Role of potassium added to Ce/NaZSM-5 catalyst in partial oxidation activity of benzyl alcohol

Article abstract of DOI:10.1039/b008093o

Cerium oxides have been used as one of the components in the three-way automobile catalyst, in which cerium oxide catalysts function as an oxygen reservoir and/or heat-proof component. The effect of the alkali metal (K) added to the cerium-supported NaZSM-5 (Ce/NaZSM-5) catalyst on the catalytic activity in benzyl alcohol vapor-phase oxidation was studied using the Ce/NaZSM-5 catalysts with various cerium contents. The main products were benzaldehyde and CO₂, together with trace amounts of benzene and toluene. The yield of benzaldehyde catalyzed by Ce/NaZSM-5 increased monotonically with the increase in the Ce loading. The K(4)/Ce/NaZSM-5 catalyst provided a maximum value in the yield of benzaldehyde at a Ce loading of around 0.2 mmol/g(NaZSM-5). Thus, the added K caused a decrease in the oxidation activity of the Ce_H (high Ce)/NaZSM-5 catalysts with the higher Ce loading. The CO₂ yield by the K(4)/Ce/NaZSM-5 catalyst were lower than those by the Ce/NaZSM-5 catalyst regardless of the Ce loading.

Full text of DOI:10.1039/b008093o

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Role of potassium added to Ce/NaZSM-5 catalyst in partial
oxidation activity of benzyl alcohol
Nao Idaka, Satoru Nishiyama and Shigeru Tsuruya*
Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University,
Nada, Kobe, 657-8501, Japan. E-mail: tsuruya=cx.kobe-u.ac.jp
Received 6th October 2000, Accepted 7th March 2001
First published as an Advance Article on the web 11th April 2001
The vapor-phase catalytic oxidation of benzyl alcohol was carried out over cerium-impregnated NaZSM-5
(Ce/NaZSM-5) and K-added Ce/NaZSM-5 catalysts. The main products were benzaldehyde and carbon
dioxide, together with trace amounts of benzene and toluene. The e†ect of the added K on the catalytic activity
in the benzyl alcohol oxidation was di†erent by the amount of supported Ce (Ce: 1.4]10~2È7.1 ] 10~1 mmol
g(NaZSM-5)~1, low Ce (Ce ): ^10~2 mmol g(NaZSM-5)~1, high Ce (Ce ): ^10~1 mmol g(NaZSM-5)~1).
L
H
The BET surface area and the temperature-programmed oxidation (TPO) of the fresh and the used K-added
Ce/NaZSM-5 catalysts was measured to obtain information on the role of the added K in the catalytic activity
of the benzyl alcohol oxidation. The amount of O uptake of the pre-reduced K-added Ce /NaZSM-5
2
H
catalysts was also measured to estimate the ability to store the oxygen species on the cerium. The main role of
potassium added to the high Ce-loaded NaZSM-5 (Ce /NaZSM-5) catalyst was to prevent the build-up of
H
coke-like materials on the catalyst surface during the oxidation and so prolong the catalytic life.
Cerium oxides have been utilized as one of the components in
the three-way automobile catalyst, in which cerium oxide
catalysts have been reported to function as a oxygen reservoir
and/or a heat-proof component. The addition of Ce to Cu-
NaZSM-5, which has been reported1 to be speciÐcally active
for NO decomposition, caused an increase in the NO
decomposition activity.2 Cerium-supported ZSM-5 catalyst
promoted by alkaline earth metals has been shown to be e†ec-
tive for the SCR of NO using propene as a reductant.3,4 The
addition of Ce to the Ag-ZSM-5 catalyst promoted the oxida-
catalytic activity in benzyl alcohol oxidation was investigated
using the Ce/NaZSM-5 catalysts with a wide variety of cerium
contents. The added K was found to have di†erent functions
in the catalysts with lower and higher contents of cerium. The
role of potassium added to the Ce/NaZSM-5 with a lower
content of cerium has been discussed previously.11 The role of
the potassium added to the Ce/NaZSM-5 catalyst with a
higher content of cerium will be mainly discussed in this
paper.
