Role of copper species in chlorination and condensation reactions of acetylene

Article abstract of DOI:10.1021/es980050s

We examined the thermally induced acetylene chlorination and condensation reactions on different types of copper salt impregnated surfaces. The System for Thermal Diagnostic Studies provided a powerful tool to study these reactions under defined reaction conditions, which were related to typical conditions in postcombustion incineration processes. Experiments were conducted with acetylene or acetylene/HCl mixtures in a quarts reactor filled with a borosilicate foam of known pore size at temperatures between 150 and 500 °C. Borosilicate was also used as the catalytic support for gas-solid reactions of acetylene and acetylene/HCl mixtures with CuCl₂ and CuO. Reaction products were trapped in-line and analyzed by GC/MS. It was shown that borosilicate is not able to catalyze acetylene condensation reactions. CuCl₂-impregnated borosilicate was a highly effective catalyst for acetylene chlorination/condensation reactions at temperatures above 150 °C. The same behavior was found for CuO- impregnated borosilicate in the presence of HCl. However, temperatures above 300 °C were required for this catalytic system. Mainly perchlorinated C-2 to C-8 hydrocarbons were trapped as reaction products in the gas phase. Maximum yields for acetylene chlorination/condensation reactions in each related catalytic system were found at temperatures between 300 and 400 °C. Results of the surface-catalyzed acetylene chlorination and condensation reactions were summarized in a global mechanism. A ligand transfer oxidative chlorination of acetylene with CuCl₂ was proposed to be the initiation of acetylene with CuCl₂ was proposed to be the initiating step. Chlorinated acetylene then condenses to higher molecular weight compounds, catalyzed by CuCl in metallacyclization reactions. We examined the thermally induced acetylene chlorination and condensation reactions on different types of copper salt impregnated surfaces. The System for Thermal Diagnostic Studies provided a powerful tool to study these reactions under defined reaction conditions, which were related to typical conditions in postcombustion incineration processes. Experiments were conducted with acetylene or acetylene/HCl mixtures in a quartz reactor filled with a borosilicate foam of known pore size at temperatures between 150 and 500 °C. Borosilicate was also used as the catalytic support for gas-solid reactions of acetylene and acetylene/HCl mixtures with CuCl₂ and CuO. Reaction products were trapped in-line and analyzed by GC/MS. It was shown that borosilicate is not able to catalyze acetylene condensation reactions. CuCl₂-impregnated borosilicate was a highly effective catalyst for acetylene chlorination/condensation reactions at temperatures above 150 °C. The same behavior was found for CuO-impregnated borosilicate in the presence of HCl. However, temperatures above 300 °C were required for this catalytic system. Mainly perchlorinated C-2 to C-8 hydrocarbons were trapped as reaction products in the gas phase. Maximum yields for acetylene chlorination/condensation reactions in each related catalytic system were found at temperatures between 300 and 400 °C. Results of the surface-catalyzed acetylene chlorination and condensation reactions were summarized in a global mechanism. A ligand transfer oxidative chlorination of acetylene with CuCl₂ was proposed to be the initiating step. Chlorinated acetylene then condenses to higher molecular weight compounds, catalyzed by CuCl in metallacyclization reactions.

Full text of DOI:10.1021/es980050s

Environ. Sci. Technol. 1998, 32, 2741-2748
phatic compounds (1-3). Acetylene (C2H2) is known to be
Role of Copper Species in
Chlorination and Condensation
Reactions of Acetylene
a major precursor in the thermal formation of condensed
hydrocarbons including aromatic structures and soot. Gas-
phase condensation reactions of acetylene have been studied
extensively since the beginning of this century and have
recently been reviewed (4, 5). These reviews concluded that
condensation of acetylene in the gas phase initiated at
temperatures above 700 °C and led to condensation products
including vinylacetylene, benzene, styrene, etc. but also gave
pyrolysis products such as vinylidene (6). Condensation
reactions in solutions of monosubstituted acetylenes were
investigated leading to the respective substituted benzenes
(7, 8). Studies of chlorinated analogues indicated that
dichloroacetylene dissolved in ether condensed to hexachlo-
robenzene (9). The temperature for condensation of chlo-
rinated acetylenes in solutions was reported to be below 130
°C (10). However, recent investigations of other C-2 aliphatic
compounds such as trichloroethylene and tetrachloroeth-
ylene reported gas-phase condensation temperatures above
700 °C and chlorinated products analogous to those observed
in acetylene condensation (11-13).
A N D R E A S W E H R M E I E R , †
D I E T E R L E N O I R , * , † S U K H S . S I D H U , ‡
P H I L I P H . T A YL O R , * , ‡ W A YN E A . R U B E Y, ‡
A N T O N I O U S K E T T R U P , † A N D
B A R R Y D E L L I N G E R ‡
Institute for Ecological Chemistry, GSF-National Research
Center for Environment and Health, Ingolsta¨dter Landstrasse
1, D-85764 Neuherberg, Germany, and Environmental Science
and Engineering, University of Dayton Research Institute,
300 College Park, Dayton, Ohio 45469-0132
We examined the thermally induced acetylene chlorination
and condensation reactions on different types of copper
salt impregnated surfaces. The System for Thermal Diagnostic
Studies provided a powerful tool to study these reactions
under defined reaction conditions, which were related
to typical conditions in postcombustion incineration processes.
