Solvothermal Synthesis of âLIT-type Zeolite - Crystal Growth & Design

Article pubs.acs.org/crystal

Solvothermal Synthesis of −LIT-type Zeolite Nagase Takako,* Ikeda Takuji, Abe Chie, Hasegawa Yasuhisa, Kiyozumi Yoshimichi, and Hanaoka Takaaki Research Center for Compact Chemical System, National Institute of Advanced Industrial Science, Technology (AIST), 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan S Supporting Information *

ABSTRACT: Lithosite (−LIT) type aluminosilicate zeolites were successfully synthesized, for the first time, by solvothermal treatment of low-silica zeolite powders. The starting materials comprised H−Y zeolite samples having different SiO2/Al2O3 ratios, together with NH3+-exchanged chabazite and NH3+-exchanged phillipsite. The low-silica zeolite powders were dispersed in alcohol containing dissolved KOH, and the mixture was solvothermally treated at 200−250 °C for 15−120 h without stirring. The crystal structure of the obtained sample was analyzed by the Rietveld method which indicated it to be single phase −LIT zeolite (monoclinic, space group P21, a = 1.513 nm, b = 1.023 nm, c = 0.842 nm, β = 89.99°). The scanning electron microscopy images show that the obtained product consists of blade-like particles. The SiO2/Al2O3 ratios of the −LIT samples varied from 4.0 to 6.7 based on the composition of the starting materials. The zeolite pores were hydrophilic in nature; however, potassium cations contained in the micropores were hardly exchanged at room temperature. Furthermore, Co-substituted −LIT zeolite was also successfully synthesized by a similar method as that used for −LIT-type aluminosilicate zeolite, using Co2+-exchanged H−Y zeolite as the starting material.

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INTRODUCTION Recent application of molecular-kinetic calculations to the structural exploration of zeolitic materials has given rise to a number of new zeolite syntheses based on the development of systematic methodologies utilizing the online system based on these calculations.1 Nonetheless, the synthesis conditions of a number of natural zeolites remain to be elucidated. Lithosite is a natural zeolite that was first discovered in a nepheline syenite rock in Murmansk District, Russia, and named by the International Mineralogical Association in 1983.2 Even though a period of over 20 years has elapsed since the discovery of natural lithosite, synthetic lithosite type zeolite has not been reported thus far.2 The crystal structure of lithosite (chemical composition: K3[HAl2Si4O13]) was initially elucidated by Pudovkina et al. (1986),3 and the new framework type code “−LIT” for lithosite was approved by the structure commission of the International Zeolite Association in 2005.4 The structure of −LIT includes characteristic isolated silanols in the framework. Although numerous zeolite structures have been synthesized for use in industrial applications and a number of studies have been conducted on individual zeolites, the phase relationship between new artificial zeolite structures and the natural zeolite structures remains vague. Many of the artificial zeolites were synthesized with the use of organic templates; nevertheless, organic template-free conditions have recently been successfully applied to the synthesis of some of these artificial zeolites (ECR-1, ZSM34, and RTH).5−7 The success of the organic © 2012 American Chemical Society

template-free syntheses suggests that new organic templates are not always necessary in order to access new structures. Zeolites undergo very sensitive structural changes in response to their environment. Therefore, new structures may be accessed by controlling the crystallization rate of the process of structural change in response to the synthetic conditions. The crystallization rate can be judiciously slowed in order to evaluate each step of the ordering process or can be intentionally accelerated to produce the disordered structure that is usually obtained at higher temperatures. During the slow crystallization process, the products are strongly affected by the solvent and other mineralizing agents, and many synthetic techniques have been applied to obtain new porous materials. Solvothermal synthesis using organic solvents such as methanol, ethanol, pentanol, ethylene glycol, etc. represents one of the major techniques for zeolite synthesis.8−11 Compared with the hydrothermal synthesis, the crystallization rate is considerably slowed during the solvothermal zeolite synthesis due to the inherently low polarity of organic solvents. In a previous study, large single crystals of all-silica type sodalite were synthesized in ethylene glycol without coprecipitation of other SiO2 minerals by maintaining the systems at 180 °C for 3−4 weeks.8,11 However, rapid crystallization of quartz precluded formation of the intended sodalite when the water Received: August 31, 2011 Revised: February 3, 2012 Published: March 8, 2012 1752

dx.doi.org/10.1021/cg201138f | Cryst. Growth Des. 2012, 12, 1752−1761

Crystal Growth & Design

Article

Table 1. Properties of the Zeolite Samples As the Starting Material for −LIT-type Zeolite Syntheses sample name

320HOAa

330HUAa

350HUAa

LSXb

structure cation type SiO2/Al2O3 (mol/mol) K2O (wt%) Na2O (wt%) CoO (atm%) crystal size (μm)

FAU(Y) H 5.5

FAU(Y) H 6.3

FAU(Y) H 10

CHA NH4+ 4.8 4.0

4

0.02

0.1

FAU(X) H 2.3 2.7 2.4

0.3

0.3

0.1

0.2−0.3

0.3

a

CHAb

PHIb PHI NH4+ 4.6 1.5

0.4−5.0

Co2+-FAUc FAU(Y) Co2+ 5.7 2.2 2.8 0.3

b

Commercial products by Tosoh Co. Prepared by the method described in Verif ied Synthesis of Zeolitic Materials, 2nd ed. (refs 13, 15, and 16). CHA was synthesized using 320HOA." cCo2+ exchanged FAU obtained by stirring 320HOA in CoSO4 solution.

