Cyclohexanone from cyclohexane and O2 in a zeolite under visible light with complete selectivity

Article abstract of DOI:10.1021/ja953273g

Irradiation of a cyclohexane and O₂-loaded zeolite NaY with green or blue light resulted in oxidation of the alkane to cyclohexyl hydroperoxide and cyclohexanone (plus water) as the sole products. The hydroperoxide was found to rearrange thermally to cyclohexanone without side reaction. Complete selectivity in terms of the final oxidation product cyclohexanone persists even at high (>40%) conversion of cyclohexane. The chemistry was monitored by in-situ FT-infrared spectroscopy and the visible reactant absorption by diffuse reflectance spectroscopy. The absorption is attributed to the cyclohexane · O₂ charge-transfer transition, which is shifted from the UV into the visible by the very high electrostatic field of the zeolite matrix cage. At elevated temperature, cyclohexane is thermally oxidized by O₂ to the same products. Irradiation of a cyclohexane and O₂-loaded zeolite NaY with green or blue light resulted in oxidation of the alkane to cyclohexyl hydroperoxide and cyclohexanone (plus water) as the sole products. The hydroperoxide was found to rearrange thermally to cyclohexanone without side reaction. Complete selectivity in terms of the final oxidation product cyclohexanone persists even at high (>40%) conversion of cyclohexane. The chemistry was monitored by in-situ FT-infrared spectroscopy and the visible reactant absorption by diffuse reflectance spectroscopy. The absorption is attributed to the cyclohexane O₂ charge-transfer transition, which is shifted from the UV into the visible by the very high electrostatic field of the zeolite matrix cage. At elevated temperature, cyclohexane is thermally oxidized by O₂ to the same products.

Full text of DOI:10.1021/ja953273g

J. Am. Chem. Soc. 1996, 118, 6873-6879
6873
Cyclohexanone from Cyclohexane and O in a Zeolite under
2
Visible Light with Complete Selectivity
Hai Sun, Fritz Blatter, and Heinz Frei*
Contribution from the Laboratory of Chemical Biodynamics, MS CalVin Laboratory,
Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720
X
ReceiVed September 25, 1995
Abstract: Irradiation of a cyclohexane and O2-loaded zeolite NaY with green or blue light resulted in oxidation of
the alkane to cyclohexyl hydroperoxide and cyclohexanone (plus water) as the sole products. The hydroperoxide
was found to rearrange thermally to cyclohexanone without side reaction. Complete selectivity in terms of the final
oxidation product cyclohexanone persists even at high (>40%) conversion of cyclohexane. The chemistry was
monitored by in-situ FT-infrared spectroscopy and the visible reactant absorption by diffuse reflectance spectroscopy.
The absorption is attributed to the cyclohexane‚O2 charge-transfer transition, which is shifted from the UV into the
visible by the very high electrostatic field of the zeolite matrix cage. At elevated temperature, cyclohexane is thermally
oxidized by O2 to the same products.
I. Introduction
serious economical impediments for large-scale applications.
Only very few attempts have been made thus far to overcome
these obstacles. Sheldon has achieved autoxidation of liquid
cyclohexane to cyclohexanone, cyclohexyl hydroperoxide, and
cyclohexanol at high (>90%) selectivity for the combined
Selective partial oxidation of alkanes by O2 is a key challenge
in the manufacture of organic building blocks and industrial
intermediates.1 The reason is that for such large-volume
chemicals, the choice of the oxidant is limited to molecular
oxygen, mainly because of economics. Yet, as radical chain
processes, autoxidations are inherently unselective. The oxida-
tion of cyclohexane by O2 plays a central role in the production
of nylons.1 Moreover, cyclohexanone, one of the two main
products of partial cyclohexane autoxidation (the other is
cyclohexanol), is an intermediate in the synthesis of a variety
of fine chemicals.2 Despite extensive research and development
in cyclohexane autoxidation, the selectivity of current liquid
phase reactions under practical conditions is limited to 70%
7
products at low (3.4%) conversion. The catalyst used was
chromium-substituted aluminophosphate-5. Jacobs found that
the cobalt-substituted version of this molecular sieve catalyst
gives high yields of the same products at 4% conversion of the
c
8
cyclohexane. Ellis and Lyons have reported a general method
for alkane oxidation by O2 that uses a transition metal complex
9
of perfluorotetraphenylporphyrin as catalyst. For isobutane-
to-tert-butyl alcohol autoxidation in benzene solution, for
example, selectivities as high as 95% were achieved.
In this paper we report, for the first time, oxidation of
cyclohexane by O2 to cyclohexanone and its precursor, cyclo-
hexyl hydroperoxide, without side reaction at high conversion
of the alkane. The approach is based on a very large red shift
of the cyclohexane‚O2 charge-transfer absorption in zeolite NaY
which moves the absorption tail from the UV into the visible
region. Access to this low-energy reactant state with visible
photons, coupled with the inhibition of indiscriminate reactions
(combined cyclohexanone plus cyclohexanol yield) at low
1a,c
conversion (9%).
