Al ratio and the synthesis method on the low-coverage adsorption properties

Microporous and Mesoporous Materials 60 (2003) 111–124 www.elsevier.com/locate/micromeso

High-temperature adsorption of n-alkanes on ZSM-5 zeolites: influence of the Si/Al ratio and the synthesis method on the low-coverage adsorption properties Ilbige C. Arik, Joeri F. Denayer *, Gino V. Baron Dienst Chemische Ingenieurstechniek, Vrije Universiteit Brussel, Pleinlaan 2, Brussel B-1050, Belgium Received 15 October 2002; received in revised form 14 March 2003; accepted 14 March 2003

Abstract The influence of the composition and synthesis method on the low-coverage adsorption properties of C5 –C9 n-alkanes on ZSM-5 zeolites was studied using the pulse chromatographic technique at temperatures between 200 and 400 °C. Experiments were performed with materials having Si/Al ratios between 12 and 400, synthesized with and without an organic template. For all ZSM-5 samples, the Henry adsorption constants increase exponentially with the carbon number, while zero-coverage adsorption enthalpies increase in a linear way. With decreasing Al content, the Henry constants and adsorption enthalpies decrease. An increase in adsorption enthalpy of 10.1 kJ/mol per added –CH2 – group is observed for an Si/Al ratio of 400, while an increase of 12.1 kJ/mol is found for an Si/Al ratio of 15. The contribution to the adsorption entropy per carbon atom depends on the ZSM-5 composition and varies between 11.2 and 14.4 J/(mol K). A significant effect of the synthesis method on the Henry constants, adsorption enthalpies and entropies is observed. All ZSM-5 samples synthesized using organic templates show the same unique relationship between adsorption enthalpy and entropy, different from that of zeolites synthesized without organic template. Ó 2003 Elsevier Science Inc. All rights reserved. Keywords: Adsorption; ZSM-5; Linear alkanes; Si/Al ratio; Synthesis method

1. Introduction Among all zeolites, ZSM-5 is one of the most frequently studied, investigated in numerous catalytic, adsorption and diffusion studies. Molecules hosted in the narrow pores of ZSM-5 have a strong interaction with the force field ex-

*

Corresponding author. Tel.: +32-2-6291798; fax: +32-26293248. E-mail address: [email protected] (J.F. Denayer).

erted by the zeolite pore walls leading to pronounced differences in the adsorption of molecules with different size, polarity, shape etc. Obviously, a good knowledge of these adsorption properties will help to understand zeolite catalysis, explaining the efforts put into the determination of adsorption equilibria on ZSM-5 zeolites. Particularly, the adsorption of n-alkanes on ZSM-5 has been studied extensively by a variety of experimental techniques (Table 1) [1–26]. Generally, a linear increase of the adsorption enthalpy with the alkane carbon number was

1387-1811/03/$ - see front matter Ó 2003 Elsevier Science Inc. All rights reserved. doi:10.1016/S1387-1811(03)00332-9

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Nomenclature A B CN L K 0 ðT Þ K00 nT ph R T vf

parameter showing the dependency of
ln K00 to DH0 (mol/kJ) parameter showing the dependency of
ln K00 to DH0 carbon number column length (cm) Henry constant at temperature T (mol/ (kg Pa)) pre-exponential factor of the vanÕt Hoff equation for adsorption (mol/(kg Pa)) number of adsorption sites standard pressure (100 000 Pa) gas constant (J/(mol K)) absolute temperature (K) superficial gas velocity (cm/s)

observed [2,5,7,8,10,14,17,19,20,22,25,27]. The linear relationship between adsorption enthalpy and carbon number is explained by the additive character of the dispersive forces invoked upon adsorption of alkanes [12,14,19,28]. Eder et al. [19] claimed that, at low-coverages, hydrocarbons are adsorbed via hydrogen bonds to the Brønsted acid sites of the zeolites. Beside dispersive interactions, this hydrogen bonding contributes to a limited extent to the total adsorption enthalpy. The strength of hydrogen bonding slightly increases with increasing size of the adsorbed alkane, caused by an increase of the mean polarizability of the alkanes with chain length. A large experimental dataset on the low-coverage adsorption of alkanes is available. In Table 1, published data on adsorption enthalpies of n-alkanes (C1 –C10 ) on ZSM-5 and silicalite-1 zeolites are summarized and shown in Fig. 1 as a function of the carbon number. Strikingly, a very large scatter in the experimental data is observed, showing differences up to 32 kJ/mol between the lowest and highest reported value for one particular n-alkane (Table 1). With respect to the increase of the adsorption enthalpy with carbon number, also different values are reported: 9.81 kJ/ mol per added –CH2 – group [17], 9.96 kJ/mol [5], 10.08 kJ/mol [10], 10.2 kJ/mol [7], 11.0 kJ/mol [24],

Greek symbols a, b parameters showing the dependency of
DH to CN (kJ/mol) c, d parameters showing the dependency of
DS to CN (J/(mol K)) adsorption enthalpy at zero-coverage DH0 (kJ/mol) h DS0;local localized adsorption entropy at zerocoverage (J/(mol K)) eext external porosity emacr porosity in macropores l first moment (s) qc crystal density (g/cm3 )

