Cobalt Encapsulated in N-Doped Graphene Layers: An Efficient and Stable Catalyst for Hydrogenation of Quinoline Compounds

Article abstract of DOI:10.1021/acscatal.6b01240

Porous nitrogen-doped graphene layers encapsulating cobalt nanoparticles (NPs) were prepared by the direct pyrolysis process. The resulting hybrids catalyze the hydrogenation of diverse quinoline compounds to access the corresponding tetrahydro derivatives (THQs), important molecules present in fine and bulk chemicals. Near-quantitative yields of the corresponding THQs were obtained under optimized conditions. Notably, various useful substituted quinolines and other biologically important N-heteroarenes are also viable. The enhanced stability of the catalyst is ascribed to the encapsulation structure, which can enormously reduce the extent of leaching of base metals and protect metal NPs from growing larger. The achieved success in the encapsulation of metal NPs within graphene layers opens an avenue for the design of highly active and reusable heterogeneous catalysts for more challenging molecules.

Full text of DOI:10.1021/acscatal.6b01240

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Article
Cobalt encapsulated in N-doped graphene layers: an efficient
and stable catalyst for hydrogenation of quinoline compounds
Zhongzhe Wei, Yiqing Chen, Jing Wang, Diefeng Su, Minghui Tang, Shanjun Mao, and Yong Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01240 • Publication Date (Web): 26 Jul 2016
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Cobalt encapsulated in N-doped graphene layers: an
efficient and stable catalyst for hydrogenation of
quinoline compounds
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Zhongzhe Wei, Yiqing Chen, Jing Wang, Diefeng Su, Minghui Tang, Shanjun Mao, Yong Wang*
Advanced Materials and Catalysis Group, ZJU-NHU United R&D Center, Department of
Chemistry, Zhejiang University, Hangzhou 310028, P. R. China.
ABSTRACT: Porous nitrogen-doped graphene layers encapsulating cobalt nanoparticles (NPs)
were prepared by the direct pyrolysis process. The resulting hybrids catalyze the hydrogenation
of diverse quinoline compounds to access the corresponding tetrahydroderivatives (THQs),
important molecules present in fine and bulk chemicals. Near quantitative yield of the
corresponding THQs were obtained under optimized conditions. Notably, various useful
substituted quinolines and other biologically important N-heteroarenes are also viable. The
enhanced stability of the catalyst is ascribed to the encapsulation structure, which can
enormously reduce the leaching of base metals and protect metal NPs from growing larger. The
achieved success in the encapsulation of metal NPs within graphene layers opens up an avenue
to design highly active and reusable heterogeneous catalysts for more challenging molecules.
KEYWORDS: encapsulated structure; chemoselective hydrogenation; metallic cobalt; N-doped
graphene layers; quinoline
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INTRODUCTION
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Catalysts, especially metallic catalysts, play a dominate role in industrial applications. The
reusability of catalysts is an important issue in green chemistry and ecological economics.1 In
this respect, development of catalysts using base metals, including Fe2,3, Co4-6 and Ni7,8, is
prevailing for their distinct electronic structures9 and magnetic property. Especially, the specific
advantage of magnetic nanomaterials is that they can be effectively recovered by an external
magnetic field (typically >99%).10 However, magnetic NPs tend to aggregate when treated
without any protection agents, thus losing their unique properties derived from their small size.10
For another, leaching of earth-abundant metals (e.g., Co) may also occur, which is prone to
irreversible deactivation.11 Therefore, the rational design of stable nano-magnetic materials based
on earth-abundant metals is challenging and attractive.
