Ind. Eng. Chem. Res. 2007, 46, 5067-5073
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Evaluation of the Main Diffusion Path in Novel Micro-Mesoporous Zeolitic Materials with the Zero Length Column Method Amir Malekian,† Hoang Vinh-Thang,‡ Qinglin Huang,† Mladen Eic´ ,*,† and Serge Kaliaguine‡ Department of Chemical Engineering, UniVersity of New Brunswick, P.O. Box 4400, Fredericton, N.B., Canada E3B 5A3, and Department of Chemical Engineering, LaVal UniVersity, Sainte-Foy, Quebec, Canada G1K 7P4
The past several years have seen a dramatic increase in research interest regarding ordered and disordered mesoporous molecular sieves. UL-ZSM5s belong to the class of disordered mesoporous materials where zeolites are intergrown in mesoporous structures. They are considered to have a great potential for adsorption separation and catalysis. The primary objective of this study is to investigate the main diffusion path of these novel materials, using the zero length column (ZLC) technique with an appropriate theoretical analysis. Two ULZSM5 samples and their precursors were investigated, employing o-xylene as a probe molecule. In addition, ZSM-5 and ZSM-12 crystals with n-heptane/toluene as probe molecules were used as reference systems. The results revealed that the structure of UL-ZSM5 and its precursor behaved approximately as a three-dimensional (isotropic) diffusion system, while ZSM-5 and ZSM-12 crystals behaved as three- (isotropic) and one-dimensional (anisotrpic) diffusion systems, respectively. The results also showed that the diffusion path will not change with the variation of temperature or nature of sorbate molecules. From a practical point of view, this simple and relatively inexpensive method can be conveniently used as an additional tool for the characterization of porous materials. Introduction Microporous materials and, more recently, mesoporous materials are of particular interest due to their specific selectivity and the relative ease of altering their sorption properties.1 One of the main limitations of zeolites is the small size of their pores, which does not allow the penetration of large molecules. Attempts to overcome this size limitation have resulted in the discovery of a new family of materials designated as the mesoporous molecular sieves (MMS) by Mobil Corporation scientists.2 The resulting materials, initially designated as M41S, have very regular arrangements of monodispersed pore size with diameters ranging from 3 to 10 nm.2 In further studies, investigators were able to synthesize similar materials with sizes up to 30 nm in diameter.3 Several studies relevant to the general classification and properties of mesoporous materials,4,5 synthesis,6,7 and potential applications4,6,8-10 have been reported. However, MMS materials are not strongly acidic and do not exhibit the similar strong catalytic properties as those of acidic zeolites due to their mainly amorphous structure. Moreover, their hydrothermal stability is relatively low. Kaliaguine and coworkers11,12 have successfully synthesized biporous micro- and mesoporous zeolites (UL-zeolites), which possess a unique combination of properties typical of zeolites, i.e., strong acid sites and hydrothermal stability, as well as relatively large pores in the 2-50 nm range which is typical of MMS. UL-zeolites form a new group of alumina-silicate mesoporous molecular sieves having semicrystalline zeolitic walls. The synthesis procedure involves a solid-state secondary crystallization using the highly dispersed amorphous material as the precursor.11,12
* To whom correspondence should be addressed. Phone: 1-506-453-4689. Fax: 1-506-453-3591. E-mail:
[email protected]. † University of New Brunswick. ‡ Laval University.
Due to the significant application of zeolitic materials in the petrochemical and hydrocarbon processing industries, the understanding of the overall kinetics behavior of molecules in these materials is of a considerable importance. In many practical cases, the diffusion step is considered to be ratecontrolling. Therefore, the diffusion pattern and diffusivities of different hydrocarbons in zeolitic materials are important. In contrast to microporous materials such as zeolites, which have been extensively investigated by various experimental techniques including both microscopic and macroscopic (transient) methods,13-15 up to the present time, mesoporous and micro-mesoporous materials have been the subject of only a rather limited number of diffusion studies.16-19 Also, to the best of our knowledge, detailed evaluations of the main diffusion pattern in biporous MMS materials have not yet been reported in the general literature. In this study, we used the zero length column (ZLC) technique20 to provide a simple and inexpensive means of distinguishing between one- and three-dimensional diffusion patterns involving micro-mesoporous and microporous materials, e.g., UL-ZSM5 and its precursor, as well as ZSM12 and ZSM-5 that were used as the reference samples.
