I n d . Eng. Chem. Res. 1993,32, 600-603
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MATERIALS AND INTERFACES Imogolite: A Possible New Type of Shape-Selective Catalyst Seiichiro Imamura: Yuki Hayashi, Kanji Kajiwara, Hiroshi Hoshino, and Chihiro Kaito Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Imogolite, a natural tubular clay mineral, was purified, and ita physicochemical properties were investigated. Strong acid sites appeared by the heat treatment at 400-500 "C. The tubular configuration was retained up to 750 "C, and the average inner diameter of the tube was 6.5 A. It was suggested that imogolite can be used as a shape-selective catalyst.
Introduction Imogolite is an aluminum-silicate polymer found in naturally occurring weathered pumice beds of volcanic ash (Yoshinagaand Aomine, 1962). It has a characteristic tubular configuration with an outer diameter of approximately 24 %I and a length of several thousand angstroms (Cradwicket al., 1972; Wada, 1978)and was characterized as a rigid rod molecule based on ita dilute solution properties (Donkai et al., 1985). The inner layer is composed of a silica tetrahedron sticking silanol groups toward inside, and this silica layer is covered by a gibbsitelike aluminum hydroxide layer: the chemical composition of this mineral is given by [Si02/A1203/2H20In. The rigid rod nature of imogolite leads to the formation of lyotropic liquid crystal mesophase, and one of the authors investigated the suprastructure formed in the mesophase of imogolite (Kajiwara et al., 1985, 1986a,b; Donkai et al., 1991). Judging from its tubular structure with an outer diameter of 25 A, imogolite must have an inner diameter less than 16 A if we assume the thickness of the aluminasilica layer is 4.51 A (Grim, 1968). Thus imogolite is expected to have a shape-selective characteristic as a molecular sieve (zeolite). Inner silanol protons will be exchanged with various metal cations, which will modify the inner environment of the tube. Imogolite can be synthesized from hydroxyaluminum cation and orthosilicic acid in the presence of Ge (Farmer and Fraser, 1979;Wada et al., 1979; Wada and Wada, 1982). Thus it is possible to prepare imogolite containing constituents other than Si and A1in ita framework,and the characteristics of these modified imogolites will be expected to differ remarkably from the starting imogolite. The above-mentioned features will make imogolite a potential new type of shape-selective catalyst. Although efforts were made to prepare new composite materials from imogolite (Donaki et al., 1989),no investigation has been carried out to utilize it as a catalyst. This paper aims to elucidate the physicochemical properties of imogolite calcined at various temperatures in an attempt to utilize it as a catalyst for vapor-phase reactions.
Experimental Section Materials. Silica-alumina N633-L with a BET surface area of 420.1 m2/g was obtained from Nikki Kagaku Co., molecular sieve 4A (MS-4A) was obtained from Fuji 0888-5885/93/2632-0600$04.00/0
Davidson Co., and Nay-zeolite TSZ-33OHUA was obtained from Toso Co. Benzene was dried with calcium chloride, followed by refluxing over sodium metal and distillation. 1-Butene,Hammett indicators, and other reagents were used without further purification. Purification of Imogolite. Soil containing imogolite was obtained from the weathered pumice beds of volcanic ash of Murasakino, Iwate Prefecture. The soil was dispersed in water, and imogolite rising to the surface was collected, The crude imogolite was purified according to the following procedure. 1. Elimination of iron: Four hundred grams of crude imogolite, 80 g of sodium citrate, and 2 g of sodium bicarbonate were mixed in 1L of deionized water. The mixture was heated to 80 "C, and 5 g of sodium hydrosulfite was added to it. After the mixture was kept at 80 "C for 15 min with stirring, it was filtered and the filtrated was washed with deionized water severaltimes. This procedure was repeated twice. 