Diffusion of Cyclic Compounds from Their

May 5, 1997 - Single-component sorption/diffusion of cyclohexane, pyridine, benzene, toluene, aniline, ethylbenzene, xylene isomers, n-propyl- and iso...
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Ind. Eng. Chem. Res. 1997, 36, 1812-1818

Single-Component Sorption/Diffusion of Cyclic Compounds from Their Bulk Liquid Phase in H-ZSM-5 Zeolite Vasant R. Choudhary,* Vikram S. Nayak, and Tushar V. Choudhary Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India

Single-component sorption/diffusion of cyclohexane, pyridine, benzene, toluene, aniline, ethylbenzene, xylene isomers, n-propyl- and isopropylbenzenes, and mesitylene in H-ZSM-5 (Si/Al ) 39.7) zeolite from their bulk liquid phase at 273-338 K has been measured volumetrically. The diffusivity and activation energy for the diffusion are found to be strongly influenced by the side groups attached to the benzene nucleus, polarity, critical molecular size, and configuration of sorbate molecules. However, the influence of configuration and/or flexibility (i.e., compressibility to the zeolite channel diameter) plays a vital role in controlling/deciding the penetration (or entry) and configurational diffusion (after penetration) of sorbate molecules in the zeolite channels. The influence of critical size and configuration of sorbate molecules, on their entry in the zeolite channel and also their configurational diffusion after sorption, can be explained by the shuttlecock-shuttlebox model for sorption/diffusion of bulky molecules in medium-pore zeolites. The influence is explained by taking into consideration the orientations in different conformations and compressibility in the movement in different directions of the sorbate molecules. Introduction ZSM-5 zeolites are medium-pore zeolites containing two types of intersecting channelssnear-circular (0.54 × 0.56 nm) zigzag channels and elliptical (0.51 × 0.55) straight-chain channels (Kokotailo et al., 1978). These medium-pore zeolites have been proved to be a very important class of catalysts for a number of catalytic processes (Scott, 1980; Chang, 1983, 1984; Kaeding et al., 1984; Chen et al., 1989). They have also shown their importance as potential sorbents in a number of separation processes (Dessau, 1980). Because of their intermediate channel diameter, configurational restrictions are imposed in the sorption and diffusion of bulky sorbate molecules having diameters comparable to that of the channels. This results in a shape-selective behavior in sorption/diffusion in these zeolites. In these shape-selective zeolites, diffusion plays a vital role in deciding the product selectivity in catalytic processes through diffusion-reaction interactions (Weisz, 1980). Diffusion also plays a very important role in deciding kinetic selectivity in the adsorption separation processes. As compared to larger pore zeolites, sorption/diffusion in the medium-pore zeolites is much more complex (Choudhary and Akolekar, 1989); it is strongly influenced by a number of sorbate factors. The importance of critical size and configuration of sorbate molecules in their diffusion in ZSM-5 has been emphasized in many studies (Olson et al., 1981; Haag et al., 1981; Wu et al., 1983; Choudhary and Singh, 1986; Choudhary et al., 1992). However, the configuration and compressibility (to the size of the channel diameter) of bulky sorbate molecules are much more important than their critical molecular size in their shape-selective sorption (Choudhary and Akolekar, 1989, 1990a, 1991; Choudhary et al., 1992). According to the shuttlecock-shuttlebox model (Choudhary and Akolekar, 1989), a bulky sorbate molecule can enter in the ZSM-5 zeolite channel only * To whom all correspondence should be addressed. Telephone: (91) 212-336451 (ext. 2163). Fax: (91) 212-333941/ 330233. Email: [email protected]. S0888-5885(96)00411-3 CCC: $14.00

