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TGA Investigation of Xylenes’ Steric Effect During Their Adsorption and Diffusion in NaX Adsorbent F. Finnouche, Y. Boucheffa,* R. Boumaza, and A. Labed EMP, BP 17 Bordj El-Bahri, 16111, Algiers, Algeria
P. Magnoux UMR 6503, Laboratoire de Catalyse en Chimie Organique, 40 Avenue du Recteur Pineau, 86022, Poitiers, France
Separation processes using selective adsorption on zeolite are widely applied in chemical industries. The X zeolite is especially used for the separation of xylenes isomers. The objective of this worksbased on NaX adsorbent known by its selectivity exclusively geometric to xylene isomerssis a contribution to the comprehension of the separation mechanisms and of the steric effect parameters. Adsorption of o-xylene, m-xylene, p-xylene, and trimethylbenzene was studied in thermobalance at 100, 250, and 420 °C and at a constant pressure of 700 Pa. The results show that the adsorption and diffusion process are strongly influenced by the molecules’ sizes. Using the Fick law plot, disruption phenomena owing to the steric effect were observed when the supercages of NaX adsorbent were filled. Introduction Xylenes are essentially produced from catalytic reforming of naphtha and ethylbenzene isomerization. The most valuable isomer among xylenes is p-xylene which is an intermediate for the production of poly(ethylene terephthalate) (PET) fibers, resins, and films. Crystallization and adsorption are both widely used to perform the xylene isomers separation. Despite the high importance of crystallization, adsorption is becoming the most appropriate technique. This is due to its high efficiency. The adsorbents used for selective adsorption of p-xylene are X or Y zeolites exchanged with adequate cations.1 Generally, the selective adsorption over faujasite (X and Y) was used at temperature close to the boiling point of xylenes (∼150 °C).2,3 A large number of experimental and theoretical studies have been performed in this field in order to understand the origin of the selectivity. Several works carried out on NaX zeolite conclude that p-xylene is preferentially adsorbed over m-xylene.4,5 At saturation, about three molecules of p-xylene should be retained in each supercage.6 The main difference between the xylene isomers is the relative position of their methyl groups and consequently their dimensions and dipolar moments. When the aperture of the zeolite cavities is close to the size of the host molecules (the case of xylene isomers over NaX zeolite), the steric effects modify the adsorption and desorption processes. On the other hand, NaX zeolite is known for not providing any selectivity owing to the sodium cation. The selectivity results exclusively from the mobility of adsorbed xylene molecules and their diffusion processes.7,8 For this reason, it is very interesting to study xylene steric effect during adsorption and diffusion in NaX zeolite. This work will focus primarily on the thermogravimetric signal disturbance which accompanies the ad* To whom correspondence should be addressed. Tel: +213 21 86 34 69. Fax: +213 21 86 32 04. E-mail: youcef_boucheffa @yahoo.com.
