Reductive hydrogenation of polycyclic aromatic hydrocarbons catalyzed by metalloporphyrins

Article abstract of DOI:10.1016/j.chemosphere.2007.01.034

The hydrogenation of polycyclic aromatic hydrocarbons (PAHs) (naphthalene, anthracene, and phenanthrene) catalyzed by metalloporphyrins based on cobalt, nickel or iron was studied in aqueous solutions at room temperature and ambient pressure. Nickel porphyrin (P1) activated by nanosized zero-valent iron (nano-ZVI) and cobalt porphyrins (P2) and (P4) activated by titanium(III) citrate as the electron donor were demonstrated to be promising catalysts for the reductive hydrogenation of PAHs. In particular, partially saturated di-, tetra-, and octahydrogenated products were obtained for anthracene or phenanthrene using a nickel porphyrin activated by nano-ZVI, while naphthalene was transformed to tetralin. Systems containing cobalt porphyrins activated by titanium(III) citrate exhibited a high selectivity and activity toward hydrogenation of anthracene, producing 9,10-dihydroanthracene. However, no formation of hydrogenated hydrocarbons was observed from naphthalene or phenanthrene using cobalt porphyrins.

Full text of DOI:10.1016/j.chemosphere.2007.01.034

Chemosphere 68 (2007) 210–217
www.elsevier.com/locate/chemosphere
Reductive hydrogenation of polycyclic aromatic
hydrocarbons catalyzed by metalloporphyrins
*
Elza Nelkenbaum, Ishai Dror , Brian Berkowitz
Department of Environmental Science and Energy Research, Weizmann Institute of Science, 76804 Rehovot, Israel
Received 2 September 2006; received in revised form 16 January 2007; accepted 17 January 2007
Available online 1 March 2007
Abstract
The hydrogenation of polycyclic aromatic hydrocarbons (PAHs) (naphthalene, anthracene, and phenanthrene) catalyzed by metallo-
porphyrins based on cobalt, nickel or iron was studied in aqueous solutions at room temperature and ambient pressure. Nickel porphyrin
(
P1) activated by nanosized zero-valent iron (nano-ZVI) and cobalt porphyrins (P2) and (P4) activated by titanium(III) citrate as the
electron donor were demonstrated to be promising catalysts for the reductive hydrogenation of PAHs. In particular, partially saturated
di-, tetra-, and octahydrogenated products were obtained for anthracene or phenanthrene using a nickel porphyrin activated by nano-
ZVI, while naphthalene was transformed to tetralin. Systems containing cobalt porphyrins activated by titanium(III) citrate exhibited a
high selectivity and activity toward hydrogenation of anthracene, producing 9,10-dihydroanthracene. However, no formation of hydro-
genated hydrocarbons was observed from naphthalene or phenanthrene using cobalt porphyrins.
ꢀ
2007 Elsevier Ltd. All rights reserved.
Keywords: PAH degradation; Reductive hydrogenation; Remediation
1
. Introduction
(Keith and Telliard, 1979), and effective remediation
techniques are required to detoxify them. One method to
transform PAHs into less toxic compounds involves
their reductive hydrogenation. For example, the compari-
son of naphthalene to its hydrogenated product, tetralin,
shows reduced oral rate LD50 levels for naphthalene
Polycyclic aromatic hydrocarbons (PAHs) form a large
class of hazardous organic compounds. These compounds
are abundant in the environment, originating naturally
from coal tar and crude oil but arising mainly from
incomplete combustion of fossil fuels and organic matter
À1
À1
(490 mg kg for naphthalene compared to 1620 mg kg
(
Menzie et al., 1992; Poster et al., 2006). Generally, PAHs
for tetralin) as well as much faster biodegradation and half
life of the hydrogenated product upon release to the envi-
ronment (Naphthalene MSDS, 2004; Naphthalene1,2,3,4-
tetrahydro MSDS, 2004). During the past decade, hydroge-
nation of PAHs over supported noble metal catalysts has
been studied extensively (Sch u¨ th and Reinhard, 1998;
Barrio et al., 2003; Jongpatiwut et al., 2004; Venezia
et al., 2004; Pawelec et al., 2005). However, hydrogenation
reactions on these catalysts typically occur under high
hydrogen pressure and/or elevated temperature, either in
the gas phase or in the presence of organic solvents. There-
fore, efforts were made to optimize hydrogenation pro-
cesses by developing procedures to allow the reaction to
proceed under mild conditions.
are known to pollute air, soil and water resources at even
low concentrations, exhibiting high thermal stability and
persistence in soil and groundwater (ATSDR, 2006;
BCME, 2006). In particular, PAHs are among the most
important contaminants of groundwater. The 2- and 3-ring
PAHs are of special concern because they are water soluble
in the 1–200 lg/l range and are transported with the
groundwater over significant distances.
