Encapsulation of Nonprecious Metal into Ordered Mesoporous N-Doped Carbon for Efficient Quinoline Transfer Hydrogenation with Formic Acid

Article abstract of DOI:10.1021/acscatal.8b01404

Ordered mesoporous N-doped carbon (OMNC) encapsulating Co nanoparticles (NPs) have been prepared under direct polymerization between [Co(NH₂CH₂CH₂NH₂)₂]Cl₂ and carbon tetrachloride through a hard template method. The catalysts (Co@OMNC) are pyrolyzed at various temperatures and characterized by elemental analysis, Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). In the quinoline transfer hydrogenation with formic acid (FA) as the hydrogen source under a base-free condition, the encapsulated Co NPs are physically isolated from the acidic reaction solution, which prevents them from poisoning or leaching. The rich mesopores and N dopants afford enhanced adsorption of quinoline. Co@OMNC-700 (pyrolyzed at 700 °C) gives the best activity (98.8% conversion) as well as >99% 1,2,3,4-tetrahydroquinoline (THQ) selectivity at 140 °C for 4 h, exhibiting significantly improved performance compared to using H₂ as the hydrogenation source. Moreover, Co@OMNC-700 is stable for recycling and exhibits high efficiency in FA dehydrogenation. Co@OMNC-700 is also a high-performance catalyst in the transfer hydrogenation of various unsaturated hydrocarbons. On the contrary, without the protection of OMNC, the exposed Co NPs in a control catalyst, Co/OMNC-700, lead to obvious Co leaching and low efficiency for the transfer hydrogenation of quinoline with FA.

Full text of DOI:10.1021/acscatal.8b01404

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Article
Encapsulation of Non-Precious Metal into Ordered Mesoporous N-Doped
Carbon for Efficient Quinoline Transfer Hydrogenation with Formic Acid
Guoqiang Li, Huanhuan Yang, Haifu Zhang, Zhiyuan Qi, Minda
Chen, Wei Hu, Lihong Tian, Renfeng Nie, and Wenyu Huang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01404 • Publication Date (Web): 27 Jul 2018
Downloaded from http://pubs.acs.org on July 27, 2018
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Encapsulation of NonꢀPrecious Metal into Ordered
Mesoporous NꢀDoped Carbon for Efficient
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Quinoline Transfer Hydrogenation with Formic Acid
Guoqiang Li,1 Huanhuan Yang,1 Haifu Zhang,1 Zhiyuan Qi,2 Minda Chen,2 Wei Hu,1 Lihong
Tian,1 Renfeng Nie,* 1,2 Wenyu Huang*2
1 Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, & Ministryꢀ
ofꢀEducation Key Laboratory for the Synthesis and Application of Organic Functional
Molecules, School of Chemistry and Chemical Engineering, Hubei University, Wuhan 430062,
P.R. China
2 Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA
ABSTRACT: Ordered mesoporous Nꢀdoped carbon (OMNC) encapsulating Co nanoparticles
(NPs) have been prepared under direct polymerization between [Co(NH2CH2CH2NH2)2]Cl2 and
carbon tetrachloride through a hard template method. The catalysts (Co@OMNC) are pyrolyzed
at various temperatures and characterized by elemental analysis, BET, XRD, XPS, and TEM. In
the quinoline transfer hydrogenation with formic acid (FA) as the hydrogen source under a baseꢀ
free condition, the encapsulated Co NPs are physically isolated from the acidic reaction solution,
which prevents them from poisoning or leaching. The rich mesopores and N dopants afford
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enhanced adsorption of quinoline. Co@OMNCꢀ700 (pyrolyzed at 700 °C) gives best activity
(98.8 % conversion) as well as 100 % 1,2,3,4ꢀtetrahydroquinoline (THQ) selectivity at 140 °C
for 4h, exhibiting significantly improved performance than using H2 as the hydrogenation source.
