UV-vis spectroscopy of iodine adsorbed on alkali-metal-modified zeolite catalysts for addition of carbon dioxide to ethylene oxide

Article abstract of DOI:10.1021/jp991091t

The basicity of alkali-metal-exchanged (Na, K, Cs) zeolites X and Y was probed by UV-vis diffuse reflectance spectroscopy of adsorbed iodine. The observed blue shift in the visible absorption spectrum of adsorbed iodine, compared to gaseous iodine, correlated well with the negative charge on the framework oxygen atoms calculated from the Sanderson electronegativity equalization principle. The blue shifts associated with iodine adsorbed on classical catalytic supports like silica, alumina, and magnesia suggest that the iodine adsorption technique for probing basicity is applicable to a wide variety of solids. Iodine was also adsorbed on X and Y zeolites containing occluded cesium oxide formed by decomposition of impregnated cesium acetate. However, the iodine appeared to irreversibly react on these strongly basic samples, possibly forming an adsorbed triiodide ion. As a complement to the adsorption studies, the activity of alkali-metal-containing zeolites for the base-catalyzed formation of ethylene carbonate from ethylene oxide and carbon dioxide was investigated. Among the ion-exchanged zeolites, the cesium form of zeolite X exhibited the highest activity for ethylene carbonate formation. The catalytic activity of a zeolite containing occluded cesium was even higher than that of a cesium-exchanged zeolite. The presence of water adsorbed in zeolite pores promoted the rate of ethylene carbonate formation for both cesium-exchanged and cesium-impregnated zeolite X.

Full text of DOI:10.1021/jp991091t

J. Phys. Chem. B 1999, 103, 6277-6282
6277
UV-Vis Spectroscopy of Iodine Adsorbed on Alkali-Metal-Modified Zeolite Catalysts for
Addition of Carbon Dioxide to Ethylene Oxide
Eric J. Doskocil, Shailendra V. Bordawekar, Brian G. Kaye, and Robert J. Davis*
Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22903-2442
ReceiVed: March 30, 1999; In Final Form: May 24, 1999
The basicity of alkali-metal-exchanged (Na, K, Cs) zeolites X and Y was probed by UV-vis diffuse reflectance
spectroscopy of adsorbed iodine. The observed blue shift in the visible absorption spectrum of adsorbed
iodine, compared to gaseous iodine, correlated well with the negative charge on the framework oxygen atoms
calculated from the Sanderson electronegativity equalization principle. The blue shifts associated with iodine
adsorbed on classical catalytic supports like silica, alumina, and magnesia suggest that the iodine adsorption
technique for probing basicity is applicable to a wide variety of solids. Iodine was also adsorbed on X and
Y zeolites containing occluded cesium oxide formed by decomposition of impregnated cesium acetate. However,
the iodine appeared to irreversibly react on these strongly basic samples, possibly forming an adsorbed triiodide
ion. As a complement to the adsorption studies, the activity of alkali-metal-containing zeolites for the base-
catalyzed formation of ethylene carbonate from ethylene oxide and carbon dioxide was investigated. Among
the ion-exchanged zeolites, the cesium form of zeolite X exhibited the highest activity for ethylene carbonate
formation. The catalytic activity of a zeolite containing occluded cesium was even higher than that of a
cesium-exchanged zeolite. The presence of water adsorbed in zeolite pores promoted the rate of ethylene
carbonate formation for both cesium-exchanged and cesium-impregnated zeolite X.