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1,2,3,4-Tetrahydroquinoline (py-THQ) and its derivatives are important building blocks in
pharmaceuticals, agrochemicals, and various fine products.12,13 The general synthesis method is
the direct hydrogenation of quinolines, which is the most convenient and hopeful way due to the
high atom efficiency.13,14 However, quinoline hydrogenation ideally involves the possibility that
py-THQ, 5,6,7,8-tetrahydroquinoline (bz-THQ) and decahydroquinoline (DHQ) may produce in
the reaction networks (Scheme 1). To achieve the chemoselective hydrogenation of pyridine ring
without byproducts remains a challenge task. Many homogeneous metal catalysts, such as Rh15,16
Ir17,18 and Ru19 have been applied in the transformation. Despite the high activity and selectivity,
the difficulties in recovering the catalysts, ligand, and additives from the reaction mixtures limit
the catalysts on industrial scale. Up to now, a series of novel heterogeneous catalytic system
using precious metal catalysts, including Au,20-22 Pd,12,13,23,24 Pt,25 Ru26-30, Ir,31 Rh32-34 and
multi-metallic catalysts35 have been developed, but the catalysts based on magnetic metal NPs
,
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are rarely reported.5,7,8 The problem often arising is that the noble metals are not selective and
perform low tolerance for functional groups.36 Particularly worth mentioning is that the catalysts
are difficult to recycle due to the presence of N-heterocycle potentially causing catalyst
poisoning through the coordination of nitrogen atoms with the metal NPs.37,38 To overcome the
limitations, the metal NPs encapsulated with porous carbon shell is an impressive strategy.6,39-41
The major advantages are that (1) the encapsulation structure can elegantly regulate the
electronic structure of metal NPs and strictly control the aggregation of NPs;42-44 (2) the structure
can stabilize the NPs and greatly reduce the leaching of base metals for the catalyst;39,43 (3) the
encapsulation structure can weaken the strong coordination between N-heterocycle and active
metal NPs. Accordingly, the life of the catalyst can be prolonged; (4) encapsulation the metal
NPs within porous carbon shells ensures that the catalytically active sites are easy to approach
and enormously inhibit the mass transfer limitation.39 Herein, we design the nano-magnetic
material featuring of nitrogen-doped carbon-coated cobalt NPs and further apply the catalysts in
hydrogenation of N-heteroarenes compounds. The hybrids serve as an efficient, selective and
robust catalyst in the transformation. Interestingly, the substrates can be smoothly converted into
the products without using commercial magnetic stirring bars due to the good magnetic response
of the catalyst.
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Scheme 1. Possible reaction pathways for quinolone hydrogenation
RESULTS AND DISCUSSION
The hybrids (denoted as CoOx@CN) were obtained in one step through pyrolysis of
D-glucosamine hydrochloride (GAH), cobalt acetate, and melamine at 800 °C. At temperature
above 450 °C, melamine gradually condensed to generate graphitic carbon nitride (g-C3N4),
acting as soft template. Meanwhile, carbon sheets formed on the interlayer of g-C3N4 through the
self-polymerization of GAH. The cobalt-based NPs were confined between the g-C3N4 and
carbon sheets. Then, g-C3N4 is fully decomposed at higher temperature and the carbon sheets on
the interlayer of the g-C3N4 were maintained.45 Finally, cobalt-based NPs were encapsulated into
the N-doped carbon sheets and produced the catalysts.46,47
.
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Figure 1. Representative HRTEM images of CoOx@CN.
High-solutiontransmission electron microscopy (HRTEM) was used to investigate the
morphology and structure of CoOx@CN. It is observed that Co NPs are well encapsulated inside
the carbon layers (Figure1,S1). It is worth to note that obvious defects like small channels can be
observed on the surrounded carbon layers as demonstrated in Figure 1b, where the reaction may
occur. An obvious hysteresis loop occurred in N2 adsorption–desorption isotherm, which is
the characteristic of mesopores (Figure S2). The application of the BET model resulted in the
surface area of 531 m2 g-1. The distribution of pore size calculated by the BJH method is around
12 nm (Figure S2). The large surface area and the mesoporous pores ensure that the substrates
access the active sites easily without suffering from high mass transfer causing by the carbon
shells. HRTEM, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) (Figure
S3-5) show that in situ generated Co0, Co3O4, and nitrogen-doped carbon shells together
constitute CoOx@CN through one-pot synthesis. Interestingly, Co0 in CoOx@CN is
exceptionally stable. Definite metallic Co reflections were displayed in the XRD pattern for the
catalyst after acid treatment (Figure S6), providing the evidence of the enhanced stability of Co0
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with the protection of nitrogen-doped graphene layers. The magnetic properties of CoOx@CN
were measured at 300 K with Physical Property Measurement System (PPMS-9). As shown in
Figure 2, CoOx@CN exhibits a soft ferromagnetic behavior at room temperature. The saturation
magnetization (MS) of the catalyst is 22.3 emu/g at 300 K. The catalyst can be readily separated
and recycled with a magnet after the reaction due to the magnetic property.