Moreover, Co@OMNCꢀ700 is stable for recycling and exhibits high efficiency in FA
dehydrogenation. Co@OMNCꢀ700 is also a highꢀperformance catalyst in the transfer
hydrogenation of various unsaturated hydrocarbons. On the contrary, without the protection of
OMNC, the exposed Co NPs in a control catalyst, Co/OMNCꢀ700, leads to obvious Co leaching
and low efficiency for the transfer hydrogenation of quinoline with FA.
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KEYWORDS: nitrogenꢀdoped carbon, cobalt, acidꢀtolerant, water, green chemistry
1. INTRODUCTION
The replacement of fossil energy sources by biomass for chemicals and fuels production is a
promising approach for generating less greenhouse gases and decreasing the impact of human
activity on nature.1 For example, catalytic hydrogenations adopt a privileged position in the
methodological toolbox for the chemical industry.2,3 Traditional hydrogenation process requires
excess (orders of magnitude more) highꢀpressure fossilꢀderived H2 that is difficult and dangerous
to control, often leading to over hydrogenated products.2 On the contrary, a liquid state hydrogen
source, formic acid (FA), has considerable hydrogen content (4.4 wt%), which could be used as a
safe and convenient hydrogen source.4 More importantly, FA can be easily obtained through
biomass oxidation or hydrolysis.5,6 The hydrogen readily releases upon catalytic decomposition
of FA, which could make it as a green hydrogen source for catalytic transfer hydrogenation
(CTH) over suitable catalysts.7
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FA could act as a hydrogen source through a catalytic dehydrogenation reaction
(HCOOH→H2+CO2).8 However, an alternative dehydration pathway (HCOOH→H2O+CO)
generates CO that could poison metalꢀbased catalysts.9 To avoid the CO pathway, the
dehydrogenation of FA with homogeneous metal compounds has been widely reported.10
However, the difficulty of recycle, the use of various additives and/or organic solvents limit their
practical largeꢀscale application.11 These disadvantages associated with homogeneous catalysts
are dramatically mitigated by heterogeneous catalysts. Among heterogeneous catalysts, precious
metals are extensively studied for FA dehydrogenation, such as palladium,8,12ꢀ18 platinum,19,20
ruthenium,9 aurumꢀbased21ꢀ23 and bimetallic7,24 systems. Moreover, most precious metalꢀ
catalyzed FA dehydrogenation reactions need the assistance of excess alkali compounds (e.g.,
Na/K salts, amines),22,25ꢀ27 which further increase the cost of products separation and
purification. Limited studies have been carried out on nonꢀprecious metal catalysts. Apart from
the poor activity of nonꢀprecious metal catalysts,28,29 the high polarity and strong acidity of FA
usually lead to severe leaching and irreversible deactivation of the catalysts.30 For all of these
reasons, it is highly desirable to design a heterogeneous catalyst that is cheap, efficient and
acid/COꢀtolerant for a baseꢀfree CTH with FA.
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Nitrogenꢀdoped carbon (NC) has superior acidꢀbase tolerance, improved basicity, and
enhanced pꢀdonor capability from nitrogen dopants. NCꢀsupported nonꢀprecious metal catalysts
have already exhibited promising performance in a variety of organic transformations.31,32 Direct
carbonization of metalꢀcoordinated polymers or MOFs under an inert gas can prepare such
catalysts, in which metal NPs were confined in a carbonaceous matrix.33ꢀ38 Although the directꢀ
carbonization method is effortless, it typically does not endow much control over the position of
the active centers and often leads to severe aggregation, which deteriorates the performance and
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stability of catalysts.33,39 Additionally, direct coarse pyrolysis could destruct the pore structure of
the original supports that hinder the access for reactants to active centers.40,41
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Effective design of the precursors to endow the NCꢀbased materials with multifunctional and
dispersed active sites that are easily reachable by substrates could significantly ameliorate the
previous problems and boost their catalytic performance.42,43 Herein, we synthesize ordered
mesoporous Nꢀdoped carbon (OMNC)ꢀencapsulated Co (Co@OMNC) catalysts, in which Co is
encapsulated in OMNC by carbonizing the rationally designed CoꢀCN polymer@SBAꢀ15
precursor. The resulted Co@OMNC catalyst has a high surface area (543.2 m2g−1) and ordered
mesopores (4.6 nm) favoring the easy access of reactants. The asꢀobtained catalysts with
multiple active sites are highly efficient for coupling FA dehydrogenation with CTH of quinoline
under a baseꢀfree condition to produce 1,2,3,4ꢀtetrahydroquinoline (THQ). Moreover, the
OMNC shell isolates the cobalt species from the acidic reaction media, and thus prevents Co
from poisoning or leaching. THQ is a key biologically active intermediate broadly used in the
production of many drugs and pesticides.44ꢀ46 Similar approaches (Scheme S1) for the synthesis
of THQ had been demonstrated using precious metal catalysts (Au, Rh, Pd), which requires large
amount of FA, high temperature (>150 °C), or assistance of excess bases (such as triethylamine,
HCO2Na).11,22,47,48 The nonꢀpreciousꢀmetalꢀbased Co@OMNC catalyst is highly efficient for the
CTH of quinoline, whose activity is comparable to the best preciousꢀmetalꢀbased catalysts.
