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Synthesis of Heterocoordinated Atom-Containing Zeotypes Utilizing a Mechanochemical Reaction Katsutoshi Yamamoto,*,† Takuji Ikeda,‡ Chiaki Ideta,† and Marie Yasuda† †

Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, Japan AIST Tohoku, National Institute of Advanced Science and Technology, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan

‡

ABSTRACT: A novel series of zeotypes, in which alkaline-earth metal atoms were introduced into a silicate framework, were successfully synthesized by utilizing a mechanochemical reaction. In the synthesis method developed in this study, an alkaline-earth metal hydroxide and silica were allowed to react mechanochemically in the solid phase, and the resulting uniformly mixed precursor was treated hydrothermally in an aqueous alkaline solution. AES-1, one of the materials obtained by this method, was analyzed in detail, and it was elucidated that the material had the same topology as a synthetic calcosilicate CAS-1 with a twodimensional eight-membered ring micropore system and zeolitic physicochemical properties. Several unknown materials have been synthesized by this method to demonstrate the validity of the incorporation of alkaline-earth metals for the synthesis of novel microporous materials.

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INTRODUCTION Microporous materials having molecular-sized uniform pores have been utilized as adsorbents and catalysts. Their micropore system greatly affects their catalytic or adsorption properties; small and medium pores restrict the penetration of large molecules to show shape-selective properties, and larger pores allow bulky molecules to enter inside the pores to show a higher catalytic activity or adsorption capacity. Therefore, it is very important to investigate new structures of microporous materials for better and more appropriate catalysts or adsorbents. There have been intense efforts to search for a new zeolite, and consequently 201 structures of natural or artificial zeolites have been discovered by now.1 In 1989, the discovery of a novel series of microporous silicates such as ETS-4 and ETS-10 was reported,2 in which octahedrally coordinated titanium species were incorporated into a tetrahedrally coordinated silicate network. In particular, ETS-10 was revealed to have 12-membered ring (12-MR) large pores,3 so it has been attracting much attention. Similarly, the incorporation of seven-coordinated lanthanoids was reported by Ferreira et al. to crystallize tobermorite-like novel microporous silicate materials.4 Such heterocoordinated atomcontaining “zeotypes” are of interest because they have a unique crystal structure which cannot be realized by zeolites that are composed only of tetrahedrons. In this regard, the incorporation of nontetrahedrally coordinated species into a silicate framework seems to be a promising approach to the synthesis of new structures of crystalline microporous silicates. Alkaline-earth metals such as calcium and strontium can be good candidates as heterocoordinated species to be introduced into a silicate framework. However, the incorporation of alkaline-earth metal species in a silicate framework seems difficult because uniformly mixed precursor hydrogel is hardly obtained in alkaline or fluoride media; alkaline-earth metals © 2012 American Chemical Society

usually precipitate separately from silicates because of the low solubility of alkaline earth metal hydroxides and fluorides. We have conceived of the idea to utilize a mechanochemical reaction in the preparation of alkaline-earth metal-containing precursors. The mechanochemical reaction is a chemical reaction promoted by the mechanical energy formed by the collision and/or the friction of milling media and can mix solid materials at the atomic level. In our previous work, we have applied the mechanochemical reaction to the synthesis of titanosilicate zeolites.5−7 In those studies, a precursor was prepared by the solid-state mechanochemical reaction of silica and titania to avoid discrete precipitation of titania. Similarly in this study, a powder precursor is prepared through the mechanochemical reaction of alkaline-earth metal hydroxide and fumed silica powder and successively converted into a crystalline material through hydrothermal treatment. Several alkaline-earth metal containing silicate materials have been successfully obtained by this synthesis method, and some products have been demonstrated to have “zeolitic” physicochemical properties.

