Supercritical water oxidation of 2-chlorophenol catalyzed by Cu2+ cations and copper oxide clusters

Article abstract of DOI:10.1021/es001062s

Experimentally, the selectivity to decomposition (S/D) (S/D = (amount of 2-chlorophenol (2CP) oxidized to CO₂ and H₂O)/(disappearance of 2CP)) ratio for oxidation of 2CP with Cu²⁺ cations in supercritical water was greater than that without any catalysts by 60%. The enhancement was due to a fast precipitation of copper chloride in the supercritical water oxidation (SCWO) process. Formation of toxic byproducts was significantly reduced. Because of a confined environment in the channels of ZSM-5, formation of undesired heavy chlorinated phenols and PAH (polycyclic aromatic hydrocarbon) byproducts in the SCWO of 2CP (catalyzed by CuO/ZSM-5) was also highly suppressed. Only trace light-PAHs with a molecule size less than 6 A were found in the SCWO of 2CP. Carcinogenic PAHs (heavy-PAHs) were not observed. The extended X-ray absorption fine structural (EXAFS) spectra of the catalyst showed that the copper oxides in the channels of ZSM-5 may form Cu₃O₂ clusters with Cu-Cu and Cu-O bond distances of 2.79 and 1.91 A, respectively. The Cu₃O₂ clusters were oxidized to Cu₃O₄ by H₂O₂ in the supercritical water. Since the diffusion coefficients of copper oxide clusters (Cu₃O₄) in the channels of ZSM-5 was greater than that of 2CP by at least 3 orders, it is possible that these clusters are relatively mobile in ZSM-5 in the SCWO process. Experimentally, the selectivity to decomposition (S/D) (S/D = (amount of 2-chlorophenol (2CP) oxidized to CO₂ and H₂O)/(disappearance of 2CP)) ratio for oxidation of 2CP with Cu²⁺ cations in supercritical water was greater than that without any catalysts by 60%. The enhancement was due to a fast precipitation of copper chloride in the supercritical water oxidation (SCWO) process. Formation of toxic byproducts was significantly reduced. Because of a confined environment in the channels of ZSM-5, formation of undesired heavy chlorinated phenols and PAH (polycyclic aromatic hydrocarbon) byproducts in the SCWO of 2CP (catalyzed by CuO/ZSM-5) was also highly suppressed. Only trace light-PAHs with a molecule size less than 6 angstroms were found in the SCWO of 2CP. Carcinogenic PAHs (heavy-PAHs) were not observed. The extended X-ray absorption fine structural (EXAFS) spectra of the catalyst showed that the copper oxides in the channels of ZSM-5 may form Cu₃O₂ clusters with Cu-Cu and Cu-O bond distances of 2.79 and 1.91 angstroms, respectively. The Cu₃O₂ clusters were oxidized to Cu₃O₄ by H₂O₂ in the supercritical water. Since the diffusion coefficients of copper oxide clusters (Cu₃O₄) in the channels of ZSM-5 was greater than that of 2CP by at least 3 orders, it is possible that these clusters are relatively mobile in ZSM-5 in the SCWO process.

Full text of DOI:10.1021/es001062s

Environ. Sci. Technol. 2000, 34, 4849-4854
pyrolysis or incomplete combustion and carbonization of
Supercritical Water Oxidation of
2-Chlorophenol Catalyzed by Cu2+
Cations and Copper Oxide Clusters
organic materials (20). Chlorine atoms may accelerate the
abstraction of aromatic H from stable PAHs molecules that
are activated for further mass growth of PAHs to form
eventually soot (21). Many PAHs are toxic and considered as
possible or probable carcinogens and/ or mutagens (20). From
an environmental viewpoint, complete destruction and
removal of hazardous compounds with a minimal release of
toxic byproducts are essential in the most waste disposal
processes (20, 21).
K U E N - S O N G L I N A N D H . P A U L W A N G *
Department of Environmental Engineering, National Cheng
Kung University, Tainan City, Taiwan 70101, R.O.C.