when the following two conditions are satisfied: (1) The molecule is oriented in its most favorable configuration (i.e., the one in which the molecule can be easily compressed to the size of the channel). (2) It possesses sufficient energy to overcome the energy barrier for its complete penetration in the zeolite channel. The rate of sorption is, therefore, expected to be controlled by the entry of sorbate molecules in the zeolite channels, by the configurational diffusion of the penetrated sorbate molecules in the channels, or by both the intracrystalline mass-transfer processes. Sorption/diffusion of various sorbates in ZSM-5 type zeolites from the gas/vapor phase has been investigated extensively (Ruthven, 1984a,b; Choudhary and Srinivasan, 1986a,b; Karger and Ruthven, 1989). However, sorption/diffusion in the zeolite from the liquid phase is relatively less investigated. Liquid-phase singlecomponent sorption/diffusion of o- and m-xylenes and mesitylene (Choudhary and Singh, 1986), C1-C4 alcohols (Lin and Ma, 1988), cumene (Choudhary et al., 1989; Choudhary and Mamman, 1990), and C2-C8 alcohols, n-hexane, and n-butylamine (Choudhary et al., 1992) in ZSM-5 zeolite has been reported earlier. The diffusion in ZSM-5 from the liquid phase is expected to differ from the gas-phase diffusion because of the lower degree of freedom for the orientation of liquid sorbate molecules and also due to the requirement of additional energy (equivalent to enthalpy of vaporization) for pulling out the sorbate molecules from their bulk liquid during their entrance in the zeolite channels. Diffusion of a particular sorbate in ZSM-5 has been found to be strongly influenced by a number of zeolite factors, such as Si/Al ratio, cations and the degree of their exchange, pretreatment conditions, and poisoning or the presence of foreign materials in the zeolite channels. These zeolite factors affect the effective channel diameter and also have an influence on the chemical environment of diffusing species (Wu and Ma, 1984; Post et al., 1984; Choudhary and Srinivasan, 1986b; Choudhary et al., 1989). Other zeolite factors like crystal defects/cracks and morphology and Si/Al distribution in zeolite can also strongly influence the intracrystalline diffusion. Hence, in order to find out © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1813

the influence of various sorbate factors on the entrance and diffusion in the zeolite, it is essential to carry out these studies with the same zeolite sample. In this investigation kinetics of liquid-phase single-component sorption of a number of cyclic compounds (viz., benzene, cyclohexane, pyridine, toluene, aniline, ethylbenzene, o-, m-, and p-xylenes, n-propyl- and isopropylbenzenes, and mesitylene) in H-ZSM-5 have been measured volumetrically for understanding the influence of various sorbate factors on the entrance and configurational diffusion of sorbate molecules in the zeolite. Experimental Section H-ZSM-5 zeolite [Si/Al ) 39.7 and H+/(Na + H+) ) 0.99] was obtained by deammoniating NH4-ZSM-5 (unit cell composition: (NH4)2.35Na0.01Al2.36Si93.64O192‚nH2O) at 773 K under static air for 4 h. The zeolite crystals were hexagonal in shape and more or less uniform in size (average crystal size: 5.6 µm). The strong acid sites of the zeolites measured in terms of the pyridine chemisorbed at 673 K were 0.19 mmol‚g-1. The preparation and characterization of the zeolite have been given earlier (Nayak and Choudhary, 1982; Choudhary and Nayak, 1984). The NH4-ZSM-5 zeolite was pressed without binder and crushed to particles of about 0.2 mm size. Entrance and diffusion of various cyclic compounds in the zeolite have been investigated by measuring volumetrically their sorption (from the liquid phase) in the empty channels of the zeolite as a function of time. The volumetric measurements were carried out by contacting an evacuated 0.5-g zeolite sample (pretreated in situ under vacuum at 623 K for 2 h) with a pure liquid sorbate at the required temperature and following the change in liquid level in a calibrated capillary, using a novel volumetric sorption apparatus. The apparatus and procedure for carrying out the volumetric sorption measurements have been described in detail earlier by Choudhary et al. (1989). All the cyclic compounds used in the work were highly pure (>99.9%) and were stored over activated 13× molecular sieves. Results and Discussion Intracrystalline Diffusion/Mass Transfer. Representative Qt/Q∞ vs t1/2 plots (where Qt and Q∞ are the amounts sorbed at time t and sorption equilibrium, respectively) for the sorption of pyridine, benzene, toluene, ethylbenzene, n-propylbenzene, isopropylbenzene, p-, m-, and o-xylenes, aniline, and cyclohexane in the H-ZSM-5 zeolite are presented in Figure 1. The values of the intracrystalline diffusion coefficients (D) for the cyclic sorbates are obtained from the slopes of the initial linear section of the Qt/Q∞ vs t1/2 plots according to the t1/2 law (Barrer, 1971):

