Sorption of hydrocarbons in silicalite-1 studied by intelligent gravimetry

Fluid Phase Equilibria 232 (2005) 149–158

Sorption of hydrocarbons in silicalite-1 studied by intelligent gravimetry Hongyan Ban a , Jianzhou Gui a , Linhai Duan b , Xiaotong Zhang b , Lijuan Song b,c , Zhaolin Sun a,b,∗ b

a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, Gansu, PR China College of Petrochemical Engineering, Liaoning University of Petroleum and Chemical Technology, Fushun 113001, Liaoning, PR China c Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK

Received 16 April 2004; received in revised form 14 March 2005; accepted 31 March 2005

Abstract Adsorption and desorption isotherms, adsorption kinetics and TG (thermogravimetry) curves of cyclopentane, cyclohexane, benzene, and p-xylene in silicalite-1 have been measured by using intelligent gravimetric analyzer (IGA). The thermodynamic and kinetic properties of the systems, such as the heats of sorption, free energies of sorption, entropy changes, and the diffusion coefficients, have been derived from these measurements. The results suggest that the size and configuration of sorbate molecules play a critical role on the adsorption and diffusion behaviour of the systems. Tiny changes in the size and the shape of sorbate molecules can result in a significant difference in the adsorption properties. © 2005 Elsevier B.V. All rights reserved. Keywords: Silicalite-1; Adsorption; Diffusion; Thermodynamics; TG

1. Introduction Silicalite-1, siliceous type of ZSM-5, consists of two types of intersecting channels of 10-membered ring openings. The sinusoidal channels are parallel to the [1 0 0] axis with elliptical apertures of 0.51 × 0.55 nm and the straight channels are parallel to the [0 1 0] axis with free near-circular apertures of 0.54 ± 0.02 nm [1,2]. The pore diameters of silicalite-1 are close to the critical dimensions of many important hydrocarbon molecules such as benzene, cyclopentane, etc. Knowledge of the structure-dependent transport properties of sorbate molecules during the adsorption and diffusion of these hydrocarbons in MFI zeolites is, therefore, very important in understanding of the sorptive separation and shape-selective catalysis processes that occur with these materials; e.g. benzene alkylation, xylene isomer∗

Corresponding author. Tel.: +86 413 6650568; fax: +86 413 6650866. E-mail address: [email protected] (Z. Sun).

0378-3812/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2005.03.024

ization, adsorptive separation of xylene isomers, etc. The effect of the features of sorbate molecules and the structures of sorbent frameworks on adsorption and diffusion behaviour have drawn more and more attentions over the last decade [3,4]. A substantial amount of work about sorption and catalytic properties of ZSM-5/silicalite and other members of pentasil zeolites has been reported. But a complete article including both adsorption, thermodynamic properties, kinetic properties, and thermogravimetry is sparse. In this study, a very accurate, completely computer controlled gravimetric technique, IGA, Hiden, UK, which can well define the adsorption behaviour of the gas–solid systems, has been applied to systematically investigated the adsorption properties and thermogravimetry properties of several sorbate molecules, which are analogous either in dimensions or in configurations, in silicalite-1. The results obtained will give us a better understanding on the essentiality of the difference in adsorption properties of these systems.