Experiments were conducted with acetylene or acetylene/
HCl mixtures in a quartz reactor filled with a borosilicate
foam of known pore size at temperatures between 150 and
500 °C. Borosilicate was also used as the catalytic
support for gas-solid reactions of acetylene and acetylene/
HCl mixtures with CuCl2 and CuO. Reaction products
were trapped in-line and analyzed by GC/MS. It was shown
that borosilicate is not able to catalyze acetylene
Historically, acetylene has been an important starting
material in the industrial synthesis of chemical compounds
(14). Since the early twentieth century a variety of catalysts
for acetylene condensation reactions were investigated; in
solution chemistry the synthesis of Reppe is of particular
significance (15). In the presence of carbonyl-nickel-
phosphine complexes acetylene condenses, forming cy-
clooctatetraene and benzene at temperatures below 100 °C.
Cuprous chloride (CuCl) dissolved in polar solvents is used
for coupling of acetylenes in preparative chemistry. These
reactions are known as the Glaser reaction for coupling of
symmetrically substituted acetylenes and Cadiot-Chod-
kiewicz coupling for unsymmetric ones (16).
Copper chlorides also play a predominant role in surface-
catalyzed chlorination reactions. In the Deacon reaction,
HCl is oxidized with oxygen in the presence of copper salts
to chlorine (Cl2). Chlorine reacts with acetylene at temper-
atures above 450 °C to yield tetrachloroethylene (17, 18).
Cupric chloride (CuCl2) can act directly as a chlorinating
agent by a ligand transfer oxidation mechanism (19). In gas-
surface reactions copper species are considered among the
most effective catalysts in thermal formation of chlorinated
combustion byproducts, e.g., polychlorinated dibenzo-p-
dioxins (PCDD) and -furans (PCDF) (20 and references
therein). Froese and Hutzinger (3) have shown that acetylene
in the presence of cupric oxide (CuO) and HCl forms
chlorinated benzenes, phenols, and PCDD/ F at 600 °C. They
have also shown that condensation of trichloroethylene (TCE)
was not catalyzed by CuO (1).
The objectives of this study were to investigate the role
of CuCl2 and CuO in thermal molecular growth reactions of
acetylene in gas-surface reactions using model borosilicate
surfaces under controlled experimental conditions. It is
shown that these surfaces reduce the reaction temperature
of acetylene to chlorinated and condensed products from
700 °C in the gas phase to 150 °C. The nature of this catalytic
reaction is discussed. A global reaction mechanism con-
sistent with observed gas-phase products is proposed based
on metallacyclization reactions involving cuprous chloride
(CuCl) intermediates.
condensation reactions. CuCl2-impregnated borosilicate
was a highly effective catalyst for acetylene chlorination/
condensation reactions at temperatures above 150 °C.
The same behavior was found for CuO-impregnated
borosilicate in the presence of HCl. However, temperatures
above 300 °C were required for this catalytic system.
Mainly perchlorinated C-2 to C-8 hydrocarbons were trapped
as reaction products in the gas phase. Maximum yields
for acetylene chlorination/condensation reactions in each
related catalytic system were found at temperatures
between 300 and 400 °C. Results of the surface-catalyzed
acetylene chlorination and condensation reactions were
summarized in a global mechanism. A ligand transfer oxidative
chlorination of acetylene with CuCl2 was proposed to be
the initiating step. Chlorinated acetylene then condenses to
higher molecular weight compounds, catalyzed by CuCl
in metallacyclization reactions.
Introduction
Recent studies indicate that copper species play an important
role in the thermal formation of chlorinated combustion
byproducts and in thermal condensation reactions of ali-
* Corresponding author (Lenoir) phone: ++49-89-3187-2960;
fax: ++49-89-3187-3371; e-mail: lenoir@gsf.de. Corresponding
author (Taylor) phone: (937) 229-2846; fax: (937) 229-2503; e-mail:
Taylorph@udri.udayton.edu.
Experimental Section
Surface-catalyzed chlorination and condensation reactions
of acetylene with copper species were performed using the
System for Thermal Diagnostic Studies (STDS) (Figure 1).
† GSF-National Research Center for Environment and Health.
‡ University of Dayton Research Institute.
9
S0013-936X(98)00050-9 CCC: $15.00
Published on Web 08/01/1998
1998 Am erican Chem ical Society
VOL. 32, NO. 18, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 7 4 1

FIGURE 1. Schematic of the System for Thermal Diagnostic Studies (STDS) as modified for this heterogeneous combustion study.