Qt/Q∞ ) 6[(Dt)/(Πr2)]1/2

(1)

Data on molecular sizes, both minimum (or critical) and maximum of the different cyclic compounds, and their sorption capacity and relative intracrystalline diffusivity (D/Dp-X) in the zeolite are given in Table 1. The minimum (critical) size of a molecule is defined as the diameter of the smallest cylinder which can circumscribe the molecule in its most favorable equilibrium conformation, whereas, the maximum size of a molecule

Figure 1. Qt/Q∞ vs t1/2 plots for the sorption of cyclic compounds from their bulk liquid phase in H-ZSM-5 at 308 K.

is considered as the length of the molecule or the diameter of the cylinder which can circumscribe the molecule in its most unfavorable conformation. The temperature dependence of the diffusivity of different cyclic compounds, according to the Arrhenius equation [D ) D0 exp(-E/RT)], is shown in Figure 2. The values of the activation energy (E) and frequency factor (D0) for the intracrystalline diffusion of the various cyclic compounds, obtained from the linear Arrhenius plots, are given in Tables 2-5. In order to be sure that the sorption rates are controlled only by intracrystalline mass transfer, the sorption of benzene at 308 K was also carried out using the zeolite particles of 0.1 mm size. The reduction in zeolite particle size from 0.2 to 0.1 mm showed no significant effect on the sorption kinetics of benzene. This shows that the intercrystalline mass transfer has no effect on the rates of benzene sorption at 308 K. Also, since the diffusion of the other cyclic compounds (except pyridine at g308 K) in the zeolite is much slower than that of benzene (Figure 2), their sorption is controlled only by their intracrystalline mass transfer. The observed high (30.2 kJ‚mol-1) activation energy for the diffusion of pyridine (fastest diffusing species) also supports the absence of intercrystalline mass-transfer resistance. Results showing the influence of various sorbate factors such as molecular weight or side chain, polarity, critical molecular size, and configuration on the diffusion of cyclic compounds are presented in Tables 2-5. In order to clearly bring out the influence of various sorbate factors, the sorbates having distinct characteristics are grouped together. Influence of the Side Chain. A comparison of the diffusivites of benzene, toluene, ethylbenzene, and npropylbenzene (Table 2), having the same critical molecular size (0.67 nm), shows that their diffusivity is

1814 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 1. Results of the Sorption of Different Cyclic Liquid Compounds in H-ZSM-5 Zeolite at 308 K molecular size (nm)

a

sorbate

maximum

minimum (critical size)

cyclohexane benzene toluene ethylbenzene p-xylene o-xylene m-xylene n-propylbenzene isopropylbenzene mesitylene pyridine aniline

0.74 0.87 0.92 0.99 0.87 0.92 1.07 0.92 0.92 0.68 0.90

0.69 0.67 0.67 0.67 0.67 0.73 0.74 0.67 0.67 0.87 0.67 0.67

sorption capacity mmol‚g-1

molecules (puc)

relative diffusivitya (D/Dp-X)

1.12 1.44 1.31 1.24 1.22 0.86 0.88 1.05 1.01 0.67 1.53 1.48

6.5 8.4 7.6 7.2 7.1 5.0 5.1 6.1 5.9 3.9 8.9 8.6

1.55 × 10-2 6.00 1.51 0.13 1.0 1.85 × 10-2 4.8 × 10-4 3.0 × 10-2 2.0 × 10-2 1.07 × 10-4 10.19 0.30

Diffusivity relative to that of p-xylene (Dp-X ) 5.13 × 10-12 cm2‚s-1 ). Table 2. Influence of the Side Chain on Diffusion of Aromatic Hydrocarbons (Critical Size: 0.67 nm) in H-ZSM-5

sorbate benzene toluene ethylbenzene n-propylbenzene

Figure 2. Temperature dependence of the diffusion coefficients for different cyclic compounds.

