Encapsulation of Porphyrin-Fe/Cu Complexes into Coordination Space for Enhanced Selective Oxidative Dehydrogenation of Aromatic Hydrazides

Article abstract of DOI:10.1002/smll.202004679

The encapsulation of specific nanoentities into hollow nanomaterials derived from metal organic frameworks has attracted continuous and growing research attentions owing to their unique structural properties and unusual synergistic functions. Herein, usin

Full text of DOI:10.1002/smll.202004679

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Encapsulation of Porphyrin-Fe/Cu Complexes into
Coordination Space for Enhanced Selective Oxidative
Dehydrogenation of Aromatic Hydrazides
Dongxu Zhang, Peiyao Du, Jia Liu, Ruizhong Zhang, Zhen Zhang, Zhengang Han,
Jing Chen, and Xiaoquan Lu*
heterogeneous
tively, compared with other solid types of
nanomaterials, thin permeable shells and
limited internal voids endow the hollow
nanomaterials with abundant accessible
reactions.[1–3]
Distinc-
The encapsulation of specific nanoentities into hollow nanomaterials derived
from metal organic frameworks has attracted continuous and growing research
attentions owing to their unique structural properties and unusual synergistic
functions. Herein, using the phase transformation of uniform rhombi
dodecahedron ZIF-67, hollow nano-shell with a well-defined morphology is
successfully prepared. Particularly, the iron-oxygen complex, that is formed
by the interaction between TCPP-Fe/Cu (TCPP = tetrakis(4-carboxyphenyl)-
porphyrin) and oxygen, can be acted as an ideal proton acceptor for practical
organic reactions. Considering the unique adaptability of hollow ZIFs (named
HZ) to the transformation of encapsulated TCPP-Fe/Cu bimetallic catalytic
active sites, a heterogeneous catalyst (defined as HZ@TCPP-Fe/Cu) through
morphology-controlled thermal transformation and rear assemble processes
is designed and constructed. Under heterogeneous conditions, HZ@
TCPP-Fe/Cu serves as a multifunctional molecular selector to promote the
oxidative dehydrogenation of different aromatic hydrazide derivatives with
high selectivity toward primary carbon among primary, secondary, and tertiary
carbons that are unachievable by other traditional homogeneous catalysts. The
high catalytic activity, selectivity, and recyclability of the catalyst proposed here
are attractive advantages for an alternative route to the environmentally benign
transformation of aromatic hydrazides to aromatic azobenzene.
catalytic active sites and approachable con-
tact space between specific reactants and
catalysts, thus achieving efficient mass dif-
fusion and reaction.[4–8] To tackle the prob-
lems of catalyst efficiency and selectivity, it
is necessary to precisely control the posi-
tions of the different functions and spatial
structures of the catalysts, but this is still a
huge challenge.
Constructing multifunctional hollow
nanostructured materials from multiple
components is considered to be one of
the promising strategies to implement
high-level applications requiring com-
plex architecture with excellent catalytic
performance.[9] As previously reported, a
wide range of materials could be used as
“host,” such as holey Fe&N codoped gra-
phene,[10] hollow mesoporous prussian
blue nanoparticles,[11] carbon-based nano-
cages,[12] and layered double hydroxide
nanocages.[13] To be more specific, layered void on the cage
walls can support each other for enhanced mechanical sta-
bility, thus regulating the mass transport and enhancing the
contact between the active catalytic components and reactant,
which could significantly improve the catalytic performances.
Accordingly, the encapsulation of nanoentities in hollow lay-
ered hydroxides materials is preferred since the properties and
accessibility of catalytic active sites are fit with those of hetero-
geneous catalysts with ideal catalytic activity and size selectivity,
showing a great potential in catalysis. Thus, we aim at ration-
ally designing a hollow nanocage that could effectively use the
layered hydroxide platform to achieve effective encapsulation of
catalytically active sites.
Nanoentities with specific activity as “guest,” such as single
atom/site, molecule, metal nanoparticles, metal-complex, etc.,
are encapsulated in the limited space of “host,” so that their
activity can be effectively guaranteed and thus show excellent
performance.[14,15] Simultaneously, the physical and chemical
properties of the “guest” are very critical factors. For instance,
metalloporphyrin compounds have been extensively studied
because of their rigidly and planar geometry, high thermal, and
1. Introduction
Encapsulation of hollow nanomaterials has proven to be an
effective strategy to make full use of active catalytic compo-
nents and achieve a higher specific reactivity for catalyzing
D. Zhang, Dr. P. Du, Dr. J. Liu, Dr. R. Zhang, Dr. Z. Zhang
Tianjin Key Laboratory of Molecular Optoelectronic Science
Department of Chemistry
School of Science
Tianjin University
Tianjin 300072, P. R. China
Dr. P. Du, Dr. Z. Han, Dr. J. Chen, Prof. X. Lu
Key Laboratory of Bioelectrochemistry and Environmental Analysis
of Gansu Province
College of Chemistry and Chemical Engineering
Northwest Normal University
Lanzhou 730070, P. R. China
E-mail: luxq@nwnu.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202004679.
DOI: 10.1002/smll.202004679
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Scheme 1. Schematic diagram of HZ@TCPP-Fe/Cu (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin) heterogeneous catalyst applied for selective
oxidative dehydrogenation of aromatic hydrazides to aromatic azobenzenes.
electronic stability.[16] And, they are also excellent candidates for
interfacial charge transfer mediators in different systems.[17,18]
Inspired by biomimetic catalysis, metalloporphyrin can activate
oxygen under mild conditions, which is incomparable with
some traditional metal salt catalysts.[19–21] In general, metal-
loporphyrin in organisms need to cooperate with some bio-
logical enzymes to display an excellent catalytic performance,
but once isolated from these environments, the catalytic activity
of individual metalloporphyrin will be greatly reduced. To solve
this problem, regulating the coordination mode and microen-
vironment of metalloporphyrin can be an effective strategy. To
be specific, an encapsulation strategy of “front introduction and
rear assembly” was proposed, which successfully overcame the
problem of metalloporphyrin inactivation.
Given the unique adaptability of self-templated zeolitic
imidazolate frameworks (ZIFs) in conversion of hollow ZIFs
(named HZ) encapsulating TCPP-Fe/Cu (TCPP = tetrakis(4-
carboxyphenyl)-porphyrin) bimetallic catalytic active sites, het-
erogeneous catalyst (defined as HZ@TCPP-Fe/Cu) within an
integral framework was designed and synthesized. Based on the
synergistic effect of TCPP-Fe/Cu bimetallic active sites and the
limited internal voids oxidative dehydrogenation of aromatic
hydrazides reaction was successfully obtained (Scheme 1). With
the appropriate integration of these encapsulation strategies
in the heterogeneous catalysts construction, the as-prepared
hollow nanomaterials are demonstrated to be the brilliant can-
didates for the rational catalyst design toward the target reac-
tions, especially for complicated industrial catalysis systems.
TCPP-Fe was encapsulated into the nanocrystals of ZIF-67 in
situ via the “bottle around ship” (named ZIF-67@TCPP-Fe).
Subsequently, by treating the ZIF-67@TCPP-Fe with Co2+ at
120 °C for 1 h, the polyhedral would ready vanish and form
hollow layered hydroxides nanocage with beneficial channel
size and well-defined morphology (named HZ@TCPP-Fe).
Then, the above hollow nanomaterial served as an ideal porous
support and TCPP-Fe as a felicitous Cu2+ supplier for TCPP-
Fe/Cu bimetallic catalytic active sites inside the layered hydrox-
ides voids with excellent dispersity, and the HZ@TCPP-Fe/Cu
heterogeneous catalyst was finally fabricated (Scheme 2).
Scanning electron microscopy (SEM) images of
as-obtained ZIF-67, ZIF-67@TCPP-Fe, HZ@TCPP-Fe, and
HZ@TCPP-Fe/Cu were shown in Figure 1. The SEM image
of ZIF-67 displayed a uniform rhombi dodecahedron struc-
ture with average crystal size of about 2 µm (Figure 1a). And
the SEM image demonstrated that the overall morphology
of the as-prepared ZIF-67@TCPP-Fe had not changed sig-
nificantly, while the average size was reduced to ≈800 nm
(Figure 1b). Besides, HZ@TCPP-Fe and HZ@TCPP-Fe/Cu
displayed hydrangea-like morphology with an average diameter
of ≈800 nm, and hollow nanocage had a thickness of ≈20 nm
(Figure 1c–f). The as-prepared HZ@TCPP-Fe/Cu heteroge-
neous catalyst presents a relatively uniform size (Figure S1,
Supporting Information). The dark-field scanning transmis-
sion electron microscopy (dark-field STEM) and the elemental
mapping images of HZ@TCPP-Fe and HZ@TCPP-Fe/Cu
confirmed that Fe and Cu elements were homogeneously dis-
persed (Figure 1g–r). The results of inductively coupled plasma
optical emission spectrum (ICP-OES) showed that Fe and Cu
were successfully doped (Table S1, Supporting Information).
The synthesis routes and the details of TCPP-Fe and H4TPP-Fe
were described in the Supporting Information (Figures S2–S8,
Supporting Information). To further gain insight into the rela-
tionship between TCPP-Fe organic ligands and different kinds
of transition metal ions (Cu2+, Ni2+, and Fe2+), the active bulk
2. Results and Discussion
A self-templated strategy to construct hydrangea-like super-
structure of hollow nanocatalyst through morphology-controlled
thermal transformation of porous ZIFs and postassembly of
bimetallic catalytic active sites were presented. To be specific,
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Scheme 2. Schematic preparation process of the HZ@TCPP-Fe/Cu (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin) heterogeneous catalyst using
an “front introduction and rear assembly” encapsulation strategy.
catalysts (TCPP-Fe/Fe bulk, TCPP-Fe/Ni bulk, and TCPP-Fe/Cu
bulk) were chosen to illustrate the applicability of the catalytic
system. Unexpectedly, the direct mixing of TCPP-Fe and transi-
tion metal ions produced a large number of irregularly shaped
precipitates, which could not further achieve the purpose of
uniform encapsulation (Figures S9–S11, Supporting Informa-
tion). More deeply, the direct introduction of transition metal
ions into ZIF-67@TCPP-Fe (named ZIF-67@TCPP-Fe/Cu,
ZIF-67@TCPP-Fe/Ni, and ZIF-67@TCPP-Fe/Fe) could cause
the skeleton structure of ZIF-67@TCPP-Fe collapsed due to the
ion exchange reaction (Figure S12, Supporting Information).[22]
In addition, H4TPP-Fe organic ligand was chosen to prepare
H4TPP-Fe/Cu bulk for comparison with the catalytic activity of
the above materials (Figure S13, Supporting Information).
The uniformity and integrity of the hollow nanomaterials
endow nanocatalysts with unexpected physical and chemical
properties.[23,24] In our work, a new strategy for preparing
hollow HZ nanocage-encapsulated nanoentities under mild
conditions was proposed. Powder X-ray diffraction (PXRD) was
measured to verify the composition and structural informa-
tion. The PXRD patterns of all the obtained ZIF-67@TCPP-Fe
and ZIF-67@TCPP-Fe/M (M = Cu, Ni, or Fe) materials are
consistent with those of bare ZIF-67, confirming their high
crystallinity and isostructural nature (Figure 2a).[25] For HZ,
HZ@TCPP-Fe, and HZ@TCPP-Fe/M, a high-intensity peak
appeared at 2θ = 10.1° related to the (003) facet of the hydrotal-
cite-like phase (Figure 2b).[26] These PXRD results also proved
that the hydroxides interlayer spacing of HZ@TCPP-Fe/M
was 8.02−9.28 Å. The Fourier transform infrared spectrometer
(FT-IR) analysis was explored to verify the construction of these
materials (Figure S14, Supporting Information).[27] For HZ@
TCPP-Fe/M (M = Cu, Ni, or Fe), the morphology of HZ@
TCPP-Fe were well maintained after modification (Figure S15,
Supporting Information). Specifically, in the thermogravimetric
analysis (TGA) profile, the typical weight loss of HZ, HZ@
TCPP-Fe and HZ@TCPP-Fe/Cu were attributed to hydrox-
ides decomposition starting at about 210 °C, which differed
with ZIF-67 (500 °C) in temperature (Figure 2c). The chem-
ical composition and oxidation states of HZ@TCPP-Fe and
HZ@TCPP-Fe/Cu were studied by high-resolution X-ray photo-
electron spectroscopy (XPS) (Figure 2d). The Co 2p spectrum
of HZ@TCPP-Fe and HZ@TCPP-Fe/Cu exhibited two distinct
peaks centered at 781.12 and 797.27 eV, which were assigned
to the 2p3/2 and 2p1/2 levels of Co2+, respectively. In addition,
two broad satellite peaks were located at 785.46 and 802.59 eV
(Figure 2e).[28] The XPS results revealed that the main oxidation
states of Fe in HZ@TCPP-Fe and HZ@TCPP-Fe/Cu materials
were +3 (Figure 2f).[29,30] The Cu 2p XPS signals appeared at
935.58 and 954.29 eV for Cu 2p3/2 and Cu 2p1/2, respectively
(Figure S16, Supporting Information).[31] The C 1s peaks of
HZ@TCPP-Fe and HZ@TCPP-Fe/Cu can be fitted into C=C
(284.43 eV), C=N (285.55 eV), C−O (286.58 eV), and C=O
(288.06 eV) (Figure 2g).[32,33] The N 1s peaks could be assigned
−
to oxidized N species (NO3 ) (406.57 eV) and 2-methylimida-
zole (399.48 eV) (Figure 2h).[34] The peaks of HZ@TCPP-Fe at
531.34 and 533.22 eV were attributed to the O 1s of the C=O and
C−OH groups, respectively. However, in HZ@TCPP-Fe/Cu,
the peak designated for C=O moved to lower binding ener-
gies (531.14 eV), thereby indicating the interaction between the
carboxyl group in TCPP-Fe and Cu2+ (Figure 2i).[35–37] Notably,
according to Irving−Williams stability series, we noticed that
the relative stability of homologous bivalent 3d metal complexes
will follow the trend: Cr2+<Mn2+<Fe2+<Co2+<Ni2+<Cu2+>Zn2+,
regardless of the structure and properties of organic ligands.[38]
The TCPP-Fe organic ligands were encapsulated in open cavi-
ties of ZIFs, and the structure would not be damaged by low-
temperature thermal treatment. Then, TCPP-Fe/Cu bimetallic
catalytic active sites were constructed by adding Cu2+ to replace
other transition metal ions (such as unquantified Co2+) in the
carboxyl group of TCPP-Fe.
The ability to prepare a kinetically oriented multivariate
hollow nanomaterials with customized layered environment
and collaborative behavior provides an unparalleled level of
control over the encapsulation and utilization of the “guest.”
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Figure 1. Scanning electron microscopy (SEM) images of a) ZIF-67, b) ZIF-67@TCPP-Fe (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin),
c) HZ@TCPP-Fe, and d) HZ@TCPP-Fe/Cu. Transmission electron microscopy (TEM) images of e) HZ@TCPP-Fe and f) HZ@TCPP-Fe/Cu. g) The
dark-field scanning transmission electron microscopy (STEM) image of HZ@TCPP-Fe. h–l) EDS elemental mapping of C, N, O, Co, and Fe signals for
HZ@TCPP-Fe. m) The HAADF-STEM image of HZ@TCPP-Fe/Cu. n–r) EDS elemental mapping of N, O, Co, Fe, and Cu signals for HZ@TCPP-Fe/Cu.
Typically, the N-containing imidazole ester on the ZIFs skel-
eton can provide active uncoordinated nitrogen sites for adsorb
“guest” molecules, which makes it more widely used in the
field of catalysis than other MOF materials.[39–41] To ascer-
tain that the TCPP-Fe “guest” molecules were indeed encap-
sulated into a porous ZIF-67 carrier, the UV−vis spectrum
of ZIF-67 and ZIF-67@TCPP-Fe were measured. ZIF-67@
TCPP-Fe displayed a new peak at 411 nm due to the Soret band
of TCPP-Fe units (Figure 3a).[42] The Raman spectra of ZIF-67,
ZIF-67@TCPP-Fe, and ZIF-67@TCPP-Fe/Cu displayed intense
bands corresponding to methyl group and imidazole ring
vibrations of the ZIFs skeleton. The strong bands at 177, 258,
423, and 686 cm−1 can be attributed to the Zn−N stretching,
C−Zn−N bending, and imidazole ring puckering (Figure 3b).[43]
Unlike rigid inorganic molecular sieves, porous metal−organic
frameworks possess good flexibility and are prone to deforma-
tion.[44] With the introduction of “guest” and postmodification,
the wrinkling peaks of the imidazole ring gradually shifted to
lower wavenumber, which was caused by the lattice expansion
process.[45] The results of time-resolved photoluminescence
(TRPL) (Figure 3c; see also Table S2, Supporting Information)
confirmed that TCPP-Fe was encapsulated in the open cavities
of ZIF-67, which result in a longer carrier lifetime (from 19.78 to
21.28 ns). The average emission lifetime of photoexcited charge
for HZ@TCPP-Fe was 21.83 ns, which was longer than that
of HZ@TCPP-Fe/Cu (12.03 ns), suggesting that excited-state
TCPP-Fe connected with Cu2+.[46]
In HZ@TCPP-Fe/Cu, the TCPP-Fe/Cu bimetallic sites
contained two five-coordinated copper cations that are
bridged in a paddle-wheel-type configuration and to which
the TCPP-Fe ligands were coordinated in a bidentate bridging
fashion.[47–49] In N2 adsorption and desorption isotherms,
compared with ZIF-67, ZIF-67@TCPP-Fe displayed a reduced
BET (Brunauer−Emmett−Teller) surface area from 1557 to
1475 m2g−1, due to the mass contributions from TCPP-Fe mol-
ecule (Figure 3d). Through the heat treatment process, the
hollow layered hydroxide shell structure was gradually formed,
and the BET surface area suddenly decreased to 126 m2g−1. For
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Figure 2. a) Powder X-ray diffraction (XRD) patterns of ZIF-67, ZIF-67@TCPP-Fe (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin), ZIF-67@
TCPP-Fe/Cu, ZIF-67@TCPP-Fe/Ni, and ZIF-67@TCPP-Fe/Fe. b) Powder XRD patterns of HZ, HZ@TCPP-Fe, HZ@TCPP-Fe/Cu, HZ@TCPP-Fe/Ni,
and HZ@TCPP-Fe/Fe. c) The thermogravimetric analysis (TGA) results of ZIF-67, ZIF-67@TCPP-Fe, ZIF-67@TCPP-Fe/Cu, HZ, HZ@TCPP-Fe, and
HZ@TCPP-Fe/Cu. d) X-ray photoelectron spectroscopy (XPS) surveys of HZ@TCPP-Fe and HZ@TCPP-Fe/Cu. e–i) The high-resolution Co 2p spectrum
(e), Fe 2p spectrum (f), C 1s spectrum (g), N 1s spectrum (h), and O 1s spectrum (i).
HZ@TCPP-Fe and HZ@TCPP-Fe/Cu, the BET surface areas
also tended to decrease, owing to the postsynthesis modifica-
tion process upon the addition of Cu2+.
hindrance, smaller aromatic azobenzene may be beneficial to
Mitsunobu reaction, so the principle of chemical selectivity
is also important. Thus, developing a novel, environmentally
friendly, recyclable, and selective catalyst for the dehydrogena-
tion of aromatic hydrazides to aromatic azobenzene is keenly
pursued.
In this regard, we focused our studies on the high reac-
tivity and selectivity oxidative dehydrogenation of aromatic
hydrazides with different substituents by heterogeneous cata-
lysts under different integration modes (Figure 4a). Being
bigger in sizes than ethyl 2-phenylhydrazine-1-carboxylate
(1) (11.5 × 4.2 × 1.8 Å3), the isopropyl 2-phenylhydrazine-
1-carboxylate (2) (11.0 × 4.2 × 3.1 Å3) and tert-butyl 2-phenyl-
hydrazine-1-carboxylate (3) (11.4 × 4.4 × 4.4 Å3) cannot be dif-
fused into HZ@TCPP-Fe/Cu heterogeneous catalyst smoothly
and then further oxidized dehydrogenation due to the steric
hindrance effect (Figure S17, Supporting Information). Firstly,
the catalytic activities toward oxidative dehydrogenation of
Aromatic azobenzene, as one of the oldest organic com-
pounds, is widely used in the field of chemical industries.[50–53]
Mitsunobu and co-workers put forward that the combination of
diethyl azodicarboxylates and triphenylphosphine causes con-
densation reactions between alcohols and acids to obtain the
corresponding esters.[54,55] Previous studies have reported that
aromatic azobenzene exhibited effective catalytic ability for Mit-
sunobu reactions, in which traditional iron(II)phthalocyanine
[Fe(Pc)] can be used as an oxidative dehydrogenation cata-
lyst.[56,57] However, the aerobic oxidation of Fe(Pc) is difficult to
recycle and reuse, has no obvious catalytic selectivity, and often
requires a large number of catalysts (10 mol%). Compared with
structure-related Fe(Pc), the catalytic performance of TCPP−Fe
complexes as the candidates for catalyzing the dehydrogenation
of aromatic hydrazides has not been fully studied. For steric
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Figure 3. a) UV–vis absorption spectra of TCPP-Fe (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin), ZIF-67 (solid samples), and ZIF-67@TCPP-Fe
(solid samples). b) Raman spectra of ZIF-67, ZIF-67@TCPP-Fe, and ZIF-67@TCPP-Fe/Cu. c) Time-resolved photoluminescence emission decay spectra
of TCPP-Fe, ZIF-67@TCPP-Fe, HZ@TCPP-Fe, and HZ@TCPP-Fe/Cu. d) Nitrogen sorption isotherms for pure ZIF-67, ZIF-67@TCPP-Fe, HZ@TCPP-Fe,
and HZ@TCPP-Fe/Cu at 77 K up to 1 bar.
aromatic hydrazides reaction were assessed with 1 mol% active
bulk catalysts (TCPP-Fe/Cu bulk, TCPP-Fe/Ni bulk, TCPP-
Fe/Fe bulk, and H4TPP-Fe/Cu bulk) in O2-saturated THF,
respectively (Figure 4b). When a mixture of 1 (0.25 mmol) and
TCPP-Fe/Cu bulk (1 mol%) in 4 mL THF was exposed to O2,
ethyl-2-phenyldiazene-1-carboxylate (4) was obtained in good
yield (>90 %) in 4 h after purification by chromatography. And
2 and 3 also had ideal yields in the same conditions. Further-
more, TCPP-Fe/Fe, TCPP-Fe/Ni, and H4TPP-Fe/Cu bulk have
good catalytic activity, but slightly inferior to that of TCPP-Fe/
Cu. For the above experimental results, we could preliminarily
determine that in the structure of the as-prepared active bulk
catalysts, porphyrin-Fe as the main component cooperates with
the different kinds of transition metal ion, which was indis-
pensable for oxidative dehydrogenation of aromatic hydrazides
reaction. When TCPP-Fe or H4TPP-Fe exists alone, the catalytic
activity was weak (Figure S18, Supporting Information). Besides
Fe, Ni, and Cu used in our previous work, the other transi-
tion metal atoms including Ag, Mn, and Cr were employed.
The SEM images of HZ@TCPP-Fe/Ag, HZ@TCPP-Fe/Mn,
and HZ@TCPP-Fe/Cr were shown in Figure S19 (Supporting
Information). From experimental results, compared with HZ@
TCPP-Fe/Cu, the above catalysts exhibited weak catalytic activity
(Table S3, Entries 1-3, Supporting Information). The combina-
tion of metalloporphyrin and electron-deficient metal salt could
not only retain the characteristics of metalloporphyrin to acti-
vate O2 and generate free radicals under mild conditions, but
also make use of the good adaptability of metal salts toward
the reaction conditions, that was, employing metalloporphyrin
as inducer and metal salt as promoter to catalyze the oxidation
reaction with high activity, which is undoubtedly of great sig-
nificance. Secondly, to further evaluate the chemoselectivity of
catalysts toward primary carbon, the same molar amounts of 1,
2, and 3 were used as the reaction substrates, respectively.
Obviously, the hollow HZ@TCPP-Fe/Cu possessed higher
catalytic selectivity than the structurally fragmented ZIF-67@
TCPP-Fe/Cu, which proved the specific internal voids inside
the nanocage successfully controlled the contact between reac-
tion substrates and the catalytic active sites (Figure 4c,d). As
the formation of internal voids in HZ@TCPP-Fe/Cu catalyst,
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Figure 4. a) Concept of a catalytic oxidative dehydrogenation of aromatic hydrazides reactions under different catalysts. b) Catalytic performance of
TCPP-Fe/Cu (where TCPP is tetrakis(4-carboxyphenyl)-porphyrin) bulk, TCPP-Fe/Ni bulk, TCPP-Fe/Fe bulk, and H4TPP-Fe/Cu bulk toward aromatic
hydrazides. c) Catalytic performance of ZIF-67@TCPP-Fe/Cu, ZIF-67@TCPP-Fe/Ni, and ZIF-67@TCPP-Fe/Fe toward aromatic hydrazides. d) Catalytic
performance of HZ@TCPP-Fe/Cu, HZ@TCPP-Fe/Ni, and HZ@TCPP-Fe/Fe toward aromatic hydrazides.
the oxidative dehydrogenation of aromatic hydrazides reaction
of 2 and 3 were very different from that of 1, the product, 4
selected for cyclic and stability tests. Although several cycles
of catalysis had been performed, HZ@TCPP-Fe/Cu exhib-
ited satisfactory activity and maintained the morphological
and structural integrity (Figure S26, Supporting Information).
The PXRD patterns of recyclable HZ@TCPP-Fe/Cu cata-
lyst remained almost unchanged after five cycles (Figure S27,
Supporting Information).
For heterogeneous catalysis in hollow nanomaterials, the
channel on the thin shell is a necessary factor for the reac-
tants (products) to diffuse into (out of) the catalyst (products).
Specifically, the active ingredients can be enriched on the outer
surface of the skeleton to form layered hydroxides as the shell
of hollow nanomaterials with excellent stability by solvothermal
treatment, and then further modified by a “postsynthesis modi-
fication” process to obtain a multicomponent heterogeneous
catalyst.[58,59]
(yield
> 90%), isopropyl-2-phenyldiazene-1-carboxylate (5)
(yield: 24%) and tert-butyl-2-phenyldiazene-1-carboxylate (6)
(yield: 18%) were generated in low yield in 4 h after purifica-
tion by chromatography. 1H NMR analysis of the reactants
and products was showed in Figures S20–S25 (Supporting
Information). HZ@TCPP-Fe/Cu and ethyl 2-phenylhydra-
zine-1-carboxylate were chosen as standard catalyst and model
substrates to explore the effect of solvent on product yield
(Table S4, Supporting Information). We found that, among the
various solvents we studied, THF exhibited the best catalytic
performance. From exprimental results, the HZ@TCPP-Fe/Cu
could act as a reaction vessel to promote the oxidative dehydro-
genation of aromatic hydrazides reaction, featuring high chem-
oselectivity toward primary carbons among primary, secondary,
and tertiary carbons. And, HZ@TCPP-Fe/Ni and HZ@TCPP-
Fe/Fe also had similar catalytic performance. Subsequently,
the resulting and highest-performing HZ@TCPP-Fe/Cu was
We presented spontaneous phase transformation of ZIF-67@
TCPP-Fe from uniform rhombi dodecahedron to HZ@TCPP-
Fe under mild solvothermal conditions. The encapsulated
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Scheme 3. Possible reaction mechanism for oxidative dehydrogenation of aromatic hydrazides catalyzed by HZ@TCPP-Fe/Cu (where TCPP is
tetrakis(4-carboxyphenyl)-porphyrin) heterogeneous catalyst.
TCPP-Fe would gradually concentrate on the spherical shell
with the formation of the hollow structure, and then coordi-
nated with Cu, increasing the density of local active sites, while
avoiding aggregation and inactivation. Specifically, the acces-
sible TCPP-Fe/Cu bimetallic catalytic active sites in the internal
voids of HZ@TCPP-Fe/Cu were located on the axial positions
porphyrin motifs, which enclosed a confined hydroxides inter-
layer spacing, imposing more steric effect than TCPP-Fe/Cu
bulk or free Fe(Pc).
oxygen to accelerate the cleavage of O−O bonds; 3) based on
the synergistic effect of TCPP-Fe/Cu bimetallic catalytic active
sites, the high reactivity for the target reaction has been estab-
lished; 4) the TCPP-Fe/Cu bimetallic catalytic active sites
are encapsulated in the hydroxide thin shell of the hollow
HZ@TCPP-Fe/Cu, which bring additional dimensional selec-
tivity and diffusion differences, thus improving the selectivity
of catalytic products.
Molecules of dioxygen at a model Fe(II/III) center are acti-
vated to generate oxometal Fe(IV) species, and the generated
highly active oxometal Fe(IV) species can act as the active inter-
mediate in many catalytic circulation systems.[60] Meanwhile,
in the activity of heme-copper oxidase analog which combines
Cu and TCPP-Fe, the non-heme metals promote the breaking
of the O−O bond.[61,62] The higher redox potential and electron
density in the d-orbital of Cu lead to a weakening of O−O bond,
which is the key to the higher activity of oxidase than TCPP-Fe.
On the basis of the literature surveys,[63–66] the mechanism
of reaction was tentatively proposed (Scheme 3). The reaction
may be initiated by single-electron transfer between aromatic
hydrazides a and Fe(III) species to give the cation radical b.
Meanwhile, the Fe(II) species capture O2 to generate the super-
oxide Fe(III) species, and superoxide Fe(III) species extract
H+ from aromatic hydrazides a to obtain intermediate c and
hydroperoxo Fe(III) species. Subsequently, the hydroperoxo
Fe(III) species leads to the heterolytic cleavage of the O−O
bond, and the formation of a highly active oxometal Fe(IV) spe-
cies. The oxometal Fe(IV) species get an electron from inter-
mediate c to transform to Fe(III) species. The dehydration of
intermediate d provided the final product e.
3. Conclusion
In summary, an advanced strategy was developed for the con-
struction of HZ@TCPP-Fe/Cu heterogeneous catalyst through
morphology-controlled thermal transformation and rear
assemble processes. For HZ@TCPP-Fe/Cu, the catalytic activity
of TCPP-Fe was significantly increased when non-heme Cu was
involved in assembly. Meanwhile, compared with conventional
homogeneous catalysts, a pronounced selectivity enhancement
was achieved in oxidative dehydrogenation. Our finding estab-
lished the potential of hollow nanostructures encapsulated with
nanoentities in designing new hybrid inorganic−organic func-
tional materials for heterogeneous catalysis and other fields.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
The promising catalytic activity and selectivity for oxida-
tive dehydrogenation of aromatic hydrazine are ascribed to
synergistic effects of bimetal catalytic active site and the lim-
ited internal voids in HZ@TCPP-Fe/Cu due to the following
reasons: 1) the intermediate tetravalent ferric oxide can be
used as an ideal proton acceptor; 2) non-heme Cu can activate
Acknowledgements
D.Z. and P.D. contributed equally to this work. The authors are thankful
to the National Natural Science Foundation of China (21575115, 21705117,
and 22001193); the Program for Chang Jiang Scholars and Innovative
Research Team, Ministry of Education, China (IRT-16R61); the Program
of Gansu Provincial Higher Education Research Project (2017-D-01);
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Special fund project for the central government to guide local science
and technology development (2020); and the Program of Tianjin Science
and Technology Major Project and Engineering (19ZXYXSY00090).
The authors thank Prof. Wenping Hu at Tianjin University for valuable
suggestions and help.
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Conflict of Interest
The authors declare no conflict of interest.
Keywords
encapsulation, heterogeneous catalysts, high selectivity, porphyrin-Fe/
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