Concurrent Hydrogenation of Aromatic and Nitro Groups over Carbon-Supported Ruthenium Catalysts

Article abstract of DOI:10.1021/cs501122h

The concurrent hydrogenation of aromatic and nitro groups poses particular challenges due to the highly differing adsorption strengths of the two chemical moieties on the surfaces of metal catalysts. In a study of the hydrogenation of nitrobenzene as a model reaction, catalysts of ruthenium supported on carbon nanotubes (Ru/CNT) provided an ideal compromise, allowing for hydrogenation of both the aromatic ring and the nitro group. The use of methyl-labeled substrates enabled tracking the pathway of specific substrates and obtaining insight into the relative rates for the hydrogenation of nitrobenzene and intermediates. Together with findings on the coadsorption of nitrobenzene and aniline on the Ru/CNT catalyst, an advanced mechanistic model for the hydrogenation of nitrobenzene emerges. (Chemical Equation Presented).

Full text of DOI:10.1021/cs501122h

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Article
Concurrent hydrogenation of aromatic and nitro-
groups over carbon-supported ruthenium catalysts
Patrick Tomkins, Ewa Gebauer-Henke, Walter Leitner, and Thomas Ernst Mueller
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs501122h • Publication Date (Web): 18 Nov 2014
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ACS Catalysis
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Concurrent hydrogenation of aromatic and nitro-groups
over carbon-supported ruthenium catalysts
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Patrick Tomkins,† Ewa Gebauer-Henke,† Walter Leitner,‡ Thomas E. Müller*,†
† CAT Catalytic Center, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany.
‡ Institut für Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany.
KEYWORDS: Nitrobenzene hydrogenation, Amine synthesis, Heterogeneous catalysis, Reaction network, Competitive
adsorption, Ru/CNT catalyst
ABSTRACT: The concurrent hydrogenation of aromatic and nitro-groups poses particular challenges due to the highly
differing adsorption strength of the two chemical moieties on the surface of metal catalysts. Studying the hydrogenation
of nitrobenzene as model reaction, catalysts of ruthenium supported on carbon nanotubes (Ru/CNT) provided an ideal
compromise allowing for both hydrogenation of the aromatic ring and the nitro-group. The use of methyl-labeled sub-
strates enabled tracking the pathway of specific substrates and obtaining insight into the relative rates for the hydrogena-
tion of nitrobenzene and intermediates. Together with findings on the co-adsorption of nitrobenzene and aniline on the
Ru/CNT catalyst, an advanced mechanistic model for the hydrogenation of nitrobenzene emerges.
pathway is ruled by the propensity of the particular group
Introduction
to be chemisorbed on the metal surface.7,19-23
Many important fine chemicals,1 agrochemicals,2,3 phar-
4
maceuticals and polymer building blocks are character-
ized by the presence of multiple functional groups. Selec-
tive hydrogenation of corresponding precursor molecules
bearing several unsaturated moieties is frequently applied
to generate these functional groups.5-10 With heterogene-
ous catalysts, the presence of a strongly coordinating
functional group poses, however, particular challenges for
the hydrogenation of more weakly adsorbing groups.
Further complexity arises when starting materials and
intermediates compete with each other for adsorption on
the metal surface.
An industrially relevant example is the hydrogenation of
nitroaromatic compounds to the corresponding cycloali-
phatic primary amines.11-13 In such nitroaromatic com-
pounds, the aromatic ring is electron-deficient due to the
strong electron-withdrawing effect of the nitro-group.14
Consequently, the aromatic ring coordinates only weakly
to metals typically employed in hydrogenation reactions.15
By contrast, the nitro-group, is strongly coordinating.16
Due to the competing adsorption modes,17 the nitro-
group has a high propensity to being hydrogenated first
(Scheme 1, Path A/B). However, also the aromatic moiety
can also be hydrogenated first (Path C/D), as reported for
substrates where aromatic ring and nitro-group are not
directly linked.18 Conceivable is also a direct hydrogena-
tion pathway that does not involve desorption of an in-
termediate from the catalyst surface. When a Langmuir
Hinshelwood-type mechanism applies, the prevailing
Scheme 1. Analysis of possible reaction pathways for the
hydrogenation of nitrobenzene and methyl-substituted ana-
logues; FG functional group.
