Energetic decomposition yields efficient bimetallic Cu MOF-derived catalysts

Article abstract of DOI:10.1039/d0ta04765a

Metal-organic frameworks (MOFs) have recently emerged as efficient self-sacrificial templates to fabricate porous carbon-supported metal nanoparticles (NPs). Due to observed increased activity, catalysts containing bimetallic NPs represent an active frontier for heterogeneous catalyst development. A strategy to synthesize active catalysts with highly dispersed bimetallic metal/metal oxides within a porous carbon matrix via rapid MOF decomposition using dopants is presented in this paper. A 2,4,6-trinitrotoluene (TNT) additive enhances the rapid thermolysis of the metal-doped Cu-based MOFs to minimize particle aggregation. Catalyst characterization reveals that a secondary metal increases dispersion of both metals over the carbon composite support. The catalyst preparation method influences both the metal particle size and oxidation state. Catalytic performance shows increased rates for 4-nitrophenol reduction even with <1 wt% of added secondary metal. Among the synthesized catalysts, the Ni-CuO?C bimetallic catalyst exhibits outstanding activity. This synthetic strategy is useful for creating highly efficient, robust, non-noble metal catalysts for development of sustainable chemical processes.

Full text of DOI:10.1039/d0ta04765a

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ARTICLE
Energetic decomposition yields efficient bimetallic Cu MOF-
derived catalysts
Anh H. T. Nguyen Sorenson,a Yu Wu,c Emma K. Orcutt,a Rosalyn V. Kent,b Hans C. Anderson,a Adam
Received 00th January 20xx,
Accepted 00th January 20xx
J. Matzger,b and Kara J. Stowers*a
DOI: 10.1039/x0xx00000x
Metal-organic frameworks (MOFs) have recently emerged as efficient self-sacrificial templates to fabricate porous carbon-
supported metal nanoparticles (NPs). Due to observed increased activity, catalysts containing bimetallic NPs represent an
active frontier for heterogeneous catalyst development. A strategy to synthesize active catalysts with highly dispersed
bimetallic metal/metal oxides within a porous carbon matrix via rapid MOF decomposition using dopants is presented in
this paper. A 2,4,6-trinitrotoluene (TNT) additive enhances the rapid thermolysis of the metal-doped Cu-based MOFs to
minimize particle aggregation. Catalyst characterization reveals that a secondary metal increases dispersion of both metals
over the carbon composite support. The catalyst preparation method influences both the metal particle size and oxidation
state. Catalytic performance shows increased rates for 4-nitrophenol reduction even with <1 wt% of added secondary metal.
Among the synthesized catalysts, the Ni-CuO@C bimetallic catalyst exhibits outstanding activity. This synthetic strategy is
useful for creating highly efficient, robust, non-noble metal catalysts for development of sustainable chemical processes.
4-aminophenol (4-AP) is a model reaction within this reaction
class to evaluate and benchmark catalytic performance.18,19
Although Cu-based catalysts are active for this reduction
Introduction
Metal-organic frameworks (MOFs) are a rapidly developing
class of porous materials due to their high surface areas,
ordered structures, and tunable functionality.1-4 Recently, there
is increasing study toward using MOFs as catalysts,5 gas storage
and separation materials,6 drug delivery agents,7 and chemical
sensors.8,9 In addition to these applications, approaches to use
MOFs as sacrificial templates to develop carbon supported
nanoparticles (NPs) and apply them as heterogeneous catalysts
has gained traction.10,11
From multiple perspectives, Cu-based catalysts are great
candidates for small molecule redox reactions due to their
accessible oxidation states, abundance and non-toxicity. An
important catalytic reduction is of nitro-aromatic compounds,
that are highly hazardous to the environment and potentially
toxic toward humans, animals, and plants.12-14 It is highly
desirable to reduce these to more useful groups. Anilines serve
as intermediates and precursors in the synthesis of a variety of
fine chemicals.15 For example, while 4-nitrophenol is a waste
product of pesticides and dye production, the reduced 4-
aminophenol is an intermediate in the synthesis of
paracetamol.16,17 The hydrogenation of 4-nitrophenol (4-NP) to
class, a second metal will often boost the catalytic performance
of monometallic catalysts by modifying the structure and
electronic effects. Often this additional metal increases
uniformity with respect to particle size and shape which results
