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steam or HF
Microporous and Mesoporous Materials 23 (1998) 211–219
Characterization of mordenites treated by HCl/steam or HF Kyong-Hwan Lee *, Baik-Hyon Ha Department of Chemical Engineering, Hanyang University, 17 Haengdangdong, Sungdongku, Seoul 133-791, South Korea Received 5 September 1997; accepted 19 February 1998
Abstract Mordenites were modified by dealumination with HCl/steam or HF treatment, and characterized by X-ray fluorescence ( XRF ), X-ray photoelectron spectroscopy ( XPS), X-ray diffraction ( XRD), infrared spectroscopy, scanning electron microscopy (SEM ) and nitrogen adsorption. All mordenites retained more than 80% crystallinity, although the lattice parameters (a, b, c) were reduced more for HCl/steam-treated mordenites than for HF-treated mordenites. In the HCl/steam-treated mordenites, framework aluminum was mainly removed, while in the HF-treated mordenites, framework aluminum was removed extensively at the beginning of treatment, and thereafter silicon and aluminum were simultaneously removed. Mesopores with a diameter of 3.7 nm developed as a result of dealumination, which did not depend on the treatment methods. The acid strength was higher on the HCl/steam-treated mordenites than on the HF-treated mordenites. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Acid/steam treatment; Dealuminated mordenite; Secondary mesopore formation
1. Introduction ˚ Mordenite has two pore channels, i.e. 6.7×7.0 A ˚ which is parallel to the c axis, and 2.9×5.7 A which is parallel to the b axis [1]. The large, straight pores do not allow molecules to diffuse easily. If the pores are blocked by coke forming around their mouths, deactivation occurs quickly [2–4]. Such a pore size and shape limits the application of mordenite to large molecules. It is therefore necessary to tailor the pore size and * Corresponding author. Fax: +82 2 9585809; E-mail: [email protected]. Present address: Environment Remediation Research Center, Korea Institute of Science and Technology, 39-1 Haweolkokdong, Sungbuk-ku, Seoul 136-791, Korea. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 11 8 - 8
shape of mordenite when considering the reactant, the product or intermediate selectivity [5,6 ]. The formation of mesopores by dealumination via thermal or steam treatment at high temperatures has been studied in relation to catalytic activity [7,8]. Such dealumination and/or steaming eliminates aluminum atoms around the pore mouth, which improves the deactivation properties. It is therefore very interesting to study the formation of secondary mesopores which provide easy migration of the reactants and/or products and also store the coke formed in hydrocarbon processing. In this paper, we treat mordenite with concentrated hydrochloric acid as well as steam at higher temperatures or with hydrofluoric acid at room temperature to form the secondary mesopores of mordenite.
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2. Experimental 2.1. Preparation of catalysts
designated as FM , in which x again indicates the x SiO /Al O weight ratio. 2 2 3 2.2. Characterization of the treated mordenites
NaM (granule type of Norton Zeolon-900) was ion-exchanged by 1 M ammonium chloride solution at 80°C for 10 h and washed with distilled water until chlorine ions were no longer detected. This was followed by drying at 120°C overnight. This process was repeated six times. The NH M 4 thus obtained was calcined at 500°C for 5 h in air to obtain HM. HM was then treated with 100% steam at 500°C for 3 h, followed by calcining at 500°C for the same period to prepare SM . 6.5 2.1.1. HCl/steam treatment SM was dealuminated in a 6 M HCl solution 6.5 (5 ml g−1 cat.) for a certain period of time at 90°C in a round flask with a reflux condenser and an agitating stirrer. The acid-treated mordenites were washed with distilled water and dried overnight at 120°C. The dried samples were heated to 600°C at a heating rate of 10°C min−1. At this temperature, slightly superheated steam was passed over the sample for 3, 6, 10 and 16 h, respectively. After steaming, the samples were immersed in 0.01 M HCl solution at 90°C for 2 h and washed with distilled water to remove extra aluminum species separated from the framework of the structure. The samples were then dried at 120°C for 2 h. These two treatments were repeated to increase the degree of dealumination. The final samples treated with HCl solution were steamed to vent free hydrogen chloride from the solids and calcined at 500°C for 3 h. The mordenite samples thus prepared are designated SM . The subscript x is x the SiO /Al O weight ratio of the mordenite. 2 2 3 2.1.2. HF treatment SM was agitated with a 0.5 M HF (4 ml HF 6.5 per g cat.) solution in polyethylene bottle for several different periods of time (192, 240, 384 and 528 h) at room temperature. The treated mordenites were washed with distilled water and dried at 120°C. To remove the free HF, the mordenites were treated in steam stream at 500°C and again dried overnight at 120°C. The dried solids were calcined at 500°C for 3 h. These samples were
The chemical compositions of prepared samples were determined by X-ray fluorescence (Philips, PW-1480). The sample was mixed with a flux of dilithium tetraborate and fused at 1000–1200°C. X-ray photoelectron spectroscopy was carried out on an electron spectrometer ( VG Scientific, MK-2) to obtain the Si/Al ratio on the external surface of the mordenite crystal. Monochromatic Al Ka (1487 eV ) radiation was used as the excitation source, and was focused to a 600 mm diameter spot at 300 W (15 kV, 20 mA). Elemental concentrations were calculated from the Si 2p, Al 2p, O 1s and C 1s peak areas, and the data were compensated for detector efficiency at different photoelectron energies. X-ray diffraction ( XRD) spectra were obtained using Cu Ka radiation (Rigaku Geigerflux, M-3A) to determine crystallinities as well as lattice parameters. Crystallinities were determined by comparing the sum of the peak heights of [330], [150], [202], [350] and [402] (19–31° 2h) of the modified mordenites with that of non-modified HM. The lattice constants a, b and c were determined by applying the d values of the diffraction peaks of [0100], [680], [004], [713], [1000], [534], [843] and [882] (43–61° 2h) to an equation of the orthorhombic form. The unit cell volumes were determined by multiplying the lattice constants (i.e. a×b×c). To determine the structure modification of the mordenites, infrared analysis was also carried out on a Fourier transform infrared spectroscope (Nicolet, Magna-IR Spectrometer 550). The modified mordenites were finely crushed and mixed in agate mortar with crushed KBr powder. A 50 mg sample containing 1.5% mordenite in KBr was pressed into a 14 mm disk. The frequency range was 400–1400 cm−1. To observe the crystal modification of the treated mordenites, scanning electron microscopy was carried out (Jeol, JSM-120EX ). Nitrogen adsorption isotherms were obtained at liquid-nitrogen temperature (Micromeritics, ASAP-2000). From the isotherms, specific surface
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areas were determined using the BET equation within a relative pressure of 0.2, and a distinction between micropores and external pores was made using the T-plot method. Total pore volumes were determined at about 0.99 relative pressure. The samples were pretreated in vacuum at 300°C for 6 h. For the infrared spectra, the samples pressed were into thin wafers (9 mg cm−2) and outgassed overnight at 400°C in vacuum (3×10−3 mmHg). The spectra were measured at room temperature in the region of 4000–3000 cm−1 for viewing hydroxyl groups. Subsequently, pyridine was admitted into the IR cell at room temperature and the samples were equilibrated at 150, 250 and 350°C for 1 h in vacuum (3×10−3 mmHg) prior to measuring the spectra in the region 2000– 1300 cm−1.
