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Ind. Eng. Chem. Res. 1996, 35, 2060-2064
Adsorption of n-Hexane and Intermediate Molecular Weight Aromatic Hydrocarbons on LaY Zeolite Douglas M. Ruthven* Department of Chemical Engineering, University of Maine, Orono, Maine 04469
Bal K. Kaul Exxon Research and Engineering, Florham Park, New Jersey 07923
Experimental equilibrium isotherms, Henry’s law constants, and heats of sorption are reported for n-hexane, benzene, toluene, p-xylene, mesitylene, naphthalene, trimethylbenzene (TMP), and hexamethylbenzene (HMB) in La-exchanged zeolite Y (Si/Al ) 1.8). Henry’s law constants and energies of adsorption are substantially smaller than those for NaX zeolite, reflecting the absence of accessible cations in LaY. These data provide a basis for the estimation of adsorbed phase concentrations of the relevant hydrocarbons on REY cracking catalysts under reaction conditions. In an earlier paper (Ruthven and Kaul, 1993) we reported the results of a detailed study of the equilibrium isotherms and isobars for a series of aromatic hydrocarbons on NaX zeolite crystals. The choice of this zeolite for our initial study of sorption of the higher molecular weight aromatics was dictated largely by the availability of extensive data for the lower molecular weight aromatics (benzene, toluene, and xylene) from earlier experimental studies in this laboratory. However, one of the objectives of the study was to provide a basis for the understanding of catalytic behavior, and, since most zeolite cracking catalysts are based on rareearth-exchanged Y zeolite, a comparative study of the adsorption equilibrium for some of the same adsorbates on LaY crystals was undertaken. The greatly increased catalytic activity of LaY, in comparison with NaX, meant that measurements could be made only with unsubstituted and methyl-substituted aromatics. Reliable equilibrium data could not be obtained for sorbates such as triethylbenzene which show significant reactivity, even at temperatures in the 200-300 °C range. Experimental Section Equilibrium isotherms and isobars were measured over a range of temperatures by standard gravimetric methods. Since only small (∼2 µm) crystals of LaY were available, comparative chromatographic measurements could not be performed to determine Henry’s law constants. Adsorption on LaY is much weaker than on NaX so measurements could be made at somewhat lower temperatures and higher pressures. The extended pressure range facilitated the experimental measurements since the need to operate at very low pressures was largely eliminated. However, the maximum attainable pressure is restricted by the saturation vapor pressure at room temperature. It was therefore impractical to measure isotherms for the higher molecular weight species such as hexamethylbenzene, and for such species only the isobar at (or below) the ambient saturation pressure was determined. The dehydration procedure and other experimental details were essentially the same as in our earlier study of NaX (Ruthven and Kaul, 1993). S0888-5885(95)00666-X CCC: $12.00
Results and Discussion The isotherms for n-hexane (for comparison), benzene, toluene, p-xylene, and mesitylene are shown in Figures 1-5. Also shown are representative Langmuir plots (1/q vs 1/p), which for all these sorbates show significant curvature. Evidently, the isotherms do not conform to the simple Langmuir expression. Virial Isotherm. If the adsorbed phase obeys an equation of state of virial form, then the equilibrium isotherm should be expressible in the form:
K′p ) q exp[A1q + A2q2 + ...]
(1)
ln(q/p) ) ln K′ - A1q - A2q2...
