Fabrication of Ruthenium Nanoparticles in Porous Organic Polymers: Towards Advanced Heterogeneous Catalytic Nanoreactors

Article abstract of DOI:10.1002/chem.201504055

A novel strategy has been adopted for the construction of a copolymer of benzene-benzylamine-1 (BBA-1), which is a porous organic polymer (POP) with a high BET surface area, through Friedel-Crafts alkylation of benzylamine and benzene by using formaldehyde dimethyl acetal as a cross-linker and anhydrous FeCl₃ as a promoter. Ruthenium nanoparticles (Ru NPs) were successfully distributed in the interior cavities of polymers through NaBH₄, ethylene glycol, and hydrothermal reduction routes, which delivered Ru-A, Ru-B, and Ru-C materials, respectively, and avoided aggregation of metal NPs. Homogeneous dispersion, the nanoconfinement effect of the polymer, and the oxidation state of Ru NPs were verified by employing TEM, energy-dispersive X-ray spectroscopy mapping, cross polarization magic-angle spinning ¹³C NMR spectroscopy, and X-ray photoelectron spectroscopy analytical tools. These three new Ru-based POP materials exhibited excellent catalytic performance in the hydrogenation of nitroarenes at RT (with a reaction time of only ≈30 min), with high conversion, selectivity, stability, and recyclability for several catalytic cycles, compared with other traditional materials, such as Ru@C, Ru@SiO₂, and Ru@TiO₂, but no clear agglomeration or loss of catalytic activity was observed. The high catalytic performance of the ruthenium-based POP materials is due to the synergetic effect of nanoconfinement and electron donation offered by the 3D POP network. DFT calculations showed that hydrogenation of nitrobenzene over the Ru (0001) catalyst surface through a direct reaction pathway is more favorable than that through an indirect reaction pathway.

Full text of DOI:10.1002/chem.201504055

DOI: 10.1002/chem.201504055
Full Paper
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Hydrogenation |Hot Paper|
Fabrication of Ruthenium Nanoparticles in Porous Organic
Polymers: Towards Advanced Heterogeneous Catalytic
Nanoreactors
John Mondal,*[a, b] Sudipta K. Kundu,[c] Wilson Kwok Hung Ng,[a] Ramana Singuru,[b]
Parijat Borah,[a] Hajime Hirao,*[a] Yanli Zhao,*[a] and Asim Bhaumik*[c]
Abstract: A novel strategy has been adopted for the con-
struction of a copolymer of benzene–benzylamine-1 (BBA-1),
which is a porous organic polymer (POP) with a high BET
surface area, through Friedel–Crafts alkylation of benzyl-
amine and benzene by using formaldehyde dimethyl acetal
as a cross-linker and anhydrous FeCl3 as a promoter. Ruthe-
nium nanoparticles (Ru NPs) were successfully distributed in
the interior cavities of polymers through NaBH4, ethylene
glycol, and hydrothermal reduction routes, which delivered
Ru-A, Ru-B, and Ru-C materials, respectively, and avoided ag-
gregation of metal NPs. Homogeneous dispersion, the nano-
confinement effect of the polymer, and the oxidation state
of Ru NPs were verified by employing TEM, energy-disper-
sive X-ray spectroscopy mapping, cross polarization magic-
angle spinning 13C NMR spectroscopy, and X-ray photoelec-
tron spectroscopy analytical tools. These three new Ru-
based POP materials exhibited excellent catalytic per-
formance in the hydrogenation of nitroarenes at RT (with
a reaction time of only ꢀ30 min), with high conversion, se-
lectivity, stability, and recyclability for several catalytic cycles,
compared with other traditional materials, such as Ru@C,
Ru@SiO2, and Ru@TiO2, but no clear agglomeration or loss of
catalytic activity was observed. The high catalytic per-
formance of the ruthenium-based POP materials is due to
the synergetic effect of nanoconfinement and electron don-
ation offered by the 3D POP network. DFT calculations
showed that hydrogenation of nitrobenzene over the Ru
(0001) catalyst surface through a direct reaction pathway is
more favorable than that through an indirect reaction path-
way.
Introduction
metal or metal oxide nanoparticles (NPs) has opened up an in-
terdisciplinary research area for the development of new heter-
ogeneous catalysts with limited agglomeration and leaching of
NPs, as well as favorable mass transport in confined cavities;
these features are desirable for catalysis.[2]
Porous organic polymers (POPs) have recently received a stag-
gering degree of attention owing to their huge surface area,
tunable pore size distribution, high hydrothermal and mechan-
ical stability, flexible synthetic strategy, and readily modifiable
functionality.[1] The assembly of nanoporous materials and
Although the porous channels and interior cavities can be
easily tuned in the case of metal–organic frameworks (MOFs),
which is an important property of platforms that encapsulate
NPs, the versatile application of MOF-encapsulated NPs in het-
erogeneous catalysis has been a difficult challenge because of
their unstable physicochemical nature towards moisture, acids,
and bases and the labile nature of metal–ligand coordination
bonds.[3] In contrast to MOFs, POPs are considered to have the
most promising molecular architecture for the encapsulation
of NPs because they possess relatively stable covalent bonds,
low skeleton density, and hydrophobic components in their
structures. Moreover, the structural integrity of POPs can be
easily tuned by varying the synthetic strategy and selection of
monomers.[4] The interaction between organic linkers in POPs
and metal NPs can be readily enhanced by introducing coordi-
nation groups into metal precursors, which provide an extra
advantage for the stabilization and improvement of the activi-
ty of metal NPs compared with other catalytic supports.
Recent reports in the literature demonstrate that heterogene-
ous catalysts for morphology-tuned aerobic oxidation,[5] hydro-
[a] Dr. J. Mondal,+ Dr. W. K. Hung Ng, Dr. P. Borah, Prof. Dr. H. Hirao,
Prof. Dr. Y. Zhao
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University, 21 Nanyang Link, 637371 (Singapore)
E-mail: hirao@ntu.edu.sg
zhaoyanli@ntu.edu.sg
[b] Dr. J. Mondal,+ R. Singuru
Inorganic and Physical Chemistry Division
CSIR-Indian Institute of Chemical Technology (IICT)
Uppal Road, Hyderabad-500007 (India)
E-mail: johncuchem@gmail.com
[c] S. K. Kundu,+ Prof. Dr. A. Bhaumik
Department of Materials Science
Indian Association for the Cultivation of Science
2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032 (India)
E-mail: msab@iacs.res.in
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504055.
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genation,[6] coupling,[7] cycloaddition reactions,[8] and so forth
can be developed through the facile fabrication and encapsu-
lation of metal NPs inside the nanoporous channel of POPs.