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Figure 2. Magnetization data analysis for CoOx@CN. Inset is the photograph that shows the
magnetism of CoOx@CN.
To exploration the catalytic properties of CoOx@CN, we select the hydrogenation of the
benchmark substrate quinoline as a model reaction. First, the degree of sensitivity of our
analytical methods was investigated. We prepared an ethanol solution including possible
reduction products including 0.0825 mg/mL DHQ, 0.1225 mg/mL 5,6,7,8-THQ and 0.1625
mg/mL 1,2,3,4-THQ as a sample. From the GC trace (Figure S7), three definitely detectable
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peaks at 8.800, 9.861 and 11.225 min are observed, which correspond to DHQ, 5,6,7,8-THQ and
1,2,3,4-THQ, respectively. The results show that the analytical method possesses a high degree
of sensitivity. Pleasingly, the evolution of the reactant and product distributions with reaction
time revealed that quinoline can be smoothly converted into py-THQ under mild conditions and
there was no bz-THQ and DHQ detected in this CoOx@CN catalytical system (Figure 3).
Quantitative yield of py-THQ was obtained in 3 h. CoOx@CN hydrogenates quinoline with a
TOF of 4.1 h-1, which is better than the reported noble metal-free catalysts5 and even comparable
to some noble metal catalysts15 (Table S3). The high activity and selectivity of CoOx@CN make
it possible in industrial applications. Interestingly, quinoline can also be converted to py-THQ
successfully when no commercial magnetic stirring bar used (Figure S8). The tantalizing results
suggest that the CoOx@CN serve as catalytic active sites as well as nanoscale magnetic stirring
bars. The reason may come from the magnetic property of CoOx@CN, which can mix the
reactant and contact with the reactant sufficiently under an external magnetic field.48
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Figure 3. Kinetic curves for quinoline hydrogenation with CoOx@CN. Reaction conditions:
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quinoline (0.5 mmol), CoOx@CN (10mg), CH3OH (5 mL), 3 MPa H2, 120 oC.
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Next, the reaction conditions were optimized for the hydrogenation of quinoline through
variation of the solvent, temperature, and pressure. Solvents make difference in heterogeneous
catalytic reactions and can influence the activity and selectivity of catalysts. As demonstrated in
Table 1, CoOx@CN promoted the chemoselective hydrogenation of pyridine ring and performed
perfect selectivity. In terms of polar and nonpolar reactants, previous studies have been reported
that more polar solvents benefit the hydrogenation of less polar substrates. However, the order of
the conversion of quinoline in the tested solvents is as follows: methanol > H2O > ethanol >
toluene > DMF > 1,4-dioxane > cyclohexane, which was not consistent with the polarity of
solvents. The results were consistent with the hydrogenation of quinoline using
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Pd@ompg-C3N4.13 It seems that CoOx@CN performs generally better in protic solvents. For
selectivity, we could readily achieve the selectivities toward py-THQ in the range 95-100% in
various solvents listed below. To explain the perfect selectivity of CoOx@CN, Raman tests of
the treated catalyst (adsorption with quinoline) and the pristine catalyst were conducted. The
interaction between the pyridine ring and the graphene layers was visible in the D- and G-band
shift in the Raman spectra, as shown in Figure S9. The downshift by 8 cm-1 of treated CoOx@CN
(adsorption with quinoline) with respect to pristine catalyst is evidence of charge transfer from N
in pyridine ring to the covered graphene layers in CoOx@CN49-51. The charge transfer suggests
the strong interaction between the pyridine ring and the N-doped graphene layers, which favors
the production of py-THQ and finally achieves the high selectivity.
