New heterometallic zirconium metalloporphyrin frameworks and their heteroatom-activated high-surface-area carbon derivatives

Article abstract of DOI:10.1021/jacs.5b00076

Four cubic zirconium-porphyrin frameworks, CPM-99(H₂, Zn, Co, Fe), were synthesized by a molecular-configuration-guided strategy. Augmentation of meso-substituted side arms (with double-torsional biphenyl rings) of tetratopic porphyrin linkers

Full text of DOI:10.1021/jacs.5b00076

Communication
pubs.acs.org/JACS
New Heterometallic Zirconium Metalloporphyrin Frameworks and
Their Heteroatom-Activated High-Surface-Area Carbon Derivatives
†
,‡
†
†
†
†
,†
Qipu Lin, Xianhui Bu,* Aiguo Kong, Chengyu Mao, Xiang Zhao, Fei Bu, and Pingyun Feng*
†
Department of Chemistry, University of California, Riverside, California 92521, United States
‡
Department of Chemistry and Biochemistry, California State University, Long Beach, California 90840, United States
*
S Supporting Information
7
contributes to the greater stability, it is of interest to design new
metallo)porphyrin ligands that could take the full advantage of
ABSTRACT: Four cubic zirconium-porphyrin frame-
(
works, CPM-99(H , Zn, Co, Fe), were synthesized by a
2
the 12-connectedness of Zr -cuboctahedron.
6
molecular-configuration-guided strategy. Augmentation of
meso-substituted side arms (with double-torsional biphenyl
rings) of tetratopic porphyrin linkers leads to a successful
implementation of zirconium-carboxylate frameworks with
cubic 2.5 nm cage. The hard-templating effect of Zr6-
polyoxo-cluster and uniformly embedded (metallo)-
An emerging application that could benefit from enhanced
stability of MOFs is their use as precursors for the synthesis of
electrocatalysts for oxygen reduction reaction (ORR), because
the resulting carbonized catalysts are more likely to retain the
high surface area of their less-collapse-prone precursors. ORR is
generally performed with Pt catalysts, however, new catalysts
8
porphyrin centers endow CPM-99 with highly desirable
properties as precursors for oxygen reduction reaction
based on earth-abundant elements are highly desirable. Noble-
metal-free electrocatalysts prepared from zeolitic imidazolate
(
ORR) catalysts. The pyrolytic products not only retain
frameworks (ZIFs) were found to show promising catalytic
9
the microcubic morphology of the parent CPM-99 but also
possess porphyrinic active sites, hierarchical porosity, and
highly conducting networks. CPM-99Fe-derived material,
denoted CPM-99Fe/C, exhibits the best ORR activity,
comparable to benchmark 20% Pt/C in alkaline and acidic
media, but CPM-99Fe/C is more durable and methanol-
tolerant. This work demonstrates a new route for the
development of nonprecious metal ORR catalysts from
stable metalloporphyrinic MOFs.
performances. The Co(Zn)N units in ZIFs allow doping of the
4
resulting carbon frameworks with metal and nitrogen to boost
10
the catalytic activity.
In comparison with ZIFs, porphyrin-MOFs have rarely been
used as precursor to make porphyrinic carbon, but they are
expected to have some advantages over ZIFs. For example, so far
ZIFs have only been made with a limited number of metal types
11
(
e.g., Zn, Co, and Cd), whereas porphyrin-MOFs can contain
more diverse metal types and oxidation states. In fact, framework
stability and active site insertion are two main concerns in the
8c,12
selection of precursors for ORR.
As shown here, ZPFs can
etal−organic frameworks based on zirconium (Zr-
excel in both aspects, because unlike tetrahedral coordination of
Zn/Co in ZIFs, macrocyclic effect allows better encapsulation of
metals in porphyrins.