Table 2. Synthetic Conditions and Identification of −LIT Zeolite Samples in Solvothermal System run no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 a

starting materialsa 320HOA

330HUA 320HOA H-LSX 320HOA CHA PHI hydrous oxide hydrous oxide Co-FAU 320HOA 330HUA 330HUA 350HUA 320HOA

alkali KOH NaOH CsOH KOH

KOH

KOH KOH TMAOH KOH KOH

Ab−Si−Al molar ratio

temp (°C)

duration (h)

obtained phases identified by XRD

sample name

2.0−2.8−1.0

200

72

LIT-72

2.2−3.2−1.0 4.0−2.8−1.0 2.0−1.2−1.0 2.0−2.8−1.0 2.0−2.4−1.0 2.0−2.3−1.0 2.0−2.5−1.0 2.0−4.0−1.0 2.0−2.9−1.0 2.0−2.8−1.0 2.2−3.2−1.0 2.2−3.2−1.0 4.2−7.0−1.0 4.0−2.8−1.0

200

72 72 73 120

−LIT Amoc + trace ECR-5 (CAN) Pollucite (ANA) Amo Kalsilite synthetic kaliophilite −LIT −LIT −LIT Amo + trace KAlSiO4 LTL −LIT −LIT −LIT FAU Amo + trace KHSi2O5 synthetic kaliophilite Kalsilite

200

250

250

15 22 22 30 1.5 21

LIT-120 LIT-C LIT-P

Co-LIT LIT-320 LIT-330

Each property is listed in Table 1. bAlkali cation. cAmorphous. and HNO3 treated low-silica X (LSX)15 zeolite (FAU-type) were prepared in order to examine the effect of the SiO2/Al2O3 ratio on the products of the syntheses. NH4+-exchanged phillipsite (PHI),16 NH4+exchanged chabazite (CHA),13 and Si−Al hydrous oxides were prepared to examine the effect of the initial structure on the syntheses. Ammonium exchange of CHA was carried out at 60 °C for 5 days and ammonium exchange of PHI zeolites was carried out overnight at room temperature. Each of the zeolite powders was stirred in 1 N (NH4)2CO3 solution until the potassium and sodium content of the zeolite was less than 5 wt %. Co2+-exchanged FAU was prepared by stirring 320HOA in 0.05 M CoSO4 solution for a few hours at room temperature. The properties of these starting materials are also listed in Table 1. Synthesis. Synthesis of the −LIT-type zeolite was attempted either from various precursor zeolites or from hydrous oxides having a similar composition to these zeolites (Table 1). The Si−Al hydrous oxides were prepared by homogenizing a solution of AlCl3·6H2O (NACALAI TESQUE, INC.) with 30% colloidal silica (SI30, JGC Catalysts and Chemicals Ltd.). The Si−Al hydrous oxides were precipitated by adding 28% NH4OH solution (Wako Pure Chemical Industries, Ltd.) and the precipitates were washed with water and ethanol and then percolated. The Si/Al ratios of the prepared hydrous oxides were determined by scanning electron microsopy−energy dispersive X-ray spectroscopy (SEM−EDX). The hydrous oxide was mixed with ethanolic KOH to produce a uniform slurry. The zeolite powders summarized in Table 1 were also dispersed in ethanolic KOH at room temperature and stirred for more than 1 h. The slurry of the hydrous oxide or the dispersion of the zeolite powder was heated in a Teflonlined steel autoclave at 200−250 °C for 15−120 h without stirring. Following solvothermal treatment, the samples were percolated and

content in the ethylene glycol systems exceeded 2 mol % due to selective dissolution of the precursor in the water cluster.11 In this study, a solvothermal method utilizing a K−Al−Si system is applied to the process of zeolite conversion in order to examine the effect of the solvent on the crystalline phases. In general, a number of aluminosilicate zeolites, for example, CHA, MER, LTL, and Linde T (ERI-OFF structural intermediate), can be synthesized using the hydrothermal K− Al−Si system without employing an organic template.12 Recently, the hydrothermal conversion of FAU to CHA, RUT (RUB-10), BEA, OFF, and LEV zeolites was reported with or without the addition of an organic template.13,14 It has been postulated that the FAU precursor dissolves to form locally ordered aluminosilicate species (i.e., nanosized parts), which promote the recrystallization process. On the basis of this background, we attempted to synthesize a new zeolitic material by combining the solvothermal method and the hydrothermal conversion. Solvothermal syntheses of −LITtype zeolites were successfully achieved using H−Y zeolite and ammonium-exchanged low-silica zeolites as starting materials. The synthesis, crystal structure, and physicochemical properties of the obtained zeolites are reported in detail herein.

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EXPERIMENTAL SECTION

Starting Materials. The chemical composition and particle size of the zeolite powders used as the starting materials for the solvothermal syntheses are summarized in Table 1. Three types of H−Y zeolites (FAU-type: HSZ320HOA, HSZ330HUA, HSZ350HUA; Tosoh Co.) 1753



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Figure 1. PXRD patterns of the solvothermally synthesized −LIT zeolites listed in Table 2. (a) LIT-72, (b) LIT1-20, (c) LIT-C, (d) LIT-P, (e) LIT320, (f) LIT-330.

