J . Phys. Chem. 1984,88, 4192-4194
4192
Kinetic Study of Ion Exchange of the Dodecyitrimethylammonium Ion for H+ in Zeolite H-ZSM-5 Using the Pressure-Jump Method Tetsuya Ikeda and Tatsuya Yasunaga* Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan (Received: August 16, 1983; In Final Form: March 26, 1984)
A single relaxation on the order of milliseconds was observed in aqueous suspensions of the zeolite H-ZSM-5-dodecyltrimethylammoniumion (DTA') system below its critical micelle concentration by using the pressure-jump relaxation method with electric conductivity detection. Also a very slow process on the order of 2 h associated with proton release was observed. The single relaxation observed was attributed to the adsorption-desorption process of DTA' onto the surface of H-ZSM-5. The slow proton release was attributed to an intracrystalline exchange of DTA' for H+ in the channels of H-ZSM-5. The forward and backward rate constants of the former and the latter processes were determined at 25 O C as follows: kl = 1.3 s-l, k-21 = 2.7 X lo-' mol-l dm3 s-l. X lo3 mol-' dm3 s-l, k-l = 2.7 8;k{ = 2.7 X
Introduction Fast reactions in aqueous suspensions of clay minerals have been studied by using the chemical relaxation method.'-' These investigations give a significant clue for the clarification of the mechanism of shape-selectivity and catalytic properties in actual catalysis. However, only ion-exchange kinetics with comparatively small ions has been performed so far. Zeolite ZSM-5 which is a new silica-rich material exhibits strong catalytic activity originating from its unique channel structures.8 It has been revealed from the application of protonated ZSM-5 (H-ZSM-5) as catalysts, e.g., the conversion of methanol to olefins and to hydrocarbons in the gasoline range, that protons bound on the negatively charged aluminosilicate framework play an important role in the catalytic and acidic activities."' In the catalysis of H-ZSM-5, the intercalation rate of the molecules entering into the H-ZSM-5 channels is governed by steric factors from the entering molecule and the channel structures and may depend on both the geometric structures of the entering molecule and the channels. In order to clarify the size and shape correlations between the channel and molecule structures in the ion-exchange reaction, one must study the kinetics of the ion exchange of various cationic surfactants for H+ in H-ZSM-5. In this paper, we present the result of pressure-jump relaxation experiments on the kinetics of ion exchange of the dodecyltrimethylammonium ion for H+ in aqueous suspensions of the zeolite H-ZSM-5. Experimental Section The details of the pressure-jump apparatus have been described previously.12 The time constant of the pressure-jump is 80 1 s . The zeolite ZSM-5 (SiO2/Al2O3= 160) which was prepared according to the directions reported in a patent (British patent 140 298 1) was supplied from Mitsubishi Heavy Industries, Ltd. (1) Ikeda, T.; Sasaki, M.; Asturdan, R. D.; Yasunaga, T. Bull. Chem. SOC. Jpn. 1981, 54, 1885. (2) Ikeda, T.; Sasaki, M.; Yasunaga, T. J . Phys. Chem. 1982,86, 1678. (3) Ikeda, T.; Sasaki, M.; Yasunaga, T. J . Phys. Chem. 1982,86, 1680. (4)3keda, T.;Yasunaga T. J. Phys. Chem. 1984, 88, 1253. (5) Ikeda, T.; Sasaki, M.; Mikami, N.; Yasunaga T. J . Phys. Chem. 1981, 85, 3896.'' (6) Ikeda, T.; Sasaki, M.; Yasunaga T. J . Phys. Chem. 1983, 87, 745. (7) Ikeda, T.; Nakahara, J.; Sasaki, M.; Yasunaga, T. J. Colloid Interface Sci. 1984, 97, 278. ( 8 ) Kokotailo, 43. T.; Lawton, S . L.; Olson, D. H.; Meier, D. H. Nature (London) 1978, 272,437. (9) Meisel, S. L.;McCullough, J. P.; Lechthaler, C. H.; Weisz, P. B. Chemi - .- . Tech - - - 197'6 -. . ,6 -, 86 ..
(10) Chang, C . D.; Silvestri, A. .IJ.. Coral. 1977, 47, 249. (11) Olson, D. H.; Haag, W. 0.;Lago, R. M. J . Curd. 1980, 61, 390. (12) Hachiya, K.; Ashida, M.; Sasaki, M.; Kan, H.; Inoue, T.; Yasunaga, T.J . Phys. Chem. 1979,83, 1866.
0022-3654/84/2088-4192$01.50/0
X-ray diffraction patterns of this powder were identical with those reported by Kokotailo et a1.8 The H-ZSM-5 used was prepared by decomposition of the ammonium-exchanged form. The cation-exchange capacity was determined to be 13.5 mequiv/l00 g from the adsorption isotherm of the dodecyltrimethylammonium ion. All chemicals were reagent grade and were used without further purification. The amount of dodecyltrimethylammonium ion (DTA+) adsorbed was determined indirectly from the concentration change in the supernatant solution by colorimetric analysis. Prior to measurements, samples of H-ZSM-5 suspensions containing DTA' were centrifuged for 30 min at 20000g in order to completely settle the particles. The particle concentration of all samples was 20 g dm-3, and the samples were equilibrated for 24 h after preparation. The temperature was controlled at 25.0 0.1 W.
*
Results and Discussion Figure l a shows a typical relaxation curve observed in aqueous suspensions of the zeolite H-ZSM-5-DTA+ system below its critical micelle concentration (cmc) of 1.6 X lo-* mol dm-3 by using the pressure-jump relaxation method with electric conductivity detection, where the direction of the relaxation signal indicates a decrease in the conductivity of the suspension during the relaxation. No relaxation was observed in the supernatant solution of the above system. The proton release profile (pH vs. time) shown in Figure 2 indicates that nearly 2 h are required for the H-ZSM-5-DTA' system to reach equilibrium. The semilogarithmic plot of the typical relaxation plot in Figure l b indicates that the relaxation curve is characterized as a single relaxation. The dependence of the reciprocal relaxation time 7-l on the concentration of added dodecyltrimethylammonium bromide (DTAB) in aqueous suspensions of the zeolite W-ZSM-5 is shown in Figure 3. As can be seen from this figure, the value of 7-l shows an increase with increasing concentration of DTA'. If we take into account the lack of relaxation in the supernatant solution and a smaller concentration of DTA' than its cmc, it is clear that the single relaxation observed does not relate to micelle formation. Furthermore, it appears from the kinetic measurements of fast and slow processes that the ion exchange may consist of at least two elementary processes. Let us consider the following mechanism of ion exchange of DTA+ for H+ in the zedlite H-ZSM-5:
DTA+
step 2
H+
step 3 where S(H) is the ion-exchange site in the channel of zeolite H-ZSM-5. Steps 1, 2, and 3 denote the adsorption-desorption step 1
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 18. 1984 4193
Exchange Kinetics of DTA+ for H+ in H-ZSM-5
2
-
0,
5
1
0
I
I
J
5
10
15
bulk concn. of DTA’ ,
lO”m0l dm-’
Figure 4. Adsorption isotherm of DTA’ (0) and the amount of H+ released by the DTA’ adsorption ( 0 ) in the zeolite H-ZSM-5-DTA’ system at a particle concentration of 20 g dm-’ and 25 OC. The solid lines represent for the theoretical curves calculated by using the equilibrium constants K,and K;.
steady-state ~ o n d i t i o n ’ ~the J ~fast and slow reciprocal relaxation - ~ 72-1are given by times T ~ and time,
s
Figure 1. (a) Typical relaxation curve in the zeolite H-ZSM-5-DTA’ system observed by using the pressure-jump relaxation method with electric conductivity detection at a particle concentration of 20 g dm-’ and 25 O C . (b) Semilogarithmic plot of a typical relaxation curve.
Under the condition a l l >> azzor a22>> all, e? 1 can be simplified, and the reciprocal relaxation times, 71- and r2-I, are approximated as follows:15 (i) all >> a22 rl-l I
0
0.5
I
1 time,
= a11
(6)
1
1.5
h
(ii) azz>> a l l
Figure 2. Time profile of the H+-releasing reaction in the zeolite HZSM-5-DTA+ system for a DTA+ concentration of 7.0 X lo-’ mol d n r 3 at a particle concentration of 20 g dm-) and 25 O C .
For case i where step 1 is much faster than steps 2 and 3 in mechanism I, the fast and slow relaxation times are given by TI-’
= ki([S(H)]
+ [DTA’]) + k-l
(10)
=
r2-1
K1([DTA+l + [S(H)l) kl’Kl([DTA+] + [S(H)]) + 1
+ k-,/([S(DTA)] + [H+]) (11)
with 0
0.5
1
1.5
added DTAB , 1 O 2 mol dm-3
Figure 3. Dependence of the reciprocal relaxation time on the concentration of added DTA+ in aqueous suspensions of the zeolite H-ZSM5-DTA’ system at a particle concentration of 20 g dm-3 and 25 O C .
process of DTA+ onto the zeolite surface, the intracrystalline exchange of DTA+ for H+ in the channel, and the adsorptiondesorption process of the exchanged H+, respectively. kf1,2,3and Kl,2,3are the rate and equilibrium constants of each step, respectively. Under the assumption that steps 2 and 3 are conducted by a
Kl = kl/k-l
(12)
k2‘ = k3kZ/(k3 + k-2) k-2‘ = k-2k-3/(k3 + k-2)
(13)
(14) The adsorption isotherm of DTA+ and the amounts of H+ released by the DTA+ adsorption in aqueous suspensions of the zeolite H-ZSM-5 are shown in Figure 4, in which [S(DTA)] and [S(H).DTA+] are equal to the amount of H+ released and the (13) Ikeda, T.;Sasaki, M.; Yasunaga, T. J. Colloid Interface Sci. 1984, 98, 192.