sorption and diffusion kinetics. This disturbance phenomenon was previously observed in adsorption and diffusion of isoalkanes over 5A zeolites,9 with the disturbance magnitude increasing with the size of isoalkanes molecules isopentane (iC5), 2 methylpentane (2MP), 3 methylpentane (3MP), 2,3 dimethylbutane (23DMB), and 2,2 dimethylbutane (22DMB) in the following order: 22DMB > 23DMB > 3MP > 2MP > iC5. It is expected that the molecular diameter of the xylene isomers (Lp-xylene ) 6.4 Å, Lm-xylene e 7.1 Å, and Lo-xylene ) 7.4 Å) will affect both adsorption and diffusion kinetics, as well as the variation in the thermogravimetric signal disturbance. The adsorption of the trimethylbenzene, as a molecule even more encumbered, will make it possible to confirm the relation between the amplitude of the disturbance and the molecule encumbrance. The effect of temperature will be studied to elucidate the evolutions of aromatics adsorption quantities, diffusion coefficients, and disruption of thermogravimetric signal. The use of thermogravimetric devices along with a computer via Cobra interface leads to curves showing a perceptible cloud of points during adsorption of xylene isomers and trimethylbenzene on NaX adsorbent. Experimental Section The adsorption isotherms were obtained at constant pressure using the MTB 10-8 Setaram microbalance with relative and absolute sensitivities of 4 × 10-8 and 4 × 10-7g, respectively (these specifications are given by Setaram). The thermobalance was linked to a computer via a Cobra interface. The Cobra software allows obtaining a curve with a number of dots close to 1008. The signal disturbances of sorption kinetics can be seen on these curves and their magnitude may be compared easily when a common scale is used. The NaX adsorbent was supplied by Fluka (about 80 wt % of NaX zeolite and 20 wt % of clay binder). The adsorption capacity, determined at -196 °C by nitrogen adsorption, was close to 0.27 cm3‚g-1. Before the introduction of xylenes (purity >99%, from Fluka), the
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sample (0.060 g) was pretreated in a vacuum (10-3 Pa) at 420 °C for 3 h. The microbalance system involves a 3 L vessel; this volume being large, the decrease in xylene pressure was negligible. To avoid adsorbate condensation, the microbalance was maintained at 40 °C. The adsorption isotherms had been monitored under a constant pressure of 700 Pa and at 100, 250, and 420 °C for 1 h. For each xylene isomer, the period of 1 h is sufficient to reach the adsorption plateau. For comparison purposes and to evaluate the effect of the third methyl group on the aromatic cycle, an adsorption kinetic of trimethylbenzene (TMB) (purity >99%, from Fluka) was also monitored and compared to that of the xylenes. After the adsorption of xylene isomers, the sample was treated in a vacuum (10-3 Pa) at the operating temperature until a constant weight was obtained (in all cases 40 min was sufficient to obtain a constant weight). Adsorption isotherms of nitrogen were then established to determine the residual adsorption capacity of the adsorbent. Thereafter, for a good understanding of the retention behavior of each xylene isomer, a programmed thermodesorption (V ) 5 °C‚min-1) under vacuum was carried out in the same apparatus from the operating temperature to 420 °C. To analyze the adsorbed phase, several samples were prepared in microbalance under identical conditions (adsorption during 1 h and 40 min of vacuum treatment). A 1.5 g aliquot of the treated adsorbent was necessary to extract a significant quantity for the analysis. Adsorbed compounds were extracted by using a method developed in the literature:10 after adsorption and vacuum treatment, the adsorbent was dissolved at ambient temperature in a hydrofluoric acid (HF) solution to release the compounds trapped in the zeolite pores. These compounds were recovered in methylene chloride and weighed, after evaporation of solvent, to determine the amount of carbonaceous compounds (coke). Furthermore, the gas released during the dissolution of zeolite in HF was recuperated and analyzed by GC (SPB capillary column). Results Kinetics of Aromatics Adsorption. The effect of time on the weight of NaX adsorbent in contact with o-, m-, or p-xylene and trimethylbenzene (TMB) monitored for 1 h is given in Figure 1. At 100 °C and at saturation (Figure 1a), the same order of uptake is approximately obtained for xylene isomers (14.7 wt % on average which corresponds approximately to 10 × 1020 molecules per gram of adsorbent). Considering the fraction composition of NaX zeolite in the adsorbent (80 wt %) and the accessibility of xylene isomers into only the supercages (8 supercages per elementary lattice and 3.6 × 1020 per gram of pure NaX zeolite), three molecules can be loaded per supercage. This number has been often reported in the literature.6 For the trimethylbenzene, the adsorption equilibrium is not established after 1 h of contact. At this moment, only two molecules of trimethylbenzene can occupy a supercage. At 250 °C (Figure 1b), the uptake in m-xylene and o-xylene decreases relative to that of p-xylene (10.5 wt % for m- and o-xylene against 12.5 wt % for p-xylene) and a light blackening coloration appears on the pellet adsorbent sample (initially white). For the trimethylbenzene, the initial rate of uptake appears very slow but an adsorbed quantity similar to that of xylene isomers was monitored after 1 h of contact. At 420 °C (Figure 1c), the amount adsorbed decreases to around 3.5 wt % and the adsorbent turns to
Figure 1. Weight increase of NaX adsorbent vs time in contact with xylene isomers and trimethylbenzene; pressure 700 Pa: (a) 100 °C, (b) 250 °C, (c) 420 °C.
