Sorption properties of silicalite-1 of pure silica form: The influence of sorption history on sorption kinetics of critically sized molecules

Sorption properties of silicalite-1 of pure silica form: The influence of sorption history on sorption kinetics of critically sized molecules H a l i m K a r s h , Ali ~ulfaz, and Hayrettin Yiicel

Chemical Engineering Department, Middle East Technical University, Ankara, Turkey Sorption properties of silicalite-1 samples of pure silica form for some parafinic and aromatic hydrocarbons of different molecular sizes were investigated. It was observed that sorption rates and apparent equilibrium capacities for critically sized molecules, m-xylene and o-xylene, depend on the sorption history of the sample. Sorption of m- and o-xylene proceeded extremely slowly on fresh samples of silicalite-1, whereas sorption rates of these molecules on silicalite-1 samples that had passed through a previous p-xylene sorption/regeneration step were significantly faster and equilibrium capacities reached about 92-95% of sorption volume of silicalite-1 when sufficient time was allowed. Uptake curves for m- and o-xylene were of the sigmoid-type, indicative of lattice distortions of silicalite framework. The results of this work, along with some recent, independent studies involving n.m.r. MAS and XRD methods strongly suggest that sorption of some critically sized molecules causes subtle lattice distortions of high-silica ZSM-5 zeolites, modifying their sorptive behavior. Keywords: Silicalite; sorption; diffusion; sorption history

INTRODUCTION ZSM-5 and its aluminum-deficient analog silicalite-1 have a unique pore system consisting of two types of intersecting channels of 10-membered ring openings.lX One channel system that is parallel to the [100] axis is sinusoidal with near-circular free apertures of 0.54 + 0.02 nm and the other one that is parallel to the [010] axis is straight with elliptical free apertures of 0.57-0.58 x 0.52 nm. These channel systems render ZSM-5/silicalite-1 zeolite pore openings that are intermediate between shape-selective small-pore zeolites (like Type A) and surface selective large-pore zeolites (X and Y). Pore sizes of ZSM-5/ silicalite-1 and molecular sizes of commercially important aromatics and substituted parafinic hydrocarbons are similar and a wide range of shape-selective adsorptive and catalytic applications of ZSM-5/ silicalite-1 have been proposed. A substantial amount of work about sorption and catalytic properties of ZSM-5/silicalite and other members of pentasil zeolites has been published, s-s9 A survey through the literature indicates that sorption equilibria and kinetics of hydrocarbons on pentasil zeolites including end-members ZSM-5/silicalite-1 are quite complex Address reprint requests to Dr. Y6cel at the Chemical Engineering Department, Middle East Technical University, 06531 Ankara, Turkey. Received 13 May 1991; accepted 10 February 1992 (~ 1992 Butterworth-Heinemann 728

ZEOLITES, 1992, Vol 12, July~August

because many factors including the method of synthesis, 3 7' crystal size, 3 crystal morphology, 1 5 15 intergrowths, silicon-to-aluminum ratio, 1 0 ' 1 3 - "and nature of cations l O ' 1 2 have been reported to influence sorptive and catalytic properties of pentasil zeolites. Furthermore, it has been shown that sorption of some critically sized molecules such as xylene isomers have peculiar sorption behavior like hysteresis 9'29 and unusual uptake curves. 7'26'ss It is noteworthy that these latter effects were more pronounced for high-silica ZSM-5 zeolites. Investigation of sorption behavior of silicalite samples of "pure" silica form would give more information about these unusual sorptive properties of ZSM-5 zeolites that might be interrelated. T h e results described in this study form part of a comprehensive study of sorption and diffusion in silicalite crystals of pure silica form.

EXPERIMENTAL Silicalite- 1 of pure silica form was synthesized according to the methods originally reported by Flanigen and Grose 39 and described as the G-procedure by Debras et al. 4° Syntheses have been done in stainlesssteel autoclaves with polytetrafluoroethylene inserts. T h e initial reaction mixtures had a molar composition of 3.25 Na20:40.0 SIO2:552 H20:2.0 TPA Br. Reagents used were sodium hydroxide (Merck), silicic acid (Merck), t e t r a p r o p y l a m m o n i u m b r o m i d e

Sorption properties of silicate-l: H. Karsh et aL Table I Equilibrium sorption capacities at 298 K and a relative pressure PIPo = 0.5

Sorbate Water Hexane Benzene Ethyl benzene

p-xylene o-xylene m-xylene

Kinetic diameter e (Ref. 41) (nm) 0.265 0.430 0.585 0.600 0.585 0.680 0.680

Volume sorbed (cma/g) 0.0474 0.183 0.142 O.138 0.180 O.175 0.176

% Filling of total volume 25 99 75 73 95 92 93

e It is assumed that the kinetic diameter of p-xylene is the same as that of benzene and the kinetic diameter of o-xylene is the same as that of m-xylene

tion of pressure in the system was 2-3 s. The response time of the balance and recording system was less than 1 s. Prior to sorption measurements, 20-50 mg of the sample was spread as a thin layer in an aluminum sample pan and treated at 673 K and a vacuum of less than 10 -2 Pa for about 4 h. The water capacity of the silicalite sample was determined gravimetrically in the microbalance. For this purpose, the calcined sample was kept over a saturated solution of calcium nitrate in a desiccator overnight and the water content was determined as the weight loss occurred when the sample was heated to 673 K under a vacuum of less than 10 -2 Pa. RESULTS AND DISCUSSION

(Kodak), and distilled water. The details of the syntheses and characterization of the samples obtained in the temperature range of 100-175°C can be found elsewhere 41 and will be the subject of another communication. T h e products were characterized by X-ray diffraction, SEM, infrared spectroscopy, and sorption capacities. T h e particular sample of silicalite1 used in this study was synthesized at 150°C for 8 h. The X-ray diffraction pattern and the electron micrograph of the sample showed a highly crystalline product and no uncrystallized gel was detectable. Electron micrographs of the sample used in this study displayed a spherical morphology of the crystal aggregates in the range of 5-10 ~tm with a mean particle diameter of 7.5 ~tm. T o remove residual organic matter in the pores of the silicalite, it was heated to 773 K at a rate of 10 K min-1 in a furnace in flowing air and held at the final temperature for about 8 h. The weight loss during this treatment was about 14.8% Sorbates were all chemically pure grade (Fluka) and were used without any further purification step. Sorption rates and equilibrium sorption capacities at 298 K for sorbates n-hexane, benzene, ethylbenzene, and 0- m-, and p-xylene were measured in a conventional constant volume, constant pressure gravimetric adsorption system. The system consisted of an electronic balance (Cahn RG), a high-vacuum pump unit (Edwards High Vacuum), doser chambers, and valves. T h e pressures were recorded by a pressure transducer and electronic manometer (Datametrics). The temperature was kept at 298 + 0.2 K with a thermostat (Lauda). The sorption chamber that had the beam of the electronic balance had a volume of about 3 dm 3. The sample pan was suspended on a nichrome wire in a hangdown tube that was sufficiently long so that it could be immersed in a furnace for regeneration and in a thermostat for sorption experiments. The sorbates were loaded with the aid of doser chambers containing the sorbate. Sorption proceeded in a closed system where the pressure remained essentially constant, since the amount of vapor sorbed by the sample was small compared to the amount in the sorption chamber. T h e time constant for equilibra-

The volume of water sorbed in the silicalite sample was found to be 0.0474 cm a g-1. This value corresponds to 25% of the theoretical sorption volume, 0.190 cm a g - l reported earlier. 2'5 This low capacity, showing the hydrophobic nature of silicalite, matches exactly the value reported by Flanigen et al. 2 Isotherms of n-hexane, benzene, ethylbenzene, and o- m-, and p-xylene at 298 K were near-rectangular, i.e., highly favorable of Type 1; thus, equilibrium sorption capacities given in Table 1 at a relative pressure of P/Po = 0.5 represent essentially the limiting sorption volumes of the silicalite for the sorbate in question. These volumes were calculated from the weights sorbed assuming that sorbates exist in their normal liquid states at the temperature of sorption. As noted by previous investigators, 9'12'2° Table I shows that n-hexane fills the theoretical sorption volume of silicalite almost completely (99%), whereas benzene and ethylbenzene only partially fill the sorption volume by 75 and 73%, respectively. It is interesting to note that p-, m-, and o-xylene fill the sorption volume approximately to the same extent of 92-95%. However, it was found that the sorption kinetics and apparent sorption capacities for sorbates 0- and mxylene depend on the sorption history of the sample, as shall be explained later. Adsorption of organic molecules on silicalite occurs by volume filling of micropores and is characterized by enhancement of the adsorption energy due to increase of dispersion forces resulting from the comparable size o f the channels and the adsorbed molecule. 2 The high affinity for hexane is attributed to the favorable packing of its molecules, allowing favorable channel-wall interaction. 9 The ability of hexane molecules to coil may contribute to this effect. The difference among benzene, ethylbenzene, and xylene isomers is due to the additional interaction forces of the methyl groups and the entropy contribution resulting from more favorable packing of p-xylene molecules. 9 Analysis of sorption data in the literature indicates that equilibrium sorption capacities for molecules whose kinetic diameters are less than 0.60 nm are relatively consistent considering many factors in-

