Decomposition of chlorofluorocarbons over metal phosphate catalysis. II. Origin of the stability of AlPO4 and the location of CE as a promoter

Article abstract of DOI:10.1016/S1381-1169(99)00324-6

Aluminum-phosphate catalysts prepared from aqueous solution of Al(NO₃)₃6H₂O and H₃PO₄ in acidic condition are effective for hydrolysis of CCl₂F₂. Active species of the AlPO₄ catalysts were studied and it was found that not Al(PO₃)₃ but pure AlPO₄ was active. AlPO₄ was amorphous when calcined at 1000°C for 5 h. But a slight crystallization of AlPO₄ took place after the reaction at 400°C for 1000 h. Crystallization of AlPO₄ proceeded only in the presence of both CCl₂F₂ and water vapor. Replacement of 5-10 at.% of Al in AlPO₄ by Ce increased the catalytic activity without changing the selectivity. The Ce-promoted catalysts were composed of the mixture of AlPO₄ and CePO₄. The compositions of the catalysts were the same as those of parent liquor in preparation and it was almost uniform in the surface and the bulk. AlPO₄ in the Ce-promoted AlPO₄ catalyst was partially crystallized after the calcination at 1000°C for 5 h. (C) 2000 Elsevier Science B.V.

Full text of DOI:10.1016/S1381-1169(99)00324-6

Journal of Molecular Catalysis A: Chemical 155 Ž2000. 111–119
www.elsevier.comrlocatermolcata
Decomposition of chlorofluorocarbons over metal phosphate
catalysis
II. Origin of the stability of AlPO and the location of Ce as a
4
promoter
Yusaku Takita a,), Hironori Wakamatsu , Gong-Liang Li , Yoshihiko Moro-oka ,
a
b
b
a
a
Hiroyasu Nishiguchi , Tatsumi Ishihara
Department of Applied Chemistry, Faculty of Engineering. Oita UniÕersity, Dannnoharu, Oita 870-1192, Japan
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-0026, Japan
a
b
Abstract
Aluminum-phosphate catalysts prepared from aqueous solution of AlŽNO . 6H O and H PO in acidic condition are
3
3
2
3
4
effective for hydrolysis of CCl F . Active species of the AlPO catalysts were studied and it was found that not AlŽPO .
2
2
4
3 3
but pure AlPO was active. AlPO was amorphous when calcined at 10008C for 5 h. But a slight crystallization of AlPO
4
4
4
took place after the reaction at 4008C for 1000 h. Crystallization of AlPO proceeded only in the presence of both CCl F
4
2 2
and water vapor. Replacement of 5–10 at.% of Al in AlPO by Ce increased the catalytic activity without changing the
4
selectivity. The Ce-promoted catalysts were composed of the mixture of AlPO4 and CePO . The compositions of the
4
catalysts were the same as those of parent liquor in preparation and it was almost uniform in the surface and the bulk. AlPO4
in the Ce-promoted AlPO catalyst was partially crystallized after the calcination at 10008C for 5 h. q 2000 Elsevier Science
4
B.V. All rights reserved.
Keywords: Chlorofluorocarbon decomposition; Metal phosphate catalysis; AlPO ; Promotion effect
4
1
. Introduction
Furthermore, CFCs are significantly responsible
for the greenhouse effect. Accordingly, many
attempts to decompose CFCs completely are
carried out. However, a safe, simple and cheap
decomposition method still remains to be devel-
oped.
Millions of metric tons of chlorofluorocar-
bons CFCs produced so far are still being
Ž .
utilized in the form of refrigerants, solvents,
etching and cleaning agents in the electronic
industry. Since CFCs are chemically stable,
those are diffused to the stratosphere and de-
stroy ozone molecules to oxygen molecules.
So far, it has been found to explore the
effective catalysts for CFCs decomposition. A
number of catalysts including Al O , activated
2
3
charcoal, Fe O ractive charcoal w1x, SiO –
2
3
2
)
Al O w2x, H- and Na–Y zeolites, H-mordenite,
Corresponding author. Tel.: q81-975-547894; fax: q81-975-
47979; e-mail: takita@cc.oita-u.ac.jp
2
3
TiO –SiO , and g-Al O w3x, 0.5–10 wt.%
5
1
2
2
2
3
381-1169r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S1381-1169Ž99.00324-6

