared to the other peaks during the 1H− 29 Si CP MAS NMR measurements (Figure S1), the resonance peak was assignable to a T2 [(H3C) (HO)Si(OSi)2] structure with hydroxyl groups. The 13C CP MAS NMR spectrum of HUS-2S revealed the presence of methyl groups on the T2 silicon derived from trichloromethylsilane, as the evidenced by the resonance peak at approximately −2.9 ppm, whereas the resonance peaks assigned to toluene and 2,2′-bipyridyl were observed at 80−160 ppm. Taking into account the fact that the 29Si MAS NMR results revealed that (1) the coverage percentage of surface silanols by silylation was approximately 100% and (2) the peak intensity ratio of T2/Q4 in HUS-2S was calculated to be 16/84, the introduced tetrahedral unit derived from trichloromethylsilane was reacted with two silanol groups on the silicate layer

Figure 5. SEM images of (A) HUS-2, (B) HUS-2(C12TMA), (C) HUS-2S, and (D) HUS-10(400).

Interlayer Dehydration and Condensation of HUS-2S. For carrying out the dehydration−condensation reaction of the remaining hydroxyl groups on the tetrahedral units immobilized on both sides of the silicate layers for construction of interlayer micropores, HUS-2S was calcined at temperatures of 200, 400, 500, and 550 °C for 6 h, affording HUS-10(200), HUS-10(400), HUS-10(500), and HUS-10(550), respectively. The SEM image of HUS-2S calcined at 400 °C [HUS-10(400)] revealed a retention of the crystal morphology, which exhibited square and plate-like particles (Figure 5D). As shown in the XRD patterns of HUS-10(200), HUS-10(400), HUS-10(500), and HUS-10(550) (Figure 6A), all the calcined samples as well as HUS-2, HUS-2(C12TMA), and HUS-2S showed a single phase, because if the interlayer distance is disordered, the basal spacing of the (010) peaks would split or broaden. Although there were no changes in the basal spacing for HUS-2S and the HUS-10(200) upon calcination at 400 °C [HUS-10(400)], the basal spacing decreased to 1.17 nm (2θ = 7.6°). When the calcination temperature was elevated to 500 and 550 °C, the change in the basal spacing of calcined products was minimal. The high-angle XRD patterns (Figure 6A) of all the calcined samples showed diffraction peaks at 10.4 and 13.4°, indicating that the d-spacing was consistent with the periodic distance of the (001) and (100) planes of the original silicate layer of HUS2, implying that the original silicate framework structure hardly changed through the silylation and subsequent calcination. The transmission electron microscope (TEM) image and electron diffraction pattern of HUS-10(400) are shown in the Figure 7. As shown in the figure, a net-like electron diffraction pattern is clearly observed in a plate-like particle with a size of 1.0 × 3.0 μm. This demonstrates that HUS-10S(400) was well-crystallized. The d-spacing of several electron diffractions were also in good agreement with the XRD measurement. For gaining insight into the detailed structure of stacked silicate layers, MAS NMR measurements of the calcined D


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Figure 6. (A) XRD patterns, (B) 29Si MAS NMR spectra, and (C) 13C CP MAS NMR spectra of (a) HUS-10(200), (b) HUS-10(400), (c) HUS10(500), and (d) HUS-10(550).

that interlayer C12TMA molecules were removed and that the environment of the immobilized tetrahedral units was changed by calcination. The presence of a T3 peak strongly indicates the dehydration−condensation reaction between the remaining silanol groups connected to tetrahedral units immobilized on both sides of the silicate layer. In addition, the peak intensity of (T2 + T3 + Q3)/Q4 for the calcined samples and HUS-2S, which reflects the number of immobilized tetrahedral units against the original silicate layer framework of HUS-2, was essentially the same, indicating that the cleavage of Si−O−Si bonds in the silicate layer framework and excess condensation reactions were negligible. It also indicates that the new resonance Q3 peak is based on the immobilized tetrahedral units owing to the decomposition and hydration of methyl groups by calcination. The peak intensity of the Q3 peak gradually increased with increasing calcination temperature and that of the T3 peak decreased monotonically, whereas a decrease in the peak intensity at −6.5 ppm in the 13C CP MAS NMR spectra and a decrease in the organic entity contents in the calcined samples were also observed (Table S1). These results strongly indicate that the methyl groups on the immobilized tetrahedral units that support the stacked silicate layers gradually decomposed and were replaced with hydroxyl groups. Figure 8 shows the proposed structural model for HUS-10 obtained by density functional theory (DFT) calculations based on the original crystal structure of HUS-2 and the XRD pattern and 29Si MAS NMR results of HUS-10(400). The simulated model shows that the porous networks of HUS-10 consist of silicate layers and interlayer pillars formed by interlayer silylation and a condensation reaction. This model exhibits newly formed interlayer Si−O−Si rings, including an 8-ring along the c axis, a 12-ring along the a axis, and Si-CH3 groups located along the hollow region in the 12-ring. Porosity and Adsorption Properties of HUS-10. The fundamental pore characteristics of HUS-10 were evaluated by

