A novel method of CCl4 disposal by disproportionation with CH4 over Pt on various supports

Article abstract of DOI:10.1246/cl.2001.264

In disproportionation of CCl₄ with CH₄ into CH₃Cl and CHCl₃, platinum supported on SrCO₃, SiO₂, MgO and MgAl₂O₄ showed stable activity and high selectivities around 700 K, providing a novel disposal method of ozone-depleting CCl₄.

Full text of DOI:10.1246/cl.2001.264

264
Chemistry Letters 2001
A Novel Method of CCl4 Disposal by Disproportionation with CH4 over Pt on Various Supports
Jong Wook Bae, Jae Sung Lee,* Kyung Hee Lee, Byeongno Lee,† and Duck Joo Yang†
Department of Chemical Engineering and School of Environmental Engineering,
Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang, 790-784, Korea
†Samsung Advanced Institute of Technology, 103-6 Moonji-Dong, Yusung-Gu, Taejeon, 305-380, Korea
(Received November 22, 2000; CL-001057)
In disproportionation of CCl4 with CH4 into CH3Cl and
CHCl3, platinum supported on SrCO3, SiO2, MgO and MgAl2O4
showed stable activity and high selectivities around 700 K, pro-
viding a novel disposal method of ozone-depleting CCl4.
introduced at 7.03 µmol/s (weight hourly space velocity (WHSV)
of 1200 L/kgcat./h based on CH4) through a mass flow controller
(Brooks), and CH4/CCl4 mole ratio of 20 was adjusted by bub-
bling helium (3.5 µmol/s) through a CCl4 saturator. Products
were analyzed by HP GC 5890 series II equipped with a 60 m
DB-5 capillary column and a flame ionization detector.
Chlorinated-carbons are known as ozone-depleting com-
pounds.1 Catalytic combustion and hydrodechlorination are useful
methods for disposal of these harmful chlorinated compounds
including CCl4.2–7 Compared to highly exothermic gas-phase
hydrodechlorination of CCl4, which has a short-coming of rapid
catalyst deactivation due to high exothermicity, disproportionation
of CCl4 with CH4 is very attractive and novel method discharging
much lower heat of reaction (∆H values of ca. –177 kJ/mol for
hydrodechlorination of CCl4, and –11.197 kJ/mol for the perfect
disproportionation of CCl4 and CH4). Chlorohydrocarbon prod-
ucts such as CHCl3, CH2Cl2 and CH3Cl, useful for solvents and
intermediates can be obtained by this reaction. Chlorination of
methane i.e., catalytic reaction of CH4 with Cl2, has been found to
show a very high selectivity to CH3Cl over solid super-acid and
supported noble metal catalysts.8–11 Although disproportiona-
tion12 of chlorofluoromethane with CCl4 and catalytic reaction13
of methane with CBrF3 were studied, there has been no report, to
our best knowledge, on the disproportionation of CCl4 and CH4 to
produce chloromethanes. In the present letters, we studied the
reaction over various supported platinum catalysts as a novel
method of CCl4 disposal.
Supported platinum catalysts were prepared by the conven-
tional wet impregnation methods with K2PtCl6 as a platinum pre-
cursor. Various supports were employed including activated car-
bon (specific surface area, Sg = 1500 m2/g), SrCO3 (4.8 m2/g),
SiO2 (300 m2/g), MgO (45.3 m2/g), ZSM-5 (350 m2/g, Si/Al =
150), TiO2 (11 m2/g), MgAl2O4 (prepared from 10 wt% Mg/Al2O3
by calcination at 1127 K, Sg = 65.1 m2/g), and γ-Al2O3 (103 m2/g).
Before initiating reaction in quartz reactor at a desired temperature,
a prepared catalyst (500 mg) was reduced at 773 K with hydrogen
flow of 21.1 µmol/s for 3 h. Methane (Matheson, 99.99%) was
As a blank test, pyrolysis of CCl4 with CH4 without catalyst
was investigated in a quartz reactor (o.d. = 1.27 mm, length = 100
mm). The reaction rates became measurable around 700 K, and
conversion of CCl4 exceeded 10% above 800 K at a total reactant
feed rate of 7.38 µmol/s. Typical product distribution (mole%) at
CCl4 conversion of 36.6% at 823 K was as follows; 42.3% for
CH3Cl, 1.2% for CH2Cl2, 28.6% CHCl3, 4.6% for C2Cl4, and
23.2% for C2HxCly. As the reaction temperature increased above
770 K, the selectivity for C2HxCly (C2H2Cl4, C2HCl3, and uniden-