Theory The ZLC desorption technique was introduced by Eic´ and Ruthven.20 For a linear equilibrium system with uniform spherical particles and negligible gas holdup in the voids of the ZLC bed, the normalized effluent gas concentration (c/co), representing the full time solution for three-dimensional geometry, is given by the following:
c co
) 2L
∞
exp(-βn2Defft/Rp2)
n)1
[βn2 + L(L - 1)]
∑
10.1021/ie061497+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007
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Table 1. Textural properties of various samples sample
SBET (m2/g)
SBJH (m2/g)
micropore volume (cm3/g)
mesopore volume (cm3/g)
mesopore diameter (Å)
Al-Meso-100 UL-ZSM5-100-6 UL-ZSM5-100-8 ZSM-5 ZSM-12
915 780 480 415 376
770 510 130 45
0.058 0.115 0.145 0.152 0.126
1.251 1.584 1.157 0.462
38 195 250
Where, Rp is particle radius, Deff is effective diffusivity, and βn are eigenvalues given by the roots of the auxiliary equation
βn cot βn + L - 1 ) 0
(2)
and
L)
FRp2 3KVsDeff
(3)
where F is the purge flow rate, Vs is the adsorbent volume, and K is Henry’s law constant. The corresponding full time solution for slab-geometry, which represents the one-dimensional model, can be expressed as follows.21
c co
) 2L
∞
exp(-βn2Defft/l2)
n)1
[βn2 + L(L + 1)]
∑
(4)
where βn are eigenvalues given by the roots of the equation:d
βn tan βn - L ) 0
(5)
Effective diffusion time constants (Deff/Rp2) or (Deff/l2) can be extracted by fitting eqs 1 and 2 or 4 and 5 with the experimental ZLC data using the SigmaPlot 2001 software tool. A fitting curve analysis based on dimensionless concentration (c/co) versus time yields diffusion time constant Deff/Rp2 or Deff/l2 and the L value. To assess the diffusion pathway, a simple method based on the short time asymptote solution of eqs 1 and 4 was proposed by Ruthven and co-workers.21-23 For a three-dimensional case, the short time solution can be expressed as
c 1 ) co L
[x
Rp2 -1 πDefft
]
(6)
and for a one-dimensional case
c 1 ) co L
[x ] l2 πDefft
(7)
According to Brandani et al.,23 these solutions are valid for the concentration range c/co < 0.2 and L > 10 (L is extracted from the full time solution method), and they provide a clear differentiation between one- and three-dimensional diffusion paths. In the former model, a plot of c/co versus 1/xt should yield a negative intercept (-1/L), whereas, in the latter model, the line should pass through the origin (zero). The shortcoming of the method is that, for the large L values, the intercept becomes too small to easily distinguish between the zero and negative intercepts. However, the consistency of these models can be verified by comparing the diffusion time constants
crystallinity (%) 77.8 88.5 100
derived from the short time model with the appropriate solutions obtained from the full time model. Experimental UL-ZSM5 was obtained using an amorphous mesoporous material as a precursor (Al-Meso-100), having a wormholelike mesoporous silica structure and thick silica walls,3 followed by a secondary templated crystallization. Tetrapropylammonium hydroxide (TPAOH) was used as a template. A detailed account of synthesis procedures is provided elsewhere.11,12,24 The structural characterization of UL-ZSM5 samples with different crystallization (aging) times was reported in our earlier communication.24 The samples are designated as UL-ZSM5-xy, where x is the Si/Al ratio and y is the crystallization (aging) time in days. The precursor sample, before crystallization, is designated Al-Meso-100. The crystallization temperature was 125 °C. Two microporous zeolite samples, e.g., ZSM-5 having a silicon to aluminum ratio of 100 and ZSM-12, were used as the 3-D and 1-D reference materials, respectively. They were synthesized according to the methods reported by Trong-On et al.25 and Bandyopadhyay et al.,26 respectively. The textural properties of