Selective catalytic hydrogenation of polycyclic aromatic hydrocarbons promoted by ruthenium nanoparticles

Article abstract of DOI:10.1039/c4cy01758g

Ru nanoparticles stabilised by PPh₃ are efficient catalysts for hydrogenation of polycyclic aromatic hydrocarbons (PAHs) containing 2-4 rings under mild reaction conditions. These compounds were partially hydrogenated with good to excellent selectivities just by optimizing the reaction conditions. The influence of the nature of substituents present in different positions of naphthalene on the selectivity of hydrogenation was also studied. Hydrogenation of products containing substituents at position 1 is slower than that of products containing substituents at position 2. In all cases, hydrogenation takes place mainly on the less substituted ring.

Full text of DOI:10.1039/c4cy01758g

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Selective catalytic hydrogenation of polycyclic
aromatic hydrocarbons promoted by ruthenium
nanoparticles†
Cite this: DOI: 10.1039/c4cy01758g
a
b
Emma Bresó-Femenia,ab Bruno Chaudret and Sergio Castillón
*
*
Ru nanoparticles stabilised by PPh3 are efficient catalysts for hydrogenation of polycyclic aromatic
hydrocarbons (PAHs) containing 2–4 rings under mild reaction conditions. These compounds were partially
hydrogenated with good to excellent selectivities just by optimizing the reaction conditions. The influence
of the nature of substituents present in different positions of naphthalene on the selectivity of
hydrogenation was also studied. Hydrogenation of products containing substituents at position 1 is slower
than that of products containing substituents at position 2. In all cases, hydrogenation takes place mainly
on the less substituted ring.
Received 31st December 2014,
Accepted 22nd February 2015
DOI: 10.1039/c4cy01758g
www.rsc.org/catalysis
Nanoparticles have been used in a wide range of reactions.
Several studies have been focused on hydrogenation of aro-
Introduction
Polycyclic aromatic hydrocarbons (PAHs) (Fig. 1) are a class of
organic compounds comprising two or more fused benzene
rings with different structural arrangements.1 PAHs have
attracted considerable attention due to their toxic, carcino-
genic and teratogenic effects.2 Different methods have been
proposed for the elimination of PAHs such as thermal treat-
ment, photo-degradation, chemical oxidation, etc., but these
processes are slow and imply complex techniques with high
energy consumption.3
In the past few years, metal nanoparticles have been
widely used in different domains such as medicine, sensors
or catalysis.4 Particularly in catalysis, nanoparticles are
advantageous because of the moderate reaction conditions
needed, the high activity obtained due to their high surface
area, their unique electronic effects and their long lifetime.5
Therefore, they can be attractive catalysts for PAH
hydrogenation.
matic compounds, which is an important step for the prepa-
ration of key intermediates in organic chemistry and for the
production of aromatic-free-fuels.8 In general, hydrogenation
of arenes is conventionally performed with heterogeneous
metallic catalysts under harsh conditions due to the stability
of the aromatic rings.9 Naphthalene has probably been the
most studied polyaromatic system in hydrogenation reac-
tions. Hydrogenation of naphthalene on some noble metals
such as platinum and palladium affords decalin (decahydro-
naphthalene).10 Classical Pd/C catalysts in the presence of an
ionic liquid reduce naphthalene to tetralin IJ1,2,3,4-
tetrahydronaphthalene) at 1 atm of hydrogen pressure.11
However, hydrogenation to decalin under mild reaction con-
ditions is still a challenge.12 Moreover, platinum-supported
catalysts were used in the reduction of naphthalene to afford
decalin at 300 °C with full conversion.13
Stabilisation of M-NPs can be achieved by the use of poly-
mers, surfactants or ligands, which allows the control of their
size, shape and dispersion. The choice of an appropriate sta-
biliser for M-NPs has an important effect on their catalytic
performance.6,7
a Laboratoire de Physique et Chimie des Nano-Objets, LPCNO, UMR5215, CNRS-
INSA-UPS, Université de Toulouse, Institut National des Sciences Appliquées, 135
avenue de Rangueil, 31077 Toulouse, France. E-mail: chaudret@insa-toulouse.fr
b Departament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili,
C/ Marcellí Domingo s/n, 43007 Tarragona, Spain. E-mail: sergio.castillon@urv.cat
† Electronic supplementary information (ESI) available: Synthesis of ruthenium
nanoparticles stabilized by P-ligands, catalytic experiments, general procedures,
gas chromatography–mass spectrometry. See DOI: 10.1039/c4cy01758g
Fig. 1 PAHs of 2, 3 and 4 condensed aromatic rings studied in this
work.