11.3 kJ/mol [8] and 12 kJ/mol [19]. Linear regression of data for pentane to decane on silicalite-1 obtained by Sun et al. [25] result in an increase of 14.7 kJ/mol per added –CH2 – group. It should be mentioned that the data of Sun et al. show a deviation from linear behavior for n-hexane and nheptane, which is attributed to the unique manner in which these molecules occupy the channels of silicalite-1. Richards and Rees [14] found a slope of 10 kJ/mol from ethane to n-butane, and only 5 kJ/mol from n-butane to n-hexane. Finally, using the values for propane to n-hexane by Narbeshuber et al. [20] an increase of 15.9 kJ/mol per –CH2 – group was calculated. Configurational-bias Monte Carlo simulations by Smit [27] result in an increase of 11 kJ/mol for short alkanes (C4 –C8 ) and 13 kJ/ mol for long alkanes (C8 –C12 ). The more pronounced increase for long alkanes is explained by the preferential sitting of the long-chain alkanes in the linear channels of silicalite-1. Molecular simulations by Maginn et al. [22] contrarily lead to a value of 10.5 kJ/mol for the C4 –C28 n-alkane series. This distribution of experimental and theoretical data for such a commonly used catalyst as ZSM-5 is remarkable. Factors that could influence the experimental data are first of all the experimental methodology and technique, but other factors such as the synthesis method, composition (Si/Al

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124 Table 1 Adsorption enthalpies of C1 –C10 n-alkanes on ZSM-5/silicalite1 from the literature
DH0 (kJ/mol)

Table 1 (continued) 40 37a 34

G FR, SU Ch

41.4 40a 40.9

Cal, V FR, SU TEOM

40.5 39.85 35a 50.6 66.9 40a

Ch V Iso S S S

C4 H10 58 62 47 65a 77.4 56a 54a 52a 51a 51a 50 53

G, Cal G, Cal G Cal S Ch Ch Ch Ch Ch G TEOM

53a 51 50.2 53a 48.27 63.4 50a 45a 48.3 47

Cal Cal Ch Cal V S S S S Ch

[15] [8]

49a

[16] [5] [12,17] [18] [10] [1] [2] [11] [19] [20] [13] [13]

Technique

Zeolite

Ref.

CH4 26.5 23.7 21 25.5 19 21.2 20.5 20

Cal S Cal S Ch Ch Ch Ch

[1] [2] [3] [3] [4] [4] [5] [6]

20.9 18.4

Cal, V Ch

18.6

TEOM

18.6

V

20.9 20.39 22.7 20a

Cal V S S

NaZSM-5 (30) HZSM-5 ZSM-5 (30) ZSM-5 (30) HZSM-5 (31.1) NaZSM-5 (31.1) Silicalite-1 Linde S115 (189b ) Linde S115 (300) Silicalite-1 (>1000) Silicalite-1 (>3000) Silicalite S115 (282b ) Silicalite-1 Linde S115 Silicalite-1 Silicalite-1

C2 H6 38 33.3 37.5 37.5 28 30 41.7 42.7 30 31.1 28.3

Cal Cal Cal G Ch Ch S G G Cal, V Ch

30a 30.7

FR, SU TEOM

30a 29.8 29 32a 32.78 31.1 36.5 30a

Iso Ch Cal Cal V Cal S S

NaZSM-5 (30) HZSM-5 (30) ZSM-5 (23) HZSM-5 (17) HZSM-5 (31.1) NaZSM-5 (31.1) HZSM-5 HZSM-5 (65) Silicalite-1 (132) Linde S115 (300) Silicalite-1 (>1000) Silicalite-1 (1230) Silicalite1(>3000) Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Linde S115 Silicalite-1 Silicalite-1 Silicalite-1

C3 H8 46 43 42.8 44

G, Cal G, Cal G G

HMFI (35) HZSM-5 (35) HZSM-5 (17) HZSM-5 (65)

[7] [4] [8] [9] [1] [10] [2] [11] [1] [1] [12] [13] [4] [4] [2] [13] [14] [7] [4]

113

Silicalite-1 (132) NaZSM-5 (135) Linde S115 (189b ) Linde S115 (300) Silicalite-1 (1230) Silicalite-1 (>3000) Silicalite-1 Linde S115 Silicalite-1 Silicalite-1 HZSM-5 Silicalite-1

[14] [15] [6] [7] [15] [8] [5] [10] [16] [2] [2] [11] [19] [20] [13] [12] [2] [21] [21] [21] [21] [21] [14] [8]

Ch

HMFI (35) HZSM-5 (35) HZSM-5 (17) ZSM-5 (23) HZSM-5 NaZSM-5 (10) HZSM-5 (10) NaZSM-5 (24) HZSM-5 (24) NaZSM-5 (44) Silicalite-1 (132) Silicalite1(>3000) Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Linde S115 Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Linde S115 (189b ) Silicalite-1

C5 H12 70 74 54 92.2 57.7 63.5 41.8 78.6 60a

G, Cal G, Cal G S Ch Cal G S S

HMFI (35) HZSM-5 (35) HZSM-5 (17) HZSM-5 ZSM-5 (137) Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1

[19] [20] [13] [2] [24] [17] [25] [2] [11]

C6 H14 82 65.1

G, Cal Cal

HMFI (35) [19] ZSM-5 (35) [26] (continued on next page)

[12] [17] [5] [18] [10] [2] [11] [22] [23] [6] [21]

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Table 1 (continued)

140


DH0 (kJ/mol)

Technique

Zeolite

Ref.