ZEOLITES, 1992, Vol 12, July/August 729

Sorption properties of silicate-l: H. Karsh et al. II0

1.0

100 9O ~-

0.8

80

-~ ~o ..~ 6o ~

8

50

0.6

.~ 4o ~ 30 ~ E 20 Io 0, o

0.4 O I

I

Io

20

I

30 V'T' min V2

I

40

50

Figure 1 Uptake rates of o-xylene: (©) on fresh sample of silicalite; (A) on silicalite sample that passed through a previous p-xylene sorption/regeneration step

0.2

0 0

I0

20 (rain)

fluencing the sorption properties of these materials. However, for molecules like 0- and m-xylene whose kinetic diameters 42 (0.68 nm) are significantly larger than pore sizes of ZSM-5/silicalite, widely different sorption capacities have been reported. This is partly due to factors affecting equilibrium sorption capacities mentioned above and partly due to the fact that sorption rates of these molecules are very slow, so that the sorbed amounts in most cases represented nonequilibrium values either explicitly stated or ignored. Sorption rates of n-hexane, benzene, ethylbenzene, and p-xylene were quite fast and equilibria were established in several minutes. The uptake curves for n-hexane and benzene followed qualitatively a pattern that is expected on the basis of a Fickian type of diffusion, whereas ethylbenzene and p-xylene were sorbed very fast initially, and their sorption rate decreased at higher degrees of loading, indicating two distinct regimes of sorption kinetics. This behavior was observed previously by Beschman et al. 26 for sorption of p-xylene on a HZSM-5(Si/AI = 35) sample at 298 K. Sorption rates of 0- and m-xylene molecules were extremely slow on a fresh sample of silicalite, and it might be concluded that these molecules are essentially excluded from the silicalite framework. However, it was interesting to observe that sorption rates of o- and m-xylene on samples that passed through a previous p-xylene sorption/desorption step were remarkably faster as exemplified for 0-xylene in Figure 1, and equilibrium could be reached in about 2 d. In the p-xylene sorption step, after degassing the sample at 670 K and in a vacuum of less than 10 -~ Pa for 4 h, the pressure of p-xylene in the chamber was increased to a pressure corresponding to a relative pressure of P/Po = 0.5 in a single step at 298 K. T h e sample sorbed 0.1198 g p-xylene/g of silicalite at equilibrium. Then, the sample was degassed at 670 K and in a vacuum of less than 10 -2 Pa for 4 h. T h e sample treated in this way sorbed 0-xylene at much

730

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I/2

30

40

Figure 2 Uptake curve of o-xylene on silicalite sample at 25°C and 2.66 x 102 Pa

faster rates than did the untreated sample degassed in the same way as above when the pressure of 0-xylene was increased to a pressure corresponding to P/Po = 0.5 at 298 K. Apparently, sorption ofp-xylene modifies the lattice of silicalite so that 0-xylene molecules are able to penetrate and can be accommodated in the silicalite structure. This effect has been confirmed by replicate experiments. At equilibrium, the sorbed volumes of xylene isomers fell in the range of 92-95% of the theoretical sorption volume. As shown in Figures 2 and 3, uptake curves for 0and m-xylene on silicalite samples were of the sigmoid 1.0

0.8

8 0.6

0.4

0.2

0 0

I0

20 (min)

:30 I/2

40

Figure 3 Uptake curve of m-xylene on silicalite sample at 25°C and 2.66 x 10z Pa

Sorption properties of silicate-l: H. Karsh et al.