Because of their carcinogenic and mutagenic properties,
PAHs are regarded as environmental priority pollutants
*
Corresponding author. Tel.: +972 8 934 4230; fax: +972 8 934 4124.
E-mail address: idror@weizmann.ac.il (I. Dror).
0
045-6535/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2007.01.034

E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
211
Catalytic hydrogenation of naphthalene and phenan-
threne using palladium over c-alumina was studied in
hydrogen-saturated deionized water at room temperature
and ambient pressure (Sch u¨ th and Reinhard, 1998).
PAHs were shown to be partially hydrogenated within
genation (Krone et al., 1989; Marks et al., 1989; Gantzer
and Wackett, 1991; Ukrainczyk et al., 1995; Dror and Sch-
lautman, 2003; Dror et al., 2005). These catalysts often func-
tion well in aqueous solutions under conditions pertinent to
groundwater environments.
1
h. For example, naphthalene was transformed to tetralin.
Hydrogenation reactions using metalloporphyrin
catalysts have been applied, for example, to saturation of
olefins (by polymer-supported metalloporphyrins) (Rong-
min and Yunpu, 1993; Rongmin et al., 1998a,b) and a,b-
unsaturated esters (Shigeyoshi et al., 1992). Recently,
catalytic hydrogenation of ethene, propene and 1-buthene
catalyzed by novel microporous rhodium(II) carboxylate
polymer complexes containing metalloporphyrins at 200 K
was described (Sato et al., 2005). In all of these cases, the
reactions were conducted in pure organic or water–ethanol
mixtures.
Based on our research investigating use of metallopor-
phyrins as catalysts for the reductive transformation of
halo-organic contaminants (Dror and Schlautman, 2003;
Dror et al., 2005), we examine here the catalytic reductive
hydrogenation of three representative PAHs – naphtha-
lene, anthracene and phenanthrene – by nickel, cobalt
and iron porphyrins.
Hydrogenation of phenanthrene yielded two isomers of
octahydrophenanthrene as the main products, with dihydr-
ophenanthrene and tetrahydrophenanthrene being tran-
sient intermediates. Recently, hydrogenation of PAHs
over palladium/c-Al O under moderate temperatures
2
3
and pressures was investigated, using a mixture of molecu-
lar hydrogen with supercritical CO as a reaction medium
2
(
Yuan and Marshall, 2005). The PAHs were readily
reduced to saturated hydrocarbons without hydrocracking.
For example, hydrogenation of anthracene at the higher
operating temperatures resulted in six completely hydro-
genated tetradecahydroanthracene isomers, two of which
were the dominant products. However, two partially
hydrogenated intermediates, tetrahydroanthracene and
octahydroanthracene, were detected in the product mix-
tures at lower temperatures.
The development of effective catalysts for PAH hydro-
genation reactions, without supplying H gas, is in many
2
cases beneficial for practical reasons. One option for such
reactions is hydrogenation of PAHs using tetralin as a
hydrogen donor, catalyzed by molten SbCl3 at 80 ꢁC
2. Materials and methods
(
Buchanan et al., 1982). Anthracene was shown to be selec-
2.1. Chemicals
tively hydrogenated to 9,10-dihydroanthracene, while no
hydrogenated products were observed when phenanthrene
underwent reaction under these conditions. In earlier stud-
ies it was indicated that anthracene can be selectively
hydrogenated in the middle ring by iron pentacarbonyl
under water–gas shift conditions at elevated temperatures
All chemicals were received in sufficient purity and
hence were used without further purification. Naphthalene
(98%), anthracene (97%), phenanthrene (97%), 5,10,
15,20-tetrakis (4-hydroxy-phenyl)-21H, 23H-porphine
(T(OH)PP), 5,10 ,15,20-tetrakis(1-methyl-4-pyridinio) por-
phyrin tetra (p-toluenesulfonate) (TMPyP), cobalt(II) ace-
tate tetrahydrate (98%), nickel(II) acetate tetrahydrate
(98%), sodium citrate dihydrate (99.9%), trizma base
(tris-(hydroxymethyl) aminomethane) (99.9%), iron(III)
chloride hexahydrate (98%) and toluene, were purchased
from Sigma-Aldrich, Israel. Titanium trichloride (15%
solution in HCl) was obtained from RdH (Riedel-de Haen,
Sleeze, Germany), and NaBH4 (96%) was received
from Merck, USA. Dimethylformamide and methyl
alcohol were obtained from Mallinckrodt Baker B.V.,
Holland and Mallinckrodt Chemicals, USA, respec-
tively.
