2534
Langmuir 1999, 15, 2534-2539
Adsorption of Branched Alkanes in Silicalite-1: A Temperature-Programmed-Equilibration Study Benoıˆt Millot,† Alain Me´thivier,*,† Herve´ Jobic,‡ Isabelle Clemenc¸ on,† and Bernadette Rebours† Institut Franc¸ ais du Pe´ trole, 1 & 4 av. de Bois-Pre´ au, 92852 Rueil-Malmaison Cedex, France, and Institut de Recherches sur la Catalyse, CNRS, 2 av. A. Einstein, 69626 Villeurbanne, France Received September 29, 1998. In Final Form: December 29, 1998 The adsorption properties of several branched paraffins from C4 to C6 on MFI type zeolite were investigated. A temperature-programmed-equilibration method was used for this study, and the analysis of the desorption rates as a function of temperature showed two desorption peaks. The interpretation of this phenomenon is discussed, and it is revealed that the high-temperature desorption occurs in conditions close to equilibrium while low-temperature desorption is subject to diffusional limitations. Furthermore, an XRD study shows that the phase transition in the crystal structure can be lowered by 70 K when 3-methylpentane is adsorbed. This property shows that the low-temperature desorption peak is not due to a change of symmetry in the structure of the zeolite. Modeling of the high-temperature peak provided adsorption isosteric enthalpy and entropy variations. These values are consistent with literature data. For the first time, adsorption isosteric entropy variations of sorption are calculated for branched alkanes in silicalite-1.
Introduction Silicalite-11,2
exhibits a hydrophobic nonswelling structure of high thermostability. It consists of tetrahedral building blocks in which the central atom is silicon and the corner atoms are oxygen. The resulting framework is composed of two groups of intersecting channels. One group of pores comprises the slightly narrower zigzag channels along the a direction with a circular cross section and a diameter of about 5.4 Å. The other group comprises the somewhat larger elliptical straight channels along the b direction with a cross section of about 5.7 Å. It is well-known that the framework symmetry of MFI type zeolite is quite flexible. As-synthesized silicalite-1 has an orthorhombic symmetry which changes after calcination and removal of the template3,4 to a monoclinic symmetry for temperatures varying between 323 and 363 K and in atmospheric conditions. Silicalite-1 shows a reversible structural phase transition and exhibits monoclinic symmetry below and orthorhombic symmetry above the transition temperature. Zeolite structural change depends on temperature, on the chemical composition of the zeolite, and on the loading of the pores with adsorbed molecules. Thus, from a study of p-xylene adsorption, Sacerdote-Peronnet et al.5 and van Koningsveld et al.6 have reported that the structure of silicalite-1 with loadings lower than 4 molecules per unit cell can be refined with the Pnma space group, while loadings of about 8 molecules per unit cell induce a P212121 space group. A * To whom correspondence should be addressed. † Institut Franc ¸ ais du Pe´trole. ‡ Institut de Recherches sur la Catalyse. (1) Grose, R. W.; Flanigen, E. M. U. S. Patent 4 061 724, 1979. (2) Flanigen, E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature (London) 1978, 271, 512. (3) Mentzen, B. F.; Sacerdote-Peronnet, M. Mater. Res. Bull. 1993, 28, 1017. (4) Mentzen, B. F.; Letoffe, J. M.; Claudy, P. Thermochim. Acta 1996, 288, 1. (5) Sacerdote-Peronnet, M.; Bouix, J.; Bosselet, F.; Mentzen, B. F. Mater. Res. Bull. 1990, 25, 443. (6) van Koningsveld, H.; Tuinstra, F.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1989, B45, 423.
