2004
H. Yu et al. / Chemosphere 68 (2007) 2003–2006
both residential and commercial applications. However,
both halocarbons are ozone depleting substances and the
phaseout of these chemicals is underway under the Mon-
treal Protocol.
3. Results and discussion
The gas phase reaction of CHClF2 with CH3Br
commences at 773 K under conditions listed in Table 1.
The major products are C2F4, CH2CF2, CH4, and HX
(X = Br, Cl and F). Minor products include CH3Cl,
C2H2, C2H4, C2H3F, CH2CFCF3, and CHF3. The prod-
ucts detected at trace amounts include CO2, CHBrF2,
C2HF3, and CH2Br2. Some unidentified species are pro-
duced at high temperatures. Coke formation is observed
on the surface of reactor and on exposed thermocouple
sheaths. Mass balances are estimated by determining the
ratio of the total number of mol of element per unit time
in the gas species at the outlet of the reactor to total num-
ber of mol of elements per unit time in the feed. Generally,
the mass balances of all elements decreases according to
the following trend H > Br ꢀ Cl > F > C. For example, at
1123 K under Condition 1 listed in Table 1, the mass
balances are estimated to be 93% for H, 91% for Br, 90%
for Cl, 80% for F, and 69% for C. This suggests that some
species including solid and liquid products, which were
formed during reaction but not included in the mass
balance analysis, are rich in C and F, probably PVDF,
(CH2@CF2)n.
Fig. 1 shows the conversion of CHClF2 and CH3Br, and
the rate of formation of important products vs temperature
under Condition 1 in Table 1. The conversion of CHClF2
is always higher than that of CH3Br until temperature
reaches 1123 K when complete conversion (>99%) of both
CHClF2 and CH3Br is achieved. The rates of formation of
C2F4 and CH4 increase initially and attain maximum levels
at ca. 950 K and 1000 K, respectively, after which the rates
decline. The rate of formation of CH2CF2, the targeted
product, increases with temperature, and a single pass yield
of 42% based on CHClF2 feed is obtained at T = 1123 K.
The rates of formation of minor products, CHF3, C2H3F,
and C2H4, increase while the rates of formation of CH3Cl,
C2H2, and CH2CFCF3 increase initially and decrease after
reaching maximum values.
2. Experimental
The experimental facility used in this study has been
described in detail elsewhere (Yu et al., 2004). Briefly, the
apparatus consists of a tubular high purity (99.99%)
alumina reactor (i.d. 8.5 mm) with the reactor zone main-
tained uniformly by a three zone furnace. Carbon contain-
ing species were identified with a GC/MS (Shimadzu
QP5000) equipped with an AT-Q column, and quantified
with a micro GC (Varian CP-2003) equipped with mole-
cular sieve 5A and PoraPLOT Q columns. Relative molar
response (RMR) factors of hydrocarbons for Thermal
Conductivity Detector detection were experimentally
obtained from standard gas mixtures and quantification
of halogenated hydrocarbons was performed with a diluted
mixture of CHClF2 (>99%, Actrol), CH3Cl (99.5%,
Aldrich) and CH3Br (>99%, BOC Gases) in nitrogen.
For other species, RMR values were estimated from pub-
lished correlations (Height et al., 1999). Hydrogen halides
formed during reaction was trapped with 0.1 M NaOH
solution and the concentration of fluoride determined with
an ion chromatograph (Dionex-100) equipped with an Ion-
Pac AS14A column (4 · 250 mm). CH3I was purchased
from Aldrich and has a purity of 99%.
The reactions of CHClF2 with CH3Br and other chemi-
cals were performed under the conditions summarised in
Table 1.
Table 1
Summary of reaction conditions considered in this investigation
(P = 101 kPa; Reactor volume = 1.35 cm3)
Condition
1
Reaction
Input molar flow
T (K)
rate (mmol hꢁ1
)
We suggest that the initial step during the reaction of
CHClF2 with CH3Br is molecular elimination of HCl from
CHClF2, producing HCl and CF2. The subsequent combi-
nation of CF2 leads to the formation of C2F4. The pyrolysis
of CH3Br has been investigated by several researchers (Kiss
et al., 1977; Barthel and Senkan, 1994). It is generally
believed that the initial steps at low temperatures include
the following reactions, which can generate CH3 radicals.
CHClF2 + CH3Br
N2:150
773–1123
CHClF2:8.1
CH3Br:8.7
2
3
CHClF2 + CH3Cl
CHClF2 + CH3I
CHClF2 + CH3Br
N2: 150
CHClF2:8.3
CH3Cl:8.7
773–1123
773–973
973–1123
N2:150
CHClF2:8.3
CH3I:13.8a
CH3Br ! CH3 þ Br
ð1Þ
ð2Þ
ð3Þ
ð4Þ
4
N2:150
CH3Br þ Br ! CH3 þ Br2
CH3Br þ CH3 ! CH2Br þ CH4
CH2Br þ CH3Br ! CH2Br2 þ CH3
CHClF2:6 0 or 4.8
CH3Br:8.7
a
During the reaction of CHClF2 with CH3I, the gas mixture of N2 and
CHClF2 was passed through a saturator which contains liquid CH3I and
cooled at temperatures between ꢁ18 and ꢁ20 ꢂC using a mixture of
23.3 wt% NaCl and 76.7 wt% of ice. Since we are not able to quantify
CH3I, the molar flow rate of CH3I is estimated, based on its vapour
pressure at ꢁ18 ꢂC. For the same reason, we did not calculate the con-
version of CH3I.
The conversion level of CHClF2 is higher than that of
CH3Br under conditions investigated, which is due to the
higher activation barrier (approximately 66.9 kcal molꢁ1
)
for (1) than that for the molecular elimination of CHClF2