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2. RESULTS AND DISCUSSION
2.1. Structural characterizations
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Scheme 1 The synthesis process of Co@OMNCꢀx composites.
Co@OMNC catalysts were constructed by using SBAꢀ15 as the template through a simple
polymerization approach (Scheme 1). During this procedure, we used ethylenediamine (EDA) to
capture Co2+ and generate [Co(EDA)2]2+. Upon polymerization between EDA and carbon
tetrachloride (CTC), CoꢀCN polymer filled into the mesopore of SBAꢀ15. After carbonization
and HF etching to simultaneously remove SBAꢀ15 and unwrapped Co species, ordered
mesoporous Nꢀdoped carbonꢀencapsulated Co catalysts were prepared.
Co@OMNC-500
Co@OMNC-600
Co@OMNC-700
(110)
Co@OMNC-800
(200)
(211)
1.0
1.5
2.0
2.5
3.0
3.5
4.0
2 Theta (o)
Figure 1. SAXRD patterns of Co@OMNCs.
The small angle Xꢀray diffraction (SAXRD) patterns of Co@OMNCꢀx shown in Figure 1
suggest the twoꢀdimensional hexagonal p6mm featured (110), (200), and (211) diffractions with
2θ below 2.5°.49 The reflections gradually shift to higher 2θ values by increasing the pyrolysis
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temperature (Table S1). As the carbonization temperature increases from 500 to 700 °C, the
(110) diffraction becomes more intense, indicating that the increase of structural order at 700 °C.
However, the (110) peak becomes less resolved at 800 °C, suggesting the slight destruction of
the ordered mesostructure at high pyrolysis temperature.
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600
0.050
Co@OMNC-500
(a)
Co@OMNC-500
Co@OMNC-600
Co@OMNC-700
Co@OMNC-800
0.045
0.040
0.035
0.030
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0.015
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0.000
Co@OMNC-600
Co@OMNC-700
Co@OMNC-800
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Relative pressure (p/p0)
Pore width (nm)
Figure 2. (a) N2 sorption isotherms and (b) the size distributions of the mesoporous
Co@OMNCs.
Table 1. Structural properties of the Co@OMNCs.
Samples
Stotal (m2/g) Pore volume (cm3/g) Average pore diameter (nm)
Co@OMNCꢀ500
Co@OMNCꢀ600
Co@OMNCꢀ700
240
420
490
0.24
0.49
0.62
3.9
4.2
4.6
Co@OMNCꢀ800
540
0.81
4.7
Figure 2a shows that all Co@OMNCꢀx samples display a type IV N2 sorption isotherm,
confirming the mesoporous nature of the catalysts.50 As the pyrolysis temperature increases from
500 to 800 °C, the isotherms upraise obviously at the range of 0.5ꢀ0.9 p/p0, indicating the
increased porosity of Co@OMNCꢀx. For instance, Co@OMNCꢀ800 exhibits the highest surface
area (540 m2/g) and pore volume (0.81 cm3/g) as shown in Table 1. Meanwhile, high pyrolysis
temperature leads to broad pore size distribution (PSD) and hysteresis loop (Figure 2b). For
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example, besides the main distribution at about 3.6 nm, Co@OMNCꢀ800 also shows an
additional distribution within the range 4ꢀ7 nm. These results indicate that high temperature
could promote pore formation in Co@OMNCꢀx but partially destroy the longꢀrange order of
their mesostructure,51 agreeing with the SAXRD results (Figure1).