step in the adsorption isotherm of aromatics has been observed by several authors.7-12 Snurr et al.13 attributed the step in the p-xylene adsorption isotherm to the change of the zeolite structure. All authors agree with the phase transition, but its correlation with the step in the adsorption isotherm continues to be a matter of debate. Lee and co-workers12,14 have used a lattice gas model with three types of adsorption sites for the representation of adsorption of aromatics in silicalite-1. A revised version15 of the three-site model of collective localized adsorption, proposed originally by Lee and Chiang,12,14 has been presented for benzene adsorption in ZSM-5 zeolite. The model predicts the two steps observed in the isotherms of benzene adsorption by taking into account that benzene molecules may be adsorbed in two different configurational states in channel intersections. Adsorption and desorption of linear or branched paraffins in silicalite-1 have been studied in detail. Jacobs et al.16 studied adsorption of C3-C10 n-paraffins, monobranched alkanes such as isobutane or isopentane, and dibranched alkanes such as neopentane. Other authors17-21 (7) Olson, D. H.; Kokotaı¨lo, G. T.; Lawton, S. L.; Meler, W. M. J. Phys. Chem. 1981, 85, 2238. (8) Guo, C. J.; Talu, O.; Hayhurst, D. T. AIChE J. 1989, 35, 573. (9) Talu, O.; Guo, C. J.; Hayhurst, D. T. J. Phys. Chem. 1989, 93, 7294. (10) Pope, C. G. J. Phys. Chem. 1984, 88, 6312. (11) Thamm, H. Zeolites 1987, 7, 341. (12) Lee, C. K.; Chiang, A. S. T.; Wu, F. Y. Presented at the AIChE Meeting, Miami Beach, FL, 1992. (13) Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1994, 98, 5111. (14) Lee, C. K.; Chiang, A. S. T.; Wu, F. Y. Presented at the Fundamentals of Adsorption Conference, Kyoto, 1992. (15) Rudzinski, W.; Narkiewicz-Michalek, J.; Szabelski, P.; Chiang, A. S. T. Langmuir 1997, 13, 1095. (16) Jacobs, P. A.; Beyer, H. K.; Valyon, J. Zeolites 1981, 1, 161. (17) Abdul-Rehman, H. B.; Hasanain, M. A.; Loughin, K. F. Ind. Eng. Chem. Res. 1990, 29, 1525. (18) Hampson, J. A.; Rees, L. V. C. J. Chem. Soc., Faraday Trans. 1993, 89 (16), 3169. (19) Stach, H.; Lohse, U.; Thamm, H.; Schirmer, W. Zeolites 1988, 6, 74. (20) Hufton, J. R.; Danner, R. P. AIChE J. 1993, 39, 954. (21) Golden, T. C.; Sircar, S. J. Colloid Interface Sci. 1994, 162, 182.
10.1021/la981344u CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999
Adsorption of Branched Alkanes in Silicalite
Langmuir, Vol. 15, No. 7, 1999 2535
Figure 1. XRD patterns of silicalite-1 in selected scanning ranges at different temperatures for sample B.
have focused on adsorption of the straight chain hydrocarbons (C1-C4) in silicalite-1. In the past few years n-hexane and n-heptane have been particularly studied22-31 because their isotherms are kinked whereas the long chain C10 alkanes exhibit simple type I isotherms. Computer simulations of the n-hexane and n-heptane adsorption isotherms have suggested that the kink is due to a phase transition because of the interplay between the length of the zigzag pores and the length of the alkanes. Smit and Maesen32 found that the molecules can freeze in a configuration that fits the pore structure. In a previous paper,26 we used a temperature-programmed-equilibration (TPE) technique for studying the adsorption of linear paraffins in silicalite-1. This technique is, in fact, a modified temperature-programmed-desorption technique conducted in quasi-equilibrium conditions. We developed a model of localized adsorption on three independent sites corresponding to the zigzag channels, the straight channels, and the intersections. This model is similar to those proposed by Chiang and co-workers.14,15 In this paper, we have extended our results to the branched alkanes. The role of the phase transition in the system is discussed, and the TPE technique is used in order to calculate adsorption enthalpy and entropy variations. These results are compared to those already published in the literature. Experimental Section 1. Materials. Three silicalite-1 samples were used in this study (see Table 1). Crystal size and shape, as determined by (22) Richards, R. E.; Rees, L. V. C. Langmuir 1987, 3, 335. (23) Lohse, U.; Thamm, M.; Noack, M.; Fahlke, B. J. Inclusion Phenom. 