Table 1. Effect of solvents on the selective hydrogenation of quinoline using CoOx@CN a
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Entry
Solvent
Conv. (%)b
Sel. (%)b
100
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pd
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7c
Cyclohexane
Toluene
1,4-Dioxane
DMF
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Ethanol
Water
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aReaction conditions: quinoline (0.5 mmol), CoOx@CN (10mg), solvent (5 mL), 3 MPa H2,
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120 °C, 4 h. Conversion (Conv.) and selectivity (Sel.) were determined by GC and GC–MS.
c3 h. d Solvent polarity.
The effects of hydrogen pressure (0.5-3.5 MPa) and temperature (60-120 oC) were also briefly
examined. The conversion of quinoline as a function of the pressure of H2 is given in Table 2.
The results show that conversion steadily increased from 0.5-3 MPa and did not change
obviously thereafter. As the H2 pressure increased, the dissolution of molecular H2 in the solvent
would be promoted, thus obtaining increased TOF value. However, when the surface of the
catalyst is mostly covered by dissociated H atoms, the TOF value would not be improved further
with the increased H2 pressure52. The catalytic activity of CoOx@CN is sensitive to the reaction
temperature. Reducing the reaction temperature to 100 oC with 3 MPa H2, obtained a marked
drop in conversion (81%) yet excellent selectivity. Further reducing the temperature to 90oC,
both of the conversion and selectivity declined. Similarly, lowering the reaction temperature to
60 oC afforded an inferior conversion, only furnishing product in 8% yield. For further studies,
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we worked at 3 MPa and 120 oC, as these conditions represented the best compromise between
activity and selectivity.
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Table 2. The influence of temperature and hydrogen pressure on the hydrogenation of quinoline
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catalyzed by CoOx@CNa
Entry
T (oC)
60
P (MPa)
Conv. (%)b
Sel. (%)b
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TOF (h-1)
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aReaction conditions: quinoline (0.5 mmol), CoOx@CN (10mg), solvent (5 mL), 3 h.
bConversion (Conv.) and selectivity (Sel.) were determined by GC and GC–MS. c1.5 h.
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To further evaluate the catalytic performance, the CoOx@CN catalyst was compared with
FeOx@CN, NiOx@CN and several commercial available catalysts including cobalt powder,
Co3O4, Ru/C, Pd/C and Pt/C in the hydrogenation of 6-chloroquinoline under identical
conditions (Table 3). Clearly, CoOx@CN was the most efficient catalyst, exhibiting fine
selectivity toward halo-substituted py-THQ with no appreciable formation of any other side
products (Table 3, entry 4). Commercial available Co3O4, cobalt powder and CoOx-AC (CoOx
NPs deposited to the outer surface of active carbon) were not active for the reaction, suggesting
the synergistic effect between CoOx and CN (Table 3, entries 1-3). Despite both of FeOx@CN
and NiOx@CN can serve as nanosacle magnetic stirring bars due to their magnetic properties,
they performed poor activity in the reaction, yielding halo-substituted py-THQ in 1% and 17%,
respectively (Table 3, entries 5-6). For Ru/C, Pt/C and Pd/C, the formation of undesirable
dehelogenation products obviously occurred. Accordingly, the reaction proceeded unselectively
in rather low yield (Table 3, entries 7-9). These results emphasize that CoOx@CN displayed
impressive selectivity in the hydrogenation of quinoline compounds with sensitive substituted
groups, much better than the precious metal catalysts.
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Table 3. Catalytic results of the hydrogenation of 6-chloroquinoline on various catalysts a
Entry
Catalysts
Conv. b (%)
Sel. b (%)
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cobalt powder
Co3O4
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CoOx-AC
CoOx@CN
FeOx@CN
NiOx@CN
Ru/Cc
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Pt/Cc
Pd/Cc
aReaction conditions: 6-chloroquinoline (0.5 mmol), Catalyst (metal: 9 mol%), CH3OH (5 mL),
b
3 MPa H2, 110 °C, 3 h. Determined by GC and GC–MS. Only the main products are listed.
cCatalyst (10 mg).
