Highly selective photochemical and thermal chlorination of benzene by Cl2 in NaZSM-5

Article abstract of DOI:10.1021/jp9814348

Loading of zeolite NaZSM-5 with benzene and chlorine from the gas phase at -100°C resulted in spontaneous reaction to form chlorobenzene and 1,4-dichlorobenzene as the sole products. Thermal reaction at elevated temperature (up to 0°C) accelerates the rates toward these products and yields, in addition, some 1,3-dichlorobenzene and a small (<3%) amount of 1,2-dichlorobenzene. No higher chlorinated benzenes or chlorocyclohexanes are formed even upon 90% conversion of the loaded benzene. Reactions were monitored by in situ FT-infrared spectroscopy. Irradiation of 1,4-dichlorobenzene in the presence of Cl₂ in the zeolite with green or blue light produced 1,2,4-trichlorobenzene as the only product at high conversion. Such selectivities of benzene chlorination by elemental chlorine are much higher than obtained currently by traditional methods. The sharp increase of the reaction rate in NaZSM-5 compared to that in homogeneous liquid or gas phase and the effect of electronic excitation by visible light are attributed to an enhanced electrophilic character of the chlorine molecule.

Full text of DOI:10.1021/jp9814348

7
106
J. Phys. Chem. B 1998, 102, 7106-7111
Highly Selective Photochemical and Thermal Chlorination of Benzene by Cl2 in NaZSM-5
Scott J. Kirkby and Heinz Frei*
Physical Biosciences DiVision, MS CalVin Laboratory, Lawrence Berkeley National Laboratory,
UniVersity of California, Berkeley, California 94720
ReceiVed: March 10, 1998; In Final Form: July 17, 1998
Loading of zeolite NaZSM-5 with benzene and chlorine from the gas phase at -100 °C resulted in spontaneous
reaction to form chlorobenzene and 1,4-dichlorobenzene as the sole products. Thermal reaction at elevated
temperature (up to 0 °C) accelerates the rates toward these products and yields, in addition, some
1
,3-dichlorobenzene and a small (<3%) amount of 1,2-dichlorobenzene. No higher chlorinated benzenes or
chlorocyclohexanes are formed even upon 90% conversion of the loaded benzene. Reactions were monitored
by in situ FT-infrared spectroscopy. Irradiation of 1,4-dichlorobenzene in the presence of Cl in the zeolite
2
with green or blue light produced 1,2,4-trichlorobenzene as the only product at high conversion. Such
selectivities of benzene chlorination by elemental chlorine are much higher than obtained currently by traditional
methods. The sharp increase of the reaction rate in NaZSM-5 compared to that in homogeneous liquid or
gas phase and the effect of electronic excitation by visible light are attributed to an enhanced electrophilic
character of the chlorine molecule.
I. Introduction
that Cl2 addition, but very little substitution, took place.7
Chlorination in the large-pore zeolites NaY or X was found to
Partially chlorinated benzenes play a key role as industrial
intermediates and fine chemicals.1-3 Chlorobenzene is an
industrial solvent and intermediate in the manufacture of
pigments, agricultural products, and pharmaceuticals. Higher
chlorinated benzenes such as 1,4-dichlorobenzene are used for
the production of thermoplastics.1 Trichlorobenzenes, espe-
yield a complex mixture of higher chlorobenzene isomers,
7
including hexachlorobenzene. The only report on a highly
selective chlorination by Cl2 in a zeolite is that of van Dijk et
1
,2
8
al. When exposing a carbon tetrachloride solution of chlorine
to a CCl4 slurry of CaX loaded with chlorobenzene at room
temperature, dichlorobenzene was formed at 95% selectivity
with a para-to-ortho ratio of 24. However, no data were reported
on the direct chlorination of benzene. High para selectivity
-4
4
cially the 1,2,4-isomer, play mainly a role as pesticides. Yet,
existing synthetic methods of benzene chlorination by elemental
chlorine are unselective. The traditional method is liquid-phase
reaction of benzene with Cl2 catalyzed by FeCl3 or other Lewis
(para/ortho ) 32) has also been achieved by chlorination of
chlorobenzene in partially decationated zeolite X, but with a
reagent other than molecular chlorine (tert-butyl hypochlorite).9
On the other hand, high selectivity using Cl2 has been observed
by Turro and co-workers upon monochlorination of terminal
methyl groups of n-alkanes occluded in pentasil zeolites by UV
1,2
acid catalysts. Since each chlorinated compound reacts further
with Cl2, the results are complex mixtures of chlorinated
benzenes which require extensive separation processes for
isolation of the desired product. To make such energy-intensive
operations unnecessary and to avoid unwanted chlorinated
products while minimizing the demand for the hazardous starting
materials, substantial improvement of the selectivity of benzene
chlorination is needed.