1987, 5, 307. (24) van Well, W. J. M.; Wolthuizen, J. P.; Smit, B.; van Hooff, J. H. C.; van Santen, R. A. Progress in Zeolite and Microporous Materials. Stud. Surf. Sci. Catal. 1997, 105, 2347. (25) Sun, M. S.; Talu, O.; Shah, D. B. J. Phys. Chem. 1996, 100, 17276. (26) Millot, B.; Methivier, A.; Jobic, H. J. Phys. Chem. B 1998, 102, 3210. (27) Van Well, W. J. M.; Wolthuizen, J. P.; Smit, B.; van Hooff, J. H. C.; van Santen, R. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 2543. (28) Smit, B.; Siepmann, J. I. J. Phys. Chem. 1994, 98, 8442. (29) June, R. L.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1990, 94, 1508. (30) Maginn, E. J.; Bell, A. T.; Theodorou, D. N. J. Phys. Chem. 1995, 99, 2057. (31) Runnebaum, R. C.; Maginn, E. J. J. Phys. Chem. B 1997, 101, 6394. (32) Smit, B.; Maesen, T. L. M. Nature 1995, 374, 42.
Table 1. Properties of Samples crystal size (µm3) Si/Al
zeolite A
zeolite B
zeolite C
2 × 1 × 0.5 380
35 × 10 × 10 >2360
100 × 50 × 50 >2360
scanning electron microscopy, are reported in Table 1. Sample A was supplied by the French Institute of Petroleum at Solaize. Samples B and C were supplied by Dr. H. Kessler and Prof. R. Le Dred from Ecole National Supe´rieure de Chimie Mulhouse. The bulk Si/Al ratio obtained by X-ray fluorescence is also reported in this table. Sample A was used for TPE experiments, and samples B and C were used for the determination of the size effect and of the heating rate effect. Sample B was used for X-ray diffraction (XRD) experiments. 2. XRD Experiments. The XRD patterns were collected on silicalite-1 powders at different temperatures with different adsorbate loadings. In situ measurements were carried out using a Siemens D501 θ-2θ powder diffractometer equipped with an Anton Paar reaction chamber and a flow-through sample holder. The powdered sample (0.1 g) was placed on a sintered glass sieve and slightly pressed. The inlet reaction flow came through the sample. Cu KR radiation was used. The explored angular range and the step size were 6-50° and 0.05°, respectively. The step time was 5 s. Alkanes were introduced by circulating a helium stream through a saturator containing liquid alkanes. Hydrocarbon partial pressure in helium (0.11 bar) was controlled via the temperature of the saturator by means of a thermostated bath. The samples were heated at 673 K in an inert gas (helium) and then cooled to 473 K, the temperature at which adsorption was performed. A first XRD pattern was taken in these conditions. The sample was then cooled in a gas atmosphere of the adsorbed species, and XRD patterns were taken at decreasing temperatures, viz. 473, 388, 313, 303, and 288 K for 3-methylpentane. Each pattern was measured after 2 h of equilibration. 3. Temperature-Programmed-Equilibration Experiments. The TPE experiments were performed with a Setaram TAG 12 thermobalance. The procedure was the same as that given in a previous paper.26 Prior to the experiment, the sample was heated at 673 K under a flow of helium for 1 h in order to remove any adsorbate or template traces. The sample was then cooled to room temperature and saturated with gas. As with the XRD experiments, the introduction of branched alkanes was performed by circulating a flow of helium (56 cm3/min) through a saturator containing the liquid alkane. The hydrocarbon partial pressure (0.11 bar) was controlled with a thermostated bath. Isobutane which was gaseous at ambient temperature and pressure was introduced in the gas phase after being mixed with helium. Once the sample was saturated, the TPE was performed
2536 Langmuir, Vol. 15, No. 7, 1999
Millot et al.