Figure 7. TEM image of HUS-10(400).

samples were carried out. Panels B and C in Figure 6 show the 29 Si MAS NMR and 13C CP MAS NMR spectra of HUS10(200), HUS-10(400), HUS-10(500), and HUS-10(550). There was no change in the 29Si MAS NMR spectra before and after calcination at 200 °C, indicating that structural changes to the silicate framework, such as cleavage of Si−O−Si bonds or condensation between surface silanol groups, were minimal. The 13C CP MAS NMR spectrum of HUS-10(200) suggests the presence of methyl groups on the immobilized tetrahedral unit derived from trichloromethylsilane and C12TMA molecules, as evidenced by the resonance peaks at −2.9 ppm and between 10−80 ppm, respectively. Upon the calcination at ≥400 °C [HUS-10(400), HUS-10(500) and HUS-10(550)], the resonance peaks at 10−80 ppm attributed to the C12TMA molecule completely disappeared, and the peak position derived from the methyl groups on immobilized tetrahedral units was shifted to −6.6 ppm. In contrast, the 29Si MAS NMR spectra of HUS-2(400), HUS-2(500), and HUS2(550) showed new resonance peaks at −65 and −100 ppm, which were assigned to the T3 [(CH3)Si(OSi)3] and Q3 [Si(OSi)3(OH)] structures, respectively. These results suggest E


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exhibited very narrow pore size distributions with an average pore diameter of 0.50 nm observed in HUS-10(400), HUS10(500), and HUS-10(550). Among the attractive properties of zeolite, the most exciting is its molecular sieving ability. It can recognize guest molecules smaller than its pore size, which is essential to catalyze or adsorb desired substrate selectively and effectively. For clarifying the actual molecular sieving ability of HUS-10, the molecular probe method using various gases with different molecular sizes was applied. The adsorption measurements were performed near the boiling point of each of the adsorbents, and the adsorbed amount was estimated as a liquid phase. Figure 10A shows the adsorption isotherms of CO2,

Figure 8. Proposed crystal structural model of HUS-10(400) along the (A) a and (B) c axes. Color bonding: yellow = Si, red = O, brown = C, and white ball = H.

N2 and Ar adsorption isotherms (Figure 9). The textural properties of HUS-10 calculated from the isotherms are listed

Figure 10. (A) Adsorption isotherms of (●) CO2, (×) C2H6, (▲) nbutane, and (■) isobutene on HUS-10(400). (B) Relationship between micropore volume as determined by the adsorption isotherms and molecular sizes of the provided molecules. Figure 9. (A) N2 adsorption isotherms and (B) pore distributions determined by Ar adsorption isotherms for (■) HUS-10(400), (◊) HUS-10(500), and (▲) HUS-10(550).

C2H6, n-butane, and isobutene on HUS-10(400). Only 0.05 and 0.03 cm3 g−1 of isobutene and n-butane, respectively, were adsorbed. Larger amounts of CO2 and C2H6 were adsorbed at 0.19 and 0.14 cm3 g−1, respectively. The micropore volumes calculated from the adsorption isotherms using the Dubinin− Astakhov (DA) method are plotted against the molecular sizes in Figure 10B. The micropore volumes from the N2 and Ar adsorption isotherms were also added. The micropore volumes estimated using larger molecules, isobutene (0.56 nm) and nbutane (0.49 nm), were only 0.005 and 0.014 cm3 g−1, respectively, whereas a higher micropore volume of 0.10 cm3 g−1 was observed on the smaller molecule C2H6 (0.42 nm). A further increase in the micropore volume was observed on Ar (0.38 nm), which was calculated to be 0.15 cm3 g−1. Although CO2 (0.28 nm) and N2 (0.30 nm) have smaller molecular sizes than Ar, the micropore volumes were consistent with that from the Ar adsorption isotherm. Furthermore, the maximum micropore volume (0.15 cm3 g−1) and the average pore

in Table 1. When HUS-2S was calcined at 200 °C [HUS10(200)], the micropore volume and Brunauer−Emmett− Teller (BET) area were calculated to be only 0.01 cm3 g−1 and 10 m2 g−1, respectively, because the presence of C12TMA molecules occluded the interlayer space. However, when the calcination was performed between 400 and 550 °C [HUS10(400), HUS-10(500), and HUS-10(550)], a steep increase in the N2 adsorption amount at low relative pressure was observed, which suggests the existence of micropores. The BET areas and micropore volumes were 385 m2 g−1 and 0.16 cm3 g−1 for HUS-10(400), 378 m2 g−1 and 0.14 cm3 g−1 for HUS-10(500), and 362 m2 g−1 and 0.13 cm3 g−1 for HUS10(550). The pore size distributions calculated from the Ar adsorption isotherms using the Saito−Foley (SF) method

Table 1. Characteristics Determined by N2 Adsorption Isotherms and 29Si MAS NMR Spectra relative intensity ratio in 29Si MAS NMR spectra a

2

−1

BET area (m g ) HUS-2 HUS-2(C12TMA) HUS-2S HUS-10(200) HUS-10(400) HUS-10(500) HUS-10(550) a

10 385 378 362

micropore volume

b

(cm3(liq.)