Introduction
blue shift in the visible absorption band of an iodine donor-
acceptor complex to its heat of formation.27,29,42
Zeolites that have been modified by ion exchange of alkali-
metal cations or by decomposition of an occluded alkali-metal
salt have emerged as interesting solid bases. They are known
to catalyze many reactions, including double-bond isomerization
of olefins, side-chain alkylation of aromatics, dehydrogenation
of alcohols, and Knoevenagel condensation of aldehydes.1-5 The
base strength of alkali-metal ion-exchanged zeolites increases
with increasing electropositivity of the exchange cation, and
occlusion of alkali-metal oxide clusters in zeolite cages via
decomposition of impregnated alkali-metal salts results in a
further increase in the basicity of these materials.1,3,6-9 The base
sites in alkali-metal-modified zeolites are commonly character-
ized by IR spectroscopy and/or temperature-programmed de-
sorption of adsorbed probe molecules such as carbon
dioxide,3,10-13 pyrrole,14-16 and chloroform.17-19
Mulliken predicted the use of iodine adsorption to probe the
donor strength of solids,28 and sorption of iodine in zeolites
has been investigated in the past.43,44 These early studies reported
on the heat of sorption and the location of iodine in various
zeolites. The results from these works confirm that iodine
interacts with the basic framework oxygen and not the charge-
balancing cation.43,44
Recently, the visible absorption spectrum of iodine has been
used to rank the donor strength of the basic oxygens in various
zeolites.45 Choi et al. have shown that a blue shift occurs in the
visible spectrum of adsorbed iodine with increasing electro-
positivity of the alkali-metal counterion from Li to K, as well
as with increasing aluminum content in the zeolite framework.45
These observations are consistent with the ranking of zeolite
basicity determined by infrared studies15 and alcohol decom-
position reactions.46
In this paper, we extend the application of iodine as a visible
probe for the determination of the basic strength for zeolites to
include cesium-exchanged samples as well as various metal
oxides. A limitation of this technique for use with strong bases
will also be presented. To correlate the characterization results
with the catalytic function, the activity of alkali-metal-modified
zeolites was examined for the reaction of ethylene oxide and
carbon dioxide to produce ethylene carbonate, an important
intermediate in the manufacture of rubber chemicals and textile
agents, as well as a precursor in the production of high-purity
ethylene glycol.47-51 The usual catalysts employed in this
reaction are homogeneous bases such as sodium hydroxide,
tertiary amines, or quaternary ammonium compounds. To ease
the separation of product from the catalyst, we have explored
the possibility of using alkali-metal-modified zeolites as het-
The nature and strength of an active site are important to
understand base catalysis on a molecular level. One of the major
problems encountered in studying surface basicity is that many
commonly used probe molecules decompose or react upon
interaction with the basic site and, therefore, do not effectively
interrogate the catalyst surface. Thus, the search for a unique,
widely applicable adsorbate molecule to probe the surface
basicity of heterogeneous catalysts is an ongoing process.
A sample that has an exposed ionizable electron acts as a
Lewis base with respect to I2,20 and the use of I2 as a molecular
probe of electron donor strength associated with the gas
phase21-26 and solvated molecules20,27-41 has been well docu-
mented in the literature. Many researchers have correlated the
* To whom correspondence should be addressed. E-mail: rjd4f@
virginia.edu.
10.1021/jp991091t CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/13/1999

6278 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Doskocil et al.
pore vol, mL g-1
TABLE 1: Properties of Zeolites Used for Iodine Adsorption Studies
zeolite
unit cell comp
no. of excess alkali metal atoms
BET surface area, m2 g-1
NaX
KX
CsX
CsOX/CsX
NaY
KY
CsY
CsOX/CsY
H8.9Na78.8Si104.3Al87.7O384
H6.8Na8.2K73.0Si104.0Al88.0O384
H14.0Na27.8Cs47.7Si102.5Al89.5O384
Na27.0Cs62.7Si108.3Al83.7O384
0.0
0.0
0.0
6.1
0.0
0.0
0.0
11.7
766
652
399
270
863
867
623
373
0.288
0.245
0.160
0.111
0.325
0.331
0.232
0.141
H
H
H
0.4Na53.6Si137.9Al54.1O384
4.8Na2.8K50.0Si134.5Al57.5O384
3.9Na14.7Cs39.0Si134.4Al57.6O384
Na15.1Cs50.1Si138.5Al53.5O384
erogeneous base catalysts for this reaction. The spectroscopic
results from iodine adsorption studies and the catalytic activities
of the zeolite catalysts have been used to rank their basicities.
after introduction of iodine vapor to the solid. The sample
containing I2 was allowed to equilibrate for at least 30 min
before spectroscopy was performed. The mass of iodine
adsorbed onto the sample was determined gravimetrically.
Desorption of iodine was accomplished by heating an iodine-
loaded sample in vacuo to 773 K at a rate of 3 K min-1 and
holding at that temperature for 5-8 h.