The main problem that prevents selective
synthesis at higher cyclohexane conversion is the overoxidation
of the alcohol and carbonyl products.3
Various new catalytic routes have been explored for cyclo-
hexane-to-cyclohexanol and cyclohexanone oxidation by O2, and
remarkable selectivities in terms of the combined products have
been reported.1
b,e
However, most of these methods either
10
of primary transients by the constraints of the zeolite nanocage
require stoichiometric coreactants (most often a reducing agent)
opens up a highly selective oxidation path. The large red shift
of the cyclohexane‚O2 contact complex is attributed to a 2-eV
stabilization of the excited charge-transfer state by the very high
electrostatic field inside the zeolite cage. We have discovered
or photosensitization with a UV source,4 which may constitute
-6
X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981; Chapters 2 and 11. (b)
ActiVation and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley: New
York, 1989. (c) Parshall, G. W.; Ittel, S. D. Homogenous Catalysis, 2nd
ed.; Wiley: New York, 1992; Chapter 10. (d) Haber, J. In PerspectiVes in
Catalysis; Thomas, J. M., Zamaraev, K. I., Eds.; IUPAC/Blackwell Scientific
Publications: London, 1992; pp 371-385. (e) Catalytic Oxidation,
Principles and Applications; Sheldon, R. A., van Santen, R. A., Eds.; World
Scientific Publishing: Singapore, 1995. (f) Lyons, J. E.; Parshall, G. W.
Catal. Today 1994, 22, 313-333. (g) Dartt, C. B.; Davis, M. E. Ind. Eng.
Chem. Res. 1994, 33, 2887-2899.
this effect recently with toluene‚O2 and olefin‚O2 complexes
11a-e
in alkali and alkaline-earth exchanged zeolite Y
and applied
it to the selective oxidation of a tertiary alkane C-H group.1
1f
II. Experimental Section
Spectroscopic instrumentation, vacuum equipment, and experimental
procedures were the same as previously described.1
1b,d
Briefly, UV-
vis spectra were obtained by diffuse reflectance measurements using a
(2) Szmant, H. H. Organic Building Blocks of the Chemical Industry;
Wiley: New York, 1989.
(7) Chen, J. D.; Sheldon, R. A. J. Catal. 1995, 153, 1-8.
(8) Vanoppen, D. L.; De Vos, D. E.; Genet, M. J.; Rouxhet, P. G.; Jacobs,
P. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 560-563.
(9) (a) Ellis, P. E.; Lyons, J. E. Catal. Lett. 1989, 3, 389-398. (b) Ellis,
P. E.; Lyons, J. E. Coord. Chem. ReV. 1990, 105, 181-193. (c) Lyons, J.
E.; Ellis, P. E. In Metalloporphyrins in Catalytic Oxidations; Sheldon, R.
A., Ed.; Marcel Dekker: New York, 1994; Chapter 10.
(
3) Tolman, C. A.; Druliner, J. D.; Nappa, M. J.; Herron, N. In ActiVation
and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley: New York, 1989;
Chapter 10.
(4) Maldotti, A.; Bartocci, C.; Amadelli, R.; Polo, E.; Battioni, P.;
Mansuy, D. J. Chem. Soc., Chem. Commun. 1991, 1487-1488.
(
(
5) Mu, W.; Herrmann, J. M.; Pichat, P. Catal. Lett. 1989, 3, 73-84.
6) Lu, G.; Gao, H.; Suo, J.; Li, S. J. Chem. Soc., Chem. Commun. 1994,
(10) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry, and
Use; Wiley: New York, 1974.
2
423-2424.
S0002-7863(95)03273-2 CCC: $12.00 © 1996 American Chemical Society

6
874 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Sun et al.
Shimadzu Model 2100 instrument with an integrating sphere set-up
Model ISR-260. Zeolite NaY (LZ-Y52, Aldrich Chemical Co. Lot No.
0
3319TX) was pressed into self-supporting wafers of 12-mm diameter
and a typical weight of 8 mg. The zeolite pellets were mounted inside
a home-built miniature stainless steel cell described previously.1 Prior
1b,d
2
to loading of cyclohexane and O from the gas phase, the zeolite pellet
was dehydrated at 200 °C in the high-vacuum cell for 10-12 h. A
turbomolecular pump Model Varian V-60 was used for evacuation
during heating. All UV-vis measurements were conducted at ambient
temperature (21 °C).
In one series of experiments the zeolite pellet was pretreated with a
small amount of HCl. The gas was prepared on a vacuum line by the
2 4
reaction of concentrated H SO with dry NaCl powder. The dehydrated
zeolite was heated to 200 °C and exposed to 0.6 Torr of the distilled
HCl for 10 min. Thereafter, the pellet was evacuated at that temperature
for 6 h.
2
For monitoring of photochemical reactions, cyclohexane and O gas
were loaded into a dehydrated zeolite pellet inside a miniature vacuum
cell equipped with KBr windows. The chemistry was followed in situ
by FT-infrared spectroscopy using a Bruker Model IFS 113 or an IBM-
Bruker Model IR 44 spectrometer. Zeolite NaY is transparent in the
2
Figure 1. Diffuse reflectance spectrum of cyclohexane and O -loaded
zeolite NaY at room temperature. The curve shows the ratio of the
reflectance of cyclohexane-loaded zeolite after and before exposure to
-
1
infrared except for the region between 1200 and 920 cm , and below
-
1
500 Torr of O gas.
8
00 cm . Photolysis was conducted with the emission of a prism-
tuned continuous-wave Ar-ion laser (Coherent Model Innova 90-5 or
0-6). The laser beam was expanded to the full size of the pellet. For
2
9
only effect was an increase of the reflectance across the entire
visible range due to the accompanying change in the refractive
index of the pellet. In a second step, 500 Torr of O2 gas was
admitted to the UV-vis cell, corresponding to one O2 molecule
experiments above or below room temperature, the infrared cell was
mounted inside a variable-temperature Oxford cryostat Model DN1714
1
1b
(77K to 200 °C).
