Comparison of Activation Parameters for Ionization Reactions within Zeolites and in Aqueous Solution

Article abstract of DOI:10.1021/ja038337z

Absolute rate constants for the ionization of chloride from the 2-chloro-1-(4-methoxyphenyl)ethyl radical are measured in aqueous methanol and in alkali-metal cation zeolites as a function of temperature. The absolute rate constants are very fast in the two distinct media. However, the activation parameters are considerably different. In solution, the reaction proceeds with low enthalpies of activation and large, negative entropies of activation, while in zeolites, the reaction is characterized by significantly higher activation enthalpies and large, positive entropies of activation. These differences reveal that the fundamental factors allowing for such rapid reactions are not the same in the two media. Copyright

Full text of DOI:10.1021/ja038337z

Published on Web 01/08/2004
Comparison of Activation Parameters for Ionization Reactions within Zeolites
and in Aqueous Solution
Norman P. Schepp, Wendy Monk, and Frances L. Cozens*
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3
Received September 4, 2003; E-mail: fcozens@dal.ca
Zeolites are microporous aluminosilicate materials made up of
[SiO4]4- and [AlO4]5- tetrahedra arranged in a defined manner to
give the material an open framework structure of cavities and pores.
Each aluminum in the framework carries a net negative charge that
must be balanced with a charge-balancing cation,1 and the presence
of these charged components produces a highly polar environment
within the cavities of zeolites.2-6 As a result, these materials have
a well-documented ability to promote charge separation reactions
of guest molecules incorporated within the cavities,7,8 including
heterolysis reactions of carbon-halide bonds that lead to the
formation of carbocations.9,10
Figure 1. Temperature dependence of the ionization of chloride from the
2-chloro-1-(4-methoxyphenyl)ethyl radical in (b) dry KY, (0) hydrated KY,
and (O) 50% aqueous methanol.
This ability to support chemical reactions that produce charged
intermediates is similar to the well-known ability of polar, ionizing
solvents to support the same kinds of reactions.11 However, given
the considerable differences between the rigid structure of zeolites
and the fluid, dynamic nature of liquid media, the factors responsible
for rapid formation of charged intermediates in zeolites and those
responsible for the same phenomena in polar solvents may be quite
different. In an effort to probe these differences, we have carried
out a study in which the activation parameters for the SN1-like
ionization reaction of 2-chloro-1-(4-methoxyphenyl)ethyl radical
to the 4-methoxystyrene radical cation, eq 1, are measured in both
Y zeolites and in solution. The results presented here represent the
first direct comparison of activation parameters for the same reaction
measured in these two distinct reaction media.
constant as the formation of the 4-methoxystyrene radical cation,
which is consistent with the radical cation being generated from
the radical. Furthermore, addition of oxygen completely suppresses
the presence of the 300 nm band of the radical as well as the 360
and 600 nm bands of the radical cation. Because the 4-methoxy-
styrene radical cation is known to be relatively inert to oxygen,14
its absence from the spectrum under oxygen purged conditions is
consistent with a mechanism for the formation of the 4-methoxy-
styrene radical cation upon photolysis of 2-chloro-1-(4-methoxy-
phenyl)ethyl acetate that involves rapid â-heterolysis of the
photogenerated â-substituted arylethyl radical.12,13
Figure 1 shows the temperature dependence of the rate constants
for ionization in dry KY, hydrated KY, and in 50% aqueous
methanol. A good linear relationship exists between the rate constant
and the inverse of the temperature, and linear least-squares analysis
leads to values for the entropy and enthalpy of activation, which
were used to calculate the free energy of activation for the ionization
reaction. The temperature dependence of the rate constants in other
alkali-metal cation exchanged zeolites under dry vacuum and
hydrated conditions, and in other methanol-water mixtures, were
also analyzed in the same manner. All of the activation parameters
obtained, as well as rate constants for ionization, are summarized
in Table 1.
The data in Table 1 show that the reaction in aqueous methanol
mixtures (entries 5-8) is characterized by low enthalpies of
activation. These activation enthalpies are sufficiently low to
overcome the unfavorable activation entropies, which in every case
for the reaction in solution are fairly large and negative due to
solvation. Thus, the rapid rate of ionization in solution is largely
driven by favorable enthalpic factors, with the polarizing and
solvating abilities of the aqueous methanol media providing the
environment required for the reaction to proceed with low enthalpic
barriers.