3. Results and discussion 3.1. Elemental analysis The SiO /Al O weight ratio, aluminum distri2 2 3 bution and surface fluorine in the modified mordenites are shown in Table 1. If it is supposed that dealumination by HCl/steam cannot eliminate the silicon component, Table 1 indicates the pro-
gressive elimination of aluminum atoms from the mordenite structure. For SM , about 85% of 52 aluminum atoms were removed on the basis of SM . However, aluminum as well as silicon could 6.5 be removed from the mordenites when treated by HF. Therefore, determination of the aluminum atoms in unit cell is impossible from the elemental analysis data due to the removal of both aluminum and silicon from the framework of the mordenites. The mordenites treated with HF contain F atoms on the surface of the mordenite because of the replacement of F with the hydroxyl group related to the framework aluminum. This plays a major role in the formation of new Lewis acid sites [10,11]. The aluminum distribution in the modified mordenites is shown in Table 1 as the average or the external surface Si/Al atomic ratio of the crystals. In the case of HCl/steam treatment, the dealuminated mordenites had higher aluminum contents on their external surfaces than within the crystals. This can be explained by the fact that the aluminum species which separate from the framework may migrate towards the external surface of the mordenite crystal in the steaming step. However, in the case of HF treatment, the aluminum content on the external surface was greater than that within the crystals at the beginning of dealumination, and for further dealuminated mordenites, more alumi-
Table 1 The elemental analysis of mordenites treated by HCl/steam (a) and HF (b) Catalyst
(a) SM 6.5 SM 15.5 SM 20 SM 39 SM 52 (b) FM 17 FM 17.5 FM 21 FM 21.5 NaM
SiO /Al O (weight ratio) 2 2 3
Si/Al atomic ratio
Na O (wt.%) 2
Averagea
Externalb
6.5 15.5 20.0 39.0 52.0
5.0 13.2 16.8 33.2 44.0
5.1 14.0 14.1 18.4 –
0.33 0.03
17.0 17.5 21.0 21.5 6.0
14.3 14.7 17.7 18.1 3.7
9.1 10.5 14.7 20.2 –
0.06 0.08 0.03 0.06 6.84
aXRF analysis. bXPS analysis.
F (surface atomic %)b
1.2 1.7 1.3 1.2 –
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num was removed from the external surface of the sample. We consider it probable that aluminum fluoride species, which are insoluble in aqueous solutions, formed on the crystal surface upon treatment with HF. 3.2. Lattice parameters The crystallinity determined by X-ray diffraction analysis for all the modified mordenites exceeded 80% of that of H-type mordenite. The lattice parameters a, b and c determined for two series of mordenites are shown in Table 2. For mordenites treated by either HCl/steam or HF, the unit cell constants a, b and c were progressively decreased by dealumination. This result indicates the same trends as a previous result [11]. The unit cell volumes (a×b×c) of mordenites treated by HCl/steam were smaller than those of mordenites treated by HF at similar SiO /Al O weight ratios. 2 2 3 3.3. Infrared spectra Infrared spectra of mordenites modified by the two kinds of treatment are shown in Fig. 1A and B as a function of the SiO /Al O weight ratio. 2 2 3 The spectra of samples treated by HCl/steam and those treated by HF are not too different. Three bands appear (at 571, 596 and 657 cm−1) which are attributed to the frequency of a chain of alternating SiO and AlO tetrahedra in the crystal 4 4 lattice. The 825 cm−1 band in treated mordenites Table 2 Unit cell parameters of mordenites treated by HCl/steam (a) and HF (b), determined by X-ray diffraction (a)
SM 6.5
SM 15.5
SM
a b c a×b×c
18.19 20.44 7.50 2789
18.07 20.31 7.48 2745
(b)
FM
FM
a b c a×b×c
18.11 20.35 7.48 2757
17
17.5
18.12 20.36 7.48 2760
SM 39
SM
18.07 20.29 7.47 2739
18.07 20.28 7.46 2734
18.06 20.27 7.46 2731
FM
FM
NaM
20
21
18.10 20.36 7.48 2756
21.5
18.07 20.32 7.47 2743
52
18.11 20.53 7.53 2800
Fig. 1. IR spectra of mordenites treated by HCl/steam (A) and HF (B) in the region of skeletal vibrations.