x
x
tion of NO to NO .5
Experimental
2
The liquid-phase oxidation of alcohol derivatives to the cor-
responding aldehydes using an oxidant is one of the environ-
mentally loaded reactions. From the green chemistry point of
view, gas-phase catalytic oxidation will be preferable because
of an easy separation of product from catalyst, no use of
solvent, and no generation of the used oxidant. The develop-
ment of a catalyst with high selectivity for the partial oxida-
tion is necessary in the gas-phase catalytic reaction. We have
reported the e†ect of the alkali metal added to Cu-supported
ZSM-5 catalysts on the gas-phase catalytic oxidation of
benzyl alcohol, in which the added alkali metals have been
reported to e†ectively promote the partial oxidation activ-
ity.6h10 The addition of alkali metals to cerium-supported
ZSM-5 catalyst causes an increase in the activity of the partial
oxidation of benzyl alcohol when the amounts of the sup-
ported cerium are lower but inversely causes a decline in the
oxidation activity using the cerium-supported ZSM-5 catalyst
with a higher cerium content.11
Catalysts
NaZSM-5 zeolite was synthesized according to a patent.12
The synthesized ZSM-5 zeolite was twice treated with 1 M
NaNO at 343È358 K for 1 h to obtain NaZSM-5 zeolite ion-
3
exchanged with 100% Na cation. The NaZSM-5 zeolite was
dried at 383 K overnight and calcined at 773 K for 5 h under
Ñowing air. The Si/Al atomic ratio determined by an atomic
absorption spectrometer (Shimazu Type AA-630-01) was 58.
HZSM-5 zeolite was prepared by ion-exchanging with 1 M
NH NO three times, drying overnight, and calcining at 773
4
3
K for 5 h. Cerium-supported NaZSM-5 (Ce/NaZSM-5) was
prepared using Ce(CH COO) É H O as a cerium source by
impregnation followed by calcination at 773 K for 5 h. Alkali
metal (K)-supported Ce/NaZSM-5 (K/Ce/NaZSM-5) was pre-
pared by impregnating CH COOK, drying at 393 K over-
night, and calcining at 773 K for 3 h in a Ñow of air. MCM-41
3
3
2
3
was synthesized by a solÈgel method at room temperature as
described previously.13 The BET surface area was 950 m2 g~1.
CeO was prepared by calcining Ce (CH COO) at 773 K for
2
2
3
3
5 h in Ñowing air. K-added CeO (K/CeO ) was prepared by
2
2
impregnating aqueous CH COOK, drying overnight, and cal-
cining at 773 K for 3 h in a Ñow of air.
3
Gas-phase catalytic oxidation of benzyl alcohol
In this study, the e†ect of the alkali metal (K) added to the
cerium-supported NaZSM-5 (Ce/NaZSM-5) catalyst on the
The gas-phase catalytic oxidation of benzyl alcohol was
carried out using a continuous Ðxed-bed Pyrex microreactor
1918
Phys. Chem. Chem. Phys., 2001 3, 1918È1924
This journal is ( The Owner Societies 2001
DOI: 10.1039/b008093o

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(id 15 mm) under atmospheric pressure. The catalyst (0.15 g)
placed in the reactor was pretreated at 773 K for 2 h in
Ñowing air. The reaction was started by feeding benzyl alcohol
from a microfeeder. Typical reaction conditions were as
Results and discussion
Catalytic activity of Ce/NaZSM-5 catalysts and K-added
counterparts in benzyl alcohol oxidation
follows: W /F \ 6.52 g(cat) min mol~1;
W
(catalyst
The main products obtained in the gas-phase catalytic oxida-
tion of benzyl alcohol over Ce/NaZSM-5 catalyst and
weight) \ 0.15 g; F (total molar Ñow) \ 0.023 mol min~1;
N : O : benzyl alcohol \ 32 : 3 : 1 (mol ratio). The products
2
2
K-added counterparts were benzaldehyde and CO . Fig. 1
were analyzed by gas chromatography; details of these
analyses have been described elsewhere.11 The conversion, the
yield, and the selectivity were deÐned7 as follows:
2
illustrates the inÑuence of the added K/Ce atomic ratio
on the yields of both benzaldehyde and CO using a
2
low Ce (Ce: ^10~2 mmol g(NaZSM-5)~1) supported
Conversion of benzyl alcohol (%)
NaZSM-5 (Ce /NaZSM-5) and the K-added counterpart
L
(K/Ce /NaZSM-5) catalysts. The yield of benzaldehyde had a
maximum value at the added K/Ce ratio of around 4, but the
CO yield did not vary by adding K to the Ce/NaZSM-5.
[Balde] ] 1[CO ] ] [Benz] ] [Tolu]
L
7
2
\
] 100
[Balco]
2
Thus the addition of K to the Ce /NaZSM-5 (Ce, 7.1 ] 10~2
[Balde]
[Balco]
L
mmol g(NaZSM-5)~1) caused both a higher yield and selec-
Yield of benzaldehyde (%) \
] 100
tivity for benzaldehyde. The apparent activation energy of
benzaldehyde formation catalyzed by K(4)/Ce /NaZSM-5
1[CO ]
[Balco]
L
7
2
(added K/Ce atomic ratio, 4; Ce, 7.1 ] 10~2 mmol g(NaZSM-
Yield of CO (%) \
] 100
2
5)~1), which was calculated using the data at 653 and 673 K,
was 170.9 kJ mol~1.