−1

c

g )

0.01 0.15 0.14 0.13

average pore diameter (nm)

T2

T3

Q3

Q4

(T2 + T3 + Q3):Q4

0.50 0.50 0.50

0 0 16 16 0 0 0

0 0 0 0 14 10 5

40 40 0 0 3 7 13

60 60 84 84 83 83 82

40:60 40:60 16:84 16:84 17:83 17:83 18:82

Determined by the BET method. bDetermined using the t-plot. cDetermined using Ar adsorption isotherms and the SF method. F


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on HUS-10(400), HUS-10(500), and HUS-10(550) at 25 °C. The amount of adsorbed H2O increased with increasing calcination temperature of HUS-10. The amounts of adsorbed H2O on HUS-10(400), HUS-10(500), and HUS-10(550) were calculated to be 83, 137, and 156 cm3 g−1, respectively. Conversely, the molar ratios of the methyl and hydroxyl groups, CH3/(CH3 + OH), connected to the introduced tetrahedral units on HUS-10(400), HUS-10(500), and HUS-10(550) were 0.82, 0.59, and 0.28, respectively. Thus, it appears that the hydrophobic methyl groups were gradually replaced with hydrophilic hydroxyl groups as a result of calcination at higher temperatures. The hydrophilicity of the interlayer micropores of HUS-10 increased, thus affording successful control of the adsorption properties. To estimate the potential of HUS-10 as an adsorption material, we evaluated the adsorption behaviors of CO2, CH4, and N2. For reference, we also used siliceous MFI with a threedimensional (0.51 × 0.55 nm, 0.53 × 0.56 nm) pore-channel system and CDO zeolite with a two-dimensional (0.31 × 0.47 nm, 0.25 × 0.42 nm) pore-channel system. The adsorption isotherms were obtained from single-component. Figure 12 shows the adsorption isotherms of CO2, CH4, and N2 at 25 °C, and Table 2 shows the textural properties of zeolite and the HUS-10s evaluated from the adsorption isotherms. HUS10(400), HUS-10(500), and HUS-10(550) adsorbed CO2 in the amounts of 23.5, 25.4, and 32.4 cm3 g−1, respectively, suggesting that the larger amounts of CO2 were adsorbed on HUS-10 with the larger number of hydroxyl groups. This may be due to the higher hydrophilicity of HUS-10 as a result of calcination at higher temperatures. The CO2 adsorption capacities of HUS-10s were comparable to those on siliceous MFI zeolite (37.4 cm3 g−1) and CDO zeolite (24.8 cm3 g−1). In contrast, the CH 4 and N2 adsorption behaviors were significantly different. Relatively large amounts of CH4 and N2 were adsorbed on siliceous MFI and CDO zeolites. The adsorbed amounts of CH4 and N2 were 13.2 and 1.8 cm3 g−1, respectively, for MFI zeolite 10.6 and 1.6 cm3 g−1, respectively, for CDO zeolite. However, when HUS-10(550) was used, CH4 and N2 were adsorbed in amounts of only 5.6 and 1.8 cm3 g−1, respectively. When the calcination temperature of HUS-10 was between 400 and 550 °C, the change in the CH4 and N2 adsorption amount was minimal; HUS-10 maintained a low adsorption amount of CH4 between 4.8 and 5.6 cm3 g−1 and of N2 between 1.5 and 1.8 cm3 g−1. To evaluate the potential for CO2 separation from a gas mixture, we compared the CO2/

diameter (0.45 nm) determined by the molecular probe method were good agreement with those from the N2 (0.16 cm3 g−1) and Ar adsorption isotherms (0.50 nm). We also confirmed that both HUS-10(500) and HUS-10(550) exhibited similar adsorption behaviors (Figure S3). These results strongly indicate that HUS-10 had micropores of uniform pore size that could recognize molecules smaller than C2H6. The structural determination using DFT calculations revealed that the pore size of the 12-ring in HUS-10 was approximately 0.44 × 0.48 nm, which is approximately the same size as that of the C2H6 molecule. These results also indicate that the atomic-scale adsorption ability, molecular sieving ability, was achieved by the accurate formation of micropores in the interlayers of layered silicate HUS-2. It is well-recognized that the adsorption properties of zeolites are based on their framework structure, which contains only an inorganic element.47,48 Therefore, ordered micropores with organic functional groups would allow for a wide range of applications because of their unique surface properties, i.e., control of the adsorption behavior of various guest molecules into the micropores. HUS-10s with different amounts of methyl and hydroxyl groups showed approximately the same pore characteristics (Table 1). For the influence of the hydroxyl and methyl groups on the adsorption properties to be clarified, the adsorption behavior of hydrophilic molecule H2O was evaluated. Figure 11 shows the adsorption isotherms of H2O

Figure 11. H2O adsorption isotherms of (■) HUS-10(400), (◊) HUS-10(500), and (▲) HUS-10(550).

