The catalytic hydrolysis of CCl4 to HCl and CO2 over magnesium oxide
Ulrike Weiss, Michael P. Rosynek and Jack Lunsford*
Department of Chemistry, Texas A&M University, College Station, Texas 77843, USA.
E-mail: lundsford@mail.chem.tamu.edu
Received (in Cambridge, UK) 29th November 1999, Accepted 1st February 2000
At temperatures > 400 °C, CCl4 reacts with H2O over a MgO
2 Torr and P(H2O) = 4 Torr. Under these conditions, the initial
catalyst to yield HCl and CO2.
conversion of 88% decreased to ca. 74% during the first hour,
but thereafter the conversion remained nearly constant. After 24
h on stream, the conversion was 68%. The surface area of the
catalyst decreased from 90 to 38 m2 g21, with most of the
decrease occurring during the first hour. At the same partial
pressures of CCl4 and H2O, but with 1.72 g MgO at 525 °C, the
conversion was > 99% (no remaining CCl4 was detected) for 72
h. Thus, nearly complete removal of CCl4 can be achieved over
a long period.
Kinetic results were obtained under differential reaction
conditions, which were achieved either by using a smaller
amount of catalyst or by operating at lower temperatures. Over
the temperature range 400–500 °C with 2 Torr CCl4 and 4 Torr
H2O, the apparent activation energy was 85 kJ mol21. At the
same initial pressures and at 450 °C, the specific activity was
0.167 mmol (g s)21 or 4.1 nmol (m2 s)21 for a catalyst having a
surface area of 38 m2. The reaction orders with respect to CCl4
and H2O are given in Fig. 2. The reaction order with respect to
H2O at 400 °C was slightly dependent on the partial pressure of
CCl4 and increased to n = 0.16 at 6 Torr of CCl4.
The amount of chloride in the sample after reaction was 4
wt%, which corresponds to a Cl/Mg ratio of 0.047. This value
may be compared with a near-surface Cl/Mg ratio of 0.12 (as
determined from XPS spectra), which was nearly the same for
samples that had been on stream for 1 h at 500 °C or for 70 h at
525 °C. After steady state was attained, there was nearly a 100%
chlorine balance between CCl4 reacted and HCl formed.
The results are consistent with the mechanism described in
Scheme 1, which is adapted from an earlier one that was
proposed by Hooker and Klabunde6 for the destructive
adsorption of CCl4. Although phosgene is a potential inter-
mediate, none was detected in the gas phase by IR spectroscopy.
The rate limiting step is believed to be the dissociative
adsorption of CCl4, although this is inhibited by the presence of
chloride ions on the surface, which is consistent with the fact
The destruction of carbon tetrachloride found in ground water
and in effluent streams is typically carried out by using
incineration1 or catalytic oxidation.2 We describe here the
catalytic reaction of CCl4 with H2O to form CO2 and HCl,
which has not been previously reported except for a brief
comment in the patent literature.3 The kinetics of the corre-
sponding uncatalyzed reaction in water was studied by Fells and
Moelwyn-Hughes,4 who observed a rather small second order
rate constant (with respect to CCl4) of k = 1.21 3 1023 L mol21
s21 at 373 K for an initial CCl4 concentration of 0.903 mmol
L21. The rate was unaffected by proton, hydroxide or chloride
ion concentration.
The use of an alkaline earth oxide catalyst is based on a non-
catalytic cycle in which CCl4 was first reacted with BaO to form
BaCl2.5 The BaCl2 was subsequently reacted with aqueous
CO322 to produce BaCO3 and aqueous HCl. The BaCO3 could
be converted to BaO at elevated temperatures (600 °C), and the
process could then be repeated. However when CCl4 and H2O
were passed over BaO at 500°C, the catalytic reaction [eqn.
(1)]
CCl4 + 2H2O ? 4HCl + CO2 DGo500 °C = 2 390kJmol21
(1)
did not occur. Magnesium oxide, however, is an effective
catalyst for this reaction, in part because magnesium chloride is
not extensively formed, and, more importantly, magnesium
carbonate decomposes at temperatures near 400 °C.
The catalyst was prepared by the decomposition of Mg(OH)2
obtained by stirring a slurry of MgO (Fisher, light) and water at
80 °C for 24 h. The Mg(OH)2 (20–40 mesh size) was
decomposed at 400 °C in flowing O2 (100 mL min21). The
reaction of CCl4 with H2O was carried out in a plug flow reactor
at a total flow rate of 80 mL min21 with He as the diluent. Gas
chromatography was used to analyze for CCl4, CO2 and H2O,
while HCl was trapped in water and subsequently titrated with
AgNO3 (aq). X-Ray photoelectron spectra (XPS) were acquired
using a Perkin-Elmer (PHI) model 5500 spectrometer.
The conversions of CCl4 and H2O are shown in Fig. 1 for the
reaction carried out at 500 °C over 0.58 g MgO with P(CCl4) =
Fig. 2 Effect of CCl4 and H2O partial pressures on the rate of CO2 formation
over 0.40 g MgO: variation in CCl4 pressure at 400 °C (5) and at 450 °C
(2) with P(H2O) = 9.7 Torr; variation in H2O pressure at 400 ° C (!) and
at 450 °C (“) with P(CCl4) = 2.1 Torr.
Fig. 1 Conversion of CCl4 (2) and H2O (!) over 0.58 g MgO at 500 °C
with P(CCl4) = 2 Torr and P(H2O) = 4 Torr.
DOI: 10.1039/a909552g
Chem. Commun., 2000, 405–406
This journal is © The Royal Society of Chemistry 2000
405
Weiss, Ulrike