92 60 72a 100.5 68.8 69a 70.3 70.5 83.8 70a 65a 70.2

G, Cal G FR, SU S Ch Cal Cal G S S S S

HZSM-5 (35) Silicalite-1 (132) NaZSM-5 (135) HZSM-5 ZSM-5(137) Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1

[20] [14] [15] [2] [24] [12] [17] [25] [2] [11] [22] [23]

120

100

−∆H0 (kJ/mol)

y = 10.13x + 4.77

80

60

40 Si/Al < 35 Si/Al>35 20

Si/Al=12,Z1,experimental Si/Al=400,Z1,experimental

0

C7 H16 83.4 79.6 101.2 101.8 80a

G Ch S S S

Silicalite-1 ZSM-5(137) Silicalite-1 HZSM-5 Silicalite-1

[25] [24] [2] [2] [11]

C8 H18 92.1 90.7 118.5 130.8 90a

G Ch S S S

Silicalite-1 ZSM-5(137) Silicalite-1 HZSM-5 Silicalite-1

[25] [24] [2] [2] [11]

C9 H20 107.7 103a

G S

Silicalite-1 Silicalite-1

[25] [11]

C10 H22 109a 120.5 115a 113a 110a

y = 10.70x + 10.92

Iso G Cal S S

Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1 Silicalite-1

[12] [25] [17] [11] [22]

Cal: calorimetry; Ch: chromatography; FR, SU: frequency response and sorption uptake; G: gravimetry; Iso: isosteric measurements; S: simulation; TEOM: tapered element oscillating microbalance; V: volumetry. a Adsorption enthalpies data were read from the graph. b Si/Al ratios were calculated based on composition.

ratio), pre-treatment procedure (calcination/activation/drying) are, among others, often crucial in the experimental determination of adsorption enthalpies. In the present work, we have determined lowcoverage adsorption properties of C5 –C9 n-alkanes using the chromatographic technique on a series of ZSM-5 zeolites from different suppliers and with different Si/Al ratios, in order to verify if there

0

2

4

6 Carbon Number

8

10

12

Fig. 1. Adsorption enthalpy of n-alkanes on ZSM-5 and silicalite-1. The solid shapes are experimental values of this study. The open shapes and  symbols are from the literature.

exists a relationship between the measured adsorption properties and the nature of the ZSM-5 sample, which could be a potential cause for the experimental scatter observed in literature. 2. Experimental section The properties of the ZSM-5 zeolite samples used in this study are given in Table 2. To discriminate between the zeolites from the two suppliers, the zeolite series will be denoted as Z1 for supplier Alsi-Penta and Z2 for supplier Zeolyst. According to the suppliers, samples Z1 (Si/ Al ¼ 150), Z1 (Si/Al ¼ 400), Z2 (Si/Al ¼ 15), Z2 (Si/ Al ¼ 25), Z2 (Si/Al ¼ 150) were synthesized using an organic template, while the two other samples Z1 (Si/Al ¼ 12) and Z1 (Si/Al ¼ 24) were synthesized without organic template. The ZSM-5 samples were calcined in a muffle oven, using a temperature ramping of 1 °C/min to a final temperature of 600 °C, which was kept overnight. Adsorption properties were determined using the pulse chromatographic technique [29]. The zeolite powders were compacted into disks by applying a pressure of ca. 350 bar, and the disks were broken into fragments and sieved. The 400–500 lm fraction was filled into 1/8 in. diameter stainless steel columns with lengths of 15–35 cm (in-

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115

Table 2 Adsorbent properties and experimental conditions Zeolite Supplier Cation Si/Al ratio Surface area (m2 /g) Mol Al/kg zeolite Mol unit cells/kg zeolite Mol straight channels/kg zeolite Mol zigzag channels/kg zeolite Mol intersections/kg zeolite Exp. temperature range (°C) Pressure range (bar) emacr þ eext

SM27 Z1 NH4 12 >400 1.28 0.173 0.69 0.69 0.69 200–350 1.27–1.37 0.68

CBV3024 Z2 NH4 15 400 1.04 0.173 0.69 0.69 0.69 200–325 1.41–1.51 0.70

SM55 Z1 NH4 24 >400 0.67 0.173 0.69 0.69 0.69 200–400 1.36–1.53 0.61

CBV5524 Z2 NH4 25 425 0.64 0.173 0.69 0.69 0.69 200–325 1.36–1.46 0.70

SH300 Z1 H 150 >400 0.11 0.173 0.69 0.69 0.69 200–375 1.66–1.87 0.59

CBV30014 Z2 NH4 150 400 0.11 0.173 0.69 0.69 0.69 200–325 1.98–2.32 0.51

Silikalit Z1 Na 400 >400 0.04 0.173 0.69 0.69 0.69 200–400 2.52–3.40 0.58

Z1: Alsi-Penta, Z2: Zeolyst.