Selectivity of benzaldehyde (%)
The dependence of the yield of both benzaldehyde and CO
on the Ce loading was investigated using both Ce/NaZSM-5
2
[Balde]
\
] 100
] 100
and K(4)/Ce/NaZSM-5 (added K/Ce atomic ratio, 4) catalysts
(Fig. 2(A)). The yield of benzaldehyde catalyzed by Ce/
NaZSM-5 increased monotonically with the increase in the Ce
loading. The K(4)/Ce/NaZSM-5 catalyst gave a maximum
value in the yield of benzaldehyde at a Ce loading of around
0.2 mmol g(NaZSM-5)~1. Thus the added K caused a
[Balde] ] 1[CO ] ] [Benz] ] [Tolu]
7
2
Selectivity of CO (%)
2
1[CO ]
7
2
\
[Balde] ] 1[CO ] ] [Benz] ] [Tolu]
7
2
decrease in the oxidation activity of the Ce /NaZSM-5 cata-
H
lysts with the higher Ce loading. The yields of CO by the
2
Where [ ] is the number of moles produced or fed,
Balco \ benzyl alcohol, Balde \ benzaldehyde, Benz \
K(4)/Ce/NaZSM-5 catalyst were lower than those by the Ce/
benzene, Tolu \ toluene and CO \ carbon dioxide. The
NaZSM-5 catalyst regardless of the Ce loading. Fig. 2(B) illus-
trates the dependence of the turnover frequencies (TOFs
(h~1); benzaldehyde formed per unit time per Ce) of both Ce/
NaZSM-5 and the K(4)/Ce/NaZSM-5 catalysts based on the
experimental results of Fig. 2(A). The TOF was deÐned conve-
niently using the total amount of impregnated Ce, in place of
the amount of surface Ce. The TOF of the Ce/NaZSM-5 did
not vary greatly with the increase in the Ce loading. The TOF
of the K(4)/Ce/NaZSM-5 catalyst had a maximum value at a
Ce loading of around 0.2 mmol g(NaZSM-5)~1, and a higher
Ce loading caused a considerable reduction in the TOF.
2
carbon balances obtained were usually more than 90%.
Measurement of BET surface area of catalysts
The BET surface area of the catalysts was measured using a
gas-adsorption apparatus equipped with a digital manometer.
Measurement of the amount of O uptake of the pre-reduced
2
catalysts
The amount of O uptake of a catalyst (0.05 g) pre-reduced by
CO (20 kPa, 773 K, 1 h) was measured using semimicro-gas-
Fig. 3(A) shows the dependence of the time course of the
yield of benzaldehyde on the added K/Ce ratio of
2
absorption equipment with a capillary-glass sample tube.
Details of the experimental procedure have been described
previously.10,11
K/Ce /NaZSM-5 (Ce: 2.9 ] 10~1 mmol g(NaZSM-5)~1)
H
catalyst. The yield of benzaldehyde catalyzed by the
Di†use reÑectance (DR) spectra of catalysts
The DR spectra of the catalysts with high cerium loading were
observed at room temperature using an electronic absorption
spectrophotometer (Hitachi type U-3210D) equipped with an
integral sphere (Hitachi 150-0902). The catalyst samples were
placed in an in situ cell and were pretreated using a vacuum
line. The observed spectra were converted into the corre-
sponding KubelkaÈMunk (KM) function using an applied
program.
Temperature-programmed oxidation (TPO) of used catalysts
The TPO of the catalysts (10 mg) used in the benzyl alcohol
oxidation was performed under Ñowing N /O (\4 : 1) in the
2
2
temperature range of room temperature to 773 K at a pro-
grammed 20 K min~1 using a thermal balance attached to a
di†erential thermal analysis (DTA) unit (Shimazu type
DTG-40). NaZSM-5 zeolite was used as a reference for the
DTA.
Fig. 1 Dependence of benzaldehyde and CO yields on the added
2
K/Ce atomic ratio using a low-Ce-supported NaZSM-5 catalyst
(Ce /NaZSM-5). Catalyst, K/Ce /NaZSM-5 (Ce \ 7.1 ] 10~2 mmol
L
L
g(NaZSM-5)~1); reaction temperature, 653 K; …, yield of benz-
aldehyde; >, yield of CO .
2
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924
1919

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Fig. 2 Dependence of benzaldehyde and CO yields (A) and turn-
2
over frequency (TOF) (B) on Ce loading. Reaction temperature, 653
K. (A) …, >, K(4)/Ce/NaZSM-5 catalyst (K/Ce atomic ratio \ 4); L,
| Ce/NaZSM-5 catalyst; …, L, yield of benzaldehyde; >, |, yield of
CO . (B) …, K(4)/Ce/NaZSM-5 catalyst (K/Ce atomic ratio \ 4); L,
2
Ce/NaZSM-5 catalyst.
