Ind. Eng. Chem. Res. 1993,32, 2053-2057
2063
Adsorption of Aromatic Hydrocarbons in NaX Zeolite. 2. Kinetics Douglas M. Ruthven’ Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3
Bal K. Kaul Exxon Research and Engineering, Florham Park, New Jersey 07923
Kinetic data are reported for the sorption of a range of aromatic sorbates in unaggregated crystals of NaX zeolite. Diffusivities (at 600 K) range from order 104-10-’ c m 2 d for benzene and p-xylene to lO-l4-lO-l5 cm2d for hexamethylbenzene and anthracene. The diffusivities decrease with increasing sorbate size, but the relationship is complex and does not follow the sequence of either molecular weight or critical molecular diameter. Rather the correlation appears to be with the moment of inertia of the sorbate molecule, suggesting that restriction of the rotational freedom of the diffusing molecule is probably a major factor. For several of the sorbates the validity of the diffusion model was confirmed by replicate measurements with different crystal size fractions.
Introduction In part 1 we reported equilibrium data for sorption of several medium molecular weight aromatic hydrocarbons in 13X zeolite crystals; we report here kinetic data for these same systems. The sorption cutoff with increasing molecular size has been clearly demonstrated for the “small pore” (type A) zeolites by Breck (Breck et al., 1956;Breck, 1974)and for the “medium pore” zeolites (silicalite/HSM5) by Leach et al. (1984). However, despite the practical importance of the “large pore” (X and Y type) zeolites, the available data for diffusion of larger molecules (aromatics in particular) in these materials are quite limited. It is clear that for sufficiently large sorbate molecules there must exist a cutoff point beyond which there is essentially no penetration of the intracrystalline pores (on any practical time scale). However, molecules that are too large to penetrate the 12-ring of the faujasite structure have high molecular weight and low volatility, making it difficult to perform accurate uptake measurements. As a result, for the X and Y zeolites, this cutoff has not been studied in any detail. Satterfield, Katzer, and their co-workers (Satterfield and Katzer, 1971;Moore and Katzer, 1974)published the results of an extensive series of liquid-phase sorption and counterdiffusion measurements. However, these studies were all made with small (1-2 pm) X and Y zeolite crystals. More recent measurements with larger crystals gave much higher diffusivity values (Culfaz and Ergun, 19861, suggesting that effects other than intracrystalline diffusion may have been important in the earlier studies. To minimize the problem of catalytic activity, which can interfere with the diffusion measurements, we used the relatively inactive sodium form of zeolite X (NaX or 13X). This material also has the advantage that it can be easily prepared in the form of relatively large crystals. However, the higher aromatics are very strongly adsorbed in NaX,and this complicates the interpretation of uptake rate measurements by introducing a possible external (bed diffusion) resistance (Ruthven, 1984). Measurements with at least two different crystal sizes are therefore necessary to confirm the absence of such effects. The majority of the sorbates used in this study were either unsubstituted or methyl-substituted aromatics containing one, two, or three benzene rings. Such species are much less reactive than the corresponding ethyl- or
propyl-substituted aromatics of comparable molecular weight. Measurements were also made with triethylbenzene, but the combination of very slow diffusion and detectable catalytic cracking makes it difficult to interpret the data for that system. Although the method of determining diffusivities from sorption rate measurements is well established (Ruthven, 1984) and the precautions necessary to obtain reliable results are well known, the present series of sorbates presented some significant experimental challenges as a result of the combination of very low vapor pressure, strong adsorption, and, for the larger sorbates, very low diffusion rates.
