Robust Iridium Coordination Polymers: Highly Selective, Efficient, and Recyclable Catalysts for Oxidative Conversion of Glycerol to Potassium Lactate with Dihydrogen Liberation

Article abstract of DOI:10.1021/acscatal.5b01782

Along with the rapid expansion of the biodiesel industry to deal with the world energy crisis, inexpensive glycerol is also produced in large scale as the main byproduct in biodiesel production via transesterification. Much attention has been paid to the development of environmentally benign technologies for the transformation of glycerol to valuable DL-lactic acid and its derivatives. Herein, a series of NHC-Ir coordination polymers were readily synthesized via reaction of some structurally rigid bis-benzimidazolium salts with iridium precursors under alkaline conditions and were successfully applied as robust self-supported catalysts in the oxidative dehydrogenation of glycerol to potassium lactate with dihydrogen liberation. Extremely high activity and selectivity were attained in open air under the mild reaction conditions even with ppm-level loadings of the catalysts, which were readily recovered after reaction by simple filtration and reused for up to 31 runs without obvious loss of activity or selectivity. Probably owing to the effective suppression of inactive binuclear iridium species in a homogeneously catalyzed reaction, the catalysts assembled via self-supported strategy exhibited high selectivity and productivity for potassium lactate, with up to 1.24 × 10⁵ turnover numbers (TON) being attained even in large-scale reactions of neat glycerol at an elevated temperature. The high catalytic activity, recyclability, and scalability of the robust self-supported catalysts highlight their potential toward the development of practical technologies for transformation of glycerol to value-added chemicals.

Full text of DOI:10.1021/acscatal.5b01782

Research Article
pubs.acs.org/acscatalysis
Robust Iridium Coordination Polymers: Highly Selective, Efficient,
and Recyclable Catalysts for Oxidative Conversion of Glycerol to
Potassium Lactate with Dihydrogen Liberation
Zheming Sun,† Yaoqi Liu,† Jiangbo Chen,† Changyu Huang,† and Tao Tu*,†,‡
†Department of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433 China
‡State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin, 300071, China
S
* Supporting Information
ABSTRACT: Along with the rapid expansion of the biodiesel
industry to deal with the world energy crisis, inexpensive glycerol is
also produced in large scale as the main byproduct in biodiesel
production via transesterification. Much attention has been paid to
the development of environmentally benign technologies for the
transformation of glycerol to valuable DL-lactic acid and its
derivatives. Herein, a series of NHC-Ir coordination polymers were
readily synthesized via reaction of some structurally rigid bis-
benzimidazolium salts with iridium precursors under alkaline
conditions and were successfully applied as robust self-supported
catalysts in the oxidative dehydrogenation of glycerol to potassium
lactate with dihydrogen liberation. Extremely high activity and
selectivity were attained in open air under the mild reaction
conditions even with ppm-level loadings of the catalysts, which
were readily recovered after reaction by simple filtration and reused for up to 31 runs without obvious loss of activity or
selectivity. Probably owing to the effective suppression of inactive binuclear iridium species in a homogeneously catalyzed
reaction, the catalysts assembled via self-supported strategy exhibited high selectivity and productivity for potassium lactate, with
up to 1.24 × 105 turnover numbers (TON) being attained even in large-scale reactions of neat glycerol at an elevated
temperature. The high catalytic activity, recyclability, and scalability of the robust self-supported catalysts highlight their potential
toward the development of practical technologies for transformation of glycerol to value-added chemicals.