The hydrogenation of nitroarenes to produce aniline deriva-
tives has attracted much attention in the field of heterogene-
ous catalysis because these compounds are the most impor-
tant intermediates for the synthesis of dyes, agrochemicals,
pharmaceuticals, polymers, and many other fine chemicals. A
number of research groups have developed various types of
heterogeneous catalysts with high efficiency and recyclability
to produce anilines through the hydrogenation of nitroarenes.
Beller and co-workers developed Co3O4@C- and Fe2O3@C-
based heterogeneous catalysts for the hydrogenation of nitro-
arenes under 50 bar of H2 at 110 8C.[9] Xia et al. designed new
functionalized carbon nanotube based Co3O4 NPs as a hetero-
geneous catalyst for the hydrogenation of nitroarene at
2508C.[10] Pd@POP has been employed as a heterogeneous cat-
alyst by Wang et al. for nitroarene hydrogenation with H2
(1 atm).[11] However, compared with traditional hydrogenation,
catalytic transfer hydrogenation (CTH) with hydrogen-donor re-
agents is more advantageous because no special experimental
setup or the use of high-pressure equipment is required, and
hydrogen-donor reagents are abundant, inexpensive, and easy
to handle. Recently, Beller et al. employed the Co3O4-NGr/C
(NGr=modified nitrogen doped graphene layer) catalyst for
the CTH reaction of nitroarenes with HCOOHÀNEt3 at 1008C
for 15 h.[12] Zeolite-supported copper NPs have been utilized as
a catalyst for the selective hydrogenation of nitroarenes in
2-propanol at 808C.[13] Compared with low-cost iron and cobalt
catalysts, ruthenium catalysts have some distinctive features
troarenes at RT, in which a shorter reaction time is required.
DFT computational studies have been performed to predict
whether the hydrogenation reaction occurs through a direct
reduction pathway or an indirect reduction pathway; these cal-
culations show that the hydrogenation of nitrobenzene over
the Ru (0001) catalyst surface through a direct reaction path-
way is more favorable than that through an indirect reaction
pathway.
Scheme 1. Synthesis of nanoporous organic polymer BBA-1 and Ru@POP
materials.
(low melting and boiling points, and binding enthalpies, of Ru0 Results and Discussion
organometallics indicates that the Ru0 (nÀ1) d electrons pro-
Synthesis and characterization of POP materials
vide less polar carbon–metal bonds than those previously re-
ported, with a high turnover frequency (TOF) for dehydrogena-
tion of a metal hydride), which promotes the ability to carry
out several hydrogenation reactions under high partial pres-
sure and an oxygen-containing atmosphere to gain insights for
designing new ruthenium-based catalysts, despite its high cost
and low availability.[13b,c]
A nanoporous organic polymer, BBA-1, with a brown color was
synthesized (Scheme 1) by Friedel–Crafts alkylation of benzyl-
amine and benzene by using formaldehyde dimethyl acetal as
a cross-linker and anhydrous FeCl3 promoter. This brown-col-
ored polymer BBA-1 is insoluble in water and common organic
solvents. Three different materials with Ru NPs embedded in
the BBA-1 polymer were developed through the reduction of
RuCl3·3H2O by following three different reduction strategies.
Coordination bonds were formed between Ru3+ ions and the
ÀNH2 groups present in the BBA-1 polymer, when BBA-1 and
RuCl3·3H2O were stirred in methanol for 24 h at RT. The Ru-A
material was prepared from the isolated solid Ru–polymer
composite by employing the strong reducing agent NaBH4,
which caused some agglomeration of Ru NPs. The Ru-B materi-
al was developed by heating the Ru–polymer composite at
reflux in ethylene glycol; ethylene glycol itself acted as a reduc-
ing agent. The hydrothermal synthesis procedure with ethyl-
ene glycol as a solvent was applied to obtain the Ru-C materi-
al, in which Ru NPs were homogenously dispersed in the inner
porous channels and on the external surface of BBA-1. The
thermal stabilities of BBA-1, Ru-A, Ru-B, and Ru-C were deter-
mined from thermogravimetric analysis (TGA) data (Figure S1
in the Supporting Information). All materials were thoroughly
characterized by XRD, TEM, SEM, EDX, XPS, N2 adsorption/de-
Herein, we have synthesized
a benzene–benzylamine-
1 (BBA-1) POP through Friedel–Crafts alkylation of benzylamine
and benzene by using formaldehyde dimethyl acetal as
a cross-linker with anhydrous FeCl3 as a promoter.[14] Rutheni-
um NPs were successfully embedded in BBA-1 to form Ru-A,
Ru-B, and Ru-C materials (Scheme 1). The BBA-1, Ru-A, Ru-B,
and Ru-C materials were characterized by employing wide-
angle powder XRD, N2 adsorption/desorption isotherms, X-ray
photoelectron spectroscopy (XPS), solid-state cross polarization
magic-angle spinning (CP MAS) 13C NMR spectroscopy, TEM,
energy-dispersive X-ray spectroscopy (EDX), field-emission (FE)
SEM, and FTIR spectroscopy. Ru@POP exhibited excellent cata-
lytic performance in the CTH of nitroarenes to produce ani-
lines, compared with traditional Ru@C, Ru@SiO2, and Ru@TiO2
catalysts, with high reusability and recyclability at RT. To the
best of our knowledge, herein, for the first time, we show that
Ru@POP-based materials act as excellent heterogeneous nano-
catalysts with high conversion and selectivity for the CTH of ni-
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sorption analysis, FTIR spectroscopy, and CP MAS 13C NMR
spectroscopy. The Ru contents in the Ru-A, Ru-B, and Ru-C ma-
terials were 0.1739, 0.0897, and 0.0548 mmolgÀ1, respectively,
as determined by inductively coupled plasma mass spectrome-
try (ICP-MS) analysis. The wide-angle powder XRD patterns of
the pure microporous polymer BBA-1 and Ru NPs encapsulated
in BBA-1 are shown in Figure 1A. The presence of only a single
broad diffraction peak for the BBA-1 polymer indicates its
amorphous nature. Ru-B shows three broad peaks at 2q=38.2,
43.3, and 68.9 due to diffractions of the (100), (101), and (110)
planes, respectively, which correspond to the hexagonal phase
of metallic Ru NPs. Ru-C exhibits peaks at 2q=38.2, 42.2, 43.6,
and 68.58 for the (100), (002), (101), and (110) diffraction
planes, respectively.[15] The Ru-A polymer shows peaks at 2q=
38.3 and 42.28 for the diffraction of the (100) and (002) planes,
respectively. Broad peaks indicate the nanosized nature of
