Crystal Structure of Tubular Na−LTA Zeolite Membrane Used for a

Nov 6, 2009 - The crystal structure of two tubular zeolite Na-LTA membranessone as-prepared membrane and a membrane used in vapor permeation (VP) ...
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Ind. Eng. Chem. Res. 2009, 48, 10870–10876

Crystal Structure of Tubular Na-LTA Zeolite Membrane Used for a Vapor Permeation Process: Unusual Distribution of Adsorbed Water Molecules Tomohiro Kyotani,† Takuji Ikeda,*,‡ Junji Saito,† Takashi Nakane,† Takaaki Hanaoka,‡ and Fujio Mizukami‡ Separation Materials Laboratory, Mitsubishi Chemical Corporation, 8-3-1 Chuo, Ami-machi, Inashiki-gun, Ibaraki 300-0332, Japan, and Research Center for Compact Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Sendai 983-8551, Japan

The crystal structure of two tubular zeolite Na-LTA membranessone as-prepared membrane and a membrane used in vapor permeation (VP) processes of bioethanolswas analyzed using Rietveld refinement with parallelbeam powder X-ray diffraction data. The crystal structure of the as-prepared membrane was consistent with that of typical Na-LTA powder. After the VP process, an unusual relative peak intensity ratio between 200 and 220 reflections was observed in the membrane. The number of adsorbed water molecules in the β-cage was reduced to half. Furthermore, detachment of sodium cations at the six-membered rings was confirmed. The electron density distributions of these Na-LTA zeolite membranes were analyzed by combining the Rietveld method and the maximum entropy method (MEM). MEM electron density images revealed that the adsorbed water molecules and Na+ cations were bonded, forming a hydrated cluster, and that the guest atoms or molecules were highly disordered. 1. Introduction Several studies have been conducted on zeolite Na-LTA membranes supported by an alumina porous tube for use in pervaporation (PV) and vapor permeation (VP) dehydration (in this study, “dehydration” refers to removal of water from not LTA zeolite but hydrous ethanol) experiments involving organic liquids such as bioethanol.1–10 Recently, these membranes have been used in actual large-commercial-scale and/or pilot-scale bioethanol dehydration plants.6,7,9,10 In the VP process of hydrous ethanol in a previous study, a small deterioration in the water permeance (Kw) was observed in the early stages of the experiment (ca. 200 h from the start of the experiment), and the Kw values subsequently were stable.7 However, this study7 only focused on the process engineering study of the VP process; the relationship between the change in the crystal structure of the membrane and the deterioration in Kw was not discussed. Generally, all dehydration processes1–10 including VP and PV do not take into account the effect of change in the crystal structure of the LTA zeolite on Kw. LTA zeolite is largely used as an ion-exchanger. Therefore, to design an optimum dehydration process and achieve a stable long-term operation, it is necessary to understand the change in crystal structure of the LTA zeolite before and after the dehydration processes. If the deterioration of the crystal structure of LTA zeolite membranes in the VP process is clarified, it would be possible to repair damaged sites and reuse the membrane. This would result in a reduction in the running cost of a dehydration plant. X-ray diffraction (XRD) is a very effective technique for analyzing the deterioration in membranes, and therefore XRD characterization has been used in studies related to tubular zeolite membranes.2,3,8 However, in many studies, a conventional parafocusing powder XRD instrument was used and therefore the quality of diffraction data was poor either due to the surface roughness of the tubular membrane or due to the * To whom correspondence should be addressed. Tel.: +81-22-2373016. Fax: +81-22-237-5217. E-mail: [email protected]. † Mitsubishi Chemical Corp. ‡ AIST.

large optical aberration resulting from a non-flat-formed specimen. Thus, XRD has been used only for qualitative analysis of the membranes. A few structural studies of tubular zeolite membranes by the Rietveld method using powder XRD data were reported by Caro and his co-workers so far,11,12 although detailed crystal structure except the change of lattice constants has not been discussed in them. While the crystal structure13 of powdered LTA zeolite has been intensively studied, the crystal structure of tubular LTA zeolite has hitherto not been discussed. In this study, a powder XRD instrument equipped with a parallel-beam optics system14 was used to analyze a commercial tubular polycrystalline zeolite Na-LTA membrane5 without crushing that is used in the VP process7 for final dehydration of sugar cane-based bioethanol used in automobile fuels. The change in the crystal structure before and after the VP process was investigated using the Rietveld method. Furthermore, the electron density distribution of the crystal structure was elucidated by the maximum entropy method (MEM), which revealed detailed structural information such as chemical bonding and distribution of adsorbed water molecules and sodium cations. 2. Experimental Section 2.1. Preparation of Tubular LTA Zeolite Membrane. Figure 1 shows the cross-sectional image of a commercial tubular LTA membrane5,7 used in this study. The image was obtained using dark field transmission electron microscopy (DFTEM; Hitachi HF-2200) in combination with the focused ion beam (FIB; Hitachi FB-2100) thin layer specimen preparation technique.15–17 To show the overall image of the cross-section prepared by the FIB, Figure 1 consists of the overlapping of four low-magnified images which were taken from different locations. Asymmetric porous alumina tubes (o.d., 16 mm; i.d., 12 mm; length, 1000 mm) were used as the substrate.5,9 The average pore size of the substrate was 0.8 µm.5,9 LTA membranes were hydrothermally synthesized on the substrates as described in our previous study.5,9,15 The present hydrothermal synthesis involves seeding and secondary growth of the