shifts from the low wave number of NaM [12]. The samples treated with HCl/steam, which have higher SiO /Al O weight ratios as compared to 2 2 3 those treated by HF, have a more intense peak than those treated with HF. Accordingly, the shifts of these bands to higher wave numbers, as well as the increase in the absorption height, appear with increasing SiO /Al O weight ratio in the treated 2 2 3 mordenites. This result indicates the removal of aluminum atoms from the framework. The broad band at 725 cm−1 corresponding to the vibration of the AlO tetrahedra in the mordenites decreased 4 sharply as the aluminum content of the samples decreased [12]. This tendency is very similar over the two series of mordenites, but the intensity of
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215
the band is less for samples treated by HF than for samples treated by HCl/steam. These results indicate that the elimination of structural aluminum is much more severe in samples treated by HCl/steam than in samples treated by HF. This agrees with the result shown in Table 1. Samples treated by HCl/steam had a higher SiO /Al O 2 2 3 weight ratio as compared to those treated by HF.
graph of SM in which the crystal shapes can be 6.5 clearly observed. However, dealuminated SM 39 shows partial cracking along the crystal edge and splitting in one direction. For FM and FM , 17.5 21.5 the crystal edges could not be observed, but erosion along the crystal edges occurred. FM contained 21.5 more amorphous species on the external surface as compared to FM . 17.5
3.4. Morphology determined by scanning electron microscopy
3.5. Nitrogen adsorption
Scanning electron microscopy showed that the crystal shapes of the treated mordenites varied considerably with the different pretreatment conditions, as shown in Fig. 2. Fig. 2a shows a photo-
For the samples treated by HCl/steam and HF, nitrogen adsorption–desorption isotherms at liquid-nitrogen temperature are presented in Fig. 3A and B, respectively. The isotherm plots show hysteresis loops which indicate the formation
(a)
(b)
(c)
(d)
Fig. 2. Scanning electron micrographs of mordenites treated by HCl/steam and HF. (a) SM , (b) SM , (c) FM , (d) FM . 6.5 39 17.5 21.5
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Fig. 3. N isotherms of mordenites treated by HCl/steam (A) 2 and HF (B).
of secondary pores. The shape of the hysteresis isotherms over two series of the samples can be classified by IUPAC as type-H2 ink-bottle pores [13]. The secondary pores are thought to be ˚ formed by opening two large pores of 6.7×7.0 A as a result of the easy elimination of aluminum atoms from sites 1 and 2 (three quarters) compared to sites 3 and 4 (one quarter) [11]. In the series of HCl/steam-treated mordenites, the adsorption capacity of nitrogen was increased, and also the hysteresis loops which were attributed to capillary condensation in the secondary pores were increased as the SiO /Al O weight ratio was 2 2 3 increased. This implies that the mesopore volume increases with increasing of SiO /Al O weight 2 2 3 ratio during the dealumination process. However, the adsorption capacity of nitrogen in mordenites treated by HF increases up to a SiO /Al O weight 2 2 3 ratio of 17.5, and then decreases at higher SiO /Al O weight ratios due to the partial collapse 2 2 3 of the crystalline structure, as shown in the SEM of FM . 21.5 The surface areas and pore volumes are given in Table 3, along with the degree of dealumination. For the HCl/steam-treated mordenites, the BET surface areas of all samples are larger than 500 m2 g−1 and increase up to 545 m2 g−1 when the SiO /Al O weight ratio increases from 6.5 to 2 2 3 52. As the degree of dealumination is gradually increased in the treated mordenites, the micro surface area decreases rapidly at a SiO /Al O 2 2 3 weight ratio of 15.5, while the external surface area increases sharply upon the transformation of
Table 3 Surface areas and pore volumes of mordenites treated by HCl/steam and HF Catalyst
SM 6.5 SM 15.5 SM 20 SM 39 SM 52 FM 17 FM 17.5 FM 21 FM 21.5
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BET
Micro-
External
Total
Micro-
Meso-
500 512 533 534 545 527 533 499 480
485 437 450 458 479 458 468 439 423
15 75 82 76 67 69 64 61 57
0.2170 0.2793 0.2942 0.3101 0.3182 0.2806 0.2939 0.2828 0.2925
0.1858 0.1686 0.1736 0.1763 0.1839 0.1772 0.1811 0.1698 0.1637
0.0312 0.1107 0.1206 0.1338 0.1343 0.1034 0.1128 0.1130 0.1288
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micropores to mesopores. However, at SiO /Al O weight ratios of above 15.5, the micro 2 2 3 surface area increases with the SiO /Al O weight 2 2 3 ratio due to the opening of micropores by the removal of aluminum oxide species which block the micropores. These phenomena show agreement with the change in micropore volume and mesopore volume. As a function of the SiO /Al O 2 2 3 weight ratio, the change in surface area and pore volume in mordenites treated by HF had a similar tendency to those treated by HCl/steam. However, above a SiO /Al O weight ratio of 21 in 2 2 3 HF-treated mordenites, the micropore surface area and the micropore volume decrease with SiO /Al O weight ratio due to the partial collapse 2 2 3 of the structure, caused by the removal of silicon as well as aluminum from the framework of the mordenite. For the mordenites treated by HCl/steam(A) or HF(B), the mesopore size distributions obtained from desorption isotherm branches are presented in Fig. 4. Mesopore size distributions are very small and have unique peaks at 3.7 nm. It is interesting to note that the pore size of 3.7 nm has some meaning, because this size is the same over two series of mordenites despite of completely different treatment conditions. It is supposed that the pore size of 3.7 nm was caused by pore destruction of two large pore contained in two unit cell over dealuminated mordenites with an average unit cell size of 1.81 nm X 2.03 nm [1]. In case of HCl/steam treatment, the mesopore volume around 3.7 nm forms slightly at the beginning of the treatment and improves gradually with dealumination. However, in case of HF treatment the mesopore volume was increased up to 17.5 of SiO /Al O weight ratio. Above this point, the 2 2 3 value was decreased due to the partial collapse of the crystalline structure by means of elimination of aluminum as well as silicon as shown in FM of SEM. 21.5 3.6. Surface acidities determined by pyridine-IR For the two series of mordenites treated by HCl/steam and HF, the acidity of the Brønsted and Lewis sites after the adsorption of pyridine at 150, 250 and 350°C are shown with the arbitrary
217
Fig. 4. Pore-size distributions obtained by desorption isotherm branches of mordenites treated by HCl/steam (A) and HF (B).