tified by-products) increased abruptly probably by free radical
reactions. Hence, performance of each catalyst for disproportion-
ation of CCl4 and CH4 was studied below 773 K where the gas
phase homogeneous reaction could be neglected.
Table 1 shows the conversion of CCl4 over 1.0 wt% platinum
supported on various carriers at 723 K, a CH4/CCl4 mole ratio of
20, and a WHSV of 1200 L/kgcat./h based on CH4. Although sup-
port itself showed some catalytic activity for disproportionation of
CCl4 and CH4, selectivity to CH3Cl was lower than Pt-supported
catalyst and catalyst deactivation occurred rapidly (not shown).
Figure 1 shows the product distribution of each catalyst at the CCl4
conversion indicated by parenthesis in Table 1. Products distribu-
tion and catalyst stability were affected strongly by the nature of
support. Carbon- and TiO2-supported platinum showed high initial
CCl4 conversions, but the conversions decreased rapidly and con-
tinuously with time on stream. Pt/ SrCO3 showed an initial rapid
decrease in the conversion but the activity was stabilized after then.
Other catalysts of Pt/MgO, Pt/ZSM-5, Pt/SiO2, and Pt/MgAl2O4
showed initial increase in CCl4 conversions followed by leveling-
off or decrease. Alumina also showed a similar behavior although
the variation was smaller. However, the catalyst formed solid
deposits at the reactor outlet probably due to the low sublimation
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
265
temperature of AlCl3 (mp = 463 K) which might be formed during
the chlorination reaction.10 After reaction, Pt/SrCO3 and Pt/MgO
also showed the change in the phase of supports from oxides to
chlorides such as SrCl2 and MgCl2·6H2O as observed XRD.
Despite this change, two catalysts showed stable catalyst activity
and product distribution at 723 K for more than 30 h. These chlo-
rides have high melting points, 987 K for MgCl2 and 1146 K for
SrCl2. In any case, platinum supported on SiO2, MgO, SrCO3 and
MgAl2O4 showed stable CCl4 conversions and could become can-
didate catalysts for a novel disposal process of ozone-depleting
CCl4 by disproportionation with CH4. Supported platinum cata-
lysts could be classified into two groups according to their prod-
ucts distribution shown in Figure 1. Pt/carbon, Pt/SrCO3, and
Pt/SiO2 showed the selectivity of CH3Cl close to 50% and compa-
rable amounts of CHCl3. Substantial amounts of C2 products
were also formed over Pt/SrCO3, and Pt/SiO2. Other catalysts
produced predominantly CH3Cl. In particular, Pt/MgAl2O4, and
Pt/Al2O3 catalysts exhibited CH3Cl selectivity values higher than
90%. An ideal disproportionation of CCl4 with CH4 should have
the following stoichiometry:
pathway should be more complicated than indicated by equations 1
and 2. As chlorine content increases in chloromethanes, the contri-
bution of CCl4 increases (30% for CH2Cl2 and 60% for CHCl3).
C2Cl4 comes exclusively from CCl4. Thus the reaction on the cata-
lyst surface is very complicated comprising reactions 1 and 2 and
others. As mentioned, dissociative adsorption of CCl4 is facile
under the current reaction conditions. The selectivity to CH3Cl
depends on the availability of adsorbed CH3 species on the surface.
When chlorine from adsorbed CCl4 is readily picked up by the CH3
species, dominant formation of CH3Cl results. Otherwise, more
chlorinated chloromethanes or dimers are preferentially formed due
to the extended residence of adsorbed CCl4 species on the catalyst
surface. This overall picture is consistent with the product distribu-
tion in Figure 1 and the amounts of methane adsorption in Figure 2.
A study of detailed reaction mechanism and optimization of reac-
tion conditions are underway.
The first group of catalyst, Pt/carbon in particular, appears to
follow this route. However, we have to consider other stoi-
chiometries in order to account for the dominant formation of
CH3Cl over the second group catalysts such as;
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