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Silicalite Systems Studied by Sorption Uptake and Frequency Response Methods
Molecular Mobility of Hydrocarbon ZSMS/ Silicalite Systems Studied by Sorption Uptake and Frequency Response Methods H. BDlow and H. Schlodder Central Institute of Physical Chemistry, Academy of Sciences of the G.D.R., 1199Berlin, Rudower Chausse 5. G.D.R. L.y.C. Rees and R.E. Richards* Physical Chemistry Laboratories, Imperial College of Science and Technology, London SW7 2AY *Present Address: Colloid Science Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames. Hiddlesex, England. Frequency response and sorption uptake measurements on the molecular mobility of hydrocarbons in ZSH5/silicalites are presented. Intracrystalline diffusion has been established for both n-hexane and benzene in ZSH5/silicalite but the data obtained for ethane and propane suggest that other processes are also involved. INTRODUCTION ZSH5/Silicalite molecular sieves have very interesting adsorptive and catalytic properties (1-3). The micropore network of intersecting straight and sinusoidal channels with distances between the intersections similar to many probe molecule sizes, and the deficiency of cations in the channels, make both sorption equilibria and molecular mobility of different sorbate species of high interest. Although a number of such studies have been reported in the literature (4-12) severely discrepent data. have been obtained. Before further progress can be made detailed information is required which accounts for possible different transport mechanisms (6,13-15), for joint application of different experimental principles (11.12.15.16) and for results obtained from the use of large single crystals. (7,14,17,18) In this paper we report a start to such investigations using both sorption uptake (s.u.) and frequency-response (f.r.) (19.20) methods for similar ZSH5 systems. EXPERIMENTAL Two types of zeolites were used in the s.u. and f.r. measurements. Details of these zeolites are given in Table 1. The zeolites were activated using standard thermal vacuum conditions. Sorption UDtake:- In the s.u. experiment a constant volume - variable pressure system with valve-effect corrections was employed. The dead-time moment of the apparatus was 0.2s. To avoid the influence of intercrystalline diffusion and to realise quasi-isothermal conditions s.u. was measured using, respectively, a monolayer of single crystals and favourable external temperature conditions (i.e. T < T , where the subscipts 0 and V represent the dosed gas and zeolite crystal pRase~ respectively (21)). For data evaluation the statistical moments theory (22,23) was used. Corrected diffusivities were calculated by the Darken equation. Freguency-response:- The f.r. method (19,20) operates under near equilibrium conditions. Although the influence of non-isothermicity on its results should be considered one may approximately compare them with corrected s.u. diffusivities and n.m.r. self-diffusivities (11,13,15). A square-wave volume perturbation (20), V, of less than 1% of the equilibrium volume. V • was applied to the sorbent/sorbate equilibrium system (designated Z) or a blank1sorbate system (designated B). The
579
580 (AD-7-l)
latter was measured to remove any phase lag introduced by the apparatus itself. The resultant pressure response, P, about the equilibrium pressure, P , of 1333 Pa was recorded. F.r. parameters were determined for an equivalent sineewave perturbation by Fourier analysis of the volume and pressure wave forms at the square-wave fundamental frequency (20). Typical f.r. parameters are presented in Figure 1 (where i = ~,e) from which the phase lag ~z-e = ~Z - ~e' and the amplitude r~tio ~e/Pz lor Pz/V) were c~lculated. ~he solution of Fick's second law for dIffusIon of a SIngle dIffusIng specIes In an IsotropIc sphere, exposed to a periodic sinusoidal surface concentration has been made (191 assuming intracrystalline diffusion to be rate-determining. The solution may be outlined as follows: (Pe/Pzl cos ~z-e
-
1
(Pe/Pzl sin ~z-e where K = (RT IV I o e
Kb Kb
3C
+
C
(1) (2 )
3S
ae lap . e e
B = number of adsorbed molecules, R = gas constant, T = 298K and C = constant t~rm to account for a second sorbate process, tentativgly assigned to intercrystalline three-dimensional diffusion (19), see Table 2. The characteristic functions for identical spheres Imonodispersel are In phase, b 3C
3
= -11
[
sinh 11 - sin cosh 11 - cos
Out of phase, 6 [ 1 b -2 3S = -11 where
11
1
~]
(3 )
sinh 11 + sin cosh 11 - cos
= (2WR 2/D)1/2, w = angular frequency, f = (w/2v) D = intracrystalline diffusion coefficient, and R (see Table 1)
( 41
frequency, particle radius
The diffusion coefficient was obtained in this study by a least-square curve fitting procedure of the Characteristic functions (solid line) to the experimental data (symbols) in Figures 2 and 3. The best fit parameters are presented in Table 2, where H represents the calculated value of K' (see Table 2 for relationship between K and K'). RESULTS AND DISCUSSION Equilibrium Studies :- Sorption isotherms were derived from the amounts sorbed in the s.u. runs at times t ~~. Sorption heats were calculated by means of isosteric plots obtained from the isotherms. Isotherm data are also required as a prerequisite of the f.r. investigations (cf. Eqs. 1 and 2). These were recorded gravimetrically (Cahn electrobalance) at different temperatures (cf. Table 2). The gradients of the isotherms at 1333 Pa required for the determination of constant K in Eqs. 1 and 2 were obtained graphically from the isotherms. The sorption heats, -dH, obtained for different hydrocarbon ZSH5/silicalite systems are given as dependences on coverage in Figure 4. The reasonable agreement between our data and those from various other sources (5.8,24-27) as well as the clear incremental dependence on hydrocarbon chain length suggests that it is reasonable to compare molecular mobility data from di.fferent sources (further investigation is needed to understand the difference between the values of -dH found for propane on ZSH5 and silicalite, respectively).