M
MOFs) have attracted increasing attention since the
first report of UiO-66 built of 12-connected [Zr (μ -O) (μ -
6
3
4
3
OH) (O C) ] cluster and terephthalic acid and its isoreticular
Here, by restoring coplanarity of four carboxyl groups through
4
2
12
1
series. Recently, efforts have been extended to other ligand types
reverse rotation of an extra inserted phenyl group in each arm of
2
13
such as bent dicarboxylates or polytopic ligands. Compared to
the parent TCPP ligand, a series of ZPFs have been made.
other MOFs, an outstanding feature of Zr-MOFs is their
exceptional stability. However, so far only a small number of Zr-
These ultrastable, single-crystalline ZPFs can be made with or
without secondary metal ions and are denoted as CPM-99X
3
MOFs have been made, in part because of the difficulty in
(CPM = crystalline porous material, X = H
feature a binodal cubic net composed of 12-connected
Zr (OH) cuboctahedra linked by longer tetrakis(4-
, Zn, Co, Fe). They
2
1a
growing large-enough single crystals for structural studies.
Metalloporphyrins are found in nature as light harvesters,
6
O
4
4
oxygen carriers, and biocatalysts. So, their incorporation into
MOFs can greatly enrich the MOF functionalities. Indeed, high
carboxybiphenyl)porphyrin (TCBPP). The porphyrinic charac-
teristics as well as cube morphology were retained in the
carbonized composites. The ORR studies show that the
optimized iron-porphyrinic carbon (denoted CPM-99Fe/C),
derived by carbonization of CPM-99Fe at 700 °C, exhibits the
excellent ORR activity comparable to the commercial benchmark
4
porosity, ultrastability, and uniform distribution of catalytic sites
have made zirconium-(metallo)porphyrin frameworks (ZPFs) of
interest for potential applications in gas storage, biomimetic
5
catalysis, and clean energy. The latest studies of the ZPFs have
2
0% Pt/C in both alkaline and acidic media.
Solvothermal reactions of TCBPP-X (X = H , Zn, Co, Fe),
led to the growth and characterization of single crystalline
materials built of Zr clusters usually with reduced connectivity
2
6
5
,6
ZrOCl ·8H O and benzoic acid in N,N-diethylformamide for 5
(8 or 6). A closer examination reveals that the reduction in
2
2
days at 120 °C yielded single crystals of CPM-99 series. With the
connectivity is due to the symmetry mismatch between 12-
connected Zr -cuboctahedron and 4 noncoplanar benzoic
6
groups of the commercially available tetrakis(4-carboxyphenyl)-
porphyrin (= TCPP) spacer. Since the higher connectivity often
Received: January 5, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b00076
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
size of ∼0.20 mm, black cube-shaped crystals (Figure 1e) allowed
the structure determination by single-crystal X-ray diffraction.
CO uptakes at 273 K for CPM-99(H , Zn, Co and Fe) were
found to be 73, 60, 61, and 76 cm g , respectively (Figure 2a),
2
2
3
−1
Figure 2. (a) CO adsorption isotherms of CPM-99X series at 273 K.
2
(
b) N sorption isotherms of CPM-99Fe and CPM-99Fe/C at 77K.
2
which are at the high end among zirconium and/or porphyrin-
16
based MOFs. The similar isosteric heats of adsorption (Qst)
toward CO for the CPM-99 family indicate a limited effect of the
2
porphyrinic metal center on the gas uptake. However, the nature
of porphyrinic-metal sites greatly influence their ORR catalytic
properties as described below.
As shown above, CPM-99X exhibits desirable features for
serving as the precursor in the synthesis of the efficient ORR
catalysts. Thus, carbonization was performed to convert the
crystals of CPM-99X into (metallo)porphyrinic carbons,
Figure 1. (a) Augmented tetracarboxylic porphyrinic linkers, TCBPP-X
(
X = H , Zn, Co, Fe). (b) 12-connected Zr (μ -O) (μ -OH) (O C)
2
6
3
4
3
4
2
12
cluster. (c) Cubic cage with 2.5 nm edge length. (d) The 3D network
with Zr clusters shown as polyhedra and 3D cubic-cavity packing in
6
CPM-99Fe. Color scheme: Zr (teal); Fe (lime); O (red); N (blue); C
(
gray). (e) Photograph of CPM-99Fe.
