Propane selective oxidation on alkaline earth exchanged zeolite Y: Room temperature in situ IR study

Article abstract of DOI:10.1039/b305777c

The effect of zeolite Y ion-exchanged with a series of alkaline-earth cations on selective propane oxidation at room temperature was studied with in situ IR spectroscopy. Isopropylhydroperoxide was a reaction intermediate and could be decomposed into acetone and water. BaY was active at room temperature. The reaction rate increased in the order BaY < MgY < SrY < CaY based on the rate of formation of adsorbed acetone. The acetone/water ratio increased with cation size, while no other products could be detected. The acetone/isopropylhydroperoxide ratio decreased with decreasing number of Bronsted acid sites. A two-step mechanism with two different active sites was proposed. Propane conversion into isopropylhydroperoxide took place on cations, while acetone decomposition occurred by Bronsted acid sites.

Full text of DOI:10.1039/b305777c

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Propane selective oxidation on alkaline earth exchanged zeolite Y:
room temperature in situ IR studyy
Jiang Xu, Barbara L. Mojet,* Jan G. van Ommen and Leon Lefferts
Catalytic Processes and Materials, Faculty of Science and Technology, University of Twente,
P.O. box 217, 7500 AE, Enschede, The Netherlands. E-mail: B.L.Mojet@utwente.nl;
Fax: (+31)53 4894683
Received 25th May 2003, Accepted 7th July 2003
First published as an Advance Article on the web 30th July 2003
The effect of zeolite Y ion-exchanged with a series of alkaline-earth cations on selective propane oxidation at
room temperature was studied with in situ infrared spectroscopy. Isopropylhydroperoxide was observed as a
reaction intermediate and can be decomposed into acetone and water. Contrary to previous studies, BaY was
found to be active at room temperature. The reaction rate increased in the order BaY < MgY < SrY < CaY
based on the rate of formation of adsorbed acetone. Surprisingly, the acetone/water ratio was found to increase
with cation size, while no other products could be detected. Moreover, the acetone/isopropylhydroperoxide
ratio decreased with decreasing number of Brønsted acid sites. Both observations mark the importance of
Brønsted acid sites for this reaction, in addition to alkali (earth) cations. A two-step mechanism with two
different active sites is proposed. Conversion of propane into isopropylhydroperoxide takes place on cations,
while the decomposition into acetone occurs by Brønsted acid sites.
was reported at conversions as high as about 35%, based on
in situ FTIR measurements.
Introduction
About a quarter of the industrial production of monomers and
chemical intermediates is made by catalytic selective oxidation
processes. Most catalysts used in these industrial processes are
based on mixed metal oxides. Typically, direct oxidation of
hydrocarbons by O2 gives poor selectivity at high conversion,
which limits conversion to a few percent in most practical pro-
cesses. The inefficiency associated with low conversion has
motivated the search for solid catalysts with higher activity
in selective oxidation of hydrocarbons.1 Driving forces for
new breakthroughs are: (i) Use of cheaper and more easily
available raw materials, like small saturated hydrocarbons
such as methane, ethane and propane. The underlying moti-
vation is the better use of natural gas and volatile petroleum
fractions.2–4 (ii) Selectivity control. Lack of selectivity is sev-
ere if conversion is pushed beyond a few percent due to over-
oxidation of partially oxidized products.4,5 (iii) Environmental
constraints. Development is needed of processes that produce
less waste and have greater safety.
If the electrostatic field is the main factor determining activ-
ity, the rate of reaction should increase with increasing electro-
static field when changing cation size and charge. Vanoppen
et al.12 found that reaction activity increased in the order
CaY > SrY > BaY > NaY during cyclohexane oxidation in
gas and liquid phase. Based on cation dependence, the results
in the gas phase were consistent with the mechanism proposed
by Frei involving the formation of an alkane–oxygen charge
transfer complex. However, a classical auto-oxidation process
was proposed after analysis of the reaction kinetics in the
liquid phase. To link the change in reaction mechanism from
gas to liquid phase, Larsen et al.13 investigated the kinetics
of thermal and photochemical oxidation of cyclohexane in
zeolite NaY and BaY. The overall conversion was found to
dramatically decrease at cyclohexane loading higher than three
molecules per supercage, attributed to limited diffusion of
cyclohexane and oxygen in the zeolite pores. Pronounced deu-
terium kinetic isotope effects indicated the proton transfer as
the rate-limiting step for both the thermal and photochemical
reaction.