strongly influenced by the side chain attached to the benzene nucleus. There is a sharp decrease in diffusivity with an increase in the side chain. The activation energy for their diffusion is also increased; the increase in the activation energy is particularly large when the side chain is changed from -CH3 to -C2H5. The critical molecular diameter of the above aromatics is larger than the channel diameter. The observed large decrease in the diffusivity with a small increase in the molecular weight indicates that the diffusion in the zeolite channels is hindered depending on the side chain; the larger the side chain is, the higher is the hindrance for diffusion. This is consistent with the earlier observation on single-component sorption/diffu-

D × 1012 E molecular (cm2‚s-1) at 308 K (kJ‚mol-1) weight 78 92 106 120

30.88 7.93 0.68 0.15

5.3 5.5 21.9

D0 (cm2‚s-1) 2.42 × 10-10 6.83 × 10-11 3.48 × 10-9

sion of toluene and n-propylbenzene from their isooctane solution in H-ZSM5 zeolite (Choudhary and Akolekar, 1989). Influence of Polarity. The results showing the influence of polarity on the diffusivity of two pairs of cyclic compounds, having nearly the same critical molecular size, configuration, and molecular weight, are given in Table 3. For the benzene-pyridine pair, benzene is nonpolar, whereas pyridine is highly polar. In this case, both the diffusivity and activation energy for diffusion of pyridine are higher than that of benzene. In the earlier studies also, the diffusivity of n-butanol, having higher polarity than that of n-butylamine, was found to be higher (Choudhary et al., 1992). On the contrary, for the toluene-aniline pair, the diffusivity of aniline (having higher polarity) is lower. This observation is, however, consistent with the reasoning that the diffusion is hindered and has a higher activation energy due to the dipole interactions with the cations in the zeolite channels (Barrer, 1971; Eberly, 1976). For the former case, as expected, the diffusion of pyridine has much higher activation energy than that of benzene. The diffusivity of pyridine at lower temperatures is also lower than that of benzene (Figure 2). However, it is much higher than that of benzene at higher temperatures (g308 K). The high activation energy for the diffusion of pyridine (which is a strong base) is because of its strong interaction with the zeolite acid sites (i.e., protons in the zeolite channels). However, the discrepancy in its diffusivity at higher temperatures indicates that apart from polarity, some other factor, probably the difference in the maximum size of benzene (0.74 nm) and pyridine (0.68 nm) molecules, plays a vital role. Influence of Critical Molecular Size. A comparison of the results in Table 4 for the C8 and C9 aromatics, having the same molecular weight but different critical molecular sizes, shows a very strong influence of the critical size on the diffusion. The increase in the critical size results in a very large decrease in the diffusivity and also causes a large increase in the activation energy

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1815 Table 3. Influence of the Polarity of Cyclic Compounds on Their Diffusion in H-ZSM-5 sorbate

molecular weight

critical size (nm)

dipole moment (D)

D × 1012 (cm2‚s-1) at 308 K

benzene pyridine toluene aniline

78 79 92 93

0.67 0.67 0.67 0.67

0.00 2.23 0.31 1.48

30.9 52.3 7.90 1.50

E (kJ‚mol-1)

D0 (cm2‚s-1)

5.3 30.27 5.5

2.42 × 10-10 7.36 × 10-6 6.83 × 10-11

Table 4. Influence of the Critical Size of C8 and C9 Aromatics on Their Diffusion in H-ZSM-5 sorbate

molecular weight

critical size (nm)

D × 1012 (cm2‚s-1) at 308 K

p-xylene o-xylene n-propylbenzene mesitylene

106 106 120 120

0.67 0.73 0.67 0.87

5.13 0.09 0.15 5.0 × 10-4

E (kJ‚mol-1)

D0 (cm2‚s-1)

18.1 35.5

6.16 × 10-9 9.1 × 10-8

E (kJ‚mol-1)

D0 (cm2‚s-1)

5.3 49.1 18.1 21.9 35.5 37.7

2.42 × 10-10 2.13 × 10-5 6.16 × 10-9 3.48 × 10-9 9.07 × 10-8 5.84 × 10-9

Table 5. Influence of the Configuration of Sorbate Molecules on Their Diffusion in H-ZSM-5 sorbate

molecular weight

critical size (nm)

D × 1012 (cm2‚s-1) at 308 K

benzene cyclohexane p-xylene ethylbenzene o-xylene m-xylene n-propylbenzene isopropylbenzene