Catalytic SCWO reactions are of interest to reduce the
high temperature and pressure requirements of conventional
SCWO processes. Because of the unique pore systems, zeolites
have excellent shape selectivities in catalytic reactions (22-
25). Since the zeolite ZSM-5 has a three-dimensional channel
structure with pore sizes of 5.1-5.8 Å, the formation of high
molecular weight hydrocarbons may be very limited (22-
24). 2-Chlorophenol with the Lennard-Jones minimum
kinetic diameter of 6.84 Å in size is tight-fit in the channels
of ZSM-5. In a separate experiment, by FTIR spectroscopy,
we have found that the channels of ZSM-5 were distorted by
adsorbed benzene molecules that possessed a high symmetry
of D6h (26).
Experimentally, the selectivity to decomposition (S/D) (S/D
)(amount of 2-chlorophenol (2CP) oxidized to CO and H O)/
2
2
(disappearance of 2CP)) ratio for oxidation of 2CP with Cu2+
cations in supercritical water was greater than that
without any catalysts by 60%. The enhancement was due
to a fast precipitation of copper chloride in the supercritical
water oxidation (SCWO) process. Formation of toxic
byproducts was significantly reduced. Because of a
confined environment in the channels of ZSM-5, formation
of undesired heavy chlorinated phenols and PAH
Reduction of undesired byproducts in the waste thermal
treatment process effected by the shape selectivity of zeolite
catalysts has not been extensively studied (22-25). Therefore,
the main objective of the present work was to investigate the
oxidation of 2CP in supercritical water and reduction of
byproducts (higher chlorinated phenols and polycyclic
aromatic hydrocarbons) in the SCWO of 2CP effected by Cu2+
cations as well as CuO/ ZSM-5. The nature of the active
oxidative species (copper oxides) in the channels of ZSM-5
was also determined by X-ray absorption, X-ray photoelec-
tron, electron paramagnetic resonance, and solid-state
nuclear magnetic resonance spectroscopies.
(polycyclic aromatic hydrocarbon) byproducts in the
SCWO of 2CP (catalyzed by CuO/ZSM-5) was also highly
suppressed. Only trace light-PAHs with a molecule size less
than 6 Å were found in the SCWO of 2CP. Carcinogenic
PAHs (heavy-PAHs) were not observed. The extended X-ray
absorption fine structural (EXAFS) spectra of the catalyst
showed that the copper oxides in the channels of ZSM-5 may
form Cu3O clusters with Cu-Cu and Cu-O bond distances
2
of 2.79 and 1.91 Å, respectively. The Cu3O clusters
2
were oxidized to Cu3O by H O in the supercritical water.
4
2 2
Since the diffusion coefficients of copper oxide clusters
(Cu3O ) in the channels of ZSM-5 was greater than that of
2CP by at least 3 orders, it is possible that these clusters
are relatively mobile in ZSM-5 in the SCWO process.
4
Experimental Section
The SCWO experiments of 2CP (Merck, purity > 99.5%) were
conducted in a high-pressure quartz-lined batch and an
isothermal, isobaric fixed-bed plug flow reactor (stainless
steel 316 L type). The system pressure was controlled by a
back-pressure regulator (Tescom, Pmax ) 41.34 MPa) and a
pressure regulator (Tescom, Pin ) 24.11 MPa; Pout ) 0.71
MPa). A safety rupture disk rated at 40.53 MPa was installed.
Oxidation of 2CP in supercritical water was conducted at
673-773 K with the reaction residence times of 0.5-5 min.
Concentrations of 2CP in the SCWO experiments were 189-
1500 mg L-1. Hydrogen peroxide (Merck, 30 wt %) was used
as the oxidant (O/ C ratio ) 0.95-2.0) in the SCWO experi-
ments. High purity copper acetate (Cu(CH3COO)2) (Merck,
purity > 99%) was also used as the cation source (equivalent
mole ratios ([Cu]/ [Cl]) were 1.2-1.3) for the abstraction of
Cl of 2CP in the SCWO process. The counteranion (CH3COO-)
was decomposed in the SCWO process (7).