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2. Experimental The adsorption or desorption measurements were carried out using IGA. The IGA apparatus is an ultrahigh vacuum system which allows the adsorption–desorption isotherms and the corresponding kinetics of adsorption or desorption at each pressure step to be determined [5]. The system consists of a fully computerized microbalance which automatically measures the weight of the sample silicalite-1 as a function of time with the gas vapor pressure and sample temperature under computer control. The microbalance had a longterm stability of ±1 ␮g with a weighing resolution of 0.1 ␮g. The equilibrium pressures were determined by three, highaccuracy, Baratron pressure transducers with ranges of 0–0.2 and 0–10 kPa and 0–1 MPa, and maintained at the set point by active computer control of the admittance/exhaust valves throughout the duration of the experiment. Buoyancy corrections were made on-line for sample and tare hang-downs and containers, and also the tare materials and adsorbed gases. The sample temperature was also monitored throughout the adsorption process and regulated to ±0.1 ◦ C by either a water bath or a furnace. The silicalite-1 sample (∼126 mg) was outgassed up to a pressure of <10−5 Pa at 400 ◦ C for about 20 h. The sorbates were purified through several freeze-pumpthaw cycles where necessary. The adsorption and desorption isotherms were measured by increasing (for adsorption) or decreasing (for desorption) the equilibrium pressure in small stepwise. The mass uptake was measured as a function of time and the approach to equilibrium monitored in real time with a computer algorithm. Thermogravimetry or temperature programmed desorption (TPD) measurements for some sorbates in silicalite-1 were run on IGA. Before starting the TPD run, the sorbent was saturated with the sorbate at room temperature at a specific pressure which was chosen from the isotherm to meet the required loading. The system was then heated at 20 ◦ C min−1 from room temperature to 400 ◦ C. The pressure was maintained constant during the heating. The weight of the sorbent was recorded as a function of temperature from which differential thermogravimetry (DTG) data could be derived. Silicalite-1 crystals used are of research grade cubic with a size of 4 ␮m × 3 ␮m × 4 ␮m and Si/Al ratio >1000. Benzene was obtained from Chemical Company of agents, Shanghai, China; Cyclohexane and p-xylene were supplied by No. 1 factory of Shanghai, China; and cyclopentane was obtained from Fluka Chemie AG. All chemicals have purity of >99+%.

3. Results and discussion 3.1. Sorption isotherms The amount of adsorbed adsorbates can be expressed in molecules of adsorbate per unit cell (m./u.c.), n, which can

Fig. 1. Adsorption (䊉) and desorption () isotherms of benzene (a), pxylene (b), and cyclopentane (c) in silicalite-1 at temperatures of (from top to bottom) 30, 50, 60, 70, and 80 ◦ C for (a), of 50 ◦ C for (b), of −19, 1, 30, 50, 80,100, and 150 ◦ C for (c).

be calculated by the following relation: n=

mMs Ma

(1)

where Ms is the molar mass of the anhydrous sorbent, Ma is the molar mass of adsorbate, m is the loading in gram of adsorbate per gram of activated silicalite-1. The isotherms of benzene, p-xylene in silicalite-1 are shown in Fig. 1(a and b). Two distinct steps can be found at ca. about 4 and 6 m./u.c., respectively, in the isotherm of benzene at 30 ◦ C, which is consistent with the results reported by Lee and Chiang [6], and by Thamm [7]. Only one step can be observed, however, at 50 ◦ C at loadings above 4 m./u.c.

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A hysteresis loop, which was reported by Thamm [7], can also be observed in the isotherm at 30 ◦ C in Fig. 1(a). The isotherm of p-xylene at 50 ◦ C shows an inflection at loading above 4 m./u.c. It is notable that a very well-defined hysteresis loop is also existent in the isotherm of p-xylene. Similar results have been reported in the literature [6–11]. The isotherms of cyclopentane which is comparable to benzene in size in silicalite-1 were also measured as shown in Fig. 1(c). Subtle differences in the shapes of these isotherms were detected. Unlike benzene, the saturation adsorption capacity of the flexible molecule, cyclopentane, is consistent with that reported in the literature [9,12], i.e. the saturation loading is up to 12 m./u.c. at 1 ◦ C. The saturation loading can be up to 9 m./u.c. at 30 ◦ C. A pronounced hysteresis loop at loadings between ca. 9 and 12 m./u.c. at −19 and 1 ◦ C can be seen and steps can also be found at loading above 4 m./u.c. in the isotherms at temperature lower than 50 ◦ C. The adsorption isotherm and desorption isotherm are complete reversibility at higher temperatures. These findings indicate that the adsorption behavior is strongly dependent on certain specific properties of the sorbate molecules, which suggests the size, configuration, and flexibility of sorbate molecules play a crucial role in the adsorption properties of these systems. The sorption data were also fitted into the Langmuir isotherm, p 1 p = + w b νm

