5184 J. Am. Chem. Soc., Vol. 120, No. 21, 1998
Tung et al.
Table 3. The Ratio of 13 to 14 in Potosensitized Oxidation of
DHB by HA in Homogeneous Solution and on the Internal Surface
of ZSM-5 Zeolites
distributions. The photolysis experiments described above were
repeated after samples were saturated by water vapor. Water
vapor was added to the samples prepared by deposition of the
substrate on dry zeolite from cyclohexane followed by evapora-
tion of the solvent. For the “high” Al content (Si/Al ) 25)
samples, the maximum amount of coadsorbed water was ca.
10% (w/w), and for the “low” Al content (Si/Al ) 55) ones,
ca. 9% (w/w). The samples were suspended in isooctane or PTE
containing sensitizer DCA or HA, and irradiated as described
above. The resulting product distributions are listed in Tables
1, 2, and 3 for DPB, TS, and DHP, respectively. The following
facts are apparent from the data in these tables: (1) The
photosensitized oxidation of DPB and TS wet samples with Si/
Al )55 gave 6 and 1 as unique products respectively as in the
case of the dry samples. Thus, for the samples with “low” Al
content (high Si/Al ratio) ZSM-5 zeolite, no effect of coadsorbed
water on the product distributions was observed. (2) On the
contrary, for the wet samples with Si/Al ) 25, the photosen-
sitized oxidation products of DPB and TS are significantly
different from those for the dry samples. The addition of water
causes the product distributions to be more like those obtained
in homogeneous solution. For example, the products for DPB
wet samples in PTE were mainly derived via the electron transfer
pathway. Furthermore, the HA-sensitized oxidation of TS wet
sample gave 10 as the main product as in the case of the
oxidation in PTE solution. (3) For the wet samples irrespective
of the Si/Al ratio, the photosensitized oxidation of DHP gave
13 and 14, and the ratio of 13 to 14 was significantly greater
than those for dry samples.
sample
isooctane solution
PTE solution
ratio of 13 to 14a
10:90
58:42
40:60
40:60
48:52
48:52
83:17
82:18
94:6
dry ZSM-5 (Si/Al ) 55)-isooctaneb
dry ZSM-5 (Si/Al ) 55)-PTEc
dry ZSM-5 (Si/Al ) 25)-isooctaneb
dry ZSM-5 (Si/Al ) 25)-PTEc
wet ZSM-5 (Si/Al ) 55)-isooctaned
wet ZSM-5 (Si/Al ) 55)-PTEe
wet ZSM-5 (Si/Al ) 25)-isooctanef
wet ZSM-5 (Si/Al ) 25)-PTEg
94:6
a Estimated error limits were 2%. b Dry sample was suspended in
isooctane. c Dry sample was suspended in PTE. d Wet sample with
water uptake of 9% (w/w) was suspended in isooctane. e Wet sample
with water uptake of 9% (w/w) was suspended in PTE. f Wet sample
with water uptake of 10% (w/w) was suspended in isooctane. g Wet
sample with water uptake of 10% (w/w) was suspended in PTE.
ratio of 13 to 14 changes from 10/90 in isooctane to 58/42 in
PTE. However, when DHP is included in ZSM-5 zeolites, the
ratio of 13 to 14 is greater than that seen in isooctane, but smaller
than that in PTE. Of particular interest is that this ratio is
independent of the solvents used, but dependent on the Al
content of ZSM-5 zeolites with the high Al content favoring
the formation of 13 over 14. The solvent-independent ratio
suggests that the oxidation reaction occurs within the internal
framework of ZSM-5. Evidence has been presented that there
exist hydrophilic centers in the internal surface of pentasils, the
most common of which are hydroxyl groups or cations
associated with a tetrahedrally coordinated aluminum, and the
remaining regions in the internal surface are hydorphobic.21 The
ratio of 13 to 14 might reflect the location of DHP, on average,
in the internal channel of ZSM-5 zeolite. It has been suggested
that within a zeolite alkene molecules are adsorbed to the surface
near Al atoms due to the interaction between the π-electron
density of the substrate and the cation of the zeolite.9e,21a,22 Based
The influence of added water is readily understood by
consideration of the character of ZSM-5 zeolite and the location
of the adsorbed substrates in the wet samples. It has been
established21 that water molecules are adsorbed at or near the
hydrophilic aluminum sites in the zeolite framework to form
clusters whose size is constrained by the size and shape
requirement and by the hydrophobic nature of the internal void
space. The portion of the ZSM-5 internal framework that is
not near the water clusters is hydrophobic. Evidently, there
are fewer sites available for water adsorption in the case of Al-
poor ZSM-5 than there are for Al-rich ZSM-5 zeolites. Thus,
the length of the hydrophobic regions within the channels of
the wet Al-poor samples is longer than that for the wet Al-rich
samples. As mentioned above, the concentration of Al is ca.