Experimental Methods
Catalyst Preparation. Potassium- and cesium-exchanged
zeolites were obtained by triply-ion-exchanging NaX (Union
Carbide, Lot No. 943191060078) and NaY (Union Carbide, Lot
No. 955089001010-S) in 1 M solutions of potassium nitrate
(Mallinckrodt, reagent grade) and cesium acetate (Aldrich,
99.9%), respectively. The exchanged samples were then washed
with an aqueous solution of the corresponding alkali-metal
hydroxide (pH ) 13.3), unless otherwise noted. The exchanged
samples were then contacted with distilled-deionized water in
an attempt to remove excess alkali metal that might have been
occluded in the zeolite cages during ion exchange. The resultant
ion-exchanged materials were dried overnight in air at 373 K.
The zeolites were then heated in flowing air at 4 K min-1 to
773 K and subsequently held at that temperature for 5-10 h. If
a sample was to be impregnated with excess cesium after heat
treatment, it was placed in a high-humidity vessel to slowly
reintroduce water to the zeolite before impregnation with an
aqueous cesium acetate solution of the appropriate concentration.
The impregnated catalyst was then calcined in an identical
manner to the ion-exchanged materials in order to decompose
the occluded acetate. The impregnated samples, which are
referred to as CsOX/CsX and CsOX/CsY, contain between 6 and
16 excess Cs atoms per unit cell. Elemental analyses of the
alkali-metal-exchanged zeolites were performed by Galbraith
Laboratories, Inc., Knoxville, TN. The additional materials used
in this study were pure mesoporous silica (Si-MCM-41)
prepared from an established procedure,52 γ-alumina (Mager
Scientific AP-312), magnesia obtained from the decomposition
of Mg(OH)2 ,53 and cesium-doped (8 wt % Cs) mesoporous
alumina.5 The BET surface areas and pore volumes were
obtained from dinitrogen adsorption isotherms measured at 77
K on a Coulter Omnisorp 100CX instrument after outgassing
for 10-12 h at 573 K.
Iodine Adsorption. Iodine adsorption was performed in a
U-shaped quartz tube with a quartz frit to support the sample.
Each catalyst sample was pressed between 2000 and 3000 psig,
ground, and size separated (+40/-60 mesh) to facilitate transfer
into a uvonic cuvette attached to the U-tube. About 0.5 g of
sample were heated in vacuo to 773 K at a rate of 3 K min-1
and held at that temperature for 10 h, unless stated otherwise.
The ultimate pressure reached at 773 K was less than 3.3 ×
10-5 mbar for each sample. A separate quartz reservoir
containing crystalline iodine (EM Science, 99.8%) was attached
by Cajon ultra-Torr fittings between the vacuum system and
the sample holder and evacuated to less than 1.2 × 10-5 mbar.
Iodine was dosed to the sample by opening the reservoir to the
evacuated assembly and subsequently exposing it to the solid
sample in the U-tube. The sample bed turned a bright color
Diffuse reflectance UV-visible spectra were recorded on a
Varian Cary 3E spectrophotometer equipped with a Labsphere
integrating sphere (DRA-CA-30) with a Labsphere Spectralon
reflectance standard (USRS-99-020). After the sample tube was
inverted to load the cuvette with sample, multiple UV-vis
spectra were recorded and averaged after each sample treatment.
Reaction of Ethylene Oxide with Carbon Dioxide To Make
Ethylene Carbonate. The reaction was carried out in a 50-mL
high-pressure batch reactor supplied by Parr Instrument Co.
About 150 mg of catalyst were loaded into the reactor, after
which a measured liquid volume of ethylene oxide was
introduced to the reactor. The vessel was pressurized with carbon
dioxide to about 400 psig at ambient conditions and then heated
to 423 K with constant stirring of the reaction mixture. After
heating, the pressure in the reactor increased to about 1500 psig.
The reaction was stopped after 3 h. The product was extracted
in acetone and analyzed by gas chromatography using a 50-m
HP-1 capillary column and decane as an external standard.
Results and Discussion
Iodine Adsorption on Zeolites. The unit cell compositions
for zeolites used in the iodine adsorption study, expressed in
terms of an ideal zeolite unit cell having 384 oxygen atoms
bridging Si and Al tetrahedra, are summarized in Table 1.