ruthenium particles. BET surface areas and pore size distribu-
tions of the pure polymer and ruthenium-loaded polymers
were measured by analyzing nitrogen adsorption and desorp-
tion isotherms at 77 K (Figure 1B and C). The y values for
BBA-1, Ru-A, and Ru-B have been shifted upwards by 80, 40,
and 10 ccgÀ1, respectively, and the values for Ru-C have been
shifted downwards by 25 ccgÀ1 for clarity (Figure 1B). The BET
surface areas were measured over the relative pressure (P/Po)
range from 0.01 to 0.1. All polymers showed an initial high
uptake, followed by a gradual increase in nitrogen adsorption
with a mild hysteresis, and the steep rise in the high P/Po
region indicates that the material consists of micro- and meso-
pores.[16]
The BET surface areas for BBA-1, Ru-A, Ru-B, and Ru-C are
607.8, 551.6, 523.2, and 543.3 m2 gÀ1, respectively. Pore size dis-
tributions were derived from the adsorption branches of the
isotherms by using the nonlocal (NL) DFT (NLDFT) pore model,
assuming a cylindrical pore geometry (Figure 1C). The calculat-
ed pore volumes for the respective polymer and Ru NP incor-
porated materials are 0.416, 0.420, 0.387, and 0.431 ccgÀ1, re-
spectively. The solid-state CP MAS 13C NMR spectra of pure
BBA-1 and the Ru NP-loaded polymers (Figure 1D) confirmed
the structural integrity of the polymers. Each spectrum exhibits
three signals at d=37.0, 129.1, and 137.0 ppm. The signal at
d=37.0 ppm could be attributed to the benzylic carbon,
whereas the signals at d=129.1 and 137.0 ppm could be as-
signed to the carbon atoms of aromatic rings and the carbon
attached to the benzylic carbon, respectively.[17]
XPS measurements were employed to elucidate the oxida-
tion state of Ru NPs and to show that the NPs were coordinat-
ed by the amine groups of the polymer. The XPS survey spec-
tra (Figure 2A) for all materials demonstrate that, in addition
to binding energy peaks corresponding to C 1s and N 1s (Fig-
ure 2B), a new peak is generated that is attributable to the Ru
3p binding energy. The binding energy peak of N1s (shown in
Figure 2B) centered at 397.9 eV signifies the presence of free
Figure 1. A) Wide-angle powder XRD patterns, B) N2 adsorption/desorption isotherms with some arbitrary shifts, C) pore size distributions, and D) solid-state
CP MAS 13C NMR spectra; the asterisks indicate spinning side bands.
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amine groups in the pure BBA-1 polymer. However, the posi-
tive shift in the binding energy for N 1s to higher energy
(399.0 eV) in the Ru NPs indicates that the metals are coordi-
nated strongly by the nitrogen atoms of the amino groups of
the polymers. In Figure 2C, the binding energy for Ru 3p1/2 in
the Ru-A, Ru-B, and Ru-C materials shows peaks at 485.96,
484.54, and 484.03 eV, respectively, and the binding energy for
3p3/2 gives peaks at 463.45, 462.2, and 462.03 eV, respectively.
These XPS spectra for Ru-A, Ru-B, and Ru-C show that Ru NPs
are in the zero oxidation state in these materials. The positive
shift in the binding energy of Ru 3p for Ru-A can be attributed
to the strong interaction between the polymer network and
Ru NPs, which suggests electron transfer from the metal to or-
ganic linkers, resulting in a partially positively charged metal
phase (Rud+). The binding energy shift of the Ru 3p core level
(ꢀ1.2 eV) in Ru-B and Ru-C, compared with Ru-A, can be ex-
plained by the interaction of surface ÀOH groups of ethylene
glycol causing changes to the electron density over Ru NPs in
these materials. Each of the XPS spectra in Figure 2D–F exhib-
its two prominent bands; the peaks are centered at binding
energies of 284.0 and 281.1 eV, 283.96 and 280.7 eV, and 284.0
and 281.2 eV for Ru-A, Ru-B, and Ru-C, respectively, and the
two peaks can be readily assigned to 3d3/2 and 3d5/2 of Ru0, re-
spectively. It is notable that the Ru0 3d3/2 binding energy peak
overlaps with the C 1s peak at around 283.9 eV.[18]
Elemental mapping and EDX patterns of carbon, nitrogen,
and ruthenium for the Ru-A, Ru-B, and Ru-C materials are
given in Figure 3. The elemental mapping images reveal that
the homogeneous distribution of all elements in the materials
and Ru NPs are homogenously dispersed in the nanoporous
polymer BBA-1. The FTIR spectra (Figure S2 in the Supporting
Information) signify characteristic absorption bands of organic
Figure 2. A) XPS survey spectra; B) N 1s spectra; C) Ru 3p spectra; and Ru 3d spectra of: D) Ru-A, E) Ru-B, and F) Ru-C.
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Figure 4. TEM images of: A–C) Ru-A, D–F) Ru-B, and G–I) Ru-C.
Ru-B and Ru-C, ethylene glycol has been used, which acts as
a stabilizer to control particle growth and prevent aggregation.
Catalytic performance of Ru0–POP-based nanocatalysts
The catalytic performance of all POP-encapsulated Ru NPs was
evaluated by performing the CTH of nitroarenes at RT (258C).
Initially, our study of this catalytic hydrogenation reaction
began by stirring nitrobenzene (0.5 mmol) with Ru-A (5 mg) in
EtOH (5 mL) at RT for 12 h (Table 1, entry 1); however, no con-
version was observed. We then utilized formic acid (2.5 equiv)
as a hydrogen source in EtOH and stirred the reaction mixture
at RT. After 8 h, 52% conversion was achieved (Table 1,
entry 2). We slowly increased the temperature of the reaction
mixture to 808C, and the progress of the reaction was ob-
served to deliver 68% conversion after 6 h (Table 1, entry 3).
iPrOH was also used as a hydrogen-donor source for this cata-
lytic hydrogenation of nitrobenzene by stirring the reaction
mixture in DMF for 6 h with 43% conversion (Table 1, entry 4).
We also heated the reaction mixture at 758C and observed
that 70% conversion was reached after 4 h (Table 1, entry 5).
Glycerol was also utilized as a hydrogen source for this catalyt-
ic hydrogenation of nitrobenzene in diglyme by heating the
reaction mixture at 1258C. After 4 h, 82% conversion could be
observed (Table 1, entry 6). We also carried out this catalytic re-
action with 1 bar of H2 gas at RT in the presence of Ru-A. After
4 h, 85% conversion was attained (Table 1, entry 7). We then
examined the CTH of nitrobenzene (0.5 mmol) with NaBH4
(1 mmol) in the presence of a mixture of THF/H2O (1:3 v/v) and
Ru-A (5 mg) at RT (258C).
