Adsorption of C7 Hydrocarbons on Biporous SBA-15 Mesoporous Silica

Adsorption of C7 Hydrocarbons on Biporous SBA-15. Mesoporous Silica. Hoang Vinh-Thang,† Qinglin Huang,‡ Mladen Eic,‡ Do Trong-On,† and...
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Adsorption of C7 Hydrocarbons on Biporous SBA-15 Mesoporous Silica Hoang Vinh-Thang,† Qinglin Huang,‡ Mladen Eic´,‡ Do Trong-On,† and Serge Kaliaguine*,† Department of Chemical Engineering, Laval University, Ste-Foy, Que´ bec, Canada, G1K 7P4, and Department of Chemical Engineering, University of New Brunswick, P. O. Box 4400, Fredericton, New Brunswick, Canada, E3B 5A3 Received January 18, 2005. In Final Form: February 23, 2005 In our recent studies (Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Langmuir 2005, 21, 2051-2057; Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Stud. Surf. Sci. Catal. 2005, in press), a series of synthesized SBA-15 materials were characterized using nitrogen adsorption/ desorption isotherms at 77 K and SEM images. In the present paper, four of them (MMS-1-RT, MMS-1-60, MMS-1-80, and MMS-5-80) were further investigated with regard to their equilibrium characteristics using n-heptane and toluene as sorbates by the standard gravimetric technique. SBA-15 materials proved to have a broad pore size distribution within the micropore/small-mesopore range in the walls of their main mesoporous channels. The adsorption capacities for toluene were found to be higher than for n-heptane. The isosteric heats of adsorption, estimated by the Clausius-Clapeyron equation, are also higher for toluene compared to n-heptane. They were found to depend on framework microporosity of the relevant SBA-15 samples. The isosteric heats of adsorption for all sorbates decrease with increased loading and approach the heats of evaporation of the respective sorbate. The adsorption capacities of SBA-15 samples are significantly higher than those of silicalite, i.e., the MFI zeolite silica analogue. In contrast to that, the isosteric heats of adsorption in the mesopore channels of SBA-15 were found to be much smaller. This result also suggests that SBA-15 can potentially be a good candidate for separation of C7 hydrocarbons.

1. Introduction SBA-15 is a member of a new family of silicate mesoporous materials recently discovered.3,4 In comparison to the M41S family,5 SBA-15 is characterized by highly ordered mesopores of larger size, higher hydrothermal stability due to their thicker walls, and lower specific surface areas. Moreover, the mesoporous channels of SBA15 are interconnected by smaller pores within its amorphous silica walls.6-8 For all these reasons, SBA-15 has attracted a great deal of attention in the field of catalysis, separation, and advanced materials.9 However, available literature data related to the adsorption of hydrocarbons, particularly bulkier ones, are limited. Up to now, only a few articles dealing with this particular topic have been published.10-12 Newalkar and co-workers * Corresponding author. Phone: (418) 656-2708. Fax: (418) 6563810. E-mail: [email protected]. † Laval University. ‡ University of New Brunswick. (1) Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Langmuir 2005, 21, 2051-2057. (2) Vinh-Thang, H.; Huang, Q.; Eic´, M.; Trong-On, D.; Kaliaguine, S. Stud. Surf. Sci. Catal. 2005, in press. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. (4) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024-6036. (5) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (6) Miyazawa, K.; Inagaki, S. Chem. Commun. 2000, 2121-2122. (7) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961-1968. (8) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105, 6817-6823. (9) Trong-On, D.; Desplantier-Giscard, D.; Danumah, D.; Kaliaguine, S. Appl. Catal. A: Gen. 2003, 253, 545-594. (10) Newalkar, B.; Choudary, N. V.; Kumar, P.; Komarneni, S.; Bhat, T. S. G. Chem. Mater. 2002, 14, 304-309. (11) Newalkar, B.; Choudary, N. V.; Turaga, U. T.; Vijayalakshmi, R. P.; Kumar, P.; Komarneni, S.; Bhat, T. S. G. Chem. Mater. 2003, 15, 1474-1479.