The STDS is a high-temperature flow reactor coupled with
an in-line gas chromatograph/ mass spectrometer (GC/ MS)
system designed to simulate the reaction conditions in a
combustor postflame zone. The setup is described in detail
elsewhere (21). Briefly, the STDS consists of four integrated
units: (1) a control console for precise adjustment of
temperature, pressure, residence time, and the respective
gas flows; (2) a thermal reactor compartment containing a
high-temperature furnace (Lindberg) housed within a gas
chromatograph (HP 5890) to allow precise sample introduc-
tion into and out of a quartz reactor; (3) a gas chromato-
graphic oven (HP 5890) containing a silicosteel trap and a
capillary column (DB-5, J & W Scientific) for separation of
organic reaction products; and (4) a mass spectrometer (HP
5970B MSD) for product identification and quantification.
A cylindrical borosilicate (BS) foam (1.5-2.0% Na2O, 1.0-
1.5% K2O, 9.5-10.0% B2O3, 9.0-9.5% Al2O3, 77-80% SiO2),
with a length of 5 cm, a diameter of 0.7 cm, and a pore size
of 200 µm, was purchased from Cercona of America, Inc.,
and used as the catalytic support. The support was coated
with different catalysts. Cupric chloride (CuCl2, 99.999%
Aldrich) and cupric nitrate (Cu(NO3)2, 99.999% Aldrich),
respectively, were applied to the borosilicate by impregnation
to incipient wetness (22). The borosilicate was impregnated
with the respective cupric salt by dipping it into an ∼20%
aqueous solution of CuCl2 or CuNO3. Foams were dried at
120 °C for 1 h. The cupric oxide (CuO)-coated foam was
prepared from the Cu(NO3)2 foam by calcinating at 480 °C
for 16 h in air. Amounts of CuO and CuCl2 on the borosilicate
varied between 1 and 9%, respectively. It should be noted
that CuCl2 needed prior cleaning because of the presence of
substantial amounts of chlorinated compounds, especially
hexachlorobenzene. Hence, powdered CuCl2 was thermally
treated at 250 °C for 4 days in a flow of oxygen-free helium
to remove all organic contaminants.
residence time inside the catalytic foam of 2.0 s ((0.06 s),
depending on the reaction volume inside the foam and the
temperature and pressure inside and outside the reactor.
Hence, reactor gas flow rates varied with temperature from
18 to 36 mL/ min. Acetylene containing an impurity of 2%
acetone (technical grade, Air Products) or acetylene and HCl
(technical grade, Air Products) were injected directly into
the reactor using a syringe pump. Flow rates of 0.065 mL/
min (1100 ppm) were used for both reagents resulting in
very fuel-lean reaction conditions for acetylene according to
relative stoichiometric oxidation. Soot formation from
acetylene was prevented. All experiments were conducted
for 1 h. Semivolatile reaction products were trapped on a
1 m long silicosteel trap positioned inside a gas chromato-
graphic oven held at 40 °C. Volatile reaction products which
were not trapped on the silicosteel were collected through
a split outside the GC oven on a Carbopack B (0.2 g, mesh
60/ 80)/ Tenax TA (0.2 g, mesh 60/ 80) trap, with the Tenax-
end connected to the silicosteel trap. This setup provided
a tool to measure the volatile reaction products, which were
expected to be formed in higher amounts with higher split
values than the semivolatile reaction products.
After each combustion experiment, the injector and
reactor flow was reduced to negligible values and the junction
flow of helium was increased. The semivolatile reaction
products were then released by heating of the silicosteel trap
to 300 °C. Separation of semivolatile gas-phase products
was achieved using a DB-5 capillary column (30 m, i.d. 0.25
mm, film thickness 0.25 µm, J&WScientific) and the following
temperature program: 40 °C / / 10 °C/ min / / 290 °C / / 5 min.
The mass selective detector (HP 5970B MSD) was operated
in the electron ionization mode, scanning all masses from
40 to 500 amu. Higher volatile reaction products trapped on
the Carbopack/ Tenax trap were analyzed in a similar system.
Products were thermally desorbed in reverse flow using
oxygen-free helium as carrier gas. The temperature program
for desorption was 35 °C / / 25 °C/ min / / 300 °C / / 15 min.
Compounds were then trapped on a DB-1 capillary column
(30 m, i.d. 0.32 mm, film thickness 0.32 µm, J &W Scientific),
held at -60 °C. Gas chromatographic separation was done
using a linear temperature program: -60 °C / / 10 °C/ min / /
The borosilicate foams were positioned inside a quartz
reactor, held in place with quartz wool, and heated to furnace
temperatures of 150, 200, 250, 300, 400, and 500 °C,
respectively. A mixture of 4% oxygen in helium was used as
reaction atmosphere. The gas flow rate for the injector was
5 mL/ min. The reactor gas flow was adjusted to an average
9
2 7 4 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 18, 1998

FIGURE 2. Predominant gas-phase products for the reaction of acetylene on CuCl2/BS at 150, 300, and 400 °C, respectively. [C H ]
)
2
2 total
4.35 mg, [CuCl2]0 ) 58 ( 11 mg, tr ) 2.0 s, reactor gas: 4% O in He.