1
12
Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
H SO rTiO , 5.0 wt.% CuOrCuSO w4x, BPO
washed by pure water and filtrated. The result-
ing powder was pressed, crushed, and sieved
into 14–32 mesh granules and finally it was
calcined at 10008C for 5 h in air. The mixed
phosphate catalyst was prepared from a mixed
solution of Al and Ce nitrates.
2
4
2
4
4
w5–7x, AurCo O w8x, AurAl O w9x, ZnO,
3
4
2
3
WO , V O , and Cr O supported on g-Al O
3
2
5
2
3
2
3
w10,11x, and WO rAl O –ZrO
w12x,
3
2
3
2
FeCl ractive charcoal w13x, are reported. How-
3
ever, no catalysts which has sufficient catalytic
activity, and long catalyst life, have been found.
Decomposition generally leads to the formation
of hydrogen halides, which are strong acids. HF
is especially strong acid and reacts with the
components of catalysts to produce fluorides,
which bring about deactivation. In previous pa-
pers w14–16x, the authors reported that AlPO4
2.2. Experimental procedure
Catalytic reactions were carried out under an
atmospheric pressure using a continuous flow
reaction system with a fixed-bed reactor as
shown in Fig. 1. The reaction conditions are
given in the captions of tables. The mixture of
CCl F , nitrogen and oxygen was fed into a
has high decomposition activity, 100% CO se-
2
lectivity, and extremely long catalyst life. For
example, 0.5 mol% CCl F in the feed gas
2
2
16-mm diameter tubular flow reactor made of
stainless steel. Water was supplied into an evap-
orator located just above the catalyst bed using
a micro-liquid-feeder. The gas effluent from the
reactor was slightly washed with distilled water
to remove HCl and HF. Analysis of the reaction
products was carried out using Shimadzu GC-
2
2
composed of about 50 mol% water vapor and
air, is converted completely at 4508C. To clarify
the catalyst life, a long-term experiment was
carried out at 4008C and it was observed that
the conversion reached to steady state expect for
the initial decrease for 50 h. After the 1000-h
reaction period, X-ray diffraction
Ž
XRD
.
mea-
8ATP gas chromatographs thermal conductiv-
Ž
surement of the catalyst revealed that AlPO4
was partly crystallized and a very small amount
of AlF was present in the catalyst.
3
Crystallization of active phases is usually
related to the activity and the life of catalysts.
The effect of water vapor on the crystallization
of AlPO , origin of the AlF present in the
4
3
catalyst after reaction and the species of the
catalyst are studied this paper.
2
. Experimental
2
.1. Preparation of catalysts
Metal phosphate catalysts were prepared by
the precipitation method described below. An
aqueous solution composed of stoichiometric
amounts of metal nitrate and 85% ortho-phos-
phoric acid, and an aqueous solution of 10 wt.%
ammonium hydroxide were simultaneously
dropped into a separate vessel keeping the pH
of the mixture as 4.5. The precipitate was well
Fig. 1. Schematic diagram of the reaction apparatus.

Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
113
ity detector, TCD
mm i.d. =7 m
mm id=3 m
.
with a Porapak Q column
and a molecular sieves 5A
Ž
Ž
4
4
3.2. Stability of AlPO structure in the presence
of water Õapor
4
Ž
.
.
.
column, and an HP5890GC with
HP5970 Mass Spectrometer. Analysis of the
products by GC was carried out 1 h after from
reaching the required temperature.
In a previous paper w15x, the authors reported
that CCl F was decomposed to CO , HCl, and
2
2
2
HF over AlPO , and high catalytic activity lasted
4
XRD patterns were obtained by a Rigaku
RINT 2500HF system. The specific surface ar-
for 1000 h at 4008C except for an initial de-
crease for 50 h.
eas
determined by BET method
using a Carlo Erba SORPTY-1750 analyzer.
Ž
SSA
.
of the fresh and used catalysts were
The AlPO catalyst before reaction, which
was calcined at 10008C for 5 h, was amorphous.
4
Ž
N2 adsorption
.
After the reaction for 1000 h, small diffrac-
tion peaks due to AlPO were observed in the
4
XRD pattern of the catalyst. This suggests that
the crystallization of the catalyst took place very
slowly. In addition to this, very small diffraction
3
. Results and discussion
.1. ActiÕe species of the aluminum phosphate
peaks due to AlF also appeared in the spec-
3
3
trum. How was this AlF formed in the cata-
3
catalyst
lyst?
It is well known that Al O is easily reacted
2
3
The aluminum phosphate catalyst which show
with HF to give AlF w16–18x. So that, there are
3
high catalytic activity was prepared from an
aqueous solution of aluminum nitrate and an
aqueous solution of ammonia, and it was amor-
phous after the calcination at 10008C for 5 h.
two possibilities for the AlF formation: one is
3
the AlF formation from Al O , which was
3
2
3
contained in the catalyst as an impurity, and
other is the formation by the reaction between
Two compounds, AlPO and Al
Ž
PO
.
have
AlPO and HF. The chemical analysis of the
4
3 3
4
been well known as crystalline aluminum phos-
phates. To examine the active species of the
catalyst, decomposition of CCl F was carried
catalyst which was used to the catalytic test for
1000 h, revealed that the catalyst had the com-
position of Al:Ps52.8:47.2. Although various
kinds of aluminum phosphates are reported so
far, no compounds which have the AlrP ratio
higher than unity is not known. It has been
reported that precipitates obtained at pH near
7.0 contained a small amount of Al O in the
2
2
out using the mixture of 80 wt.% of crystalline
Al
PO3.3 and 20 wt.% of the aluminum phos-
Ž
phate catalyst as a binder. No reaction of CCl F2
2
took place up to 5008C, but about 17.7% of
CCl F was consumed at 5508C and some CFCs
2
2
2
3
were obtained but no further identification was
carried out. The SSA of the crystalline Al
PO3.3
is about 24.9 m rg which is much smaller than
preparation of AlPO by precipitation method.
Since the catalyst used in this study was pre-
4
Ž
2
pared at pHs7, the catalyst may contain Al O .
2
3
2
1
60 m rg of the aluminum phosphate catalyst.
It can be concluded that Al PO is not an
active species for the decomposition of CCl F .
If so, the Al O may easily be converted into
2 3
Ž
.
AlF during the reaction. However, all of the
3
3
3
detected AlF may not necessarily come from
2
2
3
Since the catalyst after a long term reaction
only Al O . It has to be proved that AlPO is
2
3
4
gave the defecation peaked due to AlPO and
not changed during the reaction. Consequently,
the CCl F decomposition was carried out us-
4
the composition of the aluminum phosphate cat-
alyst determined by the chemical analysis was
closely near to Al:Ps1:1, it is reasonable to
consider that the active species of the aluminum
phosphate catalyst is amorphous AlPO4.
2
2
ing a pure AlPO catalyst was prepared by the
4
precipitation method at pHs5.0 when a com-
position of 50.5 at.% Al. The results are shown
in Fig. 2. The conversion was about 90% at the