10.1021/ie900967e CCC: $40.75  2009 American Chemical Society Published on Web 11/06/2009

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Figure 2. In-house powder X-ray diffraction apparatus using parallel beam geometry and schematic structure of tubular specimen.

Figure 1. Cross-sectional image of a tubular LTA membrane observed using DF-TEM in combination with the FIB thin layer specimen preparation technique.

membranes from a low-viscosity solution. Before hydrothermal synthesis of the membranes, the outer surface of the substrate was seeded with zeolite Na-LTA crystalline particles by the dip coating method. The membranes were synthesized hydrothermally on the seeded substrates at 80 °C for 3 h with a lowviscosity solution whose chemical composition was optimized in the range of Al2O3:SiO2:Na2O:H2O ) 1:(2-5):(2-50): (500-1000). After the hydrothermal treatment, the surface of the tubular LTA membrane was first washed with water, then brushed in order to remove the accumulated material or crystals, and finally dried in air. As reported in our previous studies,15–17 a tubular membrane comprises four parts: (I) a columnar zeolite LTA crystal, (II) an amorphous grain boundary that is several nanometers wide used as a surface layer, (III) an alumina support layer including zeolite LTA crystal particles, and (IV) an amorphous component inside the alumina support. The thicknesses of the columnar LTA layers of the membranes prepared in this study were approximately 4-5 µm, as shown in Figure 1. In this study, two tubular LTA membranes, an as-prepared membrane (abbreviated as “as-synthesized sample”) and a practical membrane used in commercial dehydration process7 of bioethanol by the VP process (abbreviated as “spent sample”), were subjected to Rietveld analysis for investigating the structural changes that occurred before and after the VP process. 2.2. VP Dehydration Process of Bioethanol. The spent sample was used in a VP process for final dehydration of sugar cane-based bioethanol for automobile fuels in a commercial plant for 1275 h.7 The run time of 1275 h was in a stable state7 after a small deterioration in Kw in the initial stage. The outline of the present bioethanol production process7 is the following. The beer with several volume percent of ethanol obtained from fermentation of sugar cane molasses was subjected to conventional distillation process, and 90-94 vol % hydrous ethanol was generated. In the VP process as the final dehydration, the vapor fed to the membrane was generated by evaporating the 90-94 vol % hydrous ethanol. The operating temperature of the VP process was 130 °C, and the product ethanol concentration was 99.8 vol %. 2.3. Physicochemical Analysis. Prior to XRD analysis, the two tubular LTA membranes were cut to a length of 20 mm. A powder X-ray diffractometer (Rigaku RINT-UltimaIII) operating

at 40 kV and 30 mA (λ ) Cu KR) was used for obtaining accurate intensity data. The diffractometer was equipped with a θ-θ configuration goniometer and parallel beam optics.14 A parabolic multilayer mirror and a Soller slit with an angular aperture and slit width of 5.0° and 5 mm, respectively, were placed on the primary beam side. A parallel-slit analyzer (PSA; angular resolution of 0.114°) and a large-area scintillation counter were placed on the receiving beam side. A standard sample stage was used for conventional powder diffraction. As shown in Figure 2, a 20 mm long tubular specimen was placed in a standard aluminum holder for typical powder sample measurement. The sample stage comprises the holder with the tubular specimen. An X-ray beam was irradiated along the long axis of the tube. For XRD measurements, a conventional θ-2θ scan was carried out. The scanning rate was set to 0.25°/min with a scan step of 0.01°. Due to the insensitivity of parallel beam optics to the sample geometry (i.e., formation and surface roughness) and displacement errors, minimal sample preparation is required for accurate measurement. An additional advantage of using parallel beam geometry is that the incident beam can be spread over a large region, leading to an increase in the number of X-ray diffracted crystals as compared to common parafocusing geometry (i.e., increasing particle statics). The chemical compositions of the two membranes (assynthesized sample and spent sample) were determined by a scanning electron microscope (SEM; Hitachi S-4800) equipped with an energy dispersive X-ray spectrometer (EDX; EDAX Genesis) with a calibration curve prepared from commercial zeolite powders on the basis of our previous study.18 2.4. Structure Analysis. The intensity data in a powder pattern were analyzed by the Rietveld method using the RIETAN-FP program package.19 During the refinement process, partial profile relaxation20 was applied to several reflections for 2θ lower than 30° for all samples. Partial profile relaxation with a modified split pseudo-Voigt function was applied to parts of nearly isolated reflections with anisotropic broadening and/or highly asymmetric profiles in the low-2θ region, which significantly improved fits between their observed and calculated profiles. A modified split-type pseudo-Voigt function was fitted to the observed profiles of these reflections. The split-type pseudo-Voigt function proposed by Toraya21 was fitted to the profiles of all of the other reflections. An 11th-order Legendre polynomial was fitted to the background of the intensity data. In the early stage of refinement, we imposed restraints on all the Si-O and Al-O bond lengths l(Si-O) and l(Al-O) and the O-T-O bond angles φ(O-T-O): l(Si-O) ) 1.60 ( 0.02 Å, l(Al-O) ) 1.74 ( 0.02 Å, and φ(O-T-O) ) 109.47 (

