8650
J. Phys. Chem. 1993,97, 8650-8655
Electron and Energy Transfer as Probes of Interparticle Ion-Exchange Reactions in Zeolite Y Elaine S. Brigbam, Paul T. Snowden,+Yeong I1 Kim, and Thomas E. Mallouk’ Department of Chemistry and Biochemistry and Center for Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 78712-1 167 Received: March 8, 1993; In Final Form: June 1, I993
Photoinduced electron transfer and energy transfer reactions of tris(2,2’-bipyridyl)ruthenium(II) (Ru(bpy)s2+) with methylviologen (MV2+)and tris(2,2’-bipyridyl)osmium(II) (0s(bpy)s2+)ion-exchanged onto/into separate zeolite Y particles were studied by emission spectroscopy. The kinetics of interparticle exchange were probed by observing the quenching of the MLCT excited state of R ~ ( b p y ) 3 ~by+ mobile MV2+or O ~ ( b p y ) 3 ~ions. + The exchange reactions occur on time scales of seconds to hours, depending on the ionic strength of the surrounding medium. The time-dependent luminescence data were fitted to a dispersed kinetics model, from which average rate constants for the exchange reactions could be extracted. Time constants for interparticle exchange of MV2+ and O ~ ( b p y ) 3 ~ions, + in the range 103-105s at electrolyte concentrations of 0.1-3 mM, are significantly longer than the time scales (lO-’-lO’ s) of most electrochemical and photochemical intrazeolitic reactions involving these and similar electroactive ions. These results argue for reaction mechanisms that invoke intrazeolite electron transfer, rather than exchange of electroactive ions followed by solution-phase electron transfer, in these systems.
Introduction Zeolites, clays, and related microporous solids are excellent host materials for guest molecules that undergo reactions and perform a variety of functions. Their structurally well-defined pore networks and ion-exchangeproperties form the basis of very significant technologies in petrochemical catalysis, water softening, and molecular separations. The same properties are responsible for several interesting potential applications in integrated optics,’ chemical sensing,2molecular electronics,3 and artificial photosynthe~is.~~s Particularly in the last two of these new areas, size-exclusion and ion-exchange effects have been exploited to create rather complex molecular assemblies that consist of spatially segregated pairs and triads of electroactive and photoactive molecules. Microstructural organization in these systemsis manifested in effects, such as current rectification and long-lived photochemical charge separation, which are not normally seen in homogeneous media. For example, zeolite- and clay-based electrochemical “diodes” can be prepared by exchanging a size-excluded electroactivecation onto the outer surface of the microporous particles and a smaller electroactivecation into the bulk. Electron transfer to and from the small cation have been found in most cases to be mediated by the size-excluded one, and current rectification occurs if the two ions have dissimilarformal potentials.6 Faulkner et al.,’ and subsequentlyDutta and co-workers,s have shown that zeolite-encapsulated Ru(bpy)32+can undergo photoinduced electron transfer reactions with intrazeolitic electron donors7 or acceptors* contained in neighboring cages. In both cases the zeolite inhibits the charge recombination reaction between oxidized donor and reduced acceptor, and especially long-lived charge separation results. Similarly, we have shown that sizeexcluded cations, such as R ~ ( b p y ) 3 ~derivatives + and metalloporphyrins, transfer electrons upon photoexcitation to intrazeolitic electron acceptors, resulting in unusually prolonged charge separation and hydrogen evolution.9 In both electrochemical and photochemical systems of this kind, the mechanism of the electron transfer reactions is incompletely understood. In some cases, time-resolved spectroscopic measurements have provided information about the rates f
Center for Fast Kinetics Research.