Water soluble metalloporphyrins T(OH)PP-Co,
TMPyP-Co and TMPyP-Ni were synthesized by the reac-
tion of metal acetate with corresponding porphyrin accord-
ing to published procedures (Fuhrhop and Smith, 1975;
Dror and Schlautman, 2003). Iron porphyrins (T(OH)PP-
Fe and TMPyP-Fe) were prepared by modifying the
method described in Warburg and Negelein (1932). Tita-
nium(III) citrate was prepared via a procedure described
by Smith and Woods (1994). Nano-ZVI was synthesized
by modifying a procedure described by Wang and Zhang
(1997).
(
Thomas et al., 1984). The addition of nitrogen heterocy-
cles as phase-transfer agents increased the yield of 9,10-
dihydroanthracene.
Anaerobic degradation of PAHs in several water–soil
systems with nitrate (Mihelcic and Luthy, 1988; Rockne
and Strand, 1998; Rockne et al., 2000), sulfate (Mihelcic
and Luthy, 1988; Coates et al., 1996a, 1997; Zhang and
Young, 1997; Meckenstock et al., 2000), or iron (Coates
et al., 1996b) as electron acceptors, and under methano-
genic conditions (Mihelcic and Luthy, 1988), has been dem-
onstrated (Meckenstok et al., 2004). Results of these studies
indicate that naphthalene and phenanthrene are amenable
to microbial remediation under denitrification and sulfate-
reducing conditions, producing naphthoic or phenanthroic
acids. Under methanogenic conditions, no transformation
of unsubstituted PAHs was found, although compounds
with polar substituents such as naphthol can be metabolized
(
Mihelcic and Luthy, 1988).
Metalloporphyrins have been employed as catalysts for a
variety of reactions, including many that are important for
environmental applications. Among the numerous available
metalloporphyrins, those bearing cobalt or nickel have been
demonstrated to be effective catalysts for reductive dehalo-

212
E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
2
.2. Hydrogenation reaction system
tial adsorption to glassware was tested by comparing
glassware to acid-washed glassware, and found to be negli-
gible. For the reaction, hydrogen is not supplied by an
external gaseous source; hydrogenation reactions were per-
formed in an anaerobic chamber containing an atmosphere
of 100% nitrogen, to mimic an anaerobic environment typ-
ical of many groundwater systems. Thus hydrogen or a pro-
ton source for the hydrogenation reaction is a result of
reaction between the solvent (water) or its dissociation
products and the substrate either directly or through the
catalysts.
All manipulations with oxygen sensitive compounds, and
all PAH hydrogenation reactions, were performed in an
anaerobic chamber (Coy Laboratory Products Inc., Michi-
gan) containing an atmosphere of 100% nitrogen at ambient
pressure and temperature. Generally, in batch systems,
2
0 ml of PAH aqueous stock solution (naphthalene
À1
À20 mg l ; anthracene and phenanthrene – saturated solu-
tions (prepared by addition of a small excess of the PAH to
warm deionized water, mixing for 48 h, followed by filtra-
tion of the solution)) was introduced into a 40 ml glass vial
equipped with magnetic stirrer, followed by an addition of
2.3. Analytical analysis
À4
metalloporphyrin aqueous solution (3.5 · 10
mmol).