78 84 106 106 106 106 120 120

0.67 0.69 0.67 0.67 0.73 0.74 0.67 0.67

30.88 0.08 5.13 0.68 0.09 2.5 × 10-3 0.15 0.10

for diffusion. This is expected because of the drastic increase in the steric hindrance for the sorption/diffusion in the zeolite because of its smaller channel diameter (0.55 nm). A similar large influence of shape or critical size of sorbate molecules has been observed for the sorption of isomers of butanol, butylbenzene, and xylene in Silicalite-I (Choudhary and Akolekar, 1990a) and of xylene isomers in H-ZSM-5, H-ZSM-8, and H-ZSM-11 zeolites (Choudhary and Akolekar, 1991) from the vapor phase and also for the diffusion of butanol isomers in H-ZSM-5 from the liquid phase (Choudhary et al., 1992). Influence of Molecular Shape/Configuration. The results showing a strong influence of molecular shape or configuration on the diffusion in the zeolite are presented in Table 5. A change in the configuration of a pair of C8 or C9 aromatics, having nearly the same molecular weight and critical size, causes a large change in their diffusivity and activation energy for their diffusion in the zeolite. The observed very slow diffusion, with very high activation energy of cyclohexane as compared to that of benzene, is attributed to the difference in the configuration of the two sorbate molecules and not to the higher weight or slightly higher critical size of cyclohexane. Benzene is a planar molecule, whereas cyclohexane is nonplanar and is most stable in its chair form. The very slow diffusion of cyclohexane is consistent with the fact that cyclohexane is generally not observed in the products of aromatization of alcohols and olefins over ZSM-5 type zeolites. This is because cyclohexane is converted to benzene before diffusing out of the zeolite crystals. It is interesting to note that, although mesitylene has a larger critical size and higher molecular weight than isooctane and 2,3-dimethylbutane, the former is sorbed in the zeolite at low temperature, but the latter two are not sorbed at all. This is consistent with earlier studies on the sorption of isooctane from the liquid phase (Choudhary and Singh, 1986; Choudhary et al., 1992) and 2,3-dimethylbutane from the gas phase (Haag et al., 1982) in ZSM-5 zeolites. The sorption of 2,3-

Figure 3. Influence of the orientation of benzene, pyridine/ toluene, aniline/ethylbenzene, and isopropylbenzene molecules on their entry and diffusion in H-ZSM-5.

dimethylbutane is, however, possible at temperatures g623 K (Chen and Garwood, 1978; Choudhary and Akolekar, 1990a). The above results reveal that the role played by the molecular configuration of the sorbate in controlling its sorption/diffusion in the medium-pore zeolite is more vital than that played by the critical size of the sorbate molecules. The importance of the molecular configuration over critical size in the sorption/diffusion is illustrated in Figures 3 and 4 by the shuttlecockshuttlebox model (Choudhary and Akolekar, 1989) for the sorption diffusion of sorbates having a critical molecular size comparable to the zeolite channel diameter. Influence of the Orientation and Flexibility of Sorbate Molecules. According to the shuttlecockshuttlebox model, the entrance and direction of diffusion in the zeolite channel of bulky sorbate molecules, whose critical size is larger than the channel diameter, are strongly dependent upon the orientation and compressibility (i.e., flexibility) of sorbate molecules (Choudhary

1816 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

Figure 4. Influence of the orientation of xylene isomers and mesitylene molecules on their entry and diffusion in H-ZSM-5.

