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Hydrothermal synthesis of cerium aluminophosphate
Journal of Alloys and Compounds 250 (1997) 532–535
L
Hydrothermal synthesis of cerium aluminophosphate ´ ´ Antonio S. Araujo*, Jaecio C. Diniz, Antonio O.S. Silva, Rogerio A.A. Melo ´ Universidade Federal do Rio Grande do Norte, Departamento de Quımica , CP 1662, CEP 59078 -970, Natal RN, Brazil
Abstract Cerium(III) aluminophosphate molecular sieve has been synthesized starting from aluminum hydroxide, phosphoric acid, hydrated cerium chloride and water, in the presence of a di-isopropylamine organic template. This material has a tetrahedral framework of aluminum (AlO 2 )2 and phosphorus (PO 2 )1 . The obtained structure was similar to that of ALPO-11. The materials have been characterized by using various physicochemical methods. The incorporation of Ce(III) ions into the aluminophosphate matrix did not change the ALPO-11 structure, but did, however, increase the surface area and total acidity. Keywords: Hydrothermal synthesis; Aluminophosphate; Cerium
1. Introduction
2. Experimental
The hydrothermal method has been used to synthesize aluminosilicate zeolites [1], phosphorus substituted aluminosilicate [2,3] and the microporous silica polymorphs [4]. The class of aluminophosphate molecular sieves (ALPOs), discovered by Union Carbide represents the first class of oxide zeolitic materials free of silica [5,6]. ALPOs can be synthesized hydrothermally in the presence of organic amines and quaternary ammonium templates. The incorporation of silicon to the ALPO generates the silicoaluminophosphate or SAPO family [7,8] and the isomorphous substitution of metals, as for example Mn, Zn, Co, Fe, can occupy different sites in the ALPO framework giving rise to the MeAPOs [9–12] and MeAPSOs [13], respectively. Many of these modified materials exhibit properties similar to those of zeolites, indicating that they may find use as adsorbents, catalysts and catalyst supports in several chemical processes [14]. In our work we have synthesized ALPO-11 and CeAPO11 molecular sieves. Their properties also have been discussed. The main synthesis parameters which affect the adsorption and activity of this material are the reactant composition, ordering of the synthesis procedure, the nature of the template agent and temperature of hydrothermal treatment [5,6,9].
The aluminophosphate (ALPO-11) and cerium modified aluminophosphate (CeAPO-11) were synthesized by the hydrothermal method, starting from aluminium hydroxide (Condea), 85% phosphoric acid (Merck), hydrated cerium chloride (Sigma) and di-isopropylamine as an organic template agent. The synthesis procedures involved the following stoichiometric portions of the reactants: ALPO-11: 1.0 i-(C 3 H 7 ) 2 NH:1.0 Al 2 O 3 :1.0 P2 O 5 :80 H2O CeAPO-11: 1.0 i-(C 3 H 7 ) 2 NH:0.05 Ce 2 O 3 :0.95 Al 2 O 3 :1.0 P2 O 5 :80 H 2 O The aluminophosphate synthesis procedure involved the following steps: (i) aluminium hydroxide was slurried in half of total volume of water; (ii) the 85% orthophosphoric acid was diluted with the rest of the water and (iii) the phosphoric acid solution was added to the aluminium hydroxide slurry. The obtained mixture was aged by 2 h, at room temperature, under continuous stirring, until pH stabilization, when the cerium chloride was added to the gel. Finally, di-isopropylamine was added to the precursor mixture, to form a reactive hydrogel, which was charged into a PTFE vessel, and autoclaved at 170 8C for a period of three days, under autogenous pressure. The product was washed with deionized water and dried at 100 8C for one day. The sample was characterized by several physicochemical methods, such as atomic absorption, X-ray diffraction,
*Corresponding author. 0925-8388 / 97 / $17.00 1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02738-7
A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535