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EXPERIMENTAL SECTION

Synthesis. In the synthesis method applied in this study, amorphous precursors for zeotypes were obtained by a mechanochemical reaction. In a typical synthesis for a calcium-containing material AES-1, fumed silica powder Aerosil 200V (Nippon Aerosil) and calcium hydroxide (Wako) were used as silicon and calcium sources, and their mixture was allowed to react mechanochemically through the milling at 400 rpm for a total milling time of 24 h with using a Fritsch P-5 planetary ball mill equipped with a milling pot and balls made of silicon nitride. The internal volume of the milling pot is

Received: October 31, 2011 Revised: January 10, 2012 Published: January 18, 2012 1354

dx.doi.org/10.1021/cg201442u | Cryst. Growth Des. 2012, 12, 1354−1361

Crystal Growth & Design

Article

45 mL, and seven balls (ϕ15) were introduced in one pot. The milling was conducted at autogenous temperature for 15 min followed by the pause for 15 min before another 15 min of milling to avoid the overheating of samples. The resultant powder was dispersed in an aqueous solution of potassium hydroxide (Wako), and the suspension with pH of 13.6 was hydrothermally treated in a Teflon-lined stainless autoclave at 150 °C for 7 days under stirring conditions. The product was filtered, washed with distilled water, and dried at room temperature. Characterization. The powder X-ray diffraction (XRD) patterns of the obtained products were collected with a PANalytical X′Pert PRO MPD diffractometer operated at 45 kV and 40 mA equipped with a high-speed X′Celerator detector. For the structure analysis of unknown materials, high quality XRD data were collected at room temperature on a Bruker D8-Advance with Vαrio1 powder diffractometer equipped with a linear position-sensitive VÅNTEC-1 detector (8° 2θ) in the modified Debye−Scherrer geometry using Cu Kα1 radiation. The diffractometer was operated at 40 kV and 50 mA. Samples were sealed into borosilicate capillary tubes with an inner diameter of 0.5 mm. The μr value (μ: linear absorption coefficient, r: sample radius) of the sample tubes, which were determined by measurement of transmittance, was 0.788, so the X-ray absorption correction is necessary. Structure analysis was carried out by the Rietveld method on the program RIETAN-FP with the visualization program VESTA.8,9 Solid-state 1H−29Si dipolar-decoupled (DD) magic-angle spinning (MAS) and 1H−29Si cross-polarization (CP)/MAS NMR spectra for AES-1 were collected with a Bruker BioSpin AVANCE400WB spectrometer with a wide-bore 9.4 T superconducting magnet operated at 79.495 MHz, using a double-resonance 4 mm MAS probe with 5 kHz spinning frequency. A π/2 pulse length was used. The chemical shifts were calibrated with tetramethylsilane. Solid-state 29 Si MAS and 1H−29Si CP/MAS NMR spectra for Ca-containing precursors were collected with a JEOL ECA-500 spectrometer with a narrow-bore 11.7 T superconducting magnet equipped with an NM93030CPM solid-state measurement unit operated at 99.367 MHz, using a double-resonance 4 mm MAS probe with 5 kHz spinning frequency. A single pulse sequence with a π/4 pulse length was used in 29 Si MAS NMR measurements. A contact time of 4 ms was applied in all CP/MAS NMR measurements. Argon gas adsorption was measured at 87 K with a Quantachrome AutoSorb-1MP instrument. The morphology of samples was observed with a Hitachi S-4100L field emission-type scanning electron microscope. Thermogravimetric and differential thermal analysis (TG-DTA) was conducted with a Rigaku ThermoPlus TG8120 at the heating rate of 5 K·min−1.

Figure 1. XRD patterns of the products obtained after the hydrothermal treatment of mechanochemically prepared Ca-containing precursors.