1
14
Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
reaction is amorphous. After the reaction, a part
of AlPO was crystallized, but no diffraction
4
peaks due to AlF were observed. In case of the
3
reaction using the catalyst of 52.8% Al, the
conversion was decreased from 100% to about
6
0% within initial 50 h. The closer the stoichio-
metric composition of the catalyst, the smaller
the decrease in the conversion. This suggests
that the deactivation be related to the AlF3
formation from Al O existing as impurity in
2
3
the catalyst. It is believed that the pure AlPO4
has never been transformed to AlF during the
3
CCl F decomposition.
2
2
The CO selectivity of the catalyst of 50.5
2
mol% Al was about 96% which was slightly
higher than that of 52.8% Al, in other words,
Fig. 2. The effect of time on stream on the CCl F decomposition
2
2
over the pure AlPO catalysts. Catalyst: AlPO ŽAl:Ps50.0:49.5.
4
4
3
4
2
.50 g; feed gas rate Žcm rmin.: 0.5 CCl F , 3.0 O , 13.5 N ,
2.1 H O; reaction temperature: 4008C.
2
2
2
2
the formation of CClF was promoted over the
3
2
catalysts containing high concentration of
Al O . This can only be explained if the halo-
2
3
start of the reaction, which is slightly decreased
within initial 50 h, and it reached to take a
steady state of about 80%. The XRD patterns of
the pure catalyst before and after the reaction
are shown in Fig. 3. Most of the catalyst before
gen exchange reaction between CCl F and F
2 2
ions of the catalyst was promoted over AlF3.
Actually, the promotion of the halogen ex-
change reaction over metal fluoride catalysts
has been reported in the literature w17–23x.
Fig. 3. XRD patterns of the catalyst before and after use for CCl F decomposition. Catalyst and reaction conditions are the same as given
2
2
in Fig. 2.

Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
115
The AlPO catalysts prepared in this study
4
2
have significantly large SSA of 120–160 m rg
after the calcination at 10008C for 5 h. Although
the AlPO catalyst has an advantage of having
4
no reactivity with HF, a slightly crystallization
of AlPO took place during the reaction. In
4
general, progress of the crystallization brings
about decrease in SSA, which is connected to
the catalytic activity. In this study, decomposi-
tion of CCl F has been carried out under high
2
2
concentration of water vapor, so that the pres-
ence of water vapor may promote the crystal-
lization of AlPO . In order to examine these
4
hypotheses, heat treatments of the AlPO cata-
4
lyst were carried out in the presence of water
vapor as high as 50 mol% and in the absence of
CCl F . However, no changes was found in the
Fig. 4. The effect of Ce content in the AlPO4 catalysts on the
3
decomposition activity. Catalyst: 4.50; feed gas rate Žcm rmin.:
0
.5 CCl F , 3.1 O , 12.1 N , 19.0 H O.
2 2 2 2 2
2
2
XRD patterns of the catalyst before and after
the treatment at 5508C for 30 h. Although the
treatment was carried out at the temperature
higher than that of the catalytic reaction, the
initial SSA of the catalyst, 125 m rg, was not
changed at all.
activity of the AlPO catalyst was increased by
4
replacement by 5% Ce, and the conversion curve
was shifted to lower temperature by about 208C.
The catalytic activity was further improved by
the 10% replacement and the temperature for
the same conversion was lowered by 408C. But
the replacement of 20% had no effect in the
conversion. The introduction of Ce into the
2
It clearly shows that no crystallization of
AlPO takes place due to the presence of water
4
vapor at 5508C. Crystallization can be pro-
ceeded in the presence of both CCl F and
AlPO catalyst affected only catalytic activity
2
2
4
water vapor. Although there is no direct evi-
dence for the presence of Al–F and P–F bonds,
presence of F ions was detected by XPS. Per-
haps F ions formed during the catalytic decom-
position may cleave the bridged Al–O–P bonds
and no influence on the product distribution was
observed. It is obvious that 10 at.% Ce is opti-
mum. The results of the catalytic activity of
CePO are also shown in Fig. 4. Catalytic activ-
4
ity of CePO was so small that the conversion
4
before removing from AlPO and this may be
reached only 16% at 4508C.
4
responsible for the gradual progress of crystal-
lization.
The SSA of the AlPO catalyst prepared at
4
2
pHs7 is about 118 m rg, which is somewhat
2
smaller than 140 m rg of the catalyst prepared
3
.3. Optimum concentration of cerium
at pHs4.5. With increasing the Ce concentra-
tion, SSA of the catalyst is decreased. The SSA
of the catalysts containing 10% and 20% Ce is
The effect of various elements as promoter
2
into AlPO on the catalytic activity was studied
67 and 43 m rg, respectively. The SSA of the
4
2
and it was found that replacement of 10 at.% Al
CePO catalyst was only 3 m rg. If the cata-
4
in AlPO by Ce brought about a marked im-
lysts are composed of simple mixture of AlPO
4
4
provement on the activity for CCl F decompo-
and CePO , SSA of the catalysts containing 10
2
2
4
2
sition. Therefore, the authors decided to further
investigate the optimum composition of the Ce.
The results are shown in Fig. 4. The catalytic
and 20% Ce would be 106 and 95 m rg. The
observed SSA values were smaller than those
values suggesting that the catalysts were not