black. This type of color change is often related to the formation of coke having polyaromatic nature.11 The quite large oscillations which appear at this temperature may be attributed to the transformation of the host molecule into heavy products. As will be shown later through the extraction of the adsorbed phase, a nonnegligible quantity of carbonaceous products is actually formed at 250 °C and increases at 420 °C. The initial rates of uptake determined from the slopes at time zero during xylene and trimethylbenzene adsorption kinetics are reported in Figure 2. For each temperature, the initial rate increases with the decrease of kinetic diameter. The increase is quasi linear at 100 °C, but this is not the case at higher temperature where the initial adsorption rates of o-xylene seem to be slowing down. Vacuum Treatment of Adsorbents. After vacuum treatment of the adsorbent sample previously in contact with xylenes, the residual adsorbate quantity, percent-
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Table 1. Results of Vacuum Treatment (10-3 Pa) at Operating Temperature of Adsorption adsorbate
o-xylene
m-xylene
p-xylene
temperature (°C)
100
250
420
100
250
420
100
250
420
adsorbed quantity after 1 h (wt %) residual quantity after vacuum treatment (wt %) percentage of removal (%) residual adsorption capacity for N2 (cm3‚g-1)
15.05 14.03 6.77 0.03
10.45 6.47 38.08 0.136
1.74 1.29 25.86 0.228
14.24 13.34 6.32 0.09
10.6 6.53 38.39 0.149
3.28 1.39 57.62 0.23
14.78 7.85 46.8 0.1
12.6 7.6 39.68 0.144
3.42 0.92 73.09 0.245
Table 2. Principal Results of Adsorbed Phase Extracted from NaX Adsorbent Treated by Xylene Isomers at 250 and 420 °C after 1 h Adsorption adsorbate
o-xylene
T (°C) quantity adsorbed (wt %) “coke” soluble in CH2Cl2 (wt %) composition of gas phase
250
m-xylene 420
250
p-xylene 420
250
420
6.47
1.29
6.53
1.39
7.6
0.92
0.3
0.98
0.22
0.74
0.16
0.68
o-x (77%); m-x (14%); light products (1.5%); heavy products (7.5%)
o-x (58%); m-x (18%); heavy products (24%)
m-x (75%); p-x (10%); o-x (12%); heavy products (3%)
m-x (16%); various heavy products
p-x (80%); light products (20%)
p-x (71%); m-x (7%); light products (11%); heavy products (12%)
Figure 2. Initial rate of xylene and trimethylbenzene adsorption on NaX at 100, 250, and 420 °C (pressure 700 Pa).
age of removal, and residual adsorption capacity for nitrogen were determined, and these are reported in Table 1. At 100 °C, results show that for the same order of uptake in xylene the vacuum treatment at temperature of adsorption leads to weak removal percentages of o-xylene (6.77%) and m-xylene (6.32%) relative to pxylene, for which the percentage is close to 46.8%. This means that for the same number of molecules per supercage (∼3 molecules/supercage), the small size molecule is easier to remove from cavities under vacuum treatment. The residual adsorption capacity for nitrogen measured at -196 °C, just after desorption, clearly shows that 13.3 and 14 wt % of o- and m-xylene totally block the pore access to the nitrogen molecule. It was surprising that after elimination of a large part of p-xylene (about 50%) the pore volume accessible to nitrogen would be so weak. One possibility will be, during vacuum treatment, the displacement of p-xylene inside the zeolite micropores from the heart of the crystallites to the pore mouth. Residual p-xylene molecules were in this way preferentially located in the pore mouth, leading, as with the other xylene isomers, to a pore blockage (about 65% of the microporosity was inaccessible to nitrogen). At 250 °C, despite the light advantage in adsorbed quantity from p-xylene, no appreciable difference in percentage of removal is observed after the vacuum treatment. At 420 °C, the percentage of removal is lower with the largest molecule (o-xylene). However, a large part
Figure 3. Standardized derivative curves of adsorbed phase removal from NaX adsorbent by programmed thermodesorption (5 °C‚min-1) after adsorption at 100 °C and vacuum treatment.
Figure 4. Thermogravimetric measurement of NaX adsorbent acidity determined by ammonia programmed thermodesorption.
of adsorbed phase can be removed after p-xylene adsorption which suggests that in this condition a very low quantity of polyaromatic compounds was formed from p-xylene. This result will be confirmed by analysis of adsorbed phase. Programmed Thermodesorption Under Vacuum. In what follows, we propose to highlight the retention force of the refractory molecules to desorption by carrying out a programmed thermodesorption under vacuum. This approach, used by various authors,12,13 allows comparison of the behavior of each xylene isomer retained in NaX adsorbent. Figure 3 shows the stan-
Ind. Eng. Chem. Res., Vol. 43, No. 21, 2004 6711 Table 3. Apparent Diffusion Coefficients of Xylene Isomers and Trimethylbenzene in NaX Adsorbent (Di/r02)‚103 (s-1) adsorbate
100 °C
250 °C
420 °C
p-xylene m-xylene o-xylene trimethylbenzene
0.80 0.20 0.15 0.11
0.88 0.39 0.32 0.12
1 1 0.98 1
dardized derivative curves of adsorbed phase removal from NaX adsorbent. Notice that the thermodesorption
(in the range from 100 to 420 °C with V ) 5 °C‚min-1) is more difficult for p-xylene. This observation leads to the conclusion that the residual p-xylene molecules can be blocked strongly in the supercage near the surface and their delocalization requires a significant thermal contribution. The same observation has been done at 250 but not at 420 °C wherein the adsorbed amount is very small and the presence of a carbonaceous compound in the supercages is probable. Analysis of the Adsorbed Phase. After recuperation of adsorbents, those which were destroyed in HF
Figure 5. Diffusion kinetic at long time (left column) and adsorption kinetic (right column) of various aromatics molecules at 100 °C: (a) p-xylene, (b) m-xylene, (c) o-xylene, and (d) TMB.
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reactions that occurred are certainly due to the presence of weak protonic sites and also to the very high contact time of the reactant and adsorbent. Diffusion and Disruption Phenomena. Table 3 shows values of the apparent diffusion coefficient (Di/ r02) obtained from the short-time approximation of Fick’s second law:14
mt/m∞ )
Figure 6. Plateau of adsorption kinetics of xylene isomers (T ) 100 °C), error bar with error bars estimated from baseline of thermogravimetric signal before adsorption (( 0.04% of error).