denoted as CPM-99X/C (X = H , Zn, Co, Fe; the detailed
2
procedure in SI). A comparative study shows that CPM-99Fe/C
exhibits the highest catalytic efficiency and is therefore described
below in detail. The optimum annealing temperature was found
to be 700 °C, and PXRD data exhibit peaks at ∼25 and 44°
corresponding to the diffractions from the (002) and (100)
graphitic carbon planes, respectively, suggesting the presence of
long-range ordering in the carbon matrix. In addition to the
graphitic peaks, the pronounced diffraction peaks (much sharper
for samples treated at the elevated temperature such as 800 and
The 3D framework contains large cubic cages with an edge
length as large as 2.5 nm (Figure 1c), which is among the largest
14
known in noninterpenetrated cube-based MOFs. Each Zr6
cluster resides on one vertex and each face of the cube is capped
by one square TCBPP spacer. Such nanocubic cavities are packed
in a primitive cubic lattice (Figure 1d). Apart from the cubic cage,
the ftw-typed structure possesses another kind of cage, slightly
distorted octahedron with a cavity diameter of ∼1.1 nm,
comprising two Zr clusters in the axial sites and four TCBPP
900 °C), matching the cubic ZrO (JCPDS card 27-0997), were
6
2
ligands in the equatorial plane. In general, the pore size expansion
for cubic structures through the longer building unit leads to
lower framework stability and, in many cases, less desired
also observed. After leaching with dilute HF (5 wt % in water), no
PXRD signals of the zirconia residues were found. This
observation was further supported by the energy-dispersive X-
ray and X-ray photoelectron spectroscopy (XPS). The cube
morphologies of the CPM-99X/C were seen in the scanning
electron microscopy (SEM) images (Figures 3b and S35). The
optical properties of CPM-99X/C and CPM-99X were also
studied using solid-state diffuse reflectance spectroscopy
(Figures 3c, S21, and S39), indicating that the pyrolytic product
did somewhat retain the porphyrinic characteristic. Moreover,
the hollow cores enclosed by the lattice fringes, in agreement
with the visible diffraction rings of the selected area electron
diffraction pattern (Figure S38), were observed in the high-
resolution transmission electron microscopy image (Figure S37).
From these studies, together with the presence of mesopore
1
4a,15
interpenetration.
Interestingly, interpenetration is avoided
in CPM-99 with ftw topology, while the high stability is retained.
For CPM-99Fe, the PLATON-calculated solvent accessible
volume is 82%, indicating its highly porous nature. Despite such
large pores, it is resistant to heat treatment. After degassing at
200°, powder X-ray diffraction (PXRD) patterns (Figure S14)
remained nearly unchanged. Thermogravimetric analysis under
nitrogen flow indicates that CPM-99Fe possesses high thermal
stability up to about 450 °C. The porosity of CPM-99X was
examined by nitrogen adsorption at 77 K. As a representative
example, the quasi-type IV isotherm of CPM-99Fe exhibits the
second increase at P/P0 = 0.02, suggesting borderline
3
−1
mesoporosity. A N uptake of 450 cm g (STP) and a
detected by the N sorption (see below), it may be concluded
2
2
2
−1
Brunauer−Emmett−Teller (BET) surface area of 1030 m g
were observed (much lower than the theoretical value of 3324 m
g , possibly due to incomplete activation and/or potential
defects in the sample). The evaluation by the density functional
that Zr -polyoxo-cluster in CPM-99X serves as the hard template
6
2
to impede the agglomeration of the resulting electrocatalytic
nanofragments and helps create the nanoscale cavity upon
removal by acid wash.
−1
theory using the N sorption curve indicates that there are two
N adsorption−desorption isotherms at 77 K for CPM-99Fe/
2
2
types of pores, with sizes of 19 and 9 nm, attributable to cubic and
pseudo-octahedral cavities, respectively, which are consistent
with the crystallographic data when van der Waals contact is
considered. Other CPM-99 materials with different metal-
loporphyrin centers showed similar results (Figure S28). The
C showed a steep increase at low relative pressures,
demonstrating microporous characteristics (the BET surface
2
−1
area of 399 m g ). The slight hysteresis of the desorption
isotherm at high relative pressure suggests the presence of
mesopores centered at 3.8 nm as calculated from the desorption
B
DOI: 10.1021/jacs.5b00076
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Journal of the American Chemical Society
Communication
Figure 3. (a,b) SEM images for CPM-99Fe and CPM-99Fe/C. (c) UV−
vis diffuse reflectance spectra of CPM-99Fe and CPM-99Fe/C. (d,e)
XPS Fe 2p and N 1s spectra of the CPM-99Fe/C.
