Unique nanocages of 12CaO·7Al2O3 boost heterolytic hydrogen activation and selective hydrogenation of heteroarenes over ruthenium catalyst

Article abstract of DOI:10.1039/c6gc02782b

The chemoselective hydrogenation of heteroarenes is one of the most important synthetic reactions for the production of key intermediates in agrochemicals, pharmaceuticals and various fine chemicals. The development of new heterogeneous catalysts for the environmentally benign synthesis of heterocycle hydrogenated products is a fundamental objective for chemists. Here, we report that 12CaO·7Al₂O₃ with a unique sub-nanocage structure loaded with Ru nanoparticles exhibits higher activity, chemoselectivity and sustainability for the hydrogenation of heteroarenes in a solvent-free system than traditional oxide-supported metal catalysts. Conversion of >99% and a selectivity close to 99% were achieved for the hydrogenation of quinoline under mild conditions. This catalyst was also successfully applied to the hydrogenation of a variety of N- and O-heteroarenes with high yields. The superior catalytic performance can be attributed to a cooperative effect between the hydrogen-storage ability and large amount of strong basic sites on the surface of the support, which promotes heterolytic H₂ cleavage and prevents poisoning of the metal surface caused by the adsorption of heteroarenes.

Full text of DOI:10.1039/c6gc02782b

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Unique nanocages of 12CaO·7Al2O3 boost
heterolytic hydrogen activation and
selective hydrogenation of heteroarenes
over ruthenium catalyst†
Cite this: DOI: 10.1039/c6gc02782b
Tian-Nan Ye,a,b Jiang Li,a,b Masaaki Kitano*a and Hideo Hosono*a,b,c
The chemoselective hydrogenation of heteroarenes is one of the most important synthetic reactions for
the production of key intermediates in agrochemicals, pharmaceuticals and various fine chemicals. The
development of new heterogeneous catalysts for the environmentally benign synthesis of heterocycle
hydrogenated products is a fundamental objective for chemists. Here, we report that 12CaO·7Al2O3 with a
unique sub-nanocage structure loaded with Ru nanoparticles exhibits higher activity, chemoselectivity
and sustainability for the hydrogenation of heteroarenes in a solvent-free system than traditional oxide-
supported metal catalysts. Conversion of >99% and a selectivity close to 99% were achieved for the
hydrogenation of quinoline under mild conditions. This catalyst was also successfully applied to the
hydrogenation of a variety of N- and O-heteroarenes with high yields. The superior catalytic performance
can be attributed to a cooperative effect between the hydrogen-storage ability and large amount of
strong basic sites on the surface of the support, which promotes heterolytic H2 cleavage and prevents
poisoning of the metal surface caused by the adsorption of heteroarenes.
Received 6th October 2016,
Accepted 25th November 2016
DOI: 10.1039/c6gc02782b
www.rsc.org/greenchem
rhodium, gold, ruthenium, and iridium.4 Among these pre-
cious metals, ruthenium is the cheapest with a price of $42
Introduction
The catalytic hydrogenation reaction is of fundamental interest per oz (BASF, Mar 2016), which is one tenth to one thirtieth
in synthetic chemistry and is applied on a large scale for the the price of the other noble metals. A noble metal-free catalyst
production of fine chemicals, agrochemicals, and pharmaceu- based on N-graphene-modified cobalt nanoparticles has very
ticals.1 Among the various hydrogenation reactions, selective recently been reported for the chemoselective hydrogenation
hydrogenation of heteroarenes is a significant scientific and of heteroarenes, with excellent chemoselectivity for numerous
technological challenge due to the high resonance stability structurally diverse substrates.5 However, this catalyst requires
and potential deactivation of the catalyst by covering of the relatively long reaction time (more than 48 h) at 120 °C.
catalyst surface with the substrate or its reduced products.2 Therefore, the relative low cost and high availability of ruthe-
The hydrogenation of quinolines often takes place on their nium makes it promising for large-scale commercial pro-
benzene ring to selectively give 5,6,7,8-tetrahydroquinoline, cesses. However, ruthenium catalysts also generally require
which is also known as a subunit in numerous medicinally severe conditions, organic solvents, and environmentally
interesting compounds.3 For the selective hydrogenation of harmful processes.6 Thus, from both economic and environ-
heteroarenes, the most efficient and practical catalysts are mental perspectives, it is imperative to design a new ruthe-
mainly based on noble metals such as platinum, palladium, nium-based catalyst that is efficient for the hydrogenation of
heteroarenes under mild conditions, and even using a solvent-
free system.