investigated the adsorption of light C1-C3 hydrocarbons on microporous-mesoporous SBA-15 materials.10,11 In these studies, the adsorption capacity of light hydrocarbons was found to be strongly dependent on the framework porosity. They also suggested that SBA-15 has some potential as an adsorbent for light hydrocarbon separation. The adsorption isotherms of n-heptane and benzene on large-pore SBA-15 materials were presented in the studies of Hartmann and Vinu.12 However, the authors used available equilibrium data in a limited way, only to estimate the pore diameter. Recently, we have reported the transport behavior of three larger hydrocarbons, i.e., n-heptane, cumene, and mesitylene, in biporous SBA-15.1,2 These materials were found to have not only an array of hexagonally ordered primary channels, but also a certain amount of smaller pores with a broad pore size distribution in the micropore and small mesopore range within their mesoporous walls. The microporosity could be controlled systematically by the synthesis conditions. These results were similar to those reported by several other authors,6-8,13-17 who indicated the existence of micropores within the walls of the primary mesopores. Furthermore, the diffusion results from previous studies1,2 showed that the intrawall micropores played a major role in controlling diffusion at a low adsorbate concentration level. The overall diffusion process of n-heptane was controlled by a combination of (12) Hartmann, M.; Vinu, A.. Langmuir 2002, 18, 8010-8016. (13) Galarneau, A.; Cambon, H.; Renzo, Di F.; Ryoo, R.; Choi, M.; Fajula, F. New J. Chem. 2003, 27, 73-79. (14) Davidson, A.; Berthault, P.; Desvaux, H. J. Phys. Chem. B 2003, 107, 14388-14393. (15) van Grieken, R.; Calleja, G.; Stucky, G. D.; Melero, J. A.; Garcia, R. A.; Iglesias, J. Langmuir 2003, 19, 3966-3973. (16) van Bavel, E.; Cool, P.; Aerts, K.; Vansant, E. F. J. Phys. Chem. B 2004, 108, 5263-5268. (17) Yang, C. M.; Zibrowius, B.; Schmidt, W.; Schuth, F. Chem. Mater. 2004, 16, 2918-2925.

10.1021/la050135o CCC: $30.25 © 2005 American Chemical Society Published on Web 04/14/2005

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Table 1. Textural Properties of Ordered Mesoporous SBA-15 Silica and Silicalite Samplesa sample

heating time (day)

synth temp (°C)

SBET (m2/g)

Vtb (cm3/g)

Vmesoc (cm3/g)

micropore vol (cm3/g)

total intrawall pore vol (cm3/g)

microporosityd (%)

primary mesopore diame (nm)

MMS-1-RT MMS-1-60 MMS-1-80 MMS-5-80 silicalite

1 1 1 5 5

RT 60 80 80 175

680 750 860 920 380

0.447 0.579 0.854 1.106

0.324 0.459 0.747 1.053

0.121f 0.119f 0.105f 0.059f 0.145g

0.19 0.22 0.33 0.32

63.2 54.5 30.3 18.8

4.6 5.5 7.1 7.8

a Data from Vinh-Thang et al.1 b Total pore volume (V ) calculated from P/P ) 0.99 of the N adsorption/desorption isotherms. c Total t o 2 mesopore volume (Vmeso) determined by the modified BJH method.20 d Percentage of micropore volume relative to the total intrawall pore e volume calculated as described in ref 1. Primary mesopore diameter determined by the modified BJH method. f Calculated from Rs-plots. g Calculated from t-plots.

Figure 2. TEM pictures of a calcined sample of MMS-5-80.

Figure 1. XRD pattern of a calcined sample of MMS-5-80.