The aromatic amino-intermediate (here: aniline) formed
during Path A/B is very different in character compared
to the original nitroaromatic compound. The aromatic
ring is electron-rich due to the electron-donating me-
someric effect of the amino-group.24 Consequently, the
aromatic ring is expected to coordinate more strongly to
the metal compared to the nitroaromatic compounds. Yet
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also the amino-group bearing a lone electron pair strongly 120 °C for 2 h. Subsequently, the ruthenium precursor was
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localized on nitrogen tends to coordinate strongly to
metal surfaces. Even more pronounced, the amino-group
in the fully hydrogenated cyclohexylamine is strongly
basic. Consequently, the amino-groups compete with the
aromatic rings and the nitro-groups for adsorption on the
coordination sites giving rise to potential product inhibi-
tion.25
reduced in a flow of hydrogen (100 mL/min) at 200 °C for
1 h.
Catalyst characterization
Prior to the hydrogenation experiments, the catalysts
were characterized in detail. The data are summarized in
the supplementary data.†
Hydrogenation experiments
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An ideal catalyst for the hydrogenation of such nitroaro-
matic compounds ought to be equally active for all moie-
ties to be hydrogenated, while non-productive adsorption
modes should have a low probability of occurring. Ac-
cordingly, all moieties to be hydrogenated should adsorb
with comparable strength to the catalyst surface. Vice
versa, the binding constant for the saturated product(s)
should be low to avoid product inhibition.
Hydrogenation reactions were carried out in a 200 mL
stainless steel autoclave equipped with gas entrainment
stirrer, heating mantel and sampling valve. The autoclave
was charged with substrate, THF, catalyst and internal
standard (dodecane in case of NB hydrogenation, tetrade-
cane when methyl-substituted substrates were used).†
The mixture was heated to the reaction temperature
(140 °C, if not stated otherwise), and the reaction was
started by pressurizing the autoclave with hydrogen to
100 bar. Samples of the liquid phase were taken during
the reaction for analysis by gas chromatography. Concen-
trations are given as the molar fraction of the particular
substance i normalized to the initial concentration of the
substrate (ci/csubstrate, t=0 ∙ 100 mol%). Rates of reaction (ri)
were calculated at 50 % of the maximum concentration of
compound i by fitting the time-concentration diagrams
with a five parameter logistic function.32
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In the synthesis of primary amines, an additional chal-
lenge arises from condensation reactions,26-29 that cause
reduced yields through formation of secondary and ter-
tiary amines,30 azobenzene and other coupling products.31
In this study, we have explored the hydrogenation of
nitrobenzene as a model system for other nitroaromatic
compounds. In an exploratory study on the choice of the
metal, ruthenium emerged as an ideal candidate. The
relative rates for the hydrogenation of nitrobenzene and
intermediates were followed in a co-hydrogenation study,
whereby the path of individual substrates was traced by
labeling certain precursors with a methyl-substituent.
This enables following the reaction pathways that lead to
condensation products and recognizing key factors that
control chemoselectivity.
Results and discussion
In an exploratory study, the most suitable metal for the
hydrogenation of nitrobenzene (NB) was explored (Table
1). Carbon was chosen as a chemically quite inert support
to skirt condensation reactions known for more acidic
oxidic supports.33-35 The catalysts Ru/C, Rh/C, Pd/C and
Pt/C showed similar activity with respect to the hydro-
genation of NB leading to 95% conversion of NB within
19-28 min. As in related hydrogenation reactions,36 two
different classes of catalysts emerged with respect to the
chemoselectivity. Ruthenium and rhodium provided cy-
clohexylamine (CA) in high selectivity. Only a low
amount of the condensation product dicyclohexylamine
(DA) was formed. By contrast, palladium and platinum
provided aniline (AN) as the main product. Thus, only
ruthenium and rhodium fulfilled the stated requirements
of a catalyst concerning high propensity for hydrogena-
tion of the aromatic ring.37,38 Ruthenium showed a higher
selectivity to the targeted primary amine, whereas a sig-
nificantly higher amount of the condensation product DA
was formed over rhodium. Therefore, ruthenium was
chosen for more detailed study.
Experimental
Materials
All chemicals were obtained from commercial suppliers
and used as received. The carbon-supported catalysts
Ru/C, Rh/C, Pd/C and Pt/C (5 wt% metal) were obtained
from Aldrich. Multiwall carbon nanotubes (CNT) from a
chemical vapor deposition process (BAYTUBES C 150 P,
Bayer MaterialScience) were used as support for the
Ru/CNT catalyst. The CNT had an average length in the
range of 1-10 µm and a mean outer diameter of 13-16 nm.