Introduction
At supercritical conditions (T > 647.3 K, P > 22.1 MPa), water
has a high solubility for both organics and oxygen (1, 2).
Properties of supercritical water such as the complete
miscibility in all proportions with oxygen, negligible surface
tension, high diffusivity, low viscosity, and low solubility of
inorganic salts are very unique especially in disposal of toxic
compounds (1-5). Most hazardous organic compounds can
be completely oxidized to CO2 and H2O in a very short
residence time in the supercritical water (6-11). The desired
destruction and removal efficiency (DRE) (>99.99%) may be
achieved in seconds or minutes in the SCWO process (12-
17).
Zeolite ZSM-5 with the Si/ Al ratio of 65 was prepared
with fumed silica and organic template agent (tetrapropyl-
ammonium bromide, TPABr) hydrothermally. TPABr was a
structure-directing agent. The zeolite syntheses were carried
out at 383 K under the autogenous pressure of 15 atm for
about 48 h. The CuO/ ZSM-5 catalyst was prepared via ion
exchange of zeolite ZSM-5 in a 0.01 N Cu(CH3COO)2 aqueous
solution for 24 h. The CuO/ ZSM-5 was dried at 378 K for 16
h and calcined at 823 K for at least 6 h. About 3.1% Cu was
loaded on ZSM-5. The CuO/ ZSM-5 catalyst (0.15 g) was used
in the SCWO of 2CP. Structures of samples were measured
by X-ray powder diffraction (XRD) scanned from 5 to 80°
2-Chlorophenol, widely used in paper, pulp, pesticide,
and herbicide industries, is one of the EPA (Environmental
Protection Agency) priority pollutants (18, 19). 2-Chloro-
phenol is very toxic and poorly biodegradable. Oxidation of
2CP in supercritical water is of practical interest since a
wastewater stream containing 2CP over 200 ppm may not
be treated effectively by direct biological methods (19).
Polycyclic aromatic hydrocarbons with two or more fused
benzene rings are frequently found in the processes of
* Corresponding author phone: 011-8866-276-3608; fax: 011-8866-
275-2790; e-mail: wanghp@mail.ncku.edu.tw.
9
10.1021/es001062s CCC: $19.00
Published on Web 10/13/2000
2000 Am erican Chem ical Society
VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 8 4 9

(2θ) with a scan rate of 4° (2θ) min-1 with monochromatic
CuKa radiation (RIGAKU Model D/ MAX III-V). The morphol-
ogy of the salt precipitate was determined by scanning
electron microscopy/ energy dispersive spectroscopy (SEM/
EDX, JEOL Model JSMr840). The average content of ZSM-5
was evaluated by atomic adsorption spectroscopy (AAS,
HITACHI Model Z-8100) and induced couple plasma/ mass
spectroscopy (ICP/ MS, ELAN model 5000). The Si/ Al ratio of
ZSM-5 was determined by X-ray fluorescence (XRF, RIGAKU
Model 3063M). The pore volume (0.19 mL g-1) and surface
area (550 m2 g-1) of ZSM-5 were measured by nitrogen
adsorption (Micromeritics ASAP 2010 Instrument) and
mercury penetration (Micromeritics Autopore II 9220),
respectively.