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dried at 65 °C overnight. CsOH, NaOH, and 25% tetramethylammonium hydroxide (TMAOH) in methanol were also used as alkali sources instead of KOH to compare the effect of various alkali cations. Physicochemical Analysis. The synthesized powders were identified using powder X-ray diffraction (PXRD) collected on a MacScience M21X powder diffractometer using Cu Kα radiation. For structural analysis, powder XRD data were also collected at room temperature on a Bruker AXS D8-Advance system equipped with V(alfa)rio-1 diffractometer, in a modified Debye−Scherrer geometry, using Cu Kα1 radiation. The samples were sealed in borosilicate capillary tubes with inner diameters of 0.5 mm. Solid-state 1H−29Si dipolar-decoupled (DD) MAS NMR spectra were measured on an AVANCE 400 WB spectrometer (Bruker BioSpin, Japan) operated at 79.49 MHz by using a spinning frequency of 5 kHz, a 4 mm MAS probe, a 90° pulse length of 3.6 μs, and a cycle delay time of 100 s. 1H−29Si cross-polarization (CP) MAS NMR spectra were also measured, with a contact time of 4 ms. 27Al MAS NMR and 1H MAS NMR spectra were measured with a spinning frequency of 12 kHz by employing a single pulse sequence operated at 78.20 MHz for 27Al and 400.1 MHz for 1H. The 1H and 29Si chemical shifts were calibrated with a standard sample of tetramethylsilane (TMS). The 27Al chemical shift was calibrated with a standard sample of 1.0 M AlCl3 aqueous solution. Crystal morphology was observed using an S-800 (Hitachi, Japan) field emission scanning electron microscope (FE-SEM). The chemical compositions of the samples were estimated by EDX spectroscopy with an EMAX EX-250 (HORIBA, Japan) system attached to the FESEM. Thermogravimetric analysis was carried out with a TGDTA2000SA analyzer (Bruker AXS, Japan) at temperatures between room temperature and 1000 °C, at a heating rate of 10 °C/min. Equilibrium water adsorption and desorption isotherms were collected at 25 °C on a BELSORP-18 volumetric sorption analyzer (BEL JAPAN, Inc.). The samples were dehydrated at 150 °C under a vacuum, for 5 h, prior to analysis. Nitrogen gas adsorption measurements were carried out at 77 K using a Quantachrome Autosorb 1-MP instrument. The local motions of the aluminosilicate framework or solvent molecules were investigated using FT-IR measurements performed on an IR Prestige-21 (Shimadzu, Japan) spectrometer, at room temperature. Additionally, diffuse reflection spectra of the Co-substituted sample were measured from 280 to 2500 nm at room temperature using an IR−vis−UV spectrometer (Cary 5000, Varian).

RESULTS AND DISCUSSION Effect of the Synthetic Conditions. The synthetic conditions and identification of the samples obtained in the solvothermal system are summarized in Table 2. Figure 1 shows the PXRD pattern of the −LIT-type zeolites listed in Table 2, which were identified based on the coincidence of the peak patterns with those of entry 37-0457 of the JCPDS card. The PXRD patterns indicate that −LIT zeolites were successfully synthesized when FAU, CHA, and PHI powders were used as starting materials in the solvothermal reaction. Addition of NaOH and CsOH as the alkali sources resulted in ECR5 (CAN type structure) and pollucite (ANA type structure) formation, which suggests that the alkali cations played the role of structure-directing agents (SDA) for the recrystallization process during the solvothermal reactions. The SEM images of the synthetic −LIT zeolite samples are shown in Figure 2. The synthetic −LIT zeolite particles have blade-like shapes with a length of 3−10 μm and a width of 0.5−2.5 μm. The particle size increased in proportion to the reaction temperature. The empirically determined chemical compositions of the synthetic −LIT zeolite samples are summarized in Table 3. Although silicon, aluminum, and alkali metal species were detected in all of the EDX spectra of the samples, the presence of the organic species was also detected. Since the K-line in carbon is nearly overlapping with the L-line in potassium, carbon content was ambiguous and excluded from Table 3. The presence of organic species, which could be also confirmed by the 1H NMR or FT-IR measurements as described later, suggests that the solvent or its fragments once were included during the crystallization process. The optimum SiO2/Al2O3 ratio for production of the −LIT zeolite fell within the range of 4.6−6.5. A reaction time in excess of 72 h was required at 200 °C for the crystallization of −LIT zeolite when a SiO2/Al2O3 ratio of 5.6 (Run nos. 1 and 7) was used. However, when the SiO2/Al2O3 ratio was increased to 6.3 (Run nos. 4 and 14), highly crystalline −LIT zeolite could not be obtained without increasing the reaction temperature and reaction time. At an even higher SiO2/Al2O3 1754



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addition to the −LIT phase were identified by PXRD measurement (Figure S1, Supporting Information). Physicochemical Properties. Figure 3 shows the thermogravimetric-differential thermal analysis (TG-DTA)

Figure 3. TG-DTA curves of the synthetic −LIT zeolite (LIT-72).

curves of the synthetic −LIT zeolite (LIT-72). A monotonous weight loss of ca. 6.5 wt % was observed in conjunction with an endothermic peak at 700 °C, the latter of which suggests internal condensation between adjacent terminal silanols. Structural collapse of LIT-72 occurred at ca. 700 °C. The obtained −LIT samples exhibit low to no gas adsorption capacity. A small Brunauer−Emmett−Teller (BET) surface area of 8.3 m2/g was obtained for the LIT-72 sample. The isotherms indicated a small hysteresis in the partial pressure range of 0.5 < P/P0 < 0.9 (see Supporting Information Figure S2). The isotherms are consistent with the presence of mesopores and the absence of micropores in the sample. The adsorption volume increased rapidly in the partial pressure range of 0.9 < P/P0, suggesting the presence of interparticle voids. The equilibrium water adsorption isotherms of the LIT-72 and LIT-330 samples shown in Figure 4 are indicative of a highly hydrophilic characteristic. The SiO2/Al2O3 ratios had little effect on the volume of water adsorbed by the samples. The water adsorption isotherms can be classified as type IV isotherms based on the IUPAC classification system. The observed differences between the adsorption and desorption curves indicate a strong adsorption of water molecules, which

