Photochemistry of 4-chlorophenol and 4-chloroanisole adsorbed on MFI zeolites: Supramolecular control of chemoselectivity and reactive intermediate dynamics

Article abstract of DOI:10.1021/ol1010906

The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactiv

Full text of DOI:10.1021/ol1010906

ORGANIC
LETTERS
2
010
Vol. 12, No. 13
062-3065
Photochemistry of 4-Chlorophenol and
-Chloroanisole Adsorbed on MFI
3
4
Zeolites: Supramolecular Control of
Chemoselectivity and Reactive
Intermediate Dynamics
,
†
‡
Jos e´ P. Da Silva,* Steffen Jockusch, Jos e´ M. G. Martinho,
§
|
M. Francesca Ottaviani, and Nicholas J. Turro
‡
Faculdade de Ci eˆ ncias e Tecnologia, UniVersidade do AlgarVe, Campus de Gambelas,
005-139 Faro, Portugal, Department of Chemistry, Columbia UniVersity, New York,
8
New York 10027, Centro de Qu ´ı mica F ´ı sica Molecular, Instituto Superior T e´ cnico,
AVenida RoVisco Pais, 1049-001 Lisboa, Portugal, and Department of Geological
Sciences, Chemical and EnVironmental Technologies, UniVersity of Urbino, Localita`
Crocicchia, 61029 Urbino, Italy
jpsilVa@ualg.pt
Received May 12, 2010
ABSTRACT
The phototransformation of aryl chlorides adsorbed on MFI zeolites is markedly different from that observed in solution and other solid
surfaces such as silica. The formation of radical cations and the constraints imposed by the channels shift the reactivity from the C-Cl bond
to the O-R bond. Irradiation generates kinetically stabilized intermediates that can be characterized using conventional steady-state spectroscopic
techniques, and these intermediates can be used as ordinary chemical reagents.
MFI zeolites are microporous crystalline aluminosilicates with
the general formula M (AlO (SiO , where M is the charge
compensation cation. Their internal surface consists of an
interconnected network of cylindrical channels (diameter ca.
5.5 Å) and intersections (diameter ca. 9 Å). The external surface
is composed of a nonporous framework and pore openings. The
pore openings on the external surface provide access to the
channels and intersections of the internal surface. As a result,
only molecules whose size/shape characteristics allow penetra-
tion into the holes and whose mobility allows diffusion inside
x
,2
2 x
)
2 y
)
1
†
Universidade do Algarve.
Columbia University.
Instituto Superior T e´ cnico.
University of Urbino.
‡
§
|
(
1) Keager, J.; Ruthven, D. M. Diffusion in Zeolites and Related
Microporous Materials; Wiley: New York, 1992; Chapter 4, pp 467-
12.
(2) Brecker, D. W. Zeolite Molecular SieVes: Structure, Chemistry and
Use; John Wiley & Sons: New York, 1974.
5
10.1021/ol1010906  2010 American Chemical Society
Published on Web 06/08/2010