TABLE 1. Trace PAH and Chlorinated Phenols Byproducts
Formed in the SCWO of 2CP with 0.02 N Cu2+ and CuO/ZSM-5
at 673 K for 1 Min in a Fixed-Bed Plug Flow Reactor (O/C
Ratio ) 1.1)
(µg (g 2CP)-1
)
minimum no
CuO/ Cu2++CuO/
sizea (Å) catal. Cu2+ ZSM-5
ZSM-5
S/Db
2CP conversion
phenol
2,4-DCP
2,4.5-TCP
2,4,6-TCP
2,3,4,6-TeCP
PCP
naphthalene
fluorene
phenanthrene
anthracene
fluoranthene
pyrene
benz[a]anthracene
chrysene
benzo[b]fluoranthene
benzo[k]fluoranthene
benzo[a]pyrene
indeno[1,2,3-cd]pyrene
dibenz[a,h]anthracene
benzo[ghi]perylene
0.43 0.97 0.86
0.98 0.99 0.99
0.99+
0.99+
7.5
5.85
6.84
7.75
7.75
7.75
7.75
5.85
5.85
9.10
5.85
9.45
9.10
9.10
9.10
9.10
9.10
9.10
9.45
9.45
9.45
1370 164.7 165.6
2590 21.9 20.5
5735 10.3 0.2
7056 11.2 N.D.c
1.7
N.D.c
N.D.c
N.D.c
N.D.c
1.0
2101 1.4
N.D.c
159 N.D.c N.D.c
Carbon dioxide produced from the SCWO of 2CP passed
through a water cooler and was measured by a totalizer and
analyzed by online FTIR spectroscopy and gas chromatog-
raphy (Perkin-Elmer Auto System) with a thermal conductive
detector (GC/ TCD) and a Supelco 60/ 80 Carboxen 1000 S.S.
column. Overall carbon balance for the SCWO of 2CP was
> 96%. Infrared spectra were recorded on a Digilab FTIR
spectrometer (FTS-40) with fully computerized data storage
and data handling capability. For all spectra reported, a 64-
scan data accumulation was conducted at a resolution of 4
25.6 4.4
6.4
1.0
3.3
4.0
3.2
1.2
0.8
1.8
0.3
0.2
0.3
0.1
N.D.c 0.1
N.D.c
0.1
0.1
1.6
N.D.c N.D.c
N.D.c N.D.c
N.D.c 0.1
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c
N.D.c N.D.c
0.1
0.1
N.D.c N.D.c N.D.c
0.5
N.D.c N.D.c
cm-1
.
0.16 N.D.c N.D.c
N.D.c N.D.c N.D.c
N.D.c N.D.c N.D.c
Trace byproducts (extracted with a dichloromethane
(Merck, purity > 99%) solvent) formed in the SCWO of 2CP
was determined quantitatively by GC (Hewlett-Packard
5890A) with a mass selective detector (GC/ MS, HP 5972) and
an automatic sampler (HP-7673A). A HP Ultra 2 capillary
column (50 m × 0.32 mm × 0.17 µm) was heated program-
mably (323 K to 373 K at 20 K min-1; 373 K to 563 K at 3.5
K min-1; and hold at 563 K for 40 min) to 563 K to obtain a
resolvable separation of PAH species. The injection tem-
perature was 583 K. One to two microliters of sample was
injected into column. Masses of primary and secondary ions
of PAHs were determined using the scan-mode for PAH
standards (Mix 610-M (Supelco, purity > 99%) and PNA-
550JM (Chem Service, purity > 99%)) and samples. Analyses
of trace byproducts of the SCWO of 2CP were also conducted
by HPLC (spectra system, SP) with a 3-D UV detector (model
UV-3000). PAHs were separated by a Spherisorb S5 PAH 5
µm column (150 mm × 4.6 mm) with a mixed acetonitrile/
water mobile phase. Chlorinated phenols were analyzed by
an Envirosep-PP column (125 mm × 3.2 mm) with a mixed
methanol/ water (both with 1% acetic acid) mobile phase. A
variable wavelength program was used in the system software
(model PC-1000) to optimize detector sensitivity and selec-
tivity.
29Si solid-state nuclear magnetic resonance (SSNMR)
spectra of the catalysts were recorded at 9.4 T and 79.0 MHz
on a Bruker Avance DSX 400 solid-state NMR spectrometer.