1
0,11
light.
We wish to report in this paper an in situ infrared spectro-
scopic study of thermal and visible light-induced reaction of
benzene-Cl2 gas mixtures loaded into zeolite NaZSM-5.
Substantially improved product selectivities compared to existing
methods were observed.
We have recently observed that large-pore alkali or alkaline-
earth zeolites (type Y or L) can be used to achieve highly
selective partial oxidation of small alkanes, olefins, or alkyl-
benzenes when the corresponding hydrocarbon-O2 mixture is
loaded into the pores from the gas phase and irradiated with
II. Experimental Section
5
NaZSM-5 was prepared from NH ZSM-5 (Zeolyst product
visible light. The strong electrostatic properties of the solvent-
4
no. CBV15014, lot no. 15014-1525-63, SiO2/Al2O3 ) 150) by
free zeolite, coupled with the diffusional and steric constraints
imposed by the micropores, might offer opportunities for
controlled reaction of benzene with molecular chlorine under
mild conditions. In fact, several groups have reported enhanced
selectivity upon benzene chlorination by Cl2 in zeolites. Most
ion exchange using sodium acetate. The exchange was repeated
+
until NH4 could not be detected by IR spectroscopy; hence,
the Na/Al ratio was inferred to be 1. No Bronsted acid hydroxyl
-
1 12
sites (OH stretch absorption at 3611 cm ) were observable
by infrared. Self-supporting pellets of NaZSM-5 (approximately
8 mg, 12.5 mm diameter) were dehydrated overnight at 200 °C
in a miniature stainless steel vacuum cell contained within a
liquid nitrogen cryostat (Oxford Instruments model DN1714).
The infrared cell was evacuated with a Varian turbomolecular
pump model V-60. Wafers were exposed to 500 mTorr of
benzene at room temperature for 0.5 h, and then the sample
6
notably, Miyake found that when the reaction is conducted in
a liquid slurry of NaZSM-5 at 100 °C, it stops at the
dichlorobenzene stage with a para-to-ortho ratio of 4 (compared
to 1.4 in the case of homogeneous liquid-phase chlorination
catalyzed by FeCl3). On the other hand, van Bekkum reported
that gas-phase chlorination at 175 °C over HZSM-5 or
1
NaHZSM-5 gave mainly hexachlorocyclohexanes, indicating
S1089-5647(98)01434-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/20/1998

Chlorination of Benzene by Cl2 in NaZSM-5
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7107
cell was evacuated to a gas pressure below 5 mTorr. The cell
was cooled to temperature and allowed to stabilize over 1.5 h.
Room-temperature chlorine was introduced into the cell in 500
mTorr increments, at 10 min intervals, for a final chlorine
pressure of 2500 mTorr. The stepwise procedure was used to
avoid a significant temperature rise upon Cl2 loading, which
might lead to uncontrolled reaction. Zeolite NaZSM-5 is
-
1
transparent in the infrared at frequencies above 1300 cm and
-
1
between 770 and 630 cm . FT-IR spectra were collected in
situ using an IBM Bruker model IR-44 spectrometer. One
-
1
hundred scans at 1 cm resolution were coadded and ratioed
against a background of the empty dehydrated pellet held at
room temperature. The cryostat and cell were fitted with KCl
windows to allow IR spectra collection and visible light
irradiation without movement of the sample.