2
280 °C / / 1 min. Products were analyzed by means of a mass
selective detector (HP 5970B MSD) operating in scan mode
from 15 to 300 amu. Quantification for both volatile and
semivolatile reaction products was accomplished using
external standard calibration curves for the respective
analyzed product or chemically similar compounds in cases
where standards were not available.
higher temperatures. The compounds C6Cl4 and the isomer
of hexachlorobenzene were only detected at 150 °C. Maxi-
mum product yields were typically produced at 300 °C.
The mass spectra together with possible molecular
structures of the unidentified compounds C6Cl4, C6Cl8 (two
isomers with identical mass fragmentation), and C8Cl9H are
shown in Figure 3. The C6Cl6 isomer and hexachlorobenzene
has identical mass spectra, but are eluted at different retention
times. The reaction products C6Cl4, the two isomers of C6Cl8,
and hexachlorobenzene all show similar mass fragmentation.
They just differ by two chlorine atoms more (C6Cl8) or less
(C6Cl4) than hexachlorobenzene. The molecular mass for
the predominant molecule isotope is 214 amu for C6Cl4 and
354/ 356 amu (similar heights) for C6Cl8. The predominant
molecule isotope of C8Cl9H has a mass of 414 amu. Its mass
fragmentation pattern is not related to hexachlorobenzene.
Figure 3 shows only a choice of possible molecular structures
of these unidentified reaction products, other structures are
as likely, e.g., besides octachlorocyclohexa-1,4-diene, another
cyclic C6Cl8 isomer, octachlorocyclo-1,3-diene, can be formed.
Adding HCl to the acetylene-CuCl2/ BS system increased
the total product yield. The same reaction products were
formed, but the distribution was shifted toward higher
chlorinated compounds, e.g., trichloroethylene was not
detected.
CuO was tested for its catalytic activity toward acetylene
condensation and chlorination in the acetylene-CuO/ BS and
acetylene/ HCl-CuO/ BS system, respectively. Gas-phase
reaction products formed in the acetylene/ HCl-CuO/ BS
system were formed in amounts similar to those in the
acetylene-CuCl2/ BS system at comparable temperatures at
and above 300 °C. In contrast to the acetylene-CuCl2/ BS
system, no reaction products were detected at temperatures
below 300 °C. The two systems investigated without adding
copper species, acetylene-BS and acetylene/ HCl-BS, did
not result in any reaction products at temperatures up to 500
°C. Figure 4 gives the total gas-phase product yields at 300
°C for the respective surfaces and reaction gas mixtures.
A few minor reaction products were not quantified, and
hence, these products are not included in sum of product
Results
Reactions of acetylene in a very fuel-lean atmosphere (fuel-
air equivalence ratio ∼0.06) on CuCl2-impregnated boro-
silicate foams produced a variety of chlorinated organic
compounds. This system was able to chlorinate acetylene
to form chlorinated C-2 compounds. Besides these, con-
densed products up to C-8 chloroorganics were analyzed in
the gas phase; higher molecular weight compounds (23) were
adsorbed on the borosilicate surface. The most striking
feature in the distribution of reaction products was the high
degree of chlorination of the organic moiety. A majority of
products were perchlorinated. An interesting observation
was the lack of unchlorinated acetylene polymerization
products. Furthermore, cracking products and compounds
with an odd number of carbon atoms were not detected
under these reaction conditions. Besides hexachloropro-
pane-2-one, which was formed in a few experiments in very
small amounts, no oxygenated reaction products were
trapped from the gas phase. Hexachloropropane-2-one was
probably the chlorination product of acetone, a 2% impurity
in the technical grade acetylene (selected experiments were
conducted where the acetone impurity was reduced to less
than 1%). The main reaction products formed in the
acetylene-CuCl2/ BS system were as follows (Figure 2):
tetrachloroethylene (C2Cl4) > hexachlorobutadiene (C4Cl6)
> trichloroethylene (C2HCl3) > pentachlorobutadiene (C4-
HCl5) > tetrachlorovinylacetylene (C4Cl4) > hexachloroben-
zene (C6Cl6) > tetrachloroethane > C6Cl8 (two different
isomers) > pentachlorobenzene (C6HCl5) > C6Cl6 (isomer of
hexachlorobenzene) > C6Cl4 > C8Cl9H.
Reaction products with a lower degree of chlorination
(e.g. pentachlorobutadiene) were predominately formed at
9
VOL. 32, NO. 18, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 7 4 3

yields. Approximately 6 mol % of acetylene was transformed
to chlorinated and condensed reaction products in the
presence of a CuCl2-impregnated borosilicate foam at 300
°C. In the presence of 1100 ppm HCl, yields rose to 9.5 mol
%.
The total yield of reaction products was a strong function
of temperature. For the acetylene-CuCl2/ BS system, reaction
started at temperatures below 150 °C and rose to maximum
yields at about 300 °C. Yields decreased at higher temper-
atures. No reaction products were produced at 500 °C.