solution, gas liberated during this operation was recuperated then analyzed. Furthermore, extract (“coke”) was recuperated by CH2Cl2 extraction. Table 2 gives the quantity of “coke” expressed in wt % and the composition of gas phase. It was noticed that, at 100 °C, only the adsorbed molecule is trapped in the pores. The main conclusions drawn from these results can be summarized as follows. (a) For similar temperature, quantities of “coke” are globally of the same order of magnitude, although we notice a perceptible decrease in the percentage when the molecule size is small (p-xylene < m-xylene < o-xylene). (b) If the adsorbed phase is entirely composed from the xylene molecule introduced in the adsorbent at 100 °C, it is not the case at 250 and 420 °C where xylene isomerization occurs. Indeed, the adsorption/desorption of ammonia of NaX adsorbent shows a weak acidity and the presence of sites able to retain ammonia at 350 °C (Figure 4). It was difficult to estimate the number of protonic sites but the
( )
6 Di t xπ r02
1/2
where mt and m∞ are the adsorbed amounts at time t and at equilibrium, respectively. Di is the initial diffusion coefficient, r0 is the radius of the zeolite crystallite, and t is the sorption time. The apparent diffusion coefficient values of xylene isomers (determined from the slope of the linear part of the curve) are of the same order of magnitude as those reported by several authors who used the quasi-elastic neutron scattering (QENS)8 and thermogravimetric method7 in a temperature range close to the operating temperatures in the present work. It is observed that the diffusion coefficient decreases as the size of the molecule increases. Because of its size, the trimethylbenzene presents the weakest diffusion at 100 and 250 °C. At 420 °C, values of the diffusion coefficient are very close because of the strong influence of heat transfer and the formation of carbonaceous products. On the other hand, these coefficients increase with temperature, in agreement with Arrhenius’ law. Considering the long time diffusion, the approximation resulting from Fick’s second law:
(
ln 1 -
) () ()
mt Df 6 ) ln 2 - 2 ‚π2‚t m∞ π r0
where Df is the final diffusion coefficient, allows visualization of an interesting phenomenon at 100 °C (the
Figure 7. Standard deviation of adsorption kinetics of xylene isomers and trimethylbenzene (T ) 100 °C) with presentation of standard deviation ((0.04).
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formation of carbonaceous compounds does not occur at this tempeature) related to the steric effect of xylenes. In fact, these diffusion kinetics monitored for 1 h exhibit a particularly large dispersion when the molecule size is small (Figure 5, left curves). In addition, we can notice that the higher the molecular encumbrance the later disturbances start (tp-xylene ≈ 0.2 h, tm-xylene ≈ 0.4 h, to-xylene ≈ 0.6 h, and tTMB ≈ 0.8 h). The start of these disturbances corresponds to the formation of an uptake plateau, i.e., when the weight approaches the equilibrium value (Figure 5, right curves). To check that the scattering effect, in the vicinity of equilibrium, is not related to the use of a logarithmic scale, a zoom of the adsorption plateau of xylene isomers is provided. The evolution of the disturbance can be observed even from adsorption kinetics data (Figure 6). In addition, the introduction of overestimated error bars in adsorption kinetics curves at saturation, determined from a magnitude of the thermogram baselines before adsorbate-adsorbent contact (error bars close to (0.04% on the wt% scale), shows that the disturbance is not solely due to experimental uncertainty (systematic error in the measurement) or to logarithmic coordinates. On the other hand, to support this assumption, the standard deviation obtained by the computation of the mean vector and its subtraction from initial data of xylene adsorption kinetics, confirms the existence of a relation between the molecular size of xylene isomers and the disturbance magnitude (Figure 7a, b, and c). However, for the more voluminous molecule (trimethylbenzene) (Figure 7d), the standard deviation increases with the molecular size as is the case for isoalkanes adsorption on 5A zeolite. According to the uptake (m∞) which is roughly identical after formation of the plateau for each of the xylene isomers, this comparison is very significant of the phenomenon taking place inside or at the entrance of NaX supercages. Because of the low dimensions of