Homogeneous catalysts are known to be catalytically active
and selective; however, they typically involve the problem of
separation. Moreover, several molecularly defined homo-
geneous catalysts generally require extra additives such as I2 to
achieve successful hydrogenation of heteroarenes.7 In contrast,
heterogeneous catalysts appear to be much more versatile for
heteroarene hydrogenation because they can be easily recycled.
Supported-metal catalysts have been studied extensively for
aMaterials Research Center for Element Strategy, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: kitano.m.aa@m.titech.ac.jp, hosono@msl.titech.ac.jp
bLaboratory for Materials and Structures, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
cACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6gc02782b
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heterogeneous catalysis.8 The availability of efficient sup-
ported-metal catalysts has been the driving force of many pub-
lications devoted to the chemoselective hydrogenation of het-
eroarenes. It is well known that the catalytic properties of
metal particles are significantly influenced by the support
materials. Hence, the development of new effective support
materials that can realize enhancement of the metal catalyst
remains a significant challenge.
The nanoporous crystal 12CaO·7Al2O3 (C12A7:O2−) is a fast
oxygen ion conductor that possesses extra framework oxygen
ions in subnanometer-sized cages with an inner free space of
ca. 0.4 nm in diameter.9 C12A7:O2− has a positively charged
lattice framework [Ca24Al28O64]4+, in which the positive charge
of the cage wall is compensated by anions that are accommo-
Fig. 1 HAADF STEM image of 2 wt% Ru/HT-C12A7. (a) Overview image
of Ru/HT-C12A7 and (b) metal particle size distribution histogram.
dated within the cages.10 The O2− ions can be replaced by estimated to be 1.9 nm on the basis of a count of more than
different anions such as hydroxide ions (OH−), hydride ions 100 individual nanoparticles (Fig. 1b).
(H−), halogen anions (F−, Cl−), sulfur ions (S2−) and electrons
The hydrogenation of quinoline to 1,2,3,4-tetrahydroquino-
(e−) via appropriate chemical/physical treatment.11 Electron- line was selected as a probe reaction to investigate the catalytic
embedded C12A7:e− is an electride in which electrons act as performance of Ru/HT-C12A7 because 1,2,3,4-tetrahydroquino-
anions.12 Our group has recently reported that the electron- line has been identified as a platform molecule with a variety
embedded C12A7:e− electride in combination with Ru func- of applications in biologically active natural products and
tions as a highly efficient catalyst for ammonia synthesis pharmacologically relevant therapeutic agents. Control experi-
under mild conditions.13 In this reaction, H− can be incorpor- ments showed that Ru/HT-C12A7 with 2 wt% Ru or more pro-
ated into the positively charged nanocages of C12A7:e−. For vides excellent conversion and selectivity to the hydrogenated
the hydrogenation of heteroarenes, the key point is to facilitate product without a solvent when Ru nanoparticles are sup-
heterolytic H2 activation at the metal-support interface.
ported on HT-C12A7 pre-annealed at 600 °C (Table 1). The
We expect that nanoporous crystal C12A7:O2− also has the optimized reaction conditions were determined to be 80 °C
potential to trap H− ions and promote heterolytic H2 cleavage. under a H2 pressure of 1.0 MPa. From the kinetic data shown
Therefore, we have examined nanometric Ru supported on in Fig. S3,† the starting material was almost fully consumed
C12A7:O2− (Ru/HT-C12A7) with a high specific surface area, after 12 h. Moreover, no intermediates could be detected by
prepared through a hydrothermal approach, as a catalyst for gas chromatography (GC) analysis during the entire reaction
the hydrogenation of various heteroarenes in a solvent-free process. Note that the Ru/HT-C12A7 catalyst exhibited signifi-
system. For the chemoselective hydrogenation of the cantly higher catalytic activity within a shorter time and under
N-heteroarene quinoline, the catalyst shows a high conversion much milder reaction conditions than other supported –
of >99% and a selectivity of close to 99% toward 1,2,3,4-tetra- metal catalysts reported to date in a solvent-free system
hydroquinoline. Furthermore, a wide range of heteroarenes (Table S1†). A tiny amount of decahydroquinoline was formed
such as N-heteroarenes and O-heteroarenes can also be exclu- as a side product. In addition, Ru/HT-C12A7 has efficient
sively hydrogenated to the desired products under mild con- activity for the hydrogenation of 6-chloroquinoline, compar-
ditions in a solvent-free system. C12A7 is demonstrated to be a able to that of Ru- or Au-loaded high surface area TiO2 cata-
new and efficient Ru catalyst support material for the hydro- lysts under extremely mild conditions (Table S1†).4b
genation of heteroarenes.