Table 1. Synthesis Conditions of Ca-Containing Silicate Materialsa code AES-1 AES-2 AES-7 AES11 AES13 AES15

precursor compositionb SiO2: 0.2Ca(OH) SiO2: 0.01Al2O3: 0.2Ca(OH)2 SiO2: 0.2Ca(OH)2 SiO2: 0.5Al2O3: 0.2Ca(OH)2 SiO2: 0.05Al2O3: 0.2Ca(OH)2 SiO2: 0.25Al2O3: 0.2Ca(OH)2

alkaline source (MIOH)

MI/Si

H2O/ Si

time (day)

KOH KOH

0.35 0.35

20 5

7 7

LiOH RbOH

0.35 0.35

20 10

7 14

KOH

0.35

2.5

7

KOH

0.35

20

7

Hydrothermally treated at 150 °C. bAfter mechanochemical reaction at 400 rpm for 24 h. a

from calcium hydroxide, and finally a mixture of these silicates and calcium hydroxide was obtained. On the other hand, when a precursor prepared through the mechanochemical reaction was hydrothermally treated under the same conditions (Figure 2B), the crystallization of a pure phase of AES-1 was observed in 3 days and practically completed in 5 days. Through the mechanochemical reaction, silica and calcium hydroxide were uniformly mixed to form an amorphous calcosilicate, which was crystallized during the successive hydrothermal treatment. However, without the mechanochemical reaction, silica had to crystallize as α-quartz or potassium silicate before being dissolved and co-condensed with calcium species. In order to understand the effect of the mechanochemical reaction on the local structure, 29Si MAS and 1H−29Si CP/MAS NMR spectra of Ca-containing precursors before and after mechanochemical treatment were collected (Figure 3). Before the mechanochemical treatment, a broad resonance peak attributable to Q4 (Si(OSi)4) was dominantly observed at −109.3 ppm in the MAS spectrum (Figure 3a). Although the CP/MAS spectrum (Figure 3b) shows a Q3 ((SiO)3SiO−) peak at −99.3 ppm, its low signal-to-noise ratio indicates that protons were scarcely included and that Q3 species were minor in the precursor, suggesting that the silica source (fumed silica) had a typical amorphous SiO2 structure. In contrast, after the mechanochemical treatment the MAS spectrum (Figure 3c) shows two broad peaks at −97.8 and

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RESULTS AND DISCUSSION Synthesis from Ca-Containing Silicate Precursors. First, calcium was employed as an alkaline-earth metal element to be incorporated into silicate matrix. Figure 1 exhibits the XRD patterns of the products obtained after the hydrothermal treatment of the precursors, and Table 1 shows the synthesis conditions of the products. Several diffraction patterns, which are not identical to those of already known zeolites, were observed. The obtained diffraction patterns were changed depending on precursor compositions such as Ca/Si and Al/Si ratios and alkaline metal species added as alkaline sources at the hydrothermal treatment of precursors. Figure 2 exhibits the XRD patterns of the products obtained from precursors with/without mechanochemical treatment, clearly demonstrating that mechanochemical reaction in the precursor preparation step is indispensable for the crystallization of AES-1. When the starting materials were just mixed without mechanochemical reaction and used as a precursor (Figure 2A), silica-based products (α-quartz and potassium hydrogen silicate) started to crystallize in 3 days separately 1355



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Figure 2. XRD patterns of the products obtained from Ca-containing precursor (A) with and (B) without mechanochemical treatment. SiO2: Ca(OH)2: KOH: H2O = 1: 0.2: 0.35: 20. Hydrothermal treatment at 150 °C for 1−7 days.

Figure 4. SEM photograph of Ca-containing silicate AES-1.

cloven, clearly observed facet proved the fine crystallization enough for the crystal structure analysis. 1 H−29Si DD MAS NMR spectrum shows Q3 and Q4 local Si atom environments in AES-1 (Figure 5a). A sharp Q4 resonance peak was observed at −109.4 ppm, and five peaks attributed to Q3 local structure were observed at −90.6, −92.0, −94.4, −95.1, and −96.8 ppm, indicating the presence of at least six independent Si sites. The relative intensities of Q3 peaks at −94.5, −95.1, and −96.8 ppm were considerably enhanced in the 1H−29Si CP/MAS NMR spectrum (Figure 5b), suggesting that these three Q3 sites are assigned to silanol groups (Si−OH) or Si−O− coordinated with H2O. Other two Q3 sites, whose resonance peaks became low and broad in the CP/MAS spectrum, could be attributed to Si−O−Ca2+. The relative peak area of Q3 against Q4, I(Q3)/I(Q4), was as high as 3.0 that AES-1 may have a layered structure. Because the XRD pattern of AES-1 looked similar to that of Ca-containing silicate CAS-1 reported by Jordá et al.,10 the Rietveld analysis for AES-1 was carried out at first on the basis of the crystal structure model of CAS-1 with a space group of C2 and lattice constants of a = 2.4158 nm, b = 0.7016 nm, c = 0.6482 nm, and β = 95.19°. The model, consisting of independent 4 silicon, 10 oxygen, 1 calcium, and 1 potassium sites, had reliability (R) factors of Rwp = 8.3%, RBragg = 3.3%, and RF = 2.6%, which seemed low enough. However, its atomic