2. Elimination of alumina and silica: The filtrate obtained above was dispersed in 1L of deionized water, and 20 g of sodium carbonate was added. The mixture was heated to simmer for 5 min. Then the solid portion was filtered and was washed several times with deionized water. 3. Elimination of carbonaceous moiety: The solid portion obtained above was heated at 60 "C in 400 mL of 30 % aqueous hydrogen peroxide. Heating was continued with occasional addition of hydrogen peroxide until no bubbles evolved. The solid was filtered and was washed with deionized water several times. 4. Soxhlet extraction: Nearly completely purified imogolite as obtained above was further purified by extracting organic materials with 0.1 vol 9% aqueous acetic acid twice for 40 h each. Imogolite was filtered and was washed until the pH of the filtrate was above 7. It was dried and kept in a desiccator, and was calcined in air at prescribed temperatures before use. Apparatus and Procedure. Isomerization of 1-butene was carried out at 50 OC with a closed circulating reactor equipped with a glass vacuum line. One gram of the catalysts was charged in the reactor, and the pressure of the initial 1-butenewas 100 Torr. 1-Butenewas analyzed by a Shimadzu 3BT gas chromatograph with a bis(2methoxyethyl) adipate column (4 m) at 50 OC. Adsorption of ammonia on imogolite was carried out with a glass vacuum line. A known amount of ammonia was introduced on imogolite in a sample tube connected 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 601 A h.
L 100
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Temp ("C) Figure 1. DTA and TG analyses of imogolite: 157 mg of imogolite dried at 80 "C was used.
to the vacuum line a t room temperature. After the sample tube was heated to prescribed temperatures, the gas-phase ammonia was condensed with a liquid nitrogen bath. The amount of ammonia remaining on imogolitewas calculated from the difference between the amount of ammonia introduced and that condensed a t liquid nitrogen temperature. Adsorption of benzene and 1,3,5-triisopropylbenzene on imogolite and other adsorbents was carried out in a 300-mL Erlenmeyer flask maintained at 25 "C in an incubator. Analyses. The composition of imogolite was determined by gravimetric analysis. One gram of imogolite dried at 120 "C overnight was mixed with 5 g of sodium carbonate and was fused in a platinum crucible. Concentrated hydrochloric acid was added to this mixture after cooling, and the mixture was heated to dryness. Then water was added to the dried residue, and the solid part was filtered. The solid containing Si was washed with water several times and was calcined to a constant weight in a form of Si02. NHdOH (3 N) was added to the filtrate obtained above, and the resultant precipitate (aluminum hydroxide) was filtered and was calcined to a constant weight (Al203). As imogolite, NaY-zeolite, and MS-4A have micropores, the adsorption behavior of nitrogen on them did not follow the BET equation, so Langmuir type isotherms were obtained. Thus the saturated amount of adsorbed nitrogen was measured for these compounds instead of BET surface area. Conventional gas adsorption apparatus connected to a glass vacuum line was used for the measurement. Acid strength was measured in dry benzene by the use of the following Hammet indicators: methyl red (pK, = 4.8), methyl yellow (pK, = 3.3), benzeneazodiphenylamine (pK, = 1.5), dicinnamalacetone (pK, = -3.01, benzalacetophenone (pK, = -5.6), and anthraquinone (pK, = -8.2). X-ray diffraction and differential thermal analyses were carried out with a Rigaku Denki Geigerflex 2012 X-ray analyzer and an Ulvac TGD-7000RH DTA analyzer, repectively. The TEM image of imogolite was obtained with a Hitachi H800 transmission electron microscope.