Fig. 3 E†ect of added K/Ce on product yield using K/Ce /NaZSM-
H
5 catalyst. Reaction temperature, 653 K. (A) Catalyst, K/Ce /NaZSM-
Ce /NaZSM-5 without added K decreased with time on
stream although the initial yield was high. The addition of K
to the Ce/NaZSM-5 catalyst inhibited the deactivation of the
oxidation activity. The yield of benzaldehyde at steady state
H
H
5
(Ce \ 2.9 ] 10~1 mmol g(NaZSM-5)~1); …, K/C \ 0; K,
K/Ce \ 1; |, K/Ce \ 2; ), K/Ce \ 4; L, K/Ce \ 8. (B) Catalyst,
K/Ce /NaZSM-5
(Ce \ 7.1 ] 10~1
mmol
g(NaZSM-5)~1);
H
…, K/Ce \ 0; =, K/Ce \ 0.4; >, K/Ce \ 4; L, K/Ce \ 0; K, K/
Ce \ 0.4; |, K/Ce \ 4; …, =, >, yield of benzaldehyde; L, K, |,
(time on stream of 2È4 h) catalyzed by the K(2)/Ce /NaZSM-
H
5 was thus higher than that catalyzed by the Ce /NaZSM-5
H
yield of CO .
2
without added K. Too high a loading of K (K/Ce \ 8) caused
a sharp decrease in the yield of benzaldehyde. The inÑuence of
added K on the yield of benzaldehyde by the K/Ce /NaZSM-
H
5 (Ce: 7.1 ] 10~1 mmol g(NAZSM-5)~1) catalyst is illustrated
in Fig. 3(B). The yield of benzaldehyde catalyzed by the
Ce /NaZSM-5 without the added K decreased with the time
H
on stream. The addition of K (K/Ce \ 0.4) caused no
improvement in the deactivation, but only a decrease in the
oxidation activity. The higher addition of K (K/Ce \ 4) did
not cause deactivation although the yield of benzaldehyde
decreased. The BET surface areas of both the Ce /NaZSM-5
H
(Ce: 7.1 ] 10~1 mmol g(NAZSM-5)~1) and the K-added (K/
Ce \ 4) counterpart were 240 and 40 m2 g~1, respectively.
The sharp decrease in the BET surface area upon the addition
of a large amount of K will be one of the reasons for the
decline in the oxidation activity.
The dependence of the yields of both benzaldehyde and
CO on the time on stream was investigated for a prolonged
2
time using both Ce /NaZSM-5 (7.1 ] 10~1 mmol g(NaZSM-
H
5)~1) and K(4)/Ce /NaZSM-5 (2.1 ] 10~1 mmol g(NaZSM-5)
H
~1) catalysts (Fig. 4). The yield of benzaldehyde produced by
Fig. 4 Plot of product yields vs. time on stream using Ce /NaZSM-5
the former catalyst monotonically decreased with the time on
H
(Ce: 7.1 ] 10~1 mmol g(NaZSM-5)~1) and K(4)/Ce /NaZSM-5 (Ce:
H
stream. The K(4)/Ce /NaZSM-5 catalyst was not deactivated
in the region of the time on stream measured. The yields of
benzaldehyde obtained by both catalysts resulted in almost
the same value at the time on stream of ca. 8 h. The yields of
2.1 ] 10~1 mmol g(NaZSM-5)~1) catalysts. Reaction temperature,
H
653 K; …, L, Ce /NaZSM-5 (Ce: 7.1 ] 10~1 mmol g(NaZSM-5)~1);
H
>, | K(4)/Ce /NaZSM-5 (Ce: 2.1 ] 10~1 mmol g(NaZSM-5)~1); …,
H
>, yield of benzaldehyde; L, |, yield of CO .
2
1920
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924

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Table 1 E†ect of support on the yield of benzaldehyde using low-Ce (Ce )-supported catalystsa
L
Yieldb (%)
Catalyst
Conversionb (%)
Benzaldehyde
CO
Benzene
Toluene
2
NaZSM-5
K/NaZSM-5
1.7
2.4
6.5
13.7
4.7
4.7
10.9
4.9
5.3
11.0
3.8
4.4
17.5
1.8
6.3
5.2
1.2
3.1
3.0
1.3
1.8
5.3
12.4
3.6
3.6
8.6
2.7
3.8
9.5
2.9
3.8
16.4
1.4
4.9
4.4
0.8
1.8
2.6
0.3
0.4
1.1
1.1
0.9
0.9
2.0
1.8
1.1
1.2
0.6
0.5
0.9
0.2
1.0
0.5
0.1
1.1
0.2
Trace
0.1
Trace
0.1
0.2
Trace
0.2
0.2
0.1
0.2
Trace
Trace
0.1
0.1
0.2
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
Trace
0.2
0.1
0.2
0.3
0.1
0.3
0.1
0.1
0.1
0.2
0.1
0.1
0.1
0.1
Ce /NaZSM-5
L
K/Ce /NaZSM-5
L
HZSM-5
Ce /HZSM-5
L
K/Ce /HZSM-5
L
NaY
Ce /NaY
L
K/Ce /NaY
L
Al O
2
3
Ce /Al O
L
2 3
K/Ce /Al O
L
2 3
SiO
2
Ce /SiO
L
2
K/Ce /SiO
L
2
MCM-41
Ce /MCM-41
L
K/Ce /MCM-41
L
a Ce (Ce ) loading, 7.14 ] 10~2 mmol g(support)~1 (1.0 wt.%); added K/Ce atomic ratio \ 4; reaction temperature, 673 K. b Average value of
L
three data obtained at the time-on-stream of 2, 3 and 4 h.