(2)

where p is the equilibrium pressure (kPa), w is the amount sorbed (mmol/g), b is constant, νm is the total sorption capacity (mmol/g). The value of the constant b and the value of the sorption capacity νm obtained from the intercepts and the slopes respectively are given in Table 1. A good correlation can be observed in the isotherms of benzene at 70 and 80 ◦ C, which predicted total sorption amount, νm , 4.5 and 4.0 m./u.c. respectively, close to the amount of adsorption measured experimentally, i.e. 4.3 and 4.0 m./u.c., respectively. A good fit is also found in the isotherm of cyclopentane at 150 ◦ C. The isotherms of benzene and cyclopentane at lower temperatures deviated from the Langmuir model, suggesting that strong sorbate–sorbate interactions induce sorbate molecules Table 1 Estimated sorption capacity of silicalite-1 by Langmuir parameters for benzene, cyclopentane Temperature (◦ C)

b

νm (mmol/g)

νm (m./u.c.)

Correlation coefficient

Benzene 30 50 70 80

0.40 0.15 0.25 0.67

0.66 1.04 0.78 0.70

3.8 6.0 4.5 4.0

0.9744 0.9850 0.9959 0.9999

Cyclopentane 30 50 80 150

0.88 0.50 0.73 5.06

1.62 1.38 0.87 0.64

9.3 7.9 5.0 3.7

0.9902 0.9952 0.9983 0.9996

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to pack effectively in silicalite-1 channels at lower temperatures. With temperature increasing, the rotational energy and translational energy increase gradually, and the interactions between the sorbates decrease accordingly. The isotherms at high temperatures can be, therefore, fitted by the normal Langmuir isotherm. There are three kinds of possible adsorption sites in the silicalite-1 channels, i.e. the elliptical sinusoidal channel segments, the near-circular straight channel segments, and the intersections between these two types of channels [13]. Heterogeneity in the potential energies of these sorption sites has been reported for various sorbates by many researchers [7,9,11–17]. Because of the heterogeneity in the potential energies, energetically favored sorption sites are created in silicalite-1 when sorbing process occurs. The sorbate molecules must be firstly adsorbed in the most energetically preferred adsorption sites. Computer simulation calculations and XRD techniques have shown that the intersections, of which there are four per unit cell in silicalite-1, are the most preferred sorption sites for these cyclic hydrocarbon molecules and the equivalent aromatics [12,14,16–18]. This finding is consistent with the fact that inflections are always found in the isotherms shown in Fig. 1 at loadings in excess of 4 m./u.c. The sorbate molecules located in the intersections at loading less than or equal to 4 m./u.c. The filling of the energetically less favourable adsorption sites is superimposed by the interactions of the adsorbed molecules with one another at loading higher than 4 m./u.c. A cooperative redistribution or reorientation of the adsorbed molecules occurs due to the strong interactions of sorbate–sorbate (for example, formation of dimmers or cluster), which can be confirmed in the isosteric heats of adsorption measured by calorimetric and isosteric methods [7,15,19]. The heats of adsorption of benzene and cyclopentane, presented in Fig. 2, have been calculated from the isotherms measured in this work. Some degrees of freedom of the sorbed molecules are retarded by these arrangements leading to an entropy loss. The larger entropy loss, the higher pressure (i.e., chemical potential) is needed to compensate it. Thus, step-like isotherms in silicalite-1 occur. The hysteresis loops found when p-xylene, benzene and cyclopentane molecules are adsorbed in silicalite-1 are of great interest. In the micropore framework system of silicalite-1, this phenomenon cannot be ascribed to capillary condensation which can only occur when the adsorbent contains meso- and macro-pores. It is obvious that these unusual hysteresis loops actually arise at high loadings where strong sorbate–sorbate interactions occur in systems, and there is a very close fit of the sorbate molecules in the micropores. The hysteresis, therefore, are actually ascribed to the strong sorbate–sorbate interactions [3]. 3.2. Sorption properties at different sorption coverage Thermodynamic sorption properties include isosteric heats (Qst ), free energy ( G), entropy change ( S), and en-