Figure 4. (a) CVs and (b) RDE polarization curves on CPM-99X/C and
20% Pt/C in 0.1 M KOH solution. (c) RDE polarization curves on
CPM-99Fe/C at different rotation rates. Inset: K-L plot of J− vs ω at
1
−1
different potentials. (d) RDE polarization curves on CPM-99X/C and
isotherm using the Barrett−Joyner−Halenda model. The surface
chemistry is of particular interest, and the electronic states of Fe,
N, and C were examined by XPS measurements (Figure S40). As
shown in Figure 3d (high-resolution Fe 2p XPS spectrum), 2p3/2
and 2p1/2 peaks occur at 710.8 and 722.9 eV, respectively, which
2
0% Pt/C in 0.1 M HClO solution. The catalyst loadings on the
4
−2
electrodes were 0.2 (in KOH) or 0.6 (in HClO ) mg·cm for the CPM-
9
mV·s .
4
−2
9X/C and 0.1 mg·cm for Pt/C (20 wt %, Alfa). The scan rate is 10
−1
12c
match well with those for Fe(II). The N 1s spectrum (Figure
e) is deconvoluted into three peaks at 398.5, 399.6, and 400.9
eV, corresponding to pyridinic, metal-coordinated, and graphitic
3
of electron being transferred was calculated to be 4.1(±0.1),
meaning that CPM-99Fe/C catalyzed a 4-electron ORR in 0.1 M
KOH solution. However, the slopes of plots for CPM-99Co/C
indicated that the electron-transfer number at 0.4 V is 3.2,
suggesting a mixed 4- and 2-electron-transfer process of ORR.
12b,17
nitrogen species, respectively.
Based on these data, the Fe
ions are believed to be associated with N atoms to form
18
catalytically active sites. In addition, the C 1s peaks for the
CPM-99Fe/C porous carbon were centered at ∼285 eV (Figure
S41) and were slightly asymmetric, which is a common
On CPM-99Zn/C and CPM-99H /C electrodes 2-electron
2
ORR process dominates, as their electron numbers transferred
12a
characteristic for nitrogen-doped graphitic carbon.
per O further reduced to 2.6 and 2.3, respectively (Figure S46).
2
To assess the electrocatalytic performance of the CPM-99-
derived materials in comparison to the benchmark carbon
supported Pt (viz. 20% Pt/C, Alfa), ORR activities were
evaluated using standard 3-electrode half-cell setup, and cyclic
voltammetry (CV), rotating ring electrode (RDE) polarization,
and chronoamperometric measurements were performed. All
potentials refer to the reversible hydrogen electrode scale. Prior
The CPM-99Fe/C catalyst exhibits long-term durability,
superior to Pt/C (Figure S47). Through continuous potential
cycling, the deterioration of CPM-99Fe/C after 10000 cycles
resulted in a slight peak current decrease with a retention of
∼93% (at cathodic peak, SI). The CPM-99Fe-derived electrode
was nearly free from the methanol crossover effect, which is
different from the significant impact of methanol on Pt/C
catalyst. Its durability is higher than Pt/C catalysts (with
retention index of 75.3%) and many nonprecious metal catalysts
at the similar measurement conditions.
to each experiment, the electrolytes were saturated with O or Ar,
2
respectively. In an Ar-saturated solution of 0.1 M KOH, the CV
curves within the entire potential range showed a featureless
slope for the cathodic current. In contrast, when the electrolyte
In addition to the basic medium (0.1 M KOH), the ORR
was saturated by O , the well-defined cathodic ORR peaks were
performance of the CPM-99-derived electrodes in O -saturated
2
2
observed, illustrating their effective ORR activities in alkaline
medium. Among the four samples, CPM-99Fe/C shows an ORR
peak at the most positive potential of 0.836 V. The electro-
catalytic activities of CPM-99X/C are in the order: Fe > Co > Zn
acidic medium (0.1 M HClO electrolyte) was investigated.