Figure 2. SEM images of the synthetic −LIT zeolites: (a) LIT-72, (b) LIT-120, (c) LIT-C, (d) LIT-P, (e) LIT-320, (f) LIT-330.

ratio of 10 (Run no. 16), a kind of polysilicate KHSi2O5 having a sheet structure17 was obtained with increasing temperature and reaction time. These results suggest that construction of the lithosite framework is promoted by a higher Al content in the starting material. On the other hand, in the case of a lower SiO2/Al2O3 ratio of 2.3 (Run no. 6), synthetic kaliophilite (KAlSiO4) was obtained.18 The synthetic kaliophilite phase tended to be obtained when the KOH content was increased rather than the Al content. Further, the kaliophilite phase was transformed into kalsilite (polymorph of kaliophilite), which consists of Si−O−Al six-membered rings (6MRs), with increasing reaction time.19 Additionally, when Si−Al hydrous oxides were used as starting materials, only a small amount of crystalline phase was obtained, even though the reaction time was extended to 120 h. The various materials obtained in

Table 3. Chemical Composition and SiO2/Al2O3 Ratio for the Obtained −LIT Zeolites sample name chemical composition Al Si K O Co total (atm%) SiO2/Al2O3

LIT-72

LIT-120

LIT-C

LIT-P

LIT-320

LIT-330

Co-LIT

7.10 19.19 16.46 57.26

6.76 19.19 17.02 57.03

8.43 19.27 14.06 58.23

8.37 18.67 15.38 57.58

7.38 19.51 15.35 57.76

6.07 20.42 15.71 57.8

6.71 19.09 15.37 57.38 1.45 100

100 5.4

100 5.7

100 4.6 1755

100 4.5

100 5.3

100 6.7

5.7



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Figure 4. Equilibrium water adsorption and desorption isotherms of the synthetic −LIT zeolite (LIT-72 and LIT-330).

may be attributed to interaction with the isolated silanol groups of the −LIT framework. Moreover, the amount of adsorbed water, especially in the low-vapor-pressure region, suggests that small water molecules were also adsorbed on the micropores and the desorption was inhibited by the hydrogen bond between silanols. These observations indicate that the micropores in the −LIT structure are unsuitable for access to guest molecules under dry conditions. Additionally, the cation exchange behavior of the obtained − LIT samples was examined by dispersing the −LIT powders in 1 N NaCl, LiCl, and CaCl2 solutions with stirring, at room temperature, for a few days. It was found, however, that potassium cations were stable in the −LIT structure and were hardly exchanged by other alkali cations. The desorption of potassium cations in the −LIT structure due to the cation exchange at high temperatures or due to acid treatment caused some structural change, accompanied by the opening up of the pores, which will be described in a future report. Solid State MAS NMR. Figure 5 shows the solid-state 1 H−29Si DDMAS and 1H−29Si CP/MAS NMR spectra of the LIT-72 sample. The assignment of individual resonance peaks proved problematic due to the presence of large amounts of Al atoms and hydroxyl groups in the −LIT framework. The shoulder resonance peak at δ = −85.3 ppm in the 1H− 29Si DDMAS spectrum (Figure 5a), is attributed to the Q4(3Al) [(SiO)Si(OAl)3] environment. Two resonance peaks observed at δ = −89.1 and −93.9 ppm are tentatively attributed to the Q 4 (3Al) or Q 4 (2Al) [(SiO) 2 Si(OAl) 2 ] or Q 3 (1Al) [(SiO)2Si(OAl)(OH)] environments. Here, the probability for the existence of the typical Q3(0Al) [(SiO)3Si(OH)] structure is low, because the presence of this configuration in the −LIT structure is inconsistent with the analytical value of the SiO2/Al2O3 ratio and would result in an elevated SiO2/ Al2O3 ratio. A resonance peak observed at δ = −98.4 ppm may be attributable to the Q4(1Al) or Q3 [(SiO)3Si(OH)] environments. The CP/MAS NMR spectrum (Figure 5b) shows a resonance peak at δ = −88.3 ppm that is derived from the ≡Si−OH environment, which can be inferred because this peak intensity was remarkably enhanced by the cross-polarization

Figure 5. Solid-state (a) 1H−29Si DDMAS and (b) 1H−29Si CP/MAS NMR spectra of the synthetic −LIT zeolite (LIT-72).

effect between Si and the H nucleus in comparison with that in the 29Si DDMAS spectrum. Conversely, the intensity of the resonance peak at −98.4 ppm was diminished, indicating the absence of protons in the vicinity of the Si site. Comparison of the 1H− 29Si DDMAS and 1H− 29Si CP/MAS NMR spectra allows the assignment of the two peaks at δ = −89.1 and −98.4 ppm, which could be assigned to the Q3(1Al) and Q4(1Al) environments, respectively. The resonance peak at δ = −88.3 ppm can tentatively be assigned to the Q3(1Al) environment. The 27Al and 1H MAS NMR spectra of the LIT-72 sample are shown in Figure 6. The 1H NMR spectrum shows six peaks in the range of 0 < δ (ppm) < 16 as shown in Figure 6(a). The two sharp signals observed at ca. 1.1 and 3.6 ppm are attributable to protons of the methyl and methylene species in the ethyl groups of the ethanol. The observation of these peaks suggested that ethanol was retained in the obtained sample. Two resonance peaks at δ = 5.9 and 7.3 ppm are attributed to the hydrogen bonding in the ≡Si−OH···H2O configuration, with d(O−O) = 2.78 Å and 2.73 Å, respectively. Moreover, the two hydrogen resonance peaks observed at δ = 13.9 and 15.4 ppm are assigned to the strong hydrogen bonds between adjacent silanols, i.e., in the ≡Si−O−H···O−Si≡ configuration, with d(O−O) = 2.51 Å and 2.46 Å, respectively. These chemical shift values can be explained by the following equation: δ/ppm =90.3 − 30.4 × d(O−H···O)/Å.20 The 27Al spectrum (Figure 6b) contains two resonance peaks at δ = 64.0 and 67.3 ppm, which are assigned to tetrahedral framework aluminum. No resonance peak attributed to octahedral Al nuclei was observed at the expected position of ca. 0 ppm. Structure of the Synthetic Lithosite. The initial structural model used in the Rietveld analysis was referred from a previous study by Pudovkina et al.3 Structure refinement and visualization of the refined structural model were carried out using the programs RIETAN-FP21 and VESTA.22 The unit cell parameters a = 1.513 nm, b = 1.023 nm, c = 0.842 nm, β = 90.01°, and the monoclinic space group P21 were adopted as a 1756