intensity of the infrared absorption band in Table 4. In HCl/steam-treated mordenites, as a function of the SiO /Al O weight ratio the 2 2 3 amount of Brønsted acid sites decreased sharply for SM as compared to SM . However, above 15.5 6.5 a SiO /Al O weight ratio of 15.5 it was almost 2 2 3 constant despite the increase in the SiO /Al O 2 2 3 weight ratio. We suppose that the sharp decrease in Brønsted acid sites in SM compared to 15.5 SM is caused by the removal of aluminum atoms 6.5 from the framework in the structure related to Brønsted acid sites [14]. Above a SiO /Al O 2 2 3 weight ratio of 15.5, the amount of Brønsted acid sites was almost constant, which we attribute to the fact that the non-framework aluminum oxide was protonated by acid treatment to form
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Table 4 Acid-site type and acid strength distributions in modified mordenites, measured by IR-pyridine adsorption Sample
Aciditya Brønsted acid site
SM 6.5 SM 15.5 SM 20 SM 39 SM 52 FM 17 FM 17.5 FM 21 FM 21.5
Lewis acid site
Acid strength
150°C (A)
250°C
350°C (B)
150°C (C )
250°C
350°C (D)
B/A
D/C
9.9 1.3 1.5 1.5 1.7 5.2 3.4 3.0 1.4
10.5 1.3 1.1 1.3 1.1 3.8 2.8 2.6 1.2
11.1 1.0 1.0 1.0 0.7 2.2 1.4 1.0 0.6
5.5 4.0 4.4 4.7 4.7 10.8 6.8 4.4 3.4
5.0 3.2 3.2 2.9 3.0 4.2 3.4 3.0 1.0
6.0 2.3 2.5 1.7 1.4 2.8 1.4 0.6 0.4
1.12 0.77 0.67 0.67 0.41 0.42 0.41 0.33 0.42
1.20 0.58 0.57 0.36 0.30 0.26 0.21 0.14 0.12
aArbitrary intensity of the absorption band of adsorbed pyridine.
Brønsted acid sites [14]. This result is consistent with OH groups, presented in Fig. 5A. A hydroxyl group appearing at 3610 cm−1, which is associated with Brønsted acid sites as a hydroxyl group bonded to framework aluminum [14], decreases sharply for mordenite with a SiO /Al O weight 2 2 3 ratio of 15.5 and then becomes constant. A terminal silanol group formed at 3740 cm−1 increased with increasing SiO /Al O weight ratio due to the 2 2 3 silanol group formed by dealumination [15]. The acidity of tthe Lewis acid sites has a similar tendency to that of the Brønsted acid sites. In HF-treated mordenites, the amount of Brønsted acid sites decreases with the SiO /Al O 2 2 3 weight ratio, while that of Lewis sites shows a maximum at a SiO /Al O weight ratio of 17. In 2 2 3 the latter, when SM was treated by HF to 6.5 prepare FM , compared to SM , more Lewis 17 6.5 acid sites were produced due to the formation of new Lewis acid sites [9,10]. According to the increase in the SiO /Al O weight ratio, the 2 2 3 decrease in the amount of Brønsted acid sites was consistent with the decrease of the 3610 cm−1 peak, which was attributed to a hydroxyl group bonded to framework aluminum in Fig. 5B. However, the number of silanol groups formed by dealumination with HF decreased with increasing SiO /Al O weight ratio. This means that treat2 2 3 ment of the mordenites with HF removed the
silanol groups by replacing F with the hydroxyl group in Si–OH [9]. The acid strength distributions of Brønsted and Lewis acid sites could be evaluated by comparing the relative intensity of the adsorbed pyridine absorption peak appearing at 350–150°C. Pyridine adsorption shows that the proportion of strong acid sites in HCl/steam-treated mordenites was higher than that in HF-treated mordenites. Brønsted acid sites especially had a higher proportion of strong acid sites than Lewis acid sites in the treated mordenites. The proportion of strong acid sites was usually high at a SiO /Al O weight 2 2 3 ratio of 20 over all the treated mordenites.
4. Conclusions The characteristics of mordenites treated by either HCl/steam or HF have been studied. In HCl/steam-treated mordenites, the framework aluminum was mainly removed, while in HF-treated mordenites, the framework silicon and aluminum were simultaneously removed. The lattice parameters were smaller on HCl/steam-treated mordenites than on HF-treated mordenites at the same SiO /Al O weight ratios. Also, HCl/steam-treated 2 2 3 mordenites had a higher proportion of strong acid sites than HF-treated mordenites.
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ter. These mesopores developed further during the dealumination process up to a SiO /Al O weight 2 2 3 ratio of 52. However, the HF-treated mordenites at SiO /Al O weight ratios of 21 or above showed 2 2 3 a decrease in the number of mesopores formed at 3.7 nm because of the partial destruction of the structure by the simultaneous removal of silicon and aluminum from the framework.
References
Fig. 5. IR spectra of OH band sites after calcination of the mordenites treated by HCl/steam (A) and HF (B).
The mordenites treated by HCl/steam and HF formed secondary mesopores of 3.7 nm in diame-
[1] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, p. 122. [2] N. Mori, S. Nishiyama, S. Tsuruya, M. Masai, Appl. Catal. 74 (1991) 37. [3] P. Magnoux, M. Guisnet, Appl. Catal. 38 (1988) 341. [4] K.H. Lee, Y.W. Lee, B.H. Ha, J. Catal., submitted for publication. [5] R.J.A. Minja, M. Ternan, Energy Fuel 5 (1991) 117. [6 ] A. Corma, P.J. Miguel, A.V. Orchilles, G. Koermer, J. Catal. 15 (1994) 181. [7] K. Seimiya, M. Hashimoto, S. Suzuki, M. Kameyama, Y. Noguchi, K. Nita, Seikiyu Gakkaishi 33 (1990) 1. [8] A.W. O’Donovan, C.T. O’Connor, K.R. Koch, Microporous Mater. 5 (1995) 185. [9] A.K. Ghosh, R.A. Kydd, J. Catal. 103 (1987) 399. [10] B.M. Lok, F.P. Gortsema, C.A. Messina, H. Rastelli, T.P. Izod, J. Am. Chem. Soc., Div. Petrochem. Prepar. 27 (1982) 470. [11] R.W. Olsson, L.D. Rollmann, Inorg. Chem. 16 (3) (1977) 651. [12] M.J. Van Niekerk, J.C.Q. Fletcher, C.T. O’onnor, J. Catal. 138 (1992) 150. [13] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London, 1982. [14] A. Corma, M. Grande, V. Fornes, Appl. Catal. 66 (1990) 45. [15] B.H. Ha, D. Barthomeuf, J. Chem. Soc., Faraday Trans. I 75 (1979) 2366.