Results and Discussion Changeof the Stateof Imogoliteby Heat Treatment. The results of DTA and TG analyses of imogolite dried at 80 "C overnight are shown in Figure 1. Imogolite gradually lost i h water (amounting to 28.5 % of the starting weight) by heating up to 600 "C. Gravimetric analysis revealed that imogolite contained alumina and silica with a molar ratio of U1.02, which is in good agreement with
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Figure 2. Change of the X-ray diffraction pattern of imogolite on heat treatment. Heat treatment; (A)dried at 80 "C overnight; (B) 250, (C)500, (D)750, (E)950, and (F)1200 "C. Heat treatments at 250 "C and above were carried out in air for 3 h. Table I. Saturated Amount of Nitrogen (SAN) of Imogolite
calcination temp ("C) 80 120 250 400 500 600 750 950 SAN (mL/g) 82.0 97.2 109.5 84.3 80.7 88.8 81.0 3.l0 a The value is inaccurate because the adsorption isotherm satisfied the BET equation rather than the Langmuir equation. The BET surface area was 3.04 m2/g.
the literature value (Donkai et al., 1985). Thus the apparent composition of imogolite dried at 80 "C was calculated as 1.06SiOz/Al203/3.67H20. In addition to the broad endothermic peak ranging from 100to 600 "C, there appeared an exothermic peak near 950 "C, which suggested that a marked change in the configuration occurred. The change in the state of imogolite on heat treatment was observed by the X-ray diffraction technique. Imogolite dried at 80 "C had a quasi-crystalline structure as shown by its diffraction pattern shown in Figure 2A. At higher temperatures ita degree of crystallinity became even poorer (Figure 2B-D, and imogolite calcined at 950 "C showed small diffraction peaks a t 28 of 16.3", 25.9", and 32.9" (Figure 2E). These peaks became stronger on further heat treatment (Figure 2F), and this diffraction pattern coincided well with that of mullite except for the peak at 21.3" (McClune, 1972). Thus the exothermic peak near 950 "C was due to the change of imogolite into mullite. The peak at 21.3" was assumed to be due to Si02 (McClune, 1982). As the composition of mullite is 2si02/3&03, excess Si in imogolite (1.06Si02/A1203/3.67H20) was crystallized as Si02 during mullite formation, although the exact form of Si02 could not be determined. Change in the saturated amount of nitrogen adsorbed on imogolite (abbreviated as SAN) on heat treatment is shown in Table I. More than 80 mL/g SAN was retained by the heat treatment up to 750 "C. Calcination at 950 "C resulted in the marked decrease in SAN in accordance with the configuration change in imogolite. As the adsorption behavior of nitrogen on imogolite calcined at 950 "C followed better the BET equation than the Langmuir type one, the SAN at 950 "C listed in Table I is an apparent and inaccurate value. Thus the BET surface area of imogolite calcined at 950 "C was measured as 3.04 m2/g. As the treatment up to 750 "C did not decrease SAN significantly, it was suggested that the tubular configuration was retained even at that temperature
602 Ind. Eng. Chem.
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Figure 3. TEM image of imogolite calcined at 650 " C in air for 3 h
although X-ray analysis suggested a partial destruction of the construction. Figure 3 exhibita a TEM photograph of imogolite calcined a t 650 "C. The presence of the tubular region is clearly shown in the photograph. The average inner diameter of the tube as observed by the TEM technique was 6.5 A. The black stripe (about 11A wide) adjacent tothe white zone (inner cavity) is the wall of the tube. As the tubes are arranged parallel toeach other, the black stripe corresponds to two side walls of the tube. Thus the thickness of the wall is calculated as 5.5 A, which is close tothe value of 4.5 A of the alumina-silica layer of kaolinite (Grim, 1968). From these data, the outer diameter of the tube is calculated as about 12 A. This value is much smaller than the reported value of 25 A (Cradwick et al., 1972; Wada, 1978). It may be due tothe heat treatment which caused the shrinkage of the tube diameter; however, the details are not known. The presence of acid sites was expected in imogolite because the components of imogolite are the oxides of silicon and aluminum which constitute silica-alumina, a typical solid acid. Thus the change in the acid property of imogolite was investigated under heat treatment. The acid strength of imogolite as measured with Hammett indicators increased from -3.0 < HO5 1.5 (imogolitedried at 100 "C) to -5.6 < HO5 -3.0 by heating at 400 and 500 OC. However, as the size of these Hammett indicators seemedtobelargerthantheinnerdiameteroftheimogolite tube, these indicators may not have monitored the acid property inside the tube correctly. The acid property was also investigated by adsorption of ammonia which can penetrate the tube of imogolite (Figure 4). As a heating process was necessary for this measurement, the state of imogolite calcined at low temperatures (e.g., 100 "C) may have changed during the measurement, which made the evaluation of the effect of calcination temperature inaccurate. However, as the largest amount of ammonia was adsorbed on the imogolite calcined at 400 and 500 'C in thewholetemperaturerange, itisclear thatthecalcination at these temperatures resulted in the formation of a large amount of strong acid site. This result, however, does not necessarily confirm that these strong acid sites are present inside the tube of imogolite.