Reaction products for the acetylene/ HCl-CuO/ BS system
were only found at and above 300 °C with maximum yields
at 400 °C. In contrast to the acetylene-CuCl2/ BS system,
significant amounts of reaction products were formed at 500
°C (Figure 5).
Changes in product yields for the two systems cor-
responded to changes of the surface. The color of dry CuCl2
is yellow-brown, and it did not change when it was loaded
onto the borosilicate. Heating this system up to 300 °C did
not produce any changes in color, but CuCl2 started to
sublime from the foam above 300 °C and the loaded
borosilicate reverted back to its original white color when it
was heated to 500 °C. This effect indicated a loss of CuCl2
from the borosilicate at elevated temperatures. The CuO-
mediated borosilicate was black. This appearance did not
change upon heating to 500 °C. However, upon addition of
HCl, the color of the foam turned yellow-brown at temper-
atures above 300 °C, indicating that CuO started to react
with HCl at 300 °C, which resulted in formation of CuCl2.
Discussion
The condensation of acetylene in the presence of HCl has
recently been studied in a fixed bed reactor on different kinds
of surfaces (1-3). Fly ash mediated reaction started at 400
°C, and gas-phase product yields rose in general up to 600
°C. Different kinds of chlorinated reaction products including
benzenes, phenols, dibenzo-p-dioxins, and -furans were
formed. Metal oxide mixtures with SiO2 were also tested for
their catalytic behavior at 600 °C. At this temperature,
chlorinated and condensed reaction products were formed
with different kinds of surfaces, including SiO2 and a SiO2/
CuO mixture.
We did not investigate acetylene chlorination and con-
densation above 500 °C, because both reaction types are
known to occur in the gas phase at elevated temperatures
(see Introduction). Our goal was to study the catalytic activity
of CuCl2 and CuO for these reactions. The catalytic support,
borosilicate, did not show any catalytic activity for the
acetylene reactions below 500 °C. Hence, in the study of
Froese and Hutzinger (1-3), reaction products found with
SiO2 as support at 600 °C may have involved a gas-phase
reaction, mediated by the surface. Furthermore, many
reaction products formed at 600 °C with CuO/ SiO2 may have
also formed by a surface-mediated, not catalyzed, gas-phase
reaction. An increase in product yields by adding CuO to
SiO2 at 600 °C was only observed for highly chlorinated
compounds. However, in our study, CuO-impregnated
borosilicate in the presence of HCl did result in acetylene
chlorination/ condensation reactions at temperatures below
600 °C. Therefore, these reactions were indeed catalyzed by
copper species.
The catalyzed chlorination/ condensation reaction of
acetylene on CuO/ BS in the presence of HCl started at 300
°C with maximum yield at 400 °C. The color of the CuO-
impregnated borosilicate in the presence of HCl changed
with rising catalytic reactivity from black to yellow-brown.
No change in the color of the foam and no reaction products
were observed for the CuO/ BS system, without adding HCl.
Hence, the reaction product of CuO and HCl was likely the
effective catalyst. This reaction is known to take part in the
FIGURE 3. Mass spectra and some possible structures of the
unidentified gas-phase products C Cl4, C Cl8, and C Cl9H.
6
6
8
9
2 7 4 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 18, 1998

FIGURE 4. Total gas-phase product yields in mole percent according to acetylene injection to the respective surfaces at 300 °C. [C H ]
2
2 total
) 167 µmol, [HCl] ) 167 µmol, [CuCl2]0) 344 ( 80 µmol, [CuO]0) 302 µmol, tr ) 2.0 s, reactor gas: 4% O in He.
total
2
FIGURE 5. Total gas-phase product yields for the reaction of acetylene on CuCl2-mediated borosilicate (gray bars) and acetylene/HCl
mixture on CuO-mediated borosilicate (black bars) at different temperatures. [C H ] ) 167 µmol, [HCl] ) 167 µmol, [CuCl2]0 ) 344
2
2 total
total
( 80 µmol, [CuO]0 ) 302 µmol, tr ) 2.0 s, reactor gas: 4% O in He.
2
Deacon reaction, where HCl is oxidized to Cl2 in the presence
of a copper(II) catalyst:
350 °C by adding a silicate support (25). This process probably
occurs in the acetylene/ HCl-CuO/ BS system. CuO and HCl
react at a temperature above 300 °C supported by borosilicate
to CuCl2, which itself reacts as the catalytic agent.
CuCl2 + 1/ 2 O2 f CuO + Cl2
CuO + 2HCl f CuCl2 + H2O
The experiments with the CuCl2-impregnated borosilicate
showed catalytic activity for the acetylene chlorination/
condensation reactions above 150 °C. The maximum product
yields for this system were found at 300 °C, but in contrast
to the CuO/ HCl system, no reaction products were formed
at 500 °C. The lack of product formation at elevated
temperatures is due to sublimation of CuCl2 from the support.