ternal diameter: 0.216 cm). In situ activation of the adsorbents was performed by raising the temperature at a rate of 3 °C/min to 400 °C and maintaining this temperature overnight. The gas flow rates were about 0.5 cm3 /s and measured by a bubble flowmeter. The GC pulse measurements were carried out using a HP 4890 D gas chromatograph with a thermal conductivity detector (TCD). An inert gas carrier (helium), dried over a 3A molecular sieve, was passed through a column filled with zeolite pellets. A mass flow controller (0–100 cm3 /min) regulated the flow rate of the helium carrier gas. The gas flow rate was about 0.5 cm3 /s. At the column inlet, a trace amount (0.01, 0.02, 0.1 ll) of a probe component was injected. The response curve at the column outlet was detected with a TCD detector. After subtraction of the baseline from the response curve, first moments were calculated by integration. The dead time of the system was negligible compared to the retention time of the injected components. Henry constants K 0 were determined using Eq. (1): L l ¼ ½ðeext þ emacr Þ þ ð1
eext
emacr ÞRT qc K 0

vf

The measurements were performed between 200 and 400 °C. Data points were taken at temperature intervals of 10, 15 or 25 °C depending on the zeolite, in order to obtain between 5 and 10 data points per component. To verify if the experiments were performed in the Henry region, volumes of 0.01, 0.02 and 0.1 ll were injected (see Fig. 2). From 0.02 ll on, no change in the retention time was observed. Experiments with different injection volumes were repeated at different temperatures, to check the

ð1Þ where L represents the column length, vf the superficial gas velocity, eext and emacr the external porosity and porosity of the macropores, R the gas constant, T the temperature, and qc the crystal density, which was calculated assuming an ideal crystal structure.

Fig. 2. Response curves obtained with different amounts of npentane injected into the zeolite column on ZSM-5 (Z2), Si/ Al ¼ 15 at 200 °C.

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effect on the adsorption enthalpy, calculated according to the vanÕt Hoff equation: K 0 ¼ K00 e
DH0 =RT

1x10-3

ð2Þ -4

1x10 K' (mol/(kg . Pa))

For the adsorption of n-pentane on zeolite CBV3024, an adsorption enthalpy of 59.4 kJ/mol was obtained for 0.1 ll, 60.0 kJ/mol for 0.02 ll and 59.9 for 0.01 ll. On zeolite SH-300, the adsorption enthalpy was 78.1 kJ/mol for 0.1 ll and 78.2 kJ/ mol for 0.01 ll for n-heptane and 89.0 kJ/mol for 0.1 ll and 89.2 for 0.01 ll for n-octane. These examples demonstrate the repeatability of the experiments and prove that the data were obtained in the linear part of the adsorption isotherm.

-5

1x10

Z1, Si/Al=12 Z1,Si/Al=24 Z1,Si/Al=150 Z1,Si/Al=400 -6

1x10

4

5

6

7 Carbon number

8

9

10

Fig. 3. Henry constants of linear n-alkanes on ZSM-5 (Z1) with various Si/Al ratios at 300 °C.

3. Results In this section, the experimental data on Henry constants, adsorption enthalpies and pre-exponential factors are presented. The data will be interpreted in the subsequent section. 3.1. Henry constants The Henry adsorption constants are shown as a function of the carbon number in Fig. 3 for the Z1 ZSM-5 series. As expected, the Henry adsorption constants increase exponentially with the carbon number. Both in the Z1 and the Z2 series, the Henry constants decrease with increasing Si/Al ratio (see Table 3), the difference becoming larger at higher chain lengths. For n-nonane, a difference of about one order of magnitude in HenryÕs constant between Z1 with Si/Al ¼ 12 and Z1 with Si/ Al ¼ 400 is observed (Table 3).

In Fig. 4, the Henry constants are depicted as a function of the zeolite aluminum content. It is noticed that all the Henry constants of n-heptane obtained on the samples synthesized with an organic template fall on a single line, while the Henry constants obtained with the samples synthesized without template are higher. 3.2. Adsorption enthalpy The temperature dependence of the Henry constants is shown in Fig. 5 for the adsorption of n-pentane to n-nonane on ZSM-5 (Z2) with an Si/ Al ratio of 150. For all vanÕt Hoff plots, correlation factors larger than 0.999 were obtained. Fig. 6 shows the zero-coverage adsorption enthalpies on the zeolites synthesized with an organic template. The data are also given in Table 4. The adsorption

Table 3 Henry constants on ZSM-5 zeolites for n-alkanes at 300 °C Carbon number

K 0 (mol/(kg Pa))

Si/Al ¼ 12

Si/Al ¼ 24

Si/Al ¼ 150

Si/Al ¼ 400

Si/Al ¼ 15

Si/Al ¼ 25

Si/Al ¼ 150

5 6 7 8 9

1.19  10
5 2.87  10
5 7.19  10
5 1.85  10
4 4.38  10
4

6.78  10
6 1.60  10
5 3.78  10
5 8.88  10
5 1.94  10
4

3.94  10
6 8.00  10
6 1.67  10
5 3.38  10
5 6.41  10
5

2.99  10
6 5.93  10
6 1.22  10
5 2.49  10
5 4.73  10
5

7.30  10
6 1.61  10
5 3.68  10
5 8.28  10
5

6.50  10
6 1.39  10
5 3.09  10
5 6.78  10
5 1.39  10
4

3.99  10
6 7.93  10
6 1.61  10
5 3.32  10
5 6.42  10
5

Z1

Z2

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124 -5

117

110

8x10

-5

7x10

100

no template -5

90 -5

5x10

- ∆ H0 (kJ/mol)

K' (mol/(kg . Pa))

6x10

-5

4x10

-5

3x10

80

70 -5

2x10

Z2,Si/Al=15 Z2,Si/Al=25 Z2,Si/Al=150 Z1,Si/Al=400

60

-5

n-heptane,Z2

1x10

n-heptane,Z1 0

50

0

0.2

0.4

0.6 0.8 mol Al/kg zeolite

1

1.2

1.4

Fig. 4. Comparison of Henry constants of n-heptane on ZSM-5 (Z1) and (Z2) at 300 °C.