Characterization of mordenites treated by HCl/steam or HF Kyong-Hwan Lee *, Baik-Hyon Ha Department of Chemical Engineering, Hanyang University, 17 Haengdangdong, Sungdongku, Seoul 133-791, South Korea Received 5 September 1997; accepted 19 February 1998
Abstract Mordenites were modified by dealumination with HCl/steam or HF treatment, and characterized by X-ray fluorescence ( XRF ), X-ray photoelectron spectroscopy ( XPS), X-ray diffraction ( XRD), infrared spectroscopy, scanning electron microscopy (SEM ) and nitrogen adsorption. All mordenites retained more than 80% crystallinity, although the lattice parameters (a, b, c) were reduced more for HCl/steam-treated mordenites than for HF-treated mordenites. In the HCl/steam-treated mordenites, framework aluminum was mainly removed, while in the HF-treated mordenites, framework aluminum was removed extensively at the beginning of treatment, and thereafter silicon and aluminum were simultaneously removed. Mesopores with a diameter of 3.7 nm developed as a result of dealumination, which did not depend on the treatment methods. The acid strength was higher on the HCl/steam-treated mordenites than on the HF-treated mordenites. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Acid/steam treatment; Dealuminated mordenite; Secondary mesopore formation
1. Introduction ˚ Mordenite has two pore channels, i.e. 6.7×7.0 A ˚ which is parallel to the c axis, and 2.9×5.7 A which is parallel to the b axis [1]. The large, straight pores do not allow molecules to diffuse easily. If the pores are blocked by coke forming around their mouths, deactivation occurs quickly [2–4]. Such a pore size and shape limits the application of mordenite to large molecules. It is therefore necessary to tailor the pore size and * Corresponding author. Fax: +82 2 9585809; E-mail: [email protected]. Present address: Environment Remediation Research Center, Korea Institute of Science and Technology, 39-1 Haweolkokdong, Sungbuk-ku, Seoul 136-791, Korea. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 11 8 - 8
shape of mordenite when considering the reactant, the product or intermediate selectivity [5,6 ]. The formation of mesopores by dealumination via thermal or steam treatment at high temperatures has been studied in relation to catalytic activity [7,8]. Such dealumination and/or steaming eliminates aluminum atoms around the pore mouth, which improves the deactivation properties. It is therefore very interesting to study the formation of secondary mesopores which provide easy migration of the reactants and/or products and also store the coke formed in hydrocarbon processing. In this paper, we treat mordenite with concentrated hydrochloric acid as well as steam at higher temperatures or with hydrofluoric acid at room temperature to form the secondary mesopores of mordenite.
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2. Experimental 2.1. Preparation of catalysts
designated as FM , in which x again indicates the x SiO /Al O weight ratio. 2 2 3 2.2. Characterization of the treated mordenites
NaM (granule type of Norton Zeolon-900) was ion-exchanged by 1 M ammonium chloride solution at 80°C for 10 h and washed with distilled water until chlorine ions were no longer detected. This was followed by drying at 120°C overnight. This process was repeated six times. The NH M 4 thus obtained was calcined at 500°C for 5 h in air to obtain HM. HM was then treated with 100% steam at 500°C for 3 h, followed by calcining at 500°C for the same period to prepare SM . 6.5 2.1.1. HCl/steam treatment SM was dealuminated in a 6 M HCl solution 6.5 (5 ml g−1 cat.) for a certain period of time at 90°C in a round flask with a reflux condenser and an agitating stirrer. The acid-treated mordenites were washed with distilled water and dried overnight at 120°C. The dried samples were heated to 600°C at a heating rate of 10°C min−1. At this temperature, slightly superheated steam was passed over the sample for 3, 6, 10 and 16 h, respectively. After steaming, the samples were immersed in 0.01 M HCl solution at 90°C for 2 h and washed with distilled water to remove extra aluminum species separated from the framework of the structure. The samples were then dried at 120°C for 2 h. These two treatments were repeated to increase the degree of dealumination. The final samples treated with HCl solution were steamed to vent free hydrogen chloride from the solids and calcined at 500°C for 3 h. The mordenite samples thus prepared are designated SM . The subscript x is x the SiO /Al O weight ratio of the mordenite. 2 2 3 2.1.2. HF treatment SM was agitated with a 0.5 M HF (4 ml HF 6.5 per g cat.) solution in polyethylene bottle for several different periods of time (192, 240, 384 and 528 h) at room temperature. The treated mordenites were washed with distilled water and dried at 120°C. To remove the free HF, the mordenites were treated in steam stream at 500°C and again dried overnight at 120°C. The dried solids were calcined at 500°C for 3 h. These samples were
The chemical compositions of prepared samples were determined by X-ray fluorescence (Philips, PW-1480). The sample was mixed with a flux of dilithium tetraborate and fused at 1000–1200°C. X-ray photoelectron spectroscopy was carried out on an electron spectrometer ( VG Scientific, MK-2) to obtain the Si/Al ratio on the external surface of the mordenite crystal. Monochromatic Al Ka (1487 eV ) radiation was used as the excitation source, and was focused to a 600 mm diameter spot at 300 W (15 kV, 20 mA). Elemental concentrations were calculated from the Si 2p, Al 2p, O 1s and C 1s peak areas, and the data were compensated for detector efficiency at different photoelectron energies. X-ray diffraction ( XRD) spectra were obtained using Cu Ka radiation (Rigaku Geigerflux, M-3A) to determine crystallinities as well as lattice parameters. Crystallinities were determined by comparing the sum of the peak heights of [330], [150], [202], [350] and [402] (19–31° 2h) of the modified mordenites with that of non-modified HM. The lattice constants a, b and c were determined by applying the d values of the diffraction peaks of [0100], [680], [004], [713], [1000], [534], [843] and [882] (43–61° 2h) to an equation of the orthorhombic form. The unit cell volumes were determined by multiplying the lattice constants (i.e. a×b×c). To determine the structure modification of the mordenites, infrared analysis was also carried out on a Fourier transform infrared spectroscope (Nicolet, Magna-IR Spectrometer 550). The modified mordenites were finely crushed and mixed in agate mortar with crushed KBr powder. A 50 mg sample containing 1.5% mordenite in KBr was pressed into a 14 mm disk. The frequency range was 400–1400 cm−1. To observe the crystal modification of the treated mordenites, scanning electron microscopy was carried out (Jeol, JSM-120EX ). Nitrogen adsorption isotherms were obtained at liquid-nitrogen temperature (Micromeritics, ASAP-2000). From the isotherms, specific surface
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areas were determined using the BET equation within a relative pressure of 0.2, and a distinction between micropores and external pores was made using the T-plot method. Total pore volumes were determined at about 0.99 relative pressure. The samples were pretreated in vacuum at 300°C for 6 h. For the infrared spectra, the samples pressed were into thin wafers (9 mg cm−2) and outgassed overnight at 400°C in vacuum (3×10−3 mmHg). The spectra were measured at room temperature in the region of 4000–3000 cm−1 for viewing hydroxyl groups. Subsequently, pyridine was admitted into the IR cell at room temperature and the samples were equilibrated at 150, 250 and 350°C for 1 h in vacuum (3×10−3 mmHg) prior to measuring the spectra in the region 2000– 1300 cm−1.
3. Results and discussion 3.1. Elemental analysis The SiO /Al O weight ratio, aluminum distri2 2 3 bution and surface fluorine in the modified mordenites are shown in Table 1. If it is supposed that dealumination by HCl/steam cannot eliminate the silicon component, Table 1 indicates the pro-
gressive elimination of aluminum atoms from the mordenite structure. For SM , about 85% of 52 aluminum atoms were removed on the basis of SM . However, aluminum as well as silicon could 6.5 be removed from the mordenites when treated by HF. Therefore, determination of the aluminum atoms in unit cell is impossible from the elemental analysis data due to the removal of both aluminum and silicon from the framework of the mordenites. The mordenites treated with HF contain F atoms on the surface of the mordenite because of the replacement of F with the hydroxyl group related to the framework aluminum. This plays a major role in the formation of new Lewis acid sites [10,11]. The aluminum distribution in the modified mordenites is shown in Table 1 as the average or the external surface Si/Al atomic ratio of the crystals. In the case of HCl/steam treatment, the dealuminated mordenites had higher aluminum contents on their external surfaces than within the crystals. This can be explained by the fact that the aluminum species which separate from the framework may migrate towards the external surface of the mordenite crystal in the steaming step. However, in the case of HF treatment, the aluminum content on the external surface was greater than that within the crystals at the beginning of dealumination, and for further dealuminated mordenites, more alumi-
Table 1 The elemental analysis of mordenites treated by HCl/steam (a) and HF (b) Catalyst
(a) SM 6.5 SM 15.5 SM 20 SM 39 SM 52 (b) FM 17 FM 17.5 FM 21 FM 21.5 NaM
SiO /Al O (weight ratio) 2 2 3
Si/Al atomic ratio
Na O (wt.%) 2
Averagea
Externalb
6.5 15.5 20.0 39.0 52.0
5.0 13.2 16.8 33.2 44.0
5.1 14.0 14.1 18.4 –
0.33 0.03
17.0 17.5 21.0 21.5 6.0
14.3 14.7 17.7 18.1 3.7
9.1 10.5 14.7 20.2 –
0.06 0.08 0.03 0.06 6.84
aXRF analysis. bXPS analysis.
F (surface atomic %)b
1.2 1.7 1.3 1.2 –
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num was removed from the external surface of the sample. We consider it probable that aluminum fluoride species, which are insoluble in aqueous solutions, formed on the crystal surface upon treatment with HF. 3.2. Lattice parameters The crystallinity determined by X-ray diffraction analysis for all the modified mordenites exceeded 80% of that of H-type mordenite. The lattice parameters a, b and c determined for two series of mordenites are shown in Table 2. For mordenites treated by either HCl/steam or HF, the unit cell constants a, b and c were progressively decreased by dealumination. This result indicates the same trends as a previous result [11]. The unit cell volumes (a×b×c) of mordenites treated by HCl/steam were smaller than those of mordenites treated by HF at similar SiO /Al O weight ratios. 2 2 3 3.3. Infrared spectra Infrared spectra of mordenites modified by the two kinds of treatment are shown in Fig. 1A and B as a function of the SiO /Al O weight ratio. 2 2 3 The spectra of samples treated by HCl/steam and those treated by HF are not too different. Three bands appear (at 571, 596 and 657 cm−1) which are attributed to the frequency of a chain of alternating SiO and AlO tetrahedra in the crystal 4 4 lattice. The 825 cm−1 band in treated mordenites Table 2 Unit cell parameters of mordenites treated by HCl/steam (a) and HF (b), determined by X-ray diffraction (a)
SM 6.5
SM 15.5
SM
a b c a×b×c
18.19 20.44 7.50 2789
18.07 20.31 7.48 2745
(b)
FM
FM
a b c a×b×c
18.11 20.35 7.48 2757
17
17.5
18.12 20.36 7.48 2760
SM 39
SM
18.07 20.29 7.47 2739
18.07 20.28 7.46 2734
18.06 20.27 7.46 2731
FM
FM
NaM
20
21
18.10 20.36 7.48 2756
21.5
18.07 20.32 7.47 2743
52
18.11 20.53 7.53 2800
Fig. 1. IR spectra of mordenites treated by HCl/steam (A) and HF (B) in the region of skeletal vibrations.