Figure 3. 29Si MAS and 1H−29Si CP/MAS NMR spectra of the Cacontaining precursors before (a, b) and after (c, d) mechanochemical treatment.

−104.6 ppm attributed to Q3 and Q4 local structures, respectively. The CP/MAS spectrum (Figure 3d) also shows a broad Q3 peak at −96.2 ppm with a high signal-to-noise ratio. These spectra clearly demonstrate that Q4 local structures were partly changed into Q3, and possibly Q2, structures through the mechanochemical reaction. Considering that terminal silanols (Si−O−H+) as well as Si−O−Ca2+ species having ionic bond with calcium cation can give Q3 local structures, the mechanochemical reaction would cleave Si−O covalent bonds and form Si−O−Ca and Si−OH local structures to give rise to an amorphous calcosilicate in the atomic level to be crystallized in the subsequent hydrothermal treatment. Structure Analysis of AES-1. Figure 4 shows the scanning electron microscopy (SEM) photograph of AES-1 with a thin rod-like morphology and the crystal size of ca. 1 μm width and ca. 5 μm length. Although the front edges of the crystals were 1356



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Table 2. XRD Experimental Conditions and Crystallographic Data for AES-1 compound name estimated chemical formula ideal structural formula FW Z space group a/nm b/nm c/nm β/° V/nm3 wavelength λ/nm 2θ range/° step size (2θ)/° counting time per step/s profile range in fwhm number of observations number of contributing reflections number of refined structural parameters Rwp RF RBragg RF 2 Rexp χ2

Figure 5. 1H−29Si (a) DD MAS and (b) CP/MAS NMR spectra of AES-1.

displacement parameters for Si sites were somewhat large, and calculated intensities of some diffraction peaks had considerable difference with observed peak intensities. In addition, the finding that the 29Si NMR spectrum suggested the presence of at least six independent silicon sites was inconsistent with the C2 model having only four independent Si sites. These facts implied that C2 symmetry of the CAS-1 model was somewhat inappropriate for our material AES-1. Therefore, the C2 model was reconstructed by systematic symmetry change between supergroup and subgroup,11 and finally a new model based on a space group Cm, consisting of independent 6 silicon, 14 oxygen, 2 calcium, and 2 potassium sites, was adopted for refinement (Figure 6). In this model, two silicate layers are contained in the unit cell, and the layers are

AES-1 K4Ca4·Si16O38·7.97(H2O) K4Ca4·Si16O38·8(H2O) 758.83 2 Cm (No. 8) 2.41933(3) 0.702297(10) 0.649078(8) 95.1437(14) 1.09840(3) 0.1540593 (Cu Kα1) 5−100.06 0.017473 21 12 5474 634 76 0.065 0.015 0.022 0.014 0.023 2.81

connected with calcium. Although the silicate layer forms an orthogonal three-dimensional (3D) micropore structure composed of 8-MR, the presence of the interlayer calcium species makes the overall micropore system two-dimensional (b and c directions). The obtained model is very similar to that of a natural mineral rhodesite (H11Ca2KSi8O24),12 which has silicate layers composing a 3D 8-MR micropore structure and calcium ions intercalated between silicate layers. The calcium forms CaO6 octahedron coordinated with water molecules, that is, Ca1−WO1 and Ca2−WO2. Because the isotropic atomic displacement parameters for WO1 and WO2