KEYWORDS: coordination assembly, iridium, lactate, oxidative dehydrogenation, self-support catalyst
progresses have been made in this vein, the selectivities and
efficiencies of most reported protocols still need to be
improved for their practical applications.18,19
INTRODUCTION
■
Due to indiscriminate consumption of fossil resources,
currently there is an ever-increasing interest in the development
of environmentally benign approaches to convert abundant
biomass into biofuels and fine chemicals.1,2 However, the direct
utilization of lignocellulosic biomass (including cellulose,
hemicellulose, and lignin) still constitutes a major challenge
in this field.3 On the basis of the recent achievements in the
oxidative dehydrogenation of simple alcohols,4 the relatively
less-explored selective conversion of sugar alcohols (polyols)
into value-added commodity chemicals along with hydrogen
generation was targeted as an upgrading strategy for the
biomass transformation.5,6 As one of the main byproducts in
the biodiesel production process and soap industry,7,8 glycerol
is a cheap bulk chemical readily available in large scale and has
been identified as one of the most important renewable carbon
sources.9 To date, a number of oxidative protocols for glycerol
transformation have been explored, to produce a variety of
value-added fine chemicals, such as acrolein,10 dihydroxyace-
tone,11−13 glyceric acid,14,15 hydroxypyruvic acid,16,17 and lactic
acid, in a cost-effective manner. Although some spectacular
As one of the most appealing biomass-derived platform
chemicals, lactic acid (LA) and its derivatives have found
numerous applications in chemical, pharmaceutical, food, and
detergent industries.20 There was a steady increase in the
annual consumption of lactic acid over the past decades, largely
as a result of the increasing global demands for biodegradable
polylactide materials.21 Whereas current industrial processes for
lactic acid production generally involve various bacteria-
mediated carbohydrate fermentation routes, they are still
plagued by disadvantages such as poor productivity/scalability,
complicated downstream process, and/or waste disposal.22
Hence the development of alternative approaches to lactic acid
production (e.g., from biomass waste products like glycerol) are
highly desirable. Although great efforts have been made to
Received: August 14, 2015
Revised: September 28, 2015
© XXXX American Chemical Society
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develop viable routes from sugars/cellulose to lactates,23−25
documented examples on the selective conversion of glycerol
into lactic acid are scarce. In 2010, Liu and co-workers reported
a bimetallic Au−Pt catalyst supported on TiO2 that successfully
attained the selective aerobic oxidation of glycerol to lactic
acid.26 Since then, many efforts have been devoted to the
development of new heterogeneous catalysts for this
reaction.27−32 However, major difficulties including low
productivities, poor selectivity, and harsh reaction conditions
have largely hampered their industrial implementation.
that preserves the structural features of its homogeneous
catalyst counterpart but is insoluble in common solvents and
thus obviates the need for an external support.38−41 Following
our recent interests in exploring the applications of novel
transition metal complexes in catalysis, supramolecular
chemistry, and material sciences,42−46 herein we would like to
report the preparation of main-chain iridium coordination
polymers from rigid bis-benzimidazolium salt precursors and
their application as self-supported catalysts in the oxidative
conversion of glycerol to lactic acid.
Inspired by the unexpected performance of bifunctional
ruthenium pincer complex 1 (Scheme 1) in the catalytic
RESULTS AND DISSCUSION
The synthesis of NHC-Ir coordination polymers 6a−e was
straightforward, as shown in Scheme 2. A number of bis-
■
Scheme 1. Representative Noble Metal Complexes for
Oxidative Dehydrogenation of Glycerol and Related
Transformations (TON = Turnover Number)
Scheme 2. Syntheses of NHC-Ir Coordination Polymers 6a−
e
benzimidazolium salts (5a−d) with a conformationally rigid
core structure were employed as precursors for the
corresponding bis-NHCs. This is to ensure that upon treatment
with a base, bis-NHCs with the two carbene sites geometrically
well-constrained and oriented opposite to each other would be
generated in situ by deprotonation.47−49 Such a bis-NHC
would bind to different Ir centers to generate an extended
supramolecular structure via coordination copolymerization
with an Ir precursor. Thus, condensation of commercial
available 1,2,4,5-tetraaminobenzene hydrochloride with formic
acid, followed by double alkylation of the resulting bis-
benzimidazole with iodomethane or n-butyl bromide, afforded
the corresponding bis-benzimidazolium salts 5a and 5b,
respectively, in nearly quantitative yields.50 Subsequent anion-
oxidative dehydrogenation of simple monoalcohols,4 recently
Beller and co-workers reported a homogeneous catalytic
reaction for conversion of glycerol to lactic acid along with
hydrogen generation, albeit with only a moderate selectivity
(67%).33 Simultaneously, in light of the successful utilization of
glycerol as both hydrogen donor and solvent in the catalytic
transfer hydrogenation of carbonyl derivatives in the presence
of N-heterocyclic carbene (NHC) iridium complexes 2
(Scheme 1),34,35 Crabtree and co-workers accomplished the
solvent-free catalytic transformation of glycerol to lactates in
very high selectivity (95%) with micromolar amount of NHC-Ir
complex 3 as the catalyst.36 It is noteworthy that a binuclear
NHC-Ir complex 4 was detected in the reaction, which slowed
the glycerol transformation. We envisaged that immobilizing
NHC-Ir complexes of the type 3 may effectively block the
formation of inactive binuclear species and enhance the
catalytic efficiency. The immobilization might also provide an
opportunity to recycle the precious iridium catalyst, which is
often essential for a large-scale transformation of biomass
feedstock. In this context, the recently developed “self-
supporting” strategy37 constitutes an attractive approach for
immobilization of homogeneous catalysts. Different from the
conventional approaches using various solid supports for
homogeneous catalyst immobilization, the self-supported
catalysts were constructed via supramolecular assembly of a
multitopic bridging ligand with an appropriate metal salt. A
coordination polymer is generated, with catalytic active sites
−
exchange of 5a and 5b with Et3OBF4 furnished BF4 salts of
bis-benzimidazolium 5c and 5d, respectively, in very good
yields. With these ditopic NHC precursors 5a−d in hand,
coordination polymerization with selected iridium(I) salts were
then carried out via a solution phase self-assembly process.
However, attempts to prepare polymer 6a via silver carbene
route51 were unsuccessful, as the precipitated Ag-NHCs species
can hardly react further with the Ir precursors. After a
comprehensive survey of reaction conditions (see Supporting
Information), lithium bis(trimethylsilyl)amid (LiHMDS) was
eventually found to be adequate for this purpose, and the in
situ-generated bis-NHC reacted smoothly with different
iridium(I) precursors in DMF, respectively, to give NHC-Ir
coordination polymers 6a−e in very good yields (93−99%).
The polymeric solids 6a−e were virtually insoluble in all tested
organic solvents and water, and their compositions were
consistent with the expected structures by IR, solid-state 13C
NMR, gel permeation chromatography (GPC), and elemental
analysis (see Supporting Information).
Morphological studies of 6b with a combined uses of
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) indicated that the bulk solids are composed
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of irregular particles with sizes of ca. 30−50 nm in diameter
Solid 6a was first tested for its catalytic activity in the
oxidative dehydrogenation of glycerol. The reaction was
conducted in water (0.3 mL) at 115 °C, with 15 mmol
glycerol, and 1.1 mol equiv of KOH in the presence of 2.3 mg
of 6a (300 ppm) as the catalyst. With evacuation of the released
dihydrogen gas, glycerol was selectively converted into
potassium lactate in very high yield (92%) and selectivity
(99%) after 36 h (Table 1, entry 1). This exciting result
prompted us to further optimize the reaction parameters,
including the water content, base additives, reaction temper-
ature, and catalyst loadings, and the results were shown in
entries 2−14 of Table 1. Only 45% yield and 90% selectivity of
potassium lactate to diol and other trace side-products were
obtained in the absence of water (entry 2), probably as a result
of the bare solubility of KOH in neat glycerol. However,
adjusting the water content to 1 or 0.1 mL, led also to less
satisfactory yields (80% and 49%, entries 3 and 4), respectively,
as compared with results in entry 1 under otherwise identical
conditions. The alkalinity of the reaction media, as modified by
the ratio of KOH base with respect to water, was found to have
a substantial impact on the outcome of the transformation.
Although the reaction with 0.1 molar equiv of KOH in 0.05 mL
of water afforded only a 16% yield of lactate (entry 5), the yield
approached 80% with 2.1 equiv of base in 1 mL of water (entry
6). No reaction occurred using K3PO4 or K2CO3 instead of
KOH under conditions otherwise identical to those in entry 1
(entries 7 and 8), suggesting that these weak bases may not be
effective in deprotonating the alcoholic OH of glycerol to the
alkoxide for iridium coordination. On the other hand, the use of
t-BuOK afforded no targeted product (entry 9). Reactivity with
NaOH is similar to that of KOH; however, the chemoselectivity
toward lactic salts formation is slightly inferior to that of KOH
(90% vs 99%, entries 10 vs 1). Thus, subsequent trials were
(Figure 1a,b). The energy dispersive X-ray (EDX) analysis
Figure 1. (a, b) SEM and TEM morphologies of the as-prepared
NHC-Ir coordination polymer 6b; (c) TEM morphology of polymer
6a; (d−f) SEM images of the recovered polymer 6b after the 2nd, 4th,
and the 15th run, respectively.