For determination of the product ratio of cyclohexanone to cyclo-
10
per five supercages. It is this difference of the spectra taken
hexyl hydroperoxide, the extinction coefficient ratio of the two
molecules was estimated by recording infrared spectra upon thermal
conversion of the hydroperoxide to the ketone. Following photoac-
cumulation of cyclohexyl hydroperoxide, the excess cyclohexane and
after and before adding O2 to the NaY pellet that is shown in
Figure 1. Three consecutive spectra showed no change of the
signal. This indicates that possible photochemical reaction
induced upon recording by the spectrometer light source has
no effect on the absorption profile. The intensity of the
absorption was found to be linear in the partial pressure of O2.
From these observations we conclude that the small but
significant absorption band of Figure 1 originates from a
cyclohexane‚O2 complex in the zeolite.
2
O were pumped off and the zeolite temperature was raised to 100 °C
for several hours. The difference spectrum of the conversion, acceler-
ated by the high temperature, allowed us to determine the relative
-1
integrated intensities of the 1367-cm cyclohexyl hydroperoxide band
-
1
and the 1705/1674-cm doublet of cyclohexanone with an accuracy
of 10%. Infrared spectra were calibrated for cyclohexanone yields by
manometric measurement of the uptake of the molecule from the gas
phase.
2
. Photooxidation with Visible Light. Upon loading of
cyclohexane into NaY, infrared bands were observed at 2932,
Cyclohexane (99%), cyclohexanone (99%), and cyclohexanol (99%,
-
1
2
851, 1452, 905, and 861 cm . Subsequent addition of 500
all from Aldrich) were distilled in vacuum before use. O
and N gas (99.997%) were obtained from Air Products and used as
received. Cyclohexyl hydroperoxide was synthesized from cyclohexyl
2
(99.99%)
Torr of O2 did not result in any notable spectral changes.
Chemical reaction was observed when irradiating the room
temperature zeolite matrix with green or blue light (514, 488,
or 458 nm emission of an Ar ion laser), but no spectral changes
were noticed when shining light on a zeolite pellet that contained
either only cyclohexane or oxygen. An infrared difference
spectrum following 120 min of photolysis at 458 nm (200 mW
2
1
2
chloride using a literature method.
III. Results
1
. Electronic Absorption. Zeolite NaY has no visible
13
absorption (the material is completely colorless). Figure 1
shows the optical absorption of zeolite NaY loaded with
cyclohexane and oxygen as measured by diffuse reflectance
spectroscopy. It was obtained by first loading the zeolite pellet
at room temperature with the alkane by exposing it to 5 Torr
of the gas. According to the room temperature adsorption
-
2
cm ) is displayed in Figure 2a. The negative features show
the consumption of cyclohexane, while the positive bands
originate from photoproducts. Recording of infrared spectra
of authentic samples of cyclohexyl hydroperoxide, cyclohex-
anone, and H2O in zeolite NaY confirmed that photolysis
product absorptions originate from these three species. No
unassigned product band remained. Specifically, no infrared
band of cyclohexanol was observed. Considering the extinction
coefficient of the alcohol absorptions in NaY in regions free of
cyclohexyl hydroperoxide and cyclohexanone and taking into
account the noise level in those regions, we calculate a lower
limit of 50 for the cyclohexanone to cyclohexanol branching
ratio. Infrared frequencies of photoproducts and of authentic
samples are presented in Table 1. Since authentic spectra of
cyclohexyl hydroperoxide loaded into NaY showed traces of
cyclohexanone, the hydroperoxide spectrum was also recorded
in CD3CN solution. There was excellent agreement between
the spectra in the two media. An exception is the ν(OH) mode
+
isotherm of cyclohexane in Na exchanged faujasite, this
corresponds to 3-4 C6H12 molecules per supercage on aver-
age.14 No optical absorption (λ > 250 nm) resulted from the
1
1
alkane loading. As in the case of olefins and toluene, the
(
11) (a) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1993, 115, 7501-7502.
(
b) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 1812-1820. (c) Sun,
H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 7951-7952. (d)
Blatter, F.; Moreau, F.; Frei, H. J. Phys. Chem. 1994, 98, 13403-13407.
(
e) Blatter, F.; Sun, H.; Frei, H. Catal. Lett. 1995, 35, 1. (f) Blatter, F.;
Sun, H.; Frei, H. Chem. Eur. J. 1996, 2, 385; Angew. Chem., Int. Ed. Engl.
996, 35.
1
(
12) Pritzkow, W.; Grobe, K. H. Chem. Ber. 1960, 93, 2156-2162.
(13) Engel, S.; Kynast, U.; Unger, K. K.; Sch u¨ th, F. In Zeolites and
Related Microporous Materials, Studies in Surface Science and Catalysis;
Weitkamp, J., Karge, H. G., Pfeifer, H., H o¨ lderich, W., Eds.; Elsevier:
Amsterdam, The Netherlands, 1994; Vol. 84, pp 477-483.
-
1
which is red shifted in the zeolite by 200 cm due to strong H
bonding. We conclude that cyclohexyl hydroperoxide, cyclo-
(14) Reference 10, p 602.