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Table 1. XRD Experimental Conditions and Crystallographic Data for Tubular Na-LTA Membranes name

as-synthesized sample

sample conditions

as-synthesized

estd chemical formula FW space group a/Å unit-cell vol./Å3 wavelength λ/Å 2θ range /deg step size (2θ) /deg scanning speed/(deg/min) profile range in fwhm fwhm/deg no. of observations no. of contributing reflcns no. of refined structural params no. of background coeff Rwp (Rietveld) RBragg (Rietveld) RF (Rietveld) Re (Rietveld) wRF (MPF) RF (MPF)

Si96Al96O384Na96 · 259.3H2O 18309.2 Fm3cj (No. 226) 24.5554(2) 14806.1(2) Cu KR 5-100 0.01 0.25 10 0.164 (at 2θ ) 7.2°) 9501 361 51 12 0.0727 0.0270 0.0208 0.0800 0.0245 0.0363

Table 2. Chemical Compositions of Tubular Na-LTA Membranes by the SEM-EDX Analysis

as-synthesized sample spent sample

Na

Al

Si

0.93 0.83

0.96 0.97

1.00 1.00

0.5°.22 Table 1 summarizes the experimental conditions and detailed crystallographic information (R factors and lattice parameters) of the samples. Furthermore, disordered atomic arrangements characteristic of guests present in zeolite frameworks can be adequately represented by their electron density distribution determined by MEM. After the first MEM analysis, the electron density distribution is redetermined by MPF (MEM-based pattern fitting),19 which is a structure refinement method that involves a combination of whole-pattern fitting (RIETAN-FP) and MEM analysis (PRIMA23). The spatial resolution was 128 × 128 × 128 pixels per unit cell. MEM analyses and whole-pattern fitting are alternately repeated until R factors in the latter stop reducing. In the present study, structural models were revised by thorough examination of density maps, and the modifications to the revised model were repeated until the revised structural model agreed well with the density maps resulting from the iterative procedure.24–26 The obtained structural models and electron density distributions were visualized by means of the program VESTA.27

spent sample after being used for 1275 h at 130 °C in a VP dehydration process of bioethanol Si96Al96.0O384Na88.2H7.8 · 241.8H2O 17812.1 Fm3cj (No. 226) 24.6065(3) 14898.8(3) Cu KR 5-100 0.01 0.25 10 0.176 (at 2θ ) 22.8°) 9501 365 51 12 0.0696 0.0136 0.0166 0.0804 0.0240 0.0345

ments since the LTA membrane is very thin. As shown in Figure 1, within the area of thickness 10 µm along the membrane crosssection, the membrane consists of a mixture of zeolite crystal, amorphous solid, and alumina support. Figure 3 shows the XRD patterns of three samples in the low 2θ angle region. Although the XRD pattern of the as-synthesized sample (Figure 3a) was similar to that of the commercial Na-LTA powder (Figure 3c), unusual relative peak intensity ratios between 200 and 220 reflections were observed in the spent sample (Figure 3b). This fact suggests that a change of sodium cation and water molecule distribution in cages took place during the VP process. The same unique XRD pattern was observed in other tubular Na-LTA membranes after the VP process. This XRD pattern change (decrease of the 200 intensity) occurred in the early stage of 50-300 h from the start of the driving, and then reached equilibrium. As already described, the water permeance of the VP process was also

3. Results and Discussion 3.1. Physicochemical Properties of Samples. The estimated Si, Al, and Na contents of the as-synthesized sample and the spent sample are listed in Table 2. EDX chemical analysis revealed that the number of Na+ cations in the spent sample was lower than that in the as-synthesized sample. Dealumination and a partial formation of the amorphous phase were not detected in both the EDX and XRD experiments. Furthermore, in both samples, the Al atom defect was scarcely observed by the Rietveld refinement (see section 3.2.2). These results indicate that although ion-exchange between Na+ to H+ partially took place, preferential dealumination did not occur during the longterm VP process. However, it is difficult to carry out chemical analysis of the tubular membrane by means of EDX measure-

Figure 3. XRD patterns for tubular Na-LTA membranes: (a) assynthesized sample; (b) spent sample; (c) XRD pattern for commercial Na-LTA powder.