-
5
the channels will be adsorbed efficiently on the internal surface.
This remarkable size/shape selectivity provides zeolites with
their selective molecular sieving properties.
initially evacuated to ∼10 mmHg (see Figure S1, Sup-
porting Information). Ground state UV-vis diffuse reflec-
tance studies of nonirradiated samples reveal only the guest;
no other absorption bands (charge transfer, products) were
detected in the spectral region of 250-800 nm. Irradiation
leads to the formation of new species that were found to
persist for several hours by UV-vis spectroscopy. This long
lifetime allowed us to study the reaction intermediates using
conventional ground state spectroscopic techniques.
2
Investigations of the photochemical reactions of molecules
adsorbed on MFI zeolites have provided insights to the
supramolecular structure and dynamics of intermediates
3
-5
produced by photolysis.
The restricted mobility within
zeolite pores limits the tendency of radicals, radical cations,
or other reactive intermediates to undergo radical-destroying
bimolecular reactions and prevents the access to adventitous
reagents (e.g., water and oxygen) that would cause their
decay in solution. As a result, the trapped reactive intermedi-
ates can have lifetimes long enough to be studied by
conventional spectroscopic techniques usually reserved for
ordinary stable molecules. Carbon centered radicals, for
example, can live several hours and be studied spectroscopi-
The photochemistry of 4-CP on silicalite was reported
previously. The formation of 4-chlorophenoxyl (4-CPO)
9
radicals was observed by diffuse reflectance laser flash
photolysis. However, no net reaction of 4-CP was observed
by product studies after photolysis, where the samples were
extracted with methanol and analyzed by GC-MS. This
suggests that 4-CP is regenerated by hydrogen abstraction
of 4-CPO radicals from the solvent (methanol) used for
extraction (eq 1).
6
cally on silica and MFI zeolite surfaces.
The phototransformation of aryl chlorides is well esthablished
7-11
in solution and on silica and cellulose surfaces.
The primary
photochemical reaction step is the homolytic cleavage of the
C-Cl bond to produce a triplet radical pair. In polar
environments, electron transfer within the caged radical pair
might occur, which leads to an ion pair (phenyl cation +
-
8
Cl ). The formation of radical cations was also observed
8
in polar solvents. The main reaction intermediates involved in
We detected similar UV-vis absorption on all studied
zeolites after Hg lamp irradiation (254 nm). The difference
the phototransformation of aryl chlorides are phenyl radicals (1),
phenyl cations (2), and radical cations (3) (Figure 1).
7-11
diffuse reflectance spectrum, F(R)irradiated - F(R)nonirradiated
,
shown in Figure 2, obtained for photolysis of 4-CP on Li-
Figure 1. Main photochemical intermediates of aryl chlorides in
2
solution. Intermediate 1 possesses a single electron in a sp orbital
localized on the 4-carbon atom. Intermediate 2 possesses an empty
2
sp orbiatal localized on the 4-carbon atom. Intermediate 3 possesses
Figure 2. Difference diffuse reflectance spectrum of 4-CP on Li-
ZSM5 before and after photolysis at 254 nm. The main peak is
assigned to the 4-CPO radical.
a half-filled π orbital delocalized over all of the carbon atoms of
the benzene ring.
We report here a UV-vis absorption, EPR, and products
study of the aryl chlorides chlorobenzene (CB), 4-chlorophe-
nol (4-CP), and 4-chloroanisole (4-CA) on silicalite, Na-
ZSM-5, and Li-ZSM-5.
ZSM5, shows a vibrational fine structure with maxima at
4
16 and 397 nm, a feature that is characteristic of 4-CPO
12
radicals. The original diffuse reflectance spectra before
subtraction are shown in Figure S2 of the Supporting
(
(
(
(
3) Scaiano, J. C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783.
4) Turro, N. J. Acc. Chem. Res. 2000, 33, 637.
5) Garcia, H.; Roth, H. D. Chem. ReV. 2002, 102, 3947.
6) Hirano, T.; Li, W.; Abrams, L.; Krusic, P. J.; Ottaviani, M. F.; Turro,
N. J. J. Org. Chem. 2000, 65, 13.
7) Grabner, G.; Richard, C.; Kohler, G. J. Am. Chem. Soc. 1994, 116,
1470.
8) Da Silva, J. P.; Jockusch, S.; Turro, N. J. Photochem. Photobiol.
Sci. 2009, 8, 210.
(
1
The adsorption of aryl chlorides on MFI zeolites was
performed by gas phase adsorption after the zeolite was
(
Org. Lett., Vol. 12, No. 13, 2010
3063