Cyclohexanone from Cyclohexane and O2 in a Zeolite
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6875
Table 1. Absorption Frequencies of C
Products and Authentic Samples in NaY (in cm
6
H
12 + O Reaction
2
-
1
)
authentic samplesa
reaction
product
cyclohexyl
hydroperoxide
cyclohexanone
water
3400
3
3
2
2
1
1
1
1
1
400b
200
941
861
705
670
644
468c
454
3200
2941
2859
2937
2863
1705
1674
1643
1469
1453
1464
1451
1
1
426
420
1
1
413d
367
1411d
1367
1
350
1
1
346
313
1346
1342
1313
1
301
1
1
1
267
259
230
1268
1259
1229
907
c
9
9
08
00
900
893
865
839
Figure 2. (a) Infrared difference spectrum of the NaY pellet loaded
892
c
8
63
862
with cyclohexane and O
58 nm (2 h, 200 mW cm ) at room temperature. (b) Prolonged
irradiation of a cyclohexane and O loaded pellet NaY at 488 nm. The
product growth corresponds to reaction of 35% of the cyclohexane
loaded into the zeolite. The trace shows the reaction products only
2
before and after Ar-ion laser photolysis at
-
2
838
4
2
a
All product absorptions are assigned except for a weak band at
-1
1
330 cm whose intensity was not reproducible from one experiment
b c
-1
to another. Very broad, fwhm ≈ 200 cm . Overlapped by decreasing
d
(
difference between spectra before loading of the pellet and after
photolysis and pumping off of excess C 12). (c) Irradiation of an HCl-
pretreated NaY pellet loaded with cyclohexane and O (750 Torr) for
h at 488 nm (500 mW cm-2). Seventy-one percent of the cyclohexane
cyclohexane band. Broad.
6
H
2
5
has been converted. The trace shows the difference between spectra
before loading of the pellet and after irradiation and pumping off of
the excess reactants.
hexanone, and H2O are the exclusive products of visible light-
induced oxidation of cyclohexane by O2 in zeolite NaY. This
holds even upon conversion of as much as 35% of the
cyclohexane loaded into the zeolite, as shown in Figure 2b.
Cyclohexyl hydroperoxide was found to rearrange thermally
to cyclohexanone without side reaction. This process is readily
observed by infrared difference spectroscopy when keeping a
hydroperoxide-loaded NaY matrix in the dark at room temper-
ature. It is strongly accelerated in acid (HCl) treated NaY.
Figure 2c shows the infrared product spectrum upon blue light-
-
2
induced oxidation of cyclohexane (488 nm, 500 mW cm ) in
an HCl pretreated zeolite for 5 h. Comparison with Figure 2a,
where the hydroperoxide to cyclohexanone ratio is about 4:1,
shows that even the most intense cyclohexyl hydroperoxide band
at 1367 cm is barely visible in spectrum 2c. Thermal
rearrangement of cyclohexyl hydroperoxide to cyclohexanone
and water is an established intramolecular heterolytic reaction
in acidic solution.15
-
1
Figure 3. Absorbance growth kinetics of the 1705-cm cyclohexanone
-1
band (curve 1) and the 1367-cm cyclohexyl hydroperoxide absorption
(curve 2) upon 458- and 514-nm photolysis at room temperature. The
size of the circles indicates the peak-to-peak noise.
-
1
diffusion of cyclohexanone into the zeolite.10 Therefore,
adsorption can occur only at the outer surface. By contrast,
Infrared spectra like the ones shown in Figure 2 constitute
direct proof that the products are generated in the interior of
the NaY particles rather than on their outer surface. If one
exposes cyclohexanone at a fixed gas pressure to a pellet of
zeolite NaY and a pellet of NaA particles, a strong infrared
spectrum is observed in NaY while only extremely weak features
appear in the case of NaA. Type Y and type A zeolites both
have a three-dimensional network of R-cages of similar size,
but the window openings of NaA are too small (4 Å) to permit
the window openings of zeolite Y R-cages have a diameter of
7
Å, hence cyclohexanone can readily diffuse into the zeolite
10
particles. We conclude that the infrared product absorptions
shown in our spectra originate exclusively from molecules that
reside inside the zeolite crystallites.
The growth kinetics of the products at two photolysis
wavelengths, 514 and 458 nm, is displayed in Figure 3. The
plot shows the infrared absorbance growth of the ν(CdO) mode
-1
of cyclohexanone at 1705 cm (curve 1) and of the CH2
-1
bending mode of cyclohexyl hydroperoxide at 1367 cm (curve
2). The extinction coefficient ratio of the cyclohexanone doublet
(15) Sheldon, R. A. In The Chemistry of Functional Groups, Peroxides;
Patai, S., Ed.; Wiley: New York, 1983; pp 161-200.

6
876 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Sun et al.
ketone growth at long times signal, again, a slow hydroperoxide
to cyclohexanone interconversion.
Comparison of the product growth curves upon thermal
(Figure 4) and photochemical oxidation of cyclohexane (Figure
3
) shows that these two reaction types can easily be distin-
guished on the basis of the kinetics. At all temperatures the
thermal reaction has an induction period with zero slope at the
start of reaction while the photochemical curves do not.
It is important to note that the heating effect which ac-
companies laser irradiation of the zeolite is far too small to
account for the observed photochemical rates. We have recently
reported several experimental observations which show that
heating by the laser amounts to 10 deg at the most for the laser
-
2
power levels used here (500 mW cm ). The most accurate
estimates for laser heating stem from the desorption effect of
small hydrocarbons monitored by FT-IR spectroscopy.1
1c,e
-
1
Figure 4. Absorbance growth kinetics of the 1705-cm cyclohexanone
IV. Discussion
-1
band (curve 1) and the 1367-cm cyclohexyl hydroperoxide absorption
curve 2) upon thermal reaction at 22, 50, and 80 °C. The size of the
circles indicates the peak-to-peak noise.
