Effect of the Crystallographic Phase of Ruthenium Nanosponges on Arene and Substituted-Arene Hydrogenation Activity

Article abstract of DOI:10.1002/cctc.201800287

Identifying crystal structure sensitivity of a catalyst for a particular reaction is an important issue in heterogeneous catalysis. In this context, the activity of different phases of ruthenium catalysts for benzene hydrogenation has not yet been investigated. The synthesis of hcp and fcc phases of ruthenium nanosponges by chemical reduction method has been described. Reduction of ruthenium chloride using ammonia borane (AB) and tert-butylamine borane (TBAB) as reducing agents gave ruthenium nanosponge in its hcp phase. On the other hand, reduction using sodium borohydride (SB) afforded ruthenium nanosponge in its fcc phase. The as prepared hcp ruthenium nanosponge was found to be catalytically more active compared to the as prepared fcc ruthenium nanosponge for hydrogenation of benzene. The hcp ruthenium nanosponge was found to be thermally stable and recyclable over several cycles. This self-supported hcp ruthenium nanosponge shows excellent catalytic activity towards hydrogenation of various substituted benzenes. Moreover, the ruthenium nanosponge catalyst was found to bring about selective hydrogenation of aromatic cores of phenols and aryl ethers to the respective alicyclic products without hydrogenolysis of the C?O bond.

Full text of DOI:10.1002/cctc.201800287

Accepted Article
Title: Effect of crystallographic phase of ruthenium nanosponges on
arene and substituted arene hydrogenation activity
Authors: Balaji Rao Jagirdar and Sourav Ghosh
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Effect of crystallographic phase of ruthenium nanosponges on
arene and substituted arene hydrogenation activity
Sourav Ghosh and Balaji R. Jagirdar*
Abstract: Identifying crystal structure sensitivity of a catalyst for a
particular reaction is an important issue in heterogeneous catalysis.
In this context, the activity of different phases of ruthenium catalysts
for benzene hydrogenation has not been investigated as yet. The
synthesis of hcp and fcc phases of ruthenium nanosponges by
chemical reduction method has been described. Reduction of
ruthenium chloride using ammonia borane (AB) and tert-butylamine
borane (TBAB) as reducing agents gave ruthenium nanosponge in
its hcp phase. On the other hand, reduction using sodium
borohydride (SB) afforded ruthenium nanosponge in its fcc phase.
The as prepared hcp ruthenium nanosponge was found to be
catalytically more active compared to the as prepared fcc ruthenium
nanosponge for hydrogenation of benzene. The hcp ruthenium
nanosponge was found to be thermally stable and recyclable over
several cycles. This self-supported hcp ruthenium nanosponge
shows excellent catalytic activity towards hydrogenation of various
substituted benzenes. Moreover, the ruthenium nanosponge catalyst
was found to bring about selective hydrogenation of aromatic cores
of phenols and aryl ethers to the respective alicyclic products without
hydrogenolysis of the C‒O bond.
Hydrogenation of benzene was first accomplished over
finely divided nickel particles by Sabatier and Senderens way
back in the 19th century.[6,7] The field of arene hydrogenation
flourished ever since.[8-10] Selective hydrogenation of substituted
arenes to their respective alicyclic derivatives is one of the most
important reactions carried out in the industry for generation of
various intermediates that are important precursors for the
production of polymers, dyes, and fine chemicals.[11-16]
Industrially, monocyclic arene hydrogenation is typically
performed under harsh conditions (high pressure and high
temperature) using heterogeneous transition metal catalysts,
such as Rh/Al2O3 and Raney nickel.[10,17] In case of metal based
catalysts, ruthenium catalysts are preferred over other precious
metal catalysts, such as rhodium, iridium, and platinum because
of its moderate cost and high catalytic activity. Among the many
homogeneous and heterogeneous ruthenium catalysts,
homogeneous small molecule based catalysts whose true
nature (homogeneous or heterogeneous colloidal particles) is
still under scrutiny, show poor activity.[9,18,19] On the other hand,
surfactant and ionic liquid stabilized ruthenium nanoparticles and
supported (MOFs, polymers, dendrimers, metal oxides)
ruthenium nanoparticles were found to be catalytically active for
arene hydrogenation reaction.[9,20-37] Although, several
heterogeneous ruthenium nanostructures were reported for
hydrogenation of derivatives of benzene, yet the influence of the
crystallographic phase of ruthenium on hydrogenation of arenes
has not been explored.
In this context, self-supported porous metal offers a unique
opportunity to eliminate the support effects and helps to
determine the true origin of the catalytic activity of different
phases of pure metal. Porous metals are of immense interest in
the field of heterogeneous catalysis because of its high surface
area and the pores which facilitate easy diffusion of the
substrates within the accessible surface active sites of the
materials.[38,39] Fabrication of three dimensional bicontinuous
porous metals in which pores are interconnected is quite
challenging.[40] Porous metals synthesized by conventional
approaches such as template synthesis and dealloying method
suffer from scalability issues and these are multistep synthetic
procedures as well as, are metal specific.[40-45] Recently, these
problems were overcome by carrying out the assembly of ex-situ
or in-situ prepared nanoparticles.[46-51] Eswaramoorthy’s group
and others reported an in-situ assembly of nanoparticles for the
synthesis of high surface area metal nanosponges.[49-51] Herein,
we present a simple template less one step synthesis of phase
selective (fcc and hcp) self-supported ruthenium nanosponge
using sodium borohydride and amine borane as reducing agents.