Experimental Section Uptake rates were measured gravimetrically using a Cahn vacuum microbalance system fitted with a Barocel capacitance manometer and an automated digital data acquisition system (providing storage of pressure and sample weight data at any desired time intervals.) The larger NaX crystals (loo-, 50-, and 30-pm mean diameter) were synthesized in this laboratory by Yucel(1979) using Charnell’s method. (Charnell, 1971). The small crystals (-2 pm) were from a sample of commercialmaterial kindly provided by Union Carbide Corporation. The Si/Al ratio for all samples was 1.2-1.3, as measured by atomic absorption spectroscopy. Prior to the experimental measurements the zeolite crystals were dehydrated under vacuum (-103 Torr) at elevated temperature. The temperature was raised slowly to 150 “C over a period of a few hours (with continuous pumping) and then raised somewhat more rapidly to 375 “C. The temperature was maintained at this level overnight to ensure thorough dehydration of the sample. The sample weight was generally about 20 mg although in initial experiments the sample quantity was varied in order to confirm the absence of significant extracrystalline resistance to mass or heat transfer. In order to minimize the intrusion of external resistances to mass and heat transfer, it is desirable to utilize as small a sample of adsorbent as possible. However, with very slowly diffusing adsorbates a larger sample is needed in order to facilitate the determination of the end point of the uptake curve. With the larger crystals and the slower diffusing sorbates, the equilibrium isotherms measured
0888-5885/93/ 2632-2053$04.00/0 0 1993 American Chemical Society
2054 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Table I. Summary of Diffusivity Data for Aromatics in NaX Crystals sorbate mesitylene
T (K) 573
603 tetramethylbenzene
547 579 610 635 635
hexamethylbenzene 599
press. step (Torr) 0.009-0.045 0.046-0.075 0.075-0.13 0.13-0.25 0.074-0.20 0.20-0.39 0.0-0.024 0.024-0.038 0.038-0.046 0.08-0.13 0.046-0.071 0-0.15 0.15-0.314 0.03-0.09 0.02-0.09 temp step p + 0.003
anthracene
triethylbenzene
639 658 546 578 608
temp step p=sx1w 0.003-0.025 0-0.024 0-0.045 0.01-0.054
av loading (wt %) 0.8 2.3 3.8 6.0 2.5 5.0 3.5 2.9 3.8 6.3 2.7 1.7 4.0 1.0 1.0 8.6" 3.2 0.8 2.6 2.3 1.5 0.5 4.0 3.8 4.5 3.2
Crystal diam (pm) 100 100 100 100 100 100 50 50 50 50 50 50 50 30 2 2 2 2 2 2 2 2 30 2 50 30
Dlr2 (s-l) 2.1 x 106 2.5 X 106 3.7 x 106 5.0 X 106 2.8 X 106 3.3 x 1 v 2.8 X 10-8 3.3 x 10-8 3.5 x 10-8 6.0 X 10-8 4.6 X 10-8 5.6 X 10-8 6.4 X 10-8 2.0 x 106 2.5 X 10-4 8 x lv7 1.1 x 10-8 f f
D (cm2.s-1) 5.25 X 10-lO 6.25 X 10-10 9.3 x 10-10 1.25 X 1W 7 x 10-10 8.3 X 1.8 x 10-11 2.1 x 10-11 2.2 x 10-11 3.75x 10-11 2.9 X 10-11 3.5 x lo-" 4.0 X 1V1l 4.5 x 10-11 210-11
8 x 10-16 1.1 x 1v14
10-8
10-14
1.1 x 10-7 1.6 x 10-7 1.9 x 10-7 2.1 x lo-' 1.3 x 1v7 7XlP 1 x 10-7 7.5 x 10-7
1.1 x 10-16 1.6 X 10-l6 1.9 x 10-16 2.1 x 10-16 3 x 10-19 7 x 10-13 6.5 X lO-'S 1.7 X W 2
This is well outside the Henry's law region.