p-xylene, the three molecules contained in the supercage permit higher molecular mobility relative to that of mand o-xylene. The free space existing in supercage induces stronger disturbance amplitude. In analogy with the diffusion of iC5, 2MP, 3MP, 23DMB, and 22DMB and in the absence of heavy compounds (T ) 100 °C), we have realized that the evolution of amplitude disturbance follows an increasing function with regard to the size of the guest molecule. This evolution was attributed to the difficult penetration of these molecules into cavities of 5A zeolite. So, the encumbrance to entering into the R cavity of various isoalkanes molecules is influenced only by geometrical factors (i.e., kinetic diameter of the guest molecule and the window diameter of the R cavity). Conclusion This thermogravimetric study highlighted the influence of the nature of xylene isomers and of methyl aromatics (xylene/trimethylbenzene) as the temperature on the kinetic parameters, during their adsorptions on NaX adsorbent (such as initial adsorption rates, adsorbed quantities, diffusion coefficients, etc.). The quantities adsorbed, for each temperature after 1 h, are in the same order of magnitude for xylene isomers. These quantities decrease notably with increase in the temperature (from 100 to 420 °C) according
to the adsorption laws. Contrary to the adsorbed quantities, the initial rates of adsorption are in favor of the p-xylene isomer whose molecular diameter is lowest. These rates also decrease by increasing the temperature from 100 to 420 °C. The study of regeneration in situ by vacuum treatment revealed a difference in retention of the molecules, as o-xylene is most strongly retained in the NaX supercages of adsorbent. On the other hand, under these conditions, p-xylene isomer is easiest to remove; showing the strong influence of the molecular obstruction on the kinetics of xylenes adsorption. Analysis of the adsorbed phase showed that reactions occur during adsorption of xylene which lead to the formation of heavy products. These reactions were certainly catalyzed by the presence of weak protonic sites but also by the high contact time between the xylenes and active sites. The curves of final diffusion (for long times) highlighted a disturbance phenomenon of the kinetics appearing when the plateau of adsorption kinetic is reached. This disturbance is more early and intense when the time of equilibrium is short, i.e., when the diffusing molecule is encumbered. This signal disturbance phenomenon observed during the diffusion of xylenes in NaX adsorbent, and confirmed by standard deviation of adsorption data, is different from those observed with the C5 and C6 isoalkane diffusion in 5A zeolite. In the case of xylenes, stronger disturbance amplitude was observed for p-xylene, the less voluminous isomer, while the reverse occurs for trimethylbenzene as in the case of isoalkanes on 5A zeolite. This difference can be related to the fact that xylenes penetrate and take their place more easily in the supercages of NaX adsorbent, whereas the isoalkanes experience difficulties entering the windows of the 5A zeolite cavities. Literature Cited (1) Me´thivier, A. Catalytic Science Series, Zeolites for Cleaner Technologies; Guisnet, M., Gilson, J. P., Eds.; Imperial College Press: London, 2002; Vol. 3, chapter 10, pp 209-221. (2) Broughton, D. B.; Neuzil, R. W.; Pharis, J. M.; Brearly, C. S. Chem. Eng. Prog. 1970, 66 (9), 70. (3) Seko, M.; Miyaket, T.; Inada, K. Hydrocarbon Process. 1980, 58, 133. (4) Mellot, C.; Simonot-Grange, M. H.; Pilverdier, E.; Bellat, J. P.; Espinat, D. Langmuir 1995, 11, 1726. (5) Mellot, C.; Bellat, J. P.; Pilverdier, E.; Simonot-Grange, M. H.; Espinat, D. Fundam. Adsorpt. 1996, 5, 611. (6) Pilverdier, E., Thesis, Bourgogne, France, 1995. (7) Goddard, M.; Ruthven, D. M. Zeolites 1985, 6, 283. (8) Jobic, H.; Be´e, M.; Me´thivier, A.; Combet, J. Microporois Mesoporous Mater. 2001, 42, 135. (9) Benaliouche, F.; Boucheffa, Y.; Magnoux, P. J. Soc. Alger. Chim. 2003, 13 (2), 207. (10) Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1. (11) Boucheffa, Y.; Thomazeau, C.; Cartraud, P.; Magnoux, P.; Guisnet, M.; Jullian, S. Ind. Eng. Chem. Res. 1997, 36 (8), 3198. (12) Herz, R. K.; Kiela, J. B.; Marin, S. B. J. Catal. 1982, 73, 66. (13) Rieck, J. S.; Bell, A. T. J. Catal. 1984, 85, 143. (14) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984.
Received for review March 1, 2004 Revised manuscript received July 3, 2004 Accepted July 19, 2004 IE0498347