in better control of activity and/or substrate selectivity.20,21 The
thermolysis of bimetallic Cu-based MOFs with a molar 2:1 ratio
of Cu:Ni had excellent performance in catalytic oxidation of
furfural to cyclopentanone.20 The addition of Ni increased
uniform dispersion to obtain an average particle size of about
15 nm and Cu particles were reduced more easily. A bimetallic
Cu-based MOF with a molar 5:2 ratio of Cu:Co also had better
dispersion of the Co and Cu with uniform oxidation states after
thermolysis.21
Due to the wide variety of building blocks available, the
sacrificial MOF template can be tuned for design of porous
carbon materials obtained under thermolysis conditions.22-26
Previously, we have shown that the unique and high energy
CuNbO-1 MOF transforms into an amorphous copper and
anisotropic carbon nanocomposite, a-Cu@C, with finely
dispersed copper NPs throughout the layers of an anisotropic
carbon structure.27 This catalyst performs well as
a
heterogeneous catalyst for oxidation of benzyl alcohol,
reduction of nitrobenzene, and N-arylation.28 The exceptional
properties of this catalyst are ascribed to its unusual
deflagrative method of synthesis, wherein rapid decomposition
creates only transient high temperatures allowing kinetic
trapping of highly active nanoparticles upon cooling; this
method contrasts with the ubiquitous high temperature
annealing approach.
a. Department of Chemistry and Biochemistry Department, Brigham Young
University, Provo, Utah, 84604, USA. E-mail: kstowers@chem.byu.edu
b. Department of Chemistry and Macromolecular Science and Engineering
Program, University of Michigan, 930 North University Ave, Ann Arbor,
Michigan 48109-1055, USA.
c. College of Chemistry and Environmental Engineering, Sichuan University of
Science & Engineering, Zigong, 643000, P. R. China.
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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In the present study, we target bimetallic Cu-based catalysts h. The explosive mixture was then carefully transferred to a
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DOI: 10.1039/D0TA04765A
and increased activity from the interactions between two copper pan and sealed. Finally, M-CuO@C was produced by the
metals. The bimetallic Cu-based catalysts (M-CuO@C with M = thermolysis procedure detailed above. The Cu wt% loading and
Ni, Co, Ag) are fabricated by one-step thermal decomposition of M (M = Ni, Co, and Ag) wt% loading was determined by
a metal-doped copper containing MOF HKUST-1 (Cu-BTC, BTC = inductively coupled plasma optical emission spectrometry from
1,3,5-benzene tricarboxylate) with the explosive 2,4,6- Perkin-Elmer Optima (8300 ICP-OES) using argon and a collision
trinitrotoluene (TNT) as an additive. The addition of an available cell filled with ultra-high-purity helium both from Airgas. Silver,
highly energetic dopant aids initiation of a rapid thermolysis, in nickel and cobalt doped M-CuO@C are designated as Ag-
much the same way CuNbO-1 does, with the considerable CuO@C, Ni-CuO@C and Co-CuO@C with further details in the
advantage that commercially available Cu-BTC can be used as supporting information.
the sacrificial MOF. Here we report the catalytic activity of the
synthesized catalysts and characterize the interaction of M-CuO
via the kinetic study of the reduction of 4-NP to 4-AP with NaBH4
as a hydride source at room temperature. The influence of a
secondary metal is investigated with respect to particle size,
Characterization of M-CuO@C
XRD Powder patterns were collected with a Panalytical
Empyrean using Cu-Kα radiation (λ = 1.54187 Å) and operating
at 45 kV and 40 mA. Samples were prepared by pressing them
onto a glass slide fitted into a sample holder to minimize height
error. Gas Sorption experiments were carried out using a NOVA
reaction time, turnover rate and rate constant.
surface area analyzer (Quantachrome Instruments, Boynton
Beach, Florida, USA). Raman spectra were collected using a
Renishaw inVia Raman Microscope in extended scan mode with
Experimental
Materials
a range of 2500-100 cm-1. Scanning transmission electron
microscopy-energy dispersive X-ray spectroscopy (STEM-EDX)
and TEM studies were performed on a Tecnai F20 TEM using an
accelerating voltage of 200 kV. X-ray photoelectron
spectroscopy (XPS) was performed using a Surface Science SSX-
100 XPS instrument (Service Physics, Bend, OR) with a
All reagents and starting materials were purchased
commercially from Sigma-Aldrich or Alfa-Aesar and used as
received without any further purification unless otherwise
noted.