1
16
Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
3
q
3q
simple mixtures of the particles of individual
phosphates.
The ionic sizes of Al , 0.047 nm, and Ce ,
0.107 nm, are significantly different. The struc-
ture of AlPO is the same as that of SiO and
It is very difficult to select a suitable temper-
ature to compare the catalytic activity. Decom-
position of CCl F over the AlPO catalysts has
4
2
3
q
2
y
the small Al ions is surrounded by 40 . On
the other hand, CePO has monazite structure
2
2
4
4
2
y
3q
also been the case. The order of the activity is
0%)5%)0%, 20% in Ce concentration from
the conversion curves. The reaction rates at
008C is 33 times larger than that over CePO4.
On the contrary, the specific reaction rate
and 9O coordinate to Ce ion in monazite.
Therefore, incorporation of Ce ions into AlPO4
matrix is thought to be difficult. However, the
incorporation of Al ions into matrix of CePO4
will be possible. The shift of the diffraction
peaks can not be found but the formation of a
1
3
q
4
2
Ž
mmolerm per min over CePO is 1.18 times
.
4
3
q
larger than that over AlPO . Therefore, the re-
small amount of CePO containing Al at the
4
4
sults of the catalytic tests can be explained as
follows. Decomposition of CCl F can proceed
grain boundary can not be ruled out. It is clear
that a major part of the catalyst bulk is com-
posed of AlPO and CePO , and the catalytic
2
2
over both AlPO and CePO and the specific
4
4
4
4
reaction rate over CePO is larger than that over
property will be depended on rather the surface
structure than the bulk structure in these cata-
lysts. Spectroscopic investigation is now in
progress.
4
AlPO . However, the increase in Ce concentra-
4
tion decreases the SSA of the catalysts, which is
related to the activity. Consequently the CCl F2
2
conversion will take a maximum at 10 at.% Ce.
The AlPO catalyst calcined at 10008C gives
3.4. Location and structure of Ce as a promoter
4
no diffraction peaks in XRD pattern but, Ce
containing catalysts gave those due to AlPO4
element in the AlPO catalysts
4
and CePO . This reveals that there are two
As shown in Section 3.3, the replacement of
4
compounds, AlPO and CePO , in the catalysts.
10 mol% of AlPO by Ce brought about a
4
4
4
Fig. 5. XRD patterns of the AlPO catalyst promoted by 10 at.% of Ce. Reaction conditions of the catalytic test Ž90 h. are the same as given
4
in Fig. 2.

Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
117
significant increase in the catalytic activity for
CCl F decomposition. Reaction temperature is
2
2
lowered by about 408C to obtain the same
CCl F conversion. The promotion effect of Ce
2
2
is quite clear. The XRD spectra of Ce promoted
AlPO catalysts are shown in Fig. 5. Although
4
the pure AlPO catalyst was amorphous after
4
the calcination at 10008C for 5 h, Ce promoted
catalysts calcined at 10008C for 5 h gave XRD
peaks due to AlPO and CePO and the XRD
4
4
pattern did not change after the catalytic test up
008C. Crystallite sizes of AlPO and CePO4
5
4
were comparable, and were 13.0 and 16.9 nm,
respectively. This promoted catalyst was pre-
pared from an aqueous solution composed of
9
0% Al and 10% Ce nitrates and an aqueous
solution containing stoichiometric amount of or-
tho-phosphoric acid. Chemical analysis revealed
that the catalyst has almost the same concentra-
3
q
3
q
3y
tion of Al , Ce , and PO
as those of the
4
parent liquor in preparation. In the catalyst
preparation, the solution containing Al and Ce
nitrates and phosphoric acid is strongly acidic
and the pH of the solution is gradually increased
by the addition of an aqueous solution contain-
ing ammonia. Since it is confirmed from sepa-
rate experiments that CePO is precipitated at
4
the lower pH than AlPO , there is possibility
4
that CePO is precipitated at lower pH and then
4
the mixture of CePO and AlPO is precipitated
4
4
in the course of the addition of ammonia. If so,
the surface concentration of Al will be higher
than that of core of the precipitates. In order to
examine the surface composition of the Ce pro-
moted catalyst, the Ce
calcined at 10008C, was analyzed by means of
XPS Fig. 6 . The mechanical mixture of sepa-
rately prepared AlPO and CePO powders were
Ž .
10% –AlPO catalyst
4
Ž
.
4
4
used as reference samples. The pure AlPO and
4
CePO gave characteristic signals at 75.5 and
4
8
86.4 eV which are assigned to the signals of
Fig. 6. XPS spectra of the AlPO catalyst promoted by 10 at.% of
Ce.
4
Al 2P and Ce , respectively. Plotting
the area ratio of these two signals against the Ce
Ž
.
Ž
3dr5r2
.
concentration of the reference catalysts with the
composition of 1, 10 and 20 at.% of CePO , a
straight line through the origin can be obtained.
Using this calibration line, the surface concen-
tration of the catalyst which was prepared from
4

1
18
Y. Takita et al.rJournal of Molecular Catalysis A: Chemical 155 (2000) 111–119
4
. Conclusions
Ž
1
.
Al
decomposition of CCl F and amorphous AlPO
4
ŽPO3.3 is not an active species for the
2
2
is thought to be an active species.
Composition of the AlPO catalysts pre-
2
Ž .
4
pared at pHs7.0 and 5.0 are Al:Ps52.8:47.2
and 50.5:49.5, respectively. Al O contained in
2
3
the catalysts as impurity, gradually changes into
AlF during the decomposition of CCl F , and
Fig. 7. Schematic plot of the structure of Ce-promoted AlPO4.
3
2 2
this change bring about a slight decrease in the
catalytic activity and promotes halogen ex-
changed reaction between CCl F and F to give
2
2
an aqueous solution containing 90 mol% of al
and 10 mol% of Ce nitrates, was determined to
CClF3.
Ž .
3 Crystallization of AlPO does not take
4
9
.5 at.%. This value was closely near to that of
place in the presence of water vapor, but it takes
place in the presence of both CFC and water
vapor.
the parent liquor in preparation, therefore it is
concluded that the composition of the catalysts
was uniform.
4 Substitution of 5–10 mol% of Al ions in
Ž .
The SSA of CePO calcined at 10008C was
4
AlPO by Ce ions bring about a promotion
2
4
only 3 m rg and it gave sharp diffraction peaks,
suggesting that CePO is tend to crystallize.
effects in the CCl F decomposition. The opti-
2
2
4
mum concentration of Ce is 10 at.%.
The catalyst prepared by the co-precipita-
When the well ground mixture of powders of
5
Ž .
CePO and AlPO dried at 2008C was calcined
4
4
tion method from solution containing Al and Ce
nitrates and ortho-phosphoric acid and an aque-
ous solution of ammonia is composed of a
mixture of fine AlPO and CePO particles, and
at 10008C for 5 h, as was expected, only small
diffraction peaks due to CePO were observed.
4
As mentioned before, in the case of the precipi-
tate obtained from an aqueous solution contain-
ing 90 mol% of Al and 10 mol% of Ce nitrates,
crystallization of AlPO and CePO took place
4
4
surface and bulk compositions of the catalyst
are almost the same.
4
4
by the calcination at the same temperature. The
reason is as follows. In the case of the pure
AlPO , extremely large network of AlO and
References
4
4
PO units is extended and it resists to the crys-
4
tallization of AlPO during calcination. On the
other hand, mixed precipitate is constituted in
4
w1x S. Okazaki, A. kurosaki, Chem. Lett. Ž1989. 1901.
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w7x S. Imamura, T. Ikeda, S. Ishida, Nippon Kagaku Kaishi 139
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3
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