M. BUlow et al.
581
Molecular Mobility Studies: Ethane-ZSM5/Silicalite:- The diffusivities obtained for this system by the f.r. method are given in Table 3, which may be compared with intracrystalline n.m.r. self-diffusivities (11). (The compatibility of these data obtained on different silicalite samples has been proved to be valid for the propane systems.) The reliability and accuracy of the n.m.r. self-diffusivities is now accepted (6,1115,22,28). Due to the severely discrepant data obtained by the two methods, it can be concluded that a further microphysical process, superimposed on the intracrystalline diffusion. must be present in the f.r. measurements. Sorption and desorption heat effects can most probably be ruled out as the f.r. experiments are carried out under near-equilibrium conditions. Although the diffusion anisotropy in ~ifferent pentasil channel systems need to be considered. it seems most likely that intercrystalline diffusion affects the intracrystalline diffusion. The weight of sample used in the f.r. measurements corresponds to a bed depth of 1.5mm. The difference between the K' equilibrium sorption values and the corresponding H values obtained from the computer fit of the f.r. data listed in Table 2 which decrease with increasing temperature suggest that the whole bed is not participating equally in the f.r. measurements. Finally the in-phase and outof-phase curves in Figure 2 should be asymptotic at high frequencies. The separation of the two curves is ascribed to sorption on the external regions of the zeolite crystals and leads to the C term in Eqn. 1. The values for C so obtained are listed in Table 2. Otherwise the values of the diffusion coefficients obtained by the f.r. method are in good agreement with previously published diffusivities for n-alkane/ silicalite-1 systems (6,7). It would seem that both the f.r. diffusivities and those presented in (6,7) should be considered to be effective long-range diffusivities. Proof of this assumption can be made by varying the crystal bed depth in f.r. experiments. ProDane-ZSH5/Silicalite and Na.H-ZSH5:- The f.r. diffusivities of propane on ZSH5/Silicalite are given in Table 4. Therein they are compared with the n.m.r. self-diffusivities from ref. (11) and corrected s.u. diffusivities obtained on ZSH5 (taken from Fig. 5). The f.r. and s.u. results are closer to each other and agree with literature data in one case (cf. (7)) but significantly larger than in a second example (9). However, the discrepancy between the f.r./s.u. diffusivities and nmr self-diffusivities in Table 4 indicates, as above for ethane, that other microphysical processes are superimposed on the intracrystalline diffusivities. Some agreement between the f.r. and s.u. data should be considered to be fortuitous (the sorption heat of propane on both samples are discrepant to each other, cf. Fig. 4). The f.r. results probably represent long range diffusivities. Intercrystalline diffusion seems to be of ever greater significance in the case of the propane-ZSH5/silicalite system. The correlation of the predicted and experimental f.r. behaviour was poor at all three temperatures. The deviation of the experimental f.r. behaviour from the predicted curve at low frequencies as shown in Figure 3 increased with the increase in the isotherm gradient at the equilibrium pressure for f.r, It would seem that at lower frequencies a greater percentage of the zeolite bed is contributing to the diffusion process. This explanation of the complex behaviour observed for propane is supported, as in the case of ethane, by the predicted H values being very much smaller than the K' values determined experimentally under equilibrium sorption conditions (see Table 2). In the determination of K' the whole bed will be contributing to the sorption. The inconsistency of the s.u, and the nmr data arises from the response-time behaviour of the s.u. apparatus (29) with possibly some non-isothermicity, also, superimposed. The s.u. experiments must be carried out using larger zeolite crystals while in f.r. experiments the bed depth must be minimized. n-Hexane/Na.H-ZSH5:- The dependences of the corrected s.u. diffusivities of this system on sorbate concentration are shown in Figure 6. Intercrystalline diffusion, sorption heat release and external gas flow problems (cf. above) are not
582 (AD-7-1) considered to be significant in these measurements. Structural surface barrier effects (30) can also be ruled out as in a diffusion study involving faster diffusion (11) to which nmr pulsed field gradient and tracer desorption (31) techniques have been applied no surface barrier problems were encountered. The high mobility of n-hexane molecules in ZSM5 micro pores is clearly demonstrated while concentration has little influence on bot~1the diffusivity and the relatively small activation energy, E , of ca. 24*2 kJ mol . Since n-hexane has a similar length to the distance bet~een two intersections in the straight channel the hexane molecules will experience a reasonably constant adsorption potential which explains the small dependence of diffusivity on concentration found. The high diffusivities are also in qualitative agreement with the diffusivities reported for n-butane in a large single silicalite crystal (7) and, significantly, larger than the diffusivities obtained with n-hexane on 2~m crystals (8-10). The activation energy obtained is also consistent with the energies reported for n-butane. Benzene/Na,H-ZSM5:- The