Figure 4. TPE curves obtained for various silicalite-1 crystals for 3-methylpentane for sample A. Figure 2. XRD patterns of silicalite-1 loaded with 3-methylpentane in selected scanning ranges at different temperatures for sample B.
Figure 5. TPE curves obtained from C4 to C6 monobranched alkanes for sample A. Figure 3. TPE curves obtained for various desorption rates for 3-methylpentane for sample A (×, 2 deg/min; 4, 5 deg/min; 0, 8 deg/min; 2, 12 deg/min; O, 15 deg/min). with a linear heating rate. Most of the experiments were performed at 5 deg/min from 298 to 673 K, but other heating rates were also used in order to study their influence. Hydrocarbon pressure was maintained during the experiment. As the amount of desorbed hydrocarbons was always small due to the low zeolite quantity (20 mg), the alkane partial pressure in the cell could be considered constant during an experiment.
Theory As already described in previous papers,26,33 we have used a quasi-chemical approach which describes the zeolite as a solid solution of structural items. This allows us to take into account the framework specificity of silicalite-1 in the modeling. As described in the introduction of this paper, the interconnection of the straight channels and zigzag channels gives rise to near spherical cavities. We have assimilated these two types of channels and the intersections as three kinds of adsorption sites, as suggested by Chiang and co-workers.12,14,15 Four occurrences per unit cell of each of these three sites must be considered. This number is called the “ magic ” number by Olson and Reischman34 because it represents the number of intersections (I), the number of straight channels (S), and the number of zigzag (Z) channels per unit cell in silicalite-1. Adsorption equilibrium on each (33) Millot, B.; Methivier, A.; Jobic, H. Proceedings of the Sixth International Conference of Fundamentals of Adsorption, Presqu’Ile de Giens, France, 1998; Meunier, F., Ed.; Elsevier: Amsterdam.
Figure 6. Comparison between experimental (9) and calculated (s) TPE curves from isopentane for sample A.
site may be described by the following quasi-chemical equilibria:
A + Si a ASi
i ) (1, ..., number of sites) (1)
A is a sorbate molecule in the gas phase, Si is a vacant site i in the solid, and ASi is an adsorbed molecule on site i in the solid. One may consider Si and ASi as the quasichemical components of an ideal solution; that is, no interaction occurs between these components in the solution. Consequently, activities may be assigned as concentrations so that the equilibrium constants are
Adsorption of Branched Alkanes in Silicalite
Langmuir, Vol. 15, No. 7, 1999 2537
Figure 7. Comparison between experimental (O) and calculated (s) TPE curves from 3-methylpentane for sample A.
Figure 9. Free energy of sorption for various C5 alkanes in silicalite-1 as a function of temperature for sample A.
Figure 8. Free energy of sorption for various C4 alkanes in silicalite-1 as a function of temperature for sample A.
expressed as
Kiads )
[ASi]
with P ) P/Ps
P[Si]
Figure 10. Free energy of sorption for various C6 alkanes in silicalite-1 as a function of temperature for sample A.
(2) and according to the Van’t Hoff law
where P is the partial pressure, Ps is the standard pressure, [ASi] is the concentration of component ASi in the solution, and [Si] is the concentration of component Si in the solution. The isotherm for a paraffin may be obtained from these equations:
KiadsP
n
q
ads
)4
∑ i)1
1 + KiadsP
(3)
where Kiads is the adsorption equilibrium constant for equilibrium i. The adsorption enthalpy variations associated with site i are deduced from
∆Hi
ads
) ∆Ui
ads
- RT
Kiads ) exp
(
) (
)
∆Siads -∆Hiads - 1 exp R RT
(4)
where ∆Hiads, ∆Siads and ∆Uiads are respectively the enthalpy, entropy, and internal energy variations associated with adsorption from the gas phase on site i in the solid. Each site exhibits a single Langmuir adsorption behavior, and competition between these sites occurs. This expression contains 2n adjustable parameters which are ∆Hiads and ∆Siads. If we now consider the desorption process, the reverse equilibrium must be considered: n
qdes ) 4
∑ i)1
P Ki
des
+P
(5)
2538 Langmuir, Vol. 15, No. 7, 1999
Millot et al.