Protons were added to the unit cell formulas in order to balance
the framework charge. Additional oxygen atoms present from
the decomposition of the occluded acetate are not included in
the unit cell composition. Based on the unit cell compositions
obtained from elemental analysis, potassium- and cesium-
exchanged zeolites appeared to contain no excess alkali metal
after ion exchange. Exchanging the Na cations with heavier
alkali metals resulted in a moderate decrease in the surface area
of the zeolites. Observed pore volumes, determined from the
liquid volume of dinitrogen adsorbed at a relative pressure of
0.3, indicated a small decrease in specific pore volume with
exchange of heavy alkali metal, mostly due to their higher
atomic weights and their larger cation radii. Decomposition of
cesium acetate in the pores of the zeolites resulted in 0.8 and
1.5 excess cesiums per supercage for CsX and CsY, respectively.
Figures 1 and 2 compare the UV-vis spectra of the various
zeolites containing adsorbed iodine. The large absorption feature
in the deep ultraviolet region is known as the charge-transfer
band associated with the formation of an iodine donor-acceptor
complex. In our work, we focus on the low-energy bands
depicted by the arrows in Figures 1 and 2. For both X and Y
zeolites, a blue shift was observed in these low- energy bands
when sodium was exchanged with potassium. Choi et al. found

UV-Vis Investigation of I2 in Zeolites
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6279
Figure 1. UV-Vis diffuse reflectance spectra for iodine adsorbed on
various X zeolites at room temperature. The spectra shown are for the
iodine doses given in Table 2, except for CsX, in which the dose is
Figure 3. Partial negative charge on oxygen as a function of energy
calculated from the blue shift of adsorbed iodine in relation to gaseous
iodine.
6.6 mg of I2 g-1
.
on KY, but a similar feature at 359 nm is found for I2 on CsOX/
CsY. For CsY and CsOX/CsY, an additional UV absorption band
occurs at 280 and 290 nm, respectively. Awtrey and Connick55
-
and Buckles et al.56 have shown that the I3 ion gives an
absorption maximum at 365 nm and another band at 295 nm,
which supports the possible formation of a cesium triiodide salt
upon iodine adsorption on CsY and CsOX/CsY. The formation
-
of the I3 ion is possible only if dissociation of I2 takes place
initially. Since complete dissociation of an iodine molecule
requires a strong base site, samples revealing an absorption
spectrum consistent with triiodide most likely expose strong
donor sites. Unfortunately, the formation of I3- limits the ability
of this technique to accurately rank the base strength by simple
comparison of UV-vis absorption bands.
Figure 2. UV-Vis diffuse reflectance spectra for iodine adsorbed on
various Y zeolites at room temperature. The spectra shown are for the
iodine doses given in Table 2.
The absorption features associated with strongly basic samples
suggest that iodine may react with the solids, so the reversibility
of iodine adsorption was investigated. After a complete iodine
dosing cycle (typically ∼100 mg of I2 g-1), zeolites NaY, KY,
CsY, and CsOX/CsY were heated in vacuum to 773 K and held
at that temperature for over 5 h under vacuum to obtain a final
pressure of less than 10-5 mbar in an attempt to remove the
adsorbed iodine from the zeolite. In each case, the visible color
initially present on the I2-dosed sample disappeared. For NaY,
the iodine was completely removed by heating the dosed sample
(53.2 mg of I2 g-1) for 5.5 h at 773 K. However, for the KY
and CsY cases, a small fraction of the iodine (0.7 and 1.5 mg
of I2 g-1 respectively) did not desorb under identical treatment
conditions; the residual iodine loading corresponded to 0.7 and
2.4 I2 molecules per 1000 alkali-metal atoms, respectively.
Because the KY and CsY zeolites were prepared using a
hydroxide wash, a very small amount of occluded alkali metal
may interact irreversibly with the adsorbed iodine. Since the
amount of residual iodine is very low compared to the total
amount of K and Cs cations in the samples, it is clear that iodine
does not interact irreversibly with the ions located in standard
ion-exchange positions. For cesium impregnated on CsY,
however, a large fraction of iodine, equivalent to 18.6 mg of I2
per g of CsOX/CsY, remained on the sample after heating at
773 K for 9 h, which corresponds to a residual loading of 1.35
I2 molecules per unit cell (0.12 I2 molecule per excess Cs atom).
