Decomposition of dichlorodifluoromethane with simultaneous halogen fixation by transition metal oxides supported on magnesium oxide

Article abstract of DOI:10.1246/cl.2003.436

CFC12 (CCl₂F₂) decomposition with simultaneous halogen fixation by MgO was significantly promoted by supporting transition metal oxides. Vanadium oxides were the most effective and over 80% of halogen was fixed as magnesium halides i

Full text of DOI:10.1246/cl.2003.436

436
Chemistry Letters Vol.32, No.5 (2003)
Decomposition of Dichlorodifluoromethane with Simultaneous Halogen Fixation
by Transition Metal Oxides Supported on Magnesium Oxide
Tsukasa Tamai, Koji Inazu, and Ken-ichi Aikaꢀ
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, 4259Nagatsuta, Midori-ku, Yokohama 226- 8502
(Received February 6, 2003;CL-030111)
CFC12 (CCl2F2) decomposition with simultaneous halogen
at total flow rate of 30 mL/min. For each run 0.2 g of pelletized
metal oxide added MgO was employed. Before the reaction, the
samples were pretreated at 873 K for 3 h under helium flow and
all the experiments were carried out under dry condition to
eliminate influences of water in the system. CFC12 and other
gas products were analyzed by GC-TCD.
fixation by MgO was significantly promoted by supporting tran-
sition metal oxides. Vanadium oxides were the most effective
and over 80% of halogen was fixed as magnesium halides in
3.3 h (stoichiometric CFC12 feeding to MgO) at 723 K.
CFC12 decomposition by MgO did not proceed without any
additive at 723 K. By supporting common transition metals (V,
Mn, Fe, Co, and Cu) as oxide onto MgO significant enhance-
ment of decomposition was observed as shown in Table 1. Ma-
jor gas-phase product was CO2 that shows minelarization of
CFC12. CCl4 was the only by-product observed. The percen-
tage of mass balance for carbon based on conversion of
CFC12 and yields of CO2 and CCl4 was substantially 100%
and no carbon deposition was observed. Since at reaction time
of 3 h, CFC12 loading became 0.45 mol/mol to MgO near stoi-
chiometric ratio of Eq 2, total MgO conversion was calculated
based on oxygen balance using the CO2 yield. The order in the
activity for CFC12 decomposition with simultaneous halogen
absorption was as follows: V ꢂ Cu = Fe > Mn > Co ꢂ Ni.
Except for VOx/MgO and NiOx/MgO the same inclination
was observed, i.e. capability for halogen absorption of the metal
oxides was considerably low compared to VOx/MgO as indi-
cated by low CO2 selectivity of about 50% and high yield of
CCl4. This observation clearly indicates that decomposition of
CFC12 by these metal oxides takes place via following reaction
as Eq 3.
Various methods for CFCs decomposition such as catalytic
hydrolysis have been reported,1{3 while in many cases forma-
tion of corrosive HCl and HF requires gases neutralizer and cor-
rosion proof system. Destructive adsorption of halocarbons onto
metal oxides has been recently reported as a novel method to
mineralize halocarbons directly to CO2 and solid halides.4{8
Klabunde and co-workers have intensively investigated CCl4
decomposition by nanosized particles of MgO or CaO shown
in Eq 1 for MgO.5{8
CCl4(g) þ 2MgO(s) ! CO2(g) þ 2MgCl2(s)
ꢀHꢁ ¼ À377 kJ/mol
ð1Þ
They reported that deposition of another metal oxides on
MgO or CaO was effective to the decomposition and almost
stoichiometric chlorine conversion to the corresponding chlor-
ides was achieved by the reaction at 698 K. Although they sug-
gested possible application of this method to decomposition of
fluorine containing halocarbons such as CFCs,7;8 no work has
been done regarding CFC decomposition using MgO or CaO
based materials.
CCl2F2(g) þ MgO(s) ! 1/2CO2(g) þ 1/2CCl4(g) þ MgF2(s)
In this paper we report the first application of this type of
destructive adsorption method to CFC decomposition using
MgO-based materials to show the feasibility of its application
to practical use and to reveal effects of transition metal oxides
addition onto MgO. CFC12, the simplest CFC containing both
chlorine and fluorine, was chosen to clarify the effects of fluo-
rine of CFCs to have very strong carbon-fluorine bondings. Em-
ploying MgO as a basic metal oxide, destructive adsorption of
CFC12 is expected to proceed to follow Eq 2.
ꢀHꢁ ¼ À289 kJ/mol
ð3Þ
Table 1. CFC12 decomposition by various transition metal
oxides supported on MgO
BET
Integrated conv.
Sample
surface area CFC12 MgOa CO2 select.
m2gÀ1
%
%
%
MgO
204
139
53
75
80
0
0
—
96
52
52
51
—
54
CCl2F2(g) þ 2MgO(s) ! CO2(g) þ MgCl2(s) þ MgF2(s)
VOx/MgO
MnOx/MgO
FeOx/MgO
CoOx/MgO
NiOx/MgO
CuOx/MgO
87
90
98
52
0
75
42
46
24
0
ꢀHꢁ ¼ À477 kJ/mol
ð2Þ
MgO used in this study was prepared from Mg(OH)2 which
was formed by suspending commercial MgO in highly purified
water. For all the transition metals (V, Mn, Fe, Co, Ni, and Cu),
MgO was impregnated with the corresponding metal acetylace-
tonato complexes dissolved in THF to have metal loading of