(
1. Electronic Absorption of Cyclohexane‚O2 in NaY.
Optical absorption of a cyclohexane‚O2 contact complex can,
in principle, originate from an O2-enhanced triplet absorption
of the hydrocarbon or from an alkane‚O2 charge-transfer
-
1
at 1705/1670 cm and the hydroperoxide absorption at 1367
-1
cm was determined to be 62. Hence, according to the two
growth curves, the cyclohexanone to hydroperoxide branching
ratio is around 0.1 at the start of photolysis. While the
hydroperoxide curve is heading toward an asymptotic limit, the
cyclohexanone growth clearly accelerates with increasing pho-
tolysis time. This behavior has its origin in the slow thermal
rearrangement of cyclohexyl hydroperoxide to cyclohexanone
under elimination of water mentioned above. It is important
to note, however, that not all cyclohexanone seems to originate
from slow thermal rearrangement of C6H11OOH at room
temperature because the cyclohexanone growth does not exhibit
an induction period with zero growth at the start of photolysis.
Comparison of the 514- and 458-nm photolysis curves shows
that the cyclohexanone to cyclohexyl hydroperoxide ratio
increases somewhat at higher photon energies.
16
absorption. The former can readily be ruled out because the
lowest triplet state of small alkanes lies at the threshold to the
17
Vacuum UV. Moreover, studies of phosphorescence spectra
have shown that the lowest triplet state of organics loaded into
zeolites do not exhibit large shifts relative to the solution phase.
18
On the other hand, our previous work on alkene + O2 and
toluene + O2 systems in zeolite Y has revealed that the
corresponding hydrocarbon‚O2 charge-transfer absorptions are
shifted to the red relative to the conventional phase by 1.5 to
19
2
.5 eV.
The onset of the cyclohexane‚O2 charge-transfer
20
absorption in the O2-saturated liquid is at 275 nm, while that
of the absorption in zeolite NaY is around 500 nm according
to Figure 1. Note that no maximum of the absorption band is
discernible in Figure 1, very similar to the case of cyclohexane‚O2
and most other hydrocarbon‚O2 charge-transfer absorptions in
The quantum yield for reaction was calculated as the product
growth per absorbed photon. In one typical photolysis experi-
ment at 458 nm, the absorption of cyclohexane‚O2 at that
wavelength was 0.04%. The incident photon flux corresponded
19-21
-1
the gas phase or solution.
The red shift of 16 000 cm
(2.0 eV) by the zeolite environment is close to that observed
previously in the case of olefin‚O2 and toluene‚O2 complexes.
We attribute the shift to the stabilization of the excited state
with its large dipole moment (C6H12 O2 ) by the high electro-
static field inside the zeolite cage.
The 13-Å supercage of zeolite Y carries a formal negative
charge of seven which resides mainly on the framework oxygen
atoms. It is counterbalanced by Na ions, three to four of which
are located in the supercage. The poor electrical shielding of
these cations by the cage oxygens gives rise to high electrostatic
fields. Such fields have been predicted by model calcula-
-
3
-6
to 8.3 × 10 mol photons (hence 3.3 × 10 mol photons
+
-
absorbed by the C6H12‚O2 pairs) over an irradiation period in
-
1
which the infrared absorbance growth was 0.016 at 1705 cm
cyclohexanone) and 0.012 at 1367 cm-1 (cyclohexyl hydro-
peroxide). Taking into account the extinction coefficient of the
(
+
-
7
absorptions, a total product growth of 1.0 × 10 mol was
10
-6
calculated for the 3.3 × 10 mol of 458-nm photons absorbed.
Hence, the quantum yield to reaction is 0.03. This efficiency
may be off by as much as a factor of 3 due to the large
uncertainty in the absorption at 458 nm as estimated from the
diffuse reflectance spectrum.
(16) Tsubomura, H.; Mulliken, R. S. J. Am. Chem. Soc. 1960, 82, 5966-
974.
5
(
17) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd ed.; Marcel Dekker: New York, 1993.
18) Ramamurthy, V. In Photochemistry in Organized and Constrained
Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; pp 429-
93.
19) Any alkane, alkene, or aromatic hydrocarbon has a contact charge-
3
. Thermal Oxidation. When keeping a cyclohexane and
O2-loaded NaY matrix in the dark at room temperature for 10
h, a very small growth of cyclohexyl hydroperoxide was noticed.
After about 20 h, cyclohexanone growth was observed as well.
An experiment with 18O2 gave exclusively cyclohexanone- O,
which confirms that no lattice oxygens are involved in the
reaction. The kinetics of the reaction at room temperature is
shown in Figure 4. While this thermal reaction is barely
noticeable at 22 °C, rates increase steadily with temperature.
Curves corresponding to thermal reaction at 50 and 80 °C are
also displayed in Figure 4. At temperatures up to 80 °C, no
products other than cyclohexyl hydroperoxide, cyclohexanone,
and water are observed. The slightly increased rates of the
(
4
(
18
transfer band with oxygen, yet no stable complex is formed. The situation
is similar to contact charge-transfer bands of alkanes and molecular iodine.
These bands appear most often as structureless tails with no distinct
absorption maximum, a direct consequence of the absence of a complex
with defined geometry. For a general reference of contact charge-transfer
absorptions, see: Mulliken, R. S.; Person, W. B. Molecular Complexes;
Wiley: New York, 1969; Chapters 1 and 14.
(
20) (a) Chien, J. C. W. J. Phys. Chem. 1965, 69, 4317-4325. (b)
Scurlock, R. D.; Ogilby, P. R. J. Phys. Chem. 1989, 93, 5493-5499.