Leaching of active components in the reaction liquid should be taken into consideration, which
resulting in the deactivation of catalyst.53 To shed light on this, we analyzed the cobalt content of
the recycled catalyst by ICP-AES. The cobalt content declined slightly to 22.1 wt% from 27.1
wt% after the 12th recycle (Table S2). In an individual experiment, 57 % conversion of quinoline
was obtained in 1 h at 120 oC with 3 MPa hydrogen. Then the reaction was hot filtered by
centrifugation to collect the liquid phase. Interestingly, no additional product yield after further
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stirring the filtrate in 1 h under the identical conditions above, indicating that the leached cobalt
in the liquid did not contribute to the activity.
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In terms of the reusability of the catalyst, we performed the selective hydrogenation of
quinoline using CoOx@CN at 120 oC and 3 MPa in successive runs and compiled the results in
Figure 4a. Some work has revealed that the catalysts used in the hydrogenation of quinoline
quickly became inactive due to the strong coordination of nitrogen atoms in quinoline and its
hydrogenation products with the metal NPs.28,29,54 Nevertheless, CoOx@CN showed a relatively
stable performance and could recycle 12 times, which has never been achieved by noble
metal-free catalysts. The result indicated that our catalytic system can greatly resist the poison of
quinolone and its hydrogenation products.
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Figure 4. Reuse of CoOx@CN (a) and CoOx-CN (b). Reaction conditions: quinoline (1 mmol),
Catalyst (20 mg), CH3OH (10 mL), 120 oC, 3 MPa H2, 3 h for each recycle.
Initially, the yield of py-THQ decreased gradually to 63% after the sixth run. Interestingly, the
activity can recover to some extent and the yield steadily increased to 89% after the tenth
recycle. Next, CoOx@CN displayed a slight inactivation in the following recycle experiments,
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indeed giving a 57% conversion after the twelfth run. The relatively high activity during the
recycle should be attributed to the N-doped graphene layers. To verify the necessity of the
encapsulated structure, we prepared the sample CoOx NPs deposited to the outer surface of CN
(denoted as CoOx-CN) for comparison (see XRD in Figure S10). The CN was obtained
according our reported work.55 CoOx-CN can also achieve the transformation of quinoline to
py-THQ selectively. However, in term of recyclability of the catalyst, the CoOx-CN exhibited a
remarkable inactivation in the process of reuse, furnishing a poor yield (35%) after five times use
(Figure 4b). The particle size distribution of the CoOx@CN after 12 cycles showed that the mean
particle size is ~12.8 nm, almost the same with the fresh catalyst (Figure S11). However, for
CoOx-CN, obvious particle aggregation occurred (Figure S12). More importantly, the cobalt
content declined slightly for CoOx@CN, approximately 2% Co leaching for a recycle in average.
Notably, the cobalt content of the CoOx-CN fell sharply to 16.7 wt% from 25.5 wt% in the fifth
reuse, approximately 10% Co leaching for a recycle in average (Table S2). Such significant
difference between CoOx@CN and CoOx-CN was ascribed to the versatile encapsulated structure.
On the one hand, the encapsulation structure can protect metal NPs from growing larger. On the
other hand, the coated N-doped graphene shells can stabilize the NPs and greatly reduce the
leaching of cobalt for the catalyst. All in all, the structure of carbon-coated cobalt NPs is
essential, which is directly related to the lifetime of CoOx@CN.
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The decreased activity of CoOx@CN at the initial phase was primarily attributed to the cobalt
leaching as mentioned above. Why the catalytic performance of the catalyst improved at the
second stage? It sparked our curiosity. We speculated that the phenomenon was the result of the
variation of the metal phase. In Figure 5, the diffraction peaks of Co3O4 and Co0 were
corroborated by XRD for the fresh catalyst. Specially, XRD patterns showed that Co3O4 NPs in
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the CoOx@CN were transformed into CoO after one use. What is more, only the diffraction
peaks of Co0 were recorded for the catalyst after 12 times use. This suggested that CoO may be
mostly converted to Co0 as the recycle progress. Density functional theory (DFT) calculations
have demonstrated that Co0 encapsulated by N-doped graphene layers can pleasingly make the
inert graphene layers active in H2 activation.47 That is, the converted Co0 can enormously
promote the H2 activation, thus improving the catalytic activity during the recycle despite
existence of leaching. Along with the leaching of the active metal, a drop in performance of the
catalyst was inevitable, corresponding to the decreased catalytic activity after the tenth recycle.