Irregularly shaped CNT were aggregated to loose agglom-
erates with 1-3 mm diameter.
Catalyst preparation
The Ru/CNT catalyst was prepared by the deposition
precipitation method. For this, CNT (20 g) were suspend-
ed in refluxing nitric acid (65 %, 150 mL) for 2 h. Subse-
quently, the CNT were filtered off, washed with deionized
water until the eluent had a neutral pH-value and dried.
The treated CNT (10.045 g) were re-suspended in an
aqueous solution (300 mL) of urea (1.462 g, 24.3 mmol),
Ru(NO)(NO3)x(OH)y (aqueous solution, ~1.5 wt% Ru) and
nitric acid (32.4288 g). The mixture was stirred under Ar
at 90 °C for 22 h. The CNT were filtered off, washed with
little water and dried in a flow of argon (100 mL/min) at
Table 1. Hydrogenation of nitrobenzene with carbon-
supported catalysts.
a
b
b
b
b
Catalyst
t95
SAN
SCA
SDA
SPC
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ACS Catalysis
the hydrogenation of NB commenced, and AN was
[min]
24
[%]
1.6
[%]
90.2
85.3
15.1
[%]
8.2
[%]
0
1
2
3
4
5
6
7
8
formed as the initial product. Notably, with Ru/CNT full
conversion was obtained more rapidly (12 min to >95 %
conversion) than with Ru/C (27 min). The AN concentra-
tion reached a maximum (64 mol%) after 9 min and de-
creased thereafter. With a time delay of 6 min, CA was
formed. After another 3 min, also the formation of DA
commenced. Hence, a consecutive reaction according to
Path A/B (Scheme 1) prevailed. This was verified by plot-
ting the ratios of the concentrations of NB/CA, NB/AN,
AN/CA and of AN/DA vs. time.42 In all cases, the ratio
extrapolated to zero time increased to infinity.† From this,
it was inferred that AN is a consecutive product of NB
(Scheme 2, Step A) and that CA and DA are consecutive
products of AN (Step B and E, respectively). The ratio of
CA/DA decreased rapidly during the reaction to level out
at a value of about 20 implying that DA is a consecutive
product of CA (Step E or F).† Closer inspection of the GC
chromatograms revealed that traces of nitrosobenzene
were formed as intermediate and that a trace amount (<1
mol%) of cyclohexanol was obtained as side-product.
Other intermediates and side-products mentioned in the
literature were not detected under our conditions.37,43,44
Ru/C
Rh/C
Pd/C
28
1.2
13.6
17.7
13.6
0
19
36.5
87.2
1.5
0.4
Pt/C
21
11.2
a
Time until 95 % conversion of nitrobenzene had been
achieved; b Selectivity after 180 min, full conversion of NB
obtained in all cases; aniline (AN), cyclohexylamine (CA),
dicyclohexylamine (DA), phenylcyclohexylamine (PC).
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Catalyst synthesis
Being a well-defined carbon support, carbon nanotubes
(CNT) were chosen.39 To anchor the ruthenium particles
on the surface of the CNT, oxidic groups were generated
by treatment in refluxing nitric acid.40 In the next step, a
ruthenium precursor was placed evenly on the surface of
the CNT by the deposition-precipitation method,41 which
was followed by reducing the precursor to metallic ruthe-
nium with molecular hydrogen. The Ru/CNT catalyst thus
obtained had a ruthenium content of 3.6 wt%. Transmis-
sion electron microscopy (TEM) measurements showed
that small ruthenium particles with a mean diameter of
1.5 nm and a standard deviation of 0.3 nm were evenly
distributed over the outer surface of the carbon nano-
tubes, giving rise to a BET surface area of 210 m2—g-1
(Figure 1). The small diameter of the Ru particles and the
absence of larger Ru crystallites were confirmed by H2-
chemisorption. Macropores formed through aggregation
of the irregularly shaped CNT to agglomerates with 1-3
mm diameter enabled rapid intraparticle transport of the
reactants.