With zeolite NaY, selective oxidation did not occur for
hydrocarbons with higher ionization potential (IP), such as
propane, as was reported by Frei et al..8 Based on the charge
transfer mechanism, they explained this by the lower electro-
static field of sodium cations. So, if the electrostatic field
governs this reaction, higher reaction rates may be achieved
by using cations with large field (e.g. Mg2+). Further, the
mechanism of thermal decomposition of the alkylhydroper-
oxide intermediate is not yet clear. While zeolite NaY and
BaY can be prepared free of acid sites, this is not possible for
CaY and SrY.14,15 The latter inevitably contain Brønsted acid
sites, but their importance for this reaction has not yet been
clarified.
Recently, Frei et al. demonstrated that oxidation of ethane,
propane, isobutane and cyclohexane with molecular oxygen to
corresponding oxygenates occurs with very high selectivity
under mild thermal and photochemical conditions in cation
exchanged zeolites, such as NaY and BaY.6–11 The reaction
mechanism proposed by Frei’s group involved a charge trans-
fer complex, [(CnH2n+2)+O2ꢀ]. The hypothesis is that the elec-
trostatic field of the exchanged cation stabilizes the charge
transfer complex. In the next step, a proton from hydrocarbon
ꢀ
cation radical is abstracted by O2 to form HO2 and a hydro-
carbon radical. Subsequently, the HO2 radical attacks the
hydrocarbon radical to form alkylhydroperoxide. Using FTIR
spectroscopy, it was observed that alkylhydroperoxide reacts
thermally to form oxygenates and water. Complete selectivity
Frei et al. have examined the propane thermal oxidation on
CaY and BaY. At room temperature the reaction was found to
y Presented at the International Congress on Operando Spectroscopy,
Lunteren, The Netherlands, March 2–6, 2003.
DOI: 10.1039/b305777c
Phys. Chem. Chem. Phys., 2003, 5, 4407–4413
This journal is # The Owner Societies 2003
4407

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occur only on CaY. The objective of the current work is to
extend propane thermal oxidation to a series of alkaline-earth
exchanged zeolites (MgY, CaY, SrY and BaY) using in situ
FTIR spectroscopy at low propane and oxygen pressure. This
approach allows us to investigate further the effect of cations,
and especially of acid sites on propane adsorption and selective
oxidation via analysis of adsorbed species on the catalyst
surface.
argon for 10 min and subsequently CDCl3 was added. The
suspension was filtered and the filtrate was analyzed on a
Varian Inova NMR spectrometer with chemical shifts relative
to CDCl3 .
Results
1. Catalyst characterization
Transmission FTIR spectra of the activated samples are given
in Fig. 1. All samples exhibited an isolated silanol peak at 3744
cmꢀ1. The band can be attributed to SiOH groups either on the
outer surface of zeolite crystals or on silica impurities.14,16–18
Further, absorption bands at 3645 cmꢀ1 for MgY, CaY, SrY
and at 3555 cmꢀ1 for MgY indicated the formation of
Brønsted acid sites, which is in agreement with literature.14–16
The intensity of Brønsted acid sites at 3645 cmꢀ1 was almost
the same for CaY and MgY, and only a small amount was
observed for SrY. BaY did not exhibit any Brønsted acid sites.
Further, additional bands at 3695 cmꢀ1 for MgY, 3590 cmꢀ1
for CaY and 3570 cmꢀ1 for SrY can be attributed to
M(OH)x species.16,18 The intensity of this species is obviously
higher on MgY zeolite.
Experimental
Materials
Alkaline-earth cation (Mg, Ca, Sr, and Ba) exchanged Y-zeo-
lites were prepared from NaY (Akzo Nobel, sample code:
1122–207). The parent zeolite was exchanged three times with
a 0.1 M solution of respectively magnesium chloride, calcium
chloride, strontium nitrate, or barium chloride (Merck) for
20 h at 90 ꢁC under stirring. The resulting alkaline-earth
exchanged zeolite Y was washed three times with distilled
water, filtered and dried at 100 ꢁC overnight. The chemical
composition of catalysts was analyzed by X-ray fluorescence
(Table 1). Power X-ray diffraction gave no indication for
collapse of the zeolite structure, even not after calcination at
650 ꢁC.