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Figure 4. Structural model comprising R-cage and β-cage. Three kinds of sodium cations are located near the center of 4-MR, 6-MR, and 8-MR.

changed as the same. Therefore, we consider that the change in XRD pattern (crystal structure) corresponds to that in the water permeance of the VP process. Furthermore, a small bump in the low-angle side of the [200] reflection at 2θ ) 6.06° (d ) 14.4 Å) was observed in the as-synthesized sample. The 2θ position of the small bump corresponds with that of the 111 reflection on the basis of the typical lattice constant of Na-LTA (ca. 24.6 Å). This fact suggests the structural symmetry of the as-synthesized sample is less than the original symmetry, if the bump is assumed to be a reflection. 3.2. Crystal Structure of Tubular LTA Membranes. It is estimated that the space group is not Fm3jc but Fm3jm (or F432), if the small bump is taken into account for the indexing. Furthermore a lower symmetry might be acceptable. However, we excluded the bump from the structure analysis because of its ambiguous broad profile shape. Furthermore, with the exception of the peaks derived from the alumina support, no other additional peaks were observed. Therefore, the initial structural model was referred to that proposed by Adams et al.,28,29 where Si and Al atoms are completely ordered using the space group Fm3jc. Cheetham et al.30 presented experimental evidence on the ordering of Si and Al, which obeyed Loewenstein’s rule.31 Na-LTA zeolite consists of two types of cages, namely, R-cage (sub-building unit 4126886) and β-cage (subbuilding unit 4668) and three sodium cation sites located at the center of the four-, six- and eight-membered rings (MRs), as shown in Figure 4. Water molecules are usually transported and adsorbed in the cages through the 8-MR pore opening. In all tubular samples, the analysis of the two phases, i.e., zeolite Na-LTA and R-alumina (space group R3jc), was carried out. 3.2.1. As-Synthesized Sample. In the case of the assynthesized sample, the lattice constant a was 24.5554(2) Å, which is less than that reported in previous studies (ca. 24.60 Å).13,28,29 Figure 5 shows the schematic structure of the models of the R-cage and β-cage determined by the Rietveld refinement of as-synthesized sample. Because the dealumination was not taken into consideration, the occupancy parameter of the Al site, g(Al), was fixed to 1.0. Sites Na1 and Na2 located near the center of the 6- and 8-MRs, respectively, were fully occupied after the refinement, although the occupancy parameter of site Na3, g(Na3), located near the center of 4-MR was fixed to the topological value (1/12) according to previous studies.28,29 Site Na2 at 96i (0, 0.23, 0.28) was regarded as a split atom configuration. The initial atomic coordinates of oxygen in the adsorbed water molecules were taken from crystal data in the literature.13,28,29 As shown in Figure 5, eight adsorbed water molecule sites were found; one was found at site WO1 located

Figure 5. Structure of R-cage and β-cage in as-synthesized sample determined by Rietveld refinement. The black and white balls denote sodium cations (Na+) and adsorbed water molecules (WO), respectively.

in the β-cage and the other seven sites (WOn, n ) 2-8) were located in the R-cage. These water molecules were situated around Na+ ions attracting negative charges. A virtual atom, which consists of one oxygen atom and two protons, was adopted at the WO site to consider the total scattering amplitude of H2O. Six WO1 molecules form the octahedral conformation in the β-cage with an interatomic distance of ca. 2.8 Å between the neighboring WO1 sites. Site WO2 at 64g position (x, x, x; x ≈ 0.15) is close to site Na1 with an interatomic distance of ca. 2.0 Å between the sodium ion and the oxygen atom of WO1. Sites WO3 and WO4, together with site WO2, form a cagelike hydrated cluster in the R-cage, which was elucidated by the MEM-electron density distribution image (see section 3.3). Sites WO5 and WO6 are distributed near site Na2, suggesting a strong ionic bond among water molecule and sodium ion. The second nearest interatomic distances of l(Na2-WO5) and l(Na2-WO6) were estimated to be 2.7 and 2.4 Å, respectively. This estimation is reasonable while considering the low occupancies of these sites. Four WO sites (WO3-WO6) were treated as a split atom model in the refinement process; nevertheless, their isotropic atomic displacement parameters B converged at a larger value of ca. 12 (Å2). This finding indicates that the distribution of water molecules is highly disordered in the R-cage. Sites WO7 and WO8 were found near the center of the R-cage, suggesting a large thermal vibration of the water molecule. In the final Rietveld refinements for the as-synthesized sample, all the B parameters for the Si and Al sites were constrained to be equal to each other; i.e., B(Si) ) B(Al). For convenience, simple approximations such as B(O1) ) B(O2) ) B(O3), B(WO2) ) B(WOn, n ) 3-6), and B(WO7) ) B(WO8) were also imposed on the B parameters of the WO sites. Refined structural parameters of the as-synthesized sample are summarized in Table 3. The structural model obtained was in agreement with that of the hydrated Na-LTA powder reported in our previous study.13 3.2.2. Spent Sample. The structural refinements of the spent sample revealed a unique distribution of the adsorbed water