100
80
NB
60
AN
CA
DA
40
20
0
0
30
60 90
Time [min]
120
150
180
Figure 2. Time-concentration diagram for the hydrogenation
of nitrobenzene (NB) over Ru/CNT (NB/Ru=160) to interme-
diary formed aniline (AN) and final products cyclohexyla-
mine (CA) and dicyclohexylamine (DA).
NH2
NO2
NH2
A
B
+3H2; ꢀ2H2O
+3H2
R
R
R
NB/NB*
AN/AN*
CA/CA*
+3H2
ꢀNH3
x2
ꢀNH3
Figure 1. Particle-size distribution of Ru/CNT used in this
study and representative scanning and bright field transmis-
sion electron microscopy image (insert and right, respective-
ly).
E
F
H
N
G
+AN/3H2 or CA;
+6H2; ꢀ2H2O; ꢀNH3
Reaction sequence of nitrobenzene hydrogenation
over Ru/CNT
R
RDA/DA*/DA**
To obtain insight into the reaction sequence, the hydro-
genation of NB over Ru/CNT was followed with time
(Figure 2). Immediately after pressurizing the autoclave,
Scheme 2. Analysis of possible reaction steps for the for-
mation of condensation products during the hydrogenation
of NB to CA.
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Coꢀhydrogenation of nitrobenzene and aniline
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40
20
0
To investigate the relative rates of the hydrogenation of
nitro-compounds and aromatic amines, an equimolar
mixture of NB and AN was hydrogenated (Figure 3). In
order to trace the pathway of the different substrate mol-
ecules, the nitrobenzene was labeled with a methyl sub-
stituent. An initial experiment on the hydrogenation of
methyl-nitrobenzene (NB*) provided a very similar profile
and showed only slightly enhanced rates compared to the
hydrogenation of NB. This is consistent with the some-
what higher electron density in the aromatic ring of NB*
caused by the inductive effect of the methyl group.
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AN
CHA
NB*
AN*
CA*
DA*
DA
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DA**
Table 2. Normalized rates for the hydrogenation of nitro-
benzene and corresponding reference reactions.
0
30 60 90 120 150 180
Time [min]
Rate [mol.(molRu.min)-1] b
Figure 3. Time-concentration diagram for the co-
hydrogenation of NB* and an equimolar amount of AN over
Ru/CNT (NB*/Ru=630). The amount of catalyst was adjusted
in such a way that comparable reaction times were achieved.
Reagents a
NB/–/–
NB
NB*
-
AN
-13.4
-
AN*
-
CA
13.1
-
CA*
-
-20.4
NB*/–/–
-
-
-
-
-
-
-22.9
-14.6
-4.0
13.1
4.0
0.8
2.9
13.6
-
Inspection of the concentration profiles of secondary
amine formation shows that there was only a small
amount of secondary amines formed as long as NB* was
present (<50 min). Thereafter DA and DA* were formed
until the corresponding aniline depleted, indicating that
the aromatic amine plays a key role in the formation of
secondary amines (Step E).
NB*/AN/–c
NB*/AN/–d
NB*/–/CA
–/AN*/CA
–/AN/–
-71.2 -7.3
7.3
34.2
-
-3.3
-35.6 -1.0
-72.4
-
-2.8
-15.0
-
-
-
-
-
-13.5
13.0
a Substrates labeled with a methyl substituent are marked
with an asterisk; b rates at 50% of the maximum concentra-
Surprisingly, the rate of NB* conversion was about three
times higher, when co-hydrogenated in the presence of
AN, compared to the hydrogenation of NB* as the sole
substrate. This goes along with an enhanced rate of for-
mation of CA and CA* during the first 30 min. Once NB*
had been consumed, also the formation of CA* from AN*
slowed down, thereby indicating that the surface coverage
with substrate molecules changes at this stage of the reac-
tion. Once a new steady-state had been established on the
catalyst surface, the reaction rate increased again. These
findings clearly show that the rate of nitroarene hydro-
genation can be increased by the addition of aromatic
amine to the initial reaction mixture. Vice versa, the rate
of aniline hydrogenation is also influenced albeit to a
smaller extend.
c
tion of the corresponding compound; initial ratio NB*/AN
1/1; d initial ratio NB*/AN 1/19.
