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Microporous characteristics of HY, H-ZSM-5 and H-mordenite dealuminated by calcination
MICROPOROUS MATERIALS ELSEVIER
Microporous Materials 4 (1995) 323 334
Microporous characteristics of H-Y, H-ZSM-5 and H-mordenite dealuminated by calcination Y. Hong, J.J. Fripiat * Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, W153201. USA Received 3 November 1994; accepted 3 December 1994
Abstract
The modifications of the microporous volume of zeolites resulting from thermal dealumination have been examined by measuring the changes in the unit cell parameters as well as the N 2 and n-pentane adsorption isotherms. For the cubic HY and orthorhombic H-ZSM-5 zeolites, the differential decrease of the microporous volume per nonframework aluminum (NFA1) can be predicted by assuming NFA1 is concentrated in pores available to N2. For the orthorhombic H-mordenite, NFA1 is homogeneously spread in all pores, available or not to N2. The Gurvitch rule (volume of adsorbed N 2 =volume of adsorbed pentane) is respected in HY and H-ZSM-5, but not in H-mordenite, irrespective of the degree of dealumination.
Keywords: Zeolite Y; ZSM-5; Mordenite; Micropore; Dealumination
1. Introduction
Thermal treatment is one of the major methods for dislodging aluminum from framework (FAI) to non-framework positions (NFA1) in zeolites [ 1]. Some dealumination procedures, such as those using EDTA, SIC14, (NH4)2SiF 6 or acid leaching, remove various amounts of aluminum from the solid, consequently the Si/A1 ratios in the bulk and in framework increase [1,2]. However, in the thermal or hydrothermal treatment of zeolites, the non-framework aluminum species stay in the zeolite channels. The Si/A1 ratio obtained by chemical analysis remains the same while the framework Si/FA1 ratio increases [1,2]. Accordingly, the unit cell contracts as observed in the X-ray diffraction *Corresponding author. 0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 6 5 1 3 ( 9 5 ) 0 0 0 3 8 - 0
( X R D ) diagram. The Si/FAI ratio is obtained from t h e 298i magic-angle spinning nuclear magnetic resonance (MAS N M R ) spectra [1,2]. Hydrothermal treatment (deep- or shallow-bed calcination in the presence or absence of water vapor) involves aluminum extraction and dehydroxylation [1,2]. It may result in two major modifications to the zeolite catalysts: ( 1 ) structure modifications in which the framework Si/FAI is changed, resulting in a change in the number and eventually the strength of the Br6nsted acid sites as well as in the number and strength of the Lewis acid sites present on the NFA1 species; (2) porosity modifications which may change the initial pore diameter and thus the availability of the acid sites. Both kinds of modifications are related [1,2]. While zeolite Y is referred to as cubic [1] and mordenite as orthorhombic [1,3,4], H-ZSM-5
324
Y Hong, J.J. Fripiat/Microporous Materials 4 (1995) 323--334
shows a reversible phase transition from monoclinic to orthorhombic symmetry at a temperature which is composition (Si/A1)-dependent [5-7]. For HZSM-5 with Si/AI=300 it occurs at 340K [5] while for Si/AI<80, the transition takes place below 300 K [7]. The monoclinic angle is 90.67 °. In the pore system of zeolite Y, the threedimensional channel system (7.8 A in diameter) links three kinds of cages, c~, fl and 6-ring prism [8]. Only the e-cage is accessible to N 2 and bigger molecules; it accounts for a pore volume of 0.35 ml/g in a perfectly crystallized zeolite Y [3]. It has been observed that there is about 20% less pore volume measured by n-hexane than that by N2 [9]. According to the Gurvitch rule [10], the volumes of the two condensed phases should be equal. The pore structure in mordenite is twodimensional. One of the channels is a 12-ring (7.0 x 6.5 A), while the other is a complex 8-ring (2.6 x 5.7 A and 3.7 x 4.8 A) [8]. The pore volume obtained for N2 adsorption is 0.21 ml/g while 0.11 ml/g only is available for pentane [3]. ZSM-5 has two kinds of channels (5.3x5.6, A, and 5.1 x 5.5 A) developed along three dimensions [8]. One straight channel goes along the (010) or b direction. The other channel zigzags between the a and c directions. The cavity at the intersection of the channels is about 9 A in diameter. The pore volumes observed from N2 and n-hexane are about 0.20 ml/g [6]. A perfect crystallized zeolite has micropores of uniform sizes. During the process of dealumination, supermicropores (7 20 A) and/or mesopores [11,12] can be produced depending on dealumination conditions. Meanwhile, the total micropore volume and the unit cell volume decrease as the dealumination degree increases [ 11 ]. The aim of this present work is to study how a dry calcination treatment (without the presence of water vapor) affects the porous properties of some commonly used hydrogen zeolites, such as H-Y, H-mordenite and H-ZSM-5. In particular, the correlation between the contraction of the unit cell measured from XRD and the decrease of the micropore volume measured by N 2 and pentane adsorption will be investigated.
2. Experimental 2.1. Materials DH Y samples Dealuminated hydrogen Y (DHY) zeolite samples were obtained by first exchanging a commercial sodium Y zeolite (PQ Corporation, CBV100) nine times with ammonium acetate, then calcining the dried ammonium Y sample as explained below. 1 M Ammonium acetate with a solid-to-liquid ratio of l g/15ml was used for cation exchange. In order to promote the degree of exchange, the sample was dried and calcined at 400°C for 2 h after the third and sixth exchange. After each exchange, the sample was washed with hot distilled water five times and dried at 120°C. In the final ammonium sample, about 98% of the sodium present in the parent sample had been exchanged. During the calcination process, the sample was first heated to 120°C and kept at the same temperature for 2 h in order to remove the adsorbed water. The temperature was then increased to 400°C at a rate of ~8°C/min and kept there for 2 h. This sample is DHY400, where the suffix 400 refers to the calcination temperature, as in later cases. Following the same procedure, DHY500, DHY600, DHY700 and DHY800 were obtained by calcining the DHY400 for 2 h at final temperatures of 500, 600, 700 and 800°C, respectively. D H M samples Dealuminated hydrogen mordenite (DHM) samples, DHM400, DHM500, DHM600, DHM700 and DHM600, were obtained by modifying a commercial product of Union Carbide, Linde Division (LZM-5, sodium form). The procedure for this preparation was described in our previous work [13,14], where these samples were designated as VG samples. D H Z samples Dealuminated hydrogen ZSM-5 (DHZ) samples were produced by calcining a parent H-ZSM-5 sample from PQ (CBV3020). DHZ400, DHZ500, DHZ600, DHZ700, DHZ800 and DHZ900 were obtained by calcining the CBV3020 sample at 400,
E Hong, J.J. Fripiat/Microporous Materials 4 (1995~ 323 334
500, 600, 700, 800 and 900°C, respectively, following the same temperature program as for the Y zeolites. The compositions of the zeolite samples were obtained by chemical analyses and 298i MAS NMR. The ratio of Si to framework aluminum (Si/FAI) and the ratio Si to total A1 were obtained. Assuming that all the Si atoms were in the framework position, from those ratios and the chemical composition, the absolute number of the framework aluminum (FA1) and non-framework aluminum (NFA1) were easily obtained.
2.2. X-Ray d(ffraction The X R D spectra of the hydrated samples were obtained by using a Scintag X-ray diffractometer: CuK~ radiation and a scan rate of 5 ° min at room temperature. A NBS standard silicon was used to calibrate the diffraction angle, and in all cases the 20 values of the silicon ( 111 ) were within _+0.01 ~ from the theoretical value (28.443°). In the case of DHY samples, the 20 positions of 111, 220, 31 l, 331, 511, 440, 533 and 642 reflections were used to calculate the unit cell parameters and the area of 533 was used to calculate the crystallinity with respect to the commercial sodium Y zeolite, CBV100, as described in Refs. [15] and [16]. For ZSM-5, the 011, 200, 012, 031, 051, 033, 053 reflections were used. For the mordenite samples, the 111, 130, 241, 002, 511 and 530 reflections were used as in Ref. [4].
2.3. Adsorption isothernls The adsorbates chosen for studying the microporous properties w e r e N 2 and pentane. N 2 is a widely used classical probe for surface area and pore size distribution measurement, while pentane is a good representative for light paraffins. All the samples were degassed under vacuum (better than 1.10 4 Torr) for 2 h at 573 K beforehand. The full sorption (adsorption and desorption) measurements of N2 and pentane were performed at 78 and 295 K, respectively, on an automated physisorption instrument (Omnisorp 100, Coulter Corporation}. The static mode was used in all
measurements which approximately 10 h.
325
were
completed
within
3. Results and discussion
The numbers of FAI, NFA1 and the ratio NFA1/A1Total, which is a good indication of the degree of dealumination, are listed in Table 1. We can see that the dealumination degree increases as the calcination temperature increases for all zeolite samples. The fraction of NFAI over the total number of aluminum atoms spreads from 13% to 79% for DHZ, 10% to 40% for DHY and 4% to 94% for the DHM.