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Ind. Eng. Chem. Res., Vol. 48, No. 24, 2009 Table 4. Refined Atomic Coordinates and B Values for Spent Samplea

Table 3. Refined Atomic Coordinates and B Values for As-Synthesized Samplea atom

site

g

x

y

z

B/Å2

Si Al O1 O2 O3 Na1 Na2 Na3 WO1 WO2 WO3 WO4 WO5 WO6 WO7 WO8

96i 96i 96i 96i 192j 64g 96i 96h 48e 64g 192j 192j 192j 192j 64g 8a

1.0 1.0 1.0 1.0 1.0 1.0 0.25 0.0833 0.822(8) 0.564(14) 0.235(44) )g(WO3) 0.203(3) )g(WO5) 0.246(7) 0.26(5)

0.0 0.0 0.0 0.0 0.0516(4) 0.1074(2) 1/4 1/4 0.0826(4) 0.1540(6) 0.1315(8) )y(WO3) 0.0709(7) )y(WO5) 0.2863(11) 1/4

0.0909(3) 0.1848(4) 0.1151(4) 0.1477(6) 0.0597(4) )x 0.234(6) 0.062(13) 0.0 )x 0.1551(7) )x(WO3) 0.7257(9) )x(WO5) )x 1/4

0.1872(4) 0.0941(3) 0.2466(8) 0.1459(6) 0.1683(2) )x 0.277(4) )y 0.0 )x 0.2729(9) )z(WO3) 0.3069(6) )z(WO5) )x 1/4

3.79(8) )B(Si1) 4.1(2) )B(O1) )B(O1) 11.4(2) 14.4(4) 5.0 10.0(11) 12.0(25) )B(WO2) )B(WO2) )B(WO2) )B(WO2) 7.1(14) )B(WO7)

a Designations: WO indicates the virtual atoms for the water molecule.g(Na3) and B(Na3) are fixed.

molecules. The lattice constant of the spent sample (a ) 24.6065(3) Å) was considerably greater than that of the as-synthesized sample. In the spent sample, the number of water molecules in the β-cage (site WO1) was reduced to half in comparison with that in the as-synthesized sample. The unusual relative intensity ratios between 200 and 220 reflections (Figure 3b) are strongly correlated to the decrease of water molecules in the β-cage, which was clarified by the XRD pattern simulation. Rehydration of the β-cage would probably be inhibited by preferential diffusion of the water molecules between the adjacent R-cages through 8-MR. The 6-MR window between the R-cage and the β-cage is almost blocked by the sodium cations. The number of water molecules per unit cell in the spent sample decreased by approximately 7% in comparison to that in the as-synthesized sample. The reason for the increase in the lattice constant is not yet clear; however, the structural relaxation might be related to the distribution of the water molecules and the decrease in the number of Na+ cations after the VP process. Furthermore, it was suggested that the total number of sodium cations decreased by approximately 8% after the refinement of g(Na), which is consistent with the results of the EDX analysis. In particular, g(Na1) decreased by over 10% and g(Na3) decreased slightly. The value of g(Na2) converged to 0.25 within analytical error σ; therefore, g(Na2) was fixed to 0.25. This finding suggests that sodium cations were easily eliminated from the framework together with H2O molecules during the long-term VP process. Furthermore, these sodium cations must be substituted by protons in order to maintain charge compensation. Additionally, we evaluated the possibility of dealumination by refinement of occupancy parameters of the Al site, g(Al). First, g(Al) was refined together with other g parameters simultaneously; however, some parameters did not converge due to strong correlation between g(Al) and g, B parameters of other sites. Second, the g(Al) value was refined after determination of almost all parameters of Na and WO sites and converged to 1.01 ( 0.03. Therefore, the g(Al) value was fixed to 1.0, which gave the smallest R factors. This fact suggests that remarkable dealumination does not occur in the VP process condition and is consistent with the result of the EDX chemical analysis. Actually, the defect of a few Al atoms must be taking place. However, neighboring Si atoms adjacent to Al atoms will remove simultaneously from the framework with Al atoms due to complete alternate ordering of Si and Al atoms. In the spent sample, simple approximations such as B(Si) ) B(Al), B(O1) ) B(O2) ) B(O3), B(WO2) )

atom site Si Al O1 O2 O3 Na1 Na2 Na3 WO1 WO2 WO3 WO4 WO5 WO6 WO7 WO8

96i 96i 96i 96i 192j 64g 96i 96h 48e 64g 192j 192j 192j 192j 64g 8a

g

x

y

z

B/Å2

1.0 1.0 1.0 1.0 1.0 0.90(2) 1/4 0.070(8) 0.399(11) 0.54(9) 0.174(15) )g(WO3) 0.244(18) )g(WO5) 0.31(5) 1.00(6)