of electron transfer events but essentially no information about the locus of these reactions. For example, in the electron transfer reaction between Ru(bpy)32+on the surface of zeolite Y particles and methylviologen (MVz+) or metallocene cations in the bulk of the zeolite, the mechanism most likely involves direct electron transfer between the two sites.6 An alternative mechanism involving exchange of both electroactive ions into the solution phase, followed by electron transfer and exchange back into/ onto the zeolite, might also explain the reduction of intrazeolitic ions, although in this case it is difficult to rationalize electrochemical current rectification. In support of the direct (intrazeolitic) electron transfer mechanism, it has been show@ that multiply charged ions on the zeolite surface block the rapid exchange of bulk MVZ+ and also that photochemical charge separation occurs almost as efficiently in the absence of a solution phase. The indirect (extrazeolitic) mechanism has been proposed for electrodes modified with zeolite A, Y, and mordenite containing Cu2+ and other electroactive ions. In these cases the shapes of the cyclic voltammetricwaves resemblethose of unmodified electrodeswith the same ions in solution,lOJ1and so it has been inferred that electron transfer occurs entirely outside the zeolite. On the other hand, Baker and Zhang have recently shown that the electrochemistry of Ag+/O in zeolite Y shows features that can be attributed to ions at different intrazeolitic sites, implying that charge transfer within the zeolite is rapid on the cyclic voltammetric time scale.I2” In related work, Baker et al. have recently found that complexing agents such as chloride and DMSO,which are excluded on the basis of charge and size, respectively, from zeolites Y and A, can be used to pinpoint the locus of reduction of Cuz+. In these cases it was established that reduction occurs outside the pore network.12b Regardless of the site of electron transfer at zeolite-modified electrodes, it is clear that exchange of solution-phase ions must accompany oxidation/reduction reductions, in order that the interior of the zeolite crystals remains electroneutral. Shaw et al. first demonstrated this effect by comparing the magnitude of currents obtained with intrazeolitic MVz+and different supporting electrolytes.10 Size-excluded tetrahexylammonium(THA+) salts gave virtually no electroactivity, whereas Li+ salts resulted in large redox waves. Interestingly, a combinationof Li+and THA+
0022-3654/93/2097-8650$04.00/0 0 1993 American Chemical Society
Ion-Exchange Reactions in Zeolite Y resulted in the highest electroactivity. This synergismmay arise from partial blocking of the zeolite windows by THA+, which prevents rapid exchange of MVZ+ out of the zeolite, combined with charge compensation by the smaller Li+ ions. In a series of elegant experiments, Baker and co-workers have shown that size exclusion effects of counterbalancing cations within a zeolite crystallite (i.e., between large and small cages) can also have a profound effect on electroactivity.12b It was shown, for example, that Ag&saY samples had little electrochemical response, because reduction of Ag+ requires motion of Cs+ from supercage to sodalitecagevia 6-ring apertures. On the other hand, reduction of Ag+ in Ag6Na~oYwas facile, because the 6-ringsdo not exclude Na+. In this paper we report measurements of the rates of exchange of large electroactive ions such as MV2+and M(bpy)j2+ (M = Ru, Os) between separate zeolite Y particles in aqueous suspensions. Quenching of the emissive excited state of Ru(bpy)32+ by electron transfer to MV2+ or energy transfer to Os( b p ~ ) was ~~+ used to follow the rate of interparticle exchange. The conditions of these experimentsare similarto those of previous photochemical and electrochemical studies. While the kinetics of these exchange reactions do not follow a simple rate law, they can be fit to a dispersed exponential model that gives an average time constant for the ion exchange. Under conditions of low ionic strength, this time constant is significantly longer than the time constant associated with either photoinduced charge separation or cyclic voltammetry. These results are consistent with a direct (intrazeolitic) electron transfer model but not with mechanisms that invoke exchange of electroactive species into the solution prior to electron transfer.
Experimental Section Materials. Zeolite Y (ideal formula Na&&i1~0~4-250H20) was purchased as a powder (LZ-Y-72) from Union Carbide, Linde Molecular Sieves Division. The zeolite powder was suspended for 24 h in a stirred 1 M aqueous NaCl solution at 50-60 "C, filtered and resuspended five times in order to remove ionexchangeable impurities, and subsequently washed with water until free of chloride ions by the silver nitrate test. Because this zeolite Y contained trace iron impurities, another sample also obtained fromUnionCarbide (95508901010-S)freeof the binder that contains most of the iron was used for encapsulation of Ru( b ~ y ) 3 ~ +This . lot was washed with NaCl as above. Methylviologen (MV2+)dichloride hydrate was obtained from Aldrich Chemical Co., and tris(2,2'-bipyridyl)ruthenium(II) dichloride hexahydrate was purchased from Strem Chemicals. These compounds were used without purification. Tris(2,2'-bipyridyl)osmium(I1) dichloride was prepared according to the method of Gaudiello et al.l3 Deionized water of resistivity 18.3 mQcm was obtained from a Barnstead Nanopure system and was used in all experiments. Sample Preparation. Zeolite Y was ion-exchanged with Ru(bpy)g2+,Os(bpy)32+,and MVZ+ as follows: appropriatevolumes of a stock solution of 4.9 X 10-4 M Ru(bpy)32+, 2.7 X 10-4 M Os(bpy)32+, and 1.8 X 10-2 M MVZ+ (as chloride salts) were added to 1.0 g of zeolite Y, which was suspended in 30 mL of water. The suspension was stirred overnight at ambient temperature. Samples were filtered, washed thoroughly with water, and dried in air at about 40 OC for 24 h. The washings and the stock solutions were examined spectrophotometrically in order to determine the loading of each ion exchanged onto/into the zeolite. Extinctioncoefficients used for these determinationswere e257 , * = 20 700 M-1 cm-l for MV2+14, e452 ,,, = 14 003 M-l cm-l for R~(bpy)3~+, and €290 ", = 73 664 M-1 cm-1 for Os(bpy)g2+.lS This procedure gave Ru(bpy)32+and Os(bpy)32+at external ionexchange sites at loadings of ca. 1.69 X 1 W and 2.91 X 1 W mol/g, respectively, and MV2+at both internal and external sites at a loading of 2.95 X 10-4 mol/g.