Table 2. Sorption Isosteric Enthalpy and Entropy Variations for Various Branched Alkanes in Silicalite-1 alkanes
pressure (bar)
loading at 303 K (molecules/unit cell)
isobutane isopentane 2-methylpentane 3-methylpentane
0.11 0.42 0.11 0.11
6 8 5.7 5.7
high-temperature desorption peak (site 1) ∆H (kJ/mol) ∆S (J/(mol K)) -50.2 - 58.4 - 61.5 - 68.4
- 88.2 - 101.5 - 116.0 - 113.0
Table 3. Sorption Isosteric Enthalpy and Entropy Variations for Various Alkanes in Silicalite-1 Given by Literaturea ∆H (kJ/mol)
alkanes -54b
N-butane
-58c
∆S (J/(mol K))
-50b
up to [19], up to -58c [22], -48 [38], -50 [30, 41, 45], -49 [29], b f -52 up to -60 [46, 47] -58 [38], -70 [39], -41.8 [25] -69.8b up to -82c [19], -62b up to -84c [22], -64b up to -76d [40], -84 [44], -72 [38], -82 [39], -70 [25, 30], -68 [29] -52 [39], -49 [11], -54.8 [45], -49 [41], -52 [42], -48,5b up to -54,5e [46, 47] -64 [39], -64 [42] -63 [29], -62.7 [43] -63 [29], - 67.7 [43]
N-pentane N-hexane isobutane isopentane 2-methylpentane 3-methylpentane
-118 [38], -65 [30], -75b up to -125f [46,47] -135 [39] -128 [38], -153 [39], -120b up to -172d [40], -85 [30] -75b up to -160e [46, 47]
a Reference numbers given in brackets. b Infinite diluted concentration in zeolite. c Concentration at saturation in the zeolite at ambient temperature. d 5.5 molecules per unit cell in silicalite-1. e 4 molecules per unit cell in silicalite-1. f 3 molecules per unit cell in silicalite-1.
where Kides is the desorption equilibrium constant for equilibrium i and Kides ) 1/ Kiads. Concerning experiments made under quasi-equilibrium conditions at constant pressure, we can use the derivative expression
∆Hides 1 -
dqdes dT
n
)4
∑ i)1
(
RT2 P
1+
1 P
exp
exp
(
(
∆Sides R
∆Sides R
) ( ) ) ( ))
- 1 exp -
- 1 exp -
∆Hides RT
∆Hides
2
RT
(6)
and ∆Si are the desorption isosteric where ∆Hi enthalpy and entropy variations associated with desorption from site i. des
des
∆Hides ) -∆Hiads ) -∆Hi and ∆Sides ) -∆Siads ) -∆Si (7) where ∆Hi and ∆Si are the sorption isosteric enthalpy and entropy variations, respectively. Results and Discussion XRD Experiments. Various X-ray diffraction patterns were obtained at different temperatures for pure zeolite and for zeolite containing 3-methylpentane. The transition from monoclinic to orthorhombic symmetry is essentially characterized by the loss of doublet splitting of the peaks at 24.4°, 29.3°, and 48.3° (2θ). For the pure activated sample B, this transition takes place between 353 and 363 K, as depicted by Figure 1 (in a selected scanning range: 22.5-25.4°). This result is in good agreement with the data published by Mentzen et al.3,4 This phase transition was also studied after preadsorption of 3-methylpentane on sample B with a procedure similar to that reported by Long et al.,35 who published XRD patterns with different loadings of p-xylene in silicalite-1. The XRD patterns were obtained at decreasing temperatures, viz. 473, 388, 313, and 303 K after adsorption of 3-methylpentane. Even if the zeolite was loaded with
more than four or five molecules per unit cell, the peaks, that is, at 24.4° (2θ), did not split (Figure 2). The structure remained orthorhombic whatever the temperatures (between 303 and 473 K) and the loadings. The phase transition was lowered between 313 and 288 K. So, the system containing 3-methylpentane was orthorhombic from 673 K to a temperature between 303 and 288 K. Similar results have been obtained by Kokotailo et al.36 with the adsorption of other molecules. Our results were also confirmed by measurements made by Hay and Jaeger.37 They reported that a reversible monoclinic T orthorhombic phase transition of their calcined silicalite-1 occurred at 318-328 K and that adsorption of linear hydrocarbons of chain length >C4 into the framework decreases the temperature of the transition. If the chain carbon number is between 7 and 12, the temperature of the phase transition is in the range 283-293 K. They obtained a decrease of approximately 30 K for this temperature. Our results are in agreement for branched molecules, but with a decrease of about 70 K. The decrease we observed is probably enhanced because of the chemical composition of the zeolite. To conclude, the symmetry of the zeolite structure loaded with 3-methylpentane does not change in the whole range of temperature (> 303 K) and taking into account the literature results, it is assumed that this behavior is also valid for the other branched alcanes. 