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Synthesis and Textural Characterization of a New Microporous Silica Material Fathi Kooli,*,† Yoshimichi Kiyozumi,† Vicente Rives,‡ and Fujio Mizukami*,† National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi Tsukuba, Ibaraki 305-8565, Japan, and Departamento de Quimica Inorganica, Universidad de Salamanca, 37008 Salamanca, Spain Received September 10, 2001. In Final Form: January 3, 2002 A new layered silicate (denoted as FLS1) has been prepared from reaction of H-magadiite in tetramethylammonium hydroxide (TMAOH) and water via hydrothermal treatment for 5 days at temperatures higher than 150 °C. When the product is heated to 500 °C, the layered silicate is converted to a three-dimensional network through condensation of hydroxyl groups and cross-linking of the layers. The calcined phase is stable up to 900 °C and has a surface area of 437 m2 g-1, more than 10 times larger than that of the starting H-magadiite (40 m2 g-1). The external surface area and micropore volumes were determined using a new method, similar to that reported previously for pillared materials. The micropore volume of FLS1 calcined at 500 °C is about 0.130 mL (liquid nitrogen) g-1. The solid-state transformation of FLS1 into a TMA-containing sodalite structure in a sodium aluminate and TMAOH‚5H2O mixture at 150 °C for 2 h is also reported.
Introduction The ability of layered silicates to undergo intercalation by a wide variety of inorganic or organic cations has been studied for many years.1,2 Among the available layered compounds, aluminosilicates have received much attention and have great potential in the area of pillared materials where pore sizes larger than that of zeolites can be obtained.2 Layered silicates are also interesting owing to their exceptional adsorption and ion-exchange properties.3,4 The particular interest of these layered silicates is based on their structural stability in acidic environments. Several layered silicates have been reported as starting materials for the preparation of mesoporous materials with narrow pore size distribution via intercalation of organic (longchain alkylammonium ions)5 and inorganic pillars.6 Namagadiite (Na2Si14O29‚11H2O) has been proposed as a good precursor to the framework materials ZSM-5, ZSM-11, and ferrierite type zeolites via intercalation of tetrapropylammonium (TPA), tetrabutylammonium (TBA), and piperidine, respectively.7,8 The synthesis of mordenite zeolite by hydrothermal transformation of Na-magadiite by using tetraethylammonium hydroxide (TEAOH) as the intercalating and/or structure directing agent has also been reported.9 We have recently investigated the transformation of Na-magadiite into a new layered structure (denoted as KLS1) via hydrothermal treatment in the presence of 1,4-dioxane, TMAOH, and H2O at different †
temperatures.10 The framework of KLS1 (which bears some similarity to that of the helix layered silicate (HLS) phase) consists of two-dimensional silicate layers containing hemihedral cages and is regarded to be an interrupted sodalite framework. TMA+ cations were incorporated into the cages, and Na+ and water were located between the anionic layered silicate sheets.11 The KLS1 phase was found to have a low thermal stability, and an amorphous phase was obtained after heating at 300 °C.12 When H-magadiite was reacted with 1,4-dioxane, tetramethylammonium hydroxide (TMAOH), and water at 150 °C for 5 days, TMA+ cations were intercalated between the magadiite layers.13 In a previous communication,14 we reported a new silica layered silicate assigned as FLS1. The new material was obtained by hydrothermal treatment of H-magadiite, TMAOH, and water (without the use of 1,4-dioxane) at 150 °C for 5 days. Calcination of this material yielded a three-dimensional structure thermally stable up to 900 °C with microporous character. Here, we describe detailed and optimum experimental conditions which lead to the formation of FLS1. The resulting products are characterized by X-ray diffraction, 29Si magic angle spinning (MAS) NMR, FTIR, and thermogravimetry. The textural properties in terms of external surface area, micropore volume, and pore size distribution are also reported. The solid-state transformation of FLS1 to a TMAcontaining sodalite (SOD) is also described. Experimental Section
National Institute of Advanced Industrial Science and Technology. ‡ Departamento de Quimica Inorganica, Universidad de Salamanca, 37008 Salamanca, Spain.
Protonated magadiite (H-magadiite) was prepared from Namagadiite (provided by Clariant Tokoyama, Japan) suspended in an aqueous solution of 0.1 N HCl under stirring for 2 h at room
(1) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (2) Ohtsuka, K. Chem. Mater. 1997, 9, 2039. (3) Eugster, H. P. Science 1967, 157, 1177. (4) Lagaly, G.; Beneke, K.; Weiss, A. Am. Mineral. 1975, 60, 642. (5) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1983, 680. (6) Daily, J. S.; Pinnavaia, T. J. Chem. Mater. 1992, 4, 855. (7) Pa´l-Borbe´ly, G.; Beyer, H. K.; Kiyozumi, Y.; Mizukami. F. Microporous Mesoporous Mater. 1997, 11, 45. (8) Pa´l-Borbe´ly, G.; Beyer, H. K.; Kiyozumi, Y.; Mizukami. F. Microporous Mesoporous Mater. 1998, 22, 57. (9) Selvam, T.; Schwieger, W. Stud. Surf. Sci. Catal. 2001, 135, 02P36.
(10) Kooli, F.; Mizukami, F.; Kiyozumi, Y.; Akiyama. Y. J. Mater. Chem. 2001, 11, 1946. (11) Ikeda, T.; Akiyama, Y.; Izumi, F.; Kiyozumi, Y.; Mizukami, F.; Kodaira, T. Chem. Mater. 2001, 13, 1286. (12) Akiyama, Y.; Mizukami, F.; Kiyozumi, Y.; Maead, K.; Izutsu, H.; Sakaguchi, K. Angew. Chem., Int. Ed. 1999, 38, 1420. (13) Kooli, F.; Kiyozumi, Y,; Mizukami, F. Mater. Chem. Phys., in press. (14) Kooli, F.; Kiyozumi, Y.; Mizukami, F. ChemPhysChem. 2001, 2, 549.
10.1021/la0114124 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/20/2002
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temperature. The solid was separated by filtration, washed with deionized H2O, and dried in air at 40 °C. To obtain FLS1, 1 g of H-magadiite powder was suspended in 2.3 g of TMAOH solution (15%) containing an additional 0.5 g of deionized water. The reaction mixture was hydrothermally treated under autogenous pressure in a Teflon-lined autoclave at 150 °C for 5 days. The product was separated by centrifugation, washed with 100 mL of acetone, and dried in air overnight at 40 °C. To study the effect of different reaction variables such as the H-magadiite, water, and TMAOH relative concentrations, each parameter was varied individually and the hydrothermal treatment repeated. The FLS1 phase was calcined at temperatures between 200 and 900 °C for 10 h in air atmosphere. The solid-state transformation of FLS1 to TMA-containing SOD was achieved by grinding a mixture of FLS1, sodium aluminate, and solid TMAOH‚5H2O (1:0.5:0.5 in weight) in an agate mortar. The mixture was ground for 10 min, and the resulting powder was loaded into a Teflon-lined autoclave and heated at 150 °C for different reaction times. The product was washed with 400 mL of deionized water and dried in air at 40 °C overnight.
Characterization Powder X-ray diffraction (XRD) patterns were collected using a Mac Science MXP18 diffractometer with Cu KR radiation. Chemical analyses of C, H, and N were performed using a CE instruments, model EA 1110, CHN analyzer. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed on a Mac-Science TGDTA 2000 analyzer, in air at a heating rate of 5 °C/min. Fourier transform infrared (FT-IR) spectra were recorded on a BIORAD FTS-45RD spectrometer, using the KBr pellet technique. Solid-state 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of the samples were measured on a Bruker AMX-500 spectrometer at 99.366 MHz with a 90° pulse length of 4.7 µs and 800 s recycle delay time. Typically, 70-100 scans were accumulated, and the chemical shifts are reported with respect to external hexamethyldisiloxane. Crystal morphology of calcined FLS1 materials was determined by transmission electron microscopy (TEM) using a JEOL TEM-2000 FXII electron microscope. Specific surface areas were calculated according to the BET method from nitrogen adsorption isotherms at -196 °C measured with a Belsorp 28 SP instrument. Samples were outgassed under vacuum for 3 h at 200 °C prior to analysis. Pore volume, t plots, and pore-size distribution were determined using a computerized program developed by Rives.15
Figure 1. Powder XRD patterns of (a) H-magadiite and as synthesized products after hydrothermal treatment for 5 days, at different temperatures: (b) 100 °C; (c) 130 °C; (d) 150 °C; (e) 170 °C; (f) 180 °C.
Results and Discussion Powder XRD patterns of H-magadiite treated with TMAOH and water at different temperatures for 5 days are presented in Figure 1. The precursor H-magadiite exhibits a basal spacing of 1.23 nm, close to that reported in earlier works.2,13,16 Upon replacement of Na+ by H+, a decrease in basal spacing compared to Na-magadiite17 (1.53 nm) was observed which is a consequence of the smaller size of the hydrated proton with respect to hydrated Na+. Peak broadening following exchange indicated increased disorder in the stacking of silicate layers. For reaction of H-magadiite with TMAOH and water at temperatures below 130 °C, the product exhibited a basal spacing of 1.85 nm which corresponds to a gallery height of 0.73 nm (since the layer thickness of magadiite is 1.12 nm17). The expansion was related to the intercalation of (15) Rives, V. Adsorpt. Sci. Technol. 1991, 8, 95. (16) Rojo, J. M.; Ruiz-Hitzky, E.; Sanz, J. Inorg. Chem. 1988, 27, 2785. (17) Dailey, J. S.; Pinnavaia, T. J. Chem. Mater. 1992, 4, 855.
Figure 2. Powder XRD patterns of (a) H-magadiite after reaction with TMAOH and water, at 150 °C for different times: (b) 1 day; (c) 3 days; (d) 4 days; (e) 5 days.
one monolayer of TMA cations and one water molecular layer between the silicate layers.18 However, at 150 °C, a formation of new crystalline product occurred that we assign as FLS1, with a basal spacing of 1.02 nm. H-Magadiite was also converted to a FLS1 phase at temperatures higher than 150 °C. Figure 2 indicates that the conversion of H-magadiite to the FLS1 phase was achieved for a reaction period of 4 days at 150 °C. At shorter (18) Liu, Z. H.; Ooi, K.; Kanoh, H.; Tang, W. P.; Tomida, T. Langmuir 2000, 16, 4154.
A Layered Silicate
Figure 3. Powder XRD patterns of (a) H-magadiite and the resulting phases after reaction with TMAOH and different amounts of water at 150 °C for 5 days: (b) 0 g; (c) 0.5 g; (d) 1 g; (e) 2 g; (f) 5 g; (g) 10 g.
reaction times, the H-magadiite was mainly intercalated with TMA cations. We noted that as the reaction temperature increased to 170-180 °C, the FLS1 phase was obtained even with shorter reaction periods (e.g., 1 day). The crystallinity of the FLS1 phase was improved by increasing the reaction time. Further investigation of reaction conditions was therefore performed at 150 °C over 5 days. Water Effect. Studies were carried out by changing the water content and keeping constant the other components of the reaction mixture. The FLS1 phase was obtained when H-magadiite was reacted only with TMAOH solution. The conversion of H-magadiite to FLS1 phase was also achieved by increasing the amount of added water up to 1 g. For amounts of water higher than 1 g, FLS1 was not obtained; only intercalation of TMA cations between silicate layers was achieved (Figure 3). Effect of TMAOH Content. TMAOH plays an important role in the transformation of H-magadiite to the FLS1 phase. For TMAOH amounts in the range from 2.3 to 3.5 g, the conversion of H-magadiite to FLS1 was achieved with highest crystallinity of the product for a TMAOH content of 3.5 g. When the TMAOH content was lower than 2.3 g, H-magadiite was intercalated with TMA cations. On the other hand, increase of TMAOH content up to 5 g in the reaction mixture led to conversion of H-magadiite to amorphous silica and other unidentified phases. Effect of H-Magadiite Content. Different amounts of H-magadiite were added to 2.3 g of TMAOH and 0.5 g of water. When using 0.5 g of H-magadiite, this was not transformed to the FLS1 phase, instead amorphous silica and unidentified phases were obtained, similar to those formed for higher amounts of TMAOH content (see previous section). The FLS1 phase was formed for added amounts of H-magadiite ranging between 1 and 1.5 g. By increase of the amount of H-magadiite up to 2 g, however, only intercalation of TMA cations between the silicate layers occurred.
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Characterization. TG measurements and DTA of products from the reaction of H-magadiite with TMAOH and water at different temperatures for 5 days are presented in Figure 4. Previous thermal analysis of H-magadiite suggested an initial mass loss below 300 °C due to water removal.17,19 The mass loss above 300 °C was attributed to the elimination of OH groups from the structure.17,19 The FLS1 material prepared at 150 °C shows three regions of mass loss that are different from those of H-magadiite. The first two losses are related to removal of water from the external surface and to removal of structural water molecules with two concomitant endothermic peaks occurring in DTA at 40 and 186 °C, respectively. A third mass loss (of 8.7%) above 290 °C is related to an exothermic effect at 428 °C observed in the DTA and is assigned to the combustion and elimination of TMA+ cations. The measured TMA mass loss increased as the reaction temperature increased from 150 to 180 °C, while the mass loss related to water molecules decreased. The increase of the hydrophobic character of the FSL1 phase could be related to a decrease in the number of surface silanol groups when the reaction temperature is raised. The DTA curves show that the decomposition temperature of the organic cations takes place in the range 290-600 °C. TMA itself decomposes in the temperature range 260-400 °C.20 The increased temperature of TMA decomposition is due to the FLS1 phase network. The empirical molar ratio deduced from C, H, and N analyses (for FLS1 prepared at 150 °C) corresponds to 4.3:16.6:1 (compared to theoretical C:H:N of 4:12:1) and indicated that TMA cations were incorporated intact in the as-synthesized FLS1 phase, in addition to other impurities which could be adsorbed on the surface of the material; the TMA/Si molar ratio is higher than the expected value of 0.1. The 29Si MAS NMR spectra of Na-magadiite, Hmagadiite, and FLS1 prepared at 150 °C for 5 days are illustrated in Figure 5. Na-magadiite shows both Q3 (HOSiO3 or Na+O-SiO3) at -99.0 ppm and Q4 (SiO4) type sites. The Q4 region showed three resonances at -109.5, -111.1, and -113.7 ppm.21 The 29Si MAS NMR spectrum of H-magadiite reveals also Q3 and Q4 silicon types with some broadening of the Q4 lines compared to those of Na-magadiite. The Q4/Q3 ratio is approximately 3.3, in agreement with that reported elsewhere.17 The FLS1 phase exhibits two signals at -99.9 and -111.6 ppm, assigned to Q3 and Q4 silicon species, respectively,22 with a Q4/Q3 ratio of 2.57. The broadness of the signals indicates a short order in the FLS1 structure. The Q3 sites may be assumed to originate from structural defects (Si-O- or Si-OH groups), which are necessary for counterbalancing the charge of TMA+.23,24 We could imagine that the silicate layer of the FLS1 phase was built from connected [SiO3OH] and [SiO4] tetrahedral units. From the Q4/Q3 ratio and mass loss data, the empirical formulas of the FLS1 product is deduced to be H0.1SiO2.1(TMA)0.1‚0.6H2O. The FT-IR spectrum of the as-synthesized FLS1 confirms the presence of TMA cations with absorption bands at about 1485, 1406, 1008, 740, and 590 cm-1 (Figure 6).25,26 (19) Sprung, R.; Davis, M. E.; Kauffman, J. S.; Dybowski, C. Ind. Eng. Chem. Res. 1990, 29, 213. (20) Valtchev, V. P. J. Chem. Soc., Chem. Commun. 1994, 261. (21) Dailey, J. S.; Pinnavaia, T. J. J. Inclusion Phenom. Mol. Recognit. Chem. 1992, 13, 47. (22) Lippmaa, E.; Magi, M.; Samoson, A.; Engelhardt, G.; Crimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889. (23) van der Waal, J. C.; Rigutto, M. S.; van Bekkum, H. J. Chem. Soc., Chem. Commun. 1994, 1241. (24) Takewaki, T.; Hwang, S. J.; Yamashita, H.; Davis, M. E. Microporous Mesoporous Mater. 1999, 32, 265.
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Figure 4. TG curves (A) and corresponding DTA traces (B) of (a) H-magadiite and the resulting phases prepared during 5 days of hydrothermal treatment at different temperatures: (b) 130 °C; (c) 150 °C; (d) 170 °C; (e) 180 °C.
Figure 5. 29Si MAS NMR of (a) Na-magadiite and (b) H-magadiite. Spectrum c corresponds to the FLS1 phase prepared at 150 °C for 5 days, then calcined at (d) 500 and (e) 700 °C.
The framework of the FLS1 phase is different from that of the H-magadiite precursor, and new bands are detected, empirically assigned to the antisymmetric Si-O-Si and Si-O stretching vibrations in the region of 950-1250 cm-1, in particular, the band at 1240 cm-1 characteristic of fivemembered rings in zeolites.27 Symmetric Si-O-Si stretching yields bands between 800 and 600 cm-1, while bending (25) Valtchev, V.; Mintova, S. Zeolites 1994, 14, 697. (26) Yang, X.; Paillaud, J.-L.; van Breukelen, H. F. W. J.; Kessler, H.; Duprey, E. Microporous Mesoporous Mater. 2001, 46, 1. (27) Kooli, F.; Kiyozumi, Y.; Mizukami, F. New J. Chem. 2001, 25, 1613.
Figure 6. FT-IR spectra of (a) H-magadiite and (b) FLS1 phase calcined at different temperatures: (c) 300 °C; (d) 400 °C; (e) 500 °C; (f) 700 °C; (g) 900 °C.
vibrations are observed below 500 cm-1.28 The band at 948 cm-1 can be assigned to TMA cations25 or possibly to the presence of Si-O- and Si-OH bonds in silicate tetrahedral units;29 such bands occur prominently in zeolite materials with a lot of defect sites, like zeolite beta.30 The latter assignment is in good agreement with the presence of Q3 silicon species detected by NMR. (28) Flanigen, E. M.; Khatami, J.; Szymanski, M. Adv. Chem. Ser. 1971, No. 101, 201. (29) Decottignies, M.; Phalippon, J.; Zarzycki, J. J. Mater. Sci. 1978, 13, 2605.
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Figure 7. Powder XRD patterns of FLS1 calcined at different temperatures (°C).
Thermal Stability. The FLS1 material was heated at 250 °C for 10 h, to remove molecular water, and the corresponding powder XRD pattern is practically unchanged compared to the starting FLS1 material (Figure 7) At temperatures above 300 °C, FLS1 is completely transformed to another new phase assigned as FLS2. The crystallinity of this phase increases as the heating temperature increased above 500 °C. The FLS2 phase is found to be stable up to 900 °C, with an amorphous phase being formed at this temperature (Figure 7). During calcination, the basal spacing of FLS1 decreased from 1.02 to 0.90 nm, which can related to the elimination of TMA cations and the loss of silanol groups via condensation of Si-OH. Indeed, the FT-IR spectrum of FLS2 (prepared via calcination at 500-700 °C) showed that TMA bands in its fingerprint region have been completely removed, as well as a decrease in intensity of the band at 954 cm-1. The detectable vibrations of FLS2 phase are found at 1217, 1086, 817, 607, 462, and 440 cm-1 (Figure 6). The shift of the band at 1068 cm-1 to a higher value of 1086 cm-1 is a consequence of the development of Q4 silicon species during calcination.31 As the calcination temperature increases, the intensity of the bands at 607 and 440 cm-1 decreases and disappears completely at 900 °C (Figure 6). The FT-IR spectrum is similar to that reported for amorphous silica.32 The 29Si MAS NMR spectrum of FLS2 calcined at 500 and 700 °C shows mainly a signal at -113.1 ppm assigned to Q4 Si species. No signal of Q3 Si type was observed (Figure 5), thus indicating that calcination allows silanol groups of FLS1 to condense and form only Q4 Si species. As a consequence, the silicate layers of FLS1 phase were cross-linked to each other.27 Similar behavior was reported for the transformation of two-dimensional layered aluminosilicate (assigned as PREFER) to a threedimensional framework of a FER-type material33 and to the layered microporous borosilicate ERB-1.34 (30) Camblor, M. A.; Corma, A.; Perez-Periente, J. J. Chem. Soc., Chem. Commun. 1993, 557. (31) Kan, O.; Fornes, V.; Rey, F.; Corma, A. J. Mater. Chem. 2000, 10, 993. (32) Kamitos, E. I.; Patsis, A. P.; Kordas, G. Phy. Rev. B 1993, 48, 12499.
Figure 8. TEM micrographs of FLS1 calcined at (a) 500 °C, (b) 700 °C; (c) 900 °C.
Examination of FLS2 crystals by electron microscopy, obtained by calcination of FLS1 at 500 °C, reveals that the crystals are square shaped with well-defined edges. As calcination temperature increases, the size of the crystals decreased and there is considerable evidence for formation of loosely bound aggregates of crystals (Figure 8). Adsorption Isotherms. Figure 9 shows the nitrogen adsorption-desorption isotherms for the FLS1 phase calcined at different temperatures. The shape of the isotherm for as-synthesized FLS1 corresponds to type IV in IUPACs classification, being characteristic of a non(33) Schreyeck, L.; Caullet, P.; Mougenel, J. C.; Guth, J. L.; Marler, B. Microporous Mater. 1996, 6, 259. (34) Millini, R.; Perego, G.; Parker, W. O., Jr.; Bellussi, G.; Carluccio, L. Microporous Mater. 1995, 4, 221.
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Figure 9. Nitrogen adsorption-desorption isotherms of (a) FLS1 calcined at different temperatures: (b) 300 °C; (c) 400 °C; (d) 500 °C; (e) 700 °C; (f) 900 °C. Table 1. Surface Areas of FLS1 Calcined at Different Temperatures Calculated Using the BET Equationa sample
SBET (m2 g-1)
CBET
corr coef
pore volc
H-magadiite FLS1 FLS1(300)b FLS1(400) FLS1(500) FLS1(700) FLS1(800) FLS1(900)
40 90 120 306 436 307 270 95
137 128 -316 -189 -180 -106 -134 282
0.999 98 0.999 85 0.999 46 0.999 39 0.999 68 0.999 55 0.999 58 0.999 88
0.141 0.114 0.212 0.317 0.205 0.181 0.115
a The corresponding CBET and correlation coefficients are also reported. b Values in parentheses indicate the calcination temperatures (°C). c mL (liquid nitrogen) g-1.
porous material.35 As the calcination temperature increases, up to 800 °C, the materials display isotherms with similar shape, characteristic of type I and corresponding to microporous materials.35 A capillary condensation assigned to interparticle void space begins at relative pressure above 0.8. The isotherms are similar to that reported for microporous zeolites.36 FLS2 heated above 900 °C gave an isotherm clearly different to that described above showing character of a nonporous material. All the isotherms show a small hysteresis loop. Surface Area and Microporosity The specific areas and total pore volumes of the FLS1 calcined at different temperatures, as estimated on the basis of the BET equation, are collected in Table 1. The corresponding CBET and the correlation coefficients of the BET plot are also listed. Specific surface areas and total pore volumes of the FLS1 calcined at temperatures in the 400-800 °C range are larger than as-synthesized FLS1, although they decreased when FLS1 was heated above 900 °C. The increase of surface area is related to elimination of TMA (35) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, T.; Siemienewska, T. Pure Appl. Chem. 1985, 57, 603. (36) Gervasini, A. Appl. Catal., A 1999, 180, 71.
cations which otherwise block the pores.27 The subsequent reduction of surface area is due to the collapse of the FLS1 structure and formation of an amorphous phase. When FLS1 was cleared of water, only a slightly increase of surface area was observed, and the adsorption occurred at the external surface of the FLS1 crystallites with no contribution of the micropores. The largest specific surface area and pore volume was obtained for FLS1 calcined at 500 °C when the TMA cations had been completely removed. The parent H-magadiite, as synthesized FLS1, and the material calcined at 900 °C, all having relatively low surface areas, give a good correlation coefficient and a positive CBET value (Table 1). However, FLS1 calcined in the range of 400-800 °C gave a low correlation coefficient and a negative CBET. According to the BET theory, CBET is related exponentially to the enthalpy (heat) of adsorption on the first adsorbed layer, and it gives an indication of the adsorbent-adsorbate interaction energy.37 Accordingly, the negative value has no physical meaning, and the low correlation coefficient suggests that the BET method is not suitable for the calculation of the specific surface area of these materials with primary micropore filling.38 A similar situation has been reported for pillared clays,39 titanates,40 and zeolites loaded with metals.36 In the case of microporous solids, very important information is the values of external surface area and micropore volume, in terms of surface area equivalent to adsorption in micropores. The t-plot of de Boer et al.41 is commonly used to calculate the micropore volume from vapor adsorption data. For nonporous samples, the t-plot (i.e., a representation of the adsorbed volume vs the statistical thickness of the adsorbed layer) would give rise to a straight line passing through the origin, and the specific surface area can be calculated from the slope of a such plot. For microporous materials, however, the plot does not pass through the origin but shows a positive zero intercept. In such a case, the external surface area (Sext) is calculated from the slope of the straight line, while the volume equivalent to adsorption in micropores (Vm) is calculated from the zero intercept according to the equation V ) Vm + 0.001Sextt. However, the exact estimation of these values should be influenced by the choice of the standard isotherm of nonporous material selected to estimate the statistical thickness of the adsorbed layer (t) and the range of the t values considered for the linear fitting.39 We have used another method to determine these parameters, which has been recently applied to pillared clays39 and titanates40 and to microporous silica material.27 On the basis of this method, the BET equation can be derived from a modified isotherm despite the absence of primary micropore filling. This procedure consists of subtracting the amount of nitrogen adsorbed on micropres (supposed to be constant in the range of 0.05 and 0.25) from the total experimental isotherm, which will give a good correlation coefficient and a positive CBET. Table 2 presents the variation of correlation coefficients with different micropore volumes subtracted from the experi(37) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London. 1982. (38) Branton, P. J.; Sing, K. S. W.; White, J. W. J. Chem. Soc., Faraday Trans. 1 1997, 92, 2337. (39) Remy, M. J.; Vieira Coelho, A. C.; Poncelet, G. Microporous Mater. 1996, 7, 287. (40) Kooli, F.; Sasaki, T.; Watanabe, M.; Martin, C.; Rives, V. Langmuir 1999, 15, 1090. (41) de Boer, J. H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; van den Heuvel, A.; Osinga, Th, J. J. Colloid Interface Sci. 1966, 21, 405.
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Table 2. Variation of Correlation Coefficient and CBET in the BET Equation with the Subtracted Micropore Volume from the Experimental Isotherm for FLS1 Calcined at 500 °C
a
subtracted micropore vola
corr coef
CBET
0 0.031 0.062 0.093 0.124 0.127 0.132 0.155
0.99968 0.99976 0.99985 0.99995 0.99999 0.99998 0.99997 0.99963
-237 -452 -109 152 51 49 37 103
mL (liquid nitrogen) g-1.
Table 3. External and Micropore Surface Areas (m2 g-1) of FLS1 Calcined at Different Temperatures Determined Using the BET Equation after Subtracting the Micropore Volume sample
SBET external
CBET
micropore volb
micropore surface
H-magadiite FLS1 FLS1(300)a FLS1(400) FLS1(500) FLS1(700) FLS1(800) FLS1(900)
40 90 56 (63) 77 (96) 170 (206) 75 (101) 78 (91) 95 (89)
137 128 82 48 45 33 37 82
-c 0.026 0.094 0.127 0.098 0.096 -
74 (44) 270 (171) 356 (215) 274 (165) 270 (144) -
Figure 10. Pore size distribution of FLS1 calcined at 500 °C, using the MP method.
a See Table 1. b mL (liquid nitrogen) g-1. The values between parentheses are obtained using the t-plot method. c - negligible.
mental isotherm for FLS1 calcined at 500 °C. The correlation coefficient improved as the subtracted micropore volume increased and then deteriorated with further subtraction. The expected micropore volume results in the best correlation coefficient. The external surface area is then determined according to the BET equation with the most appropriate correlation coefficient, and the related CBET becomes positive. Table 3 reports the external surface areas, micropore volumes, and specific micropore surface areas of FLS1 calcined at different temperatures determined by the method described above and determined by t-plot. At first, the micropore volume increased in proportion to the calcination temperature before sharply decreasing. The increase in the first stage could be ascribed to the gradual elimination of TMA cations thus making the pores accessible to the nitrogen molecules; the decrease of micropore volume is related to the collapse of the micropores and the formation of amorphous phase. In fact the TEM micrographs show the formation of particle aggregates during calcination at 900 °C which might block the access to the pores. As we mentioned above, the difference between the values obtained by the t-plot and this method could be related to the range of t considered for the linear regression. The micropore volume for FLS1 calcined at 500 °C (0.130 mL g-1) is close to that reported in the literature for ZSM-5,36 morderite,7,42 and nanocrystalline zeolite beta.43 A further characterization of the microporous character of FLS1 samples was realized by studying their micropore size distribution. The micropore analysis method, abbreviated as the MP method, has been adopted.44 Figure 10 illustrates a typical pore size distribution of FLS1 (42) Kim, G. J.; Ahn, W. S. Zeolites 1991, 11, 745. (43) Camblor, M. A.; Corma, A.; Valencia, S. Microporous Mesoporous Mater. 1998, 25, 59.