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Temp ("C) Figure 4. Adsorption of ammonia on imogolite calcined at 160 O C (A),250 OC (v),400 O C (0).500 O C (O),150 'C (O),and 1200 'C (m) inairfor3 h. TheamountofNH3adaorbedisexpressedbvmiUilitera at standard temperature and pressure. Table 11. Isometbation of L-Butene on Imegolite at 60 OC calcination temp ('C)
kb ( X l V min-lg')
12.8 52.3 32.7 22.0 6.8
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BET surface area = 420.1 mz/g; acidity -5.6 < Ho 5 -3.0. constant. a
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TableIIshowsthe resultoftheisomerizationof1-butene on imogolite calcined a t various temperatures. Isomerization proceeded effectively on imogolite calcined at 400 and 500 OC, which is in accord with the result of the acidity measurement. The ratioof cis-Qbuteneto tram-2-butene was in the range of 0.86-0.98. However, the activity was lower than that of silica-alumina which had the same acid strength of -5.6 < Ho 5 -3.0. The absorption behavior of imogolite calcined at 500 "C was compared with those of Nay-zeolite and MS-5A at 25 "C by using benzene and 1,3,5-triisopropylbenzene asadsorbates. SANwasevaluatedas80.7,247.3,and11.4
Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 603 Acknowledgment We thank Mr. K. Utani and Miss T. Kokubu of Kyoto Institute of Technology for their kind cooperation. Nomenclature MS-4A molecular sieve 4A SAN: saturated amount of nitrogen adsorbed
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Literature Cited
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Figure 5. Adsorption of benzene (A) and 1,3,5-triisopropylbenzene
(B)on MS-4A (O),Nay-zeolite (13, and imogolite ( 0 )calcined at 500 "C in air for 3 h. Saturated amount of nitrogen adsorbed (SAN);80.7 mL/g (imogolite); 247.3 mL/g (Nay-zeolite); 11.4 mL/g (MS-4A).
mL/g for imogolite, Nay-zeolite, and MS-4A, respectively. The smallest diameter of benzene was calculated as 6.2 A and that of 1,3,5-triisopropylbenzeneas 8.5 A. The diameter of the cavity of Nay-zeolite was assumed to be 7.4 A and that of MS-4A to be 4.1 A (Barrer, 1978). The absorption of benzene occurred very easily and reached equilibrium rapidly on Nay-zeolite and imogolite (Figure 5A). On the other hand, benzene was adsorbed very slowly on MS-4A, and the absorption continued for more than 30 h. It must be difficult for benzene to enter the small cavity of MS-4A. However, it is incredible that MS-4A adsorbed more than 10 w t % of benzene even at a slow rate. Benzene might have been adsorbed through the defects of the crystal into the cavity, although details are not known now. Figure 5B shows the result of the adsorption of 1,3,5-triisopropylbenzene.In this case, the rapid adsorption and saturation were seen only for Nay-zeolite: imogolite did not show enough ability to adsorb this material rapidly. It seems that 1,2,3-triisopropylbenzene (diameter of 8.5 A) cannot pass through the window of Nay-zeolite (7.4 A). However, the guest molecule (1,3,5-triisopropylbenzene)is not a hard sphere, and the lattice oxygen of zeolite is also vibrating. Thus the breathing of the window of Nay-zeolite will let 1,3,5triisopropylbenzene get inside the cavity (Barrer, 1978). At any rate, the result confirms that imogolite has a inner pore diameter larger than that of MS-4A (4.1 A) and smaller than that of Nay-zeolite (7.5 A) in accordance with the TEM observation and that it can be used m a shapeselective catalyst. Much work, however, should be done to clarifythe nature of imogolite such as the location of the acid sites, its ion exchange ability, and so on to utilize it as a shape-selective catalyst.