5 wt%. After the impregnation, the ligands of precursors were
removed at 873 K for 3 h under gentle stream of air. The pre-
pared samples were denoted as VOx/MgO for vanadium oxides
added MgO. The reaction gas was 1% CFC12 balanced with He
102
97
97
48
Reaction temperature: 723 K, Reaction gas: 1% CFC12,
30 mL/min.
Reaction time: 3 h, Metal loading: 5 wt%.
aCalculated based on oxygen balance using amount of CO2
formed.
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.5 (2003)
437
handling for XRD analysis, was observed for VOx/MgO as well
as those from MgF2 and MgO. These observations agree with
the gas-phase product analysis shown in Figure 1.
The reason why only deposition of vanadium oxides pro-
moted chlorine absorption should be related to capability for de-
composition of CCl4. Klabunde et al. reported that vanadium
oxides supported on MgO showed the highest ability for CCl4
decomposition among several metal oxides and the order in
the activity was as follows: V > Mn > Co > Fe > Cu >
Ni.6 Facile formation of unstable vanadium halides (or oxyhal-
ides) would be related to the activity for chlorine absorption, i.e.
CCl4 decomposition. After the CFC12 decomposition by VOx/
MgO, a small amount of yellowish deposition of VF3O was ob-
served in the reactor and at downstream of reaction gas flow, a
dark green hygroscopic material, which was assigned to VClO
by XRD analysis, was formed. VClO was probably originated
from VCl3O. These two oxyvanadium compounds are easily
melted or evaporated under the reaction condition employed
in this system (boiling point for VF3O and VCl3O are 753 K
and 400 K, respectively). Such unstable intermediate com-
pounds can migrate to MgO-rich sites to react with MgO to alter
back to vanadium oxides with formation of magnesium halides
until almost all of MgO are consumed. However, as available
amount of MgO decreased, they evaporated to loss from the
MgO. In this way, addition of vanadium oxides can enhance
CFC12 decomposition and the absorption of both chlorine and
fluorine as magnesium halides.
Figure 1. CFC12 decomposition by VOx/MgO (l, ꢄ) and
FeOx/MgO (n, Ã). CFC12 conv. (solid), CO2 select. (open).
Reaction temperature: 723 K.
From these results, supporting transition metal oxides on
MgO was effective for destructive adsorption of CFC12 with
halogen fixation as magnesium halides, but fixation of chloride
was found to be difficult except for VOx/MgO. Even for VOx/
MgO, which exhibited the highest activity, the complete decom-
position of CFC12 to CO2 and halides takes place until MgO
consumption becomes below 30%, and then efficiency of de-
composition decreased as more MgO is consumed. But even
when stoichiometric amount of CFC12 was loaded to the sys-
tem, MgO conversion reached over 80% and formations of by-
products were still quite small at CFC12 loading of stoichio-
metric amount to MgO. Destructive halogen fixation by
transition metal supported MgO is a promising method to de-
compose CFCs without releasing corrosive acid gases. Further
studies to reveal detail mechanism of this destructive halogen
fixation and to improve the system are under way.
Figure 2. XRD patterns of FeOx/MgO (a) and VOx/MgO (b)
after reaction with CFC12 for 3 h. MgO (ꢄ), MgF2 (n), and
MgCl2ꢃnH2O (s).
In short, selective fluorine absorption happened to proceed
throughout the CFC12 decomposition. However, VOx/MgO
showed high CO2 selectivity of 96% and negligible formation
of CCl4, hence remarkable activity to CFC12 decomposition
and simultaneous halogen fixation.
Since FeOx/MgO exhibited high activity to CFC12 decom-
position with selective fluorine absorption, VOx/MgO and
FeOx/MgO were studied further to understand details of the de-
composition of CFC12 with halogen absorption. As time course
of CFC12 decomposition shown in Figure 1, VOx/MgO exhib-
ited high activity by the reaction time of 1 h to decomposition of
CFC12 to form CO2 as the only gas-phase product. However, as
MgO was consumed to be halides, the efficiency of the decom-
position gradually decreased. FeOx/MgO showed high CFC12
conversion, but CO2 selectivity was just around 50% throughout
the reaction and CCl4 was always formed as a dominant by-pro-
duct.
XRD patterns of VOx/MgO and FeOx/MgO after the reac-
tion indicated formation of magnesium halides (Figure 2). Prob-
ably because of high dispersion and low content of vanadium
oxides and iron oxides, all observed diffraction patterns were
originated from magnesium compounds. For FeOx/MgO after
the reaction, only MgF2 and remained MgO were found and
MgCl2 was not clearly observed while diffraction pattern origi-
nated from MgCl2ꢃnH2O, which was formed by the hydration of
hygroscopic MgCl2 by exposure to ambient air during sample-
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Published on the web (Advance View) April 16, 2003;DOI 10.1246/cl.2003.4 36