4
Again, the CPM-99Fe/C-electrode showed the best electro-
catalytic activity among four samples, with an onset potential
(0.875 V) comparable to the Pt/C (0.880 V), while the CPM-
≈
H (Figure 4a), supported by the linear sweep voltammetry
99Co/C, CPM-99Zn/C and CPM-99H /C electrodes exhibited
much lower onset potential and current density (Figure 4d). The
CPM-99X/C electrodes in the acid solution showed the same
2
2
measurements on a RDE. As shown in Figure 4b, CPM-99Fe/C
displays the best ORR activity among the CPM-99-derived
electrodes, exhibiting the onset and half-wave potential (0.950
and 0.802 V, determined at the polarization curves at 1600 rpm)
close to those (0.978 and 0.818 V) of the 20 wt % Pt/C. These
results reveal that CPM-99Fe/C has the ORR catalytic activity
comparable to commercial Pt/C catalyst.
activity sequence (Fe > Co > Zn ≈ H ) and were also free from
2
the methanol crossover effect. Based on the K-L equation, the
transferred electron number per O molecule for the CPM-
2
99Fe/C in the acidic medium was calculated to be 4.0(±0.1)
(Figure S52), suggesting a 4-electron ORR pathway even in the
acid condition. The prominent performance of CPM-99Fe-
derived electrode is believed to come from unique structural and
compositional features of its precursor, such as high porosity,
Moreover, the Koutecky−Levich (K-L) plots from RDE
polarization curves (Figure 4c) for CPM-99Fe/C between 0.6
and 0.1 V exhibited good parallel straight lines, and the number
C
DOI: 10.1021/jacs.5b00076
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
high thermal stability, uniform distribution of FeN active sites,
and hard-templating effects by the rigid framework.
In summary, we have described a molecular-configuration-
based materials design strategy for fabricating a series of
noninterpenetrating ZPFs with nanoscale cubic cavity. The
resulting high porosity and stability, coupled with uniformly
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4
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9
(
9Fe-derived carbonization product exhibited high ORR activity
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1
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(
1
(
activity of nanocomposite CPM-99Fe/C can be attributed to its
unique precursor with large cavity, high proportion of heme-like
center and hard polyoxozirconate cluster template. This work
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porphyrin-functionalized zirconium-carboxylate frameworks
with desired topology and porosity and reveals a new strategy
for developing high-performance electrocatalysts.
2
(
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2
ASSOCIATED CONTENT
Supporting Information
Experimental details, characterization data, crystallographic data
■
Zelenay, P. Acc. Chem. Res. 2013, 46, 1878. (d) Wang, H.; Dai, H. Chem.
Soc. Rev. 2013, 42, 3088. (e) Katsounaros, I.; Cherevko, S.; Zeradjanin,
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*
S
(
CIF files, CCDC#1044848−1044851), and additional tables
and figures. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
10) (a) Wang, S. B.; Hou, Y. D.; Lin, S.; Wang, X. C. Nanoscale 2014,
AUTHOR INFORMATION
Corresponding Authors
pingyun.feng@ucr.edu
xianhui.bu@csulb.edu
■
6
, 9930. (b) Wang, S.; Lin, J.; Wang, X. Phys. Chem. Chem. Phys. 2014,
1
6, 14656. (c) Wang, S.; Yao, W.; Lin, J.; Ding, Z.; Wang, X. Angew.
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Chem., Int. Ed. 2014, 53, 1034.
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Notes
The authors declare no competing financial interest.
(
12) (a) Yuan, S. W.; Shui, J. L.; Grabstanowicz, L.; Chen, C.; Commet,
ACKNOWLEDGMENTS
■
S.; Reprogle, B.; Xu, T.; Yu, L. P.; Liu, D. J. Angew. Chem., Int. Ed. 2013,
52, 8349. (b) Wu, Z. S.; Chen, L.; Liu, J.; Parvez, K.; Liang, H.; Shu, J.;
Sachdev, H.; Graf, R.; Feng, X.; Mullen, K. Adv. Mater. 2014, 26, 1450.
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Chem., Int. Ed. 2014, 53, 2433.
The research was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Materials Sciences
and Engineering under award no. DE-FG02-13ER46972.
(
13) Just prior to the submission of this manuscript, a similar strategy
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DOI: 10.1021/jacs.5b00076
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX