Pd modified prussian blue frameworks: Multiple electron transfer pathways for improving catalytic activity toward hydrogenation of nitroaromatics

Article abstract of DOI:10.1016/j.mcat.2020.110967

Prussian blue analogs (PBAs) exhibit potential as low-cost and eco-friendly nanocatalysts that can be fabricated with ease. However, the PBA framework structure suffers from poor electronic conductivity, which limits the catalytic efficiency for this clas

Full text of DOI:10.1016/j.mcat.2020.110967

MolecularCatalysis492(2020)110967
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Pd modified prussian blue frameworks: Multiple electron transfer pathways
for improving catalytic activity toward hydrogenation of nitroaromatics
Kaiqiang Zhanga,b, Joo Hwan Chac, Se Yeon Jeond, Kent O. Kirlikovalie, Mehdi Ostadhassanf,
Vamegh Rasoulif, Omar K. Farhae,*, Ho Won Janga,*, Rajender S. Varmag,*,
Mohammadreza Shokouhimehra,f,**
a Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea
b Jiangsu Key Laboratory of Advanced Organic Materials, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing, Jiangsu 210023, China
c Innovative Enterprise Cooperation Center, Korea Institute of Science & Technology, Hwarangro 14-gil, Seongbuk-gu, Seoul, Republic of Korea
d Advanced Analysis Center, Korea Institute of Science & Technology, Hwarangro 14-gil, Seongbuk-gu, Seoul, Republic of Korea
e Department of Chemistry and International Institute of Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States
f Department of Petroleum Engineering, University of North Dakota, Grand Forks, ND 58202, United States
g Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University, Šlechtitelů 27, 783 71 Olomouc,
Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
Prussian blue analogs (PBAs) exhibit potential as low-cost and eco-friendly nanocatalysts that can be fabricated
with ease. However, the PBA framework structure suffers from poor electronic conductivity, which limits the
catalytic efficiency for this class of materials. Noble metals represent an alternative class of materials that display
inherent catalytic activity but suffer from aggregation, ultimately reducing the amount of accessible catalytic
sites. Herein, we demonstrate a combinatory approach that circumvents the known disadvantages with these
classes of catalytic materials in which PBA-supported nanocatalysts were synthesized. These composite materials
exhibit excellent catalytic activity for the reduction of nitroaromatics to aminoaromatics while displaying long-
term cycling stability, which is attributed to the availability of multiple electron transfer pathways. Overall, this
work opens the study on the assembly of PBA-supported heterogeneous nanocatalysts and potentially paves the
way toward future applications.
Heterogeneous catalysis
Hydrogenation
Metal organic framework
Palladium
Nitroarene
1. Introduction
include synthesis strategies that require multiple steps and the need for
the use of expensive instruments. Therefore, the development of cata-
Catalysts are quite useful for the fabrication of chemical products
[1–12]. In particular, highly active and selective catalysts play an im-
portant role in the conversion of nitroaromatics to aminoaromatics used
in the fabrication of pesticides, dyes, explosives, and pharmaceuticals
[13–15]. Thus far, the development of stable catalysts used for these
conversion reactions has remained a challenge due to many dis-
advantages, such as agglomeration, loss of activity, and poor selectivity
of the catalysts [16–22]. To date, great advances have been achieved
through the use of nanocatalysts, which exhibit highly improved cata-
lytic activity that originates from the innovative design of composite
nanocatalysts [23–28]. However, drawbacks for this class of materials
lysts targeted for use in practical applications should be designed in a
way that can address the aforementioned issues.
Prussian blue analogs (PBAs), also known as porous coordination
polymers, have been garnering extensive attention in various fields,
including batteries, gas sensors, and catalysis, among others, owing to
inherent advantages, such as high electrochemical activity, stable fra-
mework structures, and ease of fabrication [29–36]. In the chain of
cation–CN–cation, through which the electron can flow which affects
the chemical properties. In the coordination, the metal cations can in-
teract with either carbon or nitrogen atoms. Thus, the PBA performance
is comprehensively determined by the containing species [37]. PBAs
⁎ Corresponding authors.
⁎⁎ Corresponding author at: Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826,
Republic of Korea.
E-mail addresses: o-farha@northwestern.edu (O.K. Farha), hwjang@snu.ac.kr (H.W. Jang), varma.rajender@epa.gov (R.S. Varma),
mrsh2@snu.ac.kr (M. Shokouhimehr).
https://doi.org/10.1016/j.mcat.2020.110967
Received 1 March 2020; Received in revised form 5 April 2020; Accepted 14 April 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

K. Zhang, et al.
MolecularCatalysis492(2020)110967
possess a high density of uniformly-dispersed active sites and open
channels, which facilitate the transport and diffusion of both reactant
substrates and products [38–40]. Furthermore, PBAs offer the potential
for the incorporation of alternative catalytically active species to fra-
meworks, further highlighting the advantages of these heterogeneous
systems. In this context, the synergy between nanoparticles (NPs), a
class of materials commonly supported on heterogeneous frameworks,
and PBAs is of great significance in the catalytic prowess of these
composite materials [41–43]. While previous reports have partly ad-
dressed the activity of PBAs toward reduction of nitroaromatics to
aminoaromatics [44], limited catalytic activities of PBAs are attributed
to the poor electronic conductivity resulting from the cyanide bridges in
these materials. Therefore, we hypothesized that a modification of the
cyanide linkers may potentially yield more efficient electron transport
pathways, ultimately resulting in greater activity and higher perfor-
mance in these hydrogenation reactions.
adding 0.15 g of Pluronic F-127 into a 100 mL of aqueous solution
containing the as-prepared FeHCFe NPs (0.1 mol). Sequentially, 0.02 g
of K2PdCl4 was dissolved in this mixed solution under vigorous stirring
and a high-power ultra-sonication for achieving the dispersion. The
synthesized heterogeneous nanocatalyst FeHCFe@Pd was separated
from the solution via filtration and dried in a vacuum chamber. The Pd
NPs loaded on carbon support was also synthesized via a similar
method for being used as a reference.
2.3. Material characterizations
The structures of the synthesized nanocatalysts were determined
using X-ray diffraction (XRD, D8 Advance). Morphologies were ob-
served using transmission electron microscopy (TEM, JEOL JEM-F200)
combined with energy-dispersive X-ray spectroscopy (EDX). Cyanide
linkers were detected using Fourier transform infrared (FT-IR, Nicolet
iS50) spectroscopy and Raman spectroscopy (LabRAM HV Evolution,
HORIBA). Surface chemical bonding features were measured by X-ray
photoelectron spectroscopy (XPS, AXIS-HSi) using an Al Kα source.
Thermal stability was verified using thermogravimetric analysis (TGA,
Simultaneous DTA/TGA analyzer); the sample was heated from room
temperature to 700 °C at a temperature ramp of 10 °C min−1 under
nitrogen flow. Local ion (Fe and Co) environment was studied by Fe and
Co K-edge X-ray absorption near edge structure (XANES) spectroscopy,
which was recorded on the CRG-FAME (BM30B) beamline with an in-
cident radiation energy selected by using a pair of Si(220) crystals.
Small-angle X-ray scattering (SAXS) was assigned for detecting the
decorated Pd NPs. Energy-filtered transmission electron microscopy
(EFTEM) was employed for the elemental mapping. The surface area of
the FeHCFe@Pd product was analyzed with Brunauer–Emmett–Teller
BET through the gas adsorption-desorption method. The amounts of
different metal species were measured by inductively coupled plasma-
atomic emission spectrometry (ICP-AES).
Noble metal NPs, such as Pd, have been established as highly active
catalysts for the hydrogenation of nitroarenes and other reductions
[45–48]. Unfortunately, noble metal NPs are typically thermo-
dynamically unstable and tend to aggregate during catalytic reactions
due to the high surface energy of small NPs. As a result, catalytic ac-
tivities are significantly reduced due to the decrease in accessible active
sites upon aggregation of NPs. Porous skeleton materials as intrinsic
heterogeneous supports can potentially offer sufficient spatial confine-
ment to prevent NP aggregation, resulting in the generation of stable
and well-dispersed NPs.
The disadvantages of aggregation and poor conductivity related to
catalysts comprised of bare noble metal NPs and PBAs, respectively, can
potentially be simultaneously circumvented through the careful fusion
of these two classes of materials. As such, we were motivated to eval-
uate the catalytic activity of PBAs@Pd heterogeneous composite ma-
terials for the reduction of nitroaromatics to aminoaromatics in order to
probe this hypothesis. These composite materials can be easily prepared
through a wet chemical method in an aqueous bath; in a typical
synthesis, Pd ions are introduced into an aqueous solution containing
the as-prepared PBAs, followed by a reductive growth process of Pd NPs
inside and on the surfaces of the PBAs. Six types of hexacyanoferrate
(III)-based PBAs (MnHCFe, FeHCFe, CoHCFe, NiHCFe, CuHCFe, and
ZnHCFe) were prepared, and out of these materials, FeHCFe exhibits
the highest activity for nitroaromatic reduction. In order to explore the
performance of PBAs as potential nanocatalyst supports, Pd NPs were
decorated on the bare FeHCFe support in the presence of a reductant
(Pluronic F-127). Relative to FeHCFe, the synthesized FeHCFe@Pd
displays improved catalytic activity, reusability, and selectivity for the
reduction of a diverse range of nitroaromatics to the corresponding
aminoaromatics, which we attribute to the multiple electron transfer
pathways constructed in this composite material.
2.4. Catalytic characterizations
The catalytic activities of the as-prepared PBAs and FeHCFe@Pd
were assessed in a model reaction for the reduction of nitroaromatics to
aminoaromatics in the presence of the reductant NaBH4 at room tem-
perature in an aqueous medium. Solutions mentioned in these studies
were prepared with deionized water without any organic additives. The
catalysts were separated from reaction systems by centrifugation after
reaction completion. In a typical procedure, 5 mg of the as-prepared
PBAs were dispersed in 30 mL of deionized H2O, and then, 0.1 mmol of
the nitroaromatic was added into the reaction system; constant stirring
yielded a uniform solution. Next, the reductant solution (containing
NaBH4, 1.2 mmol) was added to the reaction mixture, which was stirred
for 5 min at room temperature. A study on the reusability of the com-
posite FeHCFe@Pd catalyst was conducted by reusing the separated
and well-washed catalyst for the subsequent runs. Yields of the ami-
noaromatic products were determined using gas chromatography-mass
spectrometry (GC–MS, Agilent 7890A Gas chromatograph and 5977A
Mass selective detector). Furthermore, well-controlled experiments
were conducted for 2, 5, and 10 min using 5, 10, 20 mg of catalysts for
the optimization of hydrogenation conditions.
2. Experimental
2.1. Preparation of PBAs
All reagents were purchased from Sigma Aldrich and used without
any further purification. The PBA powders were prepared by mixing 0.1
mol of FeSO4·7H2O, MnSO4·H2O, NiSO4·6H2O, ZnSO4·7H2O,
CuSO4·5H2O, or CoSO4·7H2O aqueous solutions with 0.1 mol of a K3Fe
(CN)6 aqueous solution, followed by constant stirring at 25 °C.
Precipitation was immediately observed after mixing the two solutions.
Thereafter, the suspension was subjected to high-power ultra-sonication
to uniformly disperse the formed products. The ensuing powders were
collected and washed with deionized water several times using filtra-
tion, and well-cleaned products were dried in a vacuum oven.