3.1. XRD studies Fig. 1 shows the X R D spectra of the zeolite samples at different degrees of dealumination. From the intensities of the diffraction lines observed for the D H Z samples, no significant change in crystallinity is observed. For DHY samples, by using the ASTM method D3906-91 [16], we calculated that the crystallinity of DHY800 is 53%, while for the other samples it is 100%. For all D H M samples, the crystallinity is higher than 80%. The X R D data are summarized in Table 2. Basically, we observed that the F W H M for the first diffraction lines (111, 011, 111 for DHY, D H Z and DHM, respectively) increased slowly with the calcination temperature. It did not change after calcination at 800c'C for D H Z samples. For D H M , it broadened upon calcination at 7 0 0 C while the diffraction lines of D H Y were already broadened after calcination at 50OC. The broadening seemed to follow the trend of thermal stability, that is the sequence of the initial (Si/Al)ca ratios (see Table 1). D H Z samples should be the most stable, followed by the D H M . The unit cell parameters were calculated from the 20 positions of the diffraction lines mentioned before by using a simulation program built into the Scintag software. From the variations of the unit cell parameters, one can judge whether the crystalline system chosen (for example, cubic, orthorhombic or monoclinic) is right or not.
Y. Hong, Z.L Fripiat/Microporous Materials 4 (1995) 323-334
326
Table 1 Chemical composition Sample
Si/AIcA
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
15.1 15.1 15.1 15.1 15.1 15.1
DHY400 DHY500 DHY600 DHY700 DHY800 DHM400 DHM500 DHM600 DHM700 DHM800
2.55 2.55 2.55 2.55 2.55 5.2 5.2 5.2 5.2 5.2
Si/FA 1
17.5 17.8 18.6 37.1 52.7 76.3
2.95 3.29 4.56 4.81 4.93 5.5 9.8 10 15 111a
A1TotaI (102°/g) 6.2 6.2 6.2 6.2 6.2 6.2
28 28 28 28 28 16 16 16 16 16
FA 1 (102°/g) 5.4 5.3 5.1 2.6 1.9 1.3
25 23 18 17 17 15.4 9.3 9.1 6.3 0.90
NFA 1 (102°/g) 0.81 0.90 1.1 3.6 4.4 4.9
2.9 4.9 10 11 11 0.7 6.9 7.1 9.9 15
NFA 1/AITota1 (%) 13 15 18 58 71 79
10 18 36 39 40 4.4 43 44 62 94
Si/AlcA and Si/FAL are the Si/A1 ratios obtained by chemical analysis and 298i MAS NMR, respectively.AITo~j,FAI and NFA1 are the number of total aluminium, FAI and NFAI, respectively.NFA1/Al~otalis the fraction of NFA1. aA Si/FA1 ratio higher than 50 is very uncertain. It would correspond to a 4Q(Si-I-A1) contribution smaller than 3% of the total integrated intensity of the 298i signal.
Interestingly, the D H Z spectra were better simulated by using the orthorhombic system for the samples calcined at 700°C or lower, close to the data of Refs. [5] and [6], while the DHZ900 pattern was better simulated by the monoclinic system, fl being 90.62 °, very close to the data in Ref. [7] with a H Z S M - 5 with Si/A1 ,~300. The DHZ800 spectrum exhibited the larger discrepancy with respect to both monoclinic and orthorhombic parameters. This observation suggests that phase transition occurred around r o o m temperature for DHZ800 and at a lower temperature for DHZ900. As shown later on, sharp changes in the adsorption properties between D H Z 8 0 0 and DHZ900 support this suggestion. Because mordenite and ZSM-5 are orthorhombic, the unit cell parameters a, b, c, which correspond to the dimension of the three lattice axes, m a y change differently with the severity of calcination, in agreement with Brotas de Carvalho et al. [17], who also observed heterogenous changes in offretite, another orthorhombic zeolite. The unit cell volume will be used for discussion instead of each parameter in these zeolites.
The unit cell volume decreased when the calcination temperature increased, as shown in Fig. 2, where the unit cell volume (Vuc) is plotted versus the number of NFA1 per unit cell. Indeed, the number of NFAI increased as the calcination temperature increased, as shown in Table 1. Fig. 2 a - c is for D H Z , D H Y and D H M samples, respectively. The slopes dVuc/d(NFAl), which represent the unit cell volume decrease per FA1 lost or per NFA1 gained, will be discussed later along with the adsorption data.
3.2. Microporosity and mesoporosity The N 2 adsorption isotherms for D H Z , D H Y and D H M are shown in Fig. 3a-c, while the pentane adsorption isotherms are shown in Fig. 4a-c, respectively. The curves in Fig. 3a-c and Fig. 4a and b looked like Brunauer Type I or Langmuir isotherms. Those in Fig. 4c looked more like Brunauer Type II isotherms. We have used the Langmuir and as plots for analyzing the N2 data and the Langmuir plot for pentane adsorption. Using N 2 adsorption on
E Hong, J.J. Fripiat/Microporous Materials" 4 (1995 323 334 14
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Fig. 1. XRD spectra of DHZ, DHY and DHM samples (CuK~). The x-axis is the diffraction angle 20 °, and the unit for the ),-axis is 105 counts/ps. From bottom to top: (a) spectra of DHZ400, DHZ500, DHZ600, DHZ700, DHZ800 and DHZg00; (b) CBV100, DHY400, DHY500, DHY600, DHY700 and DHY800: (c) LZM-5, DHM500, DHM600, DHMT00 and DHM800.
DHZ900 as an example, Fig. 5a and b show how the microporous volumes are obtained from the intercepts of the linear parts of the Langmuir and ~ plots. In the Langmuir plot, (P/Po)/V is plotted versus P/Po and the inverse of the slope is the monolayer volume, Vm. In this example, its value is 0.147 ml/g. The standard isotherm of non-porous silica provided by Sing and co-workers [18,19] was taken as reference in the ~ plot. The adsorbed volume is plotted versus cq. ~+ is the ratio of volume adsorbed by the sample to that adsorbed by the
327
reference at the same relative pressure (~s = 1 at P/Po = 0.4). Among the different techniques which have been suggested for measuring the microporous volume from a N 2 adsorption isotherm, the so-called t and ~s method have been competing successfully with Dubinin method [18]. In particular, on the nonporous and mesoporous silicas the specific surface areas obtained from the BET or ~+ algorithm have quasi-identical values. On microporous silicas, differences in the order of 10 20% have been reported by Bhambhani et al. [19]. In zeolites where the pore sizes are accurately known, the question posed by Bhambhani et al. [19] concerning the critical value of the pore width which "marks the boundary between filling and monolayer-multilayer adsorption" is of course of particular interest. Brotas de Carvalho et al. [17] have studied the influence of hydrothermal treatment of offretite on its pore structure and shown that the ~ plot (extrapolation of the high relative pressure linear portion) gave pore volumes in very good agreement with those obtained by applying the Dubinin equation. They also showed that the volume of adsorbed n-Co, estimated through the Dubinin method, was less than 60% the volume of N2 adsorbed and that this ratio decreased with increasing dealumination. Offretite has, as mordenite, 8-ring channels which are probably not available to n-C6 or n-Cs. As shown in Table 3 for DHZ, as well as for DHY and DHM, the N 2 micropore volume obtained from the ~s plot represents about 90 _+7% of the Langmuir monolayer volume. Thus, 12-ring pores 6.5 x 7 A (mordenite), 10-ring pores 5.3 × 5.6 A, (ZSM-5) and 12-ring pores 7.4 x 7.4A ( Y ) as well as the 12-ring channel (6.7 x 6.7 A) in the offretite are within the boundary where both the Langmuir and ~s algorithms apply. In addition, and as shown in Table 3, the ratio :q/Vm seems independent of the degree of dealumination, as long as there is no phase change (DHZ900) or collapse of the crystallinity (DHY800). Indeed, in spite of the fact that high degrees of dealumination have been reached in the present work, the integral volume occupied by the NFAI debris generally does not exceed 10% of the :~ volume.