FT-IR spectroscopy, thermogravimetry and scanning electron microscopy. In order to determine the chemical composition of the sample, it was dissolved in aqua regia and the aluminium and phosphorus content were estimated using a Varian AA-175 atomic absorption spectrometer. Infrared spectra was obtained by using the KBr wafer technique. A 0.3 mg amount of the sample was ground with 100 mg of KBr and pressed at about 500 Mpa. The spectra was recorded on a double-bean Fourier Transform Infrared spectrometer, Perkin-Elmer 16 PC, at a resolution of 2 cm 21 , in the region of 4000–300 cm 21 . X-ray diffraction pattern of the sample was recorded on a Rigaku diffactometer using Co-Ka radiation, with the diffraction angle (2Q ) ranging from 5 to 408. Thermogravimetric analysis (TG) was performed on a Perkin-Elmer TG-7 thermobalance, at a heating rate of 10 K min 21 ranging from room temperature to 800 8C, using dry air at a flow-rate of 50 cm 3 min 21 , since the synthesized compounds are calcined in flowing oxygen. The morphology and size of the crystals were determined by scanning electron microscopy, in a Zeiss DSM microscope, at 10 Kv and 77 mA. For acid sites density measurement, the sample (0.1 g) was firstly activated at 400 8C, in a 60 cm 3 min 21 nitrogen stream for 2 h. Then, the vapours of n-butylamine were directed to the sample at 95 8C, for a period of 1 h, until complete saturation of the acid sites of the ALPO-11 and CeAPO-11 samples. Afterward, the sample containing the amine, was purged with pure nitrogen at the same adsorption temperature, to remove the physisorbed base. The n-butylamine desorption was initiated by heating the saturated sample (ca. 0.01 g) in a DuPont TA-951 thermobalance, at a rate of 10 K min 21 up to 873 K in the same activation nitrogen flow. The acid sites density was quantitatively determined by means of the amount, in mmols, of n-butylamine desorbed, normalized by mass of sample, being expressed in mmol g 21 . This thermogravimetric method of acidity analysis has been described [15,16]. The adsorption isotherms of nitrogen on the calcined ALPO-11 and CeAPO-11 samples were measured with an Accusorb 2100E instrument (micrometrics) at 293 K. The sample was degassed at temperature of 673 K for 4 h, until a pressure of 10 24 Pa was attained. From the desorption branch of the hysteresis loop, the cumulative surface area (SBET ) of the mesopores was calculated. Other parameters determined were void volume and surface area, according to the BET method.
Fig. 1. Infrared lattice vibration spectra of CeAPO-11.
mixture obtained from step (i). The pH value in the first step was 2.3, due to the dissociation of the phosphoric acid in aqueous media. In the second step, these ions interact with di-isopropylamine, forming an ammonium salt, increasing the pH to 4.5. The gel was stirred 4 h, and the pH value stabilized at about 6.0. After hydrothermal treatment, the samples were washed with deionized water, and calcined at 500 8C, using N 2 and O 2 , to remove the organic template. The FTIR spectrum (Fig. 1) and X-ray diffractogram (Fig. 2) of the as-synthesized CeAPO-11 are characteristic of the type 11 family [17], which consists of columns of 4and 6-rings parallel to the c-axis outlining a unidimensional channel (Fig. 3). The spectra shows absorption bands due to –O–Al–O– and –O–P–O– bonds in the structural internal tetrahedra and external linkages. The broad band around 1000–1250 cm 21 has been assigned to the asymmetric stretch of TO 4 tetrahedra [18] (T5Al, P) and it is characteristic of all zeolites and zeolite-like materials. In the CeAPO-11, this band is shifted to a higher frequency, compared with aluminosilicate zeolites, owing to the presence of large amounts of phosphorus [19]. The
3. Results and discussion The synthesis of the ALPO-11 and CeAPO-11 materials, was basically realized in two steps: (i) addition of phosphoric acid solution to the pseudobohemite, at room temperature; (ii) addition of the organic template to the
533
Fig. 2. X-ray powder diffraction pattern of CeAPO-11.
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A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535
Fig. 3. Framework structure of the CeAPO-11 viewed along [001] projection showing the D-4 and D-6 rings and the elliptical opening.