The Journal of Physical Chemistry, Vol. 97, NO. 33, 1993 8651
Ion-Exchange Equilibrium Measurements. The equilibrium constant for the MVZ+-NaY ion-exchange reaction was determined by preparing MVZ+-NaY at relatively high loading of MV2+(ca. 100 mg MVZ+.21-stirred 1 h in 200 mL of water with N a y , 1.2 8); the product was filtered, washed, resuspended in water, filtered, washed, and air-dried at 50 OC to give a white powder. Spectrophotometricand electrochemicalanalysesof the filtrate showed that MV2+exchange was essentially quantitative. A 150-mg sample of this zeolite powder was suspended in 20.0 mL of water, and weighed amounts of NaCl were added. The suspension was stirred ca. 20 min between additions to allow for equilibration, and the concentration of MV2+in the solution was measured electrochemically. Electrochemical measurements were made in a single-compartment cell in solutions deoxygenated with N2, The working electrode was a 0.5 cm2 Sb-doped Sn02 electrode, and the reference electrode was a saturated calomel electrode (SCE). A Pine Instruments RDE4 bipotentiostat was used to obtain cyclic voltammetric i-Vcurves at scan rates of 25 mV/s. Solution concentrations of MV2+ were determined by calibrating the working electrode with known concentrations of MV2+-21- in 20 mL of 0.1 M KCl(2.27 mM). The equilibrium constant for the R~(bpy)3~+-NaY ion-exchange reaction was determined spectrophotometrically. Ru( b ~ y ) 3 was ~ + exchanged onto the external surface of zeolite Y at a loading of 1.27 X 1 W mol/g. Fifty-milligram samples of this solid were suspended in 10.0 mL of NaCl solution, with the latter at concentrations from 2.5 X 10-4 to 0.01 M. The samples were stirred 24 h at ambient temperature (23 "C) to allow for equilibration, and then centrifuged, and the concentration of R u ( b p y ) ~ ~in+the supernatant was determined from the absorbance at 452 nm. Instrumentation. Diffuse reflectance UV-vis spectra were taken with Varian DMS-300 spectrophotometer equipped with an integrating sphere attachment. Samples were referenced to the same zeolite containing no MV2+,R~(bpy)3~+, or O~(bpy)3~+ ions. UV-vis spectra of solutions were obtained with a Hewlett Packard-8452Adiode array spectrophotometer. Emission spectra were collected with a SPEX double monochromator Fluorolog fluorimeter, with detection at right angles. The intensity at the emission maximum of Ru(bpy)2+ (633 nm) was monitored as a function of time, using 452-nm excitation, in order to follow the kinetics of ion exchange. Aqueous zeolite suspensions (typically 20 mg in 2.0 mL of water, at various concentrations of NaC104) were deoxygenated with an argon purge during the measurements. The suspended samples were sonicated prior to measurement, and the sample in the cuvette was stirred with a circular stir bar for the duration of the measurements. The concentration of NaC104 solutions was measured conductometrically. The concentration (M) was assigned for all suspensions by using a calibration curve. Conductance measurements were made with a YSI Model 35 conductance meter. Synthesis and Characterizationof Materials. Ru(bpy)j2+was synthesized within the zeolite Y supercagesas described by Incavo and Dutta.8 These samples were prepared at low loading (1.6 X 10-6 mol/g, approximately one R u ( b p y ) ~ ~for + every 275 supercages) and rinsed several times with NaCl solution in order to remove R ~ ( b p y ) ~ 2ions + from external ion-exchange sites. Diffusereflectance UV-vis spectra were measured over the range 30&700 nm, using NaY as a reference sample. The spectrum of the orange-colored zeolite-encapsulated Ru(bpy)32+is characterized by a maximum at 454 nm. A shoulder at 530 nm in the diffuse reflectance spectrum, absent in samples containing only externally sited Ru(bpy)~~+, is attributed to small amounts of F e ( b ~ y ) ~in~the + zeolite. Results and Discussion
Ion-Exchange Equilibria. Because of the technological importance of zeolites and related microporous solids as ion
0652 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 exchangers,the thermodynamicsof their exchangereactions have been thoroughly investigated. Equilibrium constants for the exchange of small monovalent and divalent cations have been measured for a variety of zeolites;16to our knowledge, however, there are no data on the exchange reactions of size-excluded ions (such as M(bpy)32+) that bind at external cation-exchange sites or for internally sited electroactiveorganic cations such as M V + . In both cases the exchange reaction with a monovalent cation such as Na+ may be represented according to (l), where the k.