UL-ZSM5 samples, Al-Meso-100, ZSM-5, and ZSM-12 are summarized in Table 1. In this study, diffusion of o-xylene in UL-ZSM5-100-6, UL-ZSM5-100-8, and Al-Meso-100 samples, n-heptane and toluene in ZSM-5, and toluene in ZSM-12 samples was investigated by the ZLC measurements to assess the diffusion pathways. ZLC Measurements. About 1-2 mg of material was placed between two sintered discs in the ZLC column. Prior to measurement, the sample was activated overnight at 270 °C by purging with a small flow of helium (∼10 cm3/ min). The sample was then equilibrated with sorbate diluted in a helium flow. The sorbate concentration was maintained low enough (e.g., 0.005-0.01 Torr partial pressure) to ensure that the measurements were carried out within the linear range of the equilibrium isotherm as required by the ZLC theory. The desorption part of the measurement was performed by purging with pure helium at a flow rate high enough to maintain a very low sorbate concentration at the external surface of the particles, thus minimizing external heat and mass transfer resistance. A purge flow rate of 120 cm3/min for helium was used in this study, which provided conditions for the ZLC desorption process to be controlled by kinetics (L values were generally found to be greater than 10, revealing that the process was controlled by kinetics27). Further details of the ZLC method can be found elsewhere.19-22 The desorption curves were corrected with blank measurements to account for dead volumes, in which only two sintered discs, i.e., with no adsorbent present, were placed in the ZLC column. Results and Discussion The representative experimental ZLC response curves for all samples plotted in a standard form, i.e., ln(c/co) versus time (t) at different temperatures are shown in Figures 1-3. The three-
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Figure 2. Experimental (symbols) and theoretical (solid lines) ZLC curves (three-dimensional model) for n-heptane and toluene in the ZSM-5 sample at different temperatures.
Figure 1. Experimental (symbols) and theoretical (solid lines) ZLC curves (three-dimensional model) for o-xylene in UL-ZSM5 and Al-Meso-100 samples at different temperatures.
dimensional full time model was applied to fit experimental ZLC curves of various UL-ZSM5 samples, its precursor, and a ZSM-5 sample, while the one-dimensional full time model was applied to fit the ZSM-12 sample (solid lines in Figures 1-3) The good agreements between experimental results and theoretical fittings are clearly observed. The L values summarized in Tables 2A, 3A, and 4A are all greater than 10, which clearly indicates that desorption is kinetically controlled as required by the criteria, as is discussed in the theoretical section. The experimental ZLC desorption results at different temperatures were further applied in the analysis of diffusion patterns. Such analysis was first conducted on the two reference materials ZSM-5 and ZSM-12. The ZSM-5 crystals possess a two-channel system, i.e., straight and zigzag channels, in which the dimensions are controlled by the 10-member rings of the framework oxygen atoms. The approximately cylindrical straight channels are intersected by elliptically shaped sinusoidal channels forming a three-dimensional channel system28 thus favoring diffusion in random directions, i.e., through the straight and zigzag channels. This three-dimensional behavior of ZSM-5 crystals has also been confirmed by other researchers.21,29 On the other hand, the ZSM-12 crystal possesses one-dimensional non-interconnecting tubularlike channel structures (pore size 5.6 Å × 6.1 Å) with 12-member rings.30 The short time plots, i.e., c/co versus 1/xt for the two reference systems, are displayed
Figure 3. Experimental (symbols) and theoretical (solid lines) ZLC curves (one-dimensional model) for toluene in the ZSM-12 sample.