˚ is shorter than Al–O bond P–O bond distance (1.54 A) ˚ in zeolite materials. The absorption bands in the (1.75 A) region of 820–650 cm 21 were attributed to symmetric stretch in the internal tetrahedra. The absorption bands in
the region 560–600 cm 21 has been assigned to vibration in the double-ring region in the external linkage, which in ALPO-11, are D4R and D6R. Other bands in the spectrum of ALPO-11 between 650–750 cm 21 and at 464 cm 21 are assigned to symmetric stretches and to the T–O bend, respectively. The vibration bands due to the pore opening in the external linkages is observed in the 420–300 cm 21 region. An absorption band in 835 cm 21 was assigned to the Ce–O asymmetric stretch. Infrared spectroscopy and X-ray diffraction measurements showed that the ALPO-11 and CeAPO-11 are crystalline, with AEL structure [17]. By using a specific route of synthesis, microcrystals with typically orthorhombic morphology were obtained, as shown in the scanning electron micrograph of the sample (Fig. 4). The chemical compositions and physicochemical properties of the samples are summarized in Table 1. The high acidity value of the CeAPO-11, can be related to the Ce.OH 21 and H 1 protons, generated by thermal decomposition of [Ce(H 2 O)] 31 complexes on the material surface.
Acknowledgments Financial support was obtained by CNPq—Conselho ´ ´ Nacional de Desenvolvimento Cientıfico e Tecnologico ˜ de Aperfeicoamento de Pessoand CAPES—Coordenac¸ao ´ al de Nıvel Superior.
References
Fig. 4. Scanning electron micrograph of the CeAPO-11 microcrystals.
[1] D.W. Breck, Zeolite Molecular Sieve, Wiley, New York, 1974. [2] E.M. Flanigen and R.W. Grose, in Molecular Sieve Zeolite I, Advances in Chemistry Series No. 101, Am. Chem. Soc., Washington, DC, 1971, pp. 76–101. [3] G. Artioli, J.J. Pluth and J.V. Smith, Acta Crystallogr., C40 (1984) 214–217. [4] R.W. Grose and E.M. Flanigen, US Patent, 4 061 724, 1977. [5] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. [6] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, in D. Olson and A. Bisio (eds.), Proc. 6 th Int. Zeolite Conf., Butteworks, Guildford, Surrey, UK, 1984, pp. 77–109. [7] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Garjek, T.R. Cannan and E.M. Flanigen, US Patent, 4 440 871, 1984. [8] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Garjek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 106 (1984) 6092.
Table 1 Physicochemical properties of ALPO-11 and CeAPO-11 Property
ALPO-11
CeAPO-11
Composition BET surface area (m 2 g 21 ) Micropore volume (cm 3 g 21 ) Total acidity (mmol g 21 )
Al 1.02 P1.00 O 4.03 104.8 0.014 0.463
Ce 0.04 Al 0.97 P1.01 O 4.04 188.4 0.072 0.756
A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535 [9] S.T. Wilson and E.M. Flanigen, US Patent, 4 567 029, 1986. [10] C.A. Messina, B.M. Lok and E.M. Flanigen, US Patent, 4 544 143, 1985. [11] D.R. Pyke, P. Whitney and H. Hyoughton, Appl. Catal., 18 (1985) 173. [12] G.C. Bond, M.R. Gelsthorpe, K.S. Sing, J. Chem. Soc. Chem. Commun., (1985) 1056. [13] E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson, Pure Appl. Chem., 58 (1986) 1351. [14] J.A. Rabo, R.J. Pellet, P.K. Coughlin and E.S. Shamshoum, in H.E. Karge and J. Weitkamp (eds.), Zeolites as Catalysts, Sorbents and Builders: Applications and Innovations, Elsevier, Amsterdam, 1989.
535
˜ Paulo, Sao ˜ Paulo, [15] A.S. Araujo, DSc. Thesis, Universidade de Sao Brazil, 1992. [16] A.S. Araujo, V.J.Fernandes Jr., I. Giolito and L.B. Zinner, Thermochim. Acta, 223 (1993) 129. [17] R.B. Borade and A. Clearfield, J. Molec. Catal., 88 (1994) 249. [18] E.M. Flanigen, in J. Rabo (ed.), Zeolite Chemistry and Catalysis, ACS Monograph Series, 171, Am. Chem. Soc., Washington, DC, 1976, p. 80. [19] E.M. Flanigen and R.W. Grose, Adv. Chem. Ser., 101 (1971) 76.