21) Davis, K. M. C. In Molecular Association; Foster, R., Ed.; Academic
Press: New York, 1975; Vol. 1, pp 151-213.
(

Cyclohexanone from Cyclohexane and O2 in a Zeolite
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6877
Scheme 1
with the large dipole generated upon excitation of the
cyclohexane‚O2 pair to the charge-transfer state results in a
strong stabilization of the excited state. Assuming a separation
+
-
of 4 Å of the cycloalkane and O2 charge centers (dipole
Figure 5. Induced infrared fundamental absorption of N
2
loaded into
-1
moment µ ) 14 D), we calculate for a field of 0.4 V Å
a
zeolite NaY at -78 °C. The loading level corresponds to 1.3 molecules
per supercage.
dipole stabilization (-µ‚E) of 1.6 eV if the dipole is oriented
parallel to the field.
tions22,23 and established experimentally by several different
It is important to keep in mind that cyclohexane‚O2 contact
complexes, while existing at high steady-state concentrations
in zeolites, are very short lived. The residence time of an O2
molecule in a faujasite supercage at room temperature is
estimated to be less than a nanosecond, that of cyclohexane
tens of nanoseconds. These estimates are based on self-diffusion
coefficients of O2 and cyclohexane for the neat compounds
reported in the literature. In our experiments, there is at least
one cyclohexane molecule per supercage and, hence, an O2
hopping from one cage to the next will inevitably collide again
with a hydrocarbon molecule. Therefore, the concentration of
reactant collisional pairs is very high in this nanopore matrix.
Some fraction of the collisional pairs will be oriented in such
a way that the large dipole of the excited charge-transfer state
is parallel to the cage electrostatic field; these are the states
that will be stabilized and absorb in the visible. Other
cyclohexane‚O2 contact pairs will be oriented close to perpen-
dicular or antiparallel to the field. The excited states of these
pairs will be unaffected by the field or even destabilized.
Therefore, only a fraction of the hydrocarbon‚O2 collisional pairs
(those parallel to the cage field) will contribute to the visible
absorption tail. This is presumably the main reason why the
absorption tail is weak (Figure 1).
methods. One method consists of the measurement of induced
infrared bands of O2 or N2 stretch fundamentals. These infrared
forbidden modes become active in the presence of the cage
electrostatic field when the molecules are loaded into a cation-
exchanged zeolite, and magnitudes of the fields can be
determined from band intensities. This effect has been used
by Cohen de Lara to measure electrostatic field strengths in
3
0
2
4
zeolite A. We have applied the method to cation-exchanged
zeolite Y and found that the electrostatic fields experienced by
-
1
N2 or O2 are around 0.4 V Å . The induced absorption of N2
loaded into a NaY matrix at -80 °C is shown in Figure 5. The
-
1
band exhibits only a very small shift of 4 cm relative to the
gas-phase frequency. This shows that there is no chemical
interaction between the molecule and the zeolite, which makes
N2 an ideal probe for the cage electrostatic fields. A full report
on electrostatic field measurements in alkali and alkaline-earth
zeolite Y will be presented in a forthcoming paper.25 Another
method is measurement of hyperfine coupling constants of guest
26
radicals by ESR spectroscopy. Electrostatic fields in the range
-
1
0
.2-1.0 V Å were found in alkali-exchanged zeolite Y by
this method. Fields of one to several volts per angstrom have
been confirmed in NaA by analysis of electron densities from
X-ray data,27 in alkali and alkaline-earth zeolite X and Y by
2. Reaction Mechanism. The proposed mechanism for the
28
photochemical C H +O reaction is shown in Scheme 1. The
heats of adsorption of rare gases, and by the measurement of
6
12
2
2
9
initial step following excitation of the charge-transfer state is
very likely proton transfer from the cyclohexane radical cation
frequency shifts of the infrared fundamental of CO. The
interaction of the very high electrostatic field inside a NaY cage
-
to O2 . Cyclohexane radical cation is a spectroscopically
(
22) Dempsey, E. In Molecular SieVes; Society of Chemical Industry:
31
established transient with a lifetime of just 300 ps with respect
London, 1968; pp 293-305.
32
to deprotonation in the neat liquid at room temperature. It is
expected that the proton transfer is even faster in the presence
(
23) Preuss, E.; Linden, G.; Peuckert, M. J. Phys. Chem. 1985, 89, 2955-
2
961.
(
24) (a)Barrachin, B.; Cohen de Lara, E. J. Chem. Soc., Faraday Trans.
-
of a base like O2 . Hydrocarbon radical cations are known to
2
1986, 82, 1953-1966. (b) Cohen de Lara, E.; Kahn, R.; Seloudoux, R.
3
3
be highly acidic in general. We consider efficient proton
transfer quenching of the charge-transfer pair as the main reason
for the rather high quantum yield to reaction. Cyclohexyl and
hydroperoxy radicals so produced are expected to undergo cage
recombination to yield the observed cyclohexyl hydroperoxide.
J. Chem. Phys. 1985, 83, 2646-2652. (c) Kahn, R.; Cohen de Lara, E.;
M u¨ ller, K. D. J. Chem. Phys. 1985, 83, 2653-2660. (d) Bras, N.; Cohen
de Lara, E. J. Chem. Phys. 1995, 102, 6990-6999.
(25) Blatter, F.; Frei, H. To be submitted. Full details of experiment and
calculation will be presented in this paper.
26) (a) Coope, J. A. R.; Gardner, C. L.; McDowell, C. A.; Pelman, A.