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Reduction of polycyclic arenes requires higher tempera-
hydrogenation of one arene in naphthalene and anthracene
IJ1,2,3,4-tetrahydroanthracene as the major product) were
achieved. Furthermore, comparable selectivities towards the
partial hydrogenation of N-heterocyclic compounds and
selectivities up to 60% in S-heteroaromatics were achieved.24
Concerning triphenylene, it is important to highlight that
the central ring is very difficult to saturate.18,25 Few examples
are reported in which a totally hydrogenated product is
observed.26 With rhodium nanoparticles supported on car-
tures and pressures where mixtures of products are generally
obtained.14 Thus, the catalytic hydrogenation of some PAHs
like naphthalene, anthracene, pyrene, etc. over presulfided
CoMo/Al2O3 catalysts was carried out at 350 °C and 68 atm of
H2 pressure, and it was deduced that the reactivity decreased
with the number of aromatic rings.15 A palladium–rhodium
system embedded in a silica sol–gel matrix was used in the
hydrogenation of anthracene, phenanthrene, triphenylene,
pyrene and perylene at 80 °C and 400 psi of hydrogen pres-
sure. Mixtures of products and low selectivities were obtained
in all cases.16 For instance, in the case of anthracene, 60% of
selectivity towards 1,2,3,4,5,6,7,8-octahydroanthracene was
observed, in the case of phenanthrene, 37% of selectivity
towards 9,10-dihydrophenanthrene was observed, or in the
case of triphenylene, 27% of selectivity towards
1,2,3,4,5,6,7,8-octahydrotriphenylene was observed.
bon
nanotubes,
high
selectivities
towards
were
1,2,3,4,5,6,7,8,9,10,11,12-dodecahydrotriphenylene
obtained under mild reaction conditions (10 atm of H2 and
room temperature).26b Hydrogenation of phenanthrene and
pyrene has also been attempted in the presence of supercriti-
cal carbon dioxide.27 For instance, Pd nanoparticles stabilized
by polydimethylsiloxane (PDMS) have been used to hydroge-
nate polycyclic aromatic hydrocarbons affording total hydro-
genated products for naphthalene, anthracene, phenanthrene
and pyrene (200 atm of CO2, 10 atm of H2).28
In this context, recently, we have reported the use of
ruthenium and rhodium nanoparticles stabilized by PPh3
and dppb in the hydrogenation of several aromatic ketones.
Ruthenium nanoparticles stabilized by PPh3 were found to be
the most selective and active system for arene hydrogena-
tion,29 and we considered that these nanoparticles could also
be efficient as catalysts for polyarene reduction. Here, we
report that ruthenium nanoparticles stabilized by
triphenylphosphine are active catalysts for the selective
hydrogenation of polycyclic aromatic hydrocarbons under
mild reaction conditions. A study of the effect of the nature
of substituents in the selectivity of naphthalene reduction
has also been performed for the first time.
There have been only a few studies concerning the hydro-
genation of PAH substrates under ambient or mild reaction
conditions using nanoparticles.17 For instance, rhodium
nanoparticles were used as efficient catalysts for the hydroge-
nation of naphthalene affording tetralin as
product.18
a unique
In the case of polycyclic aromatic hydrocarbons with more
than two fused rings, nanoparticles have also been used but
totally hydrogenated products are rarely obtained.19 Rh and
Ir nanoparticles entrapped in aluminium oxyhydroxide nano-
fibers were tested in the hydrogenation of bicyclic and tricy-
clic aromatic compounds. Thus, naphthalene was reduced to
tetralin and anthracene to 9,10-dihydroanthracene at room
temperature and 1 bar of pressure. However, high catalyst
loading (10 mol% of catalyst) was needed to achieve complete
hydrogenation of anthracene.20
Supported Pd, Rh and Rh/Pd nanoparticle catalysts have
also been used to hydrogenate anthracene showing an unusu-
ally high catalytic activity.21 In all cases, moderate to high
selectivities towards partial hydrogenation of anthracene
IJ1,2,3,4,5,6,7,8-octahydroanthracene as the major product)
were obtained; however, total hydrogenation could not be
achieved even under 10 bar of H2 pressure.
Recently, a study about the use of carbon-supported Pd
nanoparticles in the hydrogenation of anthracene concluded
that ring B was initially reduced to give DHA, which then
isomerized to afford THA. From this intermediate, the hydro-
genation progressed furnishing a fully hydrogenated com-
pound (Scheme 1).22
Results and discussion
Synthesis and characterization of ruthenium nanoparticles
Soluble ruthenium NPs prepared in the presence of 0.4 eq. of
triphenylphosphine (Scheme 2) were synthesised by decom-
position of the organometallic precursors ijRuIJCOD)IJCOT)] in
THF under H2 pressure. The NPs were isolated as black pow-
ders after precipitation with pentane and characterised by
transmission electron microscopy (TEM), X-ray diffraction
(XRD), X-ray photoelectron spectroscopy (XPS), wide-angle
X-ray scattering (WAXS), elemental analysis (EA) and
thermogravimetric analysis (TGA).29
Ruthenium
nanoparticles
stabilized
by
polyIJ4-
vinylpyridine) were used in the hydrogenation of naphtha-
lene, anthracene and different N-heteroaromatic substrates. A
double mechanism for the reduction of heteroaromatic com-
pounds which involves conventional homolytic hydrogen
splitting of the simple aromatic substrates and novel hetero-
lytic hydrogenation was proposed.23 Naphthalene and anthra-
cene were also hydrogenated at 150 °C and 50 bar of H2 pres-
sure using ruthenium nanoparticles supported on
magnesium oxide. Selectivities around 80% towards the
Scheme
1 Proposed reaction pathway for the anthracene
hydrogenation using carbon-supported Pd nanoparticles as catalyst.22
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Next, the hydrogenation of polycyclic aromatic compounds
containing three conjugated arenes was attempted. Initially,
the optimized conditions for hydrogenation of naphthalene
were tested in the hydrogenation of anthracene to afford
moderate conversion (41%) and excellent selectivity of the
hydrogenation of only one aromatic ring (91% of 2a) (Table 2,
entry 1). It was clear that more drastic conditions were
needed in this case in order to improve the activity. Thus,
when pressure was increased to 20 bar, 44% of conversion
was achieved after only 0.5 hours and total selectivity towards
product 2a was observed (Table 2, entry 2). Increasing the
reaction time to 9 h, full conversion and 96% of compound
2b were obtained (Table 2, entry 3). Finally, after 16 hours, a
small proportion (10%) of the completely reduced compound
2e was detected (Table 2, entry 4).