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The scanning electron microscopy (SEM) image of Co@OMNCꢀx (Figure S1a) shows that the
void spaces and the morphology of OMNC are analogous to the parent SBAꢀ15. The highꢀ
resolution SEM image (Figure S1b) further exhibits that mesopore channels are periodically
aligned over a large domain. The transmission electron microscopy (TEM) images in Figure 3
show that all Co@OMNCꢀx catalysts have uniformly distributed mesopores. We did not observe
any obvious Co NPs in Co@OMNCꢀ500 (Figure S2). Increasing the pyrolysis temperature
a ords the formation of large Co NPs with broad size distribution. The average particle sizes in
Co@OMNCs prepared at 600, 700, and 800 °C are 5.7, 14.1 and 18.4 nm, respectively. TEM
images of Co@OMNCꢀ700 and Co@OMNCꢀ600 show that, apart from the presence of
relatively large Co NPs up to ~15 nm, ultrasmall nanoclusters (~1 nm) are well dispersed in
mesopores in most areas (as shown in Figures 3b, d, and S3). HRTEM image shows that Co NPs
are embedded in mesoporous carbon skeleton. Figure 3e shows a representative Co NP with
0.215 nm interplanar spacing corresponding to the Co(111) plane, and the Co NP is embedded
by graphitic layers with a C(002) lattice plane (0.341 nm). As observed in the elemental mapping
of Co@OMNCꢀ700 (Figure 3h), there is a good spatial overlap of C, N and Co elements,
demonstrating that N species and Co NPs distribute homogeneously over the mesoporous
carbon. The unique structure composed of ordered mesoporous Nꢀdoped carbon with
encapsulated Co NPs could favor the electron transferring, and restrain the leaching and
poisoning of Co NPs,46 and thereby improve their catalytic stability and activity.
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Figure 3. TEM images of Co@OMNCꢀ (a,b) 600, (cꢀe) 700, (f,g) 800, and (h) elemental
mapping of Co@OMNCꢀ700.
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We measured the wideꢀangle XRD patterns of Co@OMNCs (Figure S4), which show that the
intensity of C(002) peak increases at high pyrolysis temperature. At 500 °C, no Co or CoOx
diffractions can be observed, while higher pyrolysis temperatures (600, 700, and 800 °C) lead to
distinct diffraction peak from metallic Co. These results suggest that ionic Co is reduced in situ
by carbon and/or nitrogen species during the highꢀtemperature pyrolysis.52
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674
D
G
467
510
587
605
Co-O/Co-N
(d)
(d)
(c)
(b)
(c)
(b)
(a)
(a)
200 400 600 800 1000 1200 1400 1600 1800 2000
Raman shift (cm-1)
350 400 450 500 550 600 650 700 750
Raman shift (cm-1)
Figure 4. Raman spectra of Co@OMNCꢀ (a) 500, (b) 600, (c) 700, and (d) 800.
The Raman spectra (Figure 4) clearly illustrate that the G peak sharpens and upshifts to higher
wavenumber as the pyrolysis temperature increases, which indicates higher graphitization of
carbon skeleton at higher pyrolysis temperature.33,53 At the same time, we also observed Coꢀ
N/CoꢀO vibration peaks at 400ꢀ700 cmꢀ1. Comparing with Co@OMNCꢀ500 and Co@OMNCꢀ
600, Co@OMNCꢀ700 shows stronger characteristic peaks, implying the enhanced N/OꢀCo
interaction in Co@OMNCꢀ700.54 Further raising the temperature to 800 °C weakens these peaks
(Figure 4d), indicating more Co0 is formed by the decomposition of CoꢀO/CoꢀN under high
pyrolysis temperature, well agreed with the XRD results (Figure S4).
