Decomposition of chlorobenzene over phosphate and sulfate catalysts properties

Article abstract of DOI:10.1246/bcsj.79.145

The catalytic conversion of chlorobenzene (CB) over some metal phosphates and sulfate was studied in the presence of water vapor and oxygen. The conversion of CB proceeded in the order of AlPO₄, Zr ₃(PO₄)₄, CePO₄-AlPO₄, and La₂(SO₃)₃ above 673 K. The major products were CO, CO₂, and HCl. The reaction proceeded in the absence of water vapor; however, the activity was greatly reduced in the absence of oxygen. Even in the presence of oxygen, 30 mol% of water vapor inhibited the reaction. No intermediate hydration products were observed. There was no correlation between each of the catalytic activity, the acidity, and the concentration of the surface hydroxys of the catalysts. These results suggest that the essential reaction for CB conversion is oxidation. The specific surface area (SSA) of the catalysts was not changed during the CB conversion.

Full text of DOI:10.1246/bcsj.79.145

ꢀ 2006 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 79, No. 1, 145–148 (2006)
145
Decomposition of Chlorobenzene over Phosphate
and Sulfate Catalysts Properties
ꢀ
Yusaku Takita, Yoshitaka Sadatomi, Hiroyasu Nishiguchi, and Katsutoshi Nagaoka
Department of Applied Chemistry, Faculty of Engineering, University of Oita, Dannoharu, Oita 870-1192
Received June 23, 2004; E-mail: takita@cc.oita-u.ac.jp
The catalytic conversion of chlorobenzene (CB) over some metal phosphates and sulfate was studied in the pres-
ence of water vapor and oxygen. The conversion of CB proceeded in the order of AlPO4, Zr3(PO4)4, CePO4–AlPO4, and
La2(SO3)3 above 673 K. The major products were CO, CO2, and HCl. The reaction proceeded in the absence of water
vapor; however, the activity was greatly reduced in the absence of oxygen. Even in the presence of oxygen, 30 mol % of
water vapor inhibited the reaction. No intermediate hydration products were observed. There was no correlation between
each of the catalytic activity, the acidity, and the concentration of the surface hydroxys of the catalysts. These results
suggest that the essential reaction for CB conversion is oxidation. The specific surface area (SSA) of the catalysts
was not changed during the CB conversion.
The incineration or oxidation method is actually used for the
decomposition of organic compounds containing chlorine.1
This method requires a high temperature more than 1273 K,
and often produces polychlorodibenzoparadioxines (PCDDs)
and polychlorodibenzofurans (PCDFs) as by-products, which
are well known to be toxic chemicals. These compounds are
chemically and microbiologically stable because of the pres-
ence of chlorine atoms.
Experimental
Catalyst Preparation. Zirconium(IV) phosphate and alumi-
num phosphate catalysts were prepared by the precipitation meth-
od described below. An aqueous solution of 10 wt % ammonia was
slowly added into an aqueous solution composed of stoichiometric
amounts of zirconium(IV) or aluminum nitrate (0.5 M) and 85%
ortho-phosphoric acid by stirring until the pH of the solution
became 4.5. The obtained precipitate was washed well with pure
water and filtrated. The resulting powder was pressed into a cylin-
drical form, crushed, and sieved into 14–32 mesh granules, and
was finally calcined at 1000 ꢁC for 5 h in air. The AlPO4 catalyst
containing Ce was prepared using a solution containing Ce(III)
nitrate together with Al nitrate and phosphoric acid. The catalyst
containing Ce was ascertained for a mixture of CePO4 and AlPO4
by an XRD analysis. The catalyst is denoted as CePO4–AlPO4
(Ce=Al ¼ 1=9) in this paper. The specific surface areas of the cat-
alysts before and after the reaction were 106 (103) for AlPO4, 102
(101) for Zr3(PO4)4, 30.4 (31.6) for CePO4–AlPO4, and 0.6 (0.5)
for La2(SO4)3, respectively.