After transferring the set of vials into the anaerobic cham-
ber, the reaction was initiated by addition of either tita-
nium(III) citrate stock solution (1.8 ml, 250 mM in
GC/MS analysis was conducted using a Varian CP-3800
gas chromatograph fitted with a model 8200 autosampler
and a Saturn 2000 ion trap MS detector equipped with cap-
illary column (VF-5 ms 30 m; 0.25 mm; 0.25 lm) with flow
6
60 mM tris buffer) or nano-ZVI (100 mg); vials were sealed
À1
with PTFE/clear silicon septa, and the resulting mixtures
were stirred at room temperature for a specified period of
time. The pH values of the titanium(III) citrate and nano-
ZVI systems were 8.7 and 6.6, respectively. The reaction
was stopped by transferring the vial outside the chamber,
exposing the solution to air and extracting the PAHs from
the aqueous solution with 4 ml of toluene (extraction effi-
ciency above 99% compared to second and third extrac-
tions). The obtained products were identified by GC/MS.
All reactions were carried out in at least two replicates.
In all cases control experiments were performed to iden-
tify possible activity of each of the single components i.e.
only substrate (no catalyst or reducing agent), only sub-
strate and reducing agent (no catalyst), and substrate solu-
tion with catalysts (no reducing agent). It was found that
metalloporphyrins were unable to catalyze hydrogenation
of PAHs in the absence of the reducing agents. Also, no
reaction was observed when PAH aqueous stock solutions
and electron donors (titanium(III) citrate or nano-ZVI)
were used without the metalloporphyrins. Sources of poten-
tial losses of substrate were examined; for example, poten-
rate (helium) of 1.0 ml min . The analysis program was:
after an initial temperature of 80 ꢁC for 1 min ramp of
À1
15 ꢁC min to 240 ꢁC, hold for 1 min at 240 ꢁC, ramped
at 30 ꢁC min to 300 ꢁC. Injector, transfer line and detector
temperature were maintained at 270, 275 and 150 ꢁC,
respectively. Eluted components were identified by com-
paring experimental mass spectra to spectra catalogued in
the National Institute of Standards and Technology or
the Saturn mass spectral libraries.
3. Results and discussion
Five water soluble metalloporphyrins were examined
as catalysts for reductive hydrogenation of naphtha-
lene, anthracene and phenanthrene: 5,10,15,20-tetrakis(1-
methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate)
nickel (TMPyP-Ni), cobalt (TMPyP-Co), or iron
(TMPyP-Fe), denoted (P1), (P2) and (P3), respectively;
5,10,15,20-tetrakis(4-hydroxyphenyl)-21H, 23H-porphine
cobalt (T(OH)PP-Co) or iron (T(OH)PP-Fe), denoted
(P4) and (P5), respectively (Fig. 1).
OH
CH
N+
3
N
N
N
N
N
N
OH
M
N
N
N+
M
N+
CH3
O
OH
H
3
C
HO
2
H O
O
S
O-
3
H C
N+
4
CH
3
OH
M = Ni (P1); Co (P2); Fe (P3)
M = Co (P4); Fe (P5)
Fig. 1. Structures of TMPyP-Ni (P1), TMPyP-Co (P2), TMPyP-Fe (P3), T(OH)PP-Co (P4) and T(OH)PP-Fe (P5).

E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
213
To identify the active catalytic species that are directly
involved in the reductive hydrogenation of the PAHs, both
titanium(III) citrate and nano-ZVI were used as electron
donors. The role of the reducing agent is to generate
(entry 1); anthracene was transformed to 9,10-dihydroan-
thracene (DHA), 1,2,3,4-tetra-hydroanthracene (THA),
1,2,3,4,4a,9,9a,10-octahydroanthracene (OHA-1) and 1,2,
3,4,5,6,7,8-octahydroanthracene (OHA-2) (entry 2). The
hydrogenation of phenanthrene produced a mixture of
9,10-dihydrophenanthrene (DHP), 1,2,3,4-tetrahydrophe-
nanthrene (THP), 1,2,3,4,4a,9,9a,10-octahydrophenanth-
rene (OHP-1), and 1,2,3,4,5,6,7,8-octahydrophenanthrene
(OHP-2) (entry 3).
Interestingly, catalytic systems based on cobalt porphy-
rins (P2) and (P4) and titanium(III) citrate exhibited a high
selectivity and activity toward hydrogenation of anthra-
cene, yielding DHA as a sole product (entries 4 and 5 in
Table 1), while no formation of hydrogenated compounds
I
the M oxidation state, which initiates the reaction. Partial
reductive hydrogenation of all studied PAHs (naphtha-
lene, anthracene, phenanthrene) was found for systems
containing cobalt or nickel porphyrins and an electron
donor, at ambient temperature and pressure in aqueous
solutions.