type. The uptake is initially not linear in V'-F-, which would result if the random motion of sorbed molecules in the silicalite controls the rate of uptake. It is obvious that sorption of 0- and m-xylene proceeds by a different mechanism than the conventional concepts of Fickian diffusion of sorption-diffusion in zeolites. This behavior was observed by Beschman et al. 26 for 0-xylene sorption on a HZSM-5 (Si/AI = 35) sample at 298 K and by Billow et al. ~s on a (Na,H)ZSM-5 zeolite (Si/AI = 135) at 393 and 444 K. Beschman et al. 26 proposed that the observed kinetics of sorption of 0-xylene in HZSM-5 can be explained on the basis of the shrinking core model, reaction at the interface between unreacted material in the interior, and an advancing layer of reacted material controlling the observed rate. They stated "as a speculative assumption" that "this pattern would result if intruding 0-xylene changes the framework in such a way that it can be accommodated." The results of this study and some independent studies about the influence of some sorbates on the structure of ZSM-5 zeolites support this hypothesis. It has been shown that ZSM-5-type zeolites can undergo temperature- and sorbate-induced symmetry transformations. 42-5° Fyfe et al. as showed by MAS n.m.r, and XRD investigations that sorption of some sorbates including p-xylene causes symmetry changes and also some reversible lattice distortions in highly siliceous ZSM-5 zeolites. In a relatively recent study, this work was extended by Kokotailo et al. 51 to ZSM-5 zeolites having low Si/A1 ratios using high-resolution MAS n.m.r, and XRD methods. This study confirmed that room-temperature sorption of p-xylene promotes the monoclinic-to-orthorhombic phase transition and causes some changes in lattice parameters. These effects were temperature- and Si/Ai ratiodependent. T h e lattice parameters and unit cell volume at ambient temperature were found to be slightly higher for the p-xylene-loaded sample than for the unloaded sample. CONCLUSIONS The observations in this study and other independent studies indicating that i. sorption of p-xylene in ZSM-5/silicalite-I samples proceeds in two distinct sorption regimes; ii. sorption rates of 0- and m-xylene in fresh silicalite sample are extremely slow, whereas the sorption rate of these molecules increases significantly on the silicalite sample that passes through the pxylene sorption/desorption step; iii. uptake curves of 0- and m-xylene sorption in silicalite samples are of sigmoid shape; and iv. lattice distortions of high-silica ZSM-5 zeolites are indicated by recent M A S n.m.r, and XRD methods strongly suggest that sorption of xylene isomers introduce some subtle changes in silicalite that modify its sorptive behavior. This effect seems to depend on the Si/AI ratio and may be more pronounced for ZSM-5

zeolites of high Si/AI ratio and silicalite and for the critically sized molecules. These observations may also correlate with a discontinuity and the associated hysteresis in the sorption isotherm of p-xylene in high-silica HZSM-5 zeolites.