in Figures 4 and 5. The fitting analysis shows that the diffusion of n-heptane and toluene in ZSM-5 and toluene in ZSM-12 conforms to three-dimensional (negative intercept) and onedimensional (zero intercept) models, respectively, as required by the theory. The result also reveals that the diffusion pattern does not vary with the desorption temperature and the nature of sorbate molecules investigated in this study. In summary, the fitting results clearly indicate that the short time method based on ZLC theory is a promising technique to distinguish between one- and three-dimensional diffusion patterns. The short time method was further applied in investigating the diffusion path of the novel biporous UL-ZSM5 and Al-Meso-100 samples using o-xylene as a probe molecule. The plots are shown in Figures 6 and 7. All plots exhibit the negative intercepts corresponding to the value of -1/L regardless of temperatures, which is consistent with the three-dimensional diffusion path, i.e., spherical model. In the original precursor, the main mesoporous channels have a wormholelike structure.3 The curvature of these original mesopores in combination with micropores that interconnect the main channels affect the diffusion path and are plausible reasons for the three-dimensional
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Table 2. Summary of Parameters from the ZLC Analysis of (A) Toluene and n-Heptane in the ZSM-5 Sample Based on Three-Dimensional Models and (B) n-Heptane in the ZSM-5 Sample Based on One-Dimensional Models A short time analysis sorbate toluene n-heptane
T (°C) 80 90 100 110
full time analysis
slope
Deff/Rp2 (s-1)
R2 value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)a
3.42 2.86 2.10 2.01
1.61 × 10-4 2.30 × 10-4 2.00 × 10-4 3.54 × 10-4
0.9752 0.9955 0.9952 0.9933
13 13 19 15
1.36 × 10-4 2.03 × 10-4 2.22 × 10-4 3.76 × 10-4
0.9609 0.9588 0.9412 0.9588
18.5 13.5 9.9 6.8
B short time analysis
full time analysis
sorbate
T (°C)
slope
Deff/Rp2 (s-1)
R2 value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)
n-heptane
100 110
1.33 0.98
1.49 × 10-3 2.74 × 10-3
0.8266 0.6508
11 11
1.16 × 10-3 1.71 × 10-3
0.9385 0.9487
28.3 60.3
a In Tables 2-4, is calculated based on this expression: ) |(D /R 2(D /l2) from short time analysis - D /R 2(D /l2) from full time analysis)/ eff p eff eff p eff (Deff/Rp2(Deff/l2) from full time analysis)| × 100%.
Table 3. Summary of Parameters from the ZLC Analysis of (A) Toluene in the ZSM-12 Sample Based on One-Dimensional Models and (B) Toluene in the ZSM-12 Sample Based on Three-Dimensional Models A short time analysis sorbate toluene
full time analysis R2
T (°C)
slope
Deff/Rp (s-1)
value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)
80 100 120
2.55 1.96 1.63
3.40 × 10-4 5.76 × 10-4 8.32 × 10-4
0.9920 0.9856 0.9799
12 12 12
3.28 × 10-4 5.50 × 10-4 8.10 × 10-4
0.9664 0.9641 0.9589
3.7 4.7 2.8
2
B short time analysis sorbate toluene
T (°C) 80 100 120
full time analysis
slope
Deff/Rp2 (s-1)
R2 value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)
3.33 2.63 2.17
2.34 × 10-5 3.76 × 10-5 5.52 × 10-5
0.9585 0.9617 0.9710
35 35 35
2.68 × 10-5 4.49 × 10-5 6.62 × 10-5
0.8869 0.8810 0.8565
12.5 16.3 16.6
Table 4. Summary of Parameters from the ZLC Analysis of (A) o-Xylene in UL-ZSM5 and Al-Meso-100 Samples Based on Three-dimensional Models (B) o-Xylene in UL-ZSM5 Samples Based on One-Dimensional Models A short time analysis sorbate
sorbent
o-xylene
UL-ZSM5-100-6 UL-ZSM5-100-8 Al-Meso-100
T (°C) 30 40 50 70 80 90 50 60 70
full time analysis
slope
Deff/Rp2 (s-1)
R2 value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)
6.33 4.87 4.25 3.28 3.18 2.49 4.99 3.86 3.22
4.06 × 10-5 7.08 × 10-5 1.23 × 10-4 6.71 × 10-5 1.10 × 10-4 1.77 × 10-4 5.69 × 10-5 1.09 × 10-4 1.56 × 10-4
0.9858 0.9861 0.9621 0.9694 0.9952 0.9921 0.9773 0.9861 0.9652
