1398
J . Phys. Chem. 1989, 93. 1398-1401
coordinating anion the combination of LiC104 plus 18C6 in D M F does show a single ultrasonic relaxation process, indicating an interaction between Li+ and 18C6 in this solvent.18 In ethanol ( t = 24.3 and 7 = 1.1 CP at 25 "C) the LiC104 plus 18C6 combination produces a double-relaxation process.'* These relaxations in DMF and in ethanol are concentration independent, indicating that they arise from the second or third equilibrium in the Eigen-Winkler mechanism (3) as would be expected for larger values of the overall complex ion stability constant KZ than are found in 1,2-DME. Since the permittivities of D M F and EtOH are somewhat similar, some other factor or factors must be invoked to explain the difference in the number of relaxation processes observed in these two solvents. The combination of LiC104 plus 18C6 in methanol ( t = 32.6 and 7 = 0.55 CP at 25 "C) yields two observable relaxation frequencies at approximately 75 and 7 MHz that both shift with concentration.19 As in the cases of the same solutes in DMF, EtOH, and now PC, the kinetic data may be interpreted in terms of the reduced mechanism (3). When the lithium ion results are compared19 with those for NaC10, and KCIO, reacting with 18C6, dicyclohexano- 18crown-6, and dibenzo-18-crown-6 in DMF and MeOH, it becomes clear that both steps in mechanism (3) depend on the metal, ligand, and solvent with all of them participating in shaping the activation profile of the complexation process. Simplistic attributions of the faster relaxation process to partial cation desolvation and the slower
relaxation process to a ligand conformational change are not borne out by the experimental data. Instead, a concerted process occurs in which, depending on solutes and solvents, either the removal of solvent or ligand rearrangement may be rate determining. Alkali-metal cation nuclear magnetic resonance kinetic studies2S2Oof the dissociation of crown ether complexes of sodium ion in nonaqueous solvents can provide additional insights regarding the relative importance of unimolecular and bimolecular contributions to the dissociation kinetics. Graves and Detellier2 do not find a systematic relationship between Gutmann donor numbers and activation parameters in their rate study of sodium tetraphenylborate with 18C6 in PC, acetonitrile, pyridine, and acetone. Thus, they also reach the conclusion that "several factors, including conformational rearrangement of the ligand and reorganization of the solvent cage", contribute to the activation profile.2 Picosecond laser pulses and fluorescent probes2'Vz2(in this case crown ethers) may eventually prove to be more effective tools than either N M R or ultrasonic absorption for unraveling the relative importance of these various competing influences on the complexation-decomplexation activation profiles for lithium ion and crown ethers in nonaqueous solvents.
(18) Maynard, K.; Irish, D. E.; Eyring, E. M.; Petrucci, S . J . Phys. Chem. 1984, 88, 129-736 (19) Wallace. W.; Chen. C.: Eyrinn. E. M.; Petrucci. S . J . Phvs. Chem. 1985.89, 1357-1366.
(20) Strasser, B. 0.;Hallenga, K.; Popov, A. I. J . Am. Chem. SOC.1985, 107, 189-192. (21) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987,86,6221-6239. (22) Simons, J. D.; Xie, X. J. Phys. Chem. 1987, 91, 5538-5540.
I
-
Acknowledgment. This work was supported in part by the Office of Naval Research. Registry No. 18C6, 17455-13-9; PC, 108-32-7; LiCIO,, 7791-03-9.
Alternatlve Feedback Pathway in the Mixed Landolt Chemical Oscillator' Yin Luo and Irving R. Epstein* Department of Chemistry, Brandeis University, Waltham. Massachusetts 02254 (Received: July 12, 1988)
An alternative route is proposed for the negative feedback pathway in the reaction of iodate, sulfite, and ferrocyanide ions in a stirred tank reactor. This route, based upon the reaction between IO3- and Fe(CN);-, gives better agreement between simulated and experimental results for both the batch (clock reaction) behavior and the periodic oscillation observed in flow systems than do earlier mechanisms. The revised mechanism resolves discrepancies between literature values of several rate constants and those employed in previous simulations. It also accounts for the high (visible) concentrations of I2 generated during the oscillation and is consistent with recent mechanistic work on the bromate-sulfite-ferrocyanide oscillator.