The T–O–T overtone vibrations in the 1700–1900 cmꢀ1
regions showed a clear shift with changing cation, in agree-
ment with the literature.19
Infrared spectroscopy studies
2. Adsorption of propane
The zeolite powder was pressed into a self-supporting wafer
and analyzed in situ during all treatments (i.e. activation,
adsorption and reaction process) by means of transmission
FTIR spectroscopy using a Bruker Vector22 FTIR spectro-
meter with a MCT detector. A miniature cell, equipped with
transparent CaF2 windows, which can be evacuated to pres-
sures below 10ꢀ7 mbar was used for the in situ experiments.
The temperature is variable from room temperature to
500 ꢁC. Each spectrum consists of 32 scans taken at 4 cmꢀ1
resolution. The spectra were corrected for absorption of the
activated zeolite.
Fig. 2a presents the FTIR spectra of adsorbed propane for the
different alkaline-earth exchanged zeolites Y. Interestingly,
propane adsorption quantities were observed to increase with
increasing cation radius as can be seen from the band intensity
of methyl and methylene stretch (between 3100 cmꢀ1 and 2700
cmꢀ1) and deformation vibrations (between 1500 cmꢀ1 and
1300 cmꢀ1).20 Two main peaks can be observed in the range
of 3100 cmꢀ1 to 2700 cmꢀ1. The position of the high frequency
peak (2970 cmꢀ1) was not affected by exchanging cations, but
the low frequency band (from 2795 to 2849 cmꢀ1) shifted
towards higher wavenumber with increasing cation radius.
Further, the intensity ratio of the high to low frequency band
decreased with increasing the cation radius.
The samples were activated in vacuum (<10ꢀ7 mbar) at
500 ꢁC (ramp 10 ꢁC minꢀ1) for 2 h, subsequently cooled down
to 200 ꢁC (dwell 10 h), and cooled to room temperature. Load-
ing of reactants (propane and oxygen) was controlled by gas
pressure. Propane was introduced into the IR cell until equili-
brium was reached at 1 mbar in the gas phase, followed by
addition of 40 mbar of oxygen.
In Fig. 2b it can be seen for CaY that the ratio of the high to
low frequency band is a function of propane coverage. How-
ever, no shift was detected for the low frequency band upon
increasing propane pressure. Similar spectra were observed
for the other catalysts.
Blank experiments were carried out for an empty cell and
the parent NaY zeolite. In both cases, no reaction between
propane and oxygen occurred at room temperature.
Propane adsorption did not affect the vibrations in the
hydroxyl region. Moreover, after admission of oxygen, no
1
Ex situ H-NMR
Ex situ NMR spectroscopy was used to analyze product for-
mation. For this the reaction was carried out in a closed glass
reactor with 0.2 bar propane, 0.8 bar oxygen and 100 mg CaY
zeolite. After 48 h of reaction the sample was flushed with
Table 1 Sample composition (determined by XRF)a
Chemical composition (wt.%)
M2+/Al
Al2O3
SiO2
Na2O
MO
(molar ratio)
MgY
CaY
SrY
22.78
22.18
20.22
19.41
67.34
65.79
59.65
57.83
3.80
1.58
1.48
2.76
6.15
0.34
0.43
0.45
0.35
10.39
18.61
20.47
BaY
a
M is alkaline earth cation; iron content <0.02 wt.%.
Fig. 1 FTIR spectra of activated catalysts.
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Fig. 2 FTIR spectra of (a) propane adsorption on different cation exchanged Y zeolite; (b) propane adsorption as a function of partial pressure
(from 0.0013 mbar to 1 mbar) on CaY zeolite.
spectral changes of adsorbed propane were found. Evacuation
at room temperature resulted in the disappearance of the
propane signals, indicating propane was weakly adsorbed.
were the only products detected by the infrared spectrum
under the applied reaction conditions. Infrared frequencies of
products and assignment are collected in Table 2. Iꢀn1terestingly
=
the C O stretch of acetone was found at 1682 cm for CaY,
SrY and BaY while a shift to 1694 cmꢀ1 was observed on
MgY. Further, with increasing cation radius a shift to lower
frequency for water O–H stretch and a shift to higher wave-
number for H–O–H bending was observed. No CO2 or other
oxygenates were detected in the spectra or in the gas phase
as analyzed with on-line mass spectrometry for all catalysts.