Clearly a strong interaction between sorbed iodine and occluded
cesium exists, but the stoichiometry did not indicate complete
conversion of the occluded cesium species to cesium iodide.
Bulk cesium iodide (CsI, Acros, 99.9%) did not reveal any
visible absorption bands in our UV-vis spectrometer, indicating
that the visible absorption band observed after iodine adsorption
onto the zeolite is not due to formation of cesium iodide. It is
TABLE 2: Results from UV-Vis Spectroscopy of I2
Adsorbed on Zeolites
zeolite X
zeolite Y
loading,
λmax
,
loading,
λmax,
sample
NaX
KX
CsX
CsOX/CsX
mg g-1
nm
sample
NaY
KY
CsY
CsOX/CsY
mg g-1
nm
19.1
20.1
19.4
26.3
410
359
362
349
27.8
26.8
20.5
23.6
473
419
419
359
similar results for analogous zeolites.45 In addition, the peak
maximum of the X zeolites was always located at higher
energies (lower wavelengths) than analogous Y zeolites. The
wavelengths of these low-energy features are reported in Table
2 for iodine loadings near 20 mg of I2 g-1. The cesium-
exchanged X and Y zeolites had absorption features at almost
the same wavelengths as their corresponding potassium-
exchanged zeolites
Figure 3 shows the correlation of the partial negative charge
on the framework oxygen atoms and the energy shift of the
iodine absorption maximum with respect to I2 in the gas phase.
The partial negative charge on oxygen was calculated from the
Sanderson intermediate electronegativity principle, which is
based solely on the overall sample composition.54 A linear
relationship between energy shift and partial negative charge
on oxygen was observed for the alkali-metal zeolites. The
incomplete levels of Cs exchange account for the similar
absorption features for CsX and KX as well as CsY and KY.
In the case of I2 adsorbed in CsY, a peak at 350 nm is
observed together with the usual visible absorption band at 419
nm. This 350-nm band is not present in the spectrum of iodine

6280 J. Phys. Chem. B, Vol. 103, No. 30, 1999
Doskocil et al.
TABLE 4: Properties of Zeolitesa Used for Catalytic
Reaction
zeolite
unit cell comp
BET surf area (m2 g-1
)
NaX
KX
CsX
NaY
CsY
H
H
H
H
H
8.9Na78.8Si104.3Al87.7O384
766
609
398
863
502
3.4 Na6.1K79.9Si102.6Al89.4O384
8.4 Na28.2Cs50.6Si104.9Al87.2O384
0.4Na53.6Si137.9Al54.1O384
2.2 Na15.7Cs39.2Si134.9Al57.1O384
a Hydroxide washed, except for Na zeolites.
TABLE 5: Activity of Alkali-Metal-Modified Zeolites for
Ethylene Carbonate Formation
catalyst
yield of ethylene carbonate,a %
NaX
1.4
2.7
20.9
10.5
26.6
33.4
0.0
Figure 4. Effect of iodine loading on the low-energy absorption band
location for various X zeolites.
KXb
KX
CsXb
CsX
CsOX/CsXb
NaY
CsY
0.3
11.9
CsOX/CsYb
a Yield reported after 3 h at 423 K and about 1500 psig. b Not
hydroxide washed.
that observed for gaseous I2 (520 nm), indicating that the
interaction with amorphous silica is relatively weak. Similar
values have been seen as well for zeolitic materials with high
Si/Al ratios.45 Alumina exhibited a significant blue shift of the
low-energy absorption peak to 410 nm, indicating a similar
donor strength of the Al2O3 oxygen to that of framework oxygen
in NaX at a similar coverage. Microcalorimetry results obtained
in our laboratory indicate that alumina contains base sites
capable of adsorbing carbon dioxide at room temperature.57
Magnesia, a classic basic oxide, gave adsorbed I2 an even more
significant blue shift to 366 nm, confirming the high donor
strength of its surface. The low-energy absorption band for
iodine adsorbed on cesium-modified mesoporous alumina (8
wt % Cs) exhibited a significant blue shift to 360 nm compared
to that for the γ-alumina under identical pretreatment conditions.