(1) Barrer, R. M. Zeolite Frameworks, Cations and Water Molecules. In Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Press: London, 1978; p 70. (2) Cradwick, P. D. G.; Farmer, V. C.; Russell, J. D.; Masson, C. R., Wada, K.; Yoshinaga,N. Imogolite,A Hydrated Aluminum Silicate of Tubular Structure. Nature Phys. Sci. 1972,240, 187-189. (3) Donkai, N.; Inagaki, H.; Kajiwara, K.; Urakawa, H.; Schmidt, M. Dilute Solution Properties of Imogolite. Macromol. Chem. 1985, 186, 2623-2638. (4) Donkai, N.; Inagaki, H.; Kajiwara, K. Molecular Composite of Hydroxypropyl Cellulose with Imogolite. In Wood Processing and Utilization; Kennedy, J. F.; Phillips, G. O., Williams, P. A., Eds.; Ellis Horwood: Chichester, 1989; pp 285-292. ( 5 ) Donkai, N.; Hoshino, H.; Kajiwara,; Miyamoto, T.; Inagaki, H. Electron MicroscopicObservation of Suprastructure Constructed by Inorganic Rod Molecule, Imogolite. Senn-i Gakkaishi 1991,47, 412-416. (6) Farmer, V. C.; Fraser, A. R. Synthetic Imogolite, A Tubular Hydroxyaluminum Silicate. Dev. Sedimentol. 1979, 27, 547-553. (7) Grim, R. E., Ed. Clay Mineralogy; McGraw-Hill: New York, 1968; p 74. (8) Kajiwara, K.; Donkai, N.; Fjuiyoshi, Y.; Hiraki, Y.; Urakawa, H.; Inagaki, H. Some Remarks on Imogolite Mesophase. Bull. Znst. Chem. Res., Kyoto Uniu. 1985, 63, 320-331. (9) Kajiwara, K.; Donkai, N.; Hiragi, Y.; Inagaki, H. Lyotropic Mesophase of Imogolite 1, Effect of Polydispersity on Phase Diagram. Macromol. Chem. 1986a, 187, 2883-2893. (10) Kajiwara, K.; Donkai, N.;Fujiyoshi, Y.; Inagaki, H. Lyotropic Mesophase of Imogolite 2, Microscopic Observation of Imogolite Mesophase. Macromol. Chem. 1986b, 187, 2895-2907. (11) McClune, W. F., Ed. Powder Diffraction File. Sets 11-15; Joint Committee on Powder Diffraction Standards: Swarthmore, PA, 1972; p 1040. (12) McClune, W. F., Ed. Powder Diffraction File. Alphabetic Index-Znorganic Phases; International Centre for Diffraction Data: Swarthmore, PA, 1982, p 699. (13) Wada, K. Allophane and Imogolite in Salts of Japan. Dev. Sedimentol. 1978, 26, 147-187. (14) Wada, S. I.; Wada, K. Effects of Substitution of Germanium for Silicon in Imogolite. Clay Clay Miner. 1982, 30, 123-128. (15) Wada, S. I.; Eto, A.; Wada, K. Synthetic Allophane and Imogolite. J. Soil. Sci. 1979, 30, 347-355. (16) Yoshinaga, N.; Aomine, S. Imogolite in Some Ando Soils. Soil Sci. Plant Nutr. 1962, 8, 114-121.
Received for review September 16, 1992 Revised manuscript received December 1, 1992 Accepted December 14, 1992