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Figure 7. Crystal structure model of the synthetic −LIT zeolite (LIT72) obtained by Rietveld analysis. Figure 6. Solid-state (a) 1H MAS and (b) 27Al MAS NMR spectra of the synthetic −LIT zeolite (LIT-72).

pseudo orthorhombic model. In this model, Si−Al ordering in the framework can be considered by comparison with a higher symmetry model employing the space group Pnma.23 Al−O−Al ordering, which is forbidden by Loëwenstein’s rule, is not inhibited in the Pnma model.24 Figure 7 shows the crystal structure of the synthetic −LIT zeolite obtained by Rietveld analysis, viewed along three axes. The framework structure is composed of 8 Si sites, 4 Al sites, and 26 oxygen sites. The framework also includes terminal silanols (Si−OH), indicating an open-framework structure. A complicated nanoporous cage is composed of 846444 T-atom connectivity with abw type composite building units (Figure 8). Three crystallographically independent potassium sites are present in the cage. O−H···O hydrogen bonding may potentially occur among neighboring hydroxyl groups. The O7−O17 and O4−O20 bond distances between adjacent terminal oxygen atoms are 0.239(9) and 0.242(8) nm. These values are in good agreement with those from the 1H MAS NMR spectra presented above (Figure 6a). The electron density distribution map calculated from the obtained structure factors by using the maximum entropy method (MEM) indicated covalent bonding within the −LIT framework structure (Figure S3, Supporting Information). Additionally, electron density was clearly observed between the terminal oxygen atoms (O7−O17 and O4− O20) (Figure S4, Supporting Information), indicating the presence of strong O−H···O hydrogen bonds. Consequently, two H sites were added to this model, and their coordinates were fixed at the center of the terminal oxygen atoms for convenience. This hydrogen bonding might restrict the ion exchange or ion diffusion capacity within the crystal.

Figure 8. Skelton model of open-framework cage in the −LIT structure with an abw composite building unit. Hydrogen bonding suppresses effective pore size and K+ ions are encapsulated in the cage.

Virtual atom species with a mixture of Si/Al scattering amplitudes of 0.184:0.816 were adopted for all of the Al sites in order to satisfy the analytical chemical composition. The K sites were almost fully occupied; however, the total amount of K+ cation calculated from the site occupancies was slightly larger than the optimal number (Table 4). This discrepancy may be attributable to an experimental error in the estimation of the Al content or the presence of Si atom defects in the framework. The presence of residual ethanol molecules in the sample, which is suggested by the 1H MAS NMR spectra (Figure 6a), was not corroborated by this analysis. Therefore, it was concluded that residual ethanol might have been adsorbed on the surface of the crystal. The details of the experimental and crystallographic information are summarized in Table 4. The reliability factors (R-values) were acceptably low; that is, Rwp = 1757



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Table 4. PXRD Experimental Conditions and Crystallographic Data for Synthetic −LIT Zeolite sample

synthetic −LIT zeolite LIT-72

estimated chemical formula refined chemical formula Z FW space group a (nm) b (nm) c (nm) β (°) unit-cell volume (nm3) wavelength 2θ range (°) step size (2θ) (°) profile range in fwhm fwhm (°) no. of observations no. of contributing reflections no. of refined structural parameters no. of background coefficients Rwp (Rietveld) RF (Rietveld) RBragg (Rietveld) Re (Rietveld) χ2

Si8.736Al3.264O26H2K5.26 Si8.736Al3.264O26H2K5.87 2 980.99 P21 (No. 4, set. 1) 1.51324(2) 1.022575(11) 0.842153(8) 89.997(8) 1.30315(2) Cu Kα1 (l = 0.1540593 nm) 8.0−100.1014 0.017368 10 0.1367 (at 2θ = 10.433°) 5304 1474 139 11 0.030 0.012 0.018 0.023 1.68

Figure 9. PXRD patterns of the H−Y zeolite (320HOA) stirred at room temperature for 1 h in ethanol (FET), in ethanolic KOH (FKET) and in aqueous KOH solution (FKWT).