Synthesis and activation of HKUST-1
α
monochromatic Al K source (1486.7 eV) and a hemispherical
Synthesis of HKUST-1 was performed according to reported
literature methods with details give in the supporting
information.29
analyzer.
Reduction of 4-Nitrophenol – UV-Vis Studies and Batch Reactions
The kinetic studies were performed on an Agilent 8453 G1103A
Spectrophotometer. In a typical experiment, catalyst (1 mg) was
measured and transferred into a 100 mL beaker containing D.I.
water (56 mL) and a stir bar. Under vigorous stirring, sodium
borohydride solution (10 mL) was added to reaction with the
kinetic run was starting immediately. Aliquots were withdrawn
and diluted prior to taking UV-Vis spectra. Spectra were
collected in the range of 200-900 nm every 50 seconds until the
reaction reached completion. The corresponding amine
products were confirmed by the GC-MS. For a typical batch
reaction, nitrobenzene and THF:H2O solution were added,
followed by the addition of catalyst (0.01-0.05 mmol Cu loading)
to the mixture. NaBH4 dissolved in D.I. H2O was then added to
the reaction mixture under vigorous stirring. The mixture was
stirred for 2 h at 70 °C. Once the reaction was completed, it was
cooled to room temperature prior to workup. A yield of aniline
product was determined by integration using an internal
standard biphenyl in the GC-MS. Catalyst leaching was
confirmed by ICP-OES.
Synthesis of HKUST-1/TNT (CuO@C)
Activated HKUST-1 crystals (25 mg) and TNT (50 mg) were
added to a 4 mL vial in the glove box and shaken for 5 minutes.
The vial was removed from the glovebox and then heated at 90
°C for 2 h. The explosive mixture was then carefully transferred
to a copper pan and sealed before thermal initiation. A torch
was then used to pre-heat the sealed end. Once heated for a
few seconds, the copper pan was knocked into the hot region
and thermally initiated. CuO@C was captured in an Erlenmeyer
flask at the open end of the tube and collected for analysis. See
Supporting Information for more details.
Synthesis of HKUST-1/TNT/M (M-CuO@C)
Activated HKUST-1 crystals (800 mg) and M salt (typically the
nitrate) were added to a 20 mL glass vial in a nitrogen filled
glovebox. The vial was shaken by hand and then removed and
methanol (1.5 mL) was added to the mixture and then shaken
again. Finally, the suspension was placed in a shaker at room
temperature for 7 days. See Supporting Information for more
details. Methanol was then evaporated by a slow stream of air
flowing over the uncapped vial for 2h. The solid doped
Results and Discussion
M/HKUST-1 composite was collected. M/HKUST-1 (25 mg) and Characterization of CuO@C and M-CuO@C (M = Ni, Co, Ag)
TNT (50 mg) were added to a 4 mL vial in the glove box. The vial
was removed from the glovebox and then heated at 90 °C for 2
During the pyrolysis process, the organic ligands in a MOF play
two important roles: acting as a reductant to reduce framework
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Table 1 Surface and physical properties of all the Cu-based catalysts.
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DOI: 10.1039/D0TA04765A
SA
∙
Cu
(wt%)
M
(wt%)
Total metal
(wt%)
Catalyst
-1
(m2 g )
9.0
CuO@C
26.7
24.8
26.7
26.1
---
26.7
25.5
27.6
26.7
9.4
Ni-CuO@C
Co-CuO@C
Ag-CuO@C
0.8
0.9
0.6
10.3
12.9
ICP-OES analysis, the catalysts have around 25 – 27 wt% Cu. The
Cu loading of Ni-CuO@C has slightly less Cu compared to the
non-bimetallic and other bimetallic catalysts.
The sample pore structure and surface area were
investigated using N2 adsorption-desorption isotherms and all
showed similar characteristics as shown for CuO@C in Fig. 2.