0.0 0.0 0.0 0.0 0.0538(8) 0.1054(2) 0.0 1/4 0.0522(9) 0.1551(13) 0.1249(13) )y(WO3) 0.076(2) )y(WO5) 0.319(5) 1/4

0.0925(8) 0.1853(9) 0.1149(6) 0.1441(14) 0.0574(7) )x 0.237(3) 0.031(2) 0.0 )x 0.1501(12) )x(WO3) 0.751(8) )x(WO5) )x 1/4

0.1848(9) 0.0909(8) 0.247(2) 0.1471(14) 0.1705(3) )x 0.2925(13) )y 0.0 )x 0.2755(12) )z(WO3) 0.310(2) )z(WO5) )x 1/4

3.58(6) )B(Si1) 4.9(4) )B(O1) )B(O1) 6.8(5) 14(3) 5.0 14.7(25) 10.8(25) )B(WO2) )B(WO2) ) B(WO2) )B(WO2) 13.3(32) )B(WO7)

a Designations: WO indicates the virtual atoms for the water molecule.g(Na3) and B(Na3) are fixed.

Figure 6. Relative comparison between quantities of structural elements in the as-synthesized sample and the spent sample (Na and H2O in R -cage; H2O in β-cage; and H2O in total), with values of the as-synthesized sample as reference.

B(WOn, n ) 3-6), and B(WO7) ) B(WO8) were also imposed on the B parameters. The refined structural parameters of the spent sample are summarized in Table 4. Figure 6 shows the relative comparison between the quantities of the structural elements in the as-synthesized sample and the spent sample (Na and H2O in the R-cage, H2O in the β-cage, and total H2O), with the values of the as-synthesized sample as reference. It can be clearly observed that the number of water molecules in the β-cage of the spent sample decreased (Figure 6), whereas that in the R-cage was constant. Furthermore, the structure refinement of the alumina support revealed the presence of a preferred orientation along the [104] direction in both samples. The R factors after the Rietveld analyses for all the samples were significantly low (Table 1). Figure 7 shows the plots of the observed, calculated, and difference patterns for the XRD data versus 2θ for each sample. 3.3. MEM-Electron Density Distribution of Tubular LTA Membranes. The electron density distribution (EDD) of the tubular Na-LTA membrane calculated from the MPF analysis revealed the presence of local structures of guest atoms and molecules in the R- and β-cages. Figure 8 shows the threedimensional EDD images of the as-synthesized sample; a and c show the entire R- and β-cages, respectively; b and d show the water molecules and sodium cations included in each cage. A covalent bonding network between Si (or Al) and O atoms can be clearly observed in Figure 8a,c.

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Figure 8. EDD 3D image of Na-LTA phase in the as-synthesized sample obtained from MPF analysis: (a) Overall view of the R-cage, (b) guest atoms and molecules in the R-cage, (c) overall view of the β-cage, and (d) guest atoms and molecules in the β-cage. The equidensity level is set to 0.6 e/Å2 for all images.

Figure 7. Observed, calculated, and difference patterns for the Rietveld refinement of (a) the as-synthesized sample and (b) the spent sample. Tick marks indicate the Bragg positions of the Na-LTA (upper) and R-Al2O3 tubes (lower) calculated from each lattice constant.

The low electron densities derived from the four equivalent split Na2 sites spread out anisotropically parallel to the 8-MR plane; however, the electron densities of site Na1 were spherical. On the other hand, cagelike electron densities consisting of three WO sites (WO2, WO3, and WO4) were found in the R-cage (Figure 8b), indicating a disordered distribution and diffusion of water molecules in the R-cage. Furthermore, the existence of a covalent bond between Na1 and WO2 was clearly observed. These analytical results indicate that a cagelike hydrated cluster was formed in the R-cage due to connections between the neighboring H2O molecules. We assumed that the cluster structure was stabilized by a strong hydrogen bonding and the Na-O bond. An anisotropic electron density distribution derived from water molecules (corresponding to sites WO7 and WO8) was observed in the vicinity of the center of the R-cage. The six isolated electron densities (corresponding to site WO1) were observed in the β-cage, as shown in Figure 8d. Electron densities based on the chemical bonding between sites Na1 and WO1 were not present. On the basis of the results of the EDD analysis, it can be inferred that the diffusion of water molecules is considerably greater in the R-cage than in the β-cage. Figure 9 shows the EDD images of the spent sample. The EDD images of both the cages of spent sample (Figure 9a,c) resemble those of the as-synthesized sample, although the size of the electron density of site Na1 in the β-cage is smaller than that of the as-synthesized sample. Furthermore, cagelike electron densities originated from adsorbed water molecules were