Chlorobenzene (CB) has been used as a model compound for
PCDDs. The catalytic decomposition of chlorine-containing
compounds is thought to be a superior method compared with
the incineration method, because it can be achieved at much
lower temperatures. Van den Brink et al. studied the decompo-
sition of CB (500ppm) in the presence of air and C7H14 (1500
ppm) over 2% Pt/ꢀ-Al2O3, and reported that CB completely de-
composed to CO2 at 548 K,2,3 and CB (1300 ppm in 15% O2 and
85% N2) completely decomposed to carbon oxides over MnOx
(3.2wt %)/TiO2 at 673 K. These reports show that CB can be
completely decomposed using noble-metal catalysts and metal
oxide catalysts within the temperature range of 523–673 K.
The authors have studied the decomposition of chlorofluoro-
carbons (CFCs) using various catalysts and have found that
metal phosphates, especially zirconium(IV) and aluminum
phosphates, and sulfates are very active in the decomposition
of CFCs. These catalysts showed a long life in the presence of
strong acids, such as HCl and HF. The essential reaction is
hydrolysis, and oxidation proceeds at higher temperatures, via
the reaction mechanism via the bidentate surface intermediates
and so on.4–14
TPD of Water Vapor and Ammonia. The amount of irre-
versibly adsorbed water vapor was determined by the TPD (tem-
perature programmed desorption) method. The sample was evac-
uated at 823 K for 1 h, a sufficient amount of water vapor was
introduced at the same temperature, and the sample was cooled to
373 K. Next, water vapor on the sample was flushed with a He
stream and the TPD was measured in a He flow (40 cm3 minꢂ1
)
at a rate of 10 ꢁC minꢂ1 after the baseline became stable. Similar-
ly, NH3 was introduced at 373 K on the catalyst pre-evacuated at
823 K, and then flushed with a He stream.
Metal phosphates are potent and very tough catalysts, which
are able to work under severe reaction conditions in the pres-
ence of strong acids. Since wastes contain various compounds
as impurity, environmental catalysts are required to exhibit de-
composition activity for many compounds. The catalytic activ-
ity of metal phosphates for the decomposition of CB, which
has an aromatic ring and a chlorine atom, was studied in the
present work.
Reaction Procedure. Catalytic reactions were carried out at
atmospheric pressure using a continuous flow reaction system
with a fixed catalyst bed reactor, as used by Takita et al.13 The re-
action conditions were as follows. The amount of catalyst used
was 4.5 g. Standard feed gas consisted of 1.2–1.8 mol % CB,
17.8 mol % O2, 10.0 mol % H2O, and the balance N2. The feed
rate was approximately 41.0 cm3 minꢂ1. CB was supplied by a
saturator (bubbler) maintained at 298 K and O2 as a carrier gas.
Published on the web January 13, 2006; DOI 10.1246/bcsj.79.145

146 Bull. Chem. Soc. Jpn. Vol. 79, No. 1 (2006)
Catalytic Decomposition of Chlorobenzene
Water was supplied to an evaporator/mixer, which was located
just in front of the catalyst bed, by a micro-liquid pump and mixed
with the gases. The effluent gas from the reactor was analyzed by
GC. The reaction products were collected separately in a cold trap
kept at liquid-nitrogen temperature for 1 h; they were then heated
up to room temperature, and dissolved into methanol. The solution
was analyzed by Shimadzu GC-8ATP gas chromatographs (ther-
mal conductivity detector, TCD) with two Porapak Q columns
(3 mm i.d. ꢃ 5 m N2 carrier and 3 mm i.d. ꢃ 5.5 m H2 carrier),
a molecular sieve 13X (4 mm i.d. ꢃ 3 m) column, and a JEOL
JMS-AMII 150 Mass Spectrometer.
the temperature range 200–1000 K. The equilibrium conver-
sion to C6H5OH was calculated approximately to be 73% at
298 K, under conditions of 1.8 mol % CB and 10% H2O.
Decomposition of CB over Al Phosphates and La Sulfate.