Tetramethylsilane was used as chemical shift references
(external standard) for 29Si. Bruker double-bearing channel
4 or 7 mm (O.D.) MAS probes and zirconia rotors were used
and spun at a frequency of 5 kHz at the magic angle. Dry air
was used as the driving and bearing gas in the SSNMR
experiments. In all cases a background signal was subtracted
from the spectra. The recycle delay was 10 s with a pulse
width of 3.0 µs (about 60° (flip angle)) for 29Si.
a
b
The Lennard-J ones m inim um kinetic diam eter (22-24). S/D )
c
(am ount of 2CP oxidized to CO2 and H2O)/(disappearance of 2CP). N.D.
denotes “not detectable”.
The electron paramagnetic resonance (EPR) spectra of
the catalysts were recorded on a Bruker Model EMX-10 EPR
spectrometer with a maximum microwave power of 200 mW
at 77-463 K. About 30-60 mg of the catalyst samples in the
quartz tube (5 mm O.D.) were measured. The magnetic field
was modulated at 100 kHz. Diphenylpicrylhydrazyl (DPPH)
was used for determination of the absolute g-factor (g )
2.0037). All EPR spectra were normalized to the same
frequency, sample weight, and spectrometer gain.
The extended X-ray absorption fine structural (EXAFS)
spectra were collected on a bending-magnet double-crystal
monochromator (DCM) X-ray beamline in the Synchrotron
Radiation Research Center (SRRC) of Taiwan. The electron
storage ring was operated with an energy of 1.5 GeV and a
current of 100-200 mA. A Si(111) DCM was used for providing
highly monochromatized photon beams with energies of 1
to 9 keV and resolving power (E/ ∆E) of up to 7000. Data were
collected in fluorescence mode with a Lytle detector (27) for
the Cu K-edge (8978.9 eV) experiments. The absorption
spectra were collected in ion chambers filled with helium
gas. The photon energy was calibrated by characteristic
preedge peaks in the absorption spectrum of a copper foil.
The raw absorption data in the region of 50 to 200 eV below
the edge position were fit to a straight line using the least-
squares algorithms. The fitted preedge background curves
were extrapolated throughout all data range and subtracted
and normalized to minimize the effect of sample thickness.
The near-edge spectra were ranged between the threshold
and the point at which the EXAFS began (27-32). The X-ray
absorption near edge structural (XANES) spectra extend to
an energy of 50 eV above the edge. The k2-weighted EXAFS
spectra were Fourier transformed to R space over the range
between 2.5 and 10.8 Å-1. The EXAFS data were analyzed
using the UWXAFS 3.0 and FEFF 8.0 programs (29-32).
X-ray photoelectron spectra (XPS) of the CuO/ ZSM-5
catalysts were measured on a Fison ESCA 210 (VG Scientific)
spectrometer with a MgKa X-ray (1253.6 eV) excitation source.
The binding energy (Eb) of the Cu 2P3/ 2 line was calibrated
with that of O 1s (530.8 and 532.5 eV) and C 1s (284.4 eV)
lines. The Eb (copper core level binding energy) value of
electrons emitted by L3M4.5M4.5 transition was used for copper
species. The standard deviation of the binding energy (Eb)
measurement was (0.3 eV typically.
Results and Discussion
Cu(II) Cations Catalysis. Table 1 shows that oxidation of
2CP in supercritical water was enhanced (S/ D ) 0.97) by
9
4 8 5 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 22, 2000

FIGURE 2. XPS spectra of (a) O 1s and (b) Cu 2P3/2 of the used
CuO/ZSM-5 catalyst for the SCWO of 2CP at 673 K for 1 min (O/C
-1
ratio ) 1.1; concentration of 2CP ) 1500 mg L ).
SCHEME 1
FIGURE 1. (a) X-ray diffraction pattern and (b) scanning electron
microscopy/energy dispersive spectrum of CuCl2 salt precipitated
in the SCWO of 2CP in the presence of Cu2+ (fixed-bed plug flow
reactor). An asterisk (*) denotes the characteristic peaks of CuCl2.
Cu2+ cations (if compared with that without any cations).