with the small (2 pm) crystals proved to be a more useful criterion for the end point than the uptake curve itself which was, in its final stages, so slow as to be indistinguishable from baseline drift. For the lighter sorbates measurements were made stepwise over incremental pressure steps. However, for hexamethylbenzene and anthracene the vapor pressure at ambient temperature (which determines the maximum Torr) sorbate pressure in the vacuum line) is so low (that it was practically feasible to make only a single step change. Furthermore, at these low pressures the rate at which the pressure equalizes through the vacuum line is surprisingly slow (up to 40 min) and it is therefore not possible to obtain a good approximation to a step change. The temperature, however, can be changed more rapidly ( 10-15 min). Since the time scale of the uptake curves for these species is more than 1 day, a change in temperature was used to provide a reasonable approximation to a step change in surface concentration. Diffusion time constants were determined by matching the experimental uptake curves to the well-known solution of the diffusion equation for a set of isothermal spherical particles, subjected to a step change in sorbate concentration at time zero:
Table 11. Summary of Molecular Dimensions and Diffusivity Values crit max Dat600K diam len h M P X lor0 sorbate (A) tG (g.cm2) (cm2-a-1) 6.63 7.3 180 6 X lV7
Q
-
H 3 C o C H 3
6.63
9.4
360
4X10-8
7.3
8.35
340
4X10-8
6.8
9.1
530
3X10-8
8.35
8.62
540
2x10"
8.6
9.4
630
3x
N
This expression assumes that the sorbate pressure remains constant followingthe initial step. With strongly adsorbed species the uptake by the sample becomes comparable with the total quantity of sorbate in the vacuum system. However, in the present study the uptake was sufficiently slow that the pressure could be maintained constant by successive addition of small amounts of sorbate to compensate for the quantity adsorbed.
Results and Discussion The results are summarized in Figures 1-5 and Tables I and 11,which show representative uptake curves, selected to show the effects of key variables such as sorbate pressure,
10-11
H5GvH5 9.5
9.7
lo00
8.6
9.4
900
6.8
12.1
1100
(1.4X 10-'2)
ff
ff
10-14
10-16
" Moments of inertia are estimated aa the geometric mean of the values for the three principal areas I = (ZzZJz)l/s. temperature, and crystal size, together with a summary of the diffusivity data obtained for each of the sorbates. 1,3,S-Trimethylbenzene (Mesitylene). Uptake of mesitylene was relativelyrapid even in the 100-rm crystals
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 2055 TMB, 50 pm NaX
603 K -
-
0
573 K
50
100
1
2
3
4
5
6
t (hrl
P(Torr1 ( 1 1 .01-945 ( 2 1 ,016-975
0
0.2-
Figure 4. Uptake curves for tetramethylbenzene in 50-pm NaX crystals showing the effect of temperature and pressure. Pressure steps (Torr) are indicated.
150
200
t ( mins)
Figure 1. Uptake curves for mesitylene in 100-rm NaX crystals. For details see Table I. 1.0
1,2,3,5 TMB a t 547K
2gm crystals P (Torr) 0.01 -0.025 x 0.025-0.045 A 0.045-0.075 0
"
0
1
2
3
.
4 5 Time ( h r )
6
7
8
9
Figure 2. Uptake curves for tetramethylbenzene at 547 Kin 2- and 50-gm NaX crystals.
0
50
100
150
200
250
t (hr)
Figure 5. Uptake curves for (a) hexamethylbenzene and (b) anthracene in 2-rm NaX crystals.
I
0
50
I
100
I
150 t (mins)
I
I
200
250
I 300
Figure 3. Uptakecurvesfortetramethylbenzeneat 635 K indifferent size fractions of NaX crystals. Pressure steps (Torr) are indicated.