3. Reaction studies
After loading 1 mbar propane and 40 mbar oxygen on acti-
vated CaY, infrared spectra showed reactant depletion and
product growth as a function of time (Fig. 3). Bands at 1682
cmꢀ1, 1420 cmꢀ1, 1379 cmꢀ1, and 1367 cmꢀ1 can be attributed
to acetone as confirmed by comparison with the infrared spec-
trum of adsorbed acetone on CaY and by literature data.8,21
The growing band at 1634 cmꢀ1 is due to bending of adsorbed
water molecules, while at the same time a band at 3695 cmꢀ1
develops, which is attributed to the isolated OH vibration of
adsorbed water molecules.14–16,22
Fig. 5 displays the growth of acetone (nC O) (a) and water
=
(dH O) (b) infrared bands (integrated area) as a function of
2
time. The acetone formation rate was found to decrease
in the order of CaY > SrY > MgY > BaY. The water for-
mation rate turned out to decrease according to CaY >
MgY > SrY > BaY. In the case of BaY (insert Fig. 5a),
the reaction rate accelerated with reaction time. Moreover,
Fig. 5c shows the acetone/water ratio based on the data in
Fig. 5a and 5b. The ratio of acetone to water increases with
increasing cation radius. In BaY, the ratio decreases with
reaction time.
Infrared spectra of alkaline-earth exchanged zeolites after
20 h reaction are presented in Fig. 4. Oxidation of propane
to acetone was observed for all zeolites. Acetone and water
Fig. 6 shows the room temperature 1H-NMR spectrum after
extraction of the products by CDCl3 . The peaks showed that
only acetone and water could be extracted from the zeolite.23
4. Evacuation after reaction
After 20 h reaction, the samples were evacuated for 5 min at
room temperature. The infrared spectra clearly showed that
adsorbed species remained on the zeolites (Fig. 7), also no des-
orption of any product molecules was detected by on-line MS
analysis. After removal of adsorbed propane, new bands
around 2990–2980 cmꢀ1 and between 1470–1300ꢀ1 were
detected that were previously masked by propane, which could
be attributed to isopropylhydroperoxide8,10 (Table 2). No
isopropylhydroperoxide was observed for MgY after 20 h of
reaction. The peak at 2925 cmꢀ1 on all zeolites was assigned
to acetone CH3 stretch after comparison of adsorbed acetone
on activated zeolites. Evaluation of the acetone to isopropyl-
hydroperoxide ratio showed increasing amounts of isopropyl-
hydroperoxide relative to acetone with larger cation size.
Fig. 3 FTIR spectra of CaY zeolite after loading 1 mbar propane
and 40 mbar oxygen at room temperature as a function of time.
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Fig. 4 FTIR spectra of 1 mbar propane and 40 mbar oxygen oxidation at room temperature on MgY, SrY and BaY after 20 h reaction.
shows increasing amounts for MgY < CaY < SrY < BaY
(Fig. 2), which has the same order as increasing basicity of
framework oxygen and decreasing hydrolysis level as deduced
from the amount of Brønsted acid sites (see below). The differ-
ent adsorption spectra of propane also suggest that the
adsorbed molecule may be influenced by nearby cations. There
is no obvious relation between reaction rate and propane
adsorption quantity with different alkaline-earth exchanged
zeolite Y. However, the lower wavenumber band (from 2795
to 2849 cmꢀ1) of propane decreased in frequency with smaller
cation radius. Since isopropylhydroperoxide and acetone are
the reaction products, it is likely that the CH2 group of pro-
pane should be activated. As the low frequency band shows
a shift with cation, we assign this C–H vibration for the
moment to the CH2 group of propane. Additional experiments
with deuterated propane will be performed to give more
detailed information.