The enhancement was about 60%. Reduction of highly
chlorinated phenol and PAH byproducts formed in the SCWO
of 2CP with Cu2+ cations is also shown in Table 1. The
reduction of these toxic byproducts may be due to the fact
that the electron-withdrawing effect of cations was involved
in which an intermediate species ((OH)PhClδ-- - -Cuδ+) might
exist. In addition, due to the extremely low solubility of salts
in the supercritical water, abstraction of Cl from 2CP and
formation of the copper chloride precipitate may be involved
in the early stage of the SCWO process. Figure 1 shows that
by using XRD and SEM/ EDX spectroscopies, CuCl2 was the
main species in the precipitate in the SCWO of 2CP at 673
K for 1 min. Over 85% of Cu2+ cations was formed as CuCl2
in the SCWO of 2CP. Solubility of copper chloride has been
reported to decrease from its ambient solubility (298 K, 0.10
MPa) of about 10% to 3-120 ppm in supercritical water (773
K, 25.33 MPa), a decrease of 104-fold (3-5).
Byproducts Shape Selective Catalysis. The unique
byproduct selectivity for oxidation of 2CP in supercritical
water effected by the CuO/ ZSM-5 catalyst is also shown in
Table 1. In the tight-fit channels of ZSM-5, the S/ D ratio for
the SCWO of 2CP at 673 K was 0.86, that may be due to the
fact that an extremely high collision frequency of 2CP with
the active copper species may occur in the channels of ZSM-
5. In the presence of both Cu2+ cations and the zeolite catalyst,
the S/ D ratio for the SCWO of 2CP was notably enhanced
(>0.99). Without the zeolite catalyst, 2,4-dichlorophenol (2,4-
DCP) and 2,4,5-trichlorophenol (2,4,5-TCP) were the main
chlorinated phenols formed in the SCWO of 2CP at 673 K.
Due to the restricted environment in the channels of ZSM-5,
the Cl-reinsertion may occur most likely at the sites #4 and
#5 of 2CP (Scheme 1). Therefore, large molecules such as
2,3,4,6-tetrachlorophenol (2,3,4,6-TCP) and pentachlorophe-
nol (PCP) were not formed. Note that reactions of ring-
opening, in which products are ultimately oxidized to CO2
and H2O, are predominant in the overall reaction. The high-
chlorinated phenol byproducts formed in the SCWO of 2CP
were, in fact, extremely low (0.2 to 20.5 µg (g 2CP)-1).
Furthermore, due to the byproduct shape selectivity in
the channels of ZSM-5, the formation of high-molecular-
weight PAHs was also very limited (Table 1). Formation of
PAHs in the SCWO of 2CP in ZSM-5 was significantly
suppressed. Mainly low-molecular-weight PAHs (or light
PAHs) such as naphthalene (Nap), fluorene (Flu), and
anthracene (Ant) were formed in the SCWO of 2CP. Carci-
nogenic heavy PAHs including benzo[a]pyrene (BaP), benzo-
[b] fluoranthene (BbF), chrysene (CHR), dibenz[a,h]an-
thracene (DBA), and benzo[ghi]perylene (BghiP) were not
found in the SCWO of 2CP.
Nature of Copper Oxides in ZSM-5. To more thoroughly
examine the nature of the copper species in ZSM-5, XPS
spectra of the catalyst were measured and calculated (33,
34). The O 1s spectrum of the CuO/ ZSM-5 catalyst (Figure
2(a)) indicates the existence of at least two oxidation states
(Cu2O and CuO) in ZSM-5. The CuO-to-Cu2O ratio was 2.1
( 0.3. In addition, in Figure 2(b), three oxidation states such
as Cu, Cu(I) (Cu2O), and Cu(II) (CuO) are also observed in
the Cu 2P3/ 2 spectra. About 13% of metallic Cu was found in
the CuO/ ZSM-5 catalyst (Table 2). The Cu(II) species
9
VOL. 34, NO. 22, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 8 5 1

TABLE 2. Fractions of Copper Species (Cu 2P ) in the Used
TABLE 3. Fine Structural Parameters of CuO/ZSM-5 Used in the
SCWO of 2CP at 673 K for 1 Min with O/C Ratio of 1.1
3/2
CuO/ZSM-5 Catalysta
b
Cu
Cu2O
CuO
shell
CNa
R ((0.05 Å)
∆σ2 c (Å2)
bonding energy (eV)
fraction (%)
933.4
13.1
932.0
25.7
935.6
61.2
Cu3O Cluster
2
Cu-O
first
second
first
2.5
2.1
4.0
5.6
1.91
3.61
2.79
4.41
0.008
0.015
0.016
0.021
a
SCWO of 2CP at 673 K for 1 m in with O/C ratio of 1.1.