3. Results and discussion
Bivalent transition metal ion nodes with an increasing main
quantum number are deployed to construct the PBAs, and the as-pre-
pared PBAs can be mass fabricated in a facile manner. Crystal phases of
the PBAs identified using XRD are shown in Fig. 1, and the character-
istic peaks are well-indexed in the patterns. A common cubic crystal
structure is observed for the prepared products, except for the ZnHCFe,
which exhibits a different, slightly distorted monoclinic crystal struc-
ture that is likely caused by the repulsion between Zn2+ nodes and Fe
2.2. Preparation of FeHCFe@Pd
The FeHCFe@Pd heterogeneous nanocatalyst was prepared by
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Fig. 1. XRD patterns of the products: (a) CoHCFe, (b) CuHCFe, (c) FeHCFe, (d) MnHCFe, (e) NiHCFe, (f) ZnHCFe.
[(CN)6]4−. Lattice parameters of around 10.12 Å (a = b = c) are
verified for the cubic PBAs; MnHCFe has an enlarged lattice parameter
of around 10.44 Å (Table S1). The monoclinic ZnHCFe with enhanced
lattice parameters of (10.95, 13.16, 18.12 Å) is depicted in comparison
with others (Table S1).
Raman spectroscopy has been deployed to verify the vibrational
states of the cyanide functional groups in representative mono-metal
and bi-metal ion-constructed PBAs (Fig. 3d). A typical Raman shift at
2070 cm−1 (A1g) for FeHCFe is observed, and dual peaks at higher
Raman shifts of 2100 (A1g) and 2160 (Eg) cm−1 are observed in the
Morphologies of the PBAs have been observed using TEM, and fol-
lowing an ultra-sonication treatment, all products show a similar
morphology for NPs with a size of around 20 nm (Fig. 2). This small
particle size results in the generation of more accessible active sites,
which is a crucial parameter in the context of catalytic hydrogenation
reactions. Furthermore, the high specific surface area that results from
small NPs is beneficial for the additive loading of other metals, such as
Pd.
spectrum for CoHCFe. The three transform modes for cyanide are A1g
,
Eg, and T1u; the former two are Raman active, and the last one is in-
frared. A mixed breathing mode for A1g (volume-changing mode) re-
veals that the central metal ions are coordinated by the stretched and
contracted cyanides, which forms by mixing with low-wavenumber
modes. Furthermore, the Eg (shape-changing) modes form by mixing
with high-wavenumber vibrations [50].
The catalytic activities of the bare PBAs were evaluated in a model
reaction in which nitrobenzene was reduced to aminobenzene in the
presence of a reductant, aqueous NaBH4. The highest yield of 33% for
aminobenzene is achieved using the bare FeHCFe catalyst (Table S2),
demonstrating the potential for FeHCFe as a catalyst in this reaction. To
further improve the catalytic activity, Pd NPs were loaded into FeHCFe
NPs to create electron transfer pathways on the FeHCFe support.
A schematic illustration of the Pd loading method is depicted in
Fig. 4; the process is eco-friendly as the synthesis occurs in water, and
this reaction can be easily used scaled up. Upon completion of the re-
action, high-powered ultra-sonication is employed to obtain a uniform
dispersion and refinement of the particle sizes. The decorated Pd NPs
with a size of around 5 nm are observed in TEM images (Fig. 5). A
uniform decoration of Pd NPs on FeHCFe NPs is observed in TEM
images obtained for the composite material (Fig. 5c and d). EDX
The Fe K-edge XANES spectra reveal the local structure around Fe
ions inside the products, and a typical feature of the pre-region in Fe K-
edge XANES spectra is illustrated by the data for CoHCFe and FeHCFe
(Fig. 3a). In these systems, 3d5 electron orbital in high-spin Fe(III) splits
into 3deg and 3dt2g owing to the octahedral crystal field. Electron
transitions from the 1s core level to the split orbitals are demonstrated
by the peaks in the pre-region A of Fe K-edge XANES spectra (Fig. 3b)
[49]. In the edge-region (B), sharp peaks at 7131 eV for both CoHCFe
and FeHCFe suggest the same oxidation state of Fe ions in the products
(Fig. 3a), and a similar pattern for both CoHCFe and FeHCFe is also
observed in the C region. The valance state of Co ions in CoHCFe is
measured by examining the K-edge XANES spectrum (Fig. 3c); a sharp
peak at 7725 eV in the B region demonstrates the bivalent Co(II), fol-
lowed by a C region.
Fig. 2. TEM images of the synthesized products: (a) FeHCFe, (b) CoHCFe, and (c) NiHCFe.
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Fig. 3. K-edge XANES spectra of (a) Fe, (b) magnified A region in (a), and (c) Co. (d) Raman spectra of the FeHCFe and CoHCFe products.
mapping shows a better supporting effect for FeHCFe than that of the
carbon substrate, displaying larger and agglomerated Pd NPs (Fig. S1).
Furthermore, the loaded Pd NPs and uniformly distributed species have
been analyzed by EFTEM (Fig. S2). The high-power ultra-sonication of a
mixture of these small, uniform Pd NPs combined and an effective
surfactant results in the generation of more accessible active sites
during the reduction reaction.
crystal structure is observed both before and after Pd loading. The ab-
sence of characteristic Pd peaks suggests the presence of small Pd NPs,
which is corroborated by the small particle sizes observed in the TEM
images (Fig. 5), and the nanocomposite shows reduced peak intensities
after Pd loading relative to data collected for the sample before Pd
loading. The structure of the composite nanocatalyst is further con-
firmed by FT-IR spectroscopic analysis (Fig. 6b). Strong peaks at 3310
cm−1 (OHe stretching vibrations) and 1580 cm−1 (HOHee bending
vibrations) shown by both bare FeHCFe and FeHCFe@Pd are attributed
XRD analysis was employed to characterize Pd-loaded FeHCFe
(Fig. 6a), and the characteristic peaks for PBAs with a face-central cubic
Fig. 4. Schematic demonstration for the synthesis of the FeHCFe@Pd catalyst.
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Fig. 5. TEM images for FeHCFe (a, b) without and (c, d) with Pd nanocatalysts.
to residual H2O molecules [51]. In addition, sharp characteristic peaks
at 2120 cm-1 are associated with in-plane vibrations of the cyanide
bridges [52,53]. It is worth noting that the characteristic peak of
cyanide linkers significantly changed from twin peaks to a mono-peak
after Pd loading (Fig. 6c). The intensity of the peak at 2130 cm-1 re-
duced while the peak intensity at 2165 cm-1 increased after Pd loading,
Fig. 6. (a) XRD patterns, (b,c) FT-IR spectra, and (d) TGA curves of FeHCFe with (red) and without (black) Pd nanocatalysts. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Fig. 7. Deconvoluted XPS spectra of the prepared FeHCFe@Pd catalyst.
suggesting an interaction between Pd NPs and the cyanide bridges in
FeHCFe [54].
Thermal stabilities of the as-prepared FeHCFe catalyst before and
after Pd loading were ascertained using TGA (Fig. 6d) in which the loss
of absorbed water molecules (10 wt.%) was observed in the initial
range up to approximately 200 °C. In the temperature range of 200–500
°C, the as-prepared FeHCFe shows a steady thermal stability, whereas a
gradual weight decay is observed for FeHCFe@Pd, suggesting poor
thermal stability. This data is consistent with the PXRD data in which
reduced crystallinity was observed for FeHCFe@Pd relative to that of
the bare FeHCFe (Fig. 6a). Next, the surface area and porosity of the
FeHCFe@Pd were estimated using BET analysis (Fig. S3). The compo-
site material exhibits a typical adsorption-desorption isotherm curve
and displays a surface area of 294 m2 g−1 similar to the previous report
[55]. Furthermore, the FeHCFe@Pd shows a pore size distribution
comprised of meso- and micro-pores (4 nm in average pore radius and
0.3 cm3 g−1 in volume, Fig. S3).
The surface chemical properties of FeHCFe@Pd were studied using
XPS (Fig. 7). The presence of the cyanide linkers is confirmed by the
deconvoluted C 1s and N 1s spectra (Fig. 7a and b), and the Fe cation
nodes through the observation of the Fe 2p spectrum (Fig. 7c). The
bonding nature of Fe and Pd ions was verified by the deconvoluted Fe
2p and Pd 3d spectra in which the data suggest that the Pd NPs are fixed
on the frameworks. The O 1s spectrum detected in the product is il-
lustrative of the residual water molecules still present in the sample.
Furthermore, the composite product was quantitatively analyzed
through XPS measurements, where a mass ratio of 1.5% Pd relative to
FeHCFe was observed, suggesting the successful loading of Pd NPs onto
the framework (Table S3).
Pd K-edge XANS analysis of the composite nanocatalyst (FeHCFe@
Pd) was employed to verify the valance states of Pd component
(Fig. 8a). A peak at 24,366 eV (Pd2+) further affirms the bonding
feature between Pd and FeHCFe at the interface, and a peak located at
24,388 eV suggests the elemental Pd in FeHCFe@Pd. In addition, SAXS
characterization was used to analyze Pd NPs that were unable to be
detected by normal XRD analysis due to their small size, and a sharp
peak at around 0.6 degrees was observed (Fig. 8b). XANS and SAXS
measurements were also performed on commercial samples of Pd on
carbon, and the similarities between spectra for this sample and the
composite sample validate the loading of Pd NPs on FeHCFe (Fig. 8).
Next, the catalytic activity of FeHCFe@Pd was established in the
Fig. 8. (a) XANS and (b) SAXS spectra of the FeHCFe@Pd catalyst (solid line)
and commercial Pd NPs on carbon (dot line).
reduction of nitrobenzene to aminobenzene in aqueous NaBH4. A high
yield of 98% is obtained, which can be attributed to the loaded Pd NPs
(Table 1). After hydrogenation, a transparent solution can be obtained,
suggesting completion of the reaction (Fig. S4). In addition, the
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Table 1
an excellent performance for this catalyst for broad catalytic reduction
applications. Furthermore, the durability of the FeHCFe@Pd composite
nanocatalyst was investigated using the reduction of nitrophenol to
aminophenol in aqueous NaBH4 as a model reaction, and the catalytic
activity was preserved as 82% yield to the desired product was ob-
served after 5 consecutive reduction cycles (Fig. 9). One possible reason
for this reduced catalytic activity is the loss of composite catalyst,
FeHCFe@Pd, as it adheres to the walls of the tube during centrifugation
after each reaction, which is consistent with the formation of thin film
that can be observed on the tube walls. To confirm the immobility of Pd
in FeHCFe, the used FeHCFe@Pd catalyst was further analyzed by ICP-
AES for the Pd amount on the FeHCFe support, and by XPS to elucidate
the bonding environment inside the composite catalyst. The results
from these experiments indicate the composite nanocatalyst contains
0.54% Pd within the composite material, suggesting the existence of Pd
species. In addition, composite catalyst was also analyzed after a re-
action by XPS, and the data indicate the existence of Pd species in the
composite product, together with others, such as C, N, Fe (Fig. S5).
Turnover frequency (TOF), an important parameter for the evalua-
tion of catalytic activity, is calculated on the basis of the turnover
number, or amount of reactant consumed per amount of catalyst, per
unit time. The composite FeHCFe@Pd exhibits a TOF value of around
1.43 s−1, suggesting high catalytic activity for this material. In addi-
tion, full conversion of the same amount of nitrophenol (0.1 mmol) can
be obtained by a smaller amount of catalyst (5 mg) within 5 min, which
can be visually discerned according to the color change between 2 and
5 min (Fig. S6).
Reduction of nitroaromatic substrates catalyzed by FeHCFe@Pd using NaBH4
reductant.
Entry
1
Substrate
Product
Yield (%)
98
2
3
99
94
4
5
6
61
98
99
A systematic comparison of the catalytic performance between the
composite nanocatalyst in this study, FeHCFe@Pd, and previously re-
ported noble metal-containing catalytic materials is provided in Table
S4. Compared to the other catalysts, which are typically synthesized via
multiple wet-chemical and high-temperature heat treatment ap-
proaches [56–58], a relatively facile synthesis process is used for pre-
paring the composite nanocatalyst in this report. Furthermore,
FeHCFe@Pd exhibits high catalytic activity as complete conversion of
0.1 mmol of nitrophenol is achieved within a short time range of 5 min
(Table 1).