We chose to study the ionization of 2-chloro-1-(4-methoxyphe-
nyl)ethyl radical because accurate kinetics for the reaction, khet, can
be readily obtained using nanosecond laser photolysis both in
zeolites12 and in solution.13 Furthermore, the absolute rate constants
for this reaction in both environments are very similar; thus the
free energies of activation are also very similar, and attention can
be focused on the differences in the entropies and enthalpies of
activation.
The 2-chloro-1-(4-methoxyphenyl)ethyl radical was generated
by photohomolysis of 2-chloro-1-(4-methoxyphenyl)ethyl ac-
etate,12,13 eq 1, both in zeolites and in aqueous methanol at various
temperatures. Rate constants for ionization of chloride ion were
then determined by monitoring the time-resolved growth at 360
and 600 nm due to the formation of the 4-methoxystyrene radical
cation, eq 1.
Several observations are consistent with the formation of the
4-methoxystyrene radical cation by ionization of chloride. Under
all conditions, a band at 300 nm which is produced promptly within
the laser pulse can be assigned to the 2-chloro-1-(4-methoxyphenyl)-
ethyl radical.12,13 This band decays with the same first-order rate
The values for the activation parameters (entries 1-4) measured
in the dry zeolites lead to a somewhat different conclusion. In the
zeolites, the activation enthalpies are still quite low, but considerably
larger than those measured in solution. For example, ∆Hq in NaY
9
1012
J. AM. CHEM. SOC. 2004, 126, 1012-1013
10.1021/ja038337z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
increases in the same direction17 and thus nicely illustrates the
importance of the electrostatic field strength on determining ∆Hq.
The larger ∆Hq values in hydrated zeolites (entries 9-12) as
compared to dry zeolites also show that the strength of the
electrostatic field is important; under hydrated conditions, water
molecules bind strongly to the metal cations, thus reducing the
strength of the electrostatic field.6
A potentially more dynamic environment might be present in
the hydrated zeolites which could lead to a considerable decrease
in the values for ∆Sq upon going from the dry to the hydrated
zeolites. Instead, the values for ∆Sq tend to increase under hydrated
conditions. This trend is consistent with the notion that water
molecules are organized around the metal cations within the zeolite
cavities.6 As the reaction progresses, the water-cation structure is
disrupted by the presence of the charged transition state, resulting
in an overall increase in disorder.
The results described in the present work highlight that both
zeolites and aqueous media are fully capable of promoting charge
separation reactions. However, the activation parameters reveal that
the fundamental factors allowing for such rapid reactions are not
the same in the two distinct media. In zeolites, the reaction takes
advantage of the ability of the zeolite to provide favorable enthalpic
and entropic conditions, while the reaction in aqueous methanol
solutions relies entirely on charge stabilization factors.
Table 1. Observed Rate Constants and Enthalpies, Entropies,
and Free Energies (at 25 °C) of Activation for the â-Heterolysis
Reaction of 2-Chloro-1-(4-methoxyphenyl)ethyl Radical in Dry and
Hydrated Cation Exchanged Y Zeolites, and in Aqueous Methanol
Mixturesa
q
q
q
∆H
∆S
∆G
conditions
dry NaY
dry KY
dry RbY
k
het (s-1
)
(kJ/mol)
(J/mol‚K)
(kJ/mol)
1
2
3
4
5
6
7
8
9
2.1 × 107
3.8 × 106
2.3 × 106
5.0 × 105
2.0 × 107
1.4 × 107
8.0 × 106
2.2 × 106
1.4 × 107
1.1 × 106
3.2 × 105
2.6 × 105
39
42
41
52
18
25
27
28
43
50
58
64
22
23
14
32
35
36
40
31
32
33
35
32
37
42
42
dry CsY
40
70% aq. CH3OH
60% aq. CH3OH
50% aq. CH3OH
30% aq. CH3OH
hydrated NaY
hydrated KY
hydrated RbY
hydrated CsY
-45
-26
-21
-32
35
42
53
73
10
11
12
a Activation plots consisted of 8-10 rate constant measurements made
over a 30-40 °C temperature range (ca. -5 to 35 °C). In each media, one
of those rate constant measurements was obtained at a temperature between
22 and 26 °C, and that observed rate constant (khet) is shown in the table.