L
Hydrothermal synthesis of cerium aluminophosphate ´ ´ Antonio S. Araujo*, Jaecio C. Diniz, Antonio O.S. Silva, Rogerio A.A. Melo ´ Universidade Federal do Rio Grande do Norte, Departamento de Quımica , CP 1662, CEP 59078 -970, Natal RN, Brazil
Abstract Cerium(III) aluminophosphate molecular sieve has been synthesized starting from aluminum hydroxide, phosphoric acid, hydrated cerium chloride and water, in the presence of a di-isopropylamine organic template. This material has a tetrahedral framework of aluminum (AlO 2 )2 and phosphorus (PO 2 )1 . The obtained structure was similar to that of ALPO-11. The materials have been characterized by using various physicochemical methods. The incorporation of Ce(III) ions into the aluminophosphate matrix did not change the ALPO-11 structure, but did, however, increase the surface area and total acidity. Keywords: Hydrothermal synthesis; Aluminophosphate; Cerium
1. Introduction
2. Experimental
The hydrothermal method has been used to synthesize aluminosilicate zeolites [1], phosphorus substituted aluminosilicate [2,3] and the microporous silica polymorphs [4]. The class of aluminophosphate molecular sieves (ALPOs), discovered by Union Carbide represents the first class of oxide zeolitic materials free of silica [5,6]. ALPOs can be synthesized hydrothermally in the presence of organic amines and quaternary ammonium templates. The incorporation of silicon to the ALPO generates the silicoaluminophosphate or SAPO family [7,8] and the isomorphous substitution of metals, as for example Mn, Zn, Co, Fe, can occupy different sites in the ALPO framework giving rise to the MeAPOs [9–12] and MeAPSOs [13], respectively. Many of these modified materials exhibit properties similar to those of zeolites, indicating that they may find use as adsorbents, catalysts and catalyst supports in several chemical processes [14]. In our work we have synthesized ALPO-11 and CeAPO11 molecular sieves. Their properties also have been discussed. The main synthesis parameters which affect the adsorption and activity of this material are the reactant composition, ordering of the synthesis procedure, the nature of the template agent and temperature of hydrothermal treatment [5,6,9].
The aluminophosphate (ALPO-11) and cerium modified aluminophosphate (CeAPO-11) were synthesized by the hydrothermal method, starting from aluminium hydroxide (Condea), 85% phosphoric acid (Merck), hydrated cerium chloride (Sigma) and di-isopropylamine as an organic template agent. The synthesis procedures involved the following stoichiometric portions of the reactants: ALPO-11: 1.0 i-(C 3 H 7 ) 2 NH:1.0 Al 2 O 3 :1.0 P2 O 5 :80 H2O CeAPO-11: 1.0 i-(C 3 H 7 ) 2 NH:0.05 Ce 2 O 3 :0.95 Al 2 O 3 :1.0 P2 O 5 :80 H 2 O The aluminophosphate synthesis procedure involved the following steps: (i) aluminium hydroxide was slurried in half of total volume of water; (ii) the 85% orthophosphoric acid was diluted with the rest of the water and (iii) the phosphoric acid solution was added to the aluminium hydroxide slurry. The obtained mixture was aged by 2 h, at room temperature, under continuous stirring, until pH stabilization, when the cerium chloride was added to the gel. Finally, di-isopropylamine was added to the precursor mixture, to form a reactive hydrogel, which was charged into a PTFE vessel, and autoclaved at 170 8C for a period of three days, under autogenous pressure. The product was washed with deionized water and dried at 100 8C for one day. The sample was characterized by several physicochemical methods, such as atomic absorption, X-ray diffraction,
*Corresponding author. 0925-8388 / 97 / $17.00 1997 Elsevier Science S.A. All rights reserved PII S0925-8388( 96 )02738-7
A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535