Figure 3. EDX analysis and corresponding elemental mapping of: A) C, N,
and Ru for Ru-A, B) Ru-B, and C) Ru-C.
functional groups of the polymer. A TEM image of nanoporous
polymer BBA-1 is provided in Figure S3 in the Supporting Infor-
mation.[19]
TEM images of Ru-A (Figure 4A–C) clearly indicate that Ru
NPs (black spots), varying in size from 3.5 to 7.0 nm with a cer-
tain degree of agglomeration, are uniformly distributed on the
external outer surface of the porous polymer. This suggests
that the nitrogen-rich polymer cannot entirely inhibit aggrega-
tion on the external surface of the NPs through Oswald ripen-
ing. The images in Figure 4D–F also reveal uniform dispersion
of Ru NPs with sizes of 1.5 to 4.0 nm in Ru-B. The Ru NPs of
Ru-C, with spherical morphology and dimensions of 2–4 nm
(Figure 4G–I), are well dispersed in the nanoporous polymer
BBA-1. In this case, the particle size is small enough, so that
they can be easily encapsulated in the interior cavities of nano-
porous polymer BBA-1. During the course of the synthesis of
The catalytic reaction finished in 2.5 h with 92% conversion
(Table 1, entry 8). We increased the ratio of NaBH4 from 1 to
1.5 mmol (Table 1, entry 9) and observed 98% conversion after
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tions. However, conversions of only 62.5 and 81.5%
were achieved with 82 and 100% product selectivity
for the Ru-B and Ru-C catalysts, respectively (Fig-
ure 5A). For comparison, the catalytic activities of
some conventional Ru NP-based catalysts were also
assessed by performing the same hydrogenation re-
action of nitrobenzene under identical reaction con-
ditions. Conventional catalysts, such as Ru@SiO2,
Ru@C, and Ru@TiO2, were prepared by applying the
same preparation method as that used for the syn-
thesis of the Ru-A catalyst (the detailed reaction pro-
cedure is given in the Supporting Information). The
amounts of all conventional Ru catalysts were em-
ployed for catalytic purposes in such a way that the
Ru loading was 0.17 mol%. Figure 5A shows that
52.5, 55.7, and 65.3% product conversions were at-
tained for similar CTH reactions of nitrobenzene with
Ru@SiO2, Ru@C, and Ru@TiO2, respectively. The nano-
porous support matrix allows easy diffusion of organ-
ic substrates that can therefore interact with the cat-
alytically active Ru NPs present in the nanoporous
channel, as well as in the external surface area, which
leads to improvement of the catalytic activity of the
bare Ru NPs (Figure 5A). The catalytic performance of
the Ru@POP nanocatalysts is superior to that of the
Ru@C and Ru@SiO2 catalysts, although the Ru@C cat-
alyst demonstrates better catalytic performance than
the Ru@SiO2 catalyst. This result implies that the hy-
Table 1. Optimization of the reaction conditions for the catalytic hydrogenation of ni-
trobenzene by the Ru nanocatalyst.[a]
Entry
H donor
Catalyst
Solvent
t
[h]
Conv.
[%]
1
2
3[b]
no additive
HCOOH
HCOOH
Ru-A
Ru-A
Ru-A
EtOH
EtOH
EtOH
12
8
6
0
52
68
4
Ru-A
Ru-A
DMF
DMF
6
4
43
70
5[c]
6[d]
Ru-A
diglyme
4
4
2.5
2
0.5
0.5
0.5
0.5
6
82
7
8
9
10
11[e]
12[f]
13[g]
14[h]
15[i]
H2 gas (1 bar)
Ru-A
Ru-A
Ru-A
Ru-A
Ru-A
Ru-A
Ru-A
Ru-BBA-1
Ru-A
EtOH
85
92
98
98
45
25
98
32
73
NaBH4 (1 mmol)
NaBH4 (1.5 mmol)
NaBH4 (2.5 mmol)
NaBH4 (2.5 mmol)
NaBH4 (2.5 mmol)
NaBH4 (2.5 mmol)
NaBH4 (2.5 mmol)
HCOONa
THF/H2O
THF/H2O
THF/H2O
THF/H2O
THF/H2O
THF/H2O
THF/H2O
H2O
6
[a] Reaction conditions: nitroarene (0.5 mmol), Ru-A (5 mg, 8.695”10À7 mol Ru), sol-
vent (5 mL), RT (258C). [b] Reaction temperature 808C. [c] Reaction temperature 758C.
[d] Reaction temperature 1258C. [e] Ru-A (2 mg, 3.478”10À7 mol Ru). [f] Ru-A (1 mg,
1.739”10À7 mol of Ru). [g] Ru-A (10 mg, 1.739”10À6 mol of Ru). [h] Ru–BBA-1 catalyst
was prepared by physical mixing. [i] Reaction temperature 1008C; conversion [%] de-
termined by GC analysis with hexadecane as an internal standard.
2 h. When 2.5 mmol of NaBH4 was employed for this reaction,
surprisingly the reaction was complete within 30 min (Table 1,
entry 10), affording 98% conversion of nitrobenzene into ani-
line (conversion was measured by GC analysis in the presence
of hexadecane as an internal standard). The catalytic per-
formance appeared to be dependent on the amount of cata-
lyst (mol% loading of Ru), as proven by carrying out the reac-
tion with different catalyst loadings. The reaction with catalyst
loadings of 2 (3.478”10À7 mol of Ru) and 1 mg (1.739”
10À7 mol of Ru) afforded conversions of 45 and 25%, respec-
tively (Table 1, entries 11 and 12). However, no substantial
change in product conversion was observed with a drastic in-
crease in the amount of catalyst (Table 1, entry 13). Hence, we
can conclude that 5 mg is the optimum amount of catalyst re-
quired to conduct this catalytic hydrogenation reaction. The
physical mixture (Ru–BBA-1) exhibited a much lower catalytic
activity (32% conversion after 6 h) than that with the Ru-A cat-
alyst under identical conditions (Table 1, entry 14). This result
established a clear synergistic effect for the Ru NPs inside the
nanoporous channel of POP. We also examined the catalytic
hydrogenation reaction with HCOONa in the presence of Ru-A,
which afforded the corresponding amine with 73% conversion
after 6 h (Table 1, entry 15). All Ru–POP-based catalysts (Ru-B
and Ru-C) were subsequently screened for this catalytic hydro-
genation of nitrobenzene to aniline under our optimized reac-
tion conditions. All catalytic reactions were carried out in the
presence of a 0.17 mol% loading of Ru in Ru@POP catalysts.