CuCl2 begins to sublime at 300 °C and was completely
sublimed from the borosilicate at 500 °C, as indicated by the
2HCl + 1/ 2O2 f H2O + Cl2
With CuCl2 as catalyst the Cl2 liberation starts at 375 °C
and gives significant yields of chlorine above 420 °C (24).
However, reaction temperatures can be reduced down to
9
VOL. 32, NO. 18, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 7 4 5

white color of the foam at this temperature. Hence, no
catalyst was involved in the 500 °C acetylene reaction.
However, CuCl2 was formed as an intermediate in the CuO/
HCl system at 500 °C and was then able to act as a catalyst,
leading to significant yields of reaction products. The
distribution of reaction products at 300 and 400 °C was the
same for the acetylene-CuCl2/ BS and the acetylene/ HCl-
CuO/ BS system. This finding is another indication that
copper chlorides are the catalytic compounds in the acetylene
chlorination/ condensation reaction.
FIGURE 6. Structure of copper-acetylene complexes.
CuCl2 is known to be a very effective chlorinating agent
(26). Most products found in our experiments were per-
chlorinated; partially chlorinated reaction products were
more predominant at elevated temperatures. The degree of
chlorination increased by adding HCl. This indicates that
CuCl2 is consumed in the reaction to CuO, like the first step
of the Deacon reaction, or reduced to CuCl, in the presence
of acetylene as the reducing agent. According to the second
step of the Deacon reaction, CuO can react with HCl to reform
CuCl2. On the other hand, CuCl in the presence of HCl can
be oxidized by air to CuCl2.
The chlorination of acetylene is usually electrophilic in
nature (27) but may also occur by a free-radical mechanism
in cases where free-radical initiators, UV-radiation, or high
temperatures are present (28). CuCl2 and CuCl may be
involved in both mechanisms. They catalyze the formation
of Cl2 in the Deacon reaction. In a radical mechanism, Cl2
is then able to chlorinate acetylene at temperatures above
450 °C (17, 18). In addition, CuCl2 and CuCl react as Lewis
acids, which support a heterolytic Cl2 bond cleavage. In our
case, chlorination occurred at temperatures below 150 °C,
temperatures too low for the thermal dissociation of Cl2 and
the initiation of free radical reactions. Assuming that free-
radical initiators and UV-radiation did not play a role, free-
radical reactions are unlikely under our reaction conditions.
A direct transfer of chlorine in a ligand transfer oxidation
mechanism, like that proposed by Nonhebel for aromatic
compounds (ArH), appears more plausible (19):
FIGURE 7. Metallacyclization reaction of acetylene and formation
of cyclooligomers.
C2HCl3, C2Cl4, and C4Cl6, temperatures in excess of 700 °C
were necessary for C2Cl2 condensation reactions to occur
(11-13).
Chlorinated condensed reaction products could be formed
in a competitive way at temperatures below gas-phase
reaction temperatures. Dimerization of acetylene or sub-
stituted acetylenes are catalyzed by CuCl in the Glaser or the
respective Cadiot-Chodikiewicz reaction. According to
Clifford and Waters (29), the mechanism starts with the loss
of a proton:
C2H2 base8 C2H- + H+
Subsequently, the chloride in CuCl reacts with the acetylide
as the nucleophile by a SN2 reaction:
ArH + CuCl2 f ArHCl• + CuCl
ArHCl• + CuCl2 f ArCl + CuCl + HCl
C2H- + CuCl f Cu(C2H) + Cl-
In the following step, the acetylide is oxidized by CuCl2 to
a C2H radical:
CuCl2 is then regenerated from CuCl with oxygen and HCl:
CuCl + 2HCl + 1/ 2O2 f CuCl2 + H2O
Cu(C2H) + CuCl2 f C2H• + 2CuCl
Tetrachloroethane was formed as the only saturated C-2
reaction product of Cl-addition to acetylene. Even higher
yields were detected for the C-2 compounds trichloroethylene
and tetrachloroethylene. This indicates that besides Cl-
addition to acetylene HCl-elimination also occurs at the
temperatures investigated. However, no chlorinated acety-
lenes were detected. This might be due to their high
reactivity. Dichloroacetylene is known to condense, when
dissolved in ether at temperatures below 130 °C, to a variety
of chlorinated organic compounds including hexachlo-
robenzene as the main reaction product (9, 10). Hexachlo-
robenzene was detected as the main C-6 reaction product
in our experiments. Other reaction products formed in our
experiments including perchlorinated vinylacetylene, highly
chlorinated butadienes, pentachlorobenzene, and com-
pounds with the formula C6Cl8 and C8Cl9H may be the result
of a condensation of mono- or dichloroacetylene. Because
of the high reactivity of chlorinated acetylenes, catalytic
surfaces are not required for their low-temperature con-
densation in solutions of chlorinated acetylene. However,
in the gas-phase kinetic modeling of C2Cl2 formation from
The acetylene dimer is then rapidly formed from two C2H
radicals:
2C2H• f HC2-C2H
The Glaser and the Cadiot-Chodikiewicz reactions were
elucidated for acetylene coupling in dissolved liquid phases.