This result is similar to the blue shift observed for the CsOX/
CsY sample mentioned previously. Clearly, the UV-vis spec-
trum of adsorbed iodine can be used to study solids other than
zeolites.
Figure 5. Effect of iodine loading on the low-energy absorption band
location for various Y zeolites.
TABLE 3: Results from UV-Vis Spectroscopy of I2
Adsorbed on Various Oxides
sample
λ
max, nm I2 loading, mg g-1 BET surf area, m2 g-1
Si-MCM-41
γ-Al2O3
508
410
366
360
17.0
27.2
19.4
24.5
802
87
77
MgO
CsOX/Al2O3
370
possible that the occluded cesium oxide clusters are sufficiently
large to restrict an iodine interaction with each occluded cesium
atom.
A gradual red shift in the visible absorption band with
increasing adsorption of iodine on the sample has been observed
by Choi et al.45 The location of the low-energy band as a
function of iodine loading can be seen in Figures 4 and 5 for
various X and Y zeolites, respectively. Each of the Y zeolites
studied showed a red shift with increasing iodine loading. For
NaX, a significant red shift in the visible absorption band with
increasing iodine sorption was observed over the entire range
of iodine loading. For the KX and CsX zeolites, however, there
was little change in the location of the visible absorption band
at loadings exceeding 40 mg of I2 g-1. It is quite possible that,
for strongly basic materials, the iodine adsorption is not
equilibrated with the sample. From Figure 4, it is clear that KX
has a greater blue shift and thus stronger basicity than the cesium
X zeolite at small I2 doses, indicating that KX contained a few
stronger adsorption sites than the CsX zeolite.
Catalytic Activity. A separate series of basic zeolite materials
was prepared for the study of catalytic probe reactions. The
elemental compositions of these alkali-metal X and Y zeolites
can be found in Table 4. Since both water-washed and
hydroxide-washed zeolites exhibited similar elemental composi-
tions, only the results for the hydroxide-washed samples are
shown in Table 4. Cesium oxide occluded in zeolites X and Y
contained about 16 excess cesium atoms per unit cell.
The catalytic activity of alkali-metal-modified zeolites for
ethylene carbonate formation is reported in Table 5. In all of
the experiments, ethylene carbonate was the only product
detected by gas chromatography. The yield of ethylene carbonate
after 3 h of reaction at 423 K increased with increasing
electropositivity of the exchange cation. Increased electroposi-
tivity of the exchange cation results in a higher negative charge
on the framework oxygen atoms, thereby making the material
more basic. This is reflected in the higher activity of KX
compared to NaX. As seen in Figure 3, the partial negative
charge on the framework oxygen atoms of CsX is nearly
identical to that of KX, which could explain why the activities
of these two catalysts are similar. The NaY sample showed no
activity for ethylene carbonate formation, whereas CsY revealed
Iodine Adsorption on Other Catalytic Materials. The use
of iodine as a visible probe for donor strength was then extended
to other metal oxides typically used as catalyst supports. Table
3 shows the peak locations for the low-energy absorption band
for various metal oxides with similar loadings of iodine. Silica
MCM-41 had an absorption band at 508 nm, which is close to

UV-Vis Investigation of I2 in Zeolites
J. Phys. Chem. B, Vol. 103, No. 30, 1999 6281
TABLE 6: Effect of Adsorbed Water on the Activity of
Alkali-Metal-Modified Zeolitesa for Ethylene Carbonate
Formation
sample for ethylene carbonate formation. Dehydration of CsOX/
CsX (water washed) also decreased its activity to ethylene
carbonate (Table 6). These results indicate that adsorbed water
plays an important role in the catalytic activity of alkali-metal-
modified zeolites for ethylene carbonate formation. Haw et al.
have shown that coadsorbed nitromethane in HZSM-5 increased
the activity of the zeolite for 2-propanol dehydration, conversion
of methanol to hydrocarbons, and conversion of acetone to acetic
acid.59 They claim that nitromethane adsorbed on HZSM-5
behaves in a manner similar to polar solvents and promotes
proton transfer by stabilizing ion-pair structures, leading to
increased catalytic activity. In our case, adsorbed water in alkali-
metal-modified zeolites enhanced the activity for ethylene
carbonate formation, possibly through a hydroxide-mediated
path. Ongoing studies in our laboratory are probing the
interesting role of water in this reaction.