Furthermore, we have addressed phase selective catalytic
property of ruthenium nanosponges towards hydrogenation of
benzene. Additionally, we present the catalytic efficacy of
ruthenium nanosponge towards the selective hydrogenation of
several other substituted arenes and aryl ethers.
Introduction
In heterogeneous catalysis, the catalytic activity is
governed by several parameters, such as active phase
composition, particle size, crystal structure, morphology, and
support.[1] In case of self-supported metals, namely metal
nanosponges, the effect of the support can be eliminated and
therefore, for a particular morphology of the material the activity
solely depends on the crystal structure of the metal.[1] On the
other hand, crystallographic phase of a material is an important
parameter wherein, change in the crystal phase can lead to
different physicochemical properties. Over decades, identifying
the crystal structure sensitivity of catalysts in chemical reactions
to accomplish maximum productivity remains one of the most
significant yet challenging issues in heterogeneous catalysis.[2]
notable example of such structure sensitivity is the Fischer-
Tropsch synthesis (FTS) wherein, various in situ
A
characterizations and theoretical calculations were conducted to
understand the crystal structure sensitivity for cobalt, ruthenium,
and iron catalysts.[2-4] Apart from Fischer-Tropsch synthesis,
several other chemical and electrochemical reactions wherein,
the crystallographic phase of metal catalysts influence the
catalytic processes have been investigated.[5]
[a]
Dr. Sourav Ghosh, Prof. Balaji R. Jagirdar
Department of Inorganic and Physical Chemistry
Indian Institution of Science
Bangalore, Karnataka – 560012, India.
E-mail: jagirdar@ipc.iisc.ernet.in
Supporting information for this article is given via a link at the end of
the document.
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Table 1. Surface area, pore size, and pore volume of ruthenium nanosponges
Surface area
(m2/g)
Pore volume
(cm3/g)
Materials (phase)
Pore size (nm)
Ru-TBAB (hcp)
Ru-AB (hcp)
146.7
84.6
83
19.7
16.5
18.1
29.1
0.444
0.222
0.27
Ru-SB (fcc)
Ru-AB-SB (fcc)
67
0.312
small sized ruthenium nanoparticles. Despite the difference in
the reducing power of amine boranes and sodium borohydride,
in all cases formation of nanosponges was noted. Fig 3 shows
the bright field TEM (BF-TEM) images of ruthenium
nanosponges. The high resolution TEM images (HRTEM) (Fig.
3e-h) revealed lattice fringes with d spacing of 0.205 nm
corresponding to the (101) plane of hexagonal close packed
phase of Ru-AB and Ru-TBAB (Fig 3e,f). On the other hand,
samples obtained using SB and AB-SB displayed lattice fringes
with d spacing of 0.221 nm (Ru-SB) and 0.218 nm (Ru-AB-SB)
corresponding to the (111) plane of fcc phase of ruthenium (Fig
3g-h). The selected area electron diffraction (SAED) pattern
showed a ring pattern for all the samples (Fig. 3i-l). The
diffraction ring pattern in case of Ru-TBAB and Ru-AB could be
indexed to the hcp phase of ruthenium (Fig 3i-j) and, the fcc
phase of ruthenium in case of Ru-SB and Ru-AB-SB samples
respectively (Fig 3k-l).
Figure 1. Powder X-ray diffraction patterns of: a) Ru-TBAB, b) Ru-AB, c) Ru-
SB, and d) Ru-AB-SB nanosponges.
Results and Discussion
Synthesis and characterization of ruthenium nanosponge
Ruthenium nanoparticles were found to be catalytically
active for hydrogen release via B-H bond (amine boranes and
sodium borohydride) hydrolysis.[52] This prompted us to
synthesize high surface area ruthenium nanosponges using
amine boranes and sodium borohydride as reducing agents. We
prepared ruthenium nanosponge by reduction of ruthenium
chloride in water using amine boranes and sodium borohydride
in excess at room temperature. The molar ratio of ruthenium
chloride to reducing agent was maintained at 5. Powder XRD
patterns of Ru nanosponges obtained using various reducing
agents are shown in Fig. 1. In case of amine borane (ammonia
borane- AB and tert-butylamine borane- TBAB) as a reducing
agent, we obtained hcp ruthenium (JCPDS – 06-0663), whereas
fcc ruthenium was obtained when sodium borohydride (SB) was
used as a reducing agent (Fig. 1). The broadness of the peaks is
indicative of nanostructured ruthenium. In addition, when a
mixture of AB and SB was used as a reducing agent, fcc
ruthenium phase was obtained. The ruthenium nanosponges
have been labeled as Ru-TBAB, Ru-AB, Ru-SB, and Ru-AB-SB,
according to the reducing agents used for the synthesis which
are TBAB, AB, SB, and a mixture of AB and SB, respectively.
Earlier, hcp ruthenium nanoparticles were obtained using amine
borane as a reducing agent.[53] On the other hand, use of sodium
borohydride as a reducing agent led to the formation of fcc
ruthenium.[54]
The EDS spectra showed the presence of Ru(0) only (see
supporting information, SI). The featureless FT-IR spectrum
further supports that ruthenium nanosponge is comprised of only
ruthenium metal (SI). The deconvoluted 3d core level XPS
spectrum of ruthenium (Ru-TBAB) consists of four peaks having
binding energies of 280.1 eV (3d5/2) and 284.3 eV (3d3/2) which
correspond to Ru (0) and two other peaks (280.7 eV - 3d5/2
;
285.2 eV - 3d3/2) which could be ascribed to ruthenium oxide
(see SI).[55] Similarly, XPS spectra were recorded for the other
ruthenium nanosponges (see SI) and the binding energy values
for different oxidation states of various ruthenium samples have
been summarized in the SI. Formation of ruthenium oxide is
unavoidable, since the samples were stored under ambient
conditions and all the manipulations were carried out in aerial
conditions. Presence of Ru(0) and Ru(IV) were further verified
from the core level spectra of 3p states and the results have
been tabulated in the SI. The nitrogen adsorption and desorption
isotherms measured at 77 K (see SI), evidence their type IV
nature (more specifically, H3 type); hysteresis was noted for all
the isotherms which is indicative of mesoporosity.[56] The BET
surface area, pore size, and pore volume values for all the
nanosponges have been tabulated in Table 1. It was found that,
Ru-TBAB nanosponge possessed the highest surface area,
followed by Ru-AB, Ru-SB and Ru-AB-SB nanosponges.