The isotherms of all samples are type II, which is characteristic
of diameters larger than a micropore or ~ 2 nm.37 (The other
isotherms are found in the supporting information Fig. S3).
From the BET analysis of N2 adsorption-desorption isotherms,
all catalysts exhibit a surface area of ~ 9.0 – 12.9 m2/g (Table 1).
This small surface area results from particle agglomeration,
pore blockage, or collapse. The surface area increases only
slightly with addition of a second metal in the order of Ni < Co <
Ag with 12.9 m2/g as the largest surface area obtained. The
porous structure of CuO@C and bimetallic M-CuO@C catalysts
is composed of micro-, meso- and macro-pores therefore metal
species can be dispersed both on the internal and external
surface of the carbon matrix. The distinct increase in adsorbate
volume in the low P/P0 region (below 0.4) indicates the
presence of micropores, which are residual from the porous
structure in the samples (see PXRD spectra).38 The hysteresis
loop at P/P0 = 0.4 – 0.6, indicative of mesopores, has very small
N2 uptake, suggesting only a small concentration of these pores.
The strong N2 uptake at P/P0 above 0.6 confirms that the surface
of catalysts is mainly macropores.39
Fig. 1 PXRD spectra of CuO@C (black), Ni-CuO@C (red), Co-
CuO@C (blue) and Ag-CuO@C (green).
metal ions30 and to act as a precursor of the porous carbon
matrix.31 Because Cu2+ ions have a reduction potential higher
than -0.27 V, the reduced metal nanoparticle is formed through
reduction from the carbon of the ligands. This synthetic strategy
thus obviates further post-reduction synthetic steps. The TNT
additive acts to rapidly initiate thermolysis and consequently
may result in a small amount of Cu2+ left as oxidized Cu2O. The
particle sizes and crystallinity of the products were
characterized by powder X-ray diffraction (PXRD). The PXRD
patterns of all decomposed catalysts show the copper metallic
phase and copper oxide phase peaks (Fig. 1). Diffraction peaks
°
°
at 2θ = 43.5 and 50.6 are ascribed to the (111) and (200)
crystal planes of metallic Cu nanoparticles (JCPDS NO. 85-
1326).32 The Cu2O nanoparticle peaks are at 2θ = 29 , 36.5 , and
°
°
°
41.5 , assigned to the (110), (111), and (200) crystal planes
(JCPDS NO. 78-2076).33 These diffraction patterns confirm that
the derived catalysts primarily contain metallic Cu nanoparticles
in the cubic structure.
In the bimetallic M-CuO@C (M = Ni, Co, and Ag) catalyst
PXRD patterns, no obvious secondary metal peaks are detected.
This suggests a fine distribution of < 3 nm nanoparticles or
amorphous secondary metal species phase. Comparing the
°
°
width of 2θ = 43.5 and 50.6 peaks in the bimetallic catalysts
to the CuO@C catalyst slight increases indicate that bimetallic
addition has yielded smaller Cu crystallites. This may be due to
partial secondary metal ion substitution for cupric ions in the
HKUST-1 framework and/or the secondary metal ions within
HKUST-1 channels.34 Both would lead to minimized aggregation
of copper ions during framework decomposition.
The average crystallite domain sizes were calculated by the
Debye-Scherrer equation (Supporting Information, Table S1).
The synthesized CuO@C catalyst has crystallite sizes of 18.5 nm.
The addition of a secondary metal yielded an average of domain
size of 13±1 nm. Similar observations of a decrease in size with
addition of a secondary metal were obtained by Wang’s
group.20,21 The PXRD patterns of the derived catalysts also show
a few characteristic peaks of the original HKUST-1 and this likely
results from ejection of solids during the violent deflagration
process. Thus, the synthesized catalysts retain some porous
character after thermal treatment (Supporting Information, Fig.
S2). 35,36
Fig. 2 N2 adsorption-desorption isotherm for CuO@C.
The metal component ratios, loadings, and the derived-
catalyst surface areas are tabulated in Table 1. According to
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Fig. 3 XPS spectra of Cu 2p from CuO@C (left, black), Ni-CuO@C (red), Co-CuO@C (blue) and Ag-CuO@C (green).