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(d)
(c)
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Pyrrolic N
Graphitic N
Pyridine oxide-N
Pyridinic N
(a)
394
396
398
400
402
404
406
408
Binding Energy (eV)
Figure 5. N1s spectra of Co@OMNCꢀ (a) 500, (b) 600, (c) 700, and (d) 800.
Co(III)-N/O
Co(II)-N/O
Shake up
Co0
(d)
(c)
(b)
(a)
774 776 778 780 782 784 786 788 790 792
Binding Energy (eV)
Figure 6. Co2p spectra of Co@OMNCꢀ (a) 500, (b) 600, (c) 700, and (d) 800.
Table 2. Structural properties of Co@OMNCꢀx.
Relative atomic percentage of
Relative atomic percentage of N (%)
Co (%)
Co(III)ꢀ
Catalysts
Co0
Co(II)ꢀ
N/O
49.3
43.2
41.3
35.5
Pyridinic
N
Pyrrolic
N
Graphitic
N
Pyridine
oxideꢀN
0
6.2
7.58
N/O
50.7
52.6
50.8
47.7
Co@OMNCꢀ500
Co@OMNCꢀ600
Co@OMNCꢀ700
Co@OMNCꢀ800
50.0
44.4
44.5
31.2
44.1
35.6
22.8
14.1
5.9
0
13.8
25.1
39.6
4.2
7.9
16.8
15.1
The surface composition of Co@OMNCꢀx is detected by XPS. The C1s XPS peaks (Figure
S5) of Co@OMNCꢀx become narrow and show an obvious shift to lower binding energy as
elevating temperature from 500 to 800 °C, indicating the removing of nitrogen or oxygen species
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from the catalysts.55 The XPS data (Table S2, Figure S6) show that N contents of Co@OMNCꢀx
decrease gradually with raising the pyrolysis temperature. We deconvoluted the XPS N1s peaks
of Co@OMNCꢀx (Figure 5) into four types, pyridinic (398.5 eV), pyrrolic (400.1 eV), graphitic
(401.1 eV), and pyridine oxide (403.3 eV).56 The pyrrolic and pyridinic N are the dominate
species (> 80 %) in Co@OMNCꢀ500 and ꢀ600 (Table 2), but their percentages drop significantly
to 67.3 % and 45.3 % in Co@OMNCꢀ700 and ꢀ800, indicating the partial removal of nonꢀstable
pyridinic and pyrrolic N atoms under highꢀtemperature pyrolysis.52
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We fitted the Co 2p3/2 XPS spectra of Co@OMNCs (Figure 6) with three Co species, Co0
(778.7 eV), Co(III)ꢀN/O (780.6 eV), and Co(II)ꢀN/O (782.6 eV).39 The presence of CoꢀO/CoꢀN
could be due to the coordination of cobalt ions with N/O in Nꢀdoped carbon that could be
partially reserved during the carbonization process.57 The percentage of Co(III)ꢀN/O increases
slightly from 50.7 to 52.6 % with increasing the pyrolysis temperature from 500 to 600 °C, but
decreases to 47.7 % at 800 °C (Table 2). XPS could not detect any Co0 in Co@OMNCꢀ500,
while 4.2, 7.9 and 16.8 % Co0 is detected in Co@OMNCꢀ600, Co@OMNCꢀ700 and
Co@OMNCꢀ800, respectively. These results suggest that, as the pyrolysis temperature increases,
CoꢀO and CoꢀNx species are reduced in situ into Co0 by OMNC.52
2.2 Transfer hydrogenation of quinoline with FA
Table 3. Performance of different catalysts in the hydrogenation of quinoline. a
Hydrogen
source
FA
Solvent
T/
°C
140
Conversion
THQ
selectivity / %
Entry
Catalysts
/ %
ꢀ
1
2
3
4
OMNCꢀ700
H2O
H2O
H2O
H2O
ꢀ
Co@OMNCꢀ700
Co@OMNCꢀ500
Co@OMNCꢀ600
none
FA
140
140
140
ꢀ
ꢀ
19.9
84.6
66.7
>99
FA
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Co@OMNCꢀ700
Co@OMNCꢀ800
Co@OMNCꢀ700
Co@OMNCꢀ700
Co@OMNCꢀ700
Co@OMNCꢀ700
Co@OMNCꢀ700
Co@OMNCꢀ700
Co@NCꢀ700 b
FA
FA
FA
FA
FA
FA
FA
FA
FA
FA
FA
H2
H2O
H2O
H2O
H2O
H2O
140
140
130
120
110
98.8
79.5
51.7
35.6
26.8
44.5
33.8
9.4
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
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Cyclohexane 140
THF
Ethanol
H2O
140
140
140
140
140
140
25.9
4.9
Co/AC
Co/OMNCꢀ700c
H2O
H2O
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33.1
Co@OMNCꢀ700
H2O
a Reaction conditions: quinoline (0.5 mmol), Co loading 10.6 mol%, FA (200 mg), H2O (10 mL),
N2 0.5 MPa, 140 °C, 4 h.
