Catalysis Letters 68 (2000) 153–156 153
NO
x
-catalyzed deep oxidation of toxic chloroorganics by dioxygen:
possible application in environmental remediation
Christine N. Elia, Harvey Fries IV and Ayusman Sen
∗
Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
E-mail: asen@chem.psu.edu
Received 17 April 2000; accepted 26 June 2000
At 400–650
◦
C, NO
x
catalyzes the deep oxidation by dioxygen of a wide range of toxic chloroorganics, including some of the most
ubiquitous environmental contaminants, to carbon oxides, water, and inorganic chloride. The catalyst, NO
x
, is not consumed in the
reactions. Mechanistic studies suggest that the reactions are initiated by an atom abstraction from the substrate by NO
2
.
Keywords: chloroorganics, nitrogen oxides, oxidations, oxygen
1. Introduction
Chlorinated organics are an important class of toxic pol-
lutants that have become widely dispersed in the environ-
ment due to their use as solvents, degreasing agents, and
in cleaning and fabrication applications [1]. For example,
chloroorganics, such as tri- and tetrachloroethylene, are the
principal constituents of DNAPL (denser than water non-
aqueous phase liquids) that are present in 60% of the “su-
perfund” sites [2]. Hence, the design of a suitable reme-
diation procedure is of high societal importance and poses
a significant scientific challenge. One obvious solution to
the problem is bioremediation [3]. However, it is a very
slow process and many of the chloroorganics are xenobi-
otic in character. In addition, when enzymes with low sub-
strate specificity encounter such molecules, products that
are xenobiotic often result (e.g., formation of the carcino-
gen vinyl chloride from polychlorinated ethylene). Among
other remediation procedures, the most common is catalytic
dechlorination to the parent hydrocarbon [4]; however, this
is only a partial solution since the product, while less toxic,
needs to be removed in a subsequent step. Catalytic oxida-
tion procedures, such as TiO
2
-catalyzed photooxidations[5]
and the “Fenton” systems [6] involving H
2
O
2
and a soluble
transition metal catalyst are also not optimal in many situ-
ations. For example, photons are relatively expensive and
the photooxidationsystemscannot be employed wherethere
is a dearth of sunlight. The Fenton system suffers from the
requirement of separately producing and transporting H
2
O
2
,
an unstable and potentially explosive chemical. In general
all metal-catalyzed procedures face the problem of catalyst
deactivation. Herein, we report a simple NO
x
-catalyzed
process for the deep oxidation of toxic chloroorganics by
dioxygen. The system is capable of oxidizing a wide range
∗
To whom correspondence should be addressed.
of toxic chlorinated organics, including some of the most
ubiquitous environmental contaminants.
2. Experimental
The procedure uses a flow reactor system. A mixture of
NO, O
2
,andN
2
(typical composition: 4.3, 8.7, and 87.0%,
respectively) was bubbled through the desired substrate into
a quartz tube (length 36.2 cm, diameter 1.9 cm; gas flow
rate 500 ml/min) heated at temperatures ranging from 400
to 650
◦
C. The exiting mixture was passed through D
2
O
and/or CDCl
3
bubblers and finally analyzed by gas chro-
matography. The products trapped in the bubblers were
analyzed by
1
Hand
13
C NMR spectroscopy. Gravimetric
silver nitrate analysis was performed to quantify the chlo-
ride ions liberated and collected in the D
2
O bubblers.
3. Results and discussion
The oxidation procedure involves passing a mixture con-
sisting of the substrate, NO, O
2
,andN
2
through a heated
flow reactor system. For all the substrates examined, oxida-
tion did not occur at or below 400
◦
C. The results of one-
pass oxidation of selected chlorinated organics at 575
◦
C
are shown in table 1. The chlorinated substrates investi-
gated were generally oxidized in moderate to good yields.
As exemplified by 1,2-dichloropropane, more oxidation
was observed with higher hydrocarbon character of the
substrate. However, even with highly chlorinated species,
such as tri- and tetrachloroethylene, a reasonable degree
of oxidation was achieved. Note that in every case the
substrate concentration (approximately 4000–200000 ppm)
was many times that typically encountered in the environ-
mental remediation situations (e.g., in “pump and treat”
procedures) [1]. In order to ascertain whether the pro-
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