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Investigation on Pore Structure Formed in a Pillared Montmorillonite Interlayer Using Adsorption Methods and a Carbonization Technique
Journal of Colloid and Interface Science 239, 440–446 (2001) doi:10.1006/jcis.2001.7571, available online at http://www.idealibrary.com on
Investigation on Pore Structure Formed in a Pillared Montmorillonite Interlayer Using Adsorption Methods and a Carbonization Technique Tatsuya Yamazaki,1,3 Yuichiro Nakamura, and Sentaro Ozawa2 Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07, Aramaki-Aoba, Aoba-ku, Sendai, 980-8579 Japan Received October 11, 2000; accepted March 21, 2001
The structure of pores formed in the alumina- or chromia-pillared montmorillonite (PMT) interlayer was investigated by analyzing the porous solid itself and the negative carbon replica formed in the pores of the parent material. The nitrogen adsorption on these materials allowed us to estimate the pillar structure and the density of pillar sites in the montmorillonite interlayers. The amount of Al13 polycation in an exchanging solution governed the interpillar distance in the pillared clay gallery without changing the diameters of the pillar (ca. 0.6 nm). The aperture size of Al–PMT was 0.8 nm in height and 0.5–0.9 nm in width depending on Al content. On the other hand, the aperture size of Cr–PMT was 0.5 nm in height and 0.8 nm in width, and the diameter of a pillar was estimated to be ca. 0.4 nm. °C 2001 Academic Press Key Words: alumina-pillared montmorillonite; chromia-pillared montmorillonite; nitrogen adsorption; micropore structure; carbonization in micropore; dimension of pillar.
1. INTRODUCTION
Many pillared montmorillonites have been prepared by intern+ calating oligomeric metal hydroxide ions, i.e., Alx On+ y , ZrOx , n+ n+ n+ Gax O y , Fex O y , and Crx O y (1–3) into the interlayers. The pillared clays thus prepared have high surface areas and they are micropores, of which the structural parameters such as the interlayer distance and the pillar spacing can change based upon pillaring materials as well as preparation conditions. Therefore, the pillared clay may be a promising material for high-performance adsorbents and catalyst supports. In addition, the structure of the micropore, being described as galley-like, may exhibit unique functions as an adsorbent different from other porous materials such as zeolites.
1 Present address: Department of Basic Sciences, School of Science & Engineering, Ishinomaki Senshu University, 01, Shinmito, Minamisakai, Ishinomaki, 986-8580 Japan. 2 Present address: Department of Materials-Process Engineering and Applied Chemistry for Environment, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegatagakuencho, Akita, 010-8502 Japan. 3 To whom correspondence should be addressed. Department of Basic Sciences, School of Science & Engineering, Ishinomaki Senshu University, 01, Shinmito, Minamisakai, Ishinomaki, 986-8580 Japan, Fax: 81225-22-7746. E-mail: t [email protected].
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C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
The pore structure of pillared clays has often been investigated using powder X-ray diffraction (XRD), which has provided information about interlayer distance and an accumulation number of layers. However, low regularity of the pillar sites in the gallery prevents one from analyzing the interpillar distance and pillar distribution on the floor. In addition, the pillar structure (diameter, etc.) cannot be analyzed by XRD because of the small particle size. Nitrogen adsorption at 77 K has often been used for analyses of pore size distribution of micro- and mesoporous materials (4, 5). However, the technique encounters difficulty in the analysis of pillared clay in which the pore structure is not cylindrical nor slit-like. Electron microscopy is known to a powerful tool for analysis of zeolite pores (6, 7). A few studies of pillared clays have been carried out (8), but it is difficult to observe pillars shielded by silicate layers. Accordingly, to clarify the pillar structure and the pore structure formed by the pillars and the silicate layers, an appropriate characterization technique other than the conventional ones is needed to be developed. Pinnavaia et al. studied the bonding between a silicate layer and an alumina pillar for a synthetic fluorohectorite using highresolution solid NMR and showed that the SiO4 tetrahedra in the layer was inverted when the pillaring reaction proceeded (9). Tsiao et al. studied the structure of clay layers by 129 XeNMR (10). They reported that the obtained interlayer distance was coincident with that from XRD measurement. Occelli et al. used an atomic force microscope (AFM) to investigate the surface features of a pillared montmorillonite, and they provided atomicscale details of the basal planes of clays and pillared clays (11). Although studies of the pillar structure and the pore structure in the interlayers of a pillared clay were few, Ooka et al. investigated the pore structure of pillared clays by analyzing the relation between the amount of dye molecule adsorbed and the amount of pillar intercalated (12, 13). Pinnavaia et al. characterized the pillar structure in montmorillonite interlayers using the neutron scattering method and estimated the pillar spacing and the pillar lateral radius to be 2.5 and 0.4 nm, respectively (14). Recently, microporous carbons have often been prepared using zeolites or silica as a template (15, 16). The structure of obtained carbons may reverse the template structure. When the pillared clay is used as a template, the resultant carbon may have micropores corresponding to traces of the pillars in the gallery. Therefore, the characterization of the carbon obtained
440
PORE STRUCTURE OF PILLARED MONTMORILLONITE
in such a way may give significant information about the pillar. Sandi et al. have studied the carbons derived from an organic compound loaded clay using small angle neutron and X-ray scattering analyses, and they discussed the hole radius (0.4–0.8 nm), the fractal dimension, the cutoff length, and the density of the final carbons (17). In the present study, alumina- and chromia-pillared montmorillonites were first examined by a conventional adsorption technique, and second the carbon materials formed in the interlayer by carbonization of organic polymers were also investigated by an adsorption technique to microscopically investigate the pore structure in pillared montmorillonite layers.
clays and 373 K for the carbon samples for 8 h, respectively. Micropore volume and pore size distribution were analyzed using a conventional t-plot method. The isotherm reported by Lecloux and Pirard was used to prepare standard t-curves for clay samples (20); those on carbon samples were determined using a nonporous carbon prepared by carbonizing bulk poly furfuryl alcohol at 923 K for 2 h, where the polymer was prepared from liquid furfuryl alcohol using hydrochloric acid as a catalyst. The adsorption of some organic molecules on Al–PMT10 was measured at 273 K by a gravimetric method using Cahn2000 microbalance equipment after the sample was evacuated overnight at 623 K.
2. EXPERIMENTAL
Alumina-pillared montmorillonites were prepared from Namontmorillonite (Kunipia F; Kunimine Industry Co.) and ion-exchanging solutions containing Al13 polycation [Al13 ]n+ prepared according to the conventional procedure (18). The concentrations of Al13 polycation in the solution were adjusted to be 0.5, 1.0, 2.0, and 10.0 times that of the CEC of the Na– montmorillonite assuming the valence of the polycation to be +7. After the ion-exchanged samples were thoroughly washed with deionized water and dried at 373 K overnight, the samples were calcined at 673 K for 3 h in nitrogen flow. These pillared clays are designated as Al–PMT-n in this paper, where n indicates an equivalent of [Al13 ]7+ per CEC of Na–montmorillonite. A chromia-pillared montmorillonite was prepared referring to the report of Brindley and Yamanaka (2). Hydroxy–chromium polycations with basicity OH/Cr = 2 were prepared by adding Cr(NO3 )3 solution to NaOH solution. The solution was stirred for 1 week to attain equilibrium in the formation of polycations. A Na–montmorillonite was ion exchanged by this solution under stirring for 3 days. After the intercalated sample was thoroughly washed with deionized water and dried at 373 K overnight, the sample was calcined at 473 K for 3 h in a nitrogen flow. The resultant sample is named Cr–PMT in the present paper. Carbonized samples were prepared by referring to the method reported by Sonobe et al. (19). Furfuryl alcohol (Wako Pure Chemicals) was used as a starting monomer. To adsorb the monomer completely into the micropore, the pillared clays were exposed to the vapor of furfuryl alcohol at 77 K after evacuation at 623 K. Then, the loaded clay was evacuated at 353 K for 24 h to remove the furfuryl alcohol accumulated on the outer surface. The polymerization of the monomer in the pore was performed at 423 K for 6 h, and the resultant polymer was carbonized by heating in a nitrogen gas flow at 973 K for 2 h. The inorganic matrix was removed by extraction with hydrochloric acid and hydrofluoric acid. The resultant carbon sample was thoroughly washed by deionized water and dried. We named the samples Al–PMT-n-nega and Cr–PMT-nega in this paper. Nitrogen adsorption was measured at 77 K under pressures from 0.7 to 97 kPa after evacuation at 623 K for the pillared
441
3. RESULTS AND DISCUSSION
Al–PMT X-ray diffraction spectra for Al–PMT-n are shown in Fig. 1. The X-ray diffraction spectra for Al–PMT-n except for Al–PMT0.5 showed a sharp peak assigned to d001 around 5◦ (in 2θ ) and no crystalline Al2 O3 particle was detected. The interlayer distance (h; excluding the thickness of the silicate layer) of the samples estimated from the d001 peak position were 0.8–0.9 nm (Table 1) regardless of the Al content. This interlayer distance coincided well with that for Al–PMT reported in the literature (22). In addition, the Al content in the outer surface estimated by XPS
FIG. 1. XRD pattern (Cu K α) of Al–PMT-n; (a) Al–PMT-10 ; (b) Al–PMT2; (c) Al–PMT-1; (d) Al–PMT-0.5.
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TABLE 1 Specific Surface Area and X-ray Diffraction Data
Sample
d001 (nm)
Interlayer distancea (nm)
d001 width (degree)
MT Al–PMT-0.5 Al–PMT-1 Al–PMT-2 Al–PMT-10
1.26 — 1.88 1.88 1.78
0.30 — 0.92 0.92 0.82
1.1 — 1.4 1.2 1.1
a Interlayer distance was estimated by subtracting the thickness of the montmorillonite layer (0.96 nm (21)) from the basal spacing of (001).
is almost equal to that of bulk. Consequently, we judged that the samples prepared in this work, except for Al–PMT-0.5, were successfully intercalated by Al13 polycation without additional adsorption of large Al polycations or alumina-like species. The content of Al intercalated in Al–PMT-n samples measured by ICP was plotted in Fig. 2 as a function of the equivalence of Al7+ 13 polycation in the exchanging solution to the CEC of the raw montmorillonite. The figure showed that the Al content increased with the increase in the concentration of polycation in the solution. This result suggests that the amount of pillar can be changed by controlling the content of the polycation in the solution. Assuming the charge of Al13 polycation to be +7, however, the Al13 polycation contents in Al–PMT-10 exceeded the theoretical exchange capacity of the montmorillonite (2.4 mmol/g). It may mean that the charge of the Al13 polycation decreased to ca. 3–4 during the ion exchange. Vaughan (23) reported that the Al13 polycation in aqueous solution often hydrolyzed and the charge of the Al13 polycation decreased with the increase of pH. In fact, the average charge of Al13 polycations was estimated to be 3.5–4.0 from pH (3.7–4.5) of the exchange solution used.
FIG. 3. Normalized adsorption isotherms of nitrogen at 77 K on Al–PMT-1 (s), Al–PMT-2 (n), and Al–PMT-10 (u).
The XRD peak widths of d001 for the pillared clays decrease with the increase in Al13 content. Accordingly, it is necessary to control the concentration of Al ion to form the homogeneous gallery in the montmorillonite interlayer with pillaring of [Al13 ]n+ , and the concentration of the solution used for the preparation of Al–PMT-0.5 seems to be too small to form a homogeneously accumulated structure. The adsorption isotherms normalized with the weight of the raw montmorillonite (excluding pillars) were shown in Fig. 3. The weights of the raw montmorillonite were estimated from ICP data using the Si content as the internal standard in Al– PMT layers. Since each isotherm has a steep increase at the low pressure end and a plateau around the middle range, the formation of micropores by pillaring was obvious. The specific surface areas and pore volumes obtained from the N2 isotherms were summarized in Table 2. Both the surface area and the pore volume increased with the increase in Al content. This result is rather strange because the enhancement of pillar site density in the gallery should reduce the free space for nitrogen adsorption as already being reported for an iron-oxide-pillared montmorillonite (24). However, considering the fact that the specific surface area of a montmorillonite unpillared is very small (ca. 30 m2 /g) and that the maximum surface area estimated from TABLE 2 Specific Surface Area (Σ), Micropore Volume (Vmicro ), and Al13 Content (np ) in Al–PMT-n
FIG. 2. Al content in Al–PMT-n interlayer as a function of Al13 content in the ion-exchanging solution, where --- indicated the ion-exchange capacity of Al7+ 13 .
Sample
6 (m2 g−1 )
Vmicro (10−6 m3 g−1 )
np a (mmol g (MT)−1 )
Al–PMT-1 Al–PMT-2 Al–PMT-10
237 285 288
0.136 0.166 0.157
0.094 0.084 0.294
a
The values were normalized with the weight of raw montmorillonite.
PORE STRUCTURE OF PILLARED MONTMORILLONITE
FIG. 4. Fractal dimension analysis of Al–PMT-10.
lattice constants of montmorillonite layer is very large (810 m2 /g) (25), the loading of an Al13 pillar should contribute to the reduction of incomplete pillaring in the gallery rather than the enhancement of pillar density in the range of this work. Assuming the pore structure to be slit-like, the mean pore sizes in each Al–PMT-n sample were estimated to ca. 0.8 nm using the t-plot method. Although this value coincided with the results of d001 X-ray diffraction (interlayer distance), information on the lateral spacing between pillars was not obtained. To obtain additional information about pore structure, several organic compounds were used as the adsorption probe. The log– log relation between the amounts of monolayer adsorption on Al–PMT-10 and the molecular cross-sectional areas was shown in Fig. 4. Although some scatters were seen, the amount of monolayer adsorption tended to decrease with the increase in the molecular area, and the points in the figure are roughly distributed on a linear line. Twice the slope of the line, corresponding to the fractal dimension on the adsorbent (26), is about 2, which coincides with that reported in literature (27). These results indicate that the surface is almost smooth and planar on the scale of the probe molecules used, and the pore did not exhibit a molecular sieving effect for the probe molecules. Namely, it was suggested that the interpillar spacing is so large that those probe molecules can enter the gallery. To obtain information about the dimensions of the pillar and the interpillar spacing, carbon samples obtained by carbonization of a polymer inside the pillared clays were investigated. The isotherms of nitrogen on Al–PMT-n-nega normalized with the weight of the raw montmorillonite were shown in Fig. 5, where the normalization factor was defined from the thermal gravimetry profile of the carbon-pillared clay complex. Since the dimension of Al13 polycation (Keggin structure, a precursor of pillar) in an aqueous solution was reported to be 0.9 nm (28), Al–PMT-n-nega should have micropores corresponding to the trace of pillar. In fact, each isotherm has a steep increase at low pressure, implying the presence of a micropore. In ad-
443
FIG. 5. Normalized adsorption isotherms of nitrogen at 77 K on Al–PMT1-nega (s), Al–PMT-2-nega (n), and Al–PMT-10-nega (u).
dition, it can be seen that the amount of adsorption increased with the increase in Al13 content without significant change in the shape of the isotherm. The micropore volume estimated using the t-plot method were shown in Fig. 6 as a function of the amount of Al intercalated in the parent PMT. The volume of micropore in Al–PMT-n-nega is almost proportional to the introduced Al13 content in Al–PMT-n, and it shows that the micropore in Al–PMT-n-nega is formed by the Al13 pillar as the template. If all of the pillars are formed from species intercalated as [Al13 ]n+ , the slope of the line in Fig. 5 should correspond to the volume of the Al13 pillar. Then, assuming the structure of the
FIG. 6. Micropore volume as a function of Al content intercalated in Al– PMT-n-nega.
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pillar to be cylindrical, one can estimate the diameter of the pillar using the height of the pillar, h, determined by X-ray diffraction. The estimated mean diameter, dpillar , was about 0.6 nm on average, as listed in Table 3, and the values seemed to be independent of the amount of intercalated Al. Pore size distribution curves (not shown) for each Al–PMT-n-nega samples, obtained by the t-plot method, also showed a peak around t = 0.3 nm (the value corresponds to the pillar diameter of 0.6 nm). The coincidence of the pillar diameters estimated by alternative methods indicated that most of the intercalated Al composed the pillar as Al13 . Although the diameter is slightly smaller than that of the Al13 polycation (Keggin structure) in an aqueous solution, the reduction can be partially explained as dehydration of the polycation during calcination. Sandi et al. estimated the hole diameter of the carbons formed in a pillared montmorillonite interlayer to be 0.8–1.6 nm by small angle neutron and X-ray scattering studies (17). The slight discordance with that in this work might be ascribed to the carbonization condition (i.e., the monomer agent and/or its adsorption condition) as reported by Sandi et al. When the carbon is closely loaded in the Al–PMT gallery, the total volume (Vtotal ) of the gallery, including the pillar, was estimated as follows, Vtotal = Vmicro + Vpiller ,
[1]
where Vmicro is the micropore volume (free space) in Al–PMT-n and Vpiller is the total volume of the pillars, which is obtainable from the micropore volume of Al–PMT-n-nega. The value of Vtotal increased with the increase in the Al13 content, as seen in Table 4. It shows that the increase in Al13 -pillar contributes to the increase in the effective area. The effective floor area (6eff ) in the gallery (including the cross-sectional area of the pillar) can be obtained by dividing the total gallery volume (Vtotal ) by the height of the gallery (h) i.e., the interlayer distance: 6eff = Vtotal / h.
TABLE 3 Micropore Volume (Vpiller ) in Al–PMT-n-nega, Mean Pillar Volume, and Mean Pillar Diameter
a b
Al–PMT-1 Al–PMT-2 Al–PMT-10 Calcdb
Vpiller a (10−6 m3 g (MT)−1 )
Mean piller volumeb (nm3 )
dpiller (nm)
0.010 0.013 0.033
0.17 0.26 0.19
0.52 0.65 0.55
The values were normalized with the weight of raw montmorillonite. Mean pillar volume = Vpiller /n p .
Vtotal a (ml g (MT)−1 )
∗ 6eff (m2 g (MT)−1 )
δpillar (nm−2 )
dip (nm)
ds (nm)
0.068 0.085 0.128 —
84 106 160 405
0.67 0.48 1.11 0.53
1.31 1.56 1.02 1.47
0.79 0.91 0.48 —
a
The values were normalized with the weight of raw montmorillonite. Theoretical values for montmorillonite. The δpillar value was estimated by assuming the valence of Al13 polycation to be +3.7. b
fore, the value of obtained 6eff suggests that the amount of closed interlayer is not small. The density (δpillar ) of pillar sites on the effective floor can be estimated as follows, δpillar = n p × 6.02 × 1020 /6eff ,
[3]
where n p is the number of Al13 pillars, which has been calculated from Al contents in Al–PMT-n as shown in Table 2. The estimated δpillar values increased with the increase in Al content in the pillaring solution, as listed in Table 4. It means that the Al13 content in the pillaring solution changes the density of the pillar sites in the surface as well as the number of pillars. Assuming that the pillar sites are homogeneously distributed on the surface in a hexagonal arrangement, the mean interpillar distance (dip ) and the mean spacing (ds ) between the nearest neighboring pillars can also be estimated by the following equations: dip dip 1 = 6× × √ , δpillar 2 2 3 ds = dip − dpillar .
[2]
6eff increased with the increase in the content of the Al13 pillar as shown in Table 4. The ideal maximum floor area should be described as half the ideal surface area of montmorillonite (810 m2 /g) (25) when the layers accumulate adequately. There-
Al–PMT-1-nega Al–PMT-2-nega Al–PMT-10-nega
TABLE 4 Physical Parameter of the Pillar of Al–PMTs Determined Using Micropore Volume
[4] [5]
The resultant mean spacing (ds ) for pillars of Al–PMT-n samples are 0.5–0.9 nm, as listed in Table 4. Therefore, the sieving aperture of Al–PMT-n samples is 0.8 nm (height) ×0.5–0.9 nm (wide), and it confirms that the aperture cannot hinder the entering of the probe molecules used in this work. Ooka et al. (12, 13) reported 0.9 nm for the value of ds , which is in the range of our observations. Cr–PMT The adsorption isotherm of nitrogen on Cr–PMT at 77 K was shown in Fig. 7. The amount of adsorption is smaller than those on Al–PMT-n samples, and the shape of the isotherm seemed to be rather close to type-1 (Langmuir type) compared with that on Al–PMT-n. The steep increase of the isotherm at the low-pressure end again suggests the presence of micropores in Cr–PMT. The specific surface area is estimated to 194 m2 /g
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PORE STRUCTURE OF PILLARED MONTMORILLONITE
FIG. 7. Adsorption isotherm of nitrogen on Cr–PMT at 77 K.
FIG. 8. Adsorption isotherm of nitrogen on Cr–PMT-nega at 77 K.
by the BET method. Since the d001 XRD peak for the sample appeared clearly around 2θ = 6◦ , we judged that the montmorillonite layers were pillared by chromium hydroxide polycations. The interlayer distance was estimated to 0.51 nm, which was smaller than those for Al–PMT-n samples. The nitrogen adsorption isotherm for the carbon (Cr–PMTnega) obtained by carbonization in a Cr–PMT interlayer was shown in Fig. 8. The steep increase of the isotherm at low pressure also suggested the presence of micropores in Cr–PMT-nega. However, the t-plot for Cr–PMT-nega was almost linear from t = 0.3 to t = 0.8 nm. Since the interpolated line did not pass through the origin (not shown) and the intercept on the ordinate is positive, it is suggested that Cr–PMT-nega has micropores under a radius of 0.3 nm, although it cannot be analyzed by nitrogen adsorption. Nevertheless, it seems that the dimensions of the pillar are smaller than those of Al13 . The micropore volumes of Cr– PMT and Cr–PMT-nega (normalized by the weight of the raw montmorillonite) and parameters of the pillar calculated in the same manner as Al–PMT-n are listed in Table 5. The micropore volume of Cr–PMT is smaller than those of Al–PMT-n samples, mainly because of the low height of the gallery. In addition, the micropore volume of Cr–PMT-nega, which corresponds to the total volume of pillars, is also smaller than those of Al–PMT-nnega samples. One of the reasons for this small volume should also be the low height of the pillar; however, the additional cause
would be a small diameter of [Crx ]n+ pillar. It is known that the size of the hydroxy–chromium complex changes with the molar ratio of OH to Cr in the exchanging solution (2). Brindley and Yamanaka investigated the form of the hydroxy–chromium polymer prepared under a range of conditions (2). According to the report, the ratio of (OH + H2 O) to Cr for the hydroxy– chromium complex should be 3.56 under the conditions applied in this work, and the number of Cr atoms in the complex is 7. In this situation, the number of pillars, n p , was estimated by assuming that all of the Cr7 polycation was intercalated in the interlayers, as listed in Table 5. The interpillar distance and the pillar diameter were also estimated by using the total pore volume (Vtotal ) and the effective floor area (6eff ) in the Cr–PMT gallery in the same manner as those in Al–PMT-n as described before. The diameter of the pillar was estimated to be 0.4 nm, which was smaller than those of Al–PMT-n samples. Considering that the interlayer distance obtained by X-ray diffraction is 0.51 nm, the shape of the pillar was more isotropic than those of Al–PMT-n samples. The interpillar distance was estimated to be 1.16 nm, which is near that of Al–PMT-10. However, the small diameter of the pillar slightly expanded the interpillar spacing (0.76 nm) compared with those of Al–PMT-n samples. Consequently, it was found that the aperture of the sieve formed in the Cr–PMT interlayer is likely to be 0.51 nm (height) × 0.76 nm (width).
TABLE 5 Parameters of Pore Structure in Cr–PMT Cr content (mmol g (MT)−1 ) 0.919 a
(10−6
Vmicro a m3 g (MT)−1 ) 42.1
(10−6
Vpiller a m3 g (MT)−1 ) 5.17
The values were normalized with the weight of raw montmorillonite.
(m2
6eff a g (MT)−1 ) 93
np a (mmol g (MT)−1 )
dip (nm)
dpillar (nm)
0.131
1.16
0.40
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YAMAZAKI, NAKAMURA, AND OZAWA
4. CONCLUSION
The pore structures formed in aluminum-hydroxide-pillared (Al–PMT) and chromium-hydroxide-pillared montmorillonite (Cr–PMT) interlayers were clarified by analyzing the pillared clays themselves and the carbons formed in these interlayers using nitrogen adsorption. The results obtained in this work are as follows. The amount of Al13 polycation in the exchanging solution can change the interpillar distance without changing the diameter of the pillar. However, a too small amount of Al13 polycation in the pillaring solution prevented the formation of homogeneous accumulated layers. The sieving aperture sizes of Al–PMT interlayers were 0.8 nm × 0.5–0.9 nm depending on the Al13 content. The diameter of the pillar was 0.6 nm regardless of Al content. On the other hand, the sieving aperture size of the Cr–PMT interlayer was 0.5 × 0.4 nm, and the diameter of the pillar was 0.4 nm. REFERENCES 1. 2. 3. 4. 5.
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8. Occelli, M. L., Lynch, J., and Sanders, J. V., J. Catal. 107, 557 (1987). 9. Pinnavaia, T. J., Landau, S. D., Tzou, M. S., and Johnson, I. D., J. Am. Chem. Soc. 107, 7222 (1985). 10. Tsiao, C.-J., Carrado, K. A., and Botto, R. E., Microporous Mesoporous Mater. 21, 45 (1998). 11. Occelli, M. L., Gould, S. A. C., Tsai, J. M., and Drake, B., J. Mol. Catal. 100, 161 (1995). 12. Ooka, C., Osaki, T., Horio, M., and Suzuki, K., Nendokagaku 35, 170 (1996). 13. Ooka, C., Osaki, T., Horio, M., and Suzuki, K., Nendokagaku 36, 175 (1997). 14. Pinnavaia, T. J., Tzou, M. S., Landau, S. D., and Raythatha, R. H., J. Mol. Catal. 27, 195 (1984). 15. Kyotani, T., Sonobe, N., and Tomita, A., Nature 28, 331 (1988). 16. Kyotani, T., Nagai, T., Inoue, S., and Tomita, A., Chem. Mater. 9, 609 (1997). 17. Sandi, G., Thiyagarajan, P., Winans, R. E., and Carrado, K. A., Prep. Pap. -Am. Chem. Soc. Div. Fuel Chem. 42, 854 (1997). 18. Yamanaka, S., Malla, P. B., and Komarneni, S., J. Colloid Interface Sci. 134, 51 (1991). 19. Sonobe, N., Kyotani, T., and Tomita, A., Carbon 28, 483 (1990). 20. Lecloux, A., and Pirard, J. P., J. Colloid Interface Sci. 70, 265 (1979). 21. Suzuki, K., Horio, M., Masuda, H., and Moril, T., Bull. Chem. Soc. Jpn. 64, 732 (1991). 22. Brindley, G. W., and Semples, R. E., Clay Miner. 12, 229 (1977). 23. Vaughan, D. E. W., Catal. Today 2, 187 (1988). 24. Heylen, I., Vanhoof, C., and Vansant, E. F., Microporous Mater. 5, 53 (1995). 25. Dyal, S., and Heilman, S. B., Soil Sci. 69, 421 (1950). 26. Avnir, D., in “The fractal approach to heterogeneous chemistry: surfaces, collides, polymers,” p. 276. Wiley, New York, 1989. 27. Van Damme, H., and Fripiat, J. J., J. Chem. Phys. 82, 2785 (1985). 28. Rausch, V., and Bale, H. D., J. Chem. Phys. 40, 3391 (1964).
Investigation on Pore Structure Formed in a Pillared Montmorillonite Interlayer Using Adsorption Methods and a Carbonization Technique Tatsuya Yamazaki,1,3 Yuichiro Nakamura, and Sentaro Ozawa2 Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 07, Aramaki-Aoba, Aoba-ku, Sendai, 980-8579 Japan Received October 11, 2000; accepted March 21, 2001
The structure of pores formed in the alumina- or chromia-pillared montmorillonite (PMT) interlayer was investigated by analyzing the porous solid itself and the negative carbon replica formed in the pores of the parent material. The nitrogen adsorption on these materials allowed us to estimate the pillar structure and the density of pillar sites in the montmorillonite interlayers. The amount of Al13 polycation in an exchanging solution governed the interpillar distance in the pillared clay gallery without changing the diameters of the pillar (ca. 0.6 nm). The aperture size of Al–PMT was 0.8 nm in height and 0.5–0.9 nm in width depending on Al content. On the other hand, the aperture size of Cr–PMT was 0.5 nm in height and 0.8 nm in width, and the diameter of a pillar was estimated to be ca. 0.4 nm. °C 2001 Academic Press Key Words: alumina-pillared montmorillonite; chromia-pillared montmorillonite; nitrogen adsorption; micropore structure; carbonization in micropore; dimension of pillar.
1. INTRODUCTION
Many pillared montmorillonites have been prepared by intern+ calating oligomeric metal hydroxide ions, i.e., Alx On+ y , ZrOx , n+ n+ n+ Gax O y , Fex O y , and Crx O y (1–3) into the interlayers. The pillared clays thus prepared have high surface areas and they are micropores, of which the structural parameters such as the interlayer distance and the pillar spacing can change based upon pillaring materials as well as preparation conditions. Therefore, the pillared clay may be a promising material for high-performance adsorbents and catalyst supports. In addition, the structure of the micropore, being described as galley-like, may exhibit unique functions as an adsorbent different from other porous materials such as zeolites.
1 Present address: Department of Basic Sciences, School of Science & Engineering, Ishinomaki Senshu University, 01, Shinmito, Minamisakai, Ishinomaki, 986-8580 Japan. 2 Present address: Department of Materials-Process Engineering and Applied Chemistry for Environment, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegatagakuencho, Akita, 010-8502 Japan. 3 To whom correspondence should be addressed. Department of Basic Sciences, School of Science & Engineering, Ishinomaki Senshu University, 01, Shinmito, Minamisakai, Ishinomaki, 986-8580 Japan, Fax: 81225-22-7746. E-mail: t [email protected].
0021-9797/01 $35.00
C 2001 by Academic Press Copyright ° All rights of reproduction in any form reserved.
The pore structure of pillared clays has often been investigated using powder X-ray diffraction (XRD), which has provided information about interlayer distance and an accumulation number of layers. However, low regularity of the pillar sites in the gallery prevents one from analyzing the interpillar distance and pillar distribution on the floor. In addition, the pillar structure (diameter, etc.) cannot be analyzed by XRD because of the small particle size. Nitrogen adsorption at 77 K has often been used for analyses of pore size distribution of micro- and mesoporous materials (4, 5). However, the technique encounters difficulty in the analysis of pillared clay in which the pore structure is not cylindrical nor slit-like. Electron microscopy is known to a powerful tool for analysis of zeolite pores (6, 7). A few studies of pillared clays have been carried out (8), but it is difficult to observe pillars shielded by silicate layers. Accordingly, to clarify the pillar structure and the pore structure formed by the pillars and the silicate layers, an appropriate characterization technique other than the conventional ones is needed to be developed. Pinnavaia et al. studied the bonding between a silicate layer and an alumina pillar for a synthetic fluorohectorite using highresolution solid NMR and showed that the SiO4 tetrahedra in the layer was inverted when the pillaring reaction proceeded (9). Tsiao et al. studied the structure of clay layers by 129 XeNMR (10). They reported that the obtained interlayer distance was coincident with that from XRD measurement. Occelli et al. used an atomic force microscope (AFM) to investigate the surface features of a pillared montmorillonite, and they provided atomicscale details of the basal planes of clays and pillared clays (11). Although studies of the pillar structure and the pore structure in the interlayers of a pillared clay were few, Ooka et al. investigated the pore structure of pillared clays by analyzing the relation between the amount of dye molecule adsorbed and the amount of pillar intercalated (12, 13). Pinnavaia et al. characterized the pillar structure in montmorillonite interlayers using the neutron scattering method and estimated the pillar spacing and the pillar lateral radius to be 2.5 and 0.4 nm, respectively (14). Recently, microporous carbons have often been prepared using zeolites or silica as a template (15, 16). The structure of obtained carbons may reverse the template structure. When the pillared clay is used as a template, the resultant carbon may have micropores corresponding to traces of the pillars in the gallery. Therefore, the characterization of the carbon obtained
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PORE STRUCTURE OF PILLARED MONTMORILLONITE
in such a way may give significant information about the pillar. Sandi et al. have studied the carbons derived from an organic compound loaded clay using small angle neutron and X-ray scattering analyses, and they discussed the hole radius (0.4–0.8 nm), the fractal dimension, the cutoff length, and the density of the final carbons (17). In the present study, alumina- and chromia-pillared montmorillonites were first examined by a conventional adsorption technique, and second the carbon materials formed in the interlayer by carbonization of organic polymers were also investigated by an adsorption technique to microscopically investigate the pore structure in pillared montmorillonite layers.
clays and 373 K for the carbon samples for 8 h, respectively. Micropore volume and pore size distribution were analyzed using a conventional t-plot method. The isotherm reported by Lecloux and Pirard was used to prepare standard t-curves for clay samples (20); those on carbon samples were determined using a nonporous carbon prepared by carbonizing bulk poly furfuryl alcohol at 923 K for 2 h, where the polymer was prepared from liquid furfuryl alcohol using hydrochloric acid as a catalyst. The adsorption of some organic molecules on Al–PMT10 was measured at 273 K by a gravimetric method using Cahn2000 microbalance equipment after the sample was evacuated overnight at 623 K.
2. EXPERIMENTAL
Alumina-pillared montmorillonites were prepared from Namontmorillonite (Kunipia F; Kunimine Industry Co.) and ion-exchanging solutions containing Al13 polycation [Al13 ]n+ prepared according to the conventional procedure (18). The concentrations of Al13 polycation in the solution were adjusted to be 0.5, 1.0, 2.0, and 10.0 times that of the CEC of the Na– montmorillonite assuming the valence of the polycation to be +7. After the ion-exchanged samples were thoroughly washed with deionized water and dried at 373 K overnight, the samples were calcined at 673 K for 3 h in nitrogen flow. These pillared clays are designated as Al–PMT-n in this paper, where n indicates an equivalent of [Al13 ]7+ per CEC of Na–montmorillonite. A chromia-pillared montmorillonite was prepared referring to the report of Brindley and Yamanaka (2). Hydroxy–chromium polycations with basicity OH/Cr = 2 were prepared by adding Cr(NO3 )3 solution to NaOH solution. The solution was stirred for 1 week to attain equilibrium in the formation of polycations. A Na–montmorillonite was ion exchanged by this solution under stirring for 3 days. After the intercalated sample was thoroughly washed with deionized water and dried at 373 K overnight, the sample was calcined at 473 K for 3 h in a nitrogen flow. The resultant sample is named Cr–PMT in the present paper. Carbonized samples were prepared by referring to the method reported by Sonobe et al. (19). Furfuryl alcohol (Wako Pure Chemicals) was used as a starting monomer. To adsorb the monomer completely into the micropore, the pillared clays were exposed to the vapor of furfuryl alcohol at 77 K after evacuation at 623 K. Then, the loaded clay was evacuated at 353 K for 24 h to remove the furfuryl alcohol accumulated on the outer surface. The polymerization of the monomer in the pore was performed at 423 K for 6 h, and the resultant polymer was carbonized by heating in a nitrogen gas flow at 973 K for 2 h. The inorganic matrix was removed by extraction with hydrochloric acid and hydrofluoric acid. The resultant carbon sample was thoroughly washed by deionized water and dried. We named the samples Al–PMT-n-nega and Cr–PMT-nega in this paper. Nitrogen adsorption was measured at 77 K under pressures from 0.7 to 97 kPa after evacuation at 623 K for the pillared
441
3. RESULTS AND DISCUSSION
Al–PMT X-ray diffraction spectra for Al–PMT-n are shown in Fig. 1. The X-ray diffraction spectra for Al–PMT-n except for Al–PMT0.5 showed a sharp peak assigned to d001 around 5◦ (in 2θ ) and no crystalline Al2 O3 particle was detected. The interlayer distance (h; excluding the thickness of the silicate layer) of the samples estimated from the d001 peak position were 0.8–0.9 nm (Table 1) regardless of the Al content. This interlayer distance coincided well with that for Al–PMT reported in the literature (22). In addition, the Al content in the outer surface estimated by XPS
FIG. 1. XRD pattern (Cu K α) of Al–PMT-n; (a) Al–PMT-10 ; (b) Al–PMT2; (c) Al–PMT-1; (d) Al–PMT-0.5.
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YAMAZAKI, NAKAMURA, AND OZAWA
TABLE 1 Specific Surface Area and X-ray Diffraction Data
Sample
d001 (nm)
Interlayer distancea (nm)
d001 width (degree)
MT Al–PMT-0.5 Al–PMT-1 Al–PMT-2 Al–PMT-10
1.26 — 1.88 1.88 1.78
0.30 — 0.92 0.92 0.82
1.1 — 1.4 1.2 1.1
a Interlayer distance was estimated by subtracting the thickness of the montmorillonite layer (0.96 nm (21)) from the basal spacing of (001).
is almost equal to that of bulk. Consequently, we judged that the samples prepared in this work, except for Al–PMT-0.5, were successfully intercalated by Al13 polycation without additional adsorption of large Al polycations or alumina-like species. The content of Al intercalated in Al–PMT-n samples measured by ICP was plotted in Fig. 2 as a function of the equivalence of Al7+ 13 polycation in the exchanging solution to the CEC of the raw montmorillonite. The figure showed that the Al content increased with the increase in the concentration of polycation in the solution. This result suggests that the amount of pillar can be changed by controlling the content of the polycation in the solution. Assuming the charge of Al13 polycation to be +7, however, the Al13 polycation contents in Al–PMT-10 exceeded the theoretical exchange capacity of the montmorillonite (2.4 mmol/g). It may mean that the charge of the Al13 polycation decreased to ca. 3–4 during the ion exchange. Vaughan (23) reported that the Al13 polycation in aqueous solution often hydrolyzed and the charge of the Al13 polycation decreased with the increase of pH. In fact, the average charge of Al13 polycations was estimated to be 3.5–4.0 from pH (3.7–4.5) of the exchange solution used.
FIG. 3. Normalized adsorption isotherms of nitrogen at 77 K on Al–PMT-1 (s), Al–PMT-2 (n), and Al–PMT-10 (u).
The XRD peak widths of d001 for the pillared clays decrease with the increase in Al13 content. Accordingly, it is necessary to control the concentration of Al ion to form the homogeneous gallery in the montmorillonite interlayer with pillaring of [Al13 ]n+ , and the concentration of the solution used for the preparation of Al–PMT-0.5 seems to be too small to form a homogeneously accumulated structure. The adsorption isotherms normalized with the weight of the raw montmorillonite (excluding pillars) were shown in Fig. 3. The weights of the raw montmorillonite were estimated from ICP data using the Si content as the internal standard in Al– PMT layers. Since each isotherm has a steep increase at the low pressure end and a plateau around the middle range, the formation of micropores by pillaring was obvious. The specific surface areas and pore volumes obtained from the N2 isotherms were summarized in Table 2. Both the surface area and the pore volume increased with the increase in Al content. This result is rather strange because the enhancement of pillar site density in the gallery should reduce the free space for nitrogen adsorption as already being reported for an iron-oxide-pillared montmorillonite (24). However, considering the fact that the specific surface area of a montmorillonite unpillared is very small (ca. 30 m2 /g) and that the maximum surface area estimated from TABLE 2 Specific Surface Area (Σ), Micropore Volume (Vmicro ), and Al13 Content (np ) in Al–PMT-n
FIG. 2. Al content in Al–PMT-n interlayer as a function of Al13 content in the ion-exchanging solution, where --- indicated the ion-exchange capacity of Al7+ 13 .