b Prepared without SBAꢀ15.
c Prepared by impregnation method.
Table 3 shows the performance of different catalysts in quinoline CTH using FA or H2 as the
reducing agent at 140 °C (Table 3). No transformation of quinoline occurs when no metal or no
FA is added (entries 1 and 2, Figure S7). With FA as the hydrogen source, catalysts with
different carbonization temperature demonstrate a distinct difference in activity. For example,
Co@OMNCꢀ500 gives only 19.9 % quinoline conversion and 66.7 % THQ selectivity (entry 3).
As the pyrolysis temperature increases, quinoline conversion increases obviously for
Co@OMNCꢀ700 with 98.8 % conversion along with 100 % THQ selectivity (entry 5). However,
Co@OMNCꢀ800 only gives 79.5 % quinoline conversion (entry 6). We also prepared another
two control catalysts, disordered Nꢀdoped carbon supported Co (Co@NCꢀ700) prepared without
using a SBAꢀ15 template and active carbon supported Co (Co/AC) prepared by impregnation
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method. Both of the two catalysts showed much lower quinoline conversion (entries 13 and 14)
than Co@OMNCꢀ700 (entry 5) under the same condition, indicating the mesoporous structure in
Co@OMNCꢀ700 is beneficial for the substrate to access the active sites, thus improving their
catalytic activity. As for reference, OMNCꢀ700 is prepared firstly (entry 15), and then loaded by
Co NPs to give Co/OMNCꢀ700, the result shows only 36.7 % quinoline conversion (Figure S8),
which is much lower than that of embedded Co@OMNCꢀ700. Using molecular hydrogen (entry
16 and Figure S9), Co@OMNCꢀ700 affords only 33.1 % quinoline conversion, indicating that
the FA is more efficiently activated and dissociated than molecular H2 to generate adsorbed
hydrogen atoms for quinoline hydrogenation.58
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Temperature is an important parameter for this reaction (Table 3). For Co@OMNCꢀ700,
reducing reaction temperature from 140 to 110 °C leads to a substantial drop in quinoline
conversion from 98.8 to 26.7 % (entries 7ꢀ9), while the THQ selectivity keeps almost constant.
We also studied the effect of solvents, which also largely affect the reaction rate.59 The survey
shows (entries 5, 10ꢀ12) that water provides better conversion than ethanol, cyclohexane, and
THF. Water is a green solvent that is ecoꢀfriendly and inexpensive.47
8
100
Selectivity
7
80
60
40
20
0
6
5
4
3
2
1
0
Co@OMNC-700
Co/OMNC-700
Conversion
0.0
0.5
1.0
1.5
2.0
FA concentration (mg/mL)
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Figure 7. The transfer hydrogenation of quinoline and the solution pH affected by FA
concentration. Reaction conditions: quinoline (0.5 mmol), Co loading 10.6 mol%, H2O (10 mL),
N2 0.5 MPa, 140 °C, 4h
.
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The FA concentration is another important factor in this reaction. Figure 7 shows the CTH of
quinoline with different FA concentration over Co@OMNCꢀ700 and Co/OMNCꢀ700 catalysts.