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Fig. 2. Isosteric heats of adsorption, Qst , of benzene (a) and cyclopentane (b) in silicalite-1 as a function of loading.

tropy of sorbed phase (Sa ). The isosteric heat of sorption at different surface coverage was calculated from the isotherm data by the Clausisus–Clapeyron equation [20,21], i.e., the value of the isosteric heat Qst is obtained from the slopes. ln(p) = A −

Qst RT

(3)

where R is gas constant, p is equilibrium pressure at constant surface coverage, T is temperature in Kelvin and A is a constant. Further, the values of free energy, entropy changes and entropy of sorbed phase were estimated using the following relations:
p G = RT ln (4) P S = −

Qst + G T

Sa = Sg0 + S

(5) (6)

where P and Sg0 are the standard pressure (101.33 kPa) and gas phase entropy of the sorbate at standard pressure. Dependence of the Qst , G, S, Sa on sorption coverage or temperature for benzene and cyclopentane is shown

Fig. 3. Change of free energy of adsorption, − G, of benzene (a) and cyclopentane (b) in silicalite-1 as a function of loading at different temperatures.

in Figs. 2(a and b), 3(a and b), 4(a and b) and 5(a and b), respectively. It can be seen that the isosteric heat values of benzene change slightly with increasing amount sorbed at loading lower than 4 m./u.c., which shows that little or no clustering is occurring. However, after the sorption space is about half-filled, there is a rapid increase in the heat of adsorption, which is consistent with the pronounced inflection at about loading 4 m./u.c. in the isotherm of benzene. The rapid increase of the adsorption heats indicates that the fore-mentioned, the filling of the energetically less favourable adsorption sites is superimposed by the interaction of the adsorbed molecules with one another, suggesting that a cooperative redistribution or reorientation of the adsorbed molecules occurs (for example, formation of benzene dimmers). On approaching saturation capacity, isosteric heat of adsorption, Qst , of benzene begins to fall. The value of isosteric heat is close to that of the heat of condensation of the sorbate in the liquid phase. Isosteric heat of adsorption curve of benzene exhibits only one maximum at high loading, which is consistent with the result reported by Pope [20]. − G and − S for benzene sorption in silicalite-1 also changes rather slowly as saturation is approached, and this too is manifest in the somewhat unusual sorption isotherm shape.

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Fig. 4. Change of entropy of adsorption, − S, of benzene (a) and cyclopentane (b) in silicalite-1 as a function of loading at different temperatures.

Sorbed phase entropies, Sa , of adsorbed benzene also confirm the knowledge about the state of benzene sorbed in silicalite-1 obtained from the heat curve. The continuous falls in Sa up to about 3.7 m./u.c. result from the diminishing of the configurational part of the entropy of the adsorbed benzene as the occupation of the energetically preferred adsorption sites approaches saturation. The sharp decreases at loading about 4 m./u.c. to loading about 7 m./u.c. indicate abrupt changes of the configuration of the sorbate, i.e. redistribution or reorientation of the adsorbed benzene molecules due to the strong interactions between sorbates. When close to saturation capacity, the sorbed phase entropy begins to increase corresponding to the occurrence of condensation of sorbate. For cyclopentane system, the heat of adsorption curve presented in Fig. 2(b) shows some differences from that of benzene. There are three maximum at loading about 4.6, 7.4 and 8.6 m./u.c. in isosteric heat curve, respectively. Cyclopentane has two stable iso-energetic conformations, namely, the half-chair and the envelope forms [22]. At lower coverages, the half-chair conformation is expected to be favoured on the intersections of the channel to maximize interaction with the surface oxygen atoms of silicalite1. The sharp increase in sorption heat with coverage (see Fig. 2(b)) shows extensive interactions between sorbed cyclopentane molecules. With increasing sorption coverage,