(2)
or
A plot of log(q/p) vs q should therefore approach linearity in the low concentration region, and this provides a simple way to extract Henry’s law constants from measurements outside the linear region of the isotherm (Barrer and Davies, 1970). Examples of such plots are shown in Figures 1-5. For all sorbates the virial plots are essentially linear, up to about 8% by weight, showing that, within this range, the higher order virial terms are negligible. A concise correlation of the isotherm is therefore obtained in the form:
K′p ) qeA1q
(3)
The parameters K′ (the Henry constant) and A1 derived from the intercepts and slopes of the virial plots are summarized in Table 1. For all four sorbates for which sufficiently extensive isotherm data were obtained the parameter A1 appears to be essentially independent of temperature. This makes eq 3 particularly useful as a simple means of data correlation. The fit of the original isotherms to this expression using a mean value of A is shown in some of the figures. Isobars. For the higher molecular weight sorbates only isobars were measured; these are shown in Figure 6. With NaX we found that the higher molecular weight sorbates approach Langmuir behavior with a saturation © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2061 Table 1. Summary of Virial Isotherm Parameters for LaY sorbate
T (K)
K′g/100 g‚Torr)
A1 (wt %)-1
n-hexane
366 388 414 445 367 413 439 373 398 424 449 399 423 453 483 503
13.5 4.5 1.3 0.5 56 6.2 2.2 410 110 40 11 1100 200 45 19 8
0.26 0.27 0.27 0.29 0.41 0.37 0.39 0.38 0.41 0.41 0.41 0.52 0.49 0.51 0.48 0.54
benzene toluene
p-xylene
A h1 0.275
0.39 0.403
0.51
Table 2. Summary of Henry’s Law Constants and Adsorption Energies sorbate n-hexane
benzene toluene
p-xylene
mesitylene
1,2,3,5-tetramenthylbenzene naphthalene
hexamethylbenzene
T
K
K0
-∆U0 (kcal/mol)
366 388 414 445 367 413 439 373 398 424 449 399 423 453 483 503 493 517 550 574 450 481 510 534 430 452 477 510 534 428 451 476 500
5.4 × 104 1.9 × 104 5.8 × 103 2.4 × 103 2.5 × 105 3.1 × 104 1.15 × 104 1.55 × 106 4.5 × 105 1.7 × 105 6.0 × 104 2.5 × 106 7.4 × 105 1.8 × 105 8 × 104 4 × 104 1.0 × 105 4 × 104 1.55 × 104 9 × 103 1.2 × 106 3.1 × 105 1.35 × 105 5.7 × 104 1.6 × 106 9.3 × 105 2.9 × 105 9 × 104 4 × 104 5.5 × 106 1.9 × 106 7 × 105 2.9 × 105
1.0 × 10-3
12.9
2.25 × 10-3
13.5
6.4 × 10-3
14.3
8.4 × 10-3
15.4
6 × 10-3
16.5
5.2 × 10-3
17.3
5.5 × 10-3
16.8
5.8 × 10-3
17.6
Table 3. Comparative Data for Sorption in NaX and LaY sorbate
Figure 1. Equilibrium data for n-hexane-LaY: (a) isotherms; (b) Langmuir plots; (c) virial plots. The lines in a are calculated from eq 3 with the parameters given in Table 1.
capacity approaching 1 molecule/channel intersection. If we assume the same pattern of behavior for LaY, we can expect the isobar to follow the equation:
1 1 1 - ) q qs K′p
(4)
and since the temperature dependence of K′ is
n-hexane benzene toluene p-xylene mesitylene naphthalene tetramethylbenzene hexamethylbenzene
ratio of K0 energy difference (kcal/mol) for LaY/NaX (-∆U0)NaX - (-∆U0)LaY 1.34 3.2 9.7 19.1 30 46 137 1160
14.2-12.9 ) 1.3 18.0-13.5 ) 4.5 19.6-14.3 ) 5.3 21.5-15.4 ) 6.1 23.3-16.5 ) 6.8 25.5-16.8 ) 8.7 27-17.3 ) 9.7 30-17.6 ) 12.4
given by:
K′ ) K0e-∆H0/RT
(5)
one may anticipate that a plot of ln(1/q - 1/qs) vs 1/T (at constant p) should be linear with slope ∆H/R. Such plots are shown in Figure 7, and their linearity suggests
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Figure 2. Equilibrium data for benzene-LaY: (a) isotherms; (b) virial plots. The lines in a are calculated from eq 3 with the parameters given in Table 1.