During the co-hydrogenation of NB* and AN (Figure 3)
NB* was converted rapidly (-71.2 mol.(molRu.min)-1, Table
2) to para-toluidine (AN*) and was consumed entirely
within 30 min. In contrast, AN-hydrogenation com-
menced only after a short lag phase (6 min) with a rela-
tively low rate (-7.3 mol.(molRu.min)-1). Unexpectedly, the
consumption of AN slowed down even further after ap-
proximately 20 min. The period of slow AN-conversion
(ca. 20 min) was characterized by a high concentration of
AN* of ca. 45 mol%. Thereafter, the consumption of AN
as well as of AN* resumed with the concentration of AN
decreasing twice as fast as that of AN* (-7.3 and -4.0
mol.(molRu.min)-1, respectively). The two primary aliphatic
amines 4-methyl-cyclohexylamine (CA*) and CA were
obtained as the main products in a ratio of 47/53.
To corroborate that the presence of nitro-compounds
leads to an increase in the rate of aniline hydrogenation,
NB* and AN were co-hydrogenated also in a molar ratio
of 1/19 (Figure 4). After an initiation period of 6 min, AN
was hydrogenated to CA. The rate of AN consumption (-
35.6 mol.(molRu.min)-1) was enhanced significantly com-
pared to the hydrogenation of AN as the sole substrate (-
13.5 mol.(molRu.min)-1, Table 2). Once the last traces of
nitro-compound had been consumed (after 30 min) the
rate of AN-hydrogenation slowed down. Clearly, the pres-
ence of a nitro-compound had a promoting effect on
aniline hydrogenation.
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60
100
80
60
40
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0
CA
NB*
AN*
CA*
DA*
DA**
DA
AN
CA
DA
NB*
AN*
CA*
DA*
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2
0
40
20
0
0
30
60
0
30 60 90 120 150 180
Time [min]
0
30
60
90
Time [min]
120
150
180
Figure 5. Time-concentration diagram for the hydrogenation
of NB* in the presence of an equimolar amount of CA over
Ru/CNT (NB*/Ru=630).
Figure 4. Time-concentration diagram for the co-
hydrogenation of NB* and AN (molar ratio 1/19) over
Ru/CNT (AN/Ru=630). The inset shows an enlarged repre-
sentation of the profiles for the labeled compounds.
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The role played by the aromatic and aliphatic amine in
the formation of secondary amines (Steps E and F) was,
thus, explored in the next step. For this, AN* was hydro-
genated in the presence of an equimolar amount of CA
(Figure 6). After a short lag phase, the concentration of
AN* decreased exponentially. In parallel, CA* was formed
as main product. By contrast, the concentration of CA
remained nearly constant. Closer inspection of the profile
of formation of secondary amines revealed that the con-
centration of DA* and DA** increased during the initial
phase of the reaction. After 30 min the concentration of
the two secondary amines leveled off at 3 and 1 mol%,
respectively. The concentration of DA remained insignifi-
cant over the entire time-range.
Pathways to the formation of secondary amines
Insight into the role of NB in the formation of secondary
amines was obtained by the hydrogenation of NB* in the
presence of an equimolar amount of CA (Figure 5). NB*
was converted at a high rate (-72.4 molNB*.(molRu.min)-1,
Step A) to AN*. In a slow consecutive reaction, AN* was
hydrogenated to CA* (-2.8 molAN*.(molRu.min)-1, Step B),
while the concentration of CA remained nearly constant.
Thus, the direct condensation of primary to secondary
amines is of minor importance (Step F). While NB* was
hydrogenated (<20 min), there was hardly any formation
of secondary amines, showing that the nitro-aromatic
compound did not participate in the formation of sec-
ondary amines (Step G). At longer reaction times, DA*
and DA** were formed in parallel once an increasing
concentration of CA* became available in the reaction
mixture.† Thus, DA*, DA** and CA* are consecutive
products of AN*, but formation of DA* and DA** also
requires the presence of CA and/or CA* (Step E). Also the
absence of aromatic secondary amines suggests that sec-
ondary amines are formed from partially hydrogenated
AN derivatives and/or fully hydrogenated amines.
In the initial phase of the reaction, DA* and DA** were
formed in a ratio exceeding 4:1, which decreased to ap-
proximately 2:1 at longer reaction times. This confirms
that most of the secondary amine is formed by reaction of
AN* with the aliphatic amine (CA or CA*) present in the
reaction mixture (Step E). One possibility is the reaction
of adsorbed AN* or of a consecutive partially hydrogenat-
ed surface intermediate
37
with co-adsorbed aliphatic
amine. In contrast, a prevailing pathway via condensation
of two surface intermediates resulting from AN* would
result in the exclusive formation of DA**. The low
amount of DA formed corroborates that also the direct
condensation of CA was insignificant (Step F). Only at
higher temperatures (180 °C), the condensation of CA to
DA became significant once AN had been converted fully.