Sample
6 (m2 g−1 )
Vmicro (10−6 m3 g−1 )
np a (mmol g (MT)−1 )
Al–PMT-1 Al–PMT-2 Al–PMT-10
237 285 288
0.136 0.166 0.157
0.094 0.084 0.294
a
The values were normalized with the weight of raw montmorillonite.
PORE STRUCTURE OF PILLARED MONTMORILLONITE
FIG. 4. Fractal dimension analysis of Al–PMT-10.
lattice constants of montmorillonite layer is very large (810 m2 /g) (25), the loading of an Al13 pillar should contribute to the reduction of incomplete pillaring in the gallery rather than the enhancement of pillar density in the range of this work. Assuming the pore structure to be slit-like, the mean pore sizes in each Al–PMT-n sample were estimated to ca. 0.8 nm using the t-plot method. Although this value coincided with the results of d001 X-ray diffraction (interlayer distance), information on the lateral spacing between pillars was not obtained. To obtain additional information about pore structure, several organic compounds were used as the adsorption probe. The log– log relation between the amounts of monolayer adsorption on Al–PMT-10 and the molecular cross-sectional areas was shown in Fig. 4. Although some scatters were seen, the amount of monolayer adsorption tended to decrease with the increase in the molecular area, and the points in the figure are roughly distributed on a linear line. Twice the slope of the line, corresponding to the fractal dimension on the adsorbent (26), is about 2, which coincides with that reported in literature (27). These results indicate that the surface is almost smooth and planar on the scale of the probe molecules used, and the pore did not exhibit a molecular sieving effect for the probe molecules. Namely, it was suggested that the interpillar spacing is so large that those probe molecules can enter the gallery. To obtain information about the dimensions of the pillar and the interpillar spacing, carbon samples obtained by carbonization of a polymer inside the pillared clays were investigated. The isotherms of nitrogen on Al–PMT-n-nega normalized with the weight of the raw montmorillonite were shown in Fig. 5, where the normalization factor was defined from the thermal gravimetry profile of the carbon-pillared clay complex. Since the dimension of Al13 polycation (Keggin structure, a precursor of pillar) in an aqueous solution was reported to be 0.9 nm (28), Al–PMT-n-nega should have micropores corresponding to the trace of pillar. In fact, each isotherm has a steep increase at low pressure, implying the presence of a micropore. In ad-
443
FIG. 5. Normalized adsorption isotherms of nitrogen at 77 K on Al–PMT1-nega (s), Al–PMT-2-nega (n), and Al–PMT-10-nega (u).
dition, it can be seen that the amount of adsorption increased with the increase in Al13 content without significant change in the shape of the isotherm. The micropore volume estimated using the t-plot method were shown in Fig. 6 as a function of the amount of Al intercalated in the parent PMT. The volume of micropore in Al–PMT-n-nega is almost proportional to the introduced Al13 content in Al–PMT-n, and it shows that the micropore in Al–PMT-n-nega is formed by the Al13 pillar as the template. If all of the pillars are formed from species intercalated as [Al13 ]n+ , the slope of the line in Fig. 5 should correspond to the volume of the Al13 pillar. Then, assuming the structure of the
FIG. 6. Micropore volume as a function of Al content intercalated in Al– PMT-n-nega.
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YAMAZAKI, NAKAMURA, AND OZAWA
pillar to be cylindrical, one can estimate the diameter of the pillar using the height of the pillar, h, determined by X-ray diffraction. The estimated mean diameter, dpillar , was about 0.6 nm on average, as listed in Table 3, and the values seemed to be independent of the amount of intercalated Al. Pore size distribution curves (not shown) for each Al–PMT-n-nega samples, obtained by the t-plot method, also showed a peak around t = 0.3 nm (the value corresponds to the pillar diameter of 0.6 nm). The coincidence of the pillar diameters estimated by alternative methods indicated that most of the intercalated Al composed the pillar as Al13 . Although the diameter is slightly smaller than that of the Al13 polycation (Keggin structure) in an aqueous solution, the reduction can be partially explained as dehydration of the polycation during calcination. Sandi et al. estimated the hole diameter of the carbons formed in a pillared montmorillonite interlayer to be 0.8–1.6 nm by small angle neutron and X-ray scattering studies (17). The slight discordance with that in this work might be ascribed to the carbonization condition (i.e., the monomer agent and/or its adsorption condition) as reported by Sandi et al. When the carbon is closely loaded in the Al–PMT gallery, the total volume (Vtotal ) of the gallery, including the pillar, was estimated as follows, Vtotal = Vmicro + Vpiller ,
[1]
where Vmicro is the micropore volume (free space) in Al–PMT-n and Vpiller is the total volume of the pillars, which is obtainable from the micropore volume of Al–PMT-n-nega. The value of Vtotal increased with the increase in the Al13 content, as seen in Table 4. It shows that the increase in Al13 -pillar contributes to the increase in the effective area. The effective floor area (6eff ) in the gallery (including the cross-sectional area of the pillar) can be obtained by dividing the total gallery volume (Vtotal ) by the height of the gallery (h) i.e., the interlayer distance: 6eff = Vtotal / h.
TABLE 3 Micropore Volume (Vpiller ) in Al–PMT-n-nega, Mean Pillar Volume, and Mean Pillar Diameter
a b
Al–PMT-1 Al–PMT-2 Al–PMT-10 Calcdb
Vpiller a (10−6 m3 g (MT)−1 )
Mean piller volumeb (nm3 )
dpiller (nm)
0.010 0.013 0.033
0.17 0.26 0.19
0.52 0.65 0.55
The values were normalized with the weight of raw montmorillonite. Mean pillar volume = Vpiller /n p .