Morphological characterization was done using scanning
electron microscopy (SEM). Low resolution SEM images of
ruthenium nanosponges evidenced the porous nature of the
samples (Fig. 2a-d). It is evident from the high-resolution SEM
images (Fig. 2e-h) that these porous samples are made up of
To evaluate the effect of reducing agents on the porosity of
ruthenium nanosponges, we measured the hydrogen evolution
rates during the reduction reaction using different
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Figure 2. Ru-TBAB, Ru-AB, Ru-SB, Ru-AB-SB nanosponges: a-d) low resolution SEM images (scale bar- 2 μm); e-h) high resolution SEM images (scale bar- 200
nm).
Figure 3. Ru-TBAB, Ru-AB, Ru-SB, Ru-AB-SB nanosponges: a-d) BF-TEM images (scale bar- 100 nm); e-h) HRTEM images (scale bar- 5 nm); i-l) SAED
patterns
(scale
bar-
5
1/nm).
reducing agents (sodium borohydride and amine boranes). We
found that the hydrolysis rates were slower when amine boranes
(ammonia borane, tertiary butylamine borane) were used as
reducing agents as compared to, when sodium borohydride was
used (see S.I.). The faster reduction kinetics of sodium
borohydride led to the formation of irregularly shaped ruthenium
nanoparticles which eventually resulted in a low surface area
ruthenium nanosponge with metastable fcc phase. On the other
hand, amine borane derivatives reduced the ruthenium salt at a
slower rate leading to the formation of high surface area
ruthenium nanosponge with stable hcp phase.
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ruthenium catalysts. Upon decreasing the substrate to catalyst
ratio from 1000 to 250, complete conversion of benzene to
cyclohexane was noted using both hcp and fcc ruthenium
catalysts and turnover frequency (TOF) value increased. For
substrate to catalyst ratio of 250, highest TOF values were noted
as compared to other substrate to catalyst ratios. In all these
cases, it was found that the catalytic activity depends on the
crystallographic phase of ruthenium: hcp ruthenium is more
active than fcc ruthenium. Similarly, when benzene
hydrogenation was carried out using different ruthenium
catalysts with a fixed substrate to catalyst ratio of 250 in n-
heptane as a solvent instead of neat benzene, better catalytic
activity for hcp ruthenium was noted as compared to fcc
ruthenium. At this stage, the origin of the phase selectivity
towards catalysis is not clear. But we propose that, the benzene
molecules get adsorbed more strongly onto the hcp phase of
ruthenium as compared to the fcc phase. On the other hand,
both adsorption and desorption of benzene takes place on the
surface of fcc phase at a rapid rate, is reflected in a lower
hydrogenation catalytic activity in this case in comparison to that
of the hcp phase. More work will be needed to decipher the
intricate mechanistic aspects involved in this phase selective
hydrogenation activity. Since these are heterogeneous catalysts
and the catalytic processes take place at the surface of the
materials, the difference in surface area could play an important
role for defining catalytic activities. Among different hcp
ruthenium catalysts (Ru-TBAB and Ru-AB), high surface area
Ru-TBAB shows greater activity towards catalytic hydrogenation
reaction as compared to the low surface area Ru-AB (see SI).
Thermal stability of ruthenium nanosponge: Thermal stability of
the catalysts was studied by annealing them at different
temperatures under inert atmosphere. Catalysts annealed at 150
oC for 3 h were labeled as Ru-TBAB-150, Ru-AB-150, Ru-SB-
150, Ru-AB-SB-150 and the catalysts annealed at 300 oC for 3 h
were labeled as Ru-TBAB-300, Ru-AB-300, Ru-SB-300, Ru-AB-
SB-300. Morphological characterization of the annealed samples
Catalytic activity of different phases of ruthenium
Table 2. Hydrogenation of benzene using ruthenium catalysts at different
substrate to catalyst molar ratio
Catalyst
(phase)
Time
(h)
Entry
S/C ratio
%Convc
TOFi(h-1)d
1
2
Ru-TBAB(hcp)
Ru-AB(hcp)
1000
1000
1000
1000
500
500
500
500
250
250
250
250
250
250
250
250
15
15
60
50
176
60
3
Ru-SB(fcc)
15
15
52
4
Ru-AB-SB(fcc)
Ru-TBAB(hcp)
Ru-AB(hcp)
15
15
48
5
4.5
5
>99
>99
95
187
106
102
103
252
185
103
111
614
348
72
6
7
Ru-SB(fcc)
11
8
Ru-AB-SB(fcc)
Ru-TBAB(hcp)
Ru-AB(hcp)
8.5
1.33
1.5
2.75
2.5
0.58
1.25
6.5
6.75
90
9
>99
>99
>99
>99
>99
>99
>99
>99
10
11
12
13
14
15
16
Ru-SB(fcc)
Ru-AB-SB(fcc)
Ru-TBAB(hcp)
Ru-AB(hcp)
Ru-SB(fcc)
Ru-AB-SB(fcc)
75
aEntry 1-12: all reactions were carried out at a constant temperature of 75 oC
and 4 bar hydrogen gas pressure and neat benzene was used; bentries 13-16:
all reactions were carried out in 10 mL of n-heptane as a solvent (ruthenium
nanosponge (16.2 mg, 0.16 mmol) and benzene (3.56 mL, 40 mmol)), at a
constant temperature of 75 oC and 4 bar hydrogen gas pressure; cconversion
determined by GC/MS analysis; dinitial turnover frequency (TOFi)
mol(benzene)/mol(Ru nanosponge)·0.25, after 15 min (0.25 h).