dependences of corrected S.U. diffusivities of this system on sorbate concentration are given in Figure 7 and may be compared with the corresponding n-hexane diffusivities given in Figure 6. These diffusivities are truly intracrystalline for the same reasons as given in the n-hexane section above. Further support for this conclusion is given by the agreement between these diffusivities and those obtained on the same crystals under constant pressure conditions (321. These latter data are included in Figure 7 for comparison. _The activation energy of intracrystalline diffusion of benzene. E . is 28*4 kJ mol The benzene and corresponds closely with the activation energy for n-hexaAe. diffusivities are, however, significantly smaller than those for n-hexane and show Both the order of magnitude and the a strong concentration dependence. conce7§ration dependence of the benzene diffusivities are supported by conclusions from C nmr studies of benzene in silicalite which found an increase in the residence time of benzene molecules on sorption sites with increasing sorbate concentration (33). It may be assumed in this system that a free volume mechanism is controlling the diffusion process. Even the lowest benzene diffusivities reported in this present study are distinctly larger than those previously reported (8-10) otherwise they agree with the data of (5). In a similar study (32) intracrystalline diffusivities of benzene in silicalite have been found to be concentration dependent in an analagous ma~ner to that found here but shifted to larger values. It would seem that the Na ions and hydroxyl groups present in the Na,H-ZSM5 used in this present study act as interaction centres for the benzene molecules and thus increase the residence time on these centres. CONCLUSIONS (i) Intracrystalline diffusivities of hydrocarbons in ZSM5/Silicalite systems have been found to be of similar magnitude (5,6) or significantly greater (8-10) than previously reported values. Iii) Intracrystalline diffusivity decreases and the activation energy for diffusion increases with increasing n-hydrocarbon chain length. The activation energies are low compared with corresponding values for other zeolite systems. (iii) The intracrystalline diffusivity of a given n-hydrocarbon adsorbed in ZSM5/silicalite sorbents is larger than in zeolite 5A but lower than in NaX (11). (iv) There is a sparcity of information on the concentration dependence of intracrystalline diffusivity in ZSM5/silicalite systems. No clear picture of the diffusion mechanism has yet been established. ACKNOWLEDGEMENT We are grateful to Dr. S.A.I. Barri. B.P. Research Centre. Sunbury-on-Thames. England. for the synthesis of the ZSM5/silicalite sample used in the f.r. experiments. REFERENCES 1. Proc. 6th International Zeolite Conference. Eds. D. Olson. A. Bisio. Butterworths. Guildford. 1984.
M. BUlow et al. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
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T.E. Whyte, Jr., R.A. Dalla Betta, E.G. Derouane and R.T.K. Baker, Catalytic Materials: Relationship between Structure and Reactivity, A.C.S. Symp. Ser. lil, Washington, 1984. B. Imelik, C. Naccache, G. Coudurir, Y. Ben Taarit and J.C. Vedrine, Catalysis by Acids and Bases, Elsevier, 1985. D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J.Phys.Chem. ~ 2238, ( 19811 . H.-J. Doelle, J. Heering, L. Riekert, and L. Marosi, J.Catal. 11 27 (1981). D.M. Ruthven, "Principles of Adsorption and Adsorption Processes", Wiley, New York, 1984, p.160. A. Paravar and D.T. Hayhurst, cf. ref. 1, p. 217. P. Wu, A. Debebe and Y.H. Ma: Zeolites, 1, 118 (1983). A.S. Chiang, A.G. Dixon and Y.H. Ma, Chem. Engng. Sci. ll, 1461 (1984). P. Wu and Y.H. Ma, cr , ref. 1, p. 251. J. Caro, M. BUlow, J. Karger, W. Heink, W. Schirmer, H. Pfeifer and S.P. Zdanov, J.C.S. Faraday I, ll, 2541 (1985). J. Caro, M. BUlow and J. Karger, Chem. Engng. Sci. 11, 2169 (1985). J. Karger, H. Pfeifer, D. Frenck, J. Caro, M. Bulow, and G. Ohlmann, paper presented at the 7th IZC, Tokyo 1986. M. Bulow, J. Karger, M. Kocirik and A.M. Voloscuk, Z. Chem. 1.1. 175 (1981). J. Karger, H. Pfeifer and W. Heink, cf. ref. 1, p. 184. M. BUlow, P. Struve, and L.V.C. Rees, Zeolites, 2, 1.13 (1985). R.M. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves", Academic Press, London, 1978. M. Bulow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer, and J. Karger: J.C.S. Faraday I, li, 597 (1980). Y. Yasuda, J. Phys. Chem. Ai, 1913 (1982). R.E. Richards, Ph.D. Thesis, London University, 1985. M. BUlow, P. Struve, W. Mietk, and M. Kocirik, J.C.S, Faraday I, lQ, 813 (1984). M. Bulow, P. Lorenz, W. Mietk, and N.N. Samulevic, J.C.S. Faraday I, 11, 1099 (1983) . P. Struve, M. Kocirik, M. Bulow, A. Zikanova, and A. Bezus, Z.phys.Chem. (Leipzig) lSi, 49 (1983). H. Stach, H. Thamm, J. Janchen, K. Fiedler, and W. Schirmer, cf. ref. I, p. 225. R.E. Richards and L.V.C. Rees, Zeolites, S, 17, (19861. U. Lohse and B. Fahlke, Chem.Techn. 12 350 (1983). A.V. Kiselev, A.A. Lopatkin and A.A. Shulga, Zeolites 2, 261 (1985). J. Karger, H. Pfeifer and W. Heink, Advances in Nuclear Magnetic Resonance, 11, 000 (1986). R.M. Barrer, "The Properties and Application of Zeolites", Ed. R.P. Townsend, The Chemical Society, London, 1979, p.3. M. Bulow, Z. Chem. ll, 81 (1985). J. Karger, AIChE Journal, ll, 417 (1982). A. Zikanova, M. Bulow and H. Schlodder: Zeolites, S, 000 (1986). B. Zibrowius, M. Bulow and H. Pfeifer, Chem. Phys. Lett, llQ, 420 (1985).