Hence, a base-catalyzed loss of a proton as the initiating
step is conceivable. In our case of a gas-surface reaction
heterolytic bond cleavages are very unlikely. However, the
mechanism proposed by Clifford and Waters can be extended
to a surface-catalyzed mechanism, in which only single-
electron processes proceed. Solid-state CuCl is known to
react with gaseous acetylene or substituted acetylenes to
give complexes of the following composition (30):
nCuCl + C2H2 f [(CuCl)n(C2H2)]
Copper forms relative weak acetylene complexes with a
copper/ acetylene ratio below 1. These complexes contain
9
2 7 4 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 18, 1998

FIGURE 8. Global acetylene chlorination/condensation mechanism.
acetylene as well in σ-metal-carbon bonds, in which copper
is bonded to an acetylide, as in π-metal-carbon bonds (31)
(Figure 6).
in a surface-catalyzed reaction. It was shown by Stahl et al.
that reactive dichloroacetylene is stabilized in metal com-
plexes (34). These metal-dichloroacetylene complexes react
with additional dichloroacetylene to form chlorinated met-
allacyclopentadienes (35, 36), which can be intermediates in
dichloroacetylene condensation reactions. According to the
Glaser and Cadiot-Chodikiewicz reaction, Cu+ complexes
are possible catalysts (16). Chlorinated metallacyclopenta-
diene can form chlorinated butadienes in oxidative chlori-
nation reaction with CuCl2 or react with dichloroacetylene
in an insertion or Diels-Alder type reaction to form
chlorinated benzenes (35, 36).
These relatively weak complexes thermally decompose
and may lead to release of the acetylene or, most frequently
in reaction with acetylene, to formation of oligomers and
cyclooligomers. The reaction starts with the formation of a
metal ([M])-acetylene π-complex. Acetylene may react with
this complex to form metallacyclic compounds. These
complexes are able to react with additional acetylene to give
metallacycloheptatrienyl complexes or release benzene
(Figure 7) (32).
Metallacyclization reactions occur most readily in com-
plexes in which metals possess d8-d10 electron configurations
(e.g., Cu+ in d10 configuration). The formation of cyclooc-
tatetraene and other cyclic hydrocarbons is well known as
one of the Reppe-synthesis reactions with Ni(0) complexes
as catalysts (15).
This type of gas-surface reaction may occur in our
experiments since observed products are analogous to those
found in acetylene condensation reactions (4, 5). However,
the following experimental observations indicate that acety-
lene is chlorinated prior to surface-catalyzed condensation:
(1) no acetylene polymerization products were found with
either catalyst investigated, including CuO/ borosilicate; (2)
condensed reaction products were only found with a chlorine
source present; (3) yields of reaction products increased with
additional chlorine; (4) reaction products were predominantly
perchlorinated.
The compounds C6Cl8 and C8Cl9H can be formed from
the respective chlorinated metallacycloheptatrienes or met-
allacyclononatetraenes by similar oxidative chlorination
reaction with CuCl2 (for a detailed discussion of these
pathways see ref 37).
The toxic compounds formed in our experiments are
usually unwanted combustion byproducts in applied incin-
eration techniques such as municipal waste incineration.
Our mechanisms of chlorination and condensation reaction
therefore provide a tool toward minimizing these com-
pounds in the effluents of incinerators. Since copper in
incineration processes is mostly present as CuO, in addition
to the reduction of the copper feed, temperatures in the
cooling zone should be quenched immediately below 300 °C
to prevent a CuCl2 formation, which according to our model
is responsible for the formation of these toxic combustion
byproducts.
A third type of reaction, which is known in solution
chemistry and may be involved in the acetylene condensation
reaction, is the [4+2] cycloaddition or Diels-Alder reaction.
Electron-rich 4π-systems, the diene, and electron-poor 2π-
systems, the dienophile, are required for this reaction to take
part readily. However, recent results indicate that this
reaction can be catalyzed by Lewis acids such as Li+ and
transition metal complexes such as cp(CO)2Fe(THF)+ (33).
The Lewis acids CuCl or CuCl2 are then able to catalyze the
[4+2] cycloaddition of, e.g., hexachlorobutadiene, one major
reaction product with electron-poor dienophile, or dichlo-
roacetylene to hexachlorobenzene, another major reaction
product.
Consistent with our experimental results and in analogy
with mechanisms found in solution chemistry, we propose
the following global reaction mechanism for the gas-surface
chlorination/ condensation reaction of acetylene on copper
species (Figure 8): Acetylene is chlorinated by CuCl2 in a
ligand transfer oxidation mechanism to mono- and dichlo-
roacetylene and other less unsaturated chlorinated C-2
compounds. Chlorinated acetylenes can then form oligomers
Acknowledgments
This work was partially supported by the National Science
Foundation, Grant No. CTS-9525783. The authors thank
Harald Bartl for his help in the laboratory and additional
computer work and Rich Striebich for the quantification of
the volatile reaction products.