The chemical composition of the derived catalysts was also spherical nanoparticle species. The CuO@C catalyst has finely
studied by XPS analysis as shown in Fig. 3. While under N2
distributed particles with an average particle size of about 27.0
conditions, the decomposition of HKUST-1 primarily shows ± 6.8 nm both on the surface and within the pores (Fig 4a).
reduced Cu species, in our sample there is a mixture of Cu+ and There is a somewhat broad distribution of particles but the
Cu2+ species. The decomposed benzoate ligands lead to Cu2+ majority are ~ 22-26 nm (Fig. 4e). The higher average particle
reduction.40 However, the presence of oxidized states can be sizes obtained from STEM and TEM analysis compared to PXRD
from the highly oxidizing power of the TNT additive with are due to closely overlapping particles. The interplanar
residual surface oxidation of the samples from air and moisture spacings of metallic Cu (111) and Cu (200) correspond to the
exposure.
lattice fringes (0.22 and 0.18 nm spacing) that can be seen in
For all of the samples, the main peaks for Cu 2p3/2 at ~932 HRTEM and confirmed with analysis of the selected-area
eV and ~934 eV are ascribed to Cu0/+ and Cu2+ respectively.41-43 electron diffraction (SAED) pattern (Fig. S9). The diffraction
The majority of the C 1s spectrum is sp2-carbon with graphitic rings result only from Cu which is consistent all amorphous
nature (Supporting information, Fig. S4).44 There is also carbon composite as observed in the Raman spectra.
evidence of copper and/or secondary metal coordinated N,
Dispersed metal particles are observed both on the surface
graphitic N and oxidized N originating from the TNT (Supporting and within the pores of the carbon composite in the STEM/TEM
information, Fig. S5).45-47 The O 1s spectrum shows evidence for of the bimetallic catalysts M-CuO@C in Fig. 4b-d (Supporting
M-O, M-OH and oxidized carbon (Supporting information, Fig. information, Fig. S10-12). The same lattice spacing for Cu(111)
S7). The peak at ~530.4 eV is associated with M–O and M–OH and Cu(200) are present in the M-Cu catalyst with an additional
groups, while those at 533 and 535 eV are related to C–OH and Cu(311) plane observed. Overall, particles sizes and
C=O groups.48
distributions for M-CuO@C catalysts confirm that Cu has
With <1wt% addition of the second metal, it is difficult to increased dispersion with added metals to HKUST-1 prior to
observe the XPS signal (Supporting information, Fig. S6). In the decomposition. A comparison yields that the Ni-CuO@C
Ni-CuO@C sample, the doublet Ni 2p3/2 peak at 854.8 eV and Ni catalyst exhibits a more homogeneous particle distribution,
2p1/2 peak at 872 eV can be assigned to Ni(II).49 No evidence of whereas the Co- and Ag-CuO@C have a bimodal distribution of
Ni0 is present which is predicted by redox potentials.30 The Ag particle sizes.
3d signal in the Ag-CuO@C spectra has a doublet at 367.1 eV
which can be ascribed to Ag(I).38 Surprisingly, there are no XPS
peaks associated with Co species found in Co-CuO@C catalyst.
This suggests that Co nanoparticles are likely to be dispersed in
the pores or encapsulated by carbon.
The Raman spectra for all of the synthesized catalysts,
(Supporting Information, Fig. S8) show two broad bands at
~1579 cm-1 and 1352 cm-1 corresponding to the G (E2g) and D
(A1g) breathing modes, respectively.50 The significant D mode
confirms structural defects and disorders in the carbon matrix
of materials, while the G mode associates with sp2 carbon
domains.51-53
The average particle size for Ni-CuO@C is 18.6 ± 4.8 nm (Fig.
4b and f), which agrees with the PXRD smaller particle size for
the bimetallic catalyst. Analysis of SAED in Ni-CuO@C and Co-
CuO@C also show that the Ni(220) and Co(221) planes are
present. The particle size distribution for Ag-CuO@C, 6.6 ± 1.7
nm and 22.3 ± 3.1 nm (Fig. 4d and h). The Ag-CuO@C catalyst
displays the most sponge-like structure of the three with
minimal to no Ag detected on Cu particles according to EDX
analysis (Fig. S12) as well as over larger regions likely caused
from low Ag loading (0.55 wt%). Two particle sizes are present
in Co-CuO@C; 6.1 ± 3.8 nm and 45.5 ± 7.0 nm (Fig. 4c and g).