Discussion
1. Effect of cation on propane adsorption and oxidation
The interaction of propane molecules with zeolites has been
reported to be due to enhanced dispersion-force-interaction
of non-polar molecules with framework oxygen.22,24,25 When
the framework negative charge is compensated by cations with
low electronegativity, the charge on the oxygen atoms may be
high enough to create basic properties. In most zeolite struc-
tures all oxygen atoms are accessible to adsorbates.26 The
calculated average charge of lattice oxygen (ꢀd0) with different
alkaline-earth exchanged zeolite is given in Table 3. The
basicity of framework oxygen increases with increasing cation
radius. Further, a lower degree of hydrolysis of water will lead
to an increased amount of localized basic lattice oxygen.14 The
quantity of adsorbed propane at 1 mbar equilibrium pressure
A cation effect has been demonstrated previously for NaY
vs. BaY by Frei’s group8–11 and Vanoppen et al.12,27 on their
studies of photo and thermal oxidation of hydrocarbons in
zeolites. The effect of type of cation on the oxidation activity
of zeolites was attributed specifically to the electrostatic field
of the cations. The present study shows that the acetone for-
mation rate follows the order CaY > SrY > MgY > BaY
(Fig. 5a). With increasing electrostatic field, propane reaction
rate increases in order BaY < SrY < CaY. These results are
consistent with cyclohexane oxidation in alkali and alkaline-
earth zeolite Y (NaY, BaY, SrY, CaY) in gas and liquid phase
as reported by Vanoppen et al.12,27 However, MgY is less
active than CaY and SrY, even though it has the smallest
ion radius, having the highest electrostatic field.18
While the same ion-exchange procedure was carried out, ele-
mental analysis in Table 1 indicated that CaY and SrY had
higher cation concentration than MgY and BaY zeolite. More-
over, FTIR spectra of the activated zeolites showed that the
hydrolysis degree (creating hydroxide and/or Brønsted acid
sites) varies with exchanged cation according to MgY >
CaY > SrY > BaY. One source of Brønsted acid sites for
these cation-exchanged zeolites is now generally accepted to
occur via:
Table 2 Absorption frequencies of propane oxidation products on
different alkaline-earth exchanged zeolite Y (in cmꢀ1).
Reaction product
MgY
3700
CaY
3695
SrY
BaY
Assignment
Reference
3686
3430
1644
3652
3611
3683
3430
1650
H2O
H2O
16; 18
16; 18
8
1630
3652
3611
1634
3652
3611
H2O
Brønsted Acid
Brønsted Acid
Isopropylhydroperoxide
Isopropylhydroperoxide
Isopropylhydroperoxide
Isopropylhydroperoxide
Isopropylhydroperoxide
Isopropylhydroperoxide
Isopropylhydroperoxide
Acetone
16; 18
16; 18
8
3245
2983
1470
1453
1390
1346
1318
2998
2994
1396
10
8; 10
8; 10
8; 10
8
8
3013
2925
1694
1420
1379
1367
3013
2925
1682
1420
1379
1367
3013
2925
1682
1420
1379
1367
ASa
AS
2925
1682
1420
1379
1367
Acetone
Acetone
8; AS
8; AS
8; AS
8; AS
Acetone
Acetone
Acetone
a
2þ þ H2Oðad:Þ þ Si O Al $ MðOHÞþ þ Si OðHþÞ Al
AS: authentic sample.
M
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Fig. 5 Integrated intensity of (a) acetone (nC O), (b) water (dH O) and (c) acetone/water ratio as a function of reaction time during exposure of 1
=
2
mbar propane and 40 mbar oxygen on alkaline earth exchanged zeolite Y at room temperature.
This reaction is thought to occur during pretreatment or acti-
vation at elevated temperature. In the presence of low amounts
of water, M2+ ions become localized and the associated elec-
trostatic field may induce dissociation of coordinated water
molecules to produce M(OH)+ and H+. The latter combine
with lattice oxygen to give Si–O(H)–Al groups similar to
decationated samples.14,16,28 The higher the electrostatic field,
the easier the dissociation of water and the more stable the
M(OH)+ species is. In addition, the presence of a Mg(OH)x
vibration the spectra suggests that in MgY most of the cations
hydrolyzed into Mg(OH)+. Consequently, fewer Mg2+ are
present in the zeolites, explaining the lower observed activity.
Interestingly, for BaY reaction occurred at room tempera-
ture in our experiment while a reaction temperature of 55 ꢁC
was reported necessary by Frei’s group.8 This may be attri-
buted to different water content due to different activation pro-
cedures. No water is present after activation in this paper,
whereas 0.3 H2O molecules per super-cage remained after the
activation procedure of Frei et al. Residual water in the zeolite
shields the cation via coulombic interaction, resulting in a
lower activity.