=
The catalytic activities of as prepared fcc and hcp ruthenium
nanosponges were investigated using benzene hydrogenation
as a model reaction. All the catalysts were found to be active
towards hydrogenation of benzene to cyclohexane under
moderate conditions (at 75 oC and 4 bar). Blank runs in the
absence of ruthenium catalysts under similar conditions did not
result in hydrogenation. Separation of the catalysts from the
reaction mixture was achieved by simple centrifugation followed
by decantation. The catalytic activity of different ruthenium
catalysts was probed by carrying out hydrogenation of benzene
at different substrate to catalyst ratios either in the neat condition
(without any solvent) or in n-heptane as a solvent. Results of
these studies have been summarized in Table 2. In all these
cases, cyclohexane was obtained as a product; no partial
hydrogenation product (cyclohexene) was noted. In case of
substrate to catalyst ratio of 1000, incomplete conversion of
benzene to cyclohexane was noted for the two phases of
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Figure 4. Powder XRD pattern stack plots of ruthenium nanosponges before
and after annealing (150 oC and 300 oC): a) Ru-TBAB, b) Ru-AB, c) Ru-SB,
and d) Ru-AB-SB.
of benzene, 0.16 mmol of Ru catalyst); bconversion determined by GC-MS
analysis; cturnover frequency = mol(benzene)/mol(Ru nanosponge)·h.
using SEM (see SI) revealed no changes as compared to un-
annealed samples. Additionally, powder XRD patterns revealed
that both Ru-TBAB and Ru-AB samples retained their hcp phase
Table 3. Surface area and pore size of ruthenium nanosponges before and
after annealing
o
o
even upon annealing at 150 C and 300 C (Fig. 4a-b). The fcc
phases of Ru-SB and Ru-AB-SB were also retained for samples
annealed at 150 oC, but a phase change from fcc to hcp was
Entry
Catalyst (phase)
Surface area (m2/g)
Pore size (nm)
o
noted when these samples were annealed at 300 C (Fig. 4c-d).
1
2
Ru-TBAB (hcp)
Ru-TBAB-150 (hcp)
Ru-TBAB-300 (hcp)
Ru-AB (hcp)
146.7
138.9
105
19.7
31.5
21.7
16.5
20.8
14.9
18.2
22.5
27
It was reported that the fcc phase of ruthenium is a metastable
phase and has not been observed in the bulk, whereas, hcp is
the bulk phase of ruthenium.[54,57] Hence, a phase change was
expected to take place at high temperature. High temperature
annealing resulted in sintering of particles which was reflected
by a decrease in their surface areas (Table 3). Interestingly, in
case of samples exhibiting fcc phase (Ru-SB and Ru-AB-SB),
the magnitude of change in surface area was found to be
greater than that in case of hcp phase (Ru-TBAB and Ru-AB).
This could be ascribed to a thermal induced phase change (fcc
to hcp) which could lead to a structural reorganization resulting
in a considerable change in surface area.
3
4
84.6
77.7
62.6
83
5
Ru-AB-150 (hcp)
Ru-AB-300 (hcp)
Ru-SB (fcc)
6
7
8
Ru-SB-150 (fcc)
Ru-SB-300 (hcp)
Ru-AB-SB (fcc)
72.1
39.6
67
Catalytic activities of the annealed samples were assessed
using benzene hydrogenation as a model reaction. Table 4
summarizes the results of these studies. In case of hcp
ruthenium catalysts (Ru-TBAB and Ru-AB), TOF values
remained constant for samples annealed at 150 oC whereas,
90% of benzene was converted into cyclohexane for samples
9
10
11
12
29.1
40.7
36.9
Ru-AB-SB-150 (fcc)
Ru-AB-SB-300 (hcp)
57.8
35.7
o
o
annealed at 300 C. Similarly, fcc samples annealed at 150 C
brought about hydrogenation to an extent of 80% using Ru-AB-
SB-150 and 85% using Ru-SB-150 catalysts. On the other hand,
o
fcc samples annealed at 300 C (which got transformed to hcp
Table 4. Hydrogenation of benzene using as prepared and annealed
ruthenium nanosponges
phase) brought about hydrogenation to an extent of 44% using
Ru-AB-SB-300 and 50% using Ru-SB-300 catalysts. These
variations in catalytic activities could be ascribed to the different
extent of reduction in surface area for the different phases of
ruthenium nanosponges upon annealing. Thermal annealing
study and substrate to catalyst ratio optimization study show that
the Ru-TBAB (hcp phase) is the most active catalyst among
various ruthenium nanosponges. Thus, the optimum conditions
required for benzene hydrogenation was investigated using Ru-
TBAB as a catalyst by a careful change in pressure, temperature,
and solvent.