Introduction
Since its discovery over a century ago,2 the Landolt system (acidic K103-Na2S03) has been a classic example of a "clock" reaction. Edblom, OrbBn, and Epstein (EOE)3 recently demonstrated sustained oscillation in [I2], [I-], pH, and R potential when the Landolt reaction is run in a CSTR (continuous flow stirred tank reactor) with ferrocyanide ion as an additional reductant. The Landolt system alone exhibits bistability, but not oscillation, in a CSTR. Two independent s t u d i e ~arrived ~ , ~ by different routes at largely identical mechanisms that qualitatively simulated both the bistable ( 1 ) Part 46 in the series Systematic Design of Chemical Oscillators. Part 45: Edblom, E. C.; Luo, Y.;Orbin, M.; Kustin, K.; Epstein, I. R. J. Phys. Chem., accepted for publication. (2) (a) Landolt, H. Ber. Dfsch. Chem. Ges. 1886, 19, 1317. (b) Eggert, J.; Scharnow, B. Z.Elekfrochem. 1921, 27, 45. (3) Edblom, E. C.; Orbin, M.; Epstein, 1. R. J . Am. Chem. SOC.1986,108, 2826. (4) Gisplr, V.; Showalter, K. J . Am. Chem. Soc. 1987, 109, 4869. ( 5 ) Edblom, E. C.; Gyorgyi, L.; Orbln, M.; Epstein, I. R. J . Am. Chem. SOC.1987, 109, 4876.
0022-365418912093-1398$01.50/0
TABLE I: Reaction Mechanism and Rate Constants for the Mixed Landolt Reaction5
IO;
R2
HI02 HI02
R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13
--- - ---
+ HS0,- SO>- + H I 0 2 + I- + Ht 2HOI + HOI IO3- + I- + 2H+ 10; + I- + 2H+ HI02 + HOI HOI + I- + Ht I2 + H20 I2 + H 2 0 HOI + I- + H* I, + HS03- + H 2 0 21- + SOa2+ 3HC SO?- + HC HSOC HS03+ Ht I, + Fe(CN)64- IF + Fe(CN):+ Fe(CN)63- 1, + Fe(CN)64Fe(CN)>- + 1-, Fe(CN)63- + 21Fe(CN),)- + 21Fe(CN)6,- + 1,-
R1
-+
k l = 2.95 X 10-I M-'s-I k2 = 2.0 X lolo M-* s-I k3 = 1.0 X los M-I s-I k , = 3.0 X lo5 M-3 s-l kJ = 3.0 X 10l2 M-2 SKI k, = 2.2 s-I k , = 1.0 X lo6 M-I s'I k8 = 5.0 X 1OIo M-' s-I k9 = 3.0 X lo3 s-' k , , = 1.3 X 1O'M-I s-I k , , = 2.0 X lo8 M-' s-I k 1 2= 1.0 X lo8 M-I s-' k , , = 1.3 X lo-' M-2s-'
and oscillatory behaviors in the CSTR, as well as the batch (closed system) behavior of the Landolt "clock". Edblom et aL5 proposed a set of elementary or pseudoelementary reactions, as listed in Table I. GBspBr and Showalter4 suggested a set of overall processes and empirical rate laws that give stoichiometric and
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1399
Mixed Landolt Chemical Oscillator TABLE 11: Alternative Mechanism and Rate Constants for the Mixed Landolt Reaction kl = 3.0 X lo-’ M-I SKI MI IO; HSO; HI02 SO?M2 H102 I- H+ 2HOI k2 = 5.0 X lo7 M-2 s-I k3 = 3.0 X 10l2 M-2 s-I M3 HOI I- H+ 1 2 H20 k4 = 2.2 s-I M4 I, H 2 0 HOI + I- + H+ M5 2HI02 IO3- + HOI H* k5 = 5.0 X lo6 M-l s-I M6 I, HSO; H 2 0 21- SO-: k6 = 2.2 X lo7 M-l S-I
-- + + + - + -+ + + + + -+ + + + + + + + + +
M7 M8 M9
3H’ H+ SO,2- HSOC H+ HSO; 10,-
k7 = 5.0 X lolo M-I s-I k8 = 3.0 X lo3 s-I kg = 2.0 X lo2 M-2 s-’
3H+ HI02 H20 HI02 + HOI klo = 5.0 X 10, M-, s-I
2Fe(CN)64-
2Fe(CN)[ MI0 10; + I- + 2H’
+