To further examine the important role of HT-C12A7 for the
hydrogenation of quinoline, other oxide materials, such as
Al2O3, CaO, Al2O3–SiO2, MgO, ZrO2 and TiO2 were also
employed as supports for the Ru catalyst (5 wt%) for quinoline
hydrogenation (Fig. 2a). Ru-loading was conducted using the
Results and discussion
The preparation of Ru/HT-C12A7 involves a two-step method. same thermal decomposition method as that for HT-C12A7.
Firstly, HT-C12A7 was prepared by simple hydrothermal pro- Table S2† shows that the Ru particle sizes are almost identical
cessing of a ball-milled mixture of Ca(OH)2 and Al(OH)3. X-ray for each support, except for Ru/ZrO2. Therefore, the difference
diffraction (XRD) analysis (Fig. S1†) indicated that as-prepared in the catalytic performance is not due to the Ru particle size
HT-C12A7 had a cubic mayenite structure. The deposition of but most likely due to the nature of the support material. For
Ru nanoparticles was then achieved by the thermal decompo- the acidic oxide support, Al2O3–SiO2, the yield of 1,2,3,4-tetra-
sition of Ru3(CO)12, in which the metal is in the zero oxidation hydroquinoline remained only 1.3% after 12 h reaction, which
state, as demonstrated by X-ray photoelectron spectroscopy was 75 times lower than that for Ru/HT-C12A7. The Al2O3,
(XPS) measurements (Fig. S2†).14 A representative scanning TiO2 and ZrO2 supports with low acidic or basic properties
transmission electron microscopy (STEM) image of Ru/ also gave low yields. Notably, the strongly basic oxide CaO cata-
HT-C12A7 is shown in Fig. 1a. The mean Ru particle size was lyst exhibits higher catalytic activity than the other oxide
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Table 1 Ru/HT-C12A7 catalytic performance for chemoselective hydrogenation of quinolinea
Entry
Catalystb
Ru [wt%]
H2 [MPa]
Temp. [°C]
Time [h]
Conv. [%]
Sel. [%]
1
2
3
4
5
6
7
8
Ru/HT-C12A7-500
Ru/HT-C12A7-600
Ru/HT-C12A7-800
Ru/HT-C12A7-1000
Ru/HT-C12A7-600
Ru/HT-C12A7-600
Ru/HT-C12A7-600
Ru/HT-C12A7-600
Ru/HT-C12A7-600
Ru/HT-C12A7-600
2
2
2
2
0.5
5
2
2
2
1.0
1.0
1.0
1.0
1.0
1.0
0.5
1.0
1.0
1.0
80
80
80
80
80
80
80
60
80
80
12
12
12
12
12
12
12
12
6
89.9
99.2
52.6
33.3
50.7
99.6
52.5
51.5
72.5
—
96.8
98.6
96.5
95.6
91.7
98.7
92.3
92.4
98.2
—
9
10c
2
24
a Typical conditions: 5 mmol quinoline, catalyst 40 mg. b HT-C12A7-X (X: calcination temperature) was prepared by calcination of hydrothermally
produced Ca–Al mixed hydroxide at various temperatures. c Hot filtration test was carried out from the mixture after 6 h of reaction (entry 9), pro-
ceeding the reaction without catalyst for 24 h under the same reaction condition as entry 9. Conversion (Conv.) and selectivity (Sel.) were deter-
mined by gas chromatography (GC) and GC-mass spectrometry (GCMS) analyses, respectively.
conclusion that no leaching of the catalyst occurred and the
process was truly heterogeneous. Therefore, the Ru/HT-C12A7
catalyst presents impressive catalytic activity and recyclability,
and possesses a truly heterogeneous catalytic nature that has
potential application in the chemical industry.