micropore- and mesopore-diffusion resistances in the mesopore walls and depended on the relative content of micropores.1,2 However, no adsorption data concerning these sorbates have been reported yet. The present work has been undertaken with the aim of studying the adsorption of C7 hydrocarbons, i.e., n-heptane and toluene, on four biporous SBA-15 materials using a CAHN D-200 digital microbalance. Because of the microbalance measurement restrictions related to the limited vapor pressure range of heavier sorbates, such as cumene and mesitylene, n-heptane and toluene, as somewhat lighter components, were chosen in this study. In addition, n-heptane and toluene are two gas molecules involved in the dehydrocyclization of n-heptane, which is considered an important reaction in the reforming process.18 Information about the adsorption capacities, heats of adsorption, selectivities, and diffusion properties provide necessary information that leads to a better understanding of the gas separation involving mesoporous molecular sieves. 2. Experimental Section 2.1. Materials. Four hexagonal SBA-15 mesoporous silica samples (MMS-1-RT, MMS-1-60, MMS-1-80, and MMS-5-80) having different microporosities, as described in the previous paper, were used.1 A microporous silica sample having an aluminum-free MFI structure (silicalite) was synthesized according to the reported method.19 The calcined powders of these materials were characterized by nitrogen adsorption/desorption isotherms and scanning electron microscopic (SEM) images. Their X-ray diffraction (XRD) patterns were obtained using a Brucker D4 diffractometer and Cu KR radiation. Figure 1 shows the XRD pattern of a calcined MMS-5-80 sample. Transmission electron microscopic (TEM) images were obtained using a JEO JEM2011 electron microscope operating at 200 kV. An example of these (18) Szczygiel, J.; Szyja, B. Microporous Mesoporous Mater. 2004, 76, 247-254. (19) Trong-On, D.; Kapoor, M. P.; Thibault, E.; Gallot, J. E.; Lemay, G.; Kaliaguine, S. Microporous Mesoporous Mater. 1998, 20, 107-118.

images obtained for MMS-5-80 sample is depicted in Figure 2. The textural properties are summarized in Table 1. 2.2. Adsorption Equilibrium Measurements. Adsorption equilibrium measurements were carried out using a digital microbalance (CAHN Instrument, Model D-200) connected to a high-vacuum system. In the first step about 8-10 mg of powdered sample was loaded on a small pan. Under a high vacuum of 10-5 Torr or better, the powder was heated to 120 °C for at least 3 h and then the temperature was raised to 250 °C to eliminate possible adsorbed impurities and hygroscopic moisture in particular. After at least 6 h of this regeneration, the sample was cooled to the required experimental temperature. Isotherm measurements were carried out by introducing a dosed amount of hydrocarbon vapor directly to the sample chamber of the balance, and recording the weight change after reaching a stable equilibrium pressure. Further consecutive measurements were taken by incrementing the vapor pressure by steps. The organic sorbates used for the adsorption experiments were n-heptane (99 wt %, Aldrich) and toluene (99 wt %, Aldrich).

3. Results and Discussion 3.1. Adsorption Equilibrium Isotherms. The adsorption isotherms of n-heptane and toluene were obtained for four SBA-15 samples with different microporosities at 25, 30, and 35 °C. The adsorption on silicalite was carried out at 35, 50, and 65 °C. Figures 3 and 4 summarize these isotherms for n-heptane and toluene, respectively. As can be seen from the figures, all isotherms involving SBA-15 samples are of type IV according to the IUPAC classification. Similar results for the adsorption of n-heptane on SBA-15 and MCM-48 were reported by Hartmann.12,21 The capillary condensation of n-heptane and toluene in the mesopore channels of SBA-15 was recently established by van Bavel et al.22 As shown in Figures 3 and 4, the adsorption isotherms of n-heptane and toluene on the silicalite sample are similar and can be classified as type I, typical of microporous materials. Figure 5 shows the dependence of adsorption capacities (expressed in cubic centimeters of liquid sorbate per gram) (20) Kruk, M.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1997, 101, 583-589. (21) Hartmann, M.; Bischof, C. J. Phys. Chem. B 1999, 103, 62306235. (22) van Bavel, E.; Meynen, V.; Cool, P.; Lebeau, K.; Vansant, E. F. Langmuir 2005, 21, 2447-2453.

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Figure 3. Adsorption isotherms of n-heptane on SBA-15 and silicalite samples at various temperatures.

of n-heptane and toluene at 25 °C on the four SBA-15 samples with relative pressure (P/Po). The values of the total pore volume were determined from the high-pressure plateau of the isotherms. As expected according to the Gurvitsch rule,23 the maximum values of the pore volumes for the two sorbates are almost identical. Interestingly, Figure 5 shows almost the same relative pressures at the inflection point for both sorbates on the respective adsorption isotherms. Due to the differences in pore diameter, capillary condensation takes place at different relative pressures for the various samples and was observed at P/Po ranging from 0.25 to 0.55 at 25 °C. The pressure of capillary condensation increases gradually with increasing synthesis temperature and heating time, due to the increase in primary mesopore diameter. Table 2 presents the estimated values of total pore volume for the four SBA-15 samples calculated from the (23) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.