Errors (95% confidence limit) in activation parameters are e3 kJ/mol for
∆Hq, e3 J/mol‚K for ∆Sq, and e4 kJ/mol for ∆Gq.
is 2-fold (or 21 kJ/mol) greater than that in 70% aqueous methanol,
despite the rate constants for ionization in both cases being virtually
identical. If unaccompanied by a favorable change in activation
entropy, such a large increase in ∆Hq would cause a significant
reduction in the rate constant for ionization. However, the magni-
tudes of the rate constants remain the same, primarily due to a
reversal in the sign of the activation entropies. Thus, while values
for ∆Hq in zeolites are still quite low, it is the large increase in
∆Sq values that provides the necessary driving force for the rate
constants to maintain their large value.
Acknowledgment. This research is supported by funding from
the Natural Sciences and Engineering Research Council of Canada.
Supporting Information Available: Tables of rate constants as a
function of temperature in the dry and hydrated alkali-metal cation
exchanged zeolites, and in aqueous methanol (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
The larger ∆Hq values measured in the zeolites seem contrary
to a large body of data indicating that the polarity within zeolites
is greater than that of 50% aqueous methanol.5,15,16 Thus, factors
other than polarity may be responsible for the larger ∆Hq values
measured in this work for ionization within the zeolites. In
particular, unlike the situation in solution where the solvent
molecules can readily adopt new orientations to accommodate the
charges formed as the reaction progresses to the transition state,
the rigid framework of the zeolites does not possess a dynamic
“solvating” ability. This would ultimately lead to larger values for
the enthalpy of activation. At the same time, the reduced ability to
solvate negates the unfavorable entropy effects observed in solution;
as a result, within the zeolites, the activation entropies favor
ionization and nicely compensate for the less favorable enthalpy
term.
(1) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry and Use;
Robert E. Krieger Publishing Company: New York, 1974.
(2) Baretz, B.; Turro, N. J. J. Photochem. 1984, 24, 201-205.
(3) Iu, K. K.; Thomas, J. K. Langmuir 1990, 6, 471-478.
(4) Liu, X.; Iu, K. K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120-4128.
(5) Sarkar, N.; Das, K.; Natha, D. N.; Bhattacharyya, K. Langmuir 1994, 10,
326-329.
(6) Uppili, S.; Thomas, K. J.; Crompton, E. M.; Ramamurthy, V. Langmuir
2000, 16, 265-274.
(7) Zhang, G.; Kerry, T. J. J. Phys. Chem. B 2003, 107, 7254-7260.
(8) Yoon, K. B. Chem. ReV. 1993, 93, 321-339.
(9) Murray, D.; Chang, J. W.; Haw, J. F. J. Am. Chem. Soc. 1993, 115, 4732-
4741.
(10) Murray, D. K.; Howard, T.; Goguen, P. W.; Krawietz, T. R.; Haw, J. F.
J. Am. Chem. Soc. 1994, 116, 6354-6360.
(11) Streitweiser, A., Jr. SolVolytic Displacement Reactions; McGraw-Hill:
New York, 1962.
(12) Ortiz, W.; Cozens, F. L.; Schepp, N. P. Org. Lett. 1999, 1, 531-534.
(13) Cozens, F. L.; O’Neill, M.; Bogdanova, R.; Schepp, N. J. Am. Chem.
Soc. 1997, 119, 10652-10660.
(14) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc. 1993, 115, 6564-6571.
(15) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1989, 111, 1128-1130.
(16) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1988, 110, 6587-6588.
(17) Ramamurthy, V.; Turro, N. J. J. Inclusion Phenom. Mol. Recognit. Chem.
1995, 21, 239-282.
The data in Table 1 also show that activation parameters are
influenced by the composition of the zeolite. One clear trend is
that the values for ∆Hq increase as the size of the metal cation
increases from Na+ to Cs+ (entries 1-4). This trend is consistent
with previous results showing that the electrostatic field strength
JA038337Z
9
J. AM. CHEM. SOC. VOL. 126, NO. 4, 2004 1013