(see Figure 1).The time constanta and diffusivitiesderived from the uptake curves are summarized in Table I. However, since measurements were made with only one crystal size, one cannot exclude the possibility of extracrystalline diffusion limitation. 1,2,3,5-Tetramethylbenzene.Uptake rates were measured for three different crystal size fractions a t temperatures ranging from 547 to 635 K. At 547 K the rate is evidently controlled by extracrystalline resistances since the rate is independent of crystal size (Figure 2). However, since the Henry constant decreases rapidly with temperature, the influence of the external (bed diffusion) resistance becomes less pronounced a t higher temperatures so that there is a transition to intracrystalline control, a t least for the larger crystals. The pronounced effect of crystal size at 635 K is shown in Figure 3. Diffusivity values derived from the data for the 30- and 50-pm crystals
are approximately consistent (see Table I),thus providing confirmation of micropore diffusion control. The effect of temperature on the uptake rate at low pressures (close to the Henry's law region) is shown as Figure 4. Hexamethylbenzene and Anthracene. Sorption rates for both these sorbates were very low and measurable only in the smallest (2 pm) crystal size. Representative uptake curves (measured by changing the temperature at a fixed sorbate pressure) are shown in Figure 5. 1,3,&Triethylbenzene. Uptake curves for triethylbenzene in 2- and 30-pm crystals are shown in Figure 6. Rates in the 50-pm crystals were generally too low for confident measurement since, with the very slow approach to equilibrium, it becomes difficult to establish the true end polnt. Some experiments were continued for several weeks, but even then a steady end point was not achieved (with the 50-pm crystals). This led to the suspicion that a very slow catalytic isomerization/cracking reaction might be taking place and this was confirmed by mass spectral analysis of the residual gas phase (see Appendix). However, with the smaller crystals, particularly at lower temperatures there appeared to be no strong evidence of end point drift (on the relevant time scale) so we have to assume that the diffusivity data are reasonably reliable. The diffusivity for triethylbenzene lies between the values for tetraethyl- and hexamethylbenzene, but the diffusional activation energy (Figure 7) appears to be
2056 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1.c
Triethyl Benzene
0.8 2 y546 i AK .
0.6
~~0.003-0.025 Torr
a
E
'0.1 E
-
\
0.3
0.2
-
p :0.003 0.022 Torr
100
0.1
~
0
10
5
15 20 t (hr)
30
25
35
Figure 6. Uptake curves for 1,3,5-triethylbenzene in 2- and 30-*m NaX crystals showing strong effect of temperatue and crystal size. 10 - 6
I
-----I
- - -_Benzene
I
\
10-7
t
10-13
.
0-0,
10-15 1.5
I 1.0
1.6
ANTH I
1.7
1.8
1.9
2.0
2.1
1 0 ~ 1 ( ~ -~ 1)
Figure 7. Arrhenius plot showing temperature dependence of diffusivities of aromatic sorbates in NaX crystals.
significantly greater than for the other sorbates. This may not be altogether unexpected since the molecule is of a quite different shape from the other molecules which are all planar. However it is also possible that the high activation energy is an artifact, reflecting increased cracking rates at higher temperatures. Variation of Diffusivity with Molecular Size. The diffusivities determined in this study are summarized in Figure 7, which includes also the values determined previously for the lighter aromatics. The range of diffusivities spans 7 orders of magnitude, and there is obviously a general correlation with molecular size. Unfortunately, due to the rather high level of uncertainty in the experimental data, the activation energies could not be deter-
300
500 700 900 1100 M.1 ~ l O ~ ~ ( g , c r n ~ )
Figure 8. Variation of diffusivity (at 600 K)with moment of inertia of sorbate molecule.
mined with the same confidence as the actual diffusivity values. Nevertheless, it appears that the variation in activation energy is modest compared with the very large variation in the actual diffusivities. This suggests that entropic or orientation effects may be more important than energy barriers. To correlate the diffusivities with the shape and size of the sorbate molecules proved a surprisingly challenging task. It is evident from Table I1 that the sequence of diffusivities does not follow the sequence of critical molecular diameters; e.g., o-xylene, naphthalene, and anthracene have the same critical molecular diameters but their diffusivities are quite different while o-xylene and p-xylene have significantly different critical diameters but almost the same diffusivities. It is also evident that the length of the molecule plays an important role but the sequence of diffusivities does not conform to the sequence of molecular lengths; e.g., p-xylene, tetraethylbenzene, and hexamethylbenzene all have essentially the same critical lengths but very different diffusivities. However, with the exception of triethylbenzene, which is the only nonplanar molecule, the sequence of diffusivities conforms to the sequence of the molecular moments of inertia (see Table I1 and Figure 8). There can be little doubt that the rotational freedom of the larger molecules will be severely restricted both in the equilibrium adsorbed state and in the diffusional transition state. Nevertheless the rotational freedom will be transformed into various rocking and torsional oscillations, the partition functions of which will be directly related to the appropriate moment of inertia. A straightforward relationship between the entropy of activation and the moment of inertia of the molecule is therefore not unexpected.