Entry
Catalyst
Ru-TBAB
Phase
hcp
hcp
hcp
hcp
hcp
hcp
fcc
Time (h)
0.58
0.58
0.58
1.25
1.25
1.25
6.5
%Convb
>99
>99
90
TOF(h-1)c
431
1
2
Ru-TBAB-150
Ru-TBAB-300
Ru-AB
431
3
388
4
>99
99
200
5
Ru-AB-150
Ru-AB-300
Ru-SB
198
Effect of temperature, pressure, and solvent on catalytic activity
of ruthenium (Ru-TBAB) catalyst: Data for benzene
hydrogenation catalysis using Ru-TBAB catalyst are
summarized in the SI. As it is evident from the data, complete
benzene hydrogenation required shorter times when the reaction
was carried out at high temperatures. At room temperature and
4 bar of hydrogen pressure, 95% benzene was hydrogenated to
cyclohexane after 11 h whereas, >99% conversion was noted at
6
88
176
7
>99
85
38.5
32.7
19.2
37
8
Ru-SB-150
Ru-SB-300
Ru-AB-SB
Ru-AB-SB-150
Ru-AB-SB-300
fcc
6.5
9
hcp
fcc
6.5
50
10
11
12
6.75
6.75
6.75
>99
80
o
75 C within 35 min. The TOF was calculated to be 431 h-1 (75
fcc
29.6
16.3
oC, 4 bar) which is comparable to the literature report for
benzene hydrogenation using ruthenium nanoparticles under
moderate conditions.[22,58-61] In another set of experiments,
benzene hydrogenation was carried out at different hydrogen
gas pressures by keeping all the other parameters constant (0.4
mol% catalyst, 40 mmol substrate in 10 mL of n-heptane at 75
hcp
44
aAll reactions were carried out at a constant temperature of 75 oC and 4 bar
pressure in 10 mL of n-heptane at 250 benzene/catalyst molar ratio (40 mmol
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oC). It is also apparent that an increase in pressure leads to an
increase in the rate of hydrogenation. No saturation of TOF was
noted up to a pressure of 6 bar.
Hydrogenation of benzene was carried out in different
solvents and the results are summarized in the SI. It was noted
that solvent polarity does not influence the catalytic activity.
However, higher rate of hydrogenation was noted for solvents
methyl groups, steric bulkiness of the substrate increases and
subsequently the hydrogenation rate slowed down which is
reflected in the TOF values (TOFbenzene> TOFxylene> TOFmesitylene).
In case of hydrogenation of xylene, the cis product was obtained
in higher amount compared to the trans isomer. We also found
that, Ru-TBAB is capable of hydrogenating biphenyl to
bicyclohexane (an important organic hydrogen storage medium)
with a TOF of 12.5 h-1 (TON = 250).[64]
Table 5. Hydrogenation of substituted arenes using hcp Ru-TBAB
Table 6. Hydrogenation of substituted benzenes (phenol, aryl ether, aryl
nanosponge
ketone) using hcp Ru-TBAB nanosponge
Time
(h)
Time
(h)
%Conv.
Entry
Substrate
Product
TOF(h-1)a
Entry
Substrate
Product
(%Selec.)a
1
2
3
4
5
6
Benzene
Toluene
0.58
0.66
1.25
2
Cyclohexane
Methylcyclohexane
Ethylcyclohexane
Isopropylcyclohexane
t-Butylcyclohexane
Ethylcyclohexane
431
378.8
200
1
4
3
>99(99)
>99(99)
Ethylbenzene
Isopropylbenzene
t-Butylbenzene
Styrene
125
2
3
3
83.3
125
2
1,2-
7
8
9
o-Xylene
m-Xylene
p-Xylene
11
8
22.7
31.3
45.5
Dimethylcyclohexane
4
>99(99)
1,3-
Dimethylcyclohexane
1,4-
5.5
4
5
4
4
>99(98)
>99(99)
Dimethylcyclohexane
1,3,5-
Trimethylcyclohexane
10
11
Mesitylene
Biphenyl
15
20
16.7
12.5
Bicyclohexane
All reactions were carried out at a constant temperature of 75 oC and 4 bar
hydrogen gas pressure in 10 mL of n-heptane as solvent at 250
a
substrate/Ru-TBAB molar ratio (40 mmol of substrate, 0.16 mmol of Ru-TBAB
catalyst); in each case >99 % substrate to product conversion was determined
6
7
12
24
>99(93)
by GC-MS analysis; aturnover frequency
nanosponge)·h.
=
mol(substrate)/mol(Ru
which have good hydrogen solubility.[62] In case of cyclohexane,
hydrogen solubility decreases with increasing the temperature
>99(45)b
which in turn causes
hydrogenation activity was noted in acetonitrile due to
a
drop in TOF.[63] No appreciable
a
competitive adsorption of the substrate and the solvent on the
catalyst surface. Moderate activity was noted in alcohol and
water as solvents.