Encouraged by the excellent performance of Ru/HT-C12A7,
the reaction protocol was further extended to different quino-
lines with various functional groups. Table 2 shows that quino-
lines with methyl substituents either on the benzene ring or
the heteroarene ring could undergo hydrogenation of the
heteroarene ring to the corresponding products with excellent
yields above 85% (Table 2, entries 1–5). The electron-donating
group (CH3O) and electron-withdrawing functional groups
Fig. 2 (a) Catalytic activities for hydrogenation of quinoline over
various Ru-based catalysts with different supports. (b) Recycle experi-
(F, Cl) had no significant influence on the activity of this catalytic
system (entries 6–8). Although we also tried the hydrogenation
of the most challenging substrate bearing aldehyde, both
quinoline ring and aldehyde group were hydrogenated
ment for (b) hydrogenation of quinoline over Ru/HT-C12A7. Reaction
conditions: 5 mmol substrate, 40 mg catalyst, H2 (1.0 MPa), 80 °C, 12 h.
supports, and even higher than the basic oxide MgO catalyst. (Scheme 1). In the case of methyl 6-quinolinecarboxylate, qui-
However, the activity of Ru/HT-C12A7 was higher than that of noline ring was selectively reduced to the corresponding
Ru/CaO by an order of magnitude. Comparison of the catalytic product with excellent selectivity (entry 9) and it is hard to
performance indicated that the HT-C12A7 support was ben- hydrogenate ester group due to the low electrophilicity of the
eficial for enhancement of both the catalytic activity of Ru and carbonyl group.15 The main side products of part substrates
the chemoselectivity for the hydrogenation of quinoline. This with relative low selectivity (<90%) were provided in Table S3.†
can be attributed to the abundance of surface basic sites on It is noteworthy that the Ru/HT-C12A7 catalyst can also hydro-
HT-C12A7, which will be discussed later.
genate pyridine and pyrrole to the corresponding piperidine
Recycling experiments were conducted to demonstrate the and pyrrolidine, which are used as building blocks for the syn-
stability and reusability of Ru/HT-C12A7. Fig. 2b shows that thesis of chemical products and pharmaceuticals.16 In the lit-
both the catalytic activity and chemoselectivity were well erature work about the hydrogenation of pyridine and pyrrole,
retained in eight consecutive runs. TEM observation clearly harsh reaction conditions (such as high temperature and/or
demonstrated that the Ru nanoparticle size and morphology high hydrogen pressure) are often required.17 Compared to
of the support remained unchanged after recycling (Fig. 1 these processes, the Ru/HT-C12A7 catalyst allows for the syn-
and S4†), which indicates excellent stability and recyclability. thesis of piperidine and pyrrolidine with extremely high yields
To check whether the hydrogenation of quinoline over of 99.9% (entries 10, 11) under relatively mild conditions,
Ru/HT-C12A7 catalyst is a heterogeneous reaction or homo- which is advantageous in real application. These results
geneous one, the catalyst was removed from the reaction confirm that the Ru/HT-C12A7 catalyst is remarkably selective
mixture by hot filtration after 6 h of reaction, and the filtrate for the hydrogenation of N-heteroarenes under mild experi-
was heated under the same reaction condition. No further con- mental conditions in a solvent-free system.
version of quinoline was observed, even after reaction for 24 h
The high activity of the Ru/HT-C12A7 catalyst toward a
under identical conditions (Table 1, entry 10), which led to the series of N-heteroarenes encouraged us to test the catalyst for
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Table 2 Ru/HT-C12A7-catalyzed hydrogenation of various N-heteroarenesa
used for pharmaceuticals, agrochemicals, and other chemical
materials.18 The Ru/HT-C12A7 catalyst had good activity for
the hydrogenated formation of 2,3-dihydrobenzofuran from
benzofurans (Table 3, entry 1). Moreover, substituted deriva-
tives such as 2-butylbenzofuran and 3-methylbenzofuran could
be converted to the corresponding heterocycle hydrogenated
products under optimized reaction conditions (entries 2 and
3). These results also highlight the superior activity of the Ru/
HT-C12A7 catalyst for the selective hydrogenation of various
heteroarenes. Interestingly, the reaction also proceeds with
furan, even at room temperature. The expected products were
obtained in more than 93% yield from furan and its derivatives
2-methyl-, 2-ethyl-, and 2,3-dimethyl-substituted furan (entries
4–7).