(
I. Mol. Phys. 1971, 21, 1043-1055; (b) Sugihara, H.; Shimokoshi, K.;
Yasumori, I. J. Phys. Chem. 1977, 81, 669-673.
(30) K a¨ rger, J.; Ruthven, D. M. Diffusion in Zeolites; Wiley: New York,
1992, Chapter 13.
(
27) Spackman, M. A.; Weber, H. P. J. Phys. Chem. 1988, 92, 794-
7
1
1
96.
(31) Iwasaki, M.; Toriyama, K.; Nunome, K. Faraday Discuss. Chem.
Soc. 1984, 78, 19-33.
(28) Huang, Y. Y.; Benson, J. E.; Boudart, M. Ind. Eng. Chem. Fundam.
969, 8, 346-353.
(32) Sauer, M. C.; Werst, D. W.; Jonah, C. D.; Trifunac, A. D. Radiat.
Phys. Chem. 1991, 37, 461-467.
(
29) (a) Angell, C. L.; Schaffer, P. C. J. Phys. Chem. 1966, 70, 1413-
418. (b) Zecchina, A.; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnelli, L.
(33) Hammerich, O.; Parker, V. D. AdV. Phys. Org. Chem. 1984, 20,
55-189.
J. Phys. Chem. 1994, 98, 9577-9582.

6
878 J. Am. Chem. Soc., Vol. 118, No. 29, 1996
Sun et al.
While our observations clearly show that cyclohexyl hydro-
infrared and NMR spectroscopic studies of hydrocarbons in
1
8,41
peroxide rearranges to cyclohexanone in a slow thermal process,
the non-zero slope of the cyclohexanone growth curve indicates
that some of the ketone emerges concurrently with cyclohexyl
hydroperoxide as well (Figure 3). We attribute it to instanta-
neous elimination of H2O from the photochemically produced
C6H11OOH before the excess energy is drained off by the cage
environment.
faujasites.
Restriction to hopping among cations may
prevent occupation and turnover of unsaturated hydrocarbons
at radical defect sites, in contrast to alkanes which show no
4
2
preference for trajectories along cation sites.
Since we lack any independent evidence for radical defect
sites, we consider explanation in terms of supercage sites with
exceptionally high electrostatic fields more likely. Fields
calculated on the basis of point charge models were found to
A mechanism analogous to the one shown in Scheme 1 has
previously been proposed for the initial events of the UV light-
induced oxidation (λ < 260 nm) of cyclohexane by O2 in the
neat liquid.34 However, secondary photolysis of the cyclohexyl
hydroperoxide and random radical coupling reactions were
found to destroy the selectivity already at very low conversion.
The observation of a slow thermal reaction of cyclohexane
with O2 in NaY is surprising in view of the fact that
hydrocarbons with weaker C-H bonds and with lower ioniza-
tion potentials than cyclohexane lack such a dark reaction. For
example, the bond energy of the benzylic CH group of toluene
+
be extremely high in the vicinity of Na ions located at site III
10
inside the supercage. For example, the field is estimated at 3
-1
+
22,23
V Å at a distance of 1 Å from the Na ion surface.
Cages
+
with naked Na (III) sites are probably quite rare because any
remaining water molecules will be attracted to these highest
field cations and shield their charge. Cyclohexane‚O pairs
2
+
exposed to the field at a Na (III) site (and oriented parallel to
the field) may spontaneously convert to alkane radical cation
-
-1
and O2 because a 3 V Å field will cause the charge-transfer
state to fall energetically below the ground state of the neutral
contact pair. Cyclohexyl hydroperoxide and cyclohexanone
would be formed according to a path similar to Scheme 1. This
dark reaction may be blocked for unsaturated hydrocarbons such
as olefins and aromatics because molecules with π systems are
-
1
-1
35
is 85 kcal mol versus 94 kcal mol for cyclohexane. The
ionization potential of toluene is a mere 8.8 eV compared to
9
.8 eV in the case of C6H12.36 Yet, toluene does not even react
thermally with O2 in zeolite NaY and BaY at a temperature as
1
1c
+
high as 80 °C.
Similarly, propylene and cis- or trans-2-
known to be strongly attracted to the Na ions with very high
butenes are not oxidized thermally by O2 in NaY or BaY despite
the fact that the energy of the allylic C-H bond is 7 kcal lower
than that of cyclohexane35 and the ionization potentials are
fields. The result is an antiparallel orientation of the hydro-
carbon‚O2 collision contact complex with respect to the
electrostatic field, preventing stabilization of the charge-transfer
state.
3
6
smaller as well (9.7 (C3H6) and 9.13 eV (C4H8)).
This
suggests to us that the thermal reaction of cyclohexane with O2
occurs at sites, or for orientations, that are not accessible to the
olefins or toluene.