Figure 6. Crystal structure of AES-1 viewing from the (a) c-axis and (b) b-axis directions. 1357



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Table 3. Refined Atomic Coordinates and U Values for AES-1 site

M

g

x

y

z

Uiso/nm2

Si1 Si2 Si3 Si4 Si5 Si6 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 Ca1a Ca2 K1 K2 WO1b WO2b WO3b WO4b

4b 4b 2a 2a 2a 2a 2a 2a 2a 4b 4b 4b 4b 2a 2a 4b 4b 2a 2a 2a 2a 2a 2a 4b 4b 4b 2a

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.496(4) = g(WO1) 0.5 1.0

0.1361(5) 0.8684(5) 0.0672(7) 0.9346(7) 0.1447(6) 0.8523(6) 0.0016(13) 0.1186(11) 0.8820(11) 0.1860(6) 0.8080(6) 0.0854(8) 0.9170(8) 0.0891(8) 0.8898(8) 0.1428(5) 0.8811(5) 0.1990(9) 0.7923(8) 0.24224 0.7554(2) 0.1117(5) 0.8834(5) 0.2691(11) 0.7211(12) 0.0088(12) −0.0049(9)

0.2882(11) 0.7247(11) 0 0 0 0 0 1/2 1/2 0.290(2) 0.760(2) 0.190(3) 0.823(2) 0 0 0.175(2) 0.806(2) 0 0 0 0 1/2 1/2 0.059(4) 0.937(4) 0.628(3) 1/2

0.254(2) 0.737 (2) 0.480(2) 0.500(2) 0.889(2) 0.080(2) 0.464(3) 0.197(4) 0.771(4) 0.430(2) 0.616(2) 0.381(3) 0.627(3) 0.717(3) 0.288(3) 0.040(2) 0.962(2) 0.778(2) 0.145(3) 0.51349 0.4913(10) 0.719(2) 0.220(2) 0.179(4) 0.838(4) 0.853(4) 0.269(3)

1.18(14) = U(Si1) = U(Si1) = U(Si1) = U(Si1) = U(Si1) 1.5(3) =U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) = U(O1) 0.9(2) = U(Ca1) 1.87(14) = U(K1) 4.1(4) = U(WO1) 11.9(13) 7.6(8)

a

Atomic coordinates of site Ca1 were fixed as origin of translational symmetry operation. bWO sites indicate water molecules whose scattering amplitude was modified in consideration of protons.

Figure 7. Observed (red crosses), calculated (light blue solid line), and difference (blue) patterns obtained by the Rietveld refinement for AES-1. Tick marks (green) denote the peak positions of possible Bragg reflections.

were at first so large (Uiso = 12.4/nm2), a split atom model was adopted by changing the site symmetry from 2a (x, 0, z) to 4b (x, y, z) for these sites, to decrease the parameters to 4.1/nm2. Occupancies of these sites, g, were slightly lower than the ideal value (1/2), and then a simple constraint of g(WO1) = g(WO2) was adopted during refinement. A bond valence sum13 of a pseudo CaO6 octahedron, calculated from seven Ca−O distances ranging from 0.209 to 0.253 nm and a bond valence parameter of 1.967, was 2.41 for Ca1 and 2.65 for Ca2. An effective coordination number14 for Ca1 and Ca2 octahedrons

were calculated at 5.18 and 6.09, respectively. These findings suggest that WO1 may more strongly coordinate to the Ca atom than WO2. In the model of CAS-1,9 the presence of a terminal silanol group (Si−OH) was not considered. Also in our case, a structure model including silanol groups could not be constructed at first due to the contradiction in atomic valence, although the 29Si CP/MAS NMR (Figure 5(b)) spectrum suggested the presence of silanols. Then we assumed that O4, O5, O12, and O13 sites were in (SiO)3Si−O− configuration 1358



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Figure 8. Argon gas adsorption/desorption isotherm of AES-1.

and that the enhancement of three resonance peaks at −94.5, −95.1, and −96.8 ppm would be caused by H2O adsorbed around Si atom. On the basis of this assumption, Q3:Q4 calculated from the multiplicities of the Si sites should be 3:1, which coincides with an I(Q3)/I(Q4) ratio of 3.0 estimated from the 29Si DD/MAS NMR spectrum. Further, two adsorbed water molecules WO3 and WO4 were located in two-dimensional (2D) micropores, although their distributions were disordered. The split atom model was also applied for WO3 with the g(WO3) value being fixed at 1/2. In this structure model, the water content was calculated at 9.5 wt %, which is in good agreement with the experimental result of the thermogravimetric analysis. Considering the two-step water desorption observed in the thermogravimetric analysis of AES-1 (see below), WO1 and WO2 would be coordinated with a Ca atom more strongly than typical physisorption. Besides, potassium sites are positioned at the center of 8-MR viewed from the a-axis as a cation. The atomic distances K1−WO2, K1−WO3, K2−WO1, and K2−WO4 are 0.273(3) nm, 0.286(3) nm, 0.278(3) nm, and 0.269(2) nm, respectively, which are somewhat shorter than that in bulk KOH15 (0.284 nm). From the considerations above, the ideal composition of AES-1 would be K4Ca4Si16O34·8H2O.

Figure 10. XRD patterns of (a) AES-1, (b) AES-1 after the treatment in 1 M NaCl aq at 60 °C for 24 h, and (c) AES-9 crystallized in the presence of Na+.

Figure 11. XRD patterns of products obtained from Sr- or Bacontaining mechanochemically prepared precursors.

Figure 9. (A) TG-DTA profiles of a water-adsorbed AES-1 sample and (B) XRD patterns of the material (a) as-made and (b) after the second measurement of TG-DTA. 1359



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zeolites. Correspondingly in these temperature areas, a small broad and a large sharp endothermic DTA peaks were observed, implying the presence of weak and strong adsorption sites. These findings really agree with the results of the crystal structure analysis. When AES-1 was treated in 1 M NaCl aqueous solution at 60 °C for 24 h, its XRD pattern was slightly changed. Figure 10 compares the XRD patterns of AES-1 before and after the treatment. Note that the diffraction pattern of the NaCl-treated AES-1 was similar to that of AES-9, which was synthesized from the same precursor but crystallized in NaOHaq instead of KOHaq. The crystal structure analysis indicated that AES-1 and AES-9 had the same topology with different lattice constants. On the basis of the evidence above, it is deduced that potassium is involved in AES-1 as an ion-exchangeable extraframework cation and replaced with sodium during the NaCl treatment with changing of the crystal structure. Actually, the inductively coupled plasma (ICP) elemental analysis showed that ca. 90% of potassium was exchanged with sodium in the treated AES-1. On the other hand, the ICP measurement demonstrated that the calcium content was intact to prove that calcium was involved not as ion-exchangeable cations but as framework atoms. In this way, AES-1 showed reversible water adsorption/ desorption ability and ion-exchanging ability. These two abilities are typical and characteristic properties of aluminosilicates zeolites. AES-1, incorporating octahedrally coordinated calcium atoms in its framework, cannot be called zeolite, but it was proved that the material had really “zeolitic” physicochemical properties. Synthesis from Sr- and Ba-Containing Silicate Precursors. The mechanochemical reaction is so simple a solidphase reaction that it can be applied widely to various kinds of solid materials. Therefore, we employed other alkaline-earth metals, strontium and barium, as heteroatoms to be incorporated into the silicate framework. Figure 11 and Table 4 exhibit the XRD patterns and the synthesis conditions of products crystallized from precursors containing strontium or barium instead of calcium. Similarly to the case of calcosilicates, the precursor compositions and concomitant alkaline metal species changed the product phases, whose diffraction patterns were different from those of above-mentioned calcosilicates or already known zeolites. Some of them showed diffraction peaks at very low angles to indicate the formation of products having large unit cells, and some products such as strontosilicate AES17 and barosilicate AES-20 showed a reversible water adsorption/desorption ability (not shown) to demonstrate their zeolitic features. The crystal structures of these materials will be studied and reported elsewhere.