To elucidate the reaction mechanism for the chemo-
selective hydrogenation of heteroarenes over Ru/HT-C12A7,
kinetic analyses were conducted on both Ru/HT-C12A7 and
Ru/CaO by changing the substrate concentration and H2
pressure. Fig. 3a shows the reactions are zero-order with
respect to the initial reactant concentration, which indicates
that the adsorption of quinoline on the catalyst surface
becomes saturated during the reaction process, most likely
due to the strong interaction between the substrate and cata-
lyst surface. Fig. 3b shows the variation of the hydrogenation
rates as a function of the H2 pressure. The rates steadily
increased with increasing pressure on both catalysts and the
reaction order with the H2 pressure over Ru/HT-C12A7 and Ru/
CaO were estimated to be +0.87 and +0.88, respectively. These
Entry Substrate
1b
Product
Conv. [%] Sel. [%]
94.6
99.9
91.2
93.4
88.9
97.9
2c
3
4
99.9
99.9
99.9
88.3
85.7
95.5
5
6d
7d
8c
9e
99.9
99.7
99.9
93.2
85.6
95.1
Table 3 Ru/HT-C12A7-catalyzed
O-heteroarenesa
hydrogenation
of
various
10f
11f
99.9
99.9
99.9
99.9
Entry Substrate
1
Product
Conv. [%] Sel. [%]
95.2
93.8
2b
3b
96.7
91.2
a Reaction condition: 5 mmol substrate, 40 mg catalyst (2 wt%
Ru/HT-C12A7), 1.0 MPa H2, 12 h, 80 °C. b 18 h. c 15 h. d 60 °C. e 100 °C.
f 24 h. Conversion (Conv.) and selectivity (Sel.) were determined by
gas chromatography (GC), GC-mass spectrometry (GCMS), and NMR
analyses.
85.6
92.9
4c
5c
6c
7c
96.9
99.9
99.2
99.9
99.9
96.2
94.5
96.5
Scheme 1 Hydrogenation of 6-quinolinecarboxaldehyde over Ru/
HT-C12A7.
the chemoselective hydrogenation of other heteroarene com-
pounds, such as O-heteroarenes. The hydrogenation of
O-heteroarenes is also important in the biological and chemi-
cal industries. Hydrogenated O-heteroarenes derivatives can be
a Reaction condition: 5 mmol substrate, 40 mg catalyst (2 wt%
Ru/HT-C12A7), 1.0 MPa H2, 12 h, 80 °C. b 24 h. c 24 h, 25 °C.
Conversion (Conv.) and selectivity (Sel.) were determined by GC and
GCMS analyses, respectively.
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These cages can accommodate anion species such as H− or e−
ions. The soft chemical preparation process used for
HT-C12A7 and the relatively low annealing temperature cause
this material to possess abundant truncated surface frame-
work cages compared with that synthesized by the solid-state
method.23 Therefore, H− ions can enter into the truncated
surface cages much more easily than the intact cages with
higher crystallinity in C12A7.
It is widely accepted that the H+/H− pair derived from the
heterolytic cleavage of H2 preferentially reacts with polar
bonds such as CvN and CvO in chemoselective hydrogen-
ation.24 For the hydrogenation of heteroarenes, this H+/H−
pair also prevents catalyst poisoning by the strong interaction
between the heteroarene and metal catalyst through direct
binding of the heteroatoms of heteroarenes to the metallic
sites.4b,25 To confirm this, H2–D2 exchange reactions were con-
ducted over the Ru/HT-C12A7 catalyst with and without pre-
adsorbed quinoline. There was no noticeable change in the
H2–D2 exchange rate between the Ru/HT-C12A7 catalyst with
and without quinoline (Fig. 4b). In contrast, the HD formation
rate over Ru/CaO catalyst was significantly reduced after quino-
line molecules were adsorbed on the catalyst surface, which
indicates that the H2 dissociation reaction on Ru/CaO is
strongly suppressed through poisoning by quinoline adsorp-
tion. Furthermore, it can be considered that quinoline mole-
cules play an important role in the heterolytic cleavage of H2
to yield H+/H− ion pairs over Ru/HT-C12A7.
Fig. 3 Effect of (a) substrate concentration and (b) H2 pressure on the
rates of quinoline hydrogenation. Reaction conditions: (a) 5 mg catalyst,
5 mL tetrahydrofuran (THF), H2 (1.0 MPa), 80 °C, 12 h, (b) 5 mmol sub-
strate, 40 mg catalyst, 80 °C, 12 h.
results imply that a hydrogen species is involved in the rate-
determining step over these catalysts.