3.0%, RF = 1.2%, and RBragg = 1.8%. Further description of the structural analysis is provided in the Supporting Information. Crystallization Process in the Solvothermal System. Effect of the Solvent. PXRD and FT-IR measurements of the solvent-treated H−Y zeolites were undertaken in order to elucidate the effect of the solvent on the H−Y zeolite structure. The final products synthesized from each solvent were also evaluated (Table 5). Figure 9 shows the PXRD patterns of the alcohol and/or KOH treated H−Y zeolites, which correspond to Run nos. 19−21 shown in Table 5. The FAU structure was stable in alcohol and in 5 wt % aqueous KOH solution at room temperature (FET and FKWT, Run nos. 1 and 3); however, the structure collapsed readily to give an amorphous form when stirred in 5 wt % ethanolic KOH (FKET, Run no. 20). Figure 10 shows the FT-IR spectra of the various precursor materials. The spectrum of FKWT was largely similar to that of the initial FAU zeolite stirred in ethanol without KOH (FET). In the case of FKET, the two absorption bands attributed to the internal and external asymmetrical T−O−T stretching vibration of the FAU framework, which were observed at 1000 cm−1 and 1150 cm−1,25 were poorly resolved, in contrast

Figure 10. FT-IR spectra of the synthetic −LIT zeolite (LIT-72) and various precursors (FET, FKWT, FKWT200-2, FKET, FKET200-2, FKET200-12) listed in Table 5.

with the spectra FET and FKWT. Therefore, we suggest that the tetrahedral arrangement of SiO4 in the initial FAU zeolite became disordered upon ethanolic KOH treatment. The bending absorption band at 1408 cm−1 in the spectrum of FKET is attributed to the CH2 species,26 although this band was not observed in FET and FKWT. Heat treatment of FKET in ethanolic KOH solution at 200 °C (FKET200-2, FKET20012, Run nos. 22 and 23), diminished this band at 1408 cm−1; and absorption peaks at 875−925 cm−1 attributed to Si−OH stretching appeared.17 The Si−OH absorption band did not appear after treatment with aqueous KOH (FKWT and FKWT200-2, Run nos. 21 and 26), which suggests that the linkages within the SiO4 tetrahedron in the FAU structure were systematically changed to the CHA structure during heating. It

Table 5. Effect of the Solvents and KOH on the Synthesis of −LIT Zeolite run no.

sample name

starting material

19 20 21 22 23 24 25 26 27

FET FKET FKWT FKET200-2 FKET200-12 LIT-MT FKET-TMA FKWT200-2 FKWT200-72

320HOA

alkali

solvent

temp (°C)

320HOA

KOH KOH KOH

ethanol ethanol water ethanol

200

320HOA FKET 320HOA

KOH TMAOH KOH

methanol methanol water

250 250 200

1758

duration (h)

25 1 2 12 22 22 2 72

obtained phases FAU amorphous FAU amorphous amorphous −LIT amorphous CHA CHA



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is well-known that alkoxides can be produced from an alcohol in the presence of alkali metals, and these alkoxides are generally not stable in water. From the FT-IR analysis, it appears that alkoxylation (similar to the transesterification reaction) and hydrolysis of FAU zeolite with potassiumethoxide occurred in the ethanolic solution of KOH, even at room temperature. Consequently, structural transformation of FAU zeolite to the amorphous form occurred readily. In the − LIT sample (LIT-72), the absorption bands at 1000 and 1150 cm−1 due to internal and external asymmetrical T−O−T stretching vibrations of the FAU framework were observed with a shoulder peak near 900 cm−1. However, the peak at 1408 cm−1, attributable to the CH2 species, was not observed. The peak at 1408 cm−1 might be due to residual fragments of CH2 species on the crystal surface. On the basis of the considerations presented above, we concluded that the destruction of the FAU structure and the phase change in the presence of potassium alkoxide were the key requirements for the crystallization of − LIT zeolite. The −LIT zeolite was also obtained in methanolic KOH (Run no. 24). However, no zeolite phase was obtained by the solvothermal reaction in which TMAOH and FKET were used as starting materials (Run no. 25). Additionally, the SEM images of FKET and FKET200-2 (Run no. 20 and Run no. 22) shown in Figure S6 suggest that FKET dissolved during the crystallization of −LIT zeolite and coalescence growth between particles occurred. The coalescence particle, which resulted from the reaction between the zeolite and potassium ethoxide in the solvent, acted as the precursor to −LIT zeolite. Recently, Yamamoto et al.26 synthesized organic−inorganic hybrid zeolites (ZOL) having MFI, LTA, and *BEA-type structures by a hydrothermal method using bis-(triethoxysilyl) methane (BTESM) as a silicon source. That study reported that ZOL crystals have a slightly larger lattice than the pure zeolites, which may be due to the insertion of longer Si−CH2−Si fragments into the structures (d(Si−C) = 0.188 nm, d(Si−O) = 0.160 nm). The unit cell volume of the LIT-72 sample obtained in this study was 1.303 nm3, which is larger than that of the natural lithosite (1.248 nm3), whereas the Al content of the LIT-72 sample is less than that of natural lithosite.3 No direct evidence of the presence of Si−CH2−Si fragments was obtained from the structural analysis or FT-IR measurement. However, we speculated that the formation of Si−CH2−Si bonds should be indispensable to the construction of the −LIT framework structure during the crystallization process. Figure 11 shows a structural model viewed along the [001] direction as well as a schematic illustration of the construction of the −LIT structure on the a−b plane. An open-type 10 membered ring (MR) composed of two 6MRs including a Si− O−H···O−Si linkage and a closed-type 6MR are observed in the a−b plane (Figure 11a); that is, the planar framework structure consists of two kinds of 6MRs. The Si−O−Si linkage along the a-axis is only formed in the closed-type 6MR and includes a symmetrical AlO4 distribution. On the other hand, the Si−OH bonds in the open-type 6MR appear to be partially cleaved and include an asymmetrical AlO4 distribution. The typical d(Al−O) bond length of ca. 1.72 Å is longer than d(Si−O) but shorter than d(Si−C). Therefore, the distortion of the ring as a result of asymmetric Al distribution in the 6MR results in an easy deformation of the overall structure of the 6MRs; this distortion is derived from the differences in the stated bond distances. In the case of the KHSi2O5 structure that contains no Al, the SiO4 tetrahedrons in the a−c plane form