CO over both catalysts were low and almost constant with
the time on stream.
Too great an amount of Ce supported on the NaZSM-5 is
oxidation activity of the K(4)/CeO ] NaZSM-5 catalyst was
2
2
rather low. The covering of the added K on the active CeO
2
sites may cause the low catalytic activity for benzyl alcohol
supposed to cause the formation of CeO on the surface of the
oxidation.
2
NaZSM-5. CeO has been utilized as a support for CO
2
oxidation14h16 and as a catalyst for the conversion reaction of
Benzyl alcohol oxidation catalyzed by Ce-supported oxides with
and without added K
low-weight hydrocarbons.17 The catalytic activity for benzyl
alcohol oxidation of CeO itself was investigated using three
2
Benzyl alcohol oxidation was attempted using low-Ce-sup-
ported catalysts (Ce /support) on various oxides with and
without added K (Table 1). The supports without Ce and K
were also utilized as catalysts. The e†ect of added K on the
yield of benzaldehyde varied considerably depending on the
physically mixed catalytic systems of CeO ] NaZSM-5,
2
CeO ] K(4)/NaZSM-5 and K(4)/CeO ] NaZSM-5 (Fig. 5).
The amounts of Ce in the three catalysts were the same, and
the added K/Ce atomic ratios of the latter two systems were
L
2
2
kept at 4. The yield of benzaldehyde catalyzed by the CeO
] NaZSM-5 system considerably decreased with time on
stream. The deactivation was to some extent inhibited using
2
oxide support. The yield of benzaldehyde by Ce catalysts
supported on NaZSM-5, HZSM-5, NaY and Al O was pro-
L
3
2
moted with the addition of K. The Ce catalyst supported on
the CeO ] K(4)/NaZSM-5 catalytic system. The CeO
L
SiO and MCM-41 showed no promotion e†ect for benz-
2
2
2
] K(4)/NaZSM-5 catalyst used was conÐrmed to be almost
aldehyde formation on adding K, but the addition of K to the
as white as a fresh one, di†ering from the CeO ] NaZSM-5
2
Ce /MCM-41 catalyst caused inhibition of the deactivation of
catalytic system, in which the used one turned brown. The
L
both benzaldehyde formation and CO formation (Fig. 6). The
2
results in Table 1 and Fig. 6 were obtained under the condi-
tion of an added K/Ce atomic ratio of 4. However, the depen-
Fig. 5 Yield of benzaldehyde obtained using CeO ] NaZSM-5,
Fig. 6 Product yields obtained using MCM-41, Ce /MCM-41 and
2
L
K/Ce /MCM-41 catalysts. Reaction temperature, 673 K; amount of
L
CeO ] K(4)/NaZSM-5 and K(4)/CeO ] NaZSM-5 catalytic
2
2
systems. Reaction temperature, 623 K; amount of Ce, 9.7 ] 10~2
Ce, 7.1 ] 10~2 mmol g(support)~1; amount of K, 2.9 ] 10~1 mmol
mmol; amount of K, 3.9 ] 10~1 mmol (K/Ce \ 4); >, CeO
g(support)~1; =, K, MCM-41; >, |, Ce /MCM-41; …, L,
2
L
] NaZSM-5;
] NaZSM-5.
=,
CeO ] K(4)/NaZSM-5;
…,
K(4)/CeO
K/Ce /MCM-41; =, >, …, yield of benzaldehyde; K, |, L, yield of
2
2
L
CO .
2
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924
1921

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dence of benzyl alcohol oxidation on the added K/Ce may be
di†erent with the supports utilized. The optimum value of the
added K/Ce for the benzaldehyde formation may thus vary
with the support.
