T. Tamai et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1573
responds to the stoichiometry (CCl2F2/MgO ¼ 0:5 mol molꢁ1). If
complete decomposition of CCl2F2 into CO2 and magnesium hal-
ides is achieved following Eq. 1, MgO is totally turned to MgCl2
and MgF2 at this period of time. The conversion of MgO was
estimated by the amount of formed CO2, since the oxygen source
in the system was only the lattice oxygen in MgO with a small
amount of vanadium oxides. For the VM sample MgO conversion
was calculated ignoring the amount of vanadium added to MgO.
CCl4 Decomposition. CCl4 decomposition by MgO, V2O5,
and Mg3(VO4)2 was carried out using a fixed-bed pulse reaction
system directly connected to GC-TCD. Pulses of 4 mL of CCl4
(Wako Pure Chem. Ind., 99%) in a precision syringe were injected
to the same reactor as CCl2F2 decomposition at 723 K.
Characterization. X-ray diffraction patterns of the samples
were obtained with Rigaku Multiflex S with Cu Kꢀ as an incident
X-ray source in the range of 20 to 70 degrees as 2ꢁ. A specific sur-
face area of the samples was determined by nitrogen adsorption at
77 K with BELSORP 28SA (BEL Japan). Infrared spectra were
obtained with a JASCO FT-IR 350. The sample was diluted with
KBr and pressed into a thin wafer. Raman spectra were taken with
a JASCO RMP-200 at ambient atmosphere. The XPS analysis was
performed by the ULVAC-PHI 1700R ESCA system with a
monochromated Al X-ray source. NH3-TPD (temperature pro-
grammed desorption of ammonia) experiments were carried out
for MgO samples by using a Pyrex-glass system equipped with
a quartz U-tube reactor. The samples were pretreated at 873 K
for 2 h under evacuation. After the sample was cooled down to
373 K, gaseous ammonia was introduced to the system, and then
adsorption equilibrium was attained within 0.5 h. NH3-TPD was
performed at a temperature ramping rate of 10 K minꢁ1 from
373 K to 873 K with detection by TCD (Shimadzu GC-8A) after
removing physisorbed ammonia under a vacuum for 0.5 h.
stage, CCl2F2 is still converted (but the reaction path is
changed), since sufficient MgCl2 is left to undergo the halogen
exchange reaction (at the final stage of the reaction). The halo-
gen exchange is not a catalytic reaction, and continues until the
conversion of MgCl2 to MgF2 is completed.
Conclusion
CCl2F2 decomposition with halogen fixation by vanadium
oxide supported on magnesium oxide was studied by using va-
nadium acetylacetonate as a precursor of vanadium oxide sup-
ported on MgO. When the precursor sample was heated in air,
Mg3(VO4)2 was formed, which was confirmed by all of XRD,
FT-IR, Raman, and XPS, while it was not observed when the
sample was treated under helium. Mg3(VO4)2 was revealed to
be an active phase for CCl2F2 decomposition with complete
halogen fixation as MgF2 and MgCl2. The details of the reac-
tion mechanism are as follows: CCl2F2 decomposition is initi-
ated by Lewis-acid site on Mg3(VO4)2 to dissociate C–F bond-
ing accompanying fluorine fixation as MgF2. The intermediate
product VOCl3 reacts with MgO to regenerate the active
Mg3(VO4)2 phase with chlorine fixation as MgCl2. Intermedi-
ate CCl4 can also be decomposed by Mg3(VO4)2 to form
VOCl3. The turnover between Mg3(VO4)2 and VOCl3 is con-
sidered to bring the high activity of CCl2F2 decomposition by
vanadium oxide supported on MgO.
Experimental
Materials. MgO (UBE Materials Industries, 100A) was sus-
pended to highly purified water and heated to be dried up; then,
the resulting Mg(OH)2 was treated at 873 K for 3 h under helium.
MgO obtained by this method has a specific surface area of
170–210 m2 gꢁ1. Vanadium acetylacetonate complex (V(acac)3,
Aldrich) was dissolved into tetrahydrofuran (THF) and the MgO
was added to the solution. Then, THF was removed from the re-
sulting suspension with a rotary evaporator and the residue was
dried in an oven at 373 K. Ligand removal from vanadium acetyl-
acetonate was carried out at 873 K for 3 h under air or helium. The
sample was denoted as 5VM-air when the sample had vanadium
loading of 5 wt % (V/MgO ¼ 0:05) and was calcined under air.
V2O3, V2O4, and V2O5 were obtained from commercial
sources (Aldrich). Three kinds of magnesium vanadates
(Mg3(VO4)2, Mg2V2O7, and MgV2O6) were prepared by the
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