Figure 11. Schematic of the construction of −LIT structure by the hydration of Si−CH2−Si bonding in the 6MR of asymmetrically located aluminum.

symmetric hexagonal rings in the framework without cleavages. Moreover, kalsilite that has a higher Al content than −LIT zeolite comprises symmetric 6MRs with alternate ordering of Si and Al atoms, that is, Si−O−Al−O−Si−O−Al bonding in the structure. On the basis of the structural relationship of these compounds, it is surmised that the open-type 6MR in the −LIT structure results from the presence of the Si−CH2−Si linkage which relaxes the symmetrical distortion of the 6MR due to the asymmetrical ordering of Si and Al. However, the presence of the Al−O−Al linkage is ruled out in this consideration based on Loëwenstein’s rule.24 We propose that the formation of the −LIT structure is initiated by the following two processes: initially, potassium ethoxide gives rise to Si−CH2−Si bonds, which are expected to be unstable after incorporation into the hexagonal frameworks due to the strong asymmetric distortion (Figure 11b).26 Consequently, the Si−CH2−Si bonds are hydrolyzed to give Si−CH3 and Si−OH bonds, thereby forming the anomalistic hydrogen bonded 10MR. Synthesis of Co Substituted −LIT Zeolite. Co3+containing −LIT zeolite could be obtained by using Co2+exchanged faujasite as the starting material for the synthesis. Co2+-exchanged FAU has a pink color, and the Co content determined by EDX analysis was 2.8 atm% as shown in Table 1. Stirring of Co2+-exchanged FAU with potassium-ethoxide at room temperature resulted in a color change from pink to blue, which suggests that oxidation of Co2+ to Co3+ with potassium ethoxide occurred prior to the solvothermal synthesis. The Co/ Al molar ratio was 0.33 and remained constant before and after the color change. Subsequent to executing the synthetic procedure at 200 °C, blue colored Co3+-LIT zeolite was obtained as shown in Figures 12 and 13. The Co/Al molar ratio did not change significantly before and after the solvothermal reaction. The Co contained in Co3+-LIT could not be leached out by washing and also could not be exchanged by other cations. Substantiating evidence for the substitution of cobalt into the framework was provided from the photoabsorption 1759



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various low-silica zeolite (FAU, CHA, PHI-types) powders as starting materials. Structural modeling of−LIT zeolite employing the P21 space group indicated that synthetic −LIT zeolite exhibits monoclinic symmetry. However, the refined unit cell parameters indicated that the −LIT zeolite possesses almost pseudo orthorhombic symmetry because β ≈ 90°. The structural model used herein was almost the same as natural lithosite including the Si and Al ordering.3 The presence of strong O−H···O hydrogen bonding in the framework was identified on the basis of the structural analysis and 1H MAS NMR measurements. The role of alkali metal ions and of alcohols in the solvothermal system was considered as follows: the potassium-ethoxide derived from the alkaline alcohol mixture results in alkoxylation and hydrolysis of FAU zeolite, with the subsequent formation of Si−CH2−Si bonds. These two factors are indispensable to the construction of the unique −LIT topology including two kinds of 6MRs. The photoabsorption spectra of the Co3+-LIT zeolite clearly reveal the inclusion of tetrahedral Co atoms in the framework.

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ASSOCIATED CONTENT

S Supporting Information *

PXRD patterns, nitrogen gas adsorption isotherm plots; EDD images; difference patterns obtained by the Rietveld refinement; SEM images; UV−vis spectra; crystallographic information file. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 12. (a) SEM image and (b) photograph of blue-colored Cosubstituted −LIT zeolite.

*Tel: +81-22-237-3013. Fax: +81-22-237-7027. E-mail: ta. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. T. Kodaira (AIST) for performing photoabsorption measurements of the Co3+-LIT zeolite. REFERENCES

(1) Roth, W. J.; Dorset, D. L. Microporous Mesoporous Mater. 2011, 142, 32−36. (2) (a) Dunn, P. J.; Gruce, J. D.; Fleischer, M.; Pabst, A. Am. Mineral. 1984, 69, 210−215. (b) Khomyakov, A. P.; Chemitsova, N. M.; Chistyakova, N. I. Zapiski Vses. Mineral. Obsh. 1983, 112, 218−222. (3) (a) Pudovkina, Z. V.; Solovieva, L. P.; Pyatenko, Y. A. Dokl. Akad. Nauk SSSR 1986, 291, 1370−1373. (b) Pudovkina, Z. V.; Solovieva, L. P.; Pyatenko, Y. A. Sov. Phys. Dokl. 1986, 31, 941−942. (4) News from the Structure Commission of International Zeolite Association (IZA), May 20, 2005. (5) Song, J.; Dai, L.; Ji, Y.; Xiao, F. S. Chem. Mater. 2006, 18, 2775− 2777. (6) Wu, Z.; Song, J.; Ji, Y.; Ren, L.; Xiao, F. S. Chem. Mater. 2008, 20, 357−359. (7) Yokoi, T.; Yoshioka, M.; Imai, H.; Tatsumi, T. Angew. Chem., Int. Ed. 2009, 48, 9884−9887. (8) Bibby, D. M.; Dale, M. P. Nature 1985, 317, 157−158. (9) Kuperman, A.; Nadimi, S.; Oliver, S.; Ozin, G. A.; Garcés, J. M.; Olken, M. M. Nature 1993, 365, 239−240. (10) Fechtelkord, M.; Posnatzki, B.; Buhl, C. J. Zeolites 1997, 19, 334−342. (11) Yang, X.; Albrecht, D.; Caro, J. Microporous Mesoporous Mater. 2007, 100, 95−102. (12) Koa, Y. S.; Ahnb, W. S. Powder Technol. 2004, 145, 10−19.