and Akolekar, 1989, 1990a). As illustrated in Figures 3 and 4, the sorbate molecules can enter in the zeolite channel only when they are oriented in their most favorable conformation (i.e., the conformation in which the molecule can be easily compressed to the size of the zeolite channel). The symbol f in the Figures 3 and 4 indicates that the entrance of the cyclic compounds in the zeolite channel in the direction shown by the arrow is favored. This is because of the smallest possible molecular size for the sorbates (e.g., benzene-I, toluene/ aniline-I and -II, ethylbenzene/n-propylbenzene-I and -II, p-xylene-I, o-xylene-I and -II, and m-xylene-II) and also because of the bending of bonds of the side groups (attached to the benzene nucleus) in the direction leading to compression of the sorbate molecule to the size of the zeolite channel (e.g., isopropylbenzene-Ia, m-xylene-Ia, and mesitylene-Ia) for the orientation of sorbate molecules in the most favorable conformations. The symbol N indicates that the entrance and diffusion of the sorbate molecules in the direction shown by the arrow is not possible (or least favored) because of an extremely strong steric hindrance. The symbol - - f indicates that the entrance of the sorbate molecules in the direction shown by the arrow is possible but is less favored because of the high steric hindrance experienced by the sorbate molecules (e.g., penetration of benzene-II, o-xylene-III, and m-xylene-II and -IIIa). The sorption/diffusion of toluene and aniline in their orientation III is less favored than their orientations I and II. For the penetration/diffusion of o-xylene, its orientation IIIa is less favored as compared to I and II because of repulsive interactions between two close methyl groups, making the compression of the molecule very difficult. The sorption of m-xylene in its orientation Ia (effective size: 0.8 nm) is more favored than its orientation II (effective size: 0.74 nm) because of its smaller size at its methyl group ( Dbenzene and Dm-xylene > Do-xylene. This trend is opposite to that observed in the present case and also in the earlier studies (Wu and Ma, 1983; Choudhary and Singh, 1986). Apart from the sorbate factors, the diffusion in ZSM-5 zeolite has been found to be strongly influenced by a number of zeolite factors, such as Si/Al ratio, degree of cation exchange, thermal/hydrothermal treatments, presence of strongly sorbed species or foreign materials, etc. (Post et al., 1984; Wu and Ma, 1984; Choudhary and Srinivasan, 1986b; Choudhary et al., 1989; Choudhary and Mamman, 1990). Hence, the diffusivi-

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1817 Table 6. Comparison of Diffusivities of Cyclic Compounds in ZSM-5-Type Zeolites for Sorption from the Vapor/Liquid Phase at Low Temperatures

a

sorbate

zeolite (Si/Al ratio)

phase

D × 1012 (cm2‚ s-1) (temp)

ref

benzene benzene benzene benzene benzene toluene toluene ethylbenzene ethylbenzene p-xylene p-xylene p-xylene p-xylene p-xylene m-xylene m-xylene m-xylene m-xylene o-xylene o-xylene o-xylene isopropylbenzene isopropylbenzene mesitylene mesitylene

ZSM-5 (35.0) Na-ZSM-5 (24.3) silicalite-I H-ZSM-5 (36.0) H-ZSM-5 (39.7) H-ZSM-5 (36.0) H-ZSM-5 (39.7) silicalite-I H-ZSM-5 (39.7) Na-ZSM-5 (24.3) silicalite-I HNa-ZSM-5 (43.0) H-ZSM-5 (36.0) H-ZSM-5 (39.7) silicalite HNa-ZSM-5 (43.0) H-ZSM-5a (38.0) H-ZSM-5 (39.7) silicalite H-ZSM-5a (38.0) H-ZSM-5 (39.7) H-ZSM-5 (31.1) H-ZSM-5 (39.7) H-ZSM-5a (38.0) H-ZSM-5 (39.7)

vapor vapor vapor vapor liquid vapor liquid vapor liquid vapor vapor vapor vapor liquid vapor vapor liquid liquid vapor liquid liquid liquid liquid liquid liquid

230 (293 K) 7 (303 K) 8.6 (293 K) 5.7 (298 K) 30.9 (308 K) 1.6 (298 K) 7.9 (308 K) 7.5 (293 K) 0.68 (308 K) 3.6 (303 K) 9.9 (293 K) 19.1 (313 K) 28.0 (298 K) 5.13 (308 K) 4.1 (293 K) 10.6 (313 K) 0.016 (303 K) 0.003 (308 K) 2.2 (293 K) 0.024 (303 K) 0.09 (308 K) 4.5 (313 K) 0.1 (308 K) 9 × 10-4 (303 K) 5 × 10-4 (308 K)