The large particles may consist of aggregates of small
nanoparticles encapsulated in carbon. These types of
encapsulated aggregates are consistent with previously
published copper28 and cobalt54 MOF decomposition materials.
In order to examine the morphology of the catalysts, particle
distribution, and size, analysis was performed with STEM and
TEM as shown in Fig. 4 (Supporting information, Fig. S9-12). The
catalysts exhibit sponge-like porous structures with dispersed
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(a)
(b)
(c)
(d)
Cu
(111)
Cu
(200)
20 nm
20 nm
50 nm
20 nm
80
1201040
3000
45
1800
60
2500
70
d = 7 ± 2 nm
d = 22 ± 3 nm
d = 27 ± 7 nm
d = 19 ± 5 nm
d = 6 ± 4 nm
d = 46 ± 7 nm
1600
40 (e)
(f)
(g)
(h)
120
1000
2500
140500
35
30
25
20
15
10
5
60
2000
50
100
800
1200
2000
1500
1000
500
40
80
60
40
20
0
1000
1500
40
30
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0
30
800
1000
600
20
20
10
0
400
10
200
500
0
0
0
0
0
2
10 18 26 34 42 50 58
Particle Size (nm)
6
10 14 18 22 26 30
Particle Size (nm)
6
10 14 18 20 24 28
16 20 24 28 32 36 40 44 48
Particle Size (nm)
Particle size (nm)
Fig. 4 TEM images of (a) CuO@C with 50 nm scale bar with an inset of electron diffraction patterns and of M-CuO@C catalysts
where M is (b) Ni, (c) Co, and (d) Ag with 20 nm scale bar. Particle histogram of (e) CuO@C and of M-CuO@C where M is (f) Ni, (g)
Co, and (h) Ag.
Analysis with scanning transmission electron microscopy-
The CuO@C catalyzed reaction reached completion (>99 %
energy dispersive X-ray spectroscopy (STEM-EDS) and conversion) within 6.7 min, whereas M-CuO@C catalyzed
diffraction patterns from transmission electron microscopy reactions reached the same conversion at shorter times (Fig 5b,
(TEM) images (Fig. S11) confirms the small particles to be Co Fig. S14). However, Ag-CuO@C (0.876 min-1 completed at 5.0
species with small amounts of Co bound to Cu particles. No min) performed only slightly better than the original CuO@C
distribution of Co is seen on the external surface of carbon, (0.804 min-1 completed at 6.7 min). This could be due to the low
which is consistent with XPS results.
wt% Ag in the catalyst or also that Ag has little to no inherent
reduction properties. Both Co-CuO@C and Ni-CuO@C showed
higher activity than Cu alone. NiCuO@C reduced 4-NP to 4-AP
with >99 % conversion within 3.3 min (k = 1.578 min-1) resulting
in a turnover frequency of 12 mmol/gcat/min. A low wt% loading
of nickel almost doubles activity. The Ni-CuO@C catalyst also
has the least amount of Cu compared to the other catalysts. This
increase in activity would be consistent with a modified surface
electronic structure of the bimetallic catalyst compared to the
corresponding Cu catalyst alone. This electronic change may
facilitate the formation of the required surface intermediate
that allows for better hydride dispersion from the NaBH4 or
influences the binding strength between the active metal
species and the phenolate intermediate. Electron transfer
would then occur following the Langmuir-Hinshelwood
mechanism.56 A known shifting of the d-band of the bimetallic
surface to between the d-band for Cu and Ni alone may
consequently result in not too weak and not too strong binding
energy for adsorption of 4-NP and release of 4-AP which results
in enhanced reduction. This increased activity was found for a
NiCu catalyst used for ammonia borane hydrolysis.57,58 These
results contrast with usual observations that higher catalyst
surface area leads to better activity; the activity enhancement
of Ni suggests that the smaller and more homogeneous Cu
particle size in the Ni-CuO@C leads to higher active site
concentration for contact with the reactants.