Information. The peak at ∼290 nm is also assigned to the
We find that the major photo product 4-CPO radical was
only observed in significant amounts if the zeolite was dried
by calcination at 500 °C for 12 h followed by evacuating to
12
4
-CPO radical. The minimum of the difference absorption
spectrum near 275 nm is probably caused by depletion of
ground state absorption of 4-CP.
Little change in the spectra is observed after opening the
sample cell to air. In fact, 4-CPO radicals are found to be stable
-5
10 mmHg for 8 h before loading the sample onto the zeolite
(see Supporting Information). Without this temperature and
vacuum treatment before probe loading, only small amounts
of 4-CPO radicals were observed by UV spectroscopy after
photolysis. In the presence of air and water, GC-MS analysis
of the methanolic extracts showed the formation of benzo-
quinone and hydroquinone (Figure S4, Supporting Informa-
tion), which are the same photo products reported on silica
(>12 h) under atmospheric (1 atm of air) oxygen. EPR spectra
of photolyzed samples of 4-CP on Li-ZSM5 were recorded.
The observed poorly resolved EPR spectrum (Figure 3, black
1
0
surfaces. In this case, the primary reaction intermediate is
a phenyl cation (2, Figure 1). This intermediate forms a
7
,10
carbene
that can react with molecular oxygen and/or
water, leading to benzoquinone and hydroquinone, respec-
tively (eq 2).
Figure 3. EPR spectrum of 4-CPO radicals (black line) formed by
photolysis (254 nm) of 4-CP on Li-ZSM5. Simulated spectrum (red
line) using coupling constants shown in inset.
The adsorption of 4-CP into the zeolite channels changes
the chemoselectivity of reaction compared to that of solution.
For example, C-Cl cleavage is observed in the photolysis
7
of 4-CP in aqueous solution and on surfaces such as silica
1
0
line) is consistent with 4-CPO radicals immobilized on surfaces.
The coupling constants between the spin of the odd electron
and cellulose, whereas for 4-CP in zeolite channels O-H
cleavage occurs. This observation excludes transients 1 and
H
and the proton nuclear spins (a ) determined by spectral
2
(Figure 1) as main intermediates since the products would
simulation (Figure 3, red line) are in good agreement with
not possess a chlorine atom, leaving radical cations (3) as
the key plausible chemical intermediates. The formation of
radical cations of organic molecules in zeolites is quite
13,14
literature values for 4-CPO radicals in aqueous solution.
The calculated coupling constant between the electron spin
and chlorine nuclear spin (aCl) is slightly higher (3 G)
5
,16,17
common.
To best of our knowledge, the UV-vis
1
3,14
compared to the literature value in solution (1.9 G).
absorption of the 4-CP radical cation is not reported in the
literature. However, we expect an absorption at a similar
spectral region to cation radicals of 4-CA, which are known
Deviations in the coupling constants on zeolite surfaces from
published solution values are probably caused by interactions
of the adsorbed radical with the polar zeolite surface and
metal cations. Significant modifications of the spin density
in the phenyl ring by interactions between the radical and
8
to absorb near 475 nm. In Figure 2, no absorption near 475
nm was observable. The lack of detectable cation radicals
for 4-CP is probably due to a fast deprotonation process, a
commonly observed reaction for radical cations containing
15
metal ions or polar groups of the zeolite surface are known.
18
For details about the EPR analysis and simulation, see the
Supporting Information.
OH groups. Deprotonation leads to 4-CPO radicals (eq 3).
Addition of methanol vapor to the irradiated samples
quenches the 4-CPO radicals as seen by the disappearance of
its absorption at ∼400 nm (Figure S3, Supporting Information).
These radicals abstract hydrogen from methanol and regenerate
the starting material 4-CP (eq 1). Therefore, no net reaction is
observed by GC-MS analysis of these samples.
In the case of CB or 4-CA, which do not contain OH
groups for deprotonation, radical cations are readily observ-
(
9) Da Silva, J. P.; Ferreira, L. F. V.; Da Silva, A. M.; Oliveira, A. S.
J. Photochem. Photobiol. A: Chem. 2002, 151, 157.
10) Da Silva, J. P.; Ferreira, L. F. V.; Da Silva, A. M.; Oliveira, A. S.
EnViron. Sci. Technol. 2003, 37, 4798.
11) Da Silva, J. P.; Ferreira, L. F. V.; Osipov, I.; Machado, I. F. J.
Hazard. Mater. 2010, 179, 187.
12) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98,
8,19
able near 450 to 475 nm.
Photolysis of CB and 4-CA
adsorbed on dry zeolites yielded detectable cation radicals
(
(Figure 4 and S5, Supporting Information). For wet zeolites
(
(
(16) Corma, A.; Fornes, V.; Garcia, H.; Marti, V.; Miranda, M. A. Chem.
6
343.
Mater. 1995, 7, 2136
(17) Morkin, T. L.; Turro, N. J.; Kleinman, M. H.; Brindle, C. S.;
Kramer, W. H.; Gould, I. R. J. Am. Chem. Soc. 2003, 125, 14917
(18) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. Engl. 1997,
36, 2550.
(19) Mohan, H.; Mittal, J. P. Chem. Phys. Lett. 1995, 235, 444
.
(
(
13) Stone, T. J.; Waters, W. A. J. Chem. Soc. 1964, 213
.
14) Dixon, W. T.; Moghimi, M.; Murphy, D. J. Chem. Soc., Faraday
.
Trans. 2 1974, 70, 1713–1720
15) Moscatelli, A.; Liu, Z.; Lei, X.-G.; Dyer, J.; Abrams, L.; Ottaviani,
M. F.; Turro, N. J. J. Am. Chem. Soc. 2008, 130, 11344.
.
(
.
3064
Org. Lett., Vol. 12, No. 13, 2010