Figure 13. PXRD patterns of the Co-substituted −LIT structure, synthesized using Co-exchanged FAU zeolite as the starting material.

spectra of the Co3+-LIT zeolite (Figure S7), the profile of which is consistent with the presence of cobalt in the −LIT framework. The observed spectral profile was very similar to that of CoAPO-5 (AFI-type) or Co-MOR (MOR-type) or CoANA (ANA-type)27 regardless of the differences in the zeolite framework structure. The photoabsorptions at 640, 590, and 540 nm are attributed to Co−O bonds with tetrahedral geometry.28 The potassium and cobalt cations in the −LIT structure are highly stable and difficult to remove, and hence, it is expected that −LIT will exhibit good catalytic activity such as in carbon-oxidation.

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CONCLUSIONS In this study, −LIT-type zeolite and its Co-substituted metallosilicate were successfully synthesized for the first time, by employing solvothermal synthesis at 200−250 °C, using 1760



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(13) (a) Milton, R. M.U. S. Patent 3,012,853, 1961. (b) Robson, H. Verified Synthesis of Zeolitic Materials, 2nd Rev. ed.; Elsevier Science, IZA2001: Amsterdam, 2001; pp 123−125. (14) (a) Jon, H.; Nakahata, K.; Lu, B. W.; Oumi, Y.; Sano, T. Microporous Mesoporous Mater. 2006, 96, 72. (b) Jon, H.; Ikawa, N; Oumi, Y.; Sano, T. Chem. Mater. 2008, 20, 4135−4141. (c) Jon, H.; Takahashi, S.; Sasaki, H.; Oumi, Y.; Sano, T. Microporous Mesoporous Mater. 2008, 113, 56−63. (d) Itakura, M.; Oumi, Y.; Sadakane, M.; Sano, T. Mater. Res. Bull. 2010, 45, 646−650. (e) Yashiki, A.; Honda, K.; Fujimoto, A.; Shibata, S.; Ide, Y.; Sadakane, M.; Sano, T. J. Cryst. Growth 2011, 325, 96−100. (15) (a) Kühl, G. H. Zeolite 1987, 7, 451. (b) Robson, H. Verified Synthesis of Zeolitic Materials, 2nd Rev. ed.; Elsevier Science, IZA2001: Amsterdam, 2001; pp 153−155. (16) (a) Hayhurst, D. T.; Sand, L. B. Crystallization Kinetics and Properties of Na, K-Phillipsites. In Molecular SievesII; Katzer, J. R., Ed.; ACS Symposium Series 40: American Chemical Society: Washington, DC, 1977; pp 219−232. (b) Robson, H. Verified Synthesis of Zeolitic Materials, 2nd Rev. ed.; Elsevier Science, IZA2001: Amsterdam, 2001; pp 244−245. (17) Deng, Z. Q.; Lambert, J. F.; Fripiat, J. J. Chem. Mater. 1989, 1, 375−380. (18) (a) Smith., J. V.; Tuttle, O. F. Am. J. Sci. 1957, 255, 282−305. (b) Smith, J. V.; Tuttle, O. F. Am. J. Sci. 1958, 256, 571−589. (c) Okamoto, Y.; Kawahara, A. Okayama Univ. Earth Sci. Rep. 1996, 3, 57−64. (19) (a) Kawahara, A.; Andou, Y.; Marumo, F.; Okuno, M. Mineral. J. 1987, 13, 260−270. (b) Dollase, W. A.; Freeborn, W. P. Am. Mineral. 1977, 62, 336−340. (20) Izumi, F.; Momma, K. Solid State Phenom. 2007, 130, 15−20. (21) Xue, X.; Kanzaki, M. J. Phys. Chem. B 2007, 111, 13156−13166. (22) Momma, K.; Izumi, F. J. Appl. Crystallogr. 2008, 41, 653−658. (23) (a) Atlas of Zeolite Framework Types, 6th revised ed.; Baerlocher, C. H.; McCusker, L. B.; Olson, D. H., Eds.; Elsevier: Amsterdam, 2007; pp 188−189. (b) http://www.iza-structure.org/databases/ (24) Loëwenstein, W. Am. Mineral. 1954, 39, 92−96. (25) Chao, K.; Shy, D.; Sheu, S.; Lin, C. Microporous Mater. 1994, 2, 91−104. (26) (a) Yamamoto, K.; Nohara, Y.; Domon, Y.; Takahashi, Y.; Sakata, Y.; Plevert, J.; Tatsumi, T. Chem. Mater. 2005, 17, 3913−3920. (b) Su, B. L.; Roussel, M.; Vause, K.; Yang, X. Y.; Gilles, F.; Shi, L.; Leonova, E.; Edén, M.; Zou, X. Microporous Mesoporous Mater. 2007, 105, 49−57. (27) (a) Xu, Y.; Maddox, P. J.; Thomas, J. M. Polyhedron 1989, 8, 819−826. (b) Nagase, T.; Chatterjee, A.; Tanaka, A. P.; Tanco, M. L.; Tazaki, K. Chem. Lett. 2004, 33, 1416−1417. (c) Kato, M.; Ikeda, T.; Kodaira, T.; Takahashi, S. Microporous Mesoporous Mater. 2011, 142, 444−453. (28) Duran, A.; Fernandez-Navarro, J. M.; Casariego, P.; Joglar, A. J. Non-Cryst. Solids 1986, 812, 391−399.

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Solvothermal Synthesis of âLIT-type Zeolite - Crystal Growth & Design

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