Y. Hong, Z Z Fripiat/Microporous Materials" 4 (1995) 323-334
328
Table 2 XRD structural data Sample
FWHM 20 C)
a
b
c
Vuc
Crystallinity ~
(A)
(A)
(A)
(A 3)
(%)
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
0.12 0.12 0.12 0.12 0.12 0.18
20.10 20.06 20.10 20.04 19.84 19.83
20.10 20.08 20.12 20.07 20.00 19.95
13.46 13.42 13.44 13.40 13.34 13.39
5432 5407 5437 5390 5288 5264
!00 100 100 100 100 100
DHY400 DHY500 DHY600 DHY700 DHY800 DHM400 DHM500 DHM600 DHM700 DHM800
0.12 0.15 0.15 0.15 0.21 0.15 0.15 0.15 0.18 0.12
24.58 24.54 24.40 24.37 24.29 18.08 18.06 18.05 18.04 17.94
24.58
24.58
14856 14772 14534 14467 14338 2783 2764 2725 2718 2682
100 100 100 100 53 92 91 86 82 82
20.50 20.42 20.30 20.27 20.18
7.51 7.49 7.44 7.43 7.41
Full width at half m a x i m u m ( F W H M ) is for the 111, 011 and 111 reflection for D H Y , D H Z and D H M , respectively, a, b and c are the unit cell parameters. Vuc is the unit cell volume and crystallinity is calculated as indicated in the text. aThe starting materials, namely CBV3020, CBV100 and LZM-5, were taken as standards, respectively: 100% means that the sum
of intensities of selected diffractions for that sample is higher than or equal to that of the standard. Brotas de Carvalho et al. [17], who did not observe N 2 adsorption isotherms on offretite with such an accentuated Langmuir character as those shown in Fig. 3, have suggested that the 5 s plot can be divided into two parts, in order to account for the fact that the volume of 5s obtained at very low vapor pressure, instead of going to zero, intercepts the V axis at 52. They considered 52 as representing the volume of the ultramicropores. 52 and cq are shown in Table 3. Note that 51 would correspond to the classical 5~ value. F r o m our view point, the empirical character of the division of the 5~ plot into two portions does not allow one to speculate on the comparison of 52 and 51. The only notable point is that (51-52)/cq is large for D H Z and much smaller for the other zeolites. A favorable example of 51 and 52 determination is shown in Fig. 5. In Fig. 5b, the total micropore volume is 0.120 ml/g. This value is 20% lower than that obtained from the Langmuir equation (Fig. 5a). The other linear region observed in the c~ plot is at a much lower P/Po(~O.02), where N2 already fills the ultramicropores (diameter less than 0.7 nm) [17] and 52 is 0.019 ml/g. Thus, the ratio
52/51,
which is 16%, would be the fraction of the micropore volume in the narrowest or ultramicropores (less than 0.7 nm). However, the two regions, 51 and 52, are easier to distinguish for the samples calcined at higher temperature. The microporous volume for all the zeolite samples calculated by these different methods are summarized in Table 3. As shown in Table 3, the fractions of the secondary micropore, or ( 5 1 - 52 )/51, calculated from 5~ plot, increase with the fraction NFAI/AITotal for the D H M and D H Y samples (Table 1). These fractions are the highest in the ZSM-5 sample but they remain almost constant, except for DHZ900. The mesopore size distribution of D H Z samples, calculated according to Ref. [20], is shown in Fig. 6. It was obtained from the N 2 desorption isotherm, showing the appearance of mesopores with a diameter around 7.5 nm when the sample was calcined at 900°C.
3.3. Pentane adsorption Pentane adsorption on the zeolites is treated the same way a s N 2 adsorption. However, since
329
Y. Hong, J.,L Fripiat/Mieroporous Materials 4 ( 1995~ 323 334 5.45
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no "standard" isotherm is available, only the Langmuir plot is used. The results are shown in Fig. 7 and the data are listed in Table 4. The ratios of the micropore volume measured by pentane to that obtained with N 2 a r e 92% and 84% for D H Z and DHY, respectively, in agreement with the data reported for Y zeolite [9] and ZSM-5 [6]. Surprisingly, as mentioned earlier [13], the ratio for mordenite is low (31%). We have to keep in mind that for a perfectly crystallized H-mordenite, a total pore volume of 0.21 ml/g was accessible to N 2 while about only 0.11 ml/g is accessible for linear paraffin, in which case the ratio should be 55% [3]. Our experimental value is significantly
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Fig. 3. N2 adsorption isotherms at 78 K on DHZ (a), DHY (b) and D H M (c} samples. The amounts adsorbed, expressed as the volume of condensed phase (specific weight= 0.808 g/roll, V, is plotted versus the relative pressure, P./Po.
lower and this is not due to the dealumination, since the ratio of the extrapolated I~Mioro(Cs) and ~icro (N2) were obtained when the NFAI was zero. The failure of the Gurvitch rule is very pronounced for H-mordenite, while it is almost insignificant for the H-Y and H-ZSM-5 zeolites.
3.4. Relationship between unit cell volume and pore volume from adsorption studies How does the dealumination process affect the micropore volume? In order to answer this question, we plotted the N 2 micropore volumes calcu-
Y. Hong, J.J. Fripiat/Microporous Materials 4 (1995) 323-334
330
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Fig. 5. Examples of a Langmuir plot (a) and an ~, plot (b) for N2 adsorption on DHZ900. (a) (P/Po)/V, where V is the volume adsorbed (ml/g), is plotted versus relative pressure (P/Po); (b) the adsorbed volume, expressed as the volume of the condensed phase, is plotted versus as, or the ratio of adsorbed volume on the sample to that on the reference sample, (V/Vr)s. Table 3 Results of the microporous volume measurements with the ~s and Langmuir methods
0.12 0.1 0.08 E ~" 0.06
0.04 0.02 0
012
024
016
018
Fig. 4. Pentane adsorption isotherms at 295 K on DHZ (a), DHY (b) and DHM (c) samples. The amount adsorbed, expressed as the volume of the condensed phase (specific weight=0.626g/ml), is plotted versus the relative vapor pressure, P/Po.
l a t e d f r o m the ~s a n d L a n g m u i r p l o t s v e r s u s the n u m b e r o f N F A 1 , as s h o w n in Fig. 8 a - c f o r D H Z , D H Y a n d D H M s a m p l e s , r e s p e c t i v e l y . I f the d a t a points for DHZ900 (phase change) and DHY800 ( s h a r p d e c l i n e in c r y s t a l l i n i t y , T a b l e 2) a r e n o t considered, good linear regressions are obtained ( F i g s . 7 a n d 8). T h e d e r i v a t i v e s dVMicro/d(NFA1) o b t a i n e d f r o m L a n g m u i r a n d c% p l o t s a r e listed in T a b l e 5 a n d will be d i s c u s s e d l a t e r on. T h e i n t e r c e p t s are the m i c r o p o r e v o l u m e o f the s a m p l e w i t h o u t a n y d e a l u m i n a t i o n , PMi.... j u s t as the i n t e r c e p t s o f the r e g r e s s i o n s in Fig. 2 a - c a r e t h e u n i t cell v o l u m e s ( ~ c ) in the a b s e n c e o f N F A 1 .
Sample
a1 (ml/g)
c~2 (ml/g)
(cq a2)/cq (%)
Langm_uir ~l/Vm (ml/g) (%)
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900 DHY400 DHY500 DHY600 DHYT00 DHY800 DHM400 DHM500 DHM600 DHM700 DHMS00
0.163 0.162 0.158 0.155 0.147 0.120 0.307 0.273 0.263 0.255 0.184 0.148 0.147 0.145 0.141 0.134
0.093 0.097 0.096 0.108 0.096 0.019 0.291 0.259 0.225 0.194 0.122 0.137 0.135 0.131 0.126 0.116
44 40 39 30 35 84 4.6 5.1 14 24 34 7.1 7.5 9.6 11 13
0.193 0.191 0.189 0.183 0.176 0.147 0.310 0.310 0.278 0.273 0.190 0.173 0.165 0.170 0.169 0.155
84 95 84 85 84 82 99 88 95 93 97 86 89 88 83 86
~1 is the volume of the pores with diameter less than 20 ,~., a2 is the volume of the pores with diameter less than 7 ,~,., ajVm is the ratio of the micropore volume from the a s plot to the monolayer volume from Langmuir plot. L e t Vuc be the v o l u m e o f the u n i t cell, as o b t a i n e d f r o m X R D . T h e v a r i a t i o n o f Vuc w i t h r e s p e c t to the n u m b e r o f the N F A 1 , d V u c / d N F A 1 is o b t a i n e d f r o m Fig. 2. T h e s l o p e b e i n g n e g a t i v e m e a n s t h a t Vuc d e c r e a s e s w h e n the n u m b e r o f
331
K Hong, J.J. Fripiat/Mieroporous Materials" 4 ( 1995J 323 334
0.02 DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
E t.- 0.015 E
O.Ol
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0.005 t~,
.,
t
x
-.
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x
/ x /
0
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L
I
I
I
I
4
6
8
10
12
14
Pore Diameter(nm) Fig. 6. Mesopore distribution for DHZ samples. NFA1 increases. Thus, each zeolite is characterized by one value of dVMicro/d(NFA1) and one value of d Vuc/d (NFA1). Is there a relation between both derivatives? Assuming that the unit cell contraction caused by calcination is an isometric process, the decrease in volume which results from the translocation of aluminum from the framework to the nonframework positions homogeneously affects each region of the unit cell, e.g. the actual lattice and the pore region. From the X R D data, the predicted decrease of the micropore volume caused by lattice contraction should be:
volume, and the total effect of lattice contraction ( 1 ) and obstruction by NFA1 will be:
d VMicro/d(NFA1)pred.
= ~icro/~Jc x [dVuc/d(NFAl)+ 14.6 ml/mol] (3)
= ~ic~o/~Jc x dVuc/d(NFA1)
1)
where PMicro/PUCis the ratio of the initial N2 pore volume, extrapolated from Fig. 8, to the volume of the unit cell before dealumination. The experimental dVuc/d(NFA1) and correlated dgMicro/ d(NFA1) values are shown in Table 5. Let us consider the two following situations for the translocation of the aluminum from framework to non-framework position. (i) All NFA1 species are translocated into the micropores available to N2. Since the specific weight of amorphous alumina is about 3.5 g/ml, the molar volume for dislodged aluminum species (A101.5) should be 14.6 ml/atom-g A1. Thus, the NFAI species will further decrease the micropore
d Vgicro/d(NFAI)High = PMicro/l,~vc×dVuc/d(NFA1)+14.6ml/mol
(2)
(ii) the NFAI will translocate into large micropores as well as small pores which may not be accessible to N2, so only a fraction of the NFA1 will be clogging the micropore volume. In this case the decrease in micropore volume caused per NFA1 will be: d VMicro/d(NFA1)Low
The actual situation should be between Eqs. 2 and 3 which represent the two boundaries (extreme cases). The predicted value (dVMicro/dNFAl) nigh from Eq. 2 should be the high boundary, since we put all the NFA1 in the space which is available to N 2. The value (d VMi~ro/dNFA1)Low from Eq. 3 will be the low boundary, since it spreads the NFA1 all over the unit cell, including the space not available to N z (such as the/~ cage in Y zeolites for instance, or the network of the small 8-ring complex in mordenite). In Fig. 9, the predicted values of the d V~aicro/d(NFA1) are represented for the "high" (from Eq. 2) and "low" (from Eq. 3) boundaries. The high limit is very close to the experimental micropore volume variations obtained for D H Z
332
Y. Hong, J.Z Fripiat/Microporous Materials 4 (1995) 323-334
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Fig. 7. Relationship between the micropore volume (ml/g) measured by pentane adsorption from the Langmuir equation and number of NFAI atoms (x 102°/g) for (a) DHZ, (b) DHY and (c) DHM samples, respectively. The open symbol data points are removed from the regression (see text).