Figure 1. Thermal reaction of benzene with Cl
2
in NaZSM-5 at -100
°
C: (a) infrared difference spectrum before and after adsorption of
benzene (full trace); (b) difference spectrum after reaction of 90% of
the loaded benzene (9.8 h reaction time, dashed line). The small spectral
+
+
+
The samples were irradiated using an Ar , a Kr , or an Ar
-
1
changes around 1630 cm are due to changes in the band profile of
residual H O in the zeolite.
pumped dye laser (Coherent models Innova 90-5, Innova 300,
and 599-01, respectively) for a fixed interval, and then the beam
was blocked for IR spectra collection. Spectra were taken
immediately upon ceasing irradiation and again after 5 min in
the dark to monitor the effects of laser heating. Spectra for
nonirradiated samples were collected at the same intervals to
monitor the thermal reaction.
2
Fractional abundances were calculated by the following
procedure. Resolved, characteristic peaks were selected for each
-
1
-1
species: 3040 cm for benzene, 1445 cm for chlorobenzene,
-
1
-1
1
437 cm
for 1,2-dichlorobenzene, 1417 cm
dichlorobenzene, 1392 cm for 1,4-dichlorobenzene, and 1375
for 1,3-
-1
-
1
cm for 1,2,4-trichlorobenzene. Each peak was chosen for
minimal overlap with those of other species. This required
selecting peaks other than the most or even the second most
intense peak for each compound. For example, benzene,
chlorobenzene, and 1,4-dichlorobenzene all have overlapping
Figure 2. Kinetics of benzene + Cl
2
dark reaction in NaZSM-5 at
100 °C: b, benzene; 1, chlorobenzene; 9, 1,4-dichlorobenzene. For
-
1
-1
peaks around 1480 cm . The peaks at 1437 and 1417 cm
-
were selected for 1,2- and 1,3-dichlorobenzene, respectively,
because it was impossible to reliably curve-resolve their
determination of fractional abundances, see Experimental Section.
-
1
overlapping peaks at 1460 and 1458 cm . The relative
extinction coefficients were calculated from those obtained from
acetonitrile-d3 solution infrared spectra. For this purpose, a
minimum of eight solutions of each compound were prepared
and measured. Relative integrated extinction coefficients,
III. Results
Upon admission of Cl2 to benzene-loaded NaZSM-5, spon-
taneous thermal reaction was observed at temperatures as low
as -100 °C. Subsequent irradiation with green or blue light
resulted in additional reaction, furnishing products distinct from
those of thermal chemistry. We will first present the thermal
experiments, which will be followed by the results of the
photochemical studies.
-
1
referenced against the 3040 cm absorption of benzene, were
-
1
0
.69 ( 0.09 for chlorobenzene (1445 cm ), 0.9 ( 0.2 for 1,4-
-1
dichlorobenzene (1392 cm ), 1.2 ( 0.1 for 1,2,4-trichloroben-
-
1
zene (1375 cm ), 1.0 ( 0.1 for 1,2-dichlorobenzene (1437
-1
-1
cm ), and 0.5 ( 0.1 for 1,3-dichlorobenzene (1417 cm ). Gas-
phase spectra were also recorded, but the relative intensities of
peaks for a given compound varied by as much as a factor of
1. Thermal Chemistry. The infrared difference spectrum
recorded upon loading of benzene into NaZSM-5 at -100 °C
showed reactant absorptions at 673, 696, 1479, 1519, 1808,
-1
4
compared with the case of authentic species in NaZSM-5.
1951, 3040, 3074, and 3094 cm (Figure 1, trace a). Exposure
The most likely reason is the inhomogeneous local environment
in the zeolite framework. This variance was much reduced in
solution. Use of the solution extinction coefficients is justified
by a comparison of ratios in solution and zeolite, where
available. For example, infrared difference spectra of thermal
chlorobenzene to 1,4-dichlorobenzene conversion in the zeolite
of the pellet to Cl gas led to infrared product growth at 675,
685, 700, 741, 1392, 1397, 1445, 1477, 1568, and 1582 cm
2
-
1
under concurrent loss of the benzene absorptions, as shown by
trace b of Figure 1. The product bands (except for those at
-
1
1392 and 1397 cm ) have the same growth kinetics and agree
with the spectrum of an authentic sample of chlorobenzene in
-
1
-1
give a ratio I(1445 cm )/I(1392 cm ) of 0.88 (Figure 5)
-1
NaZSM-5. The 1392 and 1397 cm absorptions, on the other
-1
compared to 0.79 in solution. Or, the ratio I(3040 cm )/I(1445
hand, grew in much more slowly as can readily be seen from
the kinetic plots of Figure 2. This doublet presumably reflects
two distinct sites and coincides with a strong infrared absorption
of an authentic sample of 1,4-dichlorobenzene in NaZSM-5.
-
1
cm ) in the thermal benzene to chlorobenzene conversion is
.54 in zeolite and 0.69 in solution. These values coincide
0
within the experimental uncertainties.
-1
Benzene and all partially chlorinated benzenes were purchased
from Aldrich (99% purity or higher) and used as received.
Chlorine gas (Matheson, 99.99%) was purified by trap-to-trap
distillation before use.
The latter also has an intense band at 1477 cm which overlaps
with the strongest chlorobenzene absorption. No other chlori-
nated products were detected even upon conversion of over 80%
of the loaded benzene. Interestingly, the infrared spectrum did

7108 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Kirkby and Frei
Figure 3. Kinetics of benzene + Cl
, 1,3-dichlorobenzene.
2
dark reaction in NaZSM-5 in the temperature range -75 to 0 °C: 1, chlorobenzene; 9, 1,4-dichlorobenzene;
[
not exhibit an absorption of the HCl coproduct in the 2500-
-
1
3
000 cm region. Instead, a very broad band grew in with a
-
1
-1
maximum around 3300 cm and fwhm of 300 cm . We
attribute this band to surface OH groups formed upon spontane-
ous dissociation of HCl to H and Cl in the presence of the
+
-
+
high electrostatic fields around the Na ions. The red shift
relative to free ν(OH) and the large width are due to H-bonding
interaction with benzene and chlorobenzenes. Similar observa-
tions about the behavior of HCl in cation-exchanged zeolites
13
have been reported recently by Ozin and earlier by Angell
1
4
and Schaffer.
When raising the zeolite temperature above -100 °C, a
substantial increase of the reaction rate was observed. Figure
3
-
shows the growth behavior of the chlorinated benzenes at -75,
50, -25, and 0 °C. Aside from the two products observed at
100 °C, chlorobenzene and 1,4-dichlorobenzene, there is
Figure 4. Green light-induced reaction of 1,4-dichlorobenzene with
Cl in NaZSM-5 at -100 °C. Shown is the infrared difference spectrum
upon 90 min 514 nm irradiation at 700 mW cm
2
-
-2
.
growth of 1,3-dichlorobenzene at -75 °C and higher temper-
-1
atures as indicated by bands at 1417 and 1460 cm . Identifica-
tion is again based on comparison with the spectrum of an
authentic sample in NaZSM-5. Traces of 1,2-dichlorobenzene
were detected at -50 °C and above by the characteristic
absorption at 1437 cm , but the growth never exceeded 3%
even at >90% conversion of the benzene. Aside from these
chlorinated benzenes, no other product absorption was detected.
This is in stark contrast to the previous observation of mainly
gas adsorption in the zeolite above -50 °C, leading to
quantitative consumption of chlorine within 5 h (-25 °C) or 3
h (0 °C) after start of reaction.
2. Photochemical Reaction. Since 1,4-dichlorobenzene is
the exclusive final product of thermal benzene chlorination at-
100 °C, it would be interesting to accomplish further chlorination
to a commercially important trichlorobenzene product. We have
explored this possibility by using visible light for inducing
-
1
(
saturated) chlorocyclohexanes in experiments conducted over
reaction of 1,4-dichlorobenzene with Cl in NaZSM-5 at -100
2
7
ZSM-5 at high temperature. While not apparent below -50
C because of too small a signal, the absorbance growth of the
°C. Upon loading of the zeolite with 1,4-C H Cl and Cl (2.5
6
4
2
2
°
Torr) at -100 °C, no dark reaction was noted. When irradiating
-
2
dichlorobenzene products at T > -50 °C exhibits a clear
induction period. This confirms that they are formed by
chlorination of chlorobenzene. Comparison of Figure 2, Figure
with 514 nm CW laser light at 700 mW cm for 90 min, we
observed 70% depletion of the reactant under concurrent growth
-
-1
of a strong band at 1460 cm
and a very weak feature at
-
1
3
a, and Figure 3b shows that there is increased chlorobenzene-
to-dichlorobenzene conversion as the temperature is raised from
100 to -50 °C. However, the product growth behavior
1375 cm (Figure 4). These two product bands are assigned
to 1,2,4-trichlorobenzene according to authentic spectra of the
molecule in zeolite. No other chlorinated product could be
detected. Note that a laser heating effect is ruled out by the
fact that 1,2,4-trichlorobenzene is not formed thermally even
at 0 °C.
-
changes little from -50 to -25 °C, and the conversion of
chlorobenzene to dichloro products decreases upon further rise
to 0 °C (Figure 3c,d). This is due to a steep decrease in Cl2

Chlorination of Benzene by Cl2 in NaZSM-5
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7109
2
Figure 7. Green light-induced reaction of benzene and Cl -loaded
2
Figure 5. Thermal reaction of chlorobenzene with Cl in NaZSM-5
at -100 °C: (a) infrared difference spectrum before and after adsorption
of chlorobenzene (full trace); (b) difference spectrum after exposure
NaZSM-5 at -100 °C: (a) difference spectrum upon 60 min irradiation
at 514 nm (solid trace); (b) difference spectrum after continued
photolysis for 360 min (dashed trace).
2
to Cl for 80 min (dashed trace).
the spectrum after 60 min irradiation, one notices a net loss at
-
1
1
397/1392 cm (1,4-dichlorobenzene) under concurrent growth
-
1
at 1460 cm (1,2,4-trichlorobenzene); see trace b of Figure 6.
-
1
A very weak product band at 1431 cm , not apparent after
just 60 min photolysis, is assigned to 1,2,3,4-tetrachlorobenzene
based on comparison with authentic spectra and on the
pronounced induction period.
In the benzene photochlorination experiment, a NaZSM-5
pellet loaded with C6H6 was first exposed to chlorine in the
dark at -100 °C. The spectrum observed upon thermal reaction
was the same as shown in Figure 1, featuring growth of
chlorobenzene and 1,4-dichlorobenzene. After the bulk of the
loaded benzene had reacted thermally with Cl2, irradiation with
-2
5
14 nm laser light at 700 mW cm led to rapid chlorobenzene-
to-1,4-dichlorobenzene conversion, as can be seen in the infrared
difference spectrum taken after 60 min photolysis (Figure 7,
Figure 6. Green light-induced reaction of chlorobenzene and Cl
loaded NaZSM-5 at -100 °C: (a) infrared difference spectrum upon
0 min irradiation at 514 nm (solid trace); (b) difference spectrum taken
2
-
-1
trace a). This is signaled by the depletion at 1445 cm and
6
-1
concurrent growth at 1392 and 1477 cm . The latter overlaps
after continued photolysis for 360 min (dashed line).
with the decreasing benzene absorption due to reaction of the
remaining C6H6. As in the case of photochlorination of
chlorobenzene, we observe growth at 1368, 1356, 1338, and
A series of experiments were performed to find out whether
such high selectivity for the 1,2,4-trichloro isomer can also be
obtained by direct chlorination of benzene or chlorobenzene.