Figure 9. EDD 3D image of Na-LTA phase in spent sample obtained from MPF analysis: (a) Overall view of the R-cage, (b) guest atoms and molecules in the R-cage, (c) overall view of the β-cage, and (d) guest atoms and molecules in the β-cage. The equidensity level is set to 0.6 e/Å2 for all images.

observed in the R-cage, the same as the as-synthesized sample (Figure 9b). The linking between neighboring water molecules is weaker than that in the as-synthesized sample, although the number of water molecules included in the R-cage by Rietveld refinement was almost the same as that of the as-synthesized sample. The observed (almost spherical-shaped) electron densities around the center of the R-cage were localized in compari-

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son with Figure 8b. On the other hand, the EDD of site WO1 was displaced to the center of the β-cage (Figure 9d), and its distribution region was reduced in proportion to the decrease in the value of g(WO1). Furthermore, no chemical bonding between sites Na1 and WO1 was observed, similar to the case of the as-synthesized sample. This fact indicates a strong quantum confinement effect significantly affects the water molecules in the small β-cage. Finally, the MEM analysis of 361 structure factors for reflections in the region d < 1.0 Å yielded final R factors of RF(MEM) ) 3.63% in the assynthesized sample. For 365 reflections in a region d < 1.0 Å, the R factors were RF(MEM) ) 3.45% in the spent sample. Conclusion The crystal structures of tubular zeolite Na-LTA membranes before and after the VP dehydration process of bioethanol were determined minutely by the Rietveld refinement technique using accurate powder XRD data. The crystal structure of an asprepared Na-LTA membrane was almost consistent with that of typical Na-LTA powder. The number of water molecules was ca. 260 per unit cell. On the other hand, unusual distribution of adsorbed water molecules was observed in the membrane after the VP process. The number of adsorbed water molecules in the β-cage reduced to half, while that in the R-cage was constant. Preferential dealumination in the framework was not observed; however, detachment of ca. 10% sodium cations at the 6-MR was confirmed, indicating the occurrence of proton exchange. Furthermore, the EDD images revealed that the adsorbed water molecules were bonded to sodium cations (site Na1) by strong hydrogen bonding, forming a hydrated cluster such as a cubic-cage-like structure in the R-cage. A highly disordered distribution of sodium cations and water molecules was clearly observed in the R-cage. Literature Cited (1) Morigami, Y.; Kondo, M.; Abe, J.; Kita, H.; Okamoto, K. The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane. Sep. Purif. Technol. 2001, 25, 251. (2) Okamoto, K.; Kita, H.; Horii, K.; Tanaka, K.; Kondo, M. Zeolite NaA membrane: Preparation, single-gas permeation, and pervaporation and vapor permeation of water/organic liquid mixtures. Ind. Eng. Chem. Res. 2001, 40, 163. (3) Bowen, T. C.; Noble, R. D.; Falconer, J. L. Fundamentals and applications of pervaporation through zeolite membrane. J. Membr. Sci. 2004, 245, 1. (4) Kondo, M. Zeolite membrane for practical dehydration of organic solvents (in Japanese). J. Vac. Soc. Jpn. 2006, 49, 225. (5) Mizuno, T.; Inoue, S.; Saito, J.; Ikeda, S.; Matsukata, M. DeVelopment of 1000 mm long high performance LTA zeolite membrane for alcohol dehydration. International Symposium on Zeolites and Microporous Crystals (ZMPC2006), OA109, Yonago, Tottori, Japan, 2006. (6) Izumi, K.; Ikeda, S.; Yamaguchi, K.; Nakane, T. Energy saving in concentration and dehydration process for fuel grade bio-ethanol using zeolite membranes (in Japanese). Kagakukougaku 2007, 71, 812. (7) Aoki, K.; Ikeda, S.; Saito, J.; Nakane, T. Membrane dehydration technology for commercial bio ethanol production for fuel (in Japanese). MEMBRANE 2007, 32, 234. (8) Wee, S.-L.; Tye, C.-T.; Bhatia, S. Membrane separation processs Pervaporation through zeolite membrane. Sep. Purif. Technol. 2008, 63, 500. (9) Sato, K.; Aoki, K.; Sugimoto, K.; Izumi, K.; Inoue, S.; Saito, J.; Ikeda, S.; Nakane, T. Dehydrating performance of commercial LTA zeolite membranes and application to fuel grade bio-ethanol production by hybrid