Moreover, condensation products with one aromatic and
one aliphatic ring were not detected indicating that the
direct condensation of aniline and primary amines is
insignificant.
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hydrogenated in the presence of a small amount of nitro
1
2
3
4
5
6
7
8
aromatics, the rate of AN hydrogenation is enhanced.
Also in this case, we propose that NB and AN form a hy-
drogen-bonded adduct on the surface of the catalyst. In
this context, we found recently that the addition of
NaNO2 in the hydrogenation of toluidines results in an
increased rate,53 most likely due to formation of an adduct
of nitrite and aromatic amine on the catalyst surface.11,54
60
40
20
0
CA
AN*
CA*
DA*
DA**
DA
The main condensation reaction (key findings III - V)
most likely involves reduction of AN to the enamine.
Later tautomerises to the corresponding imine, which is
susceptible to nucleophilic attack by CA. Nitrobenzene
stabilizes the enamine by formation of a hydrogen bond-
ed aggregate on the catalyst surface (Scheme 3), thereby
reducing the probability that the condensation reaction
occurs.
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0
30
60
90
Time [min]
120
150
180
O
The nitro group
N
is reduced first.
Figure 6. Hydrogenation of AN* in the presence of an
equimolar amount of CA over Ru/CNT (AN*/Ru=630).
O
H
Aniline is formed
N
as key intermediate
H
Kinetic and mechanistic model
Free intermediates are prone to
condensation reactions
O
O
H
H
The key findings of the preceding experiments allow elu-
cidating the reaction pathways to formation of cyclohex-
ylamine and secondary amines:
H
N
N
N
Ruꢀsurface
NH2
Formation of adducts f acilitates
coordination of the aromatic ring(s)
to the ruthenium surf ace
I
The hydrogenation of NB to CA proceeds via AN
as intermediate (Path A/B), while the pathway via nitro-
cyclohexane (Path C/D) and the direct hydrogenation are
insignificant (Scheme 1).
O
O
H
H
N
N
Strong adsorption of inter-
mediates reduces the propensity
to condensation reactions
Ruꢀsurface
II
In the presence of an aromatic amine (AN) at the
H
H
N
initial stage of the reaction, NB is hydrogenated at a con-
siderably higher rate.
This adduct formation gives rise to higher rates
and strongly enhanced selectivity to primary amines
III
NB is hydrogenated quite selectively to AN (Step
Scheme 3. Adduct proposed to be formed during the hydro-
genation of nitrobenzene and stabilization of the intermedi-
ate enamine against nucleophilic attack of an amine on an
imine intermediate.
A) and does not contribute to the formation of DA
(Scheme 2, Step G).
IV
The predominant condensation reaction com-
prises AN as well as CA (Step E). Most likely, a partially
hydrogenated AN derivative reacts on the ruthenium
surface with CA.
Adsorption measurements
To clarify the interaction, the competitive adsorption of
NB and AN on the catalyst surface was explored based on
the frontal analysis method.55 For this, a chromatographic
column was filled with Ru/CNT and the breakthrough of
a mixture of NB and AN was followed (Figure 7). The
breakthrough of NB commenced shortly (at 12 min) after
the breakthrough of a non-adsorbing tracer (1,3,5-tri-tert-
butyl-benzene, at 5 min). Quantification of the break-
through curve revealed that one NB molecule was ad-
sorbed for every 8.6 Ru atoms in the Ru/CNT catalyst.
Interestingly, the concentration of NB then went through
a maximum (at 134 min) to decrease thereafter to the
steady state concentration. Such maximum in the NB
concentration clearly demonstrates that NB and AN ad-
sorb on the same sites. Quantification showed that AN
from the feed displaced 37% of the already adsorbed NB
(roll-over effect).55 Thus, a roughly equimolar amount of
NB and AN is coadsorbed on the catalyst surface fully
V
Although feasible, the condensation of two CA
molecules occurs at a relatively low rate (Step F).