We also employed Ru-B and Ru-C (0.17 mol% loading of Ru)
for the CTH of nitrobenzene under identical reaction condi-
drophobicity of the support matrix is the most important
factor that controls this CTH reaction. The hydrophobic nature
of the support matrix surface decreases in the order of porous
polymer>porous carbon>porous SiO2. This trend was estab-
lished by performing contact-angle measurements (Figure S4
in the Supporting Information). The organic substrate is ad-
sorbed onto the nanoporous matrix for the catalytic reaction
because the super-hydrophobic nature of the as-synthesized
BBA-1 POP results in strong hydrophobic and p–p stacking in-
teractions between the hydrophobic organic substrate and
support matrix. This also facilitates desorption of the hydro-
philic product into the reaction mixture. To determine the ad-
vantage of nitrogen-rich highly cross-linked porous organic
polymeric compounds as supports, a variety of non-nitrogen-
based organic supports, including organic mesoporous carbon
(OMC), commercial multiwalled carbon nanotubes (CNT), and
hydrothermal carbon (HC), were used to fabricate Ru NPs on
the supports and were examined for the catalytic efficiency of
those catalysts in the hydrogenation of nitrobenzene to aniline
under identical reaction conditions.
Among them, Ru@OMC, Ru@CNT, and Ru@HC catalysts re-
sulted in nitrobenzene conversions to aniline of 62.5, 39.1, and
28.5%, respectively (Figure 5A). These results demonstrate that
nitrogen-rich highly cross-linked POP plays an important role
when it is utilized as a support for the development of Ru-
based catalysts; thus, the Ru@POP-based materials have a high
performance and show great promise as heterogeneous cata-
lysts in hydrogenation. We also performed a comparative
study on the catalytic conversion of nitrobenzene under opti-
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compared with the other catalysts with smaller particle sizes.
Larger Ru NPs interact favorably with organic reactant mole-
cules through their more exposed, highly crystalline facets,
leading to an improvement in catalytic conversion over other
Ru–POP-based nanocatalysts (Figure 5B). We believe that,
during the course of the catalytic reaction, the first of the two
oxygen atoms of the ÀNO2 group of nitrobenzene coordinates
to the exposed Ru (001) surface. This interaction is highly fa-
vored when the positive charge density on the Ru surface is in-
creased, resulting in an enhancement of the catalytic per-
formance of the Ru-A catalyst compared with the other cata-
lysts, and this distinct catalytic performance is affected by the
electronic environment, as revealed in the XPS spectra. We
compared the catalytic performance of our newly developed
cocatalyst with other previously reported heterogeneous cata-
lysts for the catalytic hydrogenation of nitroarene (Table 2). All
catalysts required a high catalyst loading (5 wt% or 10 mol%),
high hydrogen pressure, excess use of hydrogen-donating
agent (10 mmol), and had low recyclability. The results in
Table 2 demonstrate that the Ru@POP catalyst is superior to
previously reported catalysts because we have carried out our
catalytic reactions with a very low loading of catalyst under
very mild conditions, and our catalyst could be recycled in sev-
eral catalytic runs without significant loss of catalytic activity.
The superior catalytic efficiency of the Ru@POP materials com-
pared with those of other conventional catalysts may be due
to the synergetic effect of nanoconfinement and electron don-
ation of BBA-1 POP. The amine organic backbone unit of 3D
BBA-1 POP could coordinate to Ru0 through p-electron dona-
tion, so Ru surfaces could acquire enhanced activity for the de-
hydrogenation of NaBH4, and thus, efficient H2 generation,
thereby facilitating the transfer hydrogenation of nitroarenes.
The high surface area of POP allows easy diffusion of organic
substrates, thereby involving interactions with catalytic active
centers to enhance catalytic rates, and the rigidity of the poly-
mer increases recyclability by diminishing the leaching of Ru
from backbone unit. By using the Ru-A catalyst and employing
various substituted nitroarenes with different steric and elec-
tronic characters in a mixture of THF and H2O (1:3 v/v) at RT
(258C), we examined the substrate scope of this CTH reaction.
Full conversions (100%) to the corresponding products with
high selectivities were provided by nitroarenes with electron-
Figure 5. A) A comparison of the catalytic performance of all Ru-based nano-
catalysts for the CTH of nitrobenzene to aniline. Reaction conditions: nitro-
benzene (0.5 mmol), NaBH4 (2.5 mmol), THF/H2O (1:3 v/v), Ru catalyst
(8.695”10À7 mol), for 30 min at RT (258C). B) Conversion of nitrobenzene to
aniline as a function of time for the hydrogenation reaction. Ru loadings in
all Ru catalysts were 0.17 mol%.
mized reaction conditions against time by employing different
Ru–POP-based nanocatalysts with different Ru particle sizes
(Figure 5B). To investigate the reaction performance with dif-
ferent Ru–POP-based catalysts, product evolution for the hy-
drogenation reaction was observed by continuous sampling
by GC at 10 min intervals. Ru NPs with large particle sizes
(ꢀ7 nm) and highly crystalline features embedded on the POP
support for the Ru-A nanocatalyst are more beneficial for CTH
Table 2. Comparison of catalytic efficiency for the catalytic hydrogenation of nitroarenes with various catalysts.
Catalyst[a]
Amount of catalyst
H2 source
Reaction conditions
Ref.
Ru–BBA-1
Fe2O3@C
Pd@POP
Ni-PVAm/SBA-15
FeÀNi bimetallic NPs
Pd-pol
0.17 mol% Ru
4.5 mol% Fe
0.5 mol% Pd
0.12 g catalyst
10 mol%
5 wt% Pd
1 mol%
10 mg catalyst
1 mol% Au
NaBH4 (2.5 mmol)
50 bar H2
1 atm H2
NaBH4
NaBH4
10 mmol NaBH4
50 bar H2
2 mmol N2H4
CO/H2O
THF/H2O 258C
THF/H2O 1208C
258C, 1 h
258C
258C
H2O, 2 h
THF/H2O 1108C
RT
258C, 3 h
our work
[9a]
[11]
[26]
[27]
[28]
[9b]
[29a]
[29b]
Co3O4@C
Rh NPs/SBA[b]
Au/TiO2-VS
[a] Ni-PVAm=nickel polyvinylamine, Pd-pol=polymer-supported Pd-nanoparticles, Au/TiO2-VS=very small Au Nanoparticles. [b] Catalyst can be recycled
up to six times.