To make it clear, we employed hydrogen temperature-programmed reduction (H2-TPR) to
investigate the reducibility of the NPs. Two obvious reduction peaks of Co3O4 were detected in
CoOx@CN, much lower than that of commercial Co3O4 (Figure S13), suggesting the Co3O4 NPs
in CoOx@CN was probable to be reduced with H2. The H2-TPR results concluded that Co3O4 in
the hybrid CoOx@CN can be gradually reduced to CoO and Co0 at 120 oC and 3 MPa. Such a
good stability could be ascribed to the following reasons: 1) The N-doped graphene layers
protect the cobalt NPs from the aggregation and leaching; 2) the coated graphene layers greatly
weaken the strong coordination between N-heterocycle and active metal NPs; 3) the CoOx can be
mostly converted to Co0 under the reaction conditions, which can promote the H2 activation and
prolong the life of the catalyst.
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Figure 5. XRD patterns of CoOx@CN after one use and 12 times use.
With the optimized reaction parameters in hand, the general scope of CoOx@CN in the
hydrogenation of quinoline compounds was investigated, including industrially and biologically
relevant heteroarenes. Gratifyingly, the hydrogenation process appeared to be universally valid,
and full conversions and essentially perfect chemoselectivity were observed for a series of
N-heteroarenes, better than the reported works.10 Quinolines compounds substituted with a
methyl group at the 2-, 6- and 8- position could be readily transformed to the corresponding
py-THQ, which were characterized by excellent conversions and selectivities (Table 4, entries
2-4). Notably, quinolines substituted by halogen were reduced efficiently to the corresponding
halo-substituted py-THQ without any dehalogenation (Table 4, entries 5). What should be noted
is the facile reduction of 8-hydroxyquinolines to biologically active
1,2,3,4-tetrahydro-8-hydroxyquinoline, which can inhibit leukotriene formation in macrophages
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(Table 4, entry 6).20 Also noteworthy is that other biologically important N-heteroarenes such as
acridine, quinoxaline, and isoquinoline were also converted into the target products in high
yields (Table 4, entries 7-9).
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Table 4. Chemoselective hydrogenation of various substituted quinolones and N-heterocycles a
Time
(h)
Conv.
(%)b
Yield
(%)b
Entry
Substrate
Product
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99 (95)
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100 (89)
96 (88)
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77 (65)
100
100 (92)
aReaction conditions: substrate (0.5 mmol), CoOx@CN (10mg), CH3OH (5 mL), 3 MPa H2,
120 °C. bDetermined by GC (internal standard: n-dodecane), GC–MS and 1H-NMR (Figure
S14,15). Numbers in parentheses represent isolated yields. c110 °C. dCoOx@CN (20mg), 140oC.
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CONCLUSIONS
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In summary, we have demonstrated that the Co-based hybrids featuring encapsulated Co NPs
within the porous N-doped graphene layers served as catalysts with excellent catalytic activity
and good stability for the chemoselective hydrogenation of N-heteroarenes. The considerable
catalytic performance is attributed to the following reasons: 1) the encapsulation structure can
enormously reduce the leaching of base metals and protect metal NPs from growing larger. 2)
The large surface area and the mesoporous pores enable the easy approached catalytically active
sites. 3) The better mixing effect. CoOx@CN can mix the reactant in nano size and sufficiently
contact with the reactant. 4) The coated N-doped carbon provides a layer of protection between
N-heterocycle and active metal NPs, greatly weakening the strong coordination. The achieved
success in the encapsulation of metal NPs within graphene layers pave the way for designing
highly active and reusable heterogeneous catalysts for more challenging molecules.
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EXPERIMENTAL SECTION
Preparation of Catalysts
GAH, melamine, Co(OAc)2·4H2O, cobalt powder, Co3O4, Ru/C, Pd/C, and Pt/C were
purchased from Aladdin. All chemicals used were analytical grade.
