A MOF-derived Co-CoO@N-doped porous carbon for efficient tandem catalysis: Dehydrogenation of ammonia borane and hydrogenation of nitro compounds

Article abstract of DOI:10.1039/c6cc03149h

The one-step pyrolysis of a zeolite-type metal-organic framework, Co(2-methylimidazole)₂ (ZIF-67), produces an N-doped porous carbon incorporating well-dispersed Co/CoO nanoparticles, which exhibit excellent catalytic activity, chemoselectivity and magnetic recyclability for the tandem dehydrogenation of ammonia borane and hydrogenation of nitro compounds at room temperature.

Full text of DOI:10.1039/c6cc03149h

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A MOF-Derived Co-CoO@N-doped Porous Carbon for Efficient
Tandem Catalysis: Dehydrogenation of Ammonia Borane and
Hydrogenation of Nitro Compounds†
Received 00th January 20xx,
Accepted 00th January 20xx
Xiao Ma,‡ Yu-Xiao Zhou,‡ Hang Liu, Yang Li and Hai-Long Jiang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The one-step pyrolysis of a zeolite-type metal-organic framework, solubility of hydrogen in different solvents. Sufficient contact
Co(2-methylimidazole)2 (ZIF-67), produces the N-doped porous between hydrogen gas and nitro compounds would be a
carbon incorporating well-dispersed Co/CoO nanoparticles, which prerequisite to boost the reaction efficiency. To meet this
exhibit excellent catalytic activity, chemoselectivity and magnetic challenge, in situ hydrogen production coupled with the
recyclability for the tandem dehydrogenation of ammonia borane reduction of nitro compounds would be a suitable solution, as
and hydrogenation of nitro compounds at room temperature.
the in situ-generated H2 throughout the reaction system will
readily and immediately react with the nitro group, enhancing
the reaction efficiency. In this regard, ammonia borane
(NH3BH3), having a high hydrogen content of 19.6 wt% and a
high solubility in water and methanol, is an excellent hydrogen
source.7,8 The coupling of dehydrogenation of NH3BH3 and
hydrogenation of nitro compounds in a tandem route is
expected to greatly boost the reaction rate.9
Aromatic and aliphatic amines are high-value chemicals with
wide applications for the manufacture of dyes, pigments, food
additives, pharmaceuticals, herbicides and fine chemicals.1
These amines are generally prepared by the hydrogenation of
corresponding nitro compounds, while the co-existence of
other reducible functional groups in them poses challenges for
the selective reduction.1 Currently, selective hydrogenation of
nitroarenes mostly relies on noble metal-based catalysts with
high price and scarcity limitations.2 Therefore, the
heterogeneous catalysts based on earth-abundant base metals
(for examples, Fe, Co and Ni) are highly desired,3 for which
base metal nanoparticles (NPs) in small sizes stabilized inside
stable porous matrix would be ideal strategies. In this context,
as a relatively new class of porous materials, metal-organic
frameworks (MOFs),4 usually constructed by base metal
(clusters) and organic ligands with diversified and tailorable
structures, have been primarily demonstrated to be suitable
templates/precursors to afford uniform metal (oxide) NPs
distributed throughout porous carbon via pyrolysis, in which
the high porosity and long-range structural ordering could be
partially preserved.5 The MOF-derived porous composites have
been recently reported in various heterogeneous reactions.6
On the other hand, the hydrogenation of nitro compounds
usually requires hydrogen as the reducing agent at high
pressure and/or high temperature, due to the very low
With these in mind, a zeolite-type MOF, Co(2-methyl-
imidazole)2 (ZIF-67),10 as a template and precursor, has been
pyrolyzed to form Co-CoO@N-doped porous carbon
nanocomposite with a regular shape, where crystalline Co NPs
with oxidized CoO species on the surface are evenly
distributed throughout the N-doped porous carbon. The
resultant nanocomposite exhibits excellent catalytic
performance and is magnetically recyclable toward the
tandem dehydrogenation of NH3BH3 and reduction of nitro
compounds at room temperature (Scheme 1). It is worthy to
stress that the current base metal catalyst takes advantage of
in situ-generated hydrogen from NH3BH3 to efficiently reduce
nitrobenzene, in which the catalytic rate/activity (1.5 h, 100%)
is several orders of magnitude higher than that using 1 bar H2
Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key
Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Suzhou
Nano Science and Technology, Department of Chemistry, University of Science and
Technology of China, Hefei, Anhui 230026, P.R. China
Scheme 1 Schematic illustration of the synthesis of ZIF-67 and its
pyrolyzed N-doped porous carbon encapsulating Co-CoO NPs for
tandem catalysis of dehydrogenation of NH3BH3 and hydrogenation of
nitro compounds.