O uptake of both the Ce /NaZSM-5 catalyst and the K-added
2
H
counterpart which were prereduced
The inÑuence of the added K on the amount of O uptake of
2
the prereduced catalysts was investigated using the
Ce /NaZSM-5 (Ce, 2.9 ] 10~1 mmol g(NaZSM-5)~1) cata-
H
lysts (Fig. 7). The amount of O uptake was almost constant
2
with the variation in the added K/Ce atomic ratios of 0 to 8,
the behavior of which is quite di†erent from the variation in
the yield of benzaldehyde. A similar dependence between the
Fig. 8 InÑuence of added K/Ce atomic ratio on the yield of benz-
H
benzaldehyde yield and the amount of O uptake on the
aldehyde (…) and BET surface area (=). Catalyst, K/Ce /NaZSM-5
2
H
added Rb/Ce atomic ratio, with both maximum values
(Ce: 2.9 ] 10~1 mmol g(NaZSM-5)~1); reaction temperature, 653 K.
observed at an atomic ratio of around 4, has been observed11
using low-Ce-supported NaZSM-5 (Ce /NaZSM-5) catalysts
L
reason for the benzaldehyde yield declining with the increase
in the added K/Ce ratio will be the blockage of the pore of the
zeolite support.
(Ce, ^10~2 mmol g(NaZSM-5)~1) which were prereduced
under CO and degassed. Based on the results of both the O
uptake and the reaction behavior in the absence of gaseous
2
O , one role of an added alkali metal such as Rb was sug-
2
Deposition of carbonaceous material on the Ce/NaZSM-5 used
and the K-added counterpart
gested to be in increasing the amount of the sorbed O
2
species, which had a strong correlation with the formation of
benzaldehyde.11 On the contrary, the high-Ce-supported
It was visually observed that the Ce /NaZSM-5 (Ce,
2.9 ] 10~1 mmol g(NaZSM-5)~1) catalyst during the benzyl
alcohol oxidation turned from white to dark brown. The used
NaZSM-5 (Ce /NaZSM-5) catalyst (Ce, 2.9 ] 10~1 mmol
H
H
g(NaZSM-5)~1) utilized in this study showed no similar
dependence between the amount of O uptake and the benz-
2
K(4)/Ce /NaZSM-5 (Ce, 2.9 ] 10~1 mmol g(NaZSM-5)~1)
aldehyde yield on the added K/Ce ratio, as indicated in Fig. 7.
H
catalyst was light brown, but the K(8)/Ce /NaZSM-5 (Ce,
The large Ce particle corresponding to the Ce species may
H
H
2.9 ] 10~1 mmol g(NaZSM-5)~1) catalyst remained white.
not be greatly inÑuenced by the added alkali metal because
Based on these visual observations, the added K will be
related to the amount of the carbonaceous material accumu-
lated on the catalysts during the benzyl alcohol oxidation.
Benzyl alcohol and benzaldehyde are thought to be precursors
for carbonaceous materials which will be formed through
cyclization, dehydrogenation, further alkylation, etc., leading
to polyaromatics.18,19 The sites on which coke formation is
promoted will be Na` and/or H` sites ion-exchanged with
the surface area of the Ce species will be considerably lower
H
and the contact between the added alkali metal and the Ce
H
species will be small. On the other hand, the degree of disper-
sion of the Ce species will be comparatively higher, and the
L
high surface area may contribute to the variation of the redox
properties (O uptake) of the Ce species seen upon adding
2
L
alkali metal.
H O formed during the oxidation. The TPO of the
BET surface area of K-added Ce /NaZSM-5 catalysts
2
H
Ce /NaZSM-5 used and the K added counterparts was
H
The dependence of the surface area of the K/Ce /NaZSM-5
carried out using a thermal balance attached to the DTA
H
catalysts (Ce, 2.9 ] 10~1 mmol g(NaZSM-5)~1) on the added
equipment (Fig. 9). The DTA curve of the Ce /NaZSM-5
H
K/Ce atomic ratio was investigated using a BET method. The
catalyst showed a considerably large exothermic peak starting
BET surface area of the K/Ce /NaZSM-5 catalyst, together
with the yield of benzaldehyde, decreased with added K/Ce
from around 573 K, accompanied by a large decrease in the
weight of the catalyst used (Fig. 9(A)). TPO of the
H
atomic ratios of more than 2 (Fig. 8). The results obtained
here indicates that too much added K will deposit outside of
the zeolite and narrow the mouth of the zeolite pores and/or
block completely the opening of the mouth. One important
K(4)/Ce /NaZSM-5 catalyst (Fig. 9(B)) also showed an exo-
thermic peak, with a peak area rather smaller than that of the
H
Ce /NaZSM-5 catalyst used. (The area of the exothermic area
H
generated by the K(4)/Ce /NaZSM-5 catalyst used was ca.
H
one-third of that by the used Ce /NaZSM-5 catalyst.) The
smaller weight loss of the K(4)/Ce /NaZSM-5 catalyst used
was accompanied by a smaller exothermic peak. The exother-
H
H
mic peak generated by the TPO of the K(8)/Ce /NaZSM-5
H
catalyst was very small, and the corresponding weight loss
was hardly observed (Fig. 9(C)). The results obtained in Fig. 9
indicate that the K added to the Ce /NaZSM-5 catalyst inhi-
H
bits the accumulation of carbon-like materials on the catalyst.