Brigham et al.
0.9
-
0.8
-
- &a+
0.5633
0.5 0.4 0.6
f
8
=
i
0.70
0.3
subscriptsdenote zeolite or solutionphase. Here M2+is a surface or bulk dication, Le., M(bpy)32+ or MV+. The ratio of rate constants kllk-1 is K, and can be calculated from the activities of all ions in both phases. In the case of solution species, concentrationsareeasily measured and activitiescan becalculated in a straghtforward manner. For the zeolite phase, only the loading l' (in units of moles/gram) can be measured directly. If the maximum loading for any species is r-, then the fractional coverage 8 is defined as I'/FmX, and the 8's for the exchangeable ions are related according to (2). Assuming that the activity of
a
I
I
I
I
I
I
9
10
11
12
13
14
I
15
Figure 1. Ion-exchange isotherm, at 23 'C, for MV2+/Na+in zeolite Y, plotted according to eq 5. 1.15[
a
= 0.95
(2)
the zeolite-bound ions is proportional to 8, that their activity coefficients are roughly independent of 8, and that the solution concentration of M2+ is sufficiently low so that its activity coefficient is one, then K, may be calculated approximately from (3), where ONa+ refers to the solution-phase Na+ activity. Since
0.40
(3)
rmxis not known a priori, however, the true 8 values are not measured. Rather, in exchanging zeolite-bound M2+ for Na+, one obtains a set of coverages 80, defined as I'/Fi, where is the initial loading of M2+. We define a number a,such that a = I'i/rmaX = 6/80, and then the equilibriumconstant can be expressed in terms of 80 and a according to (4):
(4) Equation 4 can be rearranged to give ( 5 ) . From ( 5 ) one can see
that a plot of 80 vs ( 8 o a ~ ~ + ~ / [ M ~should + ] ) ~ yield / ~ a straight line with intercept l/a and a slope of -(aK,)-ll2, from which both a and K, can be extracted. Figures 1 and 2 show isotherms plotted according to ( 5 ) for exchange of solution-phase Na+ with MV2+/zeolite Y and Y. Both plots are linear over the range of 80 R~(bpy)~~+/zeolite measured, indicating that the activity coefficients for zeolitebound species are approximately constant. The divalent MV2+ and Ru(bpy)s2+ions are preferentially bound, relative to monovalent Na+, consistent with the expected positive ASo (and possibly negative W ) for reaction 1. Interestingly, the equilibrium constant (1 87 M-1) for binding M V + primarily at bulk ion exchange sites is about 7 times larger than that for binding the larger R ~ ( b p y ) 3 ~ion + exclusively at external surface sites. From the a and ri values for the two ions, we calculate maximum loadings I'max for MV2+and R ~ ( b p y ) ~ of~3.4 + X 10-4 and 1.3 X 10-6mol/g, respectively. The calculated rmU for MV2+ corresponds to approximately 0.8 ion per supercage, consistent
Figure 2. Ion-exchange isotherm, at 23 O C , for external Ru(bpy)JZ+/ Na+ on zeolite Y.