Synthesis of MOx@CN (M = Co, Fe or Ni): 1 g GAH, 40 g melamine and a certain amount
of metal salt (0.81 g Co(OAc)2·4H2O, or 1.67 g Fe(NO3)3·9H2O, or 1.10 g Ni(NO3)2·6H2O) were
thoroughly mixed. First, the resulting solids were placed into a crucible and heated up to 600 oC
in 4 h, then it kept at 600 oC for 1 h. After that, we further rose the temperature to 800 oC in 2 h
and maintained for another 1 h in N2 flow. Finally, the catalysts were obtained when it cooled
down to room temperature.
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Synthesis of CoOx NPs deposited to the outer surface of active carbon (AC) and
nitrogen-doped carbon (CN): The catalysts CoOx-AC and CoOx-CN were prepared by wetness
impregnation method: 0.58 g of Co(OAc)2·4H2O was first dissolved in 5 mL of deionized water;
to this solution 0.5 g of support (AC or CN) was added. The resulting slurry was dried in an
oil-bath at 80 oC overnight. Then the resulting solids were placed into a crucible and heated up to
600 oC in 4 h, then it kept at 600 oC for 1 h. After that, we further rose the temperature to 800 oC
in 2 h and maintained for another 1 h in N2 flow. Finally, the catalysts were obtained when it
cooled down to room temperature.
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Characterization Analyses
TEM experiments were conducted on a Hitachi H7700 instrument, operating at 100 kV.
HRTEM images were taken on Tecnai G2 F30 S-Twin, working on 300 kV. The BET surface
area and pore volume were measured by N2 adsorption-desorption experiments with a
Micromeritics ASAP 2020 HD88. Powder XRD measurements were carried out on Model
D/tex-Ultima TV using Cu Ka radiation (1.54 Å). The sample was scanned in the 2 θ range from
10o and 80o. XPS characterization was performed on an ESCALAB MARK II spherical analyzer
with an aluminum anode (Al 1486.6 eV) X-ray source. The Raman spectra were recorded on a
Raman spectrometer (JY, HR 800) with a 514 nm laser. ICP-AES measurements were conducted
on PerkinElmer Optima OES 8000. H2-TPR was carried out on a FINESORB-3010 apparatus
with a thermal conductivity detector (TCD).
Catalytic Applications in the hydrogenation reaction
The hydrogenation of quinoline was studied in a 50 mL stainless steel high-pressure batch
reactor. Typically, quinoline (0.5mmol), the cobalt catalyst (CoOx@CN, 10 mg) and the solvent
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(5 mL) were loaded into the autoclave. First, the reactor was purged three times by pure H2 to
remove air. After that, the reactor was charged with 3 MPa H2 and the reaction mixture was
stirred 120 oC. The catalytic experiments were performed at agitation speed of 1500 rpm to rule
out the external mass transfer limitation. After reaction finished, the reaction products were
analyzed by gas chromatograph from Shimadzu with a Rtx-1071 column using n-dodecane as the
internal standard, as well as by GC-MS (Agilent Technologies, GC 6890N, MS 5970) and
1H-NMR. For the reuse of the catalyst, the hydrogenation of quinoline was conducted with
1mmol scale (1 mmol quinoline, 20 mg CoOx@CN, and 10 mL methanol) at 120 oC and 3 MPa.
After the reaction, CoOx@CN was recycled by centrifugation or a magnet and then washed
several times with ethanol. Finally, the recycled catalyst was dried overnight in an oven at 70 oC.
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TOF = initial rate / (metal amount × time)
ASSOCIATED CONTENT
Supporting Information. Detailed characterizations of the CoOx@CN, Raman spectra of the
treated catalyst, GC-MS, 1H-NMR results of the products and supplementary figures. This
material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel.: 86-571-88273551. Fax: +86-571-87951895. E-mail: chemwy@zju.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGEMENT
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Financial support from the National Natural Science Foundation of China (91534114,
21376208), the MOST (2016YFA0202900), the Zhejiang Provincial Natural Science Foundation
for Distinguished Young Scholars of China (LR13B030001), the Fundamental Research Funds
for the Central Universities (2016FZA3006), and the Partner Group Program of the Zhejiang
University and the Max-Planck Society are greatly appreciated.
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