adsorption isotherms of n-heptane and toluene at 25 °C. The values estimated from nitrogen adsorption data at 77 K are also included for comparison. The values obtained from n-heptane and toluene adsorption data are very similar, about 20-30% lower than those obtained from nitrogen data. Similar observations were interpreted by van Bavel et al.22 as reflecting the partial filling of the micropores of SBA-15 by n-heptane or 3-methylpentane. This assumption cannot be accepted here because the differences Vnitrogen - Vn-heptane and Vnitrogen - Vtoluene are larger than the micropore volumes. As discussed below, differences in the density of the adsorbed hydrocarbons and liquid hydrocarbons yield another explanation. The equilibrium adsorption capacities for the two hydrocarbons, expressed in millimoles per gram, at different temperatures are summarized in Table 3. The results show that the adsorption capacities for n-heptane are substantially lower than those for aromatic toluene. Similar results for these two sorbates obtained on SBA-

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Figure 4. Adsorption isotherms of toluene on SBA-15 and silicalite samples at various temperatures.

15 materials were reported by van Bavel et al.22 Furthermore, the adsorption capacities for both sorbates increase with specific surface area (SBET) (see Table 1); e.g., the adsorption coverage of each sorbates is in the following order: MMS-5-80 > MMS-1-80 > MMS-1-60 > MMS-1-RT > silicalite. In comparison to silicalite, the adsorption capacities of the two sorbates in the SBA-15 samples are much higher, corresponding to the larger pore volume of the mesoporous adsorbent. As can be seen from Table 3, the values of the adsorption capacity of SBA-15 samples are 2-7 times higher compared to silicalite. Our adsorption capacity values for C7 hydrocarbons on the silicalite are close to those reported in the literature. For example, for n-heptane, Eder and Lercher24 reported a maximum coverage of 1.05 mmol/g on silicalite at 60 °C. Newalkar et al.25 obtained a value of 0.69 mmol/g from (24) Eder, F.; Lercher, J. A. Zeolites 1997, 18, 75-81. (25) Newalkar, B.; Jasra, R. V.; Kamath, V.; Bhat, T. S. G. 12th International Zeolite Conference; 1999; pp 127-134.

the adsorption data on AlPO4-5 at 30 °C. For toluene, Song and Rees26 found a value of 1.2 mmol/g on silicalite at 50 °C. Similarly, our adsorption results for mesoporous materials are also comparable with literature data. With respect to this, the adsorption capacities of n-heptane on our SBA-15 samples are close to those reported by Hartmann and Bischof21 for cubic mesoporous MCM-48. Furthermore, results from the present study for toluene are only slightly higher in comparison to results obtained for MCM-48 by Lee et al.27 3.2. Henry’s Constants. The Henry’s constants were calculated from isotherm data at low pressures, i.e., P < 0.1 Torr, and are listed in Table 3. These values are evaluated with a precision of (1 × 103. This particular parameter reflects adsorption affinity in the linear region (26) Song, L.; Rees, L. V. C. Microporous Mesoporous Mater. 2000, 35-36, 301-314. (27) Lee, J. W.; Shim, W. G.; Yang, M. S.; Moon, H. J. Chem. Eng. Data 2004, 49, 502-509.

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Figure 5. Adsorption isotherms of n-heptane and toluene at 25 °C. The amounts adsorbed are expressed in volumes of liquid sorbate. Table 2. Total Pore Volume (Vt, cm3/g) Calculated from Adsorption Isotherms of Different Sorbates sample