To further verify the difference in catalytic activity between
Ru/HT-C12A7 and the other Ru-based catalysts, electron para-
magnetic resonance (EPR) measurements were conducted on
the Ru/HT-C12A7 catalysts before and after reaction. As shown
in Fig. 4a, an EPR signal appeared at g = 2.004, which was
superposed on the signals of the oxygen anions (O− or O2−).19
This signal is ascribed to the adsorbed product, 1,2,3,4-
tetrahydroquinoline radicals. Subsequently, irradiation of the
Ru/HT-C12A7 with ultraviolet (UV) light resulted in the appear-
ance of a new signal with an almost isotropic shape at g =
1.994, which is similar to an F+-like center where an electron is
trapped at an oxygen ion vacancy and is coordinated by Ca2+
ions (Fig. S5†).20 According to our previous studies, UV
irradiation can induce electron emission from H− ions
encaged in C12A7:H− (H− + O2− → OH− + 2e−) and a similar
EPR signal appears at g = 1.994.21 Accordingly, it can be con-
cluded that H− ions are incorporated in the cages of HT-C12A7
during the hydrogenation reaction. A plausible explanation for
this phenomenon of Ru/HT-C12A7 is that the surface frame-
work cages of C12A7 are truncated and can capture hydrogen
species as H−. The surface of C12A7:e− contains low-co-
ordinated cations associated with the truncated cages, while
the near-surface regions retain the intact cage structure.22
The incorporation of H− ions into the cages of C12A7 in the
presence of Ru often requires a relatively high temperature of
approximately 360 °C.26 In the present reaction system, the
reaction temperature was as low as 80 °C, which is not ade-
quate for the incorporation of H− ions into the intact cages of
bulk C12A7. However, H− ions were formed in Ru/HT-C12A7
after the chemoselective reaction (Fig. 4a), and these species
may be trapped in the truncated framework cages on the
surface. HT-C12A7 contains a large amount of OH− ions in the
framework cages due to preparation from the Ca–Al mixed
hydroxides. The OH− ion concentration of the Ru/HT-C12A7
catalyst was therefore compared before and after the reaction.
The amount of OH− ions remained consistent, which demon-
strates that H− ions are only incorporated into the truncated
surface cages, irrespective of the inner cages with OH− ions
(Fig. S6†).27 For comparison, EPR measurements were also
conducted on Ru/CaO without a nanocage structure as a refer-
ence sample. The EPR spectra showed no signal due to
trapped electrons centered at g = 1.994 in Ru/CaO after the
same reaction process as that for Ru/HT-C12A7 (Fig. S7†).
Accordingly, the catalytic activity of Ru/CaO was also poor, as
expected. Moreover, a clear distinction of the catalytic activity
between the Ru/C12A7 and Ru/CaO·Al2O3 catalysts emphasizes
that the truncated surface framework cages in C12A7 favor the
chemoselective hydrogenation of heteroarenes (Fig. S8†).
The heterolytic cleavage of H2 to yield H+/H− ion pairs plays
a crucial role in determining the selectivity. The truncated
surface cages could promote the heterolytic hydrogen splitting
by incorporated H−. On the other hand, when HT-C12A7 is
Fig. 4 (a) EPR spectra for Ru/HT-C12A7 catalyst before and after hydro-
genation reaction, and Ru/HT-C12A7 after reaction under UV light
irradiation (microwave frequency = 9.7 GHz, temperature = 25 °C). (b)
Rate for HD formation at 50 °C over Ru/HT-C12A7 and Ru/CaO with and
without pre-adsorbed quinoline.
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used as a support, the surface O atoms can function as strong readily spill over onto HT-C12A7 and be incorporated into the
basic sites to efficiently fix the H+ ions. Therefore, the basic truncated surface cages that have the capability to trap H−,
properties of HT-C12A7 were investigated using temperature thereby facilitating the heterolytic H2 activation at the surface
programmed desorption (TPD) with CO2 as a probe molecule. of the Ru/HT-C12A7 catalyst. However, due to the weak inter-
The TPD profile for the Ru/HT-C12A7 catalyst shows three CO2 action between H− ions and the truncated cages, the H− ions
desorption peaks at approximately 235 °C, 315 °C, and 390 °C may desorb from the truncated surface cages and then partici-
(Fig. S9†). Recent density of states (DOS) calculations con- pate in the hydrogenation reaction. As a result, a large amount
firmed that three different oxygen sites are located at or near of H+ and H− species are generated on the surface of Ru/
to the surfaces of truncated cages in C12A7, including (i) one HT-C12A7, which promotes efficient and selective hydro-
site coordinated by one Ca and one Al neighbor, (ii) one site genation of the heteroarenes to form the corresponding hetero-
coordinated by two Al neighbors, and (iii) an initial extra- cycle hydrogenated products. Additionally, Ru/HT-C12A7 cata-
framework ion that merged with the cage wall.28 These three lyst preferentially hydrogenated aldehyde group in the pres-
types of basic sites correspond well with the CO2-TPD results, ence of quinoline ring, also suggesting the heterolytic manner
which indicates the existence of a variety of basic sites of of H2 splitting on the surface of Ru/HT-C12A7 (Scheme 1).
different adsorption strengths on the surface of HT-C12A7. Moreover, these surface hydrogen species also prevent catalyst
Moreover, the average coordination number of these oxygen poisoning caused by the strong interaction between heteroar-
sites is below four which is the same as the local environment enes and the metal catalyst through the direct binding of
in the bulk, due to the truncated cage structure. Therefore, the heteroatoms (N or O) to the metallic sites.
basicity of these sites is much stronger than that in conventional
oxides. Various Ru based catalysts with different oxide sup-
ports were also measured and the order of the amount of basic
sites was determined as Ru/SiO2·Al2O3 < Ru/ZrO2 < Ru/TiO2 <
Conclusions
In summary, we have developed a highly active and chemo-
selective catalyst of Ru/HT-C12A7 for the hydrogenation of both
N- and O-heteroarenes in a solvent-free system. The key to suc-
cessful catalysis is the use of stable, durable and low-cost nano-
porous HT-C12A7 as the support. The HT-C12A7 support can
act as a special hydrogen storage material with abundant
surface basic sites to enhance the catalytic activity and selecti-
vity for the chemoselective hydrogenation of heteroarenes.
Moreover, the heterolytic ionic hydrogenation approach prevents
catalyst poisoning caused by the strong interaction between the
substrate and metallic sites. This catalyst was also successfully
recycled and reused without significant deactivation. The
unique nanoporous structure of HT-C12A7 can be extended to
guide the design and development of novel catalysts for a broad
spectrum of green syntheses of many important chemicals.
Ru/Al2O3 < Ru/MgO < Ru/CaO < Ru/HT-C12A7, which corres-
ponds well with the activity of these catalysts for the hydrogen-
ation of quinoline (Fig. S9†). These results unambiguously
demonstrate that the basic sites of HT-C12A7 presumably act
to facilitate the formation of the heterolytic hydrogen species (H+).
Based on these results, we propose a possible mechanism
for the chemoselective hydrogenation of heteroarenes over the
Ru/HT-C12A7 catalyst, as follows. As shown in Scheme 2, the
crucial factor in the hydrogenation of heteroarenes is the
unique surface configuration of Ru/HT-C12A7 with truncated
cages. These positively charged truncated cages can serve as
anchors capable of trapping anions, such as H−. The hetero-
geneous dissociation of H2 occurs preferentially at the inter-
face between a catalyst metal and a basic oxide support (H2 →
H− + H+). Therefore, in the case of Ru/HT-C12A7, it can be
expected that H− species are formed on Ru metal sites and H+
is simultaneously formed on the framework basic sites at the
surface of HT-C12A7. The H− ions on Ru surfaces would
Experimental
Preparation of HT-C12A7
HT-C12A7 was synthesized by the calcination of hydro-
thermally produced Ca–Al mixed hydroxide. Briefly, a stoichio-
metric mixture (Ca : Al = 12 : 14) of Ca(OH)2 and Al(OH)3 was
dispersed in distilled water and ball-milled at a speed of 300
rpm for 4 h. Then, the mixture was hydrothermal treated in a
Teflon-lined stainless autoclave at 150 °C for 6 h with stirring.
The product was separated by centrifugation at 3000 rpm and
dried in an oven at 70 °C overnight. The resultant powder was
calcined at 600 °C for 5 h in air, which resulted in the for-
mation of HT-C12A7 powder with a mayenite structure.
Preparation of Ru/HT-C12A7
Scheme 2 Possible pathway for the chemoselective hydrogenation of
heteroarenes over Ru/HT-C12A7. The dotted circles represent the trun-
cated surface cages.
Ru loading was conducted by chemical vapor deposition (CVD)
process using the corresponding metal carbonyl Ru3(CO)12
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precursors. In a typical catalyst synthesis, HT-C12A7 powders spectroscopy (ICP-AES; ICPS-8100, Shimadzu). X-band electron
with Ru3(CO)12 was sealed in an evacuated silica tube and paramagnetic resonance (EPR) of the sample Ru/HT-C12A7
heated under a temperature program of 2 °C min−1 up to was performed on
a spectrometer (EMX8/2.7, Bruker,
40 °C, held for one hour, 0.25 °C min−1 up to 70 °C, held for Germany). Data for UV-irradiated samples were collected at RT.