2. TPE Experiments. Prior to applying a model on desorption curves, the experimental conditions which give rise to quasi-equilibrium desorption must be checked. Several TPD experiments were performed for 3-methylpentane with various heating rates and various crystal sizes. Figure 3 shows the comparison of TPD experiments performed at various heating rates for 3-methylpentane. Some differences can be observed for the high-temperature peaks as well as for the low-temperature ones. These differences could be due to diffusion limitations. For the lowest heating rates, these differences are suppressed. (34) Olson, D. H.; Reischman, P. T. Zeolites 1996, 17, 434. (35) Long, Y.-C.; Sun, Y.-J.; Zeng, H.; Gao, Z.; Wu, T.-L.; Wang, L.-P. J. Inclusion Phenom. 1997, 1. (36) Kokotailo, G. T.; Kennedy, C. A.; Gobbi, G. C.; Strobl, H.; Pasztor, C. T.; Barlow, G. E.; Bradley, S. Pure Appl. Chem. 1986, 58 (10), 1367. (37) Hay, D. G.; Jaeger, H. J. Chem. Soc., Chem. Commun. 1984, 1433.
Adsorption of Branched Alkanes in Silicalite
Moreover, Figure 4 shows the comparison of TPD curves for samples A, B, and C. It appears that the hightemperature peaks are similar while some differences occur for the low-temperature ones. Sample C exhibits a smaller peak than samples A and B, and this difference may be related to the large dimension of sample C, which confirms the occurrence of diffusion effects. It is likely from these results that kinetic effects become predominant at low temperature. Consequently, we will consider that high-temperature desorption occurs in conditions close to equilibrium while low-temperature desorption occurs in conditions which are rate limited. From the above results, a comparison between TPE curves for isobutane (iC4), isopentane (iC5), 2-methylpentane (2MP), and 3-methylpentane (3MP) is reported in Figure 5. All sorbates show two temperature regions. The XRD results allow us to conclude that the peaks at low temperature are not due to an abrupt change of symmetry of the zeolite structure. Considering these results, it is possible to use the described model of the theoretical part. The position of the high-temperature peaks is correlated with the adsorption site strength because desorption occurs in quasi-equilibrium conditions. These high-temperature data are analyzed by fitting the peaks with eq 6. For example, we have reported a very good agreement between the experimental data and the fitting for isopentane and 3-methylpentane (Figures 6 and 7). Isopentane and 2-methylpentane results look like those of isobutane and 3-methylpentane, respectively. We have given the values of the fitting parameters (∆H and ∆S) in Table 2. For linear alkanes, we have already demonstrated that a two-site model provided a very good description.26 It was demonstrated that, in all cases, the difference between the sorption enthalpy variations of the two sites is very low while sorption entropy variations are significantly larger. So entropic factors were responsible for the weaker adsorption at higher loadings. The low-temperature TPE peak of linear alkanes results in a higher variation of entropy of sorption. The larger the chain length of molecules (from n-butane to n-heptane measured by TPE experiments), the more important the difference between the adsorption entropy variations on site 1 and on site 2. The same conclusions can be derived qualitatively from the monobranched alkane TPE spectra. Moreover, for low coverage (from 0 to 4 molecules per unit cell), the affinity of silicalite-1 for linear alkanes is not greater than that for branched molecules, as evidenced by Figures 8-10. These results are in close agreement for adsorption enthalpy with the recent literature data (Table 3). For linear alkanes, we have shown that the results were in accordance with those of Van Well et al.24,27 and Olson and Reischman34 about entropic influence. Our results agree with calorimetric and gravimetric data found by Lercher and co-workers,38,39 with data of an isosteric method applied by Yang and Rees,40 with simulations at low loadings calculated by Theodorou and co-workers,29,30 (38) Eder, F.; Lercher, J. A. Zeolites 1997, 18, 75.