with a previous measured value of l.0.17 The fact that rmu for Ru(bpy)a2+is 2 orders of magnitude smaller than that of MV2+ reflects the large ratio of bulk to surface ion exchange sites in the ca. 1-pm-diameter zeolite Y particles used in these experiments. Rates of InterparticleIon Exchange. The exchange rates for small ions have been measured for several zeolitesusing radiotracer techniques.lS In cases where diffusion of ions within the solid is rate-limiting and a large excess of exchanging ions is present in solution, the fractional conversion of the zeolite from one form to another is linear with N 2 , and the interdiffusion coefficient D m for the two ions can be calculated from the slope of the line. This behavior has been observed for aqueous Na+/Ca2+exchange in zeolite A19 and for alcoholic solutions of divalent ions in the faujasitic zeolites X and Y." The 12-ring apertures between cages in the X and Y zeolites are sufficiently large that diffusion of small hydrated cations is very rapid, and special techniques (such as temperature jump methods21) are needed to follow aqueous alkaline earth exchange reactions. Exchanges of monovalent aqueous ions such as Na+ are normally too fast to measure by these techniques in zeolites X and Y. In solvent-free systems, the motion of ions is slow enough that migration and interparticle exchange reactions can be followed by surface spectroscopictechniquesu or NMR ~pectroscopy.~~ Even in these cases, interparticle ion-exchangereactions can be quite rapid (on the order of seconds23) if mechanical contact between particles is good. The systems of interest in the present study also involve ions that diffuserapidly between supercages in zeolite Y. The diffusion coefficient of MV2+in hydrated zeolite Y has been estimated" to be ca. 1 X cm2/s at 23 OC; this is 2-3 orders of magnitude larger than Davalues measurable by normal radiotracer methods.
Ion-Exchange Reactions in Zeolite Y
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8653
SCHEME I: Interparticle Exchange of Mvz+ Ions
I
interparticle
N ~ + / M V 2+ ion exchange
2
TlmellO
66
90
sec.
Figure 3. Plot of 1 - F vs time for the internal Ru(bpy)sz+/intemal MVz+ interparticleexchange reactionat [Na+] = 1.2 X l(r, 5.1 X l e , and 3.15 X lk3M. F is calculated from luminescence data according to eq 8.
(7)
Also,the photochemical and electrochemicalzeolite-based systems we wish to model are typically operated at very low ionic strength, where the Helfferich-Plesset solution to the nonlinear diffusion problem" is not applicable becaqse of changing solution composition during the exchange. In cases where solid-statediffusion is rate limiting, the concentration of ions in solution (so long as solution-phasecations are in large excess over the exchangeable ions in the zeolite) has little effect on the rate of the exchange reaction. However, we note that for surface-confined species such as Ru(bpy),z+, diffusion of (bulk) ions clearly cannot be rate limiting. In this case the rate is expected to depend on the rate constants kl and k-1 for interfacial reaction 1,on the fractional coverage B of exchangeable ions, and on the concentrations of solution-phaseions. The data for all exchange reactions studied show that, under the conditions of these experiments, interparticle ion-exchange rates depend strongly on solution-phase Na+ concentration. The strategy we have employed for measuring the rates of interparticleexchangeof surface and bulkions is shown in Scheme I. Separate zeolite samples are prepared, one containing the luminescent R~(bpy)3~+ ion either at internal (bulk) or external (surface) sites and the other containing a luminescencequencher, either MVZ+(at bulk sites) or O~(bpy)~2+ (at surface sites). The two suspensions are mixed, and the degree of ion exchange is determined from the luminescence as a function of time. The ionic strength of the solution is varied by adding NaC104. The actual Na+ concentration (determined from solution conductivity measurements) reflects contributions from both added salt and hydrolysis of the zeolite. At higher initial Na+ concentrations, the rate increases because the equilibrium represented by (1) lies farther to the left, and therefore the concentration of mobile Mz+ ions increases. Previous experimentsgd have shown that the quenching of the R~(bpy)3~+ MLCT state by intrazeoliticviologenions is dynamic and follows Stern-Volmer behavior according to (6), where kgv
is the Stern-Volmer quenching constant in appropriate units and 6' is the coverageof quencher ions in the zeoliteparticles containing Ru(bpy)32+. 8' varies from an initial value of zero to Bi/2 at equilibrium, where 8 is coverage of MVZ+ or O ~ ( b p y ) ~in~the + particles that initially contain all the quencher ions, and I = 8i -8. The fractional conversion Fis related to 8 by (7) and therefore to the measured luminescence intensity Iby (8), where subscripts i and m denote initial and equilibrium Z and 8 values.