Since C6H6 and C6H5Cl react thermally with Cl2 at temperatures
as low as -100 °C, photochemical and thermal reaction will
proceed concurrently in these systems. Figures 5 and 6 show
the result for chlorobenzene as starting material. Trace a of
Figure 5 is the infrared spectrum of C6H5Cl in NaZSM-5 before
loading of chlorine. Admission of Cl2 over a period of 80 min
leads to spontaneous reaction to form 1,4-dichlorobenzene, as
can be seen in the difference spectrum b of Figure 5.
-
1
1
331 cm which indicates formation of chlorocyclohexanes.
The difference spectrum taken after 60 and 420 min irradiation
Figure 7, trace b) is dominated by continued chlorobenzene-
to-1,4-dichlorobenzene conversion as well as growth at 1460
(
-
1
cm . The latter is due to 1,2,4-trichlorobenzene produced by
photochlorination of the 1,4-dichloro isomer. Fractional abun-
dances in this particular experiment could not be determined
reliably because of a factor of 2 discrepancy between the relative
extinction coefficients of 1,4-dichloro- and 1,2,4-trichloroben-
zene in solution and zeolite.
-
2
Subsequent irradiation at 514 nm (700 mW cm ) results in
reaction of >90% of the remaining chlorobenzene within 60
min as shown by trace a of Figure 6. The positive band at
Photolysis at shorter, blue wavelengths led to the same
products, but at faster rates. The results are summarized in the
form of kinetic plots in Figure 8. There is a substantial
acceleration of chlorobenzene consumption and growth of 1,4-
C6H4Cl2 and 1,2,4-C3H3Cl3 as the photon energy is raised from
-
1
1
392 cm signals continued production of 1,4-dichlorobenzene,
-1
while the new absorption at 1460 cm indicates photogenera-
tion of 1,2,4-trichlorobenzene. The depletion at 1397 cm
-
1
5
14 to 488, to 458 nm. No reaction was observed when
suggests that the higher frequency site of 1,4-dichlorobenzene
reacts preferentially with Cl2 to form the 1,2,4-trichloro product.
Apart from the trichlorobenzene bands, very weak growth is
irradiating with the CW dye laser at longer wavelengths, 580
or 647 nm, using comparable laser power and irradiation times.
At the shortest wavelength, 458 nm, chlorination of 1,2,4-
trichloro- to 1,2,3,4-tetrachlorobenzene was detected by growth
-
1
detected at 1368, 1356, 1338, and 1331 cm . These features
do not increase upon continued photolysis and are in a spectral
-
1
1
5
at 1431 cm .
region characteristic for CH bending modes of alkanes.
Therefore, we assign them tentatively to hexachlorinated cy-
clohexane. As expected, prolonged irradiation leads mostly to
conversion of 1,4-dichlorobenzene to 1,2,4-trichlorobenzene. If
one records the spectrum after 420 min photolysis and subtracts
IV. Discussion
The well-established mechanism of thermal reaction of
molecular chlorine with benzene consists of electrophilic attack

7110 J. Phys. Chem. B, Vol. 102, No. 37, 1998
Kirkby and Frei
Figure 8. Photolysis wavelength dependence of benzene + Cl
2
product absorbance growth kinetics. Photolysis was preceded by 80 min dark
-
1
-1
reaction at -100 °C, leading to nearly quantitative conversion of C
H
6 6
: (1) chlorobenzene, 741 cm ; (9) 1,4-dichlorobenzene, 1392 cm ; ([)
-
1
1
,2,4-trichlorobenzene, 1460 cm .
of Cl2 on the aromatic ring to form a transient arenium ion
Wheland intermediate), followed by proton elimination to yield
chlorobenzene. Existing benzene chlorination methods use
O2 collisional complexes in alkali and alkaline-earth zeolites.5,24
However, benzene is known to reside near the extraframework
(
2
5
cations while halogen molecules tend to prefer sites at the
16,17
26
Lewis acids such as AlCl3 or FeCl3 as catalysts.