E-mail: jianglab@ustc.edu.cn
†Electronic Supplementary Information (ESI) available: Detailed Experimental
procedures and supporting figures. See DOI: 10.1039/x0xx00000x
‡These authors contributed equally to this work.
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(12 h, 0.7%). In addition, the catalyst is able to selectively lattice spacing of 0.20 nm (Fig. 1d), in consistent with the
convert a variety of substituted aromatic or aliphatic nitro powder XRD results (Fig. S2a, ESI†). The SAED pattern exhibits
compounds into corresponding amines with conversion and individual rings mainly indexed to the (111), (200), (220) and
yield up to 100% in several hours, tolerating different (311) planes of β-Co, and other rings can be assigned to the
reducible groups in functionalized nitrobenzene, including planes of CoO (marked with asterisks, Fig. 1d, inset),
nitrile, aldehyde and ketone.
supporting the surface oxidation of Co to CoO, in agreement
ZIF-67 was obtained by facilely stirring Co(NO3)2 and with the previous report.6b X-ray photoelectron spectroscopy
2-methylimidazole in water at room temperature.6b,10 The (XPS) analysis verifies the co-existence of Co, C, N and O on the
purity of ZIF-67 was confirmed by powder X-ray diffraction surface of Co-500-3 h (Fig. S6, ESI†), in which the forms of Co
(PXRD) (Fig. S1a, ESI†). N2 sorption result shows the typical include Co(0), Co2+ and Co-OH species, as revealed by the Co
type I isotherm for ZIF-67, matching its microporous character 2p spectrum (Fig. S7a, ESI†).11 The Co2+ observed is consistent
(Fig. S1b, ESI†). The pyrolysis of ZIF-67 nanocrystals at different with the above SAED results. In addition, the N 1s spectrum
temperatures under N2 provides Co-CoO@N-doped porous exhibits four distinct peaks (pyridinic, pyrrolic, graphitic and
carbon nanocomposites, denoted as Co-T-t (T and t represent oxidized nitrogen species) with binding energies of 398.6,
pyrolysis temperature and time, respectively; T = 500, 600, 399.3, 400.7 and 404.1 eV, respectively (Fig. S7b, ESI†). All
700 °C; t = 1, 2, 3 h). PXRD patterns for all products exhibit above results suggest that the pyrolysis of ZIF-67 leads to
three sharp peaks assignable to metallic β-Co at 2θ = 44.38° crystalline Co NPs, with partially oxidized CoO on the surface,
(111), 51.60° (200) and 75.68° (220) (Fig. S2a, ESI†), approving evenly distributed throughout the N-doped porous carbon.
the main component, and the peak intensity is dominated by
The Co NP-based catalysts have been reported to be active
pyrolysis temperature. The weak (002) peak at ~25° should be in many important processes, including the reduction of nitro
due to the low graphitization degree, supported by the Raman compounds,2,3b the hydrolysis of NH3BH3,12 etc. However, to
spectra (Fig. S3, ESI†), of resultant carbons obtained at the best of our knowledge, the tandem dehydrogenation of
relatively low pyrolysis temperatures. All N2 sorption isotherms NH3BH3 and hydrogenation of nitro compounds over base
are close to type-IV with a hysteresis loop and the pore size of metal catalysts remains vacant thus far. The above
Co-500-3 h mainly falls into 1-3 nm with some larger ones ZIF-67-derived high-density Co-CoO NPs are uniformly
started from 7 nm (Fig. S4, ESI†). The inducꢀvely coupled distributed in the porous carbon matrix, in which the pore
plasma atomic emission spectrometry (ICP-AES) data present structure not only facilitates the transportation of
the increased Co contents along with elevated pyrolysis substrates/products but also limits the aggregation of the
temperatures, namely, 21.6%, 29.0%, and 32.7%, respectively active Co species. These features would make the
for Co-500-3 h, Co-600-3 h, and Co-700-3 h (Table S1, ESI†).
The microstructure observation for ZIF-67 and its pyrolysis
nanocomposites ideal catalysts for the tandem conversion.