distillation/vapor permeation process. Microporous Mesoporous Mater. 2008, 115, 184. (10) Kondo, M. Present condition of zeolite membranes used in bioethanol plant (in Japanese). Zeolite News Lett. 2008, 25, 93. (11) Noack, M.; Schneider, M.; Dittmar, A.; Georgi, G.; Caro, J. The change of the unit cell dimension of different zeolite types by heating and its influence on supported membrane layers. Microporous Mesoporous Mater. 2009, 117, 10. (12) Caro, J.; Albrecht, D.; Noack, M. Why is it so extremely difficult to prepare shape-selective Al-rich zeolite membranes like LTA and FAU for gas separation. Sep. Purif. Technol. 2009, 66, 143. (13) Ikeda, T.; Izumi, F.; Kodaira, T.; Kamiyama, T. Structural study of sodium-type zeolite LTA by combination of Rietveld and maximumentropy methods. Chem. Mater. 1998, 10, 3996. (14) Fujinawa, G.; Toraya, H.; Staudenmann, J.-L. Parallel-slit analyzer developed for the purpose of lowering tails of diffraction profiles. J. Appl. Crystallogr. 1999, 32, 1145. (15) Kyotani, T.; Mizuno, T.; Katakura, Y.; Kakui, S.; Shimotsuma, N.; Saito, J.; Nakane, T. Characterization of tubular zeolite NaA membranes prepared from clear solutions by FTIR-ATR, GIXRD and FIB-TEM-SEM. J. Membr. Sci. 2007, 296, 162. (16) Kyotani, T.; Kakui, S.; Mizuno, T.; Shimotsuma, N.; Inoue, S.; Saito, J. Evaluation of fine structure of tubular zeolite NaA membrane by FTIR-ATR and FIB-TEM. Anal. Sci. 2006, 22, 1031. (17) Liu, Z.; Ohsuna, T.; Sato, K.; Mizuno, T.; Kyotani, T.; Nakane, T.; Terasaki, O. Transmission electron microscopy observation on fine structure of zeolite NaA membrane. Chem. Mater. 2006, 18, 922. (18) Kakui, S.; Kyotani, T.; Sato, K. Determination of chemical composition of zeolite membranes by SEM-EDX, 53rd Annual Meeting of the Japan Society for Analytical Chemistry, Abstract, Chiba, Japan, 2004; P2098. (19) Izumi, F.; Momma, K. Three-dimensional visualization in powder diffraction. Solid State Phenom. 2007, 130, 15. (20) Ohta, T.; Izumi, F.; Oikawa, K.; Kamiyama, T. Rietveld analysis of intensity data taken on the TOF neutron powder diffractometer VEGA. Physica B 1997, 234, 1093. (21) Toraya, H. Array-type universal profile function for powder pattern fitting. J. Appl. Crystallogr. 1990, 23, 485. (22) Bergerhoff, G.; Brandenburg, K. In International Table for X-ray Crystallography, Vol. C, 3rd ed.; Prince, E., Ed.; Kluwer Academic: Dordrecht, The Netherlands, 2004; Chapter 9.4. (23) Izumi, F.; Dilanian, R. A. In Recent Research DeVelopments in Physics, Vol. 3, Part II; Transworld Research Network: Trivandrum, India, 2002; pp 699-726. (24) Knorr, K.; Madler, F.; Papoular, R. J. Model-free density reconstruction of host/guest compounds from high-resolution powder diffraction data. Microporous Mesoporous Mater. 1998, 21, 353. (25) Takata, M.; Nishibori, E.; Sakata, M. Charge density studies utilizing powder diffraction and MEM. Exploring of high Tc superconductors, C-60 superconductors and manganites. Z. Kristallogr. 2001, 216, 71. (26) Papoular, R. J.; Cox, D. E. Model-free search for extra-framework cations in zeolites using powder diffraction. Europhys. Lett. 1995, 32, 337. (27) Momma, K.; Izumi, F. VESTA: A three-dimensional visualization system for electronic and structural analysis. J. Appl. Crystallogr. 2008, 41, 653. (28) Adams, J. M.; Haselden, D. A.; Hewat, A. W. The structure of dehydrated Na zeolite A (Si/Al)1.09) by neutron profile refinement. J. Solid State Chem. 1982, 44, 245. (29) Adams, J. M.; Haselden, D. A. The structure of dehydrated zeolite 3A (Si/Al)1.01) by neutron profile refinement. J. Solid State Chem. 1983, 47, 123. (30) Cheetham, A. K.; Eddy, M. M.; Jefferson, D. A.; Thomas, J. M. A study of Si, Al ordering in thallium zeolite-A by powder neutron-diffraction. Nature 1982, 299, 24. (31) Loe¨wenstein, W. The distribution of aluminum in the tetrahedra of silicates and aluminates. Am. Mineral. 1954, 39, 92.

ReceiVed for reView June 16, 2009 ReVised manuscript receiVed September 30, 2009 Accepted October 29, 2009 IE900967E