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Fig. 5. Sorbed phase entropy, Sa , of benzene (a) and cyclopentane (b) in silicalite-1 as a function of loading at different temperatures.

the interactions between cyclopentane molecules become stronger. Cyclopentane molecules begin to reorient themselves from a horizontal orientation (half-chair conformation) to a stacked orientation (envelope conformation), which will enlarge effectively the interaction between cyclopentane molecules and silicalite-1 channels. The stepwise increase in the isosteric heat of adsorption suggests a cooperative redistribution or reorientation of the adsorbed molecules. When the loading increases from 4 to 5 m./u.c., some sorbed cyclopentane molecules will interact with one another to form trimers, which is justified by the increases in the heat of sorption, consistent with the inflection in the isotherms at loadings of ca. 4 m./u.c. When the loading increases from 6 to 7 m./u.c., all the sorbed molecules will interact one another, which is confirmed by the second maximum at ∼7.4 m./u.c. Another redistribution and/or reorientation of the adsorbed cyclopentane molecules (for example, the formation of larger associates) is indicated by the third maximum at ∼8.6 m./u.c. Since the occurrence of capillary condensation phenomena for the adsorption systems studied can be excluded, the hysteresis loop observed in the corresponding coverage interval of the isotherm appears to result from difficulties in the dissociation of these clusters on desorption. Such a molecular rearrangement in sorbed cyclopentane also explains the sharp decrease in − G and Sa

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with coverage, and the quick increase in − S with coverage. The comparison of the heat curves of benzene and cyclopentane which exhibit energetic heterogeneity of the pore in silicalite-1 gives evidence of the existence of an energetically favoured adsorption sites with an adsorption capacity of 4 molecules per unit cell. From the fairly modest change of the isosteric heats of adsorption from loading 1 m./u.c. to loading 4 m./u.c., it follows that these high energy adsorption sites are separated sufficiently to prevent very strong sorbate–sorbate interactions. The heat of adsorption curve of benzene clearly shows the existence of two energetically different adsorption sites, and justifies the statement as mentioned above. 3.3. Kinetic properties at different sorption coverage The diffusion in the crystals is assumed to follow Fick’s equations. The mathematical solution for the transient diffusion equation for a spherical particle in terms of uptake of sorbate by the crystals assumes the well-known form given by Crank [23].   ∞ Qt − Q0 6  1 −Dn2 π2 t =1− 2 exp (7) Q∞ − Q 0 π n2 r02 n=1 where Q0 , Qt and Q∞ are the amounts sorbed at time t = 0, at time t, at sorption equilibrium, respectively, D is the diffusivity coefficient, and r0 is the radius of the particle. A simplified and convenient solution for short time is [24]  1/2 √ Qt − Q0 6 D =√ t (8) 2 Q∞ − Q 0 π r0 It can be seen from Eq. (8) that, for short time, the plot of (Qt − Q0 )/(Q∞ − Q0 ) versus the square root of time should be linear in the initial part of the curve and the slope yields the diffusion time constant (D/r02 ). Fig. 6 shows the curves of (Qt − Q0 )/(Q∞ − Q0 ) versus the square root of time of different sorbates at 30 ◦ C, indicating the good linear correlation

Fig. 7. Concentration dependence of diffusion coefficients of benzene (a), cyclopentane (b), and cyclohexane (c) in silicalite-1 at different temperatures.