(14) Ikeda, T.; Yasunaga, T. J . Colloid Interface Sci. 1984, 99, 183. (15) Bernasconi, C.F. “Relaxation Kinetics”; Academic Press: New York, 1976.
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The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 2 01
I
1
0
I
I
I
05
1
I
1.5
CS(H)I + CDTA? , 1 0 2 m o l dm-'
Figure 5. Plot of
Additions and Corrections
vs. [S(H)] + [DTA'] in eq 10.
~ 1 - l
TABLE I: Rate Constants and Equilibrium Constants of Steps
1 and
In order to determine the equilibrium constants, the values of K, and K2) were calculated from the concentration of products of each species and are listed in Table I. As is shown in Figure 4, the data fall on the theoretical curves evaluated by using the values of K, and K2), This fact indicates that the mechanism proposed is statically plausible. Furthermore, as can be seen from Table I, the value of K1 obtained kinetically is in good agreement with that obtained from the adsorption isotherm, lending additional support to the proposed mechanism. Slow Process. As discussed above the static data are consistent with mechanism I in which step 1 is much faster than steps 2 and 3. As shown in Figure 2, the slow process accompanying H+ release was found to be on the order of 2 h. If we assume that the intracrystalline difffusion rate of DTA+ in the channels is responsible for the observed slow H+release process, eq 13 can be rewritten by using the obtained equilibrium constant KzK3as
2 at 25 'C
kl, mol-' dm3s-l k-l, 5-I k i , s-l k-;, mol-' dm's-l K1,mol-l dm3 K;, mol dm3
7*-1
1.3 x 103 2.1 2.1 x 10-1 4 X lo2 (4.8 X 1 x 10"
+
+
lo2)'
difference between the amounts of DTA+ adsorbed and of H+ released, respectively. Fast Process. The plot of eq 10 shown in Figure 5 yields a straight line. The good linear correlation leads to the conclusion that the single relaxation observed may be attributed to step 1 in mechanism I. The values of the rate constants kl and k-, were obtained from the slope and the intercept of the straight line and are listed in Table I. The value of the equilibrium constant K1 was calculated from the obtained rate constants and is also listed in Table I. The static equilibrium constants K1 and K2) of mechanism I are given by
with
KI([S(H)I + [DTA+I) + Kl([S(H)] EDTA']) 1
2.1 x 10-4
"This value was calculated from the ratio of the obtained rate constants kl and k-l.
K1 = [S(H).DTA+]/([S(H)] [DTA'])
(15)
[S(DTA)][H+]/ [S(H).DTA+]
(16)
K2'
=
For a DTA+ concentration of 7.0 X lo-' mol dmm3,it was found from Figure 2 that the value of r2-I is nearly 7 X s-I. Since the value of k; can be estimated by using eq 13, the values of k-2) can be calculated from the known value of k2). The values of k i and k - i obtained are listed in Table I. From a comparison of the mobility of DTA+ and H+ in the bulk phase, one can see that the desorption rate of H+ from the zeolite surface is much faster than that of DTA+ in mechanism I. This fact indicates that the steady-state condition may be expressed as k3 >> k-2. Thus, it is plausible that the rate-determing step is the intercalation process of DTA+ into the channels followed by the DTA+ adsorption onto the H-ZSM-5 surface. Next, another case was considered, where step 2 and 3 were assumed faster than step 1. With the same approach as above, a negative value for k2) was obtained, which is a physically meaningless result. From the present experimental results, it is seen that the association of intercalated cationic surfactants is not due to formation of hemimicelles. Registry No. DTA', 10182-91-9.
ADDITIONS AND CORRECTIONS 1984, Volume 88 Eva Meirovitch,* Igal Belsky, and Shimon Vega: Deuterium Nuclear Magnetic Resonance Line-Shape Study of Structure and Mobility in a Solid Benzene-Cyclophosphazene Inclusion Compound. Page 1525. The following should be added to the caption of Figure 3: The X-ray structure diagrams were reproduced with permission from ref 1 1.