Figure 5
main reaction steps.
. Possible guest orientations in the zeolite channels and
Figure 4
0.2% w/w of water before and after photolysis at 254 nm. The
assigned chemical structures of the bands as shown.
. Difference absorption spectrum of 4-CA on silicalite with
<
1
0
leads to hydroquinone for 4-CP and 4-hydroxyanisole for
1
1
4
-CA. However, these products were not observed for
samples prepared with the addition of water (<0.2%, w/w)
Supporting Information). The photoproduct distributions
instead indicate that C-Cl bond is stabilized relative to the
O-R bond. In agreement with the main final product, 4-CPO
radicals were observed for both compounds, 4-CP and 4-CA.
This suggests that supramolecular structure II (Figure 5) is
the preferred configuration.
(
<0.2%, w/w) in addition to cation radicals, the absorption
bands of phenoxyl radicals and 4-CPO radicals were
observed for CB and 4-CA, respectively (Figure 4 and S5,
Supporting Information). Photoproduct studies show the
formation of phenol and 4-CP, for CB and 4-CA, respectively
(
(
eq 4, Figure S6, Supporting Information), thus confirming
the assignments (Figures S6-S8, Supporting Information).
Orientation II easily explains the observed spectra and
products. After the formation of the radical cation, depro-
1
8
tonation occurs (eq 3), leading directly to the 4-CPO
radical. In the presence of water and for substituents other
than H, the radical cation reacts with water (nucleophile)
and also forms 4-CPO radicals (Figure 5), probably through
2
0
an OH-adduct.
In conclusion, spectroscopic and product analysis data
showed that the photo reaction of oxygen-containing aryl
chlorides adsorbed on ZSM-5 zeolites depends on the
orientation of the guest at the adsorption sites. This orienta-
tion, together with the formation of the aryl chloride radical
cations and the constraints imposed by the channels, shifts
the photoreactivity from the C-Cl bond, well established
in solution and on surfaces such as silica, cellulose and
cyclodextrin, to the O-R bond. Orientation II (Figure 5) is
considered to be the more plausible supramolecular structure
involved in the photochemistry.
Analogous to 4-CP, the adsorption on the zeolite surfaces
shifts the photoreactivity of 4-CA from the C-Cl bond to
the O-CH bond.
3
The formation of benzene in the case of photolysis of CB
and anisole in the case of 4-CA (eq 4 and Figure S6,
Supporting Information) in methanol (minor products, Fig-
ures S7 and S8, Supporting Information) suggests the
presence of phenyl radicals in the irradiated samples. The
final reduction products are formed after hydrogen abstraction
from methanol used for extraction (eq 4).
Two plausible orientations for the initial adsorption of the
4
-substituted chlorobenzenes are considered: (I) C-Cl bond
Acknowledgment. J.P.S. thanks the Luso-American Foun-
dation for financial support (project FLAD/NSF ref 600-13/
006). The authors at Columbia thank the National Science
Foundation for support through grants NSF-CHE 04-15516
and NSF-CHE 07-17518.
outside the pore or near an aluminum/exchange cation; (II)
C-Cl bond adsorbed into the pore (Figure 5). In both cases,
it is assumed that a cation, M , is present near the pore
2
+
opening. In general, under ambient conditions, water mol-
ecules will be available through hydration of the cation.
Supramolecular structure I is considered less plausible based
on the following observations. First, interaction between the
C-Cl of the aryl chloride and the water/zeolite surface is
expected to be weaker for I than with II, the latter allowing for
hydrogen bonding. In orientation I, the C-Cl bond experiences
a polar environment, and therefore the expected photoreactivity
of 4-CP and 4-CA would be that of the phenyl cation (Figure
Supporting Information Available: Experimental pro-
cedures, diffuse reflectance spectra, GC-MS data, details on
EPR experiments and simulation. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL1010906
(
20) Quint, R. M.; Park, H. R.; Krajnik, P.; Solar, S.; Getoff, N.;
7-11
1, structure 2).
When water is present, this intermediate
Sehested, K. Radiat. Phys. Chem. 1996, 47, 835.
Org. Lett., Vol. 12, No. 13, 2010
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