shifts from the low wave number of NaM [12]. The samples treated with HCl/steam, which have higher SiO /Al O weight ratios as compared to 2 2 3 those treated by HF, have a more intense peak than those treated with HF. Accordingly, the shifts of these bands to higher wave numbers, as well as the increase in the absorption height, appear with increasing SiO /Al O weight ratio in the treated 2 2 3 mordenites. This result indicates the removal of aluminum atoms from the framework. The broad band at 725 cm−1 corresponding to the vibration of the AlO tetrahedra in the mordenites decreased 4 sharply as the aluminum content of the samples decreased [12]. This tendency is very similar over the two series of mordenites, but the intensity of
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215
the band is less for samples treated by HF than for samples treated by HCl/steam. These results indicate that the elimination of structural aluminum is much more severe in samples treated by HCl/steam than in samples treated by HF. This agrees with the result shown in Table 1. Samples treated by HCl/steam had a higher SiO /Al O 2 2 3 weight ratio as compared to those treated by HF.
graph of SM in which the crystal shapes can be 6.5 clearly observed. However, dealuminated SM 39 shows partial cracking along the crystal edge and splitting in one direction. For FM and FM , 17.5 21.5 the crystal edges could not be observed, but erosion along the crystal edges occurred. FM contained 21.5 more amorphous species on the external surface as compared to FM . 17.5
3.4. Morphology determined by scanning electron microscopy
3.5. Nitrogen adsorption
Scanning electron microscopy showed that the crystal shapes of the treated mordenites varied considerably with the different pretreatment conditions, as shown in Fig. 2. Fig. 2a shows a photo-
For the samples treated by HCl/steam and HF, nitrogen adsorption–desorption isotherms at liquid-nitrogen temperature are presented in Fig. 3A and B, respectively. The isotherm plots show hysteresis loops which indicate the formation
(a)
(b)
(c)
(d)
Fig. 2. Scanning electron micrographs of mordenites treated by HCl/steam and HF. (a) SM , (b) SM , (c) FM , (d) FM . 6.5 39 17.5 21.5
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Fig. 3. N isotherms of mordenites treated by HCl/steam (A) 2 and HF (B).
of secondary pores. The shape of the hysteresis isotherms over two series of the samples can be classified by IUPAC as type-H2 ink-bottle pores [13]. The secondary pores are thought to be ˚ formed by opening two large pores of 6.7×7.0 A as a result of the easy elimination of aluminum atoms from sites 1 and 2 (three quarters) compared to sites 3 and 4 (one quarter) [11]. In the series of HCl/steam-treated mordenites, the adsorption capacity of nitrogen was increased, and also the hysteresis loops which were attributed to capillary condensation in the secondary pores were increased as the SiO /Al O weight ratio was 2 2 3 increased. This implies that the mesopore volume increases with increasing of SiO /Al O weight 2 2 3 ratio during the dealumination process. However, the adsorption capacity of nitrogen in mordenites treated by HF increases up to a SiO /Al O weight 2 2 3 ratio of 17.5, and then decreases at higher SiO /Al O weight ratios due to the partial collapse 2 2 3 of the crystalline structure, as shown in the SEM of FM . 21.5 The surface areas and pore volumes are given in Table 3, along with the degree of dealumination. For the HCl/steam-treated mordenites, the BET surface areas of all samples are larger than 500 m2 g−1 and increase up to 545 m2 g−1 when the SiO /Al O weight ratio increases from 6.5 to 2 2 3 52. As the degree of dealumination is gradually increased in the treated mordenites, the micro surface area decreases rapidly at a SiO /Al O 2 2 3 weight ratio of 15.5, while the external surface area increases sharply upon the transformation of
Table 3 Surface areas and pore volumes of mordenites treated by HCl/steam and HF Catalyst
SM 6.5 SM 15.5 SM 20 SM 39 SM 52 FM 17 FM 17.5 FM 21 FM 21.5
Surface area (m2 g−1)
Pore volume (cm3 g−1)
BET
Micro-
External
Total
Micro-
Meso-
500 512 533 534 545 527 533 499 480
485 437 450 458 479 458 468 439 423
15 75 82 76 67 69 64 61 57
0.2170 0.2793 0.2942 0.3101 0.3182 0.2806 0.2939 0.2828 0.2925
0.1858 0.1686 0.1736 0.1763 0.1839 0.1772 0.1811 0.1698 0.1637
0.0312 0.1107 0.1206 0.1338 0.1343 0.1034 0.1128 0.1130 0.1288
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micropores to mesopores. However, at SiO /Al O weight ratios of above 15.5, the micro 2 2 3 surface area increases with the SiO /Al O weight 2 2 3 ratio due to the opening of micropores by the removal of aluminum oxide species which block the micropores. These phenomena show agreement with the change in micropore volume and mesopore volume. As a function of the SiO /Al O 2 2 3 weight ratio, the change in surface area and pore volume in mordenites treated by HF had a similar tendency to those treated by HCl/steam. However, above a SiO /Al O weight ratio of 21 in 2 2 3 HF-treated mordenites, the micropore surface area and the micropore volume decrease with SiO /Al O weight ratio due to the partial collapse 2 2 3 of the structure, caused by the removal of silicon as well as aluminum from the framework of the mordenite. For the mordenites treated by HCl/steam(A) or HF(B), the mesopore size distributions obtained from desorption isotherm branches are presented in Fig. 4. Mesopore size distributions are very small and have unique peaks at 3.7 nm. It is interesting to note that the pore size of 3.7 nm has some meaning, because this size is the same over two series of mordenites despite of completely different treatment conditions. It is supposed that the pore size of 3.7 nm was caused by pore destruction of two large pore contained in two unit cell over dealuminated mordenites with an average unit cell size of 1.81 nm X 2.03 nm [1]. In case of HCl/steam treatment, the mesopore volume around 3.7 nm forms slightly at the beginning of the treatment and improves gradually with dealumination. However, in case of HF treatment the mesopore volume was increased up to 17.5 of SiO /Al O weight ratio. Above this point, the 2 2 3 value was decreased due to the partial collapse of the crystalline structure by means of elimination of aluminum as well as silicon as shown in FM of SEM. 21.5 3.6. Surface acidities determined by pyridine-IR For the two series of mordenites treated by HCl/steam and HF, the acidity of the Brønsted and Lewis sites after the adsorption of pyridine at 150, 250 and 350°C are shown with the arbitrary
217
Fig. 4. Pore-size distributions obtained by desorption isotherm branches of mordenites treated by HCl/steam (A) and HF (B).