Literature Cited
(1) Froese, K. L.; Hutzinger, O. Chemosphere 1994, 28, 1977-1987.
(2) Froese, K. L.; Hutzinger, O. Environ. Sci. Technol. 1996, 30, 998-
1008.
(3) Froese, K. L.; Hutzinger, O. Environ. Sci. Technol. 1996, 30, 1009-
1013.
(4) Kiefer, J. H.; Von Drasek, W. A. Int. J. Chem. Kinet. 1990, 22,
747-786.
(5) Kiefer, J. H.; Sidhu, S. S.; Kern, R. D.; Xie, K.; Chen, H.; Harding,
L. B. Combust. Sci. Technol. 1992, 82, 101-130.
(6) Duran, R. P.; Amorebieta, V. T.; Colussi, A. J. J. Am. Chem. Soc.
1987, 109, 3154-3155.
(7) Breitkopf, V.; Hopf, H.; Kla¨rner, F.-G.; Witulski, B.; Zimmy, B.
Liebigs Ann. 1995, 613-617.
9
VOL. 32, NO. 18, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 7 4 7

(8) Smirnov, K. M.; Tomilov, A. P.; Shchekotikhin, A. I. Russ. Chem.
Rev. 1967, 36, 326-338.
(9) Ott, E.; Dittus, G. Chem. Ber. 1943, 76, 80-84.
(10) Riemschneider, R.; Brendel, K. Liebigs Ann. 1960, 640, 1-15.
(11) Taylor, P. H.; Tirey, D. A.; Rubey, W. A.; Dellinger, B.; Combust.
Sci. Technol. 1994, 101, 75-102.
(12) Taylor, P. H.; Tirey, D. A.; Dellinger, B. Combust. Flame 1996,
104, 260-271.
(13) Taylor, P. H.; Tirey, D. A.; Dellinger, B. Combust. Flame 1996,
106, 1-10.
(14) Parshall, G. W. Wiley: New York, 1980, p 223.
(15) Reppe, W.; v. Kutepow, N.; Magin, A.; Angew. Chem., Int. Ed.
Engl. 1969, 8, 727-766.
(16) Chodikiewicz, W. Ann. Chim. (Paris) 1957, 13, 819-869.
(17) Chloberag; Chlorbetrieb Reinfelden; DBP. 868294, 1950.
(18) Fruhwirth, O.; Walla, H.; Donau Chemie AG.; Erf.; DBP. 734024,
1940.
(19) Nonhebel, D. C. J. Chem. Soc. Special Publication 1970, 24, 409.
(20) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29, 1425-1435.
(21) Rubey, W. A.; Grant, R. A. Rev. Sci. Instrum. 1988, 59, 265.
(22) Bond, G. C.; Namijo, S. N.; Wakeman, J. S. J. Mol. Catal. 1991,
64, 305.
(26) Olah, G. A. Friedel-Crafts and related Reactions; Interscience:
New York, 1965; Vol. 4, p 127-183.
(27) de la Mare, P. B. D. Electrophilic Additions to Unsaturated
Systems, 2nd ed.; Elsevier: New York, 1982.
(28) March, J. Advanced Organic Chemistry. Reactions, Mechanisms
and Structures, 3rd ed.; Wiley: New York, 1985; p 725.
(29) Clifford, A. A.; Waters, W. A. J. Chem. Soc. 1963, 3056-3062.
(30) Pruchnik, F. P. Organometallic Chemistry of the Transition
Elements; Plenum Press: New York, 1990; p 389-412.
(31) Lang, H.; Ko¨ hler, K.; Blau, S. Coordination Chem. Rev. 1995,
143, 113-168.
(32) Peter, K.; Vollhardt, C. Acc. Chem. Res. 1977, 10, 1-8.
(33) Bonnesen, P. V.; Puckett, C. L.; Honeycuck, R. V.; Hersh, W. H.
J. Am. Chem. Soc. 1989, 111, 6070-6081.
(34) Stahl, K.; Weller, F.; Dehnicke, K.; Z. Anorg. All. Chem. 1986,
533, 73-82.
(35) Su¨ nkel, K. J. Organomet. Chem. 1990, 391, 247-257.
(36) Su¨ nkel, K. Chem. Ber. 1991, 124, 2449-2451.
(37) Taylor, P. H.; Sidhu, S. S.; Rubey, W. A.; Dellinger, B.; Wehrmeier,
A.; Lenoir, D.; Schramm, K.-W. 27th Symposium on Combustion;
The Combustion Institute: Pittsburgh, PA, 1998, in press.
(23) Wehrmeier, A.; Lenoir, D.; Schramm, K.-W.; Taylor, P. H.; Sidhu,
S. Unpublished data.
Received for review January 21, 1998. Revised manuscript
received June 4, 1998. Accepted June 23, 1998.
(24) Hisham, M. W. M.; Benson, S. W. J. Phys. Chem. 1995, 99, 6194-
6198.
(25) Hardie, D. W. F. Ind. Chemist 1951, 27, 502.
ES980050S
9
2 7 4 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 18, 1998