Table 4. Synthesis Conditions of Sr- or Ba-Containinng Silicate Materials codea AES-16 AES-17 AES-18 AES-19 AES-20

precursor compositionb SiO2: SiO2: SiO2: SiO2: SiO2:

0.1 0.1 0.1 0.2 0.1

Sr(OH)2 Sr(OH)2 Sr(OH)2 Sr(OH)2 Ba(OH)2

alkaline source (MIOH)

MI/Si

H2O/ Si

LiOH NaOH KOH KOH LiOH

0.35 0.35 0.35 0.35 0.35

18 18 18 20 20

a Hydrothermally treated at 150 °C for 7 days. bAfter mechanochemical reaction at 400 rpm for 24 h.

In the structure refinement, restraint conditions upon all bond lengths and bond angles were applied for framework, for example, Si−O bond lengths l(Si−O): l(Si −O) = 0.160 ± 0.002 nm and the O−Si−O bond angles ϕ(O−Si−O): ϕ(O− Si−O) = 109.4 ± 0.5°. Simple approximations of U(Si1) = U(Sin, n = 2−4), U(Ca1) = U(Ca2), U(K1) = U(K2), and U(WO1) = U(WO2) were applied to the isotropic displacement parameters of the Si, Ca, K, and WO sites, respectively. Further approximations of U(O1) = U(On, n = 2−13) were also applied to the isotropic displacement parameters of the O sites. Finally, R-factors were remarkably decreased to Rwp = 6.5%, RBragg = 2.2%, and RF = 1.5%. The experimental conditions and crystallographic data for AES-1 are summarized in Table 2. Refined atomic coordinates and Uiso values are listed in Table 3. Figure 7 shows observed, calculated, and difference patterns obtained by the Rietveld refinement for AES-1. As a result, AES-1 is an analogue of CAS-1, whose symmetry is lower than that of CAS-1 crystallographically. Physicochemical Properties of AES-1. Figure 8 exhibits the argon adsorption/desorption isotherm of AES-1. While the small hysteresis loop implied the adsorption into micropores, the slight adsorption step at around 0 of P/P0 suggested that microporous adsorption capacity was so little. In addition, the surface area was calculated at 85 m2 g−1, which was small for a microporous material. Presumably, although the structure analysis in the previous section indicated that AES-1 had 2D 8-MR micropores, the practical opening of 8-MR in AES-1 would not be large enough for argon molecules to enter. Also in the nitrogen adsorption measurement, a considerable adsorption into micropores was not observed. On the other hand, the TG-DTA measurement clarified that AES-1 was able to adsorb smaller water molecules to a considerable amount. Figure 9 shows the TG-DTA profiles of AES-1. The TG-DTA measurement was conducted for a sample preliminarily heated to 300 °C and kept at room temperature under saturated water vapor, and this measurement procedure was repeated two times. The TG and DTA profiles for the first and the second measurements (Figure 9A) were almost identical. In addition, the XRD pattern of the sample just after the second measurement was almost the same as that of as-made sample (Figure 9B; another experiment (not shown) proved that AES-1 was thermally stable until at least 500 °C). These findings clearly demonstrate that AES-1 can reversibly adsorb and desorb water molecules without changing its crystalline topology just like zeolites. AES-1 has two water desorbing steps, gradual desorption below 200 °C and a rapid desorption between 220 to 240 °C, and the total amount of adsorbed water was ca. 11 wt % of the sample weight, which is comparable to that of hydrophilic

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-93-695-3264. Fax: +81-93-695-3735. E-mail: katz@ kitakyu-u.ac.jp.

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ACKNOWLEDGMENTS This research work was financially supported in part by Fukuoka Industry, Science and Technology Foundation. Authors thank Prof. Y. Oumi (Gifu University) for some NMR measurements. The corresponding author thanks Nippon Aerosil Co., Ltd. for the supply of their fumed silica products. 1360



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REFERENCES

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