The catalytic decomposition of CB was carried out using
Zr3(PO4)4, AlPO4, Ce-promoted AlPO4, and La2(SO4)3 cata-
lysts, which showed very high activity with regard to CCl2F2
decomposition.5,7 The CB conversion and yield of products
over various catalysts are shown in Figs. 2 and 3.
The CePO4–AlPO4 catalyst showed the highest activity with
regard to the decomposition of CB; CB began to react at
673 K, and the conversion approached 100% at 923 K. The
carbon-containing products were CO2 and CO and no other
compounds were observed. Mass analysis of the effluent gas
showed no formation of COCl2 and Cl2. Chloride ions were re-
covered in a water trap located just after the reactor. Therefore,
not COCl2, but HCl, was formed in the reaction. The selectiv-
X-ray diffraction patterns (XRD) were obtained by a Rigaku
RINT 2500HF system. The specific surface areas of the fresh and
used catalysts were determined by the BET method (N2 adsorp-
tion) using a Carlo Erba SORPTY-1750 analyzer.
Results and Discussion
Thermodynamic Consideration of CB Decomposition.
As reported in a previous paper, in the decomposition of
CCl2F2, as being representative of CFCs, the calculations indi-
cated hydrolysis as being much more feasible than oxidation.
The hydrolysis actually proceeded as shown below:
100
80
60
40
20
0
CCl2F2 þ 2H2O ! CO2 þ 2HF þ 2HCl;
ꢀH298 ¼ ꢂ148:0 kJ; ꢀG298 ¼ ꢂ224:5 kJ;
CCl2F2 þ O2 ! CO2 þ F2 þ Cl2;
ð1Þ
ð2Þ
ꢀH298 ¼ 98:1 kJ; ꢀG298 ¼ 58:3 kJ:
Therefore, thermodynamic calculations for several reactions
of CB were carried out. Selecting of the oxidation technique is
quite difficult for CB reactions. It is meaningless to consider
the deep oxidation products, because the greater is the number
of oxygen atoms incorporated into the products, the more neg-
ative are the ꢀG298 values. The possibility for the formation of
chlorophenol can not be ruled out. However, unfortunately no
thermodynamic data for chlorophenol was obtained. Thus, the
authors tentatively calculated the oxidation for quinone forma-
tion: C6H5Cl + O2 ! C6H4O2 + HCl. The calculated ꢀG
for the reaction is ꢂ257 to ꢂ263 kJ, which is a large negative
value, as expected. No thermodynamic calculations for hydro-
gen abstraction of CB were possible due to a lack of thermo-
dynamic data. The ꢀG change for the hydrolysis of CB is
shown in Fig. 1. As can be seen from the figure, the values are
close to zero, suggesting that hydrolysis can proceed between
650
700
750
800
850
900
950 1000
Temperature / K
Fig. 2. CB conversion in decomposition over various
phosphates and sulfates. Catalyst: 4.50 g; Feed rate
(cm3 minꢂ1): CB 0.54 (AlPO4), 0.60 (Zr3(PO4)4), 0.67
(CePO4–AlPO4), 0.71 (La2(SO4)3), 7.3 O2, 29.2 N2, 4.1
H2O; ꢄ ꢄ ꢄ ꢄ ꢄ ꢄ: CePO4–AlPO4 (Ce/Al ¼ 1=9), - - - -:
Zr3(PO4)4, — —: La2(SO4)3, - - - - - -: AlPO4.
0
C6H5Cl(g) + H2O(g) = C6H5OH(g) + HCl(g)
−50
−100
−150
−200
C6H5Cl(g) + O2(g) = C6H4O2(g) + HCl(g)
−250
200
400
600
800
1000
1200
1400
Temperature / K
Temperature / K
Fig. 1. ꢀG changes for hydrolysis and oxidation of chloro-
benzene.
Fig. 3. Yields of the products.
Reaction conditions are the same as those shown in Fig. 2.
: CO2,
: CO.

Y. Takita et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 1 (2006)
147
ity for CO2 was 100% when the conversion was below 10%;
and the CO2 selectivity decreased at one instance along with
an increase in the temperature, and then increased again.