24c
24d
10e
6f
7(1)c
91(74)d
>99(87)e
>99(88)f
8
9
Hydrogenation of substituted arenes and aryl ethers using Ru-
TBAB catalyst: To understand the scope of the Ru-TBAB
catalyst, various alkylated benzenes were hydrogenated under
o
relatively mild conditions (75 C and 4 bar, n-heptane). Results
10
>99(99)
are summarized in Table 5. In all the cases, >99% conversion of
substituted arene to substituted cyclohexane was noted. The
TOF value decreases with an increase in the steric bulkiness of
378.8 h-1
>
All reactions were carried out at a constant temperature of 75 oC and 20 bar
hydrogen gas pressure in 3 mL of n-heptane as a solvent at 133 substrate/Ru-
TBAB molar ratio (4 mmol of substrate, 0.03 mmol of Ru-TBAB catalyst);
the substituent on benzene (TOFtoulene
=
TOFethylbenzene = 200 h-1 > TOFisopropylbenzene = 125 h-1 > TOFt-
= 83.3 h-1). Similarly, upon increasing the number of
butylbenzene
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10.1002/cctc.201800287
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aproduct conversion was determined by GC-MS analysis; b>99% conversion of
benzaldehyde was observed and 45% cyclohexylmethanol and 44% benzyl
alcohol was obtained; cdiphenyl ether:Ru (250), 5 Bar H2/75 oC; ddiphenyl
ether:Ru (250), 20 Bar H2/75 oC; ediphenyl ether:Ru (133), 20 Bar H2/75 oC;
fdiphenyl ether:Ru (100), 20 Bar H2/75 oC.
Figure 5. a) Catalyst activity for benzene hydrogenation by ruthenium nanosponge in n-heptane upon recycling, b) SEM image, c) powder XRD stack plot of Ru-
TBAB before and after catalysis, d) BF-TEM image, e) HRTEM image, and f) SAED pattern of the ruthenium catalyst after 7 cycles of hydrogenation.
hand, selective hydrogenation of aromatic cores of lignin derived
compounds and other oxygenated aromatics are also important
chemical transformations for the generation of alicyclic ethers,
which are key intermediates for the production of bio-fuels and
fine chemicals (Scheme 1).[72] Reports of these transformations
are rather scarce.[72] In this context, the catalytic ability of Ru-
TBAB was further assessed by carrying out the hydrogenation of
phenol at 75 oC and 4 bar which gave a poor yield of
cyclohexanol. Upon increasing the pressure to 20 bar, nearly
quantitative conversion to cyclohexanol without C‒O bond
hydrogenolysis was noted (Table 6, entry 1). Subsequently, the
hydrogenation of anisole resulted in the formation of
methoxycyclohexane (Table 6, entry 2). In case of
acetophenone, complete reduction of both arene and ketone
moieties was noted. Similarly, selective conversion of furfural
and furfuryl alcohol to the corresponding tetrahydrofurfural was
Scheme 1. Fragments derived from lignin and cellulosic biomass.
achieved; tetrahydrofurfural is an industrially important
compound. In general, benzylic compounds (benzyl ethers and
benzyl alcohols) are quite prone to hydrogenolysis during
hydrogenation because of their low C–O bond dissociation
energy.[73] Herein, hydrogenation of benzyl alcohol proceeded
efficiently to the corresponding saturated alicyclic alcohol with
93% selectivity (Table 6, entry 6). Similarly, hydrogenation of
benzaldehyde resulted in the formation of benzyl alcohol
followed by 45 % conversion into the saturated alicyclic alcohol
was noted. In case of diphenyl ether, the optimum
hydrogenation conditions were achieved by carefully varying the
substrate to catalyst ratio and hydrogen gas pressure. In case of
A
clear majority of the industrially important arene
derivatives are produced from fossil fuel feedstock. However the
rapid depletion of fossil fuels necessitates search for
a
sustainable alternatives such as biomass (lignin, cellulose,
hemicellulose) whose valorization produces oxygenated
aromatics.[65-67] For subsequent utilization of such oxygenated
aromatics, several transition metal based homogeneous and
heterogeneous catalysts have been developed which afford
mainly C‒O bond hydrogenolysis products.[68-71] On the other
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10.1002/cctc.201800287
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0.75 mol% catalyst loading at 20 bar pressure after 10 h,
dicyclohexyl ether was obtained with 87% selectivity and 13%
C–O bond hydrogenolysis products. Similarly, hydrogenation of
phenyl benzyl ether resulted in the formation of alicyclic ether
with 99% selectivity (>99% conversion). All these results clearly
suggest that hcp ruthenium is solely responsible for the catalytic
hydrogenation of aromatic ring over the C–O bond cleavage.
Catalyst recyclability and catalyst poisoning: Catalyst
recyclability was evaluated for Ru-TBAB catalysts using
benzene hydrogenation under similar conditions as mentioned
above: 0.4 mol% catalyst, 40 mmol substrate in 10 mL of n-
In summary, phase selective ruthenium nanosponge was
synthesized by chemical reduction method using amine borane
and sodium borohydride as
nanosponges were utilized as
a
reducing agent. These
catalyst for benzene
a
hydrogenation reaction wherein, hcp ruthenium shows better
catalytic activity as compared to the fcc ruthenium. Among the
two different hcp ruthenium catalysts, Ru-TBAB showed better
catalytic activity as compared to the Ru-AB due to its high
surface area. Thermal annealing study showed that the hcp
phase of ruthenium is thermally more stable and active than the
fcc phase. On the other hand, fcc phase undergoes a thermally
induced phase transition to hcp ruthenium which is associated
with substantial change in the surface area and the catalytic
activity. Using Ru-TBAB as a catalyst, several other substituted
benzenes were successfully hydrogenated under mild conditions.
Furthermore, the aromatic moieties of phenol and aryl ethers
were selectively hydrogenated using Ru-TBAB nanosponge to
their alicyclic components without C‒O bond hydrogenolysis.
The catalytic activity of these nanosponge catalysts opens new
opportunities for the valorization of lignin derived aromatic
compounds under mild conditions in a green and sustainable
manner. Hence in future, the catalytic activity of these metal
nanosponges could further be tuned for efficient and sustainable
transformation of biomass feedstock to valuable chemicals
under moderate conditions.
o
heptane at 75 C. Figure 5a shows the percentage conversion
over 7 cycles. Even after the 7th cycle, the catalyst is still active
and capable of hydrogenating ~97% benzene to cyclohexane.