F=-
1 - Ii/I 1 - IJI,
If the interfacial reaction 1 is rate-limiting, then we would expect (9) as the differential rate equation, which should give an initial exponential decay of 8 (and 1 - F) with dO/dt = kl[M2+](1 - 0)'- k-,[Na+]2B
(9)
time. At longer times, where there is appreciable MZ+ in the solution and (1 - 0) > 0, the kl term in (9) becomes important, and so the decay of 1 - F will be slower than the initial single exponential. In practice, the data may be fit to a sum of exponentialsor to a dispersed exponental form originally derived by Albery et al.25 The Albery equation (10) is useful for modeling 1 - F = (1/2r)'/2Jme~p(-x2)exp[-(k,,t) exp(yx)] dx -OD
(10) the kineticsof heterogeneoussystems such as ours that give curved semilogarithmic plots when tested for first-order kinetics. This model assumes a Gaussian distribution of the logarithm of the rate constant about some mean, k,,, and introduces a parameter, y, which is the width of the distribution. When y = 0, there is no dispersion and the system follows a single-exponential decay. The virtue of this model in the present case is that the decays can be fit to two parameters (k,,, y), and an average time constant T = 1/ k, for the interparticle exchange reactions can be extracted. These T values can then be compared to the time scales of electrochemicaland photochemical reactions in zeolite Y in order to determine whether exchange of electroactive ions can be mechanistically important in those cases. Figures 3-5 show plots of 1 - F vs time for the three cases of interparticle ion exchange studied. Table I lists the k,, T , and y values extracted from best fits to (10) of the data contained in Figures 3-5, and Figure 6 shows graphically the dependence of the rate constant on [Na+] for the three cases. The simplest case, involving exchangeof MV2+into zeolite particles containing immobile Ru(bpy)sZ+ (exclusively at internal sites), is shown in Figure 3. The time constant for interparticle MVZ+ exchange varies from ca. 10 000 to 1000 s as the Na+ concentration is increased from 0.1 to 3 mM. Exchange of external Ru(bpy)32+ and Os(bpy)32+ions, as shown in Figure 4, is slightly more rapid over the same range of [Na+]. The latter system approaches equilibrium somewhat faster than the R~(bpy)3~+(internal) / MV2+system, consistent with the larger equilibrium constant for the MVZ+/Na+ ion exchange; that is, the solution concentration
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
8654
Brigham et al. 11
0.4
I
..--
.00012 M -- -.00051 M .00315 M
2
I
I
8.0-
-X
5.0-
'0
I \,
0.2 0.
19
42
90
66
Figure 4. Plot of 1 - F vs time for the external R~(bpy)3~+/external Os(bpy)p2+interparticle exchange reaction at [Na+] = 1.2 X 10-4, 5.1 X l e,and 3.15 X lP3M.
-.....M --..00012 .00051 M
0.8
-.00315
M
0.6
I
r
0.4
0.0 Om2.
117
245
372
500
Time/l O3 sec. Figure 5. Plot of 1 - F vs time for the external R~(bpy),~+/internal MV2+interparticleexchange reaction at [Na+] = 1.2X 10-4,5.1 X 10-4, and 3.15 X M.