Table 1
Details of zeolites used
Zeolite
Si/Al ratio
Na,H-ZSM5
'" 135**
ZSM51 silica lite
"'1230
Crystal size/llm*, R 24.6*2.6 (polyhedral***) 19.1*1.35 Ispheroid)
Mass (usually used)
Crystal arrangement
s. u. : 10-20mg
one-crystal monolayer
f .r.: 2.1g for propane 3.3g for ethane
polylayer lmm bed depth 1.5mm bed depth
*R is average radius of spherical equivalents of the actual crystals. **Calculated from starting gel composition. ***Elongated hexagon.
584 (AD-7-1)
Diffusion Model Best Fit Parameters Sorbate
T/K
q/lm/uc)
!'thane
298
1.34.0.01
7.ox10
- 12
6.5x1o- 12
3.89
6.90.0.43
0.24
0.65.0.01
1. 2x10 -11
1.2)(10- 11
2.00
3.36.0.21
0.25
338
0.33.0.01
1.8)(10- 11
1.8)(10- 11
1. 28
1.73.0.11
0.13
29B
7.0300.10
1.3)(10- 11
3.5)(10- 12
2.33
8.47.0.90
0.21
4.08.0.10
1.2)(10- 11
6.3x10- 12
2.79
9.2600.92
0.26
-11
-12
2.02
5.4300.40
0.21
318
Propane
*
318
2.05.0.10
338
H
1. 5)(1 0
9.3)(10
K )( sorbent weight: ~
C
model fitted for >0.2Hz only.
Corrected f.r. diffusion and n.m.r. self-diffusion (11) data of the ethane/silicalite system on.m.r./ m2 -1 s intra
T/K 298 318 338
*
K' ""
-12 6.5)(10_ m/ucl 11(1.3 1.2xl0_ 11(0.65m/ucl 1.8)(10 10.33m/ucl
~5.0)(10_9
-9
(2.6)(10
-9
at 8m/ucl
>6)(10_9" >7)(10 "
"e)(trapolated data
.IiJllL.i Corrected f.r. (ZSM5/silicalitel and s.u. (ZSM51 diffusion as well as nmr'(silicalitel self-diffusion (111 data of propane. T/K 298 318 338
q/m/uc 7 7 8 4 4 4 2 2 2
method
2s -1 D/m
f.r. s.u. n.m.r. f.r. s.U. n.m. r. f.r. s.u. n.m.r.
. -12 3.5)(10_ 6.5)(10_ 12 10 ~5 )(10_ 12 6.3)(10_ 11 4 )(10_ 9 3 )(10_ 12 9.3)(10_ 9 ~2 )(10_ ~7 )(10 9
"The n.m.r. self-diffusivities of propane on the silicalite characterised in Table 1 have been proved to be smaller by the factor of ~1.5 smaller than on the silicalite sample of Ref.(lll.
M. BUlow et al. 2·5,.,-------------+20
cO
+10
in-phase
¢/deg
0
1'5
blank
-10
1·0
-20 0'6
0·5 N
0·3
Fig.2 Characteristic functions for ethane in ZSM5/silicalite at 318K and 1333 Pa.
~
VI
0·2 0·1 0-2 11Hz
0'02
Fig.I Frequency-response parameters for ethane in ZSM5/silicalite at 298K and 1333 Pa.
a/mmol9' 0.8
04 t, 0
E
...,
70
6
66
66
6
12
666
•
~
40 •
oX
14
:c
D
lP
JI DO
•
cO
16 00 6
50
in-phase
~ 6
~
:c
."i"_
666
12
•••••• •• •••••••• • 10
o
0
0
0
o
o
0
0
8 10
2 Fig.4
4
6
m.luc.
8
Isosteric sorption heats of ethane
( f) and propane (e) on ZSM5/silicalite and propane (0), n-hexane (t> ) and benzene (0 ) on Na,H-ZSM-5.
Fig.3 Characteristic functions for propane in ZSM5/silicalite at 318K and 1333 Pa.