nitrogena

n-heptane

toluene

MMS-1-RT MMS-1-60 MMS-1-80 MMS-5-80

0.447 0.579 0.854 1.106

0.322 0.415 0.654 0.798

0.320 0.419 0.657 0.819

a

Data from Vinh-Thang et al.1

of the adsorption isotherm. Generally, for both sorbates, the Henry’s constants increase with increasing intrawall pore volume and show the same trend as adsorption capacity, i.e., MMS-5-80 > MMS-1-80 > MMS-1-60 > MMS-1-RT. This observation logically suggests that in the linear region adsorption process is controlled by filling of both micro- and mesopores within the walls of the primary channels. The Henry’s constants of n-heptane are systematically lower than those of toluene on all SBA-15 samples. In contrast, these values on silicalite display an opposite trend. A similar sequence for adsorption of n-heptane and toluene on silicalite was reported by Szczygiel and Szyja.18 This result is obviously associated with the less polar (even hydrophobic) nature of the silicalite surface. At 35 °C the Henry’s constants of n-heptane on biporous SBA-15 were found to be 1-2 orders of magnitude lower than those on silicalite. It is obvious from these results that n-heptane is much less strongly adsorbed in the amorphous walls of SBA-15 materials than in the micropore channels of silicalite. This result reflects the relatively highly polar nature of the intrawall pore surface of SBA-15, which has a low affinity for the alkyl chain of n-heptane. Such behaviors provide favorable conditions for the separation of the n-heptane-toluene mixtures on SBA-15 materials, since the aromatic ring of toluene interacts rather strongly

with the surface silanols. Table 4 shows the values of Henry’s constant ratios, or selectivities for toluene/nheptane at different temperatures. The high values observed for all SBA-15 materials, in particular for sample MMS-1-RT, indicate that these materials will selectively adsorb toluene at moderate temperatures and at low gasphase partial pressures corresponding to chromatographic separation conditions. By contrast, the ratio of K values is lower than 1 for silicalite and indicates a less favorable selectivity. This finding reveals the potential of mesoporous silica SBA-15 as a good adsorbent for separation of C7 hydrocarbons. 3.3. Isosteric Heats of Adsorption. The isosteric heats of adsorption (∆Hads, kJ/mol) of the two C7 hydrocarbons on the four SBA-15 samples, as well as the reference silicalite sample at different sorbate loadings, were evaluated from the temperature dependence of adsorption data, using the Clausius-Clapeyron equation. Figure 6 shows the dependence of the isosteric heats of adsorption for n-heptane and toluene with respect to the coverage, expressed in volume of liquid sorbate per unit mass of adsorbent. Values corresponding to the lower and higher measures of coverage are listed in Table 3. These heats are significantly higher for toluene in comparison to n-heptane over the entire range of coverage. The strong sorption of aromatic toluene is caused by the strong interaction of π-electrons from the aromatic ring with the hydroxyl groups on the adsorbent surface. The magnitudes of isosteric heats of adsorption for all four mesoporous SBA-15 samples were found to be in the order toluene > n-heptane. Similar results were also reported by Choudhary and Mantri28 as well as Hernandez et al.29 (28) Choudhary, V. R.; Mantri, K. Langmuir 2000, 16, 7031-7037.

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Table 3. Henry’s Constants (K), Equilibrium Adsorption Capacities (Q), and Isosteric Heats of Adsorption (∆Hads) at Low and High Coverage of n-Heptane and Toluene -∆Hads (kJ/mol) sample

sorbate

T (°C)

K (10-5)

Q (mmol/g)

MMS-1-RT

n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene n-heptane n-heptane n-heptane toluene toluene toluene

25 30 35 25 30 35 25 30 35 25 30 35 25 30 35 25 30 35 25 30 35 25 30 33 35 50 65 35 50 65

0.89 0.42 0.20 8.14 4.42 2.28 1.58 0.84 0.42 8.76 4.64 2.36 1.57 0.86 0.46 8.96 5.02 2.77 1.78 0.89 0.48 9.99 5.35 3.66 21.85 11.50 6.51 7.51 4.48 2.40

2.19 2.16 2.13 3.01 2.99 2.89 2.82 2.80 2.73 3.93 3.84 3.78 4.44 4.37 4.27 6.15 6.01 5.84 5.42 5.33 4.98 7.68 7.46 7.36 1.30 1.24 1.16 1.23 1.07 0.97