TABLE I: Average Lifetimes, Rate Constants, and 7 Values for Interparticle Ion-Exchange Reactions system and [Na+] (M) T (s) kw (8') Y int Ru/int MV 1.16 x 104 8.60X 7.30X 1W 0.000 12 7.00 x 103 1.43 X 10-4 2.10 X 1W 0.000 5 1 1.16 x 103 8.62X 10-4 1.08 X 100 0.003 15 ext Ru/int MV 0.000 12 1.52 x 105 6.59 X 1od 4.00 X 10-1 0.OOO 51
0.003 15
ext Ru/ext Os 0.000 12 0.00051 0.003 15
--text
9.38 x 104 5.47 x 104
1.07X 1.83 X lW5
9.23 x 103 1.52 x 103 9.70X lo2
1.08 X 104 9.20 X 10-I 6.57 X l e 2.10 X 100 1.03 X lt3 1.87 X 10"
1.14 X 100 1.97 X loo
of MV2+ is lower than that of R ~ ( b p y ) 3 ~or+ Os(bpy)32+ at a given value of [Na+], and so interparticle exchange of MV2+is expected to occur more slowly. Interestingly, the slowest interparticle exchange occurs in the case of external R ~ ( b p y ) 3 ~ + and internal MV2+ (Figure 5). In order to attain equilibrium in this system, the rapidly equilibrating Ru(bpy)32+ needs to move out of the way as MV2+ ions pass through surface sites on their way in and out of bulk sites. The large time constants (5.5 X lo4 - 1.5 X loss) associated with this process appear to reflect blocking of the MV2+ exchange by surface Ru(bpy)32+. Similar effects have been observed previously in experiments involving exchange of MV2+through a strongly bound surface layer of zinc tetra(Nmethyl-4-pyridyl)phorphyrin4+ ions.6b The slowest reactions (those carried out at the lowest values of [Na+]) show the smallest y values; i.e., they are closest to single-exponential decays as predicted for conditions in which the kl term dominates the differential rate expression (9). We note that most of the zeolite-based photochemical systems reported to operate at very low concentrations of solutionphase cations and in general resemble the two Ru(bpy)32+/MV2+
I' -1.0 f 0
Ru/int MV
+ext Ru/ext Os
A*2.0-
Time/lO 3sec.
LL
+int Ru/int MV
2
W
-
a I
I
I
0.001
0.002
0.003
0.004
[Na+I/(M) Figure 6. Plots of average rate constant, kav, for the three cases of interparticleexchangerepresented in Figures 3-5, as a function of [Na+].
systems represented in Figures 3 and 5 . In both cases the time scale for ion exchange is much longer than the excited-state lifetime of Ru(bpy)32+ (ca. 600 ns) or the lifetime of charge separation (microseconds to seconds in most cases). While this result argues for an intrazeolitic electron-transfer mechanism in these supramolecular systems, it should be noted that the rate of interparticle exchange is still sufficiently rapid in aqueous suspensions of large-pore zeolites to ensure equilibration (Le., equal concentrations of exchangeable ions in all particles) in a matter of hours. This finding places some limitations on possible designs for systems that might contain one set of ions on some particles and another set on others (e.g., electron donors and acceptors, respectively). Such systems would rapidly equilibrate to a more homogeneous one in which all particles contained donors and acceptors. With solid zeolite samples, the rate of interparticle ion exchange is often much slower, but the time scale varies widely depending on particle size, degree of hydration, and the nature of the reactants. Quite rapid (seconds to minutes) interparticle ion exchange can occur between contacting hydrated particles; under strictly anhydrous conditions, however, the same ion-exchange reactions can require weeks to attain equilibrium.23 Most of the electrochemical systems that show current rectifying or photodiode effects6 resemble the system represented in Figure 5,which has theslowest rate ofexchange. Thesesystems typically employ supporting electrolytes at millimolar concentrations. The time scale for interparticle ion exchange in this case is much slower than that of cyclicvoltammetry. Interestingly, the time scale for reverse-bias oxidation/reduction reactions of zeolite-encapsulated ions, on the order of minutes in these systems, is still some 2 orders of magnitude faster than ion exchange. In other electrochemical reactionslOJ1 employing high (20.1 M) electrolyte concentrations and no blocking surface ions, the time scale for exchange of electroactive ions in and out of the zeolite could indeed be comparable to the time scale (seconds) of cyclic voltammetry.
Conclusions Rates of interparticle ion exchange in zeolite Y have been studied for both bulk (MV2+) and surface-confined (Os(bpy)32+, R~(bpy)3~+) cations. Electron (MV2+)and energy (Os(b~y)3~+) transfer quenching of luminescent Ru(bpy)32+ ions provides a convenient method for following these exchange reactions on time scales of seconds to hours. At low concentrations of exchangeable solution-phase ions, the interparticleexchange rates are not limited by bulk diffusion but by interfacial exchange reactions. Under conditions (0.1-3 mM electrolyteconcentration) mimetic of most electrochemical and photochemical experiments involving similar ions in zeolite Y, the time scales of these exchange reactions are slow. These results imply that electron transfer reactions in these systems must be predominantly intrazeolitic. Since the rates of
Ion-Exchange Reactions in Zeolite Y interparticle exchange increase with increasing concentration of solution-phasecations, exchange of electroactive ions in and out of zeolite Y may be mechanistically important on the time scale of cyclic voltammetry in experiments carried out at higher electrolyte concentrations.
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