585
586 (AD-7-1)
Fig.5 Concentration dependence of the intracrystalline nmr self-diffusivities Dintra nmr 0 f propane on Sl'1'rca I'rte (' , 313K (11)) and ZSM5/silicalite (e, 296K) and the corrected apparent s.u, diffusivities Doapp of propane on Na,H-ZSM5 (\1 313K,
o
298K, I:. 253K,
aQ+~ o
13
10
0.1
223K, 0 188K)
0
/mmo(gl
0.2 0.3 Oh 0.5 0.6 0.7
.00
\I>.O~
I>. •
1
o.
I>.
o~
0
0
0
o
•
1
atmmolg Q2 0.4 0.6 0.8 1.0 1.2 1h
1011 m N E ....... o
~~.6_
"'i
o
~
66
----"'--
I
I>.
.12
10
• 2
3 m.tu.c.
-0
2
3
4
5
6
7
4
(, 0
8 9
m.tu.c. Fig.6 Concentration dependence of the corrected intracrystalline s.u, diffusivities D of n-hexane on 0 Na,H-ZSM5 (1:. 373K, o 348K, 0 323K, 'V 295K, 0 273K)
Fig.7 Concentration dependence of the corrected intracrystalline s.u. diffusivities Do of benzene on Na,H-ZSM5 (0, e 423K; 1:.,. 393K; 0,. 363K; \1 , ' 333K; ¢ , • 303K; empty symbols.•• constant volume-variable pressure, full symbols... constant pressure conditions)
579
580 (AD-7-l)
latter was measured to remove any phase lag introduced by the apparatus itself. The resultant pressure response, P, about the equilibrium pressure, P , of 1333 Pa was recorded. F.r. parameters were determined for an equivalent sineewave perturbation by Fourier analysis of the volume and pressure wave forms at the square-wave fundamental frequency (20). Typical f.r. parameters are presented in Figure 1 (where i = ~,e) from which the phase lag ~z-e = ~Z - ~e' and the amplitude r~tio ~e/Pz lor Pz/V) were c~lculated. ~he solution of Fick's second law for dIffusIon of a SIngle dIffusIng specIes In an IsotropIc sphere, exposed to a periodic sinusoidal surface concentration has been made (191 assuming intracrystalline diffusion to be rate-determining. The solution may be outlined as follows: (Pe/Pzl cos ~z-e
-
1
(Pe/Pzl sin ~z-e where K = (RT IV I o e
Kb Kb
3C
+
C
(1) (2 )
3S
ae lap . e e
B = number of adsorbed molecules, R = gas constant, T = 298K and C = constant t~rm to account for a second sorbate process, tentativgly assigned to intercrystalline three-dimensional diffusion (19), see Table 2. The characteristic functions for identical spheres Imonodispersel are In phase, b 3C
3
= -11
[
sinh 11 - sin cosh 11 - cos
Out of phase, 6 [ 1 b -2 3S = -11 where
11
1
~]
(3 )
sinh 11 + sin cosh 11 - cos
= (2WR 2/D)1/2, w = angular frequency, f = (w/2v) D = intracrystalline diffusion coefficient, and R (see Table 1)
( 41
frequency, particle radius
The diffusion coefficient was obtained in this study by a least-square curve fitting procedure of the Characteristic functions (solid line) to the experimental data (symbols) in Figures 2 and 3. The best fit parameters are presented in Table 2, where H represents the calculated value of K' (see Table 2 for relationship between K and K'). RESULTS AND DISCUSSION Equilibrium Studies :- Sorption isotherms were derived from the amounts sorbed in the s.u. runs at times t ~~. Sorption heats were calculated by means of isosteric plots obtained from the isotherms. Isotherm data are also required as a prerequisite of the f.r. investigations (cf. Eqs. 1 and 2). These were recorded gravimetrically (Cahn electrobalance) at different temperatures (cf. Table 2). The gradients of the isotherms at 1333 Pa required for the determination of constant K in Eqs. 1 and 2 were obtained graphically from the isotherms. The sorption heats, -dH, obtained for different hydrocarbon ZSH5/silicalite systems are given as dependences on coverage in Figure 4. The reasonable agreement between our data and those from various other sources (5.8,24-27) as well as the clear incremental dependence on hydrocarbon chain length suggests that it is reasonable to compare molecular mobility data from di.fferent sources (further investigation is needed to understand the difference between the values of -dH found for propane on ZSH5 and silicalite, respectively).