-,........, . #
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Fig. 8. Relationship between the micropore volume measured by Nz adsorption and number of NFAI atoms (× 102°/g); solid circle for Langmuir, solid lozenge for ~, plot. The open symbol data points are removed from the regression (see text).
Table 4 Comparison between the N 2 and pentane adsorption data Sample
DHZ DHY DHM
o~2 (N2)
Langmuir (N2)
Langmuir
Langmuir (pentane)
Slope (10-23 ml/NFA1)
VOm (ml/g)
Slope (10 23 ml/NFA1)
V~ (ml/g)
Slope (10-23 ml/NFA1 )
VOm (ml/g)
(%)
3.7 4.1 1.2
0.194 0.311 0.180
3.5 5.0 1.3
0.165 0.312 0.155
3.0 2.9 1.3
0.178 0.262 0.054
92 84 20
The slope is d VN2/d(NFA 1) or d VcJd(NFA1 ). VOmand V~ are the pore volume extrapolated to the state when dealumination has not occurred from Langmuir and cq plots. The ratio /~c~/l~N2is a check of the Gurvitch rule; it should be 100% if the Gurvitch rule was obeyed.
E Hong, J.J. Fripiat/'Microporous Materials 4 (1995~ 323 334
333
Table 5 Relationship between unit cell volume and the pore volume contraction Sample
DHZ DHY DHM
1k4~,o, I~c ('! i,~
32 41 28
d Vuc/d(NFAI ) (ml/mol)
19.4 14.6 8.9
d V~licro,d ( NFA1 )Low ( Eq. 3 ) (ml/mo)
10.9 12.0 6.6
d 1"Micro/d( NFAI )High ( Eq. 2 ) (ml/mol)
21.9 21.5 17.1
d I M~,o/dNFAI ( Experimental ) Langmuir
:~
22.3 24.7 7.2
21.1 30.1 7.8
lA~i,.,o/l"~c is the ratio of pore volume accessible for N 2 to the unit cell volume, both obtained by extrapolating from the state when dealumination has not occurred, d Vuc/d(NFAI) and d FMio,o/d NFA1) are the variations of the unit cell volume and pore volume caused by lhe translocation of one FAI--*NFAI. ml/mol means ml per mole of AIO~ ~. 35 30
~'25 }20 0
P~15
_J DHZ
DHY
DHM
I I LOW • Langmuir [3 alpha [] High ] Fig. 9. Relationship between the micropore volume decrease per NFA1 by N 2 adsorption and the unit cell contraction predicted from Eq. 2 (high) and 3 (low). The units are ml/mol AIO~ 5.
and DHY samples. This match-up means that, during the dealumination process, the micropore shrinks proportionally to the contraction of the unit cell, and NFAI atoms take a fraction of the space available to N2. In the case of D H M , Fig. 9 suggests that the NFAI species are clogging the entire porous system.
4. Conclusions Dealuminated H-ZSM-5, H-Y and H-mordenite samples were obtained by calcining the corresponding H-zeolites at increasing temperatures. Upon increasing the calcination temperature, the number of NFA1 species increases while the total number of aluminum atoms stays the same. The F W H M of the first diffraction (111, 011 and 111 for DHY, D H Z and D H M , respectively) increases slowly with the calcination temperature.
The temperatures where it broadens substantially are 900, 700 and 500"~'C for the DHZ, D H M and DHY sample, respectively, in agreement with the thermal stability of these lattices. The three zeolites studied here exhibit different behaviors upon calcination. From the relationship between the unit cell shrinkage and the micropore volume decrease, it is suggested that the NFAI goes to the pores which are available to N 2 in the cases of D H Y and DHZ, while it spreads into the entire DH M lattice. Irrespective of the degree of dealumination, the Gurvitch rule (volume of adsorbed N z = v o l u m e of adsorbed n-Cs) fails to apply to DHM, but it presents a reasonable agreement for the H-ZSM-5 and HY samples that were studied. This observation demonstrates clearly that one set of channels is available to N2 and not to n-pentane in H-mordenite.
Acknowledgements The financial support through DOE Grant DT-FG02-90 ERI430 is gratefully acknowledged. We thank Dr. V. Gruver for his mordenite samples and useful discussions, Dr. A. Blumenf'eld and D. Coster for the N M R data and Dr. S. Hardcastle of the Advanced Analytical Facilities ( A A F ) of" UWM for his help in X R D measurements.
References [1] R. Szostak, in H. Van Bekkum, E.M. Flanigen and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice. Elsevier, New York, 1991, p. 153.
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Y. Hong, ,LJ. Fripiat/Microporous Materials 4 (1995) 323-334
[2] L. Kubelkova, S. Beran, A. Malecka and V.M. Mastikhin, Zeolites, 9 (1989) 12. [3] D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Robert E. Krieger Publishing, FL, 1984. [4] V.G. Chumbhale, A.J. Chandwadkar and B.S. Rao, Zeolites, 12 (1992) 63. [5] D.G. Hay and H. Jaeger, J. Chem. Soc., Chem. Commun., (1984) 1433. [6] D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J. Phys. Chem., 85 (1981) 2238. [7] (a) H. van Koningsveld, J.C. Jansen and H. van Bekkum, Zeolites, 13 (1990) 235; (b) E. de Vos Burchart, H. van Bekkum and B. Van de Graaf, Zeolites, 10 (1993) 212. [8] W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemann, London, 1992. [9] J. Pires, M. Brotas de Caralho, F. Ramoa Ribeiro and E.G. Derouane, Zeolites, 11 (1991) 345. [10] L. Gurvitch, J. Phys. Chem. Soc. Russ., 47 (1915) 805.
[11] A. Zukal, V. Patzelova and U. Lohse, Zeolites, 6 (1986) 133. [12] C. Fernandez, J.C. Vedrine, J. Grosmangin and G. Szabo, Zeolites, 6 (1986) 484. [13] Y. Hong, V. Gruver and J.J. Fripiat, J. Catal., 150 (1994) 421. [ 14] V. Gruver and J.J. Fripiat, J. Phys. Chem., 98 (1994) 8544. [15] Annual Book of ASTM Standards, D 3942-91, 15.03 (1991) 612. [16] Annual Book of ASTM Standards, D 3906-91, 15.03 (1991) 597. [17] M. Brotas de Carvalho, A.P. Carvalho, F. Ramoa Ribeiro, A. Florentino, N.S. Gnap and M. Guisnet, Zeolites, 14 (1994) 217. [18] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 2nd ed., 1982. [19] M.R. Bhambhani, P.A. Cutting, K.S.W. Sing and D.H. Turk, J. Coll. Interf. Sci., 38 (1973) 109. [20] E.P. Barret, L.G. Joyner and P.H. Halenda, J. Am. Chem. Soc., 73 (1951) 373.
Microporous Materials 4 (1995) 323 334
Microporous characteristics of H-Y, H-ZSM-5 and H-mordenite dealuminated by calcination Y. Hong, J.J. Fripiat * Department of Chemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, W153201. USA Received 3 November 1994; accepted 3 December 1994
Abstract
The modifications of the microporous volume of zeolites resulting from thermal dealumination have been examined by measuring the changes in the unit cell parameters as well as the N 2 and n-pentane adsorption isotherms. For the cubic HY and orthorhombic H-ZSM-5 zeolites, the differential decrease of the microporous volume per nonframework aluminum (NFA1) can be predicted by assuming NFA1 is concentrated in pores available to N2. For the orthorhombic H-mordenite, NFA1 is homogeneously spread in all pores, available or not to N2. The Gurvitch rule (volume of adsorbed N 2 =volume of adsorbed pentane) is respected in HY and H-ZSM-5, but not in H-mordenite, irrespective of the degree of dealumination.