Results of metalloporphyrin-catalyzed hydrogenation of
PAHs are presented in Table 1, showing that TMPyP-Ni/
Fe(0) mediated reductive hydrogenation of naphthalene
resulted only in 1,2,3,4-tetrahydronaphthalene (THN)
Table 1
Metalloporphyrin-catalyzed hydrogenation of PAHs
a
À1
Entry
1
Substrate
Catalytic system
TMPyP-Ni/Fe(0)
TOF (min (15%))
3.6
Products
Naphthalene
THN
OHA-1
OHA-2
DHA
THA
2
Anthracene
TMPyP-Ni/Fe(0)
4.2
DHP
THP
OHP-1
OHP-2
3
4
Phenanthrene
TMPyP-Ni/Fe(0)
0.42
DHA
DHA
Anthracene
Anthracene
T(OH)PP-Co/Ti(III) citrate
24
5
6
TMPyP-Co/Ti(III) citrate
TMPyP-Ni/Ti(III) citrate
156
–
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
Naphthalene
Phenanthrene
Anthracene
–
–
–
7
8
9
TMPyP-Co/Fe(0)
–
–
–
–
–
–
TMPyP-Fe/Fe(0)
TMPyP-Fe/Ti(III) citrate
T(OH)PP-Fe/Fe(0)
–
–
Traces after 96 h
–
1
1
1
0
1
2
a
T(OH)PP-Fe/Ti(III) citrate
T(OH)PP-Co/Fe(0)
–
–
À1
TOF-turnover frequency = mol of product · (mol of catalyst · min) at the time of 15% conversion. Reaction conditions as in Figs. 2a and 5 for
metalloporphyrins activated by nano-ZVI and titanium(III) citrate, respectively. Entries 6–12: three PAHs were tested individually.

214
E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
was observed from naphthalene or phenanthrene using
these catalysts.
cene were observed, in terms of TOF as shown in Table 1
and kinetic measurements as presented in Fig. 2a. Compar-
ison of TOFs for hydrogenation processes of these
three PAHs shows higher activity of the nickel catalyst (P1)
toward anthracene (entries 1, 2 and 3). These results are in
The metalloporphyrins considered here can be charac-
terized by the organic ligand and its functional groups,
and by the core metal. As shown in Table 1, comparison
of the products obtained from hydrogenation mediated
by metalloporphyrins indicates significant differences in
catalytic activity of complexes that differ only in their core
metals. For example, TMPyP-Co (P2) was found to be
active only in the presence of titanium(III) citrate, while
TMPyP-Ni (P1) exhibited catalytic activity only when
nano-ZVI was used as a reducing agent. The combination
of TMPyP-Ni and Fe(0) led to hydrogenation reactions for
all three PAHs (entries 1, 2, 3). However, the same reaction
with use of titanium(III) citrate as the electron donor failed
to promote the reaction and no hydrogenation was
observed for any of the PAHs examined during the exper-
iment (entry 6). It is further noted that nano-ZVI activated
nickel porphyrin (P1), and their combination enabled the
catalytic cycle; titanium(III) citrate did not enable this
reaction. Using the cobalt porphyrin (P2), the reverse result
was found, i.e., nano-ZVI did not reduce the catalyst, while
titanium(III) citrate enabled the catalytic hydrogenation
reaction (compare entries 5 and 7). Moreover, only anthra-
cene was involved in the reaction by TMPyP-Co/Ti(III) cit-
rate, producing only one dihydrogenated compound.
Interestingly, TMPyP-Fe (P3) was found to be inactive in
the presence of nano-ZVI (entry 8), while traces of dihydro-
genated anthracene were observed using the catalytic sys-
tem TMPyP-Fe/Ti(III) citrate after 96 h of reaction
À1
accord with resonance energy calculations (255 kJ mol
À1
for naphthalene, 347 kJ mol
mol
for anthracene, 381 kJ
À1
for phenanthrene; Morrison and Boyd, 1992).