Cu-Cu
second
Cu3O Cluster
4
Cu-O
first
second
first
2.6
2.1
4.4
5.5
1.89
3.54
2.80
4.53
0.008
0.015
0.013
0.019
Cu-Cu
second
a
b
c
CN: coordination num ber. R: bond distance. σ: Debye-Waller
factor.
Generally, the EXAFS spectroscopy can provide the
information on the atomic arrangement of catalysts in terms
of bond distance, number and kind of near neighbors, thermal
and static disorder. An over 99% reliability of the EXAFS data
fitting for Cu species in zeolite was obtained (Figure 3(b)).
The Fourier transformation was performed on k2-weighted
EXAFS oscillations over the range between 2.5 and 10.8 Å-1
.
Standard deviation calculated from the averaged spectra was
also determined. In all EXAFS data analyzed, the Debye-
Waller factors (∆σ2) were less than 0.02 Å (Table 3).
Since interactions between Cu and Si or Al (second shell)
were not observed by EXAFS, it is possible that copper oxide
clusters may be formed in the channels of ZSM-5. The
structural parameters of the copper oxide clusters obtained
from the best fit to the EXAFS data are shown in Table 3. The
postulated structure of the copper oxide clusters involved in
the SCWO of 2CP in the channels of ZSM-5 was Cu3O2. The
Cu-O and Cu-Cu species with bond distances of 1.91 and
2.79 Å, respectively, were found. Coordination number (CN)
of the Cu-O species was 2.5. The Cu3O2 clusters were oxidized
by H2O2 to form Cu3O4 clusters with Cu-Cu and Cu-O bond
distances of 2.80 and 1.89 Å, respectively. Coordination
number of the Cu-O species was 2.6. Note that chlorine-
bonded CuO species in the channels of ZSM-5 was not
observed by EXAFS spectroscopy. Thus, one may eliminate
the possibility that copper species of the clusters was involved
in the abstraction of Cl from 2CP in the SCWO process.
In addition, 29Si solid-state NMR spectra of the CuO/ ZSM-5
catalysts used in the SCWO of 2CP are shown in Figure 4. The
feature at (-114) - (-103) ppm represents the Q4 sites where
all of the second-nearest neighbor atoms are Si. Instead of
interaction with ZSM-5, copper oxide species was mechani-
cally supported on zeolites. Interactions of the copper species
with the dSi-O-Al) sites on ZSM-5 seem to be insignificant.
Electron paramagnetic resonance spectroscopy has been
used to investigate the structural environment of paramag-
netic copper species in zeolites (35-41). Determining the
motion, location, and coordination of copper species in the
channels of zeolite catalysts is acute to understanding the
role of copper species in the SCWO of 2CP. The EPR spectra
of the CuO/ ZSM-5 catalyst measured at 77, 298, and 463 K
(Figure 5) indicate that the copper oxide species has a square-
plane structure in the channels of ZSM-5. The EPR spectrum
(Figure 5(c)) of the CuO/ ZSM-5 catalyst at 463 K is broad and
structureless. On the contrary, at 77 K, the spectrum (Figure
5(a)) exhibits a sharp signal and characteristic structure of
copper oxide complexes that the hyperfine coupling between
the 3d unpaired electrons and the copper (I ) 3/ 2) nuclear
spin is well resolved. Furthermore, the broad EPR spectrum
observed at 463 K in Figure 5(c) suggests that the copper
oxide species may be mobile in the channels of ZSM-5. At
77 K, the appearance of the parallel edges in the EPR spectrum
FIGURE 3. (a) XANES and (b) Fourier transform (FT) of the Cu K-edge
EXAFS oscillation k2ø(k) of the used CuO/ZSM-5 catalyst for the
SCWO of 2CP at 673 K for 1 min (O/C ratio ) 1.1; concentration of
-1
2CP ) 1500 mg L ). The best fittings of the EXAFS spectra are
expressed by the dotted lines.