Langmuir, Vol. 15, No. 7, 1999 2539
and with simulations obtained by Smit and co-workers.28,32 For isobutane, we are in good agreement with values given by Thamm,11 Lercher et al.,39 and Hufton and Danner41 whereas isopentane data are lower than those found by Lercher et al.39 and larger than those predicted by Smit et al.42 (in absolute values). Concerning isosteric sorption enthalpy variations of 3-methylpentane and 2-methylpentane, it is of interest to note that the molecular simulation predictions29 are in reasonable agreement with our results and the experimental values obtained by Ruthven et al.43 The molecular simulations made by June et al.29 predicted a greater affinity of silicalite-1 for n-hexane rather than 2-methylpentane and 3-methylpentane, as evidenced by the larger isosteric sorption heat of n-hexane. As we have already explained, this is not so obvious when sorption entropies are taken into account. However, no comparison of experimental sorption entropies can be made because there are no available literature data for branched alkanes in silicalite-1. Except for the case of isobutane,46,47 it is the first time that sorption entropies are found for branched alkanes in this zeolite. Finally, for the location of molecules in silicalite-1, it is probable that due to their ramification the first branched alkanes are localized in the intersections of the channels. This was already suggested by molecular simulations29 and by the TEOM technique:46,47 monobranched alkanes were sorbed preferentially in the channel intersections while n-hexane tended to sit in the channels. Conclusion The XRD study of silicalite-1 showed that the adsorption of branched paraffins in the orthorhombic phase leads to the stabilization of this phase at room temperature. TPE experiments carried out with different linear or branched chain alkanes in silicalite-1 yield consistent adsorption values which agree well with published adsorption equilibrium data. As shown in a previous paper,26 the TPE technique is a very powerful technique to investigate adsorption of fast diffusing molecules in silicalite-1. This is confirmed by the present results. However, the limitation of this technique is clear when diffusion limitations become critical. This is shown by the results of branched alkanes at low temperature. In these conditions, classical adsorption equilibrium measurements are necessary. LA981344U (39) Eder, F.; Stockenhuber, M.; Lercher, J. A. J. Phys. Chem. B 1997, 101, 5414. (40) Yang, Y.; Rees, L. V. C. Microporous Mater. 1997, 12, 117. (41) Hufton, J. R.; Danner, R. P. AIChE J. 1993, 39, 954. (42) Smit, B.; Loyens, L. D. J. C.; Verbist, G. L. M. M. Faraday Discuss. 1997, 106, 000. (43) Cavalcante, C. L.; Ruthven, D. M. Ind. Eng. Chem. Res. 1995, 34, 177. (44) Titiloye, J. O.; Parker, S. C.; Stone, F. S.; Catlow, C. R. A. J. Phys. Chem. 1991, 95, 4038. (45) Chiang, A. S.; Dixon, A. G.; Ma, Y. H.Chem. Eng. Sci. 1984, 39, 1461. (46) Zhu, W.; van de Graaf, J. M.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 1966, 5, 1934. (47) Zhu, W.; van de Graaf, J. M.; van den Broeke, L. J. P.; Kapteijn, F.; Moulijn, J. A. J. Am. Chem. Soc. 1998, 120, 5599.