Conclusions The observed pattern of behavior suggests that the diffusion of large sterically hindered planar molecules within the pores of zeolite X is governed primarily by entropy effects, rather than by the energy barrier associated with penetration of the pore constrictions. The
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 2057 only sorbate which does not conform to the observed pattern is triethylbenzene. Such a difference in behavior is perhaps not surprising when the differences in molecular shape are considered. The critical dimension of the triethylbenzene molecule is relatively easily compressible since a simple torsional bending of the -CzHs side chain is all that is needed. By contrast the planar molecules are almost incompressible in the plane of the aromatic ring since for these species compression would require shortening of either a C-C or a C-H bond or compression of a methyl group. In a previous experimental study we observed apparent diffusivities for triisopropylbenzene in NaX in the range 10-8 cm2.s-1 a t 500 K (Ruthven et al., 1990). These values seem very high compared with the values observed in the present study. We therefore have to suspect that the high sorption rate may have arisen from an unnoticed catalytic decomposition reaction since the reaction rate for triisopropylbenzene would be expected to be substantially higher than for triethyl- or trimethylbenzene.
Appendix Samples of triethylbenzene which had been in contact with the 50-bm NaX crystals for about 4 weeks were removed using the cold-finger technique and compared, by mass spectrometry, with a sample of fresh triethylbenzene from the same bottle. Significant peaks corresponding to mass numbers 119 and 133 were detected in the sample from the vacuum system. These correspond to the main ions formed from C&(CzHs)z andC&(CH3)3. These same peaks were identifiable but much less intense in the fresh TEB sample. These observations therefore suggest that, in contact with the zeolite, triethylbenzene is slowly reacting:
H5C2g-C2H5
C2H5
+ C2H4
C6H4(C2H5)2
\ C6kdCHd3
-t
Literature Cited Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974;pp 633-645. Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. Structure and Properties of a New Synthetic Zeolite. J. Am. Chem. SOC.1956, 78,5963. Charnell, J. F. Synthesis of Large Crystals of A and X Zeolites. J. Cryst. Growth 1971,491. Culfaz, A.;Ergun, G. Counter-diffusion of Liquid Hydrocarbon Pairs in Ion Exchanged Forms of Zeolite X. Sep. Sci. Technol. 1986, 21,495. Leach, H. F.; Harrison, I. D.; Whan, D. A. Correlation between Sorptive and Catalytic Properties of a Series of Pentaeil Zeolites. Proceeding of the Sixth International Conference on Zeolites; Olson, D., Bisio, A.,Eds.; Butterworths: Guildford, UK, 1984;pp 479-488. Moore,R. M.; Katzer, J. R. Counter-diffusionof Liquid Hydrocarbons in Type Y Zeolite. Effect of Molecular Size, Molecular Type and Diffusion Direction. AZChE J. 1972,18,876. Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984;Chapter 6. Ruthven, D. M.; Eic, M.; Xu, 2.Diffusion of Hydrocarbons in A and X Zeolites and Silicalite. In Catalysis and Adsorption by Zeolites; Ohlmann,G., Pfeifer, H., Ficke, R., Eds.; Elsevier: Amsterdam, 1991;pp 233-246. Sattefield, C. N.; Katzer, J. R. Counter-diffusion of Liquid Hydrocarbons in Type Y Zeolites. Adv. Chem. Ser. 1971,102,193-208. Yucel, H. Diffusion in Large Crystals of 4A and 5A Zeolites. Ph.D. Thesis, University of New Brunswick, of Fredericton, 1979.
Received for review December 14, 1992 Revised manuscript received May 28, 1993 Accepted June 11, 1993. ~
~~
~
~~
~
~
~~~
* Abstract published in Advance ACS Abstracts, August 15, 1993.