MMS-1-60

MMS-1-80

MMS-5-80

silicalite

Table 4. Toluene-to-n-Heptane Ratio of Henry Constants (K) sample

25 °C

30 °C

35 °C

MMS-1-RT MMS-1-60 MMS-1-80 MMS-5-80 silicalite

9.15 5.54 5.71 5.61

10.52 5.52 5.84 6.01

11.40 5.62 6.02 0.34

50 °C

65 °C

0.39

0.37

In the case of the silicalite sample, the isosteric heats of adsorption for n-heptane were generally found to be higher than those for toluene. These differences can be explained by taking into consideration both the pore size of the adsorbent and the nature of sorbate-sorbent interactions. For a nonpolar molecule such as n-heptane, the sorption is expected to be governed by nonspecific van der Waals forces of attraction with the sorbent surface. On the other hand, the sorption of toluene is driven by the π-electrons of the benzene ring. These sorbates will orient in the channels in such a way as to maximize their interactions with the channel walls. Hence, the critical diameters of sorbate will influence the confinement effect in the channels. As a consequence, in systems of small pore size, such as silicalite, higher isosteric heats of adsorption are expected for n-heptane (critical diameter 4.3 Å) than for larger toluene (critical diameter 5.75 Å).30 In addition, the absence of silanol groups in the micropores of a well-crystallized silicalite will minimize the role of π-electron interactions with the surface groups, thus further reducing the adsorption potential of toluene. As can be seen in Table 3, the isosteric heats of adsorption for the two sorbates are lower on all biporous SBA-15 samples compared to silicalite. The values cal(29) Hernandez, M. A.; Velasco, J. A.; Asomoza, M.; Solis, S.; Rojas, F.; Lara, V. H. Ind. Eng. Chem. Res. 2004, 43, 1779-1787. (30) Choudhary, V. R.; Nayak, V. S.; Choudhary, T. V. Ing. Eng. Chem. Res. 1997, 36, 1812-1818.

low coverage

high coverage

107.2

40.4

109.4

46.7

96.0

38.8

104.5

43.7

82.8

37.3

93.5

41.3

71.8

36.8

89.1

39.7

125.8

53.9

113.3

49.9

culated from adsorption data at high coverage are remarkably lower than those reported in the literature. For example, for toluene, Choudhary and Mantri28 reported a value of 62.0 kJ/mol on highly siliceous MCM-41, while Choudhary et al.31 obtained a value of 65.7 kJ/mol on silicalite-1 by using the temperature programmed desorption method. For n-heptane, the isosteric heats of adsorption at zero coverage on Na-Y, beta, mordenite, ZSM-5, and ZSM-22 were found to be 51.9, 72.6, 77.0, 79.6, and 87.9 kJ/mol, respectively.32 Moreover, a value of 83.4 kJ/mol obtained from the adsorption data of n-heptane on silicalite was reported by Sun et al.33 At high coverage the main adsorption process is due to capillary condensation in the primary mesopores, so that the low heats of adsorption reflect the larger diameter of these pores. From the data presented in Figure 7, it can be observed that, at the high coverage, the difference between ∆Hads values and the heats of vaporization for the respective sorbate is smaller, as the primary mesopore diameter is larger. As can be seen in Figure 6, the isosteric heats of adsorption decrease with an increase in loading for the two sorbates investigated in this study. In the initial stage of adsorption, the sorbate-sorbent interactions play a dominant role. Gregg and Sing23 pointed out that the initial heats of adsorption in microporous solids are much larger than on a flat surface as a consequence of the overlap of the adsorption field from the walls of the micropores. This effect was shown to be much more significant in cylindrical micropores compared to slit-shaped ones. Hence, the high isosteric heats of adsorption at the initial stage can be attributed to the filling of ultramicropores which are (31) Choudhary, V. R.; Srinivasan, K. R.; Singh, A. P. Zeolites 1990, 10, 16-20. (32) Denayer, J. F. M.; Souverijns, W.; Jacobs, P. A.; Martens, J. A.; Baron, G. V. J. Phys. Chem. B 1998, 102, 4588-4597. (33) Sun, M. S.; Talu, O.; Shah, D. B. J. Phys. Chem. 1996, 100, 17276-17280.

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Figure 6. Dependence of isosteric heats of adsorption (∆Hads) with surface coverage for the adsorption of n-heptane and toluene on SBA-15 and silicalite samples. The vertical dashed lines indicate micropore volume and intrawall pore volume, respectively.