FT-IR spectroscopy, thermogravimetry and scanning electron microscopy. In order to determine the chemical composition of the sample, it was dissolved in aqua regia and the aluminium and phosphorus content were estimated using a Varian AA-175 atomic absorption spectrometer. Infrared spectra was obtained by using the KBr wafer technique. A 0.3 mg amount of the sample was ground with 100 mg of KBr and pressed at about 500 Mpa. The spectra was recorded on a double-bean Fourier Transform Infrared spectrometer, Perkin-Elmer 16 PC, at a resolution of 2 cm 21 , in the region of 4000–300 cm 21 . X-ray diffraction pattern of the sample was recorded on a Rigaku diffactometer using Co-Ka radiation, with the diffraction angle (2Q ) ranging from 5 to 408. Thermogravimetric analysis (TG) was performed on a Perkin-Elmer TG-7 thermobalance, at a heating rate of 10 K min 21 ranging from room temperature to 800 8C, using dry air at a flow-rate of 50 cm 3 min 21 , since the synthesized compounds are calcined in flowing oxygen. The morphology and size of the crystals were determined by scanning electron microscopy, in a Zeiss DSM microscope, at 10 Kv and 77 mA. For acid sites density measurement, the sample (0.1 g) was firstly activated at 400 8C, in a 60 cm 3 min 21 nitrogen stream for 2 h. Then, the vapours of n-butylamine were directed to the sample at 95 8C, for a period of 1 h, until complete saturation of the acid sites of the ALPO-11 and CeAPO-11 samples. Afterward, the sample containing the amine, was purged with pure nitrogen at the same adsorption temperature, to remove the physisorbed base. The n-butylamine desorption was initiated by heating the saturated sample (ca. 0.01 g) in a DuPont TA-951 thermobalance, at a rate of 10 K min 21 up to 873 K in the same activation nitrogen flow. The acid sites density was quantitatively determined by means of the amount, in mmols, of n-butylamine desorbed, normalized by mass of sample, being expressed in mmol g 21 . This thermogravimetric method of acidity analysis has been described [15,16]. The adsorption isotherms of nitrogen on the calcined ALPO-11 and CeAPO-11 samples were measured with an Accusorb 2100E instrument (micrometrics) at 293 K. The sample was degassed at temperature of 673 K for 4 h, until a pressure of 10 24 Pa was attained. From the desorption branch of the hysteresis loop, the cumulative surface area (SBET ) of the mesopores was calculated. Other parameters determined were void volume and surface area, according to the BET method.
Fig. 1. Infrared lattice vibration spectra of CeAPO-11.
mixture obtained from step (i). The pH value in the first step was 2.3, due to the dissociation of the phosphoric acid in aqueous media. In the second step, these ions interact with di-isopropylamine, forming an ammonium salt, increasing the pH to 4.5. The gel was stirred 4 h, and the pH value stabilized at about 6.0. After hydrothermal treatment, the samples were washed with deionized water, and calcined at 500 8C, using N 2 and O 2 , to remove the organic template. The FTIR spectrum (Fig. 1) and X-ray diffractogram (Fig. 2) of the as-synthesized CeAPO-11 are characteristic of the type 11 family [17], which consists of columns of 4and 6-rings parallel to the c-axis outlining a unidimensional channel (Fig. 3). The spectra shows absorption bands due to –O–Al–O– and –O–P–O– bonds in the structural internal tetrahedra and external linkages. The broad band around 1000–1250 cm 21 has been assigned to the asymmetric stretch of TO 4 tetrahedra [18] (T5Al, P) and it is characteristic of all zeolites and zeolite-like materials. In the CeAPO-11, this band is shifted to a higher frequency, compared with aluminosilicate zeolites, owing to the presence of large amounts of phosphorus [19]. The
3. Results and discussion The synthesis of the ALPO-11 and CeAPO-11 materials, was basically realized in two steps: (i) addition of phosphoric acid solution to the pseudobohemite, at room temperature; (ii) addition of the organic template to the
533
Fig. 2. X-ray powder diffraction pattern of CeAPO-11.
534
A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535
Fig. 3. Framework structure of the CeAPO-11 viewed along [001] projection showing the D-4 and D-6 rings and the elliptical opening.