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donating groups (Table 3, entries 2 and 3). The ÀOH,
ÀF, and ÀNH2 functional groups were well tolerated
for this CTH reaction (Table 3, entries 4, 8, and 10, re-
spectively). 4-Chloronitrobenzene (Table 3, entry 6)
gave 100% product conversion to 4-chloroaniline in
30 min and no dechlorinated product of aniline was
observed. 4-Nitroacetophenone underwent 98% con-
version with 92% selectivity for the corresponding
aniline derivative, but, surprisingly, some carbonyl re-
duced product was obtained with 8% selectivity
(Table 3, entry 5). Our catalyst was so active that the
hydrogenation of 2,4-dinitrochlorobenzene (Table 3,
entry 7) was achieved with 100% selectivity for the
corresponding dianiline product. 1-Nitronaphthalene
and 5-nitroindole (Table 3, entries 9 and 13) exhibited
significant conversions in the CTH reaction. Success-
ful CTH reactions were carried out for 4-nitrobenzyl
alcohol and 1-iodo-4-nitrobenzene (Table 3, entries 11
and 12). Our newly developed Ru-A catalyst was also
effective for carrying out the large-scale CTH reaction
of 4-nitrotoluene with consistent product conversion
(Table 3, entry 14). We carried out two additional cat-
alytic processes to illustrate the versatility and poten-
tial of our newly designed POP-based catalytic
system. We conducted the CTH reaction of an inter-
nal alkyne (diphenyl acetylene) to provide the alkene
with E/Z selectivity and very high conversion with
our Ru@POP nanocatalyst in the presence of formic
acid as a hydrogen source. The reaction methodolo-
gy is provided in Table S1 in the Supporting Informa-
tion. We also performed the hydrogenation of ligno-
cellulosic biomass-derived levulinic acid to give
value-added g-valerolactone (GVL) with 85% conver-
sion and 95% selectivity by employing Ru@POP cata-
lyst. The reaction methodology is provided in
Table S2 in the Supporting Information.
Table 3. The CTH reaction of different nitroarenes catalyzed by the Ru-A nano-
catalyst.[a]
Entry
Substrate
t
Conv.[b]
[%]
Selectivity
[%]
TOF
[minÀ1
]
[min]
1
2
3
4
5
6
30
45
45
60
90
30
100
100
100
100
98
100
100
100
100
92
19.1
12.7
12.7
9.5
6.26
19.1
100
100
7
8
120
60
100
100
100
100
4.79
9.5
9
60
99
98
9.4
10
11
12
45
45
80
99
98
99
100
100
88
12.6
12.5
7.1
13
60
45
99
99
99
9.4
2.5
14[c]
100
[a] Reaction conditions: nitroarene (0.5 mmol), Ru-A catalyst (5 mg, 8.695”10À7 mol),
THF/H2O (1:3 v/v), NaBH4 (2.5 mmol), RT (258C); selectivity refers to the corresponding
amine derivative. [b] Determined by GC analysis with hexadecane as an internal stan-
dard. [c] The reaction was carried out on a 10 mmol scale.
Experimental and computational DFT investigation
into the reaction mechanism
mation) to observe the formation of phenylhydroxylamine, ni-
trosobenzene (PhNO), and azoxybenzene intermediates.[23] Ac-
Adsorption of an aromatic molecule on a metal surface de-
pends on the type of molecule, the orientation, and nature of
the metal surface.[20] Among the three possible binding config-
urations of nitrobenzene on the Ru (0001) surface, the parallel
orientation has the highest adsorption energy (44.59 kcal
molÀ1; Figure S6 in the Supporting Information), with a CÀRu
distance of 2.2 Š, as supported by previously reported benzene
adsorption on Ru (0001).[21]
cording to the mechanism proposed by Pathak et al. for
a nickel catalyst,[22a] there are two possible pathways for the
formation of aniline, namely, direct and indirect pathways (Fig-
ure S7 in the Supporting Information). To determine the most
plausible pathway, we performed DFT calculations. Because
a full exploration of the potential energy surface through de-
termination of transition-state structures is computationally
prohibitive for this system, we optimized the geometries of
key intermediates in the direct and indirect pathways (Fig-
ure S7 in the Supporting Information) to compare the stability
of the intermediates by following the scheme provided by
Pathak et al.[22a] To further reduce the computational cost, hy-
dride transfer from NaBH4, which may yield a chemisorbed hy-
drogen atom, was not considered; rather, we assumed that the
hydrogen source on the Ru (0001) surface was responsible for
reduction. In addition, since the diffusion of a hydrogen atom
on the Ru (0001) surface should be fast, we focused on the
The two oxygen atoms coordinate to ruthenium atoms,
while the nitrogen atom bulges upwards, as in the cases of ni-
trobenzene adsorption on Ni (111) and Pt (111).[22] The most
stable parallel adsorption configuration was used as a starting
geometry for subsequent reaction pathway calculations. To in-
vestigate the catalytic hydrogenation pathway (Figure S7 in
the Supporting Information), the progress of the reaction was
monitored by UV/Vis (Figure S8 in the Supporting Information)
1
and H NMR spectroscopy (Figure S9A in the Supporting Infor-
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thermodynamic profile of the reacting nitrobenzene mole-
cule.[24]
dicates that the indirect pathway, especially the process after
the formation of azoxybenzene, is not energetically feasible.
PhÀNHÀNOHÀPh will then release one water molecule to form
azobenzene (PhÀNÀNÀPh) on the surface. A subsequent two-
hydrogen uptake process leads to the formation of 1,2-diphe-
nylhydrazine (PhÀNHÀNHÀPh), which is again a steeply uphill
process with a large energy increase (+26.57 kcalmolÀ1). This
energy increase also suggests that the indirect pathway is not
involved in the reduction of nitrobenzene on the ruthenium
surface. Aniline is produced after accepting two hydrogen
atoms from PhÀNHÀNHÀPh. Surprisingly, GC-MS analysis of the
reaction mixture (Figure S9B in the Supporting Information)
Nitrobenzene is first reduced to PhNO by the addition of
two hydrogen atoms to release one water molecule with
a large amount of energy (À22.95 kcalmolÀ1; Figure 6). The
PhNO thus formed would follow either the direct or indirect
pathways to be followed. In the direct pathway, there are two
possibilities for the hydrogen atom to bind to the substrate
molecule, through either the nitrogen or oxygen atoms, to
form PhNHO or PhNOH, respectively (Figure 6). In contrast to
the exothermic reaction for the formation of PhNHO, the for-
mation of PhNOH is slightly endothermic. The difference origi-
nates from a change in bonding after the addition of one hy-
drogen atom. When PhNHO is formed, the hydrogen atom in-
creases the valency of the nitrogen atom, and the NÀRu
(2.25!2.22 Š) and OÀRu (2.08!2.11 Š) distances are barely af-
fected. By contrast, in the formation of PhNOH, the OÀRu dis-
tance is increased significantly by 0.27 Š. These two intermedi-
ate states are then converted into N-phenylhydroxylamine
(PhNHOH), after the uptake of another hydrogen atom. The en-
dothermic process (Ereaction =10.69 kcalmolÀ1), when consider-
ing the formation of PhNHOH from PhNHO, should be restrict-
ed; however, this endothermocity is not considered to be as
critical as that for the indirect pathway. Subsequent sequential
uptake of two hydrogen atoms generates aniline, which is
seen to be thermodynamically very stable.