REFERENCES 1 Kokotailo, G.J., Lawton, S.L., OIson, D.H. and Meier, W.M. Nature 1978, 272, 437 2 Flanigen, E.M., Bennett, J.M., Grose, R.W., Cohen, J.P., Patton, R.L., Kirchner, R.M. and Smith, J.M. Nature 1978, 271,512 3 Anderson, J.R., Foger, K., Mole, T., Rajadhyaksha, R.A. and Sanders, J. V. J. Catal. 1979, 58, 114 4 Chen, N.Y., Kaeding, W.W. and Dwyer, F.G.J. Am. Chem. Soc. 1979, 101, 22 5 0 l s o n , D.H., Haag, W.O. and Lago, R.M.J. Catal. 1981, 61, 390 6 Derouane, E.G. and Gabelica, Z. J. Catal. 1980, 65, 486 7 Doelle, H.J., Heering, J. and Riekert, L. J. CataL 1981, 71, 27 8 Jacobs, P.A., Beyer, H.K. and Valyon, J. Zeolites 1981, 1,161 9 0 l s o n , D.H., Kokotailo, G.T. and Lawton, S.L.J. Phys. Chem. 1981, 85, 2238 10 Wu, P., Debebe, A. and Ma, Y.H. Zeolites 1983, 3, 118 11 Pope, C.G.J. Phys. Chem. 1986, 90, 835 12 Wu, P. and Ma, Y.H., in Proceedings of 6th International Zeolite Conference (Eds. D. Olson and A. Bisio) Butterworths, Guildford, 1984, p. 251 13 Wu, E.L., LandoR, G.R. and Chester, A.W., Ibid., p. 547 14 Paravar, A. and Hayburst, T. Ibid., p. 217 15 Foger, K., Sanders, J.V. and Seddon, D. Zeolites 1984, 4, 337 16 Ma, Y.H., Tang, T.D., Sand, L.B. and Hou, L.Y., in Proceedings of the 7th International Conference on Zeolites (Eds. A. Murakami et al.) Kondansha, Tokyo, 1986, p. 531 17 Wu, E.L., Landolt, G.R. and Chester, A.W., in Ibid., p. 547 18 Below, M., Schlodder, H., Richards, R.E. and Rees, L.V.C., in Ibid., p. 579 19 Karger, J., Pfeifer, H., Caro, J., Billow, M. and Chimann, G. in Ibid., p. 633 20 Choudhary, V.R. and Singh, A.P. Zeolites 1986, 6, 17 21 Richards, R.E. and Rees, L.V.C. Zeolites 1988, 8, 35 22 Choudhary, V.R. and Srinavason, K.R. Chem. Eng. ScL 1987, 42, 382 23 Zikanova, A., Below, M. and Schlodder, H. Zeolites 1987, 7, 115 24 Arbuckle, R., Scharman, G.H. and Duncan, D. Zeolites 1987, 7, 438 25 Harrison, I.D., Leach, H.F. and Whan, D.A. Zeolites 1987, 7, 21 26 Beschman, K., Kokotailo, G.T. and Riekert, L. Chem. Eng. Process. 1987, 22, 223 27 Sugimoto, M., Katsuno, HJ., Tokatsu, K. and Kawata, N. Zeofites 1987, 7, 503 28 Thamm, H. J. Phys. Chem. 1987, 91, 8 29 Richards, R.E. and Rees, L.V.C. Zeolites 1988, 8, 35 30 Shah, D.B., Hayhurst, D.T., Evanina, G. and Guo, C.J. AIChE J. 1988, 34, 1713 31 Reischman, P.T., Schmitt, K.D. and Olson, D.H.J. Phys. Chem. 1988, 92, 5165 32 Talu, O., Guo, C.J. and Hayhurst, D.T., in Adsorption: Science and Technology, NATO ASl Series, Vol. 158 (Eds. A.E. Rodrigues et ai.) Vimerio, Portugal, 1988, p. 53 33 Eic, M. and Ruthven, D.M., in Zeolites: Facts, Figures, Future (Eds. P.A. Jacobs and R.A. van Santen) Elsevier, Amsterdam, 1989, p. 897 34 Van-Den-Begin, N. and Rees, L.V.C. Ibid., p. 915 35 Yan, T.Y., Ind. Eng. Chem. Res. 1989, 28, 572 36 Bosselet, F., Sacerdote, M., Bouix, J. and Mentzen, B.F. Mat. Res. Bull. 1990, 25, 443 37 Van-Den-Begin, N.G., Rees, L.V.C., Caro, J. and B01ow, M. Zeolites 1989, 9, 287

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1984, 1433 46 Fyfe, C.A., Kennedy, G.J., De Schutter, C.T. and Kokotailo, GoT. J. Chem. Soc., Chem. Commun. 1984, 541 47 Hay, D.G., Jaegar, H. and West, G.W.J. Phys. Chem. 1985, 88, 1070 48 Fyfe, C.A., Kennedy, G.J., Kokotailo, G.T., Lyerla, J.R. and Fleming, W.W.J. Chem. Soc., Chem. Commun. 1985, 740 49 van Koningsveld, H., Jansen, J.C. and van Bekkum, Ho Zeolites 1987, 9, 564 50 Fyfe, CoA., Kokotailo, G.T., Stobl, H., Gies, H., Kennedy, G.J., Pasztor, C.T. and Barlow, G.E., in Zeolites as Catalysts, Sorbents and Detergent Builders (Eds. H. Karge and J. Weitkamp) Elsevier, Amsterdam, 1989, po 827 51 Kokotailo, G.T., Riekert, L. and Tissler, A. Ibid., pp. 821,843