results of solid 6b were consistent with its elemental
composition. The powder X-ray diffraction (PXRD) patterns
did not show any obvious peaks and confirmed the amorphous
nature of the polymeric solid 6b. Similar results were also
obtained for solids 6a (Figure 1c) and 6c−e. Additionally,
solids 6a−e were insoluble in all common organic solvents and
stable against air, moisture, and remained intact after prolonged
heating (100 °C, 120 h) in water, suggesting a potential utility
as self-supported catalysts.
a
Table 1. Oxidative Dehydrogenation of Glycerol Catalyzed by NHC-Ir Coordination Polymers
b
c
entry
[cat.] (ppm)
base (equiv)
H2O (mL)
temp (°C)
yield (%)
select. (%)
1
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 300
6a: 100
6a: 150
6a: 300
6a: 300
6b: 300
6c: 300
6d: 300
6e: 300
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (0.1)
KOH (2.1)
K3PO4 (1.1)
K2CO3 (1.1)
t-BuOK (1.1)
NaOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
KOH (1.1)
0.3
0
115
115
115
115
115
115
115
115
115
115
115
115
165
80
92
45
80
49
16
80
<1
<1
/
99
90
97
98
98
97
/
2
3
1.0
0.1
0.05
1.0
0.3
0.3
0
4
5
6
7
8
/
9
/
10
11
12
13
14
0.3
0.3
0.2
0.3
0.3
0.3
0.3
0.3
0.3
95
77
94
89
81
98
62
37
16
90
95
95
93
98
99
89
93
80
d
15
115
115
115
115
16
17
18
a
Reactions were carried out in air with glycerol (1.1 mL, 15 mmol), solid catalyst 6 (2.3 mg, 300 ppm), base (0.1−2.1 equiv), and H2O (0−1 mL) at
b
115 °C for 36 h. Yield was calculated on the basis of potassium lactate formation determined by 1H NMR spectroscopy with NaOAc as an internal
c
standard, and further confirmed by hydrogen gas evolution quantified by gas buret measurements. Determined by 1H NMR with the percentage of
d
yield/conversion. For 24 h.
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Figure 2. (a) Recycling and reuse of the NHC-Ir coordination polymer solids 6b in oxidative dehydrogrenation reactions of glycerol: 15 mmol scale
with 300 ppm solid catalyst, 1.1 equiv of KOH, and 0.3 mL of H2O at 115 °C for each run; (b) corresponding reaction profiles with recycled catalyst
6b (the volume of hydrogen evolution was measured using a gas buret).
performed using KOH as the base in the presence of water.
However, reducing the loading of catalyst 6a to 100 ppm
resulted in a decline in yield to 77% (entry 11). To our delight,
a catalyst loading of 150 ppm was adequate to achieve 94%
yield of the product with 95% selectivity, albeit at a cost of
extended reaction time (TON: 6600, 72 h, entry 12). Further
elevating or lowering the reaction temperature gave a reduced
yield in either case (entries 13 and 14).
(Figure 2a). The reaction profiles for each runs were monitored
by the hydrogen evolution with a gas buret (selected profiles
were depicted in Figure 2b; for other profiles, please see
Supporting Information). Interestingly, after a certain run-in
period, along with the growth of recycling number, the reaction
rate reached a stable state after the ninth run, in which only 12
h was required to complete the conversion of glycerol.
Investigated by SEM, the morphology of the recovered catalyst
solids 6b after the second run was similar to that of the new
prepared polymer (Figures 1a vs 1d). A similar but slightly pale
image was observed with the recovered mixture after the fourth
run (Figure 1e), and then no obvious change was found with
the catalyst samples of later runs (Figure 1f). The sustained
reactivity of catalyst 6b may be caused by its robust structure.