Anthracene and phenanthrene are reduced to the 9,10-
dihydro compounds, and are less resistant to reduction
than naphthalene. Reduction at the 9- and 10-positions
leaves two benzene rings intact; thus there is energy loss
À1
of only 50 kJ mol of resonance energy for anthracene,
À1
and 83 kJ mol for phenanthrene. However, for the case
of phenanthrene, the two remaining rings are conjugated;
to the extent that this conjugation stabilizes the product,
100
8
6
0
0
40
Anthracene
Naphthalene
Phenanthrene
2
0
(
entry 9). In addition, T(OH)PP-Co activated by tita-
nium(III) citrate was found to hydrogenate anthracene,
while the catalytic system T(OH)PP-Co/Fe(0) did not
enable hydrogenation reactions for any of the three PAHs
0
0
20
40
60
80
100
120
140
160
180
(
compare entries 4 and 12). The iron porphyrin containing
Time (h)
the same ligand was found to be inactive in the presence of
any of the reducing agents examined in the present study
5.0
(
entries 10 and 11). These phenomena can be explained
4
.5
.0
by considering the different electronic structures of the
metal ions in metalloporphyrins. Their highest occupied
molecular orbital and the lowest unoccupied molecular
orbital (HOMO–LUMO) gaps are different and as a conse-
quence, their abilities to transfer electrons, coordinate with
the substrate in the active site, and stabilize intermediates
differ accordingly (Dror and Schlautman, 2003; Dror
et al., 2005; Bhyrappa et al., 2006).
Considering Table 1, it is seen that the catalytic activity
is also influenced by the functional groups on the periphe-
ral porphyrin ligands, for complexes with the same core
metal. For example, in terms of Turnover Frequency
Anthracene
linear fit
4
3.5
3.0
2.5
2.0
1.5
1.0
(
TOF, as defined in Table 1), T(OH)PP-Co was six-fold
0
20
40
60
80
100
slower than TMPyP-Co under the same reaction condi-
tions (compare entries 4 and 5). TMPyP-Co/Ti(III) citrate
was the most active in hydrogenation of anthracene, with a
TOF of 156 min (entry 5).
Different activities of TMPyP-Ni/Fe(0) catalysts toward
transformation of naphthalene, phenanthrene and anthra-
Time (h)
Fig. 2. (a) PAH conversion vs. time. Hydrogenation reaction catalyzed by
TMPyP-Ni using nano-ZVI as an electron donor. Reaction conditions:
Ni] = 1.6 · 10 mM, pH = 6.6, T = 25 ꢁC. (b) First-order dependence of
anthracene in the reductive hydrogenation by TMPyP-Ni using nano-ZVI
À1
À2
[
as an electron donor.

E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
215
a
b
a
100
Anth
DHA
THA
OHA-1
OHA-2
DHA
THA
OHA-1
OHA-2
8
6
4
2
0
0
0
0
ANTHRACENE
THP
0
OHP-2
b 100
PHENANTHRENE
DHP
OHP-1
8
6
4
2
0
0
0
0
Phen
DPH
TPH
OHP-1
OHP-2
Fig. 3. Proposed reaction pathways for TMPyP-Ni/Fe(0) mediated
hydrogenation of anthracene (a) and phenanthrene (b).
and the energetic loss is estimated at anywhere from 0 to
À1
3
3 kJ mol (Morrison and Boyd, 1992).
Fig. 2a shows the dependence of naphthalene, anthracene
and phenanthrene conversion on the time of hydrogenation
mediated by the TMPyP-Ni/Fe(0) catalytic system. More
than 95% transformation of anthracene and naphthalene
was achieved after 168 h, while conversion of phenanthrene
was 90%. Hydrogenation of anthracene follows a first-order
0
6
21
47
72
96
168
Time (h)
Fig. 4. (a). Distribution of products resulting from hydrogenation
of anthracene by TMPyP-Ni using nano-ZVI as an electron donor.
(b) Distribution of products resulting from hydrogenation of phenan-
threne by TMPyP-Ni using nano-ZVI as an electron donor. Reaction
conditions as in Fig. 2a.
À4
À1
dependence on the substrate, yielding k = 6.5 · 10 min
,
as presented in Fig. 2b. Reaction rates for naphthalene and
phenanthrene were zero-order up to 70 and 40 h, respec-
tively, and first-order thereafter. Proposed reaction path-
ways for TMPyP-Ni/Fe(0) mediated hydrogenation of
anthracene (a) and phenanthrene (b) are presented in Fig. 3.