kinetic behavior quite similar to that found experimentally3 or simulated by Edblom et aL5 Despite the encouraging agreement both between the independently developed mechanisms and between their predictions and the results of experiment, several features of the proposed mechanisms6 would seem to merit further consideration: 1. The rate constant used for (R2) is several orders of magnitude higher than the upper limit found experimentally by Noszticzius et al.’ 2. The rate constant used for (R7) is 2 orders of magnitude lower than that measured by von Bunau and Eigen.8 3. The simulated oscillation period and batch induction period are shorter than their experimental counterparts by nearly a factor of 2. 4. Simulated levels of [I2] remain below M throughout the oscillation, while the brown I2 color is clearly visible during the experimental oscillation. 5. The mechanism suggested for the 103--S032--Fe(CN)64oscillator differs significantly from that proposed for the analogous Br03--S032--Fe(CN)64- oscillator.’ We propose here a revised mechanism for the EOE system that resolves these discrepancies by introducing a different pathway for negative feedback.
Positive Feedback Pathway In the elementary step mechanism: Table I, steps R1, R2, R3, R5 and R7 combine to generate the “clock” behavior in the closed system and bistability in the CSTR, via a process autocatalytic in H + and I- having the overall stoichiometry IO3- + 3HSO3- I- 3s042- + 3H+ (1) The autocatalysis in I- and H+constitutes a positive feedback that brings the system from SSI (low potential, high pH) to SSII (high potential, low pH) when the flow rate is The kinetics of (R3) and (R4), the rate-determining step in the Dushman reaction: have long been a matter of controversy.2b,9-12 Step R3 was insignificant in simulating the BriggsRauscher reactionI2 but had to have a value as high as 3 X lo5 M-I s-I in the model5 for the EOE system, owing to its key role in process 1. We find that in the mixed Landolt system, replacement of (R3) with the disproportionation of H I 0 2 (step M5 in Table II), an alternative route used in the Briggs-Rauscher simulation, leads to dynamical behavior identical with the previous r e s ~ l t s .The ~ rate constant for reaction M5 is adopted from ref 12. However, the rate constant for step R4 ((M10) in Table 11) used in that studyI2 (1.43 X lo3 M-3 s-l, the lowest value found in the literature) is too low to give the right shape for the batch curve in the present system. We find that k l o must be at least 5 X lo3 M-3 s-l in order to convert the simple sigmoid curve of process 1 to the special shape found in the Landolt “clock” reaction
-
+
(6)We focus here on the elementary step mechanism: since it is more detailed and gives slightly better agreement with experiment. (7) Noszticzius, 2.; Noszticzius, E.; Schelly, 2. A. J. Phys. Chem. 1983, 87, 510. (8) von Biinau, G.;Eigen, M. Z.Phys. Chem. (Munich)1962, 32, 27. (9) Dushman, S. J. J . Phys. Chem. 1904, 8, 453. (10) Bray, W. C. J . Am. Chem. SOC.1930, 52, 3580. ( 1 1) Furuichi, R.; Liebhafsky, H. A. Bull. Chem. SOC. Jpn. 1975,48,745. (12) De Kepper, P.; Epstein, I. R. J . Am. Chem. SOC.1982, 204, 49.
0
2
6
4
8
10
Time (min)
0
5
10
15
Time (min)
Figure 1. Experimental’ (a) and calculated (b) batch behavior of the Landolt reaction with [IO,-] = 0.0715 M, = 0.0865 M, and [H,SOI] = 0.005 06 M.
and depicted in Figure 1. To obtain oscillation requires further modification of the mechanism by Edblom et aL5
Negative Feedback Pathway When most of the sulfite has been consumed by process 1, the slower reduction of iodate by ferrocyanide with stoichiometry (2) becomes dominant, causing the pH to rise.