The SEM images confirm that the spongy nature of the catalyst
remained unaltered (Fig. 5b). Additionally, powder XRD pattern
showed no structural change (Fig. 5c). Further, BF-TEM image
of recycled ruthenium nanosponge revealed that the porous
nature of the samples remained intact (Fig. 5d). Moreover, the
HRTEM image exhibited lattice fringes with d spacing of 0.205
nm corresponding to the (101) lattice plane of hcp ruthenium
(Fig. 5e). The SAED pattern could be perfectly indexed to the
hcp phase of ruthenium (Fig. 5f). The mesoporous nature of the
catalyst is responsible for its long-term stability and high catalytic
activity. Thus, it is evident that the catalyst is robust and retains
its morphology and activity for several cycles. Hot filtration study
was carried out to illustrate the true heterogeneous nature of the
catalyst and to rule out the possibility of catalyst leaching. After
about 50% conversion of benzene using the Ru-TBAB catalyst,
Experimental Section
o
the hot reaction mixture was passed through a hot (~75 C) frit
Materials
and the supernatant was again subjected to hydrogenation
under similar reaction conditions (75 oC and 4 bar). No further
hydrogenation of benzene took place which demonstrates the
heterogeneous nature of the catalyst. Catalyst poisoning
experiments were performed using PCy3, a well-known catalyst
poison.[30] Hydrogenation of benzene using Ru-TBAB in
Ruthenium chloride (RuCl3.xH2O) was purchased from Arora Matthey
Limited, India. Sodium borohydride (NaBH4), ammonium sulfate
((NH4)2SO4), n-heptane, thiophene, and tetrahydrofuran (THF) were
purchased from S. D. Fine Chemicals Limited, India. Tert-butylamine
borane (TBAB) and tricyclohexylphosphine (PCy3) were procured from
Sigma Aldrich. All the arenes were used as received. THF and n-heptane
were dried over Na-benzophenone. Ammonia borane was synthesized
following literature procedure and characterized using 11B NMR and FT-
IR spectroscopy before use.[76] Double distilled water was used for all the
reactions.
o
presence of PCy3 in 1:5 molar ratio (catalyst: poison) at 75 C in
n-heptane showed no catalytic activity even after 2 h of reaction.
Similarly, poisoning experiment was carried out by addition of 5
equiv. of PCy3 after about 50% conversion which resulted in an
instantaneous suppression of catalytic activity. In this case,
strong binding of PCy3 on the catalyst surface blocks the
catalytically active sites. Additionally, carbon monoxide[74] and
thiophene[75] were used separately as catalyst poisons, wherein
suppression of catalytic activity was noted for carbon monoxide
and thiophene treated catalyst (Ru-TBAB). Since, ruthenium
nanosponge exhibits good catalytic activity towards arene
hydrogenation, it would be important to study the effect of
alloying with other metals to understand the catalytic activity at
moderate reaction conditions with lower catalyst loading.
Research in this direction are currently being pursued in our
laboratories.
Synthesis of ruthenium nanosponge
Ruthenium nanosponge was prepared by adopting a literature procedure
using NaBH4, AB (H3N·BH3), and TBAB (tBuH2N·BH3) as reducing
agents.[49] In a typical experiment, 310 mg (10 mmol) of AB was
dissolved in 100 mL of water in a 500 mL round bottomed flask.
Ruthenium chloride, RuCl3.xH2O (415 mg, 2 mmol) was dissolved in 20
mL of water and added to the aqueous solution of AB under vigorous
stirring. Strong effervescence of hydrogen gas was noted. After 5 min,
the black residue was filtered and washed thoroughly with water and
acetone and kept for drying overnight. The yield obtained was 148 mg
(73.2 %). The ruthenium nanosponge was labelled as Ru-AB. In a similar
manner, sodium borohydride (NaBH4, SB) (378.3 mg, 10 mmol) and
RuCl3.xH2O (415 mg, 2 mmol) were allowed to react in water following
the above procedure and the nanosponge obtained was labelled as Ru-
SB. The yield obtained was 156 mg (77.2 %). Similarly, tert-butylamine
borane (tBuH2N·BH3, TBAB) (869.7 mg, 10 mmol) and RuCl3.xH2O (415
mg, 2 mmol) were reacted in water to obtain the Ru-TBAB nanosponge.
Conclusions
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10.1002/cctc.201800287
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The yield obtained was 155 mg (76.7 %). Ruthenium nanosponge was
also synthesized using a mixture of AB (155 mg, 5 mmol) and SB (189
mg, 5 mmol) from RuCl3.xH2O (415 mg, 2 mmol) in water and the
catalyst was labelled as Ru-AB-SB. The yield obtained was 160 mg
(79.1 %).
4 mmol) was added to the reaction mixture. The mixture was sonicated
for 3 min and then placed in an autoclave and sealed. Subsequently, the
reaction vessel was purged with hydrogen gas for 4 times; then, the
solution temperature was raised to 75 oC and pressurized with 20 bar of
hydrogen gas. The pressure drop was monitored until gas consumption
ceased. After completion of the reaction, reaction vessel was cooled
down and depressurized carefully. Finally, an aliquot was drawn for GC-
MS analysis. Same procedure was followed for hydrogenation of the
remaining substrates in n-heptane.
Characterization
Scanning electron microscopy (SEM) images were acquired using FEI
Sirion XL30 FEG SEM. Powder samples were placed on a double-sided
conductive carbon tape supported on an aluminum stub. Powder XRD
patterns were acquired using PANalytical EMPYREAN diffractometer
using Cu Kα radiation. Bright field (BF) TEM imaging, high resolution
(HR) TEM imaging, and selected area electron diffraction (SAED)
experiments were performed using a JEOL 2100F FETEM. Energy
dispersive X-ray spectroscopy (EDS) was performed using Oxford EDS
spectrometer attached to the TEM. The operating voltage of the field
emission gun (FEG) is 200 kV. Samples were prepared by dispersing 2
mg of ruthenium sponge in 4 mL of THF and 5 µL solutions were drop
casted on a 300 mesh formvar coated copper grid and dried under a 40
Watt lamp for 12 h. FT-IR spectra were recorded using Bruker ALPHA
FTIR spectrometer. X-ray photoelectron spectroscopy was carried out
using a Kratos AXIS Ultra XPS spectrometer. BET surface area and
porosity measurements were performed using Micromeritics ASAP 2020.
NMR spectral measurements were carried out using an Avance Bruker
400 MHz spectrometer.
Thermal stability of the ruthenium catalysts
Thermal stability of the ruthenium catalyst was evaluated by annealing
the samples in argon filled ampoules and subsequently used for benzene
hydrogenation reactions. Annealing study was performed for Ru-TBAB,
Ru-AB, Ru-SB, Ru-AB-SB catalysts. The ampoules were placed
separately in a box furnace at 150 oC for 3 h and 300 oC for 3 h. The
samples annealed at 150 oC were labelled as Ru-TBAB-150, Ru-AB-150,
Ru-SB-150, and Ru-AB-SB-150. Similarly, samples annealed at 300 oC
were labelled as Ru-TBAB-300, Ru-AB-300, Ru-SB-300, and Ru-AB-SB-
300. Hydrogenation reactions were performed with these annealed
samples at 75 oC and 4 bar H2 pressure in n-heptane as a solvent. The
reaction was carried out as described above.
Catalyst recyclability
Ruthenium nanosponge (16.2 mg, 16 mmol) was dispersed in 10 mL of
n-heptane; then benzene (3.56 mL, 40 mmol) was added to the
dispersion. The reaction was carried out as described above. After one
cycle, the catalyst was separated by centrifugation. The filtrate was
characterized using 1H NMR spectroscopy. The catalyst was washed
thoroughly with n-heptane. Then, 10 mL of n-heptane was added to the
reactor and the catalyst was dispersed; subsequently, 3.56 mL of
benzene was added and the next cycle was carried out for 35 min. After
7 cycles of hydrogenation, the catalyst was separated by centrifugation
and washed thoroughly with n-heptane and acetone and dried for 12 h.
The catalyst was characterized using powder XRD, SEM, and TEM.
General procedure for hydrogenation catalysis
Ultra-high purity Ultra-high purity (99.997%) hydrogen gas was obtained
from Bhuruka Gases Limited, India. All the hydrogenation experiments
were carried out in n-heptane solvent using a Parr hydrogenation
apparatus. In a typical experiment, ruthenium nanosponge (16.2 mg,
0.16 mmol) was dispersed in 10 mL of n-heptane and then 3.56 mL (40
mmol) of benzene was added to the reaction mixture. The mixture was
sonicated for 3 min and then placed in a Parr hydrogenation apparatus
and closed with a neoprene stopper. Before starting the reaction, an
aliquot was drawn for recording 1H NMR spectrum. The bottle was
pressurized with 3 bar of hydrogen and then depressurized for four times
before each catalysis experiment. Temperature was fixed at 75 oC and
the bottle was pressurized with 4 bar of H2. The reaction was monitored
until gas consumption ceased which was evident from no further
pressure drop. The reaction mixture was cooled down and centrifuged to
separate the filtrate from the catalyst. The filtrate was characterized using
GC-MS and 1H-NMR spectroscopy. Same procedure was followed for
hydrogenation of the remaining substrates in n-heptane.
Acknowledgements
The authors gratefully acknowledge the financial support from
the Council of Scientific & Industrial Research (CSIR), India and
the Indian Institute of Science for funding the procurement of a
200 kV FETEM. Authors thank Prof. S. Ramakrishnan of IPC
department, IISc, for providing the GC-MS facility and Dr.
Subhadip Chakraborty and Mr. Sujoy Bej for their help with the
GC-MS measurements and Ms. Debdyuti Mukherjee for her help
with the electrochemical measurement.
Samples of the reaction mixture were withdrawn and analyzed by
using Shimadzu GCMS-QP2010 SE gas chromatograph-mass
spectrometer, installed with an Rtx-5MS capillary column (0.25mm x 30m
x 0.25μm) and a high-performance quadrupole mass detector. Each
reaction was performed twice for reproducibility and the mass spectra of
the products were matched with the NIST library. The percentage
conversion was calculated based on peak areas, with respect to the n-
dodecane as an internal standard.
Keywords: Heterogeneous catalyst • Ruthenium nanosponge •
HCP and FCC Phase • Arene hydrogenation • Aryl ether
hydrogenation
General procedure for hydrogenation reaction using Parr reactor
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Different phases, different
Sourav Ghosh, Balaji R. Jagirdar*
reactivity: Using one step reduction
method, by a judicious choice of the
reducing agents, two different phases
(hcp and fcc) of ruthenium were
synthesized which exhibit different
catalytic activities towards benzene
hydrogenation reaction under mild
conditions. Further, this ruthenium
catalyst was utilized for selective
hydrogenation of aromatic cores of
various oxygenated aromatics without
C-O bond hydrogenolysis.
Page No. – Page No.
Title
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