Zeolite
negatively charged cage wall. This implies that most charge-
+
-
ZSM-5 consists of intersecting channels with a diameter of 5.5
transfer dipoles (C6H6 ‚Cl2 ) are oriented antiparallel to the
1
8
+
Å. Each Al introduces a negative charge into the alumino-
silicate framework, which is mainly located on the oxygens.
The negative charge is compensated by extraframework Na+
ions. The shielding of these cations is very poor. Therefore,
electrostatic field around the Na ion. Nevertheless, due to the
mobility of benzene and chlorine molecules in the zeolite, a
small fraction may be oriented parallel to the field and absorb
visible light. However, even if complexes are excited to the
charge-transfer state by visible light, the electron distribution
of this state would be altered in a way that makes the chlorine
species less, not more, reactive toward electrophilic attack on
the benzene ring. Therefore, it is unlikely that reaction would
be triggered from that state.
+
the Na ions can act as strong Lewis acids, especially when no
solvent is present that would attenuate the effect of the positive
charges. By polarizing the Cl2, the Na+ ions activate the
molecule toward electrophilic attack on the benzene. The fast
chlorination of benzene, even at -100 °C, is attributed to the
+
Lewis acidity of the Na . It is further aided by the high
By contrast, photoexcitation of Cl2 with green or blue light
generates an electronic state with much stronger electrophilic
character than the ground-state molecule. Two excited states
are contributing to absorption of Cl2 in the visible, namely, the
concentration of benzene‚Cl2 complexes in the zeolite pores.
Benzene does not react with chlorine in gas or solution phase
even at room temperature.19
We attribute the selectivity of the second reaction step,
chlorination of chlorobenzene to 1,4-dichlorobenzene at -100
3
-1
weakly bound π0 u state with the 0-0 transition at 18 147 cm
+
2
7
1
(551 nm) and the repulsive πu state. The latter gives rise to
°
C, to a steric effect of the zeolite pores on the transition state
a broad, structureless absorption in the near-UV with a tail
20
28
(transition-state shape selectivity).
Benzene has a kinetic
extending to about 450 nm. Both excited states originate from
21
2
2
2
3
diameter of 5.8 Å.
The transition state leading to 1,4-
the electron configuration KKLL(σg3s) (σu3s) (σg3p) (πu3p) -
(σu3p) in which a πg3p electron is promoted to the σu3p orbital.
The vacancy created in the πg3p orbital renders the excited
chlorine molecule a substantially better electrophile than ground-
dichlorobenzene is not expected to be hindered sterically by
the zeolite walls if the chlorobenzene molecule is oriented with
the molecular axis parallel to the channel axis. By contrast,
the transition state leading to 1,2- or 1,3-dichlorobenzene is
sterically hindered by the channel for any orientation of the
chlorobenzene. Apparently, the activation barriers to 1,2- or
1
+
state Cl2( Σg ), resulting in enhanced reactivity toward benzene
or chlorobenzene compared to the case of the dark thermal
system. In the case of 1,4-dichlorobenzene, only the electroni-
cally excited Cl2 molecule is sufficiently electrophilic to cause
reaction. This interpretation is strongly supported by the finding
that excitation of Cl2‚benzene, chlorobenzene, or 1,4-dichlo-
robenzene complexes leads to reaction even with 514 nm light
1
,3-dichlorobenzene block these paths at -100 °C, but they
become accessible as minor reaction channels at T g -75 °C.
Complete selectivity toward 1,4-dichlorobenzene upon direct
chlorination of benzene with elemental chlorine is unprec-
edented.
-
1
-1
despite the fact that these quanta lie 560 cm (1.6 kcal mol )
below the chlorine dissociation threshold (500 nm).28 While
generation of free Cl atoms is possible at wavelengths shorter
than 500 nm, the predominant formation of benzene substitution
rather than free-radical addition products indicates that the
excited Cl2 molecule undergoes electrophilic attack even at
photon energies above the dissociation limit.