Thus the main role of the K added to the high-Ce-supported
NaZSM-5 is thought to be in impeding the deposition of car-
bonaceous materials that will cause deactivation of the cata-
lyst. Coke formation on a non-acidic silica has been shown to
be less than that on an acidic silica-alumina.20 Coke deposi-
tion has been reported to become less serious as the strong
acid sites of silica-alumina were weakened by NaOH treat-
ment.21 The di†erence in the dependence of the yield of benz-
aldehyde on the time-on-stream catalyzed by the
Fig. 7 InÑuence of added K/Ce atomic ratio on yield of benz-
H
Ce /NaZSM-5 and the K-added counterparts (Figs. 3 and 4)
aldehyde (…) and O uptake (=). Catalyst, K/Ce /NaZSM-5 (Ce:
H
2
H
is ascribed to the role of the K added to the high-Ce-sup-
2.9 ] 10~1 mmol g(NaZSM-5)~1); reaction temperature, 653 K; tem-
perature for O uptake, 653 K.
ported NaZSM-5 catalyst mentioned here.
2
1922
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924

View Article Online
Fig. 10 Di†use-reÑectance (DR) spectra of Ce /NaZSM-5
H
(7.1 ] 10~1 mmol g(NaZSM-5)~1). (A) a, Treated at 773 K for 1 h
Fig. 9 Temperature-programmed oxidation (TPO) of Ce /NaZSM-5
H
and the K-added counterparts used. Sample, K/Ce /NaZSM-5 (Ce:
H
under O followed by degassing at room temperature for 1 h; b,
2
sample a was degassed at 773 K for 1 h and treated at 773 K for 1 h
2.8 ] 10~1 mmol g(NaZSM-5)~1) catalyst used for benzyl alcohol
under CO followed by degassing at room temperature for 1 h. (B)
Enlarged KM function of (A) in the wavelength region of 400È800 nm.
oxidation at 653 K for 4 h; (A) added K/Ce \ 0; (B) added K/Ce \ 4;
(C) added K/Ce \ 8.
K/Ce atomic ratio, 0.4, 4) were measured to investigate the
inÑuence of the added K on the redox behavior of the cata-
We have previously reported11 that the alkali metal added
to a low-Ce-supported NaZSM-5 catalyst (Ce /NaZSM-5,
L
lysts. The DR spectra of the Ce /NaZSM-5 catalysts treated
Ce, ^10~2 mmol g(NaZSM-5)~1) acts as a promoter for the
H
with O and with CO at 773 K for 1 h are illustrated in Fig.
benzyl alcohol oxidation, particularly for the activity of the
partial oxidation. The added alkali metal has been
2
suggested11 to play an important role in increasing the O
2
uptake, of which the sorbed O species will participate in the
2
partial oxidation of benzyl alcohol. However, the K added to
a high-Ce-supported NaZSM-5 catalyst (Ce /NaZSM-5; Ce,
H
more than around ^10~1 mmol g(NaZSM-5)~1) caused
instead a decline in the yield of benzaldehyde and did not act
as a promoter, although the deactivation with the time on
stream was inhibited as described previously. Thus the added
K was conÐrmed to have a di†erent role in the Ce /NaZSM-5
L
and in the Ce /NaZSM-5 catalysts. The reversal of the TOF
H
of the Ce/NaZSM-5 and the K(4)/Ce/NaZSM-5 catalysts
at a Ce loading of more than around 3.5 ] 10~1 mmol
g(NaZSM-5)~1 (Fig. 2(B)) indicates the conversion of the
role of the added K depending on the Ce loading.
Di†use reÑectance (DR) spectra of the Ce /NaZSM-5and the
H
K added-counterpart
Fig. 11 DR spectra of preoxidized K/Ce /NaZSM-5 (Ce:
H
7.1 ] 10~1 mmol g(NaZSM-5)~1) catalysts. Degassed at 773 K for
The DR spectra of the Ce /NaZSM-5 (Ce, 7.1 ] 10~1 mmol
0.5 h and treated at 773 K for 1 h under O followed by degassing
H
2
g(NaZSM-5)~1) and the K-added counterparts (K/Ce /
at room temperature for
1
h; a, added K/Ce \ 0; b, added
H
H
NaZSM-5; Ce, 7.1 ] 10~1 mmol g(NaZSM-5)~1); added
K/Ce \ 0.4; c, added K/Ce \ 4.