2. Product distribution
In order to probe product distribution, ex situ proton NMR
was applied to analyze the CDCl3 extraction solution. Only
water and acetone were found as products after oxidation of
propane at room temperature. Nevertheless, isopropylhydro-
peroxide was reported as the reaction intermediate of the
propane photo-oxidation reaction over BaY zeolite by Frei’s
group.8 In the present study isopropylhydroperoxide was only
observed after evacuation because its absorption peaks were
masked by propane bands. Raising the temperature or simply
waiting in time leads to the formation of acetone and water
due to isopropylhydroperoxide decomposition. The fact that
isopropylhydroperoxide was not detected by NMR may be
due to either a too low concentration, or to possible decompo-
sition during extraction, or to strong adsorption preventing
desorption into the solvent.
The molar ratio of acetone and water from propane partial
oxidation should be constant for all catalysts if a stoichio-
metric reaction between propane and oxygen takes place.
Evaluation of IR data (Fig. 5c), however, showed that the
acetone/water ratio increases with increasing cation radius.
It was reported that the products extinction coefficients are
the same with different cation exchanged zeolite Y.8 This
suggests that water was lost during reaction. However, water
desorption was excluded by on-line MS analysis. A possible
explanation would be that produced water hydrolyzes into
Brønsted acid sites and Ba(OH)+ species. Unfortunately, the
small changes in the OH-stretch region do not allow quantifi-
cation at present. Interestingly, on BaY, the acetone/water
ratio decreases with reaction time up to 20 h, while the
other catalysts have a constant ratio after 3 h. Since BaY
has mainly bare cations, produced water will be hydrolyzed,
and consequently not be observed at 1650 cmꢀ1. Hydrolysis
of water at room temperature has been reported when read-
sorbing water in dehydrated divalent cation exchanged zeolite
Fig. 6 Proton NMR spectrum of the products extracted in CDCl3
after 48 h loading 0.2 bar propane and 0.8 bar oxygen at temperature
30 ꢁC.
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Fig. 7 FTIR spectra after 20 h propane oxidation, and subsequent 5 min evacuation.
Y.14,15,18,28 This hydrolysis leads to in situ generation of
protons that are of importance for isopropylhydroperoxide
decomposition as will be discussed below.
frequency (1694 cmꢀ1) for MgY. The methyl stretching and
deformation vibrations were identical for all samples. Fig. 8
shows the FTIR spectrum of acetone adsorbed at room tem-
perature on CaY. Two nC
are visible at 1713 cmꢀ1 and
=
O
1701 cmꢀ1. The negative band at 3640 cmꢀ1 indicates inter-
3. Product adsorption
=
action of acetone with Brønsted acid sites. Both C O bands
merged and shifted to 1683 cmꢀ1 when increasing temperature
under vacuum, where it remained after cooling to room tem-
It can be seen in Table 2 that the OH-stretch frequency shifted
gradually from 3700 cmꢀ1 for MgY to 3683 cmꢀ1 for BaY,
while the H2O bending mode frequency increased from 1630
cmꢀ1 for MgY to 1650 cmꢀ1 for BaY. Literature reporting
on H2O direct bonding to cations showed a shift in wavenum-
ber of isolated OH stretching from high frequency for LiX to
low frequency for KX, while the H2O bending mode shifted
from low frequency for LiX to high frequency for KX.29 The
identical trends of water adsorption on the present alkaline-
earth exchanged zeolites Y indicates that water is adsorbed
on cation sites.
In addition, the methyl stretch vibrations for adsorbed iso-
propylhydroperoxide also shift in wavenumber from 2998
cmꢀ1 on CaY to 2983 cmꢀ1 on BaY, showing a similar trend
as the OH stretch frequency of adsorbed water. Therefore,
we conclude that adsorbed isopropylhydroperoxide is also
related to the cation site.