Fig. 6. The diffusion kinetic curves of different sorbates in silicalite-1 at 30 ◦ C.

within the limits of short time. It proves that the Fick’s law model can be used to calculate diffusion coefficients. Therefore, the diffusivity was estimated using the Eq. (8) which considering the thermal effect or the distribution of adsorbent particle size. Dependence of diffussion coefficient of benzene, cyclopentane, cyclohexane on sorption coverage and temperature is given in Fig. 7(a–c). It can be seen that the curves of diffusion coefficients with loading show some similarities for benzene and cyclopentane, i.e. there is a sudden increase for diffusion coefficient at loading about 4 m./u.c. and then

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a decrease at higher loading when the maximum saturation amount is larger than 4 m./u.c. at low temperature (see Fig. 7(a and b)). This result reflects the diffusion coefficients correspond to the progressive stepwise adsorption, i.e. an inflection about 4 m./u.c. for the silicalite-1 over a range of loadings and temperatures, indicating two distinct regimes of sorption kinetics. For benzene system, the intersections are the energically favourable sorbed sites, as mentioned above. Sorbed benzene molecules preferentially located at the intersection sites with a fixed orientation parallel to the straight channel direction at loading less than (or equal to) 4 m./u.c. XRD and NMR studies show that benzene is an almost perfect spherical rotator at the intersections of the two channels in silicalie-1 structure [16,25,26], indicating that benzene molecules can interchange readily between the two channels resulting in an average single diffusion coefficient. The simulation calculation results also support the possibility of benzene molecules reorienting in the intersections [27]. At high loading, strong interactions between sorbate–sorbate induce redistribution and (or reorientation) of benzene molecules in silicalite-1 structure. The diffusion coefficients, therefore, change from a sudden increase to a decrease with loading increasing, which is justified by the change of the isosteric heat with loading. For cyclohexane system, the curves of diffusion coefficient with loading are different from those of benzene and cyclopentane at low temperature. There is no increase in diffusion coefficient at loading 4 m./u.c. To the contrary, the diffusion coefficients decrease with loading increasing. Diffusion in zeolites may be viewed as an activated process. Transport along the zeolite framework involves activated “jump” of the diffusing molecule between adjacent equilibrium adsorption sites [28]. The kinetic diameter of the cyclohexane molecules (0.6 nm) is slightly larger than the sizes of channels. Cyclohexane molecules will suffer strong repulsions from the walls when the diffusion “jump” occurs because the transitional jumps will involve passage of the molecules through the channels. Thus, the diffusion of cyclohexane will be retarded. The diffusivity, therefore, decreases gradually with loading increasing. The comparison of the diffusion coefficients of benzene, cyclopentane and cyclohexane at different temperatures shows that the diffusion coefficients increase with temperature increasing at specific loading. Actually the sorbates have three rotational degrees of freedom. The rotational energy and translational energy increase with temperature increasing, respectively. Entropy of sorbate increases accordingly. The rocking vibratory motion of sorbate quickens, which

155

Fig. 8. TG and DTG curves for benzene in silicalite-1 at different initial adsorbate loadings (from 1.5 to 8.0 m./u.c., linear heating rate 20 ◦ C min−1 ).

to some extent induces the interactions of sorbate–sorbate to decrease. It can be seen from the inflections broaden in isotherms at high temperatures (Fig. 1) and the entropy contribution becomes larger (Fig. 5) as temperatures increase. The diffusion coefficients, therefore, increase with temperature increasing. To correlate the diffusivities with the shape and size of the sorbate molecules proved a surprisingly challenging task [29]. It is evident from Table 2 that the sequence of diffusivities actually follows the sequence of critical molecular diam-

Table 2 Diffusion coefficients, pre-exponential factors, activation energies Ea and activation entropies S* of aromatics and cyclic hydrocarbons Sorbates

Critical diameter (nm)

Temperature (◦ C)

Loading (m./u.c.)