Analogous hydrogenation products for phenanthrene
were obtained using palladium on alumina in hydroge-
nated water (Sch u¨ th and Reinhard, 1998). Typically org-
ano-metallic catalysts are used for such reactions, which
proceed through a well-known double bond catalytic
hydrogenation mechanism. This mechanism involves dis-
sociation of hydrogen molecules, oxidative addition of
5% OHA-2, while for phenanthrene (Fig. 4b) no hydrogena-
tion products were found after the same time span. Theoret-
ically, for anthracene, DHA is expected to be the main
hydrogenation product at the first step, which then isomer-
izes to give THA, and then further hydrogenation products.
However, after 6 h, all three products appeared (i.e., DHA,
THA and OHA-2) and the reactions pathways can there-
fore not be verified. For the hydrogenation of anthracene,
as shown in Fig. 4a, DHA and THA concentrations peaked
at 41% and 31% after 47 h, respectively. This correlation
may indicate a fast isomerization between DHA and THA
and, therefore, their concentrations are synchronized, i.e.,
increasing and decreasing throughout the reaction together.
OHA-1 appeared for the first time after 47 h, as a conse-
quence of either isomerization of OHA-2 or hydrogenation
of THA or DHA. Anthracene was degraded completely
after 168 h, and DHA and THA concentrations were
reduced while the amount of OHA-2 increased.
A different product distribution was found for phenan-
threne (Fig. 4b). The transformation rate in this case is
much slower and the first product DHP (which corre-
sponds to DHA in the hydrogenation of anthracene) was
detected after 21 h (4%). In this case DHP was the only
product after 21 h, and after 47 h very small amounts of
THP (which corresponds to THA in the hydrogenation
of anthracene) appears. Here again the DHP and THP
H to the metal center to form a hydride complex, coordi-
2
nation of double bond to the metal center and insertion
into the M–H bond to form a half-hydrogenated species
followed by reductive elimination, yielding a saturated
product (Crabtree, 2005). Based on the above scheme, it
is hypothesized that the subsequent hydrogenation of di-
or tetra-derivatives proceeds stepwise. However, the inter-
mediates with partially hydrogenated rings were not
detected and therefore assumed to isomerize or react rap-
idly to produce fully saturated rings which are energetically
more stable.
The product distribution analysis of the hydrogenation
reaction for anthracene (Fig. 4a) and phenanthrene
(
Fig. 4b) with TMPyP-Ni/Fe(0) systems gives a closed mass
balance and indicates the reaction pathways. The transfor-
mation rate of anthracene is much faster than the transfor-
mation rate of phenanthrene. For anthracene (Fig. 4a) the
reaction products after 6 h were 3% DHA, 7% THA and

216
E. Nelkenbaum et al. / Chemosphere 68 (2007) 210–217
100
naphthalene; partially di-, tetra-, and octahydrogenated
compounds (DHA, THA, OHA-1 and OHA-2) from
anthracene, and similarly four hydrogenated compounds
9
0
0
0
0
0
0
0
0
0
a
b
8
7
6
5
4
3
2
1
(DHP, THP, OHP-1, and OHP-2) from phenanthrene. Sig-
nificant differences observed in product distributions and
reaction rate for anthracene and phenanthrene indicate dis-
tinct reaction pathways.
Catalytic systems based on cobalt porphyrins and tita-
nium(III) citrate exhibited high selectivity and activity
toward hydrogenation of anthracene, yielding a dihyrogen-
ated compound (DHA) as a sole product. No formation of
hydrogenated compounds was observed from naphthalene
or phenanthrene using these catalysts.
0
0
20
40
60
80
100 120 140 160 180 200
Acknowledgement
Time (h)
The financial support of the Yeshaya Horowitz Associ-
ation is gratefully acknowledged. B.B. holds the Sam Zuc-
kerberg Professorial Chair.
Fig. 5. Anthracene conversion vs. time. Hydrogenation reaction catalyzed
by TMPyP-Co (a) and T(OH)P-Co (b) using Ti(III) citrate as an electron
donor. Reaction conditions: [Co] = 1.6 · 10 mM, Co:Ti = 1:1200,
À2
pH = 8.7, T = 25 ꢁC.
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