1
also shows the signal for PhÀNÀNÀPh. However, the H NMR
spectrum of the reaction mixture does not display any signifi-
cant signals for PhÀNÀNÀPh (Figure S9A in the Supporting In-
formation), which implies that PhÀNÀNÀPh is not generated in
a significant or quantitative amount as a reaction intermediate.
An insignificant amount of PhÀNÀNÀPh could presumably be
formed from azoxybenzene in the presence of excess NaBH4. It
may also be possible that PhÀNÀNÀPh is produced from phe-
nylhydroxylamine through dehydration.[25] We examined the
conversion of pure phenylhydroxylamine and PhÀNÀNÀPh to
aniline under the same optimized reaction conditions. A high
selectivity for aniline production (92.5%) was achieved with
63.8% aniline conversion when pure phenylhydroxylamine was
used as a hydrogenation substrate. Moreover, in the case of
the direct reduction of pure PhÀNÀNÀPh to give aniline, no
aniline formation could be observed, even after a prolonged
reaction time. Thus, together with the experimental data, our
calculations suggest that the direct pathway is more favorable.
In the indirect pathway, there are two thermodynamically diffi-
cult uphill processes (Figure 7). Even if azoxybenzene is formed
on the surface (Figure 7), this species is likely to be decom-
posed because the energy of PhÀNHÀNÀOHÀPh is much
higher than that of PhNO+PhNHOH. Importantly, the reaction
of PhNHOH with a hydrogen atom to release one water mole-
cule is a highly exothermic process (Figure 6).
In the indirect pathway, PhNO and PhNHO molecules
formed in the direct pathway are involved. The reference
energy of the system is assumed to be the sum of the two in-
termediates (À22.95+(À16.45)=À39.40 kcalmolÀ1), at which
these two molecules are placed far apart on the surface. The
energy of the system is decreased by 2.52 kcalmolÀ1 when the
two molecules come close to each other. The subsequent re-
lease of one water molecule and the formation of the NÀN co-
valent bond lead to a considerable energy decrease from
À41.92 to À56.98 kcalmolÀ1, which suggests the formation of
1
azoxybenzene, as supported by H NMR spectroscopy and GC-
MS analyses (Figure S9A and B in the Supporting Information).
The coupled molecule then takes up two hydrogen atoms and
forms PhÀNHÀNOHÀPh, with a stepwise energy increase to
À27.26 kcalmolÀ1. An increase in energy by 29.72 kcalmolÀ1 in-
Recyclability and heterogeneous nature of the Ru-A nanoca-
talyst
The efficacy of the Ru0 nanocatalyst was evaluated in terms of
recyclability and reusability by carrying out the CTH reaction of
nitrobenzene with the Ru-A nanocatalyst under optimized re-
action conditions. After completion of the reaction, the nano-
catalyst was recovered by centrifugation of the reaction mix-
ture, subsequently washed with ethanol followed by acetone,
and dried at 608C before being used in the next catalytic run.
No special technique, such as the addition of acid or base or
calcinations at higher temperature, was needed for the reacti-
vation of our catalyst. Recycling tests for the Ru-A nanocatalyst
suggested that the nanocatalyst could be effectively reused for
10 consecutive catalytic cycles (Figure 8). A drop in conversion
between the 10th and 11th cycles from 87 to 80% was record-
ed; this may be due to the clogging of some catalytic active
sites with organic reagents during the course of the reaction,
although the selectivity is maintained in each catalytic run. To
Figure 6. Energy profile for PhNO formation and the direct reduction path-
way. The energy of the system with one nitrobenzene molecule adsorbed
on the surface is taken as 0.00 kcalmolÀ1
.
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the oxidation state of Ru0 NPs remains unchanged after the
catalytic reaction (Figure 9C). The wide-angle powder XRD pat-
tern of reused Ru-A nanocatalyst demonstrates that the hexag-
onal phase of metallic Ru NPs in the porous polymeric frame-
work remains unchanged (Figure 9D). All of these results signi-
fy that our newly developed nanocatalyst is stable and the
structural integrity of organic functional groups on the nano-
porous polymer backbone is preserved under the proposed
methodology for the hydrogenation reaction.
To determine whether the CTH reaction of nitrobenzene
with POP-encapsulated Ru0 NPs is indeed heterogeneous in
nature, we performed leaching tests (see the Experimental Sec-
tion) by considering nitrobenzene as a model substrate under
the optimized reaction conditions: after 15 min, 65% conver-
sion was attained. The catalytic hydrogenation reaction was
then continued for an additional 30 min with the filtrate only
after removing the ruthenium nanocatalyst by centrifugation.
However, no change in conversion was observed. Atomic ab-
sorption spectroscopy (AAS) was employed to verify whether
any leaching of ruthenium took place during the course of the
catalytic reaction. The results indicate that the ruthenium con-
tent in the filtrate was below the detection limit of the instru-
ment and the filtrate was completely colorless. The Ru content
in the Ru-A nanocatalyst after the 11th catalytic run was
0.169 mmolgÀ1, as determined by ICP-MS analysis, which was
analogous to that of the fresh sample (0.173 mmolgÀ1). This
study clearly established that no ruthenium leaching occurred
during the course of the reaction and our catalyst is truly het-
erogeneous in nature.
Figure 7. Energy profile for the indirect pathway. In the first step, there is an
energy drop as PhNHOH and PhNO come closer to each other.
explain these results, we performed N2 adsorption–desorption
isotherm studies of the reused Ru-A catalyst after 11 cycles
(Figure S10 in the Supporting Information). A decrease in BET
surface area (from 551.6 to 482.6 m2 gÀ1) led us to conclude
that the accumulation of external materials hindered N2 ad-
sorption in the pore space (reduction of pore volume from
0.416 to 0.325 ccgÀ1) and slight degradation of the supported
catalyst into less active forms (extraneous loss in catalyst re-
covery in the 11th cycle) might also be an important contribu-
tor to successive decreases in catalytic conversion, although
we were not aware of possible degradation products.