H
H
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924
1923

View Article Online
mmol g(NaZSM-5)~1) catalyst on the partial oxidation activ-
ity in the gas-phase catalytic oxidation of benzyl alcohol was
found to be quite di†erent from that of alkali metal added to
the low-Ce-supported NaZSM-5 (Ce /NaZSM-5, Ce; ^10~2
L
mmol g(NaZSM-5)~1): the K added to the Ce /NaZSM-5
H
catalyst caused a decrease in the catalytic activity of benzyl
alcohol oxidation, although the deactivation of the K/Ce/
NaZSM-5 catalyst with time on stream was inhibited. On the
other hand, the alkali metal added to the Ce /NaZSM-5 cata-
L
lyst promoted the catalytic activity, particularly the partial
oxidation activity, in accordance with our previous study. The
decrease in the amount of carbonaceous materials deposited
on the catalyst by adding K to the Ce /NaZSM-5 catalyst
H
was clearly observed using a thermal balance attached to
DTA equipment. The decrease in the yield of benzaldehyde
using the K/Ce /NaZSM-5 catalysts with K/Ce atomic ratios
H
of more than ca. 2 was related to the decrease in the BET
Fig. 12 DR spectra of prereduced K/Ce /NaZSM-5 (Ce: 7.1 ] 10~1
surface of the K/Ce /NaZSM-5 catalyst. The amount of O
uptake was hardly varied by adding K to the reduced
H
H
2
mmol g(NaZSM-5)~1) catalysts. The sample of Fig. 11 was degassed
at 773 K for 1 h and treated at 773 K for 1 h under CO followed by
Ce /NaZSM-5 catalyst, in agreement with the observation
degassing at room temperature for 1 h; a, added K/Ce \ 0; b, added
H
H
that the DR spectra of Ce /NaZSM-5 and the K-added
K/Ce \ 0.4; c, added K/Ce \ 4.
H
H
H
counterpart in both the oxidized and the reduced forms were
similar in both intensity and shape.
10. Two peaks at 280 and 295 nm and two shoulders at 210
and 260 nm were observed in the oxidized Ce /NaZSM-5
H
catalysts treated with O (Fig. 10(A), (a)). Viana et al.22 and
2
We express our thanks to Mr Kenji Nomura of Kobe Uni-
versity for his technical assistance during this work.
Bensalem et al.23,24 have identiÐed both the peaks at 280 and
294 nm as charge transfer bands between Ce4` and oxygen
and the peaks at 210 and 260È265 nm as based on a Ce3`
transition and the charge transfer between Ce3` and oxygen.
The Ce4` species will be mainly present in the oxidized
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2
3
4
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9
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2
3
H
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H
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10 Jian Xu, M. Ekblad, S. Nishiyama, S. Tsuruya and M. Masai, J.
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2
oxidized Ce /NaZSM-5 without added K, in Fig. 11. The
intensities and the shape of the three DR peaks were substan-
H
tially similar, although the peak at around 295 nm tended to
decrease and the peak at around 250 nm tended to increase in
the case of the K(4)/Ce/NaZSM-5 catalyst. The CO reduced
forms of the three catalysts decreased in their intensities but
the shape of the DR peaks at 200È400 nm were once again
almost similar, though the peak at around 580È590 nm tended
12 R. J. Araguer and G. Landort, US Pat., 3 702 886, 1972.
13 H. Fujiyama, I. Kohara, K. Iwai, S. Nishiyama, S. Tsuruya and
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15 W. Lin and M. Flytzani-Stephanopoulos, J. Catal., 1995, 153,
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H
16 M.-F. Luo, Y.-J. Zhong, X.-X. Yuan and X.-M. Zheng, Appl.
12). Based on the DR spectra of the Ce /NaZSM-5 and the
H
K-added counterparts, the addition of K to the Ce /NaZSM-
H
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18 K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solid Acids
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5 catalyst has only a small inÑuence on the redox properties.
Thus the fraction of the Ce`3 species in the
K(4)/Ce /NaZSM-5 catalyst tended to be larger than that of
19 N. Mori, S. Nishiyama, S. Tsuruya and M. Masai, Appl. Catal.,
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H
the K-free counterpart in both oxidized and reduced types.
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2
21 M. Misono and Y. Yoneda, Bull. Chem. Soc. Jpn., 1967, 40, 42.
22 B. Viana, G. Ako, D. Vivien, A. A. Lejus, J. Thery, A. Derory,
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tion activity by adding K to the Ce /NaZSM-5 catalyst can
H
be partly interpreted by the decrease in the oxidized form of
Ce, the Ce4` species.
23 A. Bensalem, J. C. Muller and F. Bozon-Verduraz, J. Chem. Soc.,
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Conclusions
24 A. Bensalem, F. Bozon-Verduraz, M. Delamar and G. Bugli,
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The inÑuence of potassium (K) as an alkali metal added to the
25 A. Kaachir and V. Perrichon, J. Chem. Soc., Faraday T rans.,
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high-Ce-supported NaZSM-5 (Ce /NaZSM-5, Ce; ^10~1
H
1924
Phys. Chem. Chem. Phys., 2001, 3, 1918È1924