Keywords: Zeolite Y; ZSM-5; Mordenite; Micropore; Dealumination
1. Introduction
Thermal treatment is one of the major methods for dislodging aluminum from framework (FAI) to non-framework positions (NFA1) in zeolites [ 1]. Some dealumination procedures, such as those using EDTA, SIC14, (NH4)2SiF 6 or acid leaching, remove various amounts of aluminum from the solid, consequently the Si/A1 ratios in the bulk and in framework increase [1,2]. However, in the thermal or hydrothermal treatment of zeolites, the non-framework aluminum species stay in the zeolite channels. The Si/A1 ratio obtained by chemical analysis remains the same while the framework Si/FA1 ratio increases [1,2]. Accordingly, the unit cell contracts as observed in the X-ray diffraction *Corresponding author. 0927-6513/95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 6 5 1 3 ( 9 5 ) 0 0 0 3 8 - 0
( X R D ) diagram. The Si/FAI ratio is obtained from t h e 298i magic-angle spinning nuclear magnetic resonance (MAS N M R ) spectra [1,2]. Hydrothermal treatment (deep- or shallow-bed calcination in the presence or absence of water vapor) involves aluminum extraction and dehydroxylation [1,2]. It may result in two major modifications to the zeolite catalysts: ( 1 ) structure modifications in which the framework Si/FAI is changed, resulting in a change in the number and eventually the strength of the Br6nsted acid sites as well as in the number and strength of the Lewis acid sites present on the NFA1 species; (2) porosity modifications which may change the initial pore diameter and thus the availability of the acid sites. Both kinds of modifications are related [1,2]. While zeolite Y is referred to as cubic [1] and mordenite as orthorhombic [1,3,4], H-ZSM-5
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Y Hong, J.J. Fripiat/Microporous Materials 4 (1995) 323--334
shows a reversible phase transition from monoclinic to orthorhombic symmetry at a temperature which is composition (Si/A1)-dependent [5-7]. For HZSM-5 with Si/AI=300 it occurs at 340K [5] while for Si/AI<80, the transition takes place below 300 K [7]. The monoclinic angle is 90.67 °. In the pore system of zeolite Y, the threedimensional channel system (7.8 A in diameter) links three kinds of cages, c~, fl and 6-ring prism [8]. Only the e-cage is accessible to N 2 and bigger molecules; it accounts for a pore volume of 0.35 ml/g in a perfectly crystallized zeolite Y [3]. It has been observed that there is about 20% less pore volume measured by n-hexane than that by N2 [9]. According to the Gurvitch rule [10], the volumes of the two condensed phases should be equal. The pore structure in mordenite is twodimensional. One of the channels is a 12-ring (7.0 x 6.5 A), while the other is a complex 8-ring (2.6 x 5.7 A and 3.7 x 4.8 A) [8]. The pore volume obtained for N2 adsorption is 0.21 ml/g while 0.11 ml/g only is available for pentane [3]. ZSM-5 has two kinds of channels (5.3x5.6, A, and 5.1 x 5.5 A) developed along three dimensions [8]. One straight channel goes along the (010) or b direction. The other channel zigzags between the a and c directions. The cavity at the intersection of the channels is about 9 A in diameter. The pore volumes observed from N2 and n-hexane are about 0.20 ml/g [6]. A perfect crystallized zeolite has micropores of uniform sizes. During the process of dealumination, supermicropores (7 20 A) and/or mesopores [11,12] can be produced depending on dealumination conditions. Meanwhile, the total micropore volume and the unit cell volume decrease as the dealumination degree increases [ 11 ]. The aim of this present work is to study how a dry calcination treatment (without the presence of water vapor) affects the porous properties of some commonly used hydrogen zeolites, such as H-Y, H-mordenite and H-ZSM-5. In particular, the correlation between the contraction of the unit cell measured from XRD and the decrease of the micropore volume measured by N 2 and pentane adsorption will be investigated.
2. Experimental 2.1. Materials DH Y samples Dealuminated hydrogen Y (DHY) zeolite samples were obtained by first exchanging a commercial sodium Y zeolite (PQ Corporation, CBV100) nine times with ammonium acetate, then calcining the dried ammonium Y sample as explained below. 1 M Ammonium acetate with a solid-to-liquid ratio of l g/15ml was used for cation exchange. In order to promote the degree of exchange, the sample was dried and calcined at 400°C for 2 h after the third and sixth exchange. After each exchange, the sample was washed with hot distilled water five times and dried at 120°C. In the final ammonium sample, about 98% of the sodium present in the parent sample had been exchanged. During the calcination process, the sample was first heated to 120°C and kept at the same temperature for 2 h in order to remove the adsorbed water. The temperature was then increased to 400°C at a rate of ~8°C/min and kept there for 2 h. This sample is DHY400, where the suffix 400 refers to the calcination temperature, as in later cases. Following the same procedure, DHY500, DHY600, DHY700 and DHY800 were obtained by calcining the DHY400 for 2 h at final temperatures of 500, 600, 700 and 800°C, respectively. D H M samples Dealuminated hydrogen mordenite (DHM) samples, DHM400, DHM500, DHM600, DHM700 and DHM600, were obtained by modifying a commercial product of Union Carbide, Linde Division (LZM-5, sodium form). The procedure for this preparation was described in our previous work [13,14], where these samples were designated as VG samples. D H Z samples Dealuminated hydrogen ZSM-5 (DHZ) samples were produced by calcining a parent H-ZSM-5 sample from PQ (CBV3020). DHZ400, DHZ500, DHZ600, DHZ700, DHZ800 and DHZ900 were obtained by calcining the CBV3020 sample at 400,
E Hong, J.J. Fripiat/Microporous Materials 4 (1995~ 323 334
500, 600, 700, 800 and 900°C, respectively, following the same temperature program as for the Y zeolites. The compositions of the zeolite samples were obtained by chemical analyses and 298i MAS NMR. The ratio of Si to framework aluminum (Si/FAI) and the ratio Si to total A1 were obtained. Assuming that all the Si atoms were in the framework position, from those ratios and the chemical composition, the absolute number of the framework aluminum (FA1) and non-framework aluminum (NFA1) were easily obtained.
2.2. X-Ray d(ffraction The X R D spectra of the hydrated samples were obtained by using a Scintag X-ray diffractometer: CuK~ radiation and a scan rate of 5 ° min at room temperature. A NBS standard silicon was used to calibrate the diffraction angle, and in all cases the 20 values of the silicon ( 111 ) were within _+0.01 ~ from the theoretical value (28.443°). In the case of DHY samples, the 20 positions of 111, 220, 31 l, 331, 511, 440, 533 and 642 reflections were used to calculate the unit cell parameters and the area of 533 was used to calculate the crystallinity with respect to the commercial sodium Y zeolite, CBV100, as described in Refs. [15] and [16]. For ZSM-5, the 011, 200, 012, 031, 051, 033, 053 reflections were used. For the mordenite samples, the 111, 130, 241, 002, 511 and 530 reflections were used as in Ref. [4].
2.3. Adsorption isothernls The adsorbates chosen for studying the microporous properties w e r e N 2 and pentane. N 2 is a widely used classical probe for surface area and pore size distribution measurement, while pentane is a good representative for light paraffins. All the samples were degassed under vacuum (better than 1.10 4 Torr) for 2 h at 573 K beforehand. The full sorption (adsorption and desorption) measurements of N2 and pentane were performed at 78 and 295 K, respectively, on an automated physisorption instrument (Omnisorp 100, Coulter Corporation}. The static mode was used in all
measurements which approximately 10 h.
325
were
completed
within
3. Results and discussion
The numbers of FAI, NFA1 and the ratio NFA1/A1Total, which is a good indication of the degree of dealumination, are listed in Table 1. We can see that the dealumination degree increases as the calcination temperature increases for all zeolite samples. The fraction of NFAI over the total number of aluminum atoms spreads from 13% to 79% for DHZ, 10% to 40% for DHY and 4% to 94% for the DHM.
3.1. XRD studies Fig. 1 shows the X R D spectra of the zeolite samples at different degrees of dealumination. From the intensities of the diffraction lines observed for the D H Z samples, no significant change in crystallinity is observed. For DHY samples, by using the ASTM method D3906-91 [16], we calculated that the crystallinity of DHY800 is 53%, while for the other samples it is 100%. For all D H M samples, the crystallinity is higher than 80%. The X R D data are summarized in Table 2. Basically, we observed that the F W H M for the first diffraction lines (111, 011, 111 for DHY, D H Z and DHM, respectively) increased slowly with the calcination temperature. It did not change after calcination at 800c'C for D H Z samples. For D H M , it broadened upon calcination at 7 0 0 C while the diffraction lines of D H Y were already broadened after calcination at 50OC. The broadening seemed to follow the trend of thermal stability, that is the sequence of the initial (Si/Al)ca ratios (see Table 1). D H Z samples should be the most stable, followed by the D H M . The unit cell parameters were calculated from the 20 positions of the diffraction lines mentioned before by using a simulation program built into the Scintag software. From the variations of the unit cell parameters, one can judge whether the crystalline system chosen (for example, cubic, orthorhombic or monoclinic) is right or not.
Y. Hong, Z.L Fripiat/Microporous Materials 4 (1995) 323-334
326
Table 1 Chemical composition Sample
Si/AIcA
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
15.1 15.1 15.1 15.1 15.1 15.1
DHY400 DHY500 DHY600 DHY700 DHY800 DHM400 DHM500 DHM600 DHM700 DHM800
2.55 2.55 2.55 2.55 2.55 5.2 5.2 5.2 5.2 5.2
Si/FA 1
17.5 17.8 18.6 37.1 52.7 76.3
2.95 3.29 4.56 4.81 4.93 5.5 9.8 10 15 111a
A1TotaI (102°/g) 6.2 6.2 6.2 6.2 6.2 6.2
28 28 28 28 28 16 16 16 16 16
FA 1 (102°/g) 5.4 5.3 5.1 2.6 1.9 1.3
25 23 18 17 17 15.4 9.3 9.1 6.3 0.90
NFA 1 (102°/g) 0.81 0.90 1.1 3.6 4.4 4.9
2.9 4.9 10 11 11 0.7 6.9 7.1 9.9 15
NFA 1/AITota1 (%) 13 15 18 58 71 79
10 18 36 39 40 4.4 43 44 62 94
Si/AlcA and Si/FAL are the Si/A1 ratios obtained by chemical analysis and 298i MAS NMR, respectively.AITo~j,FAI and NFA1 are the number of total aluminium, FAI and NFAI, respectively.NFA1/Al~otalis the fraction of NFA1. aA Si/FA1 ratio higher than 50 is very uncertain. It would correspond to a 4Q(Si-I-A1) contribution smaller than 3% of the total integrated intensity of the 298i signal.