Figure 12. (A) CO2, (B) CH4, and (C) N2 adsorption isotherms of (■) HUS-2S(400), (◊) HUS-2S(500), (▲) HUS-2S(550), (●) MFI zeolite, and (×) CDO zeolite at 25 °C. G


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Table 2. Textural Properties of Porous Materials Evaluated from the CO2, CH4, and N2 Adsorption Evaluated at 25°C

MFI zeolite CDO zeolite HUS-10(400) HUS-10(500) HUS-10(550)

adsorbed CO2 at 100 kPa (cm3 g−1)

adsorbed CH4 at 100 kPa (cm3 g−1)

adsorbed N2 at 100 kPa (cm3 g−1)

CO2/CH4 at 10 kPa/ 100 kPa

CO2/N2 at 10 kPa/ 100 kPa

37.4 24.8 23.5 25.4 32.4

13.2 10.6 4.8 5.4 5.6

4.3 2.9 1.5 1.8 1.8

5.2/2.8 4.1/2.3 7.3/4.9 7.1/4.7 10.0/5.8

18.8/8.7 22.0/8.6 22/15.7 25/14.1 35.0/18.0

CH4 and CO2/N2 adsorption ratios. MFI zeolite exhibited a CO2/CH4 ratio of 2.8 and a CO2/N2 ratio of 8.7 at high pressure (100 kPa), whereas those of the COD zeolite were 2.3 and 8.6, respectively. Thus, although MFI and CDO zeolites possess different pore-channel systems, the differences in their CO2/CH4 and CO2/N2 ratios were minimal. Conversely, the HUS-10s showed high CO2/CH4 and CO2/N2 ratios because of the adsorption inhibition of CH4 and N2. The CO2/CH4 and CO2/N2 ratios at 100 kPa were 4.9 and 15.7, respectively, for HUS-10(400), 4.7 and 14.1, respectively, for HUS-10(500), and 5.8 and 18.0, respectively, for HUS-10(550). Larger CO2/ CH4 and CO2/N2 ratios were observed at a lower pressure (10 kPa). HUS-10(550) in particular achieved high CO2/CH4 and CO2/N2 ratios of 10.0 and 35.0, respectively. The CO2 adsorption capacities of HUS-10 were lower than the values observed on the two-dimensional aluminosilicate zeolite (∼80 cm3 g−1)49,50 and the covalent organic polymer with electron donor sites (∼300 cm3 g−1).51,52 This is due to the relative low hydrophilicity and basicity of HUS-10 derived from its siliceous framework. However, adsorption molar ratios of CO2/CH4 are comparable with those on beneficial zeolites having excellent CO2 separation ability53,54 and the silylated derivative of layered silicate RUB-51.37 Taking into account the fact that HUS-10 exhibited an ordered pore size between that of the MFI and CDO zeolites, the results strongly indicate that the specific adsorption properties are derived from the unique pore structure of HUS-10 with methyl and hydroxy groups that are regularly located on the pore surfaces.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +81-82-424-7606; e-mail: [email protected]. *Tel.: +81-82-424-7607; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS A novel microporous material, HUS-10, was synthesized from the layered silicate HUS-2 through silylation with trichloromethylsilane and subsequent calcination. HUS-10 had an ordered pore structure with methyl and hydroxyl groups, which were attributed to immobilized trichloromethylsilane located on the pore surfaces. Furthermore, it was possible to control the hydrophilicity of HUS-10, and it exhibited actual molecular sieving ability and excellent CO2 adsorption ability as compared to siliceous MFI and CDO zeolites. In general, the design synthesis of zeolites with desirable structures and properties is very difficult; however, the interlayer silylation process using layered precursors makes it possible to design specific structures that cannot be achieved with conventional zeolites with T2 or Q2 structures.25−37 This is a novel process of interlayer micropore construction utilizing layered precursors, which opens the door to artificially and rationally synthesizing novel microporous materials.

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CHN elemental analysis, 29Si CP MAS NMR, DFT calculations, and molecular sizes proving the results of HUS-10s (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07996. H


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Design of Microporous Material HUS-10 with Tunable Hydrophilicity

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