D × 1015 (m2 /s)

D0 × 1011 (m2 /s)

Ea (kJ/mol)

S* (J/mol/K)

p-Xylene Benzene Cyclopentane Cyclohexane

0.664 0.664 0.64 0.69

50 50 50 50

1.3 0.8 1.0 1.1

10.10 2.74 7.50 0.11

1.20 0.37 2.33 0.20

19.0 [27] 19.4 21.6 26.4

−118.37 −128.02 −112.85 −133.10

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eter; e.g. Dcyclopentane > Dbenzene > Dcyclohexane . But as to benzene and p-xylene, the diffusivity does not follow this. Benzene and p-xylene have the same critical diameter, 0.664 nm, but their diffusion coefficients are quite different. The diffusion coefficient of p-xylene is about one order of magnitude larger than that of benzene. This result disagrees with the zero length column (ZLC) results where the diffusivity of pxylene was found to be comparable with that of benzene [30]. Frequency response (FR) studies [27] showed similar trends for diffusivities of p-xylene and benzene to those obtained in this study but the absolute values of the diffusivities derived by FR are about one order of magnitude larger than those given in this work. Enthalpy and entropy effects must be taken into account in the interpretation of the anomalous features of the diffusivities of aromatics in silicalite-1. Benzene molecules rotate freely in the intersections of the two channels and, there-

Fig. 9. TG and DTG curves for cyclopentane in silicalite-1 at different initial adsorbate loadings (from 1.1 to 9.1 m./u.c., linear heating rate 20 ◦ C min−1 ).

fore, have to orient themselves before they can jump from one intersection to an adjacent intersection through either a straight or sinusoidal channel segment [27]. The sorbed p-xylene molecules are orientated in the straight channels and the diffusion jump steps do not require the molecules to be orientated as with benzene. In addition, the energy barriers to be overcome may be higher for benzene molecules than that for p-xylene molecules because the longer p-xylene molecules can actually span cross higher and lower energy sorption sites. This assumption has been supported by the higher value of the activation energy which is estimated by Arrhenius equation (D = D0 exp(−Ea /RT)) for benzene than that for p-xylene as listed in Table 2. The entropies of activation, S∗ , calculated from the preexponential factors, D0 , using equation   kT S ∗ 2 2.72d exp D0 = h R

(9)

Fig. 10. TG and DTG curves for cyclohexane in silicalite-1 at different initial adsorbate loadings (from 1.0 to 3.8 m./u.c., linear heating rate 20 ◦ C min−1 ).

H. Ban et al. / Fluid Phase Equilibria 232 (2005) 149–158

where k and h are Boltzmann and Planck constants, respectively; R is the gas constant and d is the jump distance between the two adjacent sorption site [31] (ca. 1 nm in the case of MFI zeolites). The S∗ values at 50 ◦ C are also presented in Table 2. The entropy of activation for benzene is higher than that for p-xylene diffusing, implying that the big difference in diffusivities between benzene and p-xylene in silicalite-1 may arise from both the enthalpy and entropy effects. The diffusivities of cyclohexane are far slower than those for the corresponding sorbates, i.e. benzene, p-xylene and cyclopentane in this study, which can be seen from Table 2. The additional interactions of the extra hydrogen atoms of the sorbate molecules with the framework oxygen in silicalite-1 structure and the slightly larger dimensions of the molecules are the most probable reasons. As given in Table 2, its activation energy is the greatest in four sorbates, i.e. 26.4 kJ/mol. 3.4. Thermogravimetry Temperature programmed desorption of benzene, cyclopentane and cyclohexane from silicalite-1 at various adsorption loadings has been studied. The TG and DTG profiles of these systems are presented in Figs. 8–10 at a heating rate of 20 ◦ C min−1 . Only a single peak is observed in these DTG