Interestingly, the D H Z spectra were better simulated by using the orthorhombic system for the samples calcined at 700°C or lower, close to the data of Refs. [5] and [6], while the DHZ900 pattern was better simulated by the monoclinic system, fl being 90.62 °, very close to the data in Ref. [7] with a H Z S M - 5 with Si/A1 ,~300. The DHZ800 spectrum exhibited the larger discrepancy with respect to both monoclinic and orthorhombic parameters. This observation suggests that phase transition occurred around r o o m temperature for DHZ800 and at a lower temperature for DHZ900. As shown later on, sharp changes in the adsorption properties between D H Z 8 0 0 and DHZ900 support this suggestion. Because mordenite and ZSM-5 are orthorhombic, the unit cell parameters a, b, c, which correspond to the dimension of the three lattice axes, m a y change differently with the severity of calcination, in agreement with Brotas de Carvalho et al. [17], who also observed heterogenous changes in offretite, another orthorhombic zeolite. The unit cell volume will be used for discussion instead of each parameter in these zeolites.
The unit cell volume decreased when the calcination temperature increased, as shown in Fig. 2, where the unit cell volume (Vuc) is plotted versus the number of NFA1 per unit cell. Indeed, the number of NFAI increased as the calcination temperature increased, as shown in Table 1. Fig. 2 a - c is for D H Z , D H Y and D H M samples, respectively. The slopes dVuc/d(NFAl), which represent the unit cell volume decrease per FA1 lost or per NFA1 gained, will be discussed later along with the adsorption data.
3.2. Microporosity and mesoporosity The N 2 adsorption isotherms for D H Z , D H Y and D H M are shown in Fig. 3a-c, while the pentane adsorption isotherms are shown in Fig. 4a-c, respectively. The curves in Fig. 3a-c and Fig. 4a and b looked like Brunauer Type I or Langmuir isotherms. Those in Fig. 4c looked more like Brunauer Type II isotherms. We have used the Langmuir and as plots for analyzing the N2 data and the Langmuir plot for pentane adsorption. Using N 2 adsorption on
E Hong, J.J. Fripiat/Microporous Materials" 4 (1995 323 334 14
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40
Fig. 1. XRD spectra of DHZ, DHY and DHM samples (CuK~). The x-axis is the diffraction angle 20 °, and the unit for the ),-axis is 105 counts/ps. From bottom to top: (a) spectra of DHZ400, DHZ500, DHZ600, DHZ700, DHZ800 and DHZg00; (b) CBV100, DHY400, DHY500, DHY600, DHY700 and DHY800: (c) LZM-5, DHM500, DHM600, DHMT00 and DHM800.
DHZ900 as an example, Fig. 5a and b show how the microporous volumes are obtained from the intercepts of the linear parts of the Langmuir and ~ plots. In the Langmuir plot, (P/Po)/V is plotted versus P/Po and the inverse of the slope is the monolayer volume, Vm. In this example, its value is 0.147 ml/g. The standard isotherm of non-porous silica provided by Sing and co-workers [18,19] was taken as reference in the ~ plot. The adsorbed volume is plotted versus cq. ~+ is the ratio of volume adsorbed by the sample to that adsorbed by the
327
reference at the same relative pressure (~s = 1 at P/Po = 0.4). Among the different techniques which have been suggested for measuring the microporous volume from a N 2 adsorption isotherm, the so-called t and ~s method have been competing successfully with Dubinin method [18]. In particular, on the nonporous and mesoporous silicas the specific surface areas obtained from the BET or ~+ algorithm have quasi-identical values. On microporous silicas, differences in the order of 10 20% have been reported by Bhambhani et al. [19]. In zeolites where the pore sizes are accurately known, the question posed by Bhambhani et al. [19] concerning the critical value of the pore width which "marks the boundary between filling and monolayer-multilayer adsorption" is of course of particular interest. Brotas de Carvalho et al. [17] have studied the influence of hydrothermal treatment of offretite on its pore structure and shown that the ~ plot (extrapolation of the high relative pressure linear portion) gave pore volumes in very good agreement with those obtained by applying the Dubinin equation. They also showed that the volume of adsorbed n-Co, estimated through the Dubinin method, was less than 60% the volume of N2 adsorbed and that this ratio decreased with increasing dealumination. Offretite has, as mordenite, 8-ring channels which are probably not available to n-C6 or n-Cs. As shown in Table 3 for DHZ, as well as for DHY and DHM, the N 2 micropore volume obtained from the ~s plot represents about 90 _+7% of the Langmuir monolayer volume. Thus, 12-ring pores 6.5 x 7 A (mordenite), 10-ring pores 5.3 × 5.6 A, (ZSM-5) and 12-ring pores 7.4 x 7.4A ( Y ) as well as the 12-ring channel (6.7 x 6.7 A) in the offretite are within the boundary where both the Langmuir and ~s algorithms apply. In addition, and as shown in Table 3, the ratio :q/Vm seems independent of the degree of dealumination, as long as there is no phase change (DHZ900) or collapse of the crystallinity (DHY800). Indeed, in spite of the fact that high degrees of dealumination have been reached in the present work, the integral volume occupied by the NFAI debris generally does not exceed 10% of the :~ volume.
Y. Hong, Z Z Fripiat/Microporous Materials" 4 (1995) 323-334
328
Table 2 XRD structural data Sample
FWHM 20 C)
a
b
c
Vuc
Crystallinity ~
(A)
(A)
(A)
(A 3)
(%)
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
0.12 0.12 0.12 0.12 0.12 0.18
20.10 20.06 20.10 20.04 19.84 19.83
20.10 20.08 20.12 20.07 20.00 19.95
13.46 13.42 13.44 13.40 13.34 13.39
5432 5407 5437 5390 5288 5264
!00 100 100 100 100 100
DHY400 DHY500 DHY600 DHY700 DHY800 DHM400 DHM500 DHM600 DHM700 DHM800
0.12 0.15 0.15 0.15 0.21 0.15 0.15 0.15 0.18 0.12
24.58 24.54 24.40 24.37 24.29 18.08 18.06 18.05 18.04 17.94
24.58
24.58
14856 14772 14534 14467 14338 2783 2764 2725 2718 2682
100 100 100 100 53 92 91 86 82 82
20.50 20.42 20.30 20.27 20.18
7.51 7.49 7.44 7.43 7.41
Full width at half m a x i m u m ( F W H M ) is for the 111, 011 and 111 reflection for D H Y , D H Z and D H M , respectively, a, b and c are the unit cell parameters. Vuc is the unit cell volume and crystallinity is calculated as indicated in the text. aThe starting materials, namely CBV3020, CBV100 and LZM-5, were taken as standards, respectively: 100% means that the sum
of intensities of selected diffractions for that sample is higher than or equal to that of the standard. Brotas de Carvalho et al. [17], who did not observe N 2 adsorption isotherms on offretite with such an accentuated Langmuir character as those shown in Fig. 3, have suggested that the 5 s plot can be divided into two parts, in order to account for the fact that the volume of 5s obtained at very low vapor pressure, instead of going to zero, intercepts the V axis at 52. They considered 52 as representing the volume of the ultramicropores. 52 and cq are shown in Table 3. Note that 51 would correspond to the classical 5~ value. F r o m our view point, the empirical character of the division of the 5~ plot into two portions does not allow one to speculate on the comparison of 52 and 51. The only notable point is that (51-52)/cq is large for D H Z and much smaller for the other zeolites. A favorable example of 51 and 52 determination is shown in Fig. 5. In Fig. 5b, the total micropore volume is 0.120 ml/g. This value is 20% lower than that obtained from the Langmuir equation (Fig. 5a). The other linear region observed in the c~ plot is at a much lower P/Po(~O.02), where N2 already fills the ultramicropores (diameter less than 0.7 nm) [17] and 52 is 0.019 ml/g. Thus, the ratio
52/51,
which is 16%, would be the fraction of the micropore volume in the narrowest or ultramicropores (less than 0.7 nm). However, the two regions, 51 and 52, are easier to distinguish for the samples calcined at higher temperature. The microporous volume for all the zeolite samples calculated by these different methods are summarized in Table 3. As shown in Table 3, the fractions of the secondary micropore, or ( 5 1 - 52 )/51, calculated from 5~ plot, increase with the fraction NFAI/AITotal for the D H M and D H Y samples (Table 1). These fractions are the highest in the ZSM-5 sample but they remain almost constant, except for DHZ900. The mesopore size distribution of D H Z samples, calculated according to Ref. [20], is shown in Fig. 6. It was obtained from the N 2 desorption isotherm, showing the appearance of mesopores with a diameter around 7.5 nm when the sample was calcined at 900°C.