Figure 7. Effect of primary mesopore diameter of SBA-15 samples on the isosteric heats of adsorption (∆Hads) at high coverage.

present in the various samples used in this study. Also from Figure 6, the isosteric heats of adsorption show a sharp decrease before complete filling of the micropores. Such a decrease has often been ascribed to heterogeneity of adsorption sites. For example, surface heterogeneity has been proposed for the adsorption of ethane, ethylene, acetylene, propane, and propylene on hexagonal SBA-15 mesoporous silica.10,11 Heterogeneous sorption sites in MFI zeolites have been suggested for various sorbates by several researchers.34-36 Choudhary and Mantri28 ex(34) Reischman, P. T.; Schmitt, K. D.; Olson, D. H. J. Phys. Chem. 1988, 92, 5165-5169. (35) Thamm, H. J. Phys. Chem. 1987, 91, 8-11.

plained an initial sharp decrease of heat of adsorption for benzene, toluene, p-xylene, and mesitylene by site heterogeneity of a pure siliceous MCM-41 sample. Another plausible explanation for such behavior could be attributed to the “lateral” sorbate-sorbate interactions, in which the first adsorbed molecules are preventing the subsequent sorbate molecules to get closer to the surface; thus the latter ones are becoming less strongly adsorbed. Such an effect is consistent with the so-called confinement effect. Furthermore, the density of the adsorbed phase should then be lower than that of the free liquid phase. This would then explain why the total pore volume Vt, calculated from the assumption of the same density for these two phases, was found to be lower by using n-heptane and toluene isotherm data than the corresponding results obtained from nitrogen data (see Table 2). Moreover, the lateral interactions provide an additional explanation for the shape of the isosteric heat of adsorption curve from the consideration of Figure 8. In this figure, the various ranges of observed heats of adsorption during the filling of the volume of the micropores are plotted as functions of this volume. It is indeed seen that as the micropore volume decreases the range of ∆Hads corresponding to the filling of micropores becomes more narrow. Furthermore, the decreasing upper value at decreasing micropore volume may correspond to two possible effects. First, the micropores may become larger when the volume is decreased. Second, the nature of the micropore surface may change. As mentioned above, the decreasing micropore volume is obtained by increasing the severity of the heating synthesis step (higher temperature or longer time). These more severe conditions may cause more silanol groups to disappear by condensation, thus creating a less polar surface. This would lead to the lower heats of adsorption for n-heptane and toluene, and at the same time these heats are getting closer to each other, as can be seen from Figure 6. (36) Rudzinski, W.; Narkiewicz-Michalek, J.; Szabelski, P.; Chiang, A. S. T. Langmuir 1997, 13, 1095-1103.

Adsorption of C7 Hydrocarbons on Biporous SBA-15

Figure 8. Heats of adsorption relative to the micropore filling of the SBA-15 samples with n-heptane and toluene at 25 °C.

4. Conclusions In this study, four biporous SBA-15 mesoporous materials showing intrawall porosity ranging from mostly micropores to predominantly mesopores were further characterized by adsorption isotherms using n-heptane and toluene as probe molecules. Adsorption capacity values for these C7 molecules in the SBA-15 samples are generally much higher than in

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the reference microporous silicalite. These capacities for linear n-heptane are substantially lower than those for aromatic toluene. Furthermore, they strongly depend on specific surface areas (SBET) and intrawall pore volume of the SBA-15 materials used in this study, which can be tailored by changing the synthesis conditions. The isosteric heats of adsorption of both sorbates in the SBA-15 samples show a sharp decrease at low surface coverage. These results, when analyzed with regard to the varying intrawall porosity, suggest that this sharp decrease is more likely due to sorbate lateral interactions than to heterogeneity of adsorption sites. They also indicate changes in the size and/or surface polarity of the micropores relative to the severity of the synthesis conditions. The selectivities of these SBA-15 materials for n-heptane and toluene adsorption are relatively high when expressed as the ratio of the Henry’s constants. This suggests that these materials could be suitable as chromatographic adsorbents for the separation of linear from aromatic hydrocarbons at relatively low temperatures, i.e., room temperature. Acknowledgment. This project was supported by the Natural Science and Engineering Research Council of Canada (NSERC) through the Chair on Industrial Nanomaterials. We thank Professor Dongyuan Zhao from Fudan University and Mr. Zhang Fu Qian and Mr. Xie Song Hai from his laboratory for providing the XRD and TEM data. LA050135O