intensity of the infrared absorption band in Table 4. In HCl/steam-treated mordenites, as a function of the SiO /Al O weight ratio the 2 2 3 amount of Brønsted acid sites decreased sharply for SM as compared to SM . However, above 15.5 6.5 a SiO /Al O weight ratio of 15.5 it was almost 2 2 3 constant despite the increase in the SiO /Al O 2 2 3 weight ratio. We suppose that the sharp decrease in Brønsted acid sites in SM compared to 15.5 SM is caused by the removal of aluminum atoms 6.5 from the framework in the structure related to Brønsted acid sites [14]. Above a SiO /Al O 2 2 3 weight ratio of 15.5, the amount of Brønsted acid sites was almost constant, which we attribute to the fact that the non-framework aluminum oxide was protonated by acid treatment to form
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Table 4 Acid-site type and acid strength distributions in modified mordenites, measured by IR-pyridine adsorption Sample
Aciditya Brønsted acid site
SM 6.5 SM 15.5 SM 20 SM 39 SM 52 FM 17 FM 17.5 FM 21 FM 21.5
Lewis acid site
Acid strength
150°C (A)
250°C
350°C (B)
150°C (C )
250°C
350°C (D)
B/A
D/C
9.9 1.3 1.5 1.5 1.7 5.2 3.4 3.0 1.4
10.5 1.3 1.1 1.3 1.1 3.8 2.8 2.6 1.2
11.1 1.0 1.0 1.0 0.7 2.2 1.4 1.0 0.6
5.5 4.0 4.4 4.7 4.7 10.8 6.8 4.4 3.4
5.0 3.2 3.2 2.9 3.0 4.2 3.4 3.0 1.0
6.0 2.3 2.5 1.7 1.4 2.8 1.4 0.6 0.4
1.12 0.77 0.67 0.67 0.41 0.42 0.41 0.33 0.42
1.20 0.58 0.57 0.36 0.30 0.26 0.21 0.14 0.12
aArbitrary intensity of the absorption band of adsorbed pyridine.
Brønsted acid sites [14]. This result is consistent with OH groups, presented in Fig. 5A. A hydroxyl group appearing at 3610 cm−1, which is associated with Brønsted acid sites as a hydroxyl group bonded to framework aluminum [14], decreases sharply for mordenite with a SiO /Al O weight 2 2 3 ratio of 15.5 and then becomes constant. A terminal silanol group formed at 3740 cm−1 increased with increasing SiO /Al O weight ratio due to the 2 2 3 silanol group formed by dealumination [15]. The acidity of tthe Lewis acid sites has a similar tendency to that of the Brønsted acid sites. In HF-treated mordenites, the amount of Brønsted acid sites decreases with the SiO /Al O 2 2 3 weight ratio, while that of Lewis sites shows a maximum at a SiO /Al O weight ratio of 17. In 2 2 3 the latter, when SM was treated by HF to 6.5 prepare FM , compared to SM , more Lewis 17 6.5 acid sites were produced due to the formation of new Lewis acid sites [9,10]. According to the increase in the SiO /Al O weight ratio, the 2 2 3 decrease in the amount of Brønsted acid sites was consistent with the decrease of the 3610 cm−1 peak, which was attributed to a hydroxyl group bonded to framework aluminum in Fig. 5B. However, the number of silanol groups formed by dealumination with HF decreased with increasing SiO /Al O weight ratio. This means that treat2 2 3 ment of the mordenites with HF removed the
silanol groups by replacing F with the hydroxyl group in Si–OH [9]. The acid strength distributions of Brønsted and Lewis acid sites could be evaluated by comparing the relative intensity of the adsorbed pyridine absorption peak appearing at 350–150°C. Pyridine adsorption shows that the proportion of strong acid sites in HCl/steam-treated mordenites was higher than that in HF-treated mordenites. Brønsted acid sites especially had a higher proportion of strong acid sites than Lewis acid sites in the treated mordenites. The proportion of strong acid sites was usually high at a SiO /Al O weight 2 2 3 ratio of 20 over all the treated mordenites.
4. Conclusions The characteristics of mordenites treated by either HCl/steam or HF have been studied. In HCl/steam-treated mordenites, the framework aluminum was mainly removed, while in HF-treated mordenites, the framework silicon and aluminum were simultaneously removed. The lattice parameters were smaller on HCl/steam-treated mordenites than on HF-treated mordenites at the same SiO /Al O weight ratios. Also, HCl/steam-treated 2 2 3 mordenites had a higher proportion of strong acid sites than HF-treated mordenites.
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ter. These mesopores developed further during the dealumination process up to a SiO /Al O weight 2 2 3 ratio of 52. However, the HF-treated mordenites at SiO /Al O weight ratios of 21 or above showed 2 2 3 a decrease in the number of mesopores formed at 3.7 nm because of the partial destruction of the structure by the simultaneous removal of silicon and aluminum from the framework.
References
Fig. 5. IR spectra of OH band sites after calcination of the mordenites treated by HCl/steam (A) and HF (B).
The mordenites treated by HCl/steam and HF formed secondary mesopores of 3.7 nm in diame-
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