With the FA concentration increasing from 0 to 0.25 mg/mL, these two catalysts give similar
reaction rates. However, further increase the FA concentration leads to a significant difference in
conversion between Co@OMNCꢀ700 and Co/OMNCꢀ700, showing the feeble activity of
Co/OMNCꢀ700 under high FA concentration. Considering the strong acidity of FA, the pH of
different FA solution is tested. We found that, as the FA concentration increases, pH value
decreases obviously and reaches 1.21 when 2 mg/mL FA was used. It seems that unwrapped
Co/OMNCꢀ700 is more sensitive than Co@OMNCꢀ700 under the strong acidic condition. We
found that, after the reaction, the filtrate of Co/OMNCꢀ700 turns to purple color (Figure S10).
UVꢀvis spectrum (Figure S11) shows a significant light absorbance for Co/OMNCꢀ700 in the
range of 400ꢀ600 nm, implying Co species in Co/OMNCꢀ700 partially dissolve in an acidic
environment. On the contrary, no absorption peak appears for Co@OMNCꢀ700, which indicate
the OMNC shell prevents Co from contacting the acidic solution and thus avoids the possibility
of etching or deactivation.60 Additionally, the strong interaction between OMNC and inner Co
leads to the superior performance of Co@OMNCs in CTH.57 It is worth to point out that surface
Nꢀcoordinated Co(II/III) sites could also involve in the catalysis,61 which requires more
characterization.
Table 4. The CTH performance of Co@OMNCꢀ700 for unsaturated hydrocarbons.a
Entry
Substrate
Product
T/ °C t/h Conversion/% Selectivity/%
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160
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6
98.8
98.9
95.4
>99
>99
>99
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5
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140
140
140
4
4
4
90.9
92.3
99.2
98.5
>99
>99
NH2
CH2OH
CH2OH
7
8
140
140
4
6
96.4
100
>99
>99
a Reaction conditions: substrate (0.5 mmol), Co loading 10.6 mol%, FA (200 mg), H2O (10 mL),
N2 0.5 MPa.
The reaction scope in Table 4 demonstrates that Co@OMNCꢀ700 is a versatile catalyst for the
CTH of different unsaturated compounds in water. For Nꢀheterocycleꢀlike pyridine and 2ꢀ
picolinic acid (entries 2 and 3), the catalyst affords good catalytic activity and the yields of
piperidine are greater than 95%. In the case of benzaldehyde and furfural (entries 4 and 5), the
reaction gives >90% conversion as well as >98% alcohol selectivity. Because of the great
importance of aniline (AN), CTH of nitrobenzene (NB) is carried out (entry 6). It is found that
99.2% NB conversion as well as nearly 100% AN selectivity could be achieved within 4 h.
Additionally, we also achieved high yield for the CTH of conjugated olefins (entries 7 and 8).
To study the activity difference for the CTH of quinoline, FA dehydrogenation over various
catalysts is investigated (Figures S12 and S13). It is found that Co@OMNCꢀ500, ꢀ600, ꢀ700 and
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ꢀ800 afford 40.6, 72.5, 100 and 84% FA conversion, respectively. Comparatively, only 61.6 %
FA conversion can be observed over Co/OMNCꢀ700. The FA dehydrogenation trend agrees with
the CTH of quinoline.
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Considering the rich N species and ordered mesopore structure of Co@OMNC, the adsorption
capacity towards quinoline is measured by UVꢀvis (Figure S14). The amount of saturate
quinoline adsorption (Figure S15) over Co@OMNCx follows the order of Co@OMNCꢀ500 < ꢀ
800 < ꢀ600 < ꢀ700, reaching maximum of 57.46 mg/g for Co@OMNCꢀ700. The trend of
quinoline adsorption amount also agrees with the order of hydrogenation activity. We also used
CO2ꢀTPD to characterize the surface basicity of the Co@OMNC catalysts. The CO2ꢀTPD profiles
(Figure S16) show that there is one broad CO2 desorption peak appearing at 300–500 °C for all
samples, which represents the desorption of chemical adsorbed CO2 molecules over N species.52
Comparatively, the CO2 desorption peak becomes stronger for Co@OMNCꢀ700. For Co@OMNCꢀ
800, however, the CO2 desorption peak weakens considerably, which could be attributed to the
decreased N content (especially for pyridinic N) resulting from the high preparation temperature.