7
90
Reaction conditions: NaBH4 1.2 mmol, nitromatics 1 mmol, catalyst 5 mg, time
5 min.
The underlying mechanisms for the excellent catalytic activity of the
FeHCFe@Pd composite catalyst can potentially be ascribed to the in-
herent high activity of Pd, as well as multiple electron transfer path-
ways present in the composite system (Fig. 10). Upon incorporation of
Pd NPs on the FeHCFe framework, the number of electron transfer
pathways increase from the original pathway in FeHCFe (i.e., NaBH4 to
FeHCFe to nitroaromatics, Fig. 10a) to at least four total potential
pathways in FeHCFe@Pd (i: NaBH4 to FeHCFe to nitroaromatics; ii:
NaBH4 to Pd to nitroaromatics; iii: NaBH4 to Pd to FeHCFe to ni-
troaromatics; iv: NaBH4 to FeHCFe to Pd to nitroaromatics, Fig. 10b); a
synergetic effect is simultaneously displayed in FeHCFe@Pd, resulting
in superior catalytic performance for this system.
4. Conclusions
In summary, six hexacyanoferrate(III)-based PBAs were synthesized
through the use of an ultrasound sonication approach, including Mn-,
Fe-, Co-, Ni-, Cu-, and Zn-based materials, and characterized using
multiple spectroscopic methods. Next, these PBAs were examined for
their catalytic performance in the reduction of nitroaromatics in aqu-
eous NaBH4, and it was found that the identity of the metal in the PBA
greatly influenced the catalytic activity. FeHCFe exhibits a 33% yield
for the reduction of nitrobenzene to aniline, which is the highest cat-
alytic activity for this reaction among these PBA-based catalysts. In
order to further improve the catalytic performance for this class of
materials, catalytically active Pd NPs were incorporated in FeHCFe in
order to build more electron transfer pathways in this system.
Following characterization of the composite material and confirmation
of successful Pd loading into the robust PBA framework, this material
Fig. 9. Repeated cycling studies for the reduction of nitrophenol to amino-
phenol by using the FeHCFe@Pd catalyst.
FeHCFe@Pd composite nanocatalyst is also active towards other ni-
troaromatic substrates with a diverse range of functional groups, in-
cluding −NH2, −Br, −Cl, −CH3, and −OH (Table 1), demonstrating
the generality of this catalytic system in the context of nitrobenzene
reductions. In all cases, FeHCFe@Pd selectively reduces the nitro
functional group to the corresponding amino group (Table 1), showing
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K. Zhang, et al.
MolecularCatalysis492(2020)110967
Fig. 10. Schematic illustration for (a) mono- and (b) multiple-pathway reduction reactions.
was tested for the same catalytic reaction and exhibited a significant
improvement in catalytic activities with near-quantitative conversions
achieved. We propose the improvement in catalytic activity for
FeHCFe@Pd originates from the introduction of multiple new electron
transfer pathways, as well as the highly improved conductivity, that
results upon the combination of porous PBAs and catalytically active Pd
NPs. Overall, these results provide insight into the construction of fra-
mework materials using PBAs as supports and the engineering of elec-
tron transfer tunnels.
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Kaiqiang Zhang and Kent O. Kirlikovali: Writing- Reviewing and
Editing; Joo Hwan Cha, Se Yeon Jeon, Vamegh Rasouli, and Mehdi
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Acknowledgments
This research was supported by the Future Material Discovery
Program (2016M3D1A1027666), the Basic Science Research Program
(2017R1A2B3009135) through the National Research Foundation of
Korea, and China Scholarship Council(201808260042) are appreciated.
K.O.K. gratefully acknowledges support from the IIN Postdoctoral
Fellowship and the Northwestern University International Institute of
Nanotechnology. O.K.F. is grateful for the financial support from the
Inorganometallic Catalyst Design Center, an Energy Frontier Research
Center funded by the U.S. Department of Energy (DOE), Office of
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