While benzene and Cl2 form a weak (π-donor, σ-acceptor)
charge-transfer complex which has been well characterized in
solution,22 it is unlikely that the excited charge-transfer state is
involved in the photoinduced reaction. The benzene-chlorine
2
2
charge-transfer band in solution absorbs in the UV region. It
is to be expected that the charge-transfer state of C6H6‚Cl2
complexes with favorable orientation with respect to the strong
+
23
V. Conclusions
electrostatic field in the vicinity of Na ions will be stabilized,
resulting in a red shift of the absorption into the visible range.
We have encountered such an effect in the case of hydrocarbon‚
This infrared spectroscopic study of benzene chlorination by
Cl2 shows that 1,4-dichlorobenzene is formed as the sole

Chlorination of Benzene by Cl2 in NaZSM-5
J. Phys. Chem. B, Vol. 102, No. 37, 1998 7111
dichloro product when loading the reactant gas mixture into
zeolite NaZSM-5 at -100 °C. Such selectivity is unprec-
edented. Even at elevated temperatures (e0 °C) or high
conversion, no products other than chlorinated benzenes are
observed. Irradiation with green or blue light affords selective
conversion of 1,4-dichlorobenzene to 1,2,4-trichlorobenzene as
a unique photochemical product. At the same time, the rate of
benzene and chlorobenzene reaction with Cl2 is enhanced by
the photons. In the latter case, chlorocyclohexanes were formed
as byproducts.
(9) Smith, K.; Butters, M.; Nay, B. Synthesis 1985, 155.
10) (a) Turro, N. J.; Fehlner, J. R.; Hessler, D. P.; Welsh, K. M.;
Ruderman, W.; Firnberg, D.; Braun, A. M. J. Org. Chem. 1988, 53, 3731.
b) Turro, N. J.; Han, N.; Lei, X.; Fehlner, J. R.; Abrams, L. J. Am. Chem.
Soc. 1995, 117, 4881.
(11) For a review of photochemistry in zeolites, see: Ramamurthy, V.
In Photochemistry in Organized and Constrained Media; Ramamurthy, V.,
Ed.; VCH Publishers: New York, 1991; p 429.
12) Wang, H. P.; Yu, T.; Garland, B. A.; Eyring, E. M. Appl. Spectrosc.
990, 44, 1070.
13) (a) Ozin, G. A.; Ozkar, S.; Stucky, G. D. J. Phys. Chem. 1990, 94,
7562. (b) Ozin, G. A.; Ozkar, S.; McMurray, L. J. Phys. Chem. 1990, 94,
(
(
(
1
(
8
8
289. (c) Ozin, G. A.; Ozkar, S.; McMurray, L. J. Phys. Chem. 1990, 94,
297.
14) Angell, C. L.; Schaffer, P. C. J. Phys. Chem. 1965, 69, 3463.
The photochemical results suggest that the excited Cl2 reacts
3
in the π0+u state in which the molecule is substantially more
(
electrophilic than in the ground state. This may open up
opportunities for new synthesis based on electrophilic reactions
that cannot be accomplished by thermal means. Electronic
excitation by visible light renders the electrophile more reactive,
and the zeolite pores afford the high concentration of reactant
complexes required for efficient bimolecular photoreaction.
(
15) (a) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to
Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: Boston, 1990;
p 228. (b) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd
ed.; Chapman & Hall: London, 1975; Vol. 1, p 21.
(16) Reference 2, p 417.
(
17) March, J. AdVanced Organic Chemistry, 4th ed.; Wiley: New York,
1
992; p 531.
(
18) (a) Meier, W. M.; Olson, D. H.; Baerlocher, C. Atlas of Zeolite
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 DE-AC03-76SF00098.
Structure Types, 4th ed.; Elsevier: London, 1996; p 146. (b) Bhatia, S.
Zeolite Catalysis: Principles and Applications; CRC Press: Boca Raton,
FL, 1989; p 15.
(19) (a) Brown, H. C.; Stock, L. M. J. Am. Chem. Soc. 1957, 79, 5175.
(
b) Andrews, L. J.; Keefer, R. M. J. Am. Chem. Soc. 1959, 81, 1063.
(
20) Turro, N. J. Pure Appl. Chem. 1986, 58, 1219.
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