=
perature (Fig. 8b). The total integrated intensity of C O
absorption remained the same. Further, methyl stretch and
deformation bands showed no changes. Infrared spectroscopy
has been used to study acetone adsorption on zeolite acid
sites.30,31 Bands at 1679 ꢂ 1682 cmꢀ1 and 1690 ꢂ 1702 cmꢀ1
=
were assigned to acetone C O interaction with bridging
Brønsted OH and with Lewis acid sites respectively. Moreover,
the Brønsted acid site vibration is independent of the cation
(Fig. 1). Thus we conclude that acetone is adsorbed on or
nearby Brønsted acid sites, resulting in identical adsorption
=
For acetone the data showed the same C O stretching
wavenumber (1682 cmꢀ1) for CaY, SrY and BaY and a higher
Table 3 Zeolite calculated intermediate Sanderson electronegativity
(Sint),a calculated average charge on the oxygen of the lattice (ꢀdO)
and electrostatic potential (e/r)
Sint
ꢀdO
(e/r)/Aꢀ1
MgY
CaY
SrY
2.810
2.802
2.777
2.764
0.210
0.214
0.221
0.224
3.16
2.01
1.77
1.48
BaY
a
Intermediate Sanderson electronegativity Sint calculated from
Sint ¼ (SPp SqQSrRStT)1/(p+q+r+t) for compound PpQqRrTt ,33 for electro-
negativity S of atom P, see Sanderson.34 Calculated average charge
1/2 33
.
Fig. 8 Adsorbed acetone (0.0005 mbar) on CaY zeolite (a) At room
temperature; (b) After 15 minutes heating at 300 ꢁC under vacuum,
cooled to room temperature.
ꢀdO on the oxygen of the lattice, using (Sint ꢀ SO)/2.08SO
Electrostatic potential (e/r), see Ward.18
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frequencies for CaY, SrY and BaY, and a higher frequency
for MgY which also contains Mg(OH)x species.
rates for BaY < MgY < SrY < CaY. The observed lower acet-
one growth rate for MgY, not expected based on its higher
electrostatic field, can be explained by a higher hydrolysis
degree of the cations, resulting in a lower amount of available
Mg2+ ions. Based on the FTIR data, water and isopropylhy-
droperoxide are adsorbed on or close to the cations in the
zeolite, while acetone adsorbs on Brønsted acid sites, pointing
to a two-step mechanism in which different active sites are
involved. Moreover, surface reaction activity and selectivity
(towards hydroperoxide or acetone) are determined by the
presence of bare divalent cations as well as the number of
Brønsted acid sites.
4. Reaction mechanism
It has been proposed that the high mobility of alkane and oxy-
gen in zeolites allows easy access to poorly shielded cation
sites, resulting in spontaneous formation of an intermolecular
charge-transfer complex, stabilized by parallel orientation to
the electrostatic field, resulting in alkane radical cation and
ꢀ
O2 formation.6,7,11 After proton transfer from the hydrocar-
bon radical cation to the superoxide,13 radical recombination
is easily reached to form isopropylhydroperoxide, which is also
observed as intermediate in our reaction. Thermal rearrange-
ment of isopropylhydroperoxide to acetone and water is
well-established8 resulting in the overall reaction:
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Hydroperoxide decomposition has been reported to be
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ment of cyclohexylhydroperoxide to cyclohexanone and water
is an established intramolecular heterolytic reaction in acidic
solution.32 In the present study, the relative ratio between
isopropylhydroperoxide and acetone decreased according to
BaY > SrY > CaY > MgY (Fig. 7). On the other hand, the
Brønsted acidity increases for BaY < SrY < CaY < MgY
(Fig. 1). The different adsorption sites of isopropylhydroperox-
ide and acetone, as discussed above, suggest that the formation
of hydroperoxide and its decomposition occurred at different
active sites. The cation site directs the reaction to isopropylhy-
droperoxide while the Brønsted acid sites are involved in the
decomposition into acetone and water (Scheme 1). Additional
evidence for this reaction scheme can be found in the acetone
production curve of BaY (inset Fig. 5a). The acetone produc-
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attributed to creation of acid sites due to hydrolysis of pro-
duced water, in agreement with the high acetone to water ratio
observed for BaY. Due to the very low concentration of pro-
duced water, quantification of hydrolysis level from the spec-
trum is difficult. It is not yet clear whether the cation and
Brønsted acid sites act independently in the reaction or that
a cooperation of both sites is needed, this is presently being
investigated.
2
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Scheme 1 Proposed mechanism for propane selective oxidation.
Phys. Chem. Chem. Phys., 2003, 5, 4407–4413
4413