Zr3(PO4)4 registered the next highest activity: The conver-
sion at 773 K was only about 6%; however, it increased sharp-
ly and the conversion above 873 K was similar to that of the
CePO4–AlPO4 catalyst. La2(SO4)3 followed Zr3(PO4)4; how-
ever, the conversion above 873 K was lower than that of the
preceding two catalysts. The activity of AlPO4 was the lowest
among these catalysts. The CO2 selectivity monotonously de-
creased to 55.3% at 923 K, suggesting that the oxidizing power
of the AlPO4 catalyst was lower than that of other catalysts.
The order of the catalytic activity of the catalysts was CePO4–
AlPO4 > Zr3(PO4)4 > La2(SO4)3 > AlPO4; and the specific
surface areas of the catalysts before and after the reaction were
30.4 and 31.6 (CePO4–AlPO4), 102 and 101 (Zr3(PO4)4), 0.6
and 0.5 (La2(SO4)3), and 106 and 103 m2 gꢂ1 (AlPO4). No
correlation between the catalytic activity and the SSA of the
catalysts was observed. It is significant that the activity of
CePO4–AlPO4 was much higher than that of AlPO4, which
has the highest SSA. This suggests that the oxidizing power
of the Ce-promoted AlPO4 catalyst is higher than that of
AlPO4. It is surprising that the La2(SO4)3 catalyst, which
has only 0.6 m2 gꢂ1, showed a similar level of activity as the
AlPO4 catalyst. The SSAs of the catalysts were measured by
N2 adsorption at liquid-nitrogen temperature; however, the
molecular size of CB is larger than that of nitrogen. This
means that AlPO4 might have many small pores through which
CB could not enter. Since partial oxidation products and phe-
nol were not formed at lower temperatures, the essential reac-
tion under these reaction conditions would be oxidation.
Crystal Structure of the Catalysts. The XRD patterns of
the catalysts are shown in Figs. 4 and 5. AlPO4 and Zr3(PO4)4
catalysts are amorphous and AlPO4 was partly crystallized.
Ce-promoted AlPO4 was well-crystallized and composed of
AlPO4 and CePO4. La2(SO4)3 was also well-crystallized. The
XRD patterns remained unchanged after the reaction, indicat-
ing that the catalysts were stable at these reaction conditions.
Effects of O2 and H2O on the Decomposition of CB and
the Reaction Path. In order to understand the reaction mech-
anism for CB decomposition, the effects of the concentrations
of O2 and water vapor on the reaction were studied using
La (SO
)
4 3
unknown
2
after the reaction
before the reaction
2
θ
/ °
Fig. 5. XRD patterns of La2(SO4)3 catalyst.
100
80
a
60
d
40
20
c
b
0
500
600
700
800
900
1000
Temperature / K
Fig. 6. The effects of O2 and H2O concentration on the CB
decomposition. Catalyst: CePO4–AlPO4 4.5 g, Feed con-
centration (mol %): (a) CB 1.3, O2 18.0, N2 80.7, H2O 0,
(b) CB 0.67, O2 14.1, N2 55.1, H2O 30.1, (c) CB 1.36, O2
0, N2 88.7, H2O 10.0, and (d) CB 0.67, O2 7.3, N2 29.2,
H2O 4.1.
CePO4–AlPO4; the results are shown in Fig. 6. When no water
vapor was introduced in the feed (reaction conditions (a)), the
CB conversion was much higher than the results in the pres-
ence of 10% water vapor (c). Under the reaction conditions
(a), CO2 was formed at a higher selectivity range 65.9–88.9%,
and the rest was CO. When no oxygen was introduced (c), the
conversion was as low as 12–13%, even at higher tempera-
tures, such as 823–923 K. No CO was formed, but many partial
oxidation products, such as C2H5OH, CH3OH, and C6H6, were
formed. In particular, C2H5OH was formed at 18.6 and 32.3%
selectivity at 873 and 923 K, respectively. C6H6 was formed at
8.5 and 1.3% selectivity at 873 and 923 K, respectively. How-
ever, the formation of phenol, which is the primary product in
hydrolysis, was not observed. An explanation of the formation
of C2H5OH is very difficult, because the fragment compounds
from the phenyl group are very small. One hypothesis is that of
the formation of C2H4 by thermal decomposition at tempera-
tures higher than 773 K, followed by consecutive hydration.