˚ is shorter than Al–O bond P–O bond distance (1.54 A) ˚ in zeolite materials. The absorption bands in the (1.75 A) region of 820–650 cm 21 were attributed to symmetric stretch in the internal tetrahedra. The absorption bands in
the region 560–600 cm 21 has been assigned to vibration in the double-ring region in the external linkage, which in ALPO-11, are D4R and D6R. Other bands in the spectrum of ALPO-11 between 650–750 cm 21 and at 464 cm 21 are assigned to symmetric stretches and to the T–O bend, respectively. The vibration bands due to the pore opening in the external linkages is observed in the 420–300 cm 21 region. An absorption band in 835 cm 21 was assigned to the Ce–O asymmetric stretch. Infrared spectroscopy and X-ray diffraction measurements showed that the ALPO-11 and CeAPO-11 are crystalline, with AEL structure [17]. By using a specific route of synthesis, microcrystals with typically orthorhombic morphology were obtained, as shown in the scanning electron micrograph of the sample (Fig. 4). The chemical compositions and physicochemical properties of the samples are summarized in Table 1. The high acidity value of the CeAPO-11, can be related to the Ce.OH 21 and H 1 protons, generated by thermal decomposition of [Ce(H 2 O)] 31 complexes on the material surface.
Acknowledgments Financial support was obtained by CNPq—Conselho ´ ´ Nacional de Desenvolvimento Cientıfico e Tecnologico ˜ de Aperfeicoamento de Pessoand CAPES—Coordenac¸ao ´ al de Nıvel Superior.
References
Fig. 4. Scanning electron micrograph of the CeAPO-11 microcrystals.
[1] D.W. Breck, Zeolite Molecular Sieve, Wiley, New York, 1974. [2] E.M. Flanigen and R.W. Grose, in Molecular Sieve Zeolite I, Advances in Chemistry Series No. 101, Am. Chem. Soc., Washington, DC, 1971, pp. 76–101. [3] G. Artioli, J.J. Pluth and J.V. Smith, Acta Crystallogr., C40 (1984) 214–217. [4] R.W. Grose and E.M. Flanigen, US Patent, 4 061 724, 1977. [5] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. [6] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, in D. Olson and A. Bisio (eds.), Proc. 6 th Int. Zeolite Conf., Butteworks, Guildford, Surrey, UK, 1984, pp. 77–109. [7] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Garjek, T.R. Cannan and E.M. Flanigen, US Patent, 4 440 871, 1984. [8] B.M. Lok, C.A. Messina, R.L. Patton, R.T. Garjek, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 106 (1984) 6092.
Table 1 Physicochemical properties of ALPO-11 and CeAPO-11 Property
ALPO-11
CeAPO-11
Composition BET surface area (m 2 g 21 ) Micropore volume (cm 3 g 21 ) Total acidity (mmol g 21 )
Al 1.02 P1.00 O 4.03 104.8 0.014 0.463
Ce 0.04 Al 0.97 P1.01 O 4.04 188.4 0.072 0.756
A.S. Araujo et al. / Journal of Alloys and Compounds 250 (1997) 532 – 535 [9] S.T. Wilson and E.M. Flanigen, US Patent, 4 567 029, 1986. [10] C.A. Messina, B.M. Lok and E.M. Flanigen, US Patent, 4 544 143, 1985. [11] D.R. Pyke, P. Whitney and H. Hyoughton, Appl. Catal., 18 (1985) 173. [12] G.C. Bond, M.R. Gelsthorpe, K.S. Sing, J. Chem. Soc. Chem. Commun., (1985) 1056. [13] E.M. Flanigen, B.M. Lok, R.L. Patton and S.T. Wilson, Pure Appl. Chem., 58 (1986) 1351. [14] J.A. Rabo, R.J. Pellet, P.K. Coughlin and E.S. Shamshoum, in H.E. Karge and J. Weitkamp (eds.), Zeolites as Catalysts, Sorbents and Builders: Applications and Innovations, Elsevier, Amsterdam, 1989.
535
˜ Paulo, Sao ˜ Paulo, [15] A.S. Araujo, DSc. Thesis, Universidade de Sao Brazil, 1992. [16] A.S. Araujo, V.J.Fernandes Jr., I. Giolito and L.B. Zinner, Thermochim. Acta, 223 (1993) 129. [17] R.B. Borade and A. Clearfield, J. Molec. Catal., 88 (1994) 249. [18] E.M. Flanigen, in J. Rabo (ed.), Zeolite Chemistry and Catalysis, ACS Monograph Series, 171, Am. Chem. Soc., Washington, DC, 1976, p. 80. [19] E.M. Flanigen and R.W. Grose, Adv. Chem. Ser., 101 (1971) 76.