catalyst
yield of ethylene carbonate,b %
CsX
10.5
3.0
10.4
33.4
3.1
dehydrated CsX
rehydrated CsX
CsOX/CsX
dehydrated CsOX/CsX
a Not hydroxide washed. b Yield reported after 3 h at 423 K and
about 1500 psig.
some activity for the reaction. However, the catalytic activity
of CsY was much lower than that of CsX. The rank of catalytic
activities of the ion-exchanged X and Y zeolites compares
very well to the expected order of basic strength reported in
Figure 3.
The hydroxide-washed KX and CsX catalysts were more
active for ethylene carbonate formation than their water-washed
counterparts (Table 5). As discussed earlier, washing the ion-
exchanged zeolite with alkali-metal hydroxide is likely to
introduce trace amounts of excess alkali metal in the zeolite
cages, which can enhance activity. Indeed, Engelhardt et al. have
shown that washing alkali-metal-exchanged zeolites with water
introduced some acidity in the materials, leading to increased
selectivity for acid-catalyzed products in the alkylation of
toluene with methanol.58 However, treatment of the ion-
exchanged zeolite with alkali-metal hydroxide solution resulted
in improved selectivity to the base-catalyzed products.58
Occlusion of cesium oxide in the X and Y zeolites improved
the activity for ethylene carbonate. Tsuji et al.3 and Kim et al.7
have reported an increase in the activity of CsX for 1-butene
isomerization upon incorporation of occluded cesium oxide
clusters. Similarly, Hathaway and Davis found that occlusion
of cesium oxide clusters in CsY resulted in an increase in
selectivity to acetone, the product primarily formed over base
sites, in the decomposition of 2-propanol.1 Our activity results
for ethylene carbonate formation are consistent with these
observations.
Conclusions
The adsorption of iodine as a probe for surface basicity has
been shown to be effective in determining the relative basicity
for alkali-metal-exchanged zeolites. Blue shifts in the visible
absorption spectra of iodine correlated well with the basic
strength of alkali-metal-exchanged zeolites. Extending this
technique to supported alkali-metal species in zeolite pores and
on oxide surfaces has yielded mixed results. Occluded alkali
species apparently interacted irreversibly with iodine yet still
exhibited a visible absorption band near 360 nm. Alkali-metal
ion-exchanged zeolites were active for the reaction of ethylene
oxide with carbon dioxide to make ethylene carbonate, and the
activity increased with the electropositivity of the exchange
cation. Incorporation of the occluded cesium oxide species in
the ion-exchanged zeolites via decomposition of impregnated
cesium acetate further promoted their catalytic activity. Water
adsorbed in zeolite pores played a critical role in ethylene
carbonate formation.
Acknowledgment. This work was supported by the Depart-
ment of Energy (Basic Energy Sciences, Grant DEFG05-
95ER14549) and the Virginia Academic Enhancement program.
The activity of CsX (water washed) for ethylene carbonate
formation showed a linear increase with time, reaching a yield
of 31.6% after 10 h. For comparison purposes, the yield of
ethylene carbonate after 3 h in a homogeneously catalyzed
reaction using tetraethylammonium bromide was 76.3%. Thus,
the yields reported for alkali-metal-modified zeolites in Table
5 are not limited by thermodynamic equilibrium.
References and Notes
(1) Hathaway, P. E.; Davis, M. E. J. Catal. 1989, 116, 263.
(2) Corma, A.; Fornes, V.; Martin-Aranda, R. M.; Garcia, H.; Primo,
J. Appl. Catal. 1990, 59, 237.
(3) Tsuji, H.; Yagi, F.; Hattori, H.; Kita, H. Stud. Surf. Sci. Catal. 1992,
75, 1171.
To ensure that the reaction between ethylene oxide and CO2
was heterogeneously catalyzed, the reaction mixture from a run
with CsX (water washed) was used in a subsequent experiment
after removal of the zeolite catalyst. A fresh feed of ethylene
oxide was added to the reaction mixture, and the vessel was
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