3.3. Pentane adsorption Pentane adsorption on the zeolites is treated the same way a s N 2 adsorption. However, since
329
Y. Hong, J.,L Fripiat/Mieroporous Materials 4 ( 1995~ 323 334 5.45
"""°'- °'°'-. ~ • °=--°°o°.. °""°...
5.4 "~ 5.35
¢,
5.3
0.3
a
a • "°"-°°..° ° °'"-°°...,° ""°.° Oo-°.. ""-..°..
A0.2 E
0.1
I DHZg00OHZ800DHZ700DHZ600DHZ500DHZ400 © 0
5.250
"'"O "'"'"'"~'-.
14.8
.C
"~ 14.6
0.3
"'-,.
"'*.°
"'-=.
14.5
1'5
OH_Y400DHYSO0DHY600DH..Y-f00DHY___800 0
25
2'0
0.3
2.8
"'J....
2.76
2.74 2.72
"'°'°.o..,.
• ""°'-..... ,. ",.° ",... • °"..
°.
0.1
o 1'0
018
0.2
"~-. °. "'..°. •5
016
>
"°'~°
14.4
14.30
0.4 '
b.. ....
b "-..
14.7
2.78
0.2
0.4
14.9
C
c
012
014
0'.6
0'.8
3
A0.2 E
>
~-°,.
0.1
°"-...°°
2.7
"°,,oo,,o°°°
I DHM.._.500DHM600DHM.700OHMS00
2.6~ 26( '0
6
8
Fig. 2. Relationship between the unit cell volume from X R D (unit 1000 AS) and the number of NFAI per unit cell. (a) DHZ, (b) DHY and (c) D H M samples, respectively. The open symbol data points are removed from the regression (see text).
no "standard" isotherm is available, only the Langmuir plot is used. The results are shown in Fig. 7 and the data are listed in Table 4. The ratios of the micropore volume measured by pentane to that obtained with N 2 a r e 92% and 84% for D H Z and DHY, respectively, in agreement with the data reported for Y zeolite [9] and ZSM-5 [6]. Surprisingly, as mentioned earlier [13], the ratio for mordenite is low (31%). We have to keep in mind that for a perfectly crystallized H-mordenite, a total pore volume of 0.21 ml/g was accessible to N 2 while about only 0.11 ml/g is accessible for linear paraffin, in which case the ratio should be 55% [3]. Our experimental value is significantly
0
0.2 '
0.4 '
0'6.
0.8 '
Fig. 3. N2 adsorption isotherms at 78 K on DHZ (a), DHY (b) and D H M (c} samples. The amounts adsorbed, expressed as the volume of condensed phase (specific weight= 0.808 g/roll, V, is plotted versus the relative pressure, P./Po.
lower and this is not due to the dealumination, since the ratio of the extrapolated I~Mioro(Cs) and ~icro (N2) were obtained when the NFAI was zero. The failure of the Gurvitch rule is very pronounced for H-mordenite, while it is almost insignificant for the H-Y and H-ZSM-5 zeolites.
3.4. Relationship between unit cell volume and pore volume from adsorption studies How does the dealumination process affect the micropore volume? In order to answer this question, we plotted the N 2 micropore volumes calcu-
Y. Hong, J.J. Fripiat/Microporous Materials 4 (1995) 323-334
330
1400
0.25
1200
0.2
~
/. 0.15
1000 800
I D.Z2ooo~..sooD~.eooo%7ooD~.800O,Z2001
0.05
0:2 0.3 0.25 0.2
~ . 0.15
0.4
0:8
0.6
b
J
~.~..~- :.:.:.3.;.'%';"%';'%L'';" %L':';":';' :';" :';" ''~ ' ~ z'~:';'%'%;':
~/
............................................................
0.1
0.05
DHY400 DHY500DHY600DHYT00DHYS00
0o 0.14
C
o12
o.4
0:6
o18
I OHMG000,M_.26000,M.r000,MS00 [ .'
/:
b :'
600 40~
/ .4¢"
0.18 ~E0.16 > 0.14
./.
/'
0.2
/,
///
o Q.
E
~' 0.1
0.22
.~/
a
0.12 . . . . . . . 0.15 0.2 0.25 0.3 P/Po
0.1
#i; /' (:~2
1
1.5 O(,s
Fig. 5. Examples of a Langmuir plot (a) and an ~, plot (b) for N2 adsorption on DHZ900. (a) (P/Po)/V, where V is the volume adsorbed (ml/g), is plotted versus relative pressure (P/Po); (b) the adsorbed volume, expressed as the volume of the condensed phase, is plotted versus as, or the ratio of adsorbed volume on the sample to that on the reference sample, (V/Vr)s. Table 3 Results of the microporous volume measurements with the ~s and Langmuir methods
0.12 0.1 0.08 E ~" 0.06
0.04 0.02 0
012
024
016
018
Fig. 4. Pentane adsorption isotherms at 295 K on DHZ (a), DHY (b) and DHM (c) samples. The amount adsorbed, expressed as the volume of the condensed phase (specific weight=0.626g/ml), is plotted versus the relative vapor pressure, P/Po.
l a t e d f r o m the ~s a n d L a n g m u i r p l o t s v e r s u s the n u m b e r o f N F A 1 , as s h o w n in Fig. 8 a - c f o r D H Z , D H Y a n d D H M s a m p l e s , r e s p e c t i v e l y . I f the d a t a points for DHZ900 (phase change) and DHY800 ( s h a r p d e c l i n e in c r y s t a l l i n i t y , T a b l e 2) a r e n o t considered, good linear regressions are obtained ( F i g s . 7 a n d 8). T h e d e r i v a t i v e s dVMicro/d(NFA1) o b t a i n e d f r o m L a n g m u i r a n d c% p l o t s a r e listed in T a b l e 5 a n d will be d i s c u s s e d l a t e r on. T h e i n t e r c e p t s are the m i c r o p o r e v o l u m e o f the s a m p l e w i t h o u t a n y d e a l u m i n a t i o n , PMi.... j u s t as the i n t e r c e p t s o f the r e g r e s s i o n s in Fig. 2 a - c a r e t h e u n i t cell v o l u m e s ( ~ c ) in the a b s e n c e o f N F A 1 .
Sample
a1 (ml/g)
c~2 (ml/g)
(cq a2)/cq (%)
Langm_uir ~l/Vm (ml/g) (%)
DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900 DHY400 DHY500 DHY600 DHYT00 DHY800 DHM400 DHM500 DHM600 DHM700 DHMS00
0.163 0.162 0.158 0.155 0.147 0.120 0.307 0.273 0.263 0.255 0.184 0.148 0.147 0.145 0.141 0.134
0.093 0.097 0.096 0.108 0.096 0.019 0.291 0.259 0.225 0.194 0.122 0.137 0.135 0.131 0.126 0.116
44 40 39 30 35 84 4.6 5.1 14 24 34 7.1 7.5 9.6 11 13
0.193 0.191 0.189 0.183 0.176 0.147 0.310 0.310 0.278 0.273 0.190 0.173 0.165 0.170 0.169 0.155
84 95 84 85 84 82 99 88 95 93 97 86 89 88 83 86
~1 is the volume of the pores with diameter less than 20 ,~., a2 is the volume of the pores with diameter less than 7 ,~,., ajVm is the ratio of the micropore volume from the a s plot to the monolayer volume from Langmuir plot. L e t Vuc be the v o l u m e o f the u n i t cell, as o b t a i n e d f r o m X R D . T h e v a r i a t i o n o f Vuc w i t h r e s p e c t to the n u m b e r o f the N F A 1 , d V u c / d N F A 1 is o b t a i n e d f r o m Fig. 2. T h e s l o p e b e i n g n e g a t i v e m e a n s t h a t Vuc d e c r e a s e s w h e n the n u m b e r o f
331
K Hong, J.J. Fripiat/Mieroporous Materials" 4 ( 1995J 323 334
0.02 DHZ400 DHZ500 DHZ600 DHZ700 DHZ800 DHZ900
E t.- 0.015 E
O.Ol
E= ~
0.005 t~,
.,
t
x
-.
•
x
/ x /
0
0
L
I
I
I
I
4
6
8
10
12
14
Pore Diameter(nm) Fig. 6. Mesopore distribution for DHZ samples. NFA1 increases. Thus, each zeolite is characterized by one value of dVMicro/d(NFA1) and one value of d Vuc/d (NFA1). Is there a relation between both derivatives? Assuming that the unit cell contraction caused by calcination is an isometric process, the decrease in volume which results from the translocation of aluminum from the framework to the nonframework positions homogeneously affects each region of the unit cell, e.g. the actual lattice and the pore region. From the X R D data, the predicted decrease of the micropore volume caused by lattice contraction should be:
volume, and the total effect of lattice contraction ( 1 ) and obstruction by NFA1 will be:
d VMicro/d(NFA1)pred.
