catalyst. The second is the decomposition of AlPO by reac-
4
tion with formed HF. It is known that AlPO prepared at
4
near pH 7, contains a small amount of Al O and the AlPO
2
3
4
prepared at lower pH contains no Al O as an impurity. The
2
3
catalyst used in this experiment was prepared at pH 7 and the
composition determined by the chemical analysis was
Al : P \ 53.0 : 47.0 by atomic ratio. This result strongly sug-
gests the Ðrst case. Therefore a decomposition experiment was
carried out using the AlPO prepared at pH 4.5 which has
4
almost the stoichiometric composition Al : P \ 50.5 : 49.5.
Fig. 7 The e†ect of reaction time on the catalytic activity and the
This catalyst showed almost the same catalytic activity as that
of the former catalyst. The catalyst after 100 h reaction was
not changed from that before reaction by XRD. Although the
reaction time is somewhat shorter than the former experiment,
this result also supports the Ðrst case.
selectivity. Catalyst: AlPO , 4.50 g; feed gas concentration (mol%):
4
1.25 (0È315 h) 0.8 (315È1000 h), 7.5 O , 57.6 H O , balance N ; feed
2
2
2
rate: 40.0 cm3 min~1; reaction temperature: 400 ¡C; L conversion, |
selectivity for CO , K selectivity for CCIF .
2
3
Currently, it has been clariÐed that the AlPO catalyst after
reaction was carried out over AlPO calcined at 1000 ¡C for
1000 h. The temperature used in the reaction was 400 ¡C
4
the decomposition of CF , which is a much more stable mol-
4
4
ecule than CCl F , at 700 ¡C for about 5 h, contains no AlF
because the CCl F conversion did not reach 100%, in order
2 2
3
2 2
and no compounds derived from the catalyst eluted from the
reactor.36
to clarify the changes in the activity. The conversion decreased
by 30% for the initial 50 h, but the conversion seems to take a
steady value for the following 265 h. Although the changes in
SSA during the reaction could not be measured, the SSA of
In many cases including ZrO ÈCr O , it has been reported
2
2 3
that the Ñuorination of the components of the oxides took
place and it lead to a deactivation of the catalysts.26,27
the AlPO catalyst decreased to 88 m2 g~1 after 1000 h.
4
However, it is clear that AlPO is surprisingly inert towards
Therefore, the decrease in the CCl F conversion for the
4
2 2
HF and no Ñuorination of the AlPO catalyst takes place
initial 50 h may be due to the decrease in the SSA of the
4
during decomposition of CCl F . This excellent nature of
catalyst. The CO selectivity was kept at about 95% until 315
2 2
2
AlPO provides the long catalyst life for the reaction.
h and the other product was CClF . As can be seen from Fig.
4
3
5, the CClF selectivity decreases with the decrease in the
CCl F concentration in the feed gas, CCl F concentration
3
Conclusions
2 2
2 2
decreased from 1.25% to 0.80% to improve the CO selec-
(1) Metal phosphates are e†ective for decomposition of
2
tivity at 315 h. After the CCl F concentration change, the
2 2
CCl F because they are resistant to the hydrogen Ñuoride
2 2
conversion increased to 70% immediately and then gradually
produced by the hydrolysis of CCl F .
2 2
decreased. The CCl F conversion, however, approached to a
(2) Having a large surface area is one of the most essential
2 2
new steady value at 600 h. The reason for the decrease of
requirements for active catalysts. AlPO and ZrÈPÈO, which
4
conversion at around 550 h is not clear at the present stage.
The CO selectivity showed some scatter after the CCl F
have large surface areas, show high catalytic activity for the
decomposition of CCl F .
2
2 2
2 2
concentration change but it increased and reached nearly
100% at 700 h as expected.
(3) Weak acid sites and those of intermediate strength are
thought to participate in the CCl F decomposition. Strong
2 2
acid sites seem to promote the CClF formation because F~
3
The many kinds of adsorbed species such as Cl~, F~,
CCl F , CCl F O and so on will be present on the catalyst
ions reside on the strong acid sites preferentially.
n m
n m
surface during the decomposition of CCl F . The formation
(4) AlPO is highly resistant to halogen compounds includ-
2 2
of CO accompanies the formation of Cl~ and F~, therefore
2
4
ing hydrogen Ñuoride and has a long catalytic life of more
CClF will be formed from the coupling of F~ and CClF or
than 1000 h for the decomposition of CCl F .
3
2
2 2
the halogen exchange reaction between CCl F and F~. The
2 2
decrease of CCl F concentration in the feed gas will lead to
2 2
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2
M. Hiraoka, Nippon Kagaku Kaishi, 1991, 559.
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3
ation.
The structure of the catalyst has studied by means of XRD
and the results are shown in Fig. 8. The catalyst before reac-
tion was amorphous. The XRD pattern of the catalyst after
3
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6
7
8
9
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4
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3
There are two possibilities for AlF formation: the Ðrst is
3
the Ñuorination of Al O that is contained in the original
2
3
10 E. Jacob, Chem. Abstr., 1990, 113, 196942c.
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4
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