Figure 11. Powder XRD patterns of (a) FLS1 and TMAcontaining sodalite, after solid-state transformation in sodium aluminate (NaAlO2)-TMAOH‚5H2O mixture at 150 °C for (c) 2 h and (d) 24 h. Spectrum b was obtained at ambient temperature prior the reaction. Asterisks correspond to NaAlO2.
calcined at 500 °C. All materials calcined at temperatures higher than 400 °C presented essentially a micropore size with a 0.88 nm in diameter, with a second pore type population of 1.56 nm in diameter. For the FLS1 calcined at 800 °C, the distribution of pore size was shifted to relatively higher values. The dimensions of TMA have been estimated to be 0.78 nm,20 thus the average pore size of 0.88 nm indicated that the TMA+ cations are accom(44) Mikhail, R. SH.; Brunauer, S.; Bodor E. E. J. Colloid Interface Sci. 1968, 26, 45.
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modated easily in the FLS1 material and that TMA plays a pore-filling role. The solid state transformation of intercalated silicates was first proposed as an alternative route for the preparation of silicalite type zeolites at relatively low temperatures.45,46 Recently Kiyozumi et al.47 have reported the synthesis of TMA-containing sodalite (SOD) from a mixture of layered silicate (HLS) and sodium aluminate, via solid-state transformation with a very small amount of water at 150 °C. In our preliminary tests, we have succeeded in transforming the FLS1 material to a SOD phase without the use of water. The SOD phase was obtained after 2 h of reaction, and its crystallinity was improved for a reaction time of 24 h. (Figure 11) When FLS1 material was only reacted with sodium aluminate, the SOD phase was not obtained. This fact was due to the low amount of TMA in the FLS1 precursor. After the mixture of FLS1, sodium aluminate, and TMAOH‚5H2O was ground at room temperature, the basal spacing of the FLS1 phase increased from 1.02 to 1.66 nm prior the solidstate transformation reaction. The expansion of 0.64 nm could be related to the intercalation of TMAOH between the FLS1 layers.48 This fact might confirm that the FLS1 has a layered structure character. (45) Salou, M.; Kiyozumi, Y.; Mizukami, F.; Kooli, F. J. Mater. Chem. 2000, 10, 2587. (46) Pa´l-Borbe´ly, G.; Szegedi, A Ä .; Beyer, K. Microporous Mesoporous Mater. 2000, 35-36, 573. (47) Kiyozumi, Y.; Akiyama, Y.; Mizukami, F.; Ikeda. T. Stud. Surf. Sci. Catal. 2001, 135, 02P25. (48) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146.
Kooli et al.
Conclusions H-Magadiite has been transformed in a narrow range of TMAOH and water contents at temperatures above 150 °C, to a new layered silicate containing TMA cations. First the H-magadiite was intercalated by TMA cations, and this material was then transformed to a new phase, FLS1. The mechanism of solution-mediated transformation of TMA intercalated magadiite to FLS1 during hydrothermal treatment is not yet clear. The presence of five membered ring chains in the unknown structure of FLS1, as indicated by the FTIR absorption at 1240 cm-1 (which it was not detected in the starting H-magadiite), could result from dissolution of TMA intercalated magadiite and reorganization of silica species around the TMA cations.13 The product yields were in the range of 7085%. Further studies are ongoing to resolve the FLS1 and FLS2 crystal structures. Acknowledgment. F.K. gratefully thanks the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for a research fellowship. We thank Dr. S. Newman from Queen Mary (University of London, U.K.) for his suggestions during the preparation of the manuscript and Dr. K. Fujimoto, from the National Institute for Materials Science (Tsukuba, Japan) for his help during the measurements of infrared spectra. The TEM micrographs were obtained with the assistance of Dr. S. Niwa, AIST, Tsukuba Central 5 (Tsukuba, Japan). LA0114124