14 14 12 21 17 17 15 14 14
4.50 × 10-5 7.74 × 10-5 1.32 × 10-4 6.44 × 10-5 1.19 × 10-4 1.90 × 10-4 6.48 × 10-5 1.00 × 10-4 1.48 × 10-4
0.9630 0.9584 0.9445 0.9220 0.9536 0.9488 0.9215 0.9084 0.9028
9.9 8.5 7.2 4.2 8.4 6.4 12.3 9.0 5.9
B short time analysis sorbate
sorbent
T (°C)
o-xylene
UL-ZSM5-100-6 UL-ZSM5-100-8
30 70
full time analysis
slope
Deff/Rp2 (s-1)
R2 value
L
Deff/Rp2 (s-1)
R2 value
relative difference of Deff/Rp2, (%)
4.00 2.47
1.64 × 10-4 4.31 × 10-4
0.9328 0.9128
11 11
1.12 × 10-4 3.61 × 10-4
0.9462 0.9194
46.9 19.5
diffusion behavior of the UL-ZSM5-o-xylene adsorption system in the concentration and temperature ranges studied here. It should be noted that the three-dimensional diffusion behavior of UL-ZSM5 samples was observed in all cases, regardless of the mesopore dimension (see Table 1), which is consistent with the results reported by Courivaud et al. for a MCM-41-hexane adsorption system.31
In order to verify the above conclusions regarding the diffusion paths of various samples, further analysis was carried out with the results presented in this section. The diffusion time constants (Deff/Rp2) or (Deff/l2) for all samples determined both from the short time method and the full time method, and their relative differences, are summarized in Tables 2-4. Theoretically, if the model exactly represents the diffusion path of the
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Figure 4. Dimensional diffusion analysis of (a) toluene and (b) n-heptane in ZSM-5 samples at different temperatures.
Figure 5. Dimensional diffusion analysis of toluene in ZSM-12 samples at different temperatures.
samples, the obtained diffusion time constants derived from both short and full time methods should show reasonable agreements. Otherwise, they would show significant deviations. Tables 2A, 3A, and 4A clearly show that the differences of diffusion time constants are relatively small when the three-dimensional model was applied to the UL-ZSM5, Al-Meso-100, and ZSM-5 data and when the one-dimensional model was applied to the ZSM12 data. However, when the inappropriate model was used, e.g., the one-dimensional model applied to the UL-ZSM5, Al-Meso100, and ZSM-5 data and the three-dimensional model to the ZSM-12 data, the relative differences of diffusion time constants shown in Tables 2B, 3B, and 4B become much larger. This analysis provides further evidence that the three-dimensional model exactly represents the diffusion path of UL-ZSM5, AlMeso-100, and ZSM-5 and that the one-dimensional model represents the diffusion path of ZSM-12. The regression (R2) method was also used to measure how well the model fits the experimental data. The obtained R2-values are listed in Tables
2-4. It is evident that the full time solution provides a reasonable fit of all experimental ZLC data with R2-values higher than 0.85, as shown in Tables 2-4, thus indicating that either the one- or three-dimensional model (full time solution) could fit the experimental ZLC data. This further supports the insufficiency of the full time solution being used alone to differentiate between one- and three-dimensional models. In addition, the R2-values obtained from the short time analysis, using correct models (Tables 2A, 3A, and 4A), are superior to those obtained from inappropriate models (Tables 2B, 3B, and 4B). This provides further evidence regarding the use of an appropriate model to fit experimental data, and indirectly supports our earlier conclusions on the diffusion paths. It can also be observed from Tables 3A and 3B that all R2-values from short time analysis are greater than 0.95, indicating the good fittings using either eq 6 or 7. This is due to relatively high L values (greater than 10) as shown in Table 3B, resulting in the similar results obtained from both eqs 6 and 7.