IO3- + 6Fe(CN)t-
+ 6H+
-
I- + 6Fe(CN)63- + 3 H 2 0 (2)
This process, which can be obtained by combining (R2)
+ (R4)
+ 3(R5) + 3(R10) + 3(R12), serves as the negative feedback. If it is strong enough, it can bring the system in a CSTR back the point where process 1 predominates, thereby leading to oscillation. According to the rnechani~m,~ (R4) and (R10) are the key components of process 2 with (R4) as the rate-determining step. The major difference between our revised mechanism, listed in Table 11, and the previous one5 lies in this negative feedback pathway. In the analogous Br0> [IO3-], because with higher [IO3-], a much faster and more complex reaction makes the experiment difficult.16 In the oscillatory r e a ~ t i o n ,[IO3-] ~ always exceeds [Fe(CN),4-] by at least a factor of 2 or 3. It is therefore not surprising that the rate constant for the term first order in [H+] in the empirical rate law (32.8 M-2 S-I) is too low to generate oscillations in the present mechanism. When we optimized k9 to give the best agreement between the simulated and observed oscillatory behavior, we obtained a value about 6 times higher. That value of k9,given in Table 11, is also supported by preliminary experirnent~'~ at pH 3. Finally, we observe that, as shown in Figure 3, the calculated [I2] reaches a maximum value of about lo4 M during the oscillation, about an order of magnitude greater than that found in the earlier simulations and consistent with the observed brief appearance of the brown I2 color. Discussion This work provides an alternative elementary step mechanism for the EOE system that is consistent with that found for its bromine analogue. Both the batch behavior and the oscillation in a CSTR have been almost quantitatively reproduced, and the agreement between the simulated and experimental induction and oscillation periods has been improved considerably. Several sig(14) Birk, J. P.; Kozub, S. G. Inorg. Chem. 1973, 12, 2460. (15) Field, R. J.; Raghavan, N. V.;Brummer, J. G. J . Phys. Chem. 1982, 86, 2443. (16) Sulfab, Y.; Elfaki, H. A. Can. J . Chem. 1974, 52, 2001. (17) Edblom, E. C. Ph.D.Thesis, Brandeis University, 1988.
J . Phys. Chem. 1989, 93, 1401-1404 nificant discrepancies with rate constants from the literature are avoided in the present mechanism, and all rate constants optimized here are in good agreement with the literature values where available. The present study also provides a clearer insight into the role of the component processes in the different stages in the dynamical behavior of the system. The simplified negative feedback (M9) made possible by the higher order dependence on H+ should lead to a clearer understanding of some derivatives of the EOE system, such as the H202-S032--Fe(CN)64- reaction," in which no
1401
"Dushman-type" reaction is possible.
Acknowledgment. This work was supported by National Science Foundation Grants CHE-8419949 and CHE-8800169. We thank Gyula Ribai and Kenneth Kustin for many helpful discussions. Registry No. IO3-, 15454-31-6; SO3*-, 14265-45-3; Fe(CN)64-, 3408-63-4. (18) Ribai,
Gy.; Kustin, K.; Epstein, I. R., submitted for publication.
29SiMagic Angle Spinning NMR Spectra of Alkali Metal, Alkaline Earth Metal, and Rare Earth Metal Ion Exchanged Y Zeolites Kuei-Jung Chao* and Jer-Young Chern Department of Chemistry, Tsinghua University, Hsinchu, Taiwan, Republic of China (Received: November 12, 1987; In Final Form: August 1 1988) ~
The variation of the extraframework cation location in groups IA and IIA metals and rare earth metal (RE) Y zeolites as a function of the dehydration and the rehydration is monitored by 29SiMAS NMR. Unheated hydrated zeolites give similar 29Sispectra as they present the similar cation distributions. Upon dehydration a high-field shift is observed which correlates with the distortion of bond angles in silicon-oxygen tetrahedra. The line shapes of 29Sispectra depend on the nature and the location of the exchangeable cations and the occupancy of the different sites in dehydrated and rehydrated states. The correlation between the line shape of 29Sispectra and the migration of cations from the supercages to the sodalite cages after heating treatment was studied. The results of 29SiNMR agree with the known structure data.