V. Conclusions
We can only speculate on the nature of these sites and as to
why they do not promote thermal reaction of unsaturated
hydrocarbons. Sensitive tests37 for Br o¨ nsted or Lewis acid sites
This is, to our knowledge, the first observation of completely
selective oxidation of cyclohexane by O2 to cyclohexanone and
cyclohexyl hydroperoxide, its precursor. Several factors con-
tribute to the tight control of the reaction. One is the very strong
stabilization of the excited alkane‚O2 charge-transfer state by
the electrostatic field of the zeolite cage. This allows the use
of low-energy visible instead of UV photons to access the
excited state which, in turn, results in minimal excess energy
in the primary products (cyclohexyl, HOO radicals). It may
prevent diffusion out of the cage and subsequent random
coupling reactions, or homolytic fragmentation of the cyclohexyl
hydroperoxide intermediate. Moreover, visible photons cannot
induce secondary photolysis of the hydroperoxide. None of the
liquid phase byproducts, like cyclohexanol, are observed. Loss
of product selectivity in liquid cyclohexane autoxidation stems
from coupling of two cyclohexyl peroxy radicals and from
homolysis or secondary bimolecular chemistry of cyclohexyl
+
such as the formation of NH4 or pyridinium ions upon
adsorption of ammonia or pyridine into our NaY pellets were
1
1
negative. A very specific test for acid sites, namely oligo-
38
merization of propylene and other small olefins, also gave a
1
1
negative result. Moreover, Trifunac has demonstrated that
NaY is an inert zeolite that can stabilize species as reactive as
hydrocarbon radical cations.39 While acid sites can be ruled
out, a small concentration of radical defects such as unterminated
Si or Al bonds might be present in these pellets, and efficient
turnover at such sites could explain the slow thermal reaction
of cyclohexane. The radical sites may abstract a H from
cyclohexane, which would be followed by addition of O2 to
the cyclohexyl radical to form a cyclohexyl peroxy radical. The
latter is expected, in turn, to abstract H from cyclohexane to
yield the observed cyclohexyl hydroperoxide and another
cyclohexyl radical. This is the familiar chain propagation of
cyclohexane autoxidation in the liquid phase with an activation
1
a,c,3
hydroperoxide.
These processes are suppressed by the
positional constraints imposed by the zeolite matrix. The
importance of the positional constraints in terms of product
control is highlighted by the selectivity of the thermal cyclo-
hexane oxidation as well. This very mild method of cyclohex-
ane photooxidation by O2 opens up selective activation of
secondary C-H bonds of light alkanes.
-
1 3,40
energy of 18 kcal mol .
A plausible explanation for the
lack of a similar thermal olefin or toluene oxidation in NaY
would be the fact that these hydrocarbons reside preferentially
at cation sites in the zeolite cage. This is well established by
Successful upscaling of our experiments with micromolar
quantities reported here requires (i) a reduction of the scattering
of photolysis light and (ii) operating conditions that allow
continuous desorption of the products from the zeolite host. The
preferred solution for the light-scattering problem would be
(
34) (a) Kulevsky, N.; Sneeringer, P. V.; Grina, L. D.; Stenberg, V. I.
Photochem. Photobiol. 1970, 12, 395-403. (b) Stenberg, V. I.; Sneeringer,
P. V.; Nin, C.; Kulevsky, N. Photochem. Photobiol. 1972, 16, 81-87. (c)
The first photooxidation of an alkane by UV excitation of alkane‚O2 contact
2
0a
charge-transfer complexes was reported by Chien.
(
(
35) Kerr, J. A. Chem. ReV. 1966, 66, 465-500.
36) CRC Handbook of Chemistry and Physics, 53rd ed.; Weast, R. C.,
Ed.; The Chemical Rubber Co.: Cleveland, OH, 1972; p E-62.
(41) (a) Datka, J. J. Chem. Soc., Farad. Trans. 1 1981, 77, 1309-1314.
(b) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311-
1318. (c) Auerbach, S. M.; Henson, N. J.; Cheetham, A. K.; Metiu, H. I. J.
Phys. Chem. 1995, 99, 10600-10608.
(42) Yashonath, S.; Thomas, J. M.; Nowak, A. K.; Cheetham, A. K.
Nature 1988, 331, 601-604.
(
(
(
(
37) Corma, A. Chem. ReV. 1995, 95, 559-614.
38) Farneth, W. E.; Gorte, R. J. Chem. ReV. 1995, 95, 615-635.
39) Qin, X. Z.; Trifunac, A. D. J. Phys. Chem. 1990, 94, 4751-4754.
40) Korcek, S.; Chenier, J. H. B.; Howard, J. A.; Ingold, K. U. Can. J.
Chem. 1972, 50, 2285-2297.

Cyclohexanone from Cyclohexane and O2 in a Zeolite
J. Am. Chem. Soc., Vol. 118, No. 29, 1996 6879
translucent zeolite membranes. Such membranes have been
reported very recently for pentasil-type zeolites (ZSM-5).
Although release of small oxygenated hydrocarbons from zeolite
Y by polar organic solvents is routine,44 a solvent-free method
would be preferable. Use of a carrier gas and modestly elevated
temperatures may be sufficient to effect desorption of the polar
products at acceptable rates. A more detailed discussion of
challenges for scale up is presented elsewhere.
4
3
4
5
Acknowledgment. This work was supported by the Director,
Office of Energy Research, Office of Basic Energy Sciences,
Chemical Sciences Division, of the U.S. Department of Energy
under Contract No. DE-AC03-76SF00098.
(
43) Kiyozumi, Y.; Maeda, K.; Mizukami, F. Workshop on Cluster
Science; National Institute for Advanced Interdisciplinary Research: Tsuku-
ba, Japan, March 1995.
(44) Thibault-Starzyk, F.; Parton, R. F.; Jacobs, P. A. In Zeolites and
Related Microporous Materials, Studies in Surface Science and Catalysis;
Weitkamp, J., Karge, H. G., Pfeifer, H., H o¨ lderich, W., Eds.; Elsevier:
Amsterdam, The Netherlands, 1994; Vol. 84, pp 1419-1424.
JA953273G
(45) Frei, H.; Blatter, F.; Sun, H. CHEMTECH 1996, 26, 24-30.