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Figure 6. Dimensional diffusion analysis of o-xylene in UL-ZSM5-100-6 and UL-ZSM5-100-8 samples at different temperatures.
Figure 7. Dimensional diffusion analysis of o-xylene in Al-Meso-100 sample at two different temperatures.
It should be noted that some of the results presented in Figures 6 and 7 show minor deviations from linearity in the short time region. These deviations could be due to the uncertainty in the short time region that leads to a significant error of the corresponding response curve. This phenomenon was also reported by Cavalcante et al.21 Moreover, we should emphasize that, if the ZLC experiments are conducted with L values much larger than 10, i.e., -1/L ≈ 0, the presented methods evidently can fit experimental results equally well using either the onedimensional model (with zero intercept) or the three-dimensional model (with negative intercept), rendering the methods ineffective to distinguish the diffusion paths. Consequently, the extracted diffusivities from diffusion path analysis should also be checked with those calculated from full time analysis. Conclusions The ZLC technique can provide a simple method to distinguish one-dimensional from three-dimensional diffusion patterns. This methodology was used to study the diffusion path in novel UL-ZSM5 samples, their precursor, and two reference samples (ZSM-5 and ZSM-12). The results indicate that the diffusion path in UL-ZSM5 samples, precursor, and ZSM-5 is three-dimensional, while in ZSM-12, it is one-dimensional. It also shows that the diffusion path pattern does not change due to the variation of temperature or the nature of sorbate molecules as should be the case. Such observation is in agreement with the earlier results obtained by pulsed field gradient (PFG) NMR and frequency response measurements.28,31 We anticipate the application of this method for the fast characterization of novel nanoporous materials or as a supple-
ments to the conventional transmission electron microscopy (TEM) method for a more detailed structural analysis. Acknowledgment This project was supported by the Natural Science and Engineering Research Council of Canada (NSERC). Literature Cited (1) Laeri, F. Schu¨th, F.; Simon, U.; Wark, M. Host-Guest-Systems Based on Nanoporous Crystals; Wiley-VCH Verlag GmbH.: Weinheim, 2003. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered mesoporous molecular-sieves synthesized by a liquid-crystal template mechanism. Nature 1992, 359, 710. (3) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks. Nature 1998, 396, 152. (4) Behrens, P. Mesoporous inorganic solids. AdV. Mater. 1993, 5, 127. (5) Behrens, P.; Stucky, G. D. Ordered molecular arrays as templates - a new approach to the synthesis of mesoporous materials. Angew. Chem., Int. Ed. Engl. 1993, 32, 696. (6) Casci, J. L. AdVanced zeolite science and application; Jansen, J. C., Stocker, M., Karge, H. G., Weithamp, J., Eds.; Studies in Surface Science and Catalysis; Elsevier Science: New York, 1994; Vol. 85, pp 329-356. (7) Antonelli, D. M.; Ying, J. Y. Mesoporous materials. Curr. Opin. Colloid Interface Sci. 1996, 1, 523. (8) Ozin, G. A. Nanochemistry - sysnthesis in diminishing dimensions. AdV. Mater. 1992, 4, 612. (9) Zhao, X. S.; Lu, G. Q.; Miller, G. J. Advances in mesoporous molecular sieve MCM-41. Ind. Eng. Chem. Res. 1996, 35, 2075. (10) Corma, A. From microporous to mesoporous molecular sieve materials and their use in catalysis. Chem. ReV. 1997, 97, 2373. (11) Trong-On, D.; Lutic, D.; Kaliaguine, S. An example of mesoporous zeolitic material. UL-TS-1. Microporous Mesoporous Mater. 2001, 4445, 435.
Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 5073 (12) Trong-On, D.; Kaliaguine, S. Mesoporous zeolitic material with microporous crystalline mesopore walls. US Patent 6,669,924 B1, 2003. (13) Do, D. D. Adsorption analysis. Equilibria and Kinetics; Imperial College Press: London, 1998. (14) Karger, J.; Ruthven, D. M. Diffusion in Zeolites and other Micropores solids; Wiley: New York, 1992. (15) Snurr, R. Q.; Karger, J. Molecular simulations and NMR measurements of binary diffusion in zeolites. J. Phys. Chem. B 1997, 101, 6469. (16) Stallmach, F.; Graser, A.; Karger, J.; Krause, C.; Jeschke, M.; Oberhagemann, U.; Spange, S. Pulsed field gradient NMR studies of diffusion in MCM-41 mesoporous solids. Microporous Mesoporous Mater. 2001, 44-45, 745. (17) Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Structure and diffusion characterization of SBA-15 materials. Langmuir 2005, 21, 2051. (18) Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Effect of the intrawall microporosity on the diffusion characterization of bi-porous SBA-15 materials. Stud. Surf. Sci. Catal. 2006, 149, 591. (19) Vinh-Thang, H.; Malekian, A.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Diffusive characterization of large pore mesoporous materials with semicrystalline zeolitic framework. Stud. Surf. Sci. Catal. 2003, 146, 145. (20) Eic´, M.; Ruthven, D. M. A new experimental-technique for measurement of intracrystalline diffusivity. Zeolites 1988, 8, 40. (21) Cavalcante, C. L.; Brandani, S.; Ruthven, D. M. Evaluation of the main diffusion path in zeolites from ZLC desorption curves. Zeolites 1997, 18, 282. (22) Cavalcante, C. L.; Eic´, M.; Ruthven, D. M.; Occelli, M. L. Diffusion of n-paraffins in offretite-erionite type zeolites. Zeolites 1995, 15, 293. (23) Brandani, S.; Ruthven, D. M. Analysis of ZLC desorption curves for gaseous systems. Adsorption 1996, 2, 133. (24) Huang, Q.; Vinh-Thang, H.; Malekian, A.; Eic, M.; Trong-On, D.; Kaliaguine, S. Adsorption of n-heptane, toluene and o-xylene on mesoporous UL-ZSM5 materials. Microporous Mesoporous Mater. 2006, 87, 224.
(25) Trong-On, D.; Kapoor, M. P.; Thibault, E.; Gallot, J. E.; Lemay, G.; Kaliaguine, S. Influence of high-energy ball milling on the physicochemical and catalytic properties of titanium silicalite TS-1. Microporous Mesoporous Mater. 1998, 20, 107-118. (26) Bandyopadhyay, R.; Kubota, Y.; Sugimoto, N.; Fukushima, Y.; Sugi, Y. Synthesis of borosilicate zeolites by the dry gel conversion method and their characterization. Microporous Mesoporous Mater. 1999, 32, 8191. (27) Hufton, J. R.; Ruthven, D. M. Diffusion of light alkanes in silicalite studied by the zero length column method. Ind. Eng. Chem. Res. 1993, 32, 2379. (28) Song, L.; Rees, L. V. C. Adsorption and diffusion of cyclic hydrocarbon in MFI-type zeolites studied by gravimetric and frequencyresponse techniques. Microporous Mesoporous Mater. 2000, 35-36, 301. (29) Song, L.; Rees, L. V. C. Diffusion of propane in theta-1 and silicalite-1 zeolites. Microporous Mesoporous Mater. 2000, 41, 193. (30) Fyfe, C. A.; Gies, H.; Kokotailo, G. T.; Marler, B.; Cox, D. E. Crystal structure of silica-ZSM-12 by the combined use of high-resolution solid-state MAS NMR spectroscopy and synchrotron x-ray powder diffraction. J. Phys. Chem. 1990, 94, 3718. (31) Courivaud, F.; Hansen, E. W.; Larlsson, A.; Kolboe, S.; Stocker, M. Pulsed field gradient NMR study of the diffusion of n-hexane confined in hydroxylated and dehydroxylated MCM-41 of various pore diameters. Microporous Mesoporous Mater. 2000, 35-36, 327.
ReceiVed for reView November 23, 2006 ReVised manuscript receiVed April 2, 2007 Accepted May 7, 2007 IE061497+