To check the mechanical stability of the Ru-A nanocatalyst
after the 11th catalytic cycle, we characterized the reused Ru-A
nanocatalyst. TEM analysis of the Ru-A nanocatalyst after the
11th cycle indicates no clear aggregation of Ru NPs during the
course of the catalytic run (Figure 9A). Solid-state CP MAS
13C NMR spectroscopic analysis reveals that the structural in-
tegrity of organic functional groups on the nanoporous poly-
mer backbone was retained (Figure 9B). The XPS spectrum of
the Ru 3d region of the reused Ru-A nanocatalyst signifies that
Conclusion
We successfully developed a new POP (BBA-1) by Friedel–
Crafts alkylation of benzylamine and benzene with formalde-
hyde dimethyl acetal as a cross-linker and anhydrous FeCl3 as
a promoter. The Ru0 NPs embedded in the nanoporous chan-
nels of this POP behave as a new efficient heterogeneous
nanocatalyst for carrying out the CTH of nitroarenes with high
reusability and recyclability at RT, compared with traditional
Ru@C, Ru@SiO2 and Ru@TiO2 catalysts. This enhancement of
the catalytic performance and recyclability of the Ru-POP
nanocatalysts was observed due to the synergetic effect of
nanoconfinement and electron donation offered by the 3D
POP network. No metal leaching and the robust integrity of
POP make the present materials attractive nanocatalysts for
the CTH of nitroarenes. Results from DFT calculations suggest
that hydrogenation of nitrobenzene over the Ru (0001) catalyst
surface occurs through the direct reaction pathway rather than
the indirect reaction pathway.
Experimental Section
Synthesis of BBA-1
Figure 8. Recyclability and reusability of the Ru-A nanocatalyst for the hy-
drogenation of nitrobenzene. Reaction conditions: nitrobenzene (0.5 mmol,
61 mg), Ru-A catalyst (5 mg, 0.17 mol%), THF/H2O (1:3 v/v), NaBH4
(2.5 mmol, 94 mg), RT (258C), 30 min.
Benzylamine (0.214 g, 2 mmol) and benzene (0.156 g, 2 mmol)
were added to anhydrous 1,2-dichloroethane (10 mL), followed by
the addition of formaldehyde dimethyl acetal (0.608 g, 8 mmol).
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Figure 9. A) TEM image, B) CP MAS 13C NMR spectrum, C) Ru 3d XPS spectrum, and D) wide-angle powder XRD pattern of the reused Ru-A nanocatalyst after
the 11th catalytic run.
Then iron(III) chloride (1.298 g, 8 mmol) was added and the reac-
tion mixture was heated to 808C for 20 h under a nitrogen atmos-
phere. The reaction mixture was cooled to RT, filtered, washed
thoroughly with methanol until the filtrate became colorless, and
then dried in a vacuum oven at 808C for another 24 h. The materi-
al was again washed with methanol by Soxhlet extraction for 48 h
to give a brown material, which was designated as BBA-1.
evaporated to dryness to obtain a brown-colored solid Ru–polymer
composite material. This material was dispersed in ethylene glycol
(50 mL) and heated at 1808C for 3 h. After cooling to RT, the mate-
rial was separated by centrifugation and washed repeatedly with
MeOH to remove ethylene glycol. This black material was designat-
ed as the Ru-B catalyst.
Synthesis of Ru-C
Synthesis of Ru-A
BBA-1 (50 mg) was dispersed in MeOH (60 mL) by stirring at RT for
2 h. Then RuCl3·3H2O (15 mg) was dissolved in MeOH (5 mL) and
added to the previous polymer reaction mixture. The resulting re-
action mixture was stirred at RT for 24 h. Methanol was evaporated
to dryness to deliver a brown-colored solid material. This material
was dispersed in ethylene glycol (50 mL) and hydrothermally treat-
ed in an autoclave at 1808C for 4 h. After cooling to RT, the materi-
al was separated by centrifugation and washed repeatedly with
MeOH to remove ethylene glycol. This black-colored material was
designated as the Ru-C catalyst.
BBA-1 (50 mg) was dispersed in MeOH (60 mL) by stirring at RT for
2 h. Then RuCl3·3H2O (15 mg) was dissolved in MeOH (5 mL) and
added to the previous polymer reaction mixture. The resulting re-
action mixture was stirred at RT for 24 h. Methanol was then
evaporated to dryness to deliver brown-colored solid Ru–polymer
composite material. This material was dispersed in deionized H2O
(100 mL) by sonication and stirred at RT for 4 h. Then a solution of
NaBH4 (5 mL, 1m; cooled in ice) was added to the above reaction
mixture and allowed to stir for 15 min at RT under a N2 atmos-
phere. This material was separated by centrifugation and washed
repeatedly with MeOH followed by drying at RT. This isolated ma-
terial was designated as Ru-A.
CTH of nitroarenes with Ru-based catalysts at RT
In a typical CTH procedure, a mixture of nitrobenzene (0.0615 g,
0.5 mmol) with supported Ru@POP nanocatalyst (5 mg) mixed with
THF (1 mL) was placed in a dry 25 mL sealed tube. Then the Ru
catalyst was dispersed in the mixture by sonication. Then NaBH4
(0.094 g, 2.5 mmol) in water (3 mL) was added to the previous re-
action mixture. The tube was capped tightly and the reaction mix-
ture was allowed to stir at RT (258C) for the time reported. The re-
Synthesis of Ru-B
BBA-1 (50 mg) was dispersed in MeOH (60 mL) by stirring at RT for
2 h. Then RuCl3·3H2O (15 mg) was dissolved in MeOH (5 mL) and
added to the previous polymer reaction mixture. The resulting re-
action mixture was stirred at RT for 24 h. Methanol was then
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S.K.K. thanks CSIR, New Delhi, for his senior research fellowship.
A.B. thanks DST, New Delhi, for financial support. J.M., P.B., and
Y.Z. thank the Singapore National Research Foundation Fellow-
ship (NRF2009NRF-RF001-015), the Singapore National Re-
search Foundation and Campus for Research Excellence and
Technological Enterprise (CREATE) Programme-Singapore
Peking University Research Centre for a Sustainable Low-
Carbon Future, and the NTU-A*Star Centre of Excellence for Sil-
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Keywords: density functional calculations · heterogeneous
catalysis · hydrogenation · polymers · ruthenium
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Received: October 9, 2015
Published online on November 17, 2015
Chem. Eur. J. 2015, 21, 19016 – 19027
www.chemeurj.org
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