Encouraged by the above considerations, the tandem
product, taking Co-500-3 h as a representative, has been catalysis using nitrobenzene as
a model substrate over
conducted with scanning electron microscopy (SEM) and ZIF-67-derived nanocomposites was investigated to explore
transmission electron microscopy (TEM). Compared with the optimized reaction parameters (Table 1). Due to the great
original ZIF-67 nanocrystals in 200-500 nm (Fig. 1a), Co-500-3 h solubility of nitro compounds in methanol but not in water, a
exhibits some shrinkage with relatively uniform size and rough methanol/water mixture of 2/3 (v:v) was adopted as an
surface (~200 nm, Fig. 1b). The metallic NPs in 6-20 nm sizes optimized solvent system to form a homogeneous solution of
with high contrast are uniformly distributed in the porous reactants (Section S2, Fig. S8, ESI†). Among all nanocomposites
carbon (Fig. 1c, Fig. S5, ESI†). The high-resolution TEM(HRTEM) obtained by different pyrolysis temperatures and time lengths,
image affords the evidence of the crystalline Co with Co-500-3 h showed the best activity and 100% selectivity to
complete the reaction and give the target product within 1.5 h
(entries 1-8). Unexpectedly, Co/AC offered a very low catalytic
activity (entry 9), possibly due to the uneven catalytic sites and
low porosity. Strikingly, when NH3BH3 was replaced by 1 bar
hydrogen while other conditions remained unaltered, the
conversion sharply lowered to 0.7% even after 12 h of reaction
(entry 10). It is believed that the in situ-generated hydrogen
from NH3BH3 has higher solubility/concentration in the
reaction solution, which is beneficial to the sufficient contact
with nitro compounds, and thus accelerates the reduction of
the nitro group. Therefore, the coupling of in situ
dehydrogenation and hydrogenation reaction should be a very
efficient approach. The presence of Co-based catalyst and
NH3BH3 as a hydrogen source are necessary for the conversion
Fig. 1 Microstructure observation for ZIF-67 and Co-500-3 h. SEM
images of (a) ZIF-67 and (b) Co-500-3 h. (c) TEM and (d) HRTEM images
(inset: SAED pattern) of Co-500-3 h.
(entries 11-12). The small-size and highly active Co NPs
stabilized by the special porous structure of Co-500-3 h play
crucial roles in the high catalytic activity and selectivity.
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Table 1 Tandem dehydrogenation of NH3BH3 and hydrogenation of Table 2 Tandem dehydrogenation of NH3BH3 and hydrogenation of
nitrobenzene over different Co-based nanocomposite catalysts.a various nitro compounds over Co-500-3 h.a
Yeildb (%)
100
Entry
1c
2d
3
Catalyst
Time (h)
Yieldb (%)
Entry
1
Substrate
Time (h)
2.5
Product
Co-500-3 h
Co-500-3 h
-
-
2
72 (no further conversion)
1.5 (1st run)
100
~100
44
Co-500-3 h
4
1.5 (3rd run)
2
2
100
5
Co-500-1 h
Co-500-2 h
Co-600-3 h
Co-700-3 h
Co/AC
6
6
3
4
5
6
3
3
3
2
100
100
100
100
6
37
7
3
100
100
3
NO2
8
6
9
6
10e
11
12f
Co-500-3 h
/
12
12
12
0.7
1.4
~0
Co-500-3 h
aReaction conditions: 0.2 mmol nitrobenzene, 4 mL CH3OH, 6 mL H2O,
0.05 mmol catalyst, 0.6 mmol NH3BH3 otherwise mentioned, 1 mmol
dodecane was added as an internal standard. bDetermined by GC or
GC-MS. cUsing 10 mL H2O as a solvent. dUsing 10 mL CH3OH as a
solvent. eNH3BH3 was replaced by 1 bar H2. fWithout NH3BH3.
7
8
9
3
3
3
100
100
100
NO2
The stability and recyclability of catalysts are of great
importance for their practical applications. It can be seen that
the activity and selectivity of Co-500-3 h were well retained
during three consecutive runs without any treatment or
activation of the catalyst (Table 1, entry 4), demonstrating its
recyclability. In addition, the Co NPs retain high dispersion and
crystallinity and their sizes are almost retained after catalytic
reaction, revealing the good confinement effect of porous
carbon (Fig. 2a, S9, ESI†). To verify the heterogeneity of the
Co-500-3 h catalyst, the filtration test was carried out after 30
min of reaction. No further conversion of nitrobenzene was
observed even after 3 h of reaction under identical conditions
(Fig. 2b). In addition, the catalyst was easily separated during
the reaction by an external magnet due to the strong
magnetism of the Co species (Fig. 2b, inset). Therefore, the
Co-500-3 h catalyst presents an impressive catalytic activity
and recyclability, and possesses a truly heterogeneous nature,
showing potential application in chemical industry.
O
10
11
3
3
100
100
12
4
100
13c
14
4
100
98
1.5
15
16
1.5
2
96
86
17
18
CH3NO2
2
2
CH3NH2
95
92
CH3CH2NO2
CH3CH2NH2
aReaction conditions: 0.2 mmol nitro compound, 0.6 mmol NH3BH3,
0.05 mmol catalyst, 4 mL of MeOH, 6 mL of H2O, room temperature
otherwise mentioned. bCatalytic reaction products were analyzed and
identified by GC and/or GC-MS. c0.1 mmol substrate.