Kinetic studies of 4-nitrophenol reduction
The catalysts (CuO@C and M-CuO@C with M = Ni, Co, Ag)
were compared for the reduction of 4-nitrophenol (4-NP) to 4-
aminophenol (4-AP) as a model reaction.19 Without a catalyst,
the UV-Vis absorption spectra of 4-NP remained unchanged and
with NaBH4 added the absorption peak of 4-NP was red-shifted
as the 4-nitrophenolate species (Fig. 5a).55,56 When CuO@C
was added, the absorption intensity of 4-NP decreased while
the absorption peak of 4-AP increased showing a clear
isosbestic point between the two peaks indicating no
observable side products (Fig. 5b).33 After addition of the
catalysts, the reduction reaction started immediately without
any induction period (Fig. S13).55
The analysis of the reduction of 4-NP was carried out with a
large excess of borohydride in order to make the reaction
independent of the borohydride concentration (100:1 mol%
NaBH4:4-NP) and fitted reasonably well to nearly 100% of the
conversion of the reactions (Fig. 5c). The calculated turnover
frequencies, rate constants (k) and activity factors (K) were used
to compare the catalytic activity of the bimetallic catalysts
synthesized here with similar catalysts. The activity factors, K,
were calculated as the ratio of the rate constant (k) over the
total weight of the catalyst.
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(a)
(b)
(c)
Fig. 5 Time-dependent UV-Vis absorption spectra of the 4-nitrophenol reduced by NaBH4 without Cu-based catalysts (a) and with
CuO@C catalyst (b), and the plots of ln(Ct/C0) versus time at room temperature for all synthesized catalysts (c). The solid lines
correspond to the best straight line fit.
Reduction of nitro-aromatic compounds
reduction in the presence of the nitro group. Over time the
nitro-group is reduced first followed by the -CHO group leading
to 4-aminobenzylalcohol as the final product. However, in the
chemoselective reduction of 4-nitrobenzoic acid to 4-
aminobenzoic acid, occurring in less than 1 minute, no further
reduction was observed as confirmed by GC-MS. Catalytic
reusability was not conducted due to the difficulty in
recollection and redispersion of the catalyst. Optimization of
the catalyst usability is underway.
Based on the above kinetic studies, the Ni-CuO@C catalyst
was selected to determine the selectivity and electronic
limitations of the reaction (Fig. 6). UV-Vis absorption spectra
was used to follow the catalytic reduction of several
nitroaromatic compounds over Ni-CuO@C at 25 °C with
identification of the final products was confirmed by GC-MS
(Table S8). All nitroaromatic compounds were reduced to
complete reduction. The conversion for these substituted nitro-
compounds with Ni-CuO@C in the first 3 min were between
27% to >99% thereby undergoing different rates based on
electronic substitution (Table S8, Fig. S15). The reaction rates
and electronic sensitivity constant, ρ, were calculated and
plotted but did not show a clear electronic correlation (Fig. 6).59
In terms of selectivity, the reduction of 1,4-dintrobenzene is
fully converted to 1,4-diaminebenzene within 13.3 minutes.
Bimetallic Ni-CuO@C is limited in chemoselectivity for aldehyde
Reduction of nitrobenzene in batch reaction
The catalytic activity of CuO@C under scalable conditions
was conducted to compare Ni-CuO@C (entry 2) compared to
CuO@C (entry 1) as shown in Table 2. The synthesized catalysts
were stable during reactions since there was a negligible
concentration of Cu leached into solution as tested by ICP-OES.
The catalytic activity is maintained with half the Ni-CuO@C
catalyst loading (entry 3) and slightly decreases to 80.7% yield
even with 1 mol% catalyst (entry 4). The other synthesized
bimetallic catalysts (entries 5-6) are only slightly better than the
Cu monometallic catalyst but not as good as the Ni bimetallic
catalyst. The turnover number (TON) of our catalysts are higher
than that of similar Cu-based catalysts that others have used
(entries 7-10).60-61 We can use less catalyst while maintaining
high conversion over similar times as these catalysts.
NO2
NH2
The increased activity seen in these catalysts is due to the
inherent activity of copper as a good conductor. Despite the low
specific surface area and the limited pore size of these catalysts
there is still a significant impact for reaction enhancement. The
heterogeneous catalysed reduction reaction is in line with a
Langmuir Hinshelwood model with adsorption of the 4-NP and
borohydride.56,62 Hydrogen and electrons are then transferred
to form 4-AP which is then desorbed from the surface.