Therefore, the good catalytic activity of Co@OMNCꢀ700 could be related to the following two
reasons, 1) the favorable interaction between quinoline and the catalyst leads to the high
quinoline concentration on the surface of OMNCꢀ700; and 2) the high density of basic sites on
the surface of OMNCꢀ700 could enhance the adsorption and dehydrogenation of FA during the
CTH of quinoline.8
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0.0
0.5
1.0
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t (h)
Figure 8. Durability test for the Co@OMNCꢀ700 for transfer hydrogenation of quinoline with
FA. Reaction conditions: quinoline (0.5 mmol), Co loading 10.6 mol%, FA (200 mg), H2O (10
mL), N2 0.5 MPa, 140 °C.
The heterogeneous catalyst Co@OMNCꢀ700 can be readily reused in the CTH of quinoline.
There is no obvious loss in the conversion of quinoline over five runs with >90% conversion
(Figure S17). To further confirm the stability of the catalyst, we also evaluated the reusability of
Co@OMNCꢀ700 by kinetics in CTH run to approximately 50 % conversion. The Co@OMNCꢀ
700 shows no sign of deactivation for five runs (Figures 8 and S18). The elemental analysis
(Table S3) of the reused catalyst only shows less than 0.1 % Co leaching in the reaction solution,
indicating that the OMNC can prevent the leaching of Co even in the acidic reaction solution at
the elevated temperature. We characterized the used Co@OMNCꢀ700 using XRD and XPS.
Both the XRD pattern (Figure S19) and Co 2p XPS results (Figure S20 and Table S4) are similar
to the fresh catalyst, which indicates the Co species in Co@OMNCꢀ700 do not change
significantly after five catalytic reaction runs. This result indicates that Nꢀdoped carbon layers
could protect Co0 from etching by FA and strengthen the stability of Co NPs. Therefore,
Co@OMNCꢀ700 is indeed an acidꢀtolerant, cheap, and efficient catalyst in water media.
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3. CONCLUSION
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We demonstrated that Co NPs encapsulated in ordered mesoporous Nꢀdoped carbon
(Co@OMNC) are efficient bifunctional heterogeneous catalysts for the transfer hydrogenation of
quinoline to THQ using FA. Within all studied catalysts, Co@OMNCꢀ700 shows the best
activity and THQ selectivity using FA as the hydrogen source. A catalyst prepared by the simple
impregnation of Co on OMNCꢀ700 affords very poor performance due to severe leaching of Co
active sites in acidic media. The unique catalytic property of Co@OMNCꢀ700 for CTH of
quinoline with FA is due to the wellꢀdesigned structure composed of OMNC with encapsulated
cobalt, in which the cobalt induces OMNC shell to be new active centers for both FA
dehydrogenation and quinoline hydrogenation. With this structure, Co@OMNCꢀ700 afforded
superior acidꢀtolerance in the existence of FA and enhanced adsorption of substrates. The
Co@OMNCꢀ700 also shows high versatility for the CTH of various unsaturated hydrocarbons.
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AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: refinenie@163.com, whuang@iastate.edu
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/acscatal.
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Experimental details, Tables S1 to S4, Figs. S1 to S20 (PDF)
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ACKNOWLEDGMENTS
This work was supported by National Natural Science Foundation of China (21603066,
21606075), Natural Science Fund of Hubei Province (2015CFB232) and Natural Science Fund
of the Education Department of Hubei Province (Q2015009). R. Nie thanks the China
Scholarship Council and Hubei Chenguang Talented Youth Development Foundation for
financial support.
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Ordered mesoporous Nꢀdoped carbon encapsulated Co is an acidꢀresistant catalyst for coupling
biomassꢀderived FA dehydrogenation and quinoline hydrogenation in water solution under a
baseꢀfree condition.
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