originally in the CuO/ ZSM-5 catalyst might be reduced to
Cu(I) and metallic Cu in the SCWO process. The Cu(I) and
metallic Cu species might be reoxidized by the oxidant (H2O2)
in supercritical water.
To further investigate the structure of the active copper
species in ZSM-5, X-ray absorption spectra of the catalyst
were also measured (26-32). The XANES spectrum of the
CuO/ ZSM-5 catalyst used in the SCWO of 2CP is shown in
Figure 3(a). The preedge XANES spectrum of CuO/ ZSM-5
exhibits very weak absorbance feature for the 1s to 3d
transition which is forbidden by the selection rule in the
case of perfect octahedral symmetry. The sharp feature at
8982-8984 eV, due to the dipole-allowed of 1s to 4pxy electron
transition, indicates the existence of Cu(I). The intensity of
the 1s to 4pxy transition is proportional to the population of
Cu(I) in ZSM-5. A shoulder at 8985-8988 eV and an intense
feature at 8994-8997 eV are attributed to the 1s to 4pxy
transition that indicates the existence of Cu(II) species in the
channels of ZSM-5. The XANES spectra work particularly
well in distinguishing of Cu(I) and Cu(II) coexisted in the
channels of ZSM-5.
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the equation D ) kDc(T/ M)1/ 2 (Dc is the channel factor
depending on the zeolite pore sizes; T is temperature in K;
M is the molecular weight; and k is a parameter) (22-24), the
diffusion coefficient of the copper oxide cluster (Cu3O4, at
673 K) was 1.10 × 10-8 m2 s-1 and was greater than that of
2CP by at least 3 orders. Note it is evident that the EPR signals
were associated with perhaps all of the copper species
introduced into zeolite by ion exchange and were repre-
sentative of all the Cu(II) in ZSM-5.
In summary, due to an effective abstraction of Cl from
2CP by Cu2+ cations and an enhanced precipitation of copper
chloride in the SCWO process, formation of toxic byproducts
was significantly reduced. In the highly restricted environ-
ment of ZSM-5 channels, formation of higher chlorinated
phenols and heavy PAH byproducts was also extremely
limited in the SCWO of 2CP at 673 K. Only trace light-PAHs
with a molecule size less than 6 Å were formed in the SCWO
of 2CP. The copper oxides in the channels of ZSM-5 may
form Cu3O2 clusters that were oxidized to form Cu3O4 clusters
by H2O2. In the channels of ZSM-5, diffusion coefficients of
the copper oxide clusters were greater than that of 2CP by
at least 3 orders. Since the reaction rate in supercritical water
was extremely rapid, one may postulate that relatively mobile
clusters (in the channels of ZSM-5) are involved in the SCWO
of 2CP.
FIGURE 4. 29Si SSNMR spectra of (a) fresh and (b) used CuO/ZSM-5
in the SCWO of 2CP (O/C ratio ) 1.1; concentration of 2CP ) 1500
-1
mg L ).
Acknowledgments
The financial support (1997-1999) of the National Science
Council of Taiwan is gratefully acknowledged. We also thank
Prof. Y. W. Yang and Dr. J. F. Lee of the Taiwan Synchrotron
Radiation Research Center for their help in the EXAFS
experiments and Ms. Ru-Rong Wu of the National Cheng
Kung University for her SSNMR experimental assistance.
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Received for review March 1, 2000. Revised manuscript re-
ceived August 24, 2000. Accepted August 30, 2000.
ES001062S
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