-8

-9

ln(K'(mol/(kg . Pa)))

5

-10

-11

-12 n-pentane n-heptane n-nonane

-13

-3

1.7x10

-3

1.8x10

-3

1.9x10

-3

2x10

n-hexane n-octane

-3

2.1x10

-3

2.2x10

1/T (1/K)

Fig. 5. VanÕt Hoff plot for n-alkane adsorption on ZSM-5 (Z2) (Si/Al ¼ 150).

enthalpy appears to be dependent on the Si/Al ratio. This is clearly demonstrated in Fig. 7, where the adsorption enthalpy is plotted as a function of the Al content for both zeolite series. For n-nonane, a difference of 10 kJ/mol is observed between Z1 with Si/Al ¼ 12 and Z1 with Si/Al ¼ 400. As is generally observed, there exists a linear relationship between the carbon number and the adsorption enthalpy:

6

7 Carbon number

8

9

10

Fig. 6. Adsorption enthalpy against carbon number at zerocoverage on zeolites synthesized with organic template.


DH0 ¼ a CN þ b

-7

-14 -3 1.6x10

4

ð3Þ

The a factor, representing the increase of adsorption enthalpy with carbon number, is given in Table 4. The increase of
DH0 with carbon number depends on the zeolite composition, and is different for the two ZSM-5 series. For the Z1 series, an increase of 10.7 kJ/mol per –CH2 – group is observed for an Si/Al ratio of 12, while the enthalpy increases less strongly with the carbon number (10.1 kJ/mol per –CH2 – group) for an Si/ Al ratio equal to 400. Although there seems to be a tendency to lower a factors at low Al contents, the Z1 sample with Si/Al ¼ 150 deviates from this tendency, with a sudden increase to 10.8 kJ/mol per added –CH2 – group. For the Z2 series, an increase of 12.1 kJ/mol is observed for an Si/Al ratio of 15, which is significantly higher than the value found for Z1, with Si/ Al ¼ 12. Also for the Z2 zeolite with Si/Al ¼ 25, a higher value (11.5 kJ/mol) is found than with the Z1 ZSM-5 zeolite (10.5 kJ/mol) with a comparable Si/Al ratio of 24. Remarkably, both the Z1 and Z2 samples with Si/Al ¼ 150 have approximately the same adsorption enthalpy and show exactly the same increase in adsorption enthalpy with carbon number (10.8 kJ/mol). When only the ZSM-5 samples synthesized with an organic template are considered (Z2 (Si/Al ¼ 15), Z2 (Si/Al ¼ 25), Z2

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Table 4 Experimental adsorption enthalpies of n-alkanes
DH0 (kJ/mol)

Carbon number

Z1

Z2

Si/Al ¼ 12

Si/Al ¼ 24

Si/Al ¼ 150

Si/Al ¼ 400

Si/Al ¼ 15

Si/Al ¼ 25

Si/Al ¼ 150

62.9  0.6 75.7  0.5 85.9  0.7 95.7  0.9 106.4  2.9

62.2  0.4 73.3  0.2 82.9  0.3 94.3  0.4 104.2  0.5

56.3  0.5 67.6  0.5 78.2  0.3 89.2  0.5 99.6  1.2

55.7  0.7 66.0  0.7 76.7  0.4 86.6  0.4 96.1  0.4

59.9  0.3 72.4  0.1 83.9  0.2 96.4  0.2

59.2  0.4 70.1  0.7 81.9  0.2 93.9  0.2 105.0  0.6

56.2  0.5 67.2  0.3 78.4  0.2 89.2  0.2 99.1  0.5

10.7  0.3

10.5  0.1

10.8  0.1

10.1  0.1

12.1  0.1

11.5  0.2

10.8  0.1

0.9977

0.9995

0.9999

0.9994

0.9997

0.9997

0.9995

5 6 7 8 9 a (kJ/mol) Correlation factor (DH0 versus CN)

Si/Al ratio

165

120

82

54

41

32

27

23

20

17

16

14

13

12

11

110

(Si/Al ¼ 150), Z1 (Si/Al ¼ 150), Z1 (Si/Al ¼ 400)), a continuous evolution of a with the composition is found (see Table 6). The samples synthesized without an organic template behave differently.

−∆H0 (kJ/mol)

100

3.3. Pre-exponential factors

90 80 70 60 n-pentane n-hexane n-heptane n-octane n-nonane

50 40 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

mol Al/kg zeolite

Fig. 7. Adsorption enthalpy as a function of the zeolite Al content. Open shapes and  symbols: Z1 series; solid shapes and + symbols: Z2 series.