With these results in hand and looking for additional
insights into the selectivity of hydrogenation of anthracene
(2) we studied the evolution of the reaction with time. As it
can be observed in Fig. 2, full conversion was obtained after
ca. 1 h. During the first 30 min, the conversion reached ca.
50% and total selectivity of the formation of product 2a was
observed, as a result of the hydrogenation of ring A of anthra-
cene. After 40 min, product 2b began to form progressively
while the percentage of compound 2a decreased. After ca. 5
hours, selectivity up to 95% of product 2b was achieved. Only
traces of products 2c and 2d were detected during the reac-
tion (maximum of 5%). Product 2e was progressively formed
reaching 10% after 16 h.
Scheme
triphenylphosphine.
2 Synthesis of ruthenium nanoparticles stabilized by
The TEM micrographs of these NPs revealed the formation
of small nanoparticles with spherical shape, narrow size dis-
tribution and a diameter of ca. 1.3 nm. Diffuse peaks were
observed in the XRD pattern of these NPs, as expected for a
homogeneous distribution of very small particles with a hex-
agonal close-packing (hcp) lattice structure. No reflections
due to ruthenium oxide were observed, and coherence
lengths in agreement with TEM analysis were obtained.
Thermogravimetric analysis evidenced the presence of ca. 2%
of solvent, 30% of phosphine ligands and 70% of Ru in these
nanoparticles, in agreement with previous reports in which
the same nanoparticles with less proportion of ligand were
used.30
Catalysis: hydrogenation of PAHs
Naphthalene 1 was first used to evaluate the selectivity of par-
tial and total hydrogenation. An initial test using different
solvents showed that THF was the solvent of choice to obtain
better activities and selectivities in comparison to heptane,
pentane or acetonitrile with which really low conversions
were achieved.
When the reaction was conducted at 30 °C and 20 bar of
H2 pressure, full conversion (TON = 39) was obtained after 16
h (Table 1, entry 1). Under these conditions, total hydrogena-
tion was achieved leading to a mixture of 84% of the product
1b (cis) and 16% of the product 1c (trans). When the reaction
was repeated at 3 bar of H2 pressure, quantitative conversion
was observed yielding 74% of tetralin 1a, 24% of 1b and 2%
of 1c (Table 1, entry 2). A reduction of the reaction time to 10
h allowed the production of 1a with an excellent selectivity
(93%) and 70% of conversion (Table 1, entry 3).
Comparing these results using Ru NPs with those reported
using Pd catalytic systems22 suggests a difference in the
hydrogenation mechanisms. With palladium, reduction of
ring B to give compound 2d is initially observed (Scheme 2),
while with ruthenium catalysts, ring A is clearly reduced first
and only traces of compound 2d are observed along the reac-
tion. This can suggest that in the case of ruthenium, the reac-
tion proceeds under kinetic control, which is probably deter-
mined by the accessibility of the arene to the nanoparticle
surface. This can also suggest a different hydrogen transfer
mechanism in the case of palladium.
Selectivities up to 97% of product 1a20 and 91% of product
1b31 in the hydrogenation of naphthalene using Rh and Pd
nanoparticles and Rh nanoparticles on TiO2 have been
reported.
Aiming to compare the reactivity of the different poly-
arenes, the same reaction conditions (20 bar of H2, 30 °C and
16 hours) were applied to the hydrogenation of phenanthrene
Table 1 Hydrogenation of naphthalene 1 catalysed by ruthenium NPsa
E.
P (bar)
Time (h)
Conv.b (%)
TONc
%1ab
%1bb
%1cb
1
2
3
20
3
3
16
16
10
100
100
70
39
39
27
—
74
93
84
24
7
16
2
—
a
b
c
General conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), THF (10 ml), T = 30 °C. Determined by GC. TON was defined as the number
of moles of the substrate converted per mole of surface Ru.
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Table 2 Hydrogenation of anthracene 2 using Ru nanoparticlesa
E.
P (bar)
Time (h)
Conv.b (%)
TONc
%2ab
%2bb
%2cb
%2db
%2eb
1
2
3
4
3
16
0.5
9
41
44
100
100
16
17
39
39
91
100
—
6
—
—
1
3
—
—
3
20
20
20
—
96
90
—
—
—
16
—
—
10
a
b
c
General conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), THF (10 ml), T = 30 °C. Determined by GC. TON was defined as the number
of moles of the substrate converted per mole of surface Ru.
3 but the obtained conversion was only 6%. Products 3b–c,
resulting from reduction of terminal rings A and C, were
obtained with 58% selectivity, while 42% of the product 3a,
resulting from the reduction of ring B, was obtained in this
case (Scheme 3). Increasing the temperature to 50 °C slightly
improved the conversion to 24% but the selectivities towards
3b–c/3a remained unchanged.
Despite having the same number of fused benzene rings,
the behaviour of anthracene (2) and phenanthrene (3)
towards hydrogenation with Ru nanoparticles is really differ-
ent. TON values in the hydrogenation of phenanthrene (3)
are much lower but in both substrates the reduction of termi-
nal rings A and C is preferred compared to the reduction of
ring B. The low selectivity observed in the reduction of phen-
anthrene is in agreement with previous results dealing with
the reduction of this substrate catalysed by Rh nanoparticles
and by Mo and Fe catalysts.18,32 It seems that interaction of
the phenanthrene with the surface of the nanoparticle is
more hindered than that of the anthracene. However, the rel-
ative rate of reduction of ring B is higher than that in the
case of anthracene in which compound 2d is only observed
as traces. This most probably results from the possibility of
coordinating the accessible double bond of this B-ring
exposed to Ru atoms of the nanoparticle. It is worth noting
that good selectivities (~70%) towards compound 3a have
been reported using heterogeneous catalyst such as PtO2
(ref. 33) or niobium.34
Then, hydrogenation of compounds containing four fused
rings such as triphenylene 4 and pyrene 5 was studied. Ini-
tially, triphenylene 4 was hydrogenated under the optimized
reaction conditions (20 bar of H2 and 30 °C) and after 16
hours a conversion of 61% was achieved. Product 4a which
has only one arene hydrogenated was obtained with a selec-
tivity of 53%, product 4b (2 external rings hydrogenated) with
a selectivity of 12%, and product 4c (3 external rings hydroge-
nated) with a selectivity of 35% (Table 3, Entry 1). An increase
in temperature to 80 °C allowed full conversion of 4 and
exclusive formation of product 4c (Table 3, entry 2). Under
the conditions tested, the fully hydrogenated product was not
observed. Increasing the reaction time to 60 hours, only
traces of the fully hydrogenated product 4e were detected
(Table 3, entry 3). It is interesting to note that 10% of product
4d containing one double bond bridging two arene rings was
observed. This double bond is of course the most difficult to
hydrogenate.
The hydrogenation of triphenylene 4 was monitored by
GC-MS (Fig. 3) to look for information about the selectivity of
the formation of compounds 4a–c. All of these 3 products
were detected soon after the beginning of the reaction. After
2 h, conversion reached ca. 20% and selectivity towards prod-
uct 4a was 70%. Then, the percentage of 4a started to
decrease and product 4c, with three hydrogenated external
arenes, started to form progressively in a major proportion.
The percentage of product 4b was practically maintained
Fig. 2 Monitoring of the catalytic hydrogenation of anthracene (2).
Conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), solvent = THF, T
= 30 °C, P = 20 bar of H2).
Scheme 3 Hydrogenation of phenanthrene 3.
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Table 3 Hydrogenation of triphenylene 4 and pyrene 5a
E.
Subs.
P (bar)
T (°C)
Time (h)
Conv.b (%)
TONc
%ab
%bb
%cb
%db
%eb
1
2
3
4
5
6
4
4
4
5
5
5
20
20
20
20
20
20
30
80
80
50
80
80
16
16
60
16
16
60
61
24
39
39
7
10
17
53
—
—
93
90
86
12
—
—
7
10
14
35
100
88
—
—
—
—
—
10
—
—
—
—
—
1
—
—
—
100
100
17
25
44
a
b
c
General conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), THF (10 ml). Determined by GC. TON was defined as the number of moles
of the substrate converted per mole of surface Ru.
during the reaction. The fully hydrogenated product was not
observed showing, as previously commented, the difficulty in
reducing product 4c.
Pyrene 5 is much less reactive. Thus, when the reaction
was performed at 50 °C and 20 bar of pressure (Table 3, entry
4), only 17% of conversion was obtained but selectivity
towards product 5a was 93%. Increasing the temperature to
80 °C, the conversion slightly increased to 25% without a
substantial change in the selectivity (Table 3, entry 5) and
when the reaction was left for 60 hours, the conversion
increased to 44% and the selectivity towards product 5a
remained high (86%) (Table 3, entry 6).
The results reported show that reactivity decreases when
the number of condensed aromatic rings increases. For that
reason, harsher reaction conditions are needed to obtain
moderate conversions in compounds containing several
fused rings. Concerning the selectivity, in the case of
triphenylene 4, compound 4c can be exclusively obtained, in
agreement with the reported results using Rh and Pt nano-
particles.26a Nevertheless, compound 4a can be obtained in
~50% of selectivity with 61% of conversion, which is the
highest selectivity reported for this compound with similar
conversion.
In the case of pyrene, excellent selectivities towards prod-
uct 5a were detected at moderate conversions. Different pub-
lications have been focused on hydrogenation of pyrene
obtaining mixtures of products and high temperatures were
required in order to obtain good conversions.16,18 Using Pt
nanoparticles supported on carbon nanotubes, total selectiv-
ity towards product 5a was achieved but the best conversion
was only 7%.26a
Fig. 3 Monitoring of the catalytic hydrogenation of triphenylene (4).
Conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), solvent = THF, T
= 30 °C, P = 20 bar of H2).
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The results obtained in the hydrogenation of PAHs 1–5
leading to excellent selectivities (up to 93%) towards product
6a although conversions were low (Table 4, entries 2 and 3).
Interestingly, when MTBE was used as solvent (Table 4, entry
4), conversion up to 35% and a selectivity of 91% towards 6a
were obtained. In order to increase the conversion, the reac-
tion was performed for 16 h but the conversion was still mod-
erate (52%) and the selectivities were comparable to the ones
obtained using THF as solvent (Table 4, entry 5).
When the reaction was performed in a lower substrate/cat-
alyst ratio (0.62 mmol of the substrate) with MTBE as the sol-
vent, the selectivity was still good (81%) and the conversion
increased to 100% (Table 4, entry 6). Driving the reaction in
THF under similar reaction conditions (Table 4, entry 7), the
selectivity towards 6a remained unchanged (83%) but the
conversion reached 91% within only 2.5 hours (6 times
shorter than using MTBE). From these assays THF was
selected as the solvent.
show that high selectivities towards different partially hydro-
genated products can be obtained in all cases except for
phenanthrene (3), and forcing the reaction conditions or
increasing the reaction time, fully hydrogenated products can
also be obtained in some of the substrates, namely, naphtha-
lene (1) and anthracene (2).
As discussed above, the catalytic hydrogenation of naph-
thalene has been the most studied system among PAHs.
However, to the best of our knowledge, there are no studies
dealing with the influence of ring substituents on the selec-
tivity of arene reduction. We have shown that good results in
terms of activity and selectivity can be obtained in the reduc-
tion of naphthalene using ruthenium nanoparticles (Table 1).
For this reason, it was considered interesting to study substi-
tution effects on a naphthalenic system, considering the
nature of the substituent and the position.
Next, we reduced the pressure aiming at enhancing the
selectivity. When the reaction was performed under 10 bar of
H2 for 2.5 hours, the selectivity was found similar but the
conversion dropped to 11% (Table 4, entry 8).
Catalysis: hydrogenation of substituted naphthalenes vs.
other functionalities
As mentioned above, few examples are reported to be related
to the effect of substitution on the selectivity of polyarene
hydrogenation, as well as to the selective reduction of poly-
arenes vs. other functional groups.35 In order to gain infor-
mation about these two aspects, reduction of substituted
naphthalenes was studied. Different substitutions were con-
sidered, namely, substitution at positions α (position 1) and
β (position 2), donor and acceptor substituents, and substitu-
ents that could be competitively reduced.
Unexpectedly, when substrate 7 containing a methyl group
instead of a methoxy group was hydrogenated, a very low con-
version (6%) was achieved after 16 h, although the selectivity
(79%) towards product 7a was similar to that obtained in the
previous examples (Table 4, entry 9). Moreover, when sub-
strate 8 containing an ester group was hydrogenated, no con-
version was obtained indicating that, in the presence of an
electron-withdrawing group, the reaction slowed down
(Table 4, entry 10).
Initially, 2-methoxynaphthalene 6 was used as a model
substrate. When the reaction was performed in THF at 30 °C,
20 bar for 2.5 hours, a conversion of 31% and a selectivity of
83% in compound 6a were obtained (Table 4, entry 1). Hydro-
genation of the less substituted ring was mainly produced.
The reaction was also carried out in pentane and ethanol
In conclusion, the introduction of substituents at position
2 of naphthalene slows down the reaction compared with
that of unsubstituted naphthalene, and the reduction prefera-
bly takes place on the non-substituted ring. The observed
selectivity cannot be strictly related to the donor or acceptor
abilities of the substituents, since when a weak donor
Table 4 Hydrogenation of 2-substituted naphthalenesa
E.
Subs.
Amount (mmol)
Solvent (ml)
P (bar)
Time (h)
Conv.b (%)
TONc
%ab
%bb
%cb
1
2
3
4
5
6
7
8
9
10
6
6
6
6
6
6
6
6
7
8
1.24
1.24
1.24
1.24
1.24
0.62
0.62
0.62
0.62
0.62
THF (10)
20
20
20
20
20
20
20
10
20
20
2.5
2.5
2.5
2.5
16
31
14
18
35
52
100
91
11
6
24
11
14
27
20
39
35
4
83
93
91
91
89
81
83
83
79
—
11
3
6
5
6
6
4
3
4
5
13
6
5
7
Pentane (10)
EtOH (10)
MTBE (10)
MTBE (10)
MTBE (10)
THF (10)
THF (10)
THF (10)
THF (10)
16
6
2.5
2.5
16
11
12
14
—
2
0
16
0
—
a
b
c
General conditions: Ru-NPs (2 mol%), T = 30 °C. Determined by GC. TON was defined as the number of moles of the substrate converted
per mole of surface Ru.
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substituent such as Me is present, conversion is really low, as
well as when there is an acceptor group such as an ester. A
possible explanation can be related to the coordination of the
heteroatoms with the nanoparticle surface. Thus, while the
oxygen of the substituted arene in 6 may interact with the
metal surface upon approaching the nanoparticle, this inter-
action through the carbonyl group in the case of the ester
function of 8 will probably leave the arene far away from the
surface which could explain the lack of reactivity. The methyl
group will only provide a steric hindrance as the arene
approaches the surface.
By comparing Tables 4 and 5, it can be deduced that the
position of the substituent has more influence on the conver-
sion than on the selectivity. When the substituent is at posi-
tion 1, conversions are lower probably due to the higher ste-
ric hindrance and the consequent difficulty for the substrate
to approach the nanoparticle surface. Nonetheless, the selec-
tivity is not significantly affected and the arene which does
not contain substituents is also preferably hydrogenated.
These results agree with the necessity for the arene ring to
approach and coordinate with the nanoparticle's surface in
order to be reduced.
The study was continued by reducing naphthalenes
containing a substituent in position 1 (Table 5). When sub-
strate 9 containing a methoxy group was reduced under stan-
dard conditions (Table 5, entry 1), 85% of product 9a was
obtained with 40% of conversion. Running the reaction for
16 hours allowed full conversion but the selectivity was
shifted towards product cis-9c (65%) (Table 5, entry 2). Curi-
ously, no product 9b was observed in this case.
Interestingly, when an electron withdrawing group like –
CF3 is present in position 1 (substrate 10), the selectivity of
the hydrogenation of the more substituted arene relatively
increases leading to compound 10b with 31% selectivity at
moderate conversion (Table 5, entry 4). Changing the solvent
to MTBE (Table 5, entry 5), the conversion decreased to 15%
although the selectivity towards product 10a increased to 85%.
Finally, substrate 11 containing an amine group was
reduced for 16 h at 20 bar leading to a conversion of only
16% (Table 5, entry 6). Despite the long reaction time, total
selectivity towards product 11a was detected, indicating that
only the unsubstituted arene ring was reduced.
Hydrogenation of naphthalenes containing ketones was
then studied (Tables 6 and 7). As expected, when a ketone is
present in the substrate, there is competition between the
reduction of the arene and the reduction of the ketone.29,37
Initially we studied the hydrogenation of compound 12,
which has an acetyl group located at position 2, under stan-
dard reaction conditions leading to full conversion in 2.5 h
(Table 6, entry 1). Three products, 12a–12c, resulting from
the reduction of the less substituted ring (12a), the keto
group (12b) and both the less substituted group and the keto
group (12c) were obtained. The presence of a methyl group at
position 6 in compound 13 placed both rings with the same
substitution pattern. The increase in substitution in 13
results in a decrease in conversion and hydrogenation of the
A ring, and hence a preferred reduction of the keto group
(Table 6, entry 2). In this case, full reduction of the aromatic
rings was not achieved.
Next, we studied the hydrogenation under standard condi-
tions of compound 14, in which the acetyl group is situated
in position 1. Full conversion was observed and a complex
mixture was produced (Table 7, entry 1). The previous obser-
vation that the substitution at position 1 has a negative effect
on the arene reduction translates in this case into a higher
relative percentage of ketone reduction, compared with com-
pound 12. However, it is also worth noting that small per-
centages of product 14e resulting from the reduction of the
more substituted ring (B), or the presence of fully reduced
product 14f, were observed. These facts indicate that the
In conclusion, the selectivity is affected when an electron
donating group or an electron withdrawing group is present.
The presence of an electron-withdrawing group slightly
favours the reduction of the more substituted ring. When an
amine group is present in the substrate (11), the conversion
decreases considerably, even after several hours of reaction.
This result is in agreement with those reported in the
bibliography.36
Table 5 Hydrogenation of 1-substituted naphthalenesa
E.
Subs.
Amount (mmol)
Solvent (ml)
Time (h)
Conv.b (%)
TONc
%ab
%bb
%cb
1
2
3
4
5
9
9
10
10
11
0.62
0.62
0.62
0.62
0.62
THF (10)
THF (10)
THF (10)
MTBE (10)
THF (10)
2.5
16
2.5
2.5
16
40
100
45
15
16
15
39
17
6
85
35
63
85
100
11
—
31
15
—
4
65
6
—
—
6
a
b
c
General conditions: Ru-NPs (2 mol%), P = 20 bar of H2, T = 30 °C. Determined by GC. TON was defined as the number of moles of the
substrate converted per mole of surface Ru.
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Table 6 Hydrogenation of 2-ketonaphthalenes with Ru-NPsa
E.
Subs.
Time (h)
Conv.b (%)
TONc
%ab
%bb
%cb
1
2
12
13
2.5
2.5
100
44
39
17
52
16
26
84
23
—
a
b
c
General conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), P = 20 bar of H2, T = 30 °C. Determined by GC. TON was defined as the
number of moles of the substrate converted per mole of surface Ru.
Table 7 Hydrogenation of 1-acetonaphthone with Ru-NPsa
E.
P (bar)
Time (h)
Conv.b (%)
TONc
%ab
%bb
%cb
%db
%eb
%fb
1
2
20
10
2.5
16
100
100
39
39
38
24
—
8
24
8
25
36
9
14
4
10
a
b
c
General conditions: Ru-NPs (2 mol%), substrate (0.62 mmol), P = 20 bar of H2, T = 30 °C. Determined by GC. TON was defined as the
number of moles of the substrate converted per mole of surface Ru.
presence of the keto group at position 1 of ring B increases
the hydrogenation ability of this ring.
PAHs under mild conditions leading to good activities and
selectivities. In general, the reaction rate decreases when the
number of aromatic rings increases. The disposition of the
rings has also an influence on the reaction rate and, for
instance, phenanthrene reacts much slower than anthracene
which is in agreement with its greater difficulty in
approaching the nanoparticle's surface.
This fact was confirmed upon carrying out the reaction at
10 bar of hydrogen pressure. After 16 hours of reaction, a
similar mixture of products was observed. However, now even
the products 14b, e resulting from the exclusive hydrogena-
tion of ring B, which was the more substituted one, were
detected (Table 7, entry 2).
The main results of conversion and selectivity are
presented in Fig. 4 and can be summarized as follows: a)
naphthalene is hydrogenated to tetralin (1a) or decalin (1b),
cis/trans = 84 : 16, just by adjusting the hydrogen pressure. b)
Anthracene can be selectively hydrogenated to compound 2a
(hydrogenation of one external ring) with total selectivity and
44% conversion, or to compound 2b (hydrogenation of both
external rings) with 96% selectivity and full conversion. c)
Triphenylene has 3 equivalent rings and partial selectivity of
hydrogenation is difficult to achieve. Thus, compound 4a
(hydrogenation of one external ring) can be obtained with a
selectivity of 53% and a conversion of 61%, which in spite of
being quite low is one of the best reported in the bibliogra-
phy. The selective reduction of the 3 external rings to give
compound 4c was achieved with 100% selectivity and full
conversion. d) Pyrene and phenanthrene were difficult to
hydrogenate and 88% of selectivity toward compound 5a was
obtained with 44% conversion. e) Complete hydrogenation
From the results observed in Tables 6 and 7, it can be con-
cluded that reduction involves important competition
between the arene and the ketone groups and it is influenced
by the position of the keto group. Thus, when the keto group
is at position 2 (12) reduction of the less substituted aromatic
ring takes place principally, although significant reduction of
the carbonyl group is also observed. If the keto group is at
position 1 (14), the most relevant observation is the fact that
the most substituted ring is also reduced. The fact that
electron-withdrawing groups activate the hydrogenation of
the neighbouring ring was already observed in the case of a
trifluoromethyl derivative, compound 10.
Conclusions
In conclusion, ruthenium nanoparticles stabilized by
triphenylphosphine are good catalysts for hydrogenation of
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metal surface rather than the naphthalenic system. If the sys-
tem becomes more hindered like in 13, the ketone is hydro-
genated preferably.
Overall, this study shows evidence of the good catalytic
properties of ruthenium nanoparticles towards arene reduc-
tion even for stable compounds such as substituted PAHs.
Experimental
Reagents and general procedures
All syntheses were performed using standard Schlenk tech-
niques under N2 or Ar atmosphere. Chemicals were pur-
chased from Aldrich Chemical Co, Fluka and Strem. All sol-
vents were distilled over drying reagents and were
deoxygenated before use. The precursor ijRuIJCOD)IJCOT)] was
purchased from Nanomeps. The synthesis of the nano-
particles were performed using 1 L Fischer–Porter and pres-
surized on a high pressure line.
Fig. 4 Main results of conversion and selectivity of PAH reduction
with Ru/PPh3 NPs.
studied under mild conditions was only achieved for naph-
thalene; in the case of anthracene small amounts of the fully
reduced product were detected. f) There are only a few mech-
anisms proposed for PAH hydrogenation. In general, we have
observed that there is competition between kinetic and ther-
modynamic control, which affects the reduction of the less
substituted ring versus preservation of aromaticity. Compare
for instance compounds 2 and 3.
General procedure for the synthesis of Ru-NPs
In a typical procedure, the ijRuIJCOD)IJCOT)] (400 mg, 1268
mmol) was placed into a Fischer–Porter reactor in 400 mL of
dry and deoxygenated THF by freeze–pump–thaw cycles in
the presence of 0.4 equiv. of PPh3. The Fischer–Porter reactor
was then pressurised under 3 bar of H2 and stirred for 24 h
at room temperature. The initial yellow solution became
black after 20 minutes. After elimination of excess dihydro-
gen, a small amount (approx. 5 drops) of the solution was
deposited under an argon atmosphere on a carbon-covered
copper grid for transmission electron microscopy analysis
(TEM). The rest of the solution was concentrated under
reduced pressure to 40 ml. Precipitation and washing with
pentane (3 × 15 ml) were then carried out, obtaining a black
precipitate.
From the study of the chemoselective reduction of
substituted naphthalenes, the following conclusions can be
extracted: a) substitution has an important effect on reactivity
and selectivity. Reactions are slower in substituted naphtha-
lenes, and hydrogenation principally takes place in the ring
that does not contain substituents. b) Selectivity is influenced
by the nature of substituents. Electron donating substituents
deactivate the ring to which they are attached and, conse-
quently, the neighbouring ring is preferably reduced. The
more relevant example is the case of compound 11. Electron-
withdrawing substituents activate the ring. Then, although
the effect of substitution predominates and reduction of the
less substituted ring is mainly produced, an appreciable
extent of reduction of the more substituted ring is observed.
See for instance compounds 10 and 14. c) These effects are
not in general, and comparing the results obtained with com-
pounds 6–8 it can be observed that the best results are
obtained with compound 6, which has an electron donor sub-
stituent, while the presence of a carboxymethyl group in
8 clearly deactivates the reaction. Probably, it is necessary to
consider in 6 the effect of the coordination of the oxygen
atom with the nanoparticle that will approach the arene to
the NP surface, while the interaction with the carbonyl oxy-
gen of the ester group in 8 will put the aromatic ring away
from the NP. d) The case of the ketone derivatives is particu-
lar since both arene and carbonyl groups are reduced. Thus,
when a ketone is present in the substrate like in 12, 13 and
14, there is competition between the reduction of the
naphthalenic system and that of the ketone. If the ketone is
situated in position 1 like in 14, its reduction is favoured
probably because the ketone coordinates preferably with the
General procedure for the hydrogenation reactions
In a typical experiment, a 5-entry autoclave or an autoclave
Par 477 equipped with PID control temperature and a reser-
voir for kinetic measurements were charged in a glove-box
with 3 mg of Ru nanoparticles (the catalyst concentration
was calculated based on the total number of metallic atoms
in the NPs) and the substrate in 10 mL of solvent. Molecular
hydrogen was then introduced until the desired pressure was
reached. The reaction was stirred during the corresponding
time at the desired temperature. The autoclave was then
depressurised. The solution was filtered over silica and
analysed by gas chromatography. The conversion and the
selectivities towards the product were determined using a
Fisons instrument (GC 9000 series) equipped with a HP-5MS
column.
Conversion and selectivity were determined by GC-MS and
cis/trans selectivity was confirmed by NOE experiments in
NMR. GC-MS spectroscopy was carried out using a HP 6890A
spectrometer, with an HP-5 column (0.25 mm × 30 m × 0.25 μm).
The method used for the polyaromatic systems consisted of
an initial isotherm period at 130 °C for 10 min followed by a
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10 °C min−1 temperature ramp to 180 °C with a hold time of
35 min and a flow rate of 3.5 ml min−1.
2005, 39, 3374; (b) M. J. García-Martínez, I. D. Riva, L.
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The method used for the substituted naphthalenes
consisted of an initial isotherm period at 40 °C for 3 min
followed by a 3 °C min−1 temperature ramp to 120 °C with a
hold time of 12 min and a flow rate of 1.3 ml min−1.
The retention times for the main products detected for
each substrate are detailed below:
- Substrate 1: tr1 = 2.03 min, tr1a = 1.83 min, tr1b = 1.58
min, tr1c = 1.41 min.
- Substrate 2: tr2 = 14.91 min, tr2a = 14.22 min, tr2b =
13.15 min, tr2c = 10.73 min, tr2d = 12.76 min, tr2e = 8.37
min.
- Substrate 3: tr3 = 14.72 min, tr3a = 13.04 min, tr3b =
14.31 min, tr3c = 13.71 min.
- Substrate 4: tr4 = 46.46 min, tr4a = 44.62 min, tr4b =
41.16 min, tr4c = 36.74 min, tr4d = 20.58 min, tr4e = 21.90
min.
- Substrate 5: tr5 = 22.80 min, tr5a = 20.80 min, tr5b =
18.27 min.
- Substrate 6: tr6 = 5.56 min, tr6a = 4.91 min, tr6b = 3.68
min, tr6c = 3.02 min.
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- Substrate 7: tr7 = 27.39 min, tr7a = 25.96 min, tr7b =
23.70 min, tr7c = 20.51 min.
- Substrate 9: tr9 = 5.46 min, tr9a = 4.55 min, tr9b = 3.39
min, tr9c = 2.90 min.
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- Substrate 11: tr11 = 7.94 min, tr11a = 6.41 min.
- Substrate 12: tr12a = 11.56 min, tr12b = 10.79 min, tr12c
= 9.21 min.
- Substrate 13: tr13 = 14.06 min, tr13a = 12.55 min, tr13b =
13.61 min.
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Acknowledgements
We are grateful to the Spanish Ministerio de Economía y
Competitividad (CTQ-2011-22872) for financial support. EB
thanks Ministerio de Educación Cultura y Deporte for a grant.
We also thank Serveis de Recursos Científics (URV) for their
support. BC thanks INSA, CNRS.
The authors also acknowledge financial support from
CNRS and the European Union for ERC Advanced Grant
(NANOSONWINGS 2009-246763).
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