that the assumption of Langmuir behavior, for the higher homologs, must be approximately correct. Henry’s Law Constants and Sorption Energies. Dimensionless Henry’s law constants derived from the virial plots and isobars are summarized in Table 2, and the temperature dependence is shown in Figure 8. As with NaX there is a regular increase in Henry’s law constant with increasing carbon number, and this is associated with an increase in the adsorption energy -∆U0 (Figure 9). However, both the magnitude of -∆U0 and the variation with carbon number are much smaller for LaY. It is remarkable that the preexponential factor K0 (K ) K0e-∆U/RT) is almost constant, implying that the entropy decrease on adsorption is essentially the same for all sorbates. This is in marked contrast to the NaX system for which the values of K0 are substantially smaller and decrease with increasing carbon number. The difference in the magnitude of K0 between NaX and LaY is understandable since in LaY all cations are located within the hexagonal prisms so that the interaction with the sorbate molecules is much
Figure 3. Equilibrium data for toluene-LaY: (a) isotherms; (b) Langmuir plots; (c) virial plots. The lines in a are calculated from eq 3 with the parameters given in Table 1.
Ind. Eng. Chem. Res., Vol. 35, No. 6, 1996 2063
Figure 4. Equilibrium data for p-xylene-LaY: (a) isotherms; (b) virial plots. The lines in a are calculated from eq 3 with the parameters given in Table 1.
Figure 5. Equilibrium isotherms for mesitylene-LaY.
Figure 7. Plots of log(1/q - 1/qs) vs 1/T from equilibrium isobars.
Figure 6. Equilibrium isobars for (a) hexamethylbenzene at p ) 0.005 Torr, (b) naphthalene at p ) 0.1 Torr, (c) mesitylene at p ) 0.4 Torr, (d) hexamethylbenzene at p ) 0.005 Torr.
weaker. However, the difference in the trend with carbon number is surprising. A comparison between the preexponential factor and adsorption energies for NaX and LaY is shown in Table 3. For nonpolar sorbates such as n-hexane the differences in both energy and entropy (preexponential factor) are modest. However, for the aromatic sorbates the differences are appreciable and become progressively greater with increasing carbon number, reflecting the influence of the Na+ cations within the supercage of NaX. The differences in adsorption energies between pairs of compounds with the same carbon number (n-hexane/
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Figure 9. Variation of adsorption energy with carbon number.
Figure 8. van’t Hoff plots showing the temperature dependence of the dimensionless Henry’s law constants.
benzene and tetramethylbenzene/naphthalene) are also much larger for NaX than for LaY. This observation is qualitatively consistent with the greater specific interaction of an aromatic ring with the Na+ cations in NaX compared with the well-shielded La3+ cations in LaY. However, it is difficult to draw more precise quantitative conclusions. Conclusions The data presented here provide sufficient information to allow the equilibrium capacity of LaY zeolite for a range of intermediate molecular weight hydrocarbons to be estimated with confidence over a wide range of conditions of temperature and partial pressure. In particular, the Henry’s law constant data presented in
Figure 8 (and Table 2) permit a straightforward extrapolation to the range of temperatures used in catalytic cracking reactors, thus allowing an estimate of the adsorbed phase concentrations present under reaction conditions and hence a more rational interpretation of the kinetic rate data. Nomenclature The symbols have the same meanings as defined in our earlier paper (Ruthven and Kaul, 1993). Literature Cited Barrer, R. M.; Davies, J. A. Sorption in Decationated Zeolites. Proc. R. Soc. London 1970, A320, 289. Ruthven, D. M.; Kaul, B. K. Adsorption of Aromatic Hydrocarbon in NaX Zeolite. Ind. Eng. Chem. Res. 1993, 32, 2047-2052.
Received for review November 3, 1995 Revised manuscript received February 20, 1996X IE950666F
X Abstract published in Advance ACS Abstracts, May 1, 1996.