However, if this hypothesis is correct, the formation of C2H4
should be observed, since the hydration has an equilibrium
limitation. Under the condition of a high concentration of
water vapor, such as 30 mol % (b), the oxidation of CB was
after the reaction
before the reaction
2
θ / °
Fig. 4. XRD patterns of AlPO4 catalyst.

148 Bull. Chem. Soc. Jpn. Vol. 79, No. 1 (2006)
Table 1. Analysis of Minor Products in the Decomposition of CBaÞ
Catalytic Decomposition of Chlorobenzene
CB feed rate
/mmol minꢂ1
28.6
CB conversion
/mol %
Rate of formation/mmol minꢂ1
CO
1.9
CO2
C6H6
C6H4Cl2
7.2
9.3
tr
0.20
a) Catalyst: CePO4–AlPO4 (Ce/Al ¼ 1=9) 0.9 g, Feed rate (cm3 minꢂ1): 7.3, N2 29.2,
H2O 4.1, 773 K.
Table 2. Relation between Catalytic Activity and Physicochemical Properties
Conversion at
873 K/%
Amount of OH
/mmol gꢂ1
Acid amount
/mmol gꢂ1
Catalyst
SSA/m2 gꢂ1
Zr3(PO4)4
CePO4–AlPO4
AlPO4
87.0
86.5
42.5
101
31.6
103
380
109
102
67.1
58.1
48.6
greatly reduced, and only CO2 was formed. This may be due to
the inhibition of CB adsorption by the competitive adsorption
of water vapor. These results support the hypothesis that the
essential reaction is oxidation when O2 and approximately
10 mol % of water vapor are present. The catalysts effective
for the decomposition of halogen-containing hydrocarbons
require two functions: hydrolysis and oxidation. The decompo-
sition of CFCs requires the function of hydrolysis, rather than
oxidation. On the contrary, decomposition of CB requires ox-
idizing ability than hydrolysis.
Conclusion
(1) Decomposition of CB proceeded on AlPO4, Zr3(PO4)4,
Ce-promoted AlPO4, and La2(SO3)3 in the presence of oxygen
and water vapor above 673 K. CO, CO2, and HCl were the
major products.
(2) The major reaction of decomposition of CB is not
hydrolysis, but oxidation.
(3) Phosphate and sulfate catalysts are stable during the
decomposition of CB.
Attempt to Detect the Minor Products Formed during
the Decomposition of CB. In the case of the oxidation of
hydrogen-containing compounds, oxidation of the compounds
cannot proceed under water-free conditions, because water is
formed by the oxidation of hydrogen-containing compounds.
Therefore, all of the reaction products formed at 773 K were
collected at liquid-nitrogen temperature for 12 h, and the minor
products were carefully analyzed. Under the adopted reaction
conditions, the conversion of CB was about 5%. The collected
products were warmed to ambient temperature, after which the
two solutions, aqueous, and CB solutions, were analyzed by
GC-MS, separately. The results are summarized in Table 1.
Small amounts of benzene and dichlorobenzene were detected
in the CB solution; however, no compounds were formed in
the aqueous solution. Benzene and dichlorobenzene can be
formed by the disproportionation of CB. It is significant that
no trace amount of phenol was formed. These results showed
that the decomposition of CB did not occur by hydrolysis, but
by oxidation.
Relation between the Catalytic Activity and the Physico-
chemical Properties. In order to investigate the reaction
mechanism, some physicochemical properties of the catalysts
were studied, which are summarized in Table 2. The conver-
sion at 873 K was adopted as the index of catalytic activity.
No good linear correlation was found among the conversion
at 873 K, SSA, the amounts of surface OH, acidic sites of
the catalysts. Therefore, it is clear that neither the surface areas
nor these physicochemical properties, which are related to the
hydrolysis of CB, govern the catalytic activity.
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