= ~icro/~Jc x [dVuc/d(NFAl)+ 14.6 ml/mol] (3)
= ~ic~o/~Jc x dVuc/d(NFA1)
1)
where PMicro/PUCis the ratio of the initial N2 pore volume, extrapolated from Fig. 8, to the volume of the unit cell before dealumination. The experimental dVuc/d(NFA1) and correlated dgMicro/ d(NFA1) values are shown in Table 5. Let us consider the two following situations for the translocation of the aluminum from framework to non-framework position. (i) All NFA1 species are translocated into the micropores available to N2. Since the specific weight of amorphous alumina is about 3.5 g/ml, the molar volume for dislodged aluminum species (A101.5) should be 14.6 ml/atom-g A1. Thus, the NFAI species will further decrease the micropore
d Vgicro/d(NFAI)High = PMicro/l,~vc×dVuc/d(NFA1)+14.6ml/mol
(2)
(ii) the NFAI will translocate into large micropores as well as small pores which may not be accessible to N2, so only a fraction of the NFA1 will be clogging the micropore volume. In this case the decrease in micropore volume caused per NFA1 will be: d VMicro/d(NFA1)Low
The actual situation should be between Eqs. 2 and 3 which represent the two boundaries (extreme cases). The predicted value (dVMicro/dNFAl) nigh from Eq. 2 should be the high boundary, since we put all the NFA1 in the space which is available to N 2. The value (d VMi~ro/dNFA1)Low from Eq. 3 will be the low boundary, since it spreads the NFA1 all over the unit cell, including the space not available to N z (such as the/~ cage in Y zeolites for instance, or the network of the small 8-ring complex in mordenite). In Fig. 9, the predicted values of the d V~aicro/d(NFA1) are represented for the "high" (from Eq. 2) and "low" (from Eq. 3) boundaries. The high limit is very close to the experimental micropore volume variations obtained for D H Z
332
Y. Hong, J.Z Fripiat/Microporous Materials 4 (1995) 323-334
0.18
0.2
........... eli~ ...........
a
0.18
~0.16
~ 0.16 E
E
~'o.14
©
0.14
.......
. ......
..._
............... ~ ....... i " º-o ..........
~
0.12 J
0.120 0.3 0.25
i
2
i
3
4
;
.................. .•
b ...... ii ................. . .................. e . . . . # ....
E 0.2
0.15 0.10 0.08
4
6
8
1'0
12
i
0.10
6
1
,
i
2
3
i
4
5
6
0.32 0.3 0.28 A 0.26 ~0.24 ~" 0.22 0.2 0.18
8
0.160
4
;
8
1'o
12
0.19
C 0.06
0.16
C
0.17 "'O .........
>
~---~0.16 0.04
0.15
"'"'-.....o.......
0.14 0.02
~
;
8
1'o I'2 I~ 16
Fig. 7. Relationship between the micropore volume (ml/g) measured by pentane adsorption from the Langmuir equation and number of NFAI atoms (x 102°/g) for (a) DHZ, (b) DHY and (c) DHM samples, respectively. The open symbol data points are removed from the regression (see text).
-,........, . #
0.13 0
~
~
8
lO
12
14
16
Fig. 8. Relationship between the micropore volume measured by Nz adsorption and number of NFAI atoms (× 102°/g); solid circle for Langmuir, solid lozenge for ~, plot. The open symbol data points are removed from the regression (see text).
Table 4 Comparison between the N 2 and pentane adsorption data Sample
DHZ DHY DHM
o~2 (N2)
Langmuir (N2)
Langmuir
Langmuir (pentane)
Slope (10-23 ml/NFA1)
VOm (ml/g)
Slope (10 23 ml/NFA1)
V~ (ml/g)
Slope (10-23 ml/NFA1 )
VOm (ml/g)
(%)
3.7 4.1 1.2
0.194 0.311 0.180
3.5 5.0 1.3
0.165 0.312 0.155
3.0 2.9 1.3
0.178 0.262 0.054
92 84 20
The slope is d VN2/d(NFA 1) or d VcJd(NFA1 ). VOmand V~ are the pore volume extrapolated to the state when dealumination has not occurred from Langmuir and cq plots. The ratio /~c~/l~N2is a check of the Gurvitch rule; it should be 100% if the Gurvitch rule was obeyed.
E Hong, J.J. Fripiat/'Microporous Materials 4 (1995~ 323 334
333
Table 5 Relationship between unit cell volume and the pore volume contraction Sample
DHZ DHY DHM
1k4~,o, I~c ('! i,~
32 41 28
d Vuc/d(NFAI ) (ml/mol)
19.4 14.6 8.9
d V~licro,d ( NFA1 )Low ( Eq. 3 ) (ml/mo)
10.9 12.0 6.6
d 1"Micro/d( NFAI )High ( Eq. 2 ) (ml/mol)
21.9 21.5 17.1
d I M~,o/dNFAI ( Experimental ) Langmuir
:~
22.3 24.7 7.2
21.1 30.1 7.8
lA~i,.,o/l"~c is the ratio of pore volume accessible for N 2 to the unit cell volume, both obtained by extrapolating from the state when dealumination has not occurred, d Vuc/d(NFAI) and d FMio,o/d NFA1) are the variations of the unit cell volume and pore volume caused by lhe translocation of one FAI--*NFAI. ml/mol means ml per mole of AIO~ ~. 35 30
~'25 }20 0
P~15
_J DHZ
DHY
DHM
I I LOW • Langmuir [3 alpha [] High ] Fig. 9. Relationship between the micropore volume decrease per NFA1 by N 2 adsorption and the unit cell contraction predicted from Eq. 2 (high) and 3 (low). The units are ml/mol AIO~ 5.
and DHY samples. This match-up means that, during the dealumination process, the micropore shrinks proportionally to the contraction of the unit cell, and NFAI atoms take a fraction of the space available to N2. In the case of D H M , Fig. 9 suggests that the NFAI species are clogging the entire porous system.
4. Conclusions Dealuminated H-ZSM-5, H-Y and H-mordenite samples were obtained by calcining the corresponding H-zeolites at increasing temperatures. Upon increasing the calcination temperature, the number of NFA1 species increases while the total number of aluminum atoms stays the same. The F W H M of the first diffraction (111, 011 and 111 for DHY, D H Z and D H M , respectively) increases slowly with the calcination temperature.
The temperatures where it broadens substantially are 900, 700 and 500"~'C for the DHZ, D H M and DHY sample, respectively, in agreement with the thermal stability of these lattices. The three zeolites studied here exhibit different behaviors upon calcination. From the relationship between the unit cell shrinkage and the micropore volume decrease, it is suggested that the NFAI goes to the pores which are available to N 2 in the cases of D H Y and DHZ, while it spreads into the entire DH M lattice. Irrespective of the degree of dealumination, the Gurvitch rule (volume of adsorbed N z = v o l u m e of adsorbed n-Cs) fails to apply to DHM, but it presents a reasonable agreement for the H-ZSM-5 and HY samples that were studied. This observation demonstrates clearly that one set of channels is available to N2 and not to n-pentane in H-mordenite.
Acknowledgements The financial support through DOE Grant DT-FG02-90 ERI430 is gratefully acknowledged. We thank Dr. V. Gruver for his mordenite samples and useful discussions, Dr. A. Blumenf'eld and D. Coster for the N M R data and Dr. S. Hardcastle of the Advanced Analytical Facilities ( A A F ) of" UWM for his help in X R D measurements.
References [1] R. Szostak, in H. Van Bekkum, E.M. Flanigen and J.C. Jansen (Eds.), Introduction to Zeolite Science and Practice. Elsevier, New York, 1991, p. 153.
334
Y. Hong, ,LJ. Fripiat/Microporous Materials 4 (1995) 323-334
[2] L. Kubelkova, S. Beran, A. Malecka and V.M. Mastikhin, Zeolites, 9 (1989) 12. [3] D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, Robert E. Krieger Publishing, FL, 1984. [4] V.G. Chumbhale, A.J. Chandwadkar and B.S. Rao, Zeolites, 12 (1992) 63. [5] D.G. Hay and H. Jaeger, J. Chem. Soc., Chem. Commun., (1984) 1433. [6] D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J. Phys. Chem., 85 (1981) 2238. [7] (a) H. van Koningsveld, J.C. Jansen and H. van Bekkum, Zeolites, 13 (1990) 235; (b) E. de Vos Burchart, H. van Bekkum and B. Van de Graaf, Zeolites, 10 (1993) 212. [8] W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemann, London, 1992. [9] J. Pires, M. Brotas de Caralho, F. Ramoa Ribeiro and E.G. Derouane, Zeolites, 11 (1991) 345. [10] L. Gurvitch, J. Phys. Chem. Soc. Russ., 47 (1915) 805.
[11] A. Zukal, V. Patzelova and U. Lohse, Zeolites, 6 (1986) 133. [12] C. Fernandez, J.C. Vedrine, J. Grosmangin and G. Szabo, Zeolites, 6 (1986) 484. [13] Y. Hong, V. Gruver and J.J. Fripiat, J. Catal., 150 (1994) 421. [ 14] V. Gruver and J.J. Fripiat, J. Phys. Chem., 98 (1994) 8544. [15] Annual Book of ASTM Standards, D 3942-91, 15.03 (1991) 612. [16] Annual Book of ASTM Standards, D 3906-91, 15.03 (1991) 597. [17] M. Brotas de Carvalho, A.P. Carvalho, F. Ramoa Ribeiro, A. Florentino, N.S. Gnap and M. Guisnet, Zeolites, 14 (1994) 217. [18] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, New York, 2nd ed., 1982. [19] M.R. Bhambhani, P.A. Cutting, K.S.W. Sing and D.H. Turk, J. Coll. Interf. Sci., 38 (1973) 109. [20] E.P. Barret, L.G. Joyner and P.H. Halenda, J. Am. Chem. Soc., 73 (1951) 373.