To demonstrate the general applicability of Co-500-3 h in
the tandem catalysis, a verity of nitro compounds were
investigated under the optimized conditions. Different
aromatic or aliphatic nitro compounds were converted into
corresponding amines with excellent yields and selectivity
(Table 2). The fluoro-, chloro- and bromo-anilines were
prepared in excellent yields (100%, entries 1-3). All ortho-,
meta- and para-methyl nitrobenzenes were converted rapidly
and completely, revealing that slight space steric hindrance to
the hydrogenation of nitro group can be well tolerated (entries
4-6). Moreover, the nitro compounds involving electron-rich
groups (for examples, methyl, p-hydroxyl, p-methoxy,
p-methylol and amino, etc.) or electron-deficient substituents
(such as ester, nitro, aldehyde, ketone and nitrile) were
reduced to corresponding amine products in an efficient
fashion with absolute selectively (entries 4-16). Particularly,
the nitroarenes functionalized with the most-challenging
reducible functional moieties, such as aldehyde, ketone and
Fig. 2 (a) TEM image of Co-500-3 h (inset: size distribution of Co NPs)
after catalysis. (b) The filtration test for the reduction of nitrobenzene
over Co-500-3 h (inset: photographs recording the facile separation of
the catalyst via an external magnet).
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nitrile, were successfully reduced to aniline products in good
to excellent selectivity (entries 14-16), highlighting the great
chemoselectivity of the Co-based porous nanocatalyst and
remarkable advantage compared to that of noble metal-based
catalysts. The –CN and –C=O groups attached to the benzene
with electronic/conjugation effect might also partially
contribute to the great catalytic selectivity. To our delight, the
tandem reaction can be further extended to simple aliphatic
nitro compounds, which were converted to related primary
amines with high yields in 2 h (entries 17-18). These results
again demonstrate that, when coupling with NH3BH3
dehydrogenation to afford hydrogen source, the Co-500-3 h
catalyst is highly efficient and chemoselective for the reduction
of diverse nitro compounds to corresponding amines.
Notes and references
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With the above results, a possible mechanism for the
reduction of nitrobenzene catalyzed by Co-CoO@N-doped
porous carbon can be proposed (Scheme S1, ESI†).3c The
reduction of nitrobenzene to aniline, via nitrosobenzene and
phenylhydroxylamine, is considered to be a favorable pathway.
At first, two fast steps take place, in which nitrobenzene is
reduced to nitrosobenzenze, subsequently the hydrogenation
of nitrosobenzenze results in phenylhydroxylamine, by active
hydrogen from NH3BH3 or its derived H2. Finally, the
rate-determining step occurs by the transformation from the
intermediate to aniline. During the process, the Co-500-3 h
catalyst featuring hierarchical pores would benefit the
transportation of the substrate, intermediates and product.
In summary, the direct pyrolysis of ZIF-67 leads to
Co-CoO@N-doped porous carbon nanocomposites. When the
optimized catalyst, Co-500-3 h, was applied to a tandem
process of NH3BH3 dehydrogenation and subsequent
hydrogenation of nitro compounds, strikingly, the reduction
reaction rate increases exponentially (1100 times higher),
compared to that using 1 bar H2. Moreover, the stable and
inexpensive Co-500-3 h exhibits high chemoselectivity and
magnetic recyclability in the reduction of a variety of aliphatic
and aromatic nitro compounds. In addition to the excellence of
the catalyst, the key to the sharp enhancement in the reaction
rate is also attributed to the rapid hydrogen generation from
NH3BH3, which enables high-concentration hydrogen
distributed throughout the reaction solution, and greatly
increases the probability of the contact between hydrogen and
nitro compounds, thus boosting the reaction activity. In
contrast to the traditional reduction of nitro compounds with
high-pressure hydrogen, the current tandem route at room
temperature is not only much more efficient but also much
safer, as stored/pressurized hydrogen is not necessary. On the
whole, the pyrolysis of MOF reported here is a facile and
versatile approach to afford nanocomposite catalysts for
diverse reactions, and the tandem catalysis strategy in current
work might open up an avenue to the boosting of catalytic
efficiency in many other reactions.
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This work was supported by the NSFC (21371162, 51301159,
21521001), the 973 program (2014CB931803), NSF of Anhui
Province (1408085MB23), the Recruitment Program of Global
Youth Experts and the Fundamental Research Funds for the
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