Vtotal a (ml g (MT)−1 )
∗ 6eff (m2 g (MT)−1 )
δpillar (nm−2 )
dip (nm)
ds (nm)
0.068 0.085 0.128 —
84 106 160 405
0.67 0.48 1.11 0.53
1.31 1.56 1.02 1.47
0.79 0.91 0.48 —
a
The values were normalized with the weight of raw montmorillonite. Theoretical values for montmorillonite. The δpillar value was estimated by assuming the valence of Al13 polycation to be +3.7. b
fore, the value of obtained 6eff suggests that the amount of closed interlayer is not small. The density (δpillar ) of pillar sites on the effective floor can be estimated as follows, δpillar = n p × 6.02 × 1020 /6eff ,
[3]
where n p is the number of Al13 pillars, which has been calculated from Al contents in Al–PMT-n as shown in Table 2. The estimated δpillar values increased with the increase in Al content in the pillaring solution, as listed in Table 4. It means that the Al13 content in the pillaring solution changes the density of the pillar sites in the surface as well as the number of pillars. Assuming that the pillar sites are homogeneously distributed on the surface in a hexagonal arrangement, the mean interpillar distance (dip ) and the mean spacing (ds ) between the nearest neighboring pillars can also be estimated by the following equations: dip dip 1 = 6× × √ , δpillar 2 2 3 ds = dip − dpillar .
[2]
6eff increased with the increase in the content of the Al13 pillar as shown in Table 4. The ideal maximum floor area should be described as half the ideal surface area of montmorillonite (810 m2 /g) (25) when the layers accumulate adequately. There-
Al–PMT-1-nega Al–PMT-2-nega Al–PMT-10-nega
TABLE 4 Physical Parameter of the Pillar of Al–PMTs Determined Using Micropore Volume
[4] [5]
The resultant mean spacing (ds ) for pillars of Al–PMT-n samples are 0.5–0.9 nm, as listed in Table 4. Therefore, the sieving aperture of Al–PMT-n samples is 0.8 nm (height) ×0.5–0.9 nm (wide), and it confirms that the aperture cannot hinder the entering of the probe molecules used in this work. Ooka et al. (12, 13) reported 0.9 nm for the value of ds , which is in the range of our observations. Cr–PMT The adsorption isotherm of nitrogen on Cr–PMT at 77 K was shown in Fig. 7. The amount of adsorption is smaller than those on Al–PMT-n samples, and the shape of the isotherm seemed to be rather close to type-1 (Langmuir type) compared with that on Al–PMT-n. The steep increase of the isotherm at the low-pressure end again suggests the presence of micropores in Cr–PMT. The specific surface area is estimated to 194 m2 /g
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PORE STRUCTURE OF PILLARED MONTMORILLONITE
FIG. 7. Adsorption isotherm of nitrogen on Cr–PMT at 77 K.
FIG. 8. Adsorption isotherm of nitrogen on Cr–PMT-nega at 77 K.
by the BET method. Since the d001 XRD peak for the sample appeared clearly around 2θ = 6◦ , we judged that the montmorillonite layers were pillared by chromium hydroxide polycations. The interlayer distance was estimated to 0.51 nm, which was smaller than those for Al–PMT-n samples. The nitrogen adsorption isotherm for the carbon (Cr–PMTnega) obtained by carbonization in a Cr–PMT interlayer was shown in Fig. 8. The steep increase of the isotherm at low pressure also suggested the presence of micropores in Cr–PMT-nega. However, the t-plot for Cr–PMT-nega was almost linear from t = 0.3 to t = 0.8 nm. Since the interpolated line did not pass through the origin (not shown) and the intercept on the ordinate is positive, it is suggested that Cr–PMT-nega has micropores under a radius of 0.3 nm, although it cannot be analyzed by nitrogen adsorption. Nevertheless, it seems that the dimensions of the pillar are smaller than those of Al13 . The micropore volumes of Cr– PMT and Cr–PMT-nega (normalized by the weight of the raw montmorillonite) and parameters of the pillar calculated in the same manner as Al–PMT-n are listed in Table 5. The micropore volume of Cr–PMT is smaller than those of Al–PMT-n samples, mainly because of the low height of the gallery. In addition, the micropore volume of Cr–PMT-nega, which corresponds to the total volume of pillars, is also smaller than those of Al–PMT-nnega samples. One of the reasons for this small volume should also be the low height of the pillar; however, the additional cause
would be a small diameter of [Crx ]n+ pillar. It is known that the size of the hydroxy–chromium complex changes with the molar ratio of OH to Cr in the exchanging solution (2). Brindley and Yamanaka investigated the form of the hydroxy–chromium polymer prepared under a range of conditions (2). According to the report, the ratio of (OH + H2 O) to Cr for the hydroxy– chromium complex should be 3.56 under the conditions applied in this work, and the number of Cr atoms in the complex is 7. In this situation, the number of pillars, n p , was estimated by assuming that all of the Cr7 polycation was intercalated in the interlayers, as listed in Table 5. The interpillar distance and the pillar diameter were also estimated by using the total pore volume (Vtotal ) and the effective floor area (6eff ) in the Cr–PMT gallery in the same manner as those in Al–PMT-n as described before. The diameter of the pillar was estimated to be 0.4 nm, which was smaller than those of Al–PMT-n samples. Considering that the interlayer distance obtained by X-ray diffraction is 0.51 nm, the shape of the pillar was more isotropic than those of Al–PMT-n samples. The interpillar distance was estimated to be 1.16 nm, which is near that of Al–PMT-10. However, the small diameter of the pillar slightly expanded the interpillar spacing (0.76 nm) compared with those of Al–PMT-n samples. Consequently, it was found that the aperture of the sieve formed in the Cr–PMT interlayer is likely to be 0.51 nm (height) × 0.76 nm (width).
TABLE 5 Parameters of Pore Structure in Cr–PMT Cr content (mmol g (MT)−1 ) 0.919 a
(10−6
Vmicro a m3 g (MT)−1 ) 42.1
(10−6
Vpiller a m3 g (MT)−1 ) 5.17
The values were normalized with the weight of raw montmorillonite.
(m2
6eff a g (MT)−1 ) 93
np a (mmol g (MT)−1 )
dip (nm)
dpillar (nm)
0.131
1.16
0.40
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YAMAZAKI, NAKAMURA, AND OZAWA
4. CONCLUSION
The pore structures formed in aluminum-hydroxide-pillared (Al–PMT) and chromium-hydroxide-pillared montmorillonite (Cr–PMT) interlayers were clarified by analyzing the pillared clays themselves and the carbons formed in these interlayers using nitrogen adsorption. The results obtained in this work are as follows. The amount of Al13 polycation in the exchanging solution can change the interpillar distance without changing the diameter of the pillar. However, a too small amount of Al13 polycation in the pillaring solution prevented the formation of homogeneous accumulated layers. The sieving aperture sizes of Al–PMT interlayers were 0.8 nm × 0.5–0.9 nm depending on the Al13 content. The diameter of the pillar was 0.6 nm regardless of Al content. On the other hand, the sieving aperture size of the Cr–PMT interlayer was 0.5 × 0.4 nm, and the diameter of the pillar was 0.4 nm. REFERENCES 1. 2. 3. 4. 5.
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