Heering et al. (1982) Wu and Ma (1983) Wu et al. (1983) Nayak and Riekert (1985) this work Nayak and Riekert (1985) this work Wu et al. (1983) this work Wu and Ma (1983) Wu et al. (1983) Mao et al. (1983) Nayak and Riekert (1985) this work Wu et al. (1983) Mao et al. (1983) Choudhary and Singh (1986) this work Wu et al. (1983) Choudhary and Singh (1986) this work Choudhary et al. (1989) this work Choudhary and Singh (1986) this work

With 20% bentonite and kaolinite binder.

ties in different ZSM-5 type zeolites can seldom be compared. The diffusivities measured using ZSM-5, having even the same composition and pretreatment, may not be comparable because of the presence of defects and/or different distributions of Al or Si/Al ratio in the crystals of different zeolite samples used in the sorption/diffusion studies. In order to avoid all these problems, the zeolite samples from the same lot were used in this investigation for studying the influence of various sorbate factors on the sorption/diffusion and moreover the data were collected on a representative (or a large) sample. In general, the diffusion of any sorbate in the mediumpore zeolite, such as ZSM-5 from the liquid phase is expected to differ from its sorption/diffusion from the vapor phase in the following matters. According to the shuttlecock-shuttlebox model, the sorption rates in the zeolites are influenced because of the following two intracrystalline mass-transfer processes, operating in series (Choudhary and Akolekar, 1989): (1) entry/ penetration of sorbed molecules in the zeolite channels; (2) configurational diffusion in the zeolite channel. Thus, the sorption is controlled by either of the two intracrystalline mass-transfer processes or both. Since the sizes of the sorbate molecules are comparable or even larger than that of the channel diameter, the sorbed molecules in the zeolite channels are in the form of single strings. For entry of the sorbate molecule in the zeolite channel, it is pulled out from its bulk liquid by providing an energy equivalent to its latent heat of vaporization. This is not the case for the penetration of sorbate molecule from its vapor phase. Moreover, the degree of freedom for the orientation of the sorbate molecule in its most favorable conformation, for its entry in the zeolite channel, is less in the liquid phase because of its lower entropy. Hence, the penetration barrier for the sorption from the liquid phase is also expected to be more than that of the sorption from the vapor phase. The activation energy for diffusion from the liquid phase

is also expected to be different. Further studies are, however, necessary for clearly understanding these aspects. Conclusions From the sorption/diffusion of a number of cyclic compounds (cyclohexane, benzene, toluene, ethylbenzene, o-, m-, and p-xylenes, n-propylbenzene, isopropylbenzene, mesitylene, pyridine, and aniline) from their bulk liquid phase in H-ZSM-5 zeolite, the influence of various sorbate factors [viz., side-group(s) attached to the benzene nucleus, polarity, critical size, configuration, and flexibility] of sorbate molecules on the intracrystalline diffusion/mass transfer has been clearly brought out. Although all these sorbate factors have been found to play an important role in the sorption/ diffusion, both the penetration (or entry) and diffusion of bulky sorbate molecules in the zeolite are controlled mainly by their configuration and/or molecular flexibility. The intracrystalline sorption/diffusion of bulky sorbates in medium-pore zeolites, which involves two mass-transfer processes, entry of sorbate molecules in the channel, and configurational diffusion of sorbate molecules, could be explained well by the shuttlecockshuttlebox model. The liquid-phase sorption/diffusion is expected to differ from the sorption/diffusion from the gas/vapor phase, because of the higher penetration barrier experienced by the sorbate molecules for their sorption from the liquid phase. The higher penetration barrier experienced by the sorbate molecules for their entry in the zeolite channel from their bulk liquid phase is due to the lower entropy and hence lower degree of freedom for the orientation of sorbate molecules in their most favorable conformation and also due to the requirement of additional energy for overcoming their latent heat of condensation. Nomenclature D ) diffusion coefficient (cm2‚s-1) D0 ) frequency factor (cm2‚s-1)

1818 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 E ) activation energy (kJ‚mol-1) Qt ) amount sorbed at time t (cm3‚g-1) Q∞ ) amount sorbed at t∞ (i.e., maximum sorption capacity for a particular sorbate) (cm3‚g-1) t ) time (min) T ) temperature (K)

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Received for review July 16, 1996 Revised manuscript received November 15, 1996 Accepted November 17, 1996X IE960411H

X Abstract published in Advance ACS Abstracts, January 1, 1997.