However, CuO and Cu2O are not good conductors. The synthesis
method for CuO@C creates a mixture of copper oxidation states
as shown by XPS resulting in equal amounts of Cu/Cu2O and
CuO.
X
X
Fig. 6 The Ni-CuO@C catalyzed conversion efficiencies of the
reduction reaction plotted as a Hammett plot.
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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Journal Name
Table 2 Reduction of nitrobenzene in batch reaction.
ARTICLE
of HKUST-1 impregnated with a second metal andViimewpArreticglenOantleinde
DOI: 10.1039/D0TA04765A
with TNT. The materials possess hierarchical porous structures
mol % Cu Conv. (%)b TONc
Ref
-
Entry
1
Catalyst
CuO@C
(micro-, meso- and macropores) with highly dispersed copper
nanoparticles. The M-CuO@C catalysts shows enhanced
catalytic activity over CuO@C for reduction of 4-nitrophenol.
Characterization of the materials indicates smaller particle size
results from an added metal. Out of the bimetallic materials, Ni-
CuO@C was the most active catalyst, which showed the highest
turnover frequency of 12 mmol 4-NP.gcat-1.min-1 with full
conversion within 3.3 minutes. Further studies into applications
for the bimetallic Cu-based catalysts for other reductions and
hydrogenation reactions are underway. The rapid
decomposition of bimetallic-HKUST-1 with TNT additive is a
powerful strategy that can be generalized to other
commercially available MOF materials for preparing highly
active heterogeneous bimetallic catalysts.
5.0
5.0
72.9
100
14.6
20
2
-
Ni-CuO@C
Ni-CuO@C
Ni-CuO@C
Co-CuO@C
Ag-CuO@C
a-Cu@C
3
-
2.5
100
40
4
-
1.0
80.7
78.5
85.8
67.5
79
80.7
15.7
17.2
13.5
7.9
9.8
1
5
-
5.0
6
-
5.0
7
28
60
60
61
68
5.0
8
CuO
10.0
10.0
100.0
>100.0
9
Cu NPs
98
10
11
CuFe2O4
100
Cu np/AC
92-100
NA
a
Reaction conditions: nitrobenzene (1 mmol), NaBH4 (3 equiv.),
catalyst (1-5 mol%), and THF:H2O (1:2, 3 mL), 70 °C, 2 h.
Conflicts of interest
There are no conflicts to declare.
b Determined by GC-MS using biphenyl as internal standard.
c
Turnover number (TON)
=
[moles of converted substrate
×
(nitrobenzene)]
(moles of Cu)-1.
Acknowledgements
The composite of a Cu2O-Cu-CuO has been found to be efficient
and a highly active catalyst due to the transfer between the
ternary copper oxidation states.63 The MOF-derived ternary
composite shows similar high activity. Nickel has been shown to
increase the redox activity for Cu-Ni bimetallic nanoparticles.64
The increased effect of Ni in combination with the ternary
composite is what results in such a highly active catalyst.
The work was supported by the Simmons Research Endowment
at Brigham Young University while the synthesis of materials
was supported by the US Department of Energy [Office of Basic
Energy Sciences] DE-FG02-08ER 15997 at the University of
Michigan.
The catalytic performance of the synthesized catalysts,
CuO@C and M-CuO@C with M = Ni, Co, Ag, was compared to
that of previously reported catalysts for the same reaction
(Supporting information Table S9). In comparison with carbon
supported catalysts, CuO@C performs 4-NP reduction with a
higher turnover rate (6 mmol/gcat/min compared to 0.01-1.04)
and a higher activity factor including those supported on
graphene oxide,65 polymer-derived support,66 and a carbon
composite.67 The Cu based catalyst on activated carbon is the
only catalyst with a similar activity factor (accounts for activity
normalized to catalyst mass) compared to CuO@C (tabulated
data is also shown in Table 2 entry 11).68 A comparison of the
bimetallic Cu-based catalysts to similar bimetallic Cu-based
catalysts synthesized by others shows that in each of the cases,
M-CuO@C have much higher turnover rates, larger rate
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