one hour, 0.4 °C min−1 up to 120 °C, held for one hour, 0.9 °C The g-value at RT was calibrated using a solution of di
min−1 up to 250 °C, hold for two hours and then cooled to (phenyl)-(2,4,6-trinitrophenyl)iminoazanium
(DPPH)
in
ambient temperature. Other oxide materials, such as Al2O3, benzene with a g-value of 2.0035. The spin density in the
CaO, Al2O3–SiO2, MgO, ZrO2 and TiO2 were commercial pro- sample was calibrated using standard samples of CuSO4·5H2O.
ducts and Ru-loading was conducted by the same thermal CO2-TPD measurement was measured in a BELCAT-A instru-
decomposition method as HT-C12A7.
ment (MiccrotracBEL, Japan). The samples (50 mg), placed in
a U-shaped quartz reactor with an inner diameter of 0.5 cm,
was exposed to a flow of CO2 at 300 °C for 30 min and then
Catalytic hydrogenation of heteroarenes
All reactions were carried out in a 25 mL stainless steel auto- cooled to room temperature. Subsequently, TPD was then
clave fitted with a glass mantel, 60-bar manometer, and a mag- started, with a heating rate of 10 °C min−1 till 500 °C. The
netic stirrer. In a typical reaction, 40 mg catalyst and 5 mmol desorbed products were analyzed by a thermal conductivity
substrates were sealed in the autoclave. Then autoclave was detector (TCD) and mass spectrometer (Bell Mass, MiccrotracBEL,
flushed three times with H2, pressurized with H2 (1.0 MPa), Japan). Nuclear magnetic resonance (NMR) spectra were
and heated to 80 °C over 30 min with stirring (800 rpm). The recorded on a JEOL ECP-300 instrument (JEOL, Tokyo, Japan)
reactor was held at this temperature for 12 h. The products using tetramethylsilane as an internal standard (300 and
were analyzed by gas chromatography (GC) and the identity of 75 MHz for 1H NMR and 13C NMR, respectively).
the products was further confirmed by GC-MS.
H2–D2 exchange reaction
Acknowledgements
Before the reaction, the catalyst was heated in Ar flow
(10 mL min−1) at 200 °C for 0.5 h, followed by cooling to the This work was supported by a fund from the Accelerated
desired temperature under He flow. The H2–D2 exchange Innovation Research Initiative Turning Top Science and Ideas
reaction was conducted in a fixed bed quartz reactor that was into High-Impact Values (ACCEL) program of the Japan
supplied with an extra pure (99.99995%) mixture of H2 : D2 : Ar Science and Technology Agency (JST), and a Kakenhi Grant-in-
(1 : 1 : 3) at 50 °C. Reaction products (H2, HD and D2) were Aid (no. 15H04183) from the Japan Society for the Promotion
analyzed with an online mass spectrometer (ANELVA, Quadrupole of Science (JSPS). The authors thank Prof. Masato Sasase and
Mass Spectrometer). Trace amounts of HD formed in the Dr K. Kishida for their help with the STEM observations and
experimental setup without catalyst and the product concen- Raman measurements. We also would like to express our
trations were corrected for this. For the pre-adsorbed quinoline appreciation to Prof. Michikazu Hara and Prof. Shinsuke Inagi
to the samples, the liquid quinoline (1 ml) was heated above for use of the XPS and NMR instrument.
its saturated vapor pressure and then injected into the sample
chamber under a vacuum cycle system.
Notes and references
Characterization of catalysts
Samples for examination by scanning transmission electron
microscopy (STEM) were prepared by dispersing the dry cata-
lyst powder onto a holey carbon film supported by a 300 mesh
copper TEM grid. STEM high angle annular dark field
(HAADF) images of the supported metal nanoparticles were
obtained using an aberration corrected JEM ARM-200F STEM
operating at 200 kV. The mean Ru particle diameters were cal-
culated by determining the size of more than 100 particles per
sample using iTEM software (soft Imaging System GmbH).
X-ray photoelectron spectroscopy (XPS, ESCA-3200, Shimadzu)
measurements were carried out using Mg Kα radiation at
<10−6 Pa (applied bias voltage to X-ray source of 8 kV). The
Brunauer–Emmett–Teller specific surface areas of the samples
were determined by the measurement of nitrogen adsorption–
desorption isotherms at −196 °C using an automatic gas-
adsorption instrument (BELSORP-mini II, MiccrotracBEL)
after evacuation of the samples at 150 °C. Ru content was
determined by inductively coupled plasma atomic emission
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