Introduction Zeolites are microporous crystalline aluminosilicates of general with x / y I 1. The net formula M,,,[(A102)x(Si02),]~mH20 charge of the tetrahedral aluminosilicate framework is neutralized by exchangeable cations M& of valence n; the void space of greater than 50% of the crystal volume is occupied by m molecules of water.' Due to the similar X-ray scattering powers of Si and AI atoms, the complete order of Si,Al in the lattice cannot be extracted from X-ray diffraction. Recently high-resolution solid-state 29SiN M R with magic angle spinning (MAS) has been shown to be effective in determining the Si,Al ordering in zeolites by providing direct information of the immediate local environment of Si atom^.^-^ As 29Sipeak position shifts either to lower field by an increase of the number of AIO, tetrahedra connected to Si04tetrahedron2s3 or to higher field by increasing the average Si-0-Si or Si-0-AI bond angle^,^,^ 29SiMAS N M R was shown to be successful in determining Si/AI ratio2 and Si,Al ordering of the tetrahedral aluminosilicate framework,6 monitoring the structural reorganization of TO, ( T = Si4+or A13+) unit in dealurnination,' and detecting temperature-induced phase transition in ~ilicalite.~ (1) Breck, D. W. Zeolife Molecular Sieves; Wiley: New York, 1974; pp 319-425. (2) Fyfe, C. A.; Thomas, J. lnt. Ed. Engl. 1983, 22, 259.
M.; Klinowski, J.; Gobbi, G. C. Angew. Chem.,
( 3 ) Lippmaa, E.; Magi, M.; Samoson, A,; Engelhardt, G.; Grimmer, A. R. J . A m . Chem. SOC.1980, 102,4889. (4) Ramdas, S.;Klinowski, J. Nature (London) 1984, 308, 521. (5) Engelhardt, G.; Radeglia, R. Chem. Phys. Lett. 1984, 3, 271. (6) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J . A m . Chem.
TABLE I: Composition and Dehydration Temperature of Exchanged Na-Y' no. of cations/u.c.
symbol 78La-Y 88Ca-Y 9 1Sr-Y 77Ba-Y 52Ce-Y 99K-Y
Na 12.5 6.7
dehydb temp, OC 350 RTCto 650 350 350 350
others 14.6 La 24.8 Ca 25.6 Sr 21.7 Ba 9.8 Ce 56.3 K
5.1
13.0 27.0
300
Na-Y composition: Na56.3(Alo2)S6,3(Si02),3~,,.~H20. The temperature was raised slowly (1 OC/min) to the desired value and kept in the temperature range of room temperature 350 OC for 6-16 h or at 650 OC for 24 h under continuous outgassing. CRoomtemperature.
-
The local environment of the silicate ions in an aluminosilicate framework also depends on the cation distribution and the nature of absorbed molecule in the extraframework ~ p a c e . ~ It J ~was reported by Melchior et a1.I0 that the 29SiN M R spectra of dehydrated LiNa-A, Li-A, and Na-A samples exhibited a different chemical shift with respect to their hydrated state. On the basis of 29Si MAS N M R study of hydrated and dehydrated Ca-Y zeolite, Grobet et aL7 suggested that the effect of the cations was more localized in the dehydrated state and the 29SiNMR spectrum of the dehydrated zeolite was more complicated than that of the hydrated state. In this paper we describe a detailed study of dehydration and rehydration of a series of alkali metal, alkaline earth metal, and rare earth metal ion exchanged Y zeolite samples (IA-, IIA-, RE-Y). The chemical shift and line shape of 29Si peaks were found to depend strongly on the degree of dehydration
SOC.1982, 104, 4859.
(7) Grobet, P. J.; Mortier, W. J.; Van Genechten K. Chem. Phvs. Left.
1985, 119, 361.
(8) Engelhardt, G.; Lohse, U.; Samoson, A,; Magi, M.; Tarmak, Lippmaa, E. Zeolifes 1982, 2, 59.
M.;
0022-3654/89/2093-1401$01.50/0
~~
~
~~
~
~
~
(9) Klinowski, J.; Carpenter, T. A.; Gladden, L. F. Zeolites 1987, 7, 73. (10) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J.; Pictroski, C. E. Proc. 6th lnt. Zeolite Conf., Reno; 1984; 684-693.
0 1989 American Chemical Society