M. BUlow et al.
581
Molecular Mobility Studies: Ethane-ZSM5/Silicalite:- The diffusivities obtained for this system by the f.r. method are given in Table 3, which may be compared with intracrystalline n.m.r. self-diffusivities (11). (The compatibility of these data obtained on different silicalite samples has been proved to be valid for the propane systems.) The reliability and accuracy of the n.m.r. self-diffusivities is now accepted (6,1115,22,28). Due to the severely discrepant data obtained by the two methods, it can be concluded that a further microphysical process, superimposed on the intracrystalline diffusion. must be present in the f.r. measurements. Sorption and desorption heat effects can most probably be ruled out as the f.r. experiments are carried out under near-equilibrium conditions. Although the diffusion anisotropy in ~ifferent pentasil channel systems need to be considered. it seems most likely that intercrystalline diffusion affects the intracrystalline diffusion. The weight of sample used in the f.r. measurements corresponds to a bed depth of 1.5mm. The difference between the K' equilibrium sorption values and the corresponding H values obtained from the computer fit of the f.r. data listed in Table 2 which decrease with increasing temperature suggest that the whole bed is not participating equally in the f.r. measurements. Finally the in-phase and outof-phase curves in Figure 2 should be asymptotic at high frequencies. The separation of the two curves is ascribed to sorption on the external regions of the zeolite crystals and leads to the C term in Eqn. 1. The values for C so obtained are listed in Table 2. Otherwise the values of the diffusion coefficients obtained by the f.r. method are in good agreement with previously published diffusivities for n-alkane/ silicalite-1 systems (6,7). It would seem that both the f.r. diffusivities and those presented in (6,7) should be considered to be effective long-range diffusivities. Proof of this assumption can be made by varying the crystal bed depth in f.r. experiments. ProDane-ZSH5/Silicalite and Na.H-ZSH5:- The f.r. diffusivities of propane on ZSH5/Silicalite are given in Table 4. Therein they are compared with the n.m.r. self-diffusivities from ref. (11) and corrected s.u. diffusivities obtained on ZSH5 (taken from Fig. 5). The f.r. and s.u. results are closer to each other and agree with literature data in one case (cf. (7)) but significantly larger than in a second example (9). However, the discrepancy between the f.r./s.u. diffusivities and nmr self-diffusivities in Table 4 indicates, as above for ethane, that other microphysical processes are superimposed on the intracrystalline diffusivities. Some agreement between the f.r. and s.u. data should be considered to be fortuitous (the sorption heat of propane on both samples are discrepant to each other, cf. Fig. 4). The f.r. results probably represent long range diffusivities. Intercrystalline diffusion seems to be of ever greater significance in the case of the propane-ZSH5/silicalite system. The correlation of the predicted and experimental f.r. behaviour was poor at all three temperatures. The deviation of the experimental f.r. behaviour from the predicted curve at low frequencies as shown in Figure 3 increased with the increase in the isotherm gradient at the equilibrium pressure for f.r, It would seem that at lower frequencies a greater percentage of the zeolite bed is contributing to the diffusion process. This explanation of the complex behaviour observed for propane is supported, as in the case of ethane, by the predicted H values being very much smaller than the K' values determined experimentally under equilibrium sorption conditions (see Table 2). In the determination of K' the whole bed will be contributing to the sorption. The inconsistency of the s.u, and the nmr data arises from the response-time behaviour of the s.u. apparatus (29) with possibly some non-isothermicity, also, superimposed. The s.u. experiments must be carried out using larger zeolite crystals while in f.r. experiments the bed depth must be minimized. n-Hexane/Na.H-ZSH5:- The dependences of the corrected s.u. diffusivities of this system on sorbate concentration are shown in Figure 6. Intercrystalline diffusion, sorption heat release and external gas flow problems (cf. above) are not
582 (AD-7-1) considered to be significant in these measurements. Structural surface barrier effects (30) can also be ruled out as in a diffusion study involving faster diffusion (11) to which nmr pulsed field gradient and tracer desorption (31) techniques have been applied no surface barrier problems were encountered. The high mobility of n-hexane molecules in ZSM5 micro pores is clearly demonstrated while concentration has little influence on bot~1the diffusivity and the relatively small activation energy, E , of ca. 24*2 kJ mol . Since n-hexane has a similar length to the distance bet~een two intersections in the straight channel the hexane molecules will experience a reasonably constant adsorption potential which explains the small dependence of diffusivity on concentration found. The high diffusivities are also in qualitative agreement with the diffusivities reported for n-butane in a large single silicalite crystal (7) and, significantly, larger than the diffusivities obtained with n-hexane on 2~m crystals (8-10). The activation energy obtained is also consistent with the energies reported for n-butane. Benzene/Na,H-ZSM5:- The dependences of corrected S.U. diffusivities of this system on sorbate concentration are given in Figure 7 and may be compared with the corresponding n-hexane diffusivities given in Figure 6. These diffusivities are truly intracrystalline for the same reasons as given in the n-hexane section above. Further support for this conclusion is given by the agreement between these diffusivities and those obtained on the same crystals under constant pressure conditions (321. These latter data are included in Figure 7 for comparison. _The activation energy of intracrystalline diffusion of benzene. E . is 28*4 kJ mol The benzene and corresponds closely with the activation energy for n-hexaAe. diffusivities are, however, significantly smaller than those for n-hexane and show Both the order of magnitude and the a strong concentration dependence. conce7§ration dependence of the benzene diffusivities are supported by conclusions from C nmr studies of benzene in silicalite which found an increase in the residence time of benzene molecules on sorption sites with increasing sorbate concentration (33). It may be assumed in this system that a free volume mechanism is controlling the diffusion process. Even the lowest benzene diffusivities reported in this present study are distinctly larger than those previously reported (8-10) otherwise they agree with the data of (5). In a similar study (32) intracrystalline diffusivities of benzene in silicalite have been found to be concentration dependent in an analagous ma~ner to that found here but shifted to larger values. It would seem that the Na ions and hydroxyl groups present in the Na,H-ZSM5 used in this present study act as interaction centres for the benzene molecules and thus increase the residence time on these centres. CONCLUSIONS (i) Intracrystalline diffusivities of hydrocarbons in ZSM5/Silicalite systems have been found to be of similar magnitude (5,6) or significantly greater (8-10) than previously reported values. Iii) Intracrystalline diffusivity decreases and the activation energy for diffusion increases with increasing n-hydrocarbon chain length. The activation energies are low compared with corresponding values for other zeolite systems. (iii) The intracrystalline diffusivity of a given n-hydrocarbon adsorbed in ZSM5/silicalite sorbents is larger than in zeolite 5A but lower than in NaX (11). (iv) There is a sparcity of information on the concentration dependence of intracrystalline diffusivity in ZSM5/silicalite systems. No clear picture of the diffusion mechanism has yet been established. ACKNOWLEDGEMENT We are grateful to Dr. S.A.I. Barri. B.P. Research Centre. Sunbury-on-Thames. England. for the synthesis of the ZSM5/silicalite sample used in the f.r. experiments. REFERENCES 1. Proc. 6th International Zeolite Conference. Eds. D. Olson. A. Bisio. Butterworths. Guildford. 1984.