In contrast, the self-supported catalysts 6a could only be reused
up to 15 times, and subsequent recyclings necessitated
extended reaction time (48 h) for satisfactory yields (see
Supporting Information).
In order to get deeper insight into the nature of the catalysis,
the filtrates after every run were analyzed by inductively
coupled plasma−atomic emission spectroscopy (ICP-AES) (see
Supporting Information, Table S2) for potential iridium
leaching. In the case of reactions with solid 6b, except for the
first run wherein the concentration of leached iridium was
0.021 μg/mL (ca. 0.025% of the original Ir catalyst), no further
iridium leaching was detected in the subsequent recycling runs.
Similar results were also obtained with the self-supported solids
6a and 6c. Given the sustained reactivity and high selectivity of
these self-supported catalysts in the consecutive recycles, it
seems likely that the initial iridium leaching may be the trace
amount of the inactive iridium species (or precursors)
entrapped in the self-supported matrix cavities during metal
coordination process. Furthermore, the IR spectra of the
recovered solids are somewhat different from that of the freshly
prepared catalyst 6b. While absorption bands of CO close to
1984 and 2047 cm−1 still exist, the intensities of the absorption
decreased along with the recycling runs (Figures S36−S38).
These results suggested that CO ligands in the self-supported
solids 6b might be labile, and gradually dissociate from metal
during the recycling to reveal more vacant sites for subsequent
catalytic turnover, which may provide a rationale for the
accelerating profiles shown in Figure 2b as well as the catalytic
lag observed in the first run with solids 6b (blue dotted line). In
order to exclude the possibility of homogeneous catalysis by the
leached iridium species, a control experiment was performed
The set of reaction conditions optimized for 6a were
subsequently extended to 6b−e, to evaluate the effect of
catalyst compositions on the transformation. The results
obtained with catalyst 6b were interesting, as the catalyst was
found to be more active than solid 6a under otherwise identical
conditions, affording better yield of 99% in shorter reaction
period (24 h) with no loss of chemoselectivity (entries 15 vs 1).
The reactivity difference might reflect the subtle difference in
their structures, suggesting that the vacant sites for substrate
activation on Ir centers should probably be more easily
accessible in catalyst 6b than in 6a, because CO as a
monodentate ligand can be expected to be more labile than
the bidentate COD ligand in the NHC-Ir complexes. The
reaction outcomes with 6c−e (entries 16−18), however, were
inferior to those obtained with either 6b or 6a, presumably as a
result of the differences in the microenvironments in the solids.
−
In contrast with noncoordinating BF4 anion, the iodide may
persistently locate in the proximity of Ir center, leading to lower
yield and selectivity (entries 16 vs 15). Along with the steric
effect, possible Hofmann elimination of n-Bu group under basic
conditions led to catalyst decomposition and unsatisfactory
results (entries 17 and 18), whereas such β-H elimination is
impossible in the NHC-Ir solids containing the Me group and
may explain the higher reactivity exhibited by solid catalysts 6a
and 6b.
Given the high solubility of potassium lactate in aqueous
media, water was added after reaction completion (see
Supporting Information). After simple centrifugation, filtration,
and washing with additional water, the self-supported catalyst
solids were readily recovered quantitatively. The isolated
potassium lactate was pure enough for NMR analysis. To our
delight, the recovered catalyst solids 6a and 6b can be directly
reused in the second run of glycerol dehydrogenation, simply
by further charging with base and water, without significant loss
in selectivity or yield of the potassium lactate product. Taking
account of its high catalytic efficiency, the self-supported
catalyst 6b was selected and could be reused up to 31 times
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with 300 ppm iridium precursor [Ir(acac) (CO)2] under the
standard reaction conditions, and in this case, only a 6% yield of
potassium lactate was obtained. Since most of the absorbed
iridium species in the catalyst matrix was washed out after the
first use, the influence of metal leaching in subsequent runs was
negligible.
ICP-AES analysis of all aqueous filtrates of the large-scale
experiments still revealed no detectable leaching of Ir species.