6.3 × 10-4 and 2.3 × 10-4 M per gram of zeolite for the
samples with Si/Al of 25 and 55, respectively. The loadings
on the unit cell composition (NaxSi96-xAlxO192)
21b of NaZSM-5
zeolite, the concentration of Al is calculated to be ca. 6.3 ×
10-4 and 2.3 × 10-4 M per gram of zeolite with a Si/Al value
of 25 and 55, respectively. The loading of DHP employed in
the present study was ca. 6 × 10-4 M per gram of zeolite. Thus,
for a high Al content sample nearly every DHP molecule may
associate with a cation near an Al atom, while for a low Al
sample a large fraction of DHP has to be located in the weaker
binding hydrophobic regions within the framework. As a result,
the ratio of 13 to 14 for a high Al content sample is larger than
that for a low Al content sample. The micropolarity in the
vicinity of the cations within the zeolite is certainly much higher
than that in a PTE solvent cage. However, we note that the
ratio of 13 to 14 even for a high Al sample is smaller than that
in PTE. This observation suggests that many factors beside
micropolarity, such as steric factors and orientation of the alkene
molecules, might influence the ratio of 13 to 14.
of DPB and TS employed in this study were ca. 2 × 10-4
M
per gram of zeolite, while that of DHP was ca. 6 × 10-4 M per
gram of zeolite (each ca. 50 mg on 1 g of zeolite). Thus, before
water is added the alkene molecules are probably adsorbed in
the internal framework near Al atoms9e,21a,22 except in the case
of a DHP-Al-poor sample, where a large fraction of DHP is
located in the hydrophobic regions within the framework as
discussed above. As water is added, the hydrophobic alkene
molecules are displaced or repelled by the more strongly bound
water molecules. Thus, water has the effect of displacing alkene
molecules from near the Al atom in the internal surface to the
external surface or to the hydrophobic regions within the internal
framework. As suggested by Turro and co-workers21a for
p-methylbenzyl benzyl ketone, which possesses a molecular size
similar to those of DPB and TS, in the wet Al poor ZSM-5
zeolite (Si/Al ) 55), the hydrophobic regions would be large
enough for accommodating DPB or TS molecules. It is
expected that water would displace DPB and TS toward the
hydrophobic portions of the framework. These DPB and TS
Effects of Coadsorbed Water on Product Distribution. To
support the mechanistic interpretation of the selectivity in
photosensitized oxidation of alkenes adsorbed on ZSM-5
zeolites, we studied the effect of added water on the product
(21) Turro, N. J.; Cheng, C.-C.; Abrams, L.; Corbin, D. R. J. Am. Chem.
Soc. 1987, 109, 2449. (b) Hill, S. G.; Seddon, D. Zeolites 1985, 5, 173. (c)
Chen, N. Y. J. Phys. Chem. 1976, 80, 60. (d) Weisz, P. B. Ind. Eng. Chem.
Fundam. 1986, 25, 53. (e) Corbin, D. R.; Seidel, W. C.; Abrams, L.; Herron,
N.; Stucky, G. D.; Tolman, C. A. Inorg. Chem. 1985, 24, 1800.
(22) Hepp, M. A.; Ramamurthy, V.; Corbin, C. R.; Dybowski, C. J. Phys.
Chem. 1992, 96, 2629. (b) Staley, R. H.; Beauchamp, J. L. J. Am. Chem.
Soc. 1975, 97, 5920.