The reaction pathway NB → AN → CA (key finding I) is
consistent with a strong preference of nitrobenzene to
coordinate via the nitro-group and not via the electron-
deficient aromatic ring. In contrast, aniline has a suffi-
ciently high propensity to coordinate via the (relatively
electron-rich) aromatic ring, which allows the ring to be
hydrogenated.
To explain the enhanced reaction rate (key finding II), we
propose that NB and AN co-adsorb on the surface of the
catalyst forming a hydrogen-bonded adduct (Scheme 3).
Such adduct formation is well known for organic nitro-
compounds and aromatic amines.45-52,† The adduct for-
mation between NB and AN allows for faster hydrogena-
tion of the nitro-group. The presence of an aliphatic
amine (CA) has a similar effect. Vice versa, when AN is
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ruthenium surface in such a way that the aromatic ring is
consistent with the formation of hydrogen-bonded ad-
1
2
ducts giving rise to an ideal coverage on the catalyst’s
surface for the hydrogenation reaction to proceed.
enabled to coordinate to the surface.
Hardly any condensation reactions took place as long as
nitrobenzene was present in the reaction mixture. This
results in a strongly enhanced selectivity to primary
amines. Condensation products form only at a later stage
of the reaction. Tracking the pathway of specific sub-
strates revealed that the prevailing condensation path-
way, involves the reaction of partially hydrogenated ani-
line derivatives with cyclohexylamine on the surface of
the ruthenium catalyst.
3
4
5
6
7
8
9
NB adsorbs
AN replaces 37% of adsorbed NB
1.0
0.8
0.6
0.4
0.2
0.0
Tracer
AN
NB
0.10
0.05
0.00
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Such concurrent hydrogenation of multiple functional
groups in selected precursor molecules provides ready
access to multi-functional molecules. Detailed knowledge
of the relative reaction rates in the resulting complex
reaction networks is the basis for the rational design of
the corresponding synthetic strategies.
0.00 0.05 0.10 0.15 0.20 0.25
Solute concentration [mmol/L]
0
200 400 600 800 1000 1200
Time [min]
Author Information
Corresponding Author
Figure 7. Breakthrough curve for competitive adsorption of
AN and NB (0.005 mmol/L) compared to that of a non-
adsorbing tracer (1,3,5-tri-tert-butylbenzene) and isotherms
for the respective adsorption of AN and NB on Ru/CNT (in-
*E-mail: Thomas.mueller@catalyticcenter.rwth-aachen.de
Fax: +492418022593; Tel: +492418028594
.
-1
set, 0.31 mmolRu gcat
)
Notes
Similarly, the adsorption isotherms determined by the
single-solute breakthrough measurements (Figure 7, in-
sert) were characterized by a linear term with approxi-
mately equal slope. Most likely, this linear term of the
isotherms corresponds to adsorption on metallic rutheni-
um. Here, both NB and AN adsorb on the same sites with
a similar adsorption constant. As the breakthrough of AN
commenced much later than that of NB (75 min) and the
adsorption isotherm was characterized by a rapid increase
at low solute concentrations, a large amount of AN must
bind to a second type of adsorption sites. While both NB
and AN may interact weakly with the surface of the CNT
support,56 only the more basic AN readily binds to
Brønsted acid sites generated in the initial acid pretreat-
ment of the CNT.
The authors declare not competing financial interest.
Acknowledgements
We thank Cleopatra Herwartz (GfE) for taking the TEM
images, Xaver Hecht (TUM) for the BET measurements,
Elise Keitel (IME) for the elemental analyses, Julia
Wurlitzer (ITMC) for developing the GC method and
Daniel Hönders for implementing the frontal analysis
method at CAT Catalytic Center. The staff of the mechan-
ical workshop at ITMC is acknowledged for their support.
Supporting Information
Supporting Information. (experimental details, catalyst
characterization, complementary kinetic data, data on
adduct formation) This material is available free of charge
via the Internet at http://pubs.acs.org.
Conclusions
The hydrogenation of nitrobenzene over Ru/CNT was
studied as a model reaction for the concurrent catalytic
reduction of an aromatic ring in the presence of a strongly
coordinating nitro-group. Our exploratory experiments
showed that Ru/CNT is an ideal choice combining high
activity with excellent chemoselectivity to the targeted
primary amine. Ruthenium distinguishes itself by compa-
rable rates for the hydrogenation of the aromatic ring and
of the nitro-group.
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