157

profiles when the initial adsorption loadings were lower than (or equal to) 4 m./u.c. At higher loadings, however, two peaks, as shown in Figs. 8 and 9, were found. The first set of peaks are sharp, corresponding to abrupt changes of loading during desorption, and the size and position of the low temperature peak depend on the degree of coverage, whereas the second set of peaks are much broader. These findings suggest the corresponding two thermodesorption processes occur, indicating the heterogeneity of the adsorption sites in silicalite-1. One is in a relatively narrow range at a low temperature and the other in a broad temperature range. Although the maximum loading differs between benzene and cyclopentane, it is interesting to note that the loading at the inflection point in TG and DTG profiles is always at four molecules per unit cell at loading >4 m./u.c, which is consistent with the inflection point of isotherm at loading about 4 m./u.c. As mentioned above, there are four intersections (≈0.9 nm) per unit cell in silicalite-1 structure. The maximum length of benzene, cyclopentane and cyclohexane is 0.73, 0.65 and 0.69 nm, respectively, which is smaller than that of intersections. These sorbates locate easily intersections in a preferable orientation so that the interactions between sorbates and oxygen in silicalite-1 framework are larger than those of sorbates in other channels. Therefore, the peak temperature of sorbate desorption at loading less than or equal to 4 m./u.c. can be

Fig. 11. TG and DTG curves of benzene (a) and cyclopentane (b) in silicalite-1 (at initial adsorbate loading of benzene 4 and 6 m./u.c. preheated at 51 ◦ C for (a), initial adsorbate loading of cyclopentane 4 m./u.c. and 6 m./u.c. preheated at 64 ◦ C for (b)).

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assigned to the desorption of the adsorbates located in intersections. In order to investigate the relationship of two thermodesorption processes, preheated thermodesorption was finished at loading 6 m./u.c. of benzene and cyclopentane at a heating rate of 20 ◦ C min−1 . Firstly, the benzene (or cyclopentane) system was heated up to 51 ◦ C (or 64 ◦ C) which is the temperature of the inflection point in TG and DTG profiles at loading 6 m./u.c. in Fig. 8 (or Fig. 9), and then cooled up to 30 ◦ C, which is followed by heating newly the benzene (or cyclopentane) system up to 400 ◦ C. The TG and DTG profiles of benzene and cyclopentane at loading 4 and 6 m./u.c. (preheated) are presented in Fig. 11(a and b). The resulting TG and DTG profiles match exactly the profiles generated previously starting with sorbents loaded with 4 m./u.c, which suggests the desorption of molecules sorbed above loading of 4 m./u.c. occurs firstly. When the loading has been reduced to 4 m./u.c., the remaining four molecules relocate preferentially at the channel intersection sites. Preheating (51 ◦ C for benzene and 64 ◦ C for cyclopentane, respectively) removes the corresponding low temperature thermodesorption processes. In summary, the high temperature peaks of benzene and cyclopentane profiles, and the single peak of cyclohexane, benzene and cyclopentane at loading ≤4 m./u.c., result from the loss of ca. 4 m./u.c. down to zero from the silicalite-1 channel intersections. The low temperature peaks of benzene and cyclopentane result from the loss of >4 m./u.c. down to 4 m./u.c., and reflect the highest degree of sorbate–sorbate interaction and steric constraint [32].

4. Conclusions Sorption isotherm of benzene at 30 ◦ C in silicalite-1 shows two interesting steps which occur at loadings about 4 and 6 m./u.c. respectively. The first step occurs after the four, energetically preferred, intersections of straight and sinusoidal channels have been filled. The second step is due to the strong sorbate–sorbate interactions and decreases in the entropy of sorption when molecules, at loadings in excess of 4 m./u.c., have to occupy sinusoidal and straight channel segment sites. In the case of p-xylene, only one step occurs at loadings in excess of 4 m./u.c at 50 ◦ C. There is a distinct hysteresis loop appearing in the isotherms of benzene and p-xylene, respectively. Cyclopentane shows isotherms with three steps and a maximum loading of 12 m./u.c. at 1 ◦ C. Cyclopentane molecules, therefore, are readily adsorbed in all of the three different sets of sorption sites present in silicalite-1 and the packing of these molecules is very efficient. A pronounced hysteresis loop can be also found in the isotherms at −19 and 1 ◦ C. Thermodynamics and kinetics and thermogravimetry studies have helped greatly to elucidate the preferred sites of these various sorbates in the channels and intersections of siliclalite-1 and the occurrence of rearrangement or reori-

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