CONCLUSION
■
In summary, a series of robust NHC-Ir coordination polymers
based on the rigid bis-benzimidazolium salts has been
successfully synthesized, which exhibited extremely high
catalytic activity and selectivity in the oxidative dehydrogen-
ation of glycerol to potassium lactate with ppm catalyst loadings
under relatively mild reaction conditions. These self-supported
catalysts were readily recovered from the reaction mixture by
simple filtration and were reused up to 31 times without
obvious loss in activity or selectivity. In comparison with their
homogeneous analogue, the self-supported catalyst demon-
strated a sustained reactivity and high selectivity for potassium
lactate formation even at elevated temperature, with TON
value up to 1.24 × 105 being attained in multigram scale
reactions. The high catalytic activity, recyclability, and
scalability of our newly developed robust self-supported
catalysts are attractive features for an alternative route to the
environmentally benign transformation of glycerol to value-
added chemicals and highlight their promising potential for
practical implementation. Further studies on the conversion of
other polyols, sugars, and biomass-related materials with these
self-supported catalysts and direct transformation without base
are under way in our laboratory.
Encouraged by the remarkable performance of the self-
supported catalysts, a scale-up test was then carried out with
catalyst 6b. Although some amount of water was found
necessary for base dissolution in the microscale reaction as
described above (Table 1, entries 1, 3, 4 and 15), the reaction
with a higher relative amount of glycerol (27.6 g, 0.3 mol) in
the presence of 18 ppm solid 6b and 1.1 equiv of KOH
proceeded readily at elevated temperature (180 °C) in the
absence of water, to give a 97% yield potassium lactate with
95% selectivity amounting to a TON value of 5.39 × 104. The
turnover frequency (TOF) could reach 2800/h during the first
4 h under such larger scale, in which 1.44 g of lactate and 382
mL of H2 were produced per hour. When the reaction
temperature was decreased to 165 °C, similar results were
observed; however, the hydrogen generation rate was much
slower. A comparative study of self-supported catalyst 6b with
its homogeneous counterpart, NHC-Ir complex 7 (Figure. 3)
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b01782.
Synthetic and experimental details, 1H, 19F and 13C
NMR, IR and ESI-MS spectra for crucial compounds,
PXRD, ICP-AES, and GPC studies as well as SEM and
TEM morphologies for coordination polymers (PDF)
Figure 3. Reaction profiles of oxidative dehydrogenation of glycerol in
the presence of 18 ppm of the NHC-Ir coordination polymer solid 6b
and equimolar amount of homogeneous catalyst 7.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: taotu@fudan.edu.cn.
Notes
The authors declare no competing financial interest.
was conducted under otherwise identical conditions. However,
even under the identical reaction conditions at 165 °C for 96 h,
only 94% conversion of glycerol and an 84% yield along with
89% selectivity of potassium lactate were observed. When
homogeneous catalyst 7 was applied under the standard
conditions in a small scale (300 ppm in 150 mmol of glycerol),
only 87% yield of the lactate with 95% selectivity was obtained
after 36 h (detail see the Supporting Information, Table S1,
Entry 25). During the first 4 h of the oxidative dehydrogen-
ations, the catalytic profile of catalysts 6b and 7 were quite
similar (TOFs were 154/h and 138/h, respectively); after 24 h,
the TOFs were 125/h and 98/h, respectively. These results
further indicated that the self-supporting catalysts demon-
strated an enhanced catalytic activity and selectivity for the
transformation in terms of long runs as compared with their
homogeneous counterpart, probably owing to difficulties in
formation of inactive binuclear Ir species in solid 6 as a result of
their geometrical constraints.52 When the catalyst loading was
further decreased to 5 ppm and the reaction was carried out in
0.8 mol scale with 100 h at 180 °C, a 63% yield was obtained;
meanwhile, an unprecedented 1.24 × 105 TON was attained.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Nos. 91127041, 21172045, and
21572036), the Shanghai International cooperation Program
(No. 14230710600), the Doctoral Fund of Ministry of
Education of China (No. 20130071110032), the Changjiang
Scholars and Innovative Research Team in University (No.
IRT1117), and the Department of Chemistry, Fudan
University, is gratefully acknowledged.
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DOI: 10.1021/acscatal.5b01782
ACS Catal. 2015, 5, 6573−6578