M. BUlow et al. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.
583
T.E. Whyte, Jr., R.A. Dalla Betta, E.G. Derouane and R.T.K. Baker, Catalytic Materials: Relationship between Structure and Reactivity, A.C.S. Symp. Ser. lil, Washington, 1984. B. Imelik, C. Naccache, G. Coudurir, Y. Ben Taarit and J.C. Vedrine, Catalysis by Acids and Bases, Elsevier, 1985. D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J.Phys.Chem. ~ 2238, ( 19811 . H.-J. Doelle, J. Heering, L. Riekert, and L. Marosi, J.Catal. 11 27 (1981). D.M. Ruthven, "Principles of Adsorption and Adsorption Processes", Wiley, New York, 1984, p.160. A. Paravar and D.T. Hayhurst, cf. ref. 1, p. 217. P. Wu, A. Debebe and Y.H. Ma: Zeolites, 1, 118 (1983). A.S. Chiang, A.G. Dixon and Y.H. Ma, Chem. Engng. Sci. ll, 1461 (1984). P. Wu and Y.H. Ma, cr , ref. 1, p. 251. J. Caro, M. BUlow, J. Karger, W. Heink, W. Schirmer, H. Pfeifer and S.P. Zdanov, J.C.S. Faraday I, ll, 2541 (1985). J. Caro, M. BUlow and J. Karger, Chem. Engng. Sci. 11, 2169 (1985). J. Karger, H. Pfeifer, D. Frenck, J. Caro, M. Bulow, and G. Ohlmann, paper presented at the 7th IZC, Tokyo 1986. M. Bulow, J. Karger, M. Kocirik and A.M. Voloscuk, Z. Chem. 1.1. 175 (1981). J. Karger, H. Pfeifer and W. Heink, cf. ref. 1, p. 184. M. BUlow, P. Struve, and L.V.C. Rees, Zeolites, 2, 1.13 (1985). R.M. Barrer, "Zeolites and Clay Minerals as Sorbents and Molecular Sieves", Academic Press, London, 1978. M. Bulow, P. Struve, G. Finger, C. Redszus, K. Ehrhardt, W. Schirmer, and J. Karger: J.C.S. Faraday I, li, 597 (1980). Y. Yasuda, J. Phys. Chem. Ai, 1913 (1982). R.E. Richards, Ph.D. Thesis, London University, 1985. M. BUlow, P. Struve, W. Mietk, and M. Kocirik, J.C.S, Faraday I, lQ, 813 (1984). M. Bulow, P. Lorenz, W. Mietk, and N.N. Samulevic, J.C.S. Faraday I, 11, 1099 (1983) . P. Struve, M. Kocirik, M. Bulow, A. Zikanova, and A. Bezus, Z.phys.Chem. (Leipzig) lSi, 49 (1983). H. Stach, H. Thamm, J. Janchen, K. Fiedler, and W. Schirmer, cf. ref. I, p. 225. R.E. Richards and L.V.C. Rees, Zeolites, S, 17, (19861. U. Lohse and B. Fahlke, Chem.Techn. 12 350 (1983). A.V. Kiselev, A.A. Lopatkin and A.A. Shulga, Zeolites 2, 261 (1985). J. Karger, H. Pfeifer and W. Heink, Advances in Nuclear Magnetic Resonance, 11, 000 (1986). R.M. Barrer, "The Properties and Application of Zeolites", Ed. R.P. Townsend, The Chemical Society, London, 1979, p.3. M. Bulow, Z. Chem. ll, 81 (1985). J. Karger, AIChE Journal, ll, 417 (1982). A. Zikanova, M. Bulow and H. Schlodder: Zeolites, S, 000 (1986). B. Zibrowius, M. Bulow and H. Pfeifer, Chem. Phys. Lett, llQ, 420 (1985).
Table 1
Details of zeolites used
Zeolite
Si/Al ratio
Na,H-ZSM5
'" 135**
ZSM51 silica lite
"'1230
Crystal size/llm*, R 24.6*2.6 (polyhedral***) 19.1*1.35 Ispheroid)
Mass (usually used)
Crystal arrangement
s. u. : 10-20mg
one-crystal monolayer
f .r.: 2.1g for propane 3.3g for ethane
polylayer lmm bed depth 1.5mm bed depth
*R is average radius of spherical equivalents of the actual crystals. **Calculated from starting gel composition. ***Elongated hexagon.
584 (AD-7-1)
Diffusion Model Best Fit Parameters Sorbate
T/K
q/lm/uc)
!'thane
298
1.34.0.01
7.ox10
- 12
6.5x1o- 12
3.89
6.90.0.43
0.24
0.65.0.01
1. 2x10 -11
1.2)(10- 11
2.00
3.36.0.21
0.25
338
0.33.0.01
1.8)(10- 11
1.8)(10- 11
1. 28
1.73.0.11
0.13
29B
7.0300.10
1.3)(10- 11
3.5)(10- 12
2.33
8.47.0.90
0.21
4.08.0.10
1.2)(10- 11
6.3x10- 12
2.79
9.2600.92
0.26
-11
-12
2.02
5.4300.40
0.21
318
Propane
*
318
2.05.0.10
338
H
1. 5)(1 0
9.3)(10
K )( sorbent weight: ~
C
model fitted for >0.2Hz only.
Corrected f.r. diffusion and n.m.r. self-diffusion (11) data of the ethane/silicalite system on.m.r./ m2 -1 s intra
T/K 298 318 338
*
K' ""
-12 6.5)(10_ m/ucl 11(1.3 1.2xl0_ 11(0.65m/ucl 1.8)(10 10.33m/ucl
~5.0)(10_9
-9
(2.6)(10
-9
at 8m/ucl
>6)(10_9" >7)(10 "
"e)(trapolated data
.IiJllL.i Corrected f.r. (ZSM5/silicalitel and s.u. (ZSM51 diffusion as well as nmr'(silicalitel self-diffusion (111 data of propane. T/K 298 318 338
q/m/uc 7 7 8 4 4 4 2 2 2
method
2s -1 D/m
f.r. s.u. n.m.r. f.r. s.U. n.m. r. f.r. s.u. n.m.r.
. -12 3.5)(10_ 6.5)(10_ 12 10 ~5 )(10_ 12 6.3)(10_ 11 4 )(10_ 9 3 )(10_ 12 9.3)(10_ 9 ~2 )(10_ ~7 )(10 9
"The n.m.r. self-diffusivities of propane on the silicalite characterised in Table 1 have been proved to be smaller by the factor of ~1.5 smaller than on the silicalite sample of Ref.(lll.
M. BUlow et al. 2·5,.,-------------+20
cO
+10
in-phase
¢/deg
0
1'5
blank
-10
1·0
-20 0'6
0·5 N
0·3
Fig.2 Characteristic functions for ethane in ZSM5/silicalite at 318K and 1333 Pa.
~
VI
0·2 0·1 0-2 11Hz
0'02
Fig.I Frequency-response parameters for ethane in ZSM5/silicalite at 298K and 1333 Pa.
a/mmol9' 0.8
04 t, 0
E
...,
70
6
66
66
6
12
666
•
~
40 •
oX
14
:c
D
lP
JI DO
•
cO
16 00 6
50
in-phase
~ 6
~
:c
."i"_
666
12
•••••• •• •••••••• • 10
o
0
0
0
o
o
0
0
8 10
2 Fig.4
4
6
m.luc.
8
Isosteric sorption heats of ethane
( f) and propane (e) on ZSM5/silicalite and propane (0), n-hexane (t> ) and benzene (0 ) on Na,H-ZSM-5.
Fig.3 Characteristic functions for propane in ZSM5/silicalite at 318K and 1333 Pa.
585
586 (AD-7-1)
Fig.5 Concentration dependence of the intracrystalline nmr self-diffusivities Dintra nmr 0 f propane on Sl'1'rca I'rte (' , 313K (11)) and ZSM5/silicalite (e, 296K) and the corrected apparent s.u, diffusivities Doapp of propane on Na,H-ZSM5 (\1 313K,
o
298K, I:. 253K,
aQ+~ o
13
10
0.1
223K, 0 188K)
0
/mmo(gl
0.2 0.3 Oh 0.5 0.6 0.7
.00
\I>.O~
I>. •
1
o.
I>.
o~
0
0
0
o
•
1
atmmolg Q2 0.4 0.6 0.8 1.0 1.2 1h
1011 m N E ....... o
~~.6_
"'i
o
~
66
----"'--
I
I>.
.12
10
• 2
3 m.tu.c.
-0
2
3
4
5
6
7
4
(, 0
8 9
m.tu.c. Fig.6 Concentration dependence of the corrected intracrystalline s.u, diffusivities D of n-hexane on 0 Na,H-ZSM5 (1:. 373K, o 348K, 0 323K, 'V 295K, 0 273K)
Fig.7 Concentration dependence of the corrected intracrystalline s.u. diffusivities Do of benzene on Na,H-ZSM5 (0, e 423K; 1:.,. 393K; 0,. 363K; \1 , ' 333K; ¢ , • 303K; empty symbols.•• constant volume-variable pressure, full symbols... constant pressure conditions)