The pre-exponential factors of the vanÕt Hoff equation are given in Table 5 and plotted in Fig. 8 for the Z1 ZSM-5 series as a function of the carbon number. K00 decreases exponentially with the carbon number. The pre-exponential factor K00 is h related to the entropy of adsorption DS0;local and the number of adsorption sites nT as [30]: "  # h DS0;local nT 0 K0 ¼ exp ð4Þ þ ln R 2ph

Table 5 Pre-exponential factors of the vanÕt Hoff equation Carbon number

K00 (mol/(kg Pa)) Z1 Si/Al ¼ 12

5 6 7 8 9 c (J/(mol K))

2.23  10
11 3.58  10
12 1.06  10
12 3.47  10
13 8.39  10
14 11.23

Z2 Si/Al ¼ 24 1.47  10
11 3.37  10
12 1.06  10
12 2.29  10
13 6.21  10
14 11.32

Si/Al ¼ 150 2.98  10
11 5.69  10
12 1.28  10
12 2.55  10
13 5.50  10
14 13.05

Si/Al ¼ 400 2.64  10
11 6.07  10
12 1.29  10
12 3.25  10
13 8.41  10
14 11.99

Si/Al ¼ 15 2.55  10
11 4.10  10
12 8.38  10
13 1.36  10
13 14.38

Si/Al ¼ 25 2.61  10
11 5.90  10
12 1.07  10
12 1.86  10
13 3.76  10
14 13.76

Si/Al ¼ 150 2.94  10
11 5.90  10
12 1.15  10
12 2.47  10
13 5.96  10
14 12.95

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124

119

32

32

31 30 29 -ln(Ko'(mol/(kg . Pa)))

-ln (Ko'(mol/(kg . Pa)))

30

28

26

Z1,Si/Al=12 Z1,Si/Al=24 Z1,Si/Al=150 Z1,Si/Al=400

24

5

6

7 Carbon number

8

9

26 25

n-pentane n-hexane n-heptane n-octane

23

10

Fig. 8. Pre-exponential factors of the vanÕt Hoff equation against carbon number (ZSM-5 Z1 series).

where ph is the standard pressure. The subscript 0 refers to the zero-coverage limit. An exponential decrease of K00 thus corresponds to a linear relationship between the adsorption entropy and the carbon number, which has been observed for various other adsorption systems [19,31]: h
DS0;local ¼ c CN þ d

27

24

22 4

28

ð5Þ

22 0

0.2

0.4

0.6 0.8 mol Al/kg zeolite

1

1.2

1.4

Fig. 9. Pre-exponential factors of the vanÕt Hoff equation as a function of the Al content. Open shapes and  symbols: Z1 series; solid shapes and + symbols: Z2 series.

on a plane, as is shown in Fig. 10 for the adsorption of C5 –C9 on the Z1 ZSM-5 sample with Si/ Al ¼ 12. The parameters calculated using Eq. (6) are compared to those obtained from linear regression of the adsorption enthalpy and preexponential factors as a function of the carbon

The contribution to the adsorption entropy per carbon atom (given by the c parameter) depends on the ZSM-5 composition, and varies between 11.2 and 14.4 J/(mol K) (Table 5). Although the increase of the adsorption entropy with CN depends in a subtle way on the zeolite composition, K00 is not proportional to the Al content (Fig. 9). Eqs. (3) and (5) can be combined into the vanÕt Hoff equation to obtain a general correlation for the Henry constants as a function of the carbon number:      a c nT b d ln K 0 ¼

CN þ ln þ RT R RT R 2ph ð6Þ By non-linear regression of this equation to the Henry constants, the a and c parameters can be obtained directly without prior calculation of the adsorption enthalpies and pre-exponential factors as is done usually. The experimental data nicely fit

Fig. 10. Temperature and carbon number dependence of HenryÕs constant of n-alkanes on ZSM-5 (Z1) (Si/Al ¼ 12).

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I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124

Table 6 Coefficients a, b, c of Eq. (6) between the Henry constants, carbon number and temperature for linear alkanes Supplier

Si/Al ratio

Organic template a (kJ/mol)

Z2 Z2 Z2 Z1 Z1

15 25 150 150 400

b (kJ/mol)

c (J/(mol K))

Method 1

Method 2

Method 1

Method 2

Method 1

Method 2

12.10 11.54 10.76 10.82 10.14

11.86 11.81 11.20 11.00 10.43

)0.47 1.22 2.69 2.47 5.23

0.85 )0.47 0.004 1.32 3.44

14.38 13.76 12.95 13.05 11.99

13.98 14.23 13.71 13.35 12.45

10.96 10.48

10.31 9.96

8.88 9.86

11.23 11.32

11.63 11.30

No template Z1 Z1

12 24

10.71 10.49

Method 1: linear regression of
DH0 and K00 data from Tables 4 and 5 as a function of CN; Method 2: non-linear regression of experimental Henry constants using Eq. (6).

number in Table 6. Only slight differences are found, the relative tendencies being the same.

4. Discussion The data of this work show a clear influence of the nature of zeolite ZSM-5 on the adsorption properties. The Henry constants decrease with decreasing Al content, but in a different way for the two synthesis methods (Fig. 4). No significant effect of the Si/Al ratio on K00 was observed for the ZSM-5 zeolites (Fig. 9), whereas a pronounced effect of the Al content with, e.g., faujasite zeolites was encountered before [31]. For the faujasites, it was demonstrated that the nalkane molecules preferentially adsorb on the Brønsted acid sites or the cations of the zeolite [19]. As the number of Brønsted acid sites or cations equals the number of tetrahedral Al atoms in the framework, K00 decreases with increasing Si/Al (see Eq. (4)). No such simple relationship between the Al content and K00 is observed for ZSM-5: at an Si/Al ratio of 12, the number of Brønsted acid sites equals 1.28 mol/kg, while at an Si/Al ratio of 400, only 0.04 mol/kg Brønsted acid sites are present. It could be expected that this difference of a factor 32 between the two extremes would also be reflected in the pre-exponential factors for both zeolites, which is clearly not the case (Fig. 9, Table 5).

With respect to the number of adsorption sites in ZSM-5, its pore structure needs some consideration. Sinusoidal channels and linear channels intersect to form free cavities with an intersection
[23,32]. The length of the sinudiameter of 9 A soidal channels (between two intersections) equals
[32], while the linear channels have a length 6.7 A
. Without elaborating on which type of of 4.5 A channel is preferred by the n-alkane molecules, it is clear that the channels of ZSM-5 can very well accommodate the alkane molecules and that, depending on the length of the molecule, each intersection, linear or sinusoidal channel segment or even combinations of them can behave as an adsorption site. According to this hypothesis, the number of adsorption sites is the same for all ZSM-5 samples. In Figs. 11 and 12, the pre-exponential factors of the vanÕt Hoff equation are shown as a function of the adsorption enthalpy on the ZSM-5 series synthesized with and without a template, respectively. The parameters of relationship between the pre-exponential factors and the adsorption enthalpy, obtained by linear regression of the experimental data, are given in Table 7. Strikingly, for all ZSM-5 samples prepared using an organic template (Z1: Si/Al ¼ 150 and 400; Z2: Si/Al ¼ 15, 25 and 150), exactly the same increase of K00 with
DH0 is obtained (Table 7), which means that the relationship between the adsorption entropy and

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124

Table 7 The parameters of the relationship between the pre-exponential factors and the adsorption enthalpy (
ln K00 ¼
A:DH0 þ B)

34

32

-ln(Ko'(mol/(kg . Pa)))

121

Supplier

Si/Al ratio

Organic template A (mol/kJ)

B

Z2 Z2 Z2 Z1 Z1

15 25 150 150 400

0.143 0.143 0.145 0.145 0.143

15.8 15.9 16.1 16.1 16.4

30

28

26

No template

Z2,Si/Al=15 Z2,Si/Al=25 Z2,Si/Al=150 Z1,Si/Al=150 Z1,Si/Al=400

24

Z1 Z1

12 24

0.126 0.130

16.7 16.9

22 50

60

70

80 90 −∆H0 (kJ/mol)

100

110

120

Fig. 11. Relationship between the pre-exponential factors and adsorption enthalpies at zero-coverage on the ZSM-5 samples synthesized using an organic template.

32

-ln(Ko'(mol/(kg . Pa)))

30

28

26

24 Z1,Si/Al=12 Z1,Si/Al=24 22 50

60

70

80 −∆H0 (kJ/mol)

90

100

110

Fig. 12. Relationship between the pre-exponential factors and enthalpies at zero-coverage on the ZSM-5 samples synthesized without an organic template.

enthalpy is independent on the ZSM-5 composition for these materials. The Z1 samples synthesized without an organic template (Si/Al ¼ 12 and 24) also show a comparable behavior, but with a lower increase of K00 with DH0 . Hence, the shift of the lines in the
ln K00 versus
DH0 plot with the Si/Al ratio can be attributed to differences in the number of adsorption sites. Based on the correla-

tions given in Table 7, together with Eq. (4), it is calculated that the Z2 sample with Si/Al ¼ 15 has approximately 1.7 times more adsorption sites than the Z1 sample with Si/Al ¼ 400. This value is certainly much lower than the ratio of Brønsted acid site concentrations on these materials, which is equal to 26 as calculated from the aluminum content (see Table 2). It should be remarked that practically no difference in the number of adsorption sites between the Z2 samples with Si/Al ¼ 15 and 25 is observed (Fig. 11). Contrarily, for the two Z1 samples with Si/Al ratios of 12 and 24, a difference in the number of adsorption sites of about 1.6 is found, which is quite close to the ratio of Al contents of both zeolites (1.9). One possible explanation for the small difference in the number of adsorption sites found with the samples synthesized using an organic template is that, at the highest Al content, the preferred adsorption sites are the Brønsted acid sites, while at lower Al contents, the channel segments and intersections serve as preferred adsorption sites at low-coverage. For the Z2 (Si/Al ¼ 15) sample, the Al content equals 1.04 mol/kg, while the number of linear channels equals 0.69 mol/kg, which gives us a ratio of 1.5, close the value of 1.7 found experimentally. Alternatively, it could be argued that at high Al contents n-alkanes are adsorbed in all channel types, while at low Al contents, only one of the two channel types is used, which gives a difference of a factor 2 in nT (see Table 2). Obviously, these suggestions are open for discussion, and other explanations might exist.

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124

The adsorption enthalpy decreases with decreasing Al content (Fig. 7, Table 4). Also Stach et al. [12] found a difference of 9 kJ/mol for the adsorption of ethane on ZSM-5 (Si/Al ¼ 23) and silicalite-1 (Si/Al very large). As shown by IR measurements, n-alkanes are adsorbed on Brønsted acid sites via hydrogen bonding [19]. Gravimetric/calorimetric measurements showed that this hydrogen bonding induces a polarity in the nalkane chain, leading to an additional interaction of 10 kJ/mol compared to non-localized adsorption in the zeolite pores [19]. The chain length dependence of this induced polarization is negligible compared to the total interaction energy. Hence an additional contribution of 10 kJ/mol for all n-alkanes studied is added to the dispersive interaction between the molecule and the zeolite framework when the n-alkane is adsorbed on a Brønsted acid site. When the Si/Al ratio of the zeolite increases, the Brønsted acid site density decreases. At high Si/Al ratios, it is more probable that a molecule is adsorbed in a non-localized way in the zeolite pores than on a Brønsted acid site, as was explained before. The Z2 (Si/Al ¼ 15) sample synthesized with an organic template contains 6.0 Brønsted acid sites per unit cell, while the Z1 (Si/ Al ¼ 400) sample, also synthesized with organic template contains only 0.23 Brønsted acid sites per unit cell. For n-octane, a difference in adsorption enthalpy of 9.8 kJ/mol is found between these two zeolites, which corresponds quite well to the contribution of 10 kJ/mol of the Brønsted acid site to the total adsorption enthalpy. As the Henry adsorption constants increase exponentially with the carbon number, the probability to adsorb in a non-selective way rather than on a Brønsted acid site increases with the chain length, explaining the dependence of the a factor (see Table 4) on the zeolite composition. A discrepancy is again observed for the two Z1 samples synthesized without an organic template (Si/Al ¼ 12 and 24). Compared to the other highalumina samples, these materials show a lower increase of adsorption enthalpy with carbon number. According to the supplier of the Z1 series, the samples with Si/Al ¼ 12 and 24 were produced by direct, template-free synthesis while the other two samples (Si/Al ¼ 150 and 400) were manufac-

tured using a template. Compared to the samples synthesized with a template, the Z1 (Si/Al ¼ 12) and Z1 (Si/Al ¼ 24) samples have smaller crystal sizes (0.5–5 lm compared to >10 lm), a more homogeneous distribution of Al across the crystals and contain more defects (silanol nests) due to the removal of the organics via calcination. The samples synthesized with a template are enriched in aluminum on the outer crystal zones [33]. Moreover, the samples synthesized without a template contain, besides Brønsted acid sites, a relatively large number of Lewis acid sites [34]. The presence of defects and Lewis acid sites in the structure of the samples synthesized without a template and the different Al distribution obviously influence the adsorption properties. The interaction between an n-alkane and a Brønsted acid site is certainly different from that between an n-alkane and a Lewis acid site. A profound characterization of the investigated samples would certainly contribute to the understanding of the observed effects, but falls outside the scope of this work. To conclude, it can be stated that, depending on the synthesis method, a different adsorption behavior is observed. This is clearly shown in Fig. 13, where the c-parameter (giving the increase of adsorption entropy with CN) is shown as a function of the a-parameter (giving the increase of adsorption enthalpy with CN). The parameters of the

15

Z2, Si/Al=25, template

14.5 14

Z1, Si/Al=150, template

Z2, Si/Al=15, template

13.5 γ (J/(mol . K))

122

13 12.5

Z2, Si/Al=150, template

12

Z1, Si/Al=400, template

11.5 11

Z1, Si/Al=24, template-free

10.5

Z1, Si/Al=12, template-free

10 10

10.5

11

11.5

12

12.5

α (kJ/mol)

Fig. 13. Increase of adsorption entropy per CN versus increase of adsorption enthalpy per CN.

I.C. Arik et al. / Microporous and Mesoporous Materials 60 (2003) 111–124

ZSM-5 samples synthesized using an organic template lie on a unique line, for which higher contributions to the adsorption enthalpy are correlated with higher contributions to the adsorption entropy (per –CH2 – group). The samples synthesized without an organic template first of all have a lower increase of adsorption entropy with enthalpy, indicating that adsorption in these samples occurs in a less constrained manner. Secondly, with these two zeolites, it appears that the increase of entropy with CN is much less affected by the increase of enthalpy with CN. The present measurements show a clear dependency of the adsorption constants on the zeolite source and composition, explaining at least a part of the distribution of reported adsorption parameters in the literature. A simple statistical analysis shows that 77% of the experimental data points from the literature on adsorption enthalpy are inside the area between the two curves giving the minimal and maximal adsorption enthalpies found in this work. Experimental data from 26 publications were inside the same distribution, while experimental data from 10 other publications were still outside.

5. Conclusions Using the chromatographic technique, the influence of the zeolite composition and synthesis method on the adsorption of n-alkanes has been investigated. For all investigated ZSM-5 samples, the commonly observed tendencies are found: an exponential increase of the Henry constant with the carbon number, an exponential decrease of the pre-exponential factors and a linear increase of the adsorption enthalpy. The adsorption of n-alkanes on ZSM-5 zeolites strongly depends on the composition of the material. Henry adsorption constants and adsorption enthalpies decrease with increasing Si/Al ratio, and thus decreasing Al content. This is probably due to the interaction between the n-alkanes and the Brønsted acid sites for the aluminium-rich samples. All ZSM-5 samples synthesized using an organic template show the same relationship between the adsorption entropy and enthalpy. For the

123

materials synthesized without an organic template, the adsorption entropy increases less strongly with the adsorption enthalpy, which should be caused by the presence of Lewis acid sites and defects and the difference Al distribution in the structure.

Acknowledgements Financial support was received from the Interuniversity Attraction Poles Programme-Belgian State-Federal Office for Scientific, Technical and Cultural Affairs, IUAP-P5/03 and the F.W. O.-Vlaanderen (G.0127.99). We acknowledge the help of Dr. Arno Tißler from the company Alsi-Penta, Bonnie